First speed of gravity measurement revealed

NewScientist.com ^ | 01/07/2003 | Ed Fomalont and Sergei Kopeikin

[forsnax5]

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.

[Southack]

" Ah, c'mon, Dan, no response to this? PLEASE PLEASE I enjoy reading your responses. And if ever there were a highly inflexible, NON-bended straight line, Southhack's given you one.

Now, some will say it's a cruel sport. It's too easy. It's like hunting elephant in a zoo. But I say: Go for it!"

Dan's a little busy trying to squirm out of his "Sun doesn't move" line of reasoning (if one can call it that).

However, if your only beef with what I've said so far is that you didn't like my thumb/lightsource example, simply substitute the better-known example of a Black Hole.

The Black Hole bends light, and that was my point in mentioning the earlier thumb/lightsource example in the first place.

Gravity bends Light easily. Light does not bend Gravity easily. Such a one-sided state of events is worth pondering.

[Southack]

#194 is also for you!

[Southack]

"The fact is that the earth accelerates toward the actual position of the sun, not toward the apparent position as it would if gravity propagated at the speed of light. This isn't an opinion or an interpretation but a demonstrated phenomenon. It can't be explained by a speed of gravity that doesn't exceed the speed of light."

Precisely. Sadly, far too many people have trouble understanding that our whole Solar System is moving en masse in one direction even as our planets revolve around our Sun (which happens to be centered in the middle of the very System that is moving through space), and thus, they erroneously conclude that the Sun isn't really moving because people on Earth can observe that it "seems" to be always in the same place (not comprehending that it only seems that way because the Earth and the Sun are maintaining their relative positions to each other as they hurtle through space).

[Southack]

fyi

[Dan Day]

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.

No it doesn't, do the math. Take it up with Einstein if you're still unclear on the concept, he's on my side.

But I'm sure you know that already. So stop trolling me or I hit the abuse button.

Physicist, would you care to beat on him for a while? My arm's tired.

[Southack]

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

"No it doesn't, do the math. Take it up with Einstein if you're still unclear on the concept, he's on my side." - Dan Day

Then in that case you simply don't understand the concept that the Sun is moving, and while it is moving it takes Light 8.3 minutes to travel the distance from the Sun to the Earth, so when the Light reaches the Earth it is the Light from where the Sun was located 8.3 minutes ago, not the Light from where the Sun is currently located.

[Dan Day]

Precisely. Sadly, far too many people have trouble understanding that our whole Solar System is moving en masse in one direction even as our planets revolve around our Sun (which happens to be centered in the middle of the very System that is moving through space), and thus, they erroneously conclude that the Sun isn't really moving because people on Earth can observe that it "seems" to be always in the same place (not comprehending that it only seems that way because the Earth and the Sun are maintaining their relative positions to each other as they hurtle through space).

"Sadly", the problem with your lame troll is that you're beating a red herring.

Aruanan's argument (and Van Flandern's, which is where Aruanan seems to have gotten his from) is not based on the motion of the Solar System as a whole, period. So your repeated flogging of that irrelevant issue in an attempt to muddy the issue is moot.

His argument rests solely on the motion of the Earth around the Sun, and would be equally valid or invalid whether the Solar System were cruising through the universe at near light speed, or nailed completely stationary to a cosmic anchor.

You're trolling with a red herring, and you most likely know that.

So I repeat -- stop trolling, you're not fooling anyone. FreeRepublic's administrators don't take kindly to trolls.

[Southack]

"Aruanan's argument (and Van Flandern's, which is where Aruanan seems to have gotten his from) is not based on the motion of the Solar System as a whole, period."

That could be, but I'm not posting Aruanan's argument. Rather I'm posting my own (which happens to dovetail nicely with Newton's points on the matter).

And my own argument is that we KNOW for a scientific FACT that our Solar System is traveling through space. This fact means that our Sun is moving even though it APPEARS to be in the same place to an Earth-based observer.

Since the Sun is ACTUALLY moving, the 8.3 minute-old Light that we see from the Sun is actually always coming from the Sun's previous location. Thus, we "see" the Sun where it was located 8.3 minutes ago (i.e. the time it takes Light to travel from the Sun to the Earth) rather than where the Sun is ACTUALLY located.

My argument is further that if Gravity travels at the same speed as Light, that the orbits of the planets around the Sun will see that same 8.3 minute delay (which would give us a Solar System in which the orbits of the planets progressively "lagged" behind the actual movement of the Sun).

Yet since we see no "lag" in the orbits of the planets, we have to conclude that Gravity doesn't travel at the slower speed of Light, but rather that Gravity travels SUBSTANTIALLY faster than Light.

And this conclusion is verifiable by direct scientific observation. QED.

[Southack]

My argument is further that if Gravity travels at the same speed as Light, that the orbits of the planets around the Sun will see that same 8.3 minute delay (which would give us a Solar System in which the orbits of the planets progressively "lagged" behind the actual movement of the Sun).

Should read something along the lines of: that same 8.3 minute delay for Earth, longer delays for planets such as Jupiter that are even further (which would require Light or anything that traveled at the speed of Light to take even more time to reach) from the Sun...

[Dan Day]

"No it doesn't, do the math. Take it up with Einstein if you're still unclear on the concept, he's on my side."

Then in that case you simply don't understand

I understand it just fine. Stop trolling. Do the math, or drop it.

[Dan Day]

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. And stop trolling.

[Southack]

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

"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. And stop trolling." - Dan Day

1. It takes Light 8.3 minutes to travel from the Sun to the Earth, yes or no?
2. The Sun (as well as the rest of our Solar System) is traveling through space, yes or no?

Now, if you disagree with EITHER point #1 or point #2 above, then you are welcome to show how the entire scientific community is in error (I'll be waiting with baited breath). On the other hand, if you accept those two above points, then all you need to know is how far the Sun travels in 8.3 minutes in order to understand that by the time Light reaches Earth that the Sun is in a different, scientifically observable actual location than what you see (which is the Sun 8.3 minutes ago, of course).

[Gary Boldwater]

Thanks for the FYI. Yes, the sun is moving through space and the light path from the sun to earth is not always a straight line. However, twice a year the light path is straight and one should observe this. In one case the light would be redshifted and in the other it would be blue shifted (moving with and against the translation in space of the earth-sun system). Correspondingly, the gravity vector would show no delay at those points. The yearly variation in the spectra of the sun should yield this data. Perhaps you can find it. I'm very interested and this would go a long way in supporting your argument.

[Southack]

Our Earth revolves on it's own axis once every twenty-four hours, annually circles our Sun, one of several hundred billion suns that constitute our galaxy.

This Sun travels at the rate of 136 miles per second around the periphery of our native Milky Way galaxy, circling it once every 230 million years.

In 8.3 minutes, our Sun has moved 67,728 miles.

[Southack]

A time delay is related to speed and distance. The further away something is, and the slower something travels, the longer it takes to reach a destination. Light takes 8.3 minutes to reach the Earth from the Sun, for instance, so there will always be a delay of 8.3 minutes as long as the distance from the Earth to the Sun remains constant, regardless of the time of year.

If Gravity traveled at the same slow speed as Light, then it would experience that same delay, again, regardless of the time of year. Moreover, if Gravity was that slow, then planets would orbit around their percieved Sun (67,728 miles off-center-plane from the actual Sun in the case of the Earth, and progressively further off-center-plane for planets in more remote orbits).

That is, if Gravity was as slow as Light.

On the other hand, if Gravity was subtantially faster than Light, then one would expect far less planer orbital deviation from the actual center-plane of the Sun. In fact, one would expect the horizontal-orbiting planets to roughly line up in the same orbital plane as each other.

[Gary Boldwater]

Calculations for the perihelion of mercury, the advance of the moon's orbit from the tides all use gravity as propagating with c and they are accurate (though it may not be the cause).
Yes, there is always a delay, however if the common motion of the earth and sun affect the gravity or light path, twice a year the apparent position of the sun would be correct as the path is in line with the common motion.

[Southack]

IF Gravity traveled as slow as Light, then the orbital planes of planets would lag behind the actual position of the Sun because by the time Gravity reached them to tug on them at the old angle, the Sun would have moved. This lag would become more pronounced with the further-away orbital distances of the more remote planets.

For instance: Distance Sun - Jupiter: 5.20 AU => yields 352,185.6 miles off center plane

This is because our Sun is traveling 67,728 miles in the time that it takes Light to travel 1 AU. Thus, by the time Light has traveled 5.20 AU, our Sun has moved 352,185.6 miles in its path around our Galaxy.

Distance Sun - Neptune: 30.0 AU => yields 2,031,840 miles off center plane for the orbit of Neptune.

Of course, in reality we see Earth and Jupiter orbiting in essentially the same center plane, rather than off-center-plane by 67,728 miles for Earth and off-center-plane by 352,185.6 miles for Jupiter. This indicates that Gravity is traveling so much FASTER than Light that the Sun hasn't really moved very much by the time Gravity covers 1 AU, 5.20 AU, and 30 AU respectively.

[Southack]

"twice a year the apparent position of the sun would be correct as the path is in line with the common motion."

Perhaps you are thinking in the 2 dimensional model of a flat solar system moving horizontally through space with flat orbits of planets revolving around the Sun inside said System, in which case twice a year the Earth would be in the path of the line in which the Sun was traveling around our Galaxy, however, it still wouldn't be accurate to say that we were seeing the "actual" Sun rather than the Sun in its 8.3 minute old position even in that model (those two cases would point to a location either too far away or too close than the actual position of the Sun).

But that's really not a good model to use in the first place. A better way to grasp the situation might be to envision a flat solar system in which the entire system is moving vertically even as the individual planets revolve horizontally around the Sun.

Now consider that by the time Light travels from the Sun to the Earth, that the Sun has moved vertically 67,728 miles.

Ergo, the actual position of the Sun will be in a different position than what one sees with the 8.3 minute-old Light from it.

[Poincare]

Isaac Newton thought the influence of gravity was instantaneous, but...

Actually Sir Isaac said that any mechanism of action-at-a-distance and any speed of propagation were mysterious and unknown to him. He just put forth a mathematical system that fairly accurately described celestial mechanics up to relativistic discrepencies such as the precession of Mercury.

[Dan Day]

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

"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. And stop trolling." - Dan Day [Tangential babbling (addressed below)] - Southack

Your reply was non-responsive, Mr. Troll. You say that your "conclusion" (not your premises, your *conclusion*) is "verifiable by direct scientific observation". So verify it by citing some of that "direct scientific observation". Point us to a paper or something where an astronomer allegedly acknowledges actually having measured the Sun being "8.3 minutes south" of where it "actually" is.

Or retract your trollish claim.

1. It takes Light 8.3 minutes to travel from the Sun to the Earth, yes or no?

Yup.

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

Relative to which inertial reference frame, please?

Unless you've been asleep since 1905, you should realize that there's no such thing as absolute motion.

Using the Solar System as our inertial reference frame, the Sun is not moving at all (but the rest of the Universe is drifting hither and yon around us).

Using Alpha Centauri as the center of our inertial reference frame, the Sun is drifting slowly.

Using the center of the Milky Way as the center of our inertial reference frame, the Sun (and Earth) are traveling in a rough circle at 220 km/s.

Using a distant quasar as our inertial reference frame, the Sun is traveling at a good fraction of the speed of light in a different direction entirely.

So by your "reasoning", where should we "see" the Sun's "actual" position as it was 8.3 minutes ago? Unchanged? Slightly changed? 8.3 minutes * 220km/s changed in a *different* direction? 8.3 minutes * 0.8c in *another* direction? Which will it be, Mr. Troll?

Something's wrong with your conclusion, obviously, because it gives four different answers simultaneously.

(Hint: In 1905 Einstein discovered that the universe doesn't make sense unless you abandon notions of absolute velocity. You still haven't gotten the message.)

Now, if you disagree with EITHER point #1 or point #2 above, then you are welcome to show how the entire scientific community is in error (I'll be waiting with baited breath).

That's "bated" breath, you goof, unless you've been eating worms.

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

You, on the other hand, are invited to explain how Einstein allegedly got it wrong, since your calculation method is in violation of Special Relativity.

On the other hand, if you accept those two above points, then all you need to know is how far the Sun travels in 8.3 minutes in order to understand that by the time Light reaches Earth that the Sun is in a different, scientifically observable actual location than what you see (which is the Sun 8.3 minutes ago, of course).

No, that's not "all you need to know" -- you're missing a key notion in your analysis, which is the aberration of light.

I didn't want to reveal the punchline until you had presented your own calculations and made a fool of yourself, but you're obviously avoiding every attempt to try to get you to actually *show* your calculations (presumably because you're not able to even *make* any), so I might as well spill the beans now.

Clue for the clueless: Your analysis would only be correct if the Sun were zipping along (in a particular inertial reference frame) and the Earth were *motionless* (relative to that frame). But that's not the case, the Earth is cruising through the galaxy (or whatever) along with the Sun itself. You *can't* ignore the motion of the Earth and get the right answer.

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.

Interestingly, this gives exactly the same "answer" as you get using the Solar System itself as an inertial reference frame, wherein the Sun isn't moving at *all* and the analysis is more straightforward and obvious. ...or any *other* inertial reference frame. They all give the same answer.

I'm writing this for the benefit of lurkers, because I know you'll just try a trolling response instead of a) retracting your nonsense or b) learning something.