- #106
mathman
Science Advisor
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For the last couple days the comic strip "Dilbert" has Schrodinger's cat as a character, having escaped from the box before it was inspected. The cat is half dead and half alive.
Teslanumber1 said:Can some one explain to me how the cat can be both alive and dead, I get the principle that since we don't know it is both at the same time because we don't know but in reality it's either alive or alive we just don't know, it's the human factor of sentience just because we don't know doesn't make neither its one we just don't know. Following by that the universe exists because we observe it if we're not alive it's not real, just like the billions of other people you haven't seen aren't real. I'm not claiming anything i just want an explanation for this.
Charles Wilson;4329210 As they return to your room they stand outside for a moment. Are you alive or dead? OR ARE YOU ALIVE AND DEAD? Do you feel alive AND dead? How would you know? This has played out since QM began and the ramifications of this argument is as lively - no pun intended - today as it has been since first contemplated. CW[/QUOTE said:This would be a good way to put it... except for a fact that having you be the observer changes the experiment too much. The cat as an observer in the original experiment knows whether its alive or dead. The cat is in a super position yet at some point it is dead. For the outside world, the cat is only dead when you send in a photon of light. So having you be "the cat" for this experiment is redundant, because it wouldn't help with the experiment
Ah, nice links. thanks dude. They seem to keep putting larger and larger objects into superposition. Although I guess a cat is a lot more difficult. Another thing I remembered is the 'Penrose interpretation' which is pretty interesting. If I understand correctly, this would predict that due to general relativity, any superposition of states with an energy difference greater than the Planck mass would undergo an objective collapse. So really, it is more than just an interpretation, since it gives predictions which are different to that of the standard QM. And as I understand it, no experiment has been done yet which uses a superposition of states with an energy difference greater than the Planck mass.audioloop said:maybe not a cat
there are various schemes,
between them
BruceW said:Ah, nice links. thanks dude. They seem to keep putting larger and larger objects into superposition. Although I guess a cat is a lot more difficult. Another thing I remembered is the 'Penrose interpretation' which is pretty interesting. If I understand correctly, this would predict that due to general relativity, any superposition of states with an energy difference greater than the Planck mass would undergo an objective collapse. So really, it is more than just an interpretation, since it gives predictions which are different to that of the standard QM. And as I understand it, no experiment has been done yet which uses a superposition of states with an energy difference greater than the Planck mass.
Crazymechanic said:That's that and that's all , sometimes I really don't get why people like to make this so mystical or sometimes the other way around , so complicated.
aha! oh yeah, I didn't even look at the authors. I should really get into the habit of that. Although, there is not much discussion about the possible implications for the 'Penrose interpretation', I guess probably because the energy difference would still not nearly be great enough to cause the 'objective collapse'.audioloop said:the second link, from penrose.
I'm curious, but I don't really know what you mean.bhobba said:Although I have mentioned it many times I think with the way this thread keeps on continuing it needs to be said again - the import of Schrodinger's Cat is it points to the necessity of a fully quantum theory of measurement. That was a valid observation when it was first promulgated - but that has now largely been accomplished.
BruceW said:I'm curious, but I don't really know what you mean.
Crazymechanic said:Well I don't want to sound too rude or whatever , but let's face it there is no half alive half dead cats or other creatures.All of this is just a physical limitation that we face in terms of knowing about the state of the cat at the very moment something happens beyond "our eyes".
Just like the guy buying a lottery ticket and later becoming the winner. He won the lottery actually in the moment he bought the "lucky" ticket because the right number combination was already in the computer but because it is a lottery he had no way of knowing that so it always feels like you won in the moment you get the information.
Now before you measure you cannot tell about the state of the cat but once you measure you have a great chance of setting the outcome and hence never knowing how long the cat would have been alive if you would have stayed aside and never touched the box.
That's that and that's all , sometimes I really don't get why people like to make this so mystical or sometimes the other way around , so complicated.
BruceW said:There is no exact cut-off which we might use to define the quantum and classical worlds.
bhobba said:Errrrr. So you think a cat isn't classical?
Crazymechanic said:Well I don't want to sound too rude or whatever , but let's face it there is no half alive half dead cats or other creatures.All of this is just a physical limitation that we face in terms of knowing about the state of the cat at the very moment something happens beyond "our eyes".
Just like the guy buying a lottery ticket and later becoming the winner. He won the lottery actually in the moment he bought the "lucky" ticket because the right number combination was already in the computer but because it is a lottery he had no way of knowing that so it always feels like you won in the moment you get the information.
Now before you measure you cannot tell about the state of the cat but once you measure you have a great chance of setting the outcome and hence never knowing how long the cat would have been alive if you would have stayed aside and never touched the box.
That's that and that's all , sometimes I really don't get why people like to make this so mystical or sometimes the other way around , so complicated.
BruceW said:aha! oh yeah, I didn't even look at the authors. I should really get into the habit of that. Although, there is not much discussion about the possible implications for the 'Penrose interpretation', I guess probably because the energy difference would still not nearly be great enough to cause the 'objective collapse'.
BruceW said:I know that there is also a lot of useful research into stuff like decoherence times, and how to try to reduce interaction of quantum systems with environment.
just because it would be very difficult (in practice) to diffract a cat, then does that mean in principle it can't be done? I'm guessing when you say "a cat is classical", you mean that it is impossible (even in principle) to diffract a cat. But there is no experimental evidence to back that up. There is no evidence either way.bhobba said:Errrrr. So you think a cat isn't classical?
Thanks
Bill
Crazymechanic said:Well I could agree that for a single electron there really is a "guess what" thing involved but as you just said not for a cat or a human
BruceW said:just because it would be very difficult (in practice) to diffract a cat, then does that mean in principle it can't be done? I'm guessing when you say "a cat is classical", you mean that it is impossible (even in principle) to diffract a cat. But there is no experimental evidence to back that up. There is no evidence either way.
stevendaryl said:I don't agree. There is nothing in principle different about a cat than an electron, other than the enormous complexity.
right, OK so when you say "a cat is classical" you essentially mean that due to decoherence, quantum effects (like diffraction of the cat) are much more difficult. and the interference between alive and dead states of the cat becomes very small. The problem is that still, the interference is non-zero. And the Copenhagen interpretation 'solution' to this problem is that we impose a subjective collapse which makes the interference between alive and dead states exactly equal to zero. This gives us a way to say "what probability is the cat alive?" (in other words, we impose the subjective collapse so that we can give meaning to the probabilities of certain states).bhobba said:That's exactly what I am NOT saying. I believe everything is quantum and in principle you can demonstrate quantum effects at all levels
One point of clarification for comment. If the cat could actually be down-converted to a collection of entangled atoms and molecules. I would no longer be a cat. None of the complex (classically based) functions of the body would be possible. We would just have some very complex new experiment on a bunch of atoms we collected together from the cat.stevendaryl said:I don't agree. There is nothing in principle different about a cat than an electron, other than the enormous complexity.
I think I understand your point. Rephrasing from a different perspective - the cat is, after all, no different that the scientist, as an observer. There is no distinction between their positions inside versus outside a box (cement, cardboard, or equally, the scientist closing his or her eyes ... etc.). Forming something as entangled requires a true down-conversion mechanism (not a box!) which creates a boundary between the classical and quantum state. The functional mechanism of this "boundary" has, to date, not been identified and consequently we have no idea what our relationship to it is, at the classical level.StarsRuler said:But while the decoherence time, in spite of being very short, the cat can´t be in a superposition. ¿ How did the cat feel in a superposition? If the superposition is in external objects, no matter because we are not in a superposition, but with alive being, it is paradoxical, the collapse must be instantaneous
StarsRuler said:How did the cat feel in a superposition?
BruceW said:And the Copenhagen interpretation 'solution' to this problem is that we impose a subjective collapse which makes the interference between alive and dead states exactly equal to zero. This gives us a way to say "what probability is the cat alive?" (in other words, we impose the subjective collapse so that we can give meaning to the probabilities of certain states).
JazzGuy said:One point of clarification for comment. If the cat could actually be down-converted to a collection of entangled atoms and molecules. I would no longer be a cat. None of the complex (classically based) functions of the body would be possible. We would just have some very complex new experiment on a bunch of atoms we collected together from the cat.
If there actually is some entangled state that has meaning for cats, humans and the entire universe then we will have to solve the issue of quantum gravity first, and since we will also be in this very large entangled state we certainly will not be aware of it or able to do experiments based on it.
bhobba said:The cat was never in a superposition - decoherence prevents that for the cat or any classical object we experience day to day - ever.
stevendaryl said:I think this point needs some clarification. The way that I understand decoherence (which probably isn't very well) is that it's not a matter of destroying superpositions, but a matter of superpositions spreading to affect the whole universe. So, rather than having the cat be in a superposition of [itex]|[/itex]dead cat[itex]\rangle[/itex] and [itex]|[/itex]live cat[itex]\rangle[/itex], you have the whole world in a superposition of [itex]|[/itex]world with dead cat[itex]\rangle[/itex] and [itex]|[/itex]world with live cat[itex]\rangle[/itex]. There is still a superposition involved, but it's not the cat in the superposition.
stevendaryl said:I think this point needs some clarification. The way that I understand decoherence (which probably isn't very well) is that it's not a matter of destroying superpositions, but a matter of superpositions spreading to affect the whole universe. So, rather than having the cat be in a superposition of |dead cat⟩ and |live cat⟩, you have the whole world in a superposition of |world with dead cat⟩ and |world with live cat⟩. There is still a superposition involved, but it's not the cat in the superposition.
Getting more complicated for sure. I think I see your points is this correct? - Many worlds assumes the cat is in a state of superposition in the box. We decohere the state and the cat becomes either dead or alive - say alive. In some other world the cat is dead ... on and on it goes for everything. In every classical event this entangled coherence of all possible entangled states takes on some new direction and our classical experience builds. The universe is an infinity resource of unexpressed complexity and classical reality causes its expression to develop lines expression. Here is a question. Before we put the cat in the box it was alive, so in a state of decoherence. By putting it in the box we were able to reverse the process for both the real cat and observer back to one of coherence before collapse. There is also a future state in which the cat is again coherent. We never see his reversal in any larger classical context. This phenomenon of time reversal is purely quantum. This means that a much larger structure of possible quantum states exists that encompasses everything in which the reversal (as for the cat) could also be initiated. Thus the universe must have a "largest of all" quantum description that includes all we have decohered in our paths (which allows time reversal for this evolved decoherence). We are back to the problem of accounting for all time and gravity - where does the description end that encompasses all this potential.bhobba said:Intuitively the coherence leaks out to the environment - it interacts with with other objects that scrambles its phase so you end up with a phase of zero - that being the average of the phase of the objects that randomly changes it.
Technically you do what is called tracing over the environment which transforms a pure state to an improper mixed state.
Thanks
Bill
What leaps? you haven't said what you disagree with. The Copenhagen interpretation is a 'subjective non-unitary collapse' interpretation. subjective does not mean a human needs to be involved. It means that the state does not actually collapse for any physical reason, but we must place in the collapse 'by hand'. For example, we might choose to add in the 'subjective non-unitary collapse' at some short time interval after the Geiger counter has interacted with the particle. And conversely, the Penrose interpretation is an objective collapse because there is a physical cause for the collapse of the state, which we will one day hopefully be able to test experimentally.bhobba said:Wow. Many leaps being made here. First, while there is a bit of variation in Copenhagen, most would side with the idea collapse is simply something that occurs in theorists calculations since a state is simply something that aids in those calculations. What exists out there is if the particle detector detects a particle or not. The atom is in a superposition - but that's it - that's all.
Intuitively it seems strange, but there is nothing paradoxical about a cat (or a person) being in a superposition. Whether it is in principle possible to do this is up for debate, since there is no experimental evidence either way.StarsRuler said:But while the decoherence time, in spite of being very short, the cat can´t be in a superposition. ¿ How did the cat feel in a superposition? If the superposition is in external objects, no matter because we are not in a superposition, but with alive being, it is paradoxical, the collapse must be instantaneous
bhobba said:Intuitively the coherence leaks out to the environment - it interacts with with other objects that scrambles its phase so you end up with a phase of zero - that being the average of the phase of the objects that randomly changes it.
Technically you do what is called tracing over the environment which transforms a pure state to an improper mixed state.
BruceW said:The use of this 'subjective non-unitary collapse' does make a working theory. But conceptually it is not very nice. Also, we must always be careful to put in the subjective non-unitary collapse only when it makes a negligible difference to the outcome of the experiment. And our choice of when it happens is arbitrary in the sense that we can always make our predictions more accurate by putting in the 'subjective non-unitary collapse' later in the experiment. And of course, we will want a different accuracy depending on the experiment, so we must consider each experiment individually before we decide when to put in the 'subjective non-unitary collapse'. So for these reasons, we would hope for a better explanation of measurement.