Name: Does the Past Still Exist?
Uploaded: 2024-11-24T09:55:15.000Z
Duration: 16 min 7 s
Description: Thousands of YouTube videos with English-Chinese subtitles! Now you can learn to understand native speakers, expand your vocabulary, and improve your pronunciation...

One of the biggest mysteries of our existence is also one of the biggest mysteries of physics.

We experience time as passing with a special moment that we call now.

Half an hour ago you were doing something else.

Whatever you did, there's no way to change it.

And what you will do in half an hour is up to you.

But what physics tells us about time is very different from our perception.

The person who figured this out was none other than Albert Einstein.

What did Einstein teach us about the past, the present and the future?

The topic we're talking about today is covered in more detail in my new book Existential Physics which will be published in August.

You find more info about the book at existentialphysics.com We think about time as something that works the same for everyone and every object.

If one second passes for me, one second passes for you, and one second passes for the clouds above.

This parameter labels how much time passes and also what we all mean by now.

Hermann Minkowski was the first to notice that this may not be quite right.

He noticed that Maxwell's equations of electrodynamics make much more sense if one treats time as a dimension, not as a parameter.

Just like a ball doesn't change if you rotate one direction of space into another, Maxwell's equations don't change if you rotate one direction of space into time.

So, Minkowski said, we just combine space with time, to a four-dimensional space-time, and then we can rotate space into time, just like we can rotate two directions of space into each other.

And that naturally explains why Maxwell's equations have the symmetry they do have.

It doesn't have anything to do with electric and magnetic fields, it comes from the properties of space and time themselves.

I can't draw a flower, let alone four dimensions, but I can just about manage two straight lines, one for time and the other for at least one dimension of space.

If you just stand still, then your motion in such a diagram is a straight vertical line.

If you move at a constant velocity, your motion is a straight line tilted at some angle.

So, if you change velocity, you rotate in space-time.

The maximal velocity at which you can move is the speed of light, which by convention is usually drawn at a 45 degree angle.

In space, we can go forward, backward, left, right, or up and down.

In time, we can only go forward, we can't make a u-turn, and there aren't any driveways for awkward three-point turns either.

So time is still different from space in some respects.

But now that time is also a dimension, it's clear that it's just a label for coordinates.

There are many ways to put labels on a two-dimensional space, because you can choose your axes as you want.

Once you have made time into a dimension, the labels on it don't mean much.

So what, then, is the time that we talk about?

What does it even mean that time is a dimension?

That could explain the strange sounds you've been hearing at night?

No, that's a separate problem I'm afraid I can't help you with.

It was Albert Einstein who understood what this means.

If we also want to understand it, we need four assumptions.

The speed of light in a vacuum is finite, it's always the same, nothing can go faster than the speed of light, and all observers' viewpoints are equally valid.

This formed the basis of Einstein's theory of special relativity.

Oh, and also, the observers don't have to exist.

I mean, this is theoretical physics, so we're talking about theoretical observers, basically.

So if there could be an observer with a certain viewpoint, then that viewpoint is equally valid as yours.

What do you need to observe to deserve being called an observer, and what do you have to observe with?

Believe it or not, there's actually quite some discussion about this the scientific literature.

We'll sidestep this interesting discussion and use the word observer the same way that Einstein did, which is a coordinate system.

You see, it's a coordinate system that a theoretical observer might use, dolphin or otherwise.

Yeah, maybe not exactly what the FBI thinks an observer is, but then if it was good enough for Einstein, it'll be good enough for us.

So Einstein's assumption basically means any coordinate system should be as good as any other for describing physical reality.

These four assumptions sound rather innocent at first, but they have profound consequences.

The speed of light is finite, and nothing goes faster than light.

You're probably watching this video on a screen, a phone or laptop.

Unless you're from the future watching this video as a hologram in your space house, I'm going to assume the answer is yes.

But a physicist might point out that actually, you don't know.

Because the light that's emitted from the screen now hasn't reached you yet.

Also, if you are from the future watching this as a hologram, make sure to look at me from the right.

Maybe you hold the phone in your hand, but nerve signals are ridiculously slow compared to light.

If you couldn't see your hand and someone snatched your phone, it'd take several microseconds for information that the phone is gone to even arrive in your brain.

So how do you know your phone is there now?

One way to answer this question is to say, well, you don't know, and really you don't know that anything exists now other than your own thoughts.

I think, therefore I am, as Descartes summed it up.

This isn't wrong, I'll come back to this later, but it's not how normal people use the word now.

We talk about things that happen now all the time, and we never worry about how long it takes for light to travel.

Why can't we just agree on some now and get on with it?

I mean, think back of that space-time diagram.

Clearly, this flat line is now, so let's just agree on this and move on.

Okay, but if this is to be physics rather than just a diagram, you have to come up with an operational procedure to determine what we mean by now.

Einstein did just that in what he called Gedankenexperiment, a thought experiment.

He said, suppose you place a mirror to your right and one to your left.

You and the mirrors are at a fixed distance to each other, so in the space-time diagram it looks like this.

You send one photon left and one right and make sure that both photons leave you at the same time.

Then you wait to see whether the photons come back at the same time.

If they don't, you adjust your position until they do.

Now remember Einstein's second assumption.

This means, if you can send photons to both mirrors and they come back at the same time, then you must be exactly in the middle between the mirrors.

The final step is then to say that at exactly half the time it takes for the photons to return, you know they must be bouncing off the mirror.

You could say, now, at the right moment, even though the light from there hasn't reached you yet.

It looks like you've found a way to construct now.

Suppose you have a friend who flies by at some constant velocity, maybe in a spaceship.

Her name is Alice, she's much cooler than you, and you have no idea why she's agreed to be friends with you.

But here she is, speeding by in her spaceship left to right.

Subtitles ListPlay Video

Does the Past Still Exist?

extremely

present

constant

basically