What is Many worlds: Definition and 134 Discussions

The many-worlds interpretation (MWI) is an interpretation of quantum mechanics that asserts that the universal wavefunction is objectively real, and that there is no wavefunction collapse. This implies that all possible outcomes of quantum measurements are physically realized in some "world" or universe. In contrast to some other interpretations, such as the Copenhagen interpretation, the evolution of reality as a whole in MWI is rigidly deterministic. Many-worlds is also called the relative state formulation or the Everett interpretation, after physicist Hugh Everett, who first proposed it in 1957. Bryce DeWitt popularized the formulation and named it many-worlds in the 1960s and 1970s.In many-worlds, the subjective appearance of wavefunction collapse is explained by the mechanism of quantum decoherence. Decoherence approaches to interpreting quantum theory have been widely explored and developed since the 1970s, and have become quite popular. MWI is now considered a mainstream interpretation along with the other decoherence interpretations, collapse theories (including the Copenhagen interpretation), and hidden variable theories such as Bohmian mechanics.
The many-worlds interpretation implies that there are very many universes, perhaps infinitely many. It is one of many multiverse hypotheses in physics and philosophy. MWI views time as a many-branched tree, wherein every possible quantum outcome is realised. This is intended to resolve some paradoxes of quantum theory, such as the EPR paradox and Schrödinger's cat, since every possible outcome of a quantum event exists in its own universe.

View More On Wikipedia.org
  1. Tommolo

    A Is the QT "no-cloning theorem" suitable as "Chronological Protection"?

    Just a question: how would the wavefunction "collapse" in a time-reversed universe? Let's take Alice. If she's taking a backward time travel to -say- 2021 and finds herself in 2021, wouldn't that be a (prohibited) quantum cloning of an already measured quantum state? Say, the |Alice 2021⟩ ket is...
  2. B

    I Many-Worlds and Testable Claims

    Among the most well-known interpretations of Quantum Mechanics is that of the "Many Worlds," in which all possible outcomes of a measured quantum event occur simultaneously in some alternative universe. Now, I realize there is some manner of debate as to whether or not the different...
  3. S

    I Can Decoherence Explain the Many Worlds Interpretation?

    I've been researching the MWI, and just when I thought I was starting to get it, I got confused again. There aren't many worlds; there's one in superposition? What's the difference? When do worlds(?) split? Will someone please explain the theory in a simple way, but not so simplistically I might...
  4. S

    I Many worlds interpretation with unequal probabilities

    Let's pay a visit to one of Schrodinger's cats. In the classical statement of the case, we have to decide if the cat is alive or dead when the probability of the radio-active decay mechanism has a 50/50 chance of releasing the cyanide, most often posed as 60 minutes. If I understand the MW...
  5. S

    I Nima Arkani-Hamed's opinion on Many Worlds?

    I am writing a blog about physics and one of the sections is about the different interpretations of quantum mechanics and some of its supporters. I was wondering what was the opinion of the physicist Nima Arkani-Hamed towards the Many Worlds interpretation... I ask this because I found...
  6. S

    Nima Arkani-Hamed's opinion on Many Worlds?

    I was wondering what was the opinion of the physicist Nima Arkani-Hamed towards the Many Worlds interpretation. Is he open to the possibility of it being true? Does he support it?
  7. S

    I Multiple (Non-Interacting) Universes?

    Before anything, I would like to clarify that I am aware that this is speculative physics more than established mainstream physics (and Smoot is not claiming that his ideas are true). However I think that it is interesting to discuss these models even if they are not yet proven to be right. I...
  8. entropy1

    I Does the photon in a double slit experiment create many worlds?

    I can't find the anwer on this anywhere, so I hope I may ask it here. My question: In a standard double slit experiment, according to the Many Worlds Interpretation, does the photon create different worlds with different impact locations of the photon? So, without measuring which way...
  9. S

    I Many Worlds as Many Histories?

    I was reading this paper from George Smoot (https://arxiv.org/abs/1003.5952) where he assumes the holographic principle as true and conjectures that our universe would be encoded on the "surface" of an apparent horizon as the weighted average of all possible histories. In that way, there would...
  10. Bob Walance

    I Many Worlds, EPR and ER=EPR

    This question is not intended to invoke arguments about whether Hugh Everett's theory, now referred to as the Many Worlds Interpretation of Quantum Mechanics, is feasible or not. When I heard David Wallace say that Many Worlds does away with the so-called 'spooky action at a distance' referred...
  11. A

    A Born Rule in Many Worlds Derived?

  12. entropy1

    I The splitting into worlds in MWI

    If we have binary decoherence at spacetime location X, as far as I understand, we almost instantly have a splitting into two worlds. Now if we have a different location Y, about a lightyear apart from X, then the decoherence hasn't reached Y before a year has passed. So for Y, the split hasn't...
  13. M

    I David Deutsch's Many Worlds Interpretation and the Double Slit Experiment

    David Deutsch is a well known proponent of the Many Worlds Interpretation. His argument seems to be that a single photon in the double slit experiment must be interfering with one from another world. It is commonly held by physicists that the the photon, as a wave going through double slits, can...
  14. Q

    I What is the current status of Many Worlds?

    Despite its lackluster reception at its conception by Hugh Everett and subsequent advocacy by Bryce S. DeWitt the concept of "just take the theory seriously" and intriguing science-fiction concept of parallel worlds eventually gave it a major resurgence, much thanks to David Deutsch's pioneering...
  15. JamieSalaor

    I How Does Quantum Measurement Influence Reality Splitting in Many Worlds Theory?

    In many words a split occurs when a quantum measurement occurs, this split would then 'radiate' out into the universe. So say a quantum measurement of an electron happens on the moon. The information from the measurement would travel to let's say some people. These people would then unknowingly...
  16. EclogiteFacies

    A Does Everettian QM imply solipsism according to Travis Norsen and Sean Carroll?

    Travis Norsen in his paper Quantum Solipsism and Non-Locality seems to believe that Everettian QM implies some sort of solipsism. He falls it FAPP (for all present purposes) solipsism. (I must say that as a geologist this goes over my head a bit!) However I have recently read Sean Carrolls...
  17. G

    I How to envision particles in superposition in Many Worlds presplitting

    Should particles in superposition before quantum decoherence (many worlds) be envisioned in the same exact way as they are envisioned before wave function collapse (Copenhagen)? Clearly, the particle in Many Worlds is in a sort of superposition, but with all of the talk about one universal wave...
  18. G

    I Question pertaining to the random distribution of many worlds

    Hi everyone, I was having a conversation with my friend about the many worlds interpretation of quantum mechanics, and we couldn't figure out if many worlds implied every single last possible conceivable outcome, or if there were certain limitations that the system was confined to. For...
  19. Lunct

    How can the many worlds interpretation be deterministic?

    So the many worlds interpretation describes every quantum events as a branch point, where in which all events happen. I'll use the example of a coin toss to illustrate my question. If I flip a coin, I create a branch point in my universe from which two universes emerge, where both heads and...
  20. S

    I Wilson's RG trajectories, Lagrangians and many worlds?

    In this article [1] we can read an explanation about Wilson's approach to renormalization I have read that Kenneth G Wilson favoured the path integral/many histories interpretation of Feynman in quantum mechanics to explain it. I was wondering if he did also consider that multiple worlds...
  21. Morbert

    I Many Worlds versus Decoherent Histories

    The interpretations have very different ontological implications. In one, the branches of a wavefunction are real. In the other, the branches are not. These differences are substantive and not trivial. They lead to very different objections being levied against each interpretation. E.g...
  22. A. Neumaier

    A Many Worlds versus Thermal interpretation

    The two volume treatise Bryce DeWitt, The global approach to quantum field theory, Oxford Univ. Press 2003. which discusses the canonical approach to dynamical quantum gravity, is probably responsible for the fact that the many worlds interpretation (MWI) of quantum mechanics has a sizable...
  23. Jarvis323

    I Exploring Sparse Many Worlds: Is a Compact Representation of Existence Possible?

    Is a sparse many worlds representation possible? Or in other words, is it compatible that existence is informationally compact, in that branching is not global? As a crude example, take two worlds, one where I had coffee and one where I had tea; would it be possible that those two versions of me...
  24. Quotidian

    What is the problem that the Many Worlds Interpretation aims to solve?

    Sean Carroll's most recent book is called Something Deeply Hidden, and is premissed on the idea that the Everett interpretation of the 'observer problem' in quantum physics is correct. Carroll, and several other prominent scientific popularisers including David Deutsche and Max Tegmark, are...
  25. bland

    I Does 'many worlds', imply worlds before the Big Bang?

    I'm at this point because a whole bunch of audio books I've been indulging in all seem to be converging on Many Worlds, and this has been given extra authority now because Sean Carroll appears to be a convert. I used to wonder if this was actual physics or metaphysics, and I've given up asking...
  26. J

    Entanglement on the Many Worlds model

    Entanglement's non-locality ('spooky action at a distance') depends on the instantaneous collapse at all points in space of the entangled particles' common wave function. The Many Worlds model has no wave function collapse. Does it also have no entanglement?
  27. bland

    I Could many worlds be a solution to time travel paradoxes?

    And also an answer to 'where are the future time travellers?'. Let's say hypothetically that in the future time travel into the past is invented. To me many worlds looks like metaphysics, but it appears to be taken seriously by physicists, therefore, what if after traveling backwards in time as...
  28. M

    I Schrodinger’s cat and the Many Worlds interpretation

    Schrodinger’s Cat and the many worlds interpretation states that the wave function collapse doesn’t happen at all; every possible outcome of an observation actually comes to pass in its own separate universe. We are presented with a binary (discrete) outcome (dead or alive) but what if there...
  29. Q

    A Understanding the Debate: Many Worlds vs Modalism in Everettian QM"

    There seems to be as many branches of Everettian QM as there are branches in Everettian QM. One of the things that never seem to be clarified by anyone is whether MWI is really just Modalism or not. I.E. whether *all worlds* happen *all the time*, or if there are specific conditions necessary. I...
  30. Quantum Alchemy

    I A question about Many Worlds and my Remote Control

    I get so many different answers to this question so maybe here someone can pin this down. When I get up in the morning and I turn on my TV, I have over 3,000 channels so is there a universe with a version of me going to each channel? If not, how do I go to one channel over the other? Can my...
  31. P

    Many Worlds and the Measurement of an Electron

    Summary: How does many worlds deal with the measurement of an electron's position in space? Hi all - I am reading Sean Carroll's book on quantum mechanics and reached the end of the section on "branching and splitting" without getting an answer. I will lay out my assumptions and then get to...
  32. Hawzhin Blanca

    A Many worlds, observer and Entropy

    According to Everett-interpretation or many world interpretation of quantum mechanics, each decision an observer makes, the world splits into two parallel universes, let’s say an observer in some point in Spacetime is tests the Schrödinger’s cat experiment, in one branch of the universe the cat...
  33. entropy1

    I Is Einstein Causality Proven in Preferred Frame SR?

    If MWI and collapse-theory are both possible interpretations of QM, then both of them are not a fact, right? If MWI is a fact then collapse isn't and vice versa, you could say the least. So, shut up and calculate, i.e. the minimal interpretation, makes no inference about the realness of these...
  34. pinball1970

    I Sean Carroll podcast on many worlds interpretation

    Interpretation of quantum mechanics is something that is discussed at length on pf so it would be interesting to get views of the quantum guys on this short (30 mins) podcast. Thanks...
  35. M

    I Quantum Correlations without Many Worlds or Determinism

    I'm trying to understand the comment by bhobba below from another thread. A related followup from RUTA is provided for reference. After reviewing these I still don't understand. If I think in terms of a single-world (not Everette) and assume Alice and Bob are free to adjust their SG...
  36. J

    I David Deutsch (1985) attempt to solve the incoherence problem

    Can anyone elaborate on Deutsch's attempt to solve the incoherence problem? He postulates a continuously infinite set of universes, together with a preferred measure on that set. And so when a measurement occurs, the proportion of universes in the original branch that end up on a given branch...
  37. J

    I Energy in Everett's Many-worlds Interpretation

    Hi, Is anybody able to explain how energy is "distributed" in the many-worlds interpretation. I'm using scare quotes as I think this may be the wrong line of thought. It's tempting to imagine energy being distributed amongst subsequent branches as the wave function evolves but I'm not certain...
  38. J

    Concerning QED and the Many Worlds theory.

    In QED the maths suggests that a photon takes all possible paths, and all but one of the paths cancel out leaving the path we observe. Using this as an analogy:- The maths of quantum mechanics suggests a vast number of possible universes concurrent with our universe. Perhaps all these...
  39. D

    I Can I steer myself into one of the Many Worlds like this?

    Given an ideal "box" as used by Schrodinger; - have a quantum event occur inside it, e.g. sudden cat death with 50% probability. - have a machine in it that sends out a qubit, fully entangled with the box' internal state, at regular intervals. - the qubit is a polarised photon - outside, use a...
  40. J

    I Quantum Suicide: Exploring Copenhagen & Many Worlds Interpretations

    Is it possible to modify the original quantum suicide experiment in a way in which rather than dying you are put to sleep for someone to distinguish between many worlds and Copenhagen interpretations?
  41. J

    I Can the Many Worlds interpretation state the Born Rule as a postulate?

    Every derivation from the MWI of the born rule is circular. http://fmoldove.blogspot.com/search?q=MWI So my question is, can the MWI state the born rule as a postulate (without deriving) and still be a coherent interpretation of probability? The most famous argument against this notion is by...
  42. B

    I Quantum eraser in Bohmian Mechanics and many worlds theory

    All the literature on the quantum eraser that I've seen is grounded in the Copenhagen interpretation of quantum mechanics. It's very easy to understand the experiment in those terms. Do you know how quantum eraser experiments are interpreted by the de Broglie–Bohm theory? What is erased in this...
  43. C

    Parallel Universes: Multiverse vs Many Worlds - Splits?

    Hi, sorry - forgive me for posting (I am not much of a science guy but I am still interested). I am just watching a documentary (I know not a great source for scientific information - but it got me thinking). In the documentary it talks about the many worlds theory (which I believe is different...
  44. P

    Question about Many Worlds branching in Quantum Mechanics

    Supposing the Many Worlds interpretation of QM is true... If a branching occurs during what we perceive is a wave function collapse, why would this be perceptible to us as probabilties? Wouldn't we just branch, leaving it just as imperceviable as the passage of time? That is, it just happens...
  45. PeterDonis

    Insights The Fundamental Difference in Interpretations of Quantum Mechanics - Comments

    Greg Bernhardt submitted a new PF Insights post The Fundamental Difference in Interpretations of Quantum Mechanics Continue reading the Original PF Insights Post.
  46. andrewkirk

    I MUST every possible thing happen in a Tegmark L1 Multiverse?

    I've been looking at one of Max Tegmark's articles about his 'Mathematical Universe' hypothesis, here on arXiv. As a preliminary, note that Tegmark's framework has four 'levels' of multiverses, with each level being an infinite collection of multiverses at the level below it. The second or...
  47. J

    B M theory, holography, many worlds -- What we do know?

    Hi! Sorry for my bad English! I don't know much of physics, I know some experiments that show that relativity is real, quantum entanglement, particle and wave duality and so on... So, the only theory that explain all of it mathematically is M theory and the strings, but we can never test it to...
  48. N

    I Many Worlds Interpretation existence

    I have a few questions about the Many Worlds Interpretation. I read the article https://plato.stanford.edu/entries/qm-manyworlds/ but was having trouble understanding what the "measure of existence" is supposed to represent in the theory, and why a believer in the idea should adhere to either...
  49. S

    Butterfly Effect: How much can you do before change occurs?

    I didn't know where to put this, but sci-fi seemed fitting: Just a hypothetical regarding the Butterfly Effect that's been puzzling me, if anyone has any insight... How much could you do in the past before your very presence started affecting history? For fun, let’s say you wake up 12 years...
  50. S

    How can Many Worlds be reconciled with unequal probabilities

    If I understand the many worlds QM interpretation correctly, for every quantum event the universe divides. However not all quantum events have a 50/50 probability. How does the many worlds interpretation deal with quantum events that have,say a 1/3 2/3 probability split?