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

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?

[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?

[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?

[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!

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: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: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

[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: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...

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.

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

[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: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.

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...