Understanding the Multiverse per David Deutsch

[expos4ever]

I find David Deutsch to be interesting, however I find his writing style a little hard to follow at times. If anyone can shed light on what he means by the following, it would be appreciated:

"the multiverse is not a discrete set of universes but a continuum"

I can accept the concept of a large number of universes, or even an infinite number. But what does it even mean, conceptually, to speak of a continuum of universes? I though quantum mechanics dispensed with the notion of anything being fundamentally a continuum.

 

[trurle]

If you mean "The Beginning of Infinity" book, it is not a scientific publication, therefore question about "multiverse continuum" should be treated as "dramatic expression", not as "scientific hypothesis".
Underlying idea may be
"so many universes what you can always find universe arbitrarily close to other, with any imaginable definition of "close to""

 

[expos4ever]

Underlying idea may be
"so many universes what you can always find universe arbitrarily close to other, with any imaginable definition of "close to""

I suspect you are right - thanks for the feedback.

 

[Bandersnatch]

I haven't read the book. Is he talking about the many-worlds interpretation of quantum mechanics here, or about some form of the cosmological concept of a multiverse? (some more context please)

 

[expos4ever]

I haven't read the book. Is he talking about the many-worlds interpretation of quantum mechanics here, or about some form of the cosmological concept of a multiverse? (some more context please)

I don't understand the distinction you are drawing here. I will nevertheless answer as follows: I am highly confident he is talking about the many-worlds interpretation of QM.

 

[Bandersnatch]

In that case, I like trurle's answer.

 

[DarMM]

"the multiverse is not a discrete set of universes but a continuum"

Some recent work on the MWI suggests that the multiverse would have to contain a continuous infinity of universes, i.e. it's not just a expression.

So take the real line between zero and one, there's a universe for every real number on that line.

 

[expos4ever]

Some recent work on the MWI suggests that the multiverse would have to contain a continuous infinity of universes, i.e. it's not just a expression.

So take the real line between zero and one, there's a universe for every real number on that line.

Ok, I think I understand what you are saying. I had always conceived of infinity as necessarily referring to discrete things - an infinite number of integers, for example. I assume you are implicitly telling me that the notion of a continuous infinity is conceptually coherent.

 

[DarMM]

I assume you are implicitly telling me that the notion of a continuous infinity is conceptually coherent.

Well it's used all the time in mathematics, integration and differentiation for example require it, as do just the plain old real numbers.

 

[Michael Price]

I find David Deutsch to be interesting, however I find his writing style a little hard to follow at times. If anyone can shed light on what he means by the following, it would be appreciated:

"the multiverse is not a discrete set of universes but a continuum"

I can accept the concept of a large number of universes, or even an infinite number. But what does it even mean, conceptually, to speak of a continuum of universes? I though quantum mechanics dispensed with the notion of anything being fundamentally a continuum.

Yes, this resolves around whether anything is continuous in quantum theory. There are pointers in either direction.
The Bekenstein bound implies an upper limit to the entropy of our observable universe, which implies only a finite number of discrete states. This would certainly jive with most people's intuition that spacetime is discrete below the Planck limit (although no one knows).
OTOH, the infrared divergences tell us that an infinite number of soft photons and gravitons are emitted and absorbed during most interactions, which sort of implies a continuum at some level.
So, in conclusion, no one knows for sure.

 

[Marek Domanski]

Firstly, it is no way obvious how to post a question on this forum. I have only found out how to make a reply.
David Deutsch is a well known proponent of the Many Worlds Interpretation. His argument seems to be that a single photon in the double slit experiment must be interfering with one from another world. It is commonly held by physicists that the the photon as a wave going through double slits can produce interference. Possibly he does not believe that the photon can be treated as a wave. Is this true, or does he have another reason? I am having difficulty find this information on the internet. I read his book The "Fabric of Reality" years ago and can't remember if or how he justified his position.

 

[Marek Domanski]

Regarding the continuum of universes. Maybe Deutsch is referring to his hypothesis that all the universes interfere with each other in quantum experiments. Also, he could mean that you would have a continuum of universes where one differs from the next by the smallest degree. It is hard to pin him down sometimes, as per my question above.

 

[PeterDonis]

Moderator's note: Moved thread to QM interpretations forum.

 

[PeterDonis]

it is no way obvious how to post a question on this forum.

There is a dark blue "Post Thread" button at the top right of every forum (i.e., every page listing the threads in a forum).

 

[MathematicalPhysicist]

Some recent work on the MWI suggests that the multiverse would have to contain a continuous infinity of universes, i.e. it's not just a expression.

So take the real line between zero and one, there's a universe for every real number on that line.

It doesn't make much sense.
I think it's more an enumerable number of universes, i.e. $\aleph_0$.
I mean a cardinality of the continuum means you cannot even count them.
There's no well ordering for the set of universes if their number is $\aleph$.

 

[PeterDonis]

There's no well ordering for the set of universes if their number is $\aleph$.

I assume you mean $C$ here (the cardinality of the continuum). However, if the Axiom of Choice holds, this statement is false, because one of the consequences of the Axiom of Choice is that every set can be well-ordered.

 

[MathematicalPhysicist]

I assume you mean $C$ here (the cardinality of the continuum). However, if the Axiom of Choice holds, this statement is false, because one of the consequences of the Axiom of Choice is that every set can be well-ordered.

Forever Undecided...