# Special Relativity finally makes sense

Preface: I studied physics in college, yet there were parts of it that didn’t click with me. Special relativity with all its “a flashlight on a train” examples, that’s counterintuitive and hard. Yesterday in Surfaces & Essences, the authors explain Einstein’s thought process when he came up with it, and it makes sense this way. It’s also an interesting example about how genius can happen when we let go of what we know.

You know how when you’re on a plane, travelling smoothly at 600 mph, you can pour water the same way as when you’re standing still? If you throw a ball straight up, it falls back down into your hand.

This is called “Galilean Relativity.” It says that while you’re traveling at a constant velocity, mechanical experiments behave the same as at any other velocity (including standing still). It’s only changes in velocity — speeding up, slowing down, bumps and curves — that affect the motion of objects you have with you. This means there is no mechanical experiment you can perform to find out whether you’re flying at 600 mph or sitting on the tarmac. The water and the ball behave the same.

Einstein said, why should electromagnetic experiments be any different? Why should a lens, a magnet, or an electron care about your velocity? Your computer works the same in the air as on the ground, and there are a lot of electrons moving around in there.

The behavior of light passing through a lens or electricity moving through your processor is closely related to the constant speed of light. For your glasses and your computer to behave exactly the same in a moving plane, the speed of light inside the plane must be the same as the speed of light on the ground. The electrons in your computer move the same way, relative to you, at 600 mph or at a stop.

Speed is distance divided by time. The distance that water traveled between the pitcher and the cup looks different to you and to the watchtower, since the watchtower sees the plane moving horizontally. Same with the light from your monitor: it travels a different distance from the tower’s perspective than yours. If the distances are different, how can the speed be the same? Speed can be the same if time is different too.

Therefore time is stretched or squashed depending on velocity, so that the speed of light is the same from every perspective. No experiment can tell the difference, and electrons in your computer don’t care how fast you’re going.

Next time you sip a drink on your way across the Atlantic, appreciate time dilation. You may not notice the fraction of a second this trip adds to your life, but reliable electronics are important at every speed.

In this story, Einstein takes one belief about the world (Galilean Relativity) and broadens it. In doing so, he narrows  another belief about the world (consistent time). The result is a new, self-consistent worldview. This worldview has proven useful.

It pays to be specific about how we know something is true, and the range of experience we can be sure about.