# Many Worlds Interpretation

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1. Apr 2, 2015

### Rodrigo Cesar

"We can’t make a measurement without influencing what we measure.
before we look, there are only probabilities. When we open the box, they give way to a single actuality"

It would be more like this, all the time, Until we look?

Last edited by a moderator: May 7, 2017
2. Apr 2, 2015

### rootone

The cat in the box is a visualisation of 'the measurement problem', we can't draw any conclusions from it.
'Multiverse' is one interpretation of QM among several, and it isn't an established fact.

Your original quote is reasonable enough - before something has been measured we don't know what it's measure is!, although we may have been able to establish a range of probabilities.
In the cat example it's life or death depends on the half-life of atomic decay - which is probabalistic.
If the cat is in the box for exactly the half life of the atom, then the probability of it being alive is exactly 50%

After the experiment is finished (box opened), then the measurement has been made.
We do know what happened. its no longer a possible outcome, it's a known outcome

Last edited: Apr 2, 2015
3. Apr 2, 2015

### bhobba

That looks like a misunderstanding of Schroedinger's Cat.

You will find many threads on this forum discussing that thought experiment. The point though is in the standard Copenhagen interpretation QM is a theory about observations that occur in an assumed common sense classical world. In Schroedinger's Cat that observation occurs at the particle detector - everything is common sense classical after that. The purpose of the thought experiment was to show, while its obvious where you should put the observation, the theory doesn't force you to do that - in fact it says nothing about it. Then we have the issue of how does a theory explain the classical world when its assumed in the first place.

A lot of progress has been made in resolving those issues. If you are interested in the modern view the following, at the lay level, is a good source:
https://www.amazon.com/Understanding-Quantum-Mechanics-Roland-Omnès/dp/0691004358

Very picturesque. I don't know what the first is trying to depict, but the second one looks like Many Worlds. It's an interpretation and as such may or may not be true - but until there is a way to experimentally test it there is no way of telling. There are tons of other interpretations as well and they are all in the same boat.

Thanks
Bill

Last edited by a moderator: May 7, 2017
4. Apr 2, 2015

### Khashishi

The key point of many worlds is that there are many versions of you. But you are only conscious of one of you. One version of you sees a living cat; another sees a dead cat. The universe contains a mixture of these different scenarios, but each version of you doesn't see this mixture.

Last edited: Apr 2, 2015
5. Apr 2, 2015

### bhobba

That's precisely what MW says is not going on. The mixed state after decoherence is ∑pi |bi><bi|. Being a mixed state its no longer in superposition. Each |bi><bi| is interpreted as a world.

Nor are you entangled with the cat. The observation in Scrodinger's Cat occurs at the particle detector - that's where the splitting occurs in MW - in each world everything is common-sense from that point on - well as common-sensical as MW can be since decoherence is occurring all the time.

Thanks
Bill

Last edited: Apr 2, 2015
6. Apr 2, 2015

### Khashishi

edit: nevermind.
You say each |bi><bi| is interpreted as a world, but it's a superposition of states in the end. The result of decoherence is that the macroscopic world is only approximately diagonal with respect to the versions of you, to a very good approximation. The decoherence started back at the particle detector, so you are "well separated" from the other versions of you, but technically, it still is a grand superposition.

Last edited: Apr 2, 2015
7. Apr 2, 2015

### bhobba

Do you understand the difference between a mixed state and a pure state? Mixed states are not in superposition - that's a concept applicable only to pure states and reflects their vector space structure when mapped to such - in reality they are operators.

Thanks
Bill

8. Apr 2, 2015

### Khashishi

Yeah, I guess you are right...
I'll edit some of above to reduce confusion.

9. Apr 2, 2015

### bhobba

Thanks.

In this stuff its always wise to be careful - its tricky enough even when you are.

Thanks
Bill

10. Apr 2, 2015

### Rodrigo Cesar

"Very picturesque. I don't know what the first is trying to depict, but the second one looks like Many Worlds. "

I thought MWI was equal to multiverse, each universe would have other versions of these cats, but all cats in the same place? Where are the others, if we can see only 1? Invisible cats? As if they were ghosts?

11. Apr 3, 2015

### bhobba

The multiverse is applicable to a number of different ideas not just MW eg eternal inflation.

Thanks
Bill

12. Apr 3, 2015

### Edward Wij

Before I thought Many World was silly because in the atomic orbital like hydrogen atom, the electron has an almost infinity of position eigenstates.. so it's silly to think each position eigenvalue has its own words.. so many worlds only occurred after any measurement? How does this work in the electron orbital?

But then, if each electron position eigenvalues in the orbitals don't have separate worlds.. then what's the use of many worlds to explain QM?

Sean Carrol is convincing us many worlds may be the easiest thing to consider because the alternative is the anger interpretation (bohmians) or in denial of reality (the bellantinians).

13. Apr 3, 2015

### bhobba

That's not what it says. It says when you observe it the possible outcomes become separate worlds. You generally don't observe electrons in orbitals.

Its beauty incarnate mathematically. Like all interpretations make up your own mind.

I love mathematical beauty - but its too weird for me.

We also have the new Consistent Histories interpretation:
http://quantum.phys.cmu.edu/CQT/index.html

Many people consider it Copenhagen done right and MW without the many worlds. It does this by not even having observations - instead it is the stochastic theory of what are called histories. Each history roughly corresponds to a separate world (roughly is because the histories are course grained). There is only one history so you don't have the many worlds and there is no observation so the measurement problem is bypassed.

The link I gave details this well so I wont be going into it - if you are interested read the link.

Thanks
Bill

14. Apr 3, 2015

### Rodrigo Cesar

Denial of reality? what? lol

15. Apr 3, 2015

### Rajkovic

MWI the easiest thing? lol this idea is the most nonsense I've ever seen, mathematically beautiful, realistically bull****.. Sean Carrol must be smoking some Mushrooms

Last edited: Apr 3, 2015
16. Apr 3, 2015

### Rodrigo Cesar

It's not so strange, I think MWI would be more like that? Everytime we make a new decision, a new universe is created?

(someone correct if I'm wrong)

17. Apr 3, 2015

### rootone

Well there is a saying 'beauty lies in the eye of the beholder''
* shudders *
Let's not start bringing consciousness into it though.

18. Apr 3, 2015

### Quantumental

Except for the preferred basis and born rule problems that has yet to be solved ;p It' really not as mathetmatically beautiful when you look at the contrived attempts at solving these problems that come from David Wallace, Max Tegmark, David Deutsch, Sean Carroll. They add so many axioms that it's really no more elegant than Bohm

19. Apr 3, 2015

### craigi

In the MWI the different worlds 'exist' in an abstract mathematical space, Hilbert Space. So don't take this visualisation too literally.

20. Apr 3, 2015

### craigi

Not only Sean Carrol. The MWI is 'believed' by most leading cosmologists and string theorists.

The MWI was conceived of by Hugh Everett and had we stuck with his original name for it, the Relative State Formulation, then reactions like this every time someone new is introduced to it, would be far less common.

Last edited: Apr 3, 2015