Physicist Brian Greene Answers Quantum Physics Questions

From Doktor J: Boy, are you quantum gravity?

Because I'm relatively attracted to you.

I have no idea what you're referring to

or what you're talking about, but thanks.

Hi, I'm Brian Greene, physicist, professor, author,

and I am here today

to answer your questions from the internet.

This is Quantum Physics Support.

[upbeat music]

@peleenmziry: Do people actually understand quantum physics

or do we all just pretend that we do?

We understand the core ideas,

but they're very different from the things

that we would think should happen in the universe

based on our common experience.

So we have to reorient our thinking

to align with the discoveries of quantum physics,

and that's not easy,

but it is something that those of us

who immerse ourselves in these ideas are able to do.

So yeah, we do understand a lot about quantum physics.

Next question from @stooksman.

Has some grizzled professor, like me,

poking a pencil through a folded sheet of paper

to explain wormholes ever made anyone go 'aha!'?

Still seems like an incomprehensible magic to me.

Take two points on a piece of paper.

I can draw a line connecting them,

and you can see that that line is kind of big.

So imagine this is in space.

Maybe this is the Andromeda Galaxy

and this is our Milky Way Galaxy.

The fabric of space can be manipulated.

It can warp so that these two locations

that were very far apart are now very close together.

So if I stick a pencil through,

you now see that the orange between the two locations,

that would be a wormhole.

We don't know if wormholes are real.

They emerge from the mathematics of general relativity,

but if they do exist,

in principle, they could provide a shortcut

from one location to another.

From Do0ozy: If I ran into a wall over and over again,

would quantum tunneling allow me

to eventually pass through the wall?

And if yes, can we calculate or estimate how many times

I would need to run into the wall to pass through?

Yes, exactly. That is what would happen.

Now look, we don't do quantum mechanical experiments

with macroscopic beings like a human body,

but we do a version of you running into a wall

with a particle like an electron or a neutron or a proton

that we fire at a barrier over and over and over again.

A small fraction of the time

we do find it passes through a barrier

that classical physics says it can't.

If you scale this up, yeah, in principle,

a many-particle system like your body

could undertake the same experiment.

You'd have to hit this barrier enormous number of times,

exponentially large in the age of our current universe.

That's just how ridiculously tiny

the probability that you will pass through the barrier is.

AssusReamus: Is our universe the only universe?

You would think that our universe is the only universe

because it's the only one that we can observe

with powerful telescopes.

But there's mathematics suggesting

that the Big Bang may not have been a one-time event.

There may be many bangs

giving rise to many expanding universes

that all populate a grand multiverse.

I like to think of it as a giant bubble bath

where the individual bubbles are universes like our own,

but the grand picture contains many of these bubbles,

many of these universes.

Some people, when they encounter this idea,

they say, You're talking nonsense.

You're no longer talking science.

They have a point, this is an important criticism,

but let me just note two things.

Number one, in principle, this may be a testable idea.

If two of these bubble universes were to collide,

there is mathematics suggesting that the collision

could leave an imprint

on what's known as the microwave background radiation.

We've not yet seen it at all,

but in principle, if we did,

this would be circumstantial evidence

in favor of this idea of multiple universes.

The second point I would make is a theoretical statement.

We theorists use mathematics to try to understand reality.

For instance, the Big Bang idea has been tested

by detailed observations of space.

It has confirmed the predictions, which is wonderful.

But if the math then says there should be other universes,

we are willing to go where the math takes us

because we have confirmed the parts

of the mathematics available to our observations.

Do we know that that's the case?

We don't, but we are willing to consider it

because of the fact that the mathematics

suggests that this really may be true.

@Joonies_Girll: Do we have any updates on time travel?

'Cause please take me back.

If you wanted to see

what was happening on planet Earth a million years from now,

Einstein himself laid out a blueprint

for how in principle you could do that.

You travel out into space near the speed of light.

Say you go six months,

you turn around, you come back for six months.

You will be one year older, of course,

but because your motion was near the speed of light,

your clock was ticking slow

compared to clocks on planet Earth.

So when you step out of that spaceship,

it won't be one year into the future on planet Earth,

it may be 2 years or 5 years or 10 years or 100,000 years,

all dependent on how close to the speed of light

your ship was able to travel.

But many of us are more interested

in traveling back in time.

And that's the one where we just don't know.

There are proposals that people have put forward

using exotic things like wormholes

or interesting kinds of motions around cosmic strings,

but whether any of those ideas really work

we don't have an answer.

And so we suspect that travel to the past is not possible,

but as of today, nobody has definitively ruled it out.

@Kulupsy asks, Physics wassup?

Why is teleportation not possible?

It's not possible for macroscopic objects like you and me,

but physicists who work on teleportation

routinely teleport individual particles

from one location to another.

You set up two entangled particles that in some sense

act as one even though they're far apart.

You bring in whatever you want to teleport

and it commingles with one of the entangled particles.

And through the magic of entanglement,

its features get imprinted on the distant particle.

This person can manipulate that particle

and extract an exact copy of the original.

And in that way, effectively, you have teleported a particle

from one location to another.

@sinonghostriley: What is reality made of actually?

Like, what is this universe?

Way back in the early days,

people though it was earth, air, fire, and water

as the basic ingredients of everything.

In the early years of the 20th century,

people got on board with the idea

that matter is made of fundamental constituents,

made of atoms.

And as we examine atoms with ever greater precision,

we realize that there are other particles

that make up atoms.

You've got electrons

that are in orbital clouds around a nucleus

that typically has neutrons and protons.

And the neutrons and protons themselves

we know are made of yet smaller particles known as quarks

arranged in correct configurations and electrons.

They are the bulk of matter as we know it.

My guess is at some point we will talk about

atoms making up the fabric of space itself.

I think there will be fundamental constituents of space

and fundamental constituents of time.

What those ingredients actually are,

we are still struggling to figure out,

but my guess is that that is

where our understanding will lead us.

All right, next question is from sparshmathur.

When does quantum mechanics get weird?

Well, it gets weird almost from the get-go, right?

Quantum mechanics tells us

that the world can be in a blended mixture

of different mutually incompatible states of being.

It tells us that distant objects can be weirdly connected

through something known as quantum entanglement.

It's weird from the beginning and yet it works.

Isaac Newton described the world

in terms of billiard balls bouncing around

according to ideas and laws that he was able to intuit.

For reasons that we do not understand,

the micro world doesn't behave that way.

Instead, it plays by these different rules

called quantum physics.

And here's what's remarkable.

The rules that Isaac Newton wrote down,

we can extract them from quantum physics.

So perhaps the real way of talking about the world is,

it's quantum mechanical through and through,

and quantum mechanics suppresses the weird stuff

in the micro world as things get larger and larger,

making the world appear to play

by the rules of Isaac Newton.

But fundamentally, everything is quantum mechanical

through and through.

All right, a user at Quora asks,

If nothing can escape a black hole,

how do they eventually die out?

Stephen Hawking determined way back in roughly 1974

that black holes may not be as black

as the mathematics

of Einstein's general relativity suggests.

Particles in a sense

can leak out of a black hole quantum mechanically.

And as particles slowly radiate from a large black hole,

the black hole gets smaller and smaller,

which means the intensity of the radiation

becomes bigger and bigger

until at the very end there's a gigantic burst of radiation,

which in essence is the death throes of a black hole.

A black hole with the mass of the Sun

will take about 10 to the 68 years

to radiate enough matter

that the black hole will shrink down

to its final state of nothingness.

But what happens then? Nobody knows.

From explain like I'm five subreddit:

If an electron can be two places at the same time,

how do we know those are not two different electrons?

We fire a single electron into an experimental context

in which the results that we find can only be explained

if en route to the detector,

the electron took two different pathways

from start to finish.

Since we only put one electron into the experiment

and we only get one electron out from the experiment,

when we find in the intermediate stage that the electron

is in some sense at two places at the same time,

it's only one electron that's doing that, not two.

From @drinkmoregravy:

I just learned about the block universe theory,

and now I'm having a major existential crisis.

It's funny that you're having an existential crisis.

To me, the block universe idea

is very comforting from an existential standpoint.

So what's the basic idea?

The idea is rather than thinking of the past as being gone

and the future is yet to be,

we imagine that all of time is out there within this block

that has all of time and all of space.

From that standpoint, we always exist

because the block itself containing all events,

it never changes.

So in that sense, nothing ever dies.

Everything that exists at a given moment

always occupies that position in the block.

From @miniapeur: Is string theory dead?

Spend too much time scrolling on the internet,

you might get the impression that string theory is dead.

But the fact of the matter is,

string theory has been making enormous strides

in understanding black holes

and understanding the nature of space and time.

String theory has not been able to make any predictions

that we can test in the laboratory,

but please bear in mind, physics is more than that.

Physics is trying to understand a coherent description

of the external world.

We don't have such a coherent description.

Gravity and quantum mechanics are at loggerheads,

but string theory is the best developed

at putting the two pillars of physics

together into one consistent framework.

On that front, string theory is absolutely

an enormously important development in the history of ideas.

From DevonSkenandore:

If everything has a beginning, then when did time begin?

And if time had a start,

what was here before that beginning?

There's nobody on our planet

who does know the answer to that question.

We do have ideas.

Let me just throw out a couple of them to you right now.

It could be that there is a beginning to time.

An analogy helps grasp what that would mean.

You know, if you're walking on planet Earth

and you want to go further north,

you can pass by someone and say,

Hey, can you point me in the direction of further north?

They point north where you keep on heading,

you pass somebody else.

Yeah, point me in the direction of north.

They point you toward the North Pole.

When you get to the North Pole

and you say to someone there, How do I go further north?,

they'd look at you kind of funny and say,

The concept of further north

than the North Pole is meaningless.

The North Pole is where north begins.

Similarly, it could be the case

that you can imagine going back a hundred years,

a thousand years, a billion years.

When you get to the Big Bang,

it could be that the concept of going further back in time

is as meaningless as going further north

than the North Pole.

It could be that time itself simply began

with the Big Bang itself.

There are other ideas that people have developed

where there is a prehistory to the Big Bang.

In fact, there are some theories

that envision that our Big Bang

is simply one expansion of space event

that is one of many times that the universe

has undergone this kind of rapid outward swelling,

which means there could be other Big Bangs,

other universes populating a grand multiverse.

From Jerswar at the explain like I'm five subreddit:

What is the big deal with quantum computers?

In quantum mechanics,

a particle can be in multiple places at the same time.

A quantum computer leverages that multiplicity

to carry out multiple calculations in essence in parallel,

which gives you a multiplier effect

on how fast the computer can in principle get to an answer.

But there's a second part of quantum computation,

which is, it's not enough

that it does all these calculations in parallel.

How do you find the answer among that multiplicity

that you are looking for?

And so for a certain class of questions,

a quantum computer can give rise to an exponential speed-up.

But for other questions, the exponential speed-up

that works for other problems won't be applicable.

And so my suspicion is that quantum computers

will be a very niche-oriented kind of device

that are really good at a certain category of questions

like simulating quantum systems

or factoring numbers into their prime factors,

but unlikely to be an all-purpose machine

that sits on your desk or on your lap

and does the kinds of things that ordinary computers do,

the kinds of things that we are familiar with.

From @kero_sock: What the [beep] is quantum entanglement?

In the everyday world, if two things are far apart,

we know that they can act independently of one another.

But in the quantum world, two things can be far apart,

and yet they are locked together

from a behavioral standpoint,

even though they are far apart in space.

Imagine you've got a particle whose spin, we call it,

is pointing up or down.

It's in a mixture of both of those.

And you can have two of these particles.

If they are quantum entangled,

a measurement on one of these particles

coaxes a single definite reality.

The other particle, if they're quantum entangled,

will also snap out of the fuzzy haze

and acquire a single definite reality.

Even though they're far apart,

quantum entanglement bridges the gap between them

and allows their behaviors to be tightly choreographed

in a way that we're still struggling to fully understand.

But the laboratory experiments are clear.

What you do here quantum mechanically

can have an impact over there

if the two objects are quantum entangled.

Next question is from the change my view subreddit.

Schrodinger's cat seems like a dumb concept.

Maybe you don't fully understand the idea

of Schrodinger's cat then,

because it's anything but a dumb concept.

Back in the 1930s

when quantum mechanics was being developed,

you might have tried to argue

that the weirdness of quantum mechanics

can be sequestered into the microscopic world,

and we simply don't need to think about it

or worry about it in the macroscopic world.

Schrodinger's cat was introduced by Schrodinger

to lay that thinking to rest.

And he basically said, look, if you have an electron,

a little particle that's in a 50/50 mixture

of being here and being there,

you might say, well, okay,

I'm willing to accept a particle in some mixture

being two places at the same time

because it's in the microscopic world

and I don't need to worry about it in the macro world.

And Schrodinger says, no.

We can amplify the weirdness

of the micro world into the macro world.

All we need to do is set up a little gadget

which has poison that will be released into this box

if the electron's on the right-hand side,

but it won't be released into the box

if the electron's on the left-hand side.

And then if we put a cat in the box,

the poison is in a 50/50 mixture of being released

and not being released,

which means the cat must be in a 50/50 mixture

of being dead and alive.

Quantum physics, he was saying,

its weirdness necessarily has a trajectory

whereby it can infect the everyday reality

that we experience.

@ResonanceEnter1: Does observation create reality?

There are physical systems where if there is no observation,

there is no measurement,

the system evolves and behaves one way.

But if there is interaction and involvement,

that interaction does change how the system behaves.

A single photon,

if it sufficiently impinges on a particle like an electron,

that photon can coax a definite reality to emerge.

One of the deep questions in quantum mechanics

that we have not yet fully understood

is why is it that an observation or a measurement

can have such a radical impact

on a quantum mechanical system?

Somehow, intervention, environmental influence,

observation or measurement is important to this transition,

but exactly how this happens,

this is something that we are

still struggling to understand.

El-Kabongg: Why do atoms not run out of energy?

We've known since the 1800s that as electrons accelerate,

they radiate away energy.

And if they radiate away energy,

that electron should spiral into the nucleus and crash.

So why doesn't that happen?

The answer, according to quantum mechanics,

is that there is a lowest energy state

that's available to a system like an atom,

such that there isn't any way in the world

for the atom to achieve a lower-energy configuration.

And so quantum mechanics itself places a limit

on how close the electron can get

to the nucleus of the atom.

We can actually calculate that lowest energy state

and then we can compare it to the energy states of atoms.

Energy can be quantized.

It can come in chunks that can't be subdivided.

They can't be diminished.

From the Manuko subreddit:

At the smallest level, we have quantum physics.

At the normal level, we have relativity.

Is there another set of laws for super big things?

You're right.

For the very small, we have quantum mechanics,

but I would say for the super big,

we have general relativity.

That's Einstein's theory of gravity.

And gravity becomes ever more important

when things are ever larger, for stars and black holes.

We are the in between, the very small and the super big,

and we are necessarily a blend.

And so there are things in the world

that we are familiar with

where quantum mechanics rears itself

and flexes its muscles more strongly.

There are other things that we encounter

that have general relativity flexing its muscles,

but we haven't found that there's a new set of laws

that we have to introduce to understand the in between.

Instead, our challenge

is to put general relativity and quantum mechanics,

so the calculations that we human beings

in the middle actually do,

we want those calculations to make sense

and to make sensible predictions.

And that is a blending that we have been trying to do for,

well, I don't know, 80 years.

From @ignorethatdoor: Why does anything exist?

In the beginning, there was totally nothing,

so how did something come from nothing?

We don't know why there is something rather than nothing.

All we physicists can do

is we assume that there was something.

Energy, matter, a configuration

that can give rise to the Big Bang.

And then from that point forward,

we analyze what happens to that something,

how that something manifests as particles

that then come together to yield stars, galaxies,

and so on and so forth.

But if you push further back and ask,

Why is there anything at all?,

nobody really has an answer yet for that.

There is one idea that I can just throw out.

A state of absolute nothingness may be unstable.

A state of absolute nothingness

may quantum mechanically fall apart

into a something and an anti-something.

Do we know that that's the case?

We don't, but at least it's an interesting idea

for how you could start with nothing and yield a something.

@troll_on_patrol: What happens after the universe dies?

I wrote a whole book on it.

In fact, it's the bottom book

in this little stack under here, Until the End of Time.

We don't know for sure,

but one dominant possibility is that in the very far future,

the universe will continue its spatial expansion.

As it expands ever more widely,

material objects will continue to disintegrate.

Stars will disintegrate. Planets will disintegrate.

Even black holes disintegrate into a bath of particles.

And so that suggests that in the very far future,

the universe will be nothing but a bath of particles

wafting through an ever larger,

ever quieter, ever colder cosmos.

Long before any of these structures

existed in the early universe,

it was just particles.

Those particles come together for a brief period of time,

giving rise to the stars and galaxies and planets

and on this planet people as well.

And then in the far future, it all disperses once again.

So everything that we know about, everything we care about,

everything that matters to us

may just be a momentary aggregate of particles

that comes together for a brief period, then falls apart,

and then for effectively an eternity,

there's nothing but particles wafting through the void.

All right, that's all for today. Thanks for joining us.

Hope you learned a little bit

about quantum physics and the universe.

This is Quantum Physics Support.