So, this is a post for little grasshoppers of physics that want to understand the basic ideas of special relativity without going too much into the math…
Let’s start easy, with Bob the scientific fisherman. He’s in his boat, looking at the waves which his fishing line creates as it oscillates in the water. When his boat advances slowly, he realizes that the forward waves move more slowly with respect to him. Kind of, his boat is trying to catch the waves.
Bob the scientific fisherman thinks: “Sure, the waves move with respect to the water with a fixed velocity. I may move, also with respect to the water even faster than that, if I row hard. Then I can overtake the waves”. That’s true, and this is what happens when you break the sound barrier.
Bob thinks even more: “By measuring how fast do the waves escape from me, I may know my velocity with respect to the water… If I am at rest with respect to the water, the velocity of the waves will be homogeneous from my point of view
“While, if I move, it will not be homogeneous, the waves will escape from me with a velocity that will depend on the direction.”
Bob went home happy. He had caught no fishes, but had acquired some wisdom…
In space nobody will hear you cry… because there is no water, nor air. But there are waves out there, anyway. There is light, and we can apply the same idea. Water waves move on water. What does light move on? Just to give it a name, people used to call it aether. Michelson and Morley in 1887 designed a very beautiful experiment to measure the velocity of the Earth with respect to this aether, by measuring the speed of light in different directions, just as Bob wanted to measure the speed of the water-waves in different directions to find out his velocity with respect to the water.
The experiment of Michelson and Morley was a complete failure. The speed of light was the same, no matter which direction you looked at.
So, there were various options: (a) the Earth is really at rest with respect to the aether. That’s a funny option: we know that the Earth moves around the Sun, at approximately 30 km/s!! And the Sun moves with respect to the galactic center, even faster… What’s the other option? It is (b): the Earth drags the aether as it moves. So to speak, the aether which is near us moves with our same speed, but the aether far away from us does not.
This last option is discarded because of stellar aberration. Let us explain it in simple terms. It’s raining, and there is no wind. The rain drops just fall vertically. You’re standing in the street, just holding your umbrella vertically so as not to get wet. OK. Now, you start moving. You should lean the umbrella forward a little bit if you want to remain dry. If you move in the opposite direction, you have to lean the umbrella in the opposite direction.
Now, let us establish the terms of the metaphor. The light from a certain star is the rain, and the telescope is the umbrella. Let us say that, with the Earth at rest, you have to point your telescope in the vertical direction in order to catch the light from your star. Now, the Earth starts moving. Then, you have to “lean” your telescope a little bit, in order not to lose the star. If the Earth moves in the opposite direction, the telescope has to lean in the opposite direction. This is what happens really: when changing from summer to winter, the positions of the stars change a little bit, exactly as predicted by this little story.
So, we know the Earth doesn’t drag the aether as it moves! Our two options are, therefore, invalid. Now, the patent office employee enters the game. But that’s another story that we will tell soon…
The exposition and images are taken from a talk I gave to high school students in 2005, for the centennial of the SR paper at IES Ágora, in Madrid.