Relative Motion and Time Dependance on Velocity of Light

[JesseM]

Question: The 'ether' for which the MM device was actually designed. I would suppose the ether was up to some point in history used by some scientist to explain how light could 'move' through space? Why would the relative movement of the light through the ether be met by any restriction? I would suppose this is an energy consideration, but I would like to know, please.

What do you mean "restriction"? Light waves were assumed to be vibrations in the ether just like sound waves are vibrations in air. In the rest frame of the air, all sound waves move at the same speed regardless of the speed of the emitter, so it was assumed light waves worked the same way (the speed of vibrations in any medium depends on physical properties of the medium like density and rigidity). And if you're moving at some speed v relative to the rest frame of the air, then in your frame sound waves will move at s + v in one direction and s - v in the opposite direction, and similarly it was assumed that if you were moving at v relative to the ether, you'd measure light waves to move at c + v in one direction and c - v in the other. Since Maxwell's laws predict that light waves always move at c, it was assumed Maxwell's laws would only work exactly in the rest frame of the ether, in other frames they'd have to be modified by a Galilei transformation.

 

[DewaldS]

What do you mean "restriction"? Light waves were assumed to be vibrations in the ether just like sound waves are vibrations in air. In the rest frame of the air, all sound waves move at the same speed regardless of the speed of the emitter, so it was assumed light waves worked the same way (the speed of vibrations in any medium depends on physical properties of the medium like density and rigidity). And if you're moving at some speed v relative to the rest frame of the air, then in your frame sound waves will move at s + v in one direction and s - v in the opposite direction, and similarly it was assumed that if you were moving at v relative to the ether, you'd measure light waves to move at c + v in one direction and c - v in the other. Since Maxwell's laws predict that light waves always move at c, it was assumed Maxwell's laws would only work exactly in the rest frame of the ether, in other frames they'd have to be modified by a Galilei transformation.

OK that is clear.

 

[atyy]

Yes, the formulae do agree with what you are saying. I am a slow mover - I will only use any STR derived formulaes once I am convinced that the theory is sound.

I think you should use the STR formulas even before you are convinced they are experimentally right. The reason is that to prove STR wrong, we need to know what it predicts, and we must therefore use the theory (but we need not accept the theory if its predictions conflict with experiments).

 

[DewaldS]

As an example of an experiment since the MM experiment that gives us high confidence that the speed of light is constant is the experimental measurement of electron magnetic moment to greater than 1 part in 1000,000,000, AND the fact that this measurement matches the theoretical prediction incorporating the constancy of the speed of light as an assumption.

The experiment:
http://hussle.harvard.edu/~gabrielse/gabrielse/papers/2006/NewElectronMagneticMoment.pdf

The theoretical prediction using the constancy of the speed of light:
http://hussle.harvard.edu/~gabrielse/gabrielse/papers/2006/NewFineStructureConstant.pdf

I am not well versed enough at physics (at this stage) to understand the papers.

We are making a distinction her between two concepts

i) The speed of light through a vacuum is constant.
ii) The speed of light is constant relative to an observer regardless of the motion of the observer's inertial frame?

 

[atyy]

We are making a distinction her between two concepts

i) The speed of light through a vacuum is constant.
ii) The speed of light is constant relative to an observer regardless of the motion of the observer's inertial frame?

Those are the same, though I am somewhat unsure of what you mean by the second statement.

The correct statement is:
Consider two observers moving at constant velocity relative to each other. If both observers measure the speed of light in vacuum to be the same, then both observers have inertial reference frames. Any other observers moving at constant velocity relative to these two observers will also have inertial reference frames, and also measure the same speed of light in vacuum.

Edit: I made a slight change to the above statement from "speed of light" to "speed of light in vacuum".

 

[atyy]

It is just difficult for me to understand that the results from the MM machine could have been used to arrive at a theory like GTR and STR. The machine had to be stable and mechanically exact within 6 e-7 m.

Michelson and Morley's conclusion was "the measured velocity was approximately one-sixth of the expected velocity of the Earth’s motion in orbit and “certainly less than one-fourth.”

I got the Michelson-Morley conclusion from Wikipedia, so I'm not sure it's right. But let's work with it.

Expected velocity = 30 km/s
Measured velocity = 30/4 = 8 km/s
Fractional difference =(30-8)/30 = 0.75 = 3/4

Thus either Michelson and Morley made a 3/4 (not 1/1000,000) error in their apparatus, or the measured velocity is really different from the expected velocity by at least 22 km/s.

Edit: Obviously, that is not enough to prove STR right, nor does it prove that the aether theory cannot be modified to account for the difference, but it certainly puts STR in contention as an alternative to aether theory. If STR is in contention, one needs to use the theory to make predictions, and then check if those predictions agree with experiment (to date, there is complete agreement between STR and experiments).

 

[Constructe]

I think he has a correct point here. Space time curves in the face of mass. Thus space time becomes more cureved the more massive a planet gets. This implies that time actually slows in the face of massive object offsetting the stretching of space. If you were walking on the sun and we were watching you from space you would wonder why are we walking so slow. How we got onto this topic from the first I don't quite recall.

I agree that light moves the same speed everywhere in a vacuum with the consideration of the bending of space time from the effects of gravity. I suppose this is what the poster is wondering about. So far no one has witnessed a directional effect of either if it exists but also, no one has actually witnessed a gravity wave or graviton particle either. All we can do is look at the effects and postulate if the effects match a certain explanation.

Can something move faster than light, in theory it is possible. I suppose it would be hard to recognize since it would be perceived going backwards in time and cause and effect won't match to our perception of how we are used to seeing things. Can they go faster than light but move forward in time? Well you'd have to make a exclusion to the know physical properties of the universe as we understand them (like a wormhole where our vision of space time is warped).

 

[DewaldS]

I got the Michelson-Morley conclusion from Wikipedia, so I'm not sure it's right. But let's work with it.

Expected velocity = 30 km/s
Measured velocity = 30/4 = 8 km/s
Fractional difference =(30-8)/30 = 0.75 = 3/4

Thus either Michelson and Morley made a 3/4 (not 1/1000,000) error in their apparatus, or the measured velocity is really different from the expected velocity by at least 22 km/s.

Edit: Obviously, that is not enough to prove STR right, nor does it prove that the aether theory cannot be modified to account for the difference, but it certainly puts STR in contention as an alternative to aether theory. If STR is in contention, one needs to use the theory to make predictions, and then check if those predictions agree with experiment (to date, there is complete agreement between STR and experiments).

The way I understand it the 8km/h would be 'calculated' back form the infringement pattern. For me that could also mean that one of the mirrors have moved 670nm*8/30.

 

[DewaldS]

The way I understand it the 8km/h would be 'calculated' back form the infringement pattern. For me that could also mean that one of the mirrors have moved 670nm*8/30.

This calculation of mine is probably nonsense. I will redo it. The point is (and I ask you to check this out) - a substantial 'aether speed' would be needed to cause an asynchronous beam detection, but a REALLY SMALL mechanical deviation will do the same.

 

[JesseM]

This calculation of mine is probably nonsense. I will redo it. The point is (and I ask you to check this out) - a substantial 'aether speed' would be needed to cause an asynchronous beam detection, but a REALLY SMALL mechanical deviation will do the same.

And again, generations of physicists and engineers have looked at the MM experiment and agreed it was sufficiently well-constructed so we don't have to worry about this sort of mechanical deviation being the explanation, not to mention that the experiment has been repeated over and over and over again by subsequent experimenters with the same result.

 

[DewaldS]

You are probably right. I will however also look at it until I am satisfied. I think I should rather do it in my own space and time, and once I am equipped to learn more from or contribute to the discussion I will return.

Thanks for all the input, everyone.

 

[atyy]

Thus either Michelson and Morley made a 3/4 (not 1/1000,000) error in their apparatus

The point is (and I ask you to check this out) - a substantial 'aether speed' would be needed to cause an asynchronous beam detection, but a REALLY SMALL mechanical deviation will do the same.

I think I should rather do it in my own space and time, and once I am equipped to learn more from or contribute to the discussion I will return.

Thanks for all the input, everyone.

Yes, I am not sure I'm drawing the right inference from my calculation. Anyway, this is obviously at the limit of my knowledge of the MMX. I wish you much fun on your studies of it!