Katie Mack: Life-altering questions about the end of the universe | TED

Lily James Olds: Hi, Katie, welcome.

Katie Mack: Thank you. Thanks for having me.

LJO: So happy to have you.

I would love if, for those of us who are not astrophysicists,

you could return and help us give a little refresher

on how the universe did begin and how we know that.

KM: Right, right, yeah.

So we know actually quite a lot about the early universe,

about the beginning of the universe,

because we can actually see it.

And this is the wildest part of astronomy,

that we can see the beginning of the universe.

So the universe is about 13.8 billion years old,

and when we look out into the cosmos, we see distant galaxies.

And when we look at the distant ones, they're all moving away from us.

And so for a long time, there's been this idea that, well,

if the galaxies are moving away from us now,

they must have been closer in the past.

The universe in the past must have been smaller in some sense,

hotter and denser,

everything packed into less space.

And that's the Big Bang theory,

the idea that the universe was smaller and denser and hotter in the past.

And we got really direct evidence of that in the 1960s

when we're able to actually see the light

from the very early universe.

So let me take one more step back.

When we look at a distant galaxy,

the light from that galaxy takes some time to reach us.

So we see, you know, we see a galaxy shining.

That light might have taken a billion years to cross the space

between there and here.

We can see galaxies that are so distant

that the light took 10 billion years, even 13 billion years to reach us,

and the universe is only 13.8 billion years old.

So what happens if you look at something so far away

that the light has taken more than, you know,

more than 13 billion years to reach us?

What happens when you try and look at something even farther?

Well, there's a limit to how far you can look,

the observable universe,

and that limit is defined by how long it takes light to travel.

So if something is so far away

that the light would take 15 billion years to reach us,

we can't see it because the light hasn't gotten here yet.

But if we look at something that's, you know, so far away,

the light's taken 13.8 billion years to reach us,

then what we're looking at is a time when the universe was just beginning.

We're looking at the light from the very beginning of the universe

and what we should see, if we look at something that far away,

is fire, right?

So we take this idea

that the early universe was hot and dense,

everywhere in the cosmos was, like, filled with this sort of roiling plasma.

And so if we look far enough away, we should see it,

because we're looking so far back in time

that we're looking at the time when the whole universe was on fire.

And we do see that shockingly,

we actually do see that.

When we use microwave telescopes,

we see this background light every direction we look.

You know, at the edges of our vision, is this heat, this fire,

and we know that it's heat,

we can analyze the spectrum of the light

and we can see that this microwave light,

this radiation,

is the kind of light you get when something is just glowing

because it's hot.

And so we can see that every direction we look,

if we look far enough away, we’re looking so far back in time

that we're seeing a universe that is still on fire.

So that's the Big Bang.

Exactly what happened, you know, around that time,

how that fire got started,

that's a whole other very complicated story that we're still figuring out.

So we think that, you know,

before the fiery part there was this inflation, this rapid expansion.

Before that, maybe there was a singularity,

maybe not, we don't know.

We don't know what started that rapid expansion.

But we do know that for the first 380,000 years of the cosmos,

it was this sort of,

all of space was filled with this fire.

And we know that because we can see it.

LJO: It's amazing.

Well, let's get into some of the juicy specifics

of how exactly the universe might end.

I know that you've talked to many other cosmologists yourself

and there are a lot of different theories on this.

Where do you think we should begin?

Dealer's choice.

What's in store for us?

KM: Well, so the one that is, as far as we know, the most likely,

the one that we talk about the most in cosmology, is the heat death.

So this is what I discussed in my TED Talk,

and the idea there is that, you know, the universe is currently expanding.

Galaxies are getting farther and farther apart from each other.

When we measured the expansion,

it turned out that it was not slowing down at all,

it was actually speeding up.

And that was like if you throw a ball up into the air,

it slows down for a little while and then just shoots off into space.

It’s very similar physics,

and we didn't have any idea why that should happen.

So we still don't know why that's happening.

We attribute it to something we call “dark energy.”

We don't know what dark energy is.

It's just something that seems to be pushing things apart,

making the universe expand faster.

And because of that, it looks like we will end up

with everything, really --

you know, all the galaxies really isolated,

the stars will die away.

The universe will get very dark, very cold.

And you know, we'll end up with this basically empty,

cold, dark, lonely universe.

And that's called the heat death.

The reason it's called the heat death

is because, like ...

Everything's decaying into, like, the waste heat of creation.

So, you know, just as you can't have a machine that's perfectly efficient,

it'll always lose a little bit of energy through friction.

That's a property of physics in general,

it's called the second law of thermodynamics.

Everything sort of decays into entropy, into disorder,

and that is called heat from a physics perspective.

So the heat death is when nothing is left but the waste heat of the universe.

Which is part of why it's fun to talk about the alternatives,

because we don't know for sure that the heat death will happen.

Partially because we don't know what dark energy is.

We don't understand this stuff that's making the universe expand faster.

Maybe it's just a property of space where, you know,

space just has this sort of, expansion built in,

and it'll keep going the way it's going.

But maybe it's something that changes over time.

Maybe it'll turn around and we'll get a big crunch

and everything will come back together.

Or maybe it'll become more powerful.

And then you end up with something called a “Big Rip,”

where if the dark energy becomes more powerful,

it starts to not just move galaxies apart from each other,

but actually expand the space in galaxies and move stars away from galaxies

and then pull apart planets and stars

and eventually destroy the entire universe.

So those are other possibilities that I talk about in the book.

Because we don't know what dark energy is,

and we don't know for sure what it'll do in the future.

LJO: I want to open up to some of the questions from the audience.

Vasily asks,

"Have you ever asked the question 'If there were no universe,

what would there be?'

This leads to the question of what will be after the universe ends?"

KM: So I think that gets into tricky questions

of how do you define universe, right?

So you can define universe as being everything,

and then it becomes a less clear question.

What does it mean for something other than everything?

Then, you know, if there is anything else,

it's by definition part of the universe.

But one of the ways we often talk about the universe in cosmology,

is we talk about the observable universe,

where the observable universe is the part of the cosmos we can see,

where the light has had time to reach us since the Big Bang.

So I talked about that before.

The edge of the observable universe is where we see that Big Bang light.

The actual universe,

we think extends far beyond the edge of the observable universe.

The observable universe is just a perspective thing.

It's like a horizon when you're on Earth,

you can only see so far because of where you're standing,

but the Earth keeps going beyond the horizon.

And similarly, with the universe, we're pretty sure that it extends

much, much farther than what we can see, what we can observe.

But we can see the observable universe and we can study,

we can learn about the observable universe,

and we can't get any information about what's beyond it.

So, you know, that brings up things like a multiverse,

where you can have regions of space that are so far away from us

that they’re effectively another universe,

and those regions can have a totally different history,

a totally different future,

different laws of physics even.

So, there are possibilities for things that carry on

long after our observable universe is decayed into entropy

or maybe meets another fate.

And there are even possibilities

where there could be higher dimensions of space,

like directions that we can't conceive,

you know, space that's separated from us by some other dimension of space,

some other direction that we don't, you know,

perpendicular to all of our spatial directions,

which I can't sort of envision.

But mathematically, that makes sense in some ways.

So there are those kinds of possibilities.

And you know, you can get into really weird stuff

about the nature of space and time

with you if you really dig into it.

But in the book,

I really just talk about our observable universe in terms of the fate of that,

because that's all we can really study.

I do talk a little bit about the multiverse

and the possibilities of other parts of space.

But in terms of what happens when our universe is destroyed,

I mean, it depends on how it's destroyed,

whether there’s, you know, the observable universe is over

but there's more space beyond it or not.

And that's all the realm of speculation at the moment.

LJO: So I want to switch gears a little bit,

because one of the articles that you wrote fairly recently

talked about how time and space might not be real,

and how there might be a deeper,

more abstract mathematical reality to the universe,

and that time and space might just be what we perceive.

Can you tell us more about this?

How is this possible?

Talk about your mind doing backflips.

KM: Yeah, yeah, this is really wild.

So I first heard about this a couple of years ago

where somebody was talking about how,

if you do calculations of particles interacting with other particles,

like the kind of stuff relevant to particle collider experiments

where you're slamming protons into each other

and measuring what happens to the particles that come out,

there are ways to do those calculations

where you can kind of put them into an abstract mathematical format

and do the calculation.

And then you get the same answer

as if you do the calculation the usual way,

assuming, you know, it's actually particles moving through space

and interacting with each other in space and time.

And since there are ways to do some of these calculations

without making use of the ideas of space or time,

you just have this sort of abstract mathematical space,

it sort of suggests that maybe space and time are not helping you

and not necessary for understanding how these processes work.

And there is actually a lot that you can calculate in physics

at the sort of, subatomic scale,

where space and time are not salient variables.

They're not part of the calculation.

And you get the right answer when you do that.

And that sort of hints at this idea

that maybe space and time are not the fundamental things

that govern how the universe works,

that you don't have to assume that, you know,

everything happens in a background of a space measured by time.

If you talk to the theoretical physicists who are working in these areas

and are actually doing these calculations, doing these equations,

they will say things like,

"Oh yeah, we've known for years

that space and time are not fundamental."

And you're like, "Wait, what?"

LJO: I missed that memo.

KM: Yeah, no, totally.

And you dig down into it and they say,

"Well, you know, maybe they're emergent."

Maybe it's like, you know, they're sort of real.

Like, we live in space, we experience time.

But the actual, sort of, fabric of the universe

is some other mathematical space that just doesn't map well

to space and time.

That's not the same kind of thing, doesn't follow the same kind of rules.

But in some sense, you know, maybe we are mathematical,

you know, some kind of instantiation of mathematics

rather than objects in space existing in time.

And that's the more fundamental thing.

And it's just that because of our perspective, because of our experience,

we think we see objects in space and time.

In fact, that is not what the universe is really made of.

LJO: I love that.

You know, it turns out you are also a poet.

I don't want to put you on the spot, but I'm wondering,

I really love your poem "Disorientation,"

and I feel like it states this really beautifully, actually.

I was wondering if you'd be willing to read the last few stanzas?

KM: Sure, yeah, I can do that.

Yes, this was a poem I wrote a few years ago,

and I wrote it as a Twitter thread actually,

just because I thought it would be kind of fun.

So each stanza is a tweet.

But it sort of encapsulates how I think about the universe.

So, yeah, this is the last bit.

I want you to believe that the universe is a vast, random, uncaring place

in which our species, our world, has absolutely no significance

And I want you to believe

that the only response is to make our own beauty

and meaning and to share it while we can

I want to make you wonder what is out there.

What dreams may come in waves of radiation

across the breadth of an endless expanse.

What we may know, given time,

and what splendors may never, ever reach us

I want to make it mean something to you.

That you are in the cosmos.

That you are of the cosmos.

That you were born from stardust

and to stardust you will return.

That you are a way for the universe to be in awe of itself.

LJO: I love that.

Thank you so much, Katie.

Thank you for such a thoughtful and engaging conversation.

It's really been such a pleasure.