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]

"This idea of "gravity propagating faster than gravitational waves" is a null concept, with no basis other than a trivial misunderstanding of what a field is. It's like saying that while you may walk at a finite speed, the ground beneath your feet is infinitely fast."

No, it is not a null concept, and no, it is not like saying that the ground moves infinitely fast, as the latter analogy crucifies the distinction being made between a field covering an area that it did not cover from that of the ground already covering an area.

When you turn an electromagnet to the ON position, a magnetic field suddenly covers a new area that in the past it did not cover (as in, when the switch was OFF).

It is *valid* to inquire as to how fast that field covered the new area.

But it has not been proven to be valid to claim that the field will cover a new area at the same speed as *disturbances* to an already *existing* field propagate (which we are all in agreement that it happens at Light speed).

What we currently know is that an existing field will propagate changes at Light speed. Fine.

But not all electromagnets have been turned ON yet, so not all fields exist yet.

When turned ON, the magnetic field goes from covering ZERO area to covering some sizeable finite area.

Does that happen at the speed of Light? Well, it hasn't been conclusively shown to be that slow, yet.

[Southack]

"Sadly, if Dr. Carlip told you the above example, then you are both absolutely wrong. Mercury's orbital precession was known when there was *only* Newtonian physics, long before the General Relativity Theory, much less Special Relativity came into our knowlege." - Southack

"Again you are wrong. The phenomenon was known but Newtonian mechanics could provide no explanation of it. If you think otherwise, please post the Newtonian explanation here. We'd all be interested to see it. The world was in an uproar when Einstein's predictions were shown to match the actual measured precession. And Dr. Carlip did not tell me this."

It's good that Carlip didn't tell you any of the above, as it would only have made him as wrong as you (though he probably tosses around fewer ad hominems). Mercury's orbital precession was known *prior* to General Relativity, so GR wasn't required to solve that planetary dillema, QED.

Likewise, other such astral issues may very well be solved *without* GR.

[Southack]

Let me simply ask you *again* what specific aspect of the Sun and Earth moving through space that *you* personally assert is non-Newtonian.

[Oberon]

Gravity propagates in "waves?" I thought it was simply a curvature of space around a mass.

Yep, and the steel ball on a rubber sheet is a great model. But when you drop the ball on the sheet, the curvature of the sheet around it propagates outward at a certain rate.

That's what they're trying to measure.

[longshadow]

this might be useful:

As seen from Earth the precession of Mercury's orbit is measured to be 5600 seconds of arc per century (one second of arc=1/3600 degrees). Newton's equations, taking into account all the effects from the other planets (as well as a very slight deformation of the sun due to its rotation) and the fact that the Earth is not an inertial frame of reference, predicts a precession of 5557 seconds of arc per century. There is a discrepancy of 43 seconds of arc per century.

This discrepancy cannot be accounted for using Newton's formalism. Many ad-hoc fixes were devised (such as assuming there was a certain amount of dust between the Sun and Mercury) but none were consistent with other observations (for example, no evidence of dust was found when the region between Mercury and the Sun was carefully scrutinized). In contrast, Einstein was able to predict, without any adjustments whatsoever, that the orbit of Mercury should precess by an extra 43 seconds of arc per century should the General Theory of Relativity be correct.

[emphasis added]

from:

http://phyun5.ucr.edu/~wudka/Physics7/Notes_www/node98.html

[Oberon]

Let me simply ask you *again* what specific aspect of the Sun and Earth moving through space that *you* personally assert is non-Newtonian.

Can I answer? Can I answer? Oooh, Oooh! (Waves hand wildly)

[Southack]

Sure, you can answer for him. Boris appears to need a little help right now, anyway.

[Physicist]

When you turn an electromagnet to the ON position, a magnetic field suddenly covers a new area that in the past it did not cover (as in, when the switch was OFF).

It is *valid* to inquire as to how fast that field covered the new area.

But there's no physical way to do that with a gravitational field. Furthermore, the sun's field has been "on" for billions of years. Furthermore, there's no region of space that is not permeated by gravitational fields. Anything else you can talk about is propagated in the form of waves, as you have admitted, and regardless of what nonsense you believe about switching things on (as if any point in space has more than one vector potential). And also as you have admitted, those waves propagate at c. Nothing more has to be said.

The Einstein field equations give accurate and verified results for any physical situation we've had the opportunity to measure, and without postulating any velocities of the kind Van Flandern imagines. They are utterly superfluous to the theory. As Laplace told Napoleon, "I have no need of that hypothesis."

[Southack]

So without modification, Newton nailed the answer to within 00.7% accuracy, whereas Einstein got it precisely right.

[Southack]

"But there's no physical way to do that with a gravitational field."

Are you saying that we can't measure the difference between the angle at which the Sun's Gravity pulls the Earth and compare that angle to the angle at which Light finally reaches the Earth from the Sun?

[Physicist]

Does that happen at the speed of Light? Well, it hasn't been conclusively shown to be that slow, yet.

OK, try this. When an electron and a positron annihilate, their fields annihilate, too. We go from some field--with a vanishing monopole moment in the far field, but with a substantial dipole moment, right up to the end--to no field at all. (I hope you will agree that once the particles are annihilated, their field suddenly disappears.) The collapse of the field demonstrably travels outwards at the speed of light, because the collapse of the field physically manifests itself in the form of photons, as is demanded by quantum mechanics (no relativity involved, here).

But how do we know that those photons travel at the speed of light? You haven't actually measured their velocity, have you? Actually, we have. One useful technique in astronomy is to look at the annihilation photons coming from distant galaxies and nebulae. It tells us where the really hot regions are: antimatter is not easy to come by! If the information of the field destruction came to us at any velocity other than c, the annihilation photons wouldn't line up with the source in visible light. This is not the case.

[Oberon]

Well, I was going to point out the minor adjustments in position, velocity, and acceleration as indicated by the Lorentz equations, but after reading back over the thread I'm pretty sure that's not what you're looking for.

[Southack]

"When an electron and a positron annihilate, their fields annihilate, too. We go from some field--with a vanishing monopole moment in the far field, but with a substantial dipole moment, right up to the end--to no field at all. (I hope you will agree that once the particles are annihilated, their field suddenly disappears.) The collapse of the field demonstrably travels outwards at the speed of light, because the collapse of the field physically manifests itself in the form of photons, as is demanded by quantum mechanics (no relativity involved, here)."

Man, don't tell Boris!

OK, so the field collapses, and we see a non-field entity called a proton emit outwards from this collapse at the speed of Light.

Now, can we demonstrably show that the Field itself is collapsing at the speed of Light, rather than that the *effects* of the Field collapsing emit themselves at the speed of Light?

[Physicist]

Now, can we demonstrably show that the Field itself is collapsing at the speed of Light, rather than that the *effects* of the Field collapsing emit themselves at the speed of Light?

Yes. Because if the field out here collapsed at some earlier time, the field energy out here would have been released at that earlier time, and manifested itself in the form of photons emitted nearby. (That energy cannot be manifest in any other way, per QM.) That doesn't happen, because if it did we'd see persistent "ghost sources" on the sky that don't line up with standard (i.e. blackbody) optical sources.

[Southack]

"The collapse of the field demonstrably travels outwards at the speed of light, because the collapse of the field physically manifests itself in the form of photons..."

One more point, you make it sound as though a Field is collapsing OUTWARD, rather than inward.

Is that really what is happening? Do Fields really collapse OUTWARD? When I turn an electromagnetic OFF, the magnetic Field goes from covering some area to covering no area, yet if it collapsed outward, one might think that it went from covering some finite area to covering an infinite area.

Surely what we are seeing is that the Field collapses inward, and this action causes a photon to be emitted outward, correct?

[Physicist]

You're joking, right?

[Southack]

Not joking.

[Anthem]

Gotta tell you that you now top my list of who to search on for great posts. (But "aww shucks" will drop you a notch ;)

[MikeD]

Sorry I'm late to the party. Work does that to me...

Anyway, it seems there's some kind of brew-ha-ha about GR and the position of the Sun. A few points to keep in mind:

1. The Sun and the Earth are both moving, roughly in tandem, around the Galactic center, towards Andromeda, towards the Great Attractor, etc.

2. From the Earth's point of view, the Sun is always where it "really" was 8 minutes ago.

3. If gravity moves faster than the speed of light, we have a problem with causality. By definition spacetime moves at speed c. If gravity moved faster, then objects would affect each other without "seeing" each other. That makes no sense.

As for General Relativity, ever measurement from Mercury to stellar occultations to binary neutron stars to rocketed atomic clocks confirms GR exactly.

MD, whose PhD dissertation was about gravitational lensing

[Southack]

"3. If gravity moves faster than the speed of light, we have [our model has] a problem with causality. By [our model's] definition spacetime moves at speed c. If gravity moved faster, then objects [in our model] would affect each other without "seeing" each other. That makes no sense."

Ahhh, much better.