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."

[autoresponder]

TRUTH BULL

[RadioAstronomer]

We go around the core about every 200 million years at a distance of however far we are from the core. Somebody [preferably a 1st year engineering student] should cruch the numbers and tell us the result in feet or miles.

I am not a 1st year engineering student, but will I do?

Distance of the Earth to the center of the galaxy: about 30,000 light years
Total distance traveled for one rotation = Pi x D = 188,400 light years
Speed of light = 299,792,458 meters per second
There are 86400 seconds in one day with approximately 365.25 days per year giving us approximately 31,557,600 seconds/year.
So the distance of one light year is around 9.46 x 10¹⁵meters.

This gives us a total distance travel for one rotation of 1.78 x 10²¹meters.

Rotational period of the galaxy: 2.25 x 10⁸ years.

This gives us about 8 x 10¹²meters traveled per year.

8 x 10¹² divided by 365.24 = approximately 2.2 x 10¹⁰ meters/day or 912,600 kilometers per hour.

This then is approximately 565,837 miles per hour or 157 miles per second.

[Southack]

"157 miles per second"

Times 8.3 minutes = 78,186 miles (for some reason I had it in my head that the Sun traveled 270,000 miles in the time that it took Light to reach Earth from the Sun, bizarre).

So the question is whether the Earth rotates in an orbital plane around where the Sun was 8.3 minutes ago, which will always be 78,186 miles away from its current position (if your math is valid), or whether the Earth orbit is centered more closely to the actual position of the Sun at the present time.

And that answer will tell us the speed of Gravity.

[Aric2000]

Man, and they thought a train couldn't go more then 20 miles an hour!!

157 miles per second? Now THAT'S FAST!!!

[RadioAstronomer]

So the question is whether the Earth rotates in an orbital plane around where the Sun was 8.3 minutes ago, which will always be 78,186 miles away from its current position (if your math is valid), or whether the Earth orbit is centered more closely to the actual position of the Sun at the present time.

Remember it takes one year for the earth to orbit the sun once. Visualize it as a combined system (earth/sun) orbiting about a common point of mass moving as a whole relative to the center of the galaxy. Don't forget the galaxy is also moving linearly as a "whole" as well.

[Southack]

That's correct and fine and well and good, but either the Earth is orbiting around where the Sun was centered 8.3 minutes ago at any given point in time, or it is orbiting around a point closer to the center of the Sun at the present time, and the differnece, if any, between those two points will enable us to calculate, from observable phenomena, the speed of Gravity.

[RadioAstronomer]

Trivia Of The Day: Because their foot pads end in literally billions of microscopic filaments, geckos use Van Der Waals forces to allow them to stick to just about any surface and climb up walls and across ceilings. They do this so efficiently that the average gecko is "glued" to the wall with about 200 pounds of force.

That is just too cool! :-)

[RadioAstronomer]

Nope. The Earth and Sun are orbiting a center point of mass. Since the Sun has such a greater mass than the Earth, the point is inside the Sun itself. However, the Earth is moving along with the Sun in the direction of travel of the solar system.

A poor analogy, but should give you an idea, is you dropping a ball on a moving train. The ball falls strait down (relative to you) because it is moving horizontally at the same velocity the train is.

[RadioAstronomer]

Take a look here:

http://math.ucr.edu/home/baez/physics/Relativity/GR/grav_speed.html

[Southack]

You've taken your eye off of the ball!

It takes Light 8.3 minutes to reach the Earth. If Gravity travels as slow as Light, then it takes 8.3 minutes for Gravity to reach the Earth.

The Gravity wave is going to pull the Earth towards the center of where the Sun was when the Gravity wave left the Sun.

If that was 8.3 minutes ago, then the Earth will be orbiting, at any given moment, around a center point that is 78 thousand miles away from where the Sun is at the present if Gravity really is as slow as Light (because that's how far the Sun will have traveled in those 8.3 minutes). Gravity and the Earth aren't predicting the *future*, after all, but rather reacting to current and past events.

And yes, the Earth is *also* traveling those same 78,000 miles in those 8.3 minutes, but the angle of the plane of the Earth's orbit is still going to be centered on where the Sun was when Gravity left it.

And this angle will be different (perhaps only slightly, perhaps by a large amount, depending upon the speed of Graivty) the further a planet resides from the Sun.

[Doctor Stochastic]

But you may have Eva Marie Saint crawl down your nose.

[Southack]

"This cancellation may seem less strange if one notes that a similar effect occurs in electromagnetism. If a charged particle is moving at a constant velocity, it exerts a force that points toward its present position, not its retarded position, even though electromagnetic interactions certainly move at the speed of light. Here, as in general relativity, subtleties in the nature of the interaction "conspire" to disguise the effect of propagation delay."

Or perhaps there is no significant delay. Hmmm, rather than having nature "conspire", perhaps Newton was basically correct that Gravity (who knows, possibly even the electromagentic field effect, too) propagated at near instantaneous speeds, say, the Speed of Light squared.

[Doctor Stochastic]

Nice picture. Kind of Art Gecko.

[Doctor Stochastic]

So, the turtles are spinning all the way down?

[RadioAstronomer]

The Gravity wave is going to pull the Earth towards the center of where the Sun was when the Gravity wave left the Sun.

Where to start. Hmmmm.....Here is a really good explanation:

http://zebu.uoregon.edu/~imamura/talks/gravity_waves/gw_intro.html

[RadioAstronomer]

who knows, possibly even the electromagentic field effect, too) propagated at near instantaneous speeds, say, the Speed of Light squared.

Nope! Go read both links I provided.

[RadioAstronomer]

So, the turtles are spinning all the way down?

ROFL! Oh yea!

[Southack]

Are you trying to say that Gravity is as slow as Light, but that the universe *predicts* the future and therefor Gravity knows where the Sun and the Earth will be in 8.3 minutes and adjusts the orbit accordingly?

Come on, let's simply examine observable phenomena. The Earth orbits the Sun of either 8.3 minutes ago or the Sun of right now (or something in between).

Where ever that point resides, from the Sun of 8.3 minutes up to the Sun of this very moment, will let us calculate the speed of Gravity (unless Gravity can somehow be slow *AND* predict where the Sun will be in 8.3 minutes).

[RadioAstronomer]

From my link:

Consider two bodies -- call them A and B -- held in orbit by either electrical or gravitational attraction. As long as the force on A points directly towards B and vice versa, a stable orbit is possible. If the force on A points instead towards the retarded (propagation-time-delayed) position of B, on the other hand, the effect is to add a new component of force in the direction of A's motion, causing instability of the orbit. This instability, in turn, leads to a change in the mechanical angular momentum of the A-B system. But total angular momentum is conserved, so this change can only occur if some of the angular momentum of the A-B system is carried away by electromagnetic or gravitational radiation.

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. 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" retarded position. Similarly, in general relativity, a mass moving at a constant acceleration does not radiate (the lowest order radiation is quadrupole), so for consistency, an even more complete cancellation of the effect of retardation must occur. This is exactly what one finds when one solves the equations of motion in general relativity.

[Southack]

"This instability, in turn, leads to a change in the mechanical angular momentum of the A-B system. But total angular momentum is conserved, so this change can only occur if some of the angular momentum of the A-B system is carried away by electromagnetic or gravitational radiation."

The first part in BOLD precisely agrees with what I've been saying all along on this thread, and the second bolded part does not disagree with what I've been saying, as it is an unknown at this point.

In other words, up to this point your source *agrees* with me.