Would Headlights Work at Light Speed?

Hey, Vsauce. Michael here.

If you were driving in a car at the speed of light and you turned on your headlights, what would happen? Would light be able to come out, or would your headlights just stay dark? Or maybe light would come out, but it just pull up inside the headlight like an overflowing sink. Or maybe the light would come out but at twice the speed of light - at its normal speed plus the speed of your car. No, that doesn't sound right. None of these answers sound right because, of course, there is no answer. A car cannot travel at the speed of light. Nothing that has mass can.

But come on, you say; there are things that can travel at the speed of light. Things that have no mass, like photons. Let's build a car out of light. Okay. Let's take that suggestion because imagining what it is like to be light brings up a lot of questions about light, about why things are the way they are, and about whether or not we are real.

In order to see what I mean, we need to talk about "C". The speed of light. It is constant. As long as you aren't accelerating, you can move as fast or slow or in whatever direction you want, and you will still always see light moving at light speed. Even when light appears to slow down as it passes through different materials, the actual photons themselves are still always traveling at "C". They're just taking a longer path, which takes more time.

This law always holds true because whenever you move relative to other things, your measurements of them and their measurements of you change. When you move, everything not moving with you appears to you to be shortened in length and to be experiencing time more slowly. Whereas to everything else, it is you that these changes happen to. These changes really happen - just not noticeably or measurably at the speeds we usually travel at.

For example, while walking to your refrigerator for a snack, you will measure your fridge to be a 100 quadrillionth of a meter nearer and thinner than you would while at rest with it. You'll also notice that it experiences time more slowly than you. Each one of its seconds will be a 100 quadrillionth of a second longer than yours. But your fridge will measure that these transformations are happening to you.

These are small numbers, but at speeds nearer the speed of light, these changes can be dramatic. As I mentioned in an earlier video, to a particle racing toward Earth at 99.9999999999991%, the entire Earth would only appear to be 70 meters deep. Our whole planet.

Now, with all of this in mind, let's take a look at headlights being used by a stationary ship and one that is moving at 99 percent the speed of light. The near-light-speed ship would be flattened in the direction of its travel to us, while a stationary one would not. As their headlights come on, the light coming from both ships travels at the same rate. The speed of a light's source doesn't push it faster.

The moving ship's velocity does, however, give its headlight energy in the direction it's headed, causing a blue shift from the front and a red shift from behind. What's really cool, though, is that this is just what we would see, floating in space at rest with the stationary ship. The crew of the fast-moving ship wouldn't see their beams, only gradually gaining on them like we do; they would see exactly what the crew of the stationary ship sees. Light beams rapidly fleeing their headlights at the speed of light just as if they were moving at all.

And here's why. Let's say this distance is a light second. The distance light travels in one second, which is 299,792,458 meters. Now, after one of our seconds, sure enough, both beams will have traveled a light second. But the crew of the moving ship measures light speed to be the same as us. But how does that make sense?

I mean, light hasn't traveled a light second from them yet. No problem. You see, what they measure a light second to be is shorter than what we measure. And they wouldn't agree that a second has passed yet either. Because, of course, to us, time is slower for them. They count one second finishing later than we do, at which point their light has, indeed, traveled one of their light seconds from them.

Observers will often disagree about time and space, but those disagreements will always conspire to make sure they agree on one thing: the speed of light. It is always the same for everyone. But what if you were traveling at the speed of light? Well, the only thing that could do that would be a massless vehicle. So, fine, let's assume that we can build one.

Such a vehicle would travel with light. Light would never even pull a tiny bit ahead of it. No matter how short it thought a meter was. No matter how long it thought a second was. It would never register light ever moving ahead of it. It would say that light was stationary. It would not agree that light traveled at the speed of light. Right? Not really.

You see, at the speed of light, there is nothing to see. Not because there's nothing to see but because there is nothing to do any seeing. As we have seen, as an object's velocity approaches "C", time for everything around it approaches a standstill.

An object traveling very very very very very very near the speed of light could travel for billions of our years before a single second passed for it. But a massless vehicle, traveling at the speed of light, could travel forever before an instant even began for it. It experiences no time. And it would have no time to do anything. Certainly no time to turn on its headlights. The concept of time doesn't even really apply.

A massless vehicle couldn't even have its headlights on before it hits "C", because an object with no mass must always travel at "C". Such an object would never have a before. Its origin, its journey, and its destination are simultaneous for it. A billion light year trip from a distant quasar might take a photon a billion years to us. But it literally takes no time for the photon. And to the photon, the distance to us from that quasar is literally nothing.

Photons are how we see. How we know anything about our place in the universe, and the energy they bring our planet from the Sun is responsible for nearly all life on Earth. But despite that list of accomplishments, they don't think much of themselves. In fact, as far as they're concerned, they don't even exist.

Outside observers, of course, see them and would measure them to be moving along at the speed of light with nothing passing or gaining on them. So if you were moving at the speed of light, you'd have to be massless, like a photon, and you wouldn't notice anything because to you there would be nothing and no time to notice it. Other people would see you doing nothing and you would be unaware of yourself. You could never turn on headlights because you would feel exactly like you felt before you were conceived.

But why is the speed of light what it is? I mean, this is how far light travels in a second. Could it also travel this far, or this far, or this far? Could we conceive of a universe where light traveled that quickly? Yeah, easily. So then, why is it that the universe we live in is only one of these ways? Why are the laws of physics what they are, instead of some other way they could be?

Well, there are a lot of theories, but so far we're not exactly sure how to test any of them because we are stuck in this universe and don't yet know how to create universes of our own for experiments. But perhaps there is something we have yet to discover that compels reality to be this way, and only this way.

Or, perhaps, nothing compels it to be this way but instead there's a multiverse. Every single possible universe exists. Some of them collapsed immediately, some of them are empty and dark, some of them involve you watching this very video but Earth has two Suns. For those of you in that universe watching, that was probably very confusing what I just said because your Earth does have two Suns.

But the point is, the multiverse would be a pretty elegant explanation. Or maybe universes are born inside black holes. Universes whose physical laws are only slightly different from the universe that gave birth to them. In this way, universes could replicate according to natural selection. Universes that are more fit for creating black holes would then have more baby universes, making universes that support black holes more common, like maybe the one that we're in.

And that's pretty lucky because universes that support black holes support Suns, and us. Or - and here's my favorite theory - maybe we're just living in a simulation. And someone or something else programmed it up. That sounds pretty sci-fi. Sure. But I like how Julian Baggini puts it in "The Pig That Wants to Be Eaten". If you assume that at some point in time, virtual reality could become good enough that even its own inhabitants didn't know they were in a simulation, and if you assume creating such simulations would be way easier than creating real flesh-and-blood creatures that require an entire universe and billions of years.

Well, then it's pretty easy to conclude that many, many, many, many more simulated, faked beings could be created than real ones. Say, 999 fake ones for every one real one. What's more likely? That you're part of the 0.1% that are real, or that you are part of the 99.9% who think they're real but aren't. Perhaps we live in a simulation created by some other intelligent species.

John Gribbin, however, points out one possible way to dismiss this idea: irrational numbers. Like the square root of 2 or Pi. These numbers don't end. I mean, they contain a never-ending, never-repeating sequence of digits, which would mean that whoever programmed this universe could fit in all of, say, Pi. So if in our tireless pursuit of calculating more and more and more digits of Pi, we ever run into an endlessly repeating series or the end, that could be a pretty good sign that we live in a simulation. A universe that is not real.

But so far we haven't found that, and we don't think that we will. So, thank you irrational numbers, for keeping it real. And as always, thanks for watching.