Quantum Randomness | Quantum Mechanics 3

Category : Randomness
Quantum Randomness | Quantum Mechanics 3by wpjljron.Quantum Randomness | Quantum Mechanics 3How is quantum randomness anymore mysterious than the randomness of a coin flip? You’ll see. The homework questions and extra readings are below: The questions: 1. What if there are three…

How is quantum randomness anymore mysterious than the randomness of a coin flip? You’ll see. The homework questions and extra readings are below: The questions: 1. What if there are three…


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25 Responses

  1. Mc27 February, 2015 at 3:32 am

    1. We’ve already seen a link of how it looks if there isn’t a detector
    there (I’ve compared it to the amplitude spectrum of a square wave but a
    glance at the math revealed that a triangle wave would be a better fit,
    still need to check that fully though) and if the detector was there then
    if particle goes to the slit with the detector it would behave classically
    and if not, wavelike, so the the patter on the wall would be a combination
    of a blob and an interference pattern with the intensity given by the
    coefficients of the wave function.
    2. No, some interactions lead to entanglement (which is in a way opposite
    to measurement?) but I’m not exactly sure where the boundary is. Here’s an
    example of a big thing that still had detectable QM properties:

    3. It stays collapsed because burning a paper doesn’t really destroy the
    information, it just dissipates it into the environment but I it would be a
    nice way to test for potential information-destroying things.
    4. It’s understandable that some people didn’t like the randomness because,
    up until QM everything was Newtonially determined and if you didn’t know
    what was exactly happening you could still be comfortable in the
    reassurance that its just bit more complicated but with the same base.
    Since Newtonian mechanics is based on the macroscopic world you can
    intuitively imagine real things alongside your equations. This gets
    trickier with QM but with more thinking and effort into making simulations
    and games and bigger quantum systems I think it will get easier.

  2. Muhammed Ali Inamdar26 February, 2015 at 4:00 pm

    You actually cleared a while bunch of doubts I had regarding the
    measurement and wavefunction collapse. THANK YOU.
    Also, does the WAY of measurement affect the wavefunction? Meaning if I
    change the way a particle is measured, would it still collapse into one of
    rhe many possible situations, given that the quantum system isn’t
    influencing the particle at all?

  3. DurtyFC26 February, 2015 at 5:11 am

    Bob is an asshole

  4. Antony Hamilton25 February, 2015 at 6:25 pm

    I keep seeing people claiming that it’s proven that quantium mechanics is
    random… How do we know this? How is it possible to know if something is
    truly random? How do we know there isn’t a hidden variable? To know that
    you would have to be certain that you know everything there possibly is to
    know about the situation, and in my opinion only a foolish scientist would
    assume that he knows everything about anything. Is there really a way to
    prove that it’s truly random?

  5. Jason9360925 February, 2015 at 6:16 pm

    Homework time?
    1) The distribution will resemble a two-slit diffraction pattern (details
    depending on location of source and overall configuration) plus a
    single-slit diffraction pattern.
    2) I’d like to say yes
    3) Yes, the wavefunction stays collapsed immediately the measurement. It
    depends of course what kind of measurement it is. If the wavefn is
    collapsed to an energy eigenstate it will remain there forever in terms of
    probability. Any other state, it will evolve in time.
    4) I’m not sure how I feel about it. I just find the collapse postulate
    strange due to non-unitarity, as I’ve already said. It is interesting to
    think of fundamentals, and what the hidden vars might represent. The
    interesting point about QM is that you cannot think of separate isolate
    subsystems, all degrees of freedom must be treated in the theory.
    But, at least, if I’m late at meetings I can say that I calculated exactly
    my energy, so I couldn’t be certain about the time.

  6. Benjamin Clark25 February, 2015 at 6:08 pm

    can you do quantum tunnelling

  7. Gabriel Urquiza25 February, 2015 at 5:58 pm

    Wow, tough questions. I have no idea what happens regarding the three
    slits. I guess
    the wavefunction of the particles going through slit1 will be collapsed so
    we will have
    1/3 of the particles going through slit 1 in a batch and 2/3 of them
    spreading in an
    interference pattern behind slits 2 and 3, but that is just a (rather
    poorly) educated

    Regarding the measurement… Well, it depends on what you count as
    “interaction”.I might be stretching a metaphor here but the double-slit
    experiment can be understood as if the wavefunction interferes with
    itself…Does that mean the particle, somehow, interferes with itself? If
    so, then that is an example of an interaction that does not produce any
    kind of collapse. Also, how exactly is the particle “associated” with a WF?
    That is often left out from QM discussions because most physicists doesn’t
    even know what the hell is a WF anyways, in a physical sense anyway
    (mathematically it is the eigenfunction of a hamiltonian). So if you
    interpret that the particle somehow interacts with its wavefunction,
    therein lies another example of an interaction that does not collapse the
    wavefunction. In a more pragmatic sense, we could think of a wavefunction
    associated not with a single particle, but with a system of particles. If
    the system hasn’t been measured, then by definition the wavefunction is in
    a superposition state, which certainly includes all possible interactions
    between the particles IN the system. So it seems that what qualifies a
    “measurement” isn’t just the interaction with something… But the ability
    that such interaction has to reveal traits of the system to things outside
    of it. At least those are my thoughts, I’m not a trained physicist so I
    have no clue as to what kind of hard experimentation is being done in that

    Whether the wave function stays collapsed… Hum, I don’t think it does.
    The information we get about, say, the position of a photon going through
    the air is transitory. We know where it was at that time, but what about
    now? What happened when the photon hit the detector? What sort of
    interaction it went through inside the material, or with the electrons it
    ejected from atoms which generated the current that in turn activated the
    circuitry that made the detection possible? It seems as if time has the
    property of blurring the system again and rendering our information
    useless, which in turns will require us to make new measurements if we want
    updates. I think if you isolate a system and let it rest long enough, it
    will enter in a superposition… Which sounds funny because if you isolate
    a system its entropy will rise until a maximum value is reached. Does that
    have anything to do with superposition? I have no clue, but it is a funny
    idea to toy with. As for the last question… I play RPGs, so I’m cool with
    god playing dice, as long as it doesn’t make critical hits against me very

    Cheers o/

  8. trick017125 February, 2015 at 5:21 pm

    4:05 – It would be way more surprising if the wave function collapsed for
    Bob but not for Alice. ;-)

  9. Luapix25 February, 2015 at 4:57 pm


    1. Before the measurement, the wave function is as usual: adding up all
    three outcomes. After the measurement, if the odds are that the detector
    does detect the particle, then the wave function collapses to just that
    state. Otherwise, I’m guessing that it collapses to an addition of the two
    other states: we know the particle didn’t go through the slit with the
    detector, but we don’t know which one it went through.

    2. I think all interactions that have different results depending on a
    certain value count as measurements of this value.

    3. If this fire is a real fire, its shape, temperature, (or its state in
    general) will be influenced ever so slightly by what is written on the
    paper. And a “perfect fire” that would be always the same regardless of
    what is written is not possible, because it would destroy information, and
    I’m pretty sure that’s impossible. (2nd law of thermodynamics ?)

    4. I really like hidden variable theories more than the standard
    interpretation of quantum mechanics, but I feel like we might never know
    which one is true…

  10. Daniel Windsor25 February, 2015 at 4:43 pm

    1. I think that you’d collapse the wave form if you detected where the
    particle was, but only ~1/3 of the time.
    2. This has bugged me a lot about quantum mechanics. Does O2 act like a
    particle because it’s two atoms bonded to each other, or does the whole
    system act like a waveform. Do quarks in a proton collapse it bajillions of
    times a second from all their interactions?
    3. I think the particle bumping into the first atom of the detector it hits
    is the actual observation, and the detector is just a tool to amplify the
    observation, and most of the machine isn’t really doing any sort of
    observation on the particle.
    4. I think my main problem with quantum randomness is I can’t grab it and
    play around with it to figure out how it works. Nor can I really relate it
    to something I can do similar experiments with. So instead of using math to
    help my brain understand the concept better (like I could with say,
    Newton’s gravity), I am completely a slave to the math involved. Plus the
    fact that the math is literally complex (bad pun sorry) and the numbers
    involved tend to be so big or small that you’d go insane trying to imagine

    I hope what I wrote actually makes sense, this whole subject makes my brain
    hurt (in a good way I think)

  11. A Braker25 February, 2015 at 4:41 pm

    Since any interaction with a quantum particle results in its collapse, I am
    curious about the odds it would interact with a virtual particle in the
    vacuum of space. I would expect this number to be extremely low, but this
    would potentially collapse any entangled particles given enough time. Hmm,
    sounds something similar to the half-life of a radioisotope.

    I also favor Bohmian Mechanics with the Pilot Theory and such. I can’t
    accept the fact that there is true randomness on the quantum level. The
    fact that we claim randomness tells us we are looking at a black box that
    spits out numbers, not knowing why or how. I would love to take a look
    inside that black box someday and see how it works.

  12. PositiveANegative25 February, 2015 at 4:22 pm

    Amazingly clear, LGU, once again. Thank you !
    How about that, that I think of about the wavefunction collapse and the
    measurement: the wavefunction is not the position of the particle, but the
    space in which it is allowed to spread; and the incoming photon is not
    collapsing the electron but the space in which the electron spread, forcing
    it to appear in a point which position is defined by the photon, but energy
    defined by the electron (only certain wavelength of photons “work”).

    My assumption here is, that in the absence of photons, matter would spread
    evenly in the space in which it is allowed to spread, and that the energy
    of the electron defines the energy that will react to it but not the
    position, which would be brought by measurement.

    Is there a link, a proportionnality, between the “size” of the observed
    particle and the energy brought by the measurement?

    Hello from Belgium!

  13. TheBurek25 February, 2015 at 3:00 pm

    True randomness really bugs me. It probably has something to do with me
    being a programmer and being used to think in terms of deterministic
    systems. If you’re playing, say, Monopoly on a computer, when it’s time to
    roll the dice, if you take a snapshot of all of your computer’s memory, you
    can positively determine what “random” number on the dice is going to land
    – as is the case IRL with classical physics. You simulate, because from one
    state you can only get to a single possible next state, there’s no real
    randomness to it. I get that, and I feel comfortable thinking in those
    terms. But this “true” randomness in quantum mechanics… I just can’t help
    but feel there’s more to it, that there’s something we’re missing, another
    bunch of hidden variables that determine the outcomes, just this time at a
    much lower level.

    But then again, if the entire universe is deterministic, you could simulate
    it. Of course, it would be impossible to measure everything in the entire
    universe all at once and use it for simulation, that could then essentially
    predict the entire future, forever. But what if that could be done on a
    smaller scale? There’s an idea that bewilders me – what if you could
    simulate yourself and enough of your surroundings? That simulation would
    contain your thoughts about it and your reactions to seeing your thoughts –
    about seeing your thoughts. I can’t help but wonder what that would look
    and feel like. Similarly, if someone created a prefect copy of yourself in
    split second in front of you – what would you say to each other? Would you
    just start saying exact same things in unison, because your brains started
    from the same state and are functioning in the exact same way? I guess
    non-determinism of the universe would at least nullify such concerns. But
    then again, I love sci-fi and sometimes think too much…

    (edit: I’m aware that my “local simulation” has a bunch of problems,
    including the recursive nature of it (such a simulating machine would have
    to simulate itself for it to work), but I’m still guessing even with
    limited recursion and limited area and whatever else needs to be limited,
    it could at least theoretically be accurate enough for you to be profoundly
    perplexed for at least a few seconds)

  14. Anthony Falk25 February, 2015 at 2:48 pm

    great video! I truly enjoy them 🙂 Is there a way to angle multiple mirrors
    to observe the double slit experiment without the detector and project the
    results somewhere else? I know they are subatomic but its a way of
    observing without directly observing—

  15. Incongruent I25 February, 2015 at 2:47 pm

    All hail the Quantum Queen! (Don’t worry, my worship-speak will collapse as
    soon as you poke fun at it).

  16. simonbelgers9425 February, 2015 at 2:09 pm

    I’d think the wavefunction would collapse completely, because you still
    measured it. You might not have measured where it goes, but you did measure
    where it didn’t go. I’d think this would still count as a measurement that
    collapses wavefunctions.

  17. Spliter25 February, 2015 at 2:09 pm

    Two things I’d like to ask you: how do we know that it’s true randomness?
    What experiments did we do to prove that?
    The other question is: Alice can determine if someone is observing the
    particles going through a door, just by looking at the pattern on the wall.
    Does this carry on to entangled particles as well? If so, does this mean
    the ability of superluminal communication (say, she has a bunch of
    particles shooting at a wall, that are entangled with bob’s particles which
    are also shooting at a wall, and bob’s observing them or not resulting in a
    morse code.)

  18. Patrick Melody25 February, 2015 at 2:09 pm

    I was expecting this video to be about all possible things, but by the time
    I played it, other viewers had collapsed it’s wave function. So now it’s
    just about quantum randomness.

  19. Panic Pillow25 February, 2015 at 1:51 pm

    Thanks for the amazing video, I had no problem understanding what you where
    trying to explain (or at least I think so).

    Answers to the questions:
    1. I would think that the wave function would only collapse if the particle
    goes through the observed door and in the other case, we would still see an
    interference pattern. This would then lead to a bunch of particles behind
    door 1 (the observed one) and an interference pattern behind the other two
    doors that would partly overlap with the bunch of particles behind door 1.
    2. Whether or not every interaction is a measurement seems like something
    testable, at least hypothetically: Create an environment where there is
    nothing to interfere with and start introducing things that could interfere
    with the particle. First separated, then cumulative (maybe we need multiple
    interactions for a wave function to collapse). There is of course the
    problem that we might not know all things that have a causal relation to
    the world yet (e.g. god if we want an extreme example but it could be
    fields, or particles we are unaware of as well) so creating an environment
    where nothing interferes with the particle can only be done in light of
    current scientific knowledge, making any conclusion based on the experiment
    temporal, however in the light of pragmatism, this should not be an issue.
    I would say that interaction through stuff like gravity, doesn’t seem to
    cause the wave function to collapse (at least to the best of my knowledge),
    so it might be other non-physical forces could interact with the particle
    without causing the wave function to collapse. One thing is sure, if the
    interpretation of the particle ‘interacting with itself during a super
    position’ is correct/actually follows from the theory, there is at least
    one type of interacting that doesn’t cause the wave function of a particle
    to collapse.
    3. I would say that the wave function restores after some time (once again
    this could be tested, but this is just my current thought on the matter).
    It would be weird to think of particles that in their entire existence have
    never interfered with anything and even if they exist, how did we manage to
    obtain so many of them and how do we manage to use them without interacting
    with them. This does bank on the idea at least all physical interaction is
    causing the wave function to collapse, which it might not.
    4. Randomness is from a pragmatic point of view not problematic, as you
    said: the theory works. This does however have some implications: If
    science tells us how the world actually works (controversial but possible
    position in philosophy) i.e even outside of our human interpretation of the
    world, this means that physical determinism (the idea that if all variables
    of the world would be known, we could predict all events) is no longer
    possible. This has some implications in the free will debate, though at the
    same time, this randomness doesn’t account for the existence of free will
    in any way.
    If science tells us how the world works, in so far it applies to beings
    living in 3 spacial dimensions and (seemingly) 1 fixed time dimension, this
    has implications for some world interpreters like Kant. He said that,
    though we can not know the actual world, we can know certain things for
    sure about how we perceive the world. He made the bold claim that the
    Newtonian view on mechanics is how our human brains structure the world, in
    order for it to make sense. But Newtonian physics in general is rather
    outdated, so this claim seems rather strange.
    Personally, I don’t think randomness is problematic and I think that
    science does a good job of accepting those theories that are able to
    explain the most, in the simplest ways (Occam’s Razor) , allowing for the
    largest amount of progress. If randomness is able to explain more, or
    simpler what caused scientists to make certain observations, I don’t see a

  20. John Doe25 February, 2015 at 1:44 pm

    First of all thanks for the new video, it was brilliant as usual 🙂

    Question 1:
    I would say that before it looks like |Y> = a|door 1> + b|door 2> + c|door
    3> before
    If it goes through door 1 (the one with the detector) |Y> = |door 1>
    If it goes through another door |Y> = d|door 2> + e|door 3>
    so the interference pattern will look like a normal one but with a higher
    concentration of particles behind the first door.

    Question 2:
    I think not, as when you change the polarization of light the wave function
    doesn’t always collapse. That is one example but there are probably other
    interactions that won’t modify the wave function but only ones that would
    not allow you to know which door it went through.

    Question 3:
    Yes, because it is the act of measuring and not the act of storing the data
    that collapses it.

    Question 4:
    I personally love the idea that there are some aspects that can never be
    predicted. I find it very interesting even though I can see why it was (and
    still is) discomforting for some people and scientists.

    Disclaimer : these are all personal answers to the questions that I came up
    with on the spot. I did not do research to find them so they may be (and
    probably are) incorrect.

  21. Erik Žiak25 February, 2015 at 1:24 pm

    Last time I did not answer the questions properly, but today I am going to
    write it (actually it is a nice distraction from work):

    1. My guess on “Three Slits”: There would be a wave patter within a wave
    pattern. Something similar to amplitude modulation maybe? I know this is
    oversimplified, but for the sake of having something to imagine in my head
    I wrote this. If you measure one slit, then one of the wave “collapses” and
    you end up with an “unmodulated carrier wave” with a “hotspot” behind the
    measured slit? Hmm. Maybe I wrote something totally stupid, but at least I
    am honest with you here. I will google “Three Slits” after I post this.

    2. There are some so called “soft measurements”, they do not count as they
    tell us only that out of let us say 100 measurements, 20 went through this
    slot, but we cannot tell which 20 of the 100. At least that would be my
    understanding of it without having studied it any deeper. I hope I did not
    write total BS now. Will google it also.

    3. Yes, it stays collapsed. We discussed it already. 🙂

    4. I have no problem with the randomness. Actually it makes sense to me
    from a (pseudo-)philosophical point – as I wrote earlier, the deterministic
    “macroscopic” behavior had to break down at some point in order to have
    true randomness (out of chaos). If we could measure everything absolutely
    precisely (so there would not be any more digits left), the universe would
    be a deterministic “machine”. But it is not and that might give us the
    illusion of free will (more pseudophilosophy). But I see that there is
    something not quite right with this view, but it would be a longer
    discussion where I point out why it might be a fallacy, no time for that
    now. I guess some physicists had/have problems with it as they interpret it
    differently, have their own reasons to believe one way or another or see
    other interpretations as a threat to their own views. It is a psychological
    matter, like the preferences between colors.

  22. Animuldok25 February, 2015 at 1:18 pm

    Ill take a whack at it…
    1. If the particle is measured (at the top door) the wavefunction
    collapses. If it is not measured, then it is still in superposition
    between the middle and lower doors.
    2. Interaction only collapses the wavefunction if what it is interacting
    with is a collapsed wavefunction
    3. The wavefunction was collapsed at the point it was measured. That is
    its startingpoint and a new wavefunction would need to be calcuted based on
    that time zero.
    4. randomness from chaos makes more intuitive sense, but quantum randomness
    is a mindtwister but QM has extremely good predictive power so should be
    accepted. People typically have difficulty with comprehension of physical
    laws and theories that are outside their sensory experience with things
    that scale with us and our natural environment. The TL;DR… our brains
    didn’t evolve in a microscopic or an immensely macroscopic (eg planetary)
    scale. And we surely never even pondered the Planck scale until last

  23. Nillie25 February, 2015 at 1:11 pm

    1. I suppose you’d see particle-like behaviour from the particles that went
    through the slit with the detector and a wave-like interference pattern
    from the ones that went through the other slits.

    2. I don’t know. The fundamental forces are always affecting a particle,
    just not always very much depending on the distance, so those can’t really
    count as interactions that “must” collapse the wave-function.

    3. That particular wave-function stays collapsed, but does it really
    collapse into just a single point with 100% probability, or does it just
    collapse into a simpler wave-function? If I understand Heisenberg’s
    uncertainty principle correctly, the latter seems more likely to be the

    4. I honestly prefer the alternatives with true randomness. This is more
    for philosophical reason: if everything is deterministic, what becomes of
    free will?

  24. pablo diaz25 February, 2015 at 12:54 pm

    I really liked your explanation of why quantum randomness doesn’t usually
    happen in everyday life, i don’t think i had heard it explained that way.
    Usually its just said that it’s really unlikely, and that it. :)

  25. Looking Glass Universe25 February, 2015 at 12:36 pm

    Please let me know if you thought this one was/wasn’t clear. I spent a long
    time worrying about it. (Which still isn’t a good enough excuse for why
    it’s this late. That can be explained by laziness.)

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