Can Particles be Quantum Entangled Across Time?

[Music]

[Applause]

Isaac Newton's insights set the course

of science for hundreds of years but

there's a sense in which Newton's

insights were also deeply misleading

Newton's famous Laws of Motion codify

what we all experience in everyday life

things move and as they do they sweep

out trajectories defined by position

where something is and velocity how fast

and in what direction Something is

moving indeed reality in this framing

comprises these very trajectories by

providing equations to delineate these

trajectories how the position and

velocity of an object change over time

Newton provided an algorithm for

predicting how reality

unfolds and the algorithm Works Newton's

Laws correctly predict where the moon

should be at any moment where the planet

should be at any moment where a ball

should land when

thrown but in the early part of the 20th

century as scientists began to probe the

newly accessible realm of molecules

atoms and subatomic particles newtonium

predictions failed to describe the data

and this failure was not one of f detail

that might suggest a simple refinement

to Newton's equations the failure was

epic suggesting to some that an entirely

new paradigm might be

required that intuition proved

correct and remarkably by the late 1920s

a single generation of scientists

produced that new paradigm with the

discovery and development of quantum

mechanics

an essential feature of the quantum

Paradigm is that the theory is built

around the concept of

probability that is unlike the Newtonian

picture in which we specify how things

are now and the equations predict how

they will be later on in the quantum

picture we specify how things are now

but the equations do something entirely

different they dictate the probability

of how things will be later on and

according to our best understanding the

Reliance and probability is not a

limitation of the approach but rather is

a fundamental feature of

reality the universe in a manner that

Einstein found

unpalatable evolves according to a

mathematically precise game of

chance so why don't we see these

probabilities in the course of everyday

life well the large scales of the

everyday compared to atams and particles

skew the probabilities making one

outcome the almost certain outcome and

that outcome is indeed the Newtonian

outcome but as we consider smaller

Realms the probabilities spread more

broadly rendering the Newtonian outcome

just one among many possibilities whose

likelihoods are governed by the

equations of quantum

mechanics Einstein may have been the

most vocal critic of this direction

physics had taken but even ardin

proponents have struggled to grasp what

quantum mechanics really means for the

nature of reality although experiment

has confirmed quantum mechanics to

astounding precision and scientists have

used it to develop stunning

Technologies many of those questions are

still with us today are quantum

probabilities an intrinsic feature of

reality or an artifact of the quantum

for

formalism how does the world transition

from the haze of possibilities Allowed

by the quantum description to the single

definite reality of common

experience how do we extend quantum

mechanics from description of systems

within the universe to the universe as a

whole is quantum mechanics The Rock

Bottom theory of reality or will it

prove a mere stepping stone to a more

fundamental description still awaiting

discovery

as WE peer into the future insight into

these questions will be essential for

navigating the quantum

Universe good

afternoon thank

you all right so our our subject today

is quantum mechanics and arguably

Quantom mechanics is really the most

profound disruption to our understanding

of the physical universe that our

species has ever

encountered and part of the reason for

that is the description of the world as

we just saw in the piece and as we will

explore here today the description of

the world is so different from our

everyday experience and in a sense

perhaps we should not be surprised by

that because after all our

minds evolved in order that we could

survive and survival and the intuition

that allows us to survive it does not

need to know about the behavior of

electrons and atoms and subatomic

particles this juncture between how we

experience the world and how we

understand the world through observation

and experiment will really be what will

guide our discussion here today we've

got a number of wonderful scientists to

help us think through some of the key

issues we have Elise Kow we have Sean

Carol we have Carla relli and let us now

turn to the first of those

conversations with Elise koll who is an

associate professor of philosophy at the

City University graduate Center and City

College her research explores the

philosophical dimensions of quantum

mechanics caal models as well as

relativistic and temporal

entanglement thank you

so just just to jump right inise I think

all of us are familiar that you know

prior to say

1900 we had a pretty good understanding

of physics right through the ideas of of

Newton and Maxwell and so forth and then

it began to to crumble right and as it

began to crumble a new paradigm came on

the scene

I want to explore that Paradigm but

you've written on the history of the

subject and I think many people perhaps

don't fully appreciate how much of a

psychological and emotional upheaval

this time was for the discovers of these

ideas can you just give us a sense of

what it was like sure um well I can try

I've felt that sort of cognitive

dissonance myself so there's some

first-person experience but um yeah

there was a famous speech given by Sir

Arthur Edington or S not Edington uh

Lord Kelvin thank you Lord Kelvin um one

of the guys in the history of physics

they're all the same yes he said there

were just a few clouds on the horizon

and we've nearly solved it all you know

we have boltzman manian statistical

mechanics we have Newtonian mechanics um

we've we've got um Maxwell's

electromagnetism uh there's just a few

issues and one of those issues was black

body radiation and that's just basically

if you've seen in an oven um whe there

was a known correlation between the

color or the heat inside of it and um

and how it radiated back out and there

was no good model for it and so uh plank

uh sort of looked at the the uh

empirical data and said I don't quite

know what the underlying story is here

yet but I can Cobble together a

mathematical structure I composite uh

this idea that light acts as though

quantized um quantized little little

pieces in bits yeah not just a wave as

it had been thought um and this captured

you know the empirical data correctly

but plank hated it because he said I

don't know what my own math really means

and it took until Einstein in 1905 and

then in 1909 uh to sort of provide that

background story and Einstein wasn't

happy with the background story either

because it required two terms in the

equation to solve black body reation one

of the terms was wav like continuous the

other term had H Plank's constant it was

quantized it was about bits and there

they were sitting together in this

equation uh and that's how we know it to

be today now is there something odd

about the idea of not being happy with

the mathematics after all if the

mathematics describes the data and

that's ultimately what physics is meant

to do why why would someone be unhappy

with it oh because I think uh we're

interested in the deeper explanations

right um or at least that's what

physicists tend to be drawn to uh and

mathematical mapping like capturing of

the phenomena is a part of it surely um

but how that mathematics is supposed to

lend insight into the real behavior of

things um if that's missing then you've

solved a puzzle but you haven't

explained the nature of the universe and

that's sort of the driving uh the

driving motivation I think for many of

these and and we'll get into the details

of you know Quantum probability waves

and issues like the measurement problem

just a moment but given that well

articulated view of what physicists are

trying to do where would you say we are

right now today with quantum

mechanics uh that's a great and large

question uh so I mean I'm primarily a

philosopher so I'm I I get sort of this

perspective uh so don't take it

personally if I say something you don't

like um but I think it's actually a

really exciting time because we're

seeing exactly the limitations of models

we've been working with for nearly a

century now I mean the 100th anniversary

of quantum mechanics is in

2025 um and there's still puzzles

enduring puzzles about what uh how the

mathematics really maps onto the world

and how to explain a lot of the data we

have uh and this gets even more uh

apparent when we get to the plank scale

the very small where our very well-

confirmed theory of general relativity

no longer sits well with our very wellc

confirmed theory of uh Quantum fields

and so on uh and so and the plank scale

just to give you people a sense of how

small that is 10 Theus 33 cm is a number

that we often kick around so it's

fantastically small yeah extraordinarily

small um but I don't know it keeps me

awake at night to think that these two

theories don't don't play well at that

level um but it's it's an exciting time

as a philosopher with with physics

training

because there's more engagement between

philosophers and physicists because

we're talking about theories of quantum

gravity and theories of quantum field

theories and so on that uh not every

piece of them is empirically uh testable

or at least we haven't figured out how

or maybe maybe in principle testable so

some of this is questions of how

brilliant are engineering how clever we

can get about shielding our systems from

external fields and so on um but part of

it is just asking can we have a broader

notion of what evidence we might look

for can we think about for instance

whether there are systems we understand

very well in a different realm of

physics like hydrodynamics or something

that might yield insights into how

Quantum gravitational systems might work

but how do you an analyze the science of

a analogy right how do you know when the

explanation from this one field that's

well known whether it's really saying

something about this other unknown uh

bit of the world or whether it's just

biasing the way you're describing the

narrative that you're telling about the

world and it can go both ways sure so so

getting in a little bit to the details

in the background in the Newtonian

picture if I tell you the initial

conditions you know the the speed and

the location from which a ball say is

thrown the velocity to be more precise

Newton tells us where it will land and

quantum mechanics comes along and says

that's not the case right so in quantum

mechanics there are many locations where

say an electron could land given the

same initial conditions and that leads

to this idea of a probabilistic

description of the world where you don't

say where it's going to land you just

give the probabilities of where it might

be so one way that that scientists were

taken to this picture of matter is of

course with the famous double slit

experiment where you know you're firing

particles at a barrier with two slits

you'd think that the particles would

land on the detector screen in two lines

that are aligned with the two openings

but when you actually do the experiment

of course as we now know for over a

hundred years you don't find just two

lines on the detector screen in fact you

find many lines many bands in a very

particular pattern which scientists were

able to explain by thinking of particles

as waves and as the waves hit the two

openings and they carry on they

crisscross and they interfere with each

other and through that interference we

get a pattern just as in the data if we

interpret of course the waves as waves

of probability where the wave is Big

many of the particles will land where

the wave is small very few will land so

this this now takes us to this this new

paradigm that particles matter should be

described as undula

waves of

probability did it take people a long

time to accept that change because I

would consider that I mean we'll talk

about other things but that's like the

dominant new idea that comes into the

story well Brian I'd argue that there

are many people who still haven't

accepted that what we what quantum

mechanics are saying is that we have a

an irrevocably probabilistic Universe um

and so there are many interpretations

that are offered of this mechanics there

are supposed was to fill this Gap

explain why it is that the formalism

that's sort of shared amongst the

interpretations the sort of core bit of

explanatory work maybe what you read in

your quantum mechanics textbooks which

you all have at home and we'll study

later this evening right um but H the

story there is that yeah we get we get

we have the born rule which is this rule

that tells us how what sort of

probability to expect which outcomes

yeah but no thoroughgoing causal story

of how we get from point A to exactly

point B A well- Defined localized spot

or measurement and that that that story

that you're referring to would start

with this new

probabilistic idea electron 30% here 22%

there 19% there and so forth go from

that which we don't experience somehow

transitioning to when we measure the

electron we find it at one location it's

a kind of schematic representation where

the height of this wave is meant to

indicate you know the likelihood of the

particle being at one location or

another that's the story that we don't

experience but then we go and measure

that electron let's just do it together

3 2 1 measure that electron oo wow that

felt very powerful to do that but now

the probability has spiked because now

we've measured the electron we know it's

at that particular location how in the

world do we go from this weird

probabilistic description

upon measurement to a definite outcome

well I uh that is a deeply unfair

question he's asking me to resolve the

interpretation problem for you um and

you know or even just tell us why it's

so hard yeah well um so first of all I

just want to clarify something a little

bit I mean it's true that at the

macroscopic level of cables and chairs

and other people we do see what look to

be definite outcomes but if you're doing

measurements on smaller systems you do

you can measure what are called

interference terms and we call those

sort of the residue of the wav like

features of those systems uh and so

they're there and we're getting better

at testing like keeping interference

terms coherent to higher and higher

levels so the idea is if we had really

brilliant uh engineers and really good

shielding we could send an elephant

through a double slit experiment and see

the elephant sort of give us a a

interference pattern on screen um but

the idea is that the appearance of the

classical world and definite outcomes we

have a pretty good physics story for how

that works and it involves what's called

Quantum decoherence and it's basically

that the entanglement of two systems um

it's a way there's a way that they can

communicate with one another once

they're entangled uh and if you're in an

environment with many many uh degrees of

freedom ways of being that's sort of a

poetical way to put it I suppose uh many

parameters then those can sort of damp

the interference terms down if they

become entangled with you and so those

waves the wave Peaks that might give

rise to a smeared cat that's dead and

alive or something get damped down so

that we'd have to do measurements over

many lifetimes of the universe before we

might see something non-classical

looking so this is possibly an

explanation for why it is that the

weirdness of quantum mechanics doesn't

come up to the macro

macro

world

ofaction you have the catons arec off of

it you're maybe petting all those

interactions affect the quantum

description of the cat and and the idea

of this Quantum decoherence is those

interactions tend to suppress the very

parts of quantum probability that are at

odds with our experience which is why

our experience is as as it is yeah I

mean is that widely

accepted perspective now would you say

well it should be because it's

right um but in fact I think to be to be

less flippant about it um those who are

working seriously on realist

interpretations of quantum mechanics

will all use decoherence to explain a

huge chunk of their story and then

they'll bring in either a spontaneous

collapse of the wave function to get

from Mostly damped interest which is

kind of what we saw in that little

example that's what that you know or you

could say that many universes come out

of it and you'll hear more about these

different things later on um but yeah

uh I want to say that those interferance

terms are still there and there are

there are experiments done where we can

recover these terms and in fact our

whole hope of building quantum computers

that are you know powerful enough is

that these Quantum cubits are in

entangled states with one another and

that's how we get more than zero and one

as our values and we have a more

powerful more expressive machine but

entanglement gets destroyed by

decoherence the whole game in building

quantum computers is to Shield it from

this very thing that hides the

quantumness as it were now you mentioned

the word entanglement a couple times and

uh it'd be great to spend a little bit

of time talking about that so you've

actually written on the history of this

idea I mean just give us a thumbnail

sketch going back say to to 1935 maybe

that's a good year to focus upon we can

scoot a bit back further I mean so

something that you learn when you look

at the history of physics is not only

that there aren't Geniuses sitting alone

in a room somewhere even Heisenberg on

helgoland I know I'm sorry to break it

to you they're in communication with one

another they're bouncing ideas off

Schrodinger was having many

conversations in 26 27 uh about the

nature of his wave function he published

a series of papers in 1926 exploring

what the wave function could do for

Quantum systems uh but he was still

troubled and you see in his notebooks

were which are written in a cryptic

German

shorthand uh so a lot of fun to decode

um if anybody feels like doing a puzzle

later on there are still some notebooks

to be translated but he starts thinking

like there's this strange feature of uh

interacting systems in quantum mechanics

that doesn't appear elsewhere and we see

him talking about this and he sends

letters back and forth with Einstein in

1935 um exploring this concept more and

at the end of 1935 he publishes a paper

in which he baptizes this strange

interconnectedness of systems such that

even when they've ceased to interact

they still cannot be described without

making reference to that other system so

our Notions of Newtonian individuality

where I can give you the list of

properties that belong to this thing and

it belong that state belongs to this

object um if this is entangled with

other stuff I can't write down a state

of its properties all by itself it's

instead uh I have to describe it by

making reference to all these other

um and he calls it entanglement uh so it

gets named for the first time by

schinger the end of 1935 but the ideas

in the air and it's being talked about

by Schrodinger and Einstein and a uh

philosopher of physics Greta Herman and

others so it's floating but nobody

really wants to accept it and I think we

even have a quote Yeah of uh shinger but

you probably know it by heart but I

would not call that referring to

entanglement one but rather the

characteristic trait of quantum

mechanics that's this notion that you

can have two things that are not next to

each other yep and yet you can't

describe either independently of the

other very very strange idea we're used

to a world that's sort of local right

what happens here happens here and you

don't need to think about stuff over

there to describe what's happening over

here and then in 1935 Einstein writes a

a curious paper on this which you've

written about yeah uh well Einstein had

less to do with the writing than he

would have liked but he co-authored a

paper with Podolski and Rosen uh and

Podolski wrote it uh yes can Quantum

yeah can quantum mechanical descriptions

of physical reality be considered

complete um and people have spent a lot

of ink trying to get clear on what The

Logical problem or Paradox is there but

in a letter to schoder Einstein says

very clearly like aside from what's

printed in the paper my issue is that

you schinger your wave function doesn't

which is that spread out blue

probability Wave Y doesn't tell me like

which state will come out in the end for

a given system that I measure and

Einstein's thinking that all these

physical systems in the world even if

they're quantized or whatever have

little flags on their heads with a list

of properties that follow them around

but with Schrodinger's new mechanics uh

it looks like there's a way that the

flag has other properties of other

systems and I can't predict my flag sort

of depends on your flag if we're

entangled in this way and and that I

can't give a complete description uh at

the beginning of my experiment which

wave function will end up describing uh

one system at the beginning there are

multiple mathematical descriptions of

the final project and he wants a one

toone correlation can we give a concrete

example I think many people are are

familiar at least at one level or

another but uh before we show any visual

we'll use the so-called spin a half

particle it's a technical term but

basically I think as many people know

every particle in the world spins around

at a fixed nonchanging rate but that

rate can be either spinning clockwise or

counterclockwise we call one spinning up

the other spinning down this is a known

fact about particles but in the quantum

World much as the cat can be sort of

part dead and part alive the electron

can be sort of partly here and partly

there this spin a half particle can be

in a blend of up and down at the same

time cuz we just see an example of a

single sitting there we go right so

again much as measuring the position you

can measure the spin so if we can do

that together 3 2 one measure there it

is right and it happened to come out up

in that case but if you let it another

example if we can just do it have that

guy going let me do it this time you got

it I don't know if you got the power but

try it three two

one ah you do look at that Fant

fantastic now that's weird enough right

because this is the example that you

began with by saying you know you've got

this probabilistic Haze of possibilities

and upon measurement somehow one is

selected that's weird but let's accept

it okay because now we want to talk

about what you were focusing on a moment

which is entanglement and to do that

let's bring up two of these particles

that have been set up and I'll let you

do the honors so why don't you measure

just the particle on the right don't

touch the particle on the left okay 3 2

1 very well done sound that time okay so

the point is by measuring the particle

on the right and getting it to have a

definite quality you force the particle

on the left to have a definite quality

that's the correlation which is weird

right Einstein called

that spooky right well spooky because we

have to imagine these guys are so far