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.