Mind-Blowing Quantum Mechanics

Trying to climb out from underneath a large pile of looming (and missed) deadlines, and in the process I’m hoping to ramp back up the real blogging. In the meantime, here are a couple of videos to tide you over.

First, an appearance a few weeks ago on Joe Rogan’s podcast. Rogan is a professional comedian and mixed-martial arts commentator, but has built a great audience for his wide-ranging podcast series. One of the things that makes him a good interviewer is his sincere delight in the material, as evidenced here by noting repeatedly that his mind had been blown. We talked for over two and a half hours, covering cosmology and quantum mechanics but also some bits about AI and pop culture.

And here’s a more straightforward lecture, this time at King’s College in London. The topic was “Extracting the Universe from the Wave Function,” which I’ve used for a few talks that ended up being pretty different in execution. This one was aimed at undergraduate physics students, some of whom hadn’t even had quantum mechanics. So the first half is a gentle introduction to many-worlds theory and why it’s the best version of quantum mechanics, and the second half tries to explain our recent efforts to emerge space itself out of quantum entanglement.

I was invited to King’s by Eugene Lim, one of my former grad students and now an extremely productive faculty member in his own right. It’s always good to see your kids grow up to do great things!

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14 Responses to Mind-Blowing Quantum Mechanics

  1. Steve Ruis says:

    Um … “Mind Boggling Quantum mechanics” … ” There’s another kind?

    I hope you are excited about the WHIM discovery!

  2. KC Lee says:

    Thanks for the interesting Rogan video. Especially appreciated is your updated view on MOND and dark matter. Will definitely invest more time on your second video.

    Perhaps one should not mention the earthquake induced by the latest nuclear test on the Korean peninsula and NASA’s plan to drill into the Yellowstone volcano in the same breath. Merely a bit concerned, with no firm scientific data to back up that concern though. After all, being interested in geophysics was a long time ago.

    On understanding QM reality, one hurdle could stem from relying on a Bayesian reasoning that is based almost exclusively on our daily experience living in a physical GR world.

    Do we know if one could demonstrate “physically” (as opposed to mathematically via Bell’s Inequality) that QM reality cannot be our familiar GR physical reality?

  3. Paul Hayes says:

    You tell those poor KCL students that they’ll be taught a nonsense , textbook ‘Copenhagen’, interpretation of QM. That may well be true but given that Ray Streater was professor there it’s very sad.

  4. KC Lee says:

    Both videos have one thing in common: saving the best for last.

    Had Rogan been in the audience at King’s College, he might have exclaimed, “Damn! You mean to tell us you’re trying to use something we do not understand at all to derive something we understand well?!”

  5. Frances Day says:

    Thanks for introducing me to Joe Rogan. Really enjoyed that podcast, great fun. But also looking forward to the Oxford one. Your answer to whether you go out of your way to look at astronomical beauty disappointed me though! Go take a look!

  6. KC Lee says:

    While eagerly awaiting the details of your scheme to extract the universe from the wavefunction, just one question first.

    If one favors MWI, does the wavefunction one starts off with already contains GR spacetime? Or are you working with a non-Everettian wavefunction, especially one devoid of Einstein’s spacetime?

  7. Sean Carroll says:

    We are working with an Everettian wave function, which is simply a vector in a large-dimensional Hilbert space. The trick is to somehow map that, in the long-distance limit, to quantum fields propagating on a curved spacetime background.

  8. KC Lee says:

    Thanks, Sean.

    There was a question, sometime back, about the quantity of energy involved in entanglement. Your response was that things are not well defined enough to address the question.

    Now hearing in your video that the destruction of entanglement takes energy, wonder if the old question has a chance to be addressed now?

    A somewhat related question is about entanglement monopoly. Is a “new” entanglement necessarily accompanied by the destruction of a previous entanglement involving one of the particles? One corollary is: does that imply the new relationship uses the energy from the old one?

  9. Ara says:

    The Maxwell lecture is a masterpiece! boiling down all that stuff to their core basics in < 1 hour is amazing. I have not seen any scientist do a better job than Sean in summarizing and communicating profound concepts to the rest of us.

  10. Another Askhole says:

    Joe Rogan asks some great questions. I agree that it is hard to understand the science translations. For instance, I just read in New Scientist that astronomers discovered half of the dark missing dark matter. However, another article in Forbes claims they found the missing baryonic matter astronomers already suspected was there and could not find before. So did they find missing dark matter or not? Does that mean that any new sources of baryonic matter will be claimed to be in the second half of the missing ordinary matter we have yet to detect? I understand how they measure the density of gas using the red shift of light and the spectrum as it passes through the plasma. However, how do we know that we did not under count the number of red dwarfs or miss rocky clumps of matter? I understand the science say the universe is not old enough for black dwarfs but maybe we should we keep the possibility of completely dark baryonic matter open since we are still missing a lot of sources of ordinary matter.

    Joe mentioned the metric expansion of spacetime is traveling faster than the speed of light at the edge of the observable universe. If the metric expansion of spacetime does not have a speed limit then does that mean that spacetime can be nonlocal? If gravity changes the metric expansion of spacetime, like inside the horizon of a black hole, then would a black hole merger distort parts of spacetime and allow an escape route from a black hole? If mass can escape from a black hole would that decrease the event horizon and photon sphere to allow gamma rays to escape too?

    I like the second video. I have not thought a lot about the interpretations of quantum physics but it seems that a lot of the confusion is due to people who are unable to get past the lack of a hidden variables in quantum. Intuition would have us believe there must be something going on that we do not understand and that we are missing a piece of the puzzle, a hidden variable. Bell’s inequalities tells us that there are no local hidden variable theories and accept the copenhagen interpretation. Sean brings up an interesting point about entropy, energy, spacetime and the entanglement collapsing the wavefunction in the second video. I do wish something about spacetime was acting like a hidden variable but that would mean that the metric expansion or contraction of spacetime is nonlocal at the quantum level. What do Bell’s inequalities say about a nonlocal hidden variable theory? If there were a nonlocal hidden variable then the copenhagen interpretation is wrong and states could be transferred faster than the speed of light during an entanglement and collapse of the wavefunction when entangled with the wavefunction of the universe. Excuse my sloppy language because nothing can travel faster than the speed of light.

  11. Rinn says:

    If it is not too late to make suggestions what to include in Something Deeply Hidden, I would love to understand how is it possible that we observe a fairly deterministic world while the true nature of reality is probabilistic. As I understand it, there are lotteries going on with subatomic particles all the time. On the other hand, we in the macroscopic scale observe that F = m*a reliably. It is not F = m*a give-or-take two standard deviations. How come there are no echoes of the quantum mechanical lotteries when we look at large systems?

  12. KC Lee says:


    Knowing that it will be corrected if too simplified or plain wrong, allow me to venture one explanation why “we observe a fairly deterministic world while the true nature of reality is probabilistic”.

    Because most everything we observe are “post-measurement” already.

    Right or wrong, the implication is unmeasured, quantum properties need not exist. But once measured, such properties are no longer described by probability. Rather, those properties have been “determined” (thus part of the deterministic reality).

    Other common ways to describe the phenomenon of measurement (above) is to say that the wave function has collapsed, the Everettian worlds have branched, or decoherence of superposition has occurred.

  13. Was so excited when you said on the Josh Rogan podcast that it is false to say that atoms are mostly empty space.
    The electron is a smooth closed wave function everywhere inside the atom makes so much sense.
    The ’empty space’ idea I have read so often always seemed somewhere between meaningless and confused to me.

  14. KC Lee says:


    In addressing your question why things appear deterministic in classical while remaining probabilistic in QM, measurement was seen to mediate between those two worlds. Let’s see if measurement could shed light on the nature of their difference.

    Classical physics says:
    A. Things physically exist whether one measures them or not.
    B. Nothing can travel faster than light.

    A standard QM experiment is the double-slit. The observed interference pattern behind the slit screen corresponds to the probabilities where one can find a particle, at measurement.

    Imagine now the double-slit screen extends across the observable universe, fully matching its diameter. When the experiment is repeated using this modified double-slit screen, a photon described above in “A” will violate “B”.

    How so?

    Per QM observation, a photon is capable of appearing, at the instant of measurement, at either end of the slit screen (and anywhere in between). Why? Because that slit screen, modified or not, corresponds to the interference pattern of the photon.

    But in classical physics, for a detector located at the center of the observable universe, it takes two photons, one from each edge, at any given instant, to cover the width of the universe, now also equaling that of the modified slit screen.

    One photon, if asked to do the job of those two photons, would violate “B”.

    Conclusion: Local realism, which underpins our understanding of classical reality, when asked to explain the QM world, breaks down due to the internal conflict just noted.

    Implication: QM reality is likely not the physical reality of classical physics. Familiar as this may sound, now we know the reason lies at the heart of GR local realism. What QM reality is remains a mystery. With this thought experiment, at least we could sense one thing it is not.

    What has made this revelation possible is the scaling of a standard QM experiment to GR size. Whereupon, the existing physics of both GR and QM are brought to bear, in the same experiment, on the same particle, at the same instant, of measurement.