What makes schrodinger cat quantum?

[Demystifier]

ArielGenesis, it seems to me nobody answered your question in a way that would satisfy you. So let me try.

How do we know that Schrodinger cat is quantum and not classical? From the experimental point of view, the only way to determine whether it is quantum or classical is to try to measure interference. If there is interference, then it is quantum. If there is no interference, then it is either truly classical or apparently classical due to decoherence that destroys interference.

Now, the fact is that there is no interference. This is actually expected, because the theory predicts that decoherence should take place for macroscopic objects such as cats. To conclude, from the experimental point of view it appears classical, but from the theoretical point of view there is a good reason to believe that it is actually quantum. In fact, from the theoretical point of view everything is believed to be quantum, including coins.

 

[ZapperZ]

I think this is an unreasonable demand, because what you are effectively asking is a deviation from classical theory at the macro level which one would not expect to exist, except in exceptional cases.
And the seemingly exceptional case of the flux qubit is actually not as macroscopic as it seems to be, as explained here:

http://arxiv.org/abs/quant-ph/0609007

ZapperZ's demand is a bit like someone making the claim that objects moving under gravity always move along geodesics and not according to a Newtonian gravitational potential, even in cases where gravity is very weak. Then a ZapperZ like sceptic comes along demanding experimental proof for that in the weak gravity limit.

Clearly in the case of general relativity, this would be unreasonable because the idea of a dualistic description of Nature is a priori not seen to be tenable. In case of quantum mechanics, however, this is atitude is different, because quantum mechanics is tradionally formulated as a dualistic theory in which classical concepts play an important role.

But then, this is an effective formulation of a true quantum theory at best. The classical world has to arise from quantum dynamics, the precise way in which that happens is still under discussion. But this lack of understanding does not always preclude one from setting up a general argument, like in this case one that shows that a superposition exists in the macro-world.

Then you are also arguing that all of our current efforts at trying to detect QM effects at the macro scale is worthless because there's nothing to prove here.

What you are trying to hide is the FACT that you cannot show such effects. Again, using size as the excuse for not being able to show such a thing doesn't wash anymore.

There is a continual work on testing the "limits" of QM and testing to what extent QM effects can be detected with not only large scale, but also longer time scale. This would be a silly thing to do if this is a done deal. The FACT that work, both experimental and theoretical, continues in this area is MY proof that there's a lot more to understand and to VERIFY the question on at what point to we lose the "weird" effects of QM.

And let me correct the misconception here. I have NEVER, ever said that QM is wrong, and that it is wrong at the classical scale. I had always stressed that both the QM world and the classical world look DIFFERENT, and at some point, the QM effects are no longer evident. That's why your writing of the wavefunction of a classical system is bogus in my book, because such a system cannot be tested, whereas the waverfuction for QM system can!

And since you tried to paint a caricature of me, I'll do the same of you. Your point is similar to someone who claims that the Higgs is already a done deal, since based on the Standard Model, it should be there, and the Standard Model is such a successful theory that there's no reason to argue that cannot be found. So asking for experimental verification of such a thing is unreasonable.

The FACT remains that you made an untestable speculation.

So there!

Zz.

 

[Adrian59]

I do consider that and my knowledge will eventually evolve be at that state. But for now I am not and I hope through this forum I could achieve firm basic understanding of the foundational principle of quantum mechanic, such as superposition, with my current technical and mathematical skill.

So far, solving all the pde for 1D quantum system and hydrogen atom and doing that in terms of operators or Hilbert space or matrix have not actually help my basic understanding. Or at least I feel, or as you can see, I am a bit lost in the interpretation.

I’m going to stick my neck out here and offer ArielGenesis what I think is an acceptable handle on quantum mechanics. As a disclaimer I agree with the succeeding comment by SW VandeCarr that essentially QM is a mathematical theory that gives precise predictions. Therefore any interpretation is just that and as such is a mental construct to make sense of what appears counterintuitive.

My interpretation is that since the mathematical formulation of QM is based on interacting wave equations and since wave interference is experimentally verified, the basic building blocks (the quanta) are wave packets and not point particles. Moreover, my additional suggestion is that any particle associated with this quantum wave is an illusion created by our experimental design. When we detect a particle we have no idea what this particle is except for an interaction between something, particle or wave & our detecting apparatus. What we see is a blip on a screen, or a tract on a photographic film or even, now, a computer generated graphic but what we don’t see is an actual particle.

Furthermore, nature gives us the result we are looking for. If you design an experiment to look for presumed particles that is usually what you find. The problem comes when you try to ask where exactly is the point particle because it doesn’t exist. Of course we do detect something and since we are often dealing with single quanta we only get one interaction. If we repeat the process we may find the particle somewhere else but it isn’t it’s just a different aspect of the same wave.

For those of you who like experimental verification for any outlandish idea, can I say that a recent experiment trying to show interference and particles at the same time would back up my interpretation. I say my interpretation but I have to admit that there is one paragraph in ‘The Brief History of Time’ by Prof Stephen Hawking that would suggest he beat me to this idea by decades

 

[ArielGenesis]

Thank you to SW VandeCarr, Demistifier and Adrian59, really appreciate that.

So a particle is basically a solution to a Schrodinger equation, which is a wave or a wave function. It simply for some strange experimental reasons that what we observe is actually a blimp on a screen which looks more like a particle instead of a wave.

And the only thing that shows that this wave is a probability amplitude wave is the interference effect which usually vanishes via de-coherence in a large scale or a long run.

I hope that I am not asking too much but what is the simplest example regarding probabilistic interference. I don't really care if it exist or possible or simply theoretical or speculative or purely mathematical with no physical meaning. I just want to get the concept.

Thank you again to three of you, really appreciate that.

 

[Adrian59]

The simplest example of probabilistic interference is the double slit experiment.

 

[Count Iblis]

Then you are also arguing that all of our current efforts at trying to detect QM effects at the macro scale is worthless because there's nothing to prove here.

What you are trying to hide is the FACT that you cannot show such effects. Again, using size as the excuse for not being able to show such a thing doesn't wash anymore.

There is a continual work on testing the "limits" of QM and testing to what extent QM effects can be detected with not only large scale, but also longer time scale. This would be a silly thing to do if this is a done deal. The FACT that work, both experimental and theoretical, continues in this area is MY proof that there's a lot more to understand and to VERIFY the question on at what point to we lose the "weird" effects of QM.

And let me correct the misconception here. I have NEVER, ever said that QM is wrong, and that it is wrong at the classical scale. I had always stressed that both the QM world and the classical world look DIFFERENT, and at some point, the QM effects are no longer evident. That's why your writing of the wavefunction of a classical system is bogus in my book, because such a system cannot be tested, whereas the waverfuction for QM system can!

And since you tried to paint a caricature of me, I'll do the same of you. Your point is similar to someone who claims that the Higgs is already a done deal, since based on the Standard Model, it should be there, and the Standard Model is such a successful theory that there's no reason to argue that cannot be found. So asking for experimental verification of such a thing is unreasonable.

The FACT remains that you made an untestable speculation.

So there!

Zz.

Quantum effects at the macro scale would shed light on decoherence mechanisms, it would potentially have a lot of applications like in quantum communications, quantum computing, quantum metrology, etc.

You would expect that size does matter as far as the decoherence rate is concerned. Numerous articles point that out. That doesn't mean that you could not somehow be able to make a device that is able to circumvent decoherence effects to some extent. I think that most people who work in this field will agree that macroscopic quantum superpostions are very fragile in the sense that they'll decohere very fast if not kept isolated from the envirmonment.

In case of the Higgs, there are alternative Higgsless theories. In case of quantum mechanics, all we have is a suggestion by Penrose that gravity may cause a real state reduction.

 

[ZapperZ]

Quantum effects at the macro scale would shed light on decoherence mechanisms, it would potentially have a lot of applications like in quantum communications, quantum computing, quantum metrology, etc.

You would expect that size does matter as far as the decoherence rate is concerned. Numerous articles point that out. That doesn't mean that you could not somehow be able to make a device that is able to circumvent decoherence effects to some extent. I think that most people who work in this field will agree that macroscopic quantum superpostions are very fragile in the sense that they'll decohere very fast if not kept isolated from the envirmonment.

In case of the Higgs, there are alternative Higgsless theories. In case of quantum mechanics, all we have is a suggestion by Penrose that gravity may cause a real state reduction.

Read again! I said that there are indications that SIZE DOESN'T MATTER.

And the alternative to decoherence has been mentioned several times already beyond Penrose's model. Read about the coarse-grained effect of our measurement that I've cited several times already!

Zz.

 

[ArielGenesis]

I see, double slit experiment for one particle at a time shows interference for a probability wavelet. I understand this experiment quiet well but still I wish for something simpler.

One last question, I am always wondering, what if I do a double slit interference in a cloud chamber, will we get different result? But I could also argue that doing double slit interference in non vacuum will decohere the wavelet too?

 

[Adrian59]

One last question, I am always wondering, what if I do a double slit interference in a cloud chamber, will we get different result? But I could also argue that doing double slit interference in non vacuum will decohere the wavelet too?

A good question which deserves an answer. Of course the get out of jail answer would be that since the cloud chamber reveals the paths taken by sub-atomic by interaction between the particle and the cloud chamber molecules one suspects as you yourself conclude that the pure quantum situation will decohere. The more challenging problem would be what would you see if you could accurately view the paths taken through the apparatus without decoherence. My hunch is that the particle would go through one slit but interference would still occur. As mentioned in one of my previous posts there is some experimental evidence that you can get both interference and particle behaviour together but alas I've lost the reference.

 

[ibcnunabit]

It's been proven that hidden-variable theories can't properly describe reality - Bell's theorem, I think, and associated experiments. I don't remember the details right now...

Studies have shown that LOCAL hidden variable theories can't properly describe the full reality (well, there are actually some assumptions made which may not turn out as expected). It has NOT been shown that NON-LOCAL hidden variable theories cannot fully account for it. (This is the Bohm interpretation.)

The Schrödinger's cat thought experiment was actually devised as a sort of reduction ad absurdum to demonstrate problems with the Copenhagen interpretation of quantum mechanics, and was not intended as a true description of reality.

http://en.wikipedia.org/wiki/Bell's_theorem
http://en.wikipedia.org/wiki/Loopholes_in_Bell_test_experiments
http://en.wikipedia.org/wiki/Bohm_interpretation
http://en.wikipedia.org/wiki/Schrodinger's_cat

 

[ibcnunabit]

I see, double slit experiment for one particle at a time shows interference for a probability wavelet. I understand this experiment quiet well but still I wish for something simpler.

One last question, I am always wondering, what if I do a double slit interference in a cloud chamber, will we get different result? But I could also argue that doing double slit interference in non vacuum will decohere the wavelet too?

Probability wave interference is the classic way of explaining the double slit experiment, but I don't think it's necessary. Worse still, probability waves are, in all likelihood, nothing more than a fantasy made up by some physicists. (What is it "made of"?--not energy, and certainly not matter--it creates more problems and questions than it solves). Certainly nobody has ever seen one. They're a construct some physicists have made to provide a conceptual framework of what is happening.

The way I see it, the seemingly unusual results of the double slit experiment are a consequence of the universe having a matter perspective, as well as an energy perspective, which are both valid. This is why we find what we call the particle/wave duality of light. (Such a duality exists for more than just light, however.)

We're used to seeing the experiment from the matter perspective. We're firing photons (from the matter perspective) and they have a probabilistic chance of moving through one slit or the other. We look at it and wonder how this photon can interfere with it very self as though it went through both slits.

But look at it from the photon's perspective. For the frame of reference of a photon time does not exist. In other words, the elapsed time from when it is emitted to when it is absorbed is zero *from the photon's perspective* (an equally valid perspective as ours, but different). This also must mean that the distance between emission and absorption must also be zero *from the photon's perspective*. Therefore all paths from the emitter to the screen are zero, which is like saying the screen at all points is "in contact" with the emitter, *FTPP*. But because of the barrier and the slits, not all of these paths are equally likely. The size, shape, and location of the slits determine areas which have a much greater chance of being "hit." (A probability, but not a "probability wave".

So in effect, the photon really does pass through both slits and interferes with itself, but this spatial relation of the slits as being separate from each other, and of the photon interfering with itself--all this only happens from the matter perspective (which is, as I said before, equally valid--and in this case, much more in keeping with our ordinary perceptions).

I'd be interested in hearing thoughts anyone might have about this interpretation.

--Mike from Shreveport

 

[Adrian59]

... probability waves are, in all likelihood, nothing more than a fantasy made up by some physicists... They're a construct some physicists have made to provide a conceptual framework of what is happening.

The way I see it, the seemingly unusual results of the double slit experiment are a consequence of the universe having a matter perspective, as well as an energy perspective, which are both valid...

We're used to seeing the experiment from the matter perspective...

So in effect, the photon really does pass through both slits and interferes with itself...

--Mike from Shreveport

I, also, dislike the use of the term probability wave. That is why I see the wave nature of QM as suggesting wave packets representing an oscillator of a yet to be determined nature - see my post #93. But I view the oscillations as real. I have move difficulty visualising the matter angle. I see the double slit experiment when considered from the matter perspective as showing point particles & from the energy perspective as showing wave interference. However, at this scale we're not seeing small specks of matter. So my conjecture was that the particles were an conceptual construct to explain the experimental results.

 

[ibcnunabit]

I, also, dislike the use of the term probability wave. That is why I see the wave nature of QM as suggesting wave packets representing an oscillator of a yet to be determined nature - see my post #93. But I view the oscillations as real. I have move difficulty visualising the matter angle. I see the double slit experiment when considered from the matter perspective as showing point particles & from the energy perspective as showing wave interference. However, at this scale we're not seeing small specks of matter. So my conjecture was that the particles were an conceptual construct to explain the experimental results.

I agree that the oscillations ARE real, from our perspective. For the photon there is no time for an oscillation to occur. (Aha!) But we see what we see, and depend on it. The oscillation may be a product of a tiny spatial dimension or some or spatial or dimensional aspect of reality that we don't fully understand. And probably our understanding of the photon as this tiny particle is flawed, too. It may be much more spread out.

I've often wondered whether light is fundamentally quantum in nature, or if it only appears that way because all of its interactions with matter must be in quantum amounts in order to occur. This may seem like a difference that makes no difference, but it actually might explain some real phenomena.

 

[RUTA]

One last question, I am always wondering, what if I do a double slit interference in a cloud chamber, will we get different result?

You won't get an interference pattern because the wave function collapses to a narrow beam after the first click (bubble), so that subsequent clicks fall in a nice line ("particle trajectory"). Sorry, I forget the classic paper that describes this change in psi after the first bubble chamber event. It's rather old, 1930ish as I recall.