Quantum myth 4: The only reality is the measured reality

peter0302

Easy to say, but what is the physical mechanism for the split? Where does the energy come from that causes the split? It's just taking the locality and causality problems of CI and giving them new names.

Hurkyl

I don't understand the question -- AFAIK, unitary evolution is fundamental in the usual formulations of quantum mechanics, so it doesn't make sense to ask for the physical mechanism behind it. (Make sure you don't fall into the trap of infinite regress! Nor into the trap of letting aesthetics dictate the physics)

Why would splitting need energy?

Demystifier

Can you rephrase your question? I have no idea what exactly do you ask me.

Phycho

Hurkyl, you've turned over to MWI proponent?

I'd say if theu niverse made copies of it self each split it would need infinite energy, yes, defendable position ? hell no

Hurkyl

When you say 'copies itself', do you mean it sort of like passing the universe through a copy machine to produce a Star Trek-like parallel universe? If so, then there's no problem, because that has nothing to do with MWI.

peter0302

Every time you add a new degree of freedom, you need more energy. It's basic thermodynamics.

Phrak

Hurkyl- If I may paraphrase, what you are saying is that in the MWI, there is no physical splitting. Instead, for any system of particles, the wave equation of the system evolves unitarily. Is this correct? Though I suppose there may be more than one MWI interpretation.

-Demystifier. Sorry, it was a poor question on my part.

Phycho

Hehe, I mean more, if a particle splits, it copies itself if this is true on a macroscopic scale.
Every second there are copies of you splitting off to other universes.
Is this the MWInterpretation you support?

peter0302

What other MWInterpretation is there?

Perhaps the argument is over semantics (it usually is with people here). Each possible state of a particle has a particular probability amplitude - you might think of this as its "thickness." If there are two equal possibilities, both states are 50% as thick as the state from which the possibilities originated. And so forth. They split ad inifnitum.

So it's not as though "copies" are being made, so much as you're taking a hologram and dividing it in two (as you may or may not know, when you cut a hologram in half you get two copies of the same image, though you can view less of it in each one).

I think Hurkyl's argument is that since you're splitting - not copying - there's no energy problem. But that doesn't solve the basic thermodynamics issue that every time you add a new degree of freedom you need more energy, and there's no question that every quantum possibility is a new degree of freedom - and they quickly accumulate at gigantic rates - and so, it would seem, would the energy requirements.

Hans de Vries

If the universe would split each 3.10^-25 second (say, the lifetime of the Z-boson) in
two, then we would have something like 10^(10²⁴) parallel universes after 1 second.

Written out in decimal (with 500 digits/meter) this is a number with a length of
circa 60,000 light years. Each further second the length of this written number,
giving the total amount of parallel universes, becomes longer by 60,000 light years.

That is, the length of the written number describing the count of different parallel
universes would grow with a speed of 2000 billion times the speed of light......


Regards, Hans

Hurkyl

Yes. When MWI uses the term "splitting", it is describing a behavior that a unitarily evolving system can demonstrate. The same is true of terms like "parallel universes".

I think a good example would be based on the famous experiment with a photon and half-silvered mirrors. After striking the first half-silvered mirror, there is one "world" with a photon in a superposition of being reflected and transmitted. This photon can self-interfere, and we find that the photon is guaranteed to reach detector 2.

Now, if the photon somehow interacted thermodynamically with the environment between the first and last mirror (thus destroying its coherence), it will have "split", and we now have two "parallel worlds", one where the photon was reflected, and one where the photon was transmitted. The photon can no longer self-interfere, so when it strikes the second mirror, it will go to either detector with equal probability.

(We could force a thermodynamic interaction by, for example, putting a device along one path that measures whether or not the photon took that path)


If you mean "copy" like a copy machine in which you started with one particle and now have two particles, then I'm not aware of that being any flavor of the many-worlds interpretation.


I know that splitting is closely related to quantum thermodynamics (one buzzword: decoherence); splitting is supposed to be an irreversible process, which is only possible in the thermodynamic sense. (because unitary evolution is always theoretically reversible, even if not practical) Unfortunately, that statement nearly exhausts my hard knowledge about quantum thermodynamics.

Phrak

Thanks Hurkyl. But the truth is, now I'm really confused. MWI seems to recognize two distinct processes. The first, unitary evolution, or what might be called causal evolution, if I'm not abusing terminology. Secondly, splitting, a result of decoherence. What I'm getting at, here, is that decoherence is not recognized as causal evolution of the larger system consisting of particle plus environment, right?

Hurkyl

Actually, that's the neat part: decoherence is the result of ordinary unitary evolution of the particle + environment system!

reilly

You are describing a standard probability chain -- they have been used for many years without any ascription of reality to individual branches -- except in science fiction stories --. Why bother, when Hans has noted, conservatively, the description of the MWI universe defies physics, not to mention common sense. Where are these universes? In what sense are they real?

If, somehow, they are real, then what about inter-universe dynamics? Why not a subset of universes that conspire to create a superconducting super-universe? It all sounds to me like:if you can imagine it, it is real. But then, I could imagine that what I imagined is real isn't real. (Note that all this imagining is the direct result of physical processes, so clearly our individual rates of creating alternate universes go on unabated into mathematical territories , largely-yet- to- be -explored by humans. Think topological nightmares, non-separable subspaces, regions where the Axiom of Choice holds; regions where it does not. If I can think it, it's real, or is it?

And, by the way, is the one photon world you mention above, just one, or is it many worlds, all differing by slightly different molecular configurations of the silvered screen, differing by traffic patterns in New York City, differing by whether at cash register 12 in Store 18 in Rochester NY, Ralph asks for paper or plastic.

In fact, for the silvered screen and photon, there are huge numbers of possibilities. That is, we are effectively dealing with a scattering problem with, well more than 10^^23 target silver atoms. If we look just at the infrared portion of the emission spectrum of the silver atoms, we'll find that each atom has a probability to emit none to, say, 10^^(10^^10) or more low energy photons -- Poisson process and all that -- in any time interval you want. So, there are an arbitrarily large number of "universes" for the "one " universe; typical of Poisson processes of Poisson processes of ....... Is it not the height of irony, that MWI can only be realistically described by statistical methods -- those pesky Poisson processes certainly mess things up.
Regards,
Reilly

Hurkyl

I'm not familiar with the term "probability chain" in this context, but your comment is entirely unsurprising. The MWI invokes no new physics -- it merely uses a subset of existing physics -- it is only natural to expect its features to be familiar from other contexts.

I'm quite aware that there are many interpretations of quantum mechanics.


I thought we were talking about MWI. Why are you talking about science fiction?


They interact exactly as specified by unitary evolution. Observable effects are incredibly rare for conceptually the exact same reason that you never observe a vacuum spontaneously appear in your kitchen because all of the air evacuated into the living room: it's just too unlikely.

I can make neither heads nor tails of this question.

Let me know when you're done talking about science fiction.


I'm confused; quantum thermodynamics is the very foundation of the MWI! The whole idea is that quantum decoherence (a statistical phenomenon) closely approximates the effects of wavefunction collapse, thus obviating the need for using collapse in quantum mechanics. I don't understand how it could possibly be ironic that 'MWI can only be realistically described by statistical methods'.

peter0302

Reilly, it's odd that you would have so much hostility toward MWI when you yourself believe that consciousness causes wavefunction collapse, which is no less outlandish than MWI.

In general it's funny to watch all the different people argue over their interpretations. The only sensible interpretation is that we don't know until we can find an experiment to disprove one or the other. That's what we should be arguing about. Not whether something sounds like science fiction or not.

GiZeHy

Hi folks,

i am happy to see people in this thread, who are engaged with quantum physics and ask themselves kind of philosophical questions, because i believe that this is the main drive for many of us to understand how the universe, and all within it, behaves.

To approach the main question of the thread we must combine the knowledge of all sciences. It really is quite naive to argue that there is no reality beside the measured one. In fact, if one is intensely thinking about it, there is no direct answer to this question. I can only deduce it from experiences in this so called reality.

The main conflict thinking about it is that physics is a science to explain incidents that are measurable. It is the same with quantum physics. But how do you know that something does not exist when you cannot measure it? If one is getting results concerning a certain experiment, it does not mean that there are no other influences. Imagine that gravity cannot be explained in detail. We only create formulas of what we experience. With a grand united theory these will be expanded and used in another way.
(Beside: What about different realities because of different outcomes of experiments? Think about the particle/wave conflict. I think it is blowing enough minds that beholding an experiment or not affects the result of it)

Actually one could equate the measurements with experiences of a conscious mind, because the measurements are only a tool to enhance the perception of the senses. Think of a child and its reality. Is is aware of quarks, atoms, molecules, their forces and interactions and so on? No...probably not ;) Does it exist? What if everything exists but we cannot measure it, what if it only exists because we built something that is able to measure it? We see that there are many aspects to consider concerning this kind of question.