(This is Part 2 of a series on what it’s like to travel near the speed of light. Read Part 1 first.)
Let’s start with perspective. It’s strange enough on its own, but it’s also gentle enough to absorb, which makes it good groundwork for the heavier stuff coming later.
Have you ever walked in the rain holding an umbrella? You hold it straight overhead because the rain is falling from straight above, and presumably you’d like to stay dry. But then you realize you’re late, or you’re just sick of being out in it, and you start to hurry. Suddenly the rain isn’t hitting you from directly above anymore. It’s coming from above and a little bit in front. Not because the wind picked up, and not because the rain started falling at an angle, but because your interaction with the rain has changed. You’re moving into it, so the point where you and the rain meet has shifted to mostly above, but a little bit in front.
So you tilt the umbrella forward a bit and keep sloshing along.
It isn’t exactly the same physics with light, because relativity is relativity, but the gist carries over. As you move through space you are surrounded by light: sunlight, starlight, the cosmic microwave background, the thermal glow of planets and dust, the occasional high-energy gamma ray from some distant supernova. The usual suspects. That light arrives from every direction, more or less uniformly (let’s assume you aren’t parked right next to a star). But the moment you start moving, the meeting point of you and the light shifts. Instead of coming at you from all around, it now comes from all around and a little bit in front.
So the view behind you goes dark, and the rest of the universe, which would normally surround you evenly, gets squeezed forward into a cone beside and ahead of you. The closer you get to lightspeed, the worse it gets, until every scrap of light from the entire universe is crammed into a single disk hanging directly in front of your face, with total blackness everywhere else.
Oh, and it burns. Partly because you’re compressing an entire sphere of light down into a small patch. But also because you’re moving relative to that light, which brings in the Doppler effect. You’re racing headlong into it, which crowds the waves together, the way the peaks and troughs of the ocean arrive faster when you gun a boat straight into the swell. The light blueshifts, dragging all that mild, low-energy light up into x-rays and gamma rays.
So, yes, it’s unpleasant. The entire universe distorted, focused, compressed, and energized into a single point in front of you, like a laser-powered fist of light aimed square at your face as you approach the speed of light.
I’m not even going to try to work out how you’d survive that. I’ll leave it to the engineers. I’m just here to tell you what happens.
And keep this in mind, because I’m going to lean on it hard in a minute: nothing about the universe has actually changed. Not one photon out there is behaving any differently. Only your perspective has changed, and near lightspeed your perspective bends your view of everything outside pretty severely.
But this is all just coasting, moving at a constant speed. At constant speed, the old no-preferred-frame rule still holds up perfectly well. The baseball whizzing past you and you yourself have equally valid claims on the universe. The whole business of Doppler shifting and the compressed view is about motion relative to the light around you, and all the usual language of relativity still applies. Drop you inside a giant sphere of lightbulbs floating in space and switch them on, and you get the exact same effect whether you’re the one moving or you’re sitting still and the sphere is moving. Same Doppler shift, same compression into a searing cone of pain. What matters is the relative motion.
So let’s add a twist, because that’s the sort of thing we enjoy doing around here. The twist is called acceleration, and it does something critical to our neat and tidy picture. With plain coasting, constant-velocity motion, reference frames are swappable. You can’t say you’re moving in any absolute sense, only that you’re moving or still relative to other things. There’s no experiment you can run to prove you’re the one moving. But you absolutely can know that you’re accelerating. You can drop a ball and watch it fall oddly. You can swing a pendulum. You can feel yourself pressed back into your seat.
You can know, locally, all by yourself in the privacy of your own home or your own spaceship cabin, whether you’re accelerating. And that acceleration doesn’t break relativity. But it does hand you a brand new perspective on the universe. It changes things in a way that no amount of steady coasting ever could.
Acceleration gives you a horizon.
In Part 3, that horizon takes shape, and a simple, endless push on the throttle quietly seals off a whole chunk of the universe from you forever.
