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."

[RightWhale]

Should shifting the reference frame make any difference in measurements?

[PonyTailGuy]

That would sort of contradict the special theory of relativity, wouldn't it? I had the same reaction.

But read closely, it only says he SIMPLIFIED the calculations by computing from the Jupiter frame of reference.

Apparently the mathematics were considered intractable prior to this, and the fellow thought of a clever way around the problem.

In any event, just how central is the speed of gravity to the general theory of relativity? Is it in the hard core of the theory, is it a supporting assumption? a mathematical consequence? I'd be curious to know. I'm well versed in the special theory but only superficially knowledgeable of the general theory.

[RightWhale]

It is thought that Einstein said that nothing can exceed the speed of light. Gravity waves would therefore move no faster than c. However, I don't know that Einstein actually said that. Minkowski did say that, and much of the popular conception of special relativity is due to Minkowski. I don't see the problem, since we're talking about appearances of electromagnetic phenomena, not the reality behind them.

[AdmSmith]

In Einstein's General Theory of Gravity the speed of gravity is assumed to be equal to the speed of light. If it is found to be different then his theory must be changed for something better. That is evolution!

[martin_fierro]

Albert Einstein may have been right that gravity travels at the same speed as light

That explains why I never see this coming. <|:)~

[Southack]

"In effect, the experiment was measuring effects associated with the propagation of light, not the speed of gravity."

That is *precisely* what I said back in September here on FR, that all they were measuring was the propagation speed of Light, not Gravity.

[martin gibson]

The quasar signals may have broken the speed of the sound of loneliness...

[theDentist]

Hmmm, it seems to me (by no means a physics guru) that reversing the refernce frame would may not be viable. I mean, Eart is moving, Jupiter is moving, even the Sun is moving. And all at different speeds. It seems to me the best frame of reference (locally) would therefore be the Sun. I mean, reversing the reference from Earth to Jupiter does help in observing effects from afar, but you then still muct wonder about the Sun's effects upon gravitational attraction, and how to subtract the difference based upon Earth's motion, (plus the reference point on Earth moving in revolution).

What we gotta do, is get the smartest person on Earth (a Senator from NY, as I recall), put her is a ship, and fire it at the closest star at a constant velocity, then measure increases in velocity at predetermined ranges. She can report back her observations to a device.

[ruiner]

AFAIK nothing with MASS can exceed or equal the speed of light. Massless objects can, which is why light (massless) can go lightspeed and not really really really really close to it :)

[Brellium]

Einstein said that nothing can be accelerated beyond the speed of light in a vaccum.

[Mr. Bird]

You are correct, if Einstein had it right. I drove my physics TA nuts asking him what would happen if Einstein was wrong. I'm certainly not saying I know one way or another. I just got my book lurnin' and all...

[10mm]

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

[PonyTailGuy]

But is that really how science works? Not every proposition in a theory exposes the theory to extinction by disproof. Those in the HARD CORE of the theory certainly do, but these are the cardinal propositions in the theory, not auxiliary laws, models, etc.

For example, the field of genetics is based on the cardinal rules governing of combinations of genetic "factors". Should someone find that alleles do not in fact combine in accordance with those rules, it's bye bye to Mendel.

But the precise mechanism of the joining of alleles does not have to be hard-wired in genetic theory. Someone can find that a commonly held view of crossover and mutation is not accurate, and the hard core of Mendel's theory would still hold.

One must distinguish between these cardinal, essential propositions in a theory, and the outer ring of ad-hoc or weak principles that merely support the theory or explain its mechanisms in some cases.

This is why I framed my question in terms of the relative place of the speed of gravity proposition in the general theory.

[keithtoo]

If they really wanted to measure the propogation of gravitational waves, why would Jupiter passing thru the light waves coming from the Quasar even enter into it?

They could simply measure the timed arrival of the light waves as Jupiter approached these waves, as it passed thru these waves, and as it continued passing by these waves to see if the aberrations in the light waves are the same approaching and leaving the light wave.

If the light wave has a constant distortion before and after Jupiter passes thru, then gravity has an infinite speed. If the ligth wave has a gradual distortion as Jupiter approaches and leaves the wave, then the speed of gravity could be calculated.

[AdmSmith]

Tha assumption that the speed of gravity is equal to the speed of light is a cardinal assumption. If the experimental data are different and these data are found to be correct then the theory of gravity according to Einstein is finito and must be replaced by something better.

That is what happened with Mendel's theory as well (it has been refined as you mentioned)

Science is always evolving, no Final Theory exists.

[Flightdeck]

I don't believe in gravity. I say gravity and EM are one and the same.

[RinaseaofDs]

This may be a completely uninformed question (I'm an EE, not a physicist), but gravity is a force, right? Light is a particle, right?

Or have we moved on, and discovered that gravity is now a particle, and so is light?

I already realize that there are no such thing as particles (part waveform, part particle).

Gravity, being a force is related to acceleration, not speed. mass times acceleration equals force.

Is this stupid? (Probably)

[Sabertooth]

((((((growl)))))

[RightWhale]

gravity and EM are one

While electromagnetic, weak, and strong are similar enough to be combined mathematically, gravity is a different species mathematically and refuses to be combined with the others.

[PatrickHenry]

Cornucopia of pings. [This ping list is for the evolution side of evolution threads, and sometimes for other science topics. FReepmail me to be added or dropped.]