The mind-bending physics of time | Sean Carroll

Summary

The video discusses the concept of time, noting that while we talk about time constantly, the notion poses many philosophical puzzles. It explores how physics theories don't distinguish between past and future, yet we perceive time's arrow. This arrow arises from the low-entropy state of our universe at the Big Bang. While complex life requires increasing entropy, the journey from simplicity to complexity allows intricate systems like life to temporarily emerge. Ongoing research investigates how complex structures like life developed amid increasing entropy.

Takeaways

• 😀 Time is just a label we use to differentiate events, not a fundamental puzzle.
• 😮 Physics theories don't distinguish between past and future, but we do in daily life.
• 🤔 The arrow of time may arise from the low entropy state of the early universe.
• 😕 Increasing entropy allows complexity and life to exist.
• 🧐 Complex structures like life arise during entropy increase, not despite it.
• 🤨 We don't fully understand how complex life emerged from simple chemistry.
• 😯 Without increasing entropy, nothing interesting would happen in the universe.
• 😲 Our perception of past, present and future arises from entropy increase.
• 😀 The early universe was simple and low entropy, the far future simple and high entropy.
• 🤓 The journey from low to high entropy allows intricate structures to exist.

Outlines

00:00

🕑 Our Perception and Understanding of Time

This paragraph discusses how we commonly use and talk about time in everyday life without thinking deeply about what time actually is. It notes that the real puzzles come when we consider time's properties like past, present and future and how they differ. Questions are raised about why we can affect the future but not the past, whether time travel is possible, and why there is an asymmetry and directionality to time.

05:02

⏳ Entropy and the Arrow of Time

This paragraph explains how entropy, a measure of disorder, tends to increase in isolated systems over time. It states that the arrow of time emerges from the low-entropy conditions in the early universe after the Big Bang. Entropy has steadily increased since, which provides an explanation for the observed difference between past and future. However, the initial low-entropy state remains unsatisfactorily explained.

Keywords

💡time

The script focuses a lot on the concept of time - what it is, its properties, and mysteries around it. The narrator notes that while we use the word 'time' constantly in everyday life, the actual nature of time is quite puzzling philosophically and scientifically. He discusses phenomena like the perceived directionality or 'arrow' of time, differences between past, present and future, and whether time travel could be possible.

💡arrow of time

This refers to the observed one-way direction or 'flow' of time from past to future. The narrator notes that while our everyday experience has a clear distinction between past and future, physics does not actually distinguish between them at a fundamental level. He explains that the arrow of time is due to the low entropy starting point of the universe at the Big Bang.

💡entropy

Entropy is a measure of randomness or disorder in a system. The video explains that there is a natural tendency for entropy to increase in the universe over time. The initially low entropy state of the early universe helps explain the arrow of time and why we remember the past but not the future.

💡past hypothesis

This is the idea proposed by philosopher David Albert that the low entropy starting point of our universe at the Big Bang explains the perceived arrow of time. The script notes this helps explain why entropy increases over time in our everyday experience.

💡complex structures

The narrator discusses how complex ordered structures like living organisms can arise and exist despite the overall increase in entropy in the universe. He notes that entropy needs to increase for these intricate systems to form and exist, even though their origins are still scientifically puzzling.

💡thermal equilibrium

This refers to a state where entropy is maximum and everything is uniformly spread out and unchanging.

Highlights

Time helps us tell the difference between one moment and another.

Many questions about the nature of time are confusing, but what time is isn't one of them.

Our best theories of physics do not distinguish between the past and future.

The arrow of time exists because we live in the aftermath of the Big Bang.

Entropy is how disorganized and random a system is.

There is a natural tendency for entropy to increase in the universe.

The low entropy early universe explains the arrow of time.

Complex structures need increasing entropy to exist.

Life exists due to increasing entropy, not despite it.

In between low and high entropy, complex things like life can exist.

We don't fully understand why complex structures exist.

Information may play a role in how complexity arises.

If entropy weren't increasing, nothing interesting would happen.

Complexity arises between order and chaos.

Without increasing entropy, there is no memory or causal effect.

Transcripts

- Lexicographers will tell you that time,

the word, "T-I-M-E,"

is the most used noun in the English language.

We can't get through the day

without talking about time all the time.

I think about how we use time,

when we actually talk about it.

If you say, "Meet me at 7 p.m.,"

no one panics.

No one says like, "Oh my God, what are you talking about

with these esoteric concepts about 7 p.m.?"

We all know what to do operationally.

Time, in some sense,

is just a label on different events in the Universe.

The Universe happens over and over again

at different things we call moments,

and time helps us tell the difference

between one moment and another.

So what time is, I don't think is the problem.

The issue-

the real puzzles-

come about when we talk about the properties

that time has.

We have a past, we have a present, we have a future.

How are they different?

Are we moving through it?

We have memories of the past,

but we have no memories of the future.

Why is that?

Where does that asymmetry come from?

Why are we all born young?

Why do we all inevitably age?

Why do we think

that we can affect the future but not the past?

Could we possibly travel back into it?

Anyway, there's a lot of questions about the nature

of time that are really confusing

and many of them we don't know the answer to,

but what time is,

I don't think it's one of them.

One of the most noticeable features

of time is that it has a direction, right?

That there's a difference between the past and future.

Sometimes we think about this

as just an intrinsic feature of reality.

Like the past already happened, it's in the books-

the future is up for grabs.

It hasn't happened yet, and the present is where we live.

But then, along comes physics.

And what people notice about our best theories

of physics is that those theories do not distinguish

between the past and the future.

But in our everyday lives, nothing is more obvious.

It really requires a bit of mental discipline to say,

"Well, time could exist without an arrow."

And one way of thinking

about that is there is no intrinsic arrow of space,

but there's still space, okay?

We live in a three-dimensional world-

up, down, left, right, forward, backward-

at the level of the fundamental laws of physics,

there's no special direction in space.

And how you perceive that is imagine you're an astronaut:

you're flying around in your little spacesuit.

There wouldn't be any difference

between any direction you could look.

There's no experiment you could do

in physics that would point

out a direction in the universe, but space still exists.

Likewise, time would still exist even

if there wasn't an arrow.

But here on Earth, we do have an arrow of space.

If I pick up a coffee cup and let it go,

it will always fall down.

There's clearly a distinction between up and down.

No one is tempted to think that's a fundamental feature

of the Universe.

It's not because downness is embedded

in the laws of physics.

It's because we live

in the vicinity of an influential object-the Earth.

The arrow of time is exactly the same way.

We in our everyday lives, perceive an arrow of time

because we live in the aftermath of an influential event:

the Big Bang.

And that gets us

into a realm of the concept of 'entropy.'

Entropy is how messy, how disorganized,

how random a system is.

When things are nice and neat and tidy,

they are low entropy.

When they're all messy

and all over the place, they're high entropy.

And there's a natural tendency of things

in the Universe to go from low entropy to high entropy.

This is called the 'second law of thermodynamics.'

The real question is:

Why was the world ever low

entropy to begin with?

Why was the world lower entropy yesterday than it is today?

The explanation is not completely satisfying, to be honest.

The explanation is the following:

because it was even lower entropy the day before yesterday.

And why was the Universe even lower entropy the day

before yesterday?

Because it was even lower entropy the day before that.

And this chain of reasoning goes back 14 billion years

to the Big Bang, to the origin of our observable universe;

in a hot, dense state, a very low-entropy state,

and the Universe has been increasing in entropy ever since.

And this is called the 'Past hypothesis' by philosophers-

David Albert, who's a philosopher of physics,

gave it this name.

So now we say, "If you know that the world is made of atoms,

and you know what entropy is,

in terms of rearranging all those atoms,

and you know the past hypothesis-

that the entropy of the universe started really low-

then you can explain everything that happened after that.

There's a way of talking

about human life and entropy, which I think is misguided,

which is that we should think about life.

You know, literally living, being a biological organism,

taking in food and everything,

as a fight against increasing entropy.

I think that's wrong.

I think that we owe life

to the fact that entropy is increasing,

because what would it mean if entropy were not increasing?

It would mean that nothing is happening.

Nothing interesting is taking place.

Without entropy increasing,

there's no memory of the past.

Without entropy increasing,

there's no causal effect that we have on the future.

You'd just be in what we call 'thermal equilibrium.'

Everything would be the same everywhere.

It would be the maximally boring universe.

But what we do have as a scientific question is:

'Why do complicated complex structures come

into existence at all?'

It's clear that they need increasing entropy to exist,

because if entropy were already maxed out,

there would be no complexity.

But that doesn't mean they have to come

come into existence.

Think about a famous example there:

The perfume is all in little bottle.

It's in a big room.

You open it, and it all floats through the room.

The entropy of the perfume increases.

But if you think about it,

when the perfume is all in the bottle, it's very simple.

Once it's all spread through the room

it's also very simple.

It went from low entropy to high entropy,

but it went from simple to simple.

It's the journey from the simple, low-entropy starting point

to the simple, high-entropy ending point,

that there's a large space

of possibilities where things can be intricate.

There's more perfume here over there.

There can be swirls caused by the motion

of the wind in the room and so forth.

The Universe is just like that.

Our Universe started out simple and low entropy.

In the future, the stars will die,

the black holes will evaporate.

It'll be dark, empty, and again, simple, but high entropy.

It's in between that things like us-

complicated, intricate systems that feed

off of the increasing entropy

of the Universe-

can and do come into existence.

We don't know the whole story there.

I think it's a very fun, active, scientific research area:

Why did complex structures like living beings come

into existence and exactly the way we did?

What is the role of information?

What is the down-to-Earth chemistry that is going on here?

What is the geology that is going on here?

Could it happen on other planets?

Very interesting questions- but one thing I do know

is that if entropy weren't increasing along the way,

none of it would've come to pass.