The Speed Of Gravity Revealed 935
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."
Sorry if i'm Skeptical (Score:2, Insightful)
If you already know the answer you are looking for it's a lot easier to "tug and pull" the numbers.
gravity slowing down? (Score:3, Insightful)
Re:I'd expect... (Score:2, Insightful)
Re:Sorry if i'm Skeptical (Score:3, Insightful)
Theory of Relativity.... (Score:2, Insightful)
I'm confused...
Re:Event Horizon (Score:5, Insightful)
By the way, did anyone else find the quoted margin of error of
Re:Wow. (Score:5, Insightful)
>
> Try telling Sonny Bono that.
Au contraire! Blunt force trauma is all about electromagnetsm. (I suppose there are a few places where it's also about electroweak interactions, but that's a hell of a lot more trauma than I care to talk about. *g* :)
At any rate, gravitational forces had accelerated Sonny pretty gently, and he was doing just fine until electrostatic forces from a nearby tree intervened.
Sonny was a silly clam (silly clam? I repeat myself) who tried to make the electrons in his body occupy the same space as the electrons in aforementioned tree. (For a guy who claimed to be a great physicist, L. Ron Hubbard sure didn't teach his disciples much about the Pauli Exclusion principle or Van der Waals forces.) Sonny Bono's failure to grasp rudimentary physics can be seen as yet another case of evolution in action.
An error margin of 25%?? (Score:5, Insightful)
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.
Re:Has science gone mad? (Score:5, Insightful)
You can't prove a physical theory - you can either show that it fits experimental evidence (in which case it might be right), or that it doesn't (in which case you've disproved it).
This experiment shows that a key assumption of GR is consistent with real life. That's it. That's all we can do, and that's all that is being claimed - observations of Jupiter give (roughly) the results we'd expect if gravity travels at c.
Re:Circular arguments... (Score:5, Insightful)
Welcome to physics (Score:1, Insightful)
The experiment doesn't tell us that gravity travels at the speed of light, it tells us that the current theory which claims that gravity travles at c is consistant with experiment.
To quote Djikstra (more or less) "testing software can only confirm the presence of bugs, never the absence of them". In much the same way a physical experiment won't (in general) tell us that a theory is perfectly correct, only that it is wrong. This happens when an experiment returns results which do not include the predicted results.
This experiment tells us that all theories which predict the speed of g to be outside the range 0.7c - 1.2c are quite probably wrong whereas those in the vicinity of 0.95c are quite possibly right. GR predicts that g==c which is very close to the measured value, so it is in fact quite reasonable to claim that the theory has passed this particular test.
This experiment doesn't *prove* GR, it just goes to show that it is indeed a very good theory as it has shown throughout the past century.
Re:Sorry if i'm Skeptical (Score:3, Insightful)
From the article:
"If you could measure the gravitational field of Jupiter, while knowing its mass and velocity, you could work out the speed of gravity."
Now, let's consider those three variables.
The first, we measured by its effect on radio waves from the occlusion of a pulsar. Check.
The third only matters relative to the inertial frame of the Earth, so we can actually measure that as well. Check.
The second, however, provides a bit more of a problem, and I have pointed this out in the past with "proof" of traditional physical concepts using astronomical objects and events. Basically, our idea of Jupiter's mass results from how it interacts with other large gravitating objects (for example, Kepler's third applied to the period of and distance to one of its satellites).
Applying that same "fact" back into an equasion designed to verify something about gravity commits the classical flaw of logic known as "circular reasoning". Thus,
Jupiter weighs X because gravity acts like Y...
And gravity acts like Y because Jupiter weighs X.
Sorry, but I'd give that paper a great big "F". "Internally consistent" does not mean "externally applicable", and *certainly* does not mean "true".
Re:Dammit! (Score:1, Insightful)
Re:Circular arguments... (Score:2, Insightful)
The fact that people can't differentiate between re-working equations and performing an experiment (to see if the experimental data matches the predicted data) is actually insignificant to the fact that they are making uneducated comments based on a one page article that sums up a complicated experiment in layman's terms.
It is actually comforting to see that people such as these abound everywhere, even in a "smarter" community like Slashdot.
Re:Deriving e=mc^2 (Score:1, Insightful)
Here is a better derivation. Starting with the axiomatic formulation of SR, construct a metric with signature +++-. Dot product w.r.t. to this form corresponds to the dot product for 4-vectors, as you might have seen previously. Show that (E,p) is also a 4-vector. Take the invariant interval, Taylor expand, and you're done.
Re:Practical Applications (Score:5, Insightful)
Your definition of evil must be the common "has different priorities or beliefs than I do and isn't perfect"
There are better choices for a definition of evil, like the following that applies to Saddam Hussain:
"kills millions, brutally supresses all opposition and all human rights, hires the worst profesional torturers and rapists in history"
You know I assumed that George Senior was full of shit when he called Saddam "another Hitler".
I was wrong. The problem here is that our media doesn't care enough to actually inform us of all the slaughter and oppression around the world and our local do-gooder activists are so busy hating their republican neighbors that they couldn't be bothered to check out the possibilty that they are occasionally right.
Cognitive dissonance makes it easier to believe whatever propaganda is floating around as long as it isn't our propaganda.
The situation in the Middle east is complicated, so of course we know nothing about it. It's scary but the people currently in the White House actually know more about that issue than the activists.
I don't want the "total information awareness" geeks reading my email. But you know, I can oppose some policies of my government without doing a full "you evil bastard" hissy fit.
Rocky J. Squirrel
Re:An error margin of 25%?? (Score:4, Insightful)
Physicists have been accused of being loose with rigour, but this is really stretching it.
That's an excellent measurement for astrophysics. Recall, there was a recent announcement that astronomers are 95% certain that the age of the universe is between 11 and 20 billion (thousand million in the UK) years old. That's 15.5 plus or minus 29%.
If you read the original paper [arxiv.org] proposing the measurements back in July, the technique requires interferometric measurements timed to within picoseconds (1e-12 seconds) to give an accuracy of at best plus or minus 10%. That translates to pegging the apparent position of a little speck of light (and radio waves) in the sky to within five millionths of a second of arc. (Roughly speaking, that's the apparent width of a bacterium at twenty miles.) I think that they did a pretty good job to be able to call the number to within 25%, especially given that nobody has ever attempted this sort of measurement before.
No doubt it will be refined in the future; meanwhile, it's another piece of evidence which supports a subtle result general relativity. GR is a really neat theory, in that it made predictions and had consequences that we are still only beginning to be able to test nearly a century later. Even more interesting, it has yet to be contradicted by a reproducible experimental result. Hats off to Einstein, yet again.
Re:Wow. (Score:5, Insightful)
This is exactly the picture introduced by Einstein's general theory of relativity. An object's trajectory is simply the path of least action in warped spacetime. Spacetime is warped because of the presence of masses.
And it is the radius of the earth.
Now I don't know where you got this statement from. First of all, the radius of the earth and the curvature of spacetime near its surface can't be the same, because those two quantities have different units. It would also be very surprising if they turn out to be numerically the same, since the amount by which an object distorts spacetime is not determined by its size, but by its mass, which is size multiplied by density.
There is no gravity, only bent space time.
It seems to have eluded you that Gravity is the name we attribute to the phenomenon that mass bends spacetime. Hence your statement really says: "There is no gravity, only gravity."
Other than that, I really like your illustrative description of how gravity works!
Re:Wow. (Score:2, Insightful)
A better pair of contrasting examples would be two pieces of iron being held together by their combined gravitation (negligable, at small sizes) versus pieces of iron held together by magnetizing both and placing them together so that opposite poles are together. The chemical bonds in the metal of the nail are not at "people" scale.
Re:sigh, indeed (Score:3, Insightful)
This observation is meaningful only in hindsight. An experiment like this one has the potential to disconfirm the hypothesis as well. The fact that it did not do so is significant, albeit not so significant as the alternative.
The slur against physicists is unjustified, particularly the "elevate it to dogma" line. If you want to test a hypothesis, you first assume that it is correct, then try to prove the assumption wrong. If you have a better method, please share it.
I doubt that's a photon mass effect... (Score:5, Insightful)
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 [ucr.edu] for a more coherent and complete explanation.
Question (Score:2, Insightful)
Re:Wow. (Score:3, Insightful)
The relativistic energy/mass equivalence equation is (as was reminded to me by another poster):
I hope I got the units correct on that. In this equation m is the rest mass (m_0) and this equation is generally equivalent to the E=m_0*gamma*c^2 form, except that form doesn't work too well to describe zero rest mass particles (because you divided out a mass to transform the equation - leaving you with a 0/0 term that is fairly meaningless). Thus all the energy of a photon can be contained in its momentum energy rather than its rest energy.
The actual argument that the rest mass term is zero is based on gauge invariance of QED, and I believe the simpler argument is that a non-zero rest mass of a photon would result in a damping term in Coulomb's law which is not observed (or at least we don't believe it exists, and we've bound it to be nearly zero by our observations).
What if..... (Score:3, Insightful)
I've often wondered lately if perhaps gravity is both a repulsive and an attractive force. For local (i.e. interstellar) distances, the attractive force prevails. But for really vast (intergalactic) distances, it might act as a repulsive force. This could partly explain why the galaxies are accelerating away from each other.
Physicists don't have much of an idea what dark energy is... maybe it's just gravity, and Newton's law needs an amendment.
I've never heard this idea proposed, but it would make a certain kind of sense to me if it turned out to be the case.
Inconsistent with demonstration - maybe (Score:3, Insightful)
If the gong is reflective, the air near it gets heated both by the incoming light and by the reflected light. If the gong is sooted, only the incoming light heats the air.
At least, this seems logical to me. A way to test it would be to put a vibration sensor on the gong, and try it both in air, and in a vacuum. If you're right, the sensor should read the same, if I'm right the impact in vacuum should be much less.