In fiction, things sometimes go faster than light. Superman’s at it all the time. People are often boggled by the end of the original Superman movie. Superman circles the Earth faster and faster until he spins it backwards. Suddenly, time runs in reverse––which makes no sense! Why would spinning the Earth clockwise reverse time itself? Of course, that’s not what’s happening. Instead, he’s going so fast he’s going back in time and we get to see the world from his point of view, reversing backwards as he tunnels into the past.
It isn’t just Superman. In the Space Eagle novels there are machines which travel faster than light, turning them into time machines. In Red Dwarf’s ‘Future Echoes’ the ship manages to break the light barrier and time becomes warped, allowing the crew to catch glimpses of the future.
But why is travelling faster than light connected to time travel?
According to physics, if you start slower than the speed of light, you can’t ramp up your speed to get past the speed of light. This fact about the universe is deeply, deeply weird and the key to understanding why people connect faster than light travel with travelling back in time.
Let’s start at the beginning. Given physics, you can’t find out how fast you’re moving. No, really, you can’t. You can discover similar things, though. You can find out how fast you’re moving relative to something else. If you’re on a train, moving away, you can find out how fast you’re moving relative to the platform (and how fast the platform is moving relative to you). And you can find out if you’re accelerating. When you’re on a roller coaster or an airplane, you might feel all giddy and stomach-turny. That’s not because you’re in motion but because you’re accelerating i.e. your velocity is increasing or decreasing. When you’re at a constant velocity (that is, a constant speed in an unchanging direction) you won’t feel a thing.
You’re familiar with these things already. Imagine smoking on a plane. This used to be a thing you could do. It isn’t now––so I recommend trying this only if you actually have a time machine to take you back to the 70s. For now, just picture it in your mind. Watch the trace of smoke. It curls behind you and around you in exactly the same way whether you’re sitting on the ground or travelling at 1000mph in the air. It’s not as if, when you’re in flight, the smoke zooms to the back of the plane, gathering at the back like a bizarre cancerous wall, pinned to the bulkhead.
So you can detect whether you’re accelerating. And you can detect whether you’re moving at a certain velocity relative to, say, the surface of the Earth, or the sun, or the Milky Way. But you can’t detect what your velocity is full stop. From this, many people believe that, not only is it undetectable, but there’s simply no fact of the matter about it. You don’t have a ‘velocity full stop’; you only have ‘relative velocities’.
Then Maxwell came along. He does some research in electromagnetism. It turns out, he says, that light moves at roughly 300,000 km/s. Here’s the kicker: That’s not 300,000 km/s relative to something, that’s just 300,000 km/s. ‘Aha!’, thought some, ‘Now we have a way to determine who’s really moving or not. We’ll simply measure how fast we’re going relative to light and then we’ll know! Now we can figure out our real velocity!’.
Sketch that out a bit more. Imagine I know my velocity relative to the train platform. I know it relative to the surface of the Earth. I know it relative to the sun. I know it relative to everything else. But I don’t know what my objective, non-relative velocity is. Then someone tells me that they’ve got God on the phone. Chatting to God, He tells me that there’s a comet which is at rest––not rest relative to anything, but just simply, really, objectively at rest. At rest full stop! Now I can figure out what my velocity is! I take my velocity relative to that comet and that’s my velocity—my objective, non-relative velocity.
Maxwell seemed to be saying that we could do the same thing using light. We figure out how fast we’re going relative to light and BINGO! We’d know how fast we’re going ‘full stop’! But the experiment to do this turned up weird results. Light was going 300,000 km/s. Which meant we were at rest. And that’d be lucky, right? Then it got weirder still: Measurements showed that, no matter how fast something was going, the result was always that light was going 300,000 km/s. Stationary relative to the Earth? That was how fast it was going. On a plane at 1000 mph? Still going the same speed!
This is super bizarre. Imagine you’re watching Mr. Slow and Ms. Fast race one another. Mr. Slow is going at 20 mph. Ms. Fast is going at 200 mph. You phone Mr. Slow. Mr. Slow says that Ms. Fast appears to be going at 180 mph (for his 20 mph means that’s how fast she is going relative to him). Mr. Slow accelerates until he appears to be going at 199 mph relative to you. Again you phone him. He now says that Ms. Fast is going only 1 mph relative to him.
That all sounds fine.
But that’s not how light works. Imagine Dr. Weird is driving a weird car that works like light does. Dr. Weird appears to be going 500 mph relative to me. I phone Ms. Fast. I expect her to say that, from her point of view, Dr. Weird is going 300 mph. But she instead says he’s going 500 mph. Weird. Stranger still, when I tell her to accelerate, and she is now bombing along at 400 mph according to me—and Dr. Weird is still going only 100 mph faster—she still reports that Dr. Weird is going 500 mph to her.
How the hell does that happen?
The explanation is that when things are moving fast relative to one another, time starts to slow down (it also turns out that things change shape, size, and mass, but I’ll ignore those changes for the purpose of this post). Think how that’d work with Dr. Weird and Ms. Fast. Imagine I watch them for an hour. Dr. Weird covers five hundred miles; Ms. Fast covers four hundred; Dr. Weird outpaces Ms. Fast by one hundred miles. But imagine that time has slowed down for Ms. Fast five-fold. When she uses her watch to measure time, it takes her watch five times as long as my watch to move forward. When she measures an hour, five hours will have passed for me. And in that period of time, Dr. Weird will have moved—not one hundred miles ahead of her—but five hundred miles. So if I communicate with Ms. Fast, she’ll tell me that Dr. Weird is moving five hundred miles per hour—exactly the same rate I have!
This is what’s going on with light and things moving closer and closer to light speed. When Ms. Fast starts moving faster and faster, she appears (from my point of view) to be going slower and slower. And that’s where we get the connection to time travel! Billions of years could take a single second to pass by from your point of view if only you went fast enough! And if you managed to hit light speed, time would stand still (although you never will hit light speed—remember, no matter how fast you go, you’ll always find light outpacing you at 300,000 km/s!). And if you somehow managed to go faster still, then time would start going backwards.
I’ll talk more about superluminal travel in future blog posts. For now, let the weirdness of light speed settle in.
Before ending, consider one mistake that people make. They think that when Ms. Fast appears to be going slower and slower in time from my point of view, that when she looks back she’ll see me all sped up. (You get this sort of thing with Red Dwarf’s ‘White Hole’.) That only makes sense, right? WRONG!
Think of the train: You’re on a train moving relative to the platform but the platform is moving relative to you. There’s no way to tell who’s ‘really’ in motion. The same applies here. If I sped up and Ms. Fast slowed down, we could tell who was ‘really’ in motion—it’d be Ms. Fast! And that’s not what happens. Instead, we both see one another ‘slowing down in time’ as each of us has a high velocity relative to the other. And if that isn’t the weirdest thing you’ve thought about today, you’re either a physicist already intimately acquainted with relativity theory (and probably grinding your teeth in rage as I run roughshod over important details for the sake of a good blog post) or you take too many hallucinogenic drugs.