Berkeley Lab Physicist Challenges Speed of Gravity Claim

spacedaily.com ^ | 23 Jun 03 | staff

[RightWhale]

Berkeley Lab Physicist Challenges Speed of Gravity Claim

Berkeley - Jun 22, 2003

Albert Einstein may have been right that gravity travels at the same speed as light but, contrary to a claim made earlier this year, the theory has not yet been proven. A scientist at Lawrence Berkeley National Laboratory (Berkeley Lab) says the announcement by two scientists, widely reported this past January, about the speed of gravity was wrong.

Stuart Samuel, a participating scientist with the Theory Group of Berkeley Lab's Physics Division, in a paper published in Physical Review Letters, has demonstrated that an "ill-advised" assumption made in the earlier claim led to an unwarranted conclusion. "Einstein may be correct about the speed of gravity but the experiment in question neither confirms nor refutes this," says Samuel. "In effect, the experiment was measuring effects associated with the propagation of light, not the speed of gravity."

According to Einstein's General Theory of Relativity, light and gravity travel at the same speed, about 186,000 miles (300,000 kilometers) per second. Most scientists believe this is true, but the assumption was that it could only be proven through the detection of gravity waves. Sergei Kopeikin, a University of Missouri physicist, and Edward Fomalont, an astronomer at the National Radio Astronomy Observatory (NRAO), believed there was an alternative.

On September 8, 2002, the planet Jupiter passed almost directly in front of the radio waves coming from a quasar, a star-like object in the center of a galaxy billions of light-years away. When this happened, Jupiter's gravity bent the quasar's radio waves, causing a slight delay in their arrival on Earth. Kopeikin believed the length of time that the radio waves would be delayed would depend upon the speed at which gravity propagates from Jupiter. To measure the delay, Fomalont set up an interferometry system using the NRAO's Very Long Baseline Array, a group of ten 25-meter radio telescopes distributed across the continental United States, Hawaii, and the Virgin Islands, plus the 100-meter Effelsberg radio telescope in Germany. Kopeikin then took the data and calculated velocity-dependent effects. His calculations appeared to show that the speed at which gravity was being propagated from Jupiter matched the speed of light to within 20 percent. The scientists announced their findings in January at the annual meeting of the American Astronomical Society.

Samuel argues that Kopeikin erred when he based his calculations on Jupiter's position at the time the quasar's radio waves reached Earth rather than the position of Jupiter when the radio waves passed by that planet. "The original idea behind the experiment was to use the effects of Jupiter's motion on quasar-signal time-delays to measure the propagation of gravity," he says. "If gravity acts instantly, then the gravitational force would be determined by the position of Jupiter at the time when the quasar's signal passed by the planet. If, on the other hand, the speed of gravity were finite, then the strength of gravity would be determined by the position of Jupiter at a slightly earlier time so as to allow for the propagation of gravitational effects."

Samuel was able to simplify the calculations of the velocity-dependent effects by shifting from a reference frame in which Jupiter is moving, as was used by Kopeikin, to a reference frame in which Jupiter is stationary and Earth is moving. When he did this, Samuel found a formula that differed from the one used by Kopeikin to analyze the data. Under this new formula, the velocity-dependent effects were considerably smaller. Even though Fomalont was able to measure a time delay of about 5 trillionths of a second, this was not nearly sensitive enough to measure the actual gravitational influence of Jupiter. "With the correct formula, the effects of the motion of Jupiter on the quasar-signal time-delay are at least 100 times and perhaps even a thousand times smaller than could have been measured by the array of radio telescopes that Fomalont used," Samuel says. "There's a reasonable chance that such measurements might one day be used to define the speed of gravity, but they just aren't doable with our current technology."

[Southack]

"Now, in electrodynamics, a charge moving at a constant velocity does not radiate. (Technically, the lowest order radiation is dipole radiation, which depends on the acceleration.) So to the extent that that A's motion can be approximated as motion at a constant velocity, A cannot lose angular momentum. For the theory to be consistent, there must therefore be compensating terms that partially cancel the instability of the orbit caused by retardation."

That's all fine and well, but let's not guess that the electrodynamic *analogy* will always be consistent for Gravitational systems.

[Aric2000]

Come on, let's get real shall we?

If I am pulling a trailer, it is directly behind me and ATTACHED to my car, it is not 2 seconds behind me, it is PART of my car, so it is the same distance behind me, NO MATTER what speed I am going.

The earth and the sun are the same way.

The earth is ATTACHED to the sun at a certain distance, and it will stay at that distance no matter what.

So your analogy has NOTHING to do with the sun/earth relationship.

The orbit of the earth does not move, it remains in the same place relative to the sun, no matter what speed the sun is moving, just as the trailer goes the same speed as my car, because it is attached to it.

[Southack]

"This is exactly what happens; a calculation shows that the force on A points not towards B's retarded position, but towards B's "linearly extrapolated" [post] retarded position."

In other words, your source is claiming that the electrodynamic force in question is *PREDICTING* where the future position will be an acting accordingly.

That's tough to swallow.

A much more believeable hypothesis is that the electrodynamic force in question simply travels fast-enough to appear to act on the current, rather than on the past, position under observation.

[Aric2000]

I don't know if 122 was a good analogy or not, critique please?

[Southack]

Would you say that in your analogy, that it is Gravity that is "attaching" the Earth to the Sun?

[metalboy]

Well in this following mathematical model, one can see the formulae of the gravitational force on objects of varying amounts of mass; Both Michael Moore and Pee Wee Herman are standing on top of the Empire State building. Pee Wee jumps first but barely cracks the cement when he hits, but when Michael Moore fictitiously jumps and hits the cement with his brick like head, a force equal to that of a 20 megaton thermonuclear explosion levels every square mile of the east coast, from the top of Maine to the tip of Florida and shock waves are felt globally.

[Aric2000]

Of course it is gravity attaching the earth to the sun.

It does NOT matter the speed of gravity, and there is NO way to measure it from that relationship, because the earth is ALWAYS at a set distant. Therefore measuring the speed of gravity from it is useless, because there is NO change to measure, the earth is at a set distance and will reamin there.

[Southack]

"Of course it is gravity attaching the earth to the sun."

And if this force were to be disturbed, say by an object such as the Moon coming in between the Sun and Earth, or by moving the Earth further away from the Sun, or by dispersing the Sun's atoms in all directions, or by whatever means of disturbance that you choose to agree upon, would said disturbance in Gravity be felt *instantly* on the Earth and Sun, or would there be some time delay?

[RadioAstronomer]

Would you say that in your analogy, that it is Gravity that is "attaching" the Earth to the Sun?

A better way of looking at it is that the Earth is falling around the Sun like the moon is falling around the Earth.

[Aric2000]

It would be impossible to meaure right now, and actually, the mass, whatever mass that would be, would be felt slowly, as it got closer, it's gravitational attraction would get stronger, the larger the object, the stronger the gravity well. and again, as it got closer, it's graitational effects would get stronger, but at this pint it is IMPOSSIBLE to measure such a thing.

Until it is technically possible to do that, then we won't really know, but can take some good guesses.

As Einstein thought, C is the limit, so I would have to assume, since he was right about a LOT of things, that Gravity would travel at C as well.

But again, it is impossible to measure, therefore we will not know until the technolgy is available.

[Southack]

I'm just working with his analogy.

[Southack]

"But again, it is impossible to measure, therefore we will not know until the technolgy is available."

Really? You mean we can't measure something that travels so fast as Gravity? That we can't see the impact of a Gravitational disturbance on our tides or our instruments? That we can't observe the plane in which each planet orbits?

[Aric2000]

BUt I have NO idea if it's a good analogy, I am an amateur here, I am just using the limited knowledge I have and a healthy dose of common sense.

The ONLY way to measure something like the speed of gravity, would be to have a difference to measure, and since the earth is at a set distant from the sun at all times, measuring from that relationship, it would seem to me, would be inaccurate, and a waste of time.

[Southack]

"As Einstein thought, C is the limit, so I would have to assume, since he was right about a LOT of things, that Gravity would travel at C as well."

Perhaps, but Einstein has also been misquoted on quite a few things, too. That's something to consider.

Another thing to consider is that Newton was a rather clever fellow, too, perhaps even in Einstein's league.

...And Newton says that Gravity propagates at near instantaneous speeds.

That's something else to consider.

[Aric2000]

Sure, but without a DIFFERENCE, ANY measurement of it would be inaccurate and impossible. You NEED differences in order to get a measurement, the planets are at a set distance, the moon is at a set distance.

So you could use them as a sort of calibration, but you need something that is moving away from or towards us, in order to get a measurement, and like I said, the technology is NOT available, and the only thing really BIG enough to get that measurement, would be from a star, a comet would be too small, even a wandering planet would be too small to get an accurate measurement from, because of course you need more then one instance to get a proper calculation.

[Southack]

"since the earth is at a set distant from the sun at all times, measuring from that relationship, it would seem to me, would be inaccurate, and a waste of time."

You do realize that NO ONE on this thread disputes that the Sun itself is MOVING at 157 miles per second, correct?

If the Earth adjusts its orbit nearly instantaneously to the Sun's movement, how fast would Gravity have to be traveling?

We *can* measure where the Sun was at a set time in the past versus its position in the present, after all. Is the plane of the Earth's orbit going around the Sun's position in the past (which would indicate a certain delay in propagation), or is the Earth's orbital plane centered much closer to the Sun's actual present position?

[Old Professer]

Mirror, mirror, on the wall...

[Old Professer]

Only four? What about chocolate and menopause?

[Aric2000]

It wouldn't matter, because even if the gravity that left the sun 8 1/2 minutes ago is what is effecting it, it would have the SAME effect no matter where the earth was in it's orbit, therefore a measurement would be impossible.

[plusone]

I think we should conduct a detailed study...