First speed of gravity measurement revealed

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.

Ed Fomalont of the National Radio Astronomy Observatory in Charlottesville, Virginia, and Sergei Kopeikin of the University of Missouri in Columbia made the measurement, with the help of the planet Jupiter.

"We became the first two people to know the speed of gravity, one of the fundamental constants of nature," the scientists say, in an article in New Scientist print edition. One important consequence of the result is that it places constraints on theories of "brane worlds", which suggest the Universe has more spatial dimensions than the familiar three.

John Baez, a physicist from the University of California at Riverside, comments: "Einstein wins yet again." He adds that any other result would have come as a shock.

You can read Fomalont and Kopeikin's account of their unique experiment in an exclusive, full-length feature in the next issue of New Scientist print edition, on sale from 9 January.

Isaac Newton thought the influence of gravity was instantaneous, but Einstein assumed it travelled at the speed of light and built this into his 1915 general theory of relativity.

Light-speed gravity means that if the Sun suddenly disappeared from the centre of the Solar System, the Earth would remain in orbit for about 8.3 minutes - the time it takes light to travel from the Sun to the Earth. Then, suddenly feeling no gravity, Earth would shoot off into space in a straight line.

But the assumption of light-speed gravity has come under pressure from brane world theories, which suggest there are extra spatial dimensions rolled up very small. Gravity could take a short cut through these extra dimensions and so appear to travel faster than the speed of light - without violating the equations of general relativity.

But how can you measure the speed of gravity? One way would be to detect gravitational waves, little ripples in space-time that propagate out from accelerating masses. But no one has yet managed to do this.

Measuring the speed of gravity

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 opportunity to do this arose in September 2002, when Jupiter passed in front of a quasar that emits bright radio waves. Fomalont and Kopeikin combined observations from a series of radio telescopes across the Earth to measure the apparent change in the quasar's position as the gravitational field of Jupiter bent the passing radio waves.

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.

Their result, announced on Tuesday at a meeting of the American Astronomical Society meeting in Seattle, should help narrow down the possible number of extra dimensions and their sizes.

But experts say the indirect evidence that gravity propagates at the speed of light was already overwhelming. "It would be revolutionary if gravity were measured not to propagate at the speed of light - we were virtually certain that it must," says Lawrence Krauss of Case Western Reserve University in Cleveland, Ohio.

"Even in an inertial reference frame where the whole Solar System is zooming along at X miles per hour relative to some other reference object, it's true that the light reaching the Earth from the Sun comes from where the Sun "was" 8.3 minutes ago, HOWEVER, the matching velocity of the Earth causes the apparent position of the Sun to be shifted forward (via aberration) in a way that *EXACTLY* cancels the "lagged" view of the Sun, and from the Earth you'll see the Sun RIGHT WHERE IT IS AT THE PRESENT MOMENT." - Dan Day

Although the optical illusion (re: your "aberration" above) might be construed to cause an Earth-based observer to coincidentally be looking at the right angle and in the right direction "as if" to see the actual position of the Sun, in reality what the Earth-based observer actually views is both the Light and the image of the Sun when it was 67,728 miles back from its current path due to the 8.3 minute delay in Light reaching the Earth from the Sun.

Moreover, such an "aberration" fails to apply to Gravity. Neither the Earth nor Gravity "knows" where the Sun will be in the future. On the contrary, the Earth is being tugged around in its orbit by the Gravity from the Sun at the tme that the Gravity left the Sun. If Gravity traveled at the slow speed of Light as you maintain, then it would have an 8.3 minute delay before it reached the Earth from the Sun.

This would cause the Earth to be pulled around the location where the Sun was located 8.3 minutes ago, which is 67,728 miles off of the center of where the Sun was back then.

Furthermore, this orbital deviation around the "old" position of the Sun would increase for more remote planets such as Jupiter and Neptune, et al.

But that's not what astronomers observe. The deviation in orbits does not increase noticeably in the more remote planets; rather, the planets tend to actually be nearly in the very same level/plane of orbit around the Sun (indicating that Gravity travels so fast that the Sun and planets haven't uniformly moved very far by the time it covers the distance from the Sun to them).

"2. The Sun (as well as the rest of our Solar System) is traveling through space, yes or no?" - Southack
"Actually, I disagree with point #2, at least as it relates to the notion of absolute velocity, and so does the "entire scientific community". - Dan Day

Heck, I'd settle for you quoting a SINGLE scientific source as saying that it is in disagreement with point #2 above (hint: you can't).

Hint: You're trolling again. And dodging again.

Several times I've challenged *you* to provide an actual "scientific source" for your various silly declarations. You've dodged every time. Now you to try to divert the issue you challenge *me* to document something that's common knowledge in every physics class, and trollishly taunt that you don't think I can do it.

What motivates you to behave this way? Is acting the fool that gratifying?

Does saying things which are so ridiculous that someone can't help but laugh at you and point out the ludicrousness of your error somehow make you feel better about yourself because you're able to be such an ass, as if that's some sort of worthy accomplishment?

Please tell me why people such as yourself devote so much time to trolling, when there are so many more constructive ways you could spend your time.

But in case any lurkers are curious:

Albert Einstein then proposed, in 1905, the "principle of relativity" as a fundamental property of the universe. No matter what physical process was used, absolute motion at a constant velocity was undetectable. No loophole existed, not even through the laws of electricity and magnetism.
-- Nasa's Goddard Space Flight Center (emphasis in original)

You'd think that NASA would have noticed if, as Southack falsely asserted many times, the Sun's image lagged behind its "absolute motion" in space, since that would not only have affected GSFC's rocket trajectory calculations, it would also have violated the Theory of Relativity and won someone a Nobel prize...

But then, Southack mumbles and *admits* he was wrong (although he's careful not to come out and say it so clearly that anyone might notice unless they were paying close attention), as we'll see in the next post.

Thank you for admitting that you were wrong when you repeatedly asserted:

And if the Solar System is moving due North toward Polaris, then the Sun that we "see" is 8.3 minutes South of the actual position of the Sun itself due to that delay in Light reaching the Earth from the Sun.

...although if you were a more honorable person, you would have made it more clear that you were shifting your claim, instead of mealy-mouthing it with waffle-words like "construed" and "coincidentally" (sorry, constancy across inertial frames is real, not "construed" and it's a necessary law of nature, not a "coincidence") and putting quotes around the inarguable fact of aberration, and weaseling with "as if"...

Nor did you actually have the guts to admit that you were abandoning your original claim, you just accepted the fact (*finally*) and then used it as a springboard to launch into yet more silly trolling, hoping that no one would notice it was a shift in your position...

As for your new attempt at a troll, your error (again) lies where it has this entire thread. Hint: "Inertial frames of reference". Come back when you've caught up on the last 99 years of physics and can discuss a scenario without violating things that have been known and verified since 1905.

I wasn't put on this Earth to correct your poor education, nor amuse you by playing whack-a-mole with your amateurish trolls. I've wasted enough time on you in this thread. Now that I've established your track record for any lurkers who may be about, my work here is done. Good day.

"2. The Sun (as well as the rest of our Solar System) is traveling through space, yes or no?" - Southack

"Actually, I disagree with point #2, at least as it relates to the notion of absolute velocity, and so does the "entire scientific community". - Dan Day

Heck, I'd settle for you quoting a SINGLE scientific source as saying that it is in disagreement with point #2 above (hint: you can't). - Southack

"Hint: You're trolling again. And dodging again.
Several times I've challenged *you* to provide an actual "scientific source" for your various silly declarations. You've dodged every time. Now you to try to divert the issue you challenge *me* to document something that's common knowledge in every physics class, and trollishly taunt that you don't think I can do it.
...
But in case any lurkers are curious:

Albert Einstein then proposed, in 1905, the "principle of relativity" as a fundamental property of the universe. No matter what physical process was used, absolute motion at a constant velocity was undetectable. No loophole existed, not even through the laws of electricity and magnetism.
-- Nasa's Goddard Space Flight Center (emphasis in original)"

That's a curious non-response. You've managed to cite a scientific source, but the problem is that scientific source doesn't support your claim or refute point #2 above.

Typical. Gee, do I get to hear more name-calling from you, or are you just going to flee from yet another thread again? I can barely stand the suspense!

And if the Solar System is moving due North toward Polaris, then the Sun that we "see" is 8.3 minutes South of the actual position of the Sun itself due to that delay in Light reaching the Earth from the Sun. - Southack

"...although if you were a more honorable person, you would have made it more clear that you were shifting your claim, instead of mealy-mouthing it with waffle-words like "construed" and "coincidentally" (sorry, constancy across inertial frames is real, not "construed" and it's a necessary law of nature, not a "coincidence") and putting quotes around the inarguable fact of aberration, and weaseling with "as if"..." - Dan Day

Except, I didn't "shift" my claim. It's the same point that I've been making.

Put another way, the Sun that we "see" from here on Earth has the Sunspots of 8.3 minutes ago, as opposed to the Sunspots that the Sun has at the current/present time.

Likewise, IF Gravity were as slow as Light (which it isn't), then the Gravity that tugs at the Earth would be 8.3 minutes old by the time it traveled from the Sun to the Earth, and during those 8.3 minutes the Sun and the Earth would have both traveled at least 67,728 miles along our Solar System's route around the periphery of our Milky Way Galaxy.

Which means that IF Gravity were really as slow as Light that we would see the orbit of the Earth be centered around the center point of the Sun's position 8.3 minutes in the past, and the orbit of Jupiter would be centered even further off of this plane (by 43.16 minutes), and Neptune would be even further behind this track (by 249 minutes).

And this point would be true under that circumstance because Newton showed that Gravity behaves as though it is pulling an object toward the center of the attracting mass, which in the case of the fast-moving Sun is something that would be moved a substantial distance by the time something as slow as Light made the trip through those distances.

So because Newton showed us how Gravity behaves, and because astromers have shown us that the planets in our Solar System do NOT have orbital lags, we can easily conclude that Gravity travels SUBSTANTIALLY faster than Light.

And that's the self-same point that I've been making from moment one on this thread, contrary to your bizarre claim above that I've somehow "shifted" my claim in mid-stream.

Thus, we "see" the Sun where it was located 8.3 minutes ago [snip] And this conclusion is verifiable by direct scientific observation.

Horse manure. Document actual observations/measurements which have "verified" this by "direct scientific observation" (that should keep you busy for a while), or retract it.

Actually, there is an aberration in the apparent position of the sun that is due to the finite speed of light, and the speed of the Earth with respect to the sun. It's called the Poynting-Robertson effect.

It's easy to see this effect when you're driving in a rainstorm or snowstorm. Even if the rain or snow is falling straight down with respect to the ground, in the frame of the car, it appears to be coming from a source located somewhere ahead of the car. In the car's frame of reference, the precipitation is falling in a slanted path.

If we assume that the precipitation takes 8.3 minutes to fall from cloud to ground, it's not a wrong point of view to say that the car "sees" the source of the rain where it used to be, 8.3 minutes before, even though a "stationary" observer will point in a different direction if asked to point to the source of the raindrops. The pedestrian will point straight up, while the driver will point forwards as well as upwards, but each really is pointing to the place in the clouds from which the future drops that hit them will come.

NOTO BENE that this has nothing to do with the motion of the sun, and everything to do with the motion of the Earth with respect to the sun, and the finite speed of light. The phrase "where the sun was located 8.3 minutes ago" makes sense only from the point of view of an observer; in this case the observer is the Earth. Further complicating the issue is that the Earth is travelling in a curved path about the sun; the direction of the Earth's motion does not stay constant over 8.3 minutes. That alone will cause a minor discrepancy between "where we see the sun" and "where the sun was 8.3 minutes ago".

This argument applies to the light from the sun, but does not apply to its gravitational field. If the driver passes a telephone pole and the observers are asked to point in the direction of the telephone pole, both will point straight upwards. Van Flandern will conclude from this that the telephone pole is falling infinitely fast, and Southack will chime in in agreement, but everyone else will agree that that doesn't make much sense.

"This argument applies to the light from the sun, but does not apply to its gravitational field. If the driver passes a telephone pole and the observers are asked to point in the direction of the telephone pole, both will point straight upwards. Van Flandern will conclude from this that the telephone pole is falling infinitely fast, and Southack will chime in in agreement, but everyone else will agree that that doesn't make much sense."

Fair enough. For the little sense that it *does* make to Southack and perhaps Van Flandern, Southack would say that it would be due to the *concept* that perhaps Gravity and Light travel at, gasp, different speeds that would explain why the argument above applied to Light but not to Gravity.

For the little sense that it *does* make to Southack and perhaps Van Flandern, Southack would say that it would be due to the *concept* that perhaps Gravity and Light travel at, gasp, different speeds that would explain why the argument above applied to Light but not to Gravity.

Light is a wave; gravity is a field. Waves--changes in fields--propagate. Fields don't have to; they are already there, like the telephone pole. You don't seem to be as worried about "the speed of electromagnetism" as you are about "the speed of gravity". Why is that? Why focus so much attention on gravity? You accept without question that light travels at c; will you not accept that gravitational waves--the appropriate analogue to light--also travel at c? Why not? In your pseudo-Newtonian view, what makes gravity geometrically different from electromagnetism?

"In your pseudo-Newtonian view, what makes gravity geometrically different from electromagnetism?"

For one thing, we haven't been able to *measure* the speed of Gravity waves inside a Gravity field, much less determine how fast the field itself will travel in order to cover new territory when it is first formed, so that lack of measurement makes Gravity intriguing.

Hey, we know how fast Light travels in a vacuum, so there isn't much mystery or controversy there!

But we know these things in the case of electromagnetism. You've made a geometrical argument why these velocities should be infinite in the case of gravity, but haven't explained why they aren't infinite in the case of electromagnetism. The geometry is the same, so why does your argument fail there?

In all fairness, if I haven't made the argument for magnetism, how could it have failed there?

The geometry of gravity and electromagnetism is the same in Newtonian physics. You have to explain why the same argument wouldn't apply. That you haven't tried to apply it so is neither here nor there: just because you dance around that third rail doesn't mean it isn't there for you. What is different? Why isn't electromagnetism an acceptable analogue, if it isn't? How can the orbits of charged particles in an EM potential be stable if it is, and if your argument has any merit?

"Why isn't electromagnetism an acceptable analogue, if it isn't?"

From where I sit magnetism is based upon a different fundamental Force than is Gravity. If the Forces are different, then the analogy will be difficult to hold up to the infinite degree (as in, somewhere along the line the analogy probably breaks down).

1. Strong Force - pion (among others)
2. Weak Force - W and Z
3. Electromagnetic Force - photon
4. Gravity - graviton

But in all fairness I didn't make the argument that they weren't analogous in the first place, though on the surface it would appear that such an argument may very well have some merit.

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