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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

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.
platz 7y ago
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)?
- gizmo686 7y ago
  Haven't we tested precisly this? The fact that we observe interference patterns shows that the wavefunctions are real. As far as I can tell, Copenhagen posits that there is a scale at which wave functions stop being real, and instead collapse down to a single "macroscopic" state. Convieniently, this scale happens to be around the scale where our experiments stop being able to test for it in practice. Put another way, Copenhagen posits that the macroscopic world is something more then simply emergent behavior.
  - platz 7y ago
    > The fact that we observe interference patterns shows that the wavefunctions are real. well, not quite - remember that what we actually observe for a given particular event is something that looks like particle that is localized in space. If you're referring to the double slit experiment, the interference pattern emerges as a property of the whole collection of individual photon strikes. Yes, you need to use the wavefunction to calculate where the photon is likely to land, but each individual photon strike appears to us on the target as a single localized photon. Whether wavefunctions are "real" is a tricky question, but just the presence in interference patterns doesn't get us to answer that question yet.
    - gizmo686 7y ago
      If we put the sensor in the slits themselves, what we would observe is individual photons passing through one slit or the other. What the double slit experiment shows is that there is still something of a wavefunction (whatever that actually is) at the moment the photons pass the slit, because that is how we get the interference pattern. As before when we observe the photons, we see them as single discreet particles. However, in principle this seems to be the same situation as when we detect photons passing through a slit.
yters 7y ago
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?
- sousavdouglas 7y ago
  YES
- faissaloo 7y ago
  This whole idea of observers just brings to mind things like culling in graphics rendering, perhaps that's what's happening
  - yters 7y ago
    What is doing the culling?
    - faissaloo 7y ago
      Depends who you ask, I'd say God, others might say the computer we exist in and others might say The Universe
      - yters 7y ago
        But in a quantum universe everything is an uncollapsed wave. It doesn't seem anything can cause the collapse to happen.
        7y ago
inscionent 7y ago
If a tree falls in the woods, and no one is around to hear it, does it make a sound?
- finchisko 7y ago
  nope
monktastic1 7y ago
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.
- bonoboTP 7y ago
  Is there actually a "split"? I've always imagined it more like parallel strands that were always distinct but had overlaps. Like in the cat case, it's undetermined whether I'm on the cat dead or cat alive time-line because both are "overlapping" in the part of space-time where my brain is at the time of not knowing the cat's state. Then when I make a measurement, the two strands no longer overlap. With Wigners friend there are also these two branches all along, but they overlap where the friend hasn't yet looked in the box, they then go apart in the part where the friend makes an observation, overlap where Wigner hasn't looked at his friend yet, but then they are distinct again where Wigner has looked at the friend's state. Similarly to how particles and light take all possible trajectories at the same time (path integral formulation), we could think that in the space-time places where the branches overlap, all compatible pasts are equally valid, at other space-time locations the particular strand extends back through the past to just one of the cat states, so the state seems collapsed, but the stand also led through the overlap area, so you remember the time where you didn't yet have the "which branch you're on" information.
  - monktastic1 7y ago
    I'm not sure I've digested your whole comment, but the basic idea is this: A "split" only means that the respective branches of the wave function cannot interfere with each other any longer. Then they will evolve completely separately from each other, and can be considered distinct. The only way you can get branches to interfere is if you have precise control over all particles involved in the entanglement, which is practically impossible for any macroscopic system. Long before you entangle with the cat system, the two branches have decohered. While technically there is still an enormous superposition there, for all practical purposes it is indistinguishable from being a classical system, with classically indeterminate branches. I should say that I'm not a fan of this explanation, possibly related to your points. Regardless of "practical purposes," QM does not allow me to say that there is a well-defined answer before it entangles with me (and that there is one afterward). I'm much more comfortable using the word "split" only there. But this skirts too close to solipsism for some, since "my world" (which I also know as "the world") only comes into being upon contact with me.
triMichael 7y ago
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!

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.

platz 7y ago

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)?

gizmo686 7y ago
Haven't we tested precisly this? The fact that we observe interference patterns shows that the wavefunctions are real. As far as I can tell, Copenhagen posits that there is a scale at which wave functions stop being real, and instead collapse down to a single "macroscopic" state. Convieniently, this scale happens to be around the scale where our experiments stop being able to test for it in practice. Put another way, Copenhagen posits that the macroscopic world is something more then simply emergent behavior.
- platz 7y ago
  > The fact that we observe interference patterns shows that the wavefunctions are real. well, not quite - remember that what we actually observe for a given particular event is something that looks like particle that is localized in space. If you're referring to the double slit experiment, the interference pattern emerges as a property of the whole collection of individual photon strikes. Yes, you need to use the wavefunction to calculate where the photon is likely to land, but each individual photon strike appears to us on the target as a single localized photon. Whether wavefunctions are "real" is a tricky question, but just the presence in interference patterns doesn't get us to answer that question yet.
  - gizmo686 7y ago
    If we put the sensor in the slits themselves, what we would observe is individual photons passing through one slit or the other. What the double slit experiment shows is that there is still something of a wavefunction (whatever that actually is) at the moment the photons pass the slit, because that is how we get the interference pattern. As before when we observe the photons, we see them as single discreet particles. However, in principle this seems to be the same situation as when we detect photons passing through a slit.

yters 7y ago

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?

sousavdouglas 7y ago
YES
faissaloo 7y ago
This whole idea of observers just brings to mind things like culling in graphics rendering, perhaps that's what's happening
- yters 7y ago
  What is doing the culling?
  - faissaloo 7y ago
    Depends who you ask, I'd say God, others might say the computer we exist in and others might say The Universe
    - yters 7y ago
      But in a quantum universe everything is an uncollapsed wave. It doesn't seem anything can cause the collapse to happen.
      - 7y ago

inscionent 7y ago

If a tree falls in the woods, and no one is around to hear it, does it make a sound?

finchisko 7y ago
nope

monktastic1 7y ago

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.

bonoboTP 7y ago
Is there actually a "split"? I've always imagined it more like parallel strands that were always distinct but had overlaps. Like in the cat case, it's undetermined whether I'm on the cat dead or cat alive time-line because both are "overlapping" in the part of space-time where my brain is at the time of not knowing the cat's state. Then when I make a measurement, the two strands no longer overlap. With Wigners friend there are also these two branches all along, but they overlap where the friend hasn't yet looked in the box, they then go apart in the part where the friend makes an observation, overlap where Wigner hasn't looked at his friend yet, but then they are distinct again where Wigner has looked at the friend's state. Similarly to how particles and light take all possible trajectories at the same time (path integral formulation), we could think that in the space-time places where the branches overlap, all compatible pasts are equally valid, at other space-time locations the particular strand extends back through the past to just one of the cat states, so the state seems collapsed, but the stand also led through the overlap area, so you remember the time where you didn't yet have the "which branch you're on" information.
- monktastic1 7y ago
  I'm not sure I've digested your whole comment, but the basic idea is this: A "split" only means that the respective branches of the wave function cannot interfere with each other any longer. Then they will evolve completely separately from each other, and can be considered distinct. The only way you can get branches to interfere is if you have precise control over all particles involved in the entanglement, which is practically impossible for any macroscopic system. Long before you entangle with the cat system, the two branches have decohered. While technically there is still an enormous superposition there, for all practical purposes it is indistinguishable from being a classical system, with classically indeterminate branches. I should say that I'm not a fan of this explanation, possibly related to your points. Regardless of "practical purposes," QM does not allow me to say that there is a well-defined answer before it entangles with me (and that there is one afterward). I'm much more comfortable using the word "split" only there. But this skirts too close to solipsism for some, since "my world" (which I also know as "the world") only comes into being upon contact with me.

triMichael 7y ago

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!