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.

[MonroeDNA]

OK, here's a question I've been asking various people for over 15 years:

Einstien stated that there is no way to measurably differentiate between gravity and accelleration.

Newton invented calculus to prove that any gravitational mass can be treated as if all the gravity was coming from a single point, at the center of mass. A point source.

Let's suppose that you were in an elevator, under either accelleration or gravity, and you hung two static pendulums from the ceiling, from strings.

My question: Under gravity, wouldn't the pendulum strings angle toward each other, exactly towards Newton's common point? But under accelleration, wouldn't the strings would be perfectly parallel?

[RightWhale]

Good question. Don't know right off. We'll get back to you.

[Cool Guy]

Does gravity travel through masses? I think it does. How do we account for that if it is gravitons? What is its speed through different masses?

I think I will go with the idea of a rubber sheet for now.

[RightWhale]

Technical staff has examined the situation and wants to know the size of the elevator. Is it small -- 20 passengers or so, or is it large -- the size of the moon? Makes a difference. If it's small, it will be impossible to measure a difference. If it is large, the gravity field will have measurable curvature and the acceleration will be of course uniformly flat. So it depends on whether you want to treat the gravity field as a uniformly flat, infinitesimally small segment of the spherical gravity field.

[Gary Boldwater]

Now you're calling me a lawyer??!! :)
I'm of the school that acceleration is absolute, relative to only one preferred reference frame. Why doesn't SRT apply to accelerated frames? Isn't the entire cosmos a series of accelerated frames, starting on the rotating earth (in a gravitational field no less) revolving around the sun which is part of a revolving cluster of stars and so on. Even the velocity of light is referred to as local, it is not universal.

[RightWhale]

Gravity is a field and does not move. The field will be non-smooth where there are masses, a bumpy set of values. The thing they are measuring the speed of is the change in intensity and direction of the field at two different points with respect to time. The speed of propagation of a change in intensity, rather than a motion of the field. The field can't move.

[Physicist]

I'm of the school that acceleration is absolute, relative to only one preferred reference frame.

Consider one observer at the center of the Earth and another at the surface, in a sealed box. Is the one on the surface accelerating? According to his measurements--remember, he can't see his surroundings, so he measures his acceleration with a scale--he is (c.f. the equivalence principle). According to the observer at the center of the Earth, he is not.

Why doesn't SRT apply to accelerated frames?

I assume you mean an accelerating frame. Frames by construction don't accelerate; they refer to inertial rest. SR does apply to accelerated frames (i.e., frames at different velocities). But what you want to know is, is there a principle of relativity that relates observers under arbitrary acceleration? Yes, it's called the General Theory of Relativity.

[PatrickHenry]

My question: Under gravity, wouldn't the pendulum strings angle toward each other, exactly towards Newton's common point? But under accelleration, wouldn't the strings would be perfectly parallel?

They would gravitate toward each other under either circumstance (whether the elevator they were in was sitting on the ground or being lifted.) A passenger in the elevator wouldn't know which was the true situation.

[MonroeDNA]

Technical staff confirmed my point!

In the right situation, with the right measuring instruments, gravity and accelleration CAN be distinguished, which is against what Einstein said.

Oh, the elevator example? It's what Einstein used as an example of how they are indistinguishable.

Assume the elevator is large enough such that the subtle angle difference can be measured. With today's technology, that would be around 100 feet wide, more or less.

[MonroeDNA]

"They would gravitate toward each other under either circumstance..."

Technically true, but under gravity they would always be closer than under accelleration.

Think of a strong gravity directly under them.

[RightWhale]

Yes, depending on geometry they can be distinguished. That, of course has little to do with the STR or the GTR; it's just Newtonian physics. Since Einstein's gravity field was uniformly flat in the elevator example, in that case they could not be distinguished.

[MonroeDNA]

See my post, #124. Comments?

[MonroeDNA]

Under what circumstance can a gravity field be "flat?"

[PatrickHenry]

I'm not sure of that. The two pendulums (pendula?) would attract one another, period. If they were in a box floating in space, it's true that the field of the earth wouldn't restrict their movement, and they'd swing closely together. But I was assuming the elevator was close enough to the earth's surface that the earth's gravity wouldn't matter either way (stationary elevator or rising elevator).

[MonroeDNA]

Not trying to be arguementative, if I come across that way.

I guess my point is that accelleration is always 2-D, while gravity is 3-D. They can ALWAYS be distinguished from each other. They are not the same, and under ANY circuimstance imagined, they can be easily distinguished.

[RightWhale]

can a gravity field be "flat?"

If the mass is not a limited volume. A limited volume, if spherical, will act like a point source. Earth, for example would be such a volume, although it is lumpy, masswise, and the gravity vectors point in various directions slightly away from the center of mass.

[RightWhale]

accelleration is always 2-D

Not at all. Spacecraft have control thrusters pointing along 3 axes and they can impart a 3-D acceleration. In the superstring model there can be more than 3 dimensions, maybe even 11 in some cases, and there could be corresponding accelerations.

[Lurking Libertarian]

There are really only two possibilities being discussed here:
1. That G = C^2
2. That G = C
I.e. either Gravity travels at the speed of light (C) or else it travels at the speed of light squared (C^2).

Southack, I'm no physicist, but I think you're misinterpreting "=". "G=C" doesn't mean that "gravity travels at the speed of light" but rather that "in any equation, C can be substituted for G without changing the result."

I'm pinging a real physicist to correct me if I'm wrong.

[MonroeDNA]

Ummm....under what circuimstance can a mass be not a limited volume?

[MonroeDNA]

I phrased that wrong.

Accelleration can be defined in 1 dimension only, whereas gravity always has more than 1 dimension.

Under accelleration, it doesn't matter if you are two feet higher, two feet to the left, or two feet forward (change in X,Y, or Z location) if you are part of the object being accellerated. The force does not change, no matter how you change your location (static location!)

Under gravity, if you are two feet higher, it changes. If you are two feet to the right (not spherical, XYZ right), the force changes, and can be detected. Under gravity, a static change in location will cause a shift in the gravity vector. That's why the pendulum detection method works.

Gravity and accelleration can be distinguished, under any imaginable scenario. Even in a sealed box, or elevator, IMHO.

I've been asking this question for 15 years, and either I'm wrong (won't be the first time, for sure!), or the elevator or sealed box thing is wrong.