# B Never-ending experiments in MWI

#### cube137

I'd like to see an instance where experiments can end in mwi because it seems it never ends...

example.. if the radioactive source emits alpha, it triggers an explosion and Schrodinger is dead. If it didn't he is alive (we often use Schrodinger's Cat example with the consequence we never think what happens next because we think alive is the final state). so consider Schrodinger himself. When he entangled himself with the radioactive source, there are two terms in the superposition which is "Schrodinger dead when source emits" and "Schrodinger alive when source doesn't emit". When the experiment happens. In one term, he is alive, in one term he is dead.. but since the wave function never disappear and the unitary evolution goes on and on with more entanglements.. then the experiments never ends because in the Schrodinger dead branch, the police would be entangled with them (Schrodinger and the source) and in the Schrodinger alive branch, her lover would be entangled with them also (Schrodinger and the source). So is it correct that experiments never end in mwi? If you disagree. Please give example of one where experiment can end in mwi. Thank you.

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#### PeterDonis

Mentor
I'd like to see an instance where experiments can end in mwi
What do you mean by an experiment "ending"?

#### cube137

What do you mean by an experiment "ending"?
In the Copenhagen. an experiment ends when an outcome is chosen, for example Schrodinger alive when the radioactive source didn't emit alpha to trigger the explosive. But in MWI, there is never ending entanglements.. there is no way to cut the unitary evolution.. an experiment ends when the entanglement is cut. In the case of Schrodinger dead or alive...

quantum state = "Schrodinger alive when source didn't emit" + "Schrodinger dead when source emits"

then one includes the police....

quantum state = "Schrodinger alive when radioactive source didn't emit and police didn't arrive" + "Schrodinger dead when radioactive source emitted and police arrives"

one can include Schrodinger lover and the unitary evolution goes on and goes as the entanglement spreads.. so in mwi how do you stop or end an experiment?

#### PeterDonis

Mentor
In the Copenhagen. an experiment ends when an outcome is chosen
So your definition of "the experiment ends" is "the wave function collapses". Then by your definition, there is no "experiment end" in the MWI, because MWI has no collapse.

However, that is a problem with your definition, not with the MWI. Actual experimental results are the same under both interpretations (they have to be, since both use the same underlying math).

#### PeterDonis

Mentor
in mwi how do you stop or end an experiment?
When decoherence separates the different outcomes so that they can no longer interfere with each other.

#### cube137

When decoherence separates the different outcomes so that they can no longer interfere with each other.
I'm aware that Schrodinger is entangled with the radioactive source... so Schrodinger doesn't have a definite state... only both of them are... but even after decoherence, is it not the entanglement never breaks inside each mixed state (or branch) so Schrodinger is forever entangled with the radioactive source even when dead or alive and the entanglement can keep spreading within each mixed state or branch?

And in each branch, Schrodinger can have different numbers of children like 3 children versus none in other branch. So it's like new entanglements are formed to each mixed state, or in other words the branches would produce separate histories? We can easily imagine this where Schrodinger survived when the radioactive source didn't decay and explosive not set off and Schrodinger got out of the laboratory and went to dinner with his lover... this proves that even when it is in mixed state, the mixed state can produce more histories. Can the math produce this where mixed state can evolve on their own?

#### PeterDonis

Mentor
in each branch, Schrodinger can have different numbers of children like 3 children versus none in other branch
Not as a result of the cat experiment. The only difference between the branches in the cat experiment is that in one, Schrodinger observes the cat to be alive, and in the other, Schrodinger observes the cat to be dead. Everything else about him is the same in both.

the branches would produce separate histories?
Basically, yes, that's what decoherence means.

this proves that even when it is in mixed state, the mixed state can produce more histories
No, it doesn't. There are no "mixed states" in the MWI. There are only superpositions and entanglements, and unitary evolution of pure states.

#### cube137

Not as a result of the cat experiment. The only difference between the branches in the cat experiment is that in one, Schrodinger observes the cat to be alive, and in the other, Schrodinger observes the cat to be dead. Everything else about him is the same in both.
How do you prepare an experimental setup where Schrodinger can have different children in different branches.. for example in one branch the radioactive source emits alpha which made him sterile and unable to bear children. In another branch he has many children. I didn't mention about the cat but you mentioned it.. so I thought you thought I was thinking about the cat.

Basically, yes, that's what decoherence means.

No, it doesn't. There are no "mixed states" in the MWI. There are only superpositions and entanglements, and unitary evolution of pure states.

#### PeterDonis

Mentor
How do you prepare an experimental setup where Schrodinger can have different children in different branches..
You would have to make some aspect of his having children dependent on quantum indeterminacy.

#### cube137

You would have to make some aspect of his having children dependent on quantum indeterminacy.
like this?
quantum state = "Schrodinger healthy when radioactive source didn't emit" + "Schrodinger sick (and infertile) when radioactive source emits which activates a box in the lab that emits more gamma radiation"

so when decoherence separates the different outcomes.. then the outcomes would produce different histories...
but even without decoherence, the outcomes would evolve separately and produce separate histories too.. is it not?

#### PeterDonis

Mentor
like this?
Basically, yes.

so when decoherence separates the different outcomes.. then the outcomes would produce different histories...
Yes.

even without decoherence, the outcomes would evolve separately and produce separate histories too.. is it not?
No, because without decoherence, the outcomes can interfere with each other.

#### cube137

Basically, yes.

Yes.

No, because without decoherence, the outcomes can interfere with each other.
If the outcomes interfere with each other.. why can't it produce separate histories?

#### PeterDonis

Mentor
If the outcomes interfere with each other.. why can't it produce separate histories?
Because "separate histories" means that each branch can treat itself as self-contained, not influenced by any other branches. That is how the MWI explains our everyday observation that experiments have outcomes where, for example, Schrodinger is in a definite state. But that isn't true if the outcomes can interfere with each other.

#### cube137

Because "separate histories" means that each branch can treat itself as self-contained, not influenced by any other branches. That is how the MWI explains our everyday observation that experiments have outcomes where, for example, Schrodinger is in a definite state. But that isn't true if the outcomes can interfere with each other.
Ok.. unitary conserves information.. that is why after decoherence occurs, there is no more communication between the different branches... but if we can pump negative entropy much like how the ATP in the mitochondria can produce negative entropy to power our cells... can't another source of negative entropy from outside pump the wave function such that it can produce a link between branches? Why is this not possible?

#### PeterDonis

Mentor
if we can pump negative entropy much like how the ATP in the mitochondria can produce negative entropy to power our cells... can't another source of negative entropy from outside pump the wave function such that it can produce a link between branches? Why is this not possible?
Where are you getting this stuff about negative entropy?

#### cube137

Where are you getting this stuff about negative entropy?
If unitary conserves information.. then we can simply supply more information from outside.. in thermodynamics.. it's negative entropy. In the wave function.. I don't know the equivalent but just illustrating the idea... what's the equivalent of negative entropy (from outside) in the unitary evolution that can connect two branches together?

#### PeterDonis

Mentor
If unitary conserves information.. then we can simply supply more information from outside
No, you can't. There is no "outside" if you are doing unitary evolution correctly, because to do it correctly, the state vector has to already include everything that can possibly interact with whatever you're interested in.

in thermodynamics
In thermodynamics you don't model the system using a state vector and you don't use unitary evolution; you throw away information in your model. So thermodynamics is irrelevant to what we're discussing here.

#### cube137

Because "separate histories" means that each branch can treat itself as self-contained, not influenced by any other branches. That is how the MWI explains our everyday observation that experiments have outcomes where, for example, Schrodinger is in a definite state. But that isn't true if the outcomes can interfere with each other.
So if asked what happened after the experiment, whether Schrodinger would be healthy or sick with radiation, the answer is "both, each history occurs in different branches".. we can't say Schrodinger doesn't have definite state after the experiment.. we can't say only Schrodinger and radioactive sources have a definite state.. because in MWI, the experiment technically never end even after decoherence, because the branches evolve separately on their own.. and so Schrodinger is forever entangled with the radioactive sources for his entire lifetimes in each branch.. so we can really say Schrodinger and the radioactive sources are forever entangled right?

#### Gigaz

The line of thought is known as the quantum suicide:
https://en.wikipedia.org/wiki/Quantum_suicide_and_immortality

The thing is that it is on the edge of scientific method and quantum terminology. I mean, does dying constitute a measurement? Can a result be considered "scientific" if the experiment is subjective and can only be confirmed by each of us in person and not by proxy? How do we determine if the experiment failed and we are in fact dead?

#### kith

My experiment ends, when I decide that I have taken enough data. An experiment always involves a person who does the experiment, so there's always an inside view. QM enables me to make predictions about what I will find, if I do a certain experiment.

In MWI, the outside view -which involves my own quantum state- can be taken and there may be many terms in the quantum state of the universe which correspond to totally different events. But if it is a correct description, it needs to involve a term where I experience the end of my experiment.

#### PeterDonis

Mentor
so we can really say Schrodinger and the radioactive sources are forever entangled right?
In the MWI, yes.

#### PeterDonis

Mentor
if it is a correct description, it needs to involve a term where I experience the end of my experiment.
According to the MWI, the "you" in each branch, does experience the end of the experiment. That occurs, as I said before, when decoherence has separated all the branches so they can't interfere with each other.

#### kith

According to the MWI, the "you" in each branch, does experience the end of the experiment. That occurs, as I said before, when decoherence has separated all the branches so they can't interfere with each other.
Although I was talking about a more complex situation, I agree in principle. My intention with the statement you quoted wasn't to challenge the MWI but to show that experiments have to "end" in the MWI.

#### cube137

According to the MWI, the "you" in each branch, does experience the end of the experiment. That occurs, as I said before, when decoherence has separated all the branches so they can't interfere with each other.
The take home lessons in MWI is once entanglement forms, it keeps branching.. that means we must avoid meeting bad people or avoid certain situations because we can be forever entangled with them.. for example you meet jack the ripper.. you can form branches with him where you are his assistance or you are his victim. Remember there are quantum choices right in our body atomic processes.

What concerns me right now is the energy. Sometime I feel fatique and wondering if my other selves in other branches are using my energy. Let's go back to Schrodinger's.

quantum state = "Schrodinger healthy when radioactive source didn't emit" + "Schrodinger sick when radioactive source emits which activates a box in the lab that emits more gamma radiation"

Let's say the energy or the Hamiltonian operator has 10 Gev defined for the entire quantum state. Does it mean each term (or branches) would have 5 Gev? How do the branches partition the energy? And if there are more branches, there are less energy for each branch? And what is the right unit for energy (instead of Gev) in the Hamiltonian operator?

#### Nugatory

Mentor
The take home lessons in MWI is once entanglement forms, it keeps branching.. that means we must avoid meeting bad people or avoid certain situations because we can be forever entangled with them.
The take-home lesson from MWI is that as you're thinking about the future evolution of whatever quantum system you've just measured... You are expected to calculate the value of a whole bunch of terms that you will never care about and then ignore them, whereas with collapse interpretations you can just set these terms to zero so there's nothing to ignore.

"Never-ending experiments in MWI"

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