[ExI] Criticisms of Many Worlds Interpretation (MWI)

[Stuart LaForge]

Forwarded to the list for John Clark:

-------- Original Message --------
Subject: [ExI] Criticisms of Many Worlds Interpretation (MWI)
Date: 2023-09-14 10:22
  From: BillK via extropy-chat <extrop... at lists.extropy.org>
To: Extropy Chat <extrop... at lists.extropy.org>
Cc: BillK <pha... at gmail.com>


> One of the main criticisms of the MWI is its lack of testability and 
> falsifiability. Since the theory posits the existence of an 
> infinitenumber of parallel universes,

That is incorrect. MWI does not postulate the existence of an infinite 
number of parallel universes, instead it deduces they must exist by 
using the one and only assumption that it does make, namely that 
Schrodinger's equation means what it says. Hugh Everett didn't wake up 
one morning and say to himself, gee it would be cool if there were a lot 
of different universes, I think I'll invent a theory that has an 
infinite number of them. Instead he asked himself what would happen if 
he took Schrodinger's Equation seriously and assumed it really meant 
what it said, and when he did that those other universes just popped up. 
The only way to get rid of them is to change Schrodinger's equation as 
GRW has done, or to do what Copenhagen has done and say that for some 
vaguely defined reason a vaguely defined thing called an "observer" 
doesn't need to obey Schrodinger's Equation.  Many Worlds is just bare 
bones, no nonsense Quantum Mechanics with no silly bells and whistles 
pasted on. The only assumption Many Worlds makes is that the mathematics 
of the equation means what it says.


> each with different outcomes, it becomes impossible to experimentally 
> verify or disprove this claim.

Many Worlds logically explains a lot of stuff and yes it seems bizarre 
to us, but any successful quantum interpretation is bound to be bizarre, 
there's just no way quantum mechanics will ever seem intuitive, it will 
always be odd but odd is not the same as paradoxical. And Many Worlds is 
consistent with all of the correct predictions of quantum mechanics . 
Yes some predictions it makes have not yet been proven but it is hardly 
alone in that. For example: we can't see things further away than 13.8 
billion light years because there hasn't been enough time for light to 
reach us, and as near as we can tell on the largest scale space is flat, 
if there is any curvature at all it must be less than 0.4% so you'd need 
to go over 200 times 13.8 billion light years to form an unbounded 
sphere; but does anyone really believe there is nothing beyond 13.8 
billion light years? If so then then they must also believe the Earth 
really is the center of the universe and 13.8 billion light years away 
there is a wall with absolutely positively NOTHING on the other side.

And Many Worlds is falsifiable. For example, right now there are 
experiments underway in an attempt to prove that the GRW theory of 
objective quantum wave collapse makes predictions that Many Worlds does 
not, if they are successful it will prove that Everett was dead wrong, 
it's as simple as that.  GRW claims that Schrodinger's equation is 
incomplete and that another very complex term needs to be added to it 
because it's the only way they could think of to get rid of all those 
worlds that for some reason they dislike, there was simply no other 
reason to add that extra term. With this new term Schrodinger's equation 
is no longer completely deterministic because a random element is added 
such that the larger the wave function is (the more particles it has) 
the more likely the quantum wave function will objectively collapse. 
They carefully tuned their very complex extra term inserted into 
Schrödinger's equation in just such a way that, because an individual 
electron is so small the probability of you being able to observe one 
objectively collapse is almost but not quite zero; but the probability 
of you NOT observing something as large as a baseball NOT collapsing is 
also almost, but not quite, zero. Despite heroic efforts. up to the 
present day nobody has found a speck of experimental evidence in support 
of the GRW theory of objective quantum wave collapse, and until and 
unless they do Many Worlds must be the preferred theory according to 
Occam's razor because it makes fewer assumptions, it has no need to 
complicate matters by adding that extra term to Schrodinger's equation.

But GRW is not the only or even the most popular competitor to Many 
Worlds, that honor would have to go to the Copenhagen interpretation, 
and there is certainly no way to falsify that, but back in 1986 in his 
book "The Ghost in the Atom" David Deutsch proposed another way to 
falsify Everett's Many Worlds; the experiment would be difficult to 
perform but Deutsch argues that is not Many Worlds fault, the reason 
it's so difficult is that the conventional view says conscious observers 
obey different laws of physics, Many Worlds says they do not, so to test 
who's right we need a mind that uses quantum properties.

In Deutsch's experiment, to prove or disprove the existence of many 
worlds other than this one, a conscious quantum computer shoots 
electrons at a metal plate that has 2 small slits in it. It does this 
one at a time. The quantum computer has detectors near each slit so it 
knows which slit the various electrons went through. The quantum mind 
now signs a document for each and every electron saying it has observed 
the electron and knows which slit it went through. It is very important 
that the document does NOT say which slit the electron went through, it 
only says that it went through one and only one slit and the mind has 
knowledge of which one. Now just before the electron hits the plate the 
mind uses quantum erasure to completely destroy the memory of what slits 
the electrons went through, but all other memories including all the 
documents remain undamaged. After the document is signed the electron 
continues on its way and hits the photographic plate. Then after 
thousands of electrons have been observed and all which-way information 
has been erased, develop the photographic plate and look at it. If you 
see interference bands then the Many World interpretation is correct. If 
you do not see interference bands then there are no worlds but this one 
and the conventional interpretation is correct.

Deutsch is saying that in the Copenhagen interpretation when the results 
of a measurement enters the consciousness of an observer the wave 
function collapses, in effect all the universes except one disappear 
without a trace so you get no interference. In the Many Worlds model all 
the other worlds will converge back into one universe when the electrons 
hit the photographic film because the two universes will no longer be 
different (even though they had different histories), but their 
influence will still be felt. In the merged universe you'll see 
indications that the electron went through slot X only and indications 
that it went through slot Y only, and that's what causes interference. I 
know that what I said in the above is a fair representation of what 
Deutsch was saying because some years ago I wrote to him about this and 
he said it was an accurate paraphrase.

A theory is not judged on the basis of what predictions it makes that 
have neither been confirmed nor falsified experimentally,  instead they 
are judged by how well they conform to experiments that HAVE been 
performed, and in Many Worlds case it conforms to every physics 
experiment that has ever been made up to the present day.

> Another criticism revolves around the issue of probability. In the MWI, 
> probabilities are assigned to different outcomes based on their 
> relative frequency across all possible universes. However, this 
> approach raises questions about how probabilities can be meaningfully 
> defined when there are an infinite number of universes. Critics argue 
> that without a clear framework for assigning probabilities, the MWI 
> fails to provide a satisfactory explanation for observed phenomena.

You're talking about deriving the Born Rule, that the probability of 
finding a particle at point X  is proportional to the square of the 
amplitude of the particle's quantum wave function given by Schrodinger's 
Equation. First of all it has been proven mathematically that if you 
want unitarity, that is all the probabilities are positive real numbers, 
and all of them add up to exactly 1 (because *something* is certain to 
happen) then the Born Rule is the only one that works. So the real 
question is, since the quantum wave function is completely deterministic 
why do we need probability at all? I'll explain that with the help of 
Schrödinger's cat. The cat has been in a sealed box for one hour with 
the radioactive atom with a half life of one hour, if the atom has 
decayed an automatic mechanism will release poison gas and kill the cat, 
if the atom has decayed then the cat will be fine. I know that the 
world, and me with it, has split and that in one world the cat is dead 
and when I open the box I will find the cat is dead, and the other world 
the cat is alive and when I open the box I will find the cat is alive, 
BUT until I open that box and look inside I would have no way of knowing 
which world I'm in.

But perhaps you're wondering how if there are infinitely many worlds how 
can more complex probabilities like 1/6 exist. I cut a rope that is 7 
inches long, which contains an infinite number of points that is even 
greater than the infinity of the integers, into 1 piece that is 2 inches 
long, and contains a continuous infinite number of points, and into 5 
pieces that are 1 inch long, which also contains a continuous infinite 
number of points. I place all 6 pieces into a hat, close my eyes and 
pick out one piece at random. I can say with confidence that there are 5 
chances out of 6 that I will pick out a string that is 1 inch long, and 
one chance out of six I will pick out a string that is 2 inches long, 
this despite the fact that all the strings have the same infinite number 
of points. By way of analogy we can say that all Everett type worlds 
that are so similar that they don't make a substantial difference to 
your subjective experience are part of the same string regardless of how 
many worlds there are, even an infinite number of them.

> Furthermore, the MWI faces challenges in explaining macroscopic 
> observations and classical behavior. While it may be able to account 
> for quantum phenomena at the microscopic level, it struggles to explain 
> why we observe a classical world with definite outcomes rather than a 
> superposition of possibilities.

If two worlds split because a tiny force made them very very slightly 
different in the submicroscopic realm then it is not unrealistic to 
think there is a reasonable possibility that things could change again 
and become identical again, but once the change reaches the macroscopic 
level the probability of ever going back to the original state is so 
ridiculously small it can be ignored.

> Additionally, some critics question the philosophical implications of 
> the MWI. The theory suggests that every possible outcome exists in a 
> separate universe, leading to an infinite number of parallel realities.

If you assume that space is infinite, or even just very very large, 
those philosophical implications remain even if Hugh Everett turns out 
to be completely wrong.  Your body contains a finite number of atoms so 
it can only be in a finite number of quantum states, it follows that if 
you traveled out far enough into space you would find your exact 
doppelgänger that was in the same quantum state that you are in, 
physicist MaxTegmark has even calculated how many meters you'd have to 
travel before you had a good chance of finding him, it turns out to be a 
1 followed by a hundred thousand trillion trillion zeros. That's big but 
no nearer to being infinite than the number 1 is.

> This idea raises philosophical concerns about personal identity and 
> consciousness. Critics argue that it is difficult to reconcile our 
> subjective experience with the notion that there are countless versions 
> of ourselves in different universes.

If there are an infinite number of worlds and an infinite number of 
subjective states then you could make a one to one correspondence 
between worlds and subjective states; and you could still do so if the 
number of worlds and the number of states is not infinite but just an 
astronomically large number to an astronomically large power.

> Moreover, the MWI has been criticized for its complexity and lack of 
> simplicity. Occam's razor,

Occam's razor is about the simplicity of assumptions not the simplicity 
of outcomes. The only assumption that MWI makes is that the world is 
described by a quantum state, which is an element in vector space, and 
the quantum state evolves through time in accordance with the 
Schrödinger equation. That's it.  All those other worlds are derived 
(not assumed) from that simple assumption. In order to get rid of those 
many worlds other interpretations must monkey around with Schrodinger's 
equation and make it non-deterministic or add additional assumptions, 
such as vaguely defined stuff about collapsing the wave function and an 
insistence that observers don't obey the same rules of quantum mechanics 
that none observers do even though both are made up of the same sort of 
particles, all of which obey the same laws of quantum mechanics.

John K Clark