Why Everything You Thought You Knew About Quantum Physics is Different - with Philip Ball

00:42:47
https://www.youtube.com/watch?v=q7v5NtV8v6I

Resumo

TLDRThis lecture explores the perplexing nature of quantum mechanics, inspired by Richard Feynman's admission that it is not easily understood. The speaker explains key concepts such as wave-particle duality, superposition, and entanglement, while urging a reconsideration of traditional interpretations of quantum mechanics. Emphasizing the significance of measurements and observation, the talk proposes a perspective that places information at the center of quantum theory. It argues that quantum mechanics is less about defining reality and more about understanding the probabilistic nature of observations, reinforcing the idea that many established notions in quantum physics require nuanced interpretation.

Conclusões

  • 🔍 Quantum mechanics is fundamentally about information.
  • đź“Ź Wavefunctions provide probabilities of outcomes, not exact states.
  • 🌀 Superpositions indicate that particles can exist in multiple states.
  • đź”— Entanglement creates instant correlations between distant particles.
  • ⚖️ Heisenberg's uncertainty principle limits simultaneous knowledge of properties.
  • 🌍 Decoherence explains the transition from quantum to classical behavior.
  • đź”— Nonlocality suggests interconnectedness beyond classical physics.
  • âť“ Measurement influences the state of a quantum system.
  • đź’ˇ Feynman emphasized asking the right questions about nature.
  • 🔄 Quantum mechanics doesn't define reality; it describes potentially observable outcomes.

Linha do tempo

  • 00:00:00 - 00:05:00

    Richard Feynman famously stated that 'nobody understands quantum mechanics,' highlighting the complexity of the subject, even for experts. Quantum mechanics is known for its strange characteristics, such as wave-particle duality and superposition, which perplex both scientists and the general public. While quantum mechanics offers predictive power, understanding its implications for reality remains elusive.

  • 00:05:00 - 00:10:00

    The distinction between quantum theory as mathematical predictions and interpretations of those predictions creates confusion. In classical mechanics, the interpretation aligns closely with observable reality, whereas quantum mechanics relies on complex principles like the Schrödinger equation, which defines a particle's wavefunction—not its trajectory.

  • 00:10:00 - 00:15:00

    The wavefunction is a mathematical representation of potential outcomes and probabilities of measurement rather than a definitive description of a particle's state. Therefore, before measurement, the property of the particle exists as probabilities rather than certainties, a nuance that makes quantum mechanics challenging to intuitively grasp.

  • 00:15:00 - 00:20:00

    When measurement occurs, the wavefunction collapses, providing specific outcomes but leaving unanswered questions regarding the particle's nature prior to measurement. This perspective, known as the Copenhagen interpretation, underscores the limits of our knowledge in quantum mechanics and stresses that reality may not be objectively defined until measurement occurs.

  • 00:20:00 - 00:25:00

    The misconception of particles being in multiple places at once is clarified through quantum superposition, particularly with properties like spin, illustrating the difference between potential states before measurement and definitive states afterward. The focus shifts from physical states to the flow of information regarding potential outcomes.

  • 00:25:00 - 00:30:00

    Quantum information theory posits that quantum systems can encode and process information more efficiently through constructs like qubits. This leads to applications in quantum computing and cryptography but also to a deeper understanding of quantum mechanics as fundamentally about information sharing, rather than traditional particle-wave dualities.

  • 00:30:00 - 00:35:00

    Entanglement allows for correlations between quantum particles that seem to defy classical explanations, resulting in questions about locality and communication. Experiments reinforce that quantum mechanics consistently outperforms classical models through phenomena such as quantum nonlocality, challenging preconceived notions of independence and separation in physics.

  • 00:35:00 - 00:42:47

    Current perspectives suggest that quantum mechanics might be reconstructed from principles of information and communication, shedding old paradigms and focusing on the conditional nature of reality—how measurement and inquiry shape our understanding of quantum systems rather than revealing fixed truths.

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VĂ­deo de perguntas e respostas

  • What is wave-particle duality?

    Wave-particle duality is the principle that quantum objects exhibit both wave-like and particle-like properties depending on how they are measured.

  • What is the Copenhagen interpretation of quantum mechanics?

    The Copenhagen interpretation posits that quantum mechanics does not describe an objective reality, but rather how we can expect to see and measure quantum systems by observing them.

  • What does superposition mean in quantum mechanics?

    Superposition means that quantum particles can exist in multiple states or locations at once until measured.

  • What is entanglement?

    Entanglement is a quantum phenomenon where two or more particles become interconnected in such a way that the state of one instantly influences the state of another, regardless of the distance between them.

  • What does Heisenberg's uncertainty principle state?

    Heisenberg's uncertainty principle asserts that certain pairs of properties, like position and momentum, cannot be simultaneously known to arbitrary precision.

  • How does the wavefunction relate to quantum mechanics?

    The wavefunction is a mathematical function that provides the probabilities of finding a particle in various states or positions, rather than a description of the particle's actual state.

  • What is decoherence?

    Decoherence describes the process by which quantum systems lose their quantum properties as they interact with their environments, leading to classical behavior.

  • What is meant by quantum nonlocality?

    Quantum nonlocality refers to the phenomenon where entangled particles can instantaneously affect each other's states despite being separated by large distances.

  • How is quantum mechanics interpreted in terms of information?

    The perspective of viewing quantum mechanics through the lens of information emphasizes how measurements and outcomes relate to the information we possess and how it changes.

  • Why is quantum mechanics often seen as strange or weird?

    Quantum mechanics is considered strange due to its departure from classical intuitions about how the world works, presenting behaviors like particles being in multiple places or states simultaneously.

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  • 00:00:05
    [Music]
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    we like to start here very often I don't
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    know whether to reassure you what to
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    disconcert you but this is one of the
  • 00:00:16
    most popular sayings about quantum
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    mechanics from Richard Feynman who said
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    I think I can safely say that nobody
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    understands quantum mechanics now he
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    said this in 1965 and that was the year
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    that he shared the Nobel Prize in
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    Physics for his work on quantum
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    mechanics so at that point no one alive
  • 00:00:37
    knew more than Richard Feynman about
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    quantum mechanics what hope is there
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    then for the rest of us
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    well quantum mechanics has this
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    reputation for being and possibly hard
  • 00:00:49
    but it's not the mathematics that's the
  • 00:00:51
    problem in his son of the mathematics
  • 00:00:53
    and it doesn't look particularly easy to
  • 00:00:55
    grasp but actually Fineman was fine with
  • 00:00:57
    it he could do the mathematics just fine
  • 00:00:59
    the trouble was that's all he could do
  • 00:01:02
    what he couldn't understand is what the
  • 00:01:05
    maths meant what it tells us about the
  • 00:01:08
    nature of the world and a fine man
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    himself didn't seem too troubled by that
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    he said well we've got a theory that
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    works it makes amazingly accurate
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    predictions about how stuff will behave
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    what more do you want from a theory than
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    that some scientists feel that same way
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    today but usually we do want more we
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    want to know what scientific theories
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    tell us about what the world is like and
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    it wasn't clear then quite what quantum
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    mechanics was telling us about the what
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    the world was like and it's still not
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    clear now but I want to suggest that we
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    can do better than fineman's admission
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    of bafflement or defeat some might say
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    we don't have all the answers about what
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    quantum mechanics means but we do have
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    better questions we know we have a
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    clearer sense than we did in the 1960s
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    or even in the 1980s of what's important
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    and what isn't and I want to try to give
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    you some sense of what I think that is
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    and let me start with some of the things
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    that everyone knows
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    quantum mechanics and when I say
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    everyone I mean everyone in inverted
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    commas so if you haven't seen these
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    things before don't worry all I mean is
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    that once you start finding out more
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    about this this problem perhaps this
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    this topic perhaps by reading you know
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    popular accounts of it then pretty soon
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    these are notions that you will
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    encounter and the first of them is that
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    quantum mechanics is weird and I want to
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    show you what some of those weirdnesses
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    are the first one is that quantum
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    objects can be both waves and particles
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    and this is called wave particle duality
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    the second is that quantum objects can
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    be in more than one state at once or
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    more than one place at once they can be
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    both here and there and these are known
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    as quantum superpositions then we hear
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    that you can't simultaneously know
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    exactly two properties of a quantum
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    object and this is Heisenberg's
  • 00:03:08
    uncertainty principle quantum objects
  • 00:03:12
    can affect each other instantly over
  • 00:03:15
    huge distances this is so-called spooky
  • 00:03:18
    action at a distance and we'll hear more
  • 00:03:20
    about it shortly and it arises from a
  • 00:03:23
    phenomenon called entanglement you can't
  • 00:03:26
    measure anything without disturbing it
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    and so the human observer can't be
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    extracted from the theory it becomes
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    unavoidably subjective and then
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    everything that can possibly happen does
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    happen there are two reasons why this is
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    often said one of them comes from
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    fireman's work itself which seems to say
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    that quantum paths take all possible
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    routes through space the other comes
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    from the controversial many-worlds
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    interpretation of quantum mechanics
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    which says that every time a quantum
  • 00:04:01
    system faces a choice of what to do it
  • 00:04:04
    takes both choices okay now here's the
  • 00:04:07
    thing quantum mechanics says none of
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    these things they're attempts to explain
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    or to articulate what quantum mechanics
  • 00:04:20
    means some of them are misleading of
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    I think are just plain wrong others are
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    just unproven interpretations or
  • 00:04:29
    assumptions I'm saying that we need to
  • 00:04:32
    change the record when we talk about
  • 00:04:33
    quantum mechanics we need to stop
  • 00:04:35
    falling back on these tired old cliches
  • 00:04:38
    and metaphors and look more closely at
  • 00:04:42
    what quantum mechanics does and doesn't
  • 00:04:44
    permit us to say and the first point to
  • 00:04:47
    realize is that there's a big difference
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    between quantum theory the mathematics
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    and the mechanics that you just glimpsed
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    which scientists use daily to make
  • 00:04:56
    predictions to predict stuff that allows
  • 00:04:59
    them to build things like this laptop so
  • 00:05:02
    this is stuff that really works there's
  • 00:05:05
    a big difference between that and the
  • 00:05:07
    interpretation of the theory this is
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    what's so hard to grasp about quantum
  • 00:05:12
    mechanics because normally the
  • 00:05:14
    interpretation of a theory is kind of
  • 00:05:16
    obvious Newtonian mechanics this is the
  • 00:05:19
    old classical mechanics that tells us
  • 00:05:22
    how everyday objects move about and
  • 00:05:25
    behave so it tells us how things like
  • 00:05:27
    tennis balls and spaceships move this is
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    the interpretation here isn't difficult
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    they treat Newtonian mechanics tells us
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    what paths objects take through space as
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    forces act on them and we don't have to
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    ask what do you mean by path what do you
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    mean by objects what do you mean by
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    force it's kind of obvious well that's
  • 00:05:51
    not so for quantum mechanics and let me
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    give you a glimpse of why to predict
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    what a quantum object will do in place
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    of Newton's equations of motion so I
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    just generally use the equation devised
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    by Erwin Schrodinger in 1925 to describe
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    the idea that quantum particles might
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    act as if they were waves this is the
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    Schrodinger equation and it doesn't tell
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    us what the trajectory of a particle is
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    instead it gives us something called a
  • 00:06:20
    wave function and the wave function can
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    be used to figure out what where we
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    might find an object and what properties
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    it might have an object like an electron
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    say so the the typical shape of a wave
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    function of a particle like an electron
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    in space might look something like this
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    okay
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    so what does this mean well it's often
  • 00:06:40
    said what it means is that the particle
  • 00:06:42
    is somehow smeared out over space and it
  • 00:06:45
    does kind of look that way doesn't it
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    but it isn't this isn't showing the
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    density of the particle over space this
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    wavefunction is a purely mathematical
  • 00:06:56
    thing and what the wavefunction lets us
  • 00:06:58
    deduce is all the possible outcomes of
  • 00:07:01
    measurements that we might make on the
  • 00:07:03
    particles properties such as its
  • 00:07:05
    position along with the relative
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    probability that we'll get that
  • 00:07:10
    particular result when we make the
  • 00:07:12
    measurement so to find out the position
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    where we would observe this particle we
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    simply calculate some number from the
  • 00:07:19
    wavefunction the value of the wave
  • 00:07:21
    function at that point in space and that
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    gives us the probability that we'll see
  • 00:07:26
    the particle there if we make a
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    measurement so the wave function doesn't
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    tell us where we'll find this particle
  • 00:07:32
    it tells us the chance that we might
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    find it at a particular position if we
  • 00:07:38
    look and this is what so what about
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    quantum mechanics because it seems to
  • 00:07:43
    point in the wrong direction not down
  • 00:07:45
    towards the thing that we're supposed to
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    be studying but up towards our
  • 00:07:50
    experience of it it says nothing or
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    perhaps we should say it's there's
  • 00:07:54
    nothing obvious about what the quantum
  • 00:07:57
    system itself is like in other words the
  • 00:08:01
    wavefunction is not a description of the
  • 00:08:04
    quantum object it's a prescription for
  • 00:08:06
    what to expect
  • 00:08:07
    when we make measurements on the object
  • 00:08:10
    but it's even more peculiar than that
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    because the wavefunction doesn't tell us
  • 00:08:14
    where the particle is likely to be at
  • 00:08:17
    any instant which we can then try to
  • 00:08:19
    verify by looking rather what the
  • 00:08:22
    wavefunction tells us well it tells us
  • 00:08:24
    nothing about where the particle is
  • 00:08:27
    until we make a measurement strictly
  • 00:08:31
    speaking we shouldn't talk about what
  • 00:08:32
    the particle where the particle is at
  • 00:08:34
    all we shouldn't talk about a particle
  • 00:08:36
    at all except in terms of the
  • 00:08:38
    measurement that we make on it now this
  • 00:08:41
    account of quantum mechanics is more or
  • 00:08:43
    less the one given by the Danish
  • 00:08:44
    physicist Niels Bohr and his
  • 00:08:46
    collaborators such as Heisenberg and
  • 00:08:49
    it's known today as the köppen
  • 00:08:51
    interpretation Copenhagen was where
  • 00:08:53
    niels bohr was based now I'm not saying
  • 00:08:55
    that this interpretation is the right
  • 00:08:57
    one but what's valuable I think about it
  • 00:09:00
    is that it tells us where our confidence
  • 00:09:02
    about meaning has to stop
  • 00:09:05
    as it stands quantum mechanics doesn't
  • 00:09:07
    permit us to say anything with
  • 00:09:09
    confidence about reality beyond what we
  • 00:09:13
    can measure of it and here's what I mean
  • 00:09:16
    by that
  • 00:09:16
    one way of speaking about this
  • 00:09:19
    measurement of a quantum particle says
  • 00:09:21
    that before the measurement the
  • 00:09:23
    wavefunction either' typical sort of
  • 00:09:25
    broad spread out thing but when we make
  • 00:09:28
    a measurement on the particle suddenly
  • 00:09:33
    it collapses into this spike at one
  • 00:09:36
    particular place because we know having
  • 00:09:39
    made the measurement where the particle
  • 00:09:41
    is now this is generally called for
  • 00:09:45
    obvious reasons collapse of the
  • 00:09:46
    wavefunction the problem is that there's
  • 00:09:48
    no real physical prescription for what's
  • 00:09:52
    going on here with in quantum theory you
  • 00:09:55
    you have to sort of put in this collapse
  • 00:09:57
    by hand so that's a problem but
  • 00:10:01
    wavefunction collapse doesn't mean that
  • 00:10:04
    the particle goes from being sort of
  • 00:10:05
    smeared out before we make a measurement
  • 00:10:07
    to being sharply defined when we make it
  • 00:10:10
    all it says is that before we make the
  • 00:10:13
    measurement there were various different
  • 00:10:15
    probabilities that a measurement might
  • 00:10:18
    reveal it at particular places whereas
  • 00:10:19
    after the measurement we know for sure
  • 00:10:21
    that it's there what's changed is our
  • 00:10:26
    knowledge and some researchers think
  • 00:10:30
    that this is really what quantum
  • 00:10:32
    mechanics is that it's a theory
  • 00:10:33
    describing how our knowledge of the
  • 00:10:36
    world changes when we intervene in it
  • 00:10:39
    and we can't deduce anything from that
  • 00:10:41
    about what the world was really like
  • 00:10:43
    before we had that knowledge about it so
  • 00:10:48
    you see it's misleading to talk in this
  • 00:10:51
    situation about the particle being in
  • 00:10:53
    many places at once the situation tells
  • 00:10:55
    us only about the possible outcomes of
  • 00:10:57
    measurements it's the same thing the
  • 00:11:00
    same story with this notion of quantum
  • 00:11:03
    superpositions and now it's often said
  • 00:11:07
    that the odd thing about quantum
  • 00:11:07
    mechanics is not just that they can be
  • 00:11:09
    in two places at once but they could be
  • 00:11:11
    in two states at once and I want to
  • 00:11:13
    illustrate what that means by referring
  • 00:11:15
    to a property that quantum particles
  • 00:11:18
    have called spin and you don't need to
  • 00:11:22
    know anything about exactly what this
  • 00:11:23
    means except that for some particles for
  • 00:11:27
    an electron say the spin can have two
  • 00:11:30
    values and you could think of them as
  • 00:11:32
    spin being up or spin being down and if
  • 00:11:35
    you make a measurement on the particle
  • 00:11:36
    on the electron then you'll find one or
  • 00:11:39
    the other so it's a binary property
  • 00:11:42
    really and for that reason spins like
  • 00:11:45
    this can be used to encode binary
  • 00:11:48
    information so thee you could say the
  • 00:11:50
    spin up is 1 and the spin down is a zero
  • 00:11:53
    and that's the basis of the quantum
  • 00:11:56
    information technologies that we're
  • 00:11:58
    starting to hear about like quantum
  • 00:12:00
    computers in which spins or other
  • 00:12:02
    quantum states act as quantum bits or
  • 00:12:05
    qubits as they're called but spins can
  • 00:12:09
    be not just up or down a qubit of one or
  • 00:12:13
    a zero they can be in a superposition of
  • 00:12:15
    up and down state so what does that mean
  • 00:12:20
    well it's often said that what it means
  • 00:12:22
    is that the the the particle the
  • 00:12:25
    electron is both up and down at once at
  • 00:12:27
    the same time but that's not right
  • 00:12:30
    remember that the wavefunction tells us
  • 00:12:32
    only what to expect when we make a
  • 00:12:34
    measurement and so in this case what
  • 00:12:36
    it's saying is that in a superposition
  • 00:12:38
    state a measurement might give us an up
  • 00:12:40
    or a down spin and in fact those are the
  • 00:12:44
    only possible outcomes of a measurement
  • 00:12:46
    but what's the the qubit like before we
  • 00:12:50
    make that measurement when it's in this
  • 00:12:52
    superposition quantum mechanics doesn't
  • 00:12:54
    really tell us that well you see now I'm
  • 00:12:58
    not talking any longer about smeared out
  • 00:13:00
    particles or collapsing waves but about
  • 00:13:02
    information and how information can be
  • 00:13:04
    get encoded in quantum systems and how
  • 00:13:06
    we can read it out by making
  • 00:13:08
    measurements this is the perspective
  • 00:13:11
    that's offered by so-called quantum
  • 00:13:13
    information theory
  • 00:13:15
    which is not just a basis for making
  • 00:13:18
    these amazing quantum technologies like
  • 00:13:21
    quantum computers or quantum
  • 00:13:22
    cryptography which is a way of
  • 00:13:24
    encrypting information that it's
  • 00:13:26
    impossible to tamper with to eavesdrop
  • 00:13:29
    on without being Detective it's not just
  • 00:13:32
    that it's really also a new way of
  • 00:13:34
    talking about quantum mechanics itself
  • 00:13:37
    talking about quantum mechanics as in
  • 00:13:42
    terms of information allows us to see
  • 00:13:44
    past or the old-style paraphernalia of
  • 00:13:47
    wave functions and Schrodinger equations
  • 00:13:49
    and quantum jumps and I think to get
  • 00:13:52
    closer to the core of what the theory
  • 00:13:54
    seems to be telling us and I want to
  • 00:13:57
    tell you a story about that and I've got
  • 00:13:59
    some props here to help me now I hope it
  • 00:14:01
    will be illuminating but at the very
  • 00:14:03
    least I'm fairly sure that it's the
  • 00:14:05
    first time that you will have sync on
  • 00:14:07
    two mechanics discussed with the help of
  • 00:14:09
    Sylvain Ian's here they are so I have
  • 00:14:13
    two boxes here a and B 1 belongs to
  • 00:14:16
    Alice 1 belongs to Bob and I believe
  • 00:14:17
    you'd figure out which is which and they
  • 00:14:20
    are boxes in which they produce one of
  • 00:14:25
    these cute toys either a rabbit or a dog
  • 00:14:29
    when we put coins in and they will take
  • 00:14:34
    either a 2 pound coin or a 1 pound coin
  • 00:14:37
    so put coin in let me get one of these
  • 00:14:40
    toys out and there are rules for how
  • 00:14:42
    that works and I'm just going to
  • 00:14:43
    stipulate what some of the rules are
  • 00:14:45
    that these boxes are going to work by
  • 00:14:47
    first of all here's the here's the boxes
  • 00:14:51
    so this is what's going on and I'm going
  • 00:14:53
    to say first of all that rule number one
  • 00:14:55
    if Alice puts a 1 pound coin into her
  • 00:14:58
    box it'll produce a rabbit okay now I'm
  • 00:15:02
    going to add two other rules if Alice
  • 00:15:05
    and Bob both put in 2 pound coins then
  • 00:15:08
    the docs then the boxes between them
  • 00:15:09
    will deliver one rabbit and one dog
  • 00:15:11
    doesn't specify which way round that
  • 00:15:13
    would be but we'll we'll just get that
  • 00:15:15
    combination any other combination of
  • 00:15:17
    coins than both putting in 2 pounds will
  • 00:15:20
    produce produce either 2 rabbits or 2
  • 00:15:23
    dogs
  • 00:15:23
    I'm just stipulating these rules now I
  • 00:15:26
    want to find out what are the inputs and
  • 00:15:28
    out
  • 00:15:29
    let's have to be in order to satisfy
  • 00:15:30
    them a pound analysis produces rabbit
  • 00:15:33
    okay a pound in Bob's produces what well
  • 00:15:35
    let's think about that
  • 00:15:36
    in fact we've we kind of have a lot of
  • 00:15:39
    these answers array so we already know
  • 00:15:41
    okay Alice pound analysis box produces a
  • 00:15:45
    rabbit okay well if when you think about
  • 00:15:49
    it that means that whatever Bob puts in
  • 00:15:51
    a pound or a two pound has to produce a
  • 00:15:54
    rabbit because it could only produce a
  • 00:15:57
    dog in the case where both put in two
  • 00:16:01
    pounds that's one of our rules that's
  • 00:16:03
    the the second rule so we thought that's
  • 00:16:05
    got all the rules already all we need to
  • 00:16:07
    know now is what happens when Alice puts
  • 00:16:10
    in two pounds okay well we know that if
  • 00:16:14
    Alice puts in two pounds and Bob puts in
  • 00:16:17
    if if if Alice puts in two pounds and
  • 00:16:20
    Bob puts in two pounds we know we have
  • 00:16:22
    to get a dog and a rabbit okay that's
  • 00:16:25
    our third rule so that means if I put in
  • 00:16:27
    two pounds
  • 00:16:29
    Bob puts and two pounds we get a dog
  • 00:16:31
    that means also that you know we get a
  • 00:16:34
    dog in this case as well Alice puts in
  • 00:16:36
    two pounds it gives you a dog okay the
  • 00:16:38
    trouble here is that this doesn't work
  • 00:16:41
    because we're not meant to get a dog and
  • 00:16:43
    a rabbit in this top case only in the
  • 00:16:47
    bottom case where they both put in two
  • 00:16:49
    pounds so that one is wrong now what it
  • 00:16:54
    was so what it means is we can only
  • 00:16:56
    satisfy those rules three times out of
  • 00:16:58
    four we get 75% success rate maybe we
  • 00:17:03
    can do better well no matter how you try
  • 00:17:05
    and juggle it and to see if there are
  • 00:17:07
    any other combination that works you'll
  • 00:17:08
    find it won't this is the best you can
  • 00:17:11
    do you can only satisfy these rules
  • 00:17:12
    three times out of four okay but what if
  • 00:17:16
    Alice's and Bob's boxes could switch
  • 00:17:19
    they're out but depending on what the
  • 00:17:21
    other one put in then it's a different
  • 00:17:25
    matter you know then we could say maybe
  • 00:17:28
    Alice's Bob Alice's box gives a dog when
  • 00:17:32
    Bob puts in two pounds but a rabbit when
  • 00:17:34
    Bob puts in one pound ok well that that
  • 00:17:36
    might work the thing is then we have to
  • 00:17:38
    know what you know one has put in before
  • 00:17:41
    the other box
  • 00:17:42
    besides what it's going to give out so
  • 00:17:44
    we need to have some communication
  • 00:17:46
    between the boxes so we need to wire
  • 00:17:47
    them together and they'll send a signal
  • 00:17:49
    between them and then we can do better
  • 00:17:51
    well okay that's fine but this signal
  • 00:17:54
    has to travel down the wire and it can
  • 00:17:56
    only do that at the speed of light
  • 00:17:58
    that's fine if they're here
  • 00:18:01
    that takes you know no time at all
  • 00:18:03
    virtually but it takes some time and in
  • 00:18:05
    fact even at the speed of light if
  • 00:18:08
    Alice's box is here and Bob's box is in
  • 00:18:11
    let's say fiji on the other side of the
  • 00:18:12
    world it takes tenth of a second for the
  • 00:18:14
    the signal to travel there so we have to
  • 00:18:17
    wait that long before bob puts in his
  • 00:18:20
    coin um before Alex puts in her coin
  • 00:18:22
    whichever way around we do it so we
  • 00:18:24
    can't we can't do any better than this
  • 00:18:28
    instantaneously if Baba now is put in
  • 00:18:30
    their coins instantaneously so we're
  • 00:18:33
    kind of stuck you know this this
  • 00:18:36
    communication won't work if we're
  • 00:18:37
    looking at what how to solve this
  • 00:18:39
    problem instantaneously however these
  • 00:18:42
    are classical boxes now what happens if
  • 00:18:44
    their quantum boxes well then we can do
  • 00:18:48
    better because it turns out that the
  • 00:18:50
    rules of quantum mechanics permit us
  • 00:18:52
    what looks like a kind of communication
  • 00:18:56
    between the boxes that happens instantly
  • 00:18:58
    and which would allow the boxes to share
  • 00:19:01
    some information between them without
  • 00:19:04
    any physical connection between them I'm
  • 00:19:08
    gonna say some more about this quantum
  • 00:19:10
    effect that allows us to do this just
  • 00:19:12
    take my word for it at the moment that
  • 00:19:14
    quantum mechanics allows us to do it
  • 00:19:16
    well then we can do better then what Bob
  • 00:19:21
    puts into his box can instantaneously
  • 00:19:23
    seem to effect what Alice puts into her
  • 00:19:26
    box and then we can we could we can do
  • 00:19:27
    better so does that mean then that we
  • 00:19:29
    can satisfy these rules all the time
  • 00:19:31
    well actually we can calculate using
  • 00:19:33
    quantum mechanics how well we can do in
  • 00:19:35
    that case and it turns out that if their
  • 00:19:38
    quantum box is we can't quite get 100%
  • 00:19:41
    success rate we can get precisely well
  • 00:19:43
    not precisely roughly 85 percent success
  • 00:19:46
    rate using this quantum what seems like
  • 00:19:49
    communication between the boxes now what
  • 00:19:52
    I just told you about this mysterious
  • 00:19:53
    quantum link
  • 00:19:55
    is the quantum phenomenon called
  • 00:19:57
    entanglement and I wanted to do that
  • 00:19:59
    without any mass without any Schrodinger
  • 00:20:01
    equation and wave particle duality even
  • 00:20:03
    without any particles just with
  • 00:20:05
    Sylvanian x'
  • 00:20:05
    okay I'm on those what's going on here
  • 00:20:09
    because doesn't Einstein's theory of
  • 00:20:12
    special relativity say you can't send
  • 00:20:15
    any signals faster than light the speed
  • 00:20:18
    of light is the ultimate cosmic time
  • 00:20:21
    speed limit well that's true but you see
  • 00:20:26
    what's going on here is that Alice and
  • 00:20:28
    Bob can't actually verify that they've
  • 00:20:30
    got this eighty-five percent success
  • 00:20:32
    rate without swapping information about
  • 00:20:35
    what their box is produced and the only
  • 00:20:38
    way they can do that is by communicating
  • 00:20:40
    with each other in some normal way by
  • 00:20:41
    email by carrier pigeon by letter
  • 00:20:43
    whatever well however they do it they
  • 00:20:46
    can't do it faster than light and it
  • 00:20:48
    turns out that actually this is what
  • 00:20:51
    special relativity forbids this that you
  • 00:20:55
    can't verify that you've got this this
  • 00:20:58
    this success rate faster than light and
  • 00:21:01
    what that effectively means is that
  • 00:21:02
    Alice and Bob can't use this quantum
  • 00:21:06
    entanglement to send any information to
  • 00:21:10
    each other faster than light that it
  • 00:21:12
    turns out is fine with special
  • 00:21:15
    relativity
  • 00:21:16
    well entanglement was discovered in 1935
  • 00:21:20
    by Albert Einstein and by two younger
  • 00:21:23
    colleagues called Boris Podolsky and
  • 00:21:24
    Nathan Rosen who were perhaps ironically
  • 00:21:26
    trying to show that quantum mechanics in
  • 00:21:29
    their view had a shortcoming and so
  • 00:21:31
    Einstein Podolsky and Rosen came up with
  • 00:21:33
    a thought experiment that they believed
  • 00:21:35
    revealed a deep paradox at the heart of
  • 00:21:38
    quantum theory and which could only be
  • 00:21:39
    resolved by adding something more to it
  • 00:21:42
    and this thought experiment was later
  • 00:21:44
    put in a bright clear reform by the
  • 00:21:46
    physicist David Bohm and that's the form
  • 00:21:48
    I'm going to talk about now and what
  • 00:21:50
    Bohm envisage was something like this
  • 00:21:52
    you have a box that spits out two
  • 00:21:56
    particles in opposite directions and
  • 00:21:58
    they are entangled together that the way
  • 00:22:01
    they're produced means that they are
  • 00:22:02
    entangled what that means is that
  • 00:22:04
    there's some relation between the
  • 00:22:06
    properties of one and the properties of
  • 00:22:08
    other and that's
  • 00:22:09
    think about it in terms of spins so you
  • 00:22:11
    can entangle them in such a way that if
  • 00:22:14
    the spin of one of these particles is up
  • 00:22:16
    the other one has to be down okay and
  • 00:22:19
    then if so then if we make a measurement
  • 00:22:22
    on one of them when we see it has a spin
  • 00:22:24
    up we know that the other one will have
  • 00:22:27
    a spin down so they're correlated now
  • 00:22:32
    perhaps you can see that this is a
  • 00:22:34
    little bit like these two boxes here but
  • 00:22:37
    in the sense that the measurement here
  • 00:22:39
    is playing the same role as me putting
  • 00:22:41
    coins in and the what we see spin up or
  • 00:22:45
    spin down is a binary choice just like
  • 00:22:48
    getting a rabbit and a dog so this was
  • 00:22:51
    really an entanglement experiment now
  • 00:22:53
    actually this correlation might sound
  • 00:22:55
    unremarkable to you because you might
  • 00:22:57
    say well we could do this with a pair of
  • 00:22:59
    gloves let's say left-handed glove and a
  • 00:23:01
    right-handed glove we could send one to
  • 00:23:02
    Alice in Melbourne or something and one
  • 00:23:04
    to Bob you know in in Shepherds Bush and
  • 00:23:07
    then as soon as Bob opens his parcel and
  • 00:23:10
    sees that he's got a left-handed glove
  • 00:23:11
    he knows instantly that Alice has got a
  • 00:23:14
    right-handed glove instantly he knows
  • 00:23:16
    that must be true because they started
  • 00:23:18
    off as a pair so what's the big deal
  • 00:23:20
    well here's the big deal according to
  • 00:23:22
    the Copenhagen interpretation the
  • 00:23:25
    direction of these spins up or down for
  • 00:23:28
    these two entangled particles
  • 00:23:29
    unlike the handedness of those two
  • 00:23:32
    gloves isn't actually determined until
  • 00:23:36
    we observe them until we make a
  • 00:23:38
    measurement and if that's so then this
  • 00:23:42
    experiment by our Entertainment Podolsky
  • 00:23:43
    and Rosen seem to be saying that the
  • 00:23:46
    making a measurement of one particle
  • 00:23:47
    somehow instantly fixes the other as if
  • 00:23:50
    that the result of that measurement is
  • 00:23:52
    being spookily communicated to the other
  • 00:23:55
    particle instantly this is what Einstein
  • 00:23:57
    called spooky action at a distance and
  • 00:24:00
    again he said it can't be right because
  • 00:24:03
    special relativity seems to forbid it
  • 00:24:06
    well for a long time no one knew quite
  • 00:24:09
    how to sort of resolve this paradox what
  • 00:24:11
    they you know what the flaw in the
  • 00:24:12
    reasoning or what the problem was or
  • 00:24:14
    maybe Einstein and Podolsky and Rosen
  • 00:24:15
    were right no one knew what to do with
  • 00:24:17
    it and it was brushed under the carpet
  • 00:24:19
    that changed in 1964 when
  • 00:24:22
    Irish physicist named John Bell whose
  • 00:24:25
    day job sort of like John's I guess was
  • 00:24:28
    a particle physicist at CERN in Geneva
  • 00:24:30
    but in his spare time he turned Quantic
  • 00:24:34
    annex on its head and here before that
  • 00:24:36
    he reformulated the
  • 00:24:37
    einstein-podolsky-rosen experiment in a
  • 00:24:39
    way that showed how you could make a
  • 00:24:41
    measurement to try to figure out what
  • 00:24:44
    was going on here in fact what he's
  • 00:24:45
    drawn on the blackboard there is
  • 00:24:46
    basically a diagram of the experiment
  • 00:24:49
    that he thought of and this experiment
  • 00:24:51
    this procedure it's kind of slightly
  • 00:24:54
    analogous to these black boxes here
  • 00:24:57
    because what john bell basically showed
  • 00:25:00
    was that if you make some measurements
  • 00:25:02
    and you find that there's a certain
  • 00:25:04
    amount of correlation in fact in his
  • 00:25:06
    case again 75% correlation so you you
  • 00:25:09
    know the rules seem to be obeyed 75% of
  • 00:25:11
    the time then it shows that you've
  • 00:25:14
    you've got something like classical
  • 00:25:16
    physics or in fact something like what
  • 00:25:18
    Einstein Podolsky and Rosen thought was
  • 00:25:20
    going on which was basically saying
  • 00:25:22
    those spins must have been fixed all
  • 00:25:24
    along somehow by some variable that is
  • 00:25:27
    hidden that we can't see and can't
  • 00:25:29
    measure okay but if you get a better
  • 00:25:32
    correlation than that between the two
  • 00:25:34
    spins if you get this 85% that quantum
  • 00:25:37
    mechanics predicts then then Einstein's
  • 00:25:40
    picture doesn't hold quantum mechanics
  • 00:25:42
    must be right you must get this strange
  • 00:25:44
    what looks like communication and well
  • 00:25:46
    these experiments were done they were
  • 00:25:49
    first done in the 1970s they were first
  • 00:25:52
    done just kind of more rigorously in the
  • 00:25:54
    1980s having done countless time since
  • 00:25:56
    every time they found the same clear
  • 00:25:58
    result that quantum mechanics is right
  • 00:26:01
    you get a better correlation than any
  • 00:26:04
    kind of classical physics or any kind of
  • 00:26:06
    Einstein like hidden variables picture
  • 00:26:09
    can give you so entanglement really
  • 00:26:13
    happens but what what was wrong then
  • 00:26:17
    with the with Einsteins reasoning in
  • 00:26:19
    this experiment well he made the
  • 00:26:22
    perfectly reasonable assumption so
  • 00:26:23
    reasonable we didn't even realize it
  • 00:26:24
    wasn't assumption um that we can call
  • 00:26:27
    locality that the idea that the the
  • 00:26:29
    properties of a particle of an object
  • 00:26:32
    located on that object I mean it just
  • 00:26:34
    stands to reason this
  • 00:26:36
    his blackness is in the bot what would
  • 00:26:39
    it possibly mean to say this box's
  • 00:26:41
    blackness is also kind of partly in this
  • 00:26:44
    box but in quantum mechanics we do seem
  • 00:26:48
    to have to say things like that it seems
  • 00:26:51
    that properties of objects of quantum
  • 00:26:55
    objects when they're entangled can be
  • 00:26:57
    non-local and it's only if we make an
  • 00:27:02
    assumption that this assumption of
  • 00:27:04
    locality that everything to do with this
  • 00:27:06
    object is fixed here in this location
  • 00:27:08
    it's only in that assumption that we
  • 00:27:10
    have to start thinking about spooky
  • 00:27:11
    action at a distance and this kind of
  • 00:27:13
    affecting this instantly through space
  • 00:27:16
    what quantum mechanics really tells us
  • 00:27:19
    is that there's something else this
  • 00:27:21
    thing that is just vaguely called
  • 00:27:23
    quantum nonlocality which means that
  • 00:27:26
    there's a kind of mixing of these two
  • 00:27:29
    things that it's very hard to put into
  • 00:27:31
    into words but it means that there's a a
  • 00:27:33
    non-local influence that means in effect
  • 00:27:37
    we can no longer think of these two
  • 00:27:39
    boxes as separate objects
  • 00:27:41
    that's what entanglement means they've
  • 00:27:43
    somehow become part of the same quantum
  • 00:27:46
    entity so quantum nonlocality isn't
  • 00:27:50
    spooky action at a distance
  • 00:27:51
    it's the alternative to spooky action or
  • 00:27:54
    distance now where when throw dinner
  • 00:27:57
    when he saw what Einstein Podolsky and
  • 00:28:00
    Rosen had had had said he recognized
  • 00:28:03
    that this phenomenon of an in 7th angle
  • 00:28:05
    Montague who was pretty central to what
  • 00:28:07
    quantum mechanics was really about and
  • 00:28:09
    in fact entanglement is what happens all
  • 00:28:12
    the time when any quantum particle
  • 00:28:15
    interacts with any other they have to
  • 00:28:17
    become entangled that is the only thing
  • 00:28:20
    that can happen according to to quantum
  • 00:28:23
    physics and what this means is that as a
  • 00:28:29
    quantum object starts to interact with
  • 00:28:31
    its environment its quantumness you
  • 00:28:34
    could say or you could say if it's in a
  • 00:28:36
    superposition its superposition starts
  • 00:28:38
    to spread into the environment and it
  • 00:28:42
    becomes harder to see that quantumness
  • 00:28:44
    that superposition in the original
  • 00:28:47
    object itself it's sort of spread out
  • 00:28:49
    like an
  • 00:28:50
    drop spreading in in in water and so
  • 00:28:54
    what that effectively means is that the
  • 00:28:56
    quantumness starts to get washed away
  • 00:28:58
    this entanglement leads to a loss
  • 00:29:02
    technically the word is a decoherence of
  • 00:29:05
    quantum properties and it seems to be
  • 00:29:07
    that that ultimately leads to quantum
  • 00:29:11
    objects behaving like classical objects
  • 00:29:15
    as they start to interact with their
  • 00:29:17
    environment so what that's really
  • 00:29:20
    telling us and what we can now say is
  • 00:29:22
    that there isn't some strange situation
  • 00:29:24
    in which little things like atoms obey
  • 00:29:26
    quantum rules and then for some reason
  • 00:29:27
    big things like a semi classical rules
  • 00:29:30
    are they're just different things
  • 00:29:31
    actually we can now say that this is
  • 00:29:37
    what quantum mechanics looks like when
  • 00:29:39
    you're 6 feet tall that the weirdness
  • 00:29:42
    that we talked about in quantum
  • 00:29:44
    mechanics is just the way the world
  • 00:29:46
    works and in fact you know it's kind of
  • 00:29:48
    us that a weird because by the time
  • 00:29:51
    quantum mechanics has become this scale
  • 00:29:53
    it kind of looks different to how it
  • 00:29:56
    does when you're talking about photons
  • 00:29:57
    and electrons why those does quantum
  • 00:30:01
    mechanics only allow us 85% success why
  • 00:30:05
    doesn't it allows us 100% well it turns
  • 00:30:08
    out that the answer really is about how
  • 00:30:10
    efficiently these boxes can share that
  • 00:30:13
    information about in this case what
  • 00:30:15
    Queen was put into them it's about the
  • 00:30:17
    efficiency of information sharing if we
  • 00:30:20
    can make use of quantum entanglement
  • 00:30:23
    then we can improve the efficiency with
  • 00:30:26
    which information is shared between
  • 00:30:28
    quantum objects like qubits and this is
  • 00:30:31
    really how quantum computing gets its
  • 00:30:35
    its power by more efficiently sharing
  • 00:30:38
    the information among the different bits
  • 00:30:41
    of the system than we can use when we're
  • 00:30:44
    using classical bits like the little
  • 00:30:45
    transistors in laptops and what it also
  • 00:30:50
    tells us is that what makes quantum
  • 00:30:52
    mechanics quantum at root doesn't really
  • 00:30:54
    have anything to do with notions of wave
  • 00:30:56
    functions and perhaps and particle wave
  • 00:30:58
    particle duality it's really about what
  • 00:31:00
    can and can't be done with
  • 00:31:02
    information let me give you a sense of
  • 00:31:06
    where that's leading us because it's
  • 00:31:08
    it's meant that some researchers feel
  • 00:31:11
    that we might be able to reconstruct
  • 00:31:14
    quantum mechanics from scratch getting
  • 00:31:17
    rid of things like the Schrodinger
  • 00:31:19
    equation of waves and particles but just
  • 00:31:21
    using some simple axioms about what and
  • 00:31:25
    what is and what isn't permitted with
  • 00:31:27
    information how it can be encoded and
  • 00:31:29
    transferred and shared and read out I
  • 00:31:33
    want to give you just a flavor of one of
  • 00:31:35
    these what are now called Quantum
  • 00:31:37
    reconstructions this is one there are
  • 00:31:39
    many this is one suggested in 2009 by
  • 00:31:43
    Borya vade deca CH and cassava Bruckner
  • 00:31:45
    at the university of vienna and they
  • 00:31:49
    proposed three what they said were
  • 00:31:51
    reasonable axioms from which we might
  • 00:31:53
    try and construct quantum mechanics so
  • 00:31:55
    here they are they probably don't look
  • 00:31:56
    that reasonable or even that necessarily
  • 00:31:58
    intelligible to you but I'll just
  • 00:31:59
    briefly say what they mean
  • 00:32:00
    information capacity was the first one
  • 00:32:03
    they said let's assume that all the
  • 00:32:04
    stuff all the all the basic entities
  • 00:32:06
    whatever they are that make up the world
  • 00:32:08
    can encode just one bit of information
  • 00:32:11
    they're like those spins they can just
  • 00:32:14
    be up or down and that's it that's all
  • 00:32:16
    what they can hold let's also assume now
  • 00:32:19
    they call this assumption locality it's
  • 00:32:21
    a bit confusing because I've just told
  • 00:32:23
    you about quantum nonlocality but the
  • 00:32:24
    locality in this case means kind of
  • 00:32:26
    something a bit different all it really
  • 00:32:27
    means is that there's nothing hidden
  • 00:32:30
    behind the scenes that's allowing stuff
  • 00:32:33
    to be done with information there's no
  • 00:32:35
    secret device underneath the you know
  • 00:32:37
    here that's allowing these boxes to
  • 00:32:39
    communicate and lastly this thing of
  • 00:32:42
    reversed this idea of a reversibility
  • 00:32:43
    they said let's assume that these bits
  • 00:32:46
    that can you know hold just one bit of
  • 00:32:48
    information they can be interconverted
  • 00:32:50
    reversibly you can go from a one to a
  • 00:32:52
    zero from a spin up to a spin down and
  • 00:32:54
    back again okay they said they showed
  • 00:32:56
    that with just these three rules about
  • 00:32:59
    what can be done with information you
  • 00:33:01
    lead you you get two possible types of
  • 00:33:04
    physics out of them one is classical
  • 00:33:05
    physics one is quantum physics but just
  • 00:33:08
    these rules what's more if you tweak
  • 00:33:11
    this third axiom a little bit to say
  • 00:33:13
    that let's assume that
  • 00:33:16
    in order to do this reversible sort of
  • 00:33:18
    flipping of spins that let's assume that
  • 00:33:22
    you can do it continuously you can
  • 00:33:25
    continuously sort of rotate or spin up
  • 00:33:27
    to a spin down okay if you assume that
  • 00:33:31
    you get quantum rules if you assume it
  • 00:33:33
    has to be just one or the other without
  • 00:33:36
    this sort of continuous rotation so like
  • 00:33:39
    a flipping a coin heads or tails once
  • 00:33:41
    it's down there it's go the heads or
  • 00:33:43
    tails and you can't sort of interconvert
  • 00:33:45
    them then you get classical rules well I
  • 00:33:49
    find that kind of extraordinary you can
  • 00:33:51
    get so much out of what seemed like so
  • 00:33:53
    little and the point about these axioms
  • 00:33:56
    about information is that they can by
  • 00:33:58
    themselves lead to what looks like
  • 00:34:00
    quantum behavior and all the stuff that
  • 00:34:02
    we get out of quantum mechanics like
  • 00:34:03
    superpositions and entanglement and some
  • 00:34:05
    researchers think that these
  • 00:34:06
    reconstructions might lead us to a
  • 00:34:08
    completely different perspective on
  • 00:34:10
    quantum theory perhaps one in which the
  • 00:34:12
    physical meaning of all this seemingly
  • 00:34:15
    strange behavior is clear well that
  • 00:34:18
    remains to be seen but what's already
  • 00:34:19
    illuminating is how they focus on this
  • 00:34:22
    question of information on our on on how
  • 00:34:24
    answers or measurement outcomes are
  • 00:34:27
    contingent on the questions we ask just
  • 00:34:31
    as the outcome of these brats is what
  • 00:34:33
    comes out of these boxes it's contingent
  • 00:34:35
    on what we put in at one pound or a two
  • 00:34:36
    pound and I think this is the most
  • 00:34:39
    productive way to think about quantum
  • 00:34:41
    mechanics and there's a very nice
  • 00:34:43
    metaphor for this perspective that was
  • 00:34:46
    suggested by John Wheeler and John
  • 00:34:49
    Wheeler was a study he studied under
  • 00:34:51
    Bohr and he had actually had Fineman as
  • 00:34:53
    his student and he had this wonderful
  • 00:34:55
    metaphor for how our answers about
  • 00:34:58
    reality can emerge from the questions
  • 00:35:02
    that we ask in a way that is perfectly
  • 00:35:04
    consistent and rule-bound and non-random
  • 00:35:07
    without requiring any pre-existing truth
  • 00:35:10
    about how things were and if this is how
  • 00:35:13
    it goes it's based on the game of 20
  • 00:35:15
    questions so this is get this game I'm
  • 00:35:17
    sure you all know where you know someone
  • 00:35:19
    where everyone chooses a let's say a
  • 00:35:23
    person okay one person goes out of the
  • 00:35:26
    room and everyone else chooses a person
  • 00:35:27
    and then the
  • 00:35:28
    one person has to come back in and find
  • 00:35:30
    out who that person is by asking
  • 00:35:33
    questions and they have to be questions
  • 00:35:34
    that only have a yes-or-no answer
  • 00:35:38
    binary questions as you can see this is
  • 00:35:40
    actually a quantum game okay so let's
  • 00:35:44
    say we play it like this person goes
  • 00:35:46
    outside we all decide on a person and
  • 00:35:51
    well we all will you know do our thing
  • 00:35:54
    and then the person comes back in and
  • 00:35:55
    starts asking questions and on this
  • 00:35:59
    occasion the person who's come back in
  • 00:36:01
    and you know she starts off in the
  • 00:36:04
    normal way she says is it is this person
  • 00:36:06
    alive or dead and what no is it I should
  • 00:36:09
    say is this person dead yes okay this
  • 00:36:13
    person male yes okay and so it goes on
  • 00:36:18
    except that the questioner finds that
  • 00:36:21
    actually asks more and more questions
  • 00:36:23
    takes longer for the answer to come the
  • 00:36:26
    person she asks sort of has to think
  • 00:36:27
    about it for a while before giving the
  • 00:36:29
    answer which is kind of odd because you
  • 00:36:31
    know surely it's like that one thing all
  • 00:36:32
    the other why do you have to think about
  • 00:36:33
    it anyway the game goes on and
  • 00:36:35
    eventually she thinks she's narrowing in
  • 00:36:37
    on who it is and eventually she says I
  • 00:36:40
    know it's Richard Fineman and everyone
  • 00:36:42
    says yes it's Richard Fineman and
  • 00:36:43
    everyone laughs and the game is over why
  • 00:36:46
    did it take you so long you know each
  • 00:36:48
    time when I was asking more and more
  • 00:36:49
    questions - - to answer and everyone
  • 00:36:53
    explains that they'd played the game a
  • 00:36:55
    bit differently they decided that they
  • 00:36:57
    weren't going to decide on a person they
  • 00:37:00
    were simply going to make sure that
  • 00:37:02
    whatever answer each individual gave
  • 00:37:05
    when they're asked was consistent with
  • 00:37:08
    all the other answers in applying at
  • 00:37:11
    least to someone someone ideally someone
  • 00:37:14
    someone famous so as soon as the first
  • 00:37:16
    question you know is this person dead
  • 00:37:20
    was answered yes all the other people's
  • 00:37:23
    answers had to be consistent with that
  • 00:37:25
    had to be a dead person that they were
  • 00:37:26
    thinking of and then it had to be a dead
  • 00:37:28
    male that they were thinking of and so
  • 00:37:30
    on
  • 00:37:31
    but the first person could just equally
  • 00:37:33
    of equally well have said no to that
  • 00:37:35
    first question and then they would have
  • 00:37:37
    converged on someone else not Richard
  • 00:37:40
    Fineman
  • 00:37:41
    so the options become more and more
  • 00:37:43
    constrained as the questions proceeded
  • 00:37:45
    and it took longer and longer to figure
  • 00:37:47
    out you know who's till it's gonna work
  • 00:37:49
    who's gonna be consistent all these
  • 00:37:50
    answers so far and everyone was forced
  • 00:37:52
    by the nature of the questions to
  • 00:37:55
    converge on the same person if you would
  • 00:37:57
    ask different questions you'd have ended
  • 00:37:59
    up with a different answer so context
  • 00:38:01
    mattered there never was a preordained
  • 00:38:04
    answer you brought it into being another
  • 00:38:07
    way that was fully consistent with all
  • 00:38:09
    the questions you'd asked what's more
  • 00:38:11
    the very notion of there being an answer
  • 00:38:14
    only makes sense when you play the game
  • 00:38:17
    it's meaningless to ask who the chosen
  • 00:38:20
    person is in that situation without
  • 00:38:23
    asking the questions about them and
  • 00:38:25
    quantum mechanics is a theory a bit like
  • 00:38:28
    this I think of what is and what isn't
  • 00:38:31
    knowable and how those knowns are
  • 00:38:33
    related and how they emerge from the
  • 00:38:35
    questions we ask and I'd like to think
  • 00:38:38
    of this in terms of a distinction
  • 00:38:39
    between a theory of business and a
  • 00:38:42
    theory of Ethne s-- quantum mechanics
  • 00:38:45
    doesn't tell us how a thing is it tells
  • 00:38:48
    us what it could be
  • 00:38:50
    along with and this is crucial along
  • 00:38:53
    with a logic of the relationships
  • 00:38:54
    between those codes and the probability
  • 00:38:57
    that it could be this so if this then
  • 00:39:00
    that and what this means is that to
  • 00:39:03
    truly describe the features of quantum
  • 00:39:05
    mechanics as far as that's possible at
  • 00:39:07
    the moment I think we should replace all
  • 00:39:09
    the conventional isms of quantum
  • 00:39:11
    mechanics that I kind of started off
  • 00:39:12
    with at beginning with affirms for
  • 00:39:14
    example we didn't say here it is a
  • 00:39:18
    particle there it is a wave rather we
  • 00:39:21
    should say if we measure things like
  • 00:39:22
    this then the quantum object behaves in
  • 00:39:25
    a manner that we associate with
  • 00:39:26
    particles but if we measure it like that
  • 00:39:29
    behaves in a manner that is like a wave
  • 00:39:31
    we shouldn't say that particle is in two
  • 00:39:34
    places at once
  • 00:39:35
    we should say if we measure it if we
  • 00:39:39
    measure it we will detect this state
  • 00:39:41
    with probability X and this state with
  • 00:39:43
    probability Y now this if nurse is kind
  • 00:39:48
    of perplexing because it's not what
  • 00:39:49
    we've come to associate with science
  • 00:39:50
    we're used to science telling us how
  • 00:39:52
    things are
  • 00:39:53
    and if they're if that arise it's simply
  • 00:39:56
    because we don't know enough we're
  • 00:39:58
    partially ignorant about those how
  • 00:40:00
    things are but in quantum mechanics it
  • 00:40:02
    seems like those ifs are fundamental
  • 00:40:05
    well okay but what's the stuff that this
  • 00:40:09
    if Ness is all about quantum mechanics
  • 00:40:12
    doesn't obviously tell us anything about
  • 00:40:14
    that and all we have right now are hints
  • 00:40:17
    and guesses and to try to bring them
  • 00:40:20
    into sharper focus is a tricky business
  • 00:40:22
    which i think means we have to use
  • 00:40:25
    sometimes an almost poetic level of
  • 00:40:27
    expression the kind of thing that will
  • 00:40:29
    send a lot of physicists scurrying for
  • 00:40:31
    cover
  • 00:40:31
    take this attempt for example by the
  • 00:40:33
    physicist Chris Foote
  • 00:40:35
    he says perhaps the world is sensitive
  • 00:40:37
    to our touch it has a kind of zing that
  • 00:40:40
    makes it fly off in ways that were not
  • 00:40:42
    imaginable classically the whole
  • 00:40:44
    structure of quantum mechanics may be
  • 00:40:46
    nothing more than the optimal method of
  • 00:40:49
    reasoning and processing information in
  • 00:40:51
    the light of such a fundamental
  • 00:40:53
    wonderful sensitivity and what folks
  • 00:40:57
    means here is not the mundane truism
  • 00:41:00
    that the human observer disturbs the
  • 00:41:02
    world rather he's saying quantum
  • 00:41:05
    mechanics may be the machinery that we
  • 00:41:07
    humans need at a scale pitched midway
  • 00:41:11
    between the subatomic and the Galactic
  • 00:41:14
    to try to compile and quantify
  • 00:41:18
    information about a world that has this
  • 00:41:22
    incredibly sensitive character so it
  • 00:41:24
    embodies what we've learned about how to
  • 00:41:27
    navigate in such a place well at any
  • 00:41:31
    rate I think it's vital that we
  • 00:41:33
    understand that this if not doesn't
  • 00:41:36
    imply that the world our world our home
  • 00:41:39
    is holding anything back from us it's
  • 00:41:43
    just that classical physics has primed
  • 00:41:45
    us to expect too much from it we've just
  • 00:41:49
    become accustomed to asking questions
  • 00:41:50
    and getting answers getting definite
  • 00:41:53
    answers what color is it
  • 00:41:55
    how heavy is it how fast is it moving
  • 00:41:57
    forgetting the almost ludicrous amount
  • 00:42:00
    that we don't know about most things
  • 00:42:03
    around us in detail we figured that we
  • 00:42:07
    just go on forever asking questions and
  • 00:42:10
    being answered had ever smaller scales
  • 00:42:13
    when we discovered that we can't we felt
  • 00:42:17
    shortchanged by nature and we pronounced
  • 00:42:20
    it weird well that won't do anymore
  • 00:42:23
    Nature does its best and we need to
  • 00:42:26
    adjust our expectations we need to go
  • 00:42:28
    beyond weird thank you
  • 00:42:32
    [Applause]
Etiquetas
  • Quantum Mechanics
  • Wave-Particle Duality
  • Superposition
  • Entanglement
  • Information Theory
  • Heisenberg's Uncertainty Principle
  • Copenhagen Interpretation
  • Quantum Nonlocality
  • Decoherence
  • Quantum Computation