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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28 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.

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