Many Worlds Insanity

I was surprised to discover I’ve never posted about the Many Worlds Interpretation (MWI) of quantum physics — I would have sworn I had. I’ve mentioned it a few times, and I know I’ve discussed it in comment sections, but it seems I never tackled the subject explicitly for the record.

It’s been on my mind lately because others have talked about it. Sean Carroll’s book promoting it generated a wave of discussion. The final push for me was Jim Baggott’s Farewell to Reality, which consigns MWI to the “fairy tale physics” heap.

Since I quite agree, this seems a good followup to yesterday’s post.

I’ve never found the Many Worlds Interpretation persuasive. What little argument there is for it doesn’t come close to convincing me, and the arguments against it seem substantial.

To be frank, my intuition rebels, and, while I appreciate that intuition is far from perfect, I need hard evidence or irrefutable argument to override it. (It might help to replace the emotionally freighted word “intuition” with “rational analysis informed by known facts”. MWI just doesn’t seem logically coherent to me.)

To be clear, this is metaphysics. There is no hard evidence about the nature of reality, so MWI is a story that seeks to explain certain observations of quantum behavior.

§ §

I’ll start with what MWI is and what attracts some to it (as I understand it).

Consider a simple experiment where a laser emits a single photon towards a half-silvered mirror. The photon has a 50% chance of being reflected and then hitting detector R (reflected) and a 50% chance of passing straight through and hitting detector T (through).

In the standard formulation of quantum physics, until the photon actually hits detector R or detector T, both possibilities co-exist. The photon passes through and is reflected. This is the same situation as in Schrödinger’s Cat box: the cat is both dead and alive.

Our experience, however, is that only one detector triggers. The cat is either dead or alive (and very angry).

The standard formulation (the Copenhagen Interpretation) has a wave-function — a mathematical construct called the Schrödinger equation — that describes the “both are true” path of the photon. It tells us, for any point along either path, what the probability is of finding the photon there.

In particular, at each detector (because of the half-silvered mirror), the equation says there is a 50% chance of that detector will detect the photon.

And, sure enough, if we do this experiment a bunch of times, we find that 50% of the time, detector T triggers, and the other 50% of the time detector R triggers. Exactly what we’d expect. It’s a half-silver mirror, so half the light goes through and half the light reflects.

It’s also true if we turn up the laser and shine zillions of photons at the mirror. Half of them reflect, and half of them pass through. (Both detectors would light up like crazy.)


This seems like a very reasonable picture, so what’s wrong with it? What does MWI bring to the table to solve?

Schrödinger’s equation. All of reality?

The problem is that the Schrödinger equation follows the linear evolution of the photon, but it doesn’t have anything to say about which detector triggers. The actual measurement of the photon at one or the other is a discontinuity we can’t explain.

It also raises some serious questions about locality, but that’s another post.

The Measurement Problem — what happens when you open the cat box — is one of the great problems in modern physics. A measurement “collapses” the Schrödinger equation such that all the probabilities suddenly drop to 0% except at the point the particle is found, which jumps to 100%. We can’t account for how that happens.

Enter the Many Worlds Interpretation.

Which says that, when the photon hits the half-silver mirror, reality splits in two, and now there are two realities. In one, the photon passes through the mirror; in the other, the photon reflects. MWI posits that both these realities are equally real.

You can see where the Interpretation gets its name. The result of all this splitting is an unimaginably huge number of realities. According to MWI, there is almost an infinite number of each of us.

The attraction is that MWI is said to solve the Measurement Problem by saying the wave-function never collapses. The photon’s wave-function becomes part of the detector’s wave-function, and that expands to the system surrounding the detector including the scientist observing.

As wave-function collapse is seen as a bolted on extra in the Copenhagen Interpretation, the Many World Interpretation claims an Occam’s Razor approach of not multiplying entities.

This seems, as far as I can tell, to be the argument in its favor: It doesn’t multiply entities, and it takes the Schrödinger equation at its word.

§ §

I can’t find the quote, but I think it was John Wheeler who said something about MWI being cheap on entities but expensive on worlds.

That’s my first objection: I don’t see how MWI can claim parsimony when it so casually multiplies entire universes.

It’s not necessarily a branch to just two worlds. Imagine an experiment that uses a grid of photon detectors — think of each as a pixel. If this array is 200×100, then there are 20,000 detectors, each of which might detect the photon.

In such a case, reality must split into 20,000 new worlds. In each, a different detector triggers, and 20,000 copies of the same scientist go on to write 20,000 papers describing which detector triggered. Those 20,000 papers are identical except that each names a different detector.


My second objection is, if we take E=MC2 seriously, if we take matter and energy seriously, where does the energy for new universes come from?

How can we multiply universes at the drop of a photon? It took a Big Bang and 13.8 billion years to create the one we seem to inhabit. But unimaginable others are created instantly with no effort and no fireworks?

This one really feels like a show-stopper to me, a reason why this just can’t right.

I also have related questions about whether the new universe springs into existence in its entirety or somehow expands from the point of splitting like a crystal growing.

What about entanglement experiments where a measurement in one place affects a measurement in another place faster than light speed permits. The first measurement would seem to drag the other into a given branch.


Another aspect of this is that, in a two-slit experiment, where the particle appears to go through two doors at once, the branches of reality are allowed to interfere with each other.

The one-world view of this is that that particle’s wave nature — literally, its wave-function per the Schrödinger equation — causes constructive and destructive interference. When the wave-function collapses, the particle nature re-emerges in some random point per the probabilities.

In MWI the wave-functions of the two worlds interfere. Until they don’t, so there still seems to be some need to explain this “collapse” thing.

From our point of view, it still appears the wave-function collapses randomly. While there may be reality branches for all possibilities, we still find ourselves in a random one with no way to account for how that happened.

A lot of this involves the idea of decoherence, which is a topic all its own. It’s one I need to research a little in the context of MWI, but I get the impression decoherence never happens in MWI since the wave-function never collapses but merges with the surrounding wave-function.

It appears from our point of view that coherent particles do decohere, though, so I need to better understand what MWI says is going on there. (Knowledgeable comments welcome.)


Why do we not see any aspect of this? How is it that tiny quantum events have such a big effect in creating whole new realities?

A single photon hits a half-silvered mirror and suddenly there are two copies of reality? Do you generate trillions of new universes by walking down the street with a pair of mirror shades? Does the sunlight passing through, or bouncing off, your car window generate trillions of new worlds?

if so, how am I supposed to take the current branch seriously?


Another objection I have is exactly what MWI boasts about: taking the Schrödinger equation at its word. (By which they mean not bolting on that “collapse” thing.)

But in saying the evolution of the Schrödinger equation creates branches of reality, they are reifying a mathematical concept into, not just something real, but the only real thing.

This is essentially the view of Max Tegmark, that reality is math.

I don’t believe reality is just math. I think math is an abstraction we create based on regular patterns we observe. I think math describes those patterns.

I think MWI confuses a description for reality.


The last thing involves probability.

Let’s return to the photon and mirror experiment, but a difference: This time the mirror is silvered such that only 20% are reflected. The other 80% pass through. (We can do this the other way around, too.)

The scientist is going to send ten photons through, one at a time, and note which detector triggers. The expected result is that one detector will trigger 20% of the time and the other will trigger 80% of the time.

And this is exactly what the scientist observes.

Given that all branches always happen in MWI, how does the scientist find they are in a branch where the probabilities are as expected? Shouldn’t there be a branch in which the 20% detector fired all ten times? And one where it never fired?

Ten experiments with two outcomes gives us 1024 branches, so the odds of being in either of the two branches I just mentioned is about one-in-one-thousand. The odds of it never firing or always firing are one-in-five-hundred.

I must have this wrong, otherwise it would seem we could falsify MWI by performing this experiment and seeing what we get. Maybe with 20% odds and ten experiments, you really do get that detector always firing 1/1024th of the time.

While I can understand never with 20% odds and ten tries, always seems a stretch. MWI got ‘splaining to do about probability, is what I’m saying.


In summary:

  • Where does the energy come from?
  • I dispute the parsimony claim.
  • I don’t think reality is math.
  • How does a quantum event affect all reality?
  • What about probability?

So, for me, MWI doesn’t seem a likely theory. I definitely need some hard evidence (or much better arguments) to take it seriously.

Stay in one world, my friends!

About Wyrd Smythe

The canonical fool on the hill watching the sunset and the rotation of the planet and thinking what he imagines are large thoughts. View all posts by Wyrd Smythe

11 responses to “Many Worlds Insanity

  • Wyrd Smythe

    The probability thing really seems like another show-stopper. In any quantum event, no matter how unlikely, there has to be branch containing people who are amazed that the low-probability thing actually happened.

    But we don’t ever seem to be in those branches. Seems like the world would be a very different place if all chances always happen.

  • SelfAwarePatterns

    I’m agnostic on quantum interpretations. None of the viable ones have evidence over the others. And they’re all absurd, in their own way. Quantum physics won’t let us escape with reality as we commonly accept it. My deep suspicion is that they’re all wrong, although like Copernicus in the 1500s, some may be less wrong than others.

    That said, many worlds seems to evoke a unique visceral dislike in people. I can understand that reaction. I’ve had it myself. But I try to look past it.

    I think it helps to understand what the postulate of the Everettian interpretation actually is. Nothing happens in a quantum measurement other than the evolution of the wave according to the Schrodinger equation. That’s it. If you think there are any other postulates, then you don’t understand the interpretation.

    The question then becomes, do the consequences of that postulate explain observations? I think they do, at least as well as any other interpretation. Of course, there are lots of consequences we can’t observe, but under the interpretation we shouldn’t expect to.

    Decoherence does figure centrally in the interpretation. In fact, it’s the core mechanism of it. Decoherence and the collapse aren’t the same thing, or at least an objective collapse and decoherence aren’t. It’s all about the wave becoming entangled with the environment.

    I do agree that reality is not math, that math describes reality.

    And I think the energy issue is puzzling, and was disappointed with Sean Carroll’s answer, that the initial energy of the universe is being diluted. Seems like we’d eventually hit something like sub-Planck units of energy. But I’ve also seen it said that energy itself may branch, which amounts to energy being created, but it doesn’t violate conservation since it would be preserved in any one branch.

    I don’t really understand the probability objection. In my mind, probabilities under MWI are epistemic, just as they are everywhere else we use them.

    It may be that there is something that stops the other branches of the wave from continuing to exist. But until someone identifies something that necessarily exists, I can’t see how we can dismiss the MWI as a candidate for reality, as disturbing as many find it.

    • Wyrd Smythe

      “I’m agnostic on quantum interpretations.”

      Are you? Your arguments give me the impression you might be more pro than you might realize. 😉

      “None of the viable ones have evidence over the others. And they’re all absurd, in their own way.”

      I agree on both counts. This is metaphysics; no evidence for any of it. Reality absolutely will defy our “common sense” views. (I agree also with your further point about it being likely none of them have it completely right.)

      That said, perhaps it’s useful to look at the degree and size of absurdity in a theory. The standard view has the absurdities of non-locality, apparent randomness, the discontinuity of the WF collapse. The MW view has the absurdities of creating entire realities based on what a photon does and the idea that reality is just a mathematical expression.

      Is being absurd a binary thing — once a theory has metaphysical absurdity, it’s just absurd — or is absurdity a thing with degree and cardinality? I think one thing can be more absurd than another, what do you think? Is the standard view LT, GT, or EQ, to the MW view in terms of absurdity? 😕

      “I can understand that reaction. I’ve had it myself. But I try to look past it.”

      That leap of faith you’re talking about there; suppose it was about the Earth being flat? There is, likewise, for most, a strong visceral reaction against the idea, but one could look past that and focus only on certain data.

      (Which, now that I write that, strikes me as a good analogy, since I think that’s exactly what Everett did. Which, ha, also means I can view Sean Carroll as someone who has turned into a Flat Earther. 😀 )

      But to be serious, how do we know our looking past our gut isn’t a case of our intellect being tricked by clever argument? Both Baggott and Hossenfelder target how our intellects get so easily lost in math or logic. (Our inability to judge probabilities naturally is an aspect of that, I think.)

      The argument works both ways. Ultimately, either way, it’s just our intuition until there’s some hard evidence. In a sense, your intuition is to counter your initial intuition, to suggest remaining open-minded (possibly even slightly in favor).

      Hmmm. Put that way, I guess my ultimate intuition is to counter my countering my initial reaction, since there was a time, long ago, that I played around with MW ideas. But after giving it a lot of thought recently, I find MW is just too absurd. I need more than a single argument; there are too many counter-arguments.

      As you mention, Carroll’s answer to the energy problem isn’t satisfying, and if such a big proponent of the theory can’t give a good answer to what seems a show-stopper problem, that doesn’t bode well for the theory.

      This is all very meta. I think I’ll reply to specific arguments in another comment. There really are two threads to the discussion: The equivalent of Chalmers’ meta hard problem and the MWI itself. Might be good to keep that separate.

      Standby for reply #2! 😉

    • Wyrd Smythe

      “If you think there are any other postulates, then you don’t understand the interpretation.”

      I think there’s more to it than that. As you go on to say, there are consequences to that postulate.

      “It’s all about the wave becoming entangled with the environment.”

      Yeah, and I’m tempted to either table this until I post about decoherence or split off a third thread… For now, I agree decoherence and collapse aren’t quite the same, but they are tied together.

      “I do agree that reality is not math, that math describes reality.”

      But what is the wave-function, then? MWI requires it be some kind of (the only) reality. About the only way MWI makes any sense at all is if Tegmark was right.

      Long ago (decades!), I met a guy who believed MWI. He prefaced telling by saying he was sure I’d think he was a kook, but I was already familiar with MWI and at that time found it viable. When I mentioned the lack of parsimony in creating multiple realities his answer stuck with me:

      We find nothing strange about there being two answers to the equation SQRT(4). Other equations have even more answers; some can have infinite answers. None of it requires conservation of mass or energy.

      But of course it means everything is just math. MWI makes perfect sense in a Tegmarkian universe.

      It makes no sense in a physical universe, in huge part because:

      “And I think the energy issue is puzzling,”

      I’d go with show-stopping, myself. 😉

      Here’s the conundrum: Sean Carroll gives a talk, during which he uses his app to split reality. In one universe, Carroll jumps left, in the other he jumps right.

      Just prior to using the app, in the single branch, Carroll’s body has an energy content equal to its mass times c-squared. After the split, we measure “our” Carroll’s body to have the same mass, and since c is a constant, how can his energy content have changed?

      But there are two Carrolls now. How can E=mc2 remain true? The energy can’t be diluted without also altering either mass or c.

      That someone like Carroll can’t answer this, to me, condemns the theory.

      “I don’t really understand the probability objection.”

      Consider a single-photon experiment where the photon has a 1% chance of being reflected. Do this experiment 10 times.

      If we see this as a binary tree with the top node being the first experiment, and each following experiment as a level below, then the tree has ten levels. The bottom level has 1024 leafs (210), each of which is a separate set of results.

      Under MWI all those 1024 worlds exist. They all contain a copy of the scientist recording the result. In one of them that scientist is very surprised to discover the 1% reflection happened each time. The odds of that are 10-18%.

      It isn’t that long odds don’t sometimes pay off. It’s that under MWI they always pay off. That surprised scientist has to exist under MWI.

      The odds that we’d be in such a branch might indeed be that low, but MWI requires that it still really exist. In a single reality, low-odds things will happen sometimes, of course they will, but usually they don’t. Under MWI they always do, which seems weird.

      “But until someone identifies something that necessarily exists,”

      Isn’t it the other way around? MWI has to identify something that blocks those branches, otherwise the theory insists they exist.

      • SelfAwarePatterns

        On agnostic vs pro, my only argument is that it’s a viable candidate for reality. I find most of the arguments against it poor, rooted in intuitive incredulity.

        Today’s metaphysics may be tomorrow’s science. Remember atoms, heliocentrism, and entanglement were all untestable speculation at first. I recently read where David Deutch proposed an experiment to test MWI vs Copenhagen, involving a variation of Wigner’s friend. It’s hopelessly impossible today, but that doesn’t mean someone won’t find a way.

        Flat Earthers aren’t looking past their intuitions. That’s the problem. Listen to them and they’ll discuss their first hand experience, which is of a flat Earth. We’re just so used to overriding our intuition on this that it’s become a new intuition, but one formed from education. Intuition by itself has a poor track record.

        I think it’s a mistake to judge MWI on its consequences unless those consequences contradict observations or empirically successful theories.

        On probability, even if the MWI is false, incredibly low but non-zero probability events can happen. If the MWI is true, a version of us will experience them, but the overwhelming majority of our versions won’t. Subjectively, the probability ends up being the chance that we become one of the versions of us that do experience them.

      • Wyrd Smythe

        “On agnostic vs pro, my only argument is that it’s a viable candidate for reality.”

        Militant agnosticism, then. 😉

        “I find most of the arguments against it poor, rooted in intuitive incredulity.”

        That’s kind of ad hominem. It dismisses a range of arguments without specifics on the claim they are emotional. What’s called for is the expression of the more logical argument, not a hand wave about “intuition.”

        More importantly, this isn’t about “most arguments” but the specific arguments I made in this post or comments.

        “Today’s metaphysics may be tomorrow’s science.”

        Maybe. Maybe not. What might happen is not an argument.

        “Intuition by itself has a poor track record.”

        Of course. Which is why I tried to make logical points in the post.

        “Intuition” is a tricky word, since it can refer to gut instinct which brands it as emotional rather than rational. In fact, some intuitions turn out to be better than rational analysis. Our tendency to second-guess ourselves is exactly where we get it wrong sometimes.

        And then there is informed intuition, when one has studied a topic at length, and now those sub-conscious sub-rational thoughts have the weight of knowledge and experience. Informed intuition can be a powerful tool.

        So I’m going to ask you to please stop using “intuition” as a debate point because I see it as both a hand-wave and vaguely ad hominem. I want to focus on rational argument and specifics.

        “I think it’s a mistake to judge MWI on its consequences unless those consequences contradict observations or empirically successful theories.”

        Can we also judge when the consequences involve logical conundrums with no evidence supporting them?

        Non-locality is a perfect example. It goes beyond even informed intuition, a fundamental tenant of physics says reality is local. But the observations leave little room for doubt.

        With MWI, we’re still stuck on logical conundrums even its strongest proponents can’t answer.

        How is E=mc2 possible under MWI? How fundamental is the relationship between matter and energy?

        “On probability,…”

        Yes, that’s what I said above. Under MWI, long odds always do happen to someone. There always exists someone who is really surprised. (This is where the whole “quantum suicide” thing comes from. Some version of you always survives.)

        Without MW, then although long odds sometimes happen (at the expected frequency), they usually don’t. There’s no infinite subset of reality containing very surprised people.

  • Wyrd Smythe

    As a general aside, I find interesting the correlation between the rise of the sort of “post-empirical” physics that makes people like Smolin, Woit, Hossenfelder, and Baggott, react against it, calling it “fairy tale” versus the increase of physicists who believe in MWI.

    For the people concerned about the overall trend into speculation, I think the concern centers around the willingness to take overly seriously ideas based on small seeds.

    MWI is an almost canonical example. Everett starts with the small seed of focusing on the wave-function, and everything then wraps around that. In the absence of supporting data, it’s little more than an idea people hold up as hopeful putative truth.

    For scientific realists, this is … worrisome.

  • Wyrd Smythe

    Another thing that seems weird to me is that the wave-function is a statement about the probability of detecting the particle at any given location.

    What does it mean for the wave-function to be reality, since it only describes the probability of the particle being detected? How does that translate to the wave-function being a real thing, of being us and everything?

    The wave-function seems to be about potential not actuality.

  • Wyrd Smythe

    Something else I don’t quite get involves the two-slit experiment: Does the world split for the “particle” going through either slit even though both paths end up in the same spot?

    Consider a single-particle two-slit experiment. In the SW, the wave nature of the particle interferes with itself producing a pattern of probability for where the particle might land.

    Where that particle lands is “random” and appears to be WF collapse, so it seems at this point, MW might say reality branches into each place the particle can land.

    But in all cases, the particle lands as a result of its WF passing through both slits.

    So are there two types of reality branching, or does MWI have nothing to say about how the interference pattern is generated?

    If reality branches for the slits, and the two branches are allowed to interfere with each other (which seems a further postulate), it seems this interference stops with the apparent WF collapse or, alternately, when the particle’s WF decoheres and mixes with the environment.

    I think another pointed question I have for MWI is: What exactly is going on in the two slit experiment? How many branches and when?

    (I have similar questions about what happens in entanglement experiments designed to test Bell’s Inequality.)

  • Wyrd Smythe

    I’ve decided I need to go back to the original, so I got a copy of Everett’s original paper to read. It’s 139 pages, so it’ll take a while to read and absorb. And I’m thinking I may have to learn more about the Schrödinger wave-function equation. What does it really say?

    I will say Everett has a fun-to-read casual style:

    However, to B’s consternation, A does not react with anything like the respect and gratitude he should exhibit towards B, and at the end of a somewhat heated reply, in which A conveys in a colorful manner his opinion of B and his beliefs, he rudely punctures B’s ego by observing that if B’s view is correct, then he has no reason to feel complacent, since the whole present situation may have no objective existence, but may depend upon the future actions of yet another observer.

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