# What exactly is a world in multiple world interpretation?

1. Jan 12, 2016

I ask this question because I am confused. I have seen two contradictory definitions.

The split causes a whole new universe.

"The concept of a world in the MWI is based on the layman's conception of a world; however, several features are different. Obviously, the definition of the world as everything that exists does not hold in the MWI. “Everything that exists” is the Universe, and there is only one Universe."

So even the concept of world is in dispute. I find it hard to believe that the split of the entire universe occurs multiple times for every quantum event.. So if a photon is measured, the whole universe is split multiple times. All the millions of galaxies 5 billion light years away are also split multiple times?

It sounds like a mathematical solution in search of a physical universe. Do the physicists who think this is correct basically mathematicians who what it to be true because of the mathematical elegance of the solution?

Thanks,
John

2. Jan 13, 2016

### bhobba

You are over complicating it.

After decoherence the state of the system is Σpi |bi><bi| where the |bi><bi| are the possible outcomes of the observation and pi gives the probability of outcome i. All MW does is assume instead of outcome i being realised the |bi><bi| are interpreted as separate worlds. Everything simply continues to deterministically evolve. Its just an interpretive assumption.

Why do some like it? Its very elegant and beautiful. If you find it too weird - you are not the only one. So go for some other interpretation.

Thanks
Bill

3. Jan 13, 2016

### Strilanc

I wouldn't characterize many-worlds that way. It's more like... purification taken to the extreme. In many-worlds there is no such thing as a measurement (or collapse) at the fundamental level, only controlled operations that allow entangled information to escape into the giant entropy sink we call the environment. Measurements and even probabilities become facts derived from how the superposition state evolves: a consequence of unitarity and how we are embedded into the system, instead of a starting point.

4. Jan 13, 2016

I am confused by the 2 replies. "All MW does is assume instead of outcome i being realised the |bi><bi| are interpreted as separate worlds." This "interpreted" word sounds like a mental concept. Is every world "real" in one universe? Are the multiple worlds like a blossom of peddles in a local spot in the one universe. And will particles from outside of this blossom interact with the blossom of worlds when they pass through the blossom worlds space? Or exactly what happens? Or is the entire universe replicated multiple times per quantum event? Mind blowing thought.

Thanks,
John

5. Jan 13, 2016

### bhobba

'Real' is a very unclear concept and best to avoid it - although on occasion that's not really possible. For example we have guys like Penrose that believe the world around us is in some sense illusion, the reality being a Platonic realm where mathematical truth lies.

Call the worlds of MW real, mental construct, Jaberwocky - it doesn't actually matter. The key thing is each outcome of an 'observation' is interpreted as a separate world and no change in state occurs.

If you want to go further than that then its really philosophy and that subject, as opposed to science, never seems to agree on anything.

I know lay people that post here want to answer those BIG questions, but unfortunately science has less lofty aims seeking to describe reality without being 100% clear on what that is exactly. Personally I take reality to be what out theories describe. In MW that would be the world that we experience, which most certainly is real; well most would say it was, I certainly would.. In that sense the worlds are real. But that's my view - you will likely find those that differ, which is why I think its best to avoid terms like that if possible.

Each |bi><bi| are separate outcomes so do not interact. If you throw a dice and interpret each side of the dice as a separate world - by your interpretive assumption they do not interact. The definition of what a world is precludes such a concept - its meaningless. Note there is in decoherence an issue here - but that cant be discussed at the lay level and I wont do it - for all practical purposes they are separate worlds.

Thanks
Bill

Last edited: Jan 13, 2016
6. Jan 14, 2016

### ddd123

As I understood it, MWI rejects the notion of collapse in a way that the scientist himself enters an entangled state when he sees a specific result and not another. So the present conscious activity of the scientist seeing "spin down" exists in one basis vector of a universal pure state. Don't misunderstand me, it's not a consciousness-based interpretation, it's just that if one was a hyperbeing watching all the universe from the outside he could say "okay everything is one pure state", but since we actually see one result of measurement and not another this begs the question.

7. Jan 14, 2016

### zonde

If you look at math part of Quantum mechanics it's that you can get only probabilities of measurements out of it. But it does not say what will be the outcome for particular single measurement event. If you try to apply QM to this question it becomes even worse as illustrated by Schrodinger's cat thought experiment.
So there interpretations come into the picture. As I understand it Many worlds is interpretation that sort of accepts Schrodinger's cat paradox in a way and says that there are no definite outcomes but we (as a complex quantum systems) perceive the world as being in a definite state.

8. Jan 14, 2016

Thanks for the responses. So the multiple worlds outcome of a quantum event is local? That is, it does not extend to the entire universe for that one quantum event? I am not that skeptical of multiple worlds concept, but the idea of the split occurs for the entire universe is hard to believe.

If it is a local affect, then what is the conceptual frame work of what is "in" and "out" of the many worlds for that one quantum event? Is the affect local? This is what I was wanting clarity on.

I may be captured by the bias of existing in a certain physical world. The concept of all things have a beginning, middle, end is ingrained in my thinking. The quote below talks about "outside" also.

"if one was a hyperbeing watching all the universe from the outside he could say "okay everything is one pure state", but since we actually see one result of measurement and not another this begs the question."

If the many worlds is a local affect, then is there interaction from the "one universe" outside objects and the local many worlds objects?

Thanks,
John

9. Jan 14, 2016

### ddd123

I think you misunderstood me. The hyperbeing example is an absurd reasoning.

To understand the point you have to know what a pure state is, and how entanglement is produced in the first stages of measurement. Since the measurement instrument is a quantum object, entanglement with the measured object happens. But, according to decoherence theory, that quickly becomes so scattered in the environment that a definite state outcome occurs (simplify it by Born's rule). Though, in principle, if you followed all the numerous entanglements you could reconstruct the pure state. In this view the universe is always in a pure state, measurement is only entanglement and an outcome is one basis vector that we see. The splitting worlds view is pictorial. I guess the splitting is "local" in the sense that it's the decomposition relevant to the specific bodies that you're looking at. It's not local in the relativistic sense. I would call it contextual, not local.

10. Jan 14, 2016

### atyy

It's a difficult question.

http://arxiv.org/abs/quant-ph/0103092
Worlds in the Everett Interpretation
David Wallace
(Submitted on 16 Mar 2001)
This is a discussion of how we can understand the world-view given to us by the Everett interpretation of quantum mechanics, and in particular the role played by the concept of world'. The view presented is that we are entitled to use many-worlds' terminology even if the theory does not specify the worlds in the formalism; this is defended by means of an extensive analogy with the concept of an `instant' or moment of time in relativity, with the lack of a preferred foliation of spacetime being compared with the lack of a preferred basis in
quantum theory. Implications for identity of worlds over time, and for relativistic quantum mechanics, are discussed.

11. Jan 14, 2016

### bhobba

Its the entire universe.

The correct state is |bi>|rest of universe><rest of universe|<bi| but for simplicity is usually written as |bi><bi|.

Thanks
Bill

12. Jan 15, 2016

Thanks Bill.

So for every photon on a double slip experiment that hits the screen and splits into different worlds, all other galaxies in the universe split too? Doesn't that on the face of it seem ridiculous? Considering the mind-blowing number of wave to particle quantum events just in our world, let alone our galaxy, and all galaxies, this is hard to believe. The mathematics may be elegant, but the physics sounds ridiculous.

Thanks,
John

13. Jan 15, 2016

### Staff: Mentor

There's something not to like about every interpretation, and this extravagant multiplication of worlds is the thing not to like about MWI.

14. Jan 15, 2016

### jlcd

guys how do you distinguish this many worlds with splits producing classical worlds versus this many worlds version where all basis exist or an initial superposition soup that is pure unitary... im a bit confused by the two.. tnx

15. Jan 15, 2016

### ddd123

As I mentioned I understood it, they're the same thing, or should be but the fact that we see one thing happening and not another.

16. Jan 15, 2016

### Staff: Mentor

I don't see how, in the context of quantum mechanics, you can apply the adjective "unitary" to anything except an operator or an evolution produced by a unitary operator. My best guess is that you don't know what the word means.

But if you're asking how to distinguish between "many worlds with splits producing classical worlds" and "this many world version where all bases exist".... there's nothing to distinguish. All bases exist in quantum mechanics no matter what interpretation you choose, and classical outcomes are an experimental fact that all interpretations must produce one way or another. MWI is an interpretation, so necessarily allows for all bases and also must produce classical outcomes.

17. Jan 16, 2016

### jerromyjon

I suppose if I made it sound even more ridiculous perhaps it makes more sense?
Imagine you have a useless switch in front of you. It is simply a lever connected to nothing with a label which reads "select a world to enter: A<>B". When you decide which way to throw the switch you choose which world exists. The virtual world that you did not choose never exists.

18. Jan 16, 2016

### stevendaryl

Staff Emeritus
I'm not sure if this helps make Many-Worlds less ridiculous, or not, but in Many-Worlds, the only thing that exists is a wavefunction, and that function just merrily evolves smoothly according to the Schrodinger equation (or some relativistic counterpart). The "Many" from the many worlds is simply from the fact that you can describe a single wave function as a superposition of many other wave functions, in exactly the same way that you can describe an arbitrary function $f(t)$ as a combination of sines and cosines. So it's not that there are worlds popping into existence all the time---the worlds are just different views of one and the same wave function.