
Stephen Hawking's Final Paper Cuts the Multiverse Down to Size - jonbaer
https://www.livescience.com/62459-hawking-final-paper-physics.html
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perpetualcrayon
It still boggles my mind, but I honestly believe that there is something
beyond our own universe. The other thing that trips me up but I can
conceptualize is the idea that there may be things that had no beginning and
will have no end. The human mind is very much suited to understand things to
have beginnings and ends. Our own universe very clearly had a beginning and
will certainly have an end. But that which exists beyond our own universe. Is
it necessary that whatever is there did not exist at some point? I think it's
entirely possible that whatever exists beyond our own universe may have been
there always and will continue to be there forever. No beginning and no end.

~~~
snissn
Pascal's mugging is a really cool thought experiment. Similarly is the thought
that what if instead of the entire universe being the size of the viewable
universe it's 10^N power, so just another exponential leap in size. Then our
little corner of the multi-verse becomes such a tiny speck.

~~~
Retric
Pascal's mugging is simple failure to recognize probabilities can be and grow
arbitrarily small. If a mugger paying you 4,000 dollars is less than half as
likely as them paying you 2,000 dollars and you repeat that as the numbers
grow then there is no inflection point.

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AndrewKemendo
What is more mind boggling to me than this is the fact that Professor Hawking
could accurately and properly collaborate on such complex topics with other
researchers at this level given the limits to his communication.

I had the privilege of attending a live speech Professor Hawking give back in
2002 or so, and the most impactful thing about it was that it wasn't pre-
recorded. He would "speak" a sentence and then there would be between 10-45
seconds of silence and then another sentence. This continued for about an hour
and a half and nobody lost interest.

I have to assume that the typing, formatting etc... of this paper was handled
by someone else, however even just communicating greek characters, algorithms
and diagrams must have been a feat.

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dmayle
I don't know... something that always bothered me about multiverse theory is
that multiverses are supposed to be the explanation for why a lot of our
quantum math works out so beautifully.

Matter is continuous into other dimensions, and this is why things like the
photon/wave duality can work, interaction with other realities.

In our reality, however, our universe is so sparse as to be almost non-
existent (density of the universe is something like 1 hydrogen atom for every
5k cubic meters). This has been measured, and means that our universe is flat
(Euclidean).

In my mind, in order to connect the density of the universe with the
continuity of quantam behavior, than every reality must have the same density,
and flow continuously from ours, which would mean that every reality is
Euclidean.

This is where it gets fuzzy for me, because it's my understanding that the
multiverse is not Euclidean....

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deciplex
Isn't this post conflating multiverse hypotheses with the relative-state
formulation of quantum mechanics?

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dmayle
To be fair, isn't Everett's relative-state formulation the basis/beginning of
all multiverse hypotheses?

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deciplex
It seems like "multiverse" is used to describe a half-dozen mostly unrelated
things or more (just based on the contents of this thread) including among
them many-worlds. But, for example, eternal inflation has nothing directly to
do with relative-state formulation, nor mathematical universe, nor the
"multiverse" where you consider that portion of the universe outside our
cosmological horizon to be a separate universe (I wouldn't agree that it is,
but apparently that's a thing).

~~~
spiralx
Max Tegmark has put these ideas into a hierarchy of multiverses, where
universes outside of the boundary of the observable Universe are a Type I
multiverse, and the many-worlds of the relative-state formulation form a Type
III multiverse.

[http://space.mit.edu/home/tegmark/crazy.html](http://space.mit.edu/home/tegmark/crazy.html)

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everdev
> this proposal implied something strange: that the multiverse is infinite,
> with endless, uncountable parallel universes existing alongside our own

My biggest skepticism of the multiverse or even an infinite universe is the
amount of energy required. Here on Earth energy is finite and cannot be
recreated without using more energy in the process.

For an infinite universe or multiverse with infinite mass, you'd need infinite
energy, right?

Infinity seems more like a concept than a physical property. It's easy to
imagine an infinite number line, but harder to think about an infinite number
of intelligent beings (which I suppose would be possible in an infinite
universe).

~~~
BlackFly
> For an infinite universe or multiverse with infinite mass, you'd need
> infinite energy, right?

Given that the current best guess for the geometry of the universe is
(conformally) flat, this universe already contains infinite energy. From what
I recall, the normal definitions of total energy of a spacetime are not
possible to formulate for non-asymptotically flat spacetimes. So it is a bit
nonsensical to talk about the total mass/energy.

The easiest skeptical take on multiverses is simply that there are no
measurables defined by multiverse theories yet. I usually lump them in with
anthropic theories, which a lot of GR theorists seem to love.

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evanpw
Talking about asymptotics already assumes that the universe is infinite. Any
smooth, finite spacetime will have finite energy. The interesting thing about
asymptotically flat spacetimes is that they can be infinite in volume but have
finite energy.

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darkmighty
But our universe is believed to be isotropic, and it has positive energy
density everywhere, so it seems that should imply infinite total energy.

~~~
wuliwong
Why would this lead to infinite energy? Positive density summed over a finite
volume does not equal infinity.

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toxicFork
We can only comprehend matter that affects only the five senses that we use to
perceive "the universe".

What if there's a kind of matter that cannot be perceived by the five senses
that we use, and does not interact in any way with the matter that can be
sensed?

Could we consider each of these kinds of matter to belong to another universe?

~~~
SonOfLilit
Affects directly or indirectly.

But indeed, anything that does not affect anything that affects anything [..]
that could be perceived, can not be considered in any meaningful way to belong
to our universe.

Isn't that the meaning of "universe"? The set of all things closed under all
possible physical interactions?

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wuliwong
> "like a sphere without edges"

That's a strange analogy. I would think of an infinite cube but then I guess
I'm thinking in cartesian space. :)

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assblaster
Who/what created the multiverse?

Do matter and energy simply exist without creation?

Is proving the existence of the multiverse possible?

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jerf
"Is proving the existence of the multiverse possible?"

It's _proving_ it's nonexistence that is quite a challenge. Proving their
existence could be demonstrated by, for instance, traveling to them. I
personally see no reason to believe that is possible, but if somebody traveled
to one and thereby disproved me, that would be that.

That said, it might just about take that to prove one. Even if one produces
theories that say a multiverse might be possible, I expect there will always
be a simple transform for which the equations still work but there is only the
one universe. For instance, taking well-established existing equations, while
the QM field equations may imply that a quantum multiverse is possible, no
matter how mathematically distasteful some people may find it the Copenhagen
interpretation is still possibly the truth, and I'm not sure what possible
experiment could disprove that. (The Universe may do as it damn well pleases.)
I'm unsure what evidence we could collect other than concrete proof of
existence that would somehow prove multiverses.

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mietek
In his 1985 paper, “Quantum theory as a universal physical theory”, Deutsch
writes:

 _> I have pointed out several times in the preceding sections that the C.I.
and the Everett “interpretations” are really different formalisms for quantum
theory—in effect, different physical theories. It is usually claimed that
although their assertions as to the nature of objective facts are radically
different, the two “interpretations” agree about all subjective experiences of
observers, and cannot therefore be distinguished experimentally. That this
claim is false is shown by equations (40) and (42) which summarize the
different predictions of the two “interpretations” concerning further
measurements on subsystems one of which has completed a measurement on the
other._

[https://pdfs.semanticscholar.org/c948/44bc9128e77077abdf36a6...](https://pdfs.semanticscholar.org/c948/44bc9128e77077abdf36a6e2cd0807252ea5.pdf)

~~~
jerf
I judge by the fact that there aren't very many people doing experiments meant
to distinguish the various proposals that physicists continue to generally
believe there isn't much that can be done to distinguish the various
interpretations from each other. Given the incentives to complete such an
experiment and even just eliminating one possibility (even if one couldn't
nail down which is true), and the fact that it seems to me many of the
possible experiments that could do this are more lab-bench style experiments
with a laser and a handful of mirrors and electronics rather than Large Hadron
Collider-class experiments, I'm guessing the idea that the theories are
distinguishable is a minority view, and one that the minority has not yet
managed to do something so compelling with that the majority has no choice but
to take note.

~~~
mietek
The specific experiment proposed by Deutsch is not yet possible to perform in
practice.

