Is Quantum Reality in the Eye of the Beholder?

World Science Festival
3 May 202431:20

Summary

TLDRThe video script presents a deep dive into quantum mechanics, focusing on the quantum measurement problem and the transition from a realm of probabilities to the definite reality we perceive. It features a conversation with physicist Carlo Rovelli, who discusses his perspective on quantum mechanics, contrasting it with the many-worlds interpretation. Rovelli introduces the concept of 'relational quantum mechanics,' suggesting that quantum properties are not intrinsic but relative, based on the interaction between systems. The discussion also touches on quantum entanglement and its implications for the structure of spacetime, hinting at a connection between entanglement and the fabric of spacetime. The dialogue underscores the ongoing evolution of ideas in quantum theory and the need for further research and debate to unravel its mysteries, while acknowledging the remarkable predictive power and technological applications of quantum mechanics.

Takeaways

  • 📊 The core of quantum mechanics is probabilistic predictions, where a quantum wave function contains many possible outcomes that collapse to a single outcome upon observation or measurement.
  • 🌐 Quantum entanglement introduces nonlocality, meaning the state of one particle can instantaneously affect another, regardless of distance, including through time.
  • 🔬 Proposed solutions to the quantum measurement problem include spontaneous collapse theories and the many worlds interpretation, where every possible quantum outcome occurs in a separate universe.
  • đŸ€” Carlo Rovelli explores a different direction, suggesting that the wave function might not represent an actual physical reality but rather a calculation of probabilities for particle behavior.
  • 📉 Rovelli emphasizes the discreteness of quantum mechanics, indicating that properties like the position of an electron are not absolute but relational and contextual.
  • 🧠 He introduces relational quantum mechanics, where every interaction is a measurement and the properties of a quantum system are relative to the observer or another system it interacts with.
  • 🔍 In relational quantum mechanics, paradoxes like Schrödinger's cat are resolved by understanding that the cat is neither alive nor dead in an absolute sense but relative to the observer.
  • ⚖ The concept of granularity or discrete quanta is central to quantum mechanics, with properties like the Planck constant (ℏ) defining the scale of quantum effects.
  • ↔ Rovelli suggests that entanglement can be understood relationally, where the correlation between distant particles is not due to hidden communication but the nature of quantum measurement.
  • ⏳ Progress in quantum mechanics and quantum gravity is gradual and requires time, with Rovelli optimistic that future generations will have a clearer understanding.
  • 🌟 Despite the ongoing mysteries, quantum mechanics has been remarkably successful in making precise predictions and enabling advanced technological developments.

Q & A

  • What is the core idea of quantum mechanics?

    -The core idea of quantum mechanics is that the best we can do in reality is make probabilistic predictions before an observation or measurement. The world is described by a quantum wave function that contains many possible outcomes, and only through observation or measurement is a single definite outcome realized.

  • What is the quantum measurement problem?

    -The quantum measurement problem refers to the mysterious transition from a state where multiple outcomes are possible, as described by the quantum wave function, to a state where only one outcome is observed and experienced after a measurement is made.

  • What is quantum entanglement and its significance?

    -Quantum entanglement is a phenomenon where the state of one particle becomes instantly correlated with the state of another, no matter the distance between them. It suggests a nonlocal quality to reality, where actions in one location can have an instantaneous impact on a distant system.

  • What is the many-worlds interpretation of quantum mechanics?

    -The many-worlds interpretation proposes that every possible quantum outcome actually happens, but each occurs in its own separate quantum universe. It is a leading, albeit controversial, contender for how quantum reality operates.

  • What is Carlo Rovelli's stance on the many-worlds interpretation?

    -Carlo Rovelli does not favor the many-worlds interpretation, although he does not dismiss it as incorrect. He believes there are other possible ways of looking at the world and is exploring a different direction, which he finds more appealing.

  • What is relational quantum mechanics?

    -Relational quantum mechanics is an approach that suggests properties of any object are always relative to something else, and objects have properties only when they interact. It emphasizes that the wave function is a probability distribution of how one system will affect another.

  • How does Carlo Rovelli's approach to quantum mechanics address the concept of discreteness?

    -Rovelli's approach acknowledges the discreteness or granularity inherent in quantum mechanics. He argues that quantum mechanics tells us that things we thought were continuous are not, and that this discreteness is a fundamental aspect of the theory.

  • What is the role of the Planck constant in quantum mechanics?

    -The Planck constant (h-bar) is a fundamental constant in quantum mechanics that determines the scale of quantum effects. It represents the size of the quantum 'grains' and is essential for understanding the granularity of quantum mechanics.

  • How does Carlo Rovelli connect the relational approach to quantum mechanics with quantum gravity?

    -Rovelli suggests that the relational approach, which emphasizes the context-dependent properties of quantum systems, should help in understanding quantum gravity. He believes that the structure of spacetime, which is quantized in loop quantum gravity, is related to the way quantum systems interact and affect each other.

  • What is the significance of quantum entanglement in the context of spacetime?

    -Quantum entanglement may be a key ingredient that holds spacetime together. The idea that entanglement creates a connection between distant particles resonates with the concept of spacetime contiguity and locality, suggesting a deep relationship between entanglement and the fabric of spacetime.

  • What does Carlo Rovelli believe is needed to achieve a complete understanding of quantum mechanics?

    -Rovelli believes that time and continued work on the problem are needed to achieve a complete understanding of quantum mechanics. He emphasizes the importance of thinking about the theory, writing papers, debating, and slowly developing a more fruitful perspective that the community can agree upon.

Outlines

00:00

😀 Introduction to Quantum Reality Series

The video script begins with an introduction to the third installment of a series on quantum reality. The host provides a brief recap of the previous discussions, which covered the fundamentals of quantum mechanics, the probabilistic nature of predictions in quantum theory, the quantum wave function, and the measurement problem. It also mentions quantum entanglement and non-locality, both in space and time. The upcoming conversation with physicist Carlo Rovelli is teased, highlighting his unique perspective on the quantum measurement problem.

05:01

đŸ€” Exploring Alternatives to the Many Worlds Interpretation

Carlo Rovelli expresses his views on the quantum measurement problem and acknowledges the importance of the issue. He does not favor the many worlds interpretation, proposing instead a different direction that considers the wave function as a calculation of probability rather than an actual physical reality. Rovelli emphasizes the probabilistic nature of quantum mechanics and suggests that the wave function might be a tool to predict where a particle is likely to appear next, drawing parallels with classical mechanics. He also touches on the concept of quantum granularity and the idea that quantum mechanics reveals the discreteness of nature.

10:01

🔬 Relational Quantum Mechanics and Contextuality

Rovelli delves into relational quantum mechanics, which posits that every interaction is a measurement and that the properties of a particle are not absolute but relative to the observer or the system it interacts with. This perspective eliminates the need for additional hypotheses like many worlds or hidden variables. The concept of 'contextuality' is introduced, where the outcome of a measurement reflects a relationship between systems rather than an intrinsic property of one system. This approach is consistent with the non-locality observed in quantum entanglement and helps to resolve some of the paradoxes in quantum mechanics.

15:03

🧠 The Role of Discreteness in Quantum Mechanics

The conversation continues with a focus on the discrete or granular nature of quantum mechanics, which is a fundamental aspect of the theory. Rovelli argues that the granularity implied by quantum mechanics suggests that particles do not exist in a continuum but rather in distinct states. He connects this concept to the broader theme of relational properties in physics, where properties are defined in relation to something else, and applies this to quantum mechanics, suggesting that the properties of quantum systems are also relational.

20:06

🌌 Quantum Mechanics and the Structure of Spacetime

Rovelli discusses the application of quantum mechanics to gravity and spacetime, suggesting that the granularity of quantum mechanics should apply to the fabric of spacetime itself. He connects the concept of quantum space to the work being done in loop quantum gravity, where spacetime is quantized into discrete units. The idea that spacetime is made up of quantum 'grains' or units is central to this approach, and Rovelli suggests that understanding these quanta and their relational connections is key to making progress in quantum gravity.

25:07

đŸ•°ïž The Evolution of Understanding in Physics

The script concludes with a reflection on the process of scientific understanding. Rovelli challenges the notion that confusion is a sign of outdated or incorrect theories, arguing that confusion is a natural part of the process of discovery in science. He points to historical examples, such as the initial confusion surrounding Maxwell's equations and the Copernican revolution, to illustrate that it takes time for new theories to be fully understood and accepted. Rovelli expresses optimism that future generations will look back on our current understanding of quantum mechanics with clarity, much like we now view past scientific revolutions.

30:09

📚 Conclusion and Acknowledgment of Quantum Mechanics' Achievements

The final paragraph wraps up the discussion by acknowledging the ongoing efforts to understand the complexities of quantum mechanics. Despite the unresolved questions, it is emphasized that quantum mechanics has led to remarkably accurate predictions and has been instrumental in developing advanced technology. The host, Brian Greene, thanks Carlo Rovelli for his insights and closes the conversation on a positive note, celebrating the achievements of quantum mechanics and looking forward to future advancements.

Mindmap

Keywords

💡Quantum Mechanics

Quantum mechanics is a fundamental theory in physics that describes how the physical world works at the smallest scales of energy levels of atoms and subatomic particles. It is central to the video's theme as it provides the framework for understanding the probabilistic nature of the universe at the quantum level. The script discusses the core principles of quantum mechanics, such as the quantum wave function and probabilistic predictions.

💡Quantum Measurement Problem

The quantum measurement problem refers to the question of how or why the quantum system transitions from a superposition of states to a single, observable state. It is a vital topic in the video, as it represents the mysterious aspect of quantum mechanics that scientists are still trying to understand. The script mentions this problem and explores various theories attempting to resolve it.

💡Quantum Entanglement

Quantum entanglement is a phenomenon where particles become interconnected and the state of one particle instantly influences the state of another, no matter the distance between them. This concept is integral to the video's discussion on the nonlocal quality of quantum reality and how actions in one location can have instantaneous effects elsewhere. The script uses entanglement to illustrate the interconnectedness of quantum particles.

💡Many Worlds Interpretation

The many worlds interpretation is a proposal that suggests every possible quantum outcome actually happens, but each in a separate, non-interacting universe. This concept is significant in the video as it is one of the proposed solutions to the quantum measurement problem. The script discusses this interpretation as a leading, albeit controversial, contender for understanding quantum reality.

💡Wave Function

The wave function in quantum mechanics is a mathematical description that encapsulates the probabilities of all possible outcomes for a quantum system. It is a key concept in the video as it represents the mixture of possibilities that exist before an observation collapses it to a single outcome. The script uses the wave function to explain the probabilistic nature of quantum systems.

💡Relational Quantum Mechanics

Relational quantum mechanics is an approach that suggests properties of quantum objects are not absolute but are relative to the observer or another object they interact with. This concept is highlighted in the video as a way to understand the measurement problem and the nature of quantum reality. The script discusses this idea as an alternative perspective to the traditional understanding of quantum mechanics.

💡Contextuality

Contextuality in quantum mechanics is the idea that the properties of a quantum system are not inherent but depend on the context or the specific conditions of the measurement. This concept is important in the video as it challenges the notion of absolute properties in quantum objects. The script uses contextuality to explain how the results of quantum interactions are relative to the systems involved.

💡Quantum Gravity

Quantum gravity is a field of study that aims to describe the gravitational force within the framework of quantum mechanics. It is mentioned in the video as a key area where understanding quantum mechanics can lead to insights into the nature of space and time. The script connects quantum gravity with the relational approach and the structure of spacetime.

💡Space-Time

Space-time is the fabric of the universe, combining the three dimensions of space with the one dimension of time into a four-dimensional continuum. The concept is crucial in the video as it is the stage where quantum mechanics and general relativity intersect. The script discusses how quantum mechanics can shed light on the structure of space-time and its granular nature at the Planck scale.

💡Quantum Decoherence

Quantum decoherence is the process by which a quantum system loses its quantum behavior and starts to interact classically with its environment, causing the system to transition from a superposition of states to a single, definite state. This concept is related to the video's theme as it is a proposed mechanism for the quantum measurement problem. The script alludes to decoherence as a way to understand how quantum systems interact with the environment.

💡Hidden Variables

Hidden variables are hypothetical properties that are not currently accounted for in quantum mechanics that could determine the outcome of quantum events. The concept is mentioned in the video as one of the approaches to resolving the quantum measurement problem. The script discusses hidden variables as an alternative to other interpretations, suggesting that they might offer a deterministic underpinning to quantum phenomena.

Highlights

Quantum mechanics is based on probabilistic predictions, with the quantum wave function containing many possible outcomes that collapse to a single outcome upon observation or measurement.

The quantum measurement problem, which concerns the transition from a superposition of states to a single observed state, remains a mystery.

Quantum entanglement demonstrates nonlocality, where the state of one particle can instantaneously affect another, regardless of distance.

The many worlds interpretation proposes that every possible quantum outcome occurs in its own separate universe.

Carlo Rovelli has developed his own theory for the transition from quantum possibilities to a definite world, which he discusses in the conversation.

Rovelli suggests that the wave function might not represent an actual physical reality but rather a calculation of probabilities.

He emphasizes the importance of understanding the discreteness or 'quantum quanta' as a core aspect of quantum mechanics.

Rovelli introduces the concept of 'relational quantum mechanics', where properties of particles are not absolute but relative to the observer or another particle.

In relational quantum mechanics, a particle's properties are only revealed through interaction, which is considered a form of measurement.

The idea that quantum properties are relational could help resolve paradoxes such as Schrödinger's cat, which is both alive and dead until observed.

Rovelli discusses the potential of quantum mechanics to inform our understanding of quantum gravity and the structure of spacetime.

He suggests that the constant of quantum mechanics, Planck's constant, is key to understanding the granularity of quantum space.

Loop quantum gravity, which Rovelli contributes to, proposes that spacetime is composed of discrete, quantized elements.

Rovelli finds parallels between the mathematical structures of loop quantum gravity and string theory's concept of spacetime being held together by entanglement.

The concept of locality in physics is re-evaluated in the context of quantum entanglement, suggesting a deep connection between interaction, entanglement, and the fabric of spacetime.

Rovelli is optimistic that with time and continued research, a clearer understanding of quantum mechanics will emerge, potentially leading to a solution in a few generations.

Despite the confusion and ongoing debate, quantum mechanics has made incredibly precise predictions and enabled advanced technology, showcasing the power of the theory even as questions remain.

Transcripts

00:01

[Music]

00:10

welcome to the third in our series of

00:12

quantum reality conversations in case

00:15

you missed the first two feel free to

00:17

check them out but also feel free to

00:19

stay here as I'll now give you a brief

00:21

summary of where we have gotten so far

00:23

all right in our first conversation with

00:26

philosopher Elise Crow we discussed the

00:30

basics of quantum mechanics namely that

00:33

the theory has at its core the idea that

00:36

the best you can ever do in our reality

00:39

is make probabilistic predictions before

00:43

an appropriate observation or

00:45

measurement or interaction the world is

00:47

described by a Quantum wave function

00:50

that contains within it an unfamiliar

00:54

mixture embracing many possible outcomes

00:57

like particle here and here

01:00

you go left and you go right and only

01:04

through observation or measurement or

01:07

interaction is the reality we have

01:09

access to coaxed into a single definite

01:13

outcome but how that transition from a

01:17

world chock full of many possibilities

01:19

to one in which only one outcome is

01:22

observed and experienced how that

01:24

transition actually happens that remains

01:27

mysterious and is called the the quantum

01:30

measurement problem okay we also

01:33

explored how through quantum

01:36

entanglement reality has what we call a

01:39

nonlocal quality that is what you do

01:42

here can have an instantaneous Quantum

01:45

impact on something way over there and

01:48

such non-locality we found can thread

01:51

not just through

01:53

space but also through time all right in

01:56

our second conversation with physicist

01:59

and author Sean Carol we explored a

02:01

number of proposed resolutions to the

02:04

quantum measurement problem most notably

02:07

spontaneous collapse theories as well as

02:11

the many worlds interpretation of

02:13

quantum mechanics in which every

02:15

possible Quantum outcome actually

02:18

happens but each takes place in its own

02:21

Quantum Universe it's not a particularly

02:24

new idea being introduced way back in

02:26

Hugh Everett's doctoral dissertation in

02:29

19 57 but it remains a leading if

02:32

controversial Contender for how Quantum

02:35

reality actually

02:37

operates in this third

02:40

conversation with physicist and author

02:42

ker relli we are going to continue our

02:45

exploration of quantum mechanics and

02:48

this mysterious transition from a haze

02:50

of quantum possibilities to something

02:53

like the definite world we each

02:55

experience because relli himself has

02:58

developed his own Theory for how this

03:01

may come about all right let's jump

03:05

in Carla relli is the director of the

03:09

quantum gravity group at the center for

03:11

theoretical physics at X Marse

03:13

University in France he is a co-founder

03:15

of the loop approach to quantum gravity

03:18

and an author of several books

03:19

popularizing science please welcome

03:22

Carlo

03:23

[Applause]

03:26

relli so thanks thanks so much for

03:29

joining us really appreciate it it's a

03:32

really pleasure so I think you've heard

03:34

you know some of the discussions about

03:36

quantum mechanics the quantum

03:37

measurement problem where do you come

03:40

down is is the Quantum measurement

03:42

problem something that's vital to

03:44

understand in your view do you think you

03:46

have a solution to it where do you come

03:48

down on

03:49

it I actually agree with much what I've

03:52

heard from from Eliz and from Sean um I

03:56

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

04:00

uh problem and that we should or some of

04:03

us should uh should work on it uh I

04:07

think that we're still confused after a

04:09

century as you're saying and uh that uh

04:13

uh we do have some ideas of how to think

04:15

about

04:16

that uh in the way you were talking with

04:20

with Sean I think I agree uh there there

04:22

are viable ideas the question is which

04:25

one is going to be fruitful and useful

04:28

and take us ahead

04:30

um in in in understanding the world and

04:33

there is a discussion going on which has

04:35

evolved through the years and is still

04:38

evolving there are new ideas coming out

04:40

I am hopeful that at some point it will

04:43

converge and so you heard that sha is a

04:46

great fan of the many worlds approach

04:49

does that resonate with you or do you

04:51

look at that and you're like you know

04:53

much as he says you know grw isn't right

04:56

hidden variables is not right do you

04:57

look at many worlds and have a similar

04:59

reaction or is that something that

05:00

you're in favor of as a viable approach

05:03

I am exploring a different direction uh

05:06

which is a polite way of saying I don't

05:08

like that but it's very

05:11

nice yeah but uh but it's important I I

05:14

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

05:18

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

05:21

looking at the world uh I think there

05:23

are other possible ways of looking at

05:25

the world and I think we should we

05:27

should work through that yeah uh at the

05:29

very beginning started by saying uh the

05:33

first choice that we have is how to

05:35

think about the wave function yeah

05:37

that's uh and one option is to say okay

05:41

Shing W function of the quantum State

05:44

that's a real thing okay so if I stop

05:46

and can I just jump in just so people

05:47

have a visual image in mind that was

05:49

like that Blue Wave I'm sure you all

05:51

know we that encapsulates say for a

05:53

given electron the various probabilities

05:56

of say being at various locations you

05:58

call it probability wave or wave fun

05:59

function go from there yes that's right

06:01

so one one possibility is to say this

06:03

blue moving thing is the actual stuff of

06:05

the universe that's what going on um

06:09

there is an

06:10

alternative which uh I find it more

06:13

appealing for a number of reasons which

06:15

is to take the opposite perspective

06:18

namely that not the thing okay the thing

06:20

is a

06:21

particle the the actual particle and uh

06:26

uh that think there is just a way we

06:27

have to compute where the particles

06:30

going to show up

06:31

next uh In classical mechanics before

06:35

quantum mechanics there is a very

06:36

similar thing there's a very similar

06:39

techniques of using a wave all over to

06:41

compute where the particle is going to

06:43

go next it's called the um Hamilton jaob

06:47

and you have something in fact the

06:49

classical limit of the Shing function is

06:51

the Hamilton jacobe so one possibility

06:53

to to sort of try to get an intuition

06:55

about quantum mechanics is to think well

06:58

that just calculations our way of

07:00

thinking uh what's going to happen next

07:03

and since as you have been all

07:07

emphasizing a lot uh the first great

07:10

discovery of quantum mechanics is that

07:12

it's

07:14

probabilistic even if you believe in

07:17

underline deterministic Theory uh you

07:21

like anybody else you're not going to

07:22

say whether the spin will go up and down

07:24

right so it's intrinsically

07:26

probabilistic as a limitation of what is

07:28

going on yeah

07:30

then uh it means that what we can do is

07:32

a probability calculation and that's way

07:35

think is a probability calculation

07:37

that's what Max Bourne clearly um got

07:40

Noel prize for understanding that but

07:42

the probability calculation of course it

07:43

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

07:46

saying simply because you know more I

07:49

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

07:51

know what's it whether tomorrow I have

07:52

probability that the weather comes out

07:54

it's one of them and then my knowledge

07:55

jumps when when when tomorrow happens

07:58

yes when tomorrow happens or even you

08:00

know if I don't know who won a certain

08:02

game and just because it had already

08:04

happened yeah and I don't know it when I

08:06

know it my my knowledge jumps but it's

08:08

only your knowledge that jumps right

08:10

there are other people who perhaps were

08:12

at the game they already have that

08:14

knowledge so you're describing a

08:16

potential view of the world where

08:18

different observers would be in very

08:21

different levels of quantum let me let

08:23

me get there in two step in two steps um

08:26

so let me backtrack one second uh

08:29

because there's one thing which is

08:31

rarely said about quantum mechanics

08:33

which to me seems the core of the theory

08:35

which is granularity discreteness

08:38

quantum quanta yeah um in the discussion

08:41

about the meaning of quantum mechanics

08:43

we we tend to forget that and I think

08:45

this is this is wrong Quantum mechanic

08:47

came out as a description that things we

08:51

thought were continuous are actually not

08:54

continuous jumps are gra for instance uh

08:58

lights is made by gets to me as photons

09:01

individual photons if I little little

09:03

bundles of light that are bundles of

09:05

little little if I have a screen lights

09:07

got here if I look sufficiently

09:09

carefully just one dot here one dot here

09:12

one dot here one dot here or I don't

09:15

know atoms have discrete orbits right

09:17

and the the electron jumps from one to

09:19

the other and so on and so forth there

09:22

all this discreteness basis of quro

09:25

mechanics which means that I it tells to

09:30

me that if you want to think about

09:31

Quantum mechanic we don't have to add

09:32

things there is less so the electron is

09:35

here the electron is there this is this

09:37

is simp to me the indication so now let

09:39

me come to your question

09:42

yes one way you pose the question is

09:44

what is a measurement okay and that's I

09:47

think the right question the right you

09:49

asked to to Le you asked

09:52

Sean the way I think could be

09:55

useful think about quantum mechanics

09:57

which call relational quantum mechanics

09:59

is to try to answer this question by

10:01

saying everything is a measurement every

10:04

time two any two systems interact

10:06

they're measuring one another by

10:08

definition interaction is a me regard of

10:12

who's doing it what doing it a measure

10:14

so the screen the the particle touch it

10:16

that's a measurement but also an atom

10:19

here and the photon bonds it that's a

10:20

measurement okay and uh so uh the the

10:26

the the the wave function is just

10:27

telling us what is the way one system is

10:30

going to affect another system what's a

10:32

probability distribution of the way it's

10:35

going to affect it now this works at one

10:38

condition and that's a hard point and

10:41

that's I think what quantum mechanics is

10:43

deeply telling us um the condition being

10:46

that the actual result of the

10:48

measurement the r of interaction doesn't

10:52

display a property of say the particle

10:55

but discls a a relative property of the

10:57

particle who spting the screen

11:00

that's

11:01

contextuality now so just so I can

11:03

understand if I'm measuring like our

11:05

spin a half particle that we had before

11:07

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

11:08

saying it's not that the particle is up

11:11

that's right it's that I and the

11:12

particle stand in a particular

11:14

relationship which is what we would

11:17

normally call the particle up but it's

11:19

not a property of it it's a property of

11:20

us both that's correct that's correct

11:23

which means that every time you say the

11:24

particle up which is fine what you

11:27

really should say the particle is up

11:28

respect

11:29

exactly and if you do that I believe

11:32

things go in order so the So-Cal

11:34

paradoxes of quantum mechanics go in

11:37

order the cat uh which interacts with

11:40

the quantum staff in the Box the shingle

11:42

cat it's either Alive or Dead with

11:46

respect to himself and do you have a cat

11:49

yeah that's right so with respect to the

11:52

cat the cat is just alive or dead

11:55

because the quantum object did this or

11:57

that but with with respect to me who are

12:00

outside the box and looking at that

12:03

careful the cat is neither dead nor

12:06

alive because with respect to me I uh

12:10

neither of these two are are are

12:12

realized so whatever I see next I

12:15

shouldn't assume that we SP to me one of

12:17

the two

12:18

happens just want make sure and it's

12:20

clear in your approach obviously it must

12:22

be but that if two different observers

12:25

are are interacting with the same object

12:28

they'll never find any kind of

12:31

contradiction you know if if the spin up

12:33

is not a property of the particle itself

12:36

you might wonder that you know person a

12:39

stands in the spin up relationship but

12:41

person B might stand in the spin down

12:43

relationship that's exactly the what has

12:46

been discussed I mean this idea of

12:47

relational quantum mechanics came out in

12:49

the '90s yeah and it slowly grew um sort

12:53

of number of people interested grew

12:55

slowly now there's a lot of paper coming

12:56

out and for you started it this was a

12:59

your idea is that yeah that's that

13:01

started off with paper of mine in the

13:03

'90s but then was developed first by

13:06

philosophers number of including

13:08

important philosophers uh Bas van fren

13:11

for instance wrote the paper on it and

13:13

and others now is is getting more in the

13:16

foundation of physics um attention um

13:19

and the first part of the discussion was

13:20

always do does this create contradiction

13:24

yeah there was a lot of debate consider

13:26

this case this case this case it doesn't

13:28

that that's the point there is a

13:30

coherence in quantum mechanics itself so

13:32

it doesn't create contradiction so the

13:34

idea here is that instead of adding uh

13:38

you know manyu world or adding hidden

13:42

variables or adding a a a g g gwr um

13:49

extra collapse and other uh

13:52

takeway uh the the the properties of any

13:56

object are always relative to some

14:00

else and object have properties only

14:03

when they interact now in some sense is

14:06

there a many worlds like quality to this

14:09

descrip it's not so different from many

14:12

worlds in a sense because if the

14:14

particle isn't spin up on its own and

14:17

it's a relationship then the other

14:20

possibilities in some sense are still

14:21

there still are still there yes in fact

14:23

it's still there

14:25

um the idea that properties are

14:28

relational it's sort of all over physics

14:30

if if you think the great

14:32

um the great step in going into into

14:36

Newtonian physics in the Renaissance

14:38

with Galileo Kepler and and so on was to

14:41

answer this question uh what is the

14:43

velocity of an object right uh is this

14:46

moving no it's not okay with respect to

14:50

us with respect to us yeah but it's

14:51

moving with respect to the sun okay

14:54

so the velocity of an object is not a

14:58

property of the object it's a property

15:00

of the object and something else it's

15:03

it's a relational property and in a

15:05

sense the relation to Quantum Mechanics

15:07

is making this very general not just

15:11

velocity uh but all the properties of an

15:14

object have to be thoughted uh uh uh

15:18

relationally with respect to something

15:19

else now when you say all presumably not

15:21

really all like the mass of a particle

15:24

presumably is not in that category

15:26

contingent conent properties the one

15:27

that change

15:29

face Bas so and so in in this approach

15:31

how does this illuminate say

15:33

entanglement you know so I mean I asked

15:35

an unfair question saying you know you

15:37

got these two distant particles you

15:39

measure one you find it up the other one

15:41

is down I said how does that happen and

15:44

of course we don't really have a story

15:46

to tell that's really convincing in the

15:49

usual approach do you have a story to

15:50

tell in this relational approach that

15:53

sheds more light on it yes it is a story

15:55

um which is uh doesn't take way the

16:00

strangeness of the phenomenal phenomenal

16:01

is strange and it remains strange uh it

16:04

sort of stor that shifted into the

16:06

strangeness of quantum mechanics itself

16:08

the story is the following um if you

16:10

imagine that you measure something here

16:13

and you measure something there you're

16:16

cheating because who is seeing here and

16:20

there at the same time nobody uh to

16:24

compare that they have seen the same

16:25

thing you have to wait until they

16:28

communicate to one another and once they

16:30

communicate to one another say the

16:32

information is is is sent um then you

16:35

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

16:39

see what is the world with respect to

16:41

the the the the final Observer that get

16:44

the information there's no nonn locality

16:46

anymore because all the information is

16:49

uh is so but when I when I unravel that

16:53

story wouldn't I still need to explain

16:55

why there's this correlation that

16:57

whenever this one up that one's down

16:59

whenever this one's down that one's up

17:01

which would be surprising if they're

17:03

both just 5050 up and down and not

17:05

somehow talking to each other because

17:08

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

17:11

assigning the property to the particles

17:12

still Yes okay exactly exactly exactly

17:15

exactly so the the the the idea here is

17:19

that think that the mistake is always to

17:22

assign absolute properties to particle

17:24

instead of relative to something else

17:27

and so are are you able to push this

17:30

approach to say um uh relativistic

17:35

quantum mechanics I mean is this

17:37

something yeah yeah it's sort of in fact

17:40

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

17:45

way through quantum gravity because uh

17:47

because that's my job sure to write it

17:50

try to write a Quantum three of gravity

17:52

even if I think the problem of quantum

17:54

mechanics is separate by the problem of

17:57

quantum gravity uh and nevertheless and

17:59

I very much agree with sha in that I

18:02

think that once we understand better

18:03

quantum mechanics this should help us to

18:07

understand better quantum gravity and I

18:09

do agree with with Sean a lot that um

18:13

there is something deep to understand

18:15

about the the quantum structure of space

18:18

time

18:20

uh by somehow a clear idea about quantum

18:24

mechanics and let me step back a moment

18:28

uh I started by saying granularity

18:32

discreetness see

18:34

uh quantum mechanics has a constant H

18:38

bar yes okay unless we understand that

18:41

constant uh we don't understand quantum

18:43

mechanics so interpretation Quant

18:45

mechanics you tell me what this constant

18:46

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

18:49

six comma 6.31 whatever I like the

18:52

number one in the correct units but yes

18:54

you one in the correct unit it's like

18:56

the speed of light right but the speed

18:58

of light you know it is is the the

19:00

fastest thing which I can go in some

19:02

proper sense and uh and uh and that

19:05

number it's just the size of the

19:07

granularity it tell us how big are the

19:10

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

19:12

core story of qu of of of quantum

19:16

mechanics now when you apply quantum

19:19

mechanics to gravitational field to

19:21

gravity as you were saying we shown

19:23

before uh gity is different in the sense

19:28

that it's actually space time itself

19:30

that that that that moves so like the

19:33

light is uh if you look in this small

19:36

it's

19:37

photons space time if you look into this

19:39

small should be Quantum space and that H

19:43

bar should say how bigger this Quantum

19:45

space together with Neutron constant and

19:47

so on and one gets to the to to the

19:49

plank scale and that's a core result of

19:53

loop quantum gravity the quantum theory

19:54

of gravity in which which I'm working is

19:56

a tentative Theory you don't know if

19:57

it's right yet

19:59

so the structure of SpaceTime is all

20:02

this quanta this granular so space space

20:05

is this grain of space which are quantum

20:08

space like photons but they don't live

20:10

in space they make up space themselves

20:14

and the way they are connected to one

20:16

another it's one in relation to another

20:20

one and if we think about how we think

20:22

about space the bound SpaceTime region

20:26

has a boundary and we descri how it

20:29

affect the rest and in quantum mechanics

20:31

we take systems and we describe how the

20:35

system affect another system and the two

20:38

things should go together I believe and

20:41

so this relational way of thinking

20:43

should help us in that direction you

20:45

know we only have a few minutes left but

20:47

I can't help it following that line of

20:49

discussion a little bit further because

20:50

I think as many in our audience know you

20:54

know I work on an approach to quantum

20:56

gravity strength Theory you work one of

20:59

the of the contributor main contributors

21:01

well among many many others but you know

21:04

what we have found and again as Sean was

21:07

using it is the Royal Wii many in the

21:10

field have found that there's now

21:13

evidence that the fabric of SpaceTime in

21:16

a string theoretic approach is stitched

21:19

together by the threads of quantum

21:21

entanglement because calculations have

21:23

been done where I mean in our visuals

21:26

that we showed spin up and spin down

21:28

where connected by this sort of

21:29

invisible line of quantum entanglement

21:32

there can be regions of space that are

21:34

connected by lines of entanglement and

21:36

we mathematically can cut those lines of

21:39

quantum entanglement and the space falls

21:42

apart into little tiny pieces and then

21:45

disintegrates completely once the

21:47

entanglement is fully dissolved so from

21:50

our perspective that's given a lot of

21:53

insight I me it resonates with what

21:55

you're saying but a very concrete means

21:56

by which entanglement would be the very

22:01

ingredient that holds SpaceTime together

22:04

yeah are you finding like a Sim I mean

22:06

obviously a different language but are

22:07

you finding a similar way of thinking

22:10

about SpaceTime and loop quantum gravity

22:12

yeah yeah you know that you know we've

22:14

been working very different opposite

22:17

direction to a quantum gravity but uh

22:20

that aspect of uh String Theory found it

22:23

um very much interesting and intriguing

22:27

and uh it does resonate with something

22:29

very similar that happened in Loop quto

22:32

gravity including in the mathematics so

22:35

the structural spin networks yeah is a

22:37

structure of entanglement between little

22:39

Hill spaces here and there um and uh uh

22:44

I do believe that we have not really

22:48

clarified yet we don't have a clarity

22:50

about that but there is something

22:52

convincing in the idea that the notion

22:57

of contiguity who is ATT to whom and the

23:01

sort of

23:02

entanglement uh they're related yeah

23:06

because uh you see the the core of

23:10

modern physics after Maxwell in the last

23:13

century is locality so what is locality

23:16

locality is the idea that interactions

23:18

are local so to you don't interact with

23:21

something far away inter in some sense

23:23

dynamically interact with something

23:24

nearby but you can turn this around um

23:28

what what does it mean to be nearby it

23:30

means you can interact with it directly

23:32

okay and if you interact with you get

23:34

entangled that's exactly what the core

23:36

of uh somehow the relation

23:38

interpretation is as soon as you

23:40

interact you're entangled from from the

23:41

perspective of something right so

23:43

between entanglement interaction because

23:46

of locality and the SpaceTime contiguity

23:49

there should be a common thing and I

23:52

think that's the beautiful uh slow

23:57

understanding and r of quantum gravity

23:59

which is happening nowadays so let me

24:02

end on a question that I asked in one

24:05

way or another to both Elise and Shawn

24:08

which is you know we've been at this for

24:10

a long time we have yet to fully unravel

24:14

it at all we're making progress but what

24:17

do we lack to get to some final complete

24:21

understanding of quantum mechanics and

24:23

hopefully then be able to apply it to

24:25

issues like the quantum nature of

24:27

SpaceTime is it is it experiment is it

24:30

better mathematics do we need to use AI

24:33

systems or do we not have the brain

24:35

power what do you think it

24:37

is I think just time namely working on

24:41

it let me challenge one thing you said

24:44

at some at some point you you describe

24:46

the Vance the physics of course you were

24:48

simplyy you know by sort of we had a

24:50

clear metaphysic everything was clear

24:52

until in the old days in the old days

24:54

and then quantum mechanics experiment

24:56

come out and then in the dark of that's

24:58

not true right I mean when Maxwell wrote

25:00

his equation everybody was confused what

25:02

this is about including Maxwell himself

25:04

and they very messy equations very messy

25:07

equation was incomprehensible and he

25:09

thought that it was really D matter

25:11

pushing and pulling and rotating them it

25:14

took Einstein to understand that's

25:15

really not the case and so on I mean

25:18

there was a confusion all over right so

25:21

being in a state of confusion is not

25:24

really uh a characteristic of our time

25:27

and when Copernicus I'm even going even

25:30

before did the soal copernica revolution

25:34

is not everybody jumped up and said oh

25:36

yeah right we're moving and we have a

25:38

new world picture and that's our world

25:39

picture it took a century yeah to go to

25:42

Kepler and and and and Galileo

25:44

convincing everybody that actually makes

25:46

sense we're moving and everybody was

25:48

orent confusing exactly because of what

25:50

we were saying before we're not moving

25:52

how can we moving so you have to

25:54

completely rethink what moving means

25:58

okay then of course comes Newton and

26:01

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

26:03

think that takes time between Copernicus

26:06

and Newton finally clarify is a century

26:09

and a half yeah quantum mechanics is

26:11

only a century we we are what does it

26:13

take it takes I believe people thinking

26:17

about that writing paper debating

26:20

getting angry to one another no you're

26:21

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

26:23

how science work it developing theories

26:27

and then slowly at some point uh I

26:30

believe uh uh some uh perspective will

26:35

become more fruitful and will come to

26:39

agree on a view um I I think that my

26:44

grand grand children I don't have

26:46

children I mean think that in two two or

26:47

three generations if we don't kill one

26:50

another with the atomic bombs which is

26:52

if survive uh people will say of course

26:55

the shoting ctis is like obvious isn't

26:57

it like we say of course in syy people

27:00

are upside down right it's obvious yeah

27:02

yeah no and it's an amazing thing even

27:04

with quantum mechanics today graduate

27:07

students speak in terms that's right

27:10

that are so intuitive that's you and I

27:13

yeah like a little bit you know and go a

27:15

generation before and it would have been

27:17

even more difficult to acclimate to this

27:19

new way of thinking about things which

27:21

becomes fluid later on so in short it's

27:24

like three cheers for confusion because

27:26

that's the natural place for us and uh

27:30

let's not forget that quantum mechanics

27:33

are 100 years old but quantum mechanics

27:34

has unbelievable predictions that nobody

27:38

believed like entanglement large

27:40

distances where many people were

27:42

thinking oh yeah but that's cannot be

27:44

true yeah and the solid convincing uh

27:49

experimental support that quantum

27:51

mechanics actually right is not so old

27:53

after all I the last Nobel price with

27:56

Zing company yeah BAS basically it's a

28:00

prize for people who are saying look

28:03

Quantum mechanic is right yeah and so if

28:07

I place your prediction in more personal

28:10

terms when my grandkids are adults

28:14

perhaps we'll have this solution in hand

28:17

which would be certainly a wonderful

28:18

outcome if get there I hope so we

28:20

probably won't be there but probably not

28:22

join me in in thanking Carlo

28:28

thank

28:30

you so perhaps as Carla roelli suggests

28:34

Quantum weirdness is bound up in our

28:37

mistaking relative qualities of objects

28:40

or particles qualities that are in

28:43

relation to another object or particle

28:45

or Observer mistaking those qualities

28:48

for intrinsic qualities like mass and

28:52

charge it is a promising approach that

28:55

no doubt will continue to be developed

28:58

all right that is the third in our

29:01

series of conversations on Quantum

29:03

reality if you've not seen part one with

29:04

Elise croll or part two with Sean Carol

29:07

I encourage you to do so as those

29:08

conversations covered a lot of ground on

29:11

the basics of quantum theory the quantum

29:13

measurement problem and the many worlds

29:16

interpretation of quantum mechanics let

29:19

me leave you with one final thought in

29:22

these conversations we have focused on

29:25

the frontier of quantum mechanics the

29:28

the aspects of the theory that we are

29:31

still struggling to fully understand but

29:34

bear in mind that for all the things

29:36

we've yet to fully sort out we can use

29:39

quantum mechanics to make the most

29:42

precise and accurate predictions in the

29:45

history of human thought and exploration

29:48

while also being able to leverage those

29:50

insights into building the most

29:52

sophisticated technology that our

29:55

species has ever achieved so while there

29:59

are deep questions that remain we should

30:02

be rightly proud of all that we have

30:06

achieved all right thanks for joining us

30:08

and until next time from the world

30:10

Science Festival I am Brian Green

30:15

[Music]

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Tags associés
Quantum MechanicsMeasurement ProblemEntanglementCarlo RovelliMany WorldsQuantum GravityPhysics InsightsWave FunctionNonlocalityProbabilityRelational Quantum Mechanics
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