This is a question Einstein had asked over a hundred years ago.

Now the trouble was, he couldn't actually do the experiment

so he had to use his imagination;

he had to do a thought experiment

to figure out what a reasonable outcome would be.

I could imagine there'd be some element of nausea.

I'd imagine everything would get more intense.

Maybe it would confuse the brain; I don't really know if you'd be seeing very much.

I was on a water slide just a couple of days ago and I found that really difficult.

Einstein's intuition told him that the world should feel the same in all frames of reference moving with constant velocity.

These are called inertial frames of reference.

So it doesn't matter if you're in a train, or on a plane, or in a car, or in a room

As long as you're moving with constant velocity

[All four] The laws of physics should apply in exactly the same way.

And this is what's called the principle of relativity.

–Is there a speed limit in the universe?

–I don't think so.

–You can go as fast as you want?

–Probably

–Could we go as fast as light?

–Ah, I'm sure eventually, absolutely!

–Can you compare the speed of sound and the speed of light?

–Well, it's however many times quicker, isn't it?

because you've got the lightning and thunder.

–Roughly how much quicker is light?

–Ten times.

–Roughly ten times?

–Ten times, I think, is it ten? or a hundred, I don't know.

Something like that.

–The answer for you is—a million.

–Oh!

–Light is a million times faster than sound.(Damn)

–Perhaps thought? or...

–Thought is faster than light?

–Yeah, it's immediate! it's there.

Let's do a little thought experiment.

Let's say I'm on a train travelling the speed of light that way.

Who am I kidding, this is city rail! We're lucky to be moving.(I looking at you Chicago and New York)

But, let's use our imaginations.

So let's say I'm travelling at the speed of light this way.

If I hold up a mirror in front of my face,

will I be able to see my reflection?

Do you see the problem?

In order for me to see my reflection,

light has to bounce off my face and travel to the mirror.

But if the mirror is going forward at the speed of light,

how is the light ever going to catch up?

So, a logical answer would be that

no, you can't see your face in a mirror when you're on a train travelling at the speed of light.

But that violates the principle of relativity because

it means that I can tell I'm in a frame of reference that's going at the speed of light that way.

Aw, crap, not again!

So let's consider the other option.

Maybe I do see my reflection

which means the light must travel from my face, to the mirror,

at its regular speed—the speed of light.

But now the problem is,

someone outside the train would see light travelling at twice the speed of light

because I'm already travelling at the speed of light, and the light is travelling at the speed of light relative to me

if that makes any sense.(Not really)

So, that leads to a really inconsistent answer

because how can the speed of light be twice the speed of light in that frame of reference?

So Einstein's conclusion was that

I must see my reflection; because otherwise, it violates the principle of relativity.

But in order for me inside the train to measure the same speed of light as someone outside the train,

the train must actually contract in the direction that it's moving.

And, my time must tick by slower relative to someone outside.

So Einstein changed what we thought were absolutes—space and time—

and made something that seemed like it should be relative—the speed of light—into the true universal constant. (Mind Blown yet?)

So what would it look like to travel alongside a beam of light?

Well, according to einstein, we'll never know,

because we can't go that fast.

At that speed,

our length would become zero;

time would stop relative to an outside observer;

and perhaps worst of all, our mass would become infinite.(No Mass Effect here guys)

but a recent experimental result has suggested that neutrinos can go faster than light.

Still, most physicists think that, well, this result probably won't hold up to scrutiny

because the principle of relativity is just so well established nowadays.

But who knows! Perhaps one day,

we'll find a way to break what we thought was the universal speed limit—the speed of light.

Subtitles: Thomas Martinez