Many-worlds: When does the universe split?

[.Scott]

This idea of Penrose might be interesting for you.

Just looking at the chapter titles in his book in sections 1 and 2, it appears Penrose and myself are on the same page.
This won't be the first time. I give kudos to Penrose for being emphatic that consciousness, at its root, is an artifact of QM. I think most Physicists would agree - but it's not an allowed topic on this board.

I still have some objections. Unfortunately, your non-standard terminology makes it hard to get to the core of the issue. For example it doesn't make sense to call an event which reduces entropy a "decoherence event" although the underlying idea may well be valid.

Given my background, and specifically lack of a serious Physics background, I am certainly open to challenges on my use of proper terminology. Although I was familiar with the term before using it on this board, my use of it on this board was based of how previous posts were using it.
Having reviewed the wiki article, the term "decoherence" clearly carries some luggage that is interesting but not fundamental to how I am describing the increase in information or entropy.

What is essential to my arguments is that decoherence (or any other rose by any other name) creates a set of outcomes with no possibility, in principle, of knowing which one you will observe. The real problem is with the notion of "increasing entropy" or "adding information". I noticed that Penrose's section 2 title is "The oddly special nature of the Big Bang". It did create something special. It created an environment filled with clocks. We know the Big Bang happened about 13.8 billion years ago! In our current universe, there is no reasonable doubt about which world predates which world. Whenever there is a decoherence event, all of the possible outcomes create worlds that we have never occurred before, so our formula for added information is not challenged. However, if we are in a world when heat death has occurred and your world has limited mass and space, then you can't presume that entropy has increased - even though it is the very same type of event.
For example, in the current world a single photon leaves a flood lamp with a miniscule aperture and strikes one of a billion atoms. Each of those atoms representing the transition into a world that has never existed since the Big Bang. So, if for one particular atom, if its chance of being struck is one in a billion, then the world entered for the photon hitting that atom will have an additional 30 bits from the pre-decoherence world.
But that same scenario in a world that has, in principle, lost track of time, will add 30 bits but possibly land you in a world that has already been "created", one with a shorter time line than the world where the photon had not yet decohered. The decoherence event is fundamentally the same. The probability is the same. You still have 30 "added" bits, but they're not really new bits because they don't land you in a unique, never-before-seen world.

The most obvious point is about the Bekenstein bound. The bound takes it's maximum entropy value for a black hole. A black hole is not isolated from it's environment: it absorbs matter and emits Hawking radiation. My understanding is that the bound occurs in the first place because a region of a certain radius which contains a certain amount of matter (resp. energy) cannot be isolated better from it's surroundings than a black hole. I don't see how it makes sense to apply this bound to the universe as a whole.

/edit: Also I think we need to keep in mind that we are not talking about the MWI here but about a speculative combination of the MWI and general relativity. As far as I know, the Bekenstein bound is derived from both GR and QM. We know that the simple combination of GR and QM is impossible at least in some cases. So the bound could be an expression of this incompatibility.

The critical part of the Bekenstein bound to my argument is that it puts a cap on the amount of information that can be held by any world. If there is no such limit, then there is no upper bound on entropy and we don't have to worry as much about heat death and we don't have to worry at all about being, in principle, unable to track time.

 

[craigi]

Just looking at the chapter titles in his book in sections 1 and 2, it appears Penrose and myself are on the same page.
This won't be the first time. I give kudos to Penrose for being emphatic that consciousness, at its root, is an artifact of QM. I think most Physicists would agree - but it's not an allowed topic on this board.


Given my background, and specifically lack of a serious Physics background, I am certainly open to challenges on my use of proper terminology. Although I was familiar with the term before using it on this board, my use of it on this board was based of how previous posts were using it.
Having reviewed the wiki article, the term "decoherence" clearly carries some luggage that is interesting but not fundamental to how I am describing the increase in information or entropy.

What is essential to my arguments is that decoherence (or any other rose by any other name) creates a set of outcomes with no possibility, in principle, of knowing which one you will observe. The real problem is with the notion of "increasing entropy" or "adding information". I noticed that Penrose's section 2 title is "The oddly special nature of the Big Bang". It did create something special. It created an environment filled with clocks. We know the Big Bang happened about 13.8 billion years ago! In our current universe, there is no reasonable doubt about which world predates which world. Whenever there is a decoherence event, all of the possible outcomes create worlds that we have never occurred before, so our formula for added information is not challenged. However, if we are in a world when heat death has occurred and your world has limited mass and space, then you can't presume that entropy has increased - even though it is the very same type of event.
For example, in the current world a single photon leaves a flood lamp with a miniscule aperture and strikes one of a billion atoms. Each of those atoms representing the transition into a world that has never existed since the Big Bang. So, if for one particular atom, if its chance of being struck is one in a billion, then the world entered for the photon hitting that atom will have an additional 30 bits from the pre-decoherence world.
But that same scenario in a world that has, in principle, lost track of time, will add 30 bits but possibly land you in a world that has already been "created", one with a shorter time line than the world where the photon had not yet decohered. The decoherence event is fundamentally the same. The probability is the same. You still have 30 "added" bits, but they're not really new bits because they don't land you in a unique, never-before-seen world.
The critical part of the Bekenstein bound to my argument is that it puts a cap on the amount of information that can be held by any world. If there is no such limit, then there is no upper bound on entropy and we don't have to worry as much about heat death and we don't have to worry at all about being, in principle, unable to track time.

You misunderstand heat death. At heat death there are no flood lamps and there are no apertures. There are no atoms and there is no coherence to decohere.

 

[.Scott]

Indeed. This is where it gets very complicated.

As Bob suggested, decoherence is just a probabalistic process, in the same way as for other entropic events. In theory, we could consider a concept of recoherence, but this really brings in the question of what we mean by time.

We have time from the relativistic space-time continuum, time as a parameter in the Schrodinger equation and time as increased entropy.

I'm hoping that you can see from my last post that I really was saying that decoherence can cause a reduction in entropy. The issue isn't with the local particles involved in the decoherence but in the "other-worldly" circumstances.
In the MWI, your space-time continuum is assembled with tiny instants of time, each leading to two or more others. The time part of the space-time continuum is what drives the laws of physics and transitions you from one world to the next. An increase in total entropy is what provides us with a sense of the long-term direction of time. If you were dropped into a world with heat death, you could still perceive time, but after a googol years, you would die and blend in with the heat - and all of your time-keeping potential would be lost.

There are even concepts of time being directed by the expansion of the unvierse, and suggestions that time and space may extend beyond the universe (observable or otherwise). The reason that we, as humans, observe time in the direction that we do is due to the entropic direction of time.

Sounds good to me.

Entropy is at the heart of all the chemical and biological processes that give rise to our consciousness.

I would be more specific and say "our human consciousness", only because I believe that consciousness is a basic part of all physics.[/QUOTE]

If we talk of decoherence happening backwards, then perhaps we're actually talking about microscopic time reversing for the particle (or entangled particles even), but then does this arrow of time match the direction given from the parameter in the Schrodinger equation? It would seem not.

I am not saying that the time is reversed at the particle level. There is still an arrow of time. It is simply that heat death can lead to a situation where every step forward in time brings you to a world with less history.

Scott's idea of "remergence" is based upon his serial number concept. In that, the same serial number emerges from different branches. I'm not even sure how it can occur, because as he describes his serial number, when a branch occurs he splits a bit to generate 2 new ones. Nevertheless, presuming that there's an extra mechanism to his serial number generation, I don't see how this could map to a reversal of decoherence or entropy. I would expect it just to map to probabilties of occurance of a particular macrostate, rather than anything more interesting.

You can only avoid remergence in a world with unlimited information capacity. That "extra mechanism" is the limit, such as the Bekenstein limit. If there is no limit, remergence can be avoided.

 

[craigi]

I'm hoping that you can see from my last post that I really was saying that decoherence can cause a reduction in entropy. The issue isn't with the local particles involved in the decoherence but in the "other-worldly" circumstances.
In the MWI, your space-time continuum is assembled with tiny instants of time, each leading to two or more others. The time part of the space-time continuum is what drives the laws of physics and transitions you from one world to the next. An increase in total entropy is what provides us with a sense of the long-term direction of time. If you were dropped into a world with heat death, you could still perceive time, but after a googol years, you would die and blend in with the heat - and all of your time-keeping potential would be lost.
Sounds good to me.

I would be more specific and say "our human consciousness", only because I believe that consciousness is a basic part of all physics.
I am not saying that the time is reversed at the particle level. There is still an arrow of time. It is simply that heat death can lead to a situation where every step forward in time brings you to a world with less history.
You can only avoid remergence in a world with unlimited information capacity. That "extra mechanism" is the limit, such as the Bekenstein limit. If there is no limit, remergence can be avoided.

Sorry man, none of that is physics. You'd be better off taking it somewhere like David Icke's site.

You seem to presume that I have a similar resilience to a heat death scenario as a black hole might. I can assure you that I don't. There are few estimations that you could make in physics that are so far out.

If you're interested in physics it's time to take reading more seriously and put your homegrown theories and postmodern philosophies to bed for a while.

 

[kith]

The critical part of the Bekenstein bound to my argument is that it puts a cap on the amount of information that can be held by any world.

The Bekenstein bound is about a region of space. You either have to show that the worlds are confineable to such regions of space or that the Bekenstein bound applies to the universe as a whole. I don't think either of these statements is correct.

Also some points about terminology as well as about relevant concepts:
-In QM, the entropy of a subsystem can be higher than the entropy of the whole system (this is because of entanglement).
-Decoherence is simply the evolution from a state with zero entropy towards a state with maximal entropy wrt to some constraints.
-Decoherence occurs only if an environment is present and fully decohered states are only approximately stable.
-Decoherence is not the cause of anything, it is the result of entanglement in the whole system.
-The same thing which leads to entanglement/decoherence can lead to disentanglement/recoherence
-This thing is simply the Schrödinger equation for the whole system
-The Schrödinger equation preserves entropy (note that the Schrödinger equation isn't valid in the subsystem)
-All of this can be observed in experiments, so nothing of this is specific to the MWI
-Difficulties arise only if you ask what is the relation between the QM system and the experimenter
-There are no decoherence "events". If you talk about selecting outcomes you are not talking about decoherence but about collapse

 

[.Scott]

The Bekenstein bound is about a region of space. You either have to show that the worlds are confineable to such regions of space or that the Bekenstein bound applies to the universe as a whole. I don't think either of these statements is correct.

I never presumed they were. If the universe is either allowed to increase in mass without limit or is allowed to increase spatially without limit, then there will always be a "clock".

Also some points about terminology as well as about relevant concepts:
-In QM, the entropy of a subsystem can be higher than the entropy of the whole system (this is because of entanglement).
...
-The same thing which leads to entanglement/decoherence can lead to disentanglement/recoherence
-This thing is simply the Schrödinger equation for the whole system
-The Schrödinger equation preserves entropy (note that the Schrödinger equation isn't valid in the subsystem)
...
-There are no decoherence "events". If you talk about selecting outcomes you are not talking about decoherence but about collapse

Thanks. I actually picked that term up from you. In post 2 you said "In the MWI, decoherence is what splits the worlds...". From what you're saying now, I suspect that collapse is what splits them in a more permanent fashion.

 

[kith]

I never presumed they were. If the universe is either allowed to increase in mass without limit or is allowed to increase spatially without limit, then there will always be a "clock".

Also if this isn't the case: it is far from obvious that the Bekenstein bound should apply to the isolated system of the whole universe. As I said: the bound is reached for a black hole which is an open system.

Thanks. I actually picked that term up from you. In post 2 you said "In the MWI, decoherence is what splits the worlds...". From what you're saying now, I suspect that collapse is what splits them in a more permanent fashion.

Yes, this may have been a little misleading. In the MWI, full decoherence during an observation is interpreted as a splitting of worlds. In the Copenhagen interpretation, a fully decohered state is interpreted as a set of possibilities from which one "true" observation outcome is chosen by an additional process called collapse. In the MWI, this process is simply absent because all outcomes are equally "true".

The splitting of worlds gets its significance only from the inside perspective of an observer who perfroms a series of observations. If there was an entropy limit, there would be a configuration where all subsystems you consider important are fully decohered and recoherence would be inevitable. The worlds would remerge. But at this point, the observer, who can only exist in a low entropy state, would be gone. Once his inside perspective is lost, why should we continue to take such a viewpoint?