 I got a fantastic print of Einstein for Christmas, but I can't decide how much of a mat I want to put on it. I need some sort of reference frame. To celebrate the arbitrary date of changing an arbitrary four-digit number to another arbitrary four-digit number, I'd like to talk a little bit about how arbitrary time itself is. It's the 100th anniversary of Einstein's first lecture describing his theory of relativity, and I feel like I'd be remiss if I let the year go by without discussing one of the most important revolutions in scientific thought. Relativity is sometimes pegged as being super weird and vastly unintuitive, but by any measure, it appears to be true. It's very successful as far as scientific theories go, making all sorts of accurate predictions about the behavior of the universe. It's absolutely essential for the operation of GPS, and where would we be without that? Also, importantly, it's really not that complicated. There's a cultural stigma surrounding it that you have to be some sort of rocket surgeon to be able to get it, but it really only requires that you remember two assumptions, neither one of which is particularly crazy. The weirdness only happens when those assumptions bounce off of each other. Assumption one is that there's no way to tell what's moving and what's standing still, because there's really no difference between those. Now we usually talk about movement as though there was some absolute stationary point of the universe that everything else is moving relative to. I mean, if I'm walking across a room, then the room is standing still, and I'm the one who's moving, right? That's an easy conception to maintain when we have a giant thing like the Earth that we can compare other smaller things like ourselves to. We call the Earth still and describe all motion relative to it. But the Earth isn't really still, is it? I mean, it's spinning in space, which is why the sun rises and sets, and we just finished a year-long loop around the sun, hurtling along at 30 kilometers a second. So what we think of as not moving isn't really not moving at all, it's just convenient to think of things that way because the Earth is big and we're usually stuck to it. If you were to subtract the Earth from the picture for a moment, just imagine yourself floating in the void of space. If you were slowly approaching someone else, you couldn't really say, oh I'm still and they're moving towards me, or oh they're still and I'm moving towards them. The distance between you is getting smaller, that's all that you really know. There's another reason to assume that there's no real difference between moving in a straight line or standing still. If you're flying in a jet, moving at over a thousand kilometers an hour, you can take a sip of your drink and put it down on your tray table, and as far as the drink's concerned, you might as well be sitting on the runway. The drink just sits there, it doesn't say, oh my god we're going so fast and then rock it off the tray to spill on you. Just because with respect to physics, so long as you're not moving relative to each other, it really doesn't matter. Okay, so that's the first assumption. You can't really say that thing is moving and that thing is standing still. You can only say these things are moving relative to each other. Not too bad, right? I mean it is called the principle of relativity. The second assumption is something that you've probably heard before. The speed of light in a vacuum is always, always the same. Now this assumption wasn't just a shot in the dark. Light is an electromagnetic wave, just like X-rays or microwaves, and we have some equations from James Maxwell that describes how electromagnetic waves work. Now if you manipulate these equations, as Maxwell did, just substituting variables and adding them to each other, then the speed of light just sort of falls out of them as a plain numeric quantity, like three, with no variables attached to its definition. Not three in any particular situation, not three if you're an electron that's moving this fast in that direction, just three. End of story. Which is a little weird. Because it doesn't have any variables attached to it, no matter what crazy stuff you're doing, whether you're flying in a jet or floating through space, if you take an accurate measurement of the speed of light in a vacuum, then it is always this number. It's pretty big number, but it's still just a number. Okay, so two things, and neither one is particularly crazy. I can remember two things. The weird stuff only really happens when you try to bash them against each other to make something break. Let's try to do exactly that. Let's say that I'm flying in a plane, and I'm standing against one wall. I shine a laser pointer at the opposite wall and time how long it takes to get from here to there. So because me, the plane, and the laser pointer are all moving at the same speed, then it shouldn't be any different than if I were doing this experiment on the ground. So I'm going to get the result that the light is traveling at the speed of light. Duh. But let's say that someone else with really good eyesight is watching this experiment from the ground. From their perspective, the laser doesn't go straight across the plane like this because we're all moving along at a constant speed. For them, it starts here and ends up here. It traverses this diagonal line instead. Because speed is defined as the distance traveled on a certain amount of time, like miles per hour, for the observer, if light travels that distance in the same amount of time, that means that it's going faster than the speed of light. Bam. Relativity Disproven. Unless something really weird were true. What if we don't just disagree about how far the light has to travel, but also how long it takes to get across the plane? I mean, we could still theoretically agree that light travels at light speed, but only if the person who's standing outside the plane thinks that it takes longer for light to make that cross-plane trip. In other words, they might look through the window and see me timing the trip and say, Josh, you screwed this up. Your watch is running slow. You timed it as only taking one second, but I timed it as taking two. Now, it's easy to think that this is just some sort of weird, funhouse mirror, optical illusion that when people move relative to each other, they see some things that aren't actually there. But the truly incredible part about this is that these aren't just things that we see. This is what's actually physically happening. Just like the observer outside the plane thinks it does, time actually runs slower for things than moving reference frames. We've done experiments with super-accurate synchronized clocks where one is accelerated and then when they're brought back together, it has experienced less time than the other one. And again, we didn't go fishing for some sort of weird statement about how people in different reference frames experience time differently. We just made two assumptions. This is just a necessary implication of those assumptions. So special relativity has some quirks to it. But that quirkiness is just the result of two very simple assumptions interacting. Now, this is just special relativity, which just covers the special case where nothing is changing speed or being affected by gravity. I'll cover those in some future episode on general relativity. But even this much was a huge boon for the development of physics, realizing that time isn't an absolute thing, that it moves differently for different observers, was an absolute game-changer. And the fact that one dude worked through all these implications in his head is pretty frickin' cool, relatively speaking. What do you think of special relativity? Do you think that it's as hard as everyone makes it out to be? Please leave a comment below and let me know what you think. Thank you very much for watching. Don't forget to blah, blah, subscribe, blah, share, and don't stop thunking.