Today I watched through the rather delightful youtube series Things You Might Not Know. This video in particular, along with currently reading through Greg Egan's Orthogonal trilogy, made me wonder something about Einstein's Special Relativity: how did it occur to Einstein to set the speed of light as a constant and then allow time and space to change instead? There is no a priori reason to think that the speed of light isn't variable, and certainly not one to think that the dimensions of space and time are variable (time spent bored in waiting rooms notwithstanding). Of course, it could have been that Einstein just decided to mess around with various combinations of variant and invariant parameters; but what actually caused him to think that a non-variable speed of light and variable time had any relation to the physical universe?
So I went on the internet and I found this. And here is my sophomoric, not technical, probably completely wrong interpretation of that thread, in the way it made sense out of my question:
Everyone assumed that Newtonian mechanics, with its absolute and invariable time frame, was correct. After all, it agreed with every experiment they had thought to put it to until then. Everyone also assumed that Maxwell's equations for electromagnetism (once they thought to combine electricity and magnetism, a development I need to do a lot more reading on) were correct. The problem was that there were certain situations in which these two systems were incompatible.
Another assumption at the time was that the universe operated under something called Galilean invariance. This stated that the laws of physics were the same under all inertial reference frames. That is, if you got an experimental result while at rest, you would get the exact same result if you were on a train moving at 100 miles an hour. Or in a space ship moving at a million miles an hour. This worked perfectly for Newtonian mechanics. However, this did not work for Maxwell's equations. (Here I am very fuzzy — read: entirely ignorant — of the actual math and physics; see disclaimer above.)
Here's what happened: imagine you were floating in an empty void with a space suit, flashlight, and notebook, and you noticed another experimenter flying past you at 100,000 meters per second with same. Under the Newtonian/Galilean view that space and time are absolute and not variable, you would see the following. When you measured the speed of light shining out of your flashlight, you'd get some number n meters per second. The person flying past you would measure the speed of the light coming out of their flashlight and get n as well. When you then looked over and measured the speed of the light coming out of their flashlight you would, naturally, measure 100,000 + n. That is, just like someone throwing a ball on a moving train, its apparent speed would be different depending on ones frame of reference.
However, Maxwell's equations demanded that the speed of light be the same under all reference frames. That is, they contain a quantity called c, the speed of light, which appears in them independently of the inertial reference frame of the observer (for mathematical reasons I am not nearly smart enough to figure out). So if one follows Maxwell's equations instead of Newtonian mechanics, then you should measure the speed of your light as n, the other experimenter should measure theirs as n, and you should measure theirs as... also n! But clearly this doesn't make sense! Therefore, some assumption is wrong. Einstein spent a while trying to figure out how to make Maxwell's equations work with Newtonian mechanics and its Galilean relativity.
However, a Dutch physicist named Handrik Lorentz had a jump on Einstein. He wondered what the world would look like if we ignored the implications of Galilean invariance and assumed that the speed of light really was constant for all observers. This had various strange implications such as clocks running at different speeds, distances shrinking as speed increased, and general chaos and malarkey. However, Lorentz was only interested in electromagnetism, and looked at all these effects as quaint mathematical tricks to get Maxwell's equations to make some sort of physical sense. It wasn't until Einstein took Lorentz's work and really thought through its implications that we arrived at Special Relativity.
An 1887 experiment by Michelson and Morley had attempted to detect variance in the speed of light moving through space. The theory at the time was that EM waves were just like any other wave, and had to be propagated through a medium, just like sound propagates through air, or a ripple propagates through the surface of a pond. One of the consequences of waves moving through a medium, though, is that an observer moving through that medium will measure wave fronts as moving slower away from it in front of itself, and faster away from it behind itself, and at some in between speed orthogonal to its motion. This implied that it should be possible to detect a variation in the speed of light from the Earth because the Earth would always be moving through the "aether", the medium which pervaded space and allowed for the propagation of light. This experiment showed no such variation. Light traveled at the same speed regardless of which direction you measured it from.
Although not the main motivation for Einstein to adopt the views that would lead to Special Relativity, it was a strong impetus for him to take the concept of a physically invariant speed of light seriously. Once he did so, it was obvious that Lorentz's thought experiments could be interpreted literally, and that if one did so, time dilation and length contraction would fall out as real phenomena that happened as one approached the speed of light. Such phenomena, though, were completely at odds with the orderly, stable Newtonian view of the Universe. Part of Einstein's genius was the willingness to provisionally jettison Newtonian physics and its Galilean relativity, privileging Maxwell's equations and Lorentz's mathematical view, and trying to rebuild mechanics based on that. He, of course, succeeded. And once you had a mechanics that explained all the same experimental data that Newton's had and also agreed with Maxwell's equations, that was clearly the theory to pick. Hence, the 1905 paper called, fittingly, "On the Electrodynamics of Moving Bodies".
I'm sure much of what I said above is confusing, muddled, or just plain wrong, despite it taking far longer than I expected for me to write. But part of the point of this blog is me finding topics that interest me, and writing about them such that I am forced to actually clearly understand them to my own satisfaction. Very easy to convince yourself that a soup of vague ideas in your own head is understanding. Much harder to do when you serve that soup to an audience.
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