Special Relativity and Michelson-Morley Experiment (1 Viewer)

[superstar12]

How did Einstein's Theory of Special Relativity explain the result of the Michelson-Morley experiment? (4 marks, 2001 HSC)

Could someone help answer this question? I'm not really sure how the Einstein's theories and the MM experiment are interconnected. Thanks!

 

[Kaido]

i would be assuming:
-talk about intentions of MM exp (i.e. earths velocity through aether etc.)
-explain that even with repetition (diff loc, time), still null
-challenged sci thinking and einstein resolved this through theory of special rel. explaining that c is constant to all observers
-rendering aether superfluous (and thus null result incurred) as only length, time and mass blah blah

 

[superstar12]

i would be assuming:
-talk about intentions of MM exp (i.e. earths velocity through aether etc.)
-explain that even with repetition (diff loc, time), still null
-challenged sci thinking and einstein resolved this through theory of special rel. explaining that c is constant to all observers
-rendering aether superfluous (and thus null result incurred) as only length, time and mass blah blah

But how does the constancy of c (speed of light) explain the null result of the Michelson-Morley experiment (i.e. non-existence of the aether)?

 

[superstar12]

bump

 

[klee98]

But how does the constancy of c (speed of light) explain the null result of the Michelson-Morley experiment (i.e. non-existence of the aether)?

It's the other way around.

 

[iStudent]

But how does the constancy of c (speed of light) explain the null result of the Michelson-Morley experiment (i.e. non-existence of the aether)?

Constancy of c means that the light rays always arrive at the detector at the same time (since the distances at which the light rays travel is equal). The MM experiment was designed to measure a change in the differences in times at which the light rays meet at the detector (by rotating the apparatus etc.). But since there was no change, there is no "interference pattern" that was supposed to be detected (if aether existed), as hence a null result. Does that sort of make sense?

I don't think the theory disproved aether or anything, rather it made it unnecessary.

This isn't a solid 4 mark response, but just trying to explain it so that it makes sense first. The actual answer you would write on paper is quite different ...

 

[superstar12]

Constancy of c means that the light rays always arrive at the detector at the same time (since the distances at which the light rays travel is equal). The MM experiment was designed to measure a change in the differences in times at which the light rays meet at the detector (by rotating the apparatus etc.). But since there was no change, there is no "interference pattern" that was supposed to be detected (if aether existed), as hence a null result. Does that sort of make sense?

I don't think the theory disproved aether or anything, rather it made it unnecessary.

This isn't a solid 4 mark response, but just trying to explain it so that it makes sense first. The actual answer you would write on paper is quite different ...

But if the speed of light was constant, why would it matter if the aether existed or not?

 

[iStudent]

But if the speed of light was constant, why would it matter if the aether existed or not?

The theory of the aether existed quite a while before einstein came up with special relativity (i.e. that c was constant). If the idea that c was constant was proposed first then obv there would be no need for aether.
You can check the dates on when those two ideas were first proposed.

 

[Kaido]

Constancy of c means that the light rays always arrive at the detector at the same time (since the distances at which the light rays travel is equal). The MM experiment was designed to measure a change in the differences in times at which the light rays meet at the detector (by rotating the apparatus etc.). But since there was no change, there is no "interference pattern" that was supposed to be detected (if aether existed), as hence a null result. Does that sort of make sense?

I don't think the theory disproved aether or anything, rather it made it unnecessary.

This isn't a solid 4 mark response, but just trying to explain it so that it makes sense first. The actual answer you would write on paper is quite different ...

i thought there was interference, just no change in pattern observed

 

[iStudent]

i thought there was interference, just no change in pattern observed

Ah yes, my bad. Was meant to say "no change in the interference pattern". It's been too long since I touched on this

 

[superstar12]

It's the other way around.

Wait, what?!?! How come the question implies otherwise? Can someone please clarify this for me? The general points for a response would be greatly appreciated.

 

[superstar12]

Bump

 

[Fizzy_Cyst]

- Describe results of MM experiment (no shift in interference pattern as interferometer was rotated)
- Explain cause of these results (difference in speed of light due to motion of the aether)
- Desribe SR (two postulates)
- Explain how constancy of the speed of light renders point 2 impossible, thus rendering the MM expt invalid -- even if the aether exists, the MM expt would not be able to detect it as the speed of light remains unchanged, regardless of the motion of its frame of reference. Seeing as speed of light is always constant, there should be no change in speed of light as device is rotated, hence no shift in interference pattern, which is what was observed.

 

[superstar12]

- Describe results of MM experiment (no shift in interference pattern as interferometer was rotated)
- Explain cause of these results (difference in speed of light due to motion of the aether)
- Desribe SR (two postulates)
- Explain how constancy of the speed of light renders point 2 impossible, thus rendering the MM expt invalid -- even if the aether exists, the MM expt would not be able to detect it as the speed of light remains unchanged, regardless of the motion of its frame of reference. Seeing as speed of light is always constant, there should be no change in speed of light as device is rotated, hence no shift in interference pattern, which is what was observed.

Thanks!

 

[braintic]

I don't think the theory disproved aether or anything, rather it made it unnecessary.

It most definitely disproved the existence of the ether. Although many scientists tried to talk themselves around that inevitability for a number of years afterwards.

 

[Kaido]

It most definitely disproved the existence of the ether. Although many scientists tried to talk themselves around that inevitability for a number of years afterwards.

Most Textbooks and trials said it rendered the model superfluous

 

[Fizzy_Cyst]

For HSC sake, we must say that it 'brought into question' the existence of the aether

Einstein himself said the following:

"The next position which it was possible to take up in face of this state of things appeared to be the following. The ether does not exist at all. The electromagnetic fields are not states of a medium, and are not bound down to any bearer, but they are independent realities which are not reducible to anything else, exactly like the atoms of ponderable matter. This conception suggests itself the more readily as, according to Lorentz's theory, electromagnetic radiation, like ponderable matter, brings impulse and energy with it, and as, according to the special theory of relativity, both matter and radiation are but special forms of distributed energy, ponderable mass losing its isolation and appearing as a special form of energy.

More careful reflection teaches us, however, that the special theory of relativity does not compel us to deny ether. We may assume the existence of an ether; only we must give up ascribing a definite state of motion to it, i.e. we must by abstraction take from it the last mechanical characteristic which Lorentz had still left it. We shall see later that this point of view, the conceivability of which I shall at once endeavour to make more intelligible by a somewhat halting comparison, is justified by the results of the general theory of relativity."

 

[braintic]

For HSC sake, we must say that it 'brought into question' the existence of the aether

Einstein himself said the following:

"The next position which it was possible to take up in face of this state of things appeared to be the following. The ether does not exist at all. The electromagnetic fields are not states of a medium, and are not bound down to any bearer, but they are independent realities which are not reducible to anything else, exactly like the atoms of ponderable matter. This conception suggests itself the more readily as, according to Lorentz's theory, electromagnetic radiation, like ponderable matter, brings impulse and energy with it, and as, according to the special theory of relativity, both matter and radiation are but special forms of distributed energy, ponderable mass losing its isolation and appearing as a special form of energy.

More careful reflection teaches us, however, that the special theory of relativity does not compel us to deny ether. We may assume the existence of an ether; only we must give up ascribing a definite state of motion to it, i.e. we must by abstraction take from it the last mechanical characteristic which Lorentz had still left it. We shall see later that this point of view, the conceivability of which I shall at once endeavour to make more intelligible by a somewhat halting comparison, is justified by the results of the general theory of relativity."

Now ... can you explain the meaning of the last paragraph?

 

[Thunderstorm]

Does it have something to do with the fact that the evidence for special relativity is limited due to testable conditions? Or maybe that the aether did not have the properties first thought? Pure guesses here.

The first post made sense, is this additional info or..?

 

[PhysicsMaths]

Does it have something to do with the fact that the evidence for special relativity is limited due to testable conditions? Or maybe that the aether did not have the properties first thought? Pure guesses here.

The first post made sense, is this additional info or..?

Limited evidence for special relativity? Did you mean limited evidence for the existence of the aether?
The apparatus they constructed was quite precise at the time, and if there was any shift in interference pattern i.e. changes in the speed of light when the apparatus was rotated through 90 degrees, it would have detected it.

As for special relativity, there are multiple experiments that were conducted to support Einstein's claims (refer to airplane experiment / time dilation for moving particles) that suggest that the speed of light is constant regardless of the observer's motion