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."
Just set up an interferometer, and measure the speed of light forwards and backwards along three coordinates. The difference, divided by two, will give you the absolute velocity along that coordinate. Add the three vectors, and you have the 'real' velocity.
I'm surprised no one has tried this :-)
We know that the cars are moving in reality because reality is superior to my fictitious model. In my fictitious model, the cars *appear* to be stationary. In reality, the cars are traveling at 70 miles per hour, as stated in the initial pre-condition above in this thread. Check their speedometers!
Do you agree with the pre-conditions in Post #172?
Indeed. One could even create a *model* in which the two cars were stationary.
But outside of that model, jumping out of one of the cars would soon inform you that you were traveling at 60 mph, with an Ouch!
But they're running on a big treadmill. Of course their speedometers are going to say that.
What I was trying to point out, rather snidely, and mostly for the amusement of Physicist, is that your 'reality' is an idea that was tested and discarded at the end of the 19th Century. The Michelson Morley experiment (which I rather crudely described) was an attempt to measure the absolute movement of the earth through space, and it failed. There is no single reference frame that is 'real', as opposed to any others. You can write a self consistent set of equations for any inertial frame you want, and they'll be just as good as those in any other inertial frame.
You're not travelling at 60 miles an hour. You're travelling at several hundred thousand miles an hour. Running into a meteorite would tell you that. Mega-ouch!
FWIW, I did emit a loud guffaw, at some velocity.
Indeed. The new frame of reference would have been "reality" instead of the fictitious model of a portion of reality.
Your cars are moving at 60 mph w.r.t the earth's surface. But the earth's surface is moving at 1000 mph. around the earth's center. The earth's center is moving at 60,000 mph around the center of mass of the solar system. But the solar system is embedded in a spiral arm of the galaxy, and is moving at some velocity I can't remember around the center of the galaxy, and the galaxy itself is rotating around the center of mass of the local cluster, which itself is moving relative to other clusters, all of which are moving relative to the center of mass of the entire universe, which is moving relative to what?
In other words, which frame of reference is reality?
Rather than toy around forever with neither of you agreeing to the pre-conditions in Post #172, let's try a different approach.One of two things could explain why the orbital planes of our planets are centered upon the Sun's actual location:
1. The Sun and the Earth are actually *not* moving in tandem, but rather, are motionless save for the Earth's orbit around the Sun itself.
or
2. Gravity travels so fast that there is almost no real delay, as in: the Sun hasn't moved very far relative to *anything* by the time Gravity covers the distance in question.
Wrong question. It isn't *reality* that changes based upon your frame of reference. Reality is simply what exists.
What changes (when you change your frame of reference) is your *model* of reality.
Forget the sun for a moment. Consider only the earth-moon system. Although the earth is in motion (around the sun, etc.) the moon manages to keep up with us. We never outrun the moon, even though, from your viewpoint, the moon should be struggling to stay in orbit around a runaway target.
I think the answer to this awesome problem is that the two components of the earth-moon system are both in orbit around a center of gravity. And that is what orbits the sun. Likewise, the earth-sun system has a center of gravity (probably within or very near to the sun) and that is what is roaming around the galaxy.
As always, if I've goofed it up, Physicist or someone else who knows this better than I do will straighten me out.
There's a coordinate system where that's true. There are also coordinate systems where that's not true. The same physical result must be achieved in all possible coordinate systems. (After all, a coordinate system is merely a theoretical construction.) Work it out in the heliocentric coordinate system and you'll have the universal answer.
"There's a coordinate system where that's true. There are also coordinate systems where that's not true. The same physical result must be achieved in all possible coordinate systems." - Physicist
Bingo! If the same physical result isn't true in all possible coordinate systems for that precondition, then that precondition must be false.
Guess what that leaves us with to explain the orbit of the planets around the Sun?!
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