
The Many-Worlds Interpretation Has Many Problems - ykm
https://www.quantamagazine.org/why-the-many-worlds-interpretation-of-quantum-mechanics-has-many-problems-20181018/
======
ZhuanXia
>What can you do with this power to generate worlds and selves? You could
become a billionaire by playing quantum Russian roulette.

Quantum suicide isn't exploitable, for reasons I went over in a thread on
reddit:

>One thing about these quantum immortality (or modal realism, dust theory,
etc.) arguments is they imply one should expect to end up in those proportion
of worlds that have the highest measure in which "you" exist. And the most
probable actions you should expect to remember are the highest-measure actions
that leads to the perpetuation of your conscious experience.

>In this way quantum suicide is not really exploitable, as the vast majority
of "your" measure will be in worlds where you find quantum suicide to be
unappealing. Even failed attempts will be of much smaller measure then those
in which you never try. So stories like Permutation City are sort of the
equivalent of stories in which magic is the result of quantum fluctuations.
They don't violate any laws of physics but they focus on such a ridiculously
myopic slice of possibility space that it is a little absurd, even when you
condition on the continuation of conscious experience.

Even if it were exploitable, this is not a reason to doubt many worlds, any
more than time dilation is a reason to doubt relativity.

This great comment on that piece by Sengachi is a must read:
[http://disq.us/p/1wnl71q](http://disq.us/p/1wnl71q)

~~~
tbabb
I think there is a still more straightforward and robust argument against
quantum immortality: The same reasoning predicts you can never fall asleep.

If futures where you are not conscious are inaccessible by the anthropic
principle, then dreamless sleep cannot be in your future.

Most sleep is dreamless. At 4:00am this morning I will almost certainly be in
this state. But being unconscious is not an experience I can have, so by the
reasoning of quantum immortality, I should expect with certainty to find
myself awake at 4:00am if there are any Everett universes where this is the
case. And of course the same argument applies to every other moment of my
entire life, and yet somehow I haven't been trapped in a state of wakefulness
since the day I was born...

(If sleep is breaking the rules, the substitute it in with "temporary death"
if you prefer. Why I am I allowed to "fork into" the branch where I die as
long as I wake up later? How does the information about my eventual revival
make it to the branching point, earlier in time? Or does the revived version
of me exist with measure zero? These seem plain absurdities to me).

Think carefully about why many-worlds doesn't prevent you from sleeping. I
think this leads to the correct answer.

I think of it in terms of a phase space diagram. You are conscious in some
parts of phase space. Those regions of phase space have certain measures. You
should expect to find yourself in parts of phase space with large measures,
and not in the parts with small measures. Like this: [1]

In the diagram, increasing purple is "increasing probability consciousness."
You should expect to find yourself in the purple area, not 10,000 years off
the top of the diagram, deep in a white region. Quantum immortality supposes
the latter, and that should be a self-evidently bad hypothesis.

[1] [https://i.imgur.com/ETOqKs9.png](https://i.imgur.com/ETOqKs9.png)

~~~
comex
I don't think that's accurate. The many worlds interpretation is fundamentally
an interpretation of something unobservable; if there were a way for you to
observe it (by noticing you haven't been able to sleep) and 'report back' to
all/most universes, it wouldn't be an interpretation anymore. (If you did a
quantum suicide experiment, you might be able to 'report back' to the subset
of universes where you still exist, like in Permutation City. I'm not sure
that would actually be a valid demonstration of many worlds – the classical
equivalent is just "something unlikely happened", and you can justify that
with the anthropic principle. But in any case, that's very different from
reporting back to all universes.)

As for the diagram… Even in a classical universe (or just a non-many-worlds
one), if you set a mechanism to have a 50% chance of killing you, you should
expect to 'find yourself' alive afterwards – in the sense that if you 'find
yourself' anything, that anything can only be alive. It's just that you have
an equal chance of not having any further conscious experience. That doesn't
apply to sleep or even temporary death, since the resulting universe would
still be observable by you at some point in the future. Now, even in a
classical universe, there is a possible set of values under which
participating in a 'suicide lottery' could be rational! That would be, "I
fundamentally _don 't care_ whether I exist or not, but I do prefer existing
while being rich to existing while being poor". The many-worlds interpretation
just lets you amend that to: "I don't care whether I exist or not in 'my'
future, as long as someone else identical to me exists in an alternate
universe." There is no reason you must or should care about someone
benefitting in an inaccessible universe just because they're (very) similar to
you; but there is also no reason you must not. To whose benefit you strive is
an arbitrary choice, after all.

~~~
liberte82
One day when we're able to use quantum technology to clone ourselves, we'll be
able to test the theory. You split your consciousness into 1000 clones of
yourself and kill 999 of them, do "you" always seem to end up in the one that
survived?

~~~
kgwgk
[https://en.m.wikipedia.org/wiki/No-
cloning_theorem](https://en.m.wikipedia.org/wiki/No-cloning_theorem)

~~~
liberte82
Thanks!

------
wildermuthn
Quantum suicide is a fascinating thought experiment. If consciousness is a
property of a certain type of brain (or calculation?), then the concept of
‘’’me” is just a variation of the anthropic principle: I believe my subjective
experience to be something unique, but there’s nothing unique or special about
a human brain and the conscious experiences it produces.

So even if there are billions of worlds, there aren’t billions of “me”s out
there. There are simply billions of consciousnesses which share common
memories and thought patterns. Each variation would believe itself to be the
true “me”, when in fact they are all wrong: there is no “me”. Just the
consciousnesses that all those brains create.

So in quantum suicide, there is no transfer of any sort. One consciousness is
simply extinguished forever. The other continues.

This aligns somewhat with Buddhist teachings about the illusion of “self”. I
don’t believe in the supernatural, but one could consider the idea of
reincarnation as merely the fact that there are not instances of
consciousness, but instead manifestations of one phenomenon. Which could
possibly mean that if you are a conscious being reading this comment at this
moment, you can rest assured that your death will not stop “you” from
continuing to experience subjective experiences as another person, in another
place, at another time. You just won’t realize it, because you won’t be “you”
anymore.

Or maybe it’s all nonsense.

I have a hunch there is a variation of quantum suicide that would be less
lethal, but no less measurable.

~~~
JProthero
>One consciousness is simply extinguished forever. The other continues.

If there is one instance of a widely-distributed program installed on a
particular computer, and the instance of the program on that particular
computer is deleted, but the program continues to exist in identical form on
many other computers, then the program continues to exist in spite of the fact
that one of its instances has been deleted.

If consciousness is thought of as a process inextricably tied to a particular
physical substrate, then our own consciousnesses are constantly being
extinguished, and we all die countless 'deaths' every day as the matter in our
brains is recycled and replaced.

If, on the other hand, consciousness is viewed as an information process that
can be realised on any suitable physical substrate, then extinguishing one
instance of consciousness in one particular universe by means of quantum
suicide doesn't necessarily mean that that particular consciousness is
extinguished forever, as long as an identical instance, or instances, of it
continue to exist somewhere in the wavefunction. The Many Worlds
Interpretation suggests there may be an infinity of such instances.

There is a tendency to think of these instances as copies, and to think of
each such copy of a consciousness as a separate doppelganger existing in some
unreachable reality. But if the instances are truly identical in every
respect, then Leibniz' principle of the identity of indiscernibles [1]
suggests that it might not be right to think of them as being separate: in
some deep sense, they're all actually the same single thing.

The philosopher Derek Parfit developed a now-famous thought experiment in the
phiolosophy of identity [2] in which he encouraged people to think about a
scenario in which a malfunctioning teleporter creates two identical copies of
a person, one of which must then be destroyed by the machine. Obviously we
tend to sympathise with the 'copy' faced with imminent destruction, and that
might lead us to reject the idea that the continuation of a physically
identical copy of our self is a sufficient condition for survival.

There is a problem with this thought experiment though, in my view, and
quantum suicide is in a sense a type of 'fix' for it. If there is a situation
in which two copies of a person exist, and one copy is about to be destroyed,
then the two copies are necessarily no longer 'indiscernible' in Leibniz'
sense. Parfit's example is actually a striking case of indiscernibility,
because at the point at which one of the copies is about to be destroyed, the
two copies actually have some very different memories and thoughts. This
version of Parfit's thought experiment therefore really involves two similar,
but not identical, people, one of whom is about to be killed.

The quantum suicide thought experiment, in contrast, necessarily ensures that
no conscious entity ever experiences its own imminent death in the same manner
as Parfit's version, because the 'suicide' is designed to take place faster
than the brain can process a thought. The other major difference is of course
that the 'copies' that survive the suicide are not created by a teleporter,
but are a necessary feature of the wavefunction.

An alternative to saying that one consciousness is extinguished forever in
quantum suicide would be to say that the single consciousness existing across
the 'copies' in the wavefunction no longer has conscious access to events in
the universes in which the suicide occurs, but survives in, and continues to
have conscious access to, the universes where the suicide does not take place.

It only makes sense to think of one consciousness as having been
'extinguished' if that consciousness is thought of as being inextricably
linked to one particular physical substrate. If consciousness is thought of as
an information processing pattern that can be instantiated on any suitable
substrate, then all that matters is that such a substrate exists somewhere. If
the substrate is physically possible, then the Many Worlds Interpretation
requires it to exist.

[1]
[https://en.wikipedia.org/wiki/Identity_of_indiscernibles](https://en.wikipedia.org/wiki/Identity_of_indiscernibles)

[2]
[https://en.wikipedia.org/wiki/Teletransportation_paradox](https://en.wikipedia.org/wiki/Teletransportation_paradox)

~~~
int_19h
For any of this to have meaning, you need to define what "consciousness" even
is. I'm not aware of any definition that is not hopelessly metaphysical.

~~~
vokep
The continued experience that you exist. You literally have never known
anything else, only the representations which consciousness allows.

The stuff itself is hopelessly metaphysical, the only good way I know to
define is the above: since you have are consciousness, its easy enough to just
say 'look around'. Its difficult to think of any material definition when
there's not much of an understanding of how it relates to anything material.
Obviously the images you see are dictated by physical rods and cones in the
eyes, but what the hell are colors?

~~~
int_19h
> The continued experience that you exist. You literally have never known
> anything else, only the representations which consciousness allows.

Right, but that's what makes it so meaningless. We generally define things by
specifying how they're different from other things. But you can't do it with
consciousness, because you can't really know what the absence of consciousness
is like. And if you can't do that, the definition of consciousness becomes
null - and nothing meaningful can be derived from it.

With colors, at least, you can define different ones in terms of different
sensations - red is not-green etc. And because they're derived from
observation of phenomena, you can compare notes with other people to make sure
that what you consider red is also what they consider red etc - it's not a
perfect system, but it's "good enough". I don't think we have anything even
remotely resembling such a "good enough" definition of consciousness, though.

------
nabla9
Philip Ball explores the problems that many world interpretation poses to the
concepts of self and meaningfulness of life in the sense that he understands
them.

Nothing in the article clearly points to problems in physics or even
philosophy of physics in many world interpretation.

Makes me feel uncomfortable is not very deep argument.

~~~
pantonia
Agreed - the idea that selfhood and duality are illusions of perception have
been around at least as long as Buddhism, and philosophically well
articulated. They are hardly grounds for dismissing a theory of the
fundamental universe.

------
Veedrac
Here's the article in short.

1\. "Roland Omnès says the idea that every little quantum “measurement” spawns
a world “gives an undue importance to the little differences generated by
quantum events, as if each of them were vital to the universe.” This, he says,
is contrary to what we generally learn from physics: that most of the fine
details make no difference at all to what happens at larger scales."

2\. "What can it mean to say that splittings generate copies of me? In what
sense are those other copies “me?”" / "Consciousness relies on experience, and
experience is not an instantaneous property: It takes time, not least because
the brain’s neurons themselves take a few milliseconds to fire. You can’t
“locate” consciousness in a universe that is frantically splitting countless
times every nanosecond"

3\. "You could become a billionaire by playing quantum Russian roulette. Your
quantum splitter is activated while you sleep, and if the dial says Up then
you’re given a billion dollars when you wake. If it shows Down then you are
put to death painlessly in your sleep." / "But a committed Everettian should
have no hesitation about doing so using the quantum splitter. For you can be
certain, in this view, that you’ll wake up to be presented with the cash. Of
course, only one of “you” wakes up at all; the others have been killed. But
those other yous knew nothing of their demise. Sure, you might worry about the
grief afflicted on family and friends in those other worlds. But that aside,
the rational choice is to play the game."

4\. "Alice is put to sleep before the measurement is made, knowing she will be
wheeled into one of two identical rooms depending on the outcome. Both rooms
contain a chest. Inside one is twice her stake, while the other is empty." /
"But what if Alice were to say, “The experience I will have is that I will
wake up in a room containing a chest that has a 100-percent chance of being
empty”? The Everettian must accept this statement as a true and rational
belief too, for the initial “I” here must apply to both Alices in the future."
/ "In other words, Alice Before can’t use quantum mechanics to predict what
will happen to her in a way that can be articulated — because there is no
logical way to talk about “her” at any moment except the conscious present
(which, in a frantically splitting universe, doesn’t exist)."

Sengachi's reply in the comments below basically summarises my reaction.

~~~
EGreg
People ridicule the nonlocality of Pilot Wave theory. But I’d rather question
locality than embrace Everett’s many-worlds theory. If you need to postulate
such amazing complexity just to say basically nothing in terms of realism,
then you’ve already lost.

Meanwhile as one good physics professor pointed out, none of these
alternatives to PWT even rule out nonlocality. So why not just assume it and
be done with the weirdness?

~~~
gotocake
The evidence against Pilot Waves keeps stacking up.
[https://www.quantamagazine.org/famous-experiment-dooms-
pilot...](https://www.quantamagazine.org/famous-experiment-dooms-pilot-wave-
alternative-to-quantum-weirdness-20181011/)

~~~
n4r9
That experiment really has nothing to do with actual pilot wave theories. It
just rules out a fluid-mechanical analogue of quantum mechanics (which anyone
could predict anyway since fluid mechanics isn't non-local in the same way).

------
calebh
Could the anthropic principle taken into conjunction with our continued
existence be used to show that the MWI is more probable?

Here's the situation that I'm thinking about: it's well known that we've come
close to nuclear annihilation multiple times, either by accident or by
tensions between the US and the USSR in the Cold War. If the MWI is correct,
it is probable that a good chunk of these worlds have been reduced to ash.
However, we currently find ourselves in a world where that has not happened,
presumably due to the anthropic principle. Could we then give a numerical
probability that MWI is correct, based on this fact?

I'm not a philosopher or physicist - just an interested layman.

~~~
philwelch
I don't think that's how the anthropic principle works, but I'm not actually
sure how the anthropic principle works.

To my understanding, the anthropic principle is basically: among the near-
infinite set of possible physical laws, we can immediately rule out the ones
that would prevent us from being around trying to figure out which physical
laws govern the universe.

I think there's two varieties of this. The "weak anthropic principle" says,
that doesn't mean those laws _couldn 't exist_, there's no weird metaphysics
around it, it's just that if the underlying physical laws didn't allow us to
exist, you'd just have a universe that didn't have anyone in it and so the
physics of that universe would be a moot point that didn't matter to anyone.

The "strong anthropic principle" says, wait, no, there _is_ some metaphysical
significance to this, and it's somehow not even logically possible for a non-
anthropic universe to exist. I find that a little self-absorbed on the part of
humanity as a whole, but that would tie in a little better to your quantum
suicide idea. The weak anthropic principle, in contrast, would just kind of
say, "yeah, there's a chance we all blow ourselves up, and if we do, I guess
nobody will be around to care about physics anymore".

Even the strong anthropic principle doesn't prove much here. "Nuclear
annihilation" probably wouldn't actually result in complete human extinction.
Some people would survive. Individual quantum immortality doesn't stop you
from shooting a hole through your brain and being neurologically disabled for
the rest of your life, so collective quantum immortality also wouldn't stop
you from living in a post-apocalyptic hellhole. Also, even the strong
anthropic principle doesn't actually mean that humanity can't go extinct;
after all, it didn't prevent the universe from existing for billions of years
before humans started to exist. Maybe the real strong anthropic principle
doesn't even apply to us, it applies to a species of avian hive-minded
critters on a planet circling an unnamed star in the Andromeda galaxy, and as
a weird side effect of living in a universe that is metaphysically required to
permit for the existence of the Andromeda flying hivemind, humanity kind of
shows up for a few hundred millennia and gets to do some physics before
getting wiped out by a swarm of paperclip maximizers.

~~~
calebh
If the MWI is true, then there must be some universes that would experience
multiple highly improbable events. These events would be a hint to residents
of that universe that the MWI is correct. Why wouldn't the usual Bayesian
analysis apply here?

~~~
philwelch
Your personal probability of observing a universe that experiences multiple
highly improbable events is the same whether or not MWI is true.

------
jhpriestley
MWI seems more elegant than Copenhagen in many formulations because the
problem of the relative magnitudes of the worlds is swept under the rug. If I
perform a quantum coin-flip with 2/3 possibility of heads, then it creates two
worlds - but I end up in one of them twice as often - why? The only solutions
I've seen to preserve the relative probabilities are: 1) sufficiently unlikely
worlds sometimes just disappear, or 2) some kind of appeal to consciousness.
These are at least as ad-hoc and hand-wavy as waveform collapse.

~~~
spiritcat
Or it actually creates three worlds, two heads worlds and one tails.

~~~
Jweb_Guru
Being able to count worlds in the manner you describe is apparently not easy
to justify. If I recall correctly, this is actually a pretty thorny
epistemological problem with the theory. IIRC the current response among Many
Worlds people is that either (1) it doesn't really matter because there is no
reason to believe that probabilities are "real" in the first place, or (2)
attempting to show that if you did insist on finding a metric that looked like
probability, it would provably have to be the Born probability (see
[https://arxiv.org/abs/1405.7577](https://arxiv.org/abs/1405.7577)). All this
stuff is way over my head so I have no way of evaluating whether it's
persuasive or not, but it does at least seem to be contentious among
physicists.

------
noahdesu
I enjoyed reading this article. But at the beginning the author has this to
say about objections to the MWI that are based on feeling:

> "This objection is rightly dismissed by saying that an affront to one’s
> sense of propriety is no grounds for rejecting a theory. Who are we to say
> how the world should behave?"

But really at the end of the article the conclusion about the problems with
MWI feel similar:

> "But an idea that, when we pursue it seriously, makes that view inchoate and
> unspeakable doesn’t fulfill the function of science."

------
jblow
Of course it seems weird to think of Many Worlds as somehow manufacturing
universes for each fluctuation. That seems weird and wrong ... because it
probably is.

A better way to think of it is as the equivalent of the relativistic block
universe. All these different spaces already exist in some superspace, and
‘random’ events take you from one space to a neighboring one. Nothing is
manufactured.

~~~
JProthero
Well put; this is more or less what Everett actually proposed, and it's the
interpretation that mainstream Everettians like Sean Carroll and Max Tegmark
subscribe to.

~~~
r_c_a_d
Yes, before reading Tegmark I thought that MW was silly because "splitting"
the whole universe for every tiny event just seemed crazy. Thinking of the
multiverse as more like a probability distribution where everything exists in
a kind of overlay seemed to make sense.

------
rladd
Unless I'm missing it, his argument about why MWI is untenable in terms of
physics is:

\- If the universe splits with each measurement (or interaction), then every
possible outcome does occur

\- Therefore nothing could be predicted based on probabilities, and that
proves MWI is wrong

This seems to have a few logical errors:

Just because everything that can happen does, in MWI, happen in at least one
universe, that doesn't mean that the probability of each outcome is identical.

Probabilities still apply: higher probability outcomes mean that one is more
likely to find one's self in a universe in which that outcome takes place.

In this way of describing things, it's still a very low (but non-zero!)
probability that all the air in your room will move to the corner, leaving you
to suffocate in a vacuum. But the way we state it would be, "it's a very low
probability that you will find yourself in one of the universes in which this
happens".

------
n4r9
Philip Ball has previously written very similar articles on his objections to
Many Worlds:

[https://aeon.co/essays/is-the-many-worlds-hypothesis-
just-a-...](https://aeon.co/essays/is-the-many-worlds-hypothesis-just-a-
fantasy)

Whilst I share his frustration with how readily it is being taken up and the
dogmatic glamour surrounding it, I feel like he consistently misses the
primary issues with it.

Copy-pasting a previous comment trying to describe succinctly what I see as
the central problem:

> Although there are many versions of MWI, the general claim is that a single
> axiom (a unitarily evolving wavefunction) is sufficient to explain our
> observations of the universe. Without getting into what it means to
> "explain" something, it seems fair to demand that a sufficiently intelligent
> agent with no prior knowledge of our physics should be able to predict what
> the theory says about their future observations. However no one would be
> able to make any experimental predictions based on the above axiom. There's
> nothing there to suggest that if the agent becomes entangled with a quantum
> system then they would only experience a single branch of the entanglement.
> Even if you add that in, there's nothing to suggest which branch the agent
> will experience (this is roughly the preferred basis problem). Even if you
> add that in, there's nothing to suggest with what probability the agent will
> experience that branch.

> You could always add in the above as additional axioms. You could postulate
> some physical content to the Born-rule, or to something weaker from which to
> deduce the Born rule. But this would break the illusion that MWI requires
> fewer axioms. Instead, a great deal of energy has been expended into
> deducing the Born rule purely from that single axiom. There have been many
> very clever attempts at this, but none convincing enough to settle the
> matter.

If you're interested in reading more, Adrian Kent has written some fairly
thorough critiques on arxiv:

[https://arxiv.org/abs/0905.0624](https://arxiv.org/abs/0905.0624)

~~~
rain1
> there's nothing to suggest which branch the agent will experience

isn't this accurate modelling though? as far as we can tell the collapse picks
a branch at random. I just don't see how this is a problem of the model.

~~~
n4r9
My description there was pretty crude, apologies. The measurement problem is
often loosely expressed in terms of metaphors like "live cat vs dead cat". But
if you actually look at the quantum state of such a complicated system, it's
this huge tangled mess of Hilbert dimensions lumped together into one big
global wave function. The "alive vs dead" separation is just one possible
perspective (technically it's a slicing of the Hilbert space into mutually
orthogonal sub-spaces). There are infinitely many possible such ways of
splitting the space up, i.e. infinitely many possible basis sets. It's not
directly clear why, when we physically interact with the system, one specific
basis is chosen for us to "collapse" into one branch of.

Perhaps it could be better explained as "there's nothing to suggest which
_branching_ the agent will _experience splitting into_ ".

I should note that the process of decoherence goes some way towards removing
this obstacle (although not the following question of explanation of
probabilities).

------
trainingaccount
Some counterpoints:

[https://www.hedweb.com/manworld.htm](https://www.hedweb.com/manworld.htm)

[https://www.lesswrong.com/posts/9cgBF6BQ2TRB3Hy4E/and-the-
wi...](https://www.lesswrong.com/posts/9cgBF6BQ2TRB3Hy4E/and-the-winner-is-
many-worlds)

------
JProthero
The Many-Worlds Interpretation is seriously mischaracterised in the summary at
the top of the article (presumably not written by Ball himself) when it says
this:

" _The idea that the universe splits into multiple realities with every
measurement has become an increasingly popular proposed solution to the
mysteries of quantum mechanics._ "

This is a misrepresentation of what the Many-Worlds Interpretation actually
suggests, and it is contradicted in the article itself by Ball:

" _In effect, this implies that the entire universe is described by a gigantic
wave function that contains within it all possible realities. This “universal
wave function,” as Everett called it in his thesis, begins as a combination,
or superposition, of all possible states of its constituent particles. As it
evolves, some of these superpositions break down, making certain realities
distinct and isolated from one another. In this sense, worlds are not exactly
“created” by measurements; they are just separated. This is why we shouldn’t,
strictly speaking, talk of the “splitting” of worlds (even though Everett
did), as though two have been produced from one. Rather, we should speak of
the unraveling of two realities that were previously just possible futures of
a single reality._ "

The error in the summary is a common misrepresentation, and it leaves many
people (including me, when I first heard of the Everett Interpretation) with
the mistaken impression that the interpretation proposes some mysterious new
mechanism that causes the entire universe to 'split' every time a physical
interaction or observation occurs. This is pretty much the opposite of what
the interpretation actually suggests, since its purpose is to dispense with
the significance of observation and 'wavefunction collapse' in the Copenhagen
Interpretation.

The physicist Max Tegmark (an adherent of the Everett Interpretation, who made
a film[1] about Everett with Everett's son; a sad story, as Everett was unable
to make a career for himself in physics due to the unpopularity of his most
famous idea) I think puts it best when he says this about the Many-Worlds
Interpretation in his book, Our Mathematical Universe:

" _The rumours I 'd heard suggested that Everett proposed crazy-sounding stuff
like parallel universes and that our Universe would split into parallel
universes whenever you made an observation. Indeed, even today, many of my
physics colleagues still think that this is what Everett assumed. Reading
Everett's book taught me a lesson not only in physics but also in sociology: I
learned the importance of going back and checking the source material for
yourself rather than relying on secondhand information. It's not only in
politics that people get misquoted, misinterpreted and misrepresented, and
Everett's Ph.D. thesis is a great example of something that, to first
approximation, every-one in physics has an opinion about and almost nobody has
read.[2]_"

" _The notion that at certain magic instances, reality undergoes some sort of
metaphysical split into two branches that subsequently never interact isn 't
only a misrepresentation of Everett's thesis, but also inconsistent with
Everett's postulate that the wavefunction never collapses, since the
subsequent developments could in principle make the branches interfere with
each other._"

[1]
[https://en.wikipedia.org/wiki/Parallel_Worlds,_Parallel_Live...](https://en.wikipedia.org/wiki/Parallel_Worlds,_Parallel_Lives)

[2] [https://www-
tc.pbs.org/wgbh/nova/manyworlds/pdf/dissertation...](https://www-
tc.pbs.org/wgbh/nova/manyworlds/pdf/dissertation.pdf) (Everett's original
thesis)

~~~
mathgenius
Thankyou for clearing this up. I've been confused about this for a long time.
It also explains why the decoherence theory fits in with this story.

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Upvoter33
My own problem with many worlds is (a) it seems to me to require a ridiculous
amount of energy and (b) what happens when two world splits occur "at the same
time" on opposite sides of our planet? Also (c), it just doesn't appeal to
intuition, but it seems like much of physics does not :)

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prmph
One thing that others me about the MWI: Is the so-called "set of all
possibilities" a well-defined set?

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n4r9
Yes - the universe is still described by a single, pure wavefunction. I think
this kind of confusion is inevitable when people talk about different
"universes" or "worlds" being created, when nothing of the sort is occurring
in the theory.

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ElBarto
This feels a bit like JIT processing.

There may be a number of states possible but there is no need to 'process'
which one it is until that information is needed.

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cleanyourroom
In some universes it has no problems.

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ThePhysicist
This is a very philosophical / meta-physical discussion of quantum mechanics,
and not in line with our modern understanding of the measurement process. For
example, there really is no sudden wave function collapse in quantum
mechanics, as every measurement process can be described as a purely quantum-
mechanical process that produces an apparent "collapse" of a wave function
through entanglement and decoherence. There's nothing magical or abrupt about
it, today we can even perform quantum measurements that precisely control the
amount of decoherence that is introduced to a system (see e.g. some of Irfan
Siddiqi's seminal quantum feedback experements like
[https://www.nature.com/articles/nature11505](https://www.nature.com/articles/nature11505),
or the earlier work done in Serge Haroche's group). Taking aside the
philosophical problems that people have with "pure" quantum mechanics (i.e. an
interpretation that does away with the second postulate of wave function
collapse) it perfectly describes our experimental results.

The largest problem with wave function collapse (which the author only briefly
skims) is that you need a way to explain how it works: Basically when the
wavefunction collapses we go from a fully reversible, deterministic quantum
system to an irreversible, stochastic system. At which scale is this supposed
to happen? Let's say one day we can build huge quantum computers (think
billions or trillions of qubits) that we can perfectly control. We could then
prepare a single qubit in a superposition state. We can entangle this qubit
with the other qubits (the measurement system) and manipulate the state of the
measurement system using a deterministic but highly chaotic control program.
If we'd then measure the state of the single qubit (e.g. by preparing many
identical systems and performing quantum state tomography) we'd find that the
wave function has collapsed, i.e. there is no more coherence between the two
qubit states. Now, after entangling the qubit and evolving the large system we
could just reverse its deterministic evolution (as we have perfect control
over it) and bring it back to the state it was in directly after entangling it
with the qubit. If we then perform quantum state tomography of the qubit we
should see that the coherence is back. Now, if we believe that wave function
collapse is a phenomenon that occurs independently and not as an effect of the
evolution of the large quantum system, we would expect to observe a loss of
coherence in the qubit even after perfectly reversing the state of the large
quantum system. The question is then of course: At which scale does this
happen, and what is the physical theory that governs this behavior? Are one
billion qubits enough to produce it? One quadrillion? To my knowledge, no one
came up with even an idea of how to describe this. If someone finds a good
theory for describing wave function collapse I'll consider it as valid theory,
until then I'll stick with the "many worlds" interpretation, though I'd really
prefer calling it "plain" quantum mechanics instead.

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gpm
> This, he says, is contrary to what we generally learn from physics: that
> most of the fine details make no difference at all to what happens at larger
> scales."

Oh? Since when have we disproved the butterfly effect?

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ars
> Since when have we disproved the butterfly effect?

There never was such a thing, not the way you are using it.

The butterfly effect is an observation in chaos theory that small changes in
initial conditions can make large changes in output.

The "popular" usage of butterfly effect "A butterfly flapping its wings can
eventually cause a hurricane" is not a thing that ever existed.

The difference is that one is directly fed into the output, the other is not,
the other is filtered by huge amounts of noise.

