You mean this Einstein?
Posted on 06/23/2003 9:25:12 AM PDT by 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."
There are four fundamental forces in nature:
Strong force
Weak force
Electromagnetism (EM) Light is an electromagnetic wave
Gravity
All of the fundamental forces are considered Exchange Forces. In other words the force involves an exchange of one or more particles.
The exchange particles are as follows:
Strong – The pion (and others)
Electromagnetic (EM) – The photon
Weak – The W and Z
Gravity – Believed to be the graviton
An addition by Physicist:
Note: The pion does mediate the inter-nucleon force. That force isn't fundamental, however. The fundamental force is the inter-quark force that binds the quarks into hadrons (such as protons, neutrons and pions), and that is what we usually mean by the strong force, nowadays. The force between hadrons is a residual color dipole interaction that is analogous to the Van der Waals force in electromagnetism.
Exactly - that's the whole basis of the miracle bra / underwire theory.
If the speed of gravity were infinite you could use gravitational effects to synchronize the clocks in two different frames. This leads to contradictions to the principle of relativity, even in special relativity.
For example, if spaceships pass each other moving in opposite directions, each appears to be contracted relative to the other.
However, by stationing observers at opposite ends of both ships with synchronized clocks, we could determine which ship was "really" contracted and which merely appeared contracted by observing the times at which observers on each ship passed their counterparties.
The argument has superficial plausibility but has been convincingly refuted by (e.g.) Prof. Carlip at UC Davis.
The basic argument relies on something like the inverse of the Poynting-Robeson (spelling?) effect. Light moving radially from the sun encounters Earth moving 'forward' and thus falls at a slight angle relative to the radius vector from Earth to Sun. The effect of this is to exert a slight retrograde pressure due to Earth's motion relative to the radial lines from the Sun.
Gravity, it is argued, would have the same vector triangle except that it would be of opposite sign (since gravity 'sucks' and light 'pushes'). This, says the argument, should cause an acceleration of Earth relative to the Sun, which in a remarkably short period of time (a few thousand years) would result in a doubling of Earth's orbital distance, and in millions of years would fling us out of the solar system entirely. Therefore gravity must propagate at infinite speed, QED.
The problem is that it can be shown that under General Relativity, gravity waves are radiated which exactly equal the increased energy due to the 'couple'. The extra momentum, in other words, is leaked away via gravity waves and the solar system remains stable.
--Boris
Glad you cleared that up.
Assuming this is going on, should we expect to be able to detect these gravity waves somehow? They would be coming from all directions and we ought to notice them if they are 'bright' enough and perhaps even form images. Or are they so weak and smooth that we can't devise instruments sensitive enough to register them?
Maybe gravity waves propagate at a finite speed. However, they are unlike light in that: light is generated by various reactions and must be generated or it goes out, ceases, whereas gravity is always just there, static, not necessarily propagating at all. Maybe gravity waves are possible, but just an ancillary phenomenon, something that gravity can do but doesn't need to do.
You'll probably be just like Einstein and give all the money to your first wife.
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