There is nothing that we experience that does not take place in some duration of time.

So if you can understand time, you're on your way to understanding reality.

Hello.

Hello.

What's your name?

Kayla.

How old are you, Kayla?

I am nine years old.

So if you're nine years old, what does that mean about the earth?

How many times has it gone around the sun?

Nine times.

Nine times.

So there's a relationship between motion through space, the earth is going through space, and the passage of time.

They're kind of connected in some way.

Have you heard of Albert Einstein?

Yes.

What do you know about him?

He has crazy hair.

He does have crazy hair, and I think I may be heading in that direction, actually.

He showed us an approach to travel to the future.

You want me to tell you how you do it?

You build a spaceship, you go out into space really quickly, you turn around and you come back to planet earth, and he showed us that when you're on that ship, your clock will tick off time more slowly.

You will age more slowly.

So that journey may only take you, say, a year, six months out and six months back, but you know what?

When you step out of the ship, it'll be 100 years into the future, or 1,000 years, a million years into the future.

Would you do that if you could?

I would probably be dead by then.

No, you'd be alive.

That's the amazing thing.

I'd be dead.

Everybody else would be dead who stayed on earth, but your body would only age one year, and yet it would be 1,000 years into the future.

That same guy with the crazy hair, Albert Einstein, showed that there's actually a limit to how fast things can go, and you know what the limit is?

The limit is the speed of light, because light travels 671 million miles per hour.

That is fast enough to go around the entire earth seven times in one second.

So if we could build a spaceship that would go as fast as light, we'd be able to do what Einstein noted.

If I was to ask you what is time, what would you say?

Time is kind of strange, because it's almost a man-made idea.

There is the tangible of how the earth revolves around the sun, or how we orbit around ourselves.

It's almost in a way, does it exist if the way that we measure it is man-made?

Before there was any life on planet earth, I think we all agree the universe existed, did things change before there was life on earth?

Yes.

And how would you talk about that change without invoking this concept of time?

It's difficult to talk about something without adding time into it.

Even if it is a human-made concept, that the universe evolved, developed, changed through time, ultimately giving rise to galaxies, stars, planets, and on this particular planet, life.

That conception of time gives a feel that it's universal, that it's out there, it's the same for everybody, it's independent of our actions or activities.

Do you know that Albert Einstein shattered that view of time?

He found that if you and I, say, have identical wristwatches, I'm sitting still and I'm watching you move, I will find that your clock is ticking off time more slowly than my clock.

Back with Einstein, we wondered, does time change with speed?

And that's another change with that before we didn't think possible.

Not only did we find that time changes with speed in special relativity, but we also found that time changes with gravity.

Einstein showed that the rate at which a clock ticks slows down based upon the stronger gravitational field or gravitational potential, actually, that it is experiencing.

I think you mentioned interstellar before.

Do you remember the scene in interstellar?

They're going to a planet that's near a black hole.

They go down to the planet and they spend just a couple hours there.

But when they go back to the ship, it's 23 years later on the ship because time is elapsing slowly near the strong gravitational field, comparatively quickly far away.

And that's not science fiction.

That's actually how time behaves.

I've always heard people say, oh, general relativity, you know, it might not seem applicable, but, you know, GPS due to satellites, we could synchronize those clocks by accounting for relativity.

But that's even a really, really good point.

The GPS would become completely inaccurate in a very short period of time if the satellites weren't taking account of, or the software wasn't taking account of the fact that time elapses differently for the clocks on the satellite compared to the clocks down here on Earth.

So we walk around with general relativity in our pockets, even though most of us perhaps don't really know that.

I've heard a few different theories that people posit.

Like maybe it is back to the future and you really change your own universe.

I've also heard people say that you could have multiple universes spawn from this event or something along these lines.

That's, if you're going to be able to change the past, that's the one that resonates most with me.

I think the same.

Yeah.

So you go into the past and maybe you can prevent your parents from meeting, but you're preventing them from meeting in a parallel reality, which means that you will never be in that reality, but the origin of your birth is still completely understood.

It was in the universe from which you originated.

And when you think about past and future on a cosmological scale, there was a long period when there were no human beings in the universe.

The fact of the matter is there will be these two long stretches with our presence being sort of a flicker in between.

Does that thought inform anything about how you live?

I rage against thinking like that because it might be a brief flicker on a single moat of dust floating in a cosmic eternity, but it's everything.

There's nothing else that I'll ever experience.

And so in a way, there's nothing else to me.

There's an eternity, but I'm never going to see it.

I'm never going to feel it.

It can be debilitating to imagine an eternal future of sort of nothing where none of what we do sort of persists.

On the other hand, if you flip your perspective around and say, how remarkable is it that we have this brief moment that allows us to think and feel and love and explore and illuminate?

Wow.

How wonderful is that?

Yeah.

One of the things that relativity also sheds a light on is what exists.

That if someone's moving relative to me, what they consider now might be in my past.

What they consider now might be in my future, which would suggest that all of time exists.

Does that hold water?

Actually, it resonates with me for various reasons.

So this idea that you have a continuum and time should not be made to disappear as soon as it's gone is very practical.

It's also what is behind the idea of histories in quantum mechanics.

I mean, the only thing that really makes you not fear what will happen or your own mortality is that every instant is eternal.

Absolutely.

It will always be there.

Yeah.

Yeah.