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."

[RightWhale]

Van Flandern says that Mars was once bigger and something happened and the asteroid belt is associated with that event. This is one time when I agree with van Flandern. Mars sure looks like it got hit hard. So does earth. Both planets look like they are missing almost a hemisphere of crust. In Alaska we are on the edge of the Pacific Ocean, which may have been encrusted early in the prehistory of earth. Mars has a similar basin and the idea that the moon is made of crustal metarials that once were where the Pacific Ocean is now is popular again. Where did the missing crust of Mars go? Asteroids, probably: a lot of asteroids seem to be made of crustal material. Anyway, van Flandern is a good astronomer most of the time, but in matters of gravity he may be somewhat outside his area of specialization.

[Southack]

"And as the radar dish sweeps around, shining its beam on regions untouched by magnetic field, is this not the same thing as "turning on the field" there?"

No, I'd say that analogy simply obfuscates the issue. We already know the speed at which electromagnetic waves propagate. Those electromagnetic waves propagate at the same speed as do disturbances inside an existing magnetic field.

On the other hand, what we are debating is not how fast those disturbances or electromagnetic waves propagate, but rather how fast the magnetic field itself propagates when it is first formed, as well as when it is turned off.

Waves can be found *inside* the field, but waves are not the field itself. Waves are known travel at a measurable and accepted speed, something that is still under debate for the field itself.

[GOPJ]

Are you saying it's difficult to come up with a unified theory? What a strange field...

While electromagnetic, weak, and strong are similar enough to be combined mathematically, gravity is a different species mathematically and refuses to be combined with the others.

[GOPJ]

Are you saying it's difficult to come up with a unified theory? What a strange field...

While electromagnetic, weak, and strong are similar enough to be combined mathematically, gravity is a different species mathematically and refuses to be combined with the others.

[Physicist]

On the other hand, what we are debating is not how fast those disturbances or electromagnetic waves propagate, but rather how fast the magnetic field itself propagates when it is first formed, as well as when it is turned off.

OK, so what does it mean to "turn on" or "turn off" a gravitational field? Physically, how do you do it? How long has it been since we've had to worry about the gravitational field from the sun "turning on"?

And since all charge is conserved, what does it mean to "turn on" an electric field? Oh, you can charge up a capacitor, sure, but you're simply moving existing electric fields into a new orientation. The only thing you're ever doing is changing and reorienting the existing field, so what exactly needs to propagate, here?

And since all magnetic fields are contingent upon point of view, what does it mean to "turn on" a magnetic field?Every charge is moving from somebody's point of view. Furthermore, even electrons have to change velocities continuously: the accelerations they undergo may be large, but they aren't infinite, so all changes to any magnetic field must ultimately be continuous. There's no such thing as a truly "sudden" turn-on for a magnetic field; it's always gradual on some timescale.

[Southack]

Great questions, but you seem to be getting progressively further and further away from figuring out how to discern the speed of Gravity.

Going back to the basic question of the speed of Gravity, we have so far listed TWO acceptable *potential* answers for the known, repeatable, and observable fact that the planets orbit in planes that are centered upon the actual position of the Sun.

1. The Sun might be *absolutely* motionless,

or

2. Gravity might propagate so fast that the Sun fails to move any significant distance in the time that it takes for Gravity to move from the Sun to the Earth.

[Old Professer]

If you have a flat sheet of rubber and you place a large ball on it, you will see a curve. If you roll a smaller ball near it the smaller ball will curve due to the bend in the sheet. This is a good analogy of a gravity field. This sheet is there all the time. However, a wave can be created in that sheet which will then propagate (but the original curve will still be there) along the sheet. If this is a gravitational field and a gravity wave, that wave will propagate at the speed of light according to General Relativity.

In an earlier life, when I still possessed all my faculties a journalism teacher once described to me the task of a writer:

"You must learn to paint pictures with words,"

That's one mighty find picture you just painted, reminds me of the time I set my beer down on the trampoline so I could stand up.

[Old Professer]

Yet another picture, I may go blind.

[Old Professer]

That's all well and good, but it brings you full circle; we can no more imagine the static state than we can the infinite range.

The very presence of objects of mass construe the field while they constitute it.

Can we agree that, in the absence of mass there is no gravity and in the absence of heat there is no light?

We have all marveled at the art of the juggler, his balls cycling their silent paths, mesmerizing us as we watch in fascinated awe absorbed in their motion to the point where the juggler disappears from our view; but, take away the juggler and the balls all fall.

My point, I guess, is to always keep your eyes on the juggler and let the balls fall where they may.

[Old Professer]

It's my ball, and I'm going home!

[Old Professer]

I once stood at the finish line at a dragstrip in Fremont, Ca.; when a AA/F dragster went past at 250+ MPH my eyes watered and my socks fluttered.

Man, was he moving!!!

[Doctor Stochastic]

Impressive. I watched the Indy 500 right near the pits a few years ago. There is a big difference in sound between a car doing 180 and 220.

[Old Professer]

Well, that doesn't quite work either, because they are using different motive forces and subject to different frictional forces, I doubt you could successfully glue the two togeter, even if both were computer-controlled.

Faraday once made a comment that space was completely empty or it was completely full for it could not tolerate any condition in between; I have never quite understood what he meant.

[Old Professer]

If there is a failure in relativity, it surely must lie in the observer, for without him nothing would be.

[Old Professer]

In other words, which frame of reference is reality?

In other words, Humpty Dumpty was right.

[Old Professer]

One life at a time and a lot of faded smudges of ink.

[Old Professer]

Air pollution.

[Old Professer]

Why do so many treat ignorance as a disease; consider us sterile cells here who sit idly by in the midst of this battle of infection of agents all hostile to our blissful comfort, what to do about our confusion?

[Old Professer]

If I may suspend levity for a moment; how would one show an acceleration greater than c?

[Old Professer]

Now you're back to the static state.