4 ms·

If you place a detector in front of (or behind!) the slits the wave function collapses and there is no interference pattern. This is all part of the mystery of

by mbell 7y ago

If you place a detector in front of (or behind!) the slits the wave function collapses and there is no interference pattern. This is all part of the mystery of it, as soon as you 'observe' a particle, in any way, it stops acting like a wave. See the Delayed Choice Quantum Eraser experiment for even more weirdness, a good video on it: https://www.youtube.com/watch?v=8ORLN_KwAgs https://www.youtube.com/watch?v=8ORLN_KwAgs

aeternum 7y ago
Only the Copenhagen interpretation suffers from the mysterious collapse and time travel issues. Interpretations like Many Worlds make it significantly less 'mysterious': https://www.preposterousuniverse.com/blog/2019/09/21/the-notorious-delayed-choice-quantum-eraser/ https://www.preposterousuniverse.com/blog/2019/09/21/the-not...
- TheOtherHobbes 7y ago
  Many Worlds is just Copenhagen trying to pitch a science fiction TV series. The issues don't disappear at all, they just get obscured by story telling about other universes. The more you try to nail down MWI on specifics - like exactly when and how universes appear, and how many there are, and how this process is supposed to map back to a probability distribution in one of the universes - the less and less plausible it becomes.
  - Erlich_Bachman 7y ago
    Thank you, I thought it was just me. Like in what possible state of mind is reasoning about endless universes apparently being born out of thin air all the time (and also dependent on observation and interaction of partciles, but not if there is no interaction, there is also no way to hop between them) easier than somewhat tricky but still quite logical wavefunction collapse? It only requires to stop thinking about these particles as physical balls of matter and instead of a bit different entities which can behave like a particle and like a wave, and like something in between, dependent on what they are forced to interact with each moment...
    - meowface 7y ago
      I'd recommend watching this talk by Sean Carroll: https://youtu.be/F6FR08VylO4 https://youtu.be/F6FR08VylO4 I think it's a much simpler interpretation than the Copenhagen interpretation. Here's an awesome discussion/debate between Carroll and a major MWI skeptic, David Albert: https://youtu.be/AglOFx6eySE https://youtu.be/AglOFx6eySE In this discussion, they cover what they consider "silly" and "important" criticisms of MWI. Respectfully, some of what you said in your comment is placed by them in the "silly" box.
      - Erlich_Bachman 7y ago
        The main criticism I have is that if we have no ability to ever travel between the different worlds or traverse them or transfer information between them, if we are for all intents and purposes stuck in one of these worlds, in our own "fate" so to speak, what practical purpose does a definition serve at all? It just seems like a handwaivy way of explaning all the weirdness that goes on in QM, to say "it happened in the other worlds from which we have split". I have watched some other Carroll's explanations and it is not clear that he sees this problem... To me it seems there can be two purposes for creating a good interpretation: 1) make it more in line with most people's intuitive thinking about the world, to make them understand the theories better and faster and 2) make it easier to produce new scientific results, or predict other things. So far I don't see how the MWI manages either of those better than the other models.
        aeternum 7y ago
        Consider that much of the universe is expanding away from us at greater than the speed of light. That is effectively another world already (without any possible information transfer) and all it requires is that you accept relativity + the expanding universe. Many worlds isn't the universe forking. It is more straightforward to think of the other worlds as probability spaces. The probability space simply divides as we call each division a "world".
        Erlich_Bachman 7y ago
  - lmm 7y ago
    > The more you try to nail down MWI on specifics - like exactly when and how universes appear, and how many there are, and how this process is supposed to map back to a probability distribution in one of the universes - the less and less plausible it becomes. On the contrary. When you look at the specifics it's all very obvious. Look at the wavefunction, not the TV-show verbal explanation, and the answer to your first two questions are very obvious, as is the ill-formedness of your final question.
  - imtringued 7y ago
    Your assumption is that wave function collapse creates new universes but MWI is the extreme opposite of that, it arises when you ask yourself "What if the wave function never collapses and every single potential outcome is equally real?" It's a pretty simple concept. If you create a random hypothetical universe that follows all the laws of physics what reason is there for it to not exist? What constraints beyond the laws of physics ensure that our universe is the only one that can possibly exist?
    - mbell 7y ago
      MWI replaces the spookiness of wave function collapse with the spookiness of quantum decoherence and adds in an equally spooky universal wave function. Whether this is simpler than wave collapse or hidden variable interpretations is an almost purely subjective opinion.
- tyfon 7y ago
  There are no time travel issues, it's a relativistic problem. From the perspective of the photon the time stands still, it arrives instantly at the destination. You can see this in that the quantum eraser experiment only works with photons and not particles with mass like electrons.
  - bookofjoe 7y ago
    "If time travel is possible, then there is no such thing as time."—Kurt Gödel
  - aeternum 7y ago
    The quantum eraser does work with particles with mass. Here is a paper that performs the experiment with electrons (specifically a transmon qubit): https://advances.sciencemag.org/content/3/5/e1603159 https://advances.sciencemag.org/content/3/5/e1603159 It works with even larger particles as well such as molecular ions or buckyballs.

by mbell 7y ago

aeternum 7y ago

Only the Copenhagen interpretation suffers from the mysterious collapse and time travel issues. Interpretations like Many Worlds make it significantly less 'mysterious': https://www.preposterousuniverse.com/blog/2019/09/21/the-notorious-delayed-choice-quantum-eraser/ https://www.preposterousuniverse.com/blog/2019/09/21/the-not...

TheOtherHobbes 7y ago
Many Worlds is just Copenhagen trying to pitch a science fiction TV series. The issues don't disappear at all, they just get obscured by story telling about other universes. The more you try to nail down MWI on specifics - like exactly when and how universes appear, and how many there are, and how this process is supposed to map back to a probability distribution in one of the universes - the less and less plausible it becomes.
- Erlich_Bachman 7y ago
  Thank you, I thought it was just me. Like in what possible state of mind is reasoning about endless universes apparently being born out of thin air all the time (and also dependent on observation and interaction of partciles, but not if there is no interaction, there is also no way to hop between them) easier than somewhat tricky but still quite logical wavefunction collapse? It only requires to stop thinking about these particles as physical balls of matter and instead of a bit different entities which can behave like a particle and like a wave, and like something in between, dependent on what they are forced to interact with each moment...
  - meowface 7y ago
    I'd recommend watching this talk by Sean Carroll: https://youtu.be/F6FR08VylO4 https://youtu.be/F6FR08VylO4 I think it's a much simpler interpretation than the Copenhagen interpretation. Here's an awesome discussion/debate between Carroll and a major MWI skeptic, David Albert: https://youtu.be/AglOFx6eySE https://youtu.be/AglOFx6eySE In this discussion, they cover what they consider "silly" and "important" criticisms of MWI. Respectfully, some of what you said in your comment is placed by them in the "silly" box.
    - Erlich_Bachman 7y ago
      The main criticism I have is that if we have no ability to ever travel between the different worlds or traverse them or transfer information between them, if we are for all intents and purposes stuck in one of these worlds, in our own "fate" so to speak, what practical purpose does a definition serve at all? It just seems like a handwaivy way of explaning all the weirdness that goes on in QM, to say "it happened in the other worlds from which we have split". I have watched some other Carroll's explanations and it is not clear that he sees this problem... To me it seems there can be two purposes for creating a good interpretation: 1) make it more in line with most people's intuitive thinking about the world, to make them understand the theories better and faster and 2) make it easier to produce new scientific results, or predict other things. So far I don't see how the MWI manages either of those better than the other models.
      - aeternum 7y ago
        Consider that much of the universe is expanding away from us at greater than the speed of light. That is effectively another world already (without any possible information transfer) and all it requires is that you accept relativity + the expanding universe. Many worlds isn't the universe forking. It is more straightforward to think of the other worlds as probability spaces. The probability space simply divides as we call each division a "world".
        Erlich_Bachman 7y ago
- lmm 7y ago
  > The more you try to nail down MWI on specifics - like exactly when and how universes appear, and how many there are, and how this process is supposed to map back to a probability distribution in one of the universes - the less and less plausible it becomes. On the contrary. When you look at the specifics it's all very obvious. Look at the wavefunction, not the TV-show verbal explanation, and the answer to your first two questions are very obvious, as is the ill-formedness of your final question.
- imtringued 7y ago
  Your assumption is that wave function collapse creates new universes but MWI is the extreme opposite of that, it arises when you ask yourself "What if the wave function never collapses and every single potential outcome is equally real?" It's a pretty simple concept. If you create a random hypothetical universe that follows all the laws of physics what reason is there for it to not exist? What constraints beyond the laws of physics ensure that our universe is the only one that can possibly exist?
  - mbell 7y ago
    MWI replaces the spookiness of wave function collapse with the spookiness of quantum decoherence and adds in an equally spooky universal wave function. Whether this is simpler than wave collapse or hidden variable interpretations is an almost purely subjective opinion.
tyfon 7y ago
There are no time travel issues, it's a relativistic problem. From the perspective of the photon the time stands still, it arrives instantly at the destination. You can see this in that the quantum eraser experiment only works with photons and not particles with mass like electrons.
- bookofjoe 7y ago
  "If time travel is possible, then there is no such thing as time."—Kurt Gödel
- aeternum 7y ago
  The quantum eraser does work with particles with mass. Here is a paper that performs the experiment with electrons (specifically a transmon qubit): https://advances.sciencemag.org/content/3/5/e1603159 https://advances.sciencemag.org/content/3/5/e1603159 It works with even larger particles as well such as molecular ions or buckyballs.