One of the things I never fully grasped in college was the ‘Michelson Morley’ experiment. I understood what it was for (proving the constancy of the speed of light) – and understood that it proved that very well – but I never understood the concept of the ether or the ‘ether wind’ or what that had to do with the speed of light. To explain the ‘constancy of’ the speed of light, two separate theories were proposed. One was the ‘sound analogy’ (or the ‘ether theory’). The second theory proposed that light’s speed, though constant, should add and subtract from the speed of the emitter of light. So – light speed measured from a moving flashlight should be different from a stationary flashlight. This was called the ‘emitter’ theory or the ‘bullets analogy’ (the speed of bullets relative to the ground would depend on whether the gun was moving or not).
These two books (The Geometry of ) and ‘Asimov on Physics’, helped shed light on the issue.
The first conjecture – Light is like sound – the constancy of its speed is due to the ‘medium’ a.k.a – The Ether Theory
Consider sound waves. When you fire a gun, the wave travels through the air at a certain speed. This speed is independent of what the gun is doing – if the gun were moving straight ahead – the sound wave would still travel at the same speed. This is because the wave is dependent on a medium – and the medium is what limits its speed.
Now – people already knew that the speed of light was a constant – and it did not depend on whether a flashlight was moving or not . So – based on the reasoning above, it seemed logical to assume that it was similar to a sound wave – that light too, required a medium. This was a logical assumption. The only problem was that the speed was so high that the medium would need to be very dense. At the same time, the medium could not interact with any matter (since light traveled without any problem through space devoid of matter). How do you construct a super-dense medium that does not interact with matter?
Michelson and Morley decided to put the existence of such a ‘medium’ to the test. The ether was assumed to be a completely stationary, non-moving object.
If the earth is moving through this stationary ether, then the earth should experience an ether ‘wind’ – the same way that a man sitting atop a train experiences a moving wind. If we now shine a flashlight in the direction of earth’s motion (against the wind), that light should take longer to reach its destination. Remember – it is the ETHER that is assumed to be ‘carrying’ the light wave. That is why – going against the ether should take longer. Similarly, going in the same direction should take less time. To paraphrase a quote in this detailed article:
The whole point of bringing in the aether was to give a picture for light resembling the one we understand for sound, compressional waves in a medium.
They devised an experiment measuring the time it took light to go ‘against’ the ether wind – and then ‘with’ the ether wind. They found that there was no difference in the times. This meant that there was no ‘ether wind’. So – light was NOT like sound – it’s constancy of speed did not have to do with any medium.
The Second Theory – Light is like bullets – it’s speed should add/subtract from the source (the emitter theory)
Even though it was understood in Einstein’s day that the speed of light DID NOT in fact, add or subtract from the speed of the source, this could not be measured accurately in the days when Michelson Morley did their experiment. Recently, however, light emitted from moving pions (moving close to the speed of light) was measured – and found to always move at the speed ‘c’. Thus, the moving source had no effect on the speed of light.
The actual experiment
This post discusses the experiment with great clarity. The analogy of the two swimmers in a moving river helps understand the experiment better. Two swimmers decide to have a race. A river is flowing at a speed of 3 m/s. Both swimmers swim at the same speed – 5 m/s. Swimmer A will try and swim directly across the river – and come back to the same point (he will obviously have to aim at a higher point on the opposite bank to be able to return to his original starting point). Swimmer B – will simply swim upstream (along the same bank that they both start at) – and then downstream to return to the starting point.
Who finishes first?
Note: They both swim the same distance (the width of the river is 50 m – swimmer A obviously swims 100 m, swimmer B will swim upstream 50 m and back again to complete his 100m).
Swimmer A’s effective speed will be 4 m/s (sqrt of 5 squared – 3 squared). So – A completes 100 m in 25 minutes. B covers the first 50 m (upstream) at a speed of 2 m/s (5 –3) = 25 minutes. He covers the second 50 m (downstream) at a speed of 8 m/s – in 50/8 = 5.2 minutes. Overall, A finishes in 25 minutes, B in 30.2 minutes. A wins.
Similarly, if two beams of light were to ‘race’ – one going diagonally across the ether (river) – and the other upstream and back, the one going across should return first. This would prove that there is such a thing as an ‘ether’ that carries the light wave.
It turns out no matter how precisely this experiment is carried out, both the light beams return at the same time. This proves that the river is not flowing (there is no ether).
It was thought that light was like sound. This led to the construction of an ether. The ether was experimentally proved to be fiction. It was also thought that light should behave like bullets from a moving gun – it’s speed should depend on the motion of the source. This was also disproved. So – light doesn’t behave like ordinary matter (bullets), it doesn’t behave like well-known waves (sound). Yet – it’s speed was always a constant – regardless of the motion of the source. How was this possible?
Einstein postulated that light did not just travel in space – it traveled in something called space time. If you think of spacetime as your ‘ether’, you may be closer to the original conception of the ether.