Time, Relativity, Spacetime and the Arrow of Entropy

Name: What Newton and Einstein agreed on that our society doesn’t | Sean Carroll
Uploaded: 2026-05-12T13:00:11+00:00
Duration: 16 min 56 s
Channel: Big Think
Description: Summary and key takeaways on Time, Relativity, Spacetime and the Arrow of Entropy, covering The Nature of Time and Physics Newtonian mechanics treats space and

Big Think

May 12, 2026

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Newtonian mechanics treats space and time as absolute, separate entities that every observer agrees upon. Maxwell’s equations introduced a constant speed of light, contradicting the Newtonian view that all velocities are relative. Einstein’s 1905 paper required the speed of light to be constant for all observers, forcing a fundamental re‑thinking of space and time. Hermann Minkowski then combined space and time into a single four‑dimensional “spacetime,” allowing different observers to slice the dimensions in distinct ways.

Gravity and Geometry

Newtonian gravity cannot be reconciled with Special Relativity, prompting Einstein to seek a new description. Gravity is unique because every particle responds to it identically, regardless of mass. General Relativity (1915) states that gravity is not a force acting on spacetime; instead, mass and energy curve spacetime itself, and objects follow the resulting geometry.

The Experience of Time

Time is not universal; it depends on the trajectory an observer follows through spacetime. The twin paradox illustrates that a traveler moving at high speed or near a strong gravitational field accumulates less elapsed time than a twin who stays at rest. Time does not “slow down” in the sense of a changing rate—the rate of time remains 1 second per second; only the total accumulated time differs along different paths.

“What is the rate at which time moves? It is 1 second per second.”

The Arrow of Time

Fundamental physical laws are time‑symmetric, working equally well forward and backward. The apparent directionality of time—memory of the past, aging, and the flow of events—arises from the Second Law of Thermodynamics, which states that entropy increases with time. High‑entropy states are statistically more probable than low‑entropy ones, so entropy naturally grows. The universe began in a very special low‑entropy state, a mystery that remains central to cosmology.

“The universe started in a very special low entropy state. Nobody knows why that is true.”

Quotable Insights

“There's no objective true fact about when I snap my fingers now what's happening light years away.”
“Gravity is not a force on top of spacetime. It's a feature of spacetime itself.”
“The twin who traveled is now younger. The twin who traveled has experienced less time than the twin who stayed home.”

Takeaways

Newtonian physics treats space and time as absolute, but the constant speed of light forces a unified four‑dimensional spacetime.
General Relativity describes gravity as the curvature of spacetime caused by mass and energy, not as a separate force.
Time dilation arises from different spacetime trajectories, as shown by the twin paradox, while the rate of time stays 1 second per second.
Fundamental laws are time‑symmetric, but entropy’s steady increase creates the observed arrow of time.
The universe began in an exceptionally low‑entropy state, a condition that remains unexplained in modern cosmology.

Frequently Asked Questions

Why does gravity act as curvature of spacetime rather than a force?

Gravity appears as curvature because all particles respond identically to mass‑energy, making a universal force unnecessary. General Relativity models this by letting mass and energy bend spacetime, and objects follow the resulting geometric paths, eliminating the need for a separate force acting on spacetime.

How does entropy explain the arrow of time?

Entropy increases because high‑entropy configurations are statistically more likely than low‑entropy ones. This statistical tendency gives a preferred direction to processes, producing the experience of time flowing from past to future even though underlying physical laws are time‑symmetric.

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Helpful resources related to this video

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A Brief History Of Time Stephen Hawking Recommended

This classic book provides a foundational exploration of spacetime, relativity, and the arrow of time, mirroring the lecture's core themes.

Amazon →

The Big Picture Sean Carroll

Written by the lecturer himself, this book expands on the relationship between entropy, the arrow of time, and the laws of physics.

Amazon →

Einstein's Relativity And The Twin Paradox Book

Provides a detailed breakdown of the mechanics behind time dilation and the twin paradox mentioned in the lecture.

Amazon →

Entropy And The Second Law Of Thermodynamics

Explains the statistical nature of entropy, which is the primary driver for the arrow of time discussed in the presentation.

Amazon →

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Full Transcript YouTube

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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May 15, 2026

Gravity and Geometry

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The Arrow of Time

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