The Many Worlds of Quantum Mechanics

Greetings from Sihanoukville, Cambodia, or at least the waters immediately off. I’m here as part of Bright Horizons 19, a two-week cruise on the Holland American ship Vollendam, in collaboration with Scientific American. We started in Hong Kong and have been working our way south, stopping a few times in Vietnam, and after this we’ll briefly visit Thailand before finishing in Singapore. A fascinating, once-in-a-lifetime experience, even if two weeks is an amount of time I can’t honestly afford to be taking off. Been getting a touch of work done here and there, but not as much as I would have liked, in between dashes ashore to sample the local cuisine. Although the local cuisine has been pretty spectacular, I have to admit.

My job here is to give a few talks about physics and cosmology to the folks who signed up for the package — a public audience, but the kind of people whose idea of a good time while sailing the South China Sea is hearing talks about molecular biology or world history. Mostly my talks are variations of themes I’ve spoken on frequently before — the Higgs boson, the arrow of time, dark matter and dark energy. But to spice things up I decided to throw in something new, so I wrote up a talk on The Many Worlds of Quantum Mechanics.

And here it is — the slides, at least. The content is roughly based on my explanation in From Eternity to Here, with a few improvements thrown in.

Two basic goals here. One is to introduce QM to people who don’t know much more about it than a vague notion of “uncertainty” or “fluctuations.” And in particular, to focus on the conceptual foundations, rather than any of the other perfectly legitimate angles one could take: the historical development, the calculational basics, the experimental evidence, the role in modern technology, and so on. Hey, it’s my talk, I might as well concentrate on the parts I’m most fascinated by. So there’s a discussion of entanglement and decoherence that is a bit more specific and detailed than one would often get in a talk of this type, even if it is enlivened by silly pictures of cats and dogs.

The second goal was to give a subtle sales pitch for the Many-Worlds interpretation. Really more damage control than full-on hard sell; the very idea of many worlds is so crazy-sounding and counterintuitive that my job is more to let people know that it’s actually quite a natural implication of the formalism, rather than a bit of ad hoc nonsense tacked on by theorists who have become unmoored from reality. I’m happy to bring up the outstanding issues with the approach, but I do want people to know it should be taken seriously.

Comments welcome, especially since I’ve never tried this approach in a talk before. Of course by only seeing the slides you miss all the witty asides, but the basic substance should come through.

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67 Responses to The Many Worlds of Quantum Mechanics

  1. Forgot to say – excellent coverage above, Mike, thanks for the many references. Science likes to look at the world, but not at itself.

  2. DEL, I have caught up with your comments. I would definitely say we are philosophically opposed in our views of what physics “is”: I say math merely describes three dimensional and temporal objects that have literal mechanical interfaces to achieve their phenomena (but too small and intricate even for Horatio’s philosophy). You appear to like mathematics without real collapses and so on. I keep an open mind to waves extending and concertinaing on themselves when collapsing – loops can do it if made of a flexible material. Which philosophy is true? Who knows? – I have read about the “calculator school” of physics where folk are told to just calculate and don’t worry about what is creating the momentum.

  3. DEL says:

    Marcus, I don’t even try to catch up with your comments—they are so numerous, lengthy and, to me, incomprehensible. I would definitely say your physics is alien to what most physicists and science philosophers would think physics “is.”

  4. DEL, no doubt, but physics should welcome input from disparate sources (in plain concepts) to help solve its undoubted problems. Uncertainty, Pure Isotropic expansion resulting in a Holographic surface, double slit, redshift, and others are mentioned in this thread alone. An attempt should be made to lay them out in plain language for understanding rather than simply calculating various “momenta”.

    I have plenty of time to read and respond to posts for the next little while. I have an ongoing project in a book at my site that I am continually trying to improve, and I find these blogs help freshen my memory. They are great way to share resources and exchange ideas, as I’m sure you will agree (like Mike’s list of references above, half of which I had never read) . John is a bit abstract for me, but otherwise I understand quite well what others write here, including you.

  5. phayes says:

    I really can’t see the attraction of the MWI or any other of the “psi-ontic” interpretations. As RF Streater says, “quantum theory is a generalisation of probability, rather than a modification of the laws of mechanics” and, for me at least, “psi-ontology” reeks of what ET Jaynes called the “mind projection fallacy” and/or of forgetting what a (pedantically) “honest” classical theory would look like:

    It is misleading to compare quantum mechanics with the deterministically formulated classical mechanics; instead, one should first reformulate the classical theory, even for a single particle, in an indeterministic, statistical manner. After that some of the distinctions between the two theories disappear, [while] others emerge with greater clarity. Amongst the first is the feature of quantum mechanics, that each measurement interrupts the automatic flow of events and introduces new initial conditions (so called “reduction of probability”); this is true just as well for a statistically formulated classical theory. [–Max Born]

  6. John Duffield says:

    DEL: I can’t speak for Jeff Lundeen I’m afraid. But I would hazard a guess that yes he measures both psi’s real and imaginary parts, the latter being related to rotation. And that he measures a ψ wave rather than some matrix. And that the ψ wave is something real, but that it’s more like “spacewarp” than a “concrete physical object”. Re “…my basic QM education did include the trick of turning a localized particle into a spread wave, and vice versa…” I’d say the thing to appreciate is that particles are “spread waves”, and that nothing is every really localized. Like the electron’s field is what it is. You might not like to imagine the photon doing the Fourier trick, but I like the many-worlds multiverse even less. And wavefunction collapse. I share your sentiment on that.

  7. Pingback: Damage control for the Multiverse | The Great Vindications

  8. Meh says:

    Joe Dickinson, get ready for some pseudo-science.

    The way I visualize it is that anything “smaller” than the wave function is not able to be described by the currently accepted laws of physics or mathematics. That means that anything smaller than the wave function (if there is anything beyond the wave function) does not exist for all intents and purposes. Maybe one day String Theory or some other multidimensional theory will have some sort of formulation that proves it should be a fundamental part of our understanding of physics, but at the moment, the wave function is what a sensible person would call the ‘known’ limit of reality.

    If everything we are composed of is just a superposition of wave functions (which is true); then reality doesn’t exist until the wave function collapses and a boundary has been defined (an electron or atom could be…what’s the word I’m looking for…pseudo-scientifically declared a defined boundary). Understand that the collapse of the wave function would be the fastest possible occurring event in the universe (if you accept the idea by 2 very respected physicists that entanglement is just quantum wormholes). So from a biological understanding, it takes far longer for the first electron to be generated for a single electrical pulse traveling between 2 neurons than it does for the wave function to collapse.

    If you accept that wave function collapse is the fastest possible event producible by the universe, then you must also accept that reality does not exist until the wave function collapses. You cannot know that the wave function has collapsed until the collection of recursively collapsing wave functions, which compose your instrumentation, are used to make a measurement.

    Since everything which exists is composed of the same medium, then every action effects every other action. It’s the probability(I) of probability (A) occurring, yet probability (A) effects the result of probability (I). Quantum mechanics uses so many ideas used in fluid mechanics and aerodynamics because they all study the way that equivalent waves effect each other’s behavior. Weather systems are a great example of large scale “wave mechanics”…though there are too many variables to tell you what the weather will be with 100% certainty, and meteorological certainty changes as we watch the weather; we can still make very good predictions of what will happen and when it will happen.

  9. Meh says:

    to no particular person in general,

    It is not that science is poorly explained; as though it is a scientist’s job to make you understand it (to be clear IT ISN”T, it is their job to do science…and it is impossible to “MAKE” you understand something; that is the ultimate example of laziness). It is that this stuff is very difficult to understand. You have to fight your ego and accept that while you may be smart in one way, you probably aren’t smart in other ways. I know plenty of great mathematicians who would be horrible accountants. Many great scientists are horrible engineers because application rarely 100% conforms to theory. Many engineers who are horrible scientists.

    It’s hard to accept, but even after putting in years of effort trying to understand something, the simple truth may be that you just aren’t smart enough to understand it or don’t possess the drive to learn it.

  10. Meh says:

    another short way to explain it, then I’ll stop.

    If you are part of an unknown area within an unknown shape, and that area has no measurable upper or lower boundaries, then there is no known way to determine where you are located within that shape or know with any certainty how you are distributed throughout it. To add to that, you are a percentage of the area; that’s quantum mechanics, the geometry of probability. Luckily, we experience other phenomenon which help us deduce how we are distributed and where the boundaries are. If our understanding of Physics started from quantum mechanics and we had to deduce our way to classical mechanics, we’d be fu…screwed.

  11. Meh, interesting ideas, and mostly true so far as our limits to actual measurement are concerned. The distinction you have failed to make in an otherwise useful analysis of what scientists can actually measure, is between measurement itself and what is measured.

    Meh, the measurer is limited by the reality of measurement – you cannot determine position and motion at the same time to determine momentum at any instant. Particles and fields reduce to wave functions as the briefest intervals when one is fixed – the other is a “wave-function” smear and no better dues to the above limit. Read my post above – this is a limit to measuring – it does not mean that matter cannot “exist” other than as a wave function = it means we cannot measure it any more accurately.

    So, don’t worry about ego and so on. Just lift your analysis to take into account a distinction you have may not be aware of. That’s an ego issue, but easily solved by grasping the distinction. There is no ego issue in this generally for humans understanding matter, just an inability to measure it exactly at all times.

    Give it some thought and let me know if I can help further.

  12. Meh says:

    marcus, I can’t understand your sentence or paragraph structure, so I don’t know what you’re trying to tell me. I wasn’t going to point fingers, but my comment on ego was a result of reading your post above. It’s directed at you. Chill out dude.

    I’m attempting to present a simplified understanding of a very complex subject. A subject so complex that there is no way to simplify it without crossing into pseudoscience, hence the problem with physicists being able to communicate it to the general public in a way that they can understand it. Physicists don’t want to loose credibility by dumbing it down into an innacurate description, something about solutions tending to be boolean or whatevs, but there doesn’t seem to be a better way for the modern 120 character attention span. Visualizing the wave function as real has always worked for my communication purposes.

  13. DEL says:

    Meh, how could psi be real if it’s mathematically a complex quantity?

    And as to understanding dudes, mystics have always used obscure and obscurantist language—it’s in the trade. If they are clear they might be contradicted. Try Nostradamus.

    But they do have one immeasurably huge advantage over scientists: what takes the latter decades of hard work, millions of man-hours, innumerable sleepless nights, billions of taxpayer dollars in experimental facilities and space observatories, and once-in-a-lifetime ingenious idea, takes them just a snap of a finger followed by a bout of disconnected keypad drumming. And, boy! they can produce several new such discoveries every day!

  14. Brett says:

    Hey, I warned you to get ready for some pseudo-science didn’t I?!

    For the sake of explaining the concept of “reality doesn’t exist until the wave function collapses during measurement” to the layperson, I honestly feel it’s better to just bend the truth and tell them that it’s a real thing even if that’s a serious stretch. If LQG becomes a fundamental part of physics, which I think it will be in my life, then sure, http://lanl.arxiv.org/abs/1111.3328v1. But you’re right, conjecture is just that.

  15. No one can do better than plain language Meh. If you come to grips with what I wrote (perhaps re-read it?) then I would be happy to read about ago and such. Otherwise, you are in limbo taking a guess. The logic here is quite plain, or not?

  16. Brett, I agree with you part the way, but it’s not much of a stretch. Read my post above, which some people here find difficult to understand, so you may need patience. We have limits to measurement itself. The act of measuring by a measurer is constrained – limited. What he measures might not have those same limitations. His knowledge about it is limited by his measurements, so he cannot know everything about it.

    In fact this is very simple. A position in space OR time units is not the same as motion in space AND time units. When one is measured it is, by definition literally impossible to measure the other at the same time. Try it. Freeze frame a position 2 meters from a table when moving a hand and tell me the rate of motion AT that position – you cannot have frozen motion – the best you can do is approximate its motion around that frozen point when measuring a fixed position.

    Now Brett, somehow this has been egotistically interpreted as matter being what WE can measure – ignoring the above limitations to OUR measurements, and matter becomes exactly that and no more – and therefore (if my sentence structure is not getting too complex in logical steps) WE say it smears all over the place as a wave functions that exist in “probabilities”.

    In fact, they may exist in their own right. They may be three dimensional and temporal objects that are literally emitted and absorbed, with clearly flexible capacities. they are probabilistic waves to the extent we are limited in measuring them, but otherwise they are quire real and interacting as objects when we cannot measure them due to smears. The relation of position and motion in inverse – the better you know one the less the other. Hopefully that is clear enough to understand – whether you agree that scientists could be so egotistical as to limit nature to exactly what we can measure and no more and then frame conjectures around that. I think that is the exact case.

    It is a preference for deduction – as with Sean and many physicists – where you only know what you can measure and you do not conjecture beyond that to hypothesize what might be happening in the “smears”. Is matter really a “smear” of probabilities? I think you have to go beyond deductive certainty of measurement by introducing the equally deductive certainty of limitation to measurement itself – then match the two principle and have a bit of humility about what we know about nature. Start conjecturing.

  17. Baby Bones says:

    Thanks, Sean.

    I realize this comment is late, but thanks anyway for the post. I hadn’t imagined the double slit experiment could be described in terms of cat trajectories. My feeling is that your explanation adds something to my understanding of QM. That is, I have usually distinguished the case of where the cat does not go and where the electron does not go in terms of behaviors and events. In the macroscopic world, a cat that never takes a nap under the table is a behavior that would be due to some preference of the cat itself, whereas where the electron does not go is determined by the experimental setup. My conclusion is that no-go areas do not require a probability calculation in principle to determine the chance event of a cat sleeping there or an electron hitting the screen. In that sense, we know something completely deterministic about electron trajectories; a non-event is one which we don’t have to calculate the probability and the wave function itself is all that we need to express with certainty that it will not happen. Now instead of imagining the different worldviews for a moment, I am thinking about the uncertainty principle. That is, in an experiment, does the Uncertainty Principle play a part in smearing the zero amplitude areas so that after all there could be electrons striking the screen or cats tracks under the table?