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.

[Oberon]

No, by making that assertation, I am pointing out that Reality doesn't care whether we can "tell" or not.

Thus asserting that you've got the goods on "Reality." That's why I referred to praying to you, because if you've really got a lock on Reality, theyn you're God.

Look, I've said some foolish things in the past and later failed to back down from them. In this case, you've done likewise, and no amount of hedging or obfuscatory argument will erase it. I'm going to drop it and go on, because I've got better things to do.

Meanwhile, according to the best theory we have to date:

All motion is relative.

Accept it now; understand it later, if necessary.

[Southack]

"All motion is relative."

No, that's not accurate.

Two cars can be driving along at identical speeds, and the windows may be so blocked up that they can only see each other. From their relative perspectives, they aren't moving.

Reality, however, cares not a whit for their relative perspectives, and as the two cars cruise through the roadblocks and off the broken bridge, the occupants will soon learn that painful lesson.

So you can claim that the appearance of motion is relative, but the reality is that appearances don't matter to the actual facts.

[RightWhale]

From their relative perspectives, they aren't moving.

Being stationary is contained in the total spectrum of movement, zero velocity. Samne math, same equations, same physics, same continuum.

[m18436572]

Gravity is my friend!

[Southack]

"Being stationary is contained in the total spectrum of movement, zero velocity. Samne math, same equations, same physics, same continuum."

Being "stationary" works for **models**, and that's why we say what we say about relative motion.

But our models aren't reality. We may perceive something to be stationary, and we may make scientific, mathematical, and geometric models of that thing in which we show that it is stationary, but such models (as well as our perception) may not always be accurate.

Just as in the example above where the two cars **seemed** to be stationary from a certain frame of reference, a different perspective might more closely resemble reality.

[Liberal Classic]

The Face on Mars is wearing a really miffed expression, right about now.

Sorry, the Face on Mars is smiling:

[Oberon]

[crickets]

[RightWhale]

our models aren't reality

It brings home the bacon. Maybe bacon isn't reality either.

[m18436572]

bump. flash light <=> gravity ?

[Southack]

"It brings home the bacon. Maybe bacon isn't reality either."

Oh no, bacon's real, it's just that the picture/model of bacon on the package that doesn't accurately represent the reality of "bacon".

< GRIN! >

[SGCOS]

[crickets]

and the occasional bull frog..

lol

[UCANSEE2]

Man, talk about opening a can of worms.

Movement, for a long time, has been measured and based on the concept that the measurer is 'still'. This is not true.

Movement of the galaxies is based on the concept that the measurer is still. This is not true. The Earth is moving, the solar system is moving, the galaxy itself is moving. (or not moving, but we can't know which--see later argument)

You say, well, we can take all those movements and use them to actually calculate movement (such as the idea all galaxies are moving AWAY from US, making us essentially the center of the universe.)

First, if the galaxy has no edge, it has no center.
Second, the only way to calculate the movement of OUR galaxy is to have a NON MOVING spot in the UNIVERSE to base that on. Again, no such thing.

THIRD, and the hardest to understand, is that the entire UNIVERSE may not be sitting still either.

All movement is calculated relevant to another object, and there is not any object we know of that is not moving/or that we can actually determine whether it is moving or not.

ANY ARGUMENTS to my PROPOSITIONS ? I love to debate the subject as it is the most fascinating, and the most educating subject I can think of.

[UCANSEE2]

PS. Light, like GRAVITY, is not a particle, as far as we know. (we know nothing, actually).

Scientists theorize that LIGHT is a particle, or/and a wave. If a particle, it's speed could surely be limited. If a wave, would it's speed be limited (or even calculable)? Gravity, surely is not a particle. Therefore it seems likely it's speed is not limited, to anything. How can an object have speed if it is not moving?

The EFFECT of a force in the universe may have a speed. At least the measurement of that force can be translated into speed.

In the ocean, the waves move fairly fast. Except, nothing is moving, at least laterally. Nothing material is moving laterally. The EFFECT or FORCE is moving laterally via an actual movement (physical) of the water VERTICALLY.

Electricity, Gravity, Light, the key is the Vertical wave movement, not the horizontal. Though the terms vertical and horizontal are again, dependent on point of view, and only a mental visualization of what actually happens. The movement is actually circular, as are all movements in the universe. That they are seen as horizontal, or vertical, is due to being seen from one viewpoint, and two dimensions.

[RightWhale]

First, if the galaxy has no edge, it has no center.

Say it is the universe rather than just the galaxy. The universe has no edge, but rather than having no center, each entity at whatever scale, quark to atom, to molecule, to biosphere, to noosphere is it's own center, all sharing the same coordinates of space and time from center to as far as it goes. UCANSEE2, both UCANSEE2-- bio-entity with trillions of Kreb cycles per second, and UCANSEE2--cybercreature, is the center of all. There are other centers, each particle, whether elementary particle or higher construct is a center. The center is a plurality. It's not 'no center', it's all centers.

[UCANSEE2]

pOST 254. I beleive you, Oberon, agree with my points. If there is such a thing, we could not detect it.

[UCANSEE2]

Sorry, I made a mental typo. First, if the galaxy has no edge, it has no center.

Statement should have been: First, if the UNIVERSE has no edge, it has no center.

Second, to answer the questions posed in your response, if all entities have a common center, then the question of distance between them, or measurement of their speed relative to us, or anything else, is pointless.

[Gary Boldwater]

If the cosmic microwave background radiation is "stationary" we can detect motion relative to it in the dipole anistropy: csep10.phys.utk.edu/astr162/lect/cosmology/cbr.html

It all comes down to the nature of space. Is it a vacuum, a fluid or what else? If there are effects produced by motion relative to absolute space then yes. If one could measure the one way speed of light in every direction, this may show absolute motion through non congruent light paths (very similar to the Sagnac effect, except in a linear translation).

[SouthParkRepublican]

Gravity ping for heavy reading!

[Oberon]

Yes, your post made perfect sense to me. Thank you.

[Oberon]

It all comes down to the nature of space. Is it a vacuum, a fluid or what else? If there are effects produced by motion relative to absolute space then yes. If one could measure the one way speed of light in every direction, this may show absolute motion through non congruent light paths (very similar to the Sagnac effect, except in a linear translation).

What would the implications be, then, for a relativistic understanding of the universe? Would Lorentz calculations require adjustment based on absolute velocity, then?