Simultaneity... Not!

You’re hanging out at a train station, savoring each sip of your coffee as you watch the trains go by.

This is no ordinary train station, though. While the coffee is at the scale of human existence, everything else runs a lot faster. Every train that goes by is nearly moving at the speed of light, but your brain allows you to process the trains as though you’re watching them at a much lower speed.

You watch as one of these trains whooshes past you. As it does, lightning strikes both ends of the train. To you, standing still on the platform, the bolts appear to hit simultaneously.

This really gets you thinking: what would a person sitting in the middle of the car see? They’re speeding by at 99% of the speed of light. What do they see?

The somewhat shocking conclusion is that they see things differently. Flying forward at such an insanely fast pace, they see the lightning strike the front of the car well before they see it strike the back of the car. After all, the train car is moving forward near the speed of light, so the information—the light from the strike—will reach them first from the front of the train. They’ll run right into it.

There’s no such thing as “at the same time.” It depends on who you ask.

This is a thought experiment, and the train station lived inside Albert Einstein’s mind.

Einstein understood that light had a finite speed. Just 20 years earlier, Albert Michelson and Edward Morley had made incredibly precise measurements with their famous experiment. Their idea was to prove that a substance called “luminiferous aether” existed.

This aether was the medium through which light traveled. Light was, after all, a wave, as everyone knew. And, waves always traveled through a medium—water was the medium for an ocean’s waves, while sound travels through the air.

The luminiferous aether was thought to be stationary and all-pervading. If the Earth was moving through the aether, like a boat through water, the speed of light in the direction of Earth's motion would be slightly different from the speed of light at right angles to it. If you’re curious, you can read a little about how this experiment worked here.

So, Michelson and Morley set off to prove that there was, in fact, such a luminiferous aether.

They failed.

This perplexing result left scientists grappling with questions about the fundamental nature of light and motion. Did light not need a medium through which to travel?

Even more perplexing, Michelson and Morley knew the earth rotated at a fixed speed. Why wouldn’t the light traveling at that fixed speed go a little faster than the light traveling perpendicular to it? If you shot a bullet moving 400 miles per hour out of a train moving 100 miles per hour, most middle school kids will tell you right away that the bullet will move at 500 miles per hour.

Not so with light.

You can be on a train going 99% of the speed of light, turn on a flashlight and point it toward the front of the train, and that light is going to come out at the same speed as if the train was sitting still.

This is where Einstein’s head was on that fateful day at the train station. A shocking conclusion formed in his mind, one that contradicted our very understanding of physics, and how everything works.

If the speed of light didn’t change, then time and space had to change instead.

This principle, central to his Special Theory of Relativity, shattered the existing notions of time and space. Neither of these were static and eternal, as Newton had assumed, and as the entire scientific world understood reality to be.

Speed is just distance divided by time. By insisting that light was the constant, but space and time could sort of bend around it, Einstein turned Newton’s world upside down.

Simultaneity is a funny thing. Two things seem to happen at the same time, only they don’t. Einstein took notice of this and thought about it in a way nobody had before, and now we have GPS satellites, cutting-edge scientific experiments with particle lasers, and so much more.