Hakeem Oluseyi: Quantum physics breaks every human intuition | Full Interview

I am Hakeem Oli.

I am an astrophysicist

and author of the new book,

Why Do We Exist?

The Nine Realms of the Universe

That Make You Possible.

Today on Big Think, we're going to discuss three of these realms.

The quantum realm, the cosmological

realm, and the multiverse

>> Chapter one, the strange world of quantum physics.

Quantum physics is weird because it breaks every intuition

of the physics that

we normally experience in our regular world.

The rules break, certainty

breaks, things become probabilistic.

Things come into existence, out of existence,

objects pass through walls. It is strange.

How should we think about quantum particles?

They are not things.

They are not things like the world around us.

Quantum particles are the fundamental

constituents of matter that come together

to build up the world around us.

So protons and neutrons are made up of fundamental quantum particles called quarks.

Light is made up of fundamental quantum particles called photons.

Electrons are a fundamental quantum particle.

And so they come together

in ways that cause

new properties to emerge

that give us a reality

that is fundamentally different from the reality that created them.

When we look at an artist's description of what an atom is, you typically

see something that looks like a solar system in three dimensions, right?

You have these lines of electrons orbiting a nucleus.

But that is not what's happening, right?

Planets that are orbiting the sun, they're basically falling around the sun.

Just like an object falls here in Earth's atmosphere.

It's the same process.

Electrons are not falling around the nucleus of the atom.

And the other thing is that they're not these little tiny spheres that we imagine.

When you look at them at their fundamental basis,

they have a particular property.

And that is is that every electron is identical.

For example, you can't tell one from the other.

So what that tells me is that gives us a hint of what they are.

So I like to make the analogy that they're kind of like musical notes.

If I hear a musical note, a C,

every C is a C. They're identical.

But is the C a real thing?

What the real thing is is the instrument or the voice that vibrated

and created this vibration in the air.

That vibration in the air is a real thing.

But that perception of the sea

is, you know, comes out of that vibration.

And so, how do you create a vibration?

You add energy to the string or to the instrument that you're blowing.

Well, reality does the same thing.

At the most foundational

level, there is this concept of quantum fields.

And these quantum fields

permeate all of spaceime.

And what we call a particle

is energy injected into one of those quantum fields.

We we call electrons

exitations in the quantum electron field.

And you know so

in essence we are a symphony of musical notes in quantum fields.

And what's really interesting is is that if you take that analogy to its extremes, right?

If I have a musical instrument like a guitar that has a string that

vibrates, if I don't pluck the string,

the string isn't vibrating, right? Well, not right.

It turns out that if I zoom in and look at that string really

closely, you'll see that it's always vibrating.

And in the same way,

those quantum fields are always fluctuating.

And that means that there's two types of vibrations.

The ones where you insert energy and get a exitation

that results in a note,

but then it still vibrates at the same frequency, but really tiny.

Those are what we call virtual particles in quantum mechanics.

So they're both can exist

on these quantum fields.

So the thing about quantum fields that makes them really uncomfortable

for me and I thought you know maybe this is just a mathematical

analogy is that they don't have some source

like a magnetic field you think oh here's a star here's a planet

that's the source of the field.

If you have an electric field,

you think, "Oh, here's a

an amount of electric charge.

That electric charge creates that electric field."

And you know, most of the fields that we're normally

concerned with, electric, magnetic,

gravity, they're associated with matter.

These quantum fields have no source.

They're not necessarily associated with matter in the same way

as far as they generate,

you know, some matter

with some property generates the quantum field.

they just seem to be there,

just exist throughout all space, right?

And so I thought for, you know, I'm like, man, this just can't be real.

This just can't be.

And then in 2012,

we discover the Higs field,

this scalar field that permeates

all space and imbuss

mass to these quantum particles.

And at that point, it's undeniable now.

The quantum fields are real

and they are you know the word field

in physics means something really simple.

It just means it has a value everywhere right?

So if I have an electric field

for example you know I can characterize

it by the strength of the magnet and direction of that electric field

everywhere in the room.

And if I put a electric charge in the room, that charge is going

to fill a force

from that force field that we call an electric field.

So it's a real thing.

And when we do physics calculations

like when we do quantum mechanics calculations

or you know we say we use this word all the time, potential energy. Potential energy.

What does that mean?

Well, here's the thing.

A field has different values at different locations in space.

So potential energy is an energy that's associated

with where you're located, right?

That's which is different from

kinetic energy that has to do with how you're moving, right?

How fast you're moving, how much mass you have or collectively

how much momentum you have.

So quantum fields are

appear to be actual

real physical entities that's

are just there and are

So, how do you think of these particles?

Man, physicists have been asking that question since we discovered them.

Sometimes they're particles, sometimes they're a wave,

but when it comes down to it,

humans ask these questions of what things are

because we want to make a calculation

to get something understood or accomplished.

And based on what you're trying to accomplish,

that's how you model it.

So, sometimes you model it as a wave.

Sometimes you model the the the atom as a electron in a nucleus connected by a spring.

Sometimes you do think of it as a little sphere.

Sometimes you think of it as spread out throughout an entire material.

And each time you make a calculation,

you get the right answer

using the physics of quantum mechanics.

So what is it really? We don't know.

What is a wave function?

The wave function is a mathematical

description of a quantum entity.

And it lives in this funky mathematical

space, this funky vector space.

And so at the basic level,

we make measurements of things.

Where is it located?

How fast is is it moving?

And this wave function,

even though it does not have the same sort of tangible

reality that we're accustomed to in classical mechanics,

it gives us the right answers to the measurements that we make with incredibly high precision.

So it is a incredibly

valuable tool, but it it it has very little intuitive

basis for a human that lives

outside of the quantum realm.

You know, the big problem with quantum mechanics and and

translating it to others

is that it's made up of, you know, it exists in this funky mathematics

that will make your eyes glaze over

and that is these these vector math, right?

So a lot of us have encountered vectors.

So the typical place that we do is you know you have your xy

plane you have some arrow that is a vector

and you can draw

a line straight down to the x-axis

and say oh here's where it intersects that axis

at three and here's where it intersects the y ais and I can say

that value is four for example

so I could write that vector as

3x + 4 y

all right well when we describe

quantum entities we use this idea called

a wave function And the wave function

is a vector in very much the same way.

But instead of having an x-axis

and a yaxis, you have axes

that are physical observables,

locations, momenta, energies, these sorts of things.

And what happens is is that

when you when you write down that wave function,

if you have 3x

and 4 y, what you know is

if I make a measurement

of that system, I'm going to get either three for x or four for y.

That's the only possibilities.

But before I make a measurement,

the state of the system is a combination

of 3x and 4 y, right?

So it's that vector

sitting there at some angle.

But now I make a measurement and boom, it's only on three or boom,

it's only on four.

But the analogy that I'm making is not accurate because my hand moved from

the original vector space that's a combination of X and Y

to being either all X or all Y, right?

It it it rotated.

That's not what happens appears to happen in the quantum world.

It's like it disappears

from being X and Y and becomes all X

or it disappears from a combination of X and Y and becomes all Y.

And what's really freaky about that is

is that when you write down this wave function,

let's say for example, five different locations.

All right, every possible

location is going to have a probability assigned to it.

And so if it's five different locations,

then you can imagine there's five axes.

And so instead of just a 2D XY, imagine XYZ.

And you have that vector sitting out somewhere in this space, right, of this XYZ space.

And when I make a measurement, it pops to one of those axes.

When I start making measurements of what is the value of of of of

the of this thing when I measure it, I get this value of X,

I get this value of Y, I get this value of Z.

Each value is going to occur

at the probability that that initial state predicts.

But you can never know

before you make the measurement

which one you're going to get. Right?

So let me make a similar

When we model as physicists

a block of iron,

right, every atom is in a particular location, we call it a lattice. All right?

And when you write down those lattice locations,

they play an important role in calculations

of what happens in reality.

But if you were to actually look at those atoms,

each little atom has a range of motion.

That lattice location is going to be where it is located the least amount of time.

It's not where it's at.

It's where it's almost is never located. Here's another analogy.

Suppose I have a pendulum, right?

Suppose I have a series of pendulums

and I want to say where each pendulum is.

If I don't energize

them by lifting the arm and letting it swing,

the best thing to say is that wherever it hangs straight down in that

equilibrium position, that's where it is, right?

That's where that pendulum is located.

But now if I lift that arm

and let it swing,

what you're going to see is for the range of motion of that pendulum,

that's where it's going to be located the least amount of time, right?

It's going to go here and slow down, and it's going to speed through

the bottom and go back up there and slow down.

So, it's going to be at its

its extremes way more often than it is right here.

But that location is what takes place.

That's what shows up in the equations.

So, are these quantum

state vectors, these wave functions, the same thing, right?

Is it that that vector

is moving around that space at all times and what we call the state

vector is just the average location

where it is or

way to characterize that vector

even though it's constantly moving around.

The problem is we have no idea.

That's that's the problem.

And not only that,

you know, when I talked about a pendulum,

we're talking about locations in space, angles,

speeds, stuff that makes sense.

The vectors in which these quantum

wave functions live are this crazy thing called a Hilbert vector space, right?

That has no physical

manifestation in our regular world.

So when we talk about

how we describe the world with physics,

typically it's an equation that says

a physical thing changes with time

based on the circumstances it's in.

And those circumstances are either circumstances

of energy or circumstances of force.

for the Schroinger equation and the wave function.

The thing that changes with time is some vector in some weird Hbert

space, but from that you get

out what you would actually measure if you made a measurement with a higher

precision than any other science.

Do quantum fields exist

in spaceime or does spaceime

emerge out of quantum fields?

Spacetime is kind of the combination

of every wear and every win in the universe.

And you know the wares and the winds

exist separately but they also exist in a combination

of a phenomenon that we call spaceime

where it's expressed by a mathematical

equation that includes you know distance

it includes time and it includes

the curvature of spaceime

as well as the change

in the space-time coordinates with time. Right?

We call that a metric.

So it's essentially uh a way of how do you measure distances in the universe?

You use this notion known as spacetime.

Are our descriptions of reality

are they really representative

of the underlying reality

or are they tools

that give us what's going to happen in certain situations?

If spacetime is a quantum field,

then you should be able to quantize it.

And physicists have been attempting

to do that, come up with a quantum theory for spacetime.

And we haven't been able to so far, especially in high energy regimes.

Um, so you know, it's a situation

where it's more like a superimposition, right?

So all these quantum fields

exist in the same space at the same time, right?

So I if you think of a mesh,

you know, one way I like to I like to imagine it is take

a swimming pool and fill it up with green

but also fill it up with red jell-o

and blue jell-o and yellow jell-o

and purple jell-o all at the same time. Right?

All of these different jellos

are there filling the same space.

Some of them interact with each other, some of them don't interact with each

other, but they coexist throughout this space.

And so you know you would think

that they are existing

in a spaceime right

and you know when we get to the multiverse realm you'll see that spacetime

exists in a spaceime

right so reality is multi-layered

and quantum fields appear to be fundamental

spaceime appears to be fundamental

some argue that spacetime is emergent

I argue that spacetime is fundamental

because again fields If they are fundamental, which seems to be the case,

they require a geometry

in which to exist.

That geometry is space.

Energy appears to be fundamental.

Energy is that which creates change.

For change to exist,

there needs to be a before and an after.

So time must exist.

So space and time

must exist for quantum fields and energy to exist.

So, is it that they come into existence at, you know, simultaneously?

It's kind of like

super massive black holes in galaxies.

We see that they are both necessary and they're both there

and we can't tell which one came first, which is the chicken and which

is the egg, right?

It, you know, it's that kind of a problem.

Can I explain entanglement to you?

Well, thank you for your confidence in me.

Quantum entanglement is a phenomenon

that occurs in quantum mechanics

when two or more entities

behave as if they are a single entity

and there are rules that these entities

must follow if they are

a single entity and the general statement is that they must be in complimentary states.

This is one of those things where

you know among physicists

a lot of us

are convinced oh there is no

mystery here it's just a correlation

and we see correlations around us all the time right

and then others are like oh no no no no

there's something really weird going on here

I'm I'm a fence sinner

in this argument right because it is a correlation

um but the thing that

makes it really weird and strange

is that you can measure

these correlations and So the analogy I normally give is

you know take a pair of twins

you know they they they are correlated

in the sense that

when one is sitting and the other the other is always standing

and you know you separate them by light years take them to opposite sides

of the universe and give each one a set of scribes

that records every time each one is sitting and standing.

What you will find

is that you know

years later when when the scribes from opposite sides of the universe come together

and compare they note their notes

they'll see that the correlation

held and there was no way

for them to actually

have a signal go from one to the other

in that instantaneous amount of time.

But there's a big problem with that

and that is is that it kind of rests on making measurements at around the same time.

And so what is the same time?

Well, that goes into the definition of what does it mean to to have now, right?

And so we define now as a set of simultaneous

events that are occurring, right?

That is what defines now.

All these things are happening at the same time.

Well, Albert Einstein showed us

that events that are simultaneous

to one observer are not simultaneous

to another observer if they're moving differently and they're if they're in a different

state of gravitational energy, right?

And then you know it was that was expanded to something called the Andromeda

paradox that shows that

this effect compounds over distance.

So different observers looking at things in the great very distance, right?

like say our two twins separated by some great distance.

Whereas one may see events as occurring as simultaneous,

the other may see those same two events

separated by days or years or centuries if the distances are great enough.

So observation breaks down at great distances.

So yeah, we've measured quantum entanglement

across, you know, distances that are great, like from the surface of Earth to

a satellite, but we haven't done it across these super vast distances

in which the universe actually exists.

So, you know, it's weird. It's strange.

What is it really the underlying reality?

We haven't figured that out.

Some folks convinced they have, and there's a big paradox there.

I sit in the ignorance.

I sit in the mystery, in the question, and in the curiosity.

When I talk about the quantum realm, I'm sure your mind goes to,

you know, some YouTube video you've watched or some movie you've watched.

You know, Ant-Man going into the quantum realm, right?

When we hear new information,

we try to understand it through

analogies or relating it to things that we already know.

But there's nothing that we already know that is like the quantum realm.

So as your brain tries to make sense of it,

it actually confuses you more.

You kind of have to empty your brain, throw it away,

and build this whole new reality from the ground up in this strange math, right?

And so that makes it opaque

to most of us.

Even us physicists, when we get into the math,

again, we live in a world that makes sense to us.

The quantum realm does not.

So, you're not alone if you have no idea

what this is about.

>> Chapter 2, the cosmological realm.

>> When you think of the cosmological

realm, think of dynamic spacetime.

And those dynamics manifest in two ways.

Spacetime curvature and space-time

motion, primarily stretching and waving.

So the realm of cosmology

is the realm of dynamic spaceime.

When I was a graduate student,

my PhD adviser turns to me.

He goes, "Hey, the solar radiance is like well something 10 to the 30s

per second or some some number 10 the 30th."

And I go, "I don't know."

And he looks at me and he go, "What?

You call yourself a solar physicist and you don't know the solar radiance?" Right?

this look of almost disgust.

And I was like, yo,

bro, you need to step up your game.

You need to get these numbers embedded in your head.

And so I started to understand

reality at the astronomical

scale in terms of these numbers.

I came to understand like, hey, you know, the the moon

is 1% the Earth's mass,

but one quarter the Earth's diameter.

Mars, it's 10% the Earth's mass,

but 1/ half the Earth's diameter.

The sun is a hundred times the earth's diameter.

And as I had these numbers in my mind,

you know, I was going about my business of being a student.

And I taught observational

astronomy at the observatory.

And one night I was leaving the observatory

very late at night, you know, 4:00 a.m. or so.

And it was a clear night.

And I looked at the moon

and I contemplated on the fact that the moon was one quarter the earth's

size and the fact that it is around

just over 60 earth radi away from us. Right?

So if you ask a person if this the size of the earth you know

how big is the moon and how far away it is.

You know you get something like this right?

But it's really 60 some odd earth radi away.

So 60 times that distance away is on the other side of the wall over here, right?

You know, and so when I realized that the thing was

60 some odd radi

away from me and I saw how big it was in the sky,

I could suddenly see the sky in 3D.

Also in the night sky

was Jupiter and I knew it was on the opposite side of the sun

and it's 10 times bigger than Earth.

and seeing how big it appeared in my night sky.

Knowing these distances, right, you know, things look smaller the farther away they are.

Knowing these distances, I was like, "Holy,

how massively large Jupiter is, right?"

But then one night,

I had this buddy who

he had not done well in school.

Let's just say it that way.

He wasn't an academic dude,

but he had crazy curiosity.

And one day he invites me up to his father's home

in the coastal mountains of California

outside the town of Elk.

And he's like, "Hey brother, bring a couple of telescopes."

And so I bring up two telescopes.

And so I'm there doing my thing, kind of ignoring him.

And he says to me, he goes, "Hey bro,

what's that fuzzy thing up there in the sky?"

And I'm like, "What?"

He's like, "Yeah, man.

I can see it with even without the telescope."

And I'm like, get out of here." Right.

in, you know, but more profane in real time.

And he's like, "Yeah, man."

And he had the sincerity in his voice.

I'm like, you know, the only naked eye blob I know

is the Andromeda galaxy.

And where we're looking, the time of year it is, it ain't Andromeda time.

What is this dude talking about?

So, I'm thinking he's making some kind of error.

So, I go and look through his eyepiece

and sure enough, there it is.

And I'm like, he said he could see it without the naked eye.

So, I pull away and look along the barrel of the telescope to the

sky and there it is.

And then it struck me.

I had always known

that the Andromeda galaxy could be seen with the naked eye in a dark

enough place, but I knew it had to be really dark and I hadn't really tried it.

You know, I hadn't had the opportunity.

But there it was.

Now, you can only see the bulge of it,

but in the night sky,

right, it's bigger than the full moon by several times, right?

And it's 3° across, right?

The moon is like half a degree.

That's six times bigger.

And that thing is 2 1/2 million lighty years away.

So now again, knowing

that I'm looking at something that's 2 and a half

billion light years away.

It's looking that big in my night sky.

My brain shattered because I could feel how big that thing was.

And it was just impossibly huge.

And that's the crazy thing about the cosmological realm, right?

An atom is 10us 10 m across.

A nucleus 10 theus5.

That sounds crazy tiny and it is.

But just our galaxy,

you know, 10 theus

15 going in smallness,

10 to the 21 going in bigness.

And I'm just like,

bro, man, where where am I?

What is this cruel joke reality has played on me?

You want me to

wrap my my mind around this bigness? Whoa, man.

Albert Einstein gets a lot of the credit for this idea of spacetime, but

it originates with Minowski.

And the idea here is that

there is a relationship

between space and time.

But it's more fundamental

than I think that we get in our everyday lives.

We have an understanding for we know

that you know there's relationship between space and time.

I can say you know I used to live on the space coast or

Orlando is this particular distance away or I can say it's an hour away right?

I could say that space or time

and we do the same thing using light.

Oh, you know that the the sun is 93 million miles away or it's

8 point, you know, four or something light seconds away.

So, space and time, you know, it's intuitive

that there is a relationship between it.

If I want to describe

a a line, the length of a line vector

in a 2D xy space, right?

I'll say, you know, the Pythagorean

theorem, a^2 + b^2= c^2, right?

We're familiar with that.

When you add that time coordinate,

it's not a plus anymore.

It turns into a minus.

So space and time kind of tug at each other, right?

And the consequence of that is that we end up with

this idea that you have a speed through space and a speed through time at all times.

And just like you have a^2

plus b^2= c^2 in the pythagorean

theorem, you have that your speed through space 2 plus your speed through time

squared equals the speed of light squared.

And so you're never at rest in time, are you?

Unless you're, you know, because you can never move at the speed of light.

Um, and you you're, you know, so

what what does this say to us?

This says to us

that at all times

everything in the universe

is moving through spaceime

because everything is moving through time

and relative to each other,

we have speeds, right?

If if there's only one thing in the universe, what does speed even mean?

But if there are two things now

you have and they're in motion you have a speed relative to something else.

What follows from that

is that your speed through space and your speed through time

will be different for each one.

So each one by having a different speed through space

has a different speed through time

necessarily and that completely

breaks intuition for us humans.

Time seems to be something that just ticks along at a regular rate. Right?

The earth spins on its axis once a day, goes around the sun on

its orbit once a year, right? That's the same.

Our year is a year.

A second is a second.

A minute is a minute. Actually, not.

And it's all because of this fourdimensional

spaceime where space and time tug against each other.

You are moving through spaceime

at the speed of light.

You're not moving through space at the speed of light.

You may be moving through time at near the speed of light, which is the maximum speed.

But at all times,

you and everything else in the universe are moving through spaceime

at the speed of light.

Not through space at the speed of light,

not through time at the speed of light, but through spaceime.

The combination, it's kind of like Galileo discovered.

An object at restoring motion remains at restoring motion less acted upon by an outside force. Right? Why is that?

Well, at the same way, your emotion through spaceime is a constant.

It is is unchanging,

but it kind of shifts

between space and time.

So, for example, you know, if you're out there

in space all alone,

you're going to be moving through time

at the maximum speed.

But suppose you come next to a gravitating

body like a planet or a star.

Your direction through motion will change.

So some physicists like to say

gravity turns speed through time into speed through space, right?

That's why you accelerate near gravitational body.

So you know it's it's difficult to wrap your mind around.

It's super easy and it's clear as day in the mathematics of relativity.

But you know when you are told this and you discover this for the

first time, you know

it it it is a brain breaker.

But it also solves mysteries.

Like you might wonder

why is the speed of light considered

a speed limit in the universe?

Well, think about it this way.

If I'm at rest relative to you, right?

Relative to each other, we are not moving through space.

So together we are moving through

time at the speed of light.

Now suppose I start moving rapidly through space.

That means I must now move more slowly through time.

So in order to move faster through space,

I have to borrow.

I have to subtract

from my speed through time.

Well, how much speed through time did I start off with?

The speed of light amount. All right.

So that's that's the maximum amount I can borrow.

If I bring my speed through space up and my speed through time down,

eventually I get all of that speed through time and I'm moving at the

speed of light through space.

Now, of course, nothing with mass can do that.

And at the same time,

anything that doesn't have mass like gravitational

waves, light, they must do that.

But here's the thing that's really weird.

If you look at light with that analogy,

right, you have this phenomenon

that, you know, the faster things go, the more time gets dilated, right?

So the less it travels through time.

And not only that, distances shrink.

So the faster you go, the shorter distances become.

So you would think that light does not experience

neither space nor time.

But that's not true.

That is not the case. Right? Why is that?

The reason why is

when I say that I move through time at the speed of light.

If we're at rest relative to each other together, we move through time at

the speed of light,

I just invoke the concept known as the rest frame. Well, guess what?

Light does not have a rest frame.

So, you can't make those definitions. It's nonsensical.

You can't say light doesn't experience space or time.

It doesn't have a rest frame.

So, you know, this

the consequences of moving through spaceime at the speed of light and living in

a fourdimensional spaceime is definitely a brain breaker, but you got to be careful

with the analogies and and how you think about it because there are subtleties

all along the way that could lead you to incorrect conclusions.

How is the universe

both curved and So by flat we mean no curvature.

Well, you know, it's kind of like saying the Earth is both a sphere

and not a sphere, right?

Or, you know, for my pedantic

nerds out there, an oblate

spheroid has somewhat egg shape.

But here here's how that works.

You can think of the universe and space-time

curvature on two scales.

There's a large scale universe

and then there are there is the local universe.

So yeah, next to

planets like the Earth, spacetime is curved.

Next to the sun, spaceime is curved.

Next to the galaxy, spacetime is curved. Right?

So I like to think, for example,

you know, if you think about our galaxy,

we have this giant halo that extends like, you know, 10 times

the Milky Way's radius around it, right?

So if you think about the the dip in the sheet or the funnel

model, that big giant

halo of dark matter contains most of the mass.

So, it's going to create this deep depression, right?

Then you got the concentration

of the galactic matter there.

There's going to be another deep depression.

And then right in the center, because density matters,

you have our super massive black hole. It's like, right?

And so, it's like, you know, it's almost like you have a drill drilling

into spaceime because it's all orbiting, right?

It's all spinning and orbiting.

So, locally, there's a lot of curvature and a lot of character

to the nature of spaceime.

But you know, just like the earth, it has mountains, it has sink holes,

it has gullies, it has valleys.

There's a lot of character

to the surface of Earth.

But if I step back and look at Earth, it looks like a smooth

blue and white ball, right?

And the universe is the same way.

Even though there's all of this local character,

if you step out and look back far enough,

it looks really smooth.

There's a large scale

global curvatureless spacetime and there's local

highly curved spaceime and curvature

of various sizes and intensities

based on mass and density

and energy and pressure.

So this overall space-time

behavior takes place in a in a universe that is not static.

The universe is expanding.

So when you start when you start talking

about the universe at the cosmological scale.

You have to take into account

curvature of spacetime and the expansion of spacetime

and that again completely breaks our intuition.

The only manifestation of space-time

curvature that we experience on earth we call that gravity

and you know we just characterize it with a single number.

But in spacetime things are very different. Space is flowing. Space is waving. Space is stretching.

And there is no analogy

here in our everyday existence

that prepares us for that.

And that's why it takes a long time to understand cosmology.

Once you get it, it seems kind of simple.

But getting your mind around this stuff that is so

foreign to us, you know, out there at the cosmological

scales and then you take into effect that there are gravitational

effects of phenomena that remains hidden.

It it really breaks your brain.

Quite often what we do when we want to create an analogy

for uh learners to understand how space-time expansion works.

We take a sphere

and we put lines of latitude and longitude on it and we place galaxies

at the intersections of those lines of latitude and longitude

and then we expand the sphere. All right?

So the the the

line segments between the intersections

of longitude and latitude they get longer

but the galaxies themselves

still remain at those intersections. Right?

So space has expanded

but they haven't moved

in that particular coordinate system.

And that analogy works really well

because you know the universe is really big.

So most of the universe is far away from us.

So most of the universe is expanding really fast.

So that means that most of the motion of galaxies

that we observe is not due to their

motion within that coordinate grid,

but really it's it's due to the expansion of the grid itself. Right?

So we have an idea that we call expansion drag.

It's almost like you know the faster it's expanding

the the the more it slows down its intrinsic motion.

in in comparison to that space-time motion.

So, an example I like to give is tadpoles in a river.

So, if I'm standing at the riverbank

and I'm looking from the

edge of the river to the center of the river,

what you're going to find is that the water near the river's edge

is interacting with the bank.

And that makes it move more slowly than the water out at the center.

So, if I'm looking at some tadpoles

right here, I see them moving around relative to the water, right?

They're moving back and forth, going back and forth, right?

The water, you know, is kind of like this background in which they exist.

But if I look at tadpoles

toward the center of the stream,

they're carried away down the stream.

Of course, they're flipping their little tails and they're moving around relative to some,

you know, local coordinate system.

But from my perspective,

standing on the bank far away,

the stream speed is what characterizes their motion.

And I would see them as basically at rest

relative to the parcel of water that holds them, even though they're flapping around in there.

Well, spacetime acts the exact same way.

Galaxies are moving, right?

They're in orbits around each other.

They're whizzing around, but because most of them are farther away,

it is that motion through spaceime

that characterizes that dominates their motion.

And so, you know, it's like they're

nailed down to that

location in space and it's just being carried away by this elevator, right?

Or this or this conveyor belt.

So, in this expansion

scenario, if you go far enough in the distance, you get objects that are

moving away from you

faster than the speed of light

and even faster, right?

It breaks our intuition that nothing can move through spaceime

faster than the speed of light.

But stretching is kind of unlimited

in terms of how fast it can make things move away from you.

But locally, nothing is moving

faster than the speed of light.

You know, when we talk about

the universe as a whole,

how does it break our notion of a universal present, right?

This is the age of the universe.

This is what the universe is right now.

Well, man, it it it breaks it in multiple ways, right?

So the first way is

we know that the rate at which time travels

for every entity in the universe

depends on its energy

situation right how deep of a gravitational

well is it in

right gravity slows time so that means when you're in an airplane or in

orbit around earth that your clock will move faster

than someone here on the ground

and also speed matters.

The faster you move relative to something else, the slower your clock goes

relative to that other entity.

So if you look around the universe,

the gravitational landscape varies.

You know, there are these vast voids

between galaxies where there's very little matter.

Then you have the the so-called whim,

the this filamentary structure where dark matter and matter come together to form what

we call the cosmic web.

Well, in the cosmic web,

time is going to travel more slowly

than it does inside these voids.

So, how do we even have a notion of the age of the universe

or now if everywhere has its own clock?

Well, it turns out that there are these two

things that we can use.

One is the evolution of stuff in the universe.

Stars are cooking up elements into other elements.

So, you know, complexity is arising, right?

So you could get different minerals

um after some time that you couldn't get before that time because they had

to be cooked up in a particular sequence

to get to to to exist. Right?

And the same way

there is light that fills the universe

that has a property of averaging

out things in the universe.

It's called the cosmic microwave background radiation.

And so there's a property

that light has, right?

Light is strongly coupled to spacetime.

So as spaceime expands,

light that is traveling through that spaceime,

it gets its wavelength

stretched out by the same amount

that space expanded while the light was traveling through it.

And so all of the CMBB,

even if along different directions,

lines of sight, the the the

rate of expansion is slightly different,

the cosmic microwave background radiation

kind of averages out over these nuisance parameters.

And so it can serve as a convenient clock.

Emergence can serve as a convenient clock.

So even though the fundamental reality is

there is no universal

now, we do have these convenient

clocks that give us a bearing, give us a fidial

reference for us to speak in this way.

Given this lack of a universal noun,

cosmologists can still talk about the universe at any given time slice

because of the cosmological

principle which says the universe

is homogeneous and isotropic.

That means it's made of the same stuff everywhere

and there are no special locations or orientations or directions. Right?

The thing about that is is that you know that homogeneity

is homogeneity in space.

Everywhere is kind of the same even though it's not. Right?

You know the being in the sun is not the same thing as being

on Earth or being in intergalactic space, right?

But over large enough averages,

we can say it's all made of the same stuff.

But every time is unique, right?

There is this pattern of emergence that occurs, right?

Forces freeze out in the universe.

You go from a universe of

nothing but plasma to a universe of nothing but gas.

And then you get stars and galaxies

and, you know, and and and structures form.

So as you look around the universe,

you can say, okay,

this has evolved to this degree.

So I can attach this time to it.

And you also have the cosmic microwave background radiation

whose light is stretching uniformly in all directions.

That does give you an average clock.

And here's the other thing.

When we model the universe

as it when we're doing

you make these assumptions.

You assume that the the the

universe is not lumpy

with filaments and all of this.

You assume it's a it's a uniform

gas and that allows you to write down equations for

conservation of energy for the universe, right?

And the acceleration equation for the universe.

You know, these equations

allow us to perform calculations

to look at the past and look at the future. They're very useful.

Some people say that astronomy

is a science of history

because we're always looking in the past.

Well, I'd say that, you know, if you're looking at your

device in front of you, you're looking into the past.

All looking is looking into the past

because light has to travel to you.

Now, to us, light moves really fast,

but that's only because we're so small.

The universe is big.

Light takes 2 and a half million years

to get to the nearest galaxy.

So, light ain't fast as far as the universe is concerned.

You have to take that into account

when you think about the universe is really

mindboggling to to to

know that I can look at the universe

as it exists today.

I go a certain distance away from us and I can look at the

universe as it was

a 100 million years ago or

30 300 billion years ago or a billion years ago or 5 billion years

ago or 10 billion years ago.

I can do that

because of the time it takes the light to travel to us for us

to do those observations.

Now, what I can't do is look at myself

a million years ago, right?

I can't look at the sun a million years ago because that light, you

know, I'd have to move faster than the speed of light,

travel beyond the light that the sun emitted a million years ago and then

turn around and look at it and say, "Ah, that's what the sun looked

like a million years ago." Right?

You know, if you had wormholes

in science fiction, you could do that sort of thing, but we can't.

So, you know, it's one of those limitations of life.

You know, I used to play a lot of basketball.

And I used to get a lot of basketball injuries.

And I used to say to myself, man, imagine if we didn't get injuries,

how fun this sport would be.

So, I just love colliding with people, right?

I just I just love to be physical.

Like, if I can just run at full speed, just blow,

you know, life would be fun.

But we have these limitations and we have to live with them.

And the speed of light as a speed limit

is a limitation we have to live with.

And so if you want to understand the universe and how you observe it

and the fact that time and distance

are related in that way

is just something you got to live with.

One consequence of living in an expanding universe

is that there are multiple horizons

that uh define how far away we can see.

So the universe is expanding

and the rate at which the universe is expanding

varies for every observer with distance.

So the farther you go away, everybody in the universe will see the same thing. Right?

Everybody is at rest relative to themselves.

I am at rest relative to myself at all times.

No matter how fast I'm moving relative to you,

I'm at rest relative to myself.

If light is traveling to me

from the great distant beyond,

right, it's going to arrive

at my location based on a relationship between

the distance it's traveled

and the speed at which it's moving, the speed of light, right?

If you're moving at 50 miles hour,

after 1 hour, you will have gone 50 miles. Okay?

So, what does that mean?

That means that as time moves forward,

light from more and more distant

reach re reaches are going to reach me.

So if I'm an observer

and I'm looking at the limit of my observations,

it's going to be a sphere around me that's going to grow with time, right?

Just because light from more distant reaches

are going to come to me, arrive at my location as time goes on.

But now we take into the account the fact that

the universe is expanding.

And that means that,

you know, if I double the distance, things are moving twice as fast, three

times the distance, four, three times as fast.

100 times the distance, 100 times as fast.

Eventually, you're going to get to a point

where the expansion rate

reaches the speed of light. Right?

And we define that as the hub sphere.

So if something is moving away from you at the speed of light and

it emits light in your direction,

will you be able to see that light?

And the answer is yeah, you will be able to see that light eventually.

But there is a point just beyond that

where you won't right that Hubble sphere also expands with time.

But then you get to the

cosmic event horizon where this is the boundary by which objects that emit their

light today, anything beyond that distance, their light will never reach us because it

will travel through a region of spaceime

that is moving so rapidly

away from us that even as it's trying to like come to us, right,

that expansion is pulling it back.

So if you imagine

that you're on a treadmill

and the treadmill is moving you away, right?

You know, you can imagine a slow treadmill, a faster one, a faster one,

a faster one, right?

So as a human runner,

if you move forward

faster than a treadmill pulls you back, you make forward progress.

But if you ramp up the speed of that treadmill,

eventually you won't be able to make forward progress,

and eventually you're going to start getting carried away.

And that's how the cosmic event horizon works.

Eventually, you get to a point

where spaceime is expanding away from you so fast

that whatever light you admit today,

it's never going to make it to you.

But notice I say a light emitted

If you look at the most distant objects

whose light is reaching us today,

they were most they were much closer to us

in the distant past

and that's when their light started traveling to us.

But then the expansion of spacetime

took them beyond our cosmic event horizon.

So those objects even though their light

is coming to us and will eventually

reach us, the objects themselves

are beyond the cosmic event horizon.

And so that defines

a third horizon called the particle horizon.

It's not for objects

whose emitting light today.

It's for objects whose light is arriving at us today.

And so those objects, if you look at the cosmic event horizon,

it's around 16 billion lighty years away.

The hubosphere is around 14 billion lighty years away.

The particle horizon is more like 46

billion lighty years away.

So we can see the light from objects that are now 46

billion lighty years away from us.

That expanding universe changes everything.

In the far future, which is, you know, far is relative.

you know, as far as the age of the universe as we can extrapolate

it, the universe is going to be around for a very, very long time.

So, as far as I'm concerned, we're still in our universe's infancy.

If I look at how long the universe is

thought to exist, it's going to exist,

you know, with with

stars in it, for example.

And I say, okay, let's make the analogy that that

universe existing in the into the future

known quantity is equivalent to a human being that's 100 years old.

How old is the universe now in comparison to that?

It's like a couple of days, two, three days old right now. Right?

So the universe is still at its infancy.

And one thing that I like to say is you know how the universe

is still at its infancy

because you can observe it.

only a new universe

is observable because that expansion

is going to take all the galaxies

except for the ones that are gravitationally

bound to us like Andromeda

outside of our cosmic event horizon

and eventually we won't be able to see any of them

in the distant future

and the Andromeda galaxy and the Milky Way galaxy are going to combine right

they're going to collide and combine to form the Hakee galaxy

or you know Milkometer

And at that point,

our entire observable universe

is going to consist of

this galaxy and these cosmic backgrounds.

Everything else will be swept out of our line of sight.

We can still make measurements.

You know, we can use like pulsars

to measure gravitational waves

that are passing through this this universe and do some cosmology.

We can still measure the cosmic microwave background radiation,

its polarizations, its fluctuations

and variations and do some cosmology.

But as far as looking at galaxies,

that game's >> Chapter 3,

the two multiveres we might live in.

>> What do physicists mean when they say a multiverse?

It is derived from what we mean when we say a universe.

And a universe has characteristics.

And one characteristic is sort of like defining

a living cell, right?

The thing that defines a living cell

is that it has it contains a volume

that encloses it and separates it from the rest of existence.

So a universe is that it's a volume separate from the rest of other existences.

And inside that universe,

there are specific physical

constants that characterize the physics within that universe and the interactions within that universe.

things like the gravitational

constant, the action constants that we call plank's constant

or um you know the constant that

uh combines energy and temperature that we call the Boltzman

constant right then there are some other more obscure constants in there like the

fine structure constant but these

constants these values we don't derive them

they just are right and so every universe

will have its own set of constants

that define how physics takes place within that universe

and it will be a volume cut off from the rest of other

such What is the problem that the

many worlds interpretation of quantum mechanics

is trying to solve?

It's trying to solve what we call the measurement problem.

And the idea that

you know when we make a quantum

measurement that vector that describes the state of the system

goes from being expressed

as a sum of the possible

config states of the system

right when it's measured it will only be found in one state

but in the pre-measurement

state of the state

we describe it as

being a combination of all the possible states

but once you make a measurement

is always found to be only in one of the possible states.

So what's going on there?

In the first instance in the initial state, you have a state vector that's

made up of all these sums

and then after measurement, the state vector just ends up having one component of that sum.

So what does that transition look like?

Well, is the measurement

changing the state of the system in this way?

And some physicists what they call the Copenhagen

uh interpretation say yeah the system

collapses into that one state upon measuring.

But some other physicists came up with a different idea

and they said what's happening here is that

every state actually exists

and every state kind of corresponds

to an its own universe.

So the measurement that you get tells you which universe you're in. Right?

So all of the possibilities,

all of the possible states

occur at every measurement, but they just occur in different universes,

which means that there are different copies of

everything in our universe.

Every time a measurement is made, you're popping off these new universes.

You know, it's it's it's weird.

It's very, very weird and strange. It seems counterintuitive.

It seems to go against conservation of energy.

And the other thing is

we don't even really define what it means to make a measurement, right?

Is it just an interaction?

So for example, if I have a

photon and electron interacting, right?

An X-ray that is traveling towards some

atom, that atom has an electron.

So there's a famous experiment in physics

known as the Compton scattering experiment.

And this is the experiment that shows

a particle property of light

that it exchanges momentum

the way particles do like billion balls colliding.

So in that interaction

the before the interaction

both the electron and the light

are described as waves

but in the interaction

they both behave as particles right.

So does that interaction

create a new universe

where each goes from a superp position state

into a definite state and you so think about that

many galaxy clusters are full of this hot

X-ray emitting gas and

all these galaxies which contain

gazillions upon unfathomable numbers of electrons

are orbiting within that hot million degree gas they're being bombarded

by X-rays is every interaction

spinning it off a universe, right? It's nuts.

And even then, you know, when you talk about

the fact that that state vector

in the state, that wave function, the pre-measurement

state has probabilities built within it.

What does the probability of a measurement even mean?

If every measurement has a probability of one, right?

Every measurement has a probability of certain in some universe.

So, what do these probabilities even mean?

Some people think, you know,

on the fringes that that might be real

and some, you know, because they think that that

idea of the wave function collapsing is too unphysical.

So there must be some other explanation.

And this is one of those interpretations.

And I find both explanations to be okay.

I think we're missing something.

I think there's something

going on that uh we're not quite interpreting

right, we're not understanding right.

And you know, I I love to live in mystery.

I I you know, I love ignorance, right?

Once you know you're ignorant,

you can fix it.

You can you can you know, it's a provocation.

You can design an experiment.

You can design a a thought experiment.

You can design mathematics

to get to the bottom of it.

And I, you know, I think that

this problem has young curious people and old curious people doing exactly that.

But if you accept one or the other,

you think you got it solved.

Now suppose you know we can speculate.

The thing about having a human mind is you can imagine

things that have never been observed that you know may never be observed can

maybe never exist but you can imagine it.

You can imagine a universe

in which the mini world's

holds up to some sort of experimental

scrutiny and we're like, "Oh my goodness, it's real."

What does that mean for our reality?

Does that mean that now

we have the possibility of accessing those other universes?

Well, what you find that happens is

a new sort of

insight into nature is kind of like a new technology, right?

The internet created a whole new economy.

Then smartphones on top of the internet created

a whole new economy.

So whenever you have these new ideas like straight theory, right?

It didn't work out for physics

but mathematically it created

whole new areas of exploration, right?

A very rich area of mathematical understanding and exploration.

So the first thing is you don't know what you don't know.

So if you find

that this multiverse idea

of many worlds interpretation

turns out to be true,

does that mean that we're forever isolated

from these other universes and can never access them?

That is the current

way that most physicists think that yeah, if this were true,

that would be the case.

But if it turned out to be true, people are really going to start

thinking about things differently.

And in that exploration,

some clever group of

researchers may very well find ways

of uh interacting between universes, right?

They can come up with experiments

that, you know, say,

oh, you know, there was once this idea

that gravity is so much weaker

than all the other forces

because it leaks out into this parallel

And what we see as dark matter in our universe is a gravitational

pull of matter in a nearby parallel universe.

And they came with an experimental idea.

Well, well, you know, if you can measure gravity

at a short enough distance,

then what would happen is you'll see suddenly this increase

because that's before it has leaked out into the other parallel universe.

It was never measured.

But the point is is that they came up with a way to measure it.

And I think that

if we do evolve to a state

of finding that, yeah, there's something to this,

we're not going to stop there.

We're going to keep exploring.

And who knows where that's going to lead.

Another way physics invokes the idea of multiverse is in cosmology.

And this came about

via a mechanism that was

hypothesized to solve a couple of problems

in cosmology that researchers

began to notice in the late 20th century.

And those problems are called the flatness problem and the horizon problem.

And the solution is something called inflation.

And inflation is this notion that at very very early time in the very

earliest moments of our universe

the universe doubled in size

over and over and over and over again

in a tiniest you know 10 to the minus some double digit number of

seconds right a billionth of a billionth of a billionth of a billionth of

a second something like that

and the consequence of that

is that today regions in the universe that appear

that there would have been outside of each other's

cosmic event horizon back in those times

and could have never communicated

with each other could have never

exchanged energy with each other when we observe those regions today

they appear to have the same temperature as if they were in contact

right so if I look at the cosmic microwave background radiation coming from that

direction and compare it to

the the radiation coming from that direction

is roughly indic indicative of the universe being at the same temperature

how could that possibly

be that should only

you know if if you do the calculation assuming a standard hot big bang

model it's only about twice the size of the full moon

regions that should have about the same temperature

right but you know it's the same temperature across the entire sky

what that means is

regions of the universe that today

appear as if they were forever

outside of each other's horizons

were at one point within each other's horizons

and that rapid expansion of the universe iverse

took them so far away that they have that appearance today.

So what do I mean by horizon? Right?

So in relativity, every future

possibility is bounded by the speed of light. All right?

So if you move at the speed of light,

there's what you can reach and you can't reach anything beyond that. Right?

So you have what is known as a future light cone.

So if you look at the

future light cones of all the regions of the early universe,

if we take the those regions today and you go backwards,

they would appear to be outside of each other's light cones.

But clearly they were inside of each other's light cones.

And this inflationary event

is what allows that to

uh manifest in a physical way.

But then you need a reason for it.

How could that have happened?

When we started studying how that could happen,

that led directly to the idea

of multiverses being real

in the cosmological realm.

The types of universes

that exist in the multiverse

are fundamentally different than the ones we see in the many worlds interpretation.

So what that means is,

you know, you can think of it like baking a loaf of bread, right?

You bake the loaf of bread, the bread expands,

and inside that bread there are little

bubbles of air pockets, right?

So every air pocket has its own history

and yet the bread itself has its own history

and every bubble is separate from every other bubble.

So they have their own separate histories.

So that's similar to what an inflationary universe would create.

But the difference is is that

the main background the whole loaf

that universe right that thing is there.

It's expanding super rapidly.

It's doing its thing.

And when it pops off bubbles, it doesn't pop them off at the same time, right?

These bubbles where they go out of this inflationary

state and then nucleate

into a new universe,

they pop off, right?

You know, one happens here.

If another one was to happen very near it because that

bigger space is expanding so rapidly,

it will be so far away by the time it nucleates

out that these bubbles are forever separate from each other.

If the multiverse of the cosmological

realm that is generated by this process called eternal inflation

is true then there are multiple spacetimes.

There is the bulk

spacetime and I don't want to use that word bulk because there was this

old model of braids and bulks right but truly

there is a bigger spaceime

out there and every universe

has its own spaceime with its own history

that evolves separately from that bigger spaceime

once that bigger spaceime

um you know births

it the properties in each bubble universe

will be slightly different each have its own history

each have its own space time

I am so impressed with

this animal on Earth

that we call homo sapiens, right?

You know, not long ago,

you were eating the leftovers

of of of predators on the plains of Africa.

At a certain point, you decide you're going to break rocks and turn them into tools.

And you go from that to melting sand and turn it into quantum technology

and performing experiments and

understanding that we live in a universe that is evolving.

And you make measurements of this light left over from the origin of the

universe called the cosmic microwave background radiation.

And your physics tells you that hey, it should have fluctuations of every sort.

And if this multiverse

thing is real, then there should be this thing called a super horizon fluctuation.

And you make this careful measurement.

You get to the point where you send up the plunk satellite

and there it is clean as day.

The signal of the super horizon fluctuations

and you say, "Wow,

eternal deflation looks like it's real and looks like we are in a universe."

So, we have reached

the point of experimental

verification of the multiverse.

But that is a big claim.

So, it is not a conclusive

piece of evidence, but it is a strongly

circumstantial piece of evidence.

Some physicists do say it's conclusive.

Some physicists that I respect greatly

say that it's for their thinking, right, is is it's conclusive.

I'm not convinced that it's conclusive because I don't quite understand it all yet.

Uh but I'm working on it in the midst of everything else I'm working on.

So, you know, we've reached that experimental point.

And you know, man,

what a fascinating place

nature and existence is.

What we see in physics over and over is that

stuff pops out of the equations

that leads us to new discoveries, right?

Maxwell discovering that light is an electromagnetic

wave by manipulating the so-called Maxwell equations.

But, you know, more often than not, stuff pops out of the equations that just isn't true.

And you got to find some way of saying, "Oh, here's how we use that." Right?

It turns out this part of the equation is reality.

That part of the equation is just mathematical junk. that's left over.

At the same time though,

we have people like Albert Einstein

that comes around and he writes down equations for say general relativity

and it and it creates

phenomena like black holes and gravitational

waves that won't be measurable

for decades to come if not a century to come. Right?

So there are things that our equations

predict that may be just complete nonsense

and there are other things that

may be insights into the universe

but we have no way of measuring them now because we just haven't

developed to that level of technology yet.

What that tells me is

we need to take these predictions

seriously and we need to continue

to as a society

fund people to pursue these questions

because every time we find out some

new fundamental nature of reality,

it leads to new engineering

that allows us to take advantage of this stuff.

Research at the edges,

research into these avantguard

ideas is the fertile ground for discovery

and that is where we must explore.

Don't just dismiss it as that can't be true.

The universe has never cared what you think can be true.

It is up to us to listen to the universe and ask the universe,

hey universe, what are you?

And our observations and our calculations

are the way that the universe

answers those questions when we

Want to support the channel?

Join the Big Think Members community where you get access to videos early and free.