You can do a really simple thought experiment that provides a persuasive argument in favor of the many worlds theory. Or at least, it shows that wave function collapse is relative to an observer, not universal everywhere.
Start with Schroedingers cat: the cat is in a box, it’s in a state of quantum superposition. It’s 50/50 alive or dead. Now a researcher opens the box: the cat is found to be either alive, or dead, and according to the Copenhagen interpretation we say that the wave function has collapsed.
But, now take the cat and the researcher, and put the whole first experiment inside another box. Now the researcher inside the second (outer) box opens up the box with the cat. From outside the second box, we still must say that the first researcher is in a state of superposition, but when the second box is opened, the first researcher will say, oh, I’ve known the cat was dead for 10 minutes or whatever.
So, before we open the second box, has the wave function collapsed, or not? It depends on which researcher you are. But Every part of the universe is in some sense “in a box” until information passes to it from another part of the universe. I give you: many worlds.
This is also known as the Wigner's Friend paradox.
Your explanation is not the standard understanding of Many Worlds today, however. The party line is that world splitting happens when decoherence has become "sufficiently irreversible" (for some arbitrary definition of "sufficiently", since there's no evidence of genuine irreversibility anywhere in physics), which in almost all real-world setups happens almost immediately.
The answer to this which makes the most sense to me is that the wave function collapses when the information exists that determines the quantum state. In Schrodinger's cat, this would be when the Geiger counter detects the quantum state, but by the time the cat dies/does not die and the researcher opens the box, everything is already in the classical world.
Schrodinger's cat was made to be a thought experiment to show how absurd some of the ideas about the quantum world are. We are unable to prove whether the cat does go into a superposition or does not. Personally, the ideas that superposition is tied to whether consciousness observes it doesn't make sense to me. After reading about some of the stranger experiments such as the quantum eraser experiment, I've gravitated towards information theory being the missing link. If that is true, then the astounding conclusion would be that the fabric of our universe is information!
The problem with many worlds is why do I think I'm in a particular timeline? If many worlds is correct, there is no "I", there are many rapidly branching "I"s, which intuitively doesn't make sense. Of course, each "I" will have the perception of being the sole "I", but the overall view that there is no singular "I" is very counter to my self conception. I prefer to preserve intuitions over conjectures, so I count that as a strike against the many worlds interpretation.
So, it seems like both the Copenhagen and the many worlds interpretations of quantum physics are wrong, given a preference for non arbitrary reference point and preserving intuitions. Is there a reason that those two must be the only possible interpretations?
The wavefunction is the thing that is in a state of quantum superposition.
It all depends on whether you think the wavefunction describes the macroscopic world (MWI), or the wavefunction merely calculates amplitudes&probabilities for things that happen in the macroscopic, classical world (Copenhagen)
Is the electron it's wavefunction (MWI), or is the electron a particle/field that is influenced by the wavefunction (Copenhagen)?
But surely the first researcher is also in a state of quantum superposition and they are 50/50 opening/not opening the box.
So how does time come into the equation? And does that mean that any observation is localised and there's no need for many worlds except in unobserved areas of the universe?
Started reading this and realized it was Sean... he gave a great talk on this last night at Symphony Space on the UWS. Believe he's doing a full book tour - worth checking out.
That characterization of Woit does not appear quite accurate. Woit's claim may even be bolder, in that he claims that there is no functional difference between MWI and Copenhagen.
> What strikes me when thinking about these two supposedly very different points of view on quantum mechanics is that I’m having trouble seeing why they are actually any different at all.
"Let me address another issue with many-worlds. It is a deterministic theory, even a hyper-deterministic theory, i.e. determinism applies to everything in the entire universe. Indeed, since there can’t be any influences coming from outside and since the Schroedinger equation - the only dynamical equation of the theory - is deterministic, everything that happens today, e.g. what I am writing, the way each reader reacts, the details of all solar eruptions, etc, was all encoded in some “quantum fluctuations” of the initial state of the universe. Given the complexity of the (many-) worlds, it had to be encoded in some infinitesimal digits of some quantum state, possibly in the billionths of billionths decimal place. I am always astonished that some people seriously believe in that."
This seems wrong to me? A lot of "things that happen" seem to be created by randomness generated along the way, from only observing subsystems.
I.e., suppose I create a random bit generator by shining photons on a half-silvered mirror and then measuring which way they go. I might then decide what to write depending what the bit is---if it's 0 I will write a HN comment, if it's 1 I will reply to an email.
In this case, the things that happen are determined deterministically, but it's not the case that there is some quantum fluctuation in the initial state of the universe which determines what I write. Rather, when I do the measurement of the bit, I create two branches, one with a HN comment and one with an email. Observers entangled with the first branch can only see the HN comment, but this complexity was created at the time of the measurement, not at the time of the big bang...
The benefit is that instead of two rules for quantum behavior 1) the Schrodinger equation 2) the born rule , that contradict each other, you just have 1) the Schrodinger equation, which you already accepted.
There is also a higher level goal that also motivates him
> the quest for quantum gravity is being held back by physicists’ traditional strategy of taking a classical theory (such as Albert Einstein’s general relativity) and ‘quantising’ it. Presumably nature doesn’t work like that; it’s just quantum from the start.
2. The benefit is that it's a very direct translation of the math into concepts. If you actually build the mathematical model, the "other worlds" are right there in the model, as the other half of a superposition.
It doesn't go away. You just can't get to it. Nor can you ignore it, because it's sitting there as a term in the equation, persisting as you evolve it over time. It just becomes smaller and smaller without ever becoming zero. That's why you observe a classical universe even though it's really quantum: the odds of the other parts of a superposition having an effect are on the order of Avogadro's-number-to-1 against.
Copenhagen works by just arbitrarily defining that number as zero, which is easy to work with but conceptually incomplete. MWI keeps it there in another "world", which is more in keeping with the math but unedifying because it sounds so unfalsifiable.
Hope that helps. (Which is always code for "... but it probably doesn't", I'm afraid.)
Quantum Immortality. Basically, if you keep splitting and you make an assumption like "My perception of the world is relative to me", then when a split causes you to die you don't really split - because you must ignore branches where you don't exist.
So then your view always follows the branching path where you live and you should expect to live 1M years from now. If there is only one world you shouldn't expect that.
As scld touched on, this "article" is really an excerpt from Sean Carroll's new book, Something Deeply Hidden: Quantum Worlds and the Emergence of Spacetime.
[+] [-] fallingfrog|6 years ago|reply
Start with Schroedingers cat: the cat is in a box, it’s in a state of quantum superposition. It’s 50/50 alive or dead. Now a researcher opens the box: the cat is found to be either alive, or dead, and according to the Copenhagen interpretation we say that the wave function has collapsed.
But, now take the cat and the researcher, and put the whole first experiment inside another box. Now the researcher inside the second (outer) box opens up the box with the cat. From outside the second box, we still must say that the first researcher is in a state of superposition, but when the second box is opened, the first researcher will say, oh, I’ve known the cat was dead for 10 minutes or whatever.
So, before we open the second box, has the wave function collapsed, or not? It depends on which researcher you are. But Every part of the universe is in some sense “in a box” until information passes to it from another part of the universe. I give you: many worlds.
[+] [-] monktastic1|6 years ago|reply
Your explanation is not the standard understanding of Many Worlds today, however. The party line is that world splitting happens when decoherence has become "sufficiently irreversible" (for some arbitrary definition of "sufficiently", since there's no evidence of genuine irreversibility anywhere in physics), which in almost all real-world setups happens almost immediately.
[+] [-] triMichael|6 years ago|reply
Schrodinger's cat was made to be a thought experiment to show how absurd some of the ideas about the quantum world are. We are unable to prove whether the cat does go into a superposition or does not. Personally, the ideas that superposition is tied to whether consciousness observes it doesn't make sense to me. After reading about some of the stranger experiments such as the quantum eraser experiment, I've gravitated towards information theory being the missing link. If that is true, then the astounding conclusion would be that the fabric of our universe is information!
[+] [-] yters|6 years ago|reply
So, it seems like both the Copenhagen and the many worlds interpretations of quantum physics are wrong, given a preference for non arbitrary reference point and preserving intuitions. Is there a reason that those two must be the only possible interpretations?
[+] [-] platz|6 years ago|reply
It all depends on whether you think the wavefunction describes the macroscopic world (MWI), or the wavefunction merely calculates amplitudes&probabilities for things that happen in the macroscopic, classical world (Copenhagen)
Is the electron it's wavefunction (MWI), or is the electron a particle/field that is influenced by the wavefunction (Copenhagen)?
[+] [-] smackay|6 years ago|reply
So how does time come into the equation? And does that mean that any observation is localised and there's no need for many worlds except in unobserved areas of the universe?
[+] [-] inscionent|6 years ago|reply
[+] [-] drcode|6 years ago|reply
[+] [-] erdleerdle|6 years ago|reply
[+] [-] aklein|6 years ago|reply
[1] https://www.amazon.com/Einsteins-Unfinished-Revolution-Searc...
[+] [-] jcims|6 years ago|reply
[+] [-] chasingthewind|6 years ago|reply
He had a recent post [1] about Everett that's interesting.
[0] https://www.math.columbia.edu/~woit/wordpress/ [1] http://www.math.columbia.edu/~woit/wordpress/?p=11202 [2] https://www.math.columbia.edu/~woit/wordpress/?p=10522
[+] [-] platz|6 years ago|reply
> What strikes me when thinking about these two supposedly very different points of view on quantum mechanics is that I’m having trouble seeing why they are actually any different at all.
[+] [-] donatj|6 years ago|reply
[+] [-] tim333|6 years ago|reply
[+] [-] jjjbokma|6 years ago|reply
[+] [-] kgwgk|6 years ago|reply
- Nicolas Gisin "Collapse. What else?" https://arxiv.org/abs/1701.08300
[+] [-] vilhelm_s|6 years ago|reply
I.e., suppose I create a random bit generator by shining photons on a half-silvered mirror and then measuring which way they go. I might then decide what to write depending what the bit is---if it's 0 I will write a HN comment, if it's 1 I will reply to an email.
In this case, the things that happen are determined deterministically, but it's not the case that there is some quantum fluctuation in the initial state of the universe which determines what I write. Rather, when I do the measurement of the bit, I create two branches, one with a HN comment and one with an email. Observers entangled with the first branch can only see the HN comment, but this complexity was created at the time of the measurement, not at the time of the big bang...
[+] [-] platz|6 years ago|reply
[+] [-] lordnacho|6 years ago|reply
Suppose we're eternally branching, as in the example about the up/down spin. If there's no way to visit other branches, how will we ever confirm this?
Second, what is the benefit of the branching interpretation? It seems to simply be a way to think about probabilities?
[+] [-] scld|6 years ago|reply
....and I didn't realize that Sean Carroll actually wrote this article! Still, he does some good explanations on his podcast.
[+] [-] platz|6 years ago|reply
There is also a higher level goal that also motivates him
> the quest for quantum gravity is being held back by physicists’ traditional strategy of taking a classical theory (such as Albert Einstein’s general relativity) and ‘quantising’ it. Presumably nature doesn’t work like that; it’s just quantum from the start.
[+] [-] jfengel|6 years ago|reply
2. The benefit is that it's a very direct translation of the math into concepts. If you actually build the mathematical model, the "other worlds" are right there in the model, as the other half of a superposition.
It doesn't go away. You just can't get to it. Nor can you ignore it, because it's sitting there as a term in the equation, persisting as you evolve it over time. It just becomes smaller and smaller without ever becoming zero. That's why you observe a classical universe even though it's really quantum: the odds of the other parts of a superposition having an effect are on the order of Avogadro's-number-to-1 against.
Copenhagen works by just arbitrarily defining that number as zero, which is easy to work with but conceptually incomplete. MWI keeps it there in another "world", which is more in keeping with the math but unedifying because it sounds so unfalsifiable.
Hope that helps. (Which is always code for "... but it probably doesn't", I'm afraid.)
[+] [-] akvadrako|6 years ago|reply
So then your view always follows the branching path where you live and you should expect to live 1M years from now. If there is only one world you shouldn't expect that.
[+] [-] excalibur|6 years ago|reply
https://www.amazon.com/Something-Deeply-Hidden-Emergence-Spa...
If you wish to read further, you may want to consider picking up a copy. Mine is waiting for me at home, I'm looking forward to it!
[+] [-] unknown|6 years ago|reply
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[+] [-] edmoffo|6 years ago|reply