Q: Which is a better approach to quantum mechanics: Copenhagen or Many Worlds?

Physicist: The Copenhagen interpretation requires that new laws be created that, in addition to being impossible, are completely unnecessary, physically unfeasible, and utterly unjustifiable.  The basic laws of quantum mechanics, when applied at all scales, give you the Many Worlds interpretation.  No fancy rules, no awkward questions.  Even better, what seems to be wave function collapse in Copenhagen is actually described by the Many Worlds approach.  So why choose the Copenhagen interpretation over Many Worlds?  No damn reason at all.

Many Worlds vs. Copenhagen. On the one hand there are different versions of you, and on the other you've got vaguely defined mental powers.

If you’re already familiar with the basic ideas behind the measurement problem, and the divide between Copenhagen and MW, feel free to skip down to the word “catawampus“.

Bell’s theorem demonstrated that either information travels faster than light (Bohm) or particles exist in many states simultaneously (everybody else).  There is a lot wrong with Bohm’s theories, not the least of which is that it doesn’t have a relativistic formulation (which may not sound too bad, but that’s really damning in physics circles).

The best example is the double slit experiment: shine coherent light on two slits and look at the pattern that’s projected onto a screen.  You get a nice interference pattern of dark and light bands on the screen, which is fine since light is a wave (don’t stress about wave/particleness here).  What’s messed up is that even when the light intensity is turned way down to just one photon at a time, the photon still tends to land where the bright bands were, and almost never lands where the dark bands were.

You have to repeat the single-photon double-slit experiment a lot to see the pattern, but there it is. In order to get these interference fringes from two slits the photon must interfere with itself. It's going through both.

Alright, so everybody has come to accept that very small things have no problem being in several states simultaneously (if you don’t buy it, then just take it as read).  But here’s the weirdness.  It turns out that particles only exhibit the super-position properties when “you” can’t tell the difference between the various states.  As soon as “you” can single one out, then suddenly the super-positionness goes away.  I’m putting quotes (“”) around “you”, because “you” could be a person, or a computer, or even another particle.  Just a weakness of the language.

This is the measurement problem.  Things can be in several states at the same time, but only if nothing observes* them.  The moment they’re observed* they are found to be in only one state, and they continue on their merry way in only that one state.  The most straight forward (brute force) example is covering one slit and noting that the interference bands disappear.  By covering one slit you’re “observing*” that the photon is going through the other.  Most examples are more complex and subtle.

(* this is another weakness of the language.  “Observe” would seem to imply a hierarchy; “this observes that”.  A better term might be “interact with”.)

Here’s where the split comes up.  Copenhagenists will say that something, usually size, or complexity, or random collapse, or, worst of all, consciousness, causes “wave function collapse” (all but one state disappears).

Many Worlds adherents will say that wave function collapse never happens, and that the trick is realizing that both the particles involved and the observer* are in multiple states.  After the observation* the multiple states of the particle are now entangled with the multiple states of the observer*.  In the double-slit test you’d have the two states: “photon goes right and observer sees it go right” and “photon goes left and observer sees it go left”.  Both states persist, and there’s no collapse.


Catawampus: You’ll often hear that there’s no experiment that can be done to prove which approach is the correct one.  I’m of the opinion that the experiments have already been done, but that most people (myself included) don’t like the results.  However, among people who have stopped to consider the options (and there aren’t many good reasons to do so), most of us have decided to accept the results and move on.

The big advantage behind the Copenhagen interpretation is that it makes people (like you!) important, and different from particles.  <sarcasm>Sure, they may be in multiple states, but I’m definitely in exactly one state.  Unlike particles, people can tell the difference.</sarcasm>

It’s creepy to think that there are different versions of yourself “out there” doing “stuff”, but it’s awesome to assume that you’re special and that your mind (not brain) has some kind of power over reality.

There is a version of the Copenhagen interpretation of quantum mechanics where consciousness (usually human, sometimes God, sometimes Gaea, …) plays a key role.  I feel pretty comfortable dismissing it out of hand.  There’s a post here that talks about it a little.

Time and again, we’ve managed to show that larger and larger objects can be in multiple states, using the double slit experiment or variations of it.  At last check, the double slit experiment was successfully preformed on C60F48, which has fully 108 atoms, or 2,424 protons, neutrons, and electrons.  The entire molecule (actually, thousands of them) actually interfered with itself, demonstrating the ability to be in multiple states.

Which raises the question: what’s the damn problem?  Everything that can be tested has demonstrated quantum superposition, so why not just extend that to “everything obeys the same quantum mechanical laws, including superposition.”?  Why not indeed?

One may be tempted to say “the physics at small scales is just different!”.  Fair enough.  However, there are no physical laws that work differently on different scales.  For example, at very small scales water acts like honey, and to swim you need to use things like flagella.  At the other end of the scale (our scale) water behaves… like water, and things like fins and flippers suddenly work really well, but flagella don’t.  However, the same physical laws (specifically, the Navier-Stokes equation) govern everything.

More generally, all laws apply at all scales, it’s just a question of degree.  Relativity works at all velocities, but you don’t notice the weird effects until you’re moving really fast.  What we call “Newton’s laws” are just an approximation that work at low speeds.

If the Copenhagen “size argument” (that larger objects somehow have different laws) holds up, it’ll be the first of its kind.

So how can the many worlds hypothesis hold up?  Either we’re in multiple states or we’re not, and (most) people don’t feel like they’re in more than one state.

In very much the same sense that relativity includes Newtonian dynamics as a special case, the Many World hypothesis actually contains Copenhagenism.

Normally in quantum mechanics, when you’re trying to predict the behavior of a system, you just let all the components evolve in time according to the Schrödinger equation.  If you follow a particular state or object, then you’ll find that it experiences wave function collapse all over the place.

That is to say: If you require (artificially) that some particular thing must remain in one state (or small set of states) by disregarding its other states, then that object will (seem) to see wave function collapse.

I should repeat that, because it bears repeating:

According to the Many Worlds approach, the individual states of an object witness wave function collapse all the time, but taken as a whole, there is no collapse.

Here’s an simplified example with very little physical basis, but that hopefully gets the point across.

In what follows the \square are collision points.  Either the particles pass each other, or bounce off each other and change direction.  They can’t both go down the same path.  The \boxminus are splitters.  When a particle hits this it has a 50% chance of passing, and 50% chance of reflecting.

To calculate the probabilities of how the two quantum particles will come out of the machine you have to sum over ever possible path. You have to take into account, not only every "choice", but every interaction.

This map shows how two particle move through some machine.  Either they bounce off each other at the first \square and you have blue-up/red-down, or they pass each other and you have blue-down/red-up.  These two states then go on to the splitters and so on.  There is no collapse, and every possibility (every path, every interaction) is included (like the double slit).

But what if you were the to track the blue particle?  Put your foot down and insisted that it remain in just one state and take just one path through the machine?  Even more profound, what is the effect on the red particle (from the new one-path-point-of-view of the blue particle)?

Easy enough, pick one of the blue paths (worlds) above.

1) The particles start. 2) In this world they pass each other. They didn't have to, they just happened too. 3) Since we're insisting the blue particle stays on one path, it must pick a path: reflect. The red particle, however, is free to take multiple paths, and does. 4) In this world the blue particle bounces off the red particle at the second box. But this interaction means that the blue particle now knows where the red particle is. Collapse!

This story is just one of the stories encompassed in the Many Worlds picture.

Here’s a slightly different one.  What would happen if the blue particle went through the splitter instead of reflecting?

1&2) Same as last time. 3) Since we insisted that the blue particle must be in one state, it must either reflect or pass. In this case it passes. The red takes both available paths. 4) The blue particle can't interact with the red, so this time the red particle is free to take even more paths simultaneously (as far as this version of the blue particle is concerned).

So the big point is that the Copenhagen wave function collapse is strictly an illusion created by restricting your attention to a particular state of one object.  So why is our attention stuck on just one state?  We’re not special, just victims of conservation laws.

Looking down on the double slit experiment from outside you can ask questions like “what is the probability that the photon will go through each slot?”.  You have no “givens” to affect your probabilities so you say “50/50”, and you’re right.  The photon goes through both, but since there’s only one photon (conserved number of photons), it does it in a particular (some what obvious) way: it combines the states “left/not right” and “not left/right”.

Now say you’re presented with two doors.  You also can’t be in the state “right and left”.  Now when you go through one door, because you’re interacting with yourself, you have givens that affect the probabilities.  Ask yourself, “What is the probability that I went through the left door, given that I just went through the right door?”  Zero, baby!

The version of you that when through the left door will be able to make a very similar calculation.

Finally (without going into too much detail), the Copenhagen interpretation also violates a number of very straight forward physical laws.  Conservation of information (supported by everything else, including logic), time reversibility (again, everything else), and information flowing backward through time (spacelike information exchange or “spooky action at a distance”) are only the most direct and grievous examples.

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49 Responses to Q: Which is a better approach to quantum mechanics: Copenhagen or Many Worlds?

  1. Pingback: Linkblogging For 16/12/10 « Sci-Ence! Justice Leak!

  2. Bruce says:

    If MWI is true, then there should be many more copies of me in the future than there are in the year 2010. Shouldn’t I be extremely surprised to find myself in 2010? This is not as surprising for Copenhagen.

  3. The Physicist Physicist says:

    MWI is a little worse than that, you also have many histories. There are already a whole lot of versions of you, each thinking they’re the only one. That’s beside the point, but still interesting.
    Even if there were many more versions of yourself in the future, how many of them will think they’re in 2010? Taking the year as a given restricts which versions you’d find there.
    To put it another way, this question should be exactly as ponderable as: “If I live for a hundred years, what’s the chance I’d find myself in this year?”
    Let me know if I misunderstood your point.
    (%00⇂ :suɐ)

  4. Jerry Kronenfeld says:

    Which is right, Copenhagen or Many Worlds? Neither. I believe the correct answer is the DeBroglie-Bohn Pilot Wave theory. A variant of hidden variables that was investigated and pushed by Bell.

  5. tonyf says:

    “Copenhagen”, extremely problematic as it is, is probably still better than MWI at present, because we are still uncertain if we get any predictions at all from MWI (problems like “derivations” of Born probabilities in MWI just handwaving (but I do not claim to know all the litterature, so please give a referenve if you know a good (rigorous) one)). Maybe those problems will be solved by further theoretical developements, but that seems to be the problem right now. And then you are showing a false dichotomy, there are many more proposals for solutions than “Copenhagen” and MWI (and bohmian mechanics). Compare also the recent discussions over at Rationally Speaking:



  6. The Physicist The Physicist says:

    The big advantage of MWI is that it includes the world we see (which is frankly pretty Copenhagenish looking) as a special case, but it doesn’t involve the massive violations, and goofy “patches” of Copenhagen. I’ve never heard another physicist talking about another approach (outside of the three you mention, and variations of them), so I’d love to hear about more.
    Saying that MWI and Copenhagen make the same predictions is akin to saying that automotive technology and “invisible gremlins do it” predict the same car behavior. They do, and no experiment will ever be able to tell the difference. But, one follows the rules, and one makes stuff up.

  7. christopher neilsorn says:

    i dont know if i buy the whole mwi thing. Wouldnt that mean that everytime that a system exists in a superposition that a virtual copy of the universe (and all its energy) has to manifest (where? seperated from this universe by what? by not space-time? whats that?) once for each possible state of the superposition? take a photon in one of the four ‘bell states’ (not sure exactly what that means), now, when i measure this photon and it undergoes a waveform collapse (from the this universes me, perspective) i have simultaneously ‘generated’ three other universes, identical except for the state of this single photon. now, where did the energy for this entire universe come from? nowhere? does that not break the first law of thermodynamics or something?

  8. The Physicist The Physicist says:

    There’s a big weakness in the language, in that the words “universe” and “world” get used a lot for all kinds of wildly different things. One can’t help but picture galaxies and planets and people and puppies and whatnot when somebody starts talking about “other worlds”, but that’s not what’s meant here.
    Two things are in different “universes” (I’d use another word if there was a better one) if they can’t interact. So for the photon in the double slit experiment the versions that take the different paths are in different universes. However, we can see an interference pattern caused by a sum of the two paths, and can infer that each of the paths were taken. They made not be in the same universe as each other, but they were in the same universe we’re in.
    While the total amount of energy in any version of the universe must be conserved, the distribution of that energy needn’t be. In some universes the energy is one the left, in some it’s on the right, and in some (the universes that don’t absolutely have to take a stand one way or the other) it’s divided between left and right. In fact, the ability to spread energy across multiple states is extremely important in most physical processes.
    Many Worlds is commonly described as the theory that “new universes are created every time anything is observed”, which (thinking of universes as planets and stars and everything) is pretty silly.
    MWI boils down to saying: “everything we measure ultimately behaves like a quantum wave, what are the implications of that?”. Copenhagen boils down to: “everything we can measure ultimately behaves like a quantum wave, but I don’t think that applies to me, so what are the implications of that?”

  9. christopher says:

    i am not quite sure i understand. in one reality photon goes to the right, in one reality the left. what is separating these two realities? the best sense i have been able to make of it so far is, the universe itself is in a superposition of states, constantly in all of its possible configurations. the easiest way to swallow this is if, say the universe will ultimately have 10^10000 possible configurations. then, at the very birth of the universe 10^10000 universes spawned and were identical. and since that point in time they have been branching off as divergent paths are taken, and internal quantum superpositions arise. ultimately leading to 10^10000 different universes. but i still have difficulty understanding what separates each reality. is it something like m-brane theory where they are ‘right there’, but separated by micro dimensions or something? or am i still just completely missing the idea? or are they actually separated by a ‘not-space-time’ nothingness or something?

  10. christopher says:

    but even then, the universe would have to start ‘knowing’ how many configurations was ultimately possible. which may not make sense.

  11. The Physicist The Physicist says:

    Why’s it gotta know, and what’s “knowing” in this case?

  12. christopher says:

    because if the universe is, in its heat dead state, going to be in a superposition of say ’10^10000′ (off the top me head) configurations, than, at its inception it would need to have “predicted” (for lack of a better word) this number in order to begin in 10^10000 identical configurations that would later diverge as internal superpositions arise.

  13. The Physicist The Physicist says:

    I dig!
    There doesn’t (so far as we know) need to be a finite number of states, nor even a conserved number of states. Even worse, there’s no clear indication that the number of states needs to be countable, although this is an open debate (that can never be resolved). That is to say, a “state” could be “the particle is at point x”. But, you can vary x as little as you like and get different states. There’s no way to count the points in space.
    Even more worser, the branching touted in the usual out-of-hand description of MW (“every event makes the universe branch into…”) goes both forward and backward in time. Not only does every event give rise to every possible outcome (branching forward), but is in turn produced by every possible cause (branching backward). You can think of this in terms of the double slit experiment by considering what happens only after the double slits are passed. A far more dramatic example can be found in the Franson experiment, which demonstrates a photon interfering with versions of itself created at distinctly different times in the past.
    The point is; the universe doesn’t need to conserve states, start with a certain number, or have all of its initial states the same.

  14. Pingback: Q: What is a “measurement” in quantum mechanics? | Ask a Mathematician / Ask a Physicist

  15. Andrew says:

    This made me think again about the implications of the double-slit experiment and superposition. A molecule going through both slits “interferes with itself, demonstrating the ability to be in multiple states.” So, for the MWI, are these “multiple states” referring to the different worlds of the MWI? In other words, would the CI’s “collapse” of the wave function be the same as MWI’s decoherence?

  16. The Physicist The Physicist says:

    That’s the long and the short of it!
    The different states of the photon see each other as being in “different worlds”, despite the fact that we can see them interfere (so both of those worlds are part of ours). The line between “worlds” is pretty fuzzy and subjective.

  17. Gill says:

    The problem I have with MWI is that if all options happen, my children would be suffering constant pain in a world somewhere.
    Physicist, I always thought the interference pattern was the wavefunction going through both slits at once

  18. David says:

    The real problems with MWI is the Born Rule issue which has proven insoluble and the preferred basis issue.

    Without solving these MW cannot even be called a functioning interpretation…

    I definitely agree that Copenhagen is also wrong and I believe some other deterministic and realist theory will either replace QM in the end (after solving quantum gravity etc.) or there is some good interpretation we have simply yet to construct. (Gerard ‘t Hooft got some very interesting papers).

  19. Candice says:

    Okay… So i was watching a documentary on the topic that there are many worlds and were making not one choice but were making both at the same time like an alternate universe. In one life you had the decision to to go the movies,in another one of these worlds you did,well I don’t understand how series of events could make this possible that wed have such similar decisions to make at the same time…The only thing I did not understand or maybe they explained it a bit wrong.Although I do believe that this would make time travel possible like if you experienced Déjà vu like something was changed that wouldve created a ripple effect…something like that,but two places at the same time making an alternate decision?i think series of events would effect the persons next chioce.Could you shed some light on this very confused mind?:)

  20. Candice says:

    sorry one more thing to add….what im saying is Theres this guy that was on a documentary called superhumans,where they search for skilled people and special attributes and this one guy on there shot a gun so fast at a set of two balloons next to each other and when he shot the balloon both poped.He really shot them both one at a time.But to the naked eye looks like he shot them both at one time but he was just so fast it looked like it was done at once and i think this is what this theory might intail,maybe it has to do with speed at unexplainable forces and not just what we see……thanks for reading:)

  21. The Physicist The Physicist says:

    That quick draw guy is pretty amazing.
    I’m afraid I didn’t quite catch what the question was. One of the most common misconceptions about the Many Worlds Interpretation is that somehow human consciousness or decisions are involved, and that the “splitting” ripples out (good word!) and affects the entire universe. But, like one might expect, the whole process is a lot more mundane.
    There’s a post here that talks a little more specifically.

  22. Vinicius says:

    I still don’t get why the photon goes through both slits at once. Would it not go through one in one “world” and through the other one in other “world”?

  23. hammm says:


    “why the photon goes through both slits at once.”

    Because the photon is a particle and has wave-like properties also:

  24. Andrew says:

    It seems to me that MWI is like the “round earth” theory of centuries ago. Most of the struggles people have with MWI aren’t actually with MWI, but with a reduced, corrupted version of MWI that’s been infected with Copenhagen assumptions.

    MWI is simple in concept and complex in its implications. Copenhagen is complex and incomplete in its concept, and has no valid implications. Occam’s Razor is on the side of MWI.

    MWI and Copenhagen are engaged in the exact same intellectual battle as evolution and creationism.

  25. James says:

    Looking at the sky, we see the Sun orbit the Earth. According to Occam’s Razor, that is what happens – even though we now know the reverse is true. According to MWI, the wavefunction (described by Schrödinger’s (deterministic) time-dependent equation) does not collapse when a measurement is made. Instead, a ‘splitting’ occurs (somehow) and the observer sees one possible result of the experiment/event. The alternative outcome(s) occur(s) in (a) ‘parallel’ (but real) universe(s).
    This would make the universe (or ‘multiverse’) entirely deterministic, and free will would be an illusion. The branching would happen all the time, everywhere, meaning that all conceivable events, no matter how bizarre, would occur in some part of the multiverse. Think of some horiffic crime. You will have committed it in some reality, according to MWI.
    That MWI has become so popular suggests that physicists must be desperate, if they really believe in this ‘theory’ that can never be proved (or disproved).
    Although it avoids the measurement problem, it seems likely that a more satisfactory understanding of quantum mechanics will eventually arrive (in this universe).

  26. Peter says:

    @The Physicist

    To explain Bruce’s comment better. Sense the big bang the number of universes should be continually increasing under the many worlds theory. So there should be many more universes later than earlier. This makes it extremely unlickly that you would be in an relativly early universe like our own. There are many historical version of ourselves but there are vastly more future ones.

    Not sure I like personally like this type of reasoning but trying to explain it better.

  27. Frank says:


    It seems as though you’re objecting to MWI on purely philosophical grounds. (“Think of some horrific crime. You will have committed it in some reality, according to MWI.”) I’d just like to say that although a person might find a theory distasteful as a matter of philosophical implications, that fact alone does not detract from the theory’s validity.

    Although I agree that MWI is unprovable (at least until we come up with some breakthrough in the future), the same holds true for Copenhagen. This being the case, it seems as though your statement, “it seems likely that a more satisfactory understanding of quantum mechanics will eventually arrive” is a meaningless one: maybe, maybe not.

    In addition, there’s nothing in MWI that suggests any sort of “splitting” occurring at the time of the measurement. In fact, there’s no limit on the number of possible “parallel universes”; there could very well be an infinite number of them–and they could have existed from the very beginning. In other words, there need be no bijection between the number of possible worlds and the number of possible results of possible measurements (whew, that was a mouthful). However, MWI is simply suggesting that the results of any measurement do not “collapse” a wavefunction through the act of observation, but rather distinguish “which” universe we as the “observer” reside in.”

    This being the case, I don’t really see anything to your near-immediate dismissal of MRI.

  28. Brandon says:

    Okay, I have limited knowledge of physics. But the problem that I have with MWI is, going with the basic die-rolling example, this leads to extremely large number of completely identical universes. Going with the die-rolling example, you start with one and end with six universes. Okay, well, one of those universes won the die roll while five lost, assuming that the odds of winning are 1:5. Well, the ultimate outcome simply being won vs lost, these five worlds are identical excusing the one moment in time when the die landed on a different number.

    If we then consider the possibility that each of these worlds were then given the opportunity to roll again, that leads to a 1:1 decision. The five identical worlds then move on to ten, five which accepted another roll and five which did not. The five that roll again then cause each another five worlds, leading to a total of thirty worlds. the odds of winning still being 1:5, five of these worlds win, while twenty-five lose, thus leading to twenty-five worlds identical except for the one detail of the die number. If continuously given the chance to roll again, and only considering the worlds that lost the die roll, after 5 rolls the total is then 3,125 worlds that are identical in having lost all five die rolls. Are there any considerations given to this in the theory or would it just mean that there were thousands to possibly millions or even billions of identical doppelgangers in existence completely identical to us?

    I know this is a little off topic by my internet search led me here. Lol.

  29. Prime Gonzola says:

    I am not a physicist at all, hardly studied any of them so bare with me and my probably flawed reasoning.

    I just can’t accept the Copenhagen interpretation just due to loss of information when the wave collapses. Information never disappears at worst it transforms into something else but does not disappear. Look at black holes, whatever drops in, the information flows back through Hawkins radiation and entangled with whatever dropped in.

    MWI looks interesting but hard to grasp. The infinite worlds/realities that could spawn is not really limiting assuming the “reality” is infinite. The reality in this case is not ours but more the one Bell is referring to which is non local and where entanglement is playing in.

    How I see MWI is straightforward by using a flatlander analogy. Assume a photon and the two slits only exists in a two dimensional plane, like a sheet of paper. The photo can only move left or right and moves through the slit by moving forward. The wave goes through it with the know results.

    Now imagine multiple reality each represented by a sheet of paper but stack on each other as an exercise and we assume that information as a total over the complete stack can never be lost. When we do a measurement in one plane i.e. extracting information then the conservation needs to be checked hence in another plane/sheet of paper the counterpart will occur. So when we look at one slit, we see the photon as we took information, the photon will go through the other slit in the other plane to balance and conserve the total amount of information.

    In our reality stepping away from the flatlanders, I see “reality” in a higher dimension where the total information needs to be conserved. Each plane, local reality is interfering directly with each other. Whatever information is added/extracted needs to be balanced elsewhere.

    How it works I have no idea, but I imagine the real non local reality be filled by a “quantum” field comprising all local realities filled by quantum waves interfering with each other creating the necessary realities and relations to it. We live a local reality interfering with ourselves but also with the other realities.

    Hope it makes sense somehow.

  30. Tetrazphere says:

    Here’s my problem/question with the MWI -If the universe is branching out into innumerable other universe’s, forget about where the doubling of energy/mass is coming from even in just one branching off, or where this new universe is located as compared to it’s original branch…since the Big Bang branching has been occurring and there are an incalculable number of universe’s now, (most unrecognizable) and many not just recognizable but some with “us” in them, and again countless universe’s with an infinite number of ridiculous phenomenon occurring. Like in one universe…I choose to write 52 letters all with the letter “T” drawn one time on the paper and mailed them to random people, but in another branched off universe I wrote 51 letters with the letter “T” and one letter with a smiley face, another world 17 letter’s mailed with the word “marsupial” and so on and so on ad infinitum….and many other universe’s with 1/3 or 1/2 or 5/6 etc.. of the population doing this but with changing nonsensical content on the letter’s, I’m not receiving any of these absurd letter’s. Now take this highly specialized example and apply it to all of the variable’s in this universe…why are we not seeing any of these absurdities? Shouldn’t this already be seen in our physical universe (use your own endless imagination for countless examples)…sort of like how ‘junk DNA’ used to be viewed or the Fermi’s Paradox (“where are they?”) But we are not seeing any of these absurd phenomenon, there should atleast be enough that we could infer this is happening. And the chances we just so happen to be in a universe that doesn’t have any of these absurdities manifesting (from our “logical” pov) is beyond astronomical, and continues to become more and more astronomical every moment we continue to be on one of the non-absurd branches. To say that our universe is just common because it’s a probable universe is really just a cop-out. To assume that it so happens that the “probable” standard for a universe is one where life, consciousness, and higher mind can evolve to become aware of this phenomenon is as bad as one feeling ‘special’ about ‘their’ consciousness causing a wave function collapse with the Copenhagen Interpretation. Where are these absurdities in our random non-particular branch of a world?

    Prime Gorgonzola – How does the other sheets of paper and the photons on them, only effect our photons on our single piece of paper when we do the experiment, why don’t we see this occurring outside of experiments? That sounds like a MWI inside of a Copenhagen Interpretation framework.

    -Thanks for any insight into this puzzling and important subject.

  31. Daniel says:

    The problem I have with Many Worlds is that it seems to be the opposite of parsimonious. It is the pure antithesis of Occam’s Razor. I mean what could be more messy than the entire universe branching a zillion different ways every zillionth of a second (a huge number of particles collide in every instant) and each of those universes branching a zillion times in a zillionth of a second. In one second, our universe would surely have divided into more universes than there are particles in this universe. And after not much time, these would start to be quite different from each other. What a mess. How are you conserving anything that way? And not so much as a hint of observational support.

    Copenhagen is by far the most parsimonious. It doesn’t attempt to explain why anything happens. It just says, here is what we observe, full stop. Maybe Copenhagen makes no sense, but it is on far more solid footing because it cuts off discussion with what we observe. It isn’t wrong, it just leaves a giant question mark in the middle. You don’t get closer to being right by just filling in the blank without more observations.

    Copenhagen draws a tight box around what we know. Many Worlds is an attempt to draw extensions that tell a fabulous story, but there is not one thing it can predict which you can test that is not already part of QM.

    Many Worlds seems very conveeenient because anything you want to be there that you don’t find can simply be put into another universe. Sounds an awful lot like a spirit world. The medieval theologians talked about this first, you know.

  32. Daniel says:

    “Here’s where the split comes up. Copenhagenists will say that something, usually size, or complexity, or random collapse, or, worst of all, consciousness, causes “wave function collapse” (all but one state disappears).”

    The Copenhagen interpretation doesn’t say that all but one state disappears. They were never states to begin with. They were only probabilities.

    It just says,

    (1) Here is this probabilistic wavefunction that works for the time leading up to an event. (True)

    (2) Now we make an observation and it has one state only (True)

    That’s it. The Copenhagen interpretation takes what we know so far and stops cold. It can’t possibly be wrong if it doesn’t say anything beyond these simple things.

    Copenhagen says the wavefunction collapses, and people say it is vain to think I caused that, but such talk misses the point. The wavefunction wasn’t real to begin with. It is only probabilities, but as soon as there needs to be a real answer, there is one. Maybe the word ‘collapse’ was an unfortunate word choice by Bohr or whoever coined it, because it makes people think that something real is collapsing, but that was never what was meant.

    The Copenhagen Interpretation says that the wavefunction is not a real thing. It is just a mathematical tool. It happens that the particles in the world follow this mathematical pattern. That is our observation.

    It is the Many Worlds people who say that the wavefunction is real and that every bit of probably a particle had in its past must have a universe attached to it (or something like that), but the Copenhagenists never said the wavefunction was real. They are probabilities, but probabilities are okay as long as you aren’t presently interpreting things.

  33. Daniel says:

    Copenhagen just says that the wavefunction works, and that your observation is real. We have high observational confidence on both of these. Copenhagen goes no further than what we already pretty much know.

    Well what happened to the wavefunction? Where is it going forward? This is not even a sensical question to ask because the wavefunction was never real. It was just a mathematical equation that worked really well in giving probabilities.

    There are other examples of this. Imaginary numbers have great usefulness in some parts of physics mid-calculation but typically don’t have real world meaning and those parts which drop out for your final physical result. Nobody constructs elaborate worlds for the tragically lost imaginary terms to dwell in. Maybe Schrodinger should have labeled his equation imaginary to remind the whole world that it is only a mathematical tool.

  34. Daniel says:

    I thing the mistake made by the Copenhagen people was to call it an ‘interpretation.’

    There really was no interpretation: it was just the math and that’s it. It should have been called the math-only, results-only, non-interpretation approach.

  35. Tetrazphere says:

    But isn’t the interpretation the “Consciousness causes the wave function collapse?” That understandably sounds mystical. Scientific reductionist disbelieve in consciousness altogether, they really don’t like it. What makes it worse is New Age books and movies about how we can manifest our destiny through the law of attraction, which is that since our consciousness collapses the wave function, we can consciously co-create our world through thoughts. Even Ken Wilber, a modern integral philosopher specializing in consciousness says that crap is “bad science and fruit loops mysticism”.

    Also, even if the wave function was never a reality, there is still an unknown process/event/factor/something that is reflecting an important absence in our understanding of reality. Possibly extra-dimensional causes? I don’t know, it’s worse than a zen koan.

  36. Laurelion says:

    I have a question about the MWI. According with the most serious interpretation: we don’t have new universes that have been “create” every time, we already have “all” universe growing up in the same time, I have a doubt: how it can possible that two universe are “literally” the same universe except for one, single quantum event?
    I mean: our universe is the result of billions and billions and billions and billions (ecc) of elements that reactive each other in a billions and billions and billions and billions of different way for a very, very, very, very, very long time. How is possible that two universe product the same result?

    Now I try to see the problem in this way.
    In the Schrodinger experiment I put in the box George W. Bush (enough cat, poor animal… let him alone), with poison, electron and measurement tool. With spin positive I have a Bush live, with spin negative I have Bush not very well (I don’t wanna write “dead” cause I’m scared of the agency :look: ).
    Ok the experiment begin and in our universe the spine is positive, Bush is stile alive. This means that there is another universe with a Bush “not very well”?
    No, means that in another universe there is an electron with a negative spin, but in a different position or state, very probably there is no bush, no box, no poison and no earth.

    This way to see is compatible with the MWI? If not I return to first question, how is possible that so many variation are almost perfectly the same?

    Sorry for my broken English.

  37. dude says:

    The mathematics of QM is the stuff of ‘mathematics’. Interpreting a mathematical model into reality components can be useful …. and can also be very ‘unuseful’. After all mathematical models are ‘fixed’ once they are set up. They have no ‘intrinsic’ time (or space), else the symbols would change in time (or space). Nevertheless the way a mathematical model changes with respect to a variable can be interpreted as ‘this variable is time (or space) in the model’. That may be a useful interpretation …. or unuseful.

    The many worlds interpretation i can’t see as being useful, unless it is an interpretation that stops further ‘interpreting’ in a universe where it would be best to do that under the circumstances. The ‘wave collapse’ interpretation is potential useful because it sites some sort of further investigation as to the cause. In a world where further interpretation of the QM mathematical model may be worth pursuing, the copenhagen is more useful because it sites a challenge that may inspire confirmation ….. or refutation that leads to some other interpretation (other than many worlds). Whereas the many worlds interpretation is a signal to stop interpreting …. and that may be useful.

    The fixed nature of the symbolic language of mathematics means that for a physics theory to be complete the universe must be deterministic and expressible through reductionism. But it does not follow that if physics cannot be complete that we should stop trying to use mathematical models and their interpretations of the universe ….. because even such an incomplete model is obviously useful. The inability of mathematics to fully describe ‘the thing in itself’ may be the reason why we have to stop interpreting useful mathematical models….. but not to stop developing them!

    If ‘movement and change’ for example have qualities in the universe that the language of mathematics cannot fully model (because a mathematical model does not move or change) then it does not mean that we should stop trying to mathematically model ‘movement and change’. ie completeness of physics is secondary to its usefulness.

    At the moment there is a push in physics to reduce everything to ‘information’. Why? because physicists instinctively feel that the best way to make a perfect complete mathematical model of the universe is to make the universe and the model out of the same ‘stuff’ …. ie information. It also explains the push for ‘holographic’ modelling too. Thus a subset holographic mathematical model (information) within a universe can potentially perfectly model a larger universe (the containing set) …… because it is also made of information.

  38. Tony Fleming says:

    According to Stefanovich in his book Relativstic Quantum Dynamics, “In the rest of this book we will try to stay out of this fascinating philosophical debate and follow the fourth approach to quantum mechanics attributed to Feynman: “Shut up and calculate!”

    At the heart of our current understanding of light is the double slit experiment that Young first used in 1801 to show that light is a wave. The modern double-slit experiment was repeated for quantized light early on in the evolution of quantum theory. There emerged the ‘many worlds’ and Copenhagen interpretations that were both unphysical as well as metaphysical almost demanding the unswerving faith of the practitioner. New ideas based on a revised EM theory and its mathematics may lead to new vistas of scientific understanding about light, physics and life.

  39. Jeff says:

    “Bell’s theorem demonstrated that either information travels faster than light (Bohm) or particles exist in many states simultaneously (everybody else).”

    Huh? Let’s suppose that everybody else is right and particles exist in many states at once. Then you measure both ends and get the same result. Isn’t the information on which entangled measurement was measured on one side still travelling faster than the speed of light? Am I misunderstanding the term “information” here?

  40. The Physicist The Physicist says:

    A little. Entanglement is basically a quantumy version of correlation, which doesn’t require communication but does require some form of previous interaction.

  41. Jeff says:

    So, you are saying there is no way to make the wave function collapse in a particular way for both particles if a stock price goes up and collapse a different way for both particles if a stock price goes down?

  42. David says:

    Does the MWI imply that “quantum immortality” is possible, or am I just getting confused with a thought experiment that uses MWI as it’s scientific background?

  43. avis says:

    i’m coming in years late on this, *physicist*, and i’m not a particularly enthusiastic supporter of the MWI, but i have to tip my hat and give due credit for what has got to be the most lucidly articulated, cogently explained and effectively structured defense of one of the ivory-tower, witch-doctor-mummery interpretations for something so technically confounding and ostensibly absurd as quantum theory.

    well done, good sir. if only our luminaries in other ‘advanced’ fields had such a surfeit with the gift of gab.

  44. Lish Lash says:

    “Bell’s theorem demonstrated that either information travels faster than light (Bohm) or particles exist in many states simultaneously (everybody else). There is a lot wrong with Bohm’s theories, not the least of which is that it doesn’t have a relativistic formulation (which may not sound too bad, but that’s really damning in physics circles).”

    This is an facile mischaracterization of both Bell’s Theorem and Bohmian Mechanics, coupled with a relativistic dismissal of BM that would apply equally to Copenhagen and MW interpretations as well.

    Bell’s Theorem nowhere states that “particles exist in many states simultaneously”; that is an interpretation assumed by MWI proponents. BT simply claims that entangled local action at distance would require superluminal propagation of information that would violate the predictions of QM. Subsequent experiments have validated Bell’s claims.

    Bohmian Mechanics does not claim that “information travels faster than light”. It explains correlation of entangled particles as a result of both particles being guided by the same pilot wave, according to QM predictions identical to those of Copenhagen and MW interpretations. BM is also consistent with Bell’s Theorem, because rather than requiring local action at a distance, it incorporates the implicit non-local determinism of the QM wave function itself. Non-locality means that the pilot wave propagates not through space from one particle to the other, but evolves simultaneously thoughout the entire universe, as dictated by wave function.

    MWI instead postulates that all possible quantum states manifest simultaneously, producing a multiplicity of superimposed universes. It is the simultaneous quantum manifestations of BM and MW that characterize both as non-relativistic interpretations of QM.

  45. Ernie says:

    It seems to me that in much of the discussion here there is a confusion of two concepts of time. In a relativistic “block universe” time is a coordinate and there is no sense in which it passes; events are simply at one time or at another. Nonetheless, physicists speak of the “passage” of time in describing events because it’s easier to talk that way since it corresponds to our personal, mental experience of time, and the equations don’t care one way or the other. Thus, it is not necessary to think that at its inception the universe needed to “know in advance” how many states it was going to have to have at its end, because the whole time coordinate of the universe just “is,” at all times. (Sorry, the language makes it hard to say that properly.) Keeping straight the difference between coordinate time and our mental experience of the passage of time helps to sort out some of the apparent problems. When particles “split” in WMI we don’t need to worry about where the energy comes from, etc., because the whole of all the universes and all their energy at all times is there no matter whether you are looking at the initial point or the end point of the time coordinate – at least according to relativity. (Of course it also points up the fact that physics has NO explanation for why time passes for our mental experience, which is a HUGE problem normally kept locked in a dark closet.)

  46. Eric says:

    “Now take this highly specialized example and apply it to all of the variable’s in this universe…why are we not seeing any of these absurdities?”

    My own gut feeling, and I’m certainly no expert, is that “reality” – as we perceive it – is an aggregate of the highest probabilities contained within all the wave functions. Even with a multiverse interpretation the superimposed alternate universes would radiate out from the one probable aggregate like light intensity diminishing around a light source. In what sense they “exist” I’m not sure, but what we actually experience is the cumulative weight of this aggregate probability – the focal point where the “light” is most intense.

  47. Jeremy says:

    There is no copenhagen interpretation. Only copenhagen statement, which describes the logical implications of quantum theory.

    Many Worlds is a crackpot theory with no logical foundation, because it regards a nonphysical mathematical abstraction (wave function) as physical. Mathematical abstractions are not physical. Therefore anything derived from that false assumption has no rational backing.

  48. Adrian S says:

    Multiverse can be easily refuted by all major 7 arguments:


    Hence, you are kinda stuck with Copenhagen.

  49. John Bayers says:

    Most answers on this site are very accurate but in this answer the Physicst comes off as a partisan wacko for MWI and skews the Cophenhagen interpretation as a straw man. Copenhagen does not require new laws as this physicist claims. Decoherence as it explains the measurement “collapse” can be applied to Copenhagen just as it can be applied to MWI. Who is this physicist who answered this question? He should have been more balanced in his presentation. Many eminent physicists would side with Copenhagen because it is fundamentally probablistic and MWI is deterministic and the born probabilities are hard to justify deterministically. I just want the readers on this question to not take Copenhagen as the straw man that this physicist makes it out to be because the consensus is that Copenhagen is as valid an interpretation as MWI while hidden variable theories are DOA.

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