It may seem odd that this blog is called “Many Worlds Theory” and yet I have not yet blogged about, well, many worlds theory. Well, I am doing so now.

The arguments that lead *a sizable portion of theoretical physicists to postulate a multiverse* are subtle and complicated. I hope to eventually cover these arguments, but I’d like to start with a discussion of what many worlds *is not*. Therefore, with much fanfare, I present to you:

The Top 5 Misconceptions About the Many-worlds Interpretation (MWI) of Quantum Mechanics

**1. ****Every time you do a quantum experiment, the universe branches into multiple universes.**

This is a popular notion, as seen in TV shows like Nova and as presented by Martin Gardner. Unfortunately, it is the wrong way to look at things. It is much better to imagine that all of the possible universes already exist, and that doing an experiment just tells you (the experimenter) which universe you happen to be *in*.

Suppose you’re watching *Star Wars*. You have no idea whether you are watching the original, or the retconned 1997 version. You finally realize which version you’re watching when you see Han shoot and kill Greedo without Greedo ever getting a shot off. You conclude you’re watching the original version.

Of course, up until that point, the movies are exactly the same. (Rather, let’s just *say* they are the same. I haven’t actually checked this.) You wouldn’t conclude, when you got to that scene, that two movies were created from one, would you? There were two movies all along; watching Han shoot first just told you *which movie you were watching*. A physicist who performs a Stern-Gerlach experiment doesn’t split the universe in two; doesn’t create a whole universe; instead she has gained some new information: “Oh, so *that’s* what universe I am in.” No new physics of universe-creation is needed, and we need not violate conservation of energy.

**2. ****The existence of a multiverse is a postulate of a strange kind of quantum mechanics.**

There is a formulation of quantum mechanics often called universally valid quantum mechanics, which was first described by Hugh Everett III in 1957. It involves (see this for details) just one postulate: isolated systems evolve according to the Schrödinger equation. That’s it. A multiverse is a *prediction* of this postulate, not a postulate in and of itself. So if you believe that isolated systems evolve according to the Schrödinger equation, you will be led to the MWI, unless you invent new postulates to make yourself feel better (see #3).

**3. ****The Many-worlds Interpretation has a lot of baggage.**

This obviously depends upon what you mean by baggage, but the claim is often made that the MWI is horribly antithetical to Occam’s razor. That is, how could anyone seriously believe that countless billions upon billions of universes exist, when believing in one universe is much, much simpler?

If you feel this way, I have two responses.

One: how can you believe that countless billions upon billions of *stars* exist, when believing in just one star is much, much simpler? Shouldn’t you be Earth-centric, and call for Galileo’s excommunication? Or what about the integers? Mathematicians claim that there are an infinite number of them, but infinity is too hard to fathom, so why don’t you just say that there are a lot of integers, but that there is only a *finite* number of them? There. I bet you feel better.

Two: like it or not, Occam’s razor cuts both ways, and can be used to *defend* MWI. The idea is whether Occam’s razor applies to the number of universes, or the number of postulates in your physical theory. As Max Tegmark pointed out, universally valid quantum mechanics leads to a multiverse *as a consequence*. In order to get the Copenhagen interpretation (for years, the most popular flavor of quantum mechanics) and rid yourself of those pesky many worlds, you have to take Everett’s quantum mechanics *and add one additional postulate*: that wave functions collapse according to random and ultimately unknowable criteria. That is, the MWI is *simpler* in the number of postulates required. As Tegmark put it, which way you use Occam’s razor depends upon whether you prefer many worlds, or many words.

**4. ****The Many-worlds Interpretation is not falsifiable and therefore not science.**

The jury’s still out on this one, but many people (including David Deutsch) think that the MWI is misnamed: that it is actually a theory in and of itself, and that it is falsifiable. I haven’t made up my mind on this.

I tend towards Tegmark’s view that MWI is an untestable prediction of quantum mechanics, which *is* testable. Because we take quantum mechanics seriously, we have to take one of its children (MWI) seriously. As Tegmark says, it’s like black holes. We can never see inside a black hole, so what goes on in there is never falsifiable; yet we take black holes seriously and call black holes “science” because general relativity (the theory that predicts black holes) is so successful.

**5. The ****Many-worlds Interpretation is fringe science and only believed by kooks.**

These kooks include Stephen Hawking (who said the MWI was “trivially true”), David Deutsch, Bryce DeWitt, and Max Tegmark, among others. They also include a sizable number of theoretical physicists working today. In 1995 one poll (published in the French periodical *Sciences et Avenir* in January 1998) showed that 58% favored MWI; see also this informal 1997 poll.

Maybe we *are* all kooks. But there are a lot of us, and the number is growing.

[Note: my book Why Is There Anything? is now available for download on the Kindle! This book examines the many-worlds interpretation from a philosophical perspective.]

on March 23, 2013 at 7:20 PM |Blogging Anniversary with Post No. 63. Equal to: 42 Plus (42 Divided by 2) | Theory and Practice of Trying to Combine Just Anything[…] the wave function is collapsing into right now (This is a sloppy statement – please read Matthew’s post on Many Worlds Theory) it is definitely a a first-class blog on explaining physics. Matthew proves that you can use […]

on March 11, 2014 at 1:14 PM |If Many-Worlds Theory Is Correct, Then Why Don’t We Ask Ourselves To Prove it? | Third News[…] Misconceptions about the Many-worlds Interpretation of quantum mechanics The Top 5 Misconceptions About the Many-worlds Interpretation (MWI) of Quantum Mechanics […]

on November 1, 2014 at 2:20 AM |Shobhit Gupta (@shbt_lightyear)Bryce DeWitt and Graham have stated – ” By the temporal development of dynamical variables the state vector decomposes naturally into orthogonal vectors, reflecting a continuous SPLITTING of the universe into a multitude of mutually unobservable but equally real world “.

Their account is quite contrary to your claim- “It is much better to imagine that all of the possible universes already exist”.

on November 1, 2014 at 9:27 AMMatthew RaveThat language is out-dated, old, and unfortunate.

on November 2, 2014 at 6:15 AM |Shobhit Gupta (@shbt_lightyear)If we consider all the branches of universe as co-existent then does the process of decoherence (which was supposed to be responsible for ‘splitting’) merely results in gain of information in a particular part of the wave function?

Though it is practically impossible, in principle we can reverse the environmental decoherence to regain the superposed state of a quantum system. But that would amount to ‘fusion’ of once different branches of wave function after the process.