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  • 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 a 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 accelarating, 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 lights' source doesn't push it faster.

  • The moving ships 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 it exactly what the crew the stationary ships 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 then 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 with 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 travelling

  • 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 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 travelled

  • 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 past for it.

  • But a massless vehicle, travelling 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 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 say

  • 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 it.

  • 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 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.

Hey, Vsauce. Michael here. If you were driving in a car

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