Is Quantum Reality in the Eye of the Beholder?

[Music]

welcome to the third in our series of

quantum reality conversations in case

you missed the first two feel free to

check them out but also feel free to

stay here as I'll now give you a brief

summary of where we have gotten so far

all right in our first conversation with

philosopher Elise Crow we discussed the

basics of quantum mechanics namely that

the theory has at its core the idea that

the best you can ever do in our reality

is make probabilistic predictions before

an appropriate observation or

measurement or interaction the world is

described by a Quantum wave function

that contains within it an unfamiliar

mixture embracing many possible outcomes

like particle here and here

you go left and you go right and only

through observation or measurement or

interaction is the reality we have

access to coaxed into a single definite

outcome but how that transition from a

world chock full of many possibilities

to one in which only one outcome is

observed and experienced how that

transition actually happens that remains

mysterious and is called the the quantum

measurement problem okay we also

explored how through quantum

entanglement reality has what we call a

nonlocal quality that is what you do

here can have an instantaneous Quantum

impact on something way over there and

such non-locality we found can thread

not just through

space but also through time all right in

our second conversation with physicist

and author Sean Carol we explored a

number of proposed resolutions to the

quantum measurement problem most notably

spontaneous collapse theories as well as

the many worlds interpretation of

quantum mechanics in which every

possible Quantum outcome actually

happens but each takes place in its own

Quantum Universe it's not a particularly

new idea being introduced way back in

Hugh Everett's doctoral dissertation in

19 57 but it remains a leading if

controversial Contender for how Quantum

reality actually

operates in this third

conversation with physicist and author

ker relli we are going to continue our

exploration of quantum mechanics and

this mysterious transition from a haze

of quantum possibilities to something

like the definite world we each

experience because relli himself has

developed his own Theory for how this

may come about all right let's jump

in Carla relli is the director of the

quantum gravity group at the center for

theoretical physics at X Marse

University in France he is a co-founder

of the loop approach to quantum gravity

and an author of several books

popularizing science please welcome

Carlo

[Applause]

relli so thanks thanks so much for

joining us really appreciate it it's a

really pleasure so I think you've heard

you know some of the discussions about

quantum mechanics the quantum

measurement problem where do you come

down is is the Quantum measurement

problem something that's vital to

understand in your view do you think you

have a solution to it where do you come

down on

it I actually agree with much what I've

heard from from Eliz and from Sean um I

do think that it's a it's a crucial open

uh problem and that we should or some of

us should uh should work on it uh I

think that we're still confused after a

century as you're saying and uh that uh

uh we do have some ideas of how to think

about

that uh in the way you were talking with

with Sean I think I agree uh there there

are viable ideas the question is which

one is going to be fruitful and useful

and take us ahead

um in in in understanding the world and

there is a discussion going on which has

evolved through the years and is still

evolving there are new ideas coming out

I am hopeful that at some point it will

converge and so you heard that sha is a

great fan of the many worlds approach

does that resonate with you or do you

look at that and you're like you know

much as he says you know grw isn't right

hidden variables is not right do you

look at many worlds and have a similar

reaction or is that something that

you're in favor of as a viable approach

I am exploring a different direction uh

which is a polite way of saying I don't

like that but it's very

nice yeah but uh but it's important I I

I don't think the many world it's wrong

uh I think it's a it's a possible way of

looking at the world uh I think there

are other possible ways of looking at

the world and I think we should we

should work through that yeah uh at the

very beginning started by saying uh the

first choice that we have is how to

think about the wave function yeah

that's uh and one option is to say okay

Shing W function of the quantum State

that's a real thing okay so if I stop

and can I just jump in just so people

have a visual image in mind that was

like that Blue Wave I'm sure you all

know we that encapsulates say for a

given electron the various probabilities

of say being at various locations you

call it probability wave or wave fun

function go from there yes that's right

so one one possibility is to say this

blue moving thing is the actual stuff of

the universe that's what going on um

there is an

alternative which uh I find it more

appealing for a number of reasons which

is to take the opposite perspective

namely that not the thing okay the thing

is a

particle the the actual particle and uh

uh that think there is just a way we

have to compute where the particles

going to show up

next uh In classical mechanics before

quantum mechanics there is a very

similar thing there's a very similar

techniques of using a wave all over to

compute where the particle is going to

go next it's called the um Hamilton jaob

and you have something in fact the

classical limit of the Shing function is

the Hamilton jacobe so one possibility

to to sort of try to get an intuition

about quantum mechanics is to think well

that just calculations our way of

thinking uh what's going to happen next

and since as you have been all

emphasizing a lot uh the first great

discovery of quantum mechanics is that

it's

probabilistic even if you believe in

underline deterministic Theory uh you

like anybody else you're not going to

say whether the spin will go up and down

right so it's intrinsically

probabilistic as a limitation of what is

going on yeah

then uh it means that what we can do is

a probability calculation and that's way

think is a probability calculation

that's what Max Bourne clearly um got

Noel prize for understanding that but

the probability calculation of course it

jumps right um if I don't know you're

saying simply because you know more I

didn't know enough I mean I I I don't

know what's it whether tomorrow I have

probability that the weather comes out

it's one of them and then my knowledge

jumps when when when tomorrow happens

yes when tomorrow happens or even you

know if I don't know who won a certain

game and just because it had already

happened yeah and I don't know it when I

know it my my knowledge jumps but it's

only your knowledge that jumps right

there are other people who perhaps were

at the game they already have that

knowledge so you're describing a

potential view of the world where

different observers would be in very

different levels of quantum let me let

me get there in two step in two steps um

so let me backtrack one second uh

because there's one thing which is

rarely said about quantum mechanics

which to me seems the core of the theory

which is granularity discreteness

quantum quanta yeah um in the discussion

about the meaning of quantum mechanics

we we tend to forget that and I think

this is this is wrong Quantum mechanic

came out as a description that things we

thought were continuous are actually not

continuous jumps are gra for instance uh

lights is made by gets to me as photons

individual photons if I little little

bundles of light that are bundles of

little little if I have a screen lights

got here if I look sufficiently

carefully just one dot here one dot here

one dot here one dot here or I don't

know atoms have discrete orbits right

and the the electron jumps from one to

the other and so on and so forth there

all this discreteness basis of quro

mechanics which means that I it tells to

me that if you want to think about

Quantum mechanic we don't have to add

things there is less so the electron is

here the electron is there this is this

is simp to me the indication so now let

me come to your question

yes one way you pose the question is

what is a measurement okay and that's I

think the right question the right you

asked to to Le you asked

Sean the way I think could be

useful think about quantum mechanics

which call relational quantum mechanics

is to try to answer this question by

saying everything is a measurement every

time two any two systems interact

they're measuring one another by

definition interaction is a me regard of

who's doing it what doing it a measure

so the screen the the particle touch it

that's a measurement but also an atom

here and the photon bonds it that's a

measurement okay and uh so uh the the

the the the wave function is just

telling us what is the way one system is

going to affect another system what's a

probability distribution of the way it's

going to affect it now this works at one

condition and that's a hard point and

that's I think what quantum mechanics is

deeply telling us um the condition being

that the actual result of the

measurement the r of interaction doesn't

display a property of say the particle

but discls a a relative property of the

particle who spting the screen

that's

contextuality now so just so I can

understand if I'm measuring like our

spin a half particle that we had before

and you see it up and I see it up you're

saying it's not that the particle is up

that's right it's that I and the

particle stand in a particular

relationship which is what we would

normally call the particle up but it's

not a property of it it's a property of

us both that's correct that's correct

which means that every time you say the

particle up which is fine what you

really should say the particle is up

respect

exactly and if you do that I believe

things go in order so the So-Cal

paradoxes of quantum mechanics go in

order the cat uh which interacts with

the quantum staff in the Box the shingle

cat it's either Alive or Dead with

respect to himself and do you have a cat

yeah that's right so with respect to the

cat the cat is just alive or dead

because the quantum object did this or

that but with with respect to me who are

outside the box and looking at that

careful the cat is neither dead nor

alive because with respect to me I uh

neither of these two are are are

realized so whatever I see next I

shouldn't assume that we SP to me one of

the two

happens just want make sure and it's

clear in your approach obviously it must

be but that if two different observers

are are interacting with the same object

they'll never find any kind of

contradiction you know if if the spin up

is not a property of the particle itself

you might wonder that you know person a

stands in the spin up relationship but

person B might stand in the spin down

relationship that's exactly the what has

been discussed I mean this idea of

relational quantum mechanics came out in

the '90s yeah and it slowly grew um sort

of number of people interested grew

slowly now there's a lot of paper coming

out and for you started it this was a

your idea is that yeah that's that

started off with paper of mine in the

'90s but then was developed first by

philosophers number of including

important philosophers uh Bas van fren

for instance wrote the paper on it and

and others now is is getting more in the

foundation of physics um attention um

and the first part of the discussion was

always do does this create contradiction

yeah there was a lot of debate consider

this case this case this case it doesn't

that that's the point there is a

coherence in quantum mechanics itself so

it doesn't create contradiction so the

idea here is that instead of adding uh

you know manyu world or adding hidden

variables or adding a a a g g gwr um

extra collapse and other uh

takeway uh the the the properties of any

object are always relative to some

else and object have properties only

when they interact now in some sense is

there a many worlds like quality to this

descrip it's not so different from many

worlds in a sense because if the

particle isn't spin up on its own and

it's a relationship then the other

possibilities in some sense are still

there still are still there yes in fact

it's still there

um the idea that properties are

relational it's sort of all over physics

if if you think the great

um the great step in going into into

Newtonian physics in the Renaissance

with Galileo Kepler and and so on was to

answer this question uh what is the

velocity of an object right uh is this

moving no it's not okay with respect to

us with respect to us yeah but it's

moving with respect to the sun okay

so the velocity of an object is not a

property of the object it's a property

of the object and something else it's

it's a relational property and in a

sense the relation to Quantum Mechanics

is making this very general not just

velocity uh but all the properties of an

object have to be thoughted uh uh uh

relationally with respect to something

else now when you say all presumably not

really all like the mass of a particle

presumably is not in that category

contingent conent properties the one

that change

face Bas so and so in in this approach

how does this illuminate say

entanglement you know so I mean I asked

an unfair question saying you know you

got these two distant particles you

measure one you find it up the other one

is down I said how does that happen and

of course we don't really have a story

to tell that's really convincing in the

usual approach do you have a story to

tell in this relational approach that

sheds more light on it yes it is a story

um which is uh doesn't take way the

strangeness of the phenomenal phenomenal

is strange and it remains strange uh it

sort of stor that shifted into the

strangeness of quantum mechanics itself

the story is the following um if you

imagine that you measure something here

and you measure something there you're

cheating because who is seeing here and

there at the same time nobody uh to

compare that they have seen the same

thing you have to wait until they

communicate to one another and once they

communicate to one another say the

information is is is sent um then you

can compare the so now let's see let's

see what is the world with respect to

the the the the final Observer that get

the information there's no nonn locality

anymore because all the information is

uh is so but when I when I unravel that

story wouldn't I still need to explain

why there's this correlation that

whenever this one up that one's down

whenever this one's down that one's up

which would be surprising if they're

both just 5050 up and down and not

somehow talking to each other because

you are oh you're get saying I'm I'm

assigning the property to the particles

still Yes okay exactly exactly exactly

exactly so the the the the idea here is

that think that the mistake is always to

assign absolute properties to particle

instead of relative to something else

and so are are you able to push this

approach to say um uh relativistic

quantum mechanics I mean is this

something yeah yeah it's sort of in fact

it's uh I got to there through in a long

way through quantum gravity because uh

because that's my job sure to write it

try to write a Quantum three of gravity

even if I think the problem of quantum

mechanics is separate by the problem of

quantum gravity uh and nevertheless and

I very much agree with sha in that I

think that once we understand better

quantum mechanics this should help us to

understand better quantum gravity and I

do agree with with Sean a lot that um

there is something deep to understand

about the the quantum structure of space

time

uh by somehow a clear idea about quantum

mechanics and let me step back a moment

uh I started by saying granularity

discreetness see

uh quantum mechanics has a constant H

bar yes okay unless we understand that

constant uh we don't understand quantum

mechanics so interpretation Quant

mechanics you tell me what this constant

is H bar right this is a it's a number

six comma 6.31 whatever I like the

number one in the correct units but yes

you one in the correct unit it's like

the speed of light right but the speed

of light you know it is is the the

fastest thing which I can go in some

proper sense and uh and uh and that

number it's just the size of the

granularity it tell us how big are the

the Quant okay that's that's why is the

core story of qu of of of quantum

mechanics now when you apply quantum

mechanics to gravitational field to

gravity as you were saying we shown

before uh gity is different in the sense

that it's actually space time itself

that that that that moves so like the

light is uh if you look in this small

it's

photons space time if you look into this

small should be Quantum space and that H

bar should say how bigger this Quantum

space together with Neutron constant and

so on and one gets to the to to the

plank scale and that's a core result of

loop quantum gravity the quantum theory

of gravity in which which I'm working is

a tentative Theory you don't know if

it's right yet

so the structure of SpaceTime is all

this quanta this granular so space space

is this grain of space which are quantum

space like photons but they don't live

in space they make up space themselves

and the way they are connected to one

another it's one in relation to another

one and if we think about how we think

about space the bound SpaceTime region

has a boundary and we descri how it

affect the rest and in quantum mechanics

we take systems and we describe how the

system affect another system and the two

things should go together I believe and

so this relational way of thinking

should help us in that direction you

know we only have a few minutes left but

I can't help it following that line of

discussion a little bit further because

I think as many in our audience know you

know I work on an approach to quantum

gravity strength Theory you work one of

the of the contributor main contributors

well among many many others but you know

what we have found and again as Sean was

using it is the Royal Wii many in the

field have found that there's now

evidence that the fabric of SpaceTime in

a string theoretic approach is stitched

together by the threads of quantum

entanglement because calculations have

been done where I mean in our visuals

that we showed spin up and spin down

where connected by this sort of

invisible line of quantum entanglement

there can be regions of space that are

connected by lines of entanglement and

we mathematically can cut those lines of

quantum entanglement and the space falls

apart into little tiny pieces and then

disintegrates completely once the

entanglement is fully dissolved so from

our perspective that's given a lot of

insight I me it resonates with what

you're saying but a very concrete means

by which entanglement would be the very

ingredient that holds SpaceTime together

yeah are you finding like a Sim I mean

obviously a different language but are

you finding a similar way of thinking

about SpaceTime and loop quantum gravity

yeah yeah you know that you know we've

been working very different opposite

direction to a quantum gravity but uh

that aspect of uh String Theory found it

um very much interesting and intriguing

and uh it does resonate with something

very similar that happened in Loop quto

gravity including in the mathematics so

the structural spin networks yeah is a

structure of entanglement between little

Hill spaces here and there um and uh uh

I do believe that we have not really

clarified yet we don't have a clarity

about that but there is something

convincing in the idea that the notion

of contiguity who is ATT to whom and the

sort of

entanglement uh they're related yeah

because uh you see the the core of

modern physics after Maxwell in the last

century is locality so what is locality

locality is the idea that interactions

are local so to you don't interact with

something far away inter in some sense

dynamically interact with something

nearby but you can turn this around um

what what does it mean to be nearby it

means you can interact with it directly

okay and if you interact with you get

entangled that's exactly what the core

of uh somehow the relation

interpretation is as soon as you

interact you're entangled from from the

perspective of something right so

between entanglement interaction because

of locality and the SpaceTime contiguity

there should be a common thing and I

think that's the beautiful uh slow

understanding and r of quantum gravity

which is happening nowadays so let me

end on a question that I asked in one

way or another to both Elise and Shawn

which is you know we've been at this for

a long time we have yet to fully unravel

it at all we're making progress but what

do we lack to get to some final complete

understanding of quantum mechanics and

hopefully then be able to apply it to

issues like the quantum nature of

SpaceTime is it is it experiment is it

better mathematics do we need to use AI

systems or do we not have the brain

power what do you think it

is I think just time namely working on

it let me challenge one thing you said

at some at some point you you describe

the Vance the physics of course you were

simplyy you know by sort of we had a

clear metaphysic everything was clear

until in the old days in the old days

and then quantum mechanics experiment

come out and then in the dark of that's

not true right I mean when Maxwell wrote

his equation everybody was confused what

this is about including Maxwell himself

and they very messy equations very messy

equation was incomprehensible and he

thought that it was really D matter

pushing and pulling and rotating them it

took Einstein to understand that's

really not the case and so on I mean

there was a confusion all over right so

being in a state of confusion is not

really uh a characteristic of our time

and when Copernicus I'm even going even

before did the soal copernica revolution

is not everybody jumped up and said oh

yeah right we're moving and we have a

new world picture and that's our world

picture it took a century yeah to go to

Kepler and and and and Galileo

convincing everybody that actually makes

sense we're moving and everybody was

orent confusing exactly because of what

we were saying before we're not moving

how can we moving so you have to

completely rethink what moving means

okay then of course comes Newton and

everything it's it's uh and so on so I

think that takes time between Copernicus

and Newton finally clarify is a century

and a half yeah quantum mechanics is

only a century we we are what does it

take it takes I believe people thinking

about that writing paper debating

getting angry to one another no you're

wrong no you're wrong that's fine that's

how science work it developing theories

and then slowly at some point uh I

believe uh uh some uh perspective will

become more fruitful and will come to

agree on a view um I I think that my

grand grand children I don't have

children I mean think that in two two or

three generations if we don't kill one

another with the atomic bombs which is

if survive uh people will say of course

the shoting ctis is like obvious isn't

it like we say of course in syy people

are upside down right it's obvious yeah

yeah no and it's an amazing thing even

with quantum mechanics today graduate

students speak in terms that's right

that are so intuitive that's you and I

yeah like a little bit you know and go a