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."

[boris]

"That delay would permit the Sun to travel 178,000 miles in those 8.3 minutes.

That would mean that the Earth's orbital plane would be centered *not* on the actual location of the Sun, but rather on the location of the Sun from 8.3 minutes back in time, a measurable difference of 178,000 miles."

Tiresome and tendentious. Loudly asserting Newtonian physics in a non-Newtonian situation proves only your ignorance.

--Boris

[Physicist]

A disturbance to a field may very well propagate at Light speed, but that's an entirely different action, in my opinion, than the field *itself* propagating.

But once the field is there, it's there. What aspect of the field needs to propagate, other than any changes to it?

[Southack]

"Going from no field to some field is a change, and that change is what is propagating. It's all that has to propagate for a complete description of what's going on."

That train of thought very likely misses the right track.

It's too easy to think along those lines and confuse the field itself changing or springing into existence with that of an existing field being disturbed.

When we turn ON the electromagnet, the magnetic field suddenly covers a sizeable area where it did not cover in the past when our electromagnet was OFF.

How *fast* did the field cover this new area?

Yet what you seem to be saying is that this "change" in going from "no field" to "some field" is akin to a disturbance in an existing field.

I'm not convinced that's entirely valid or useful. It might be a division by zero event.

Yes, if we have an *existing* magnetic field, then a disturbance in that field should propagate through the field itself at the speed of Light. That's well-known and not under dispute so far as I'm aware.

But what hasn't been conclusively proved or accepted is how fast the field itself covers an area when the field first forms, likewise for when the field ends as to how fast it ceases to cover an area.

[Southack]

"But once the field is there, it's there. What aspect of the field needs to propagate, other than any changes to it?"

That's rich! It's also a bit beyond the scope of this thread.

We're simply trying to answer *how fast* the field gets there, not *why* it needs to continue to do anything.

[sit-rep]

Why, does an object fall only about 145+- MPH to earth?

[Southack]

"Loudly asserting Newtonian physics in a non-Newtonian situation proves only your ignorance."

Proves, conclusively proves beyond dispute my ignorance?! My, my, my...

OK, rather than respond in kind, I'll simply ask you what specific aspect of the Sun and Earth moving through space that *you* assert is non-Newtonian.

[RightWhale]

Why, does an object fall only about 145+- MPH to earth?

Could you rephrase the question? An object might fall at that speed, another might fall at a different speed.

[aruanan]

Van Flandern alternates in a random pattern between reality and kookism, so it would be well to read the paper for oneself or wait for further letters to the editor.

Ha ha. Sometimes kookism is more a measure of the degree to which the expectations by which one interprets kookism are out of line with reality than the degree to which the presumed kookism actually departs from reality. I haven't seen in any of the Mars photos anything that looks definitively artificial. But then, astronomers with a lot less to go on than Flandern has have thought they saw canals on Mars where none existed. Still, the "seeing faces in the carpet" phenomenon in the interpretation of Mars photographs shouldn't detract from otherwise cogent arguments on the speed of gravity.

[Physicist]

Yet what you seem to be saying is that this "change" in going from "no field" to "some field" is akin to a disturbance in an existing field.

Exactly! Because consider: an observer at rest with respect to an electrical charge will see an electric field and NO magnetic field, while a moving observer (passing arbitrarily close to the first observer's position) will see both an electric and a magnetic field. The two observers will not agree on whether there is a magnetic field or not; they will, however, agree on the dynamics of locally moving charged particles (i.e., the physics works out the same).

So you see, you can practically never say that any region of space is free from a magnetic field, because the magnetic field in that region will be different for different observers. One man's zero field may be another man's strong field. There can't be anything special about turning on a field; it's the same thing as a change to the existing field. The 4-potential is defined everywhere in space. (Homework: look up the terms "Gauge Principle" and "Gauge Invariance".)

[Geek alert 1: The reason that different observers will see different fields is because of special relativity. From a moving frame of reference, time moves more slowly and space is contracted along the direction of motion. If the field is the same for both observers, the motions of a charged particle can't be agreed upon by different observers. The magnetic force is the force that arises that compensates for the difference in motion between the two versions of spacetime. (It's tempting to call the magnetic force an "apparent force", such as the coriolis force, but it is possible--easy, in fact--to construct magnetic fields that can't be zeroed out by a Lorentz boost.)]

[Geek alert 2: Woah! Wait a second! If the relative distortion of spacetime causes an apparent--no, a real--change in the electromagnetic field for relatively moving observers, why doesn't it do the same thing for the gravitational field? It does. It's called the gravitomagnetic effect. Well, doesn't that just prove what Van Flandern is saying? Isn't that just a different formulation of the same effect? No, because first, unlike the "time delay" canard, the gravitomagnetic effect depends on the speed of the observer and not on the distance from the source, and second, because any "time delay" effects (such as the Poynting-Robertson effect, which really does pertain to the light from the sun) would be orders of magnitude greater than the gravitomagnetic effect.]

(I have used the blue font color to denote scientific ignorance, as is customary on FR.)

[Physicist]

We're simply trying to answer *how fast* the field gets there

If it didn't get there at the speed of light, radar wouldn't work.

[sit-rep]

Sorry about that.

I think I am assuming the gravitational force the article is talking about is the speed in which an object will be pulled...

IOW... why, when an apple falls from the tree, it does not leave a 24" crater because it was doing 130,000 MPS and gaining??

[Southack]

"an observer at rest with respect to an electrical charge will see an electric field and NO magnetic field, while a moving observer (passing arbitrarily close to the first observer's position) will see both an electric and a magnetic field. The two observers will not agree on whether there is a magnetic field or not; they will, however, agree on the dynamics of locally moving charged particles (i.e., the physics works out the same). So you see, you can practically never say that any region of space is free from a magnetic field, because the magnetic field in that region will be different for different observers. One man's zero field may be another man's strong field. There can't be anything special about turning on a field; it's the same thing as a change to the existing field."

Ahhh, but there *can* be something special about turning on the field (more below).

Your *observers* in your above example depend upon frames of reference, yet you and I both know that if behavior isn't explained consistently in different frames of reference that someone's equation is wrong. One observer might see a magnetic field, and another observer might not, but that's *not* the same thing as turning on a field in the first place. The two observers won't agree on whether there is or is not a magnetic field in your example, this is true, yet we both know that the field does exist! It just isn't *observable* to one of the participants.

Likewise, the frame of reference doesn't determine the magnetic field of our electromagnet. Regardless of the position of the observer, in reality there is NO field when the electromagnet is OFF, while there is a field that covers a sizeable area when our electromagnet is turned ON.

And what is so special about that fact? Well, that fact means that a magetic field has to propagate outwards over an area in a given amount of time (once we switch the electromagnet ON).

Thus, it is a valid question to ask *how fast* does the field cover that area.

[Southack]

"If it didn't get there at the speed of light, radar wouldn't work."

I don't see where you dug that statement up. I'm not arguing about the speed of *disturbances* in an existing electromagnetic field, but rather how fast a magnetic field covers an area.

[Southack]

"unlike the "time delay" canard, the gravitomagnetic effect depends on the speed of the observer" frame of reference.

Now what do we say about a phenomenon that is not consistently explained in *every* frame of reference, not just one?!

[PatrickHenry]

Because consider: an observer at rest with respect to an electrical charge will see an electric field and NO magnetic field, while a moving observer (passing arbitrarily close to the first observer's position) will see both an electric and a magnetic field. The two observers will not agree on whether there is a magnetic field or not; they will, however, agree on the dynamics of locally moving charged particles (i.e., the physics works out the same).

And because Maxwell's equations showed that they will both agree the charge propagates at c, a certain patent clerk drew some rather interesting conclusions.

[Physicist]

The two observers won't agree on whether there is or is not a magnetic field in your example, this is true, yet we both know that the field does exist! It just isn't *observable* to one of the participants.

By that standard, the field always exists, everywhere. Zero is a perfectly good value for the field to have, as you point out. So if you want to say that a zero-field region of space is not a field-free region of space, I'll accept that.

Now, show me a region of space that is free from all gravitational fields.

[Physicist]

I'm not arguing about the speed of *disturbances* in an existing electromagnetic field, but rather how fast a magnetic field covers an area.

Oh, aye. 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?

[Physicist]

Now what do we say about a phenomenon that is not consistently explained in *every* frame of reference, not just one?!

The phenomenon in question is the motions of physical bodies, and those motions will be agreed upon for all observers. The gravitomagnetic effect guarantees that, even in the face of different spacetime perspectives.

[Physicist]

the charge propagates at c

Smoke what?

[RightWhale]

why, when an apple falls from the tree, it does not leave a 24" crater because it was doing 130,000 MPS and gaining??

That's a good question. We know it doesn't because we haven't seen it happen, but then again, we haven't seen all apples falling. Maybe some make craters.