Slashdot Mirror
The Speed Of Gravity Revealed
redwolfoz writes "New Scientist is reporting that the speed of gravity has been measured for the first time. 'The landmark experiment shows that it travels at the speed of light, meaning that Einstein's general theory of relativity has passed another test with flying colours.' Researchers made the measurement of the fundamental physical constant with the help of the planet Jupiter. One important consequence of the result is that it will help constrain the number of possible dimensions in the Universe."
Wow, that's pretty cool. Now if we could only figure out why and how gravity works, we'd be in business.
Every year during my review, I just pray the words "slashdot.org" aren't mentioned.
But of course, travelling at the speed of light, all the flying colours just appeared red due to the red-shift.
Does this make my brain look big?
Sure, this experiment will "help constrain the number of possible dimensions in the Universe" ... but will it lead to new weapons?
-kgj
I'm sorry, I don't mean to ask the stupidest question ever, but how does gravity have speed? The last I was taught on the subject (and believe me, it was a while ago) was that gravity was a force, but didn't have mass. Doesn't something need to have mass in order to have speed?
...a topic like this to be a bit more precise in the summary. There's a signifigant difference between .95 times the speed of light, and the speed of light. Not to mention the large .25 margin of error. Which theoretically shouldn't be able to get to +.25 anyhow.
Light has mass? no it does not.. the energy of a photon has a mass equivalence, but it does not have mass.
You're confusion arises because you were taught elementary Newtonian physics. In general relativity, one learns that any "information" cannot travel faster than light. Gravity is considered information because if you feel a gravitational force on you, you know that there is a body out there acting on you. That is, you have information about it (you could even estimate its mass by measuring the tug it exerts on you).
In Newtonian physics, lots of things are assumed to happen instantaneously (like gravity) so they don't have a speed per se. But in general relativity, everything has a speed -- and that speed is no greater than the speed of light.
GMD
watch this
"Kopeikin found another way. He reworked the equations of general relativity to express the gravitational field of a moving body in terms of its mass, velocity and the speed of gravity. If you could measure the gravitational field of Jupiter, while knowing its mass and velocity, you could work out the speed of gravity."
The theory of relativity was appearantly used to detect the speed of gravity. This would be fine if the theory of relativity didn't assume a speed of gravity. Basically, all he did was prove his given. So, if eggs are green, then eggs are green!
Karma Clown
The same way an electric field doesn't have a charge, but affects objects that do have a charge. Gravitational/electric fields are -created- by masses/charges. And don't confuse gravity with gravity waves (the speed of which being what are measured here).
.25 to be kinda ridiculous? So based on their measurements, the speed of gravity could actually be anywhere from 30% slower to 20% faster than light. I mean, the article makes it sound like they're just assuming the real number is 1.0 c because anything else would be really surprising. Or maybe the article is wrong. Or I'm mis-reading it. But at the moment, it doesn't sound like "passing with flying colors" to me.
By the way, did anyone else find the quoted margin of error of
The enemies of Democracy are
Well, now we have the speed of light AND the speed of gravity! If we can find out the Speed of Darkness, we'll be all set!
The sun couldn't suddenly disappear, although that scenario works for the purpose of explaining the speed of gravity. Consider this alternative.
Take the sun and instantly accellerate it to almost the speed of light, toward a collision course with Earth. For most of the 8 minutes between acceleration and collision, nobody would notice anything, as light, all other energy, and gravity would all present the sun as occupying its original location.
However, brief moments before the collision, the sun's change of accelleration toward earth will be noticed. Of course, you're noticing the change that happened 93 million miles away, even though the sun is about to impact. However, one second later, the sun will appear to be almost 186000 miles closer, and it will FEEL like it's 186000 miles closer. Suddenly the gravitational accelleration has increased to reflect the new position of the sun. But within that second, you get all the accumulated influences of gravity over a much larger stretch of space than just the 186000 miles it travelled in that time. Since the sun is moving at almost the speed of light, let's say 99% of it, after 99 seconds, the influence of the sun's gravity will only be 1 second ahead of the sun. However, within that one second between the position of the sun and the gravitational influence of the sun is contained the gravitational influence of the sun over the last 99 seconds. You get the combined force in 1 second that you normally would have gotten in 99. So when the Sun's influence is finally felt by Earth, you will not get a force that implies a steady rise in gravitational force of a sun massed object until impact, you'll get a very quick rise in force of an object that is, generally, about 99 times as large as the sun.
And if you remember relativity, when an object is travelling near the speed of light, the mass increases. So the theory at least makes sense. Here's another thing to ponder. If an object the size of the sun suddenly acquired the 99x its mass, would it not either collapse upon itself, or expand rapidly, nova, and the core would collapse upon itself, causing the same result, a singularity, with a small event horizon. And it will be this singularity that will collide with Earth, ripping through it in a fraction of a second, and the sudden, combined gravitational effect on earth will cause it to very suddenly pull out of it's orbit toward the origninal center of gravity of the sun, with a nice city sized hole carved through it.
Ok, this had no purpose at all, but it was interesting to think about. Go on with your business... nothing to see here. Rant over.
-Restil
Play with my webcams and lights here
I prefer to think of it as having no loose ends...
___
If you think big enough, you'll never have to do it.
From that they worked out that gravity does move at the same speed as light. Their actual figure was 0.95 times light speed, but with a large error margin of plus or minus 0.25.
So, really, they're triumphantly announcing that the speed of the light is somewhere between 0.7 c and 1.2 c, and just supposing it has to be c for everything to make sense.
Physicists have been accused of being loose with rigour, but this is really stretching it.
What next? The speed of magnetism?
Yes, the speed of magnetism. The particle which mediates electromagnetic interactions is the photon which propagates at the speed of light. So if a magnet is suddenly given a push in one direction then there is a delay before distant particles notice a change in the field of that magnet.
This is an analogous result for gravity and the postulated graviton particles.
It's one thing to not understand something, we all have our fields of expertise. But assuming you know everything based on some limited high schooling makes you the saddest kind of idiot.
:wq
"In general relativity, one learns that any "information" cannot travel faster than light"
What about quantum pairs? Move them apart, and a change in one is reflected intantly in the other.
That's why I specifically said "In general relativity...". Quantum pairs are from the theory of quantum mechanics, not general relativity. Physicists have been working hard to try to combine relativity and quantum into a single unified theory. However, problems arise when the two theories predict different things -- such as the quantum pairs example you listed. According to relativity, there would be a finite time lag for the change to be reflected in the second entity of the pair whereas quantum would say that the change is instantaneous.
Incidently, I heard that a few years ago an experiment was performed on quantum pairs and, sure enough, the change was indeed instantaneous. Can anyone else corroborate this?
GMD
watch this
YESSS! In your FACE, Infinite-dimension Universe theorists! You SUCK!
Yeaaaaah! Gimme one up top, bro!
pr0n - keeping monitor glass spotless since 1981.
the energy of a photon has a mass equivalence, but it does not have mass
Bastard. I'm going to have sleepless nights trying to figure out what the hell you mean by that. This is not the kind of concept mere mortals like me should have to deal with while sober.
People couldn't type. We realized: Death would eventually take care of this.
Gravity waves have been used in many stories as a FTL communication system, now that's all out of date.
Venus is a big swampy planet, eh guys ?
I think we should assume more of the author. Think of it this way: Using General Relativity, you can predict what the gravitational field will be. His experiment measured what the field actually was. If the predictions match the measurement, the theory is confirmed (or at least not disproven).
Relativistic speeds are usually measured in terms of gamma, not meters per second. Gamma is a value that represents the amount of time dilation and mass increase an object has; if you're moving at 86% of the speed of light (~206257211 m/s) then gamma is ~2.0, meaning that time would run twice as fast for you, and to a relatively stationary observer, your mass would be double what it is at rest. Gamma is calculated thusly:
y = 1 / sqrt(1 - (v^2 / c^2))
Gamma can rise unbounded; as your velocity approaches light, gamma rises exponentially, reaching infinity when your velocity is equal to that of light. I'm assuming that the original paper used values of gamma for measurement, rather than meters per second.
More about gamma here.
"Destroy science and religion. Science would re-emerge exactly the same; but not religion." - Penn Jillette, paraphrased
Okay, I'll bite.
A photon delivers an impulse when it is fired or when it is destroyed on impact with matter - but when it is in transit in space it has no mass.
Imagine a giant cluster of light, like fired by a superlarge pulse laser. It will transfer momentum to whatever it hits, but it does not actually have mass, so when its in transit this massive ball of light will not suck in anything with its gravity.
- Strong
- Electromagnetic
- Weak
- Gravitational
I even fetched a URL on a whim, just in case you disagree for some reason.How about this: a photon has zero rest mass. However, it is never at rest, but travelling at C. It does have energy, which translates to a very little mass and does warp space time, but when it hits something, that energy goes away, and so does the photon.
I wonder if a sufficient density of photons would collapse into a black hole.
"We returned the General to El Salvador, or maybe Guatemala, it's difficult to tell from 10,000 feet"
...you see, it's all component forces. If you look at the free body diagram of my car travelling on the road, you'll see the normal force, force of gravity, and my velocity in the x direction. As I mentioned, one of these component forces is gravity, labelled FsubG. It was recently discovered, and posted on slashdot, that the speed of said force is 3x10^7m/s.
And THAT'S why, officer, your radar reported that I was going 60 in a 40 zone!
I asked my physics teacher the very same exact question- "if the sun disappeared, would the earth fly off from its orbit instantly, or would it take about 8 minutes?" He goes "it would be like snipping a cord- instantaneous". Discouraged, I went into the slashdot-posting, linux compiling netadmin that I am today, never knowing the true path of lab coats, leather gauntlets, and welding glasses that is physics- How dare you Stockwell! You stole my life with an assumption and I want my five years back!!!
slashdot: where everyone yells sarcastic metaphors to themselves to understand the issue
To perform the experiment, numerous (probably several thousand) measurements are taken, but due to imprecision in the process of taking the measurements (imperfect measuring equipment, human error, etc) you get a variety of results. These answers could vary from well below c to well above it. If Einstein was right and nothing propogates faster than c, the higher results could only be attributed to imprecise measuremements, but you can't throw those measurements out if you are trying to be objective.
At the end of the process, you have something vaguely resembling a normal bell curve, where the height of the curve at a point along the x axis (velocity) is a measurement of the relative frequency with which that speed of gravity was obtained as a measurement. The total area under the curve will be exactly 1. In many cases, the curve may not be symmetric, but for an experiment such as this, you are unlikely to obtain an assymetric curve (Central limit theorem of statistics, or some such thing). A line right down the middle of the curve shows the measured average result (.95c).
A confidence interval is then picked (it is a shame that this interval is not mentioned in the article, but it is almost assuredly at least 95%, probably even 99%, or 99.9%). This percentage is converted to decimal (95%=.95, 99%=.99, etc), and a symmetric region around the average score with that area is blocked off. This blocked off area has a minimum X component of .7125c, and a maximum X component of 1.1875c, the difference between each of these and the average measured velocity being .2375c, which is 25% of .95c.
And that's where the 25% margin of error comes from -- for their desired level of confidence, the variance in measured results was off by no more than 25% of the value that was actually obtained as the mean.
Since the value of 'c' lies WELL within the bounds of the margin of error of the experiment, and pre-existing theories support the speed of gravity being c, this experiment supports those theories. It is important to note that this experiment did not prove anything, it only failed to disprove that the speed of gravity is anything other than something very close to c.
File under 'M' for 'Manic ranting'
Strange, 20 years ago I was taught other people had experimental evidence agreeing with a prediction that the effects of gravity move at light speed:
In 1882 Simon Newcomb observed an excessive perturbation in precession of the orbit of mercury, to the tune of 43 seconds of arc per century. In 1915, Albert Einstein showed this could be explained by the propogation of gravitic wave effects at the speed of light...
But thanks for playing.....
"Remember the Unified Field Theory? Well, forget it. Physicists have pretty much thrown in the towel on unifying gravity with the other elemental forces, so now we have the Standard Model, which says that everything works together in intricate harmony except gravity, which is on holiday in Tasmania and need not concern us further."
- Jon Carroll on the Higgs Boson
Nerd: Derogatory term typically directed at anybody with a lower Slashdot ID than you.
If Gravity's speed is equal to that of the speed of Light, then how do you explain the pull descrepencies between blackholes and low graivty environments? Go to the moon, you'll notice that the gravitational pull there is much lower than that of Earth's. And Earth's is far far less then a blackhole's gravitational pull.
:)
So how can one say that Gravity's pull is as fast as the speed of Light when Gravity itself doesn't stay constant in different environments? I never heard light not traveling the "speed of light" so it's a bit confusing.
Ao, from what I gather, blackholes have so much gravitational pull that even light can't escape. Which suggests to me that Gravtiy is stronger than light. It would also suggest to me that gravity is is faster than light because of this. I don't have any sources to back this up, all of this is just my train of thought in words here.
I'd appreciate a simple-as-possible answer as to why my train of thinking is wrong, as i said, i'm no scientist, but this topic is interesting none the less
A Penny for my thoughts? Here's my two cents. I got ripped off!
Actually, you got the order wrong.
:).
Strong, Electromagnetic, Weak, Gravitational
This depends strongly on the distance you choose to measure the force at. At a distance of 1m, as opposed to 1e-15m, the original ordering may be correct.
And as long as we're being nit-picky, I'll point out that human-observable phenomena tend to be larger than 1e-15m
The mass of photons is very real. Try this experiment, which a professor did at one of my Engineering Physics classes:
Take a relatively large gong. Make sure it is reasonably well polished.
Next, take a professional-class camera flash and set the intensity to "fry".
Third, fire the flash at the gong. As the photons bounce off the (polished) gong, it will resound as if having been struck with a solid object.
This was a very awakening demonstration to me...
Flintstones (Hanna Barbera): Sep 30 1960 (ABC)
Road-runner (Warner Bros.): 1949
It's much more likely the ringing comes from the air right next to the polished gong surface suddenly heating up.
There's a similar confusion about what drives those "solar radiometer" things - you know, a little black-and-white paddlewheel inside an evacuated glass ball that spins when you shine a light on it? People often say the reason they run is photon momentum, when the actual explanation is that the black sides of the paddles are hotter than the white sides, so when the few gas molecules left inside the ball hit the paddles, they leave the black sides going faster than the white sides.
The proof of this is the direction the paddlewheel turns - it turns white-side-first, and a photon-mass explanation would have the paddle turning black-side-first. If you put a paddlewheel inside a REAL hard vacuum, with a REAL low friction bearing, and REALLY isplate it from outside vibration, it turns the right way. See here for a more coherent and complete explanation.
To a Lisp hacker, XML is S-expressions in drag.