# Does the Many Worlds interpretation of quantum mechanics necessarily imply every world exist?

I know the Many Worlds interpretation is controversial among physicists, but it's been a pop culture hit nonetheless. I frequently see people making statements like, "Well in another universe I'm a rock star", where you can substitute rock star for any given fantasy.

But one thing that's always bothered me about that kind of statement: does Many Worlds really imply Every World? You can have an infinite set of numbers that doesn't include every number.

So even assuming MWI is true, is it necessarily true that one of those universes contain a rockstar version of myself?

• The answer is yes, however there's a huge "however". The cosmic wave function assigns a probability to every possible universe. Can you comprehend a probability like $1/{10^{10^{10^{10}}}}$? I can't, but it's not impossible! Oct 24 '16 at 17:45
• But what determines whether a universe is possible? We assume that just because it is imaginable that it must be possible. How is this a safe assumption? Oct 24 '16 at 17:47
• If it's not logically impossible, then it's possible. It's not possible for $2+2=3$, but it is possible that the entire earth suddenly relocates in a different galaxy. Oct 24 '16 at 17:50
• @NathanFig The possible universes are constrained by quantum mechanics. MWI says that each quantum mechanically possible measurement is realized in an actual universe. The Earth's wavefunction, technically speaking, does have some amplitude in a distant galaxy; therefore MWI implies there is a universe in which it is observed there. Indeed even in standard quantum mechanics this relocation is in principle possible. We know this wouldn't violate physical laws because it is quantum mechanically consistent.
– AGML
Oct 24 '16 at 19:21
• If you attempt to widen the notion of "you" then you have to consider why some present day rock star in (this universe) doesn't qualify as "you". Oct 25 '16 at 0:00

No, it doesn't. For example, since charge is conserved, every "world" in the wavefunction must have the same charge. This goes for any other conserved quantity, too.

(This doesn't rule out you being a rockstar, though.)

• Comments are not for extended discussion; this conversation has been moved to chat. Nov 22 '16 at 17:33

The crucial word is "interpretation".

In physics it means that the results of calculations, hard numbers to be checked with data for interactions and processes that we can measure in the lab, are the same in all interpretations of a theory.

There is no difference in the hard data.

One can see the integrals as "many worlds splitting off", but it is just mathematics; going further than that it is metaphysics, not physics.

I will give an example:

Take an apple. One can fit it with a fourier series in three dimensions. The series has an infinity of terms with sines and cosines and the sum of the terms gives the apple's shape. Is the apple full of sines and cosines? One could make an interpretation that each term is real and exists in some space, coming together to make the apple.

• But we were told that before: just accept wave function collapse it gives the right answers. Then the decoherence program came along and QM started to make a bit more sense. Once bitten twice shy. Oct 25 '16 at 4:36
• @BruceGreetham that is because the "collapse" concept is stupid, ab initio. The problem comes in confusing mathematics with physics,the wave function is a mathematical concept, it is not a balloon. Oct 25 '16 at 5:16
• @BruceGreetham yes, mathematics is important from the time of euclidean geometry, but from that time also there was the idea that "mathematics formed reality" and not "reality was modeled by mathematics". The former is metaphysical ( the Platonic ideals) the latter is physics. Interpretations fall in the middle of the two, fine if it is easier to understand the mathematics, but can fall into metaphysics if not careful. Oct 25 '16 at 7:03
• Your apple argument is common but wrong headed. The components of the Fourier transform of an apple don't contain independent patterns of information processing, which do occur in the multiverse described by quantum mechanics: arxiv.org/abs/quant-ph/0104033. Oct 25 '16 at 9:11
• @annav If you don't know what information processing is you can't judge my argument since you don't understand what it is saying. You also don't understand the word interpretation. In the case of quantum mechanics, some'interpretations' are actually distinct physical theories with distinct implications, e.g. - the Bohm interpretation. Others are complete nonsense: the copenhagen interpretation. One of the interpretations, the many worlds interpretation, is just quantum mechanics taken seriously as an account of how the world works. Oct 25 '16 at 11:35

Any alternate present must be the result of a whole possible alternate history. Assuming our decision making is not sensitive to quantum effects in our rather noisy brains (and you did not make your career choice as a random outcome of a quantum experiment, with rockstar already being on the table), significant alterations might be necessary to make rockstar-you happen. The alternate-reality person might be rather different from present you. There also might be careers that are unavailable in any history, though I have no idea how one would go about proving such a thing.

• Thanks, this is closer to what I'm asking. That last sentence is really my question: Are there states that are simply not achievable in any scenario? Oct 24 '16 at 18:10
• my tentative answer is probably so, but I'll have to think some more on that one... Oct 24 '16 at 18:18
• @NathanFig Think about it in a different way - what makes you you? If you detest politics in all its forms (for the sake of the argument), would an alternate you that became the president of the Earth be in any way you? How? And if you can wiggle the "you" definition enough, are there more of "you" in your own universe? The common sci-fi approach basically always looks at a "parallel" universe where everything is exactly the same except for a few (human-level) decisions made in the past. Now, that would be possible in MWI, but just consider how you would even find such a universe! :) Oct 26 '16 at 8:12
• However, we're surrounded by things like atmosphere where small deviations can be amplified immensely. Gust of wind slapping a child in the face with \$100 note can both alter his decision-making and practical means, resulting in different actions, possibly with lifelong consequences, all astronomically unlikely but not forbidden. Oct 27 '16 at 10:57

The MWI interpretation of quantum mechanics does not introduce any substantially new answer to this question than already exists in any other interpretation of quantum mechanics or indeed classical statistical mechanics.

In the MWI interpretation of quantum mechanics (as originally conceived by Everett), viewed at the microscopic level there is only one universe evolving according unitary deterministic dynamics.

The MWI interpretation provides an explanation for the measurement problem by saying that whenever an entangled quantum system undergoes decoherence and approximately forms a mixture then each member of that mixture forms its own evolving branch effectively independent of the evolution of the other members.

Within this emergent branching structure we can ask questions like "what is the probability of ending up in a branch with all the atoms in a box ending up in the left half" but such a question is not greatly different from the similar question in classical statistical physics - given enough time it will happen.

• Well MWI is different in one respect. If you decide to do something based on a quantum-level result (for example to kill a cat locked in a box), MWI says there is no such thing as "quantum risk". You should have been more careful - this is not a "risk", because the harm is sure for one of futures. And that future is also about real you and a real cat. You don't observe the alternate future post factum, but it exists as a world. And you are responsible for creating it. Oct 26 '16 at 11:40
• @kubanczyk Yes you are right. However notions such as "responsibility" are even higher level emergent notions than that of the emergent multiverse. We are forbidden to discuss such things on Physics SE! Oct 26 '16 at 12:23

Basically yes, there are infinite worlds with different "outcomes" in each one. However, the Many Worlds interpretation says that some are less probable than others. So your rock star you might be less probable than, say, your normal you. (Just an example, obviously.)

The other important thing to note here is basically that every option of every decision or thing in the world has been carried out. So the Declaration of Independence might have been signed five minutes later than it was in our world. These changes then might lead to further changes and so on.

So, if you are feeling especially philosophical one day, go to the fridge and drink some chocolate milk or lemonade, whichever you want. The decision you make will (according to the MWI) create multiple new alternate universes (depending on whether you choose to ignore me, mix them, choose one or the other, or choose something entirely different).

Hope this helps!

Edit: NathanFig brought up a good point in the comments that needed clarifying; these universes would, for instance, have to follow the laws of physics. There would only be a universe for every outcome for which a "decision" has been made in our universe. So the answer would be yes, every outcome possible from the decisons made in our universe (if that makes sense).

The other thing worth mentioning here is that the data is the same whether there is an alternate universe where something else happens or there isn't. It is merely an interpretation.

• But how does an infinite set of outcomes imply every outcome? Wouldn't that universe have to be at least logically self-consistent somehow? How do we know that there exists a logically self-consistent universe in which the Declaration of Independence is signed 5 minutes later? Oct 24 '16 at 17:46
• @NathanFig, perhaps I worded this poorly: every outcome for which a "decision" has been made in our universe. So the answer would be yes, every outcome possible from the decisons made in our universe (if that makes sense). I was by no means implying that, for example, the laws of physics are different, or something like that. Does that make sense? Oct 24 '16 at 23:17
• @heather how do we know that decisions are quantum mechanical though? Would every possible output of my computer be a different quantum world. Oct 25 '16 at 14:14
• If every option of every decission is carried out (leading to as many worlds as possible options), ¿how are some worlds less probable? It would seem that all worlds which arise from a plausible series of options would have a 100% chance of existing while the other worlds (no matter what options are taken that world does not arise) would have a 0% chance of existing. Oct 25 '16 at 19:04
• Why would "rock star you" be less probable than "normal you"? What makes you think "normal you" isn't the statistical outlier, and most of the others aren't all rock stars? Also, they don't need to follow our laws of physics - maybe they're different based on some minute change during inflation? Oct 25 '16 at 21:15

But one thing that's always bothered me about that kind of statement: does Many Worlds really imply Every World? You can have an infinite set of numbers that doesn't include every number.

Quantum mechanics places restrictions on the set of possible worlds.

Worlds are structures in the quantum multiverse where information is copied, see

The version of me that is typing this is in the same universe as you because it is possible for information to be copied between this version of me and your eyes, brain etc. That has implications for the set of possible universes, e.g. - they are a discrete set that can be labelled by numbers 1,2,3... not a continuous set:

More generally, there are no universes in the multiverse in which the laws of physics are broken. For example, there are no universes in which energy is not conserved.

So even assuming MWI is true, is it necessarily true that one of those universes contain a rockstar version of myself?

It is not necessarily true that there is a universe in which you are a rock star. I can't see any particular reason why that would be physically impossible so I would guess there is such a universe.

• Is there not a quantum state from which information can be copied to either "world A" or "world B", but information from world A cannot be copied to B and vice versa? Or, where information can be copied from world A to B and back again but only at extreme cost, in the limit approaching impossible but never actually reaching it? Is there a physical mechanism that makes two quantum states perfectly "orthogonal"? Second, QM permits entropic reversal with low probability, including matter spontaneously becoming a nuke. Rockstar seems more likely.
– Yakk
Oct 25 '16 at 18:17
• On your first two questions, the answer is no. Read the linked papers. On entropic reversal it is not clear to me that is possible since when information is copied in quantum mechanics entropy always increases. So how would any one measure such an event? I don't know how likely a rockstar is since I can't calculate the Hamiltonian of the whole universe. Oct 26 '16 at 7:31

Many Worlds is really a misnomer, despite being the most commonly used name for Everett's interpretation. Everett initially called it the Relative State Interpretation. The "many worlds" and "universe splitting" terminology was introduced later by DeWitt.

There are no multiple universes or splitting in the formal statement of Everett's interpretation any more than in any other interpretation. The dimensionality of the state space remains constant throughout the evolution of any system and the state consists of a single state vector. Any apparent "splitting" is just the representation of the state-vector in physically meaningful bases becoming less sparse due to entanglement. In Everett's interpretation measurement is just a form of entanglement if you include the experimental apparatus in the system description. The odd thing that is brought out by the Everett interpretation, but I believe is intrinsic to QM in general, is that there appears to be a preferred basis for macroscopic objects. For example we do not seem to be able to observe cats in the $$|alive+dead \rangle$$, $$|alive-dead \rangle$$ basis, though there is nothing in the quantum formalism that would make this any harder than the usual $$|alive \rangle$$, $$|dead \rangle$$ basis.

Everett's interpretation is really just the Copenhagen interpretation without observers, measurement & state collapse.

The Stanford Encyclopedia of Philosophy has accessible explanations of these issues:

I think this question comes down to determinism. Our universe might or might not be deterministic, so I think the answer to this question is "nobody knows for sure".

If the universe is deterministic, then two universes that are identical at any given point in time will continue to be identical for the remainder of all time. So, if another universe exists that is exactly the same as this one at the moment of your birth, then it is impossible that you became a rockstar in that other universe, because you did not become one in this universe.

So, the next question is, is there a universe similar enough to ours that a person with your exact DNA was born at the exact moment and place that you were born, but with sufficient differences that you became a rockstar? Maybe or maybe not. If the universe is deterministic, then only one timeline is possible given a set of inputs, so a different universe would require a different set of inputs. A universe with different inputs might necessarily be quite different from our own. This is debatable.

If the universe is not deterministic, then it is probabilistic. In which case I'd have to say anything is possible within the laws of physics.

• within the laws of physics and within the phase space accessible from the initial state of the universe; there might very well be inaccessible potential realities Oct 24 '16 at 18:20
• This is helpful, thank you. I am curious about what states are accessible. If the phase space is limited at all in any way, then it seems that it is not safe to make any assumptions about what universes exist. Oct 24 '16 at 18:31
• This answer does not seem to tie to MWI of quantum physics. In a sense MWI is deterministic: every possible quantum "observation" happens, so it is meaningless to ask if things can go differently with same "input": in MWI they already go every possible way.
– hyde
Oct 25 '16 at 4:57
• OP asks about MWI. MWI obeys Pauli exclusion principle. So your answer becomes wrong here in the third sentence "two universes that are identical at any given point in time will continue" bzzz forbidden. Also MWI says you can set up a quantum experiment that really splits the world each time you make it (in your terminology "one world does deterministically become two worlds, easily, all the time"). Oct 26 '16 at 10:10

No. Keep in mind that all universes where you are you, everything up to your conception was identical (or very, very close). Otherwise due to chaos theory/butterfly effect, it's likely a different sperm won the race, or the... event happened at a different time, and you lost your chance entirely. So even if they are named Nathan Fig, the genetics might be slightly different (or maybe they are named Natalie. Who knows).

So out of that relatively small subset of the Many Worlds where your specific genetics exist, most will be substantially the same. As an easy counter example to your question, if you were born after 1945, there is almost certainly no universe with you in it where the Axis powers won WW2. So right off the bat, that eliminates a large number of potential jobs that are no longer available to you in any universe.

In case of eternal inflation theories, one can show that the MWI ensemble is realized physically such that the relative frequencies are given by the Born rule. There is then no doubt about the reality of the alternate MWI Wolds even if the MWI is false. One can then argue like is done in this article that all physically possible histories of the universe are realized:

Some readers will be pleased to know that there are infinitely many O-regions where Al Gore is President and - yes - Elvis is still alive.

• @BobBee Believing is what you do in Church (if you're not an atheist like me), in physics this doesn't work. Take e.g. Einstein not believing in quantum mechanics. Oct 25 '16 at 4:18
• Agree. That's why it is not a good idea to believe in MWI or any other nonscientific explanations. In science, actually talking about many worlds. That used to be religion. Is there a single equation you can test in MWI? Oct 25 '16 at 16:55
• @BobBee The MWI is motivated by the problems you get when invoking collapse. That it leads to "many worlds" is not the reason why people support MWI, it's just that a law of physics shouldn't have ad hoc exceptions (unitary time evolution gives way to collapse because I'm taking a look at the system, despite the fact that I'm just a collections of atoms that should evolve and get entangled with the system just like any other collection of atoms would). What one can test is if time evolution is unitary, one can falsify MWI by demonstrating collapse in a perfectly isolated system. Oct 25 '16 at 18:56
• Well, collapse is seen as a just 'statistical' explanation for what really happens, which is that it interacts with another, macroscopic system designed to measure an observable and thus, with some probability get one result out of many. The combined measurement apparatus plus measured entity still follows a unitary evolution. It is so complex to follow each particle in a macrospsopic system and all its interactions that we describe it as the statistical density matrix. Decoherence is about what you can measure. Don't get confused with popular and false interpretations. Oct 26 '16 at 0:00
• @BobBee When you emit an entangled photon and measure it far away (EPR), you claim the measurement destroys an alternate world and experiments do show it happens far above the speed of light. If you believe this weird FTL_destruction_theory, please explain the underlying mechanism and experimenters will test it. The MWI says the alternate world only becomes undetectable; nothing more; this humble prediction is easily confirmed. Oct 26 '16 at 9:51

In MWI, universes branch off all the time, with every possible quantum "observation". So let's say a version of "you" to exist or at least existed in any universe which branched off after your conception.

So, question becomes: Is there a series of quantum "choices" which leads to you becoming a rock star?

I think that is certain. Even if that was impossible under "normal" circumstances, consider radioactive decay causing brain tumor, which in turn causes behavioral changes. It could happen to a version of you, but also to an artist agent or your music teacher or parents, leading you to become a rock star. This is just an example to demonstrate that there almost certainly exists MWI universes where quantum "choises" and events unfolding from them lead to you becoming a rock star.

Not only that, but by same logic, in countless MWI universes branching off after this moment, you will become a rock star, and then later wonder how that relates to you having asked this question here.

• I like how this answer inserts a known quantum-level phenomenon where others offer dubious assertions of how everyday neuron action depends (or not) on world splits. Oct 26 '16 at 11:17

The answer to your question is basically depending on if you decide to:

1. allow all other worlds to have totally different physical laws then ours, different combination of them, maybe no laws at all
2. all other worlds must obey the same exact physical laws

In the case of 1. the answer to your question is yes, all of them exist. In this case all kinds of combinations would be existing, since there is no limit for example on the amount of energy, speed of expansion etc. The size of these parallel worlds would not be limited either.

In the case of 2. the answer would not be so simple. In this case the number of parallel worlds would still be infinite. But all these infinite worlds would just be versions of our current one. All of the infinite ones would start with a big bang like ours, just they would end up differently, depending on little changes in the starting setups, like the allocation of energy, particles differently allocated, (though the amount of energy would be the same as our world's), but maybe the size of the starting point would be different (where everything is bunched into before starting the big bang), and so the energy of explosion would be different. maybe the era of inflation would be different too, longer/shorter then ours. So this way all those worlds would exist, but maybe the arrangement of the particles/energy would be different in them. A little background on this to understand the question: (Imagine a 6D world, where our 3D space would be then extended infinitely in a 4th dim., can call it time. Then we have a infinitely long (both directions) line, with every point of the line having one of our current 3D spaces. So then we have the 4D timeline, and infinitely replicate it on the 5th dim. That looks more like a flat surface filled with our 3D spaces in 2 dimensions now, the 4th and 5th. And then you just replicate this 5D world infinitely into a 6th dim. So then basically you have Our 3D world infinitely expanded in 3 more dimensions. Each coordinate of this 4th,5th, 6th dim system has one 3D world on it. Those are our parallel worlds. Now imagine that the timeline as we imagine it now is not linear, but moving in 3 dimensions, the 4,5,6th. So basically our world is currently in this 3D sytem somewher and as we pass through time, we move on this 3D grid. our timeline describes the possible line through this system based on our starting settings and out laws. Nobody can change it, since that wouldre quire too much energy. But those other worlds on the other spots of this 6D grid are all existing parallelly)