Sagnac, Michelson & Co
Relativity to "touch"
RuedigerRodloff
Background image - DLR - Experimental Laser Gyro - ELSY
Two experiments that have changed the image of our world!
Michelson experiment
Sagnac experiment
The Michelson experiment was the starting point for the theory of relativity and thus the basis for a total orderroll our idea of space and time! The Sagnac experiment is called "Einstein opponents" as Evidence cited to the contrary.
In the historical Sagnac experiment, of course, the route was not round but polygonal; the signal or the light wave train was sent into a closed orbit with mirrors or deflection prisms. Today this orbit is realized with optical fibres, so I have indicated a circular path in the schematic picture above.
Well, actually Michelson and Sagnac intended to find out something about the "nature" of light, but it didn't work out. Nevertheless, the experiments "made history" - you will soon see why!
What do you see ?
You will see two runtime experiments:
-in both cases a light wave is used for the experiment. (We will see later that the type of signal does not matter in the Sagnac experiment!)
-in both cases the light wave, represented here by a blue dot, is split into two partial waves (brown and light blue dots) at a semi-transparent mirror and sent off in different directions;
-in both cases is the idea that the light wave has a longer path to travel in the direction of motion than the second, which is perpendicular (Michelson) or opposite (Sagnac) to it;
You have to imagine it like this:
Michelson experiment in a uniform motion
Sagnac experiment in a rotary motion
- the difference in path between the two light waves - Michelson and Sagnac hoped - is a measure of the speed or rotational speed of the system.
- In both cases, it was to be expected that there would be a significant difference in path between the two signals, and this should be greater the faster the apparatus moves.
It could look something like this in the Michelson interferometer:
....the Expected travel route effects in the Michelson experiment:
Michelson experiment in a slow motion
Michelson experiment in a faster motion
And now comes the bang!
- all attempts to achieve a measuring effect with the Michelson apparatus have failed. The Michelson experiment is still used today constantly repeated, but a path difference between the light waves cannot be detected even with the most modern devices! (A You can find an overview of the large number of experiments on this topic here.)
but .... the Sagnac experiment shows exactly the difference in walking distance that the classical calculation predicts!
You're floored now - aren't you?
The same model, the same calculation that gives the correct result in the Sagnac experiment fails completely in the Michelson experiment!
What a contradiction! -There will be a lot to be clarified in the next chapters!