Specific Relativity/Light-Speed Question

[Erasmus00]

Now there's sound, independant reasoning if I ever heard it. Einstein's box is deep.

 

You act as if everyone simply takes Einstein's word for it. Einstein suggestions are elegant, but ultimately experiment had to make the final confirmations, and it did.

-Will

[CraigD]

I should point out that there is no proof that gravity travels at the speed of light. Some models show that it needs to travel at the speed of light, and others insist that it is instantaneous.

 

If there is proof, then please cite a reference to it.

http://news.bbc.co.uk/2/hi/science/nature/2639043.stm

Good quick find, Tormod.

 

However, as summarized by articles such as this one, Fomalont and Kopeikin have been having difficulty convincing the scientific community that their experiment actually measures the speed of gravity, so much so that their papers are still being reviewed for publication by “Astrophysical Journal Letters”. While everybody agrees that the experiment is an excellent measurement of the GR effect on light, even the professionals are having difficulty believing in this esoteric experimental design.

 

The lack of incontrovertible evidence that gravity propagates at the speed of light is annoying, especially since very simple, easy to understand experiment to test the hypothesis are so easy to imagine. Eg:

1. Measure the gravitational force on object A due to object B;
   
2. Move object B such that the distance between A and B changes;
   
3. Measure the time elapsed before the change in force due to the change in distance is measured at A.

This experiment seems especially easy, when you consider that “move object B such that…” can be satisfied by rearranging its matter so that its combined gravitational effect on A changes, which can be done simply by exploding it.

 

It’s just that gravity is such a weak force, there’s no practical way to measure the tiny effect this experiment would produce using available experimental apparatuses!

 

I recall reading that LIGO and VIRGO, 2 big laser/mirror interferometers, were created for an experiment only slightly less direct than this, with Supernovae remnants and neutron stars playing the role of object B.

 

While massive objects at interstellar distances are promising, I’m hopeful that the next century may see our engineering ability increase to the point where a compelling speed-of-gravity experiment can be done closer to home. A rough, naive analysis of masses and distances for big stars at interstellar distances (10^32 kg @ 10^17 m) vs. big rocks at clock-measurable light-traverse distance (10^20 kg @ 10^6 m) shows that a asteroid-exploding experiment would need only about 10^10 the sensitivity of a distant star exploding experiment, well within current capabilities.

 

:lol: Even this may not be necessary. From what I just read, the numbers for large tractor movable objects at big vacuum chamber distances (10^10 kg @ 10^2 m) could be withing the capabilities of current high-precision timers and nuclear detonators.

[EWright]

Good quick find, Tormod.

 

However, as summarized by articles such as this one, Fomalont and Kopeikin have been having difficulty convincing the scientific community that their experiment actually measures the speed of gravity, so much so that their papers are still being reviewed for publication by “Astrophysical Journal Letters”. While everybody agrees that the experiment is an excellent measurement of the GR effect on light, even the professionals are having difficulty believing in this esoteric experimental design.

 

The lack of incontrovertible evidence that gravity propagates at the speed of light is annoying, especially since very simple, easy to understand experiment to test the hypothesis are so easy to imagine. Eg:

1. Measure the gravitational force on object A due to object B;
   
2. Move object B such that the distance between A and B changes;
   
3. Measure the time elapsed before the change in force due to the change in distance is measured at A.

This experiment seems especially easy, when you consider that “move object B such that…” can be satisfied by rearranging its matter so that its combined gravitational effect on A changes, which can be done simply by exploding it.

 

It’s just that gravity is such a weak force, there’s no practical way to measure the tiny effect this experiment would produce using available experimental apparatuses!

 

I recall reading that LIGO and VIRGO, 2 big laser/mirror interferometers, were created for an experiment only slightly less direct than this, with Supernovae remnants and neutron stars playing the role of object B.

 

While massive objects at interstellar distances are promising, I’m hopeful that the next century may see our engineering ability increase to the point where a compelling speed-of-gravity experiment can be done closer to home. A rough, naive analysis of masses and distances for big stars at interstellar distances (10^32 kg @ 10^17 m) vs. big rocks at clock-measurable light-traverse distance (10^20 kg @ 10^6 m) shows that a asteroid-exploding experiment would need only about 10^10 the sensitivity of a distant star exploding experiment, well within current capabilities.

 

:eek: Even this may not be necessary. From what I just read, the numbers for large tractor movable objects at big vacuum chamber distances (10^10 kg @ 10^2 m) could be withing the capabilities of current high-precision timers and nuclear detonators.

 

:lol: Um, you get right on that exploding experiment there. But, ah, wouldn't the shock wave from the explosion influence the experiment? Or no, because it'd travel well behind the speed of gravity? And how do we synchronize, from an emence distance, the timing of the detonation relative to the result? And doesn't LIGO have a big sister named LISA somewher? or is that still in the works?

[EWright]

You act as if everyone simply takes Einstein's word for it. Einstein suggestions are elegant, but ultimately experiment had to make the final confirmations, and it did.

-Will

 

You might be more selective in your use of the word "ultimately". His theories only hold true to an extent. If I'm wrong, please direct me to the forumlas that tie relativity and quantum theory together. The Grand "Ultimate" Theory has yet to be discovered.

[Erasmus00]

You might be more selective in your use of the word "ultimately". His theories only hold true to an extent. If I'm wrong, please direct me to the forumlas that tie relativity and quantum theory together. The Grand "Ultimate" Theory has yet to be discovered.

 

For SR those formulas encompass quantum field theory. For GR, you are right, that hasn't happened yet. My point was simply that before scientists accepted Einstein's theories, experiments had to be done. They have been. So far, GR has not been disproved via experiment.

-Will