Berkeley Lab Physicist Challenges Speed of Gravity Claim

spacedaily.com ^ | 23 Jun 03 | staff

[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.