What does high-speed motion through the universe do to your perspective? What is the Rindler horizon, and how does it appear? Would you get vaporized by a high-energy bath of radiation? I discuss these questions and more in today’s Ask a Spaceman!

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EPISODE TRANSCRIPT (AUTO-GENERATED)

Imagine you were traveling at the speed of light, racing alongside a single photon, the fastest possible thing in the universe. What would you see? What would the universe look like to you? Einstein envisioned the exact same thing. As a teenager, he wondered what it would be like to race in a bicycle next to a beam of light. Listen, we didn't have rockets yet, so bicycles was the best he got. After decades of toil, he came to the following conclusion. What's it like to travel at light speed? You can't. You can't. You don't ever get to know what the universe is like from a photon's point of view. It's not that it's a bad or stupid question. It's just a malformed one. And this insight reveals something very weird about the universe. Your experience of reality is shaped by your speed. A photon has a different conception of reality that does not map onto our own. And the best part is, we don't even have to reach light speed to see the weird stuff come out. Let me get one thing out of the way.

In special relativity, which honestly is my favorite kind of relativity and how we're going to approach today's question, when we talk about perspectives and views, we're talking about rest frames. The best way to think about a rest frame is your own point of view. In relativity, there is no such thing as perfect absolute stillness. All motion is relative against all other objects. You see a baseball whizzing by you, and from your perspective, you are perfectly still while the baseball is doing the whizzing. You are in your own rest frame. In fact, you always are because you are always you. But the baseball has its own rest frame, which is its own POV. And from its perspective, it is perfectly still and you are rushing past it in the opposite direction. Who's right? Who's wrong? Which counting of the universe, yours or the baseball's, is correct? The answer from relativity, it's all relative. Both perspectives are perfectly valid and neither is more correct than the other. It just means that when we talk about motion, we first have to specify which frame of reference, which perspective we're working from.

I am still in the baseball is going really, really fast. Dot, dot, dot from my reference frame. The same goes for stillness. If you and me are standing next to each other, I can say that you are at rest with respect to my frame of reference. So when we ask about the point of view of a particular object, like you or me or a baseball or a photon, We're really asking what's the universe like from a frame of reference that is at rest with that object. You know, like racing your bicycle to catch up with a beam of light. I know I'm in the relativity jargon weeds here and I'm doing it for two reasons. One, it's cool stuff. And two, I need this language to explain what happens with light itself. When we ask, what does the universe look like to a photon, we're really asking, what is the perspective from a frame of reference at rest with respect to a photon? And the answer is light has no rest frame. Light has no rest frame. There is no frame of reference that is at rest with respect to the speed of light.

They're just distant. The entire machinery of relativity is constructed from this singular insight. Einstein realized that it was impossible to catch up with light. There are different arguments you can employ here, and here's the one that he used. Light is made of waves of electricity and magnetism. And if you caught up with light, the waves would appear to be frozen in place. But then they're not exactly waving, are they? And so you don't have light anymore, do you? Which was the whole point of the exercise. We can't talk about what the universe is like from the point of view of a photon, because strictly speaking, A photon has no point of view. It doesn't even have a sense of time or space or duration or length or measurement or speed or anything else we attach to the usual concept of point of view. Look, I know this is weird. But everything about relativity is weird. Just some things we get used to. The cost of relativity is that measurements in time and space are, well, relative to your speed and point of view.

Moving clocks run slow. Moving rulers shrink. This is all needed to make the real prize work. What stays the same are physical laws. I've done a lot of episodes on relativity. We're all used to it by now. Oh yeah, cute, if one twin goes on a rocket ship ride and comes back to Earth, they'll be younger than their twin. Neat. But we're a little less used to taking relativity to its conclusion. All these effects like time dilation and length contraction get worse the closer you get to light speed, which means at light speed itself, they break. They stop. Time and space effectively have no meaning at light speed because our very conception of space-time relies on clocks and rulers obeying the laws of relativity that were explicitly designed to operate below the speed of light. But that doesn't mean we can't get close to the speed of light and see what happens when we do. And the best part about almost but not quite the speed of light is that I don't have to spend the entire rest of the episode telling you that the question is impossible to answer, which is nice.

And your speed doesn't just change measurements of time and space. It literally edits the universe you experience. Let's talk about perspective first. It's weird enough already as it is, but somewhat easy enough to absorb so it'll lay the right groundwork for the heavy stuff that's about to come. So you ever walk in the rain and hold up your umbrella? You hold it right above you because the rain is coming from directly overhead and presumably your goal in holding the umbrella is to stay dry. But then you realize you're a little late for your meeting or you're just really sick of walking through the rain. So you start to hurry. This changes where the rain hits you. It's still coming from mostly above now, but now also a little bit in front of you. Not because the wind is picked up or the rain is literally going diagonal, but because your interaction with the rain, your perspective of the rain has changed. You're moving into it. So the rain plus you intersection point is now mostly above you.

but a little bit in front. So you tilt your umbrella slightly in front of you and you keep sloshing forward. It's not exactly the same physics with light because relativity is relativity, but the gist is the same. As you move through space, you're surrounded by light. Sunlight, starlight, cosmic microwave, background light, thermal emission from planets and dust, random high energy gamma rays from distant supernovae, the usual suspects. That light is coming to you from all directions pretty much uniformly. Let's say for the purposes of this discussion that you aren't like right next to a star or something. But once you start moving, the intersection point of you plus light changes. Instead of being all around you, it's all around you and a little bit in front of you. So the view behind you looks blacked out. And the rest of the universe, which would normally be coming at you in all directions, compresses into a cone beside you and in front of you. And the closer you get to light speed, the worse it gets, until at some point all the light from the entire universe is concentrated in a single disk directly in front of your face and complete blackness everywhere else.

Oh, and it burns. Not just because we're compressing an entire sphere of surrounding light into a small patch, but because you're moving relative to the light, which introduces a Doppler effect. You're racing head-on into this light, which makes the waves compressed. Like a boat racing into the swell, the peaks and valleys happen sooner than if you were standing still. The light blue shifts, shifting all the low-energy light into high-energy stuff like X-rays and gamma rays. So yeah, it's kind of unpleasant. The entire universe is distorted, focused, compressed, and energized right in front of you like a laser-powered fist of light aimed squarely at your face as you try to approach light speed. Listen, I'm not even going to try to examine how to survive something like that. That's for the engineers to figure out. I'm just here to explain what would happen. Keep in mind, and I'm doubling down on this because it's going to get important, nothing about the universe has actually changed. But your perspective has.

And your perspective near light speed really warps your outside view. But this is just coasting. Maintaining constant speed. At constant speeds, the language of no preferred reference frame still holds just fine, the baseball whizzing by you and you yourself both have equally valid perspectives of the universe. The whole thing about the Doppler shifting and the compressed view is about motion relative to light surrounding you, and all the language of relativity still holds. If I put you inside a giant sphere of lights in the middle of space and turn them on, you get the same effect whether you are the one moving or you are standing still and the sphere is moving. The same Doppler effect, the same compression to a blazing cone of pain right in front of your eyes. What matters is relative motion. So let's add a little twist because that's kind of the thing we like to do here. And the name of that twist is acceleration. Acceleration does a very critical thing to our neat and tidy picture.

With just coasting constant velocity motion, reference frames are swappable. You can't really say you're moving in an absolute sense. You can only say you're moving or still relative to other stuff. There's no experiment you can perform that tells you that you're moving in an absolute sense. But you can know that you're accelerating. You can drop a ball. You can swing a pendulum. You can do a Patreon ad. Patreon.com slash PM Sutter is how you contribute to this show and keep it going. I truly do appreciate it. You can know, locally, just you and the privacy of your home and or cabin on board a spaceship if you're accelerating. And that acceleration doesn't break relativity, but it does give you a new perspective on the universe. Acceleration changes things in a way that no mere constant motion does. You get to know that you're accelerating, and that knowledge gives you a horizon. There's a guy. His name was Wolfgang Rindler. He was born in Vienna, shipped out to escape the Nazis as a kid.

Expert in relativity. Such an expert that he became, at least for the purposes of our episode, The Horizon God. He named event horizons as event horizons before they were just called, well, they didn't have a catchy name, just Schwarzschild radii for the surfaces of black holes. It's through his work that we recognize that the radius of a black hole is a very special thing in spacetime. It separated what different observers could see and access. We can't look inside a black hole, but we could go in if we wanted to, pro tip, don't. If you're inside a black hole, however, you can look out, but you can't leave. That's a horizon. It's a boundary marking the visible and the accessible. What kinds of signals you can send and receive. An event is in there because the word event is very special in special relativity. An event is a location, an address in both space and time. For example, a coffee shop is a place. A meeting is a date. A date at a coffee shop is an event. The black hole's horizon separates what kinds of future events you can access.

Once you cross through, you can get signals from the folks still outside, but you can't visit them. You're going to miss your coffee shop date. A whole big chunk of the universe's events are cut off from you. An event horizon. An end. A border. To events. Now, with black holes, we get it. Their gravity is so strong that nothing, not even light, can escape. That's what creates an event horizon. But what if you went so fast that light could never catch you? I know, I know. It's impossible to go faster than the speed of light. And that's not how this is going to work. This isn't about speed. It's about acceleration, which is a different animal altogether. Let's say you maintain constant acceleration, not speed, acceleration. You're always getting faster and faster. You will never ever go faster than light, but you'll get closer. 0.9C, 0.99C, 0.999C, 0.999999C. I think you get the idea. Now let's say your coffee date sends a signal, a single pulse of light, just to let you know they're wondering where you are.

You're already somewhere out in space. If you were standing still, the signal would reach you in a finite amount of time and you would realize that you're way out in space when you're supposed to be meeting for coffee. If you were cruising outwards at a fixed speed, the light would still reach you. It would take longer because by the time it got to where you were when the signal was sent, you'd already be further away so it would have to catch up with you. But still, after a finite amount of time, it would catch up with you because you can never go faster than light. Now the fun part. Now the acceleration. You totally forgot about your coffee date. You're out here to explore a distant galaxy. You fire your rockets. Now if the signal wants to catch up with you, it has to be two things. It has to beat both your speed... and your acceleration. Let's say you start your journey at Alpha Centauri. Let's say you started at 90% of the speed of light. By the time the light signal reaches Alpha Centauri, you're long gone.

You're in the next spiral arm over. Okay, fine. The light signal keeps marching outwards, chasing you. But by the time the signal reaches the next spiral arm, not only are you also not there, you're now on the edge of the Milky Way, you're also not going 90% light speed anymore. That's the spiral arm. You were going 99% the speed of light. So the signal has to close the distance to you and it has to work even harder to do it. The difference between light speed and you is now only 1% when it used to be 10%. We wait a bit. The chase continues. The signal finally reaches the outer edges of the Milky Way. Of course, you're far away. You're in intergalactic space. Not as far away as you used to be. The light signal is gaining on you. But now, by the time the light signal reaches the edges of the Milky Way, you used to be going 99% light speed. Now you're going 99.99% light speed, and the signal chasing you has just a little bit more work to do. The chase continues. The light reaches where you were only to find that because of constant acceleration, it always has a little bit more catching up to do.

The light starts off millions of kilometers behind you, then a kilometer, then a meter, then a millimeter, then a femtometer. With every passing second, the light gets closer to you, but it can't quite finish the job. Every time it thinks it's finally closed the gap, you're just a tiny bit faster than you were before. Eventually, the light will catch you after an infinite amount of time. But an infinite amount of time is also never, so the light never catches you. And you never know that you missed your coffee date, by the way. And any beams of light that get sent out later or start off even farther away never even get the chance. Now, this doesn't work for every single beam of light. If the signal is close enough to you when you start, it will catch up. And if you ever slow down or stop, then yeah, the light is going to get to you. But as long as the signals start from far enough away and you maintain constant, endless, perfect acceleration, then you will never receive signals from a big chunk of the universe around you.

Their events will be closed off to you. It's a horizon built entirely from acceleration. You can't see past the horizon on the Earth, and you can't see outside the horizon of your accelerating spaceship. The simple act of accelerating through the universe closes it off from you. We call it the Rindler Horizon because Rindler was a horizon guy, and he played a huge role in studying and clarifying the physics and math behind it. So motion compresses your view of the outside universe, and acceleration hides it. And just for fun, acceleration does one more thing. Makes you question the fundamental nature of reality itself. Quick question. How many particles exist around you right now? There are probably, uh, a lot. But accountable a lot. Work hard enough and you can get the number. And yeah, yeah, I know, there are all these quantum fields wiggling and vibrating with a lot and possibly infinite amount of energy, but we're not going to talk about that today. Quantum fields permeate all of space and time, and one way to view the energy contained in the quantum fields is to imagine particles popping in and out of existence, borrowing energy against nothing for a brief amount of time before giving that energy back before anybody notices.

If you've listened to this show long enough, you know that I'm not entirely a fan. of the so-called virtual particles, both as a name and a concept. It's much more natural to me to think of the quantum fields as vibrating, and it's only the vibrations that stick around that we count as particles. But no matter what, counting the particles around you is still an achievable goal. And to be honest, despite my sneering contempt for virtual particles, they're a useful way to think about what's going to happen next. Okay, we're in our spaceship. We know we can't reach the speed of light. There's no frame of reference there. Nothing for us to talk about. But we can get close. And close is cool enough on its own. We've been coasting along at constant speed. The universe is compressed into a small disk in front of us. And it's been blue-shifted. Uncomfortable, but we're managing. Then we started accelerating. And when we did that, we introduced a Rindler horizon. And relativists sometimes now also tag this accelerating frame of reference as Rindler observers, but that seems self-indulgent.

Anyway, that Rindler horizon acted as horizons tend to do. They cut off regions of the universe from causal contact, preventing us from receiving signals. They also chop up virtual particles. You remember Hawking radiation? Virtual particles come in pairs of matter and antimatter. It's the only way to maintain balance, by the way. Because while you can borrow energy from the vacuum, you can't just manifest charge. Turns out the universe cares more about charge than mass and energy. That's cool. Anyway, outside a black hole, the virtual particles meet and return back to pure energy. But if the pair arrive on the scene straddling the event horizon of a black hole, one gets locked inside while the other wanders away. For observers outside black holes, which is where observers should really want to be, we see a glow of radiation emitted by the black hole. Hawking radiation. Well, that was the event horizon of a black hole. Now we're at the Rindler horizon of an accelerated observer. we have the same deal.

Virtual particles appear, straddling the horizon. One goes off into the universe, unable to reach you. The other gets trapped inside your bubble. They don't get to meet. They don't get to annihilate. They don't get to return to energy. Here they are. They're real now, and we have to deal with them. And if this picture is confusing for you and you start wondering what happens to the wandering particle, this is exactly why I prefer the wiggling fields picture. Because we just get to say that the Rindler horizon changes what fields are allowed inside the bubble, and that makes some of the wiggles persist, and persistent wiggles are what we call particles, and we can move on with our lives. But no matter how you describe it, the math is unambiguous. Accelerated observers get a bath of particles and radiation in their little bubbles. It's called the UNRWA radiation, after William UNRWA, a former student of John Wheeler, along with the folks... along with folks like Richard Feynman and Kip Thorne, who looked at Hawking radiation and thought that there's no way that this is the end of the story.

And honestly, it's even weirder than Hawking's black hole version. There are no black holes here, no curvature, no exotic dance with space-time, just a rocket ship accelerating through space and boom, this weird quantum effect. So the answer to a very, very basic question, how many particles live around you, depends on your acceleration. More acceleration, It means more particles. You're changing a seemingly fundamental property of reality, something that should have an easy answer, to something that depends on movement. And movement that you can measure. This is not a relative thing. This is an absolute thing because local observers know when they're accelerating. They can feel it. They can measure it. And then they can look around and they can see it. And this acceleration doesn't just change your view of the universe, your ability to receive signals and the appearance of horizon. It changes the stuff right under your nose, like the particles right next to your body when you're accelerating.

There are more of them than when you're standing still. The more you accelerate, it's like the vacuum of space-time comes alive and cooks you. It's like the wind. If the air is perfectly still, it's just you and air molecules. Once you start moving, the molecules push against you. You feel the wind. Nothing about the air has changed. Nothing about your body has changed. What changed was your body's motion through the air. That gave rise to something brand new. Wind. The Anra effect, also known as Anra radiation, is the quantum wind of space-time. And you only feel it through acceleration. This is what relativity has done to us. It removed simultaneity. Nobody can agree on what now is. Fine. It took duration. Moving clocks run slow. Fine. It took length. Moving rulers shrink. Fine. We gave up things like when and how long and how far. We assumed we could hold on to things, like what is actually there. We may disagree about what the clock on the wall says, but we should at least agree that there's actually a clock on the dang wall.

Not anymore. Thanks to Ben J. for the question that led to today's episode. Thank you to all my Patreon contributors. That's patreon.com slash pmsutter. Thank you for dropping reviews on your favorite podcasting platform. It really helps the show visibility. And of course, thank you for all the questions. That's what really powers this show. You can send questions to askaspaceman at gmail.com or the website askaspaceman.com. And I'd like to thank my top Patreon contributors this month. They're Justin G, Chris L, Alberto M, Duncan M, Corey D, Michael P, Nyla, Sam R, Joshua, Scott M, Rob H. Scott M., Louis M., John W., Alexis, Gilbert M., Rob W., Jessica M., Jules R., Jamel, David S., Scott R., Heather, my guests, Pete H., Steve S., Lisa R., Kevin B., Eileen G., Deb A., Michael J., Phil Bell, and Stephen B. Thank you again for all of your support. Keep those questions coming and I will see you next time for more Complete Knowledge of Time and Space.

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