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. Want to support the channel? Join the Big Think   members community where you get  access to videos early ad free.