What Newton and Einstein agreed on that our society doesn’t | Sean Carroll

One thing that is true even in Newtonian ways of thinking about space and time or

in Einsteinian ways of thinking about space and time is that these fundamental laws work forward

and backward in time. Knowing everything about the universe at one moment predicts the past

as well as the future. That's because what we think of as the fundamental laws of physics do

not have a directionality to time. They treat the past and future the same. But there's clearly a

direction to time in the world. I remember what I was doing yesterday. I might guess what I'm going

to do tomorrow, but I don't remember it in the same way. I have no photographs or memory books

of the future. What is going on with that? And the answer is it's not the fundamental laws of

physics. It's the collective behavior of many, many things in the universe that start out in a

special state. I'm Sean Carroll. I am a physicist and philosopher at Johns Hopkins University,

host of the Mindscape podcast, and also author of a bunch of books, most recently the biggest

ideas in the universe series, including spacetime in motion and quant and fields.

What is the nature of time? Isaac Newton, you may have heard, was a smart fellow. And one of the interesting things when you

invent a whole new way of doing physics, if you're right and it becomes successful, then later on

people kind of take it for granted. They're like, "Yeah, this is how the world works or whatever."

But at the time, you're still very careful. And and Newton was himself super duper careful about

all the assumptions that went into his theory and what their implications were and so on.

One part of classical mechanics is the idea of space and time both separately existing and being

absolute. There is a meaningfulness to that. There is no preferred position in the universe. You can

be anywhere you want. The laws of physics work the same. There's not even a preferred velocity to the

universe. This was figured out by Galileo and Newton kind of took it on board. If you started

everything moving at 1 mile per hour to the left, the world will look exactly the same. There's no

actual frame of rest that you can measure. But there is space and there is time and everyone

agrees on what those two things mean. When I say I am one mile away from a certain other point,

everyone in the universe agrees you are one mile away. Yes, that is correct. When I snap my fingers

and say at the moment I snap my fingers a certain thing is happening in Los Angeles everyone agrees

that indeed at that moment that's a well- definfined concept to say what is happening

at some point far away not just Los Angeles but Alpha Centtory or the Andromeda galaxy. Turns out

those assumptions are not quite right and it was a journey to get there as it often is. It started in

the 1800s with the invention of electromagnetism. So there are all these new phenomena that people

were thinking about since Ben Franklin flew his kite and studied lightning coming down. It

was James Clark Maxwell who put the whole story together after work by people like Faraday and

Aier and so forth. And what he realized is there's two fields pervading the universe, an electric

field and a magnetic field. and he wrote down some equations that these fields obey and they sort of

play with each other and push around charged particles and things like that. People were

very happy at the existence of electromagnetism. They started thinking about what it all meant and

what they realized is that the sort of way that space and time are treated in Maxwell's theory

of electromagnetism is different than the way they are apparently treated in Newton's theory.

In particular, Maxwell's equations predicted a special velocity. There's no special velocity in

Newtonian mechanics. Every velocity is created the same. Maxwell says there is something called the

speed of light. It is the speed at which waves in the electromagnetic fields move. And naively, you

look at the equations and everyone measures the same value for the speed of light. It's a constant

of nature. How can it possibly be the case that everyone measures the same speed for light even if

they're moving with respect to each other? So for a long time, for decades, people physicists bashed

their heads against this problem. They came with very elaborate schemes to get rid of it. And it

was Einstein, Albert Einstein in his great paper in 1905 who first said you should get rid of the

idea of these waves traveling through a medium. You should think of the electromagnetic waves as

really being the thing that exists. And when the equations tell you everyone measures the speed

of light the same, that's because they do. Take that seriously. All you have to do is entirely

rejigger your thoughts about what space and time are. And in fact, it wasn't until two years later

when Herman Mmanovsky, who was a mathematician who had been one of Einstein's professors, said,

"You know, the right way to think about Einstein's theory is to say that space and time aren't

separate anymore. To imagine there's one thing called spacetime, and different people, different

observers moving in different ways through the universe will divide it up into space and time

differently. There's no objective true fact about when I snap my fingers now what's happening light

years away. That's going to depend on who's doing the observing and who is doing the measuring. It

can all be explained very beautifully by imagining a single four-dimensional spaceime instead of

separate space and time. Einstein himself was not impressed by this move. Einstein was a hilarious

character because he was a physicist's physicist. He was very mathematically adept. You know, don't

believe the stories that Einstein wasn't good at math in school. He was very good at it, but

he wasn't in it for the math. He was in it for the physics. So, he learned as much math as he needed.

And when Benovsky says, "I have some new math that unifies space and time based on Einstein's

theories," Einstein himself is like, "I don't need that. That's like extra mathematical nonsense."

He soon changed his mind because it turns out that that move from space and time being separate to

being combined is super useful going forward, including 10 years later, he would invent his

general theory of relativity that include gravity into the space-time story. When Einstein put

together what we now call the special theory of relativity, the idea that there's no preferred

standard of rest in the universe, but also everyone thinks the speed of light is the same.

All you have to do is imagine ultimately that space and time are glued together. That was a

radical reworking of the framework of physics. You know, Newton's idea of separate space and separate

time absolute and agreed upon by everyone had been there for hundreds of years. And when you do that,

when you say, okay, I'm going to completely invent space and time in part because I want

to match this wonderful theory. We have Maxwell's theory of electricity and magnetism. You have to

go back to everything that was a success in your previous way of doing things and say does it still

work? The biggest success of Newtonian classical mechanics was gravity. The famous inverse square

law of gravity. Newton posited that if you have two objects with two different masses,

they have a gravitational force that will pull them together that diminishes as one over the

square of the distance between them. And that simple rule plus the framework of Newtonian

mechanics is enough to match exactly what you see in the sky in terms of the planets moving around.

It's enough to launch a rocket and get it to the moon. So Einstein comes along and says, "Well,

okay, can I make a version of Newton's theory of gravity that is compatible with my new theory of

special relativity?" And after trying, he said, "No, I can't. You have to do something much more

dramatic." And what he realized is that gravity is a special force of nature. You know, Maxwell talks

about electricity and magnetism. If I want to know what the electric field is at one point in space,

it's very easy to do. I put a positively charged particle, a negatively charged particle,

they get pushed in opposite directions by the electric field. But Einstein realized that

every particle reacts the same way to gravity. If I have a very heavy particle and a very light

particle and I drop them, Galileo showed that they drop at exactly the same rate. They're not pushed

around in a different way. So because of that, gravity seems to disappear if you only look at it

in a tiny region of the universe. If you were in a sealed box and you were dropping things and going,

"Oh, I have gravity here." You don't know that for sure. Maybe you're in a rocket ship and the rocket

is accelerating and you're being tricked into thinking you have gravity. So Einstein, because

he's Einstein, he's very smart. You know, you or I would go, "Huh, that's interesting." But he says,

"I think what that means is that gravity is not a force on top of spacetime. It's a feature of

spacetime itself." What feature could it be? Well, my ex-professor Minkovsky says that spacetime has

a geometry. It's one combined thing, and there are equations telling me how particles move in it.

Maybe that geometry is curved. Maybe it's not like a flat tabletop like uklitian geometry. Maybe it's

warped and bent and dynamical and changes in response to the existence of mass and energy

and things like that. It's a it's a good idea to have. It takes you a lot of effort and a lot of

mathematical work to figure it out. So 10 years later in 1915, Einstein finally completes what

we call the general theory of relativity. In the general theory of relativity says spaceacetime

is a four-dimensional thing. That four-dimensional thing has a geometry. It's pushed around by matter

and energy. And we experience the curvature of spacetime as the force of gravity. When Einstein

and Minkovsky figured out that space and time are both two different ways of slicing up spaceime,

what does that mean? What does that mean like in our guts, right? What does it visually

or measurably imply? You know, in space there's something called the distance between two points.

If you say, you know, I'm here in Washington DC and a friend of mine is in Los Angeles, there's

a distance between those two cities and we all agree on what that distance is because implicitly

we're imagining the shortest distance path, right? The straight line that connects these two points.

But of course, if you actually travel between these two cities, you won't exactly necessarily

take that much distance because you're going to go right and left. You're not going to go exactly

on a straight line. So in space, we're all very very used to the idea that different paths have

different lengths, even if they start and end at the same point. In special relativity, now that

space and time are unified, what that means is that time is like that. The time you personally

measure on your wristwatch is very analogous to the distance that you travel moving on some

path. What that means is that rather than being a universal thing that everyone agrees on, time

depends on the trajectory you take through the universe. The most famous example of this is the

twin paradox. You imagine two twins. They don't have to be twins, but it's more vivid if they are

because you think of twins as being the same age. Okay? One twin just doesn't move, just stays home.

This is the lazy twin. And they get older like the rest of us all do. The other twin hops in a

rocket ship that moves out very close to the speed of light. You need to move close to the speed of

light to feel the effects of relativity and then comes back. And so they left at the same time.

They were the same age. They come back to the same point in space and the same point in time. But the

twin who traveled is now younger. The twin who traveled has experienced less time than the twin

who stayed home. And the reason why is because they took different paths through spaceime.

Space and time are similar to each other but not exactly the same. That's why in space the shortest

distance path is a straight line. But in time the longest time path is the straight line. The twin

who stays stationary and doesn't move, that's moving in a straight line through spacetime,

that's the one that feels more time pass before the other twin comes back. When people hear

this stuff about relativity and moving through space and things, what they want to say is, "Oh,

so you're saying that time moves more slowly when you're traveling near the speed of light?" No,

I do not want to say that. I very much do not want to say that. What is the rate at which time moves?

It is 1 second per second. You're being tricked by your use of the English language because you

move through space and it makes perfect sense to say I am moving at 1 meter/s or 2 meters/s

or whatever. The rate at which you move is the number of distance you travel per unit time. But

the amount of time you travel per unit time is always one. Now that accumulated time along two

different trajectories can be different. That's the origin of something like the twin paradox. Or

when gravity comes into the game, the amount of total time you experience will be less if you're

deep in a gravitational field than if you're out there in interstellar space where gravity is not

that important. So in general relativity, being in a strong gravitational field is much like

moving out there close to the speed of light. If you had someone stay back here on Earth,

someone else go near a black hole, for example. A black hole is the strongest kind of gravitational

field you can have. Don't go in to the black hole because then you can't come back out. But

if you go near it and then you come back, you will be younger than the person who just stayed

behind. You will have experienced less time. Your wristwatch is still clicking at one second

per second, but the accumulated amount of time is different because you have taken a different path

through curved spacetime. In Interstellar, the Christopher Nolan movie, this was wonderfully

illustrated. Kip Thorne, who is a Nobel Prize winning physicist, was the executive producer

and one of the instigators of that movie, and he put all of his physics knowledge in there about

wormholes and black holes and gravity and time travel. So up until the very last scenes when

they're in the library and everything goes haywire, all the physics in that movie is

completely respectable. One thing that is true even in Newtonian ways of thinking about space

and time or in Einsteinian ways of thinking about space and time is that these fundamental laws work

forward and backward in time. Knowing everything about the universe at one moment predicts the past

as well as the future. That's because what we think of as the fundamental laws of physics do

not have a directionality to time. They treat the past and future the same. But there's clearly a

direction to time in the world. I remember what I was doing yesterday. I might guess what I'm going

to do tomorrow, but I don't remember it in the same way. I have no photographs or memory books

of the future. I was younger. I will always be older in the future. What is going on with that?

And the answer is it's not the fundamental laws of physics. It's the collective behavior of many

many things in the universe that start out in a special state. It goes back to the idea of entropy

from the 1800s. The idea of the disorderliness of a system, the randomness, the disorganization. And

entropy increases with time. That's the famous second law of thermodynamics. Why does entropy

increase with time? Because there are more ways for a system to be arranged in a high

entropy configuration than a low entropy one by definition. And the universe started in a very

special low entropy state. Nobody knows why that is true. This is a mystery to cosmology. But the

entropy of the universe was very very low to start and it's been increasing ever since. and us having

memories of the past but not the future. The ability to have records, the fact that we age in

the same direction. This is all because entropy is increasing in one direction rather than the other.

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