6 ms·

The multiverse usually refers to multiple universes each with their own big bang, existing in regular space, but outside our light cone. It is a completely unsc

The multiverse usually refers to multiple universes each with their own big bang, existing in regular space, but outside our light cone. It is a completely unscientific theory, in the sense that there is no chance of ever proving or denying it, by its very construction. The many worlds interpretation is completely unrelated, and it doesn't suffer from this problem of being unfalsifiable even in principle. However, I would also not say that the MWI is a straightforward deduction from the laws of QM. It rejects the most visible effect of those laws (the Born rule, i.e. that they predict probabilities of classical events and not certainties) and replaces it with an infinity of "worlds". Even worse, it still doesn't explain WHY we can only "see" one such "world", while elementary particles can "see" all of them (i.e. it doesn't solve the measurement problem any more than Copenhagen interpretation). And there is also debate on how plausible it is to even define the "probability of a world", so whether MWI is actually consistent with the Born rule is somewhat in doubt.

DavidSJ 6y ago
it still doesn't explain WHY we can only "see" one such "world" It's just that there's one of us in each world. The one in this world sees this one. The one in that world sees that one.
- TheOtherHobbes 6y ago
  Define "sees." That's just one of the many problems that MWI fails to solve.
  - ummonk 6y ago
    The local wavefunction part corresponding to each world has a configuration in it that involves a human being with eyes and optical nerves and a brain registering neural impulses in response to light stimuli to perceive its world.
  - DavidSJ 6y ago
    In MWI, the problem doesn't exist to begin with. In e.g. the Copenhagen interpretation, observation plays a special role, because it's the moment of wave function collapse. In MWI, there is no wave function collapse, so there's no need to identify certain special events called "observations" which are somehow different from the rest of physics. It's just our eyes, and brains, and instruments, interacting and becoming entangled with the other things in the world. When we "observe" Schrödinger's cat, we entangle ourselves with the cat just as the cat was already entangled with the cesium atom. Now the world is in a superposition of two states: in one, the cat is alive and we see it as alive; in the other, the cat is dead and we see it as dead. This is exactly what the equations say should happen. Nothing special.
    - simiones 6y ago
      And yet, photons can keep interacting with both versions, while we are forever unable to see the other version of the cat. The problem doesn't go away. Even more interestingly, say you want to predict the outcome of a quantum experiment. In the MWI, that outcome is deterministic, and it is a superposition in Hilbert space. However, in your own space, you still need to compute a probability for that outcome. What is this a probability of in the MWI? The concept of probability across worlds is not really rigorously defined, especially since you need to arrive at very specific results to be consistent with observations.
      - DavidSJ 6y ago
        I address your first point here: https://news.ycombinator.com/item?id=25999766 https://news.ycombinator.com/item?id=25999766 (in brief, photons are treated no differently than people). I agree that the meaning of probability in the MWI context is an interesting open question. Mathematically, it's the squared amplitude of part of the wave function. But why this corresponds to our personal experience of frequency is something I'm unclear on.
- simiones 6y ago
  Why doesn't that apply to photons and electrons, then? At what scale do things stop interacting with the other worlds and stay in their own world only? This is exactly the measurement problem, essentially identical between Copenhagen and MWI.
  - DavidSJ 6y ago
    It does apply to photons and electrons. As I understand it, it becomes ever more difficult to maintain a coherent quantum state among larger and larger collections of particles (this is why making large quantum computers is difficult), so for most practical purposes large aggregates of particles behave like a single "world". But there is no sharp dividing line, and the same math applies at all scales. Unfortunately, this is where the imperfect mapping to which I referred above — between the math and our intuition — becomes misleading, and why I don't like the term "world" to begin with.
    - simiones 6y ago
      MWI actually postulates that coherence of the system is always preserved, even up to the level of the entire universe. It's only locally that decoherence seeps in, at some scales. But still, the problem I was questioning was different: you are saying "there is one of us in each world". In fact, that is not what QM predicts: in QM, there is only one of us in the entire system described by the wave function, with different amplitudes in different states. A system with 1 electron with some amplitude < 1 in state A and some amplitude < 1 in state B is not the same as a system with 2 electrons, one with amplitude 1 in state A and another with amplitude 1 in state B. So, if we took a detector that could measure state A or state B, we should expect that it detects state A with some amplitude and, simultaneously, state B with some other amplitude. Instead, MWI postulates that there are 2 detectors, one which measures state A with amplitude 1, and the other state B with amplitude 1 (in fact, it postulates an infinity of detectors, out of which some number measure state A and some measure state B, such that the Born rule predictions by "counting" detectors). This is the part that remains unexplained.
      - DavidSJ 6y ago
RobertoG 6y ago
Reading Sean Carroll (1) and David Deutsch convinced me that Everett is the straightforward interpretation of what the data is saying. Copenhagen is adding unnecessary concepts to the interpretation and making it more complex, so, if we are using the scientific method, the burden of proof should be with the backers of Copenhagen. (1) - https://www.youtube.com/watch?v=F6FR08VylO4 https://www.youtube.com/watch?v=F6FR08VylO4
- simiones 6y ago
  I dislike both interpretations, because they both throw in the towel and consider that the measurement problem is not resolvable. Even in MWI, you still to compute probabilities (of what is actually much harder to say) if you want to actually use it for predictions. Instead, the more interesting thing is to stop ignoring the measurement problem and find a theory that actually solves it somehow; until such a thing exists, waxing philosophically about many worlds or un-reality is not really interesting.
  - slowmovintarget 6y ago
    Everettianism does not ignore the measurement problem. It fully acknowledges it as decoherence. The problem you have instead is the location identification problem, and for that you need to calculate probabilities. Objective Collapse theories, and Bohmian Mechanics (hidden variables / pilot waves) are the only other games in town for being actual working theories. (Copenhagen is not a theory, it is an assertion that one should stop trying.) Of these, Everettianism is actually the simplest. That said, the multiverse, often referred to as the Bulk is a horse of different color. It has nothing to do with branches of the wave function in Everettian mechanics. Instead it seems to be a way to explain the fine-tuning problem. If there is no Bulk, and our comoving patch of the universe is representative of the whole shebang, then the conditions at the time of the Big Bang were unimaginably improbable, extraordinarily special, just exactly right to produce the stars in the sky, and us. A single solar mass black hole has more entropy than the entirety of the universe did at the time of the Big Bang. One way to explain that "problem" is to think that the universe in which our comoving patch exists is a small pocket arising from quantum fluctuations in a much more vast "bulk" of true vacuum that exists at high entropy. It does seem like a lot of mental gymnastics, and mostly it seems to be directed at elimination of every whiff of the idea that a creator was required for the improbable conditions in the early universe that give us the arrow of time. I'm not sure it's at all useful, nor would it have any predictive power were the idea to be true. Our comoving patch still expands to an infinite nothingness where even the black holes have evaporated away.
onethought 6y ago
I thought the multiverse as you describe it has been falsified, logically: if you could observe it then it’s part of our universe, therefore not a multiverse. If it cannot be observed, then it’s not a thing. Perhaps that is just playing with the meaning of words or semantics to some, but it’s compelling enough for me. The other description of quantum equations predicting many “worlds” I think is a different and more acceptable proposition. Like we’ve observed the shadow of an object, but can’t figure out how to observe the object directly.
canjobear 6y ago
> it still doesn't explain WHY we can only "see" one such "world", while elementary particles can "see" all of them For that you need quantum decoherence. Big creatures like us only ever see one or the other outcome of a two-outcome experiment because we get entangled with the outcome.

DavidSJ 6y ago

it still doesn't explain WHY we can only "see" one such "world" It's just that there's one of us in each world. The one in this world sees this one. The one in that world sees that one.

TheOtherHobbes 6y ago
Define "sees." That's just one of the many problems that MWI fails to solve.
- ummonk 6y ago
  The local wavefunction part corresponding to each world has a configuration in it that involves a human being with eyes and optical nerves and a brain registering neural impulses in response to light stimuli to perceive its world.
- DavidSJ 6y ago
  In MWI, the problem doesn't exist to begin with. In e.g. the Copenhagen interpretation, observation plays a special role, because it's the moment of wave function collapse. In MWI, there is no wave function collapse, so there's no need to identify certain special events called "observations" which are somehow different from the rest of physics. It's just our eyes, and brains, and instruments, interacting and becoming entangled with the other things in the world. When we "observe" Schrödinger's cat, we entangle ourselves with the cat just as the cat was already entangled with the cesium atom. Now the world is in a superposition of two states: in one, the cat is alive and we see it as alive; in the other, the cat is dead and we see it as dead. This is exactly what the equations say should happen. Nothing special.
  - simiones 6y ago
    And yet, photons can keep interacting with both versions, while we are forever unable to see the other version of the cat. The problem doesn't go away. Even more interestingly, say you want to predict the outcome of a quantum experiment. In the MWI, that outcome is deterministic, and it is a superposition in Hilbert space. However, in your own space, you still need to compute a probability for that outcome. What is this a probability of in the MWI? The concept of probability across worlds is not really rigorously defined, especially since you need to arrive at very specific results to be consistent with observations.
    - DavidSJ 6y ago
      I address your first point here: https://news.ycombinator.com/item?id=25999766 https://news.ycombinator.com/item?id=25999766 (in brief, photons are treated no differently than people). I agree that the meaning of probability in the MWI context is an interesting open question. Mathematically, it's the squared amplitude of part of the wave function. But why this corresponds to our personal experience of frequency is something I'm unclear on.
simiones 6y ago
Why doesn't that apply to photons and electrons, then? At what scale do things stop interacting with the other worlds and stay in their own world only? This is exactly the measurement problem, essentially identical between Copenhagen and MWI.
- DavidSJ 6y ago
  It does apply to photons and electrons. As I understand it, it becomes ever more difficult to maintain a coherent quantum state among larger and larger collections of particles (this is why making large quantum computers is difficult), so for most practical purposes large aggregates of particles behave like a single "world". But there is no sharp dividing line, and the same math applies at all scales. Unfortunately, this is where the imperfect mapping to which I referred above — between the math and our intuition — becomes misleading, and why I don't like the term "world" to begin with.
  - simiones 6y ago
    MWI actually postulates that coherence of the system is always preserved, even up to the level of the entire universe. It's only locally that decoherence seeps in, at some scales. But still, the problem I was questioning was different: you are saying "there is one of us in each world". In fact, that is not what QM predicts: in QM, there is only one of us in the entire system described by the wave function, with different amplitudes in different states. A system with 1 electron with some amplitude < 1 in state A and some amplitude < 1 in state B is not the same as a system with 2 electrons, one with amplitude 1 in state A and another with amplitude 1 in state B. So, if we took a detector that could measure state A or state B, we should expect that it detects state A with some amplitude and, simultaneously, state B with some other amplitude. Instead, MWI postulates that there are 2 detectors, one which measures state A with amplitude 1, and the other state B with amplitude 1 (in fact, it postulates an infinity of detectors, out of which some number measure state A and some measure state B, such that the Born rule predictions by "counting" detectors). This is the part that remains unexplained.
    - DavidSJ 6y ago

RobertoG 6y ago

Reading Sean Carroll (1) and David Deutsch convinced me that Everett is the straightforward interpretation of what the data is saying. Copenhagen is adding unnecessary concepts to the interpretation and making it more complex, so, if we are using the scientific method, the burden of proof should be with the backers of Copenhagen. (1) - https://www.youtube.com/watch?v=F6FR08VylO4 https://www.youtube.com/watch?v=F6FR08VylO4

simiones 6y ago
I dislike both interpretations, because they both throw in the towel and consider that the measurement problem is not resolvable. Even in MWI, you still to compute probabilities (of what is actually much harder to say) if you want to actually use it for predictions. Instead, the more interesting thing is to stop ignoring the measurement problem and find a theory that actually solves it somehow; until such a thing exists, waxing philosophically about many worlds or un-reality is not really interesting.
- slowmovintarget 6y ago
  Everettianism does not ignore the measurement problem. It fully acknowledges it as decoherence. The problem you have instead is the location identification problem, and for that you need to calculate probabilities. Objective Collapse theories, and Bohmian Mechanics (hidden variables / pilot waves) are the only other games in town for being actual working theories. (Copenhagen is not a theory, it is an assertion that one should stop trying.) Of these, Everettianism is actually the simplest. That said, the multiverse, often referred to as the Bulk is a horse of different color. It has nothing to do with branches of the wave function in Everettian mechanics. Instead it seems to be a way to explain the fine-tuning problem. If there is no Bulk, and our comoving patch of the universe is representative of the whole shebang, then the conditions at the time of the Big Bang were unimaginably improbable, extraordinarily special, just exactly right to produce the stars in the sky, and us. A single solar mass black hole has more entropy than the entirety of the universe did at the time of the Big Bang. One way to explain that "problem" is to think that the universe in which our comoving patch exists is a small pocket arising from quantum fluctuations in a much more vast "bulk" of true vacuum that exists at high entropy. It does seem like a lot of mental gymnastics, and mostly it seems to be directed at elimination of every whiff of the idea that a creator was required for the improbable conditions in the early universe that give us the arrow of time. I'm not sure it's at all useful, nor would it have any predictive power were the idea to be true. Our comoving patch still expands to an infinite nothingness where even the black holes have evaporated away.

onethought 6y ago

I thought the multiverse as you describe it has been falsified, logically: if you could observe it then it’s part of our universe, therefore not a multiverse. If it cannot be observed, then it’s not a thing. Perhaps that is just playing with the meaning of words or semantics to some, but it’s compelling enough for me. The other description of quantum equations predicting many “worlds” I think is a different and more acceptable proposition. Like we’ve observed the shadow of an object, but can’t figure out how to observe the object directly.

canjobear 6y ago

> it still doesn't explain WHY we can only "see" one such "world", while elementary particles can "see" all of them For that you need quantum decoherence. Big creatures like us only ever see one or the other outcome of a two-outcome experiment because we get entangled with the outcome.