Michelson-Morley (A farewell to arms)

[Slaihne]

Hi there,

 

I’ve been reading up a bit on the Michelson-Morley experiment (thanks to CraigD), the results of which disproved the existence of a medium that light traversed through.

 

A couple of things strike me about this experiment though...

 

A glaringly obvious one, but i can’t find a mention of. The light is split into two beams perpendicular to each other and then bounced back from a mirror along the same (or similar) path that it took to get to that mirror. If there were some medium that ‘pushed’ the light on the outward leg, wouldn’t it slow light on the return leg? I haven’t seen a reference to an experiment that considered this effect. I also can’t come up with an arrangement of the two light beams paths so that you couldn’t be sure to remove this effect since the beams have to be brought back together for an interference pattern to be measured. If this effect were present i know it wouldn’t be an equation of the type c + speed of aether since this would cause the pattern to change as the angle to the aether altered. I think it would have to follow some type of logarithmic relationship for the math to work out correctly. This one is so obvious that it must have been spotted before. I gather i’m missing something here that takes this effect into account? I bet it's my crap maths

 

I’ve read that the Sagnac experiment and the Michelson-Gale-Pearson experiment actually produced a positive result and the effect measured by both experiments is used successfully today within GPS systems. This fact would presume that the above paragraph is incorrect although i still don’t understand why they got a positive result and many others didn’t.

 

The second issue is that of Aether Drag. From what i’ve read, only the fluidic type of drag has been considered. The type of thing where a boat moves through water and water near the hull of the boat almost ‘clings’ to it. At a further distance there are eddies where the moving water, clinging to the hull, merges itself into the stationary body of water. Would it be possible for a completely different type of drag to be occur, similar to that of air. A room full of air on Earth moves round Earth’s axis and also moves round the Sun for total velocities of 10’s of times the speed of sound, yet it is possible to carry out sound based interference experiments and achieve expected results without having to factor that movement in. Could the aether behave in a similar fashion and would it lead to a null result, effectly being static over the whole apparatus. Although i still have no idea if this could lead to the above two experiments giving a positive result.

 

Cheers again CraigD for the link to this, some very interesting reading. I remember this being mentioned in Physics classes years ago, but my impression was that it was a ‘done deal’ and aether definitely didn’t exist and had been disproved for 100’s of years. From

 

Timeline of luminiferous aether - Wikipedia, the free encyclopedia

 

it seems there’s still a slight touch of contention there.

 

Rgds

 

David

[Mr. Peterman]

Thank you for reminding me about that. I haven't thought about that for quite some time when I just thought I didn't understand things correctly. But that does make sense. It seems like the design would have guaranteed close to a null result. I don't know if the experimenters took that into account. It seems like no matter how you position the experiment the light should take the same time to get back where it started from. Maybe I'm wrong too, but it wouldn't surprise me too much if this was exactly the problem.

[Janus]

Hi there,

 

I’ve been reading up a bit on the Michelson-Morley experiment (thanks to CraigD), the results of which disproved the existence of a medium that light traversed through.

 

A couple of things strike me about this experiment though...

 

A glaringly obvious one, but i can’t find a mention of. The light is split into two beams perpendicular to each other and then bounced back from a mirror along the same (or similar) path that it took to get to that mirror. If there were some medium that ‘pushed’ the light on the outward leg, wouldn’t it slow light on the return leg? I haven’t seen a reference to an experiment that considered this effect. I also can’t come up with an arrangement of the two light beams paths so that you couldn’t be sure to remove this effect since the beams have to be brought back together for an interference pattern to be measured.

 

Okay, Let's say the the velocity of the medium relative to our apparatus is v, and light travels at c relative to the medium. Then on the outbound leg the light travels at c+v relative to our apparatus. If the length of our apparatus is L, then the time for the light to travel the length of our apparatus on the outbound leg is

[math]t1= \frac{L}{c+v}[/math]

 

On the return trip, the light is traveling against the motion of the medium, so its speed relative to the apparatus is c-v, and the time to make the return trip is

 

[math]t2= \frac{L}{c-v}[/math]

 

The time for the round trip is therefore:

 

[math]T=\frac{L}{c+v}+ \frac{L}{c-v}[/math]

 

which reduces to

 

[math]\frac{2L}{C-\frac{v^2}{c}}[/math]

 

However, if there is no medium, and the light simply travels at c relative to the apparatus, the round trip time would be

 

[math]T=\frac{2L}{c}[/math]

 

Which is what you get with the M&M experiment.

 

 

 

I’ve read that the Sagnac experiment and the Michelson-Gale-Pearson experiment actually produced a positive result and the effect measured by both experiments is used successfully today within GPS systems. This fact would presume that the above paragraph is incorrect although i still don’t understand why they got a positive result and many others didn’t.

 

The Sagnac experiment deals with a rotating frame of reference, which is also an accelerated frame. The behavior of light in accelerated frames is different than it is in the Inertial reference frames dealt with in SR.

 

The second issue is that of Aether Drag. From what i’ve read, only the fluidic type of drag has been considered. The type of thing where a boat moves through water and water near the hull of the boat almost ‘clings’ to it. At a further distance there are eddies where the moving water, clinging to the hull, merges itself into the stationary body of water. Would it be possible for a completely different type of drag to be occur, similar to that of air. A room full of air on Earth moves round Earth’s axis and also moves round the Sun for total velocities of 10’s of times the speed of sound, yet it is possible to carry out sound based interference experiments and achieve expected results without having to factor that movement in. Could the aether behave in a similar fashion and would it lead to a null result, effectly being static over the whole apparatus. Although i still have no idea if this could lead to the above two experiments giving a positive result.

 

 

David

 

The problem with the ether drag theory is that it doesn't jive with astronomical observations. At some point there has to be a transition between the ether traveling with the Earth (no matter how static in the near region around the Earth) and the normal ether of space. Light reaching us from far off stars would have to pass through this transition. But the light reaching us from these stars shows no indication of this.

[Mr. Peterman]

Whoa. I better correct myself. I said I could be wrong and I am. I was idiotically assuming that a speed of c+v and a speed of c-v on the way back would make an average speed of c, but it doesn't. I think I'm sure that c(1-v^2/c^2) would be the average speed. Because the light would travel slower through the aether in one direction, the aether would have more time to contribute to it. So as far as I don't know, this should work.