# Arguments for and against Many Worlds?

10,186

## Solution 1

For:

1. Collapse is an awkward non unitary concept in the C.I. and likely to be the source of most of the so called paradoxes (like Schrodinger's Cat etc.). Many worlds does not have collapse and thus avoids those paradoxes. It frees one from assuming the "add on" of collapse.

2. Quantum entanglement finds a "local" meaning.

3. Removes randomness from quantum theory (although for an observer it does not remove any randomness).

Against:

1. Huge proliferation of universes for explaining the observations of an observer. In a way it does not respect the Occam's razor. (Many worlders claim MWI respects it since this interpretation has an economy of principles).

2. The problem of preferred basis.

3. Defining a suitable measure of probability to achieve Born rule.

4. Other universes can not be observed. (A variation of saying it does not respect Occam's razor)

5. It is rather a psychological way of thinking about Q.T. rather than a real ontology.

## Solution 2

in my opinion, the best argument is that removes the need to postulate a collapse of wavefunction, and explains the source of the randomness of quantum events as a single observer splitting in multiple version of the same observer entangled to each eigenstate of the observed system

the weak aspect of the many worlds interpretation is that it doesn't give a natural explanation of probabilities as the magnitudes-squared of amplitudes, this is still required to be assumed

## Solution 3

Everettians are trying to pull wool over your eyes... If it only were that simple that our world keeps branching with time with a preferred basis, and we subjectively find ourselves in one branch. There are two major problems with this interpretation.

• Different branches can and will recombine in the future. This is seldom emphasized, but this throws a wrench in the interpretation. In fact, for a system in thermal equilibrium, branching and recombination happens at an equal rate. It's only thanks to the fact that locally, we are out of equilibrium that one-way branching makes any approximate sense at all.

• There's no canonical preferred basis in general, not even macroscopically. Decoherence works most of the time at macroscopic scales, but with many important exceptions. If decoherence were universal at macroscopic scales, do you think we'd be able to observe double slit experiments or superconductivity or quantum optics? Even more troubling is the fact that the basis to be chosen can depend contextually upon future decisions, as in the delayed choice experiment.

• In the many minds interpretation, subjectively fixing the conscious state of the observer still leaves most of the rest of the universe in an indeterminate superposition. Only those coarse-grained properties of the "world out there" corresponding to our internal conscious states will be determined by entanglement. Those Everettians who try to tell you subjective experiences are what causes the entire universe to split into many worlds are bluffing.

• If the other worlds out there have some objective existence, how come we can't extract information from them, except in very special cases where we have a coherent variation in the phase and amplitudes between the many branches which then recombine? Not only that, after recombination, the separate worlds lose their separate identities. Besides, a coherent variation rules out the possibility of a complex intelligent observer, at least in the part of the wave function which varies coherently.

Do you really see?

## Solution 4

Has the MWI produced any new physics? What I mean is not people who subscribe to it doing new physics, but people doing new physics because they were thinking within that interpretation. If the answer is nothing, then this, in my eyes, is a very strong argument against it. Of course this could apply to other interpretations.

I am not sure if I am clear. What I mean is for example, is there a calculation or a derivation or anything like that, which would have not been done (at least for some time) had there been no MWI.

Edit: I take the comment about David Deutsch as an example of what I was asking for, hence my answer should be viewed as a point in favour for the usefulness of MWI.

## Solution 5

Having studied some David Deutsch material recently here are some other points:

For

1. A Multiverse framework for considering whether there is "something beyond" traditional quantum theory. EDIT Thus providing a framework for investigating quantum gravity. Deutsch sees the Multiverse as an as yet undiscovered theory, motivated by Everett, Quantum Computation and certain properties of General Relativity.

2. A possible "explanation" for Quantum Computation speedups (EDIT ADDITION) That is for those quantum computations in the BQP Complexity class.

Against

1. Those Probability and Preferred Frame problems

2. The fact that the theory has split into many sub-theories (e.g. Decoherence, Many Minds, Multiverse, etc) that there is no single model to evaluate any more.

3. (Related to 2 perhaps) Everett did not publish enough to clarify exactly what the theory actually said in several respects.

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### Qurious

Updated on September 12, 2020

I would like to hear the best arguments for and against the Many Worlds interpretation of QM.

• HDE over 11 years
Perhaps any "the best" question should be a community wiki..
• Justin L. over 11 years
MW is inherently untestable/unfalsifiable by its nature, and I would venture to say that is less Physics and more Philosophy.
@JustinL. That is precisely what I don't like about it.
@JustinL.: MWI is philosophy, but it might be better to say that all interpretations of quantum mechanics are philosophy.
@JustinL. recently scientist proved there have been four clash between ours and another universe. So it is now a days MW become testable. phys.org/news/2010-12-scientists-evidence-universes.html#nRl‌​v
• Mark Mitchison about 9 years
@KaziarafatAhmed The popular article you linked and the research it describes has nothing to do with many-worlds whatsoever!!
• Warren Dew over 7 years
@JustinL. Everett (many worlds) predicts that interference will occur in all cases specified by the wave equations. Copenhagen predicts that interference will not occur after "observation". This difference is testable, and thus far, all tests have favored Everett.
• Lars over 11 years
@Vladimir: I don't agree at all. I accept it is rather psychological. But it is one of the best ways to think about quantum theory.
• Brien Malone over 11 years
@Roy: I don't understand your "something beyond" point. MWI is nothing beyond Q.T.
• Vladimir Kalitvianski over 11 years
To sb1: I disagree. In physics we must deal with observable things, not with out fantasies.
• Roy Simpson over 11 years
@sb1 : If you follow the Deutsch discussions you will see in more detail. His argument seems to be that "Multiple Universes" is a bit classical ie simplistic view of what is really happening. What is really needed is a "Quantum Multiple Universe" concept which he believes needs further equations to fully define. So the Multiverse becomes a research framework into QM, in my view (of his view).
• kame over 11 years
@Vladimir But maybe there is something more than observable things and fantasy.
• Brien Malone over 11 years
@Roy: It may be David Deutsch's view (I have huge admiration for him). But as far as MWI is concerned it does not go beyond quantum theory in my humble opinion.
• Vladimir Kalitvianski over 11 years
Yes, and this something is called "hand waiving".
• Lars over 11 years
@Vladimir: "In physics we must deal with observable things" can you observe an atomic orbital or the wave function itself? In my humble opinion, we can think of anything as long as it has some observational consequences.
• Vladimir Kalitvianski over 11 years
Yes, we can. Learn atomic form-factors, for example.
• Roy Simpson over 11 years
@sb1 : It all depends on what "beyond" means of course. I have added some further EDIT points on that.
• HDE over 11 years
I think Decoherence remove collapse need too
• HDE over 11 years
real ontology, could you explain it further?
• HDE over 11 years
Quantum Wave is not observable neither and physics deal with it anyway because of its observable effects
• Lagerbaer over 11 years
@sb1 The key here is "observational consequences". So far, nobody could point out observational consequences that would be different for a Many World universe and a Single World universe.
@HDE: Since it is not a philosophy community, I shall restrain myself from explaining it deeply. Just would like to mention that the word "ontology" means "the nature of being" that is the "thing in itself" rather than our knowledge and suppositions which is called "epistemology".
• HDE over 11 years
I see, then the word ontology is a synonym of realism, and as I understood ralism is questioned since Bell theorem..
@HDE: As far as MWI is concerned it does not question "realism". Bell's theorem is about different predictions of local hidden variable theories and quantum theory. Experimental verdict favored quantum theory. It means local hidden variable theories are out.
• HDE over 11 years
@Lagerbaer, that's a better path, then for any given observation a set of theories could explain them if they doesn't find a contradiction, the idea that could exist a single theory (TOE believers) that explain everything without experimental contradiction and I repeat a SINGLE theory, perhaps is just human illusion
• Lars over 11 years
@Lagerbaer: In my humble opinion, if in order to explain the observations we have to make theoretical construct in the most economical way possible then all those things which are part of that construct are part of our reality. If in order to explain our universe we need multiple universes in the most economical way then all those other universes are part of reality even if we can't directly observe them.
• Lagerbaer over 11 years
Sure. The question is: Which does Occam's razor favor? Proponents of both the Statistical Interpretation and the Many World Interpretation could claim that their concept is the more economical one. The former doesn't need infinitely many universes, the latter doesn't need the collaps of the wavefunction. As far as I am concerned, I believe it's (currently, until more evidence comes up) a matter of taste.
• lurscher over 11 years
interesting. Is there a reference you want to share that explains this?
• Lars over 11 years
@Lagerbaer: I agree.
@lurscher: Yes, the "decoherent histories" or "consistent histories" approach enjoys the virtues of both worlds, C.I. and M.W.I. although there are problems of this approach too. You have to make some additional assumptions (like maximality etc.) to make it work.
• Vladimir Kalitvianski over 11 years
I know experimental physicists who took "photographs" of wave functions squared of slow neutrons in the gravitational field. They obtained $|\psi(z)|^2$ on the photo film.
• lurscher over 11 years
this is not a place to do follow-up questions. However the answer is clearly no; MWI is just an alternative axiomatic framework, if you like, for copenhague quantum mechanics, which is shown to be entirely equivalent in predictions
• Lars over 11 years
;-) photograph of the probability? ;-) then be sure some day some experimental "physicists" will come with a photo graph of a multiverse ;-) Can those "physicists" take photograph of irritation?
• MBN over 11 years
I didn't mean it as an actual question. I phrased it as a question because I cannot be 100% sure that there aren't any examples. So it is a statement with a reasonable amount of uncertainty.
• HDE over 11 years
• HDE over 11 years
@Vladimir Kalitvianski, she is photoshopped ;)
• Vladimir Kalitvianski over 11 years
You do not believe me. OK, let us take an atom $H^0$ in a highly excited state |n>, n >> 1. Such an atom is very "large". If you "throw" this atom on a photo film, you will get two points - one from electron and another from the nucleus. Doing so many times you will obtain a photo of the excited atom probability to find the electron and the nucleus here or there. It will be the projection of $|\psi_n (\vec{r})|^2$ on the photo film plane.
• Peter Shor over 11 years
David Deutsch invented (co-invented?) quantum computation by thinking about how you could experimentally test the many-worlds interpretation.
• Peter Shor over 11 years
• Qurious over 11 years
About this, I'd love to hear Peter Shor explain further what he means by misleading intuition, because like the original answerer said: quantum computation is often used as a argument for MWI.
• Qurious over 11 years
What does this "photo" really mean in regards to MWI though?
• Qurious over 11 years
I've heard the preferred basis problem many times, but it seems people have different views on how it is a problem for MWI. Some claim it mean you can't have ontology in MWI and hence doesn't explain our observations. Other's say it put MWI in problems with relativity.
• Vladimir Kalitvianski over 11 years
It means that the wave function describes ensemble of measurements, not one event. Each separate measurement (a point on the screen) is not a collapse of the wave function but a bit of information about the wave function. One needs many points (bits) because the system is not simple. Any system state is not simple and there fore needs many "points" to be described properly.
• Qurious over 11 years
I've read about the Ensemble Interpretation, but I seem to remember that the main objection against this is that you can then never use it for single particles?
• Qurious over 11 years
but decoherent histories, require indeterminism, right?
• Peter Shor over 11 years
If you are thinking about the many-worlds interpretation and quantum computation, then the obvious (wrong) conclusion is that quantum computers should be able to do anything that is possible with parallel polynomial time. This is much too big a complexity class.
• Roy Simpson over 11 years
@Qurious : there is a Complexity Class called BQP which is (assumed to be) larger than P, but different from all the better known non-P complexity classes, that represent "parallelism". So if this is real, then maybe MWI has something to say about it, but the story of parallelism seems to be wider than Quantum Computation as currently understood.
• Qurious over 11 years
Sorry as a complete layman in quantum computation I didn't quite understand your statements. It seems Peter Shor says that MWI gives a misleading picture of QC, while you Roy Simpsons think the opposite? Correct me if I'm wrong
• Roy Simpson over 11 years
@Qurious : I have edit my sentence on that. In layman's terms Peter is pointing out that the Multiverse view is not able to explain all of Computational Parallelism. (This is another issue.) I think that the best way to explain Deutsch here is that there is a subset of Parallelism called BQP which could be explained via the Multiverse. So Peter is concerned that simplistic attempts to explain the Multiverse in terms of quantum computation with inevitable mentions of "parallelism" might fail to point out that quantum and classical parallelism are (or seem to be) quite different.
• Roy Simpson over 11 years
As I happens the Shor Algorithm is believed to be in the BQP class itself, so he might have more to say on this but that may have to be in a different question.
• Vladimir Kalitvianski over 11 years
A single particle, how does it look like? As one point on a screen? No, there is no single particles but many. In Classical Mechanics where the "particles" are macroscopic we replace the many-particle system with a three coordinates of its center of mass. Obviously it is a very approximate description (no shape, size , etc. are present in 3 coordinates). In "microscopic" world we believe our particles are "single". Who said that? A single point-like electron has very long "hands", it is felt everywhere, it is very sticky. Is it point-like? No. According to QED, it is not. It is complicated.
• Piotr Migdal over 11 years
@lurscher Actually, the Born's rule (i.e. $p(x)=|\psi(x)|^2$) can be derived in a very natural way, see Zurek's paper arXiv.org/abs/quant-ph/0405161.
• Piotr Migdal over 11 years
Sorry, by I see little sense it your post. Actually MWI eliminates measurement (which is very not natural in many aspects and non ), which Ockham's razor. It's better to calculate rules than naively calculate beings (whatever they mean). Well, as we are familiar with classical world, it's intuition puts as biased against MWI, not for (BTW: talk with non-physicits, and they will cal MWI quirky; talk with quantum physicists, and for them MWI is natural).
• Vladimir Kalitvianski over 11 years
The most important lesson of elementary particle physics is that there is no elementary (single, free) particles in empty space but elementary excitations (quasi-particles) within compound systems.
• Qurious over 11 years
Pretty certain that paper has been refuted
• Qurious over 11 years
About this tho, what makes the particles go where they go? If no guidin wave like pilot wave?
• Vladimir Kalitvianski over 11 years
When I draw a line on a blackboard, can we state that it is the chalk who makes this line? Am I involved in a visible way in this line?
• Sklivvz over 11 years
• Sklivvz over 11 years
Whilst this may theoretically answer the question, it would be preferable to include the essential parts of the answer here, and provide the link for reference.
• MBN over 11 years
@Skivvz: Was that comment for me? This answer was written more than a month ago, why now? And why are you telling me that!
• anna v over 11 years
@ Roy Simpson Feb 28 at 15:58 if this : "What is really needed is a "Quantum Multiple Universe" concept" would be a metaphysicist's dream come true in physics :). The multiple universe has caught the imagination from metaphysics to fantasy fiction, but it needs a framework of "interactions" and "gradations" between universes, tree structures, like the "trousers of time" in Terry Pratchett's discworld. Any experimentally detectable effect would need something like that.
• wnoise over 11 years
Yes, you are. The overall energy over all "worlds" is still conserved, because it is weighted by measure -- just like standard superpositions, because that's all MWI is. Within each "world", there is a local "relative energy" that also appears to be conserved.
• John Alexiou over 11 years
But then an observer following one world line would see a shift in energy from 100% to 50% after the bifurcation. In the end after billions of bifurcations the total energy available to any one observer is going to approach zero. MWI still makes no sense to me.
• wnoise over 11 years
Not at all. An observer in each world is also cut down in measure, so the effect of a given energy is the same, so is measured the same.
• Roy Simpson over 11 years
@Anna, yes if this QMU is even physics there is still the question as to whether it is "observable physics". It could be that just as we have non-observable parts of the (GR) Universe, Everett-Deutsch predicts non-observable consequences of QM. On the other side any (potential) observable consequence would be a challenge for experimentalists to try to identify. Deutsch has a new book coming out in the summer.
• Ron Maimon almost 11 years
Dear Coward, this is actually relatively accurate as a criticism, except for the conscious state argument. The conscious state will fix the wavefunction all the way throughout the universe relatively sharply, and this is not a bluff. The reason your intuition is failing is that every particle is entangled with every other (for example, every atom here collapses the Andromeda galaxy, the wavefunction is global)
• Anixx almost 11 years
Many minds in NOT many worlds. Many minds is completely invalid, pseudo-scientific "interpretation".