In Search of Schrodingers Cat*

[peter]

[*Stolen from the eponymous book title]

We all know about the famous Schrodingers Cat thought experiment - the cai is placed in the box with a radioactive isotope that, if it decays will kill the cat. While the box remains unopened the cat is said to be neither alive nor dead, but in an intermediate state of 'livedeadness', and it is only the opening of the box that precipitates the cat into one or other of the states allowable in the (macro?) world - ie live or dead. Which state will be precipitated we cannot predict, we can only observe the state resulting once we have opened the box.



But I have some questions about this thought experiment - things that may be effecting my understanding of what quantum physics is - and what it isn't. I'll try to number the queations but the areas may well overlap.

i) First, are we to understand the thought experiment only as an allegory of the situation pertaining to light. The experiment was formulated early in the days of what I would call 'modern physics', when experiments had been performed that could show light as a particle (a photon) or a wave, depending upon your experimental set up. [problems already here in that we can predict the outcome of these expts purely by the type of equipment we use for the detection ie Look for a particle and you see a particle, look for a wave and you see a wave.

ii) Are we to understand this 'quantum uncertainty' to extend to the macro world as well as to the micro - or is the use of the cat just allegory. ie I'm upstairs, my wife may be downstairs or may be off in the town; is she in a state of 'downstairsin the towness that only my going down to see will precipitate into one case or the other, or as I said, was the use of a macro object (the cat) just an allegorical device to illustrate the point.

iii) If the above was the case why was it necessary. Schrodinger might just as well have used the radio isotope on it's own and caused less confusion. The isotope having decayed, or not decayed is itself a quantum event(?) and would have saved the confusion above.

iv) Assuming the thought expt refers only to events at the quantum scale how are we to view these intermediate states - as true existant states or as predictive (or not as the case may be) instruments covering a hole in our knowledge of how the quantum world really is. Is the 'not-knowing' state we are in before we open the box the 'uncertainty' of Werner Heisenburg's Uncertainty Principle (ie we can never know when an isotope will decay because it is an event without 'a cause' (ie no causal event occurs to trigger the next cause in the cause and effect chain).

It will be interesting to see if you guys exhibit a conformity of view as to your collective understanding of this most famous thought experiment - and I might learn something in the process!

[I'm Murrin]

Schroedinger used the cat because he was highly skeptical of the concept of quantum uncertainty - the cat experiment is a reductio ad absurdum, an attempt to show how ridiculous the idea is by extending it to a macro scale. It just happened to also be seen as useful for teaching people the concept.

[wayfriend]

The thing that always bothered me about The Cat was: no one is demonstrating that the cat is neither alive nor dead; they are just demonstrating that such an imaginary state is indistinguishable from the real state, from an observer's point of view, if the observer cannot see into the box.

It is rather a quantum leap, if you will, to go from it's as if the cat were in a neither-nor state to the cat is in a neither-nor state.

The fallacy of "as if" is that analogies only go so far, they only apply within a specific set of circumstances; turning them into "is" lets us believe that it applies in any circumstance we care to consider. This is a fallacy.

[Zarathustra]

The underlying principle has been shown to be real, if not with an actual cat.

The experiment as described is a purely theoretical one, and the machine proposed is not known to have been constructed. However, successful experiments involving similar principles, e.g. superpositions of relatively large (by the standards of quantum physics) objects have been performed.[12] These experiments do not show that a cat-sized object can be superposed, but the known upper limit on "cat states" has been pushed upwards by them. In many cases the state is short-lived, even when cooled to near absolute zero.

en.wikipedia.org/wiki/Schr%C3%B6dinger% ... _and_tests

This is a demonstrable feature of reality. It's not an analogy. But our current upper limit (as tested) doesn't yet include cat-sized objects. However, that's not the same as saying that the upper limit can't include such objects. We'll have to wait and see.

[Hashi Lebwohl]

The cat in the box--cats always seems to wind up in boxes, don't they?--was posed in the earlier days of quantum mechanics and served as a great mental exercise to let non-physicists think about uncertainty and how observation collapses potential realities into only one reality.

We are now at the point where we can set up experiments to observe the effects of new states of matter. Both Bose-Einstein condensates and non-Newtonian liquids are great fun. The only problem with this linked article is that it is 4 years old--times update quickly in cutting-edge physics.

The one question that I haven't seen answered is this: suppose there in an experiment in a variety of non-collapsed states. If two different observers who are unaware of each other both view the event at the same time does it collapse the same way for both viewers?

[wayfriend]

How did they create an experiment to show what something that is unobserved does, without observing it?

[Hashi Lebwohl]

*shrug* Your guess is as good as mine. If I were a theoretical physicist I could probably design some experiments, though.

[I'm Murrin]

The double slit experiment is the classic. If you set up something to detect which slit a particle passes through, you only ever see it pass through one. If you don't detect at the slits, the result you see after the particle passes them indicates it went through both at the same time, and interfered with itself. The key is what and when you measure.

[wayfriend]

... Sort of like opening the box without looking, and detecting if the live cat starts gnawing on the dead cat.

My immediate problem with swallowing that is, not only does this require the cat be in two states, it requires that there now be two cats.

[I'm Murrin]

Well, that's the thing about that experiment. A particle shouldn't be able to be in two places at once, but it seems to behave that way, because the particle is actually a distributed wavefunction occupying a certain probability space and not a discrete object.

[Avatar]

Hahaha, beautifully put Murrin.

...we can predict the outcome of these expts purely by the type of equipment we use for the detection ie Look for a particle and you see a particle, look for a wave and you see a wave.

Seems more of a proof than a problem...here too it's the observation that determines the outcome.

--A

[peter]

OK - let's put it this way. Has our understanding of quantum mechanics developed to the point where Scrodingers cat expt is now too simplistic a way of viewing things; ie that what was initially intended to be metaphorical is actually turning out to be actual as quantum effects are demonstrated at the larger and larger scale.

(re The Young's slits expt Murrin, is the 'distibuted wave function' that is the particle different from the elecrtmagnetic wave that is the ...er wave.... if we assume light to be a wave?)

[wayfriend]

Well, that's the thing about that experiment. A particle shouldn't be able to be in two places at once, but it seems to behave that way, because the particle is actually a distributed wavefunction occupying a certain probability space and not a discrete object.

It's more than being in two places at once - I can grasp that. It is the place-1 self interacting with the place-2 self that defies my credibility.

[Hashi Lebwohl]

If time flows differently at the quantum level this could explain a particle/wave being able to interact with itself.

[TheFallen]

The issue with quantum is that it's completely counter-intuitive to our perception of everyday reality.

Although absolutely no scientist, I don't feel too bad about just not getting it - especially when revered academics have opined as follows:-

The Universe is not only queerer than we suppose, but queerer than we can suppose.

If you think you understand quantum mechanics, you don't understand quantum mechanics.

The quantum world is just plain weird and freaky - so much so that you pretty much have to be batshit crazy as a starting point in order to attempt to wrap your mind round it without your brain slowly leaking out of your ears. How else to explain the observable phenomenon of quantum entanglement for example, where a pair of particles that have interacted in the past can be separated by considerable distance - 8 miles I think is the current experimental maximum achieved - and then, if one of the erstwhile pair is observed to start exhibiting a property like clockwise spin, the other will apparently instantaneously take on the opposite property (counter-clockwise spin). It'll apparently instantaneously "know" what its distant partner is doing. I say "instantaneously", but that may not be entirely correct - but if particle A is having some sort of undetectable effect on particle B, then the transmission transference of such effect must occur at least at 10,000 times the speed of light.

Again I'll recommend Bill Bryson's "A Short History Of Nearly Everything" to any enquiring non-expert mind. He deals with quantum as enjoyably as he deals with all major scientific topics (including the start of the universe, the start of life, evolution and pretty much nearly everything - the book does exactly what it says on the tin).

And as we're talking about Schroedinger's cat, I'll also recommend another of my favourite authors to you, namely Terry Pratchett. Why the latter, you may ask? Well, this from "Lords & Ladies", describing Greebo, the temperamental cat of one of Pratchett's witches:-

The elf looked at Magrat's innocent expression, and opened the box.

Greebo had spent an irritating two minutes locked in that box. Technically, a cat locked in a box may be alive or it may be dead. You never know until you look. In fact, the mere act of opening the box will determine the state of the cat, although in this case there were three determinate states the cat could be in: these being Alive, Dead, and Bloody Furious.

Shawn dived sideways as Greebo went off like a claymore mine.

"Don't worry about him," said Magrat dreamily, as the elf flailed at the maddened cat. "He's just a big softy."

Sheer brilliance...

[Hashi Lebwohl]

The quantum world is just plain weird and freaky - so much so that you pretty much have to be batshit crazy as a starting point in order to attempt to wrap your mind round it without your brain slowly leaking out of your ears. How else to explain the observable phenomenon of quantum entanglement for example, where a pair of particles that have interacted in the past can be separated by considerable distance - 8 miles I think is the current experimental maximum achieved - and then, if one of the erstwhile pair is observed to start exhibiting a property like clockwise spin, the other will apparently instantaneously take on the opposite property (counter-clockwise spin). It'll apparently instantaneously "know" what its distant partner is doing. I say "instantaneously", but that may not be entirely correct - but if particle A is having some sort of undetectable effect on particle B, then the transmission transference of such effect must occur at least at 10,000 times the speed of light.

I suspect entanglement must be quantum-level particle/waves existing not only in our usual four-dimensional spacetime but in some other "higher" dimension where space doesn't exist like it does here. If you study topology you can examine topological spaces where any two distinct points are equidistant from each other. If entanglement is like this then we might think the particles are really far apart but from their point of view they are not.

[sgt.null]

I found him.

[peter]

Just found this Sarge - great pic!

[wayfriend]

[peter]

Like it Wayfriend.

Just seen a Stephen Hawking program on 'the History of Everything'. He concluded with what I would consider a weird sort of statement for a cosmologist who has previousely stated his belief that it is unlikely that there is any creator/designer/God behind the existence of the Universe; he said that it was his belief that in time scientists would answer the most fundamental questions of existence, namely how will the universe end and why is it here in the first place?.

I italicise this strange (to me) question because it was my understanding that the prevailing view was that to ask 'why' there was a universe as opposed to just asking 'how' was a meaningless exercise. ie That there is no reason to suppose that there is a 'why' at all - that the rules governing the quantum behavior of space, time and matter are perfectly consistant with a Universe coming into existence with no prior causitory factor - in fact they demand it to be so. How is this squared with questions of 'why are we here'? I would like to ask Hawking to explain that comment to me.