Slowly Pulling Facts from Black Holes

lee1 writes "Astronomers have proven the existence of the event horizon, the 'point of no return' that surrounds black holes. An MIT and Harvard team said they showed its existence by looking for X-ray bursts from neutron stars and more compact objects thought to be black holes." Relatedly beuges writes "IOL is reporting that by tracking the death spiral of cosmic gas at the center of a galaxy called NGC1097, scientists figured that material moving at 177 000km an hour would still take eons to cross into a black hole. 'It would take 200 000 years for gas to travel the last leg of its one-way journey,' Kambiz Fathi of Rochester Institute of Technology told reporters at a meeting of the American Astronomical Society."

Comments

What is inside a black hole?

[filenavigator]

The thing I find amazing about research with black holes is that effectivly we will never be able to prove this by direct observation. I guess scientists could tell us that superhuman monkeyoid hippies live at the bottom - and we can vist them - woot woot!

Re:What is inside a black hole?

[LordNightwalker]

I wouldn't be too sure about that. For centuries man thought it impossible to prove the existence of atoms, things so small one could never discern them not even with the best of microscopes. Right now we know about the existance of even smaller things in our universe...

So what makes you think that we'll never be able to prove the existence of places we could never visit in physical form, not even in the strongest and most powerful of spaceships? ;)

Re:What is inside a black hole?

[wass]

The thing I find amazing about research with black holes is that effectivly we will never be able to prove this by direct observation.

And also without direct observation of the poster by the scientists in question, since they already presented their poster the day before yesterday at the poster session at AAS. When I first read in TFA that they'd be presenting their results at the AAS Winter Meeting , I thought it might be cool to see the talk or poster, but alas, it already happened.

Although I'm actually a condensed-matter physicist, as per the other posts on the phase-transitions thread I just typed up. But my girlfriend will be at AAS, so that's always a good excuse to anonymously crash the festivities :-)

Re:What is inside a black hole?

[Combas]

What makes you so certain Atoms really do exist?

Oh sure, we have mathematical formulas that prove that "something" does exist and we can even do some manipulation, but what if it turns out that all this stuff thats so tiny that we can't see it isnt at all what we think it is even though it does respond in predictable ways that we can understand and measure.

Its like an old programming analogy I once read, picture a man in a box who only understands english, but has a big book on Chinese. Now programmers feed the box instructions written in chinese and the man inside is able to look up words in his book, and then he is also able to write chinese replys using the book, but he doesnt actually understand the chinese language itself. The people on the outside would think he understands chinese because they send in chinese messages and get chinese replys in return.

Maybe atoms are like that as well. We poke atoms, and they respond in a predictable fasion, but since we cant actually see them how do we really know what we're poking?

Re:What is inside a black hole?

[Anonymous Coward]

That's a classic defense of dualism you're quoting, and the fatal flaw is only considering the programmer as an individual. If the book contains enough information to trick outsiders into thinking a chinese speaking person is inside, the book itself can be considered an individual, living of course only in the environment created by the actions of the programmer, but an individual none the less.

So, essentially, even if the atoms are only emulating on something more complex or different, we are still working with what we conceive of as atoms, simply at a lower level. The intermediate stage may be interesting or it may not, especially if it remains undetectable. For instance, assume a perfect simulation of the universe on a computer, the inhabitants of the universe are, philosophically, no less real than we are, they see the same phenomena as we do and think they live in a universe of atoms despite the fact that they're just data structures and algorithms emulating what our atoms would do.

Re:What is inside a black hole?

[helioquake]

It's hard to resolve the event horizon of a black hole spatially in X-rays, but NASA has one or two R&D dedicated to address such issues. Note that the angular resolution required to "see" such small angular size is far better than what the HST can achieve today (although such comparison may not be so prudent).

Re:What is inside a black hole?

[Travis Mansbridge]

Things that were predicted about atomic interaction were proved true much later. I assure you that scientists do not assume something is completely true without rigorous attempts at discounting its nature, and atomic theory as it is presented today is generally fully accepted worldwide. Of course, there are still things we don't understand, but we wouldn't try to slap a name on them and call it factual at the first sign. That's medieval.

Re:What is inside a black hole?

[Combas]

Im was not trying to suggest that scientists are guessing, obviously they do real scientific studys and check and recheck data and the results.

Maybe this example will help to clarify what I'm talking about, and this is also in reply to the parent poster who said that the programmers chinese book would have to be thought of as an individual.

Every human cell contains enough DNA information to make a complete copy of the entire human being. A cell with its DNA can be thought of as a chinese book.

Now what if every atom has atomic DNA and we just dont know it? What if there is something internal to every atom that is even smaller than electrons. This atomic DNA would be to an atom what regular DNA is to a human cell.

Perhaps this atomic DNA is even more complex than cellular DNA. Perhaps there is enough information in the Atomic DNA of a single atom to make a perfect copy of the entire universe.

Ok, pretty far out on the limb with that one, but hey, just trying to say that we really dont know and there's a chance we never will know, but meh.. uh, what was my point again?

Re:What is inside a black hole?

[LegendLength]

I don't really understand the chinese book riddle, because anyone who learns a foreign language knows that you cannot form proper replies from a dictionary alone (you need a grammar reference, list of exceptions and current slang etc.). So it seems to depend on how you define the dictionary.

Now what if every atom has atomic DNA and we just dont know it?

I think most scientists would agree with you that this is even likely (strings vibrating at certain frequencies are thought to be electrons at one frequency and protons at another etc.).

just trying to say that we really dont know and there's a chance we never will know, but meh.. uh, what was my point again?

I think your point was that we should not say that atoms are 'proven' without also adding that their make-up is still being researched at smaller scales.

Re:What is inside a black hole?

[fbjon]

Don't get stuck on the details, or the analogy breaks down. The book is simply an large, imaginary tome on the Chinese language, containing everything needed to translate to/from English within.

Re:What is inside a black hole?

[Anonymous Coward]

The book doesn't translate from Chinese, it just magically has the appropriate response to any inquiry. The thought experiment is actually designed to pose questions about artificial intelligence.

http://en.wikipedia.org/wiki/Chinese_room

Re:What is inside a black hole?

[LegendLength]

Still after reading that link I get stuck on the first line, where he introduces a magic dictionary/program into the room that somehow outputs correctly formed replies. Such a dictionary does not exist and if it did then it really would describe the language entirely.

So as I see it, Searle invents a hypothetical program which, if it actually existed, would prove him exactly wrong! (prove that AI is possible, i.e. the program would pass the turing test). I don't get it.

Re:What is inside a black hole?

[lgw]

I think you do get it! Turing's test is just a better way of stating the problem than the Chinese Room.

The often overlooked and very insightful aspect of Turing's test is that it doesn't just apply to "artificial" intelligence. In his formulation, by communicating with an unknown entity, you can determine by conversation whether or not it is at least as intelligent as you.

While that's an interesting test for an AI (though many AI researchers have problems with it today) it also makes a very different point: given two people, both smarter than you, you can't tell which of the two is the smartest through direct interaction! Much of the scientific method and the culture of science can be understood from this simple observation.

Re:What is inside a black hole?

[quanminoan]

The HST doesn't really do too much x-ray observations as far as I know- the Chandra telescope has amazing x-ray resolution on the other hand.

Re:What is inside a black hole?

[helioquake]

The HST cannot do any X-ray observation; and my point is that the spatial resolution of an X-ray telescope must be a few order of magnitude better than that of Chandra.

Re:What is inside a black hole?

[Anonymous Coward]

i see - my bad

Re:What is inside a black hole?

[Anonymous Coward]

Who would have thoughe the words Condensed Matter Physicist and Girlfriend would be used in the same post. Go Dude!

Slowly Pulling Facts from Black Holes?

[Anonymous Coward]

Not another story about SCO...

Re:Slowly Pulling Facts from Black Holes?

[Anonymous Coward]

A troll goatse story?

It all depends.

[Phariom]

"...material moving at 177 000km an hour would still take eons to cross into a black hole."

Unless you happen to throw in some McAfee stock...in which case it will only take a few days.

Great!

[Voltageaav]

So even if God does answer my prayers and my boss gets sucked into a black hole, it'll take forever.

Re:Great!

[Firehed]

Nah. If he's fat enough, his own gravity will pull the black hole closer to him as well. He could be the first thing to ever break the speed of light. That or he tears the fabric of space-time and opens up a gateway to a parallel universe. Either way, he's no better off for it, and humanity may very well be.

Re:Great!

[Reapman]

I don't know why, but I SERIOUSLY misread that as: "So even if Google does....." I think I need to take a break from the Internet.

Re:Great!

[Nefarious Wheel]

It's good to know that a single black hole can contain an infinite number of bosses ... they're not just packed tightly into the space, they're spread out over 17,000 years too.

Hmm... better order two.

orbit?

[HermanAB]

Actually, I don't understand it at all. Why would the matter spiral in. Why won't it stay in orbit? What is slowing the matter down to make the orbit decay?

Re:orbit?

[Anonymous Coward]

Friction.

Re:orbit?

[Rickler]

The greater the gravity the slower time is, for the viewer that is.

Re:orbit?

[Daniel Dvorkin]

Um ... friction? Even interplanetary space in the orbit around a normal star isn't a perfect vacuum, after all; the Earth's orbit is decaying, just veeery slowly (and it would take much, much longer than the projected lifetime of the Sun for such decay to make us spiral appreciably inward.) Around a black hole, you can expect the gas to be relatively dense, and friction to have a relatively fast effect on its orbit. I say "relatively" because, of course, the density is still very low and the time scales are still very large by terrestrial standards -- at least until you get right up near the event horizon, at which point things just get weird.

Re:orbit?

[HermanAB]

Makes me think of this little ditty about high falutin English:

"Father, I have spilt some butter. What shall I do?"

"Rub it briskly with a woollen cloth my son. For friction generates heat which quickly volatizes the sterine matter."

Re:orbit?

[HermanAB]

Hmmm - So friction in the revolving gas could will cause it to heat up and possibly glow, while slowing down its rotation, causing it to cross the event horizon and fall in?

Re:orbit?

[Sterling Christensen]

I tried to look up "volatize" and "sterine", and discovered they were misspelled:
"Rub it briskly with a woollen cloth, my son. For friction generates heat which quickly volatilizes the stearine matter."

Also, I don't get the joke... can you explain?

Re:orbit?

[csrster]

Iirc, in general relativity orbits decay slowly due to the emission of gravitational radiation. (Does this explain why the Enterprise can't remain in orbit around a planet for more than a couple of hours without engine power?)

Re:orbit?

[Gaccm]

Hmmm - So friction in the revolving gas could will cause it to heat up and possibly glow, while slowing down its rotation, causing it to cross the event horizon and fall in?

Yes. The only way something can fall into a black hole is by losing energy to fall it. If it doesn't lose any energy it will keep revolving around the black hole. Same thing with Earth, if Earth was in the middle of a cloud of gas that could eat away at very large amounts of the Earth's momentum, then the Earth could spiral into the Sun. Since that gas isn't there our Earth keeps revolving around the Sun, which is good for us.

Re:orbit?

[Anonymous Coward]

It sound like a mnemonic for something, maybe some recondite astrophysical constant.

Re:orbit?

[Anonymous Coward]

If you're set on being a pedantist, no-one English, let alone someone high-falutin', would use a z in such an ugly manner. The correct word is 'volatilises'.

Re:orbit?

[radu124]

Nope, that won't work, you can't expect the black hole to have an atmosphere, any gas that is near the black hole and is not orbiting itself would just fall into the black hole.

What there is, is a continuous flow of matter and light (which also has mass) towards the black hole, I believe (though I'm no expert) that that would push things into the black hole.

I think some theories say that there are also some high energy particles coming from the event horizon of the black hole, but I think they're not enough to balance what is flowing in.

Re:orbit?

[amateur bore]

If you're going to send me off to dictionary.com with your post. At least get your spelling right please :) http://dictionary.reference.com/search?r=2&q=stear ine

Re:orbit?

[mangu]

The correct word is 'volatilises'

No, it's not. The general rule is: if the word is derived from another that has an 's', the derived word will have an 's', otherwise it's spelled with a 'z'. Since 'volatile' has no 's', then it's 'volatilize'.

Re:orbit?

[Cyberax]

Because of some obscure effects of general relativity, and not because of gravity waves as some people think. I can write you differential equations of, but I'm not going to write them here in ASCII art.

These effects are extremely weak in our Solar System, but they can be observed in perihellion precession of planets ( http://en.wikipedia.org/wiki/Precession#Precession _of_planetary_orbits ). Right now Gravity Probe B ( http://einstein.stanford.edu/ ) is in the final stage of experiment which aims to check the gravitomagnetic effect which is another manifestation of GR (and is partially responsible for decay of black hole orbits).

Re:orbit?

[DiamondGeezer]

Because there's a critical distance away from the black hole below which matter cannot orbit because the orbital speed would be greater than the speed of light. So anything orbiting that reaches the critical orbital radius (which depends on the black hole's mass) will be sucked in.

In that sense, it shows how differently General Relativity is compared to Newtonian Mechanics.

See this site for a visual demonstration and an explanation.

By the way, I've no idea where "the 200,000 years to hit the event horizon" comes from. According to GR, from our frame of reference it would take an infinite amount of time to hit the event horizon.

Re:orbit?

[volatilises]

Since 'volatile' has no 's', then it's 'volatilize'.

In English the correct spelling is 'volatilises'. Worldwide, the majority of English speakers use British English, not American English.

Please keep your spelling, 'culture', and 'cuisine' to yourselves. The rest of the world isn't interested... just ask Baghdad.

Re:orbit?

[Phanatic1a]

Relativity.

The closest stable orbit around a black hole is at a distance three times the Schwarzchild radius. Closer orbits exist, but they're unstable, the slightest perturbation in them will result in either an escape to infinity or an intersection with the event horizon. At 1.5 Schwarzchild radii, you have the photon sphere; at this distance, orbital velocity equals c, and it's unstable so nothing stays there. Anything closer than 1.5 radii, there are no orbits possible.

Re:orbit?

[Anonymous Coward]

Oooh! Look at all the 4s!

Re:orbit?

[Bloke down the pub]

The general rule is: if the word is derived from another that has an 's', the derived word will have an 's', otherwise it's spelled with a 'z'.

I call bullshit. If you disagree, I'll neutralise, pulverise, and even atomise you.

If there is a rule, it's that British use -ise and Americans (living in areas colonised by them) use -ize. The latter is becoming more accepted right of the pond, though.

Re:orbit?

[Bloke down the pub]

A much more likely explanation is storyarc radiation causing a change in polarity of the plotdevicicon field.

Re:orbit?

[astralbat]

Why then is the moon's orbit decaying? As I understand it, we'll eventually lose the moon altogether

Re:orbit?

[Anonymous Coward]

The Moon loses orbital energy due to tidal friction via its interaction with the Earth (the same reason why it is now tidally locked with the Earth).

Re:orbit?

[Anonymous Coward]

The 4th. dimension (time) is warped by black holes.

Gravity is slightly less at the top of the earth - everest etc. and according to relatvity of you represent the earths magnetic field with a plane - this plane has a big bowl shape for the Earth. The boundaries of this bowl are like a slope - Earths gravity is not that intense (and so the change in time is insignificant).

But, Black holes are an enormous source of gravity so it's not really slipping into a bowl in ths case, it's more like going off a steep ravine. Here, the plane is not dented by a bowl - but rather a "hole".

The change in time is of course very big here. Just as time is slightly quicker with low gravity- everest is slightly quicker, the moon is a bit quicker etc..

so too is this the case with a black hole. The gravity is huge and it occurs at this "ravine" so that time becomes much slower BUT it becomes much slower over a very quick period in time.

Scientists simulate this with an elsatic trampoline like surface. Roll a big heave ball in there and it makes a dent. Roll a huge one in an it bursts the fabric!

So, when you go towards a black hole, time rapidly changes - we acall this the event horizon.

Also, this is why Russian Scientists don't refer to black holes as black holes - they call them frozen stars. Why? Becasuse time appears to freeze at the event horizon. Once you cross the event horizon time rapidly slows down or "freezes" Anyway we would advance greatly if we could to harness this energy!

Re:orbit?

[CrimsonAvenger]

Because there's a critical distance away from the black hole below which matter cannot orbit because the orbital speed would be greater than the speed of light. So anything orbiting that reaches the critical orbital radius (which depends on the black hole's mass) will be sucked in.

True, so far as it goes.

But...

The Event Horizon marks the surface where Escape Velocity is lightspeed. Circular Orbital Velocity is only sqrt(0.5) times Escape Velocity, so orbital velocity at the Event Horizon is less than lightspeed.

In other words, that "critical orbital radius" you mentioned in beyond the Event Horizon, thus anything there has already been sucked in....

Re:orbit?

[DiamondGeezer]

The Event Horizon marks the surface where Escape Velocity is lightspeed. Circular Orbital Velocity is only sqrt(0.5) times Escape Velocity, so orbital velocity at the Event Horizon is less than lightspeed.

Yes, but there's a big difference between escape velocity which is radial, and orbital speed which is tangential.

As my link above shows, above the critical limit for being able to orbit is outside the event horizon (actually its at 3MG/c^2 expressed in metres from the singularity, whereas the Event Horizon for a non-spinning black hole is at 2MG/c^2)

If you think about it, the vector sum of tangential and radial velocity cannot be greater than c, the speed of light.

Re:orbit?

[Anonymous Coward]

You've got it backwards. The critical orbital radius (the innermost stable circular orbit, or ISCO) is outside the horizon, not inside. It is located at a radius 3 times larger than the Schwarzschild radius. There is no orbital velocity at the horizon; no orbits exist there, unless you count "falling into the black hole".

Incidentally, your formula for circular orbit velocity in terms of escape velocity is Newtonian and doesn't apply in strong field relativistic gravity.

Re:orbit?

[Tango42]

it's root(2), not root(0.5), i think, so it's outside the event horizon.

Re:orbit?

[roman_mir]

Since that gas isn't there our Earth keeps revolving around the Sun, which is good for us. - speak for yourself! Where I am right now, I wouldn't mind getting closer to the Sun, maybe even falling in (assuming it is the night time.)

Re:orbit?

[1shoonya0]

No, I think that is because of the drag near the planets orbit.

Re:orbit?

[smithmc]

  Please keep your spelling, 'culture', and 'cuisine' to yourselves.

A Brit, making comments about cuisine? Holy Pot-Kettle-Black, Batman. That's rich. And as for culture? Besides The Office, what have you guys done for us lately (i.e. post-Shakespeare; well, OK, post-Python)? Page 3? The Spice Girls?

Re:orbit?

[Anonymous Coward]

How we will laugh when Iran nukes your silly ass. Sorry, arse. Only a real man has an ass.

Re:orbit?

[lgw]

Yes! That is, in fact, the usual way in which we see a black hole. The gas is so amazingly hot from friction that it's glowing in the range of X-Rays.

Re:orbit?

[lgw]

But note that we'll never lose the Moon altogether, as the Moon's primary is the Sun, not the Earth. In terms of orbital mechanics, the Moon and Earth act much like two planets sharing the same orbit, and unlike any other moon in the Solar System.

Re:orbit?

[WaterBreath]

a continuous flow of matter and light (which also has mass)

Well, that really depends on who you ask, and how you phrase the question.

Light, like everything else, has momentum/energy. Mass is one of the observable manifestations of that, but it is not the only one. The reason light is generally considered to be massless is fairly simple: When you push against the motion of moving bowling ball, or anything else with mass, you remove kinetic energy from it. When the kinetic energy is gone, the thing stops moving. But it still exists, because it has mass potential energy as well. Light, however, ceases to exist once the kinetic energy is gone. That's all the energy it has, which would indicate it has no mass.

There's still the question of "relativistic mass", which IMHO is just an artifact of trying to make relativistic effects mesh better with our Newtonian perception. The real test of whether "relativistic mass" is actual real mass would be to see if the gravity well around a massive object is stronger when it is moving than when it is stationary.

Re:orbit?

[lgw]

Qualitatively, when you get close enough to the event horizon of a black hole, gravity is messing with *direction*. At the event horizon, there is no direction which is away from the center. Near the event horizon (but still pretty close) there's no way to "fall sideways" and thereby orbit, because moving in the direction that would be "sideways" in classical mechanics is actually moving (somewhat) towards the black hole in general relativity.

Quantitatively, I can't explain it at all!

Re:orbit?

[Anonymous Coward]

What a sterling example that Americans don't really hate the French specifically, they just can't stand criticism and opposition from anyone and instinctively will use put-downs against any such critics. This type of knee-jerk reaction is actually quite amusing.

BTW, some people would claim that 20th century british popular music (the Beatles, the Who, Queen, David Bowie, Radiohead, and many others) created a significant impact on American culture (and vice versa). I doubt it's the only example of british influence on the portion of the US populace willing to accept foreign influences, although perhaps you are so egotist as to be unable to do so.

Re:orbit?

[DM9290]

There's still the question of "relativistic mass", which IMHO is just an artifact of trying to make relativistic effects mesh better with our Newtonian perception. The real test of whether "relativistic mass" is actual real mass would be to see if the gravity well around a massive object is stronger when it is moving than when it is stationary.

When an object is moving relative to you, you will measure its mass to be greater than if it were standing still. This includes its intertia and gravitational attraction to other things as well as time dilation related to gravity. These are all products of mass. Gravity has no meaning without mass, and mass without gravity is nonsensical in the theory of relativity, as well as in all observations we have ever made.

In the theory of relativity, nothing has an absolute mass and there is no absolute frame of reference (nor have we detected one in the real world).

Re:orbit?

[Anonymous Coward]

I believe the Moon gains energy from the tidal interaction with Earth; as Earth's rotation slows, the Moon gains some of that energy as orbital velocity. I could be making that up, though.

Re:orbit?

[smithmc]

  What a sterling example that Americans don't really hate the French specifically, they just can't stand criticism and opposition from anyone and instinctively will use put-downs against any such critics.

If the critic in question is unwilling to admit to and accept American influences, why should I give him the satisfaction vice versa? "Just can't stand criticism?" Why should anyone, American or otherwise, have to take that kind of crap?

Re:orbit?

[panthro]

The only way something can fall into a black hole is by losing energy to fall it. If it doesn't lose any energy it will keep revolving around the black hole.

Something has to be in orbit to begin with for this to be true. Yes, the gas is rotating around the black hole, but that doesn't mean it's in orbit. It's just falling (being gravitationally attracted) toward the black hole.

In other words, the tangential velocity of the gas is not high enough to maintain a circular orbit at the same distance. Technically, anything falling with nonzero tangential velocity could be considered to be in a decaying orbit, but you're talking about a stable orbit which would require some loss of energy to start decaying, which TFA says nothing about.

Re:orbit?

[WaterBreath]

When an object is moving relative to you, you will measure its mass to be greater than if it were standing still.

But how is the mass being measured? The primary ways of measuring mass deal with measuring changes in momentum. And momentum is a function of both mass and velocity (kinetic energy). So how can we say for certain how much of that momentum is coming from mass and how much from kinetic energy? We need a corroborating measurement to say for sure. One that is not dependent on velocity. If no such method of measurement exists (i.e., if gravitational effects do indeed depend on kinetic energy), then we can safely say that mass does indeed increase with velocity, because all mass effects are affected by changes in velocity.

You seem to indicate that this is, in fact, the case:

This includes its intertia and gravitational attraction

But I am unaware of any experiments that have been carried out to verify this. If there have been, I would happily be pointed to them so I can correct my understanding. As far as I know, GR does not predict this. I was under the impression that the stress-energy tensor is compartmentalized in a way that kinetic energy does not directly affect measured gravitational effects.

Re:orbit?

[drn8]

Since 'volatile' has no 's', then it's 'volatilize'.

In English the correct spelling is 'volatilises'. Worldwide, the majority of English speakers use British English, not American English.

Please keep your spelling, 'culture', and 'cuisine' to yourselves. The rest of the world isn't interested... just ask Baghdad.



I will, thanks. Something about British "'culture', and 'cuisine'" has always turned my stomach. Nasty little island with an inbred populous with bad teeth and ugly women eating goat testicles and such, freaking gross.

Re:orbit?

[drn8]

What a sterling example that Americans don't really hate the French specifically, they just can't stand criticism and opposition from anyone and instinctively will use put-downs against any such critics. This type of knee-jerk reaction is actually quite amusing.

You do realize(hehe) that you are making a blanket statement about over 350,000,000 people right? Just wanted to make sure you are aware of the ass you are making of yourself.

Re:orbit?

[Anonymous Coward]

Whether mass changes with velocity depends on how you're defining "mass". The modern definition of mass is the invariant mass (= the rest mass, for massive bodies). The older "relativistic mass" is now equated with "energy", or rather energy/c^2.

If you want to discuss inertial responses to forces, mass becomes a tensor (matrix), which decomposes into longitudinal and transverse masses—it's no longer described by a number.

If you want to describe the source of gravity, that's a tensor (matrix) too: the stress-energy tensor that you mention, of which relativistic mass(-energy) is one component.

Kinetic energy contributes to the relativistic mass-energy and therefore the stress-energy tensor; this issue is discussed in a pedagogical paper.

Re:orbit?

[WaterBreath]

Whether mass changes with velocity depends on how you're defining "mass". The modern definition of mass is the invariant mass (= the rest mass, for massive bodies). The older "relativistic mass" is now equated with "energy", or rather energy/c^2.

If you want to discuss inertial responses to forces, mass becomes a tensor (matrix), which decomposes into longitudinal and transverse masses--it's no longer described by a number.

If you want to describe the source of gravity, that's a tensor (matrix) too: the stress-energy tensor that you mention, of which relativistic mass(-energy) is one component.

Kinetic energy contributes to the relativistic mass-energy and therefore the stress-energy tensor; this issue is discussed in a pedagogical paper [arxiv.org].

Much appreciated, Mr. Anonymous. Now that I know there is some info out there on this, I will certain look into it. Hopefully I can find arguments for both sides and get a sense for whose arguments are stronger. I have to admit, I did not know that GR as formulated does treat inertial mass (measured by collisions) and gravitational mas (measured by gravity) as identical. That's probably a gross oversight on my part but I am, after all, just an "armchair physicist".

I'm not sure which conclusion I'd like to see validated right now. The absence of experimental evidence (or at least discussion of it) had in the past led me to the position I stated in my earlier post. Obviously I'm still somewhat attached to it. But what I've read recently about theories on the "origin" of mass has opened my mind more to Dr. Carlip's argument. Especially if there is experimental evidence to support him.

Thanks again!

Re:orbit?

[CrimsonAvenger]

it's root(2), not root(0.5), i think, so it's outside the event horizon.

Orbital Velocity is HIGHER then Escape Velocity??

I think you need to re-evalute here.

Re:orbit?

[Anonymous Coward]

You can also read the text Gravitation and Inertia by Ciufolini and Wheeler for discussions of the weak and strong forms of the equivalence principle between inertial, passive gravitational, and active gravitational masses.

Re:orbit?

[CrimsonAvenger]

Yes, but there's a big difference between escape velocity which is radial, and orbital speed which is tangential.

Umm, no. Escape Velocity is misnamed, really. Properly, it's Escape Speed. Any velocity vector whose magnitude is equal to Escape Speed at a given point in space will escape, as long as the direction of the vector doesn't hit anything (like the object you're escaping, for instance).

For that matter, orbital velocity is similarly misnamed. Any velocity vector whose magnitude is less than Escape Speed will "orbit", given the limits that it doesn't hit anything (like the object you're orbitting). Even then, it orbits until the impact.

above the critical limit for being able to orbit is outside the event horizon (actually its at 3MG/c^2 expressed in metres from the singularity, whereas the Event Horizon for a non-spinning black hole is at 2MG/c^2)

Let's see, Orbital Speed is sqrt(Rg), where R is distance from the point you're orbitting, and g is the local acceleration due to gravity. g = Gm/(R^2), so orbital velocity is sqrt(Gm/R). If orbital velocity is c, then R = Gm/c^2.

Escape Speed is sqrt(2Rg). Which reduces down to R = 2Gm/c^2, for the Event Horizon of a black hole.

Looks to me like the radius of orbit of an object moving at c is inside the Event Horizon.

Note, of course, that I am using strictly Newtonian orbital mechanics here. The region near an Event Horizon is nowhere near flat enough for Newtonian physics to apply all that well.

Nonetheless, the assumption that you made reduces down to orbital speed > escape speed, which is absurd on the face of it.

Re:orbit?

[Anonymous Coward]

As you say, your Newtonian arguments are inapplicable. The other poster is correct. The Schwarzschild radius is at r=2M (in G=c=1 units). The "photon sphere" is at r=3M, where you may orbit (unstably) at the speed of light. The innermost stable circular orbit (ISCO) is at r=6M.

The OP was using "orbital velocity" in the sense of "circular orbital speed". Escape velocity is not terribly well defined in general relativity, but there is a certain sense in which the escape velocity of the event horizon could be said to be c.

Re:orbit?

[Anonymous Coward]

I'm guessing this has been proven mathematically, right? I mean on paper. At >1.5 Sradius, would that outcome be independent of the orbiting body mass?

Does that hold true, w/ regard to 1.5 Sradius, for Binary stars, where the secondary(smaller) star orbits the primary(larger) at extremely high frequencies? Else star collision ensues ...

Been a few years since physics and astronomy ...

Re:orbit?

[the chao goes mu]

To be a bit pedantic, orbits can be stable while not being circular. Eliptical orbits are also stable. (Hey, everyone else was being so pedantic, I had to join in.)

Re:orbit?

[panthro]

Yes, I know, but the stable orbit requiring the minimum tangential velocity to remain stable would be circular... as TV increases, orbit gets more elliptical and eventually parabolic (open)... it's like increasing the slope of a conical cross-section...

Re:orbit?

[mfrank]

Actually, Russians don't refer to them as "black holes" because, in Russian, "black holes" can't be used in polite company. Like Uranus, only more harsh :)

Re:orbit?

[bcrowell]

Yes. The only way something can fall into a black hole is by losing energy to fall it. If it doesn't lose any energy it will keep revolving around the black hole. Same thing with Earth, if Earth was in the middle of a cloud of gas that could eat away at very large amounts of the Earth's momentum, then the Earth could spiral into the Sun. Since that gas isn't there our Earth keeps revolving around the Sun, which is good for us.
That's incorrect. You're right about the Earth, but it doesn't work the same way for a black hole. There are some possible orbits around a black hole, the ones that don't get too close, that are well approximated by the classical ellipses, hyperbolas, etc. But there are also orbits that are qualitatively different, and you can have orbits that start at infinity, and yet result in capture by the black hole (which isn't possible in nonrelativistic gravity). It's true that friction in the accretion disk can turn an escaping orbit into a non-escaping one, just like in nonrelativistic cases.

Re:orbit?

[Anonymous Coward]

A binary system, or any other thing that is not a black hole, does not have a Swartzchild radius.

Re:orbit?

[Anonymous Coward]

For a star, the surface of the star is always at a radius greater than 1.5 times the Schwarzschild radius; stars are not as compact as black holes. So there is never a region about a star where stable orbits can't exist. However, if two stars get close enough, they will either touch, or if further apart, usually don't have exactly the velocity necessary to maintain a stable orbit (especially in light of tidal friction and such), and will collide.

Re:orbit?

[rwise2112]

Collisions with other matter?

Re:orbit?

[Doctor Death Utah]

All the stuff in orbit around a black hole keeps bumping in to each other. With each collision, some energy is lost to heat and so the "stuff" falls into a lower orbit. Eventually, MANY thousand years later, the gas and other cosmic debris either falls across the event horizon or gets blasted away from the black hole in an entirely seperate process. But that's another story.

Re:orbit?

[Phanatic1a]

Yes, it does, of 2Gm/c^2. It's just that that radius is so small that it's *inside* the object. Schwarzchild radius of the sun, for example, is about 3 kilometers. Doesn't mean the sun's a black hole, it means that if you somehow compressed the entire sun's mass into a sphere of 3 km radius, *then* you'd have a black hole.

Re:orbit?

[Phanatic1a]

At >1.5 Sradius, would that outcome be independent of the orbiting body mass?

Yes. 1.5 Sradii is the distance at which you need to travel at c just to stay in orbit. Since that means you're a photon, this distance is called the photon sphere. As I said, orbits at this distance aren't *stable*, but they're *possible*, at least for massless objects. For massive ones, then the closest possible distance at which to orbit is slightly further away.


Does that hold true, w/ regard to 1.5 Sradius, for Binary stars, where the secondary(smaller) star orbits the primary(larger) at extremely high frequencies? Else star collision ensues ...

If the object's not a black hole, then the Sradius is smaller than the object itself.

Re:orbit?

[StikyPad]

Earth keeps revolving around the Sun, which is good for us.

Good for you maybe.. Some of us are getting very dizzy.

Re:orbit?

[volatilises]

How we will laugh when Iran nukes your silly ass. Sorry, arse. Only a real man has an ass. Not many people they want to nuke at the moment, pal.

Here in the free world the general population has a sweepstake going as to when New York will get nuked. I have $20 riding on July 4, 2015, by Iran.

You guys haven't won a war in over sixty years.

Have a nice day.

Re:orbit?

[Anonymous Coward]

Hmm, that gives me an idea for creating an evil planet destroying device. Just blow up asteroids/comets, whatever in the path of a planet so that the friction causes the orbit to decay. Then sit back and watch as your victims get hotter and hotter. Hmm, I think I'm going to hijack some nuclear weapons and hold the earth hostage.

"We will raise your planet's temperature by one million degrees per day, for five days, until you bring us McNeill."

Metric

[MacDork]

177000 Km/hr... or 49166.67 m/s... why is there no metric second?

Re:Metric

[jfengel]

Well, you can have kiloseconds, if you want, which is on the same order of magnitude as an hour (well, a bit over a quarter of an hour).

As you note, we use metric fractions of a second (mill, nano, femto, etc) all the time. Why megaseconds (about 11.6 days) and gigaseconds (31.7 years never caught on), I can't say. Maybe it's because we're all so familiar with hours, minutes, and days, and unlike other metric/English conversions the conversion factors are at least integers, and well known integers at that (e.g. 60 seconds to the minute, 60 minutes to the hour, etc.)

I'll admit I find the European speed limits in "km/hr" somewhat disconcerting, since the latter is such a non-metric unit. Hey, let's all try to convince the EU to standardize on km/kilosec, aka meter/second.

Google units conversion, BTW, does know about megaseconds and gigaseconds.

Re:Metric

[Anonymous Coward]

They may not be metric, but they are SI units.

Re:Metric

[mbrother]

Most X-ray satellites allocate time by the kilosecond. I just got awarded 60 ksec on Chandra recently, for instance.

Re:Metric

[ichin4]

Fundamentally, the reason we have no metric unit of time is that there are two lengths of time we really care about a lot -- the day and the year -- and they are not seperated by a power of 10.

Actually, those metric-crazy revolutionary Frenchmen did try it. They picked the day as the fundamental unit. They then divided the year into 12 30-day months, plus a 5-6 day party at the end.

Re:Metric

[Anonymous Coward]

Yeah, miles/hour is really much better! No metric stuff in either miles or hours! Yay!

Re:orbit at greater than c

[Harmonious Botch]

You can't orbit a black hole inside the event horizon without going faster than the speed of light.

Pardon

[MacDork]

why is there no metric second?

Pardon, hour-ish metric units of time. Here comes the onslaught of nanosecond wisecracks...

Facts

[Anonymous Coward]

A few interesting facts about black holes that some people aren't aware of:

• Black holes emit x-ray radiation and get smaller and smaller until they disappear, or "evaporate".

• Most black holes are formed from the death of large stars (larger than the sun) that run out of fuel and cannot sustain its nuclear reaction. The star loses the force pushing itself outward and is overcome by the force of its own gravity pulling inward. Eventually, the star has so much gravity and is so compacted that it "eats itself" until there is nothing left but a hole in the "fabric" of space-time, created by the gravity left over from the star.

• The gravity around the "hole" of a black hole is so strong that NOTHING can make its way back out after a critical distance.

• Even before crossing the event horizon, though possible to travel away from the black hole, it is not easy. Even light has a hard time getting out, so light being emitted from something almost at the Event Horizon but not yet inside the threshold takes a much longer time to escape and be seen by someone then it would in normal space going at 186,000 miles per second.

• The Singularity is the true point of destruction, the actual hole part of the black hole, although any object, especially a person, would be long dead before they reached the Singularity.

• Some black holes are spinning and have several event horizons called the "Ergosphere", "Outer Event Horizon", and "Inner Event Horizon".

Stephen Hawking's recent concession that black holes do not irretrievably eradicate information after all has garnered much attention. In my opinion, it is refreshing to see the public focused, if just for a moment, on an important conundrum that has fascinated theoretical physicists for three decades, and prompted much conceptual progress. The scientific issues, however, remain much less settled than Dr. Hawking's celebrated wager on the question. He most recently pronounced: "If you jump into a black hole, your mass energy will be returned to our universe, but in a mangled form, which contains information about what you were like but in an unrecognizable state." These ideas are profound and will have a lasting effect on our scientific theories as well as life as we know it.

Re:Facts

[Shimmer]

so light being emitted from something almost at the Event Horizon but not yet inside the threshold takes a much longer time to escape and be seen by someone then it would in normal space going at 186,000 miles per second.

Not true. The speed of light is a constant, even near a black hole.

As I understand it, what actually happens to the light emitted by an object approaching an event horizon is that it gets increasingly red-shifted. So an observer at a safe distance would see the object "fade" into infrared and then into ever-longer radio waves until it crosses the horizon.

Re:Facts

[Anonymous Coward]

"Black holes emit x-ray radiation and get smaller and smaller until they disappear, or "evaporate"."

Don't confuse the x-ray radiation (emitted outside of a black hole) with Hawking radiation, which is the true cause for black hole evaporation.

Re:Facts

[Anonymous Coward]

> Black holes emit x-ray radiation and get smaller and smaller until they disappear, or "evaporate".

Not really. The black hole itself emits black body radiation and the temperature of a reasonably large hole will be very low, so it's emitting radio waves. As it gets smaller, the temperature goes up, so the emissions will pass through visible light, x-ray and so on. However most large black holes have gas streaming into them from their surroundings, which gets really hot while spiralling into the black hole and this is the part that usually emits x-rays.

> Most black holes are formed from the death of large stars.

Unknown. Unproven.

Re:Facts

[plasmacutter]

"The gravity around the "hole" of a black hole is so strong that NOTHING can make its way back out after a critical distance.The gravity around the "hole" of a black hole is so strong that NOTHING can make its way back out after a critical distance." youre wrong.. because FACTS can apparently make their way out of a black hole after a critical distance.

Re:Facts

[scaryjohn]

The speed of light is a constant, even near a black hole.

The speed of light is constant with respect to inertial frame of reference (e.g. whether I'm looking at the light standing still, or you're looking at the light travelling down the road at some appreciable fraction of the speed of light), not constant with respect to the medium. While I gave up on Physics after my freshman year of college, I'm willing to accept that the pull of the black hole on photons falls more under the heading of medium than inertia.

Re:Facts

[Black Parrot]

> The Singularity is the true point of destruction, the actual hole part of the black hole

No, the event horizon is the methaphorical "hole in space".

Lots of physicists doubt that singularities even exist. Singularity essentially means "the math broke", a result of applying GR at scales where QM effects almost certainly dominate. If we ever get a theory that unifies GR and QM, we might discern what actually happens at the center of a black hole.

Re:Facts

[Scott Carnahan]

Medium in this case refers to a material through which the light is passing, such as air, glass, water, etc. The atoms of these media interact with the light passing though, slowing its propagation. Gravitational fields do not qualify, since they do not couple with electromagnetic fields to any appreciable extent (except perhaps if the black hole is extremely small).

Re:Facts

[Anonymous Coward]

I am in China; Your link is useless to me, you insensitive clod.

Re:Facts

[Markus Registrada]

As I understand it, it should take infinitely long for any bit of matter to cross the event horizon. Where does this 200,000 years come from? And, why don't we expect to find a shell of matter frozen around the outside of the event horizon?

I could guess (though they never seem to say) that they're expecting the event horizon itself to grow each time any bit of matter crosses (by whatever means), causing it to engulf just a bit more of the stuff collected just outside, in a positive feedback loop. That seems pretty fragile, particularly for an especially massive hole.

Who works these things out? Are they actually making sense, or is it all flimflam that it is to nobody's advantage to call shenanigans on? I.e., is black hole astrophysics really just science's equivalent of academic literary criticism?

Re:Facts

[Rik Sweeney]

Black holes emit x-ray radiation and get smaller and smaller until they disappear, or "evaporate".

OK.

The gravity around the "hole" of a black hole is so strong that NOTHING can make its way back out after a critical distance.

Haven't you just contradicted yourself?

Re:Facts

[strider44]

I think you and the grandparent are getting a bit confused here on this issue (though it has been a few years since I studied Cosmology, and I may be wrong here, but I *do* try to keep abreast of the journals and recent discoveries when I have time).

A black hole itself has no temperature and emits no light. It literally can't. Hawking radiation comes from particles from before the event horizon. The actual amount of radiation is insignificant for astronomical black holes since they absorb more radiation from just the cosmic background microwave radiation than is let go through Hawking radiation. It's only really important for quantum black holes.

Stellar-mass black holes pretty much have been proven to come from the death of larger stars, more than about 3-4 stellar masses. Whether it's proven depends on how strict you are with the word "proof". Supermassive black holes *probably* started as stellar black holes, a long long time ago, maybe not. I'm not sure if anyone knows or has given proof, but if they have then I haven't heard about it. I'd like to though!

Re:Facts

[aarku]

There is a question I have been wondering for some time about black holes, so karma be damned I shall ask it in hopes of some response. Is it possible for light to actually orbit a black hole at a certain radius? Could energy "build up" around this radius and slam into and annihilate whatever crosses into the black hole?

Re:Facts

[MillionthMonkey]

As I understand it, it should take infinitely long for any bit of matter to cross the event horizon.

No, that's to a distant observer. As you fall in you appear more and more redshifted to him until you reach an infinite redshift at the event horizon and your image appears frozen there. In practice you'd still disappear quickly from the visible spectrum, although in theory you might be visible a few microseconds longer to any observers with an ELF antenna. But in the proper time aboard the ship you fall right through the horizon and reach the singularity in seconds.

Where does this 200,000 years come from?

Probably standard Newtonian stuff: the decay of angular momentum. The gas won't fall in if it has significant angular momentum around the hole. You have to wait for that to be lost somehow, and one way it's lost is by friction. The gas heats up and emits X-rays, which carry off energy and angular momentum. A similar mechanism will make Saturn's rings fall into Saturn in a few hundred million years (as the particles bump into each other, etc). Even without friction, if there is ionization the gas will radiate as the hole flings it around. The losses from X-ray emission are what make the gas eventually fall in.

And, why don't we expect to find a shell of matter frozen around the outside of the event horizon?

Because it fell in and its ghost redshifted away.

I could guess (though they never seem to say) that they're expecting the event horizon itself to grow each time any bit of matter crosses (by whatever means), causing it to engulf just a bit more of the stuff collected just outside, in a positive feedback loop. That seems pretty fragile, particularly for an especially massive hole.

It doesn't grow by that much. The Schwarzchild radius has a linear dependence on black hole mass, about 3 km per solar mass. Unless there's more than one solar mass within 3 km of the horizon it should reach a stable radius.

Re:Facts

[freedom_india]

A black hole itself has no temperature and emits no light.

Correction: A black hole "emits" heat, but since heat is transmitted as infra-red travelling at speed of light, it is never "emitted". So you don't get to "feel" the heat or the light. That doesn't mean the blackhole is cold.

from the death of larger stars

Stars are dense and hot, and once they shrink/collapse they will be hotter. My guess is the inside of a black hole must be much hotter than the temperate of the core of the star from which it collapsed.

Re:Facts

[penguinoid]

Not true. The speed of light is a constant, even near a black hole.

But spacetime is bent quite badly near the event horizon. Light emitted in the appropriate direction would orbit the black hole several times before entering/leaving the black hole, so while the speed of light may be chugging along at 299,792,458 m/s, the distance it travels might not be what you expected...

Re:Facts

[Gleng]

The scientific issues, however, remain much less settled than Dr. Hawking's celebrated wager on the question.

Um, I think he's a Professor.

Re:Facts

[retro128]

Black holes emit x-ray radiation and get smaller and smaller until they disappear, or "evaporate".

Does all the matter of a black hole bleed off as X-Ray radiation? Or is all of it just folded up into the singularity, which should be theoretically impossible to get to since spacetime is infinitely warped around it?

Could the "big bang" have occurred when a singularity in another universe isolated itself and folded into this dimension? Could the whole universe be a spacetime bubble? Stuff to think about...

Re:Facts

[Wolfbone]

It doesn't actually matter what direction the light is emitted in - the "slowdown" of light occurs along radial paths too. It is, as you say, due to the badly bent spacetime that this effect is observed, but it's because of the metric structure of the spacetime itself, not the path of the light. In a "normal" (t,r,theta,phi) coordinate system - appropriate to a far away observer - where the Schwarzschild metric describes the spacetime structure surrounding a massive body at the origin, the radial coordinate speed of light (dr/dt) turns out to be (c - 2GM/rc). You get a better picture of what is happening near black holes with more suitable coordinate systems but at least here you can see that if we describe spacetime with a set of coordinates appropriate to "normal", speed of light = c conditions, "the speed of light" really is affected by the massive body.

Re:Facts

[at_18]

Is it possible for light to actually orbit a black hole at a certain radius?

Yes, but you are unlikely to ever see it. The only possible orbital radius is exactly the same as the event horizon. Larger, and it will spiral out of the hole. Smaller, it's already in and will fall in the singularity.

Re:Facts

[somersault]

nothing in the universe is every 'annihilated', it only changes from one thing to another.. if matter was 'annihilated' it would be converted into heat/light/whatever. I know that from when I was in Science when I was 11 years old or so. I'm not sure about 'energy' being in orbit, but I guess that since energy itself would be made of particles (?) then it wouldnt be able to hold a steady orbit forever since it would slow down because of friction (though I'm guessing that for friction the particle has to come into contact with another particle, and they'd probably slow each other down and lose orbit anyway). I've not done physics for ages.

Re:Facts

[ObsessiveMathsFreak]

"If you jump into a black hole, your mass energy will be returned to our universe, but in a mangled form, which contains information about what you were like but in an unrecognizable state." These ideas are profound and will have a lasting effect on our scientific theories as well as life as we know it.

It all sounds a bit string-theory-ish to me.

Re:Facts

[Anonymous Coward]

"Does all the matter of a black hole bleed off as X-Ray radiation? Or is all of it just folded up into the singularity, which should be theoretically impossible to get to since spacetime is infinitely warped around it?"

None of the matter bleeds off as x-ray radiation. Everything inside the black hole (which you say is impossible to get to) will eventually evaporate away through Hawking radiation.

Re:Facts

[Shimmer]

Light emitted in the appropriate direction would orbit the black hole several times before entering/leaving the black hole, so ... the distance it travels might not be what you expected

That's true, but it's not really germane to the OP's claim that light slows down near a black hole. Assuming for simplicity that the object is directly between the observer and the black hole, then the observer will simply see it fade into infrared.

Re:Facts

[Anonymous Coward]

"Could the "big bang" have occurred when a singularity in another universe isolated itself and folded into this dimension? Could the whole universe be a spacetime bubble? Stuff to think about..."

Except that these are all just catch phrases that don't currently mean anything. "folded into this dimension"??? "spacetime bubble"??? Sure, it sounds nice when you don't know anything about physics, but in reality these probably aren't even observable.

Re:Facts

[Wolfbone]

"A black hole itself has no temperature"

Actually it is considered to have a temperature, though it's not the same thing as the temperature of ordinary matter. The analogy of black holes as thermodynamic systems (which I think arose from the study of rotating black holes and Penrose processes) is what motivated Bekenstein historically to suggest that a black hole /should/ emit black body radiation. Hawking set out to prove him wrong and - ironically - discovered that they do indeed.

Re:Facts

[johno.ie]

Your comment is purely based on the theories of Stephen Hawkings. I've read a lot of his work and I think he is mostly full of shit. For some reason he has been put on an intellectual pedestal by his peers in the field of theoretical physics. I don't believe that this is for the right reasons though, but I won't speculate what the reasons actually were. I think his theories are having a harmful effect on physics as a whole, because (1) like i said above, he's full of shit, and (2) any new breakthroughs that contradict his work are usually ignored, because they contradict him. Science advances generation by generation because its often very hard to refute a great scientists work until after he has died. I sincerely hope that he has written his last book. I sick of hearing dumbasses quoting from A Brief History of Time to support some bullshit they are trying to make me believe in.

Re:Facts

[Shimmer]

>> The Singularity is the true point of destruction, the actual hole part of the black hole

> The event horizon is the methaphorical "hole in space"

Yeah, but an object isn't destroyed just by crossing the event horizon. In fact, for a sufficiently large black hole, you could cross the event horizon without noticing anything disruptive. Inside the black hole, GR works fine until you get near the singularity. What would actually kill you on the way down is the stretching effect of tidal forces.

Re:Facts

[fred_sanford]

If the speed of light is a constant, how can, almost 6 years ago, physicists slow the speed of light?

Re:Facts

[Shimmer]

Because the speed of light is constant in a vacuum. If you choose another medium, you get another (constant) speed of light.

Please read up on Special Relativity for more details.

Re:Facts

[fred_sanford]

so, you answered my point exactly. Since the light in question is no longer unimpeded in a vacuum but traveling in the medium of a black hole, it would travel slower, no?

Re:Facts

[Anonymous Coward]

No. A black hole is a vacuum; it's just a region of space, not a solid body like a star. (There might be matter at the central singularity, a point of zero size, but the rest of the interior is vacuum, except for any objects briefly falling through it to the singularity.)

Re:Facts

[Wolfbone]

"The speed of light is constant with respect to inertial frame of reference"

That is the essential point (if meant as the observation that the constant 'c' isn't the notion of speed useful to describe what the light in question is doing in the curved spacetime it's moving in) but the "slowing" of the light has nothing to do with any "medium" the light is moving in.

Re:Facts

[regen]

What actually happens is that time slows down near the event horizon as observed from a distance away (outside the gravity well of the hole). It slows down so much that it would taken an infinite amount of time to cross the horizon. This is also the cause of the red-shift.

Re:Facts

[twifosp]

The speed of light would not be at its C constant near a black hole. GR states that light is affected by large bodies of mass. Large bodies of mass warp both space AND time. This experiment was validated (I hate to use the term prove) by the radio transmissions the Cassini space probe shot across the sun towards Earth. The radio transmissions took a longer amount of time than predicted using the C Constant and the distance from the trasnmitter and Earth. This and other experiments, as well as General and Special Relativity theory math, suggests that gravity does in fact curve and slow spacetime. And that radio waves (traveling at the speed of light as well as much of the same behavoir as a light wave) are in fact affected by this spacetime warping.

More information is available about this experiment at wikipedia.

http://en.wikipedia.org/wiki/Cassini_probe#Test_of _Einstein.27s_theory_of_general_relativity

Re:Facts

[tgrigsby]

To clarify:

* Black holes emit x-ray radiation and get smaller and smaller until they disappear, or "evaporate".

The time taken to do this for any appreciably large black hole is on the order of trillions of years. Theoretically speaking, of course. Black hole evaporation only really makes sense in high energy particle collisions.

* Most black holes are formed from the death of large stars (larger than the sun) that run out of fuel and cannot sustain its nuclear reaction. The star loses the force pushing itself outward and is overcome by the force of its own gravity pulling inward. Eventually, the star has so much gravity and is so compacted that it "eats itself" until there is nothing left but a hole in the "fabric" of space-time, created by the gravity left over from the star.

The "hole" is a mathematical construct. There is no actual hole, just a point beyond which information cannot escape, including information contained in light.

Re:Facts

[Anonymous Coward]

No, Hawking radiation comes from a PAIR of particles, virtual particles no less, wherein the pair pops into existence straddling the event horizon, such that one of the pair is outside it and escapes, while the other is inside and is trapped. Because its mate is inaccessable to cause the mutual destruction normal for virtual particles, the exterior particle CONTINUES TO EXIST, outside the event horizon. Thus, an entire particle has been created, and energy conservation DEMANDS that the energy for that to be must come from somewhere else in the universe. In this case, it is contributed from within the event horizon, which is how black holes lose mass even though none of the mass inside can travel outward.

Hawking radiation certainly does NOT come from particles before the event horizon. If it did, there would be no way for them to "extract" mass from the black hole.

If you meant that the radiation emitted simply was from a particle that was never inside the event horizon, okay, you're correct. But I don't think that by "emitting" radiation, it is meant that particles exit the event horizon, I think rather it is meant that radiation is emitted from the surface of the event horizon AND takes with it mass/energy away from the black hole.

Re:Facts

[Jamu]

The (measured) speed of light is slower in a medium because the medium will absorb and emit the photons. This results in the average speed being slower. The photons will also become dispersed after passing through a medium; but any front edge is limited by the speed of light, c. The true speed of the photons is c regardless of medium.

The grandparent is also correct when he states that the speed of light is constant. It's a basic principle of not just SR (inertial frames), but also GR (accelerating frames). The "pull" of gravity on light results in a red-shift. The speed of light doesn't change.

Of course all this depends on how you measure the speed of light (The group velocity of light, for example, can exceed c.). But in the context of SR and GR the speed of light is always a constant.

Re:Facts

[Jamu]

GR says the speed of light is invariant and that the light traveled further. Only in the sense that the light took longer than expected (without using GR) can you say that the light was slowed.

Re:Facts

[Shimmer]

That makes sense, thanks.

Re:Facts

[clem9796]

And how many theories have you published lately? I'm an athiest but I don't call other peoples' views of religion bullshit. Science is a process of failures, we accept a theory until something better comes along. And that's just it, it's theoretical, look it up, it's a best guess based on what we see. Feel free not to believe it but have some contradictory informationsupporting you, you can't sluff off one of the best minds we have as a bullshitter without backing up what you're saying.

Re:Facts

[frgough]

No, that's to a distant observer.

We are distant observers. The parent's question still stands.

Re:Facts

[twifosp]

Think more relative. The old formula speed = distance over time doesn't fully apply to light since it doesn't care about time. The fact that it took longer due to a curve in space, also means a curve in team, which means technically, in both spaces, light was slowed. Remember, the same phenomenom that changed the space vector also curved time. So while you could technically say: The light beam, or in this case radio beam, was still a relative C velocity, it's momentum and therefore overall speed in our frame of reference was slowed by the time and vector change. I guess the net of it is, we're both right.

Re:Facts

[Frostalicious]

Not true. The speed of light is a constant, even near a black hole.

So how come Lene Hau claims to be able to reduce the speed of light?

Re:Facts

[Deidog]

considering the medium in described in the article was produced by packing atoms in a super cooled envirement creates a new medium of matter. Could it not be argued that the black hole itself creates another unkown medium or mediums at certain distance (such as the event horizon itself)? The scenarios do seem somehwat comparable I think.

  The extreme gravity of the black hole pulls in gasses and other matter wich at some point could combine under the pressure to create a medium simular to that of the experiment.

  Different combinations of particles sucked in as well as differnt suround temperatures could very well cause this medium to be slightly different from black hole to black hole... theoretically slowing light to different speeds at varyng distances from the black hole untill it reaches a point of density actually capable of if not stopping it then slowing it beyond our perception.

  Could that point not be the event horizon itself?

Re:Facts

[Anonymous Coward]

Could it not be argued that the black hole itself creates another unkown medium or mediums at certain distance (such as the event horizon itself)?

There are singularity theorems in general relativity which state that once an event horizon forms, anything inside the event horizon cannot stay at rest forming some kind of stable structure, but must inevitably collapse to a singularity. So no, that can't happen, at least in ordinary general relativity. (Maybe in some different theory of gravity, but we don't currently have evidence for any such.)

Re:Facts

[Clith]

The effect of light being warped into orbit is the same as other light-bending effects - you will get distortions and possibly multiple images, like a mirage. Still, the light will be at a lower energy level, which means its wavelength will be shifted towards infrared. How much depends on lots of other stuff that is not in the scope of this reply. :-)

Re:Facts

[Anonymous Coward]

Could the "big bang" have occurred when a singularity in another universe isolated itself and folded into this dimension? Could the whole universe be a spacetime bubble? Stuff to think about...

Nah, the universe is just exhibiting periodic motion, like a spring. The Big Bang happens, the Universe expands for a while, then contracts (big crunch), then the cycle repeats itself.

Re:Facts

[XzeroR3]

Does all the matter of a black hole bleed off as X-Ray radiation? Or is all of it just folded up into the singularity, which should be theoretically impossible to get to since spacetime is infinitely warped around it?

Could the "big bang" have occurred when a singularity in another universe isolated itself and folded into this dimension? Could the whole universe be a spacetime bubble? Stuff to think about...

If I were high right now your post would be awesome to try to expound on.

Re:Facts

[retro128]

That is my understanding of it as well...But where did all that matter come from to begin with?

Re:Facts

[retro128]

It means something to someone.

http://www.space.com/scienceastronomy/white_hole_0 30917.html
http://www.astronomy.pomona.edu/Projects/moderncos mo/Sean's%20mutliverse.html

Additionally, a singularity is defined as a point in space that has such intense gravitation that spacetime is curved infinitely around it (a theory about gravity states that it does not "pull" things towards it, but rather warps space, and objects caught in gravitation are just following the curvature of said space) It seems to me that when this happens, nothing can enter or leave. That sounds like a spacetime bubble to me.

The use of the words "folded into this dimension" sounds a little cliche, I agree. But we can also agree that this universe has 3 dimensions, but has no detectable edge.

Consider what we thought about the earth a relatively short time ago...It was thought the earth was flat (2-D), because that's the way it looked from our perspective. Obviously we know better now and the earth is actually spherical (3-D). In this sense, does it defy logic that our universe exists in a 4-D or greater system?

Re:Facts

[retro128]

I'll share, all you had to do was ask.

Re:Facts

[Shimmer]

The speed of light in a given medium, such as the vacuum of space, is constant. In a different medium, the speed of light is different, but still constant. Some exotic media (such as the one you link to) slow light remarkably, but that's not really relevant to this discussion.

Re:Facts

[StikyPad]

That's an old theory. Recent evidence suggests that the rate of expansion of the universe is actually increasing, not decreasing as you'd expect in a spring-type scenario.

http://www.google.com/search?hs=bMo&hl=en&lr=&c2co ff=1&client=opera&rls=en&q=universe+expansion&btnG =Search
http://en.wikipedia.org/wiki/Universe#Fate_of_the_ universe

Big distance but useless figures

[Red Flayer]

310316400000000 km is the last leg of the journey?

FYI, that's 2,074,335.22 Astronomical Units, or 32.8 Lightyears, or about the distance from Sol to the Cepheids. Dang.

Too bad they don't specify how far out (radially) from the event horizon the last leg starts. Or even loosely define what 'last leg' means in this case.

Re:Big distance but useless figures

[Hobbex]

You think so three dimensionally.

(Consider relativity...)

Re:Big distance but useless figures

[bogado]

I imagine that those particles are probably closer to the blackhole, but since they are rotating arround and loosing "altitude" more slowly then their actual speed it takes a long time to reach the blackhole.

This is much like a artificial satelite that looses altitude due to several factors and if their route is not correct they would slowly fall into earth. If I am not mistaken that was what happened with the skylab, wasn't?

Re:Big distance but useless figures

[Phanatic1a]

They're referring to the effects of relativity as an infalling object approaches the event horizon. The object is accelerating towards c, so as it approaches the event horizon (from our external frame of reference), its clock is moving slower and slower, and it takes longer to travel a given distance.

Re:Big distance but useless figures

[Red Flayer]

Of course, but I'm still curious what absolute distance the 'last leg' covers.

Re:Big distance but useless figures

[bogado]

Well I would define this "last leg" as the last revolution arround the event horizont, but I did not define it so I can't be sure.

One thing would help is to know the actual diameter of the event horizont, in my distant memory this depends on the mass of the blackhole.

It was never in orbit

[Gorimek]

What makes you think it is in orbit in the first place?

It's just basic gravity. Things fall down. Even gas. Just look at Earth.

Re:It was never in orbit

[Anonymous Coward]

The star off which the black hole is feeding was in orbit around the black hole. That includes all of the contents of the star.

Re:It was never in orbit

[MillionthMonkey]

What makes you think it is in orbit in the first place? It's just basic gravity. Things fall down.

You're drawing a distinction where there is none. That's what an orbit is.

Re:It was never in orbit

[UserGoogol]

No, an orbit is when you fall sideways.

Re:It was never in orbit

[Intron]

Damn. I just bought that star from the International Star Registry, too.

Re:It was never in orbit

[Anonymous Coward]

Being in orbit is just like flying; You just fall down to Earth and miss it.

Re:orbit at greater than c

[Anonymous Coward]

In fact, circular motion around a black hole would require v>c if the radius of the orbit is less than 1.5 R, where R is the black hole radius, and such motion is unstable at distances below 3.0 R.

And, of course, don't forget that an object travelling around the black hole emits gravitation waves and loses energy (but that applies to our Solar system too).

Wikipeding?

[bubulubugoth]

So, if there is a "googling" action, also is there a wikipedin action?
The Wikipedia entry about Event Horizon has an interesting "faq" about, orbitig the event horizon and sticking you hand into the event...
Also the wikipedia companion, talks about Stephen Hawking saying that no "event horizon" can be formed at a black hole... This article needs edition... :)
Good reading before a good sleep...
Btw, there is a neat animation about a neutron star X-ray burst


enough of karma whoring...

Re:Wikipeding?

[Anonymous Coward]

The cool kids don't use 'Wikipeding' they use 'Wicked'

Example:
Cool Kid 1: I read this 'Wicked' article about Steven Hawkings today.

Cool Kid 2: For real? I read that 'Wicked' shit yesterday. You notice where it says "no event horizon can be formed at a black hole"? I edited the 'Wicked' entry to say that.

Cool Kid 1: You punk-ass bitch, that wasn't in the spirit of 'Wickedness'.

Re:Wikipeding?

[Travis Mansbridge]

It's a Wikipedia article. You don't need to tell us it needs revision.

Marketing "Facts"

[Anonymous Coward]

Pulling facts out of black holes?

I don't see what the big deal is. The marketing department of (insert company name here) seems to do this all the time.

~The Anadromous Cowherd

"pulling facts from black holes"

[jayloden]

Wait...I'm confused. Is this another John Dvorak article, or what?

If you could destroy a black hole...

[Centurix]

You'd be messing with the Phantom Zone, and we'd perish under the rule of the great General Zod!

It hurts less...

[Black Parrot]

...if you pull the facts out quickly.

Re:It hurts less...

[pintomp3]

but then it's less fun. btw, the pull out method does not work.

Re:It hurts less...

[Anonymous Coward]

but then it's less fun. btw, the pull out method does not work.

It does too! How many pregnant pr0n starz have you seen?

Why the long time?

[ichin4]

The article isn't very clear on why matter traveling rapidly toward a black hole would still take a long time to fall in. I assume they are refering to the gravitational time dilation effect. For someone looking from far away, clocks near the black hole appear to run slower, and in fact to stop at the event horizon. Conversely, someone falling into the black hole (ignoring for the moment that he would in fact be ripped apart by tidal forces) would see the entire history of the universe played out above himself as he fell in.

Re:Why the long time?

[KrizDog]

Perhaps the gas doesn't lose much orbital energy as it orbits the hole. In addition, collisions happen at random sending many atoms away from the hole.

Perhaps its something like the random walk of a photon from the suns core which can take millions of years.

Just my midnight speculations...

Re:Why the long time?

[massivefoot]

Conversely, someone falling into the black hole (ignoring for the moment that he would in fact be ripped apart by tidal forces) would see the entire history of the universe played out above himself as he fell in.

Well not quite. Whether he'd be ripped apart would depend on the rate of change of the gravitational field strength with distance. He'd last longer if it were a larger black hole, and if it were spinning. Also he'd see the entire future of the universe play out before him. It would also appear, well, rather blue...

Re:Why the long time?

[ichin4]

Thanks for the correction. By history, I did mean future history, but I agree that I was not at all clear. And you are, of course, completely correct that the derivative of the field strength becomes infinite only at the singularity, not at the event horizon.

Re:Why the long time?

[Anonymous Coward]

The article isn't very clear on why matter traveling rapidly toward a black hole would still take a long time to fall in. I assume they are refering to the gravitational time dilation effect.

Actually, they're referring to how long it takes the matter's orbit to decay substantially. (If they were referring to time dilation, that's infinite to reach the horizon—as you note—not merely "a long time".)

Conversely, someone falling into the black hole (ignoring for the moment that he would in fact be ripped apart by tidal forces) would see the entire history of the universe played out above himself as he fell in.

This is not correct, and the "correction" below that he'd see the entire future of the universe played out is also not correct. You can see this by inspecting the light cone structure of a Kruskal-Szekeres diagram of the Schwarzschild solution (if you know how to interpret it, as least!). The observer merely sees some past events, sped up (blueshifted). The issue is discussed briefly in this FAQ.

Prostitute Schedule in San Francisco

[Anonymous Coward]

The prostitute schedule in San Francisco is now updated on a daily basis. The prostitutes work at the Mitchell Brother's O'Farrell Theater.

Spread the word.

minor error

[Combas]

This is a bit off topic, so my karmas' probably going to take a hit for saying this..

But what I don't understand is how anyone can claim that light slows down as it travels in near proximity to the even horizon.

Light is an immutable substance, that's one of lights unique property's.

For example: If you travel in a space ship towards Pluto at near the speed of light and someone travels in another ship from Pluto to earth at the same speed, and if as they travel towards one another they each flick on a powerful light, when each of them measures the speed of the light coming from the other ship they will find that although the speed of the lights transmitter is incredibly high the light itself is still travelling at "the speed of light" 186,000 miles per second. Weird, but true, the speed of light just isnt affected by anything, including gravity, or perhaps I was away that day in school.

Please correct me if I'm wrong.

Re:minor error

[promatrax161]

The question is: What slows down? It is true that light is always moving at the speed of light (tautology), but not always along the same straight lines that an observer away from the gravitational influence of the black hole sees.

I think the issue is what happens with the matter. We expect to see all clocks on a spaceship approaching the event horizon slow down (provided we are located further away from the horizon than that spaceship is). As for the speed of light: if a spaceship is moving at the speed of light, than, for an external observer, the clock on that spaceship stands still. In other words, a photon has no internal clock, there is nothing that can "slow down" there.

Re:minor error

[DarkKnightRadick]

Light moves, generally, at c.

The problem is though, that light can be slowed down. According to several sources, light can be slowed down, although they all seem to agree that a photon travels at the speed of light no matter what, just the absorption/release/re-absorption process can slow down how quickly it crosses a given distance.

Re:minor error

[promatrax161]

But that is another question: how fast is the (radiation) energy transferred. Of course it depends on the optical thickness of the material etc...

Re:minor error

[Anonymous Coward]

The answer is that while the light moves locally at c the distance between the photon and a point in its path isn't necessarily shrinking/growing at that speed. While this seems counterintuitive, it happens because gravity deforms the space itself between the black hole (or any other gravitating object) and an observer. An 2x2x2 cube of space will eventually become a 4x1x1 cube and so on.

Re:minor error

[odohvare]

But light is effected by things such as gravity. It bends around planets, and other stellar objects. Light exhists as both a particle and a wave. I think that is how I read it several years ago. Light is however just a photon.

"# Light acts like particles--little light bullets--that stream from the source. This explains how shadows work.

# Light also acts like waves--ripples in space--instead of bullets. This explains how rainbows work. In fact, light is both. This "wave-particle duality" is one of the most confusing--and wonderful--principles of physics. "

The Above was taken from a site that I think is built for the much younger crowd. You can take a look here : http://www.learner.org/teacherslab/science/light/

Re:minor error

[Eivind]

Yes. A photon does indeed travel at the speed of ligth. This is true for the same reason that it is true that a cow generally travels at the speed of cow.

Hint: what is the *definition* of "speed of ligth" ?

Re:minor error

[ichin4]

Please correct me if I'm wrong.

You are wrong. But in a very understandable way.

In special relativity, the speed of light is a constant in all frames of reference. But special relativity only applies to non-accelerated frames.

In general relativity, the speed of light does vary in accelerated frames. And by extension, in frames with gravity, since general relativity requires that frames with gravity act just like accelerated frames.

In the 20s and 30s, a few people made fun of Einstein for first saying that the speed of light is a constant, then saying that gravity could change the speed of light.

Re:minor error

[arevos]

I believe the GP post was merely noting that photons have a constant speed of c.

Re:minor error

[Anonymous Coward]

But what I don't understand is how anyone can claim that light slows down as it travels in near proximity to the event horizon.

ichin4's post here is the correct answer to your question. Light travels at a constant speed only in a local inertial frame; in a noninertial frame, it can travel at any speed.

It is true that light's interaction with (charged) matter slows it down from its vacuum speed, but this has nothing to do with light emission near black holes, which are largely in vacuum.

(It's also true and may be relevant, depending on what you're thinking of, that light gets redshifted by gravity as well, and its frequency is slowed down.)

Re:minor error

[DarkKnightRadick]

too true. I have heard (but cannot quickly find) that they had "slowed" light down to the speed of a man on a bicycle.

Re:minor error

[Eivind]

He's talking bull. Plain and simple.

Look:

The problem is though, that light can be slowed down. According to several sources, light can be slowed down, although they all seem to agree that a photon travels at the speed of light no matter what, just the absorption/release/re-absorption process can slow down how quickly it crosses a given distance.

• It's not a "problem" that light can be slowed down, he is confusing the fact that the speed of light *in*vacum* is a constant, with the equally well-known fact that ligth moves slower in all other materials, in some very much slower. (this is the reason we've got difraction, what makes fiber-optics possible and so on.
• Several sources my ass. This is something only someone who understands nothing of it, and knows nothing of it himself would say. It's sorta like saying that "according to several sources" there is gravity on earth. While technically correct it's not exactly a insigthful comment.
• They *don't* all "seem" to agree that a photon travels at the speed of light, no matter what. Infact that is absolutely totally wrong. It's only correct in vacuum. In *anything* else the speed of ligth (and thus photons) is lower. (the higher the index of refraction of the material, the slower)
• He seems to be saying that speed, time and distance is *not* connected to eachother according to distance=time*speed. He says that a photon travels always at C (FALSE!), but that it still can use longer than distance/C time to cross that distance. This leads to the question what exactly he means by "speed" when he explicitly says he does *NOT* mean distance/time.

Ok. I'm being hard on the poor fella. It's just -- sometimes it'd be beneficial if people would recognize the limits in their own understanding. Discussing black holes when you don't know what "speed" is, have no idea what causes refraction. Thinks photons always move at C and so on is sorta pointless.

For the same reason you need to know what "multiply" and "add" means before you can fruitfully discuss second order heterogenous differential equations.

Re:minor error

[arevos]

Whilst the GP is a little confused, most of your rebuttals are worse, because they're completely incorrect.

Current physics says that photons are massless particles that travel at a constant speed c. This speed is independant of the motion of the observer, and independant of the forces acting upon the photons. Incidentally, we have pretty good evidence for all this.

Light waves slow down in a solid medium, but the individual photons do not. The GP is correct in asserting that the slowdown of a light wave is caused by a process of absorption/readmission of photons. This is due to the quantum mechanical processes of excitation and de-excitation. If you've pursued physics past high school level, you'll certainly have done some experiments on this phenomenon.

You assert that photons don't always travel at c, but think about this. If this were true, then the photons are being decelerated by a force. This force could equally be applied to photons in empty space, which would mean that the speed of light in a vacuum was variable. Whilst this may be what occurs, current experimental evidence suggests otherwise.

As for the GP's muddle over distance and time, I can only assume he refers to relativistic time dilation.

Re:minor error

[Eivind]

Light waves slow down in a solid medium, but the individual photons do not. The GP is correct in asserting that the slowdown of a light wave is caused by a process of absorption/readmission of photons.

How do we know ? If the new photon is identical to the old, and the absorption/readmission has no observable effect, how do we know it took place at all ? We certainly cannot observe individual photons on their way "between" absorbtion/readmission events, because to do that we'd need to let them interact with some sort of sensor inbetween, and... you see where this leads.

Re:minor error

[arevos]

How do we know ? If the new photon is identical to the old, and the absorption/readmission has no observable effect, how do we know it took place at all ?

A number of reasons, not all of which I know enough about to comment on. The photoelectric effect demonstrates that light can liberate electrons from atoms; high enough frequencies of light create small amounts of static electricity in metals. We also know the opposite can occur; that an electric current can produce photons, which is how LEDs work.

So from these two examples, we know that atoms can absorb and readmit photons. We also know that photons, as the carriers of light, cannot be slowed down by a force, according to Relativity. If the photons themselves are not slowing down, then something else needs to explain this activity. And though the patch of light within a solid sometimes isn't a straight line, this change in direction isn't enough to account for the decrease in speed. Further, most transparent mediums don't bend light at all, and yet we still see a decrease in speed.

If we rule out all that, the only thing left is absorption/readmission, and coincidentally enough, we know that atoms can do just this.

There are likely a lot more reasons, but that's beyond the scope of my knowledge. Suffice to say that no scientist seems to be questioning this idea, suggesting that it has some solid grounding and evidence.

Too many jokes

[JakeD409]

There are way too many jokes to make about reaching the "point of no return" while being in a "black hole" to choose just one.

Wait a second....?

[Anonymous Coward]

Ummm wasn't Event Horizon a movie about a ship being lost in deep space, where it came back abandoned but it did not come back..........alone!!

What would happen..

[Durinthal]

..if someone tried to steer a spaceship into a black hole, from their perspective? Would the rest of the universe seem to freeze to them until they died of old age? Or would they actually be destroyed by some force inside the hole itself?

Re:What would happen..

[Anonymous Coward]

No. Theoretically time would pass normally for them. They would however, appear to be moving more slowly to the rest of the universe if we could view them from afar.

Re:What would happen..

[Gleng]

Or would they actually be destroyed by some force inside the hole itself?

Have a read about Spaghettification.

Re:What would happen..

[bmo]

From: Syd Midnight
Subject: Re: FAQ this shit!
Date: 1999/07/13
Message-ID: #1/1
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Content-Transfer-Encoding: 7bit
References:
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X-Trace: news.onlynews.com 931856756 216.144.10.111 (Tue, 13 Jul 1999 02:05:56 PDT)
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MIME-Version: 1.0
NNTP-Posting-Date: Tue, 13 Jul 1999 02:05:56 PDT
Newsgroups: alt.tasteless

GRay wrote:
>
> Dave wondered:
>
> > ObT: MotorGimp Stephen Hawking says that for an astronaut who falls inside
> > the event horizon of a black hole time would slow down at an exponentially
> > increasing rate, such that eventually it would appear to stop altogether.
> > Imaging having itchy balls just as you pass through: You'd start to
> > scratch, maybe get you hand halfway there in the first second after you
> > pass the event horizon. In the second after that, you cover another half of
> > the remaining distance. The second after that, half again. But you'd never
> > ever reach 'em. Spending eternity with itchy balls, and never being able to
> > get close enough for a good rub. Now *that's* hell. Take note, christians.
> >
>
> Or, bliss!
>
> Imagine, the eternal cum:) Just as you start to cum, you fall into the
> event horizon. The intense first spasm slows as you fall forward,
> eternally cumming...
>
> What a way to go, er, cum.

    ObScienceNerd: [Geek Voice] Actually, according to the Theory of
Relativity, the astronaut would not experience the time slowing, and
would be able to enjoy one final ball scratching/orgasm before being
ripped apart atom by atom by tidal forces, and utterly annihilated.
    The time-slowing would be seen by observers, however. So if they were
watching an attractive astronaut removing her clothes, wanking furiously
as she slipped her thumbs into the waistline of her panties, when she
crossed the event horizon they would be disappointed as she would appear
to suddenly slow and stop, getting redder and dimmer before finally
fading from sight, forever denying the observers a glimpse of her silken
pubic thatch. They'd have to close their eyes and use their
imaginations.

    ObT: Ejaculation on zero gravity. If an astronaut were able to
concentrate enough during orgasm, they could shoot their load across the
cabin and nail a fellow astronaut in the back of the head. I imagine
that this has been done on Mir, because 4 or 5 months in space station
the size of a trailer truck would get pretty fucking boring. Of course
on a leaky deathtrap like Mir, they'd be lucky just to get a hard-on
before the obligatory "You're Fucked" klaxons started honking,
announcing the life threatening crisis-of-the-hour.

    ObMir: 3 astronauts, one decrepit space station, and a 2-man escape
pod. What do you want to bet that the US and Russians each hid a pistol
somewhere in the station "for emergencies", and that each astronaut made
sure, every minute of every day, that they they knew EXACTLY the
quickest route to the pistol/escape pod?

--

      Rev. Syd Midnight - Remove SPAM from my address to reply

"God had some serious quality-control problems."
          -- Superior Court Judge Leslie Light

Re:What would happen..

[CODiNE]

Heh... and here I was thinking the link would explain the Flying Spaghetti Monsterism explanation of Black Holes.

Aha!

[Anonymous Coward]

So THAT'S what the goatse guy is doing!

Reuters was only off by 99.99%

[xmark]

"The one-way journey from the heart of a galaxy into the oblivion of a black hole probably takes about 200,000 years..."

"...scientists figured that material moving at 177 000km an hour would still take eons to cross into a black hole."

Eons are the largest division of geologic time. There have been just four of them since the formation of the Earth. In rough terms, that's a billion years each.

Maybe the reporter can get a job working on unit conversion for the next Mars probe. (*cough*)

Re:Reuters was only off by 99.99%

[Phanatic1a]

From our frame of reference, matter moving at 177,000km an hour would still take eons to cross into a black hole. In fact, we'd never observe it doing so, by the time it crosses the event horizon any radiation from it will be redshifted to infinity.

From the frame of reference of that matter, entry into the black hole will take but an instant.

This is relativity. Always specify your FOR.

Scientists have what????

[fcummins]

Scientists don't prove anything. Ever. Please. That is not the nature of science. Every claim that 'scienctists have proven...' is simply false. Science works differently. Less likely theories are distinguished from more likely ones, and the most likely theory is typically taken as the working hypothesis of the day. This isn't nitpicking. Any paragraph that begins 'scientists have proven...' consigns itself to the bin of tabloid nonsense immediately.

Re:Scientists have what????

[Jesus_666]

Scientists can prove that a certain observation fits a certain theory. They can't prove a theory, but they certainly can't not prove anything.

Re:Scientists have what????

[stridebird]

I don't think so. Observations can never prove a theory. All observations - ALL observations - include uncertainty and the best that can be shown is a good fit of the observational data to the theories predictions. And trying to calculate the EXACT uncertainty unfortunately produces another measure of uncertainty. Mathematical proofs can be derived to prove a means of calculating the degree of "fit" of observational data but that still doesn't get around the uncertainty problems encountered when handling any observational data.

Re:Scientists have what????

[Jesus_666]

Note that I explicitly admitted that scientific theories can't be proved. However, scientific theories aren't "everything" and the assertion that it's impossible for a scientist to prove everything is (in my opinion) wrong, even when we only look at science. For example, why do you think that mathematical proofs may introduce uncertainty? Probably because some mathematician (= a scientist) successfully argued that they do. Now, is "mathematical proofs may introduce uncertainty" provable? You appear to say no, because a scientist said it.

Re:Scientists have what????

[DM9290]

the word "prove" bears defining. Technically nothing is ever proven in a courtroom either, if the standard of proof you expect is "beyond any conceivable doubt", because in court you only need to prove guilt "beyond all reasonable doubt" or many other things "on the balance of probabilities". But this is not the standard used by science. Scientists hold them selves to a much higher standard.

On the otherhand... engineers dont require proof beyond all conceivable doubt. But are quite happy to build machines based on what science has "proved". Because by the standards of engineering, something merely need to work in the vast majority of cases (say 99.999% or 99.9999% of the time for high quality work). Scientists are trying to find solutions to prove things far beyond 5 or 6 significant digits of reliability, but to an infinite degree of reliability.

So, while nothing is ever proven by scientists according to the standard of mathematicians (i.e. absolute perfection), according to other standards.... science proves all sorts of things.

Making a claim like "scientists never prove anything", is misleading, because it causes one to confuse your position with being "truth is subjective", and such notions are what allow certain idealogues to claim the earth is 5000 years old (based on the scientific fact that there is perhaps a 0.00000000000000001% chance or so that the earth might actually be only 5000 years old) or that global warming is a myth based on the perhaps 1% chance that it is. Scientists prove things to the limits of our technology and knowledge and scientists also document the margin of error to which it is proven, and that is a hell of a lot MORE proof than anyone in any other field provides, except pure maths.

In almost all fields of human endeavor, except the very highly technical, such as medicine or advanced engineering, we can treat science as if it were proven with absolute perfection because our own human error is so much more likely to be the cause of difficulty than the margin of error in most main stream sciences in trying to account for why things go wrong.

There is a slight chance that your automobile does not actually need gasoline. It may be a perpetual motion machine. But are you going to disregard the science which has "proved" that your car is not? Are you going to invest your time and money trying to invent a perpetual motion machine yourself?

Are you worried that your car may transmute into a thermonuclear bomb and cause the sun to implode the next time you start it? Science "proved" those things wont happen well enough for you to treat those things as impossibilities. And yes, scientist to scientist... I can't be certain you wont destroy the solar system tonight, but I science has "proven" I'm more likely to simply burst into flame myself in the next 30 seconds, so I have more urgent matters to attend to.

"100 million times the mass of the sun."

[Anonymous Coward]

hey dudes, that's alot!
so, is it just like a small super duper dense dot, or ...
might it lead somewhere ... wormholes and stuff ... is there a .. whitehole
too? "100 million times the mass of the sun." doesn't that like eh? have
a space-time weather influence on me too?

km/h !

[Anonymous Coward]

Please, it's km/h
not Km/Hr
not kph
not Km/H
not anything else...

Please get it right, it's not *that* hard!

Re:km/h !

[Anonymous Coward]

BZZZZT WRONG!

it is kmh^-1

HAND

WRONG!!!

[mangu]

The black hole itself emits black body radiation

Nope. You are confusing black holes with black bodies. A "black body" is an object which reflects no light, but it emits its own radiation. A "black hole" is an object which emits no radiation at all.

A black body emits radiation unless its temperature is absolute zero. It need not be black to the human eye, black bodies used in laboratories often are glowing red hot. Black bodies used in labs are usually formed by a cavity with a very small hole. The energy emitted by the hole is the black body radiation. It's simple to see why that body is black. Make a small hole in a big cardboard box and shine a flashlight on the hole. Seen from the outside, the light from the flashlight will be lost inside the big box and the hole will still be seen as black. Confusing names, yes, because a hole in a black body is not a "black hole"...

The study of black body radiation was one of the problems that started the science of quantum physics. The spectrum a black body emits does not follow the shape that classical physics predicted. The German scientist Max Planck created a theory that correctly predicted the black body spectrum by postulating that energy is not emitted in a continuous way, but in lumps which he called "quanta".

Black holes, OTOH, emit no radiation at all, no light can escape from it because its escape velocity is the speed of light. However, since no material body can accelerate all the way to the speed of light, the stuff that falls into the black hole will take forever, as seen from our external referential. Since we see that stuff going away from us at a very high speed, any radiation it emits is redshifted. But the blakc hole itself emits no radiation, it's the matter falling into it that emits radiation.

The evaporation of black holes is a different thing. In our universe, due to Heinsenberg's uncertainty principle, pairs of particles and anti-particles may be created spontaneously and recombine a very short time after. But if that happens very close to a black hole, the pair may not have time to recombine because one of the particles will be caught by the black hole. In this way, the anti-particles will be going into the black hole, slowly cancelling its mass while the particle radiates away. Given enough time, the black hole disappears completely.

Re:WRONG!!!

[Anonymous Coward]

"The black hole loses mass by what is radiating away, not by what is dropping back into it."

Actually, nothing can radiate away that has already crossed the event horizon. The earlier comments from mangu pretty much perfectly describe what is known as hawking radiation. The particles we see radiating away from the black hole were never really inside it.

Re:WRONG!!!

[Wolfbone]

"Even anti-particles still have a positive mass. They cannot cancel out anything. The black hole loses mass by what is radiating away, not by what is dropping back into it."

In the presence of an event horizon, though, occasionally one member of a virtual pair will fall into the black hole while its partner escapes to infinity. The particle that reaches infinity will have to have a positive energy, but the total energy is conserved; therefore the black hole has to lose mass. (If you like you can think of the particle that falls in as having a negative mass.) We see the escaping particles as Hawking radiation. It's not a very big effect, and the temperature goes down as the mass goes up, so for black holes of mass comparable to the sun it is completely negligible. Still, in principle the black hole could lose all of its mass to Hawking radiation, and shrink to
nothing in the process.

[Lecture notes on G.R, Sean Carroll]

Re:WRONG!!!

[nacturation]

Still, in principle the black hole could lose all of its mass to Hawking radiation, and shrink to
nothing in the process.

If the black hole loses enough mass, wouldn't it get to a point where it no longer has critical mass so it expands again?

Re:WRONG!!!

[Wolfbone]

It's a black hole when all its mass is contained within a radius r = 2GM/c^2. When it loses mass due to the Hawking effect, that radius (the radius of its event horizon) shrinks too and it stays a black hole.

Re:WRONG!!!

[WaterBreath]

In the presence of an event horizon, though, occasionally one member of a virtual pair will fall into the black hole while its partner escapes to infinity. The particle that reaches infinity will have to have a positive energy, but the total energy is conserved; therefore the black hole has to lose mass. (If you like you can think of the particle that falls in as having a negative mass.) We see the escaping particles as Hawking radiation.

There's something that always bothered me about this idea....

Whenever one of these spontaneous particle pairs pops out of the vacuum, there is always one matter particle and one antimatter particle. Shouldn't there be an equal probability that either one could fall into the black hole (unless it's a charged black hole, of course)? If equal numbers of matter and antimatter particles are going in, then shouldn't they balance each other out and result in a net effect of zero?

And speaking of charged black holes.... If the black hole has a predominant positive charge, then it seems to me that it would be predisposed to pulling in negatively charged particles as opposed to positively charged ones. So, when an electron-positron pair pops up, the electron is more likely to be "eaten", and contribute to mass, rather than negate it. Does the lower probability of proton-antiproton pairs popping up, combined with the larger mass of anti-protons exactly balance out the contributions of the electrons? If not, then either either the positive or negative black hole will have steady growth, instead of evaporation.

And what about anti-matter black holes? Granted, anti-matter is so rare that it's extremely unlikely to arise in nature. But if one did arise, then by absorption of positive matter, would we not have the exact opposite dilemmas that we have with regular black holes? E.g. if a "regular matter" black hole evaporates naturally, then anti-matter black holes should naturally grow, and vice-versa. Man, I only had just the one quandry to start with, but once I got thinking, all this stuff popped up!

Re:WRONG!!!

[Anonymous Coward]

Whenever one of these spontaneous particle pairs pops out of the vacuum, there is always one matter particle and one antimatter particle. Shouldn't there be an equal probability that either one could fall into the black hole (unless it's a charged black hole, of course)?

Yes. Hawking radiation emits equal numbers of particles and antiparticles.

If equal numbers of matter and antimatter particles are going in, then shouldn't they balance each other out and result in a net effect of zero?

No. Whichever particle escapes (be it matter or antimatter) always has positive energy, and whichever particle falls in (be it matter or antimatter) always has negative energy. This has to do with how the timelike coordinate behaves inside and outside of the event horizon. This discussion has more information.

And what about anti-matter black holes? Granted, anti-matter is so rare that it's extremely unlikely to arise in nature. But if one did arise, then by absorption of positive matter, would we not have the exact opposite dilemmas that we have with regular black holes? E.g. if a "regular matter" black hole evaporates naturally, then anti-matter black holes should naturally grow, and vice-versa.

No. The composition of the black hole is entirely irrelevant (the "no-hair theorem"). A black hole of a given mass, charge, and spin will be indistinguishable from any other with those same three properties, regardless of how it was formed. (With some caveats...)

Re:WRONG!!!

[Wolfbone]

"There's something that always bothered me about this idea....

Whenever one of these spontaneous particle pairs pops out of the vacuum, there is always one matter particle and one antimatter particle. Shouldn't there be an equal probability that either one could fall into the black hole (unless it's a charged black hole, of course)? If equal numbers of matter and antimatter particles are going in, then shouldn't they balance each other out and result in a net effect of zero?"

You're probably right to be bothered by it! My understanding is that the particle pair description is only a crude (and potentially misleading) picture of a result that involves doing quantum field theory calculations on a classical curved spacetime. Not only is such a calculation difficult, it may not be appropriate to the physical conditions and I believe that the Hawking result isn't universally considered completely sound (and likely won't be without a working quantum theory of gravity). Even if it held up it'd still only be a theoretical prediction of course.

Re:WRONG!!!

[nacturation]

Cool, thanks for the explanation.

Scanning Tunneling Microscope

[Shashvat]

You can now take pictures of atoms with a scanning tunneling microscope.

Researchers at IBM even move individual atoms around to create artwork.

More here: http://www.almaden.ibm.com/vis/stm/corral.html

Re:Scanning Tunneling Microscope

[Combas]

Dont get me wrong, very interesting, but these pictures proves nothing.

FTFA: "These spatial oscillations are quantum-mechanical interference patterns caused by scattering of the two-dimensional electron gas off the Fe adatoms and point defects."

Oscillations and interference patters, _not_ atoms, although they are pretty pictures I suppose.

Re:Scanning Tunneling Microscope

[Anonymous Coward]

Yes, they're pictures of atoms, taken with electrons instead of photons. What we see with our eyes, after all, is nothing but the oscillations produced by the scattering of photons off atoms.

Unless you are a Heechee.

[ivaldes3]

Spoiler warning: If you are a Heechee then black holes make great hiding places. You could take your spaceship in and out as well as rescue your girlfriend trapped in the event horizon. -- IV

That's a lot better than pulling them from...

[Anonymous Coward]

... Uranus!

Oh wait....

WRONG!!!

[Ihlosi]

In this way, the anti-particles will be going into the black hole, slowly cancelling its mass while the particle radiates away.

Even anti-particles still have a positive mass. They cannot cancel out anything. The black hole loses mass by what is radiating away, not by what is dropping back into it.

Hey, Samantha Carter!

[mpitcavage]

You guys could totally go back and rescue those guys from season 1! heck, you'll even be able to wait till season 12.

NOTHING, except..

[Sigg3.net]

The gravity around the "hole" of a black hole is so strong that NOTHING can make its way back out after a critical distance.

I believe you meant "NOTHING, except for Chuck Norris, can make its way back".

Not exactly

[Anonymous Coward]

A black hole "emits" heat, but since heat is transmitted as infra-red travelling at speed of light, it is never "emitted". So you don't get to "feel" the heat or the light.

That makes no sense. Ordinary objects radiate heat via infrared rays travelling at the speed of light, and you can certainly feel their heat. (Also, heat is not restricted to IR radiation; all forms of EM radiation transmit heat.)

Black holes emit heat due to Hawking radiation which, roughly speaking, is produced at the event horizon. (It is actually not completely possible to "localize" the source of the radiation, and you can equivalently describe it in terms of the popular particle-antiparticle pair production near the horizon, or quantum tunnelling through the horizon.)

Stars are dense and hot, and once they shrink/collapse they will be hotter. My guess is the inside of a black hole must be much hotter than the temperate of the core of the star from which it collapsed.

The inside of a black hole is a vacuum, with the possible exception of the central singularity. (We don't know whether anything still exists there.) Plus of course any transient matter that may happen to be in the process of falling to the singularity, although that doesn't last long. The interior of a black hole bears no resemblance to the core of a star.

HELL is inside a black hole!

[SoldierOfGod]

Well the thing that I find amazing is that so many people waste their time with this scientific devil-worship, drawing their attention away from the one true God! By wasting their lives with these futile pursuits of such nonsense, they spend no time studying the Truth of God's pure Word! Put down that telescope and pick up your Bible, sinners!

The Converse isn't necessarily true

[Solr_Flare]

Black Holes are just that, holes in space-time. While, to an outside observer, time appears to stop at the event horizon that doesn't mean that time stops for the person going into the black hole. All it means is at that point the object is no longer moving forward in time with the rest of us. The person/object could just as easily be moving backwards in time or broken free of time, both of which are plausible explainations if you look at the structure of space-time in an Einsteinian manner.

nitpick

[aepervius]

The GP probably means you can't measure the temp of the black hole itself (as a balck body) like you can do for stars, because no radiation from the black hole itself would come out. At best what you measure is the black body temp of the acretion disk or hawking radiation which is NOT the BB temp of a black hole.

What they don't mention

[merlin_jim]

Is the scientific reason that gas can take 200,000 years to be pulled in by the most gravitationally massive type of object possible in the universe.

It is for two reasons; first off, gravitational time dilation - time gets slower the closer you get. The gas is orbitting the black hole, which also adds relativistic time dilation.

The gas, in fact, probably orbits at just under escape velocity - thanks to a fun little effect called (IANAA) relativistic frame dragging - basically the black hole drags the fabric of spacetime around itself - and objects within about 1.5 radii of the event horizon start feeling the effect - effectively locking them into a particular path. One way to look at this is to say that time is swallowed by the black hole same as mass - and therefore objects in the vicinity of the black hole fall in because their time arrow points to its dark, dark heart.

This frame dragging should happen at speeds approaching the speed of light - and require comparable amounts of energy to change your frame. There's even some theory that infalling matter will follow gravitational field lines, like you get around a magnet - but I'm not sure how much I believe that...

nitpick nitpick

[Anonymous Coward]

The Hawking radiation temperature is the blackbody temperature of the black hole. For very deep reasons in the quantum field theory of curved spacetimes, horizons themselves do have a blackbody temperature, and that temperature is equal to that of the radiation (Hawking for a black hole event horizon, Unruh for a Rindler horizon) emitted from that horizon. See, for instance, chapter 4 (IIRC) of the text by Birrell and Davies.

There's a joke in there somewhere...

[Catastrophator]

I didn't think people pulling facts out of (their) "black holes" was anything new :)

Wormhole

[Anonymous Coward]

Some scientists belive that Black Holes are infact a gateway, that bend space and time. As most /. ers know id imagine all objects with mass have an effect on the universe, black holes have so much gravity they may actually bend space time. Creating a wormhole effect. Now this cannot be proved at our current stage of technology but one day we may be able to prove this. Some scientists also belive that as energy can never be destroyed and black holes would seem to do this as they "eat things" that there must be a so called white hole, pushing stuff back into the universe, although one has never been observed.

What I found intersting

[AviLazar]

Within a boundary of about 50 miles from the black hole center, gravity is so strong that not even light can escape its pull

I found this to be the most interesting...and here I always thought if you were anywhere near (as in hundreds of thousands, if not millions of miles) of a black hole you would be screwed...but apparantly, it is only on the 50 mile marker that you are totally hosed...I mean with our technology we would be hosed at much farther distances...but if we ever have a chance to travel at just below the speed of light then we could get within say 75 miles of a black hole? :) Just food for thought...well my thought.

Re:What I found intersting

[twifosp]

Even if we could travel at say, .75c, anything "near" a black hole any living thing would be hosed. The g forces of orbit insertion would likely tear our fragile bodies and crafts apart. Additionally the Xrays, gamma particles, and other nasty radiation would cook and fry everything.

Achieving neccesary velocity to not fall into a black hole would be easiest part. Even if we got into some kind of orbit, at near C speeds, you'd never leave. You'd need even closer to C speeds, and if you were near the event horizon, you'd need to actually achieve C to escape, should you ever want to extend your orbit beyond the black holes sphere of influence. IE come home. Which would require infinite energy.

SO in short, I suggest we just stay the hell away from black holes :D

Re:What I found intersting

[mfrank]

Just use a puppeteer General Products hull. Make sure you stay near the center of mass, though, unless you want to be mistaken for strawberry jam.

Orbit is...

[CarpetShark]

As I understand it (which is not well :), an orbit is when something falls, but keeps missing what it's falling towards, because of their different speeds/directions.

Re:Orbit is...

[LouisZepher]

...but keeps missing what it's falling towards... That is, of course, the art, or rather the knack, to learning how to fly...

More to an orbit

[snowwrestler]

An orbit is not just things falling down, it also requires a tangential velocity within a specific range. Gas spiraling into a black hole does have a tangential velocity, but it's not within the range create a orbit. In other words, yes, it was never in orbit.

Re:More to an orbit

[twifosp]

Well I hate to nitpick a nitpicker, but actually it is in orbit. Everything everywhere, including galaxies are in some form of orbit. There is some evidence of some black holes orbiting super massive black holes. It just so happens that the gases tangential velocity and/or momentum (gas would have very little) puts it in an orbit where it's perigee (the minimum distance of the orbital path and the orbital body) actually crosses the mass itself. So yes, it is in fact in orbit. Just not a stable one with good eccentric circle :D

Time

[Turn-X Alphonse]

Maybe someone could answer this for me please. I'm not much into this sort of thing but now I think about it I'm curious.

If you did get stuck in a black hole (I know you can't, you'd be dead from the tidal forces), how would time seem to you? For example if time slowed down to next to nothing would you notice it had slowed or would just everything speed up around you?

Thanks for any help guys.

Re:Time

[twifosp]

Depends on which branch of phsyics you want to use. If you use general relativity, then the math says that the space time curvature would be so great, that time would curve back in on itself. Since we think of time as linear, this would indicate time would stop all together when compared to any other frame of reference. This is one of the major reasons why GR breaks down under black hole conditions.

Qauntum mechanics gives us a different picture. It basically would describe any matter crossing the event horizon being destroyed and converted into what basically amounts to energy waves with mass. Then, really crazy shit happens. Or, uhh quantum shit rather. It's theorized that the electrons that form these waves cross the event horizon traveling backward in time, scatter, and then radiate out of the black hole traveling forward in time. This is what causes the "black hole evaporation" that Hawking now recongnizes as information leaving a black hole.

Re:Facts.

[Old Grey Beard]

...most large black holes have gas streaming into them from their surroundings...

Do we "know" this, or is it simply possible that we can more easily find those black holes that emit X-rays?

yeah

[Anonymous Coward]

but why can't we just chuck a big super computer into an anti-black whole event horizon with a wireless antenna (where time moves faster instead of slower) and solve all our endless computations in no time. Forget quantum computers..

Re:you mean velecity not energy

[vertinox]

Same thing with Earth, if Earth was in the middle of a cloud of gas that could eat away at very large amounts of the Earth's momentum, then the Earth could spiral into the Sun. Since that gas isn't there our Earth keeps revolving around the Sun, which is good for us.

It's not the earth's energy, but rather its velocity in orbit that keeps it from free falling into the sun.

In fact, everything in orbit around earth is in free fall. The reason it does go straight towards the earth is because of its velocity in the horizontal direction which makes it fall vertically really slow.

Yes, this velocity does require a great deal of energy (aka rocket fuel) to get something in orbit, but you don't have to constantly generate it once you are up there.

Gas and various other objects would affect the earths velocity so it would crash into the sun sooner, but as all things in the orbit, the Earth is on a death spiral towards the sun as we speak.

But don't worry, it will be a couple thousand million years and by then the sun will have burt off enough fuel to expand itself way past our present location so it would be a moot point regardless.

Re:you mean velecity not energy

[scarletire]

In a classical sense, energy = 1/2 mv^2. So orbital velocity and energy are equivalent. I think the point you're trying to make is that you don't have to do work to remain in orbit.

You keep using that word

[aminorex]

> Astronomers have proven the existence of the event horizon

You keep using that word. I do not think it means what you think it means.

Orbit is best described...

[Anonymous Coward]

As falling, but missing the ground.

where does the tangential velocity come from?

[Anonymous Coward]

If the black hole is sucking gas from a neighboring star, why isn't the velocity vector of the gas stream pointing towards the center of the black hole instead of tangential to it?

Re:orbit at greater than c

[Anonymous Coward]

No, you have to go as faster than 'c' below the 'event horizon'. The escape volocity AT the event horizon is 'c' according to current theory. Now to orbit at the event horizon requires faster speeds yet. The idea that it would take '200K years' to cross the horizon once so close is preposterous. More probably that is what the 'observer from outside the system would see as 'time dilation' is witnessed by only the outsider as events slowly catch up to what has happened long, long ago. I won't go into the math as most slashdotters seem to be of a lower brow as witnessed by all the asenine posts that I have read on serious subjects. Rest assured that vector calculus in three dimensions is involved for the simple de-orbit calcs. The other time dilation crap I will leave in the hands of the successors to Dr Heim to whom the world will one day owe a debt of gratitude for handing mankind the keys to interstellar travel and trade.

Any SF stories on mining the neutron stars?

[mi]

From the article:

Stars of about 10 to 25 solar masses will collapse into compact spheres about 10 miles across, called neutron stars. These objects have a hard surface and no event horizon.

I'd imagine there being a lot of very useful stuff in those compact spheres. Is it theoretically possible to mine these things?

Getting even a small bit off is going to be very hard, of course, but if the light can escape, it should still be possible, no?

Re:Any SF stories on mining the neutron stars?

[twifosp]

Technically possible? Yes. Anywhere near practical? Probably not.

The body would still contain a great amount of mass, and therefore gravity.

Problem one: Pretty much any matter would still be ripped apart when it touches down on the surface (or anywhere near it). Good luck creating equipment that can survive. Basically the only matter that would probably be strong enough to stay cohesive, would be the type of dense matter you could actually get from a neutron star.

Problem two: Assuming you could actually create equipment capable of surviving the gravitational forces of the star, the energy or resources you pull out of the star would not be greater than the energy required to lift it off the star and escape the star's gravity for use. So you'd have to somehow convert it and transfer the energy from afar. Basically the only way to transfer energy from a large body of mass is how all stars do it. Light. So to mine energy from a dead neutron star, you'd want to ship a nebulous cloud of hydrogen right next to it. Wait a good eon or too for the pressure and gravity forces to start up another fusion reaction inside the star. Then you could mine the star from afar... with solar energy. :D

Basically an active star is about the best energy resource phsyics can imagine using the elements we know about. Fusing hydrogen.

A few photos..

[dimension6]

...of the star in question.

The same effects

[Anonymous Coward]

can be produced by immense electrical stress. No need for exotic physics, or even black holes. Feed matter through a massive plasma "pinch" and see what happens.

Re:The same effects

[Anonymous Coward]

No need for immense electrical stress: just apply a strong mass and a bit of relativity. Get close enough and you need to exceed c to escape again, thus leading to capture.

The Equality of Inertial and Gravitational Mass

[DM9290]

I'm afraid you aren't going to find any physicists out there arguing that GR doesn't claim that gravitational mass and inertial mass are equal. It has nothing to do with what any experiment proves. The theory is the theory.

Anyone claiming that General Relativity does not purport that Gravitational Mass and Inertial mass are the same, simply doesn't know the theory.

Here is a direct quote from Albert Einstein (the guy who discovered GR, and widely respected by scientists):

"The gravitational mass of a body is equal to its inertial law"

- source: Relativity: The Special And General Theory, chapter 19.

I just googled that exact phrase and found it at http://www.marxists.org/reference/archive/einstein /works/1910s/relative/ch19.htm

I wonder what Chapter 20 is called....

"The Equality of Inertial and Gravitational Mass as an argument for the General Postule of Relativity"

do you think its part of the theory?

Re:The Equality of Inertial and Gravitational Mass

[WaterBreath]

I'm afraid you aren't going to find any physicists out there arguing that GR doesn't claim that gravitational mass and inertial mass are equal. It has nothing to do with what any experiment proves. The theory is the theory. You misunderstood me. I'm not looking for arguments about whether GR includes this postulate. I'm looking for arguments about where this postulate isn true in reality. Which, ideally, would be something we can test by experiment. If it were not true, obviously that would mean GR has a flaw.

Re:The Equality of Inertial and Gravitational Mass

[DM9290]

Good luck with that.

Alternatively if you are interested, the current special edition of Scientific American "The Frontiers of Physics" has an article called "The Search for Relativity Violations" on page 13. It is on news stands now until February 20, 2006.

If you want to design your own experiments and stick it to the "man", then what you may want to look for are substances which FALL at different speeds in a vacuum.

Since the force of gravity is proportional only to gravitational mass of both bodies, and acceleration due to gravity is proportional to force and inversly proportional to intertial mass: Something with a lower inertial mass than gravitational mass will accelerate faster in free fall. Something high inertial mass compared to gravitational mass will accelerate slower in free fall.