11 ms·

How the Many-Worlds Theory of Hugh Everett Split the Universe

83 points by jonbaer 7y ago

lordnacho 7y ago
I'm not sure if the Aeon article explains what Everett really meant, at least I still have questions. 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 7y ago
  I'd recommend reading some of the explanations given by Sean Carroll on the subject. https://www.preposterousuniverse.com/ https://www.preposterousuniverse.com/ ....and I didn't realize that Sean Carroll actually wrote this article! Still, he does some good explanations on his podcast.
- platz 7y ago
  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.
- akvadrako 7y ago
  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.
- excalibur 7y ago
  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. https://www.amazon.com/Something-Deeply-Hidden-Emergence-Spacetime/dp/1524743011/ 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!
- jfengel 7y ago
  1. No, there is no way to visit other branches. 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.)
donatj 7y ago
Fun fact, Hugh Everett is “E” Mark Oliver Everett’s father, lead singer of The Eels.
- jjjbokma 7y ago
  Came here for this :-D.
  - lukeclewlow 7y ago
    :-D
- tim333 7y ago
  Good documentary on that. (https://en.wikipedia.org/wiki/Parallel_Worlds,_Parallel_Lives https://en.wikipedia.org/wiki/Parallel_Worlds,_Parallel_Live...)
chasingthewind 7y ago
Peter Woit in his blog "Not Even Wrong" [0] sometimes deals with the Many Worlds interpretation. He doesn't seem to be a fan. He had a recent post [1] about Everett that's interesting. [0] https://www.math.columbia.edu/~woit/wordpress/ https://www.math.columbia.edu/~woit/wordpress/ [1] http://www.math.columbia.edu/~woit/wordpress/?p=11202 http://www.math.columbia.edu/~woit/wordpress/?p=11202 [2] https://www.math.columbia.edu/~woit/wordpress/?p=10522 https://www.math.columbia.edu/~woit/wordpress/?p=10522
- platz 7y ago
  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.
  - lisper 7y ago
    The main difference is one of rhetoric: Copenhagen insists that experiments have single outcomes. MWI insists that experiments have multiple outcomes in point of metaphysical fact. Another difference is that the MWI has to explain the Born rule (which is problematic), whereas Copenhagen simply takes it as an axiom. http://blog.rongarret.info/2019/07/the-trouble-with-many-worlds.html http://blog.rongarret.info/2019/07/the-trouble-with-many-wor...
    - doubleunplussed 7y ago
      It should be noted though that few interpretations have an explanation for the born rule.
      - lisper 7y ago
        Right, most of them just take it as an axiom. But the MWI explicitly rejects the Born rule as an axiom, insisting that the Schrödinger equation is all the actual physics there is.
fallingfrog 7y ago
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.
- smackay 7y ago
  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?
  - lmm 7y ago
    > But surely the first researcher is also in a state of quantum superposition and they are 50/50 opening/not opening the box. In this simple thought experiment we're assuming they're not. Imagine that the first researcher is definitely going to open the inner box after 10 minutes and then the second researcher is definitely going to open the outer box after 20 minutes. > And does that mean that any observation is localised and there's no need for many worlds except in unobserved areas of the universe? Well the whole problem with non-many-worlds is that it has this idea that observed areas and unobserved areas are different. Which can work in very simple thought experiments but gets crazy as soon as you try to apply it to the real world: any interaction between two particles can be seen as an "observation", every part of the universe is gradually "observing" every other part, how do you draw a line between which parts are in superposition and which parts aren't? Many worlds simplifies all this and says that everyone is always in a superposition, there's never a "collapse", and observing outcome A or outcome B with 50% probability is just what being in a superposition of A or B feels like from the inside.
deleted 7y ago
[deleted]
drcode 7y ago
Sean Carroll's new book on quantum physics that came out yesterday is the clearest laymen's exposition of quantum physics I have ever read: https://www.audible.com/pd/Something-Deeply-Hidden-Audiobook/0525592474 https://www.audible.com/pd/Something-Deeply-Hidden-Audiobook...
- erdleerdle 7y ago
  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.
- jcims 7y ago
  Halfway through his latest Mindscape podcast episode that drills down into this topic in a very approachable way.
- aklein 7y ago
  Another book I'm enjoying is "Einstein's Unfinished Revolution" [1] by Lee Smolin, which also has a deep dive into foundations of QM. [1] https://www.amazon.com/Einsteins-Unfinished-Revolution-Search-Quantum/dp/1594206198/ref=sr_1_1?crid=1TMC2EZDUXMZ0&keywords=einsteins+unfinished+revolution&qid=1568216306&s=gateway&sprefix=einsteins%27+unfin%2Caps%2C315&sr=8-1 https://www.amazon.com/Einsteins-Unfinished-Revolution-Searc...
  - drcode 7y ago
    I'm not familiar with that book, but I can say that Dr. Carroll's book is far better than any previous book on the subject I've read, in terms of getting rid of the confusion inherent in QM.
kgwgk 7y ago
"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." - Nicolas Gisin "Collapse. What else?" https://arxiv.org/abs/1701.08300 https://arxiv.org/abs/1701.08300
- vilhelm_s 7y ago
  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...
  - kgwgk 7y ago
    There is no "randomness generated along the way" in the MWI. Schroedinger's equation is deterministic and according to many-worlders is the only thing that "makes things happen". Everything that happens in every "world" was bound to happen since the beginning of the evolution of the "multiverse".
    - 7y ago
edmoffo 7y ago
Haha... Anything is in some sense "in a box" until information passes to it from another thing. Philosophical points only...

83 points by jonbaer 7y ago

I'm not sure if the Aeon article explains what Everett really meant, at least I still have questions. 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 7y ago
I'd recommend reading some of the explanations given by Sean Carroll on the subject. https://www.preposterousuniverse.com/ https://www.preposterousuniverse.com/ ....and I didn't realize that Sean Carroll actually wrote this article! Still, he does some good explanations on his podcast.
platz 7y ago
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.
akvadrako 7y ago
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.
excalibur 7y ago
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. https://www.amazon.com/Something-Deeply-Hidden-Emergence-Spacetime/dp/1524743011/ 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!
jfengel 7y ago
1. No, there is no way to visit other branches. 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.)

donatj 7y ago

Fun fact, Hugh Everett is “E” Mark Oliver Everett’s father, lead singer of The Eels.

jjjbokma 7y ago
Came here for this :-D.
- lukeclewlow 7y ago
  :-D
tim333 7y ago
Good documentary on that. (https://en.wikipedia.org/wiki/Parallel_Worlds,_Parallel_Lives https://en.wikipedia.org/wiki/Parallel_Worlds,_Parallel_Live...)

chasingthewind 7y ago

Peter Woit in his blog "Not Even Wrong" [0] sometimes deals with the Many Worlds interpretation. He doesn't seem to be a fan. He had a recent post [1] about Everett that's interesting. [0] https://www.math.columbia.edu/~woit/wordpress/ https://www.math.columbia.edu/~woit/wordpress/ [1] http://www.math.columbia.edu/~woit/wordpress/?p=11202 http://www.math.columbia.edu/~woit/wordpress/?p=11202 [2] https://www.math.columbia.edu/~woit/wordpress/?p=10522 https://www.math.columbia.edu/~woit/wordpress/?p=10522

platz 7y ago
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.
- lisper 7y ago
  The main difference is one of rhetoric: Copenhagen insists that experiments have single outcomes. MWI insists that experiments have multiple outcomes in point of metaphysical fact. Another difference is that the MWI has to explain the Born rule (which is problematic), whereas Copenhagen simply takes it as an axiom. http://blog.rongarret.info/2019/07/the-trouble-with-many-worlds.html http://blog.rongarret.info/2019/07/the-trouble-with-many-wor...
  - doubleunplussed 7y ago
    It should be noted though that few interpretations have an explanation for the born rule.
    - lisper 7y ago
      Right, most of them just take it as an axiom. But the MWI explicitly rejects the Born rule as an axiom, insisting that the Schrödinger equation is all the actual physics there is.

fallingfrog 7y ago

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.

smackay 7y ago
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?
- lmm 7y ago
  > But surely the first researcher is also in a state of quantum superposition and they are 50/50 opening/not opening the box. In this simple thought experiment we're assuming they're not. Imagine that the first researcher is definitely going to open the inner box after 10 minutes and then the second researcher is definitely going to open the outer box after 20 minutes. > And does that mean that any observation is localised and there's no need for many worlds except in unobserved areas of the universe? Well the whole problem with non-many-worlds is that it has this idea that observed areas and unobserved areas are different. Which can work in very simple thought experiments but gets crazy as soon as you try to apply it to the real world: any interaction between two particles can be seen as an "observation", every part of the universe is gradually "observing" every other part, how do you draw a line between which parts are in superposition and which parts aren't? Many worlds simplifies all this and says that everyone is always in a superposition, there's never a "collapse", and observing outcome A or outcome B with 50% probability is just what being in a superposition of A or B feels like from the inside.

deleted 7y ago

[deleted]

drcode 7y ago

Sean Carroll's new book on quantum physics that came out yesterday is the clearest laymen's exposition of quantum physics I have ever read: https://www.audible.com/pd/Something-Deeply-Hidden-Audiobook/0525592474 https://www.audible.com/pd/Something-Deeply-Hidden-Audiobook...

erdleerdle 7y ago
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.
jcims 7y ago
Halfway through his latest Mindscape podcast episode that drills down into this topic in a very approachable way.
aklein 7y ago
Another book I'm enjoying is "Einstein's Unfinished Revolution" [1] by Lee Smolin, which also has a deep dive into foundations of QM. [1] https://www.amazon.com/Einsteins-Unfinished-Revolution-Search-Quantum/dp/1594206198/ref=sr_1_1?crid=1TMC2EZDUXMZ0&keywords=einsteins+unfinished+revolution&qid=1568216306&s=gateway&sprefix=einsteins%27+unfin%2Caps%2C315&sr=8-1 https://www.amazon.com/Einsteins-Unfinished-Revolution-Searc...
- drcode 7y ago
  I'm not familiar with that book, but I can say that Dr. Carroll's book is far better than any previous book on the subject I've read, in terms of getting rid of the confusion inherent in QM.

kgwgk 7y ago

"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." - Nicolas Gisin "Collapse. What else?" https://arxiv.org/abs/1701.08300 https://arxiv.org/abs/1701.08300

vilhelm_s 7y ago
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...
- kgwgk 7y ago
  There is no "randomness generated along the way" in the MWI. Schroedinger's equation is deterministic and according to many-worlders is the only thing that "makes things happen". Everything that happens in every "world" was bound to happen since the beginning of the evolution of the "multiverse".
  - 7y ago

edmoffo 7y ago

Haha... Anything is in some sense "in a box" until information passes to it from another thing. Philosophical points only...