# Many Worlds Interpretation and act of measuring

"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?

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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

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bhobba
Mentor
"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"
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/dp/0691004358/?tag=pfamazon01-20&tag=pfamazon01-20

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

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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.

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bhobba
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Mathematically, there's a superposition of yous.
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

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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.

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bhobba
Mentor
You say each |bi><bi| is interpreted as a world, but it's a superposition of states in the end.
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

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

bhobba
bhobba
Mentor
Thanks.

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

Thanks
Bill

"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?

bhobba
Mentor
I thought MWI was equal to multiverse,
The multiverse is applicable to a number of different ideas not just MW eg eternal inflation.

Thanks
Bill

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).

bhobba
Mentor
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.
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.

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).
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

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).
Denial of reality? what? lol

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

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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
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)

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

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

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)

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

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
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.

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In the MWI the different worlds 'exist' in an abstract mathematical space, Hilbert Space. So don't take this visualisation too literally.
Sorry my ignorance, but What is an abstract mathematical space? where are the other cats, I can't understand it
I just wonder if the reality remains the same, we live in a UNIverse only, and even if MWI is true, It has nothing to do with our Universe, we can see only 1 outcome, the others are in others Universes, etc,etc.. Invisible for us..?
If this is true , we move on, nothing special or mystic about it..

bhobba
Mentor
Denial of reality? what? lol
Yea - there is a lot of confusion about this stuff. To deny reality you need first to figure out what reality is and have everyone agree - as I say - good luck with that.

Thanks
Bill

bhobba
Mentor
Sorry my ignorance, but What is an abstract mathematical space? where are the other cats, I can't understand it
The mathematical formulation of QM requires an abstract mathematical space called a Hilbert space:
http://en.wikipedia.org/wiki/Hilbert_space

These days in math space and set are generally synonymous and set is rather an abstract idea.

Some harp on such math cant be reality bla bla bla. The modern view of the math is the following:
http://arxiv.org/pdf/quant-ph/0101012.pdf

It's simply an extension of probability theory to allow continuous transformations between so called pure states which, if you think about it, is what's required to model physical systems. If a system transforms to a state after 1 second it went through another state after 1/2 second. This is the generalised probability view of QM. It explains the math very elegantly. What it means of course is where interpretations come in - but the math is no more or less abstract than ordinary probability theory.

I just wonder if the reality remains the same, we live in a Universe only, and even if MWI is true, It has nothing to do with our Universe, we can see only 1 outcome, the others are in others Universes, etc,etc.. Invisible for us..?If this is true , we move on, nothing special or mystic about it..
Forget this reality stuff. There is no agreement on what that is. Physical theories describe it without getting into exactly what it's describing. Guys like me believe that's the best we can do. There is nothing in QM that FORCES anyone to adopt a weird mystical new age inspired view of reality. That's what common sense people need to take from it. However there is no way of proving, without experimental support, the weird views of QM are incorrect - its in the nature of science. Also weirdness is in the eye of the beholder.

Thanks
Bill

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bhobba
Mentor
Not only Sean Carrol. The MWI is 'believed' by most leading cosmologists and string theorists.
I have zero idea where you got that from.

I saw a poll that was took at some conference, it may have even been a string theory one, and that most definitely was NOT the view of most physicists - Copenhagen was still the most favoured one.

Brian Green, for example, ascribes to Qbism (interesting discussion as well):

However MW is one of the most popular interpretations - as I said its mathematical elegance is striking.

Thanks
Bill

bhobba
Mentor
Except for the preferred basis and born rule problems that has yet to be solved
As I have pointed out to you before the issue has yet to be resolved one way or the other. Certain key mathematical theorems are lacking.

I have seen papers where for simple models it is shown the results do NOT depend on the factorisation. They however need to be extended, and that hasn't been done yet.

Thanks
Bill