The Many Worlds Interpretation of quantum mechanics is classified by Max Tegmark as a level-3 multiverse hypothesis. This means that in the universes that it will predict, there could be different constants of nature, but the fundamental laws of physics will remain the same

(...) while the worlds of the many-worlds interpretation all share the same physical laws (...)

As it is said here: https://en.wikipedia.org/wiki/Many-worlds_interpretation

But is this right?

Couldn't MWI predict universes with different fundamental laws of physics (as a level-4 multiverse hypothesis would do, like string theory)? For example, string theory is a quantum theory and it predicts universes where fundamental laws of physics would be different. Couldn't MWI be applied to string theory quantum processes? Also, I've read that quantum computing is closely related to MWI and that quantum computing could be strong evidence for the validity of MWI. Also, I've read that MWI can be applied to everything computable by a quantum computer. Does it mean that all universes/systems that a quantum computer could simulate could be predicted by MWI? Quantum computers could simulate universes with radically different fundamental laws and nature. Wouldn't that mean that MWI could predict universes with those characteristics? I've also been told that "MWI can be applied to the interpretation of any quantum system" by a physicist I know.

Finally, I've seen some pages that say that Quantum Mechanics would give rise to level-3 and 4 multiverse theories

Quantum Mechanics, which gives rise to Tegmark's Level 3 and Level 4 multiverses (From https://millenniumconjectures.com/2015/08/21/quantum-weirdness-108-many-interacting-worlds/)

So, in summary, can't MWI be applied to Level-4 universes? Is it there any version of MWI that can do this? If yes, then, why is it usually classified only as a Level-3 multiverse hypothesis?

  • There are several ideas about multiple universes. One is that there are infinitely many almost identical ones in which all causality plays out. And others that are different because of quantum fluctuation differences in the first instant of the big bang made huge differences. Some of those may contain no mass.. or electrons etc.
    – Richard
    Commented Dec 12, 2018 at 22:13

4 Answers 4

Couldn't MWI predict universes with different fundamental laws of physics (as a level-4 multiverse hypothesis would do, like string theory)?

No. To understand why, you need to really grok what MWI is. You seem to be under the impression that MWI posits that each time there's a quantum event of a certain type, the universe actually splits into two or more pieces and each possibility is manifest in a parallel universe. You can certainly be forgiven for this impression, on three points; first, this is the general pop sci account of MWI. Second, it's in the name, "Many Worlds Interpretation". And third, you're reading people who outright say this; from Mark Sackler's blog:

Everett postulated that at each quantum “dice roll” the universe would split into alternate universes for each outcome. These universes are forever separated and cannot communicate with or influence each other.

But this isn't really what MWI is about; MWI does not posit that multiple worlds come about. The many worlds in MWI are already embedded in mainstream QM.

Think of it this way; let's forget MWI and just invoke a Copenhagen Interpretation. Under CI, there are two fundamental processes in QM. Process 1, the Born Rule, is indeterministic; it takes a quantum state and, by pure probability, gets out a classical state from it. By contrast, Process 2, the Schrodinger Equation, is deterministic, and takes a quantum state to another quantum state. Process 2 gives you many worlds, and CI has it.

Taking Schrodinger's cat as an example, let's say we have a cat (A) placed in a box by Schrodinger (B), along with a lump of radioactive material (S) that has 50% chance of decaying after some time t. Along with the cat is a detector which, if it detects decay, will break a vial of poison killing the cat (consider that equipment part of the cat). So now Schrodinger closes the box isolating the cat+lump (A+S) from the rest of the universe. Now we wait for time t. The evolution of (A+S) is a Process 2 function; it takes that initial state and evolves it to a quantum mix between a living cat that observed no decay, and a dead cat that observed decay. But now Schrodinger opens the box and looks inside; that causes a different process, Process 1, to occur. Schrodinger then winds up either seeing a live cat and an undecayed lump, or a dead cat and a decayed lump. Mind you, we're still doing Copenhagen.

Before Schrodinger opens the box, but after time t has passed, you have a quantum mix of two worlds, even under CI. Those are bona fide worlds; they are exactly the kind of worlds you have under MWI. Taking an Everettish view, though, this is a bit of a paradox, because Schrodinger really must describe (A+S) using Process 2, but that implies that the cat, if he were a physicist, should also describe his observation of lump decay using Process 2, after time t before the box is opened. So if observations of systems use process 2 for the cat, why do they suddenly use process 1 for Schrodinger?

I've given this example the same labels that Everett uses in his paper, The Theory of the Universal Wave Function. But the point is this; MWI doesn't posit that there are many worlds as a result of QM; QM already has that in it. MWI instead posits that the wave function is ontic; that when Schrodinger opens the box, nothing happens that doesn't happen when the cat observes the lump. You still get many worlds either way, but it's a conclusion, not a proposition.

So if you want to add a proposition of there being many worlds to multiple other theoretical areas, knock yourself out. But it would be erroneous to say that you're applying MWI to it, because MWI doesn't add process 2; it subtracts process 1.


Yes, we can imagine that two worlds with distinct physical laws both exist, but that's not part of what's usually meant by many-worlds interpretation of quantum mechanics.

The basic idea is that

the worlds of the many-worlds interpretation all share the same physical laws

because when you "open the box to look at Schrödinger's cat", the two worlds

  • d in which the cat is dead and
  • a in which the cat is alive

are identical except for the dead/alive status of the cat. In particular they share the same physical laws.

  • But if the two universes share the exact same physical laws, how could the outcome be different? My understanding is that randomness is only one interpretation, but you seem to be assuming it. I'm not expert on these ideas, would appreciate clarification.
    – user4894
    Commented Dec 13, 2018 at 7:09
  • @BjørnKjos-Hanssen But I'm not saying that just by imaging two different worlds in MWI would make them to exist. I already knew that in MWI all universes are considered to have the same set of laws. I'm asking how can the universes in MWI all have the same physical laws, if MWI can be applied to every quantum interpretation (for example, to string theory, which predicts universes with fundamentally different laws of physics)
    – Forsete
    Commented Dec 13, 2018 at 8:29
  • 1
    @Forsete That's not what you asked. What you asked is if the many-worlds interpretation of quantum mechanics supports the idea of universes with different laws. It doesn't. The MWI applies to quantum interactions that have nonzero probabilities of two different outcomes, roughly speaking, and not to every quantum interpretation. There is no quantum interaction that can make the gravitational constant larger or smaller, for example. Commented Dec 13, 2018 at 21:50
  • @DavidThornley But what do you mean with "It does not apply to every quantum interpretation"? There are alternative quantum theories (String theory, loop quantum gravity, holographic principle...) where Many Worlds Interpretation could be applied...
    – Forsete
    Commented Mar 30, 2019 at 1:26

The Many Worlds Interpretation cannot predict universes with different laws because it is explicitly crafted to not do so.

The Many Worlds Interpretation (MWI) is an interpretation of quantum physics which is designed to provide an interpretation of what quantum mechanics could mean to a classical (non-quantum) observer. It is formulated to do that and only that. In particular, there is one time evolution operator which is used for the entire ensemble of subjects. MWI is really just a nearly-literal translation of the QM wave function into a language which makes some sense classically by using superposition and abandoning realism (the idea that all un-observed states have one value, we just don't know it).

MWI predicts universes with different physical constants because there are indeed theories which suggest that the physical constants we know today were the result of a symmetry breaking very early on in the universe's life. That symmetry breaking could have gone differently for different observers, so different observers could indeed see diffrentphysical constants.

There is no reason you could not extend MWI to reach out to larger classes of universes. However, the moment you do so it would cease to be MWI as "the entity physicists refer to as the Many Worlds Interpretation." It would be a new interpretation, which just happens to also predict many worlds. It might end up being called f-MWI, for Forsete's Many Worlds Interpretation. It just wouldn't be "MWI."

  • what do you mean with "There is no reason you could not extend MWI to reach out to larger classes of universes."? As far as I'm concerned MWI could be applied to every quantum theory, for example string theory which predicts the existence of universes with different laws @CortAmmon
    – Forsete
    Commented Mar 12, 2019 at 11:14
  • @Forsete MWI is specifically a quantum mechanical interpretation which interprets a wavefunction as a superposition of waveforms where each describes the world from one classical observer's subjective perspective and classical observations "split" the world into multiple universes. You can consider larger multiverses, they're simply not MWI at that point.
    – Cort Ammon
    Commented Mar 12, 2019 at 15:22
  • well, it would be MWI applied to other types of quantum theory @CortAmmon
    – Forsete
    Commented Mar 19, 2019 at 18:28
  • @Forsete Yes, if you define Many Worlds Interpretation to means something different, it can be applied to other theories.
    – Cort Ammon
    Commented Mar 19, 2019 at 22:52
  • "using superposition and abandoning realism (the idea that all un-observed states have one value, we just don't know it)" - Observables, not states, have values. But anyway, there's nothing particular in MWI about realism in the sense you say, or lack thereof, that isn't present in many other interpretations of QM. In a different philosophical sense (realism as opposed to being just epistemic) we can say MWI is absolutely realist: it is supposed to describe how reality is (which according to MWI amounts to describing the wave function of the universe), not just what we know about reality.
    – Qfwfq
    Commented Nov 17, 2020 at 0:26

The hypothesis of physical laws being fundamental on a theoretical alternative universe is completely naive and meaningless from a philosophical perspective, because, while having similar objects (e.g. stars), it assumes that the the subject (the observer) is also similar.

Physical laws depend not only on the object, but also on the subject, which in this universe is invariably a human being with scientific knowledge.

In order to show you the fallacy, I assume that Immanuel Kant was correct, not only because his Critique of Pure Reason is one of the most representative definitions (if not the best) of the meaning of human knowledge, but also because my personal research in the systems theory conducted me to the same conclusions. Among them, that space and time are knowledge a priori of experience, and not absolute properties of the physical world. They would be absolute only if we consider that our rational understanding would be the universal absolute way of assessing nature by all entities that can not only get knowledge but interact with other systems (things) (including all the level 3 and 4 universes in the theory).

For example, thermodynamics is a beautiful theory, but it works only if we accept that objects (things) are fundamental physical natural manifestations. Put it simple, thermodynamics laws are correct only if we assume that our perception is absolutely equivalent to truth. We know that things are just an appearance, that the physical universe is not made of things. So, thermodynamics is an example of a set of physical laws that work perfectly... if we assume that the universe is made of the objects of our perception. More details on my answer here: https://philosophy.stackexchange.com/a/57718. Attention! This does not mean that science is wrong. Science is valid for the objects of our perception. [Personally, I think that quantum mechanics is perhaps the only science that faces such fallacy elegantly, although we're far from understanding the reasons that the subject defines the behavior of the object; QM assumes that our perception is not the truth].

This is a typical list of properties of fundamental physical laws, obtained from Wikipedia. Along with each property, a refutation of its meaning on a fundamental level.

[physical laws are...]

True, at least within their regime of validity. By definition, there have never been repeatable contradicting observations.

If we assume that perception is truth, ok. "Observations" require a human observer.

Another refutation to this concept of truth is given by Kant: there's no ultimate rule able to validate the rest of rules. "Within their regime of validity" implies "accepting that perception defines truth".

Universal. They appear to apply everywhere in the universe.

Absolutely naive and subjective. Universal does not mean only "everywhere" but also "everywhen". How would it be possible to be sure that physical laws worked near the big bang? By sending a scientist with a tester there and then? Again, this assumes we can extrapolate the laws that result from our perception to situations where perception is impossible.

Simple. They are typically expressed in terms of a single mathematical equation.

Obviously subjective. Ohm's law can be extremely complex for people with disabilities. Simple if the observer is a scientist with a degree. That would not be universal (or multiversal) at all. This implies that physical laws must be simple also for bidimensional neptunians.

Absolute. Nothing in the universe appears to affect them.

Obviously subjective. Depends on perception ("appears").

Stable. Unchanged since first discovered (although they may have been shown to be approximations of more accurate laws—see "Laws as approximations" below),

Physical laws would be stable within a subjectively assumed framework. Science is not the truth. But it becomes a truth sustained as a law since we perform subjective assumptions. Newton's laws can be true within some framework of assumptions (e.g. slow speeds), or can false in another (high speed). And if you think that Relativity is the ultimate law, be careful: there's some inconsistency between QM and gravity, which could lead relativity to be false within another set of assumptions.

Another refutation to this property would be this: how can a scientist can be sure that this is the same river than yesterday? How can he even be sure that the person he looks at on the mirror is the same person as yesterday?

Omnipotent. Everything in the universe apparently must comply with them (according to observations).

Works fine... if we accept that things are universal features.

Generally conservative of quantity.

Quantities depend on perception. This is a way of telling "hey, I see x things here and now". Ok, if dogs have 42 parts. Or if two persons can count the same amount of apples in a truck of apples in different states or maturation.

Often expressions of existing homogeneities (symmetries) of space and time.

Space and time would be "knowledge a priori of experience". Ok, if martians the size of a quark have the same knowledge.

Typically theoretically reversible in time (if non-quantum), although time itself is irreversible.

Time again, c.f. previous observation.

So, physical laws would be fundamental only if the subjects assessing such laws in others universes are also persons. With scientific knowledge. And with a similar set of assumptions (a priori knowledge). Can MW interpretation have universes with different laws? Yes, if persons can survive traveling to other universes, gathering observations performed using the scientific method, and bringing them back here. For a traditional scientist that assumes perception is the truth, this could have some meaning. But it does not from a philosophical perspective. Perception is not reality.

  • Ohm's law works just fine on people with disabilities. They have electrical resistance that can be measured, and that shows the current flow with a given voltage quite nicely. Do NOT pierce the skin and run current across the chest, but that will kill a person who's not disabled just as easily. If you're saying that it's difficult for a person in a coma to conduct a scientific experiment, well, that seems trivially true. What do you mean by that statement? Commented Dec 13, 2018 at 21:56
  • Not at all my statement. You talk about the electrical resistance of a person with disabilities, which is absurd, ridiculous. The property is simplicity, which is subjective. A physical law definition can be simple for some and complex for others, that's the statement.
    – RodolfoAP
    Commented Dec 13, 2018 at 22:05
  • In which you should try to be more clear. I've been disabled (disability can be temporary). I've known disabled people. Ohm's law always appeared to be simple. Ohm's law is simple in its expression. It can be represented in five symbols and three definitions. If your hypothetical disabled person would have trouble understanding that from his or her wheelchair, said person would not understand other things at all well. Again, if what you're saying is that people with extreme difficulty understanding things won't understand Ohm's Law, that's trivial. Commented Dec 15, 2018 at 18:15

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