
How the Many-Worlds Theory of Hugh Everett Split the Universe - jonbaer
https://aeon.co/essays/how-the-many-worlds-theory-of-hugh-everett-split-the-universe
======
fallingfrog
You can do a really simple thought experiment that provides a persuasive
argument in favor of the many worlds theory. Or at least, it shows that wave
function collapse is relative to an observer, not universal everywhere.

Start with Schroedingers cat: the cat is in a box, it’s in a state of quantum
superposition. It’s 50/50 alive or dead. Now a researcher opens the box: the
cat is found to be either alive, or dead, and according to the Copenhagen
interpretation we say that the wave function has collapsed.

But, now take the cat and the researcher, and put the whole first experiment
inside another box. Now the researcher inside the second (outer) box opens up
the box with the cat. From outside the second box, we still must say that the
first researcher is in a state of superposition, but when the second box is
opened, the first researcher will say, oh, I’ve known the cat was dead for 10
minutes or whatever.

So, before we open the second box, has the wave function collapsed, or not? It
depends on which researcher you are. But Every part of the universe is in some
sense “in a box” until information passes to it from another part of the
universe. I give you: many worlds.

~~~
yters
The problem with many worlds is why do I think I'm in a particular timeline?
If many worlds is correct, there is no "I", there are many rapidly branching
"I"s, which intuitively doesn't make sense. Of course, each "I" will have the
perception of being the sole "I", but the overall view that there is no
singular "I" is very counter to my self conception. I prefer to preserve
intuitions over conjectures, so I count that as a strike against the many
worlds interpretation.

So, it seems like both the Copenhagen and the many worlds interpretations of
quantum physics are wrong, given a preference for non arbitrary reference
point and preserving intuitions. Is there a reason that those two must be the
only possible interpretations?

~~~
finchisko
>"I" is very counter to my self conception

it'a because of human nature, ego and self-worth. It's hard to accept that "I"
might have many siblings living in different branches.

~~~
yters
But perhaps those are valid constraints. Given that all of science is built
from fundamental subjective perceptions, it seems inconsistent to extrapolate
scientific theories that deny fundamental subjective perceptions.

------
drcode
Sean Carroll's new book on quantum physics that came out yesterday is the
clearest laymen's exposition of quantum physics I have ever read:
[https://www.audible.com/pd/Something-Deeply-Hidden-
Audiobook...](https://www.audible.com/pd/Something-Deeply-Hidden-
Audiobook/0525592474)

~~~
aklein
Another book I'm enjoying is "Einstein's Unfinished Revolution" [1] by Lee
Smolin, which also has a deep dive into foundations of QM.

[1] [https://www.amazon.com/Einsteins-Unfinished-Revolution-
Searc...](https://www.amazon.com/Einsteins-Unfinished-Revolution-Search-
Quantum/dp/1594206198/ref=sr_1_1?crid=1TMC2EZDUXMZ0&keywords=einsteins+unfinished+revolution&qid=1568216306&s=gateway&sprefix=einsteins%27+unfin%2Caps%2C315&sr=8-1)

~~~
drcode
I'm not familiar with that book, but I can say that Dr. Carroll's book is far
better than any previous book on the subject I've read, in terms of getting
rid of the confusion inherent in QM.

------
chasingthewind
Peter Woit in his blog "Not Even Wrong" [0] sometimes deals with the Many
Worlds interpretation. He doesn't seem to be a fan.

He had a recent post [1] about Everett that's interesting.

[0]
[https://www.math.columbia.edu/~woit/wordpress/](https://www.math.columbia.edu/~woit/wordpress/)
[1]
[http://www.math.columbia.edu/~woit/wordpress/?p=11202](http://www.math.columbia.edu/~woit/wordpress/?p=11202)
[2]
[https://www.math.columbia.edu/~woit/wordpress/?p=10522](https://www.math.columbia.edu/~woit/wordpress/?p=10522)

~~~
platz
That characterization of Woit does not appear quite accurate. Woit's claim may
even be bolder, in that he claims that there is no functional difference
between MWI and Copenhagen.

> What strikes me when thinking about these two supposedly very different
> points of view on quantum mechanics is that I’m having trouble seeing why
> they are actually any different at all.

~~~
lisper
The main difference is one of rhetoric: Copenhagen insists that experiments
have single outcomes. MWI insists that experiments have multiple outcomes in
point of metaphysical fact.

Another difference is that the MWI has to explain the Born rule (which is
problematic), whereas Copenhagen simply takes it as an axiom.

[http://blog.rongarret.info/2019/07/the-trouble-with-many-
wor...](http://blog.rongarret.info/2019/07/the-trouble-with-many-worlds.html)

~~~
platz
> The main difference is that the MWI has trouble explaining the Born rule.

To re-phrase, the claim in that article is that once MWI has done away with
the Born rule, it has to deal with how the probabilities that we find in
experiment are calculated under MWI.

.. Which is not obvious at first glance, because MWI seems to suggest
everything happens with probability = 1.

Carroll acknowledges it is one of the challenges against MWI that needs to be
answered.

Carroll offers "self-location uncertainty" as a way to recover the
probabilities predicted by the Born Rule despite the determinism of the
Schrodinger Equation

> But even if both people know the wave function of the universe, there is now
> something they don’t know: which branch of the wave function they are on.
> There will inevitably be a period of time after branching occurs but before
> the observers find out what outcome was obtained on their branch. They don’t
> know where they are in the wave function. That’s self-locating uncertainty,
> as first emphasized in the quantum context by the physicist Lev Vaidman.
> ([https://news.ycombinator.com/item?id=20927410](https://news.ycombinator.com/item?id=20927410))

* edit - OP's post was edited but i'm leaving this post as-is.

> MWI insists that experiments have multiple outcomes in point of metaphysical
> fact.

That doesn't seem right. under MWI there is only one Wafefunction and one big
quantum state. There are not additional 'worlds'

~~~
lisper
> OP's post was edited

For the record, the original just said (as quoted in the above response) "The
main difference is that the MWI has trouble explaining the Born rule." Upon
reflection I decided that wasn't really the _main_ difference, that the main
difference is the difference in ontology. But the fact that the MWI has to
recover the Born rule from the linear dynamics is a major consequence of this
difference in ontology.

The jury is still out about whether or not self-location uncertainty actually
allows the Born rule to be recovered without begging the question. If it
works, it would be a major breakthrough. Personally, I'll give long odds
against.

[UPDATE]:

> There are not additional 'worlds'

Um, what do you think that the "MW" in "MWI" stands for?

~~~
ncallaway
The label "Many Worlds Interpretation" is a popular label for the theory, but
it's not a particularly accurate one.

The original paper by Hugh Everett was titled '"Relative State" Formulation of
Quantum Mechanics'.

The underlying principle of Many Worlds isn't actually that there are many
worlds. Is that there is a singular wave function, which is the true reality.
The things we refer to as "worlds" are massively-complicated entanglements and
superpositions within that wave function, of which we—for some reason—can only
observe one position of that superposition.

~~~
lisper
Everything you say is true. Nonetheless, if you read papers written by self-
identified advocates of Everett, they invariably refer to it as "many-worlds"
and they talk about "the branching structure of the multiverse" and "copies"
or "versions" of yourself. So regardless of what you or I think Everett meant,
the present-day advocates of his theory are clearly trying to advance the idea
that there are, in point of metaphysical fact, many worlds.

~~~
ncallaway
Sure, but I would think every one of those advocates would also agree with
what I wrote.

That the "world" described by many worlds is a convenient short-hand for a
single position in a massively entangled system of particles in a complex-
superposition.

It's the "Many Worlds" is common moniker, and the rhetoric does use the
concept of worlds all the time, but whenever you get down to the specifics of
the theory there really aren't many worlds.

I think it's unfair to hold the shorthand rhetoric against people when they
attempt to get to that more precise level. It would be like using the plain
english definition of "observation" against someone trying to provide a more
rigorous definition of what an "observation" is under the Copenhagen
interpretation.

Ultimatley, this discussion all stems from the original statement: "MWI
insists that experiments have multiple outcomes in point of metaphysical
fact." It's a fairly moot point at this point since that statement was
removed.

~~~
lisper
> I would think every one of those advocates would also agree with what I
> wrote.

No, they wouldn't. They don't. See my response to lmm here:
[https://news.ycombinator.com/item?id=20941803](https://news.ycombinator.com/item?id=20941803)

> "MWI insists that experiments have multiple outcomes in point of
> metaphysical fact." It's a fairly moot point at this point since that
> statement was removed.

I'll stand by that with one minor modification: contemporary advocates of MWI
insist that experiments have multiple outcomes in point of metaphysical fact.

~~~
ncallaway
Sorry for the slow response. Been a busy couple of days :)

> No, they wouldn't. They don't. See my response to lmm here

I don't think lmm and I disagree.

I said "[The underlying principle of Many Worlds] is that there is a singular
wave function, which is the true reality". lmm says: "The only thing that MWI
demands we take as metaphysically real is the wavefunction".

I think this is expressing the same thing in different terms.

I said: "The things we refer to as "worlds" are massively-complicated
entanglements and superpositions within that wave function". lmm says: "Given
a wavefunction like 1/sqrt2(|experimenter who observed a live cat> \+
|experimenter who observed a dead cat>) ...". Again, I think our descriptions
are in fairly close agreement.

I think lmm's description is a good summary of how the "worlds" in Many Worlds
are viewed by Many Worlds advocates: "in the same way that people who hear a
particular pattern of sound might find it convenient to think of it as a chord
of two or three notes". The "worlds" aren't actually fundamental to the
theory, but rather an emergent phenomena from them.

The reason for the label "Many Worlds" is not because they are the fundamental
concept. It's because it's the most... interesting element from a pop-sci
marketing angle.

------
donatj
Fun fact, Hugh Everett is “E” Mark Oliver Everett’s father, lead singer of The
Eels.

~~~
jjjbokma
Came here for this :-D.

~~~
lukeclewlow
:-D

------
kgwgk
"Let me address another issue with many-worlds. It is a deterministic theory,
even a hyper-deterministic theory, i.e. determinism applies to everything in
the entire universe. Indeed, since there can’t be any influences coming from
outside and since the Schroedinger equation - the only dynamical equation of
the theory - is deterministic, everything that happens today, e.g. what I am
writing, the way each reader reacts, the details of all solar eruptions, etc,
was all encoded in some “quantum fluctuations” of the initial state of the
universe. Given the complexity of the (many-) worlds, it had to be encoded in
some infinitesimal digits of some quantum state, possibly in the billionths of
billionths decimal place. I am always astonished that some people seriously
believe in that."

\- Nicolas Gisin "Collapse. What else?"
[https://arxiv.org/abs/1701.08300](https://arxiv.org/abs/1701.08300)

~~~
vilhelm_s
This seems wrong to me? A lot of "things that happen" seem to be created by
randomness generated along the way, from only observing subsystems.

I.e., suppose I create a random bit generator by shining photons on a half-
silvered mirror and then measuring which way they go. I might then decide what
to write depending what the bit is---if it's 0 I will write a HN comment, if
it's 1 I will reply to an email.

In this case, the things that happen are determined deterministically, but
it's not the case that there is some quantum fluctuation in the initial state
of the universe which determines what I write. Rather, when I do the
measurement of the bit, I create two branches, one with a HN comment and one
with an email. Observers entangled with the first branch can only see the HN
comment, but this complexity was created at the time of the measurement, not
at the time of the big bang...

~~~
kgwgk
There is no "randomness generated along the way" in the MWI. Schroedinger's
equation is deterministic and according to many-worlders is the only thing
that "makes things happen". Everything that happens in every "world" was bound
to happen since the beginning of the evolution of the "multiverse".

~~~
vilhelm_s
Right, but my point is that this doesn't mean that the initial state contains
very much information.

Maybe a different example makes this clearer. Suppose you use a quantum random
number generator to choose letters from A to Z, and write out a full book this
way. If you consider the entire (multiple) world, it's basically Borges'
Library of Babel: it has non-zero amplitudes for every possible book. So one
possible "thing that can happen" is that Shakespeare's Hamlet gets written.

But that doesn't mean that the initial state of the universe somehow encodes
the text of Hamlet. The initial state of the universe doesn't have any
information at all, in quantum fluctuations or otherwise. You only see Hamlet
emerge if you restrict your attention to some particular branches of the
world. It was "bound to happen" that Hamlet would get written, but only
because every book would get written.

~~~
kgwgk
The initial state contains as much information as the current state if the
evolution is unitary (only Schroedinger’s equation matters according to the
MWI, right?).

And surely the wavefunction of a system of 10^80 particles (or whatever) does
contain some information about what happens... or do you think that “every
book would get written” independently of the form of the universal
wavefunction?

~~~
vilhelm_s
But the "current state" of the entire wavefunction doesn't necessarily contain
very much information. You only get most of the complexity (solar flares,
novels) if you look at subsystems of it.

I think the situation is exactly analogous to the Library of Babel in Borges'
story. If someone sends you the _entire_ library in the mail, they have given
you zero bits of information. If they only send you a particular room of it
you get more information, and narrowing it down to sending a particular book
gives you more bits still.

It's the same with the multiverse. If we start out with the photon about to go
through the half-silvered mirror, the state looks like |photon>⊗|me>, and then
over time it evolves into

(1/√2)|photon went left>⊗|me observing left> \+ (1/√2)|photon went right>⊗|me
observing right>

and the latter state contains no more information than the former one (you can
run the time evolution backwards to recover the original state). However, if
you restrict your attention and only consider one of the branches, e.g.
|photon went right>⊗|me observing right>, then you _have_ added (one more bit)
of information. And most macroscopic processes can be explained "one branch at
a time", since in practice decoherence means that there are no noticable
interactions between branches. I think in the MWI this is the source of most
complexity in the universe: not detailed information encoded in the initial
state, and not objective collapse, but the bits of information you implicitly
create if you only consider one branch of an experimental outcome.

I imagine the same thing happens in general: we start with some uniform soup
(with ~zero information), in a bunch different MWI branches it self-gravitates
slightly differently so we get a different set of galaxies and stars in each
branch, then for each branch that has a planet, different quantum randomness
may cause different intelligent life to develop, and eventually they may write
different books. Of course, it's also possible that some small initial state
gets magnified, as a separate mechanism, but that's the same in objective-
collapse and MWI. The random bits created by objective collapse are the same
same the ones you get by looking at particular branches in MWI.

~~~
kgwgk
> If we start out with the photon about to go through the half-silvered
> mirror, the state looks like |photon>⊗|me>,

But in the MWI what we start with is the universal wavefunction |Psi>. Say
there are 10^80 particles in the universe and they may have a spin and could
be in infinite locations. That makes for an ininite-dimensional Hilbert space.
If we consider that the precision in the location variables is limited by
Planck's length there are still around 10^180 possible locations in the
universe for each particle. The Hilbert space where the quantum state of the
universe is defined is huge.

Now, you say "the state looks like |photon>⊗|me>". To keep things simple and
in the realm of Schroedinger's equation I will consider an electron instead of
a photon, prepared in the state spin z=+1/2.

Is "|up>⊗|me>" shorthand for "(1 |up> \+ 0 |down>)⊗|me, the electron at that
position, the rest of the universe>" which would be the "in-branch" quantum
state of the universe?

If indeed |Psi> can be written as a superposition of that state and an
inconceivable huge number of orthogonal states, are all those states (when
their coefficient is non-zero) the multiple worlds?

But given |Psi> there are infinitely many (or at least a lot of) possible
basis to express the pure state as a decomposition (including of course the
one where the only non-zero "world" in the superposition is the one described
by the wave function |Psi>). Do we count as "worlds" as well the states
appearing in other basis?

Is this slicing of |Psi> into other states just a mathematical trick or does
it have a physical meaning? Even if we can redo the operation at different
times on the evolving |Psi(t)>, how are the "emergent substates" at different
times related?

The answer to all these questions is far from trivial.

~~~
vilhelm_s
Indeed not, but is any of this relevant to the original quote, that
deterministic dynamics means that the initial state of the universe must have
lots of information hidden in the far-off decimals? I would hope that one
could consider a simplified setting for that and then extrapolate back to the
continuous case.

~~~
kgwgk
I agree the original quote doesn’t present a very strong argument against the
MWI. In the same way that arguments against determinism in classical physics
are not very convincing. But seemed worth pointing out.

In the MWI case it’s not clear how sensitive the “worlds” are to the in
initial state. It’s not even clear how sensitive they are to the current
state! Better understanding of how (and how many) worlds “emerge” would be
needed to say if the “problem” is or not as bad as in the classical case.

Is the number of worlds stable? Is it increasing? In any case, those worlds
are the inevitable consequence of the initial conditions even though we can’t
even estimate how much fine-tuning is needed. I don’t think “randomness
generated along the way” is a satisfactory counter-argument.

------
lordnacho
I'm not sure if the Aeon article explains what Everett really meant, at least
I still have questions.

Suppose we're eternally branching, as in the example about the up/down spin.
If there's no way to visit other branches, how will we ever confirm this?

Second, what is the benefit of the branching interpretation? It seems to
simply be a way to think about probabilities?

~~~
scld
I'd recommend reading some of the explanations given by Sean Carroll on the
subject.
[https://www.preposterousuniverse.com/](https://www.preposterousuniverse.com/)

....and I didn't realize that Sean Carroll actually wrote this article! Still,
he does some good explanations on his podcast.

------
edmoffo
Haha... Anything is in some sense "in a box" until information passes to it
from another thing. Philosophical points only...

