Universe Today
(This is Part 3 of a series on what it's like to travel near the speed of light. Read Part 1 and Part 2 first.)
There's a guy. His name was Wolfgang Rindler. Born in Vienna, shipped out as a child to escape the Nazis, and grew up into an expert on relativity. Such an expert that he became, for the purposes of this article, the Horizon Guy. He's the one who named event horizons. Before him they didn't have a catchy label, just the Schwarzschild radius. It was through his work that we came to see the radius of a black hole as a genuinely special place in spacetime: a boundary separating what different observers can see and reach. We can't look inside a black hole, but we could go in if we really wanted to (pro tip: don't). Once you're inside, you can look out, but you can't leave. That's a horizon. It's a boundary marking off the visible from the accessible, dividing which signals you can send from which you can receive. And "event" is in the name because "event" is a loaded word in relativity: an event is a location in both space and time, a full address. A coffee shop is a place. A meeting is a time. A meeting at a coffee shop is an event.
A black hole's horizon separates which future events you're allowed to reach. Once you cross it, you can still receive signals from the folks outside, but you can never visit them again. You are going to miss your coffee date. A whole enormous chunk of the universe's events is simply cut off from you.
With black holes, we get why. Their gravity is so ferocious that nothing, not even light, can climb back out.
But what if you went so fast that light could never catch you? I know, I know, you can't go faster than light. That's not how this works. This isn't about speed. It's about acceleration, which is an entirely different animal. Say you hold a constant acceleration. Not a constant speed. Acceleration. You are always getting faster. You will never, ever pass the speed of light, but you'll keep creeping toward it. 0.9c. 0.99c. 0.999c. 0.999999c. You get the idea.
Now say your coffee date sends a signal, a single pulse of light, just to ask where on Earth you are. You're already out in space somewhere. If you were sitting still, the pulse would reach you in some finite time and you'd realize you were light-years from your date. If you were coasting outward at a fixed speed, the pulse would still catch you eventually. It would take longer, since by the time it reached your old position you'd have moved on, so it would have to chase you down. But because you can never outrun light at a fixed speed, it would get there in the end.
Now the fun part. Now the acceleration. You've completely forgotten the coffee date. You're out here to explore a distant galaxy, and you fire your rockets. Now, for the signal to catch you, it has to beat two things at once: your speed and your acceleration.
Say you begin at Alpha Centauri, already moving at 90 percent of lightspeed. By the time the pulse reaches Alpha Centauri, you're long gone, off in the next spiral arm over. Fine, the pulse keeps marching after you. By the time it reaches that spiral arm, you're not only somewhere else, you're out at the edge of the Milky Way, and you're not doing 90 percent of lightspeed anymore. You're doing 99 percent. So the signal has to cover more distance and work harder to do it, because the gap between your speed and lightspeed has narrowed to a sliver.
We wait. The chase goes on. The pulse finally reaches the outskirts of the galaxy. You're far away, out in intergalactic space, though not as far ahead as before, so the signal is gaining. But now you're doing 99.99 percent of lightspeed, and the pulse has just a bit more to do. The light reaches where you were, only to find that thanks to your constant acceleration it always has a little more catching up left. It starts millions of kilometers behind you. Then a kilometer. Then a meter. Then a millimeter. Then a femtometer. Every second it closes in, and every second it can't quite finish, because every time it thinks it's finally caught you, you're a touch faster than you were a moment ago.
Eventually the light does catch you. After an infinite amount of time. But "an infinite amount of time" is just a fancy way of saying "never," so the light never catches you at all. And you never even learn that you stood up your coffee date.
Any beams sent later, or from farther away, don't even get the chance.
Now, this doesn't apply to every beam of light. If a signal is close enough when you start, it will catch you. And if you ever slow down or stop, the light gets you for sure. But as long as the signals start from far enough away, and you hold that constant, endless, perfect acceleration, there's a whole region of the universe whose signals will never reach you. Its events are closed off.
It's a horizon, built purely out of acceleration. You can't see past the horizon on Earth, and you can't see past the horizon of your accelerating ship. The simple act of accelerating through the universe walls part of it off. We call it the Rindler horizon, after our Horizon Guy, who did so much to work out the physics and the math behind it.
So motion compresses your view of the universe. And acceleration hides part of it.
And, just for fun, acceleration does one more thing. It makes you question the fundamental nature of reality.
In Part 4, that questioning pays off in the strangest way yet, as the empty vacuum around an accelerating ship quietly bursts into a glow of real particles.
