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Speed of Gravity measured

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From NewScientist


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.


Hazel Muir
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How does this limit the possible number of universes :confused:
Theoritical Physics was never my strong point...
 
Chriswsm said:
How does this limit the possible number of universes :confused:
Theoritical Physics was never my strong point...
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Technically, I'm not sure it does, necessarilly. What the "brane" theorists were proposing was that, in addition to the three space dimensions of up, forward, and sideways, and the one time dimension that we can directly perceive, there may be other dimensions that are wrapped up around themselves.

So instead of living in a 4 dimensional space-time, we could be living inside a 4 dimensional subset of a larger space. (The classic example is of two-dimensional beings living on the surface of a sphere. They perceive only a 3 dimensional universe of space + time, but they are living in a 4 dimensional, or possibly higher, universe, but only using 3 dimensions of it.)

The extra dimensions, that we are unable to see, could explain two things about gravity. Firstly: why gravity is so much weaaker on the micro scale, and yet dominant on the macro scale (particularly as compared to the Strong, Weak, and Electro-Magenetic forces). This could be explained by gravity leaking into the smaller dimensions. This would make gravity, as a force, much weaker close in, but stronger once you get away from the object.

Secondly, it could also explain how gravity could move faster than light, without actually doing so, by taking a shortcut through some of the smaller dimensions. Now that the speed of gravity has been measured (and assuming the results hold), the need to explain this particular possibility goes away.

In summary, the dimensions they are talking about are just axes on the great chart of the universe, not seperate universes. The fact that gravity seems to travel at light speed (and I hope it does, as it does make things a tad more consistent), does not mean that these dimensions (or any hypothetical universes that may exist) don't exist, it just means that their effects on the viewable universe are different to one of the possibilities that had been considered.

Finally, remember that I am not a professional physicist, and I am only posting my understanding of what this means. I may be wrong about some of the details, and any of the above that looks like opinion should not be quoted as being fact, 'cause you'll make both of us look silly.

Nevertheless, I hope that helps.
 
Considering gravity waves can be used in theory to detect distant high mass objects and cataclysmic events

they would also be able to transmit information from one end of the galaxy to the other (not easily, but possible)
if gravity waves propagated any faster than light there could be a possible causality problem...
but they don't
phew!
 
Urm... Hold on, they used light to measure the speed of the propigation of gravity and discovered that it was the speed of light?

Does anyone else see the problem here or shall I suggest using Sonar instead of Radar in a plane travelling at the speed of sound?

If gravity propigates faster than light then the maximum speed you could detect using light would be light speed...
 
Don't tell a certain board member this. He still maintains that gravity is a government conspiracy....or something like that anyway..
 
Niles Calder said:
Urm... Hold on, they used light to measure the speed of the propigation of gravity and discovered that it was the speed of light?

Does anyone else see the problem here or shall I suggest using Sonar instead of Radar in a plane travelling at the speed of sound?

If gravity propigates faster than light then the maximum speed you could detect using light would be light speed...
Click to expand...


They didn't measure the speed of gravity with light, they used an equation to work out the speed using measurements of Jupiter's mass, velocity and gravitational field.


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.
Click to expand...
 
Is there anything faster than the speed of light ?

Could gravity be faster/slower under any circumstances. I read somewhere that the speed of light is not constant. If that is true then neither is the speed of Gravity
 
We hope not, that's why it's good to see that gravity seems to be restricted to the speed of light as well.

The speed of light is actually a consequence of one of the fundamental constants, Alpha: the fine structure constant. This sets the limits at which radiation (like light or gravity) propogate.

There have been a set of observations that can be interpreted that Alpha is changing. Some work has been done on what this would mean, but some of those involved have attempted to distance themselves from the initial supposition. (Some of those who did the work in Sydney have explained that they were working on it as a "What If..." on the suggestion of Paul Davies. Paul Davies, on the other hand, has been trying to convince people that it is true.)
 
Tom van Flandern believes the speed of gravity is many times light speed. (Details are on his web site.)

His comments on this latest research are on this page.

It's quite a long article, and I won't attempt to summarise it, or van Flandern's theories, except to mention that the earth could not orbit the sun if gravity was light-speed - the phenomenon of abberation would mean the force of the sun's gravity would act towards the apparent, not the true, position of the sun (because of the Earth's orbital motion, which is a small fraction of the Speed of Light), and this would continually expand the earth's orbit.

TvF may not be right in his theories, but he does a good job of pointing out serious flaws in conventional theory.
 
Right! Simple experiment ot find out. We'll just remove the sun from the solar system.

now where did I put that sticky - back plastic?
 
I've had a look at the van Flandern website, and he failed to convince me, as it looked like he was saying that 1: the speed of gravity was not what the experiment measured; and 2: it wasn't measured correctly.

I've referred it off to someone I know who keeps more up to date on this sort of thing, and it turns out that this is a well known phalacy in General Relativity. What van Flandern has failed to take into account is that there is not only the gravitational effect from the mass of the Sun, but also an effect from the Sun's momentum. The momentum component neatly cancels out the light-speed propogation delay. (There is apparently an identical effect with Magnetic Fields, which are generally accepted to travel at light-speed.)

Full details can be read in the .Usenet Relativity FAQ , which is a combined effort from a large number of people on the net who work in this area professionally. The relevant topic is here.
 
Yes, an interesting article, Anome.

But, in the last paragraph, there still seems to be some confusion between the speed of gravitational forces and the speed of gravitational waves. Even van Flandern accepts that gravitational waves travel at c.

But the maths of GR is too much for my old brain!
 
rynner said:
Yes, an interesting article, Anome.

But, in the last paragraph, there still seems to be some confusion between the speed of gravitational forces and the speed of gravitational waves. Even van Flandern accepts that gravitational waves travel at c.

But the maths of GR is too much for my old brain!
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Despite what van Flandern claims, they are actually the same thing. That is to say that gravitational waves are the way that the gravitational forces propogate. Hence if the waves travel at c, then the forces also travel at c.

I'm with you on the maths thing. SR is at leat fairly straightforward, and Newtonian gravity is simple enough to work with. I need to do some more reading on GR, though.
 
As a thought experiment, imagine a middling sized black hole popped into a solar system through an Einstein-Rosen bridge, a.k.a. a wormhole-
the local asteroids, planets and the system star(s) would progressively start to be attracted or perturbed by this object, as the initial gravity wave spread out across the system at the speed of light... the outer system might be unaffected for hours or days and act as if the intruder did not exist until the gravity wave arrived.
Mind you the bridge itself would have to be very large and stable for this trick to be carried off...
 
Nice site Anome.

I guess that one way of viewing the problem is to think about what SR (and hence GR as this effectively contains SR as an underpinning theory) has as a basic postulate, re: inertial reference frames.

Take the following example.

A large mass, with a small "test mass" suspended above it (somewhat like a pendulum.)

(For the purposes of the thought experiment the support has negliglable mass, and the two masses are static w.r.t. each other.)

In the rest frame of this system the test mass will experience a force in the direction of the "true" position of the large mass. In the rest frame case the "true" and "retarded" positions of the large mass are identical.

Now lets consider an inertial frame moving relative to the experimental setup. In this frame, if Van Flandern was correct, the test mass will experience a force in the direction of the "retarded" position of the large mass. (Not the "true" position.) Hence if the test mass was the bob of a pendulum, the pendulum would no longer hang vertically. This also means the experimental system would experience a torque, which if allowed to evolve would lead to the test mass oscillating around the large mass. This behaviour, I suggest, would not be consistent with the fundamental postulates of SR (and hence GR).

As these postulates are "hardwired" into the theory, then I believe that the theory must "conspire" (as detailed calculation appears to show) to produce a force in the direction of the "true" (linearly extrapolated) postion, and not the "retarded" postion.

Hopefully this example shows (without any horrendous GR calculations) why Van Flandern is wrong on this point. (If it doesn't then I'll blame it on the fact that I'm writing this after a truly horrendous car journey... ;) )
 
wasn't Einstein's TOR slightly disproven last year(or the year before last?) i remember a flap about something he overlooked and was only noticed recently.
 
Well, I remember some australian scientists coming up with this-

http://news.bbc.co.uk/1/hi/sci/tech/2181455.stm

but then a different lot of australian scientists are heavily involved in the quantum teleporting shtick as well
they seem determined to make all the star trek physics reality in that part of the world
 
A colleague of the chaps, Fomalont and Kopeikin, who measured the speed of gravity now says their results are pants
So it seems possible that Niles Calder and others on this board were correct.
In order to measure the speed of gravity surely you need a large alteration in a massive object- like a supernova, or an antimatter explosion, or a star falling into a black hole.
You can't measure it using just light- no matter how far it has travelled, as far as i can understand it.
 
An interesting article - quite a cat-fight! So it's not just Tom van Flandern who doubts their results!

Science should be impartial, but as this story shows, people invest a lot of energy and belief in their theories, and are not easily budged.
 
It may be that the interpretation of the experiment is incorrect, however I suspect that the true correct status is that the jury is still out. An interesting view in the article is that the measurements were considered so difficult to make that few people gave serious consideration as to what they mean. It reminds me of an old adage with GR that it can take a week to perform a calculation, but that it can take a year to understand what the answer meant. (42 anyone? ;) )

I believe, however, that at least some of Tom Van Flandern's arguments for the none speed of light value for the "speed of gravity" are wrong. (see recent post)

My own bias is to the view that the speed of gravity is the same as the speed of light. If the experiment has measured something else, then this doesn't invalidate that bias.
 
what?

what if they had the mesurement or time itself. which all other followed,i mean what time is itself and its infuence on all mesured.
so we have what it is not what it infuences
 
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