One of the most common criticisms of the many-worlds interpretation (MWI) of quantum mechanics is that it is absurdly complicated, and therefore violates Occam’s razor. Most people’s first reaction, on hearing of MWI, think that the theory is (to quote Martin Gardner) “bizarre”, “monstrous”, “fantastic”, “radical”, “appalling”, “nonsense”, “frivolous”, and “low”. And many people seem to think that theorists who ascribe to MWI have their heads in the clouds to believe such nonsense. MWI seems to be taken, in fact, as evidence that physics has lost its way—as if (supposedly) blind belief in such frivolity is indicative of a philosophical rot that pervades theorists like me.
There are so many refutations of such criticisms that I don’t know where to start. First of all, although MWI is popular, it is by no means canon, and I daresay that a majority of physicists reject it still. So there! We’re not all sheep. Still, MWI has become almost mainstream (especially with cosmologists) so maybe it’s the cosmologists and the ivory-tower theorists who should be singled out for criticism?
People who think this have probably never met a theoretical physicist before in their life. Getting such people to agree is like herding cats; every theory one puts forth (in a journal article or in a conference talk) is debated, criticized—dare I say, attacked. And that is as it should be. There is not, contrary to popular belief, some holy scripture that every theorist quotes verbatim. We are all different, and have basically come to interpret quantum mechanics in our own personal way…not at the behest of some lord on high.
How do I know this? Because I was never taught about interpretations of quantum mechanics. Ever. Everything I know about such things, I learned on my own since graduation. Thinking of taking a quantum mechanics class at your local university? Guess what: they will probably not talk about MWI, or the Copenhagen interpretation, or Schrodinger’s f***ing cat. Why not? Because those are philosophy topics, not physics. You can do quantum mechanics without ever interpreting a single thing. There’s no crying in baseball, and there’s no philosophy in quantum mechanics. It is a purely mathematical theory, that undeniably works, and most people just leave it at that. The idea that thousands of physicists subscribe to one particular world-view just because they constitute a single monolithic conformist society is ludicrous. Invite a physicist to lunch if you don’t believe me.
But I still haven’t addressed the idea that MWI is obviously absurd. It is absurd, right? I mean, come on!
But wait. Let’s think back to the Copernican revolution. It’s obvious that the Earth is stationary, no? I bet people thought that Copernicus and Galileo and their ilk were bizarre, monstrous, fantastic, radical, appalling, nonsensical, frivolous, and low.
And what about the idea that there are billions and billions of galaxies, each with billions and billions of stars? We forget now, but this idea was radical when first presented and wasn’t settled until the 1920’s. Why are we OK with a multiplicity of stars, but not a multiplicity of universes? Why aren’t people complaining about the absurd notion (fact) that there are more stars in the observable universe than there are grains of sand on Earth’s beaches?
So, Occam’s razor. MWI just seems to have too much baggage, right? For a lot of people MWI is too high a cost to bear to have a mathematically simple interpretation of quantum mechanics. And let’s be clear: MWI is a simpler theory than (say) the Copenhagen interpretation (CI). For you can start with three postulates, and add a fourth about wave-function collapse, and you get CI. Or you can start with just three, and say nothing of wave-function collapse, and you get MWI. Which interpretation seems simpler now? MWI is a consequence of accepting the three basic postulates of quantum mechanics. If you don’t like that, then you must introduce a fourth postulate ex nihilo to make yourself feel better.
But wait! you say. 10100 universes doesn’t seem simpler. It’s a huge number! It’s ridiculous!
OK. You wanna go there? I’ll turn the argument around. By that rationale, you probably believe that there are only a finite number of integers, because any finite number is simpler than infinity. There. That makes sense, right?
The truth is that an infinite set is often simpler than a single member of that set. Take the natural numbers. I can write a computer program in BASIC that writes every natural number. Here it is:
10 x=1
20 PRINT x
30 x=x+1
40 GOTO 20
On the other hand, if I want to print out the number
5679200359662711389685023885761799
then my computer program is longer:
10 PRINT “5679200359662711389685023885761799”
Count the keystrokes. The second program requires more typing. And note that the first (simpler) program will eventually print this number—the long arbitrary program is “contained” within the first.
In information theory, the information “content” of something is related to its algorithmic complexity—roughly speaking, how easy it is to write a computer program that “specifies” the object. By that measure, “all the natural numbers” is a simpler concept than the number
5679200359662711389685023885761799
Similarly, “all possible universes” is a much simpler concept than one specific arbitrary universe. You want to recreate this universe? Good luck…you’ll have to specify the position and momentum of every particle in the universe. That’s a long computer program. However, if you just say “create all possible universes” then eventually this one will pop up…
Do I believe in the MWI? Yes. Why? It’s not because I was indoctrinated into such belief; I don’t think a single professor in graduate school ever mentioned MWI. It’s because I’ve looked at the evidence over a number of years, and (tentatively) decided that it fits the data best. That is the only reason to ever believe something, ever. It fits the data best. But I stress that my conclusion is tentative because, hey, it’s science. There is no dogma. There is just stuff that we are 99.44% sure of.
Like evolution by natural selection, or heliocentrism, or the existence of ghosts. I mean, seeing’s believing, right?
[Note: more Americans believe in ghosts than evolution. Sigh.]
If you enjoyed this post, you may also enjoy my book Why Is There Anything? which is available for the Kindle on Amazon.com.
I am also currently collaborating on a multi-volume novel of speculative hard science fiction and futuristic deep-space horror called Sargasso Nova. Publication of the first installment will be January 2015; further details will be released on Facebook, Twitter, or via email: SargassoNova (at) gmail.com.
“it fits the data best. That is the only reason to ever believe something, ever. It fits the data best.”
Matt, do you have a post on hand which details how the data fits best? This subject comes up a lot on my travels and my tool belt is treacherously bare in the matter.
The short answer is that MWI solves, basically, every “problem” in quantum mechanics. If you don’t want to admit the possibility of multiple universes, then (philosophically) you’re backed into a corner since you instead have to believe in wave-function collapse, spooky action at a distance, bizarre probability affects, even stranger “superposition” states such as in Schrodinger’s cat, and the supremacy of chance in our universe. Maybe I’ll post on this soon. Basically, Bell’s theorem says that either God plays dice, you can teleport across the universe instantly, or there are multiple universes. One of these MUST be true. I find the third the least strange, and the one with the least amount of (mathematical) hoops to jump through.
Spooky action can’t also exist in MWI? (Pity, i was quite fond of this idea)
I haven’t written code for a while, but in your example above, if from Line 40 you “GOTO 10” wouldn’t x be reset to 1 and you would end up printing a whole lot of 1s? I would have intuitively thought that 40 should say “GOTO 20”
Pardon me for my very rusty skills…..
The post by the way is fantastic!
Yup, typo…I’ll fix it
Reblogged this on Ignostic Atheist.
Thanks!
I love it when someone covers the very questions I’m asking.
David Deutsch talks quite a lot about MWI in his book “The beginning of infinity” but I didn’t understand what he was saying, despite having studied QM at degree level. He seemed to be implying that MWI could be tested. Is that your understanding?
There’s two answers. The first is I don’t know; at least, I am not sure it can be tested in the manner Deutsch implies. The second is that any test of QM is really a test for MWI. It’s analogous to black holes. We accept General Relativity, because there’s so much evidence for it; that forces us to accept black holes since they are a consequence of GR. Similarly we accept QM, because there’s so much evidence for it; QM implies many-worlds, so we should accept them to. If QM were ever invalidated in the lab then MWI would be finished. MWI is a consequence of accepting all the QM that there’s evidence for; it is not a separate theory but a consequence of a theory (I think I’m paraphrasing Tegmark here).
Deutsch also points out that quantum interference is really unexplained except through the lens of MWI. We can exploit this by making a quantum computer. If the quantum computer works, you have basically proven MWI since you’ve harnessed the computer power of countless other parallel computers.
Well, quantum computers do work, but that doesn’t prove MWI. It proves QM.
What I’m getting at is, is there a prediction that MWI makes that other interpretations of QM don’t?
Yes. That quantum computers would work if built (none has ever been built to my knowledge). No other interpretation explains quantum interference, or quantum computers, at all. Specifically: Shor’s algorithm should be able to perform 10^100 calculations in a microsecond. If there are NOT parallel universes then we’re hard pressed to explain how this works at all.
Note that I’ve been lazy in my use of “theory” and “interpretation”. MWI is an “interpretation” of QM. QM is iron-clad; no one doubts it. MWI is one way for meager humans to “interpret” what is happening. Shor’s algorithm has no satisfactory explanation apart from MWI.
As for falsifiability, anything that falsifies QM falsifies MWI as well. Typical Copenhagen QM involves *one more postulate*, so since that is a more complicated theory, the burden of proof is on the Copenhagenists to justify their fourth axiom (about wave-function collapse).
Well, D-Wave have sold two quantum computers and many other researchers have demonstrated working quantum calculations, so your statement puzzles me.
Quantum computers rely on the principle of quantum superposition. A qubit is in two states at once. That’s consistent with any interpretation of QM.
And the Copenhagen interpretation is surely the idea that human-initiated measurements cause the collapse of the wavefunction? That’s clearly untenable, and quantum computers demonstrate that by spontaneously moving from mixed states to pure states, thus terminating their algorithm.
From Wikipedia with comments added: “This approach was liked by investors more than by some academic critics [including me], who said that D-Wave had not yet sufficiently demonstrated that they really had a quantum computer. Such criticism softened once D-Wave published a paper in Nature giving details, which critics said proved that the company’s chips did have some of the quantum mechanical properties needed for quantum computing. [I agree that it has SOME properties of a quantum computer but in my mind we’re not there yet.]
We’re talking about semantics I think. You say the quantum computer works using quantum superposition, I say it uses the parallel processing capability of many worlds. Those are mathematically equivalent statements. The only difference is how we INTERPRET this mathematical framework. Which makes more sense to you: that objects can exist in superposition of states, like the alive/dead cat, OR that multiple universes exist? Many people find the second statement less strange.
Does MWI not explain “anything”. And once you have a theory of “anything” (instead of a theory of everything) aren’t you making unfalsifiable claims?
I don’t know enough QM to have an actual opinion. Just curious.
I don’t think I understand.
Sorry.
Isn’t the MWI a model that could explain ‘anything’, regardless of whether we observe it or not. Is there an idea (that is logically possible) which the MWI couldn’t explain?
If not, it is different from other scientific models where there are things that could falsify them.
Your wrote above “Basically, Bell’s theorem says that either God plays dice, you can teleport across the universe instantly, or there are multiple universes. One of these MUST be true.”
Pardon my ignorance, but how likely is it that Bell’s Theorem is false and none of the three alternatives are true?
Bell’s inequality has been demonstrated in the lab. So it’s true.
If a theorem has one of three bizarre consequences, there might be something wrong with the theorem. In this case, the theorem doesn’t seem to have been demonstrated to everyone’s satisfaction yet:
From Abner Shimony’s 2009 article “Bell’s Theorem” at the online Stanford Encyclopedia of Philosophy:
Bell’s Theorem is the collective name for a family of results, all showing the impossibility of a Local Realistic interpretation of quantum mechanics. …. The incompatibility of Local Realistic Theories with Quantum Mechanics permits adjudication by experiments, some of which are described here. Most of the dozens of experiments performed so far have favored Quantum Mechanics, but not decisively because of the “detection loophole” or the “communication loophole.” The latter has been nearly decisively blocked by a recent experiment and there is a good prospect for blocking the former.
From a brief 2013 article at phys.org:
A new grant will allow [Dr. Le Luo] to work with researchers at the University of Science and Technology in China, Harvard University and several European universities to conduct a “loophole-free test” of the Bell Inequalities—one of the most fundamental questions in quantum mechanics…. Luo said the new collaboration would, for the first time, be using several different quantum systems—including photons, ions, quantum dots and solid-state ensembles—to test the theory across large distances and hopefully eliminate all possible loopholes. “It is very important that such Bell Inequality tests be implemented at large distances, across distances such as tens of miles, so that our measurement can be loophole-free by eliminating the possibility the two objects can communicate with one another,” Luo said.
http://phys.org/news/2013-02-physics-effort-quantum-theory.html#jCp
Not to everyone’s satisfaction…just 99% of physicists.
Matthew, I think you are cheating a little by introducing Schrodinger’s Cat. I don’t know any physicist who believes that Schrodinger’s Cat can be both alive and dead. The cat is a red, um, herring.
A quantum computer works by accessing multiple quantum states simultaneously. Whether those quantum states exist in this universe or in parallel universes is open for debate. You could equally argue that those parallel universes are mere mathematical constructs.
Or is there some distinction that can be measured???? If so, what?
If a superposition of states “exists in this universe” (whatever that means) then Schrodinger’s cat can be alive/dead. You’re right in saying that no one believes this; what they believe instead is that (a) there are other universes, which solves the problem, or (b) there are strange unmotivated rules that we have to introduce to get rid of the cat, such as wave collapse or quantum consciousness. (I’ll point out that Copenhagenists talk of observers as fundamental without ever defining an observer).
But ultimately, you might be right: you can’t “prove” an interpretation; you just accept the simplest. And MWI is the simplest explanation in terms of information theory and the number of required postulates.
Well I didn’t claim that MWI is wrong. I just asked if it can be tested.
Reblogged this on oogenhand.
Regarding the two computer programs. The one that lists the integers will only print out the extremely large integer in question after an extremely long period of time. The other one prints it instantly.
Also, the listing program doesn’t know when to stop.
Time is irrelevant with regard to information theory.
So… when are you going to write about the preferred basis / factorization problem that has been raised for decades? Up until recently people just spewed bullshit about “decoherence, decoherence and more decoherence”. But even the biggest defenders of MWI (Max Tegmark and David Wallace) has finally succumbed to the problem and are now desperately trying to solve it by giving consciousness a special treatment
I am unaware of any Tegmark arguments involving “consciousness”. And to call decoherence bullshit is to dismiss the consensus opinion that decoherence describes reality, even among MWI opponents. The fact that there are still wrinkles to work out is not a proof that an interpretation is wrong; it means more work is to be done. Every other interpretation has baggage, too…in my opinion, more baggage.
Matt, i finished your book last week, it was great, truly great, but have only now had the time to sit down and pen the review. It should be up. Hope you don’t mind, but i included an anecdote you posted on my blog about an experiment you devised at age 12.
Again, great work. You should, deservedly so, feel well-proud.
Any thoughts on this idea of “many interacting worlds”?
View at Medium.com
I’ll have to look at the source paper, but I’ve always had a hunch that reality was like this…so I am encouraged…
The arguments here seem to support the cosmological version of a multiverse — that our universe is just far away from all the other universes. And you seem to be using this multiverse interchangeably with the quantum mechanical many worlds multiverse, which I always took to be very different. Now I come across this post by Sean Carroll (originally from Tegmark and collaborators), suggesting these two ideas might be the same: http://bit.ly/1eT8ePr . I suppose you too think the two senses of multiverse might be the same, and using similar reasoning?
Yes.
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