Theoretical Physicist Brian Greene Explains Time in 5 Levels of Difficulty | WIRED

I'm Brian Greene and today I have been

challenged to explain a topic in five

levels of difficulty

we're going to be talking about the

nature of time the most familiar and the

most mysterious quality of the physical

Universe there is nothing that we

experience that does not take place in

some duration of time so if you can

understand time you're on your way to

understanding reality

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hello hello what's your name Kayla how

old are you Kayla I am nine years old so

if you're nine years old what does that

mean about the Earth how many times has

it gone around the Sun nine times nine

times so there's a relationship between

motion through space the Earth is going

through space

and the passage of time they're kind of

connected in some way but there are

differences right

if I asked you to move through space you

could do it freely right can you get up

and let's see how easy it is to move

through space can you move over to that

location and can you come back

anything getting in your way easy to do

yep if I were to ask you to sit

perfectly still in space can you do that

I mean hold perfectly still

that's good but if I ask you to hold

still in time

to not go to the next second or the next

second

can you do that no so there's definitely

this difference between space and time

some fundamental quality that

distinguishes how freely we can move

through space versus how freely we can

move through time have you heard of

Albert Einstein yes what do you know

about him yes please

here and I think I may be heading in

that direction actually he showed us an

approach to travel to the Future you

want me to tell you how you do it

you build a spaceship you go out into

space really quickly you turn around and

you come back to planet Earth and he

showed us that when you're on that ship

your clock will tick off time more

slowly you will age more slowly so that

Journey may only take you say a year six

months out and six months back but you

know what when you step out of the ship

it'll be a hundred years into the future

or a thousand years a million years into

the future would you do that if you

could I would probably be dead by that

no you'd be alive that's the amazing

thing I'd be to everybody else would be

dead who stayed on Earth but your body

would only age one year and yet it would

be a thousand years into the future the

question though is could you get back

and I don't know the answer to that

nobody does we don't know if you can

travel back but we certainly know that

you can travel forward has anyone ever

tried to go forward and back I don't

think so that same guy with the crazy

hair Albert Einstein showed that there's

actually a limit to how fast things can

go and you know what the limit is the

limit is the speed

light because light travels

671 million miles per hour that is fast

enough to go around the entire Earth

seven times in one second so if we could

build a spaceship that would go as fast

as light we'd be able to do what

Einstein noted there's something else

that's really curious about time things

tend to go in One Direction and we call

it the arrow of time it sort of points

from the past into what we call the

future if you'd ask me why is there an

arrow to time ask me why is there an

error at the time I'm not really sure I

have some ideas but I'd say we've still

not completely nailed it down Kayla what

have you learned about time from talking

about it here that you can't really

travel back through time and can you

travel to the Future in Prince maybe

maybe that's absolutely right I think

it's unlikely we'll learn how to travel

to the past but it's not been ruled out

that's kind of exciting that it's still

at least an open possibility

if I was to ask you what is time

what would you say time is

kind of strange because it's almost a

man-made idea there is the tangible of

you know how the Earth revolves around

the Sun or how we orbit around ourselves

it's almost in a way does it exist if

the way that we measure it is man-made

before there's any life on planet Earth

I think we all agree the universe

existed did things change before there

was life on Earth yes and how would you

talk about that change without invoking

this concept of time it's difficult to

talk about something without adding time

into it even if it is a human-made

concept that the Universe evolved

developed changed Through Time

ultimately giving rise to Galaxy stars

planets and on this particular Planet

life that conception of time gives it

feel that it's like Universal that it's

out there it's the same for everybody

it's independent of our actions or

activities do you know that Albert

Einstein shattered that view of time he

found that if you and I say have

identical wrist watches I'm sitting

still and I'm watching you move I will

find that your clock is taking off time

more slowly than my clock you know it's

really remarkable you can figure out

this quality of time if you know one

fact that the speed of light is constant

have you ever heard that phrase my

freshman year physics class yeah if

you're clever you can use that with high

school algebra maybe even a little high

school trigonometry to make it even

easier to derive that clocks take off

time at different rate do you want me to

show you how that goes yes please

right so to figure out the effect of

motion on time I'm going to use a really

simple clock it's called a light clock

it's two mirrors

that are facing each other and what we

do is we have a little ball of light

called a photon right that goes up hits

the top mirror then comes back down and

hits the bottom mirror and every time it

does that you go Tick Tock that's one

unit of time imagine now we have another

one of these light clocks but I'm going

to have it in motion now what do you

notice

about that path it's much longer it's

much longer right this one's going tick

tock tick this is going to go tick

tick tock in fact we can figure out the

ratio let's consider time on the

stationary clock compared to time on

this moving clock well that ratio is

going to be the ratio of the lengths

then this would be given by D over L

more time on the stationary because it's

longer distance on the moving clock well

this length over here

that's the same as this L over here

right so we want D over L now you may

have recalled that in trigonometry

there's a name for L over d

sine of theta

opposite is l

hypotenuse is d right and so this ratio

is just 1 over

sine of theta so if we can figure out 1

over sine Theta we'll have our beautiful

formula for the ratio of time on the

stationary clock to time on the moving

clock we just need one other fact the

speed of light along this diagonal is

equal to what we call C C equals speed

of light

but in order for this ball of light to

hit that point on the mirror the

component of the speed of light in the

horizontal Direction better be keeping

perfect Pace with the speed of the clock

itself so let's assume that this clock

is moving in this Direction with the

speed equal to V so c times cosine Theta

must be equal to

the speed of the clock in motion and

from this we learn that cosine Theta is

equal to V over C

there's another beautiful identity you

may recall from your study of

trigonometry that sine squared theta

plus cosine squared theta

is equal to one this is really just a

Pythagorean theorem in Disguise and from

that we can now solve for sine squared

theta equals 1 minus V over c squared

and if for sine Theta is the square root

of this and now we're basically done

because

we already had over here that this ratio

is 1 over sine Theta which now is 1 over

square root of 1 minus V over C

squared so you see as V gets very close

to C this gets very close to one one

minus something very close to one is

very close to zero one over something

close to zero is huge which means the

ratio of time on the stationary to time

the moving that can be a huge number as

the speed of the moving clock approaches

the speed of light now I did this for a

light clock but it's true for any clock

and this is what Einstein discovered in

1905 with his special theory of

relativity do you think that in the near

or foreseeable future of humans as we

know ourselves now will there be a time

where we are using these formulas and

these Concepts in our daily lives as

technology progresses the barrier

between the limitations of experience

and the truth of how the world behaves

in extreme environments will be moved in

the very same way that you know we can

toss a pack of gum and we know where to

put our other hand to catch it well we

have that kind of intuition about these

ideas I think it's quite possible

what are you studying right now I'm

doing physics and computer science so

have you spent any time thinking about

this weird quality of the laws of

physics that there's no mathematical

distinction in the laws between forward

and time and backward in time is that

something you're familiar with yeah and

the one thing that really confuses me

there I'm thinking about uh one of the

most basic things we learn I guess from

Interstellar is that the universe is

expanding or space is expanding and so

I'm thinking how does that square with

gravity and electromagnetism which is

like kind of predicated on the density

of charges or masses the fact that space

expands is perfectly compatible with our

understanding of all the forces of

nature because all of them continue to

operate in a somewhat more subtle way

but we have a beautiful

prescription for taking any law that we

understand in the simpler context a flat

space time and juicing it up so that it

works in a curved space-time the more

philosophical question is in any of

those formulations if you replace T by

minus t and you do it properly in the

equations the equations still work but

if past and future are kind of treated

on equal footing in the fundamental

equations why are they so different from

the perspective of experience when

you're talking about the perspective

experience could you is that just human

subjective experience or actual

observation for physics well certainly

it starts with human subjective

experience but then we are able to

elevate it to a more objective

description for instance when we

introduce words like disorder and order

an entropy in the second law of

Thermodynamics and the equation usually

the way we say it is s equals K log W

entropy equals boltzmann constant times

log of a particular quantity which is

ultimately counting the number of

distinct configurations that A system

can be in what boltzmann and others

showed is that entropy

tends to increase toward the future but

the key word there is tends to increase

so this Arrow of time going from the

past to the Future rests on a curious

Foundation it's a statistical Foundation

which says that it's more likely for

eggs to Splatter than unsplatter it's

more likely for glasses to smash than

unsmash but not that it's impossible for

those things to happen

you just have to wait a really long time

for there to be a reasonable chance of

it ever taking place when you said that

um you know it's more likely for an egg

to smash or for glass to smash and

that's probably because there's so many

atoms so much stuff going on yes but I'm

thinking if we zoom in on like a single

thing I guess do we have variations that

are extremely unintuitive because you

know things can happen in a way that

isn't a statistical average if I take a

film of that electron it's a little

fanciful to describe it that language

because you know about quantum mechanics

and so forth but I take a film of a

little particle moving around and I show

you that film you'll be really

hard-pressed to determine whether I've

shown you the film running forward in

time or whether I've shown you the film

going in reverse time if I had two or

three or four or a gazillion particles

into the mix then it will be much easier

for you to determine whether the film is

going forward in time or backward in

time but order and disorder don't have

much meaning if there's only one

particle and that's why the fundamental

laws don't draw a distinction but the

macroscopic experience does but it

raises a key question where'd the order

of the egg come from if everything goes

toward disorder how do I get this

orderly collection of atoms called an

egg well you probably would say from a

chicken but then I say to you where'd

the chicken come from and you say from

an egg but there's actually some real

insight we can draw from this because if

we keep going back with the chicken and

egg story we'll go back through the

evolutionary lineage of Life we'll go

back to early moments of the sun and the

Galaxy and ultimately the universe each

step taking us to greater and greater

order so we believe that the ultimate

source of order is the Big Bang itself

highly ordered beginning called the bank

and we have been living through the

degradation of that order ever since we

still don't really have a solid

explanation for why the Big Bang had to

be or was high highly ordered at the

moment it's really a deep assumption

Einstein we wondered does time change

with speed and that's another change

with that before we didn't think

possible but I guess we found out

eventually some of the fanciful ideas I

guess there's a tiny sliver of Hope yeah

not only did we find that time changes

with speed and special relativity but we

also found that time changes with

gravity Einstein showed that the rate at

which a clock ticks slows down based

upon the stronger gravitational field or

gravitational potential actually that it

is experiencing I think you mentioned

Interstellar before do you remember the

scene in interstellar they're going to a

planet that's near a black hole they go

down to the planet and they spend just a

couple hours there but when they go back

to the ship it's 23 years later on the

ship because time is elapsing slowly

near the strong gravitational field

comparatively quickly far away and

that's not science fiction that's

actually how time behaves I've always

heard people say oh general relativity

you know it might not seem applicable

but you know GPS due to satellites yeah

we could synchronize those Clocks by

accounting for relativity well but

that's even a really really good point

the GPS would become completely

inaccurate in a very short period of

time if the satellites weren't taking

account of or the software wasn't taking

account of the fact that time elapses

differently for the clocks on the

satellite compared to the clocks down

here on Earth so we walk around with

general relativity in our pockets even

though most of us perhaps don't really

know that

have you started any projects yet or

that's I'm just starting one right now

I'm trying to figure out how stars in

the galaxy are moving based on what

they're made of which is interesting

clearly time comes in to what you're

doing to what extent do you have to

Grapple with some of the subtle features

yeah with my research in particular I

really want to know what happened in the

past and what happened in the future but

you only get a single snapshot when you

look up at the night sky right but if

it's officially far back you're right

and so we can learn a lot by looking at

other galaxies and seeing what they were

doing in in their present I suppose yeah

just figuring out what's gonna happen

next is is part of part of the issue

yeah that's looking into the future I

suppose and so have you taken general

relativity are you taking that now or I

took a course on general relativity yeah

but you learned about black holes sure

one of the weird things and wonderful

things about wormholes is that they are

tunnels if you will shortcuts from one

point in space to another point in space

but once you have a shortcut from here

to there the beautiful thing is if you

move the openings

time will elapse differently at the

different openings so there's a

possibility of wormholes as time

machines go One Direction you're going

into the future go the other direction

you'd be going into the past but that of

course raises philosophical and logical

Paradox paradoxism sort so what do you

think what do you I throw it to you yeah

I've heard a few different theories that

people pause it maybe it is Back to the

Future and you really change your own

Universe I've also heard people say that

you could have a multiple universes

Spawn from this event or something yeah

that's that's if you're going to be able

to change the past that's the one that

resonates most with me I think the same

yeah so you go into the past and maybe

you can prevent your parents from

meeting but you're preventing them from

meeting in a parallel reality which

means that you will never be born in

that reality but the origin of your

birth is still completely understood it

was in the universe from which you

originated another one that's more

subtle is the laws of physics May

prevent you from interceding right and

that raises uncomfortable issues for

many people having to do with like free

will I'm immediately uncomfortable yeah

so that one there's actually some people

like Joe

who did some wonderful studies of

billiard ball tables where you imagine a

billiard ball goes into a wormhole comes

out and hits the very ball that was

going into the hole and in that way if

it could knock it of course we seem to

be in some logical Paradox absolutely

but the finding was the ball can come

out and just sort of graze the other one

but it can't affect it enough to prevent

the sequence of events from happening

and the way I like to think about it

frankly is if there's one Universe not

parallel universes like in the other

solution moments in time

just are they don't change the whole

point of time is the variable along

which change can happen so if you have

the atoms of time the individual events

there's no conception of them changing

so whatever collection of influences

were in play that allowed your parents

to meet they will always be in play

because you were always part of that

moment do you think travel back to the

past is impossible because of a deep

physical like mathematical reasoning or

just because of all of these problems

that you've been talking about I suspect

that when we fully understand the

mathematics of the final physical laws

if we ever come upon them I think

there's going to be something built in

okay that prevents this kind of free

travel to the past but sometimes I I

wonder if that's just coming from a more

emotional place where I sort of want the

world to be safe from this kind of

paradoxes so what pretty clear based on

any of the hypothetical proposals for

traveling to the past that have come out

of physics you can't

travel to a moment in the past before

the first time travel machine is built

sure you know the twin paradox yeah sure

where you know you fly off looks like

you're moving fast but to you it looks

like the other guy is moving fast so who

actually ages more who ages less yeah a

resolution I've heard is that because

you have to be going away and then

coming back you had to accelerate at

some point and this breaks the the

ambiguity yeah what if say the universe

isn't flat what if the universe is

curved and you go off in One Direction

and then you come back in the same

direction you pass by the Earth who's

older then yeah do we have an answer

yeah we do have an answer so the

simplest version of that is imagine that

the Universe has a shape of a donut

imagine I'm on the circular part of this

donut Universe all right and imagine I

turn on two laser beams sending a beam

of light going to my right and to my

left and these beams will go around the

entire entirety of space and will both

come back and at some point they will

hit me imagine they hit me at the same

moment from my perspective now imagine

someone's moving relative to my frame of

reference say to my left they do the

same experiment they fire the beam of

light left and right notice that the

beam that they fired to their left will

have to travel farther to reach them

because they're moving away from it

whereas the beam that they fired to

their right will not have to travel as

far because in some sense they're moving

toward it the two beams of light will

not hit that moving Observer at the same

moment to you or to them to them wow

which means that there's a preferred

frame of reference in this universe

everybody is not on equal footing as

they are when we teach to freshmen the

special theory of relativity that's

right exactly no everybody else is

moving relative to me and it's real

emotion so everybody else will be like

the moving twin they will be younger and

I will be older and when you think about

past and future on a cosmological scale

there was a long period when there were

no human beings in the universe the fact

of the matter is there will be these two

long stretches with our presence being

sort of a flicker in between to that

thought inform anything about how you

live in your brief flicker within that

brief flicker I rage against thinking

like that

yeah I think that's the perfect way to

put it yeah because it might be a brief

flicker on a single moat of dust like

floating in a cosmic eternity yeah but

it's everything yeah there's nothing

else that I'll ever experience and so in

a way there's there's nothing else to me

yeah there's an eternity but I'm never

gonna see it I'm never gonna feel it it

can be debilitating to imagine an

eternal future of sort of nothing or

none of what we do sort of persists on

the other hand if you flip your

perspective around and say how

remarkable is it that we have this brief

moment that allows us to think and feel

and love and explore and illuminate wow

how wonderful is that yeah

so often when we try to give the basic

idea of what time is I'm fond of saying

look space is the language that allows

us to say where events take place and

time is the language that allows us to

specify when they take place where would

you jump off from there and trying to

give a deeper understanding of the

basics of time maybe part of the

distinction in between time and space

it's that you have a clear irreversible

Evolution so how to explain what time is

well it's a parameter that is measured

by clocks I mean at the end this is what

we know and allows us to talk about

change and also that we know how to

describe in terms of some set of

equations which gives a clear sense of

causality to note our recent paper where

we were thinking about Einstein's ideas

of special relativity but in a setting

where the global shape of the space-time

if you imagine there might be a curled

up dimension of space a circular

Dimension and thought about how special

relativity Works in that setting we came

to result in this very basic setting you

can send signals into the past not in a

way that will violate causality did that

surprise you that by going from the

usual topology that doesn't have a

closed part of space but making that one

change you could have this radical

impact I found quite surprising that you

could have this exotic behavior of a

signal propagation in a system that was

extremely simple totally classical there

was nothing weird except one dimension

that was compactified was identified on

a circle actually even the standard

vanilla

causal structure of special relativity

may give some very unexpected behaviors

when you combine it with other simple

modifications of just the flat space

time and the beauty of it is it's not

like there is some high-powered

mathematical methodology

straightforward algebra that a high

school kid would know is all that you

need to extract these unusual results

and they were unusual because even if

you do not violate causality we

discovered that a fast moving Observer

that could be us on a rock it could send

signals verifier and then back in a very

small time so the old idea that of

course we always hear about that if we

ever make contact with extraterrestrial

life they're far away we can't really

have a conversation because we'll say

hello and then like 10 000 years or a

hundred thousand years later they'll

answer us because it'll take that long

for the signals but at least in this

setup which we don't know is true of our

universe but if it were then you could

have a real time Conversation Over

arbitrarily large distances yes which is

that was unexpected yeah and so that

shows how even ideas that seem to be

well settled and well understood have

surprises like why didn't Einstein

realize this yes yeah maybe the idea of

living on a Subspace or being confined

in this extra dimension on a Surface may

have been looked exotic but definitely

did not look exotic after the 1990s many

people have begun thinking about the

possibility that space and time may be

so-called emergent quantities that

they're not as fundamental as perhaps

Newton or Einstein would have thought

where do you stand on that idea it seems

not Unthinkable saying that something is

emergent will make full sense only when

we have some concrete model in which

space and time emerging in which we make

sense of this non-space non-time

description of a theory I don't know if

we are still at this stage in which I

would say we begin to understand this

scenario because often I tell my

students the greatest scientific

revolution has been not in the 20th

century it has not been quantum

mechanics nor general relativity no

special relativity it has been the

passage from the qualitative description

of nature to quantitative one when you

pass from asking how to how much right

then you understand something and who do

you credit with as that Newton do you go

a little bit below I would say Galileo

Newton of course the the completion of

this idea is Newton one of the things

that relativity also sheds a light on is

what exists that if someone's moving

relative to me what they consider now

might be in my past what they consider

now might be in my future which would

suggest that all of time exists much

says we're willing to accept that all of

space exists does that hold water

actually it resonates with me for

various reasons one is that

professionally we use space-time

diagrams Penrose diagrams a lot of

diagrams where space and times are just

two axis on a board and when we describe

a particle we have a line that goes up

in time by the way this image also is

the last image improves the last page

there is this beautiful sentence which

is also true saying that if he has the

time he would like to describe people as

being monstrous beings that extend in

time much longer than in space if it's

equal to one that's very true so this

idea that you have a Continuum and the

time should not be made to disappear as

soon as it's gone it's very practical

it's also what is behind the idea of

histories in quantum mechanics when in

the approach of various people including

heart or collaborated a lot with Hawking

there is this idea that what you

describe is a history it's not a

particular moment in time but it's an

evolution this history treats space and

time on a more equal basis but would you

say there's more to it than the

technical more to it than the diagrams

more to it than sort of an

interpretation of the mathematical

equations when you go so far as the text

solace in the fact that that in a sense

that you will always exist because you

will always be at the moments of space

and time that you have occupied

throughout your life well actually

that's probably the only way in which

you can take solace because all the rest

right it's almost children's stories

yeah I mean of course it's something

that you come by yourself you arrive at

this conclusion which is totally not

objective and is part of your personal

history by yourself but it was

interesting to see that for instance

Kurt Vonnegut had exactly the same thing

I said no I mean the only thing that

really makes you not fear what will

happen or your own is that every instant

is eternal instances exactly as each

point in space it's not the space

disappears it will always be there yeah

the idea that something is irretrievable

maybe it's an accident and we go back

again to order the initial conditions in

which we started absolutely

foreign

of time I hope that you have appreciated

the subtlety and the richness of this

quality of the world that we experience

all the time thanks so much for watching

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