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."

[PatrickHenry]

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

Relative to me, the sun is motionless. It stays right where I want it, one AU from the earth. Nothing else matters. All your computations are for naught! Mrruuuhahahahahaha!

[Physicist]

You are stating the analogy just fine, but the limits of analogies come into play here because the sheet has no propagation speed and it has no movement of its own, either (nor does the big ball at rest in its center).

Those are the aspects of the situation that are analogous to the sun.

The sun, insofar as it is not subjected to accelerations, is at a fixed location in its own reference frame. The sun's gravitational field, as long as the sun hasn't undergone an acceleration for more than 8.3 minutes, is fixed to all points in space, at least as far as the Earth's orbit is concerned. The field, like the sheet, does not need to propagate; it's already where it needs to be. The Earth will "see" the same field at every point, regardless of how fast it's moving (neglecting the "gravitomagnetic" effect, which is a known relativistic correction that is vanishingly small).

Should the sun undergo an acceleration (or, God forbid, disappear), the changes in the field will propagate from the sun outward. These changes are gravitational waves, and they propagate at c. Again: waves propagate, fields do not.

The situation is exactly analogous to the electromagnetic field vs. electromagnetic waves. The same geometrical argument applies. Van Flandern now says that while electromagnetic waves propagate at c, electromagnetic fields propagate infinitely fast. Whatever. It's like saying that while cars drive at finite speeds, the road goes infinitely fast. I suppose that's one way to describe it...just not a very useful way.

(I had promised myself not to respond to this Van Flandern crackpottery any more, but RadioAstronomer has inspired me. For the future, I'll put together a boilerplate response.)

[Doctor Stochastic]

Or in the words of Tom T. Hall: "Faster Horses, Younger Women, Older Whiskey, More Money."

[boris]

"Relative to its former position, how far will the Sun have moved in 8.3 minutes?"

Define your coordinate system. In a heliocentric system, it does not move at all.

The whole solar system is moving around the center of the galaxy at about 230 km/s. If you use a galactic-centered coordinate system, the Sun moves 114,678 kilometers in orbit around the galaxy in 8.3 minutes.

However, the galaxy is moving toward the "Great Attractor":

"Detailed observations of the galaxies around us indicate that there is superposed on the Hubble flow a large-scale streaming motion of about 600 km/s in the general direction of the constellation Centaurus.

"This mass migration includes the Local Group, the Virgo Cluster, the Hydra--Centaurus Supercluster, and other groups and clusters for a distance of at least 60 Mpc up and downstream from us. It is as if a great river of galaxies (including our own) is flowing with a swift current of 600 km/s toward Centaurus.

"Location of the Great Attractor

"Calculations indicate that ~10^16 solar masses concentrated 65 Mpc away in the direction of Centaurus would account for this. This mass concentration has been dubbed the Great Attractor. Detailed investigation of that region of the sky (see adjacent image of the galaxy cluster Abell 3627) finds 10 times too little visible matter to account for this flow, again implying a dominant gravitational role for unseen or dark matter. Thus, the Great Attractor is certainly there (because we see its gravitational influence), but the major portion of the mass that must be there cannot be seen in our telescopes."

So the Sun moves ~299,196 kilometers toward the Great Attractor in 8.3 minutes.

However, there is another frame of reference, that of the gross motion of the Solar System with respect to the cosmic background radiation field. This motion has been found to be ~371 km/s. So the sun moves 184,980 km in 8.3 minutes relative to the cosmic background radiation.

Satisfied?

--Boris

[sandydipper]

Hello! Hello! Can you hear me now??

[RadioAstronomer]

And thus, speculation is that our Sun is a third-generation star...because we have heavier elements.

Absolutely. :-)

[RadioAstronomer]

Thanks for the help! I was running out of analogies to describe our system. :-)

[RadioAstronomer]

Nice post. I wasn't even going to go there here on this discussion. :-) (We could also add the effects of the multiple bodies just in this solar system alone, Precession and all the various nutations of our Earth, precession of the Sun, etc.)

[Junior]

A "baby among giants" placemarker.

[RadioAstronomer]

Good morning Junior! :-)

[longshadow]

(I had promised myself not to respond to this Van Flandern crackpottery any more, but RadioAstronomer has inspired me. For the future, I'll put together a boilerplate response.)

Someone has to drive a stake through its heart every so often to make sure it doesn't creep about, snagging unsuspecting, scientifically-challenged lurkers.

;-)

[Southack]

"The sun, insofar as it is not subjected to accelerations, is at a fixed location in its own reference frame. The sun's gravitational field, as long as the sun hasn't undergone an acceleration for more than 8.3 minutes, is fixed to all points in space, at least as far as the Earth's orbit is concerned."

That's an oversimplification, and it misses reality.

The Sun is moving relative to its previous position. It isn't fixed. Likewise, the Earth is moving relative to its previous position, it isn't fixed, either.

The Light that we on Earth see from the Sun is actually from where the Sun was located 8.3 minutes ago. Yes, both the Earth and Sun have moved in tandem during those 8.3 minutes, but that doesn't mean that the Earth and Sun are in a "fixed" location. The *angle* at which we see the Sun from here on Earth is distorted from where the Sun is actually located.

Instead of seeing precisely where the Sun resides, we see where the Sun was located 8.3 minutes ago (some 78,000 to 300,000 miles away, depending upon how fast one can show that the Sun moves).

If *both* the Sun and the Earth are moving Northward at 157 miles per second, even as the Earth simultaneously revolves around the Sun, then the Light that we see from the Sun will be from the location that the Sun was at 8.3 minutes ago, which is 78,186 miles *SOUTH* of its actual current position (because it takes Light 8.3 minutes to reach the Earth).

So forget Gravity. Until we can agree that:

1. The Sun is *moving* relative to it previous position,
2. Light takes 8.3 minutes to travel from the Sun to the Earth

Then we aren't ready to discuss Gravity.

We have to be able to agree that the Sun is moving and that Light takes time to reach the Earth before we can go any further in this debate, and that's because we have to build a framework wherein we agree or disagree that we are viewing the Sun in its old location of 8.3 minutes ago.

[Southack]

C#172

[Southack]

C#172

[Southack]

C#172

[Physicist]

Until we can agree that:

1. The Sun is *moving* relative to it previous position,

There is no such thing as absolute motion. If we don't accelerate the sun, then I am at liberty to choose a coordinate system in which the sun is at rest. So no, the sun does not move, according to my choice of coordinate system...and your arguments have to work in that coordinate system.

[Southack]

"There is no such thing as absolute motion. If we don't accelerate the sun, then I am at liberty to choose a coordinate system in which the sun is at rest."

No, you are not at liberty to change reality.

True, you can create a fictitional *model* in which the Sun and Earth do not "move", just as I could create a fictitious model for two cars that are both traveling together at 70 miles per hour, but it would be erroneous to say that simply because my *model* of those two cars had unchanging reference points that the cars weren't physically moving.

[Physicist]

How do you know the cars are moving?

[Southack]

The cars themselves *are* moving in reality, it is in my fictitious *model* of the cars that they appear to be stationary.

Modeling moving objects to make them appear stationary doesn't make the actual objects themselves stationary, QED.

[Physicist]

How do you know the cars are moving? Moving with respect to what? How fast are they going?