someone explain schrodinger's cat

[Gonzo]

it's alive. it's dead. head assplode.


please explain in donkey terms. kthxbai

[Len Myers]

Cats can't be explained, but they can be petted.

[crabber]

Watch The Princess Bride again.

Miracle Max explains it pretty well.

[Len Myers]

Sorry for the silly response. Let's start over with the basic Heisenberg equation below...

In a nutshell the equation states that for small objects -- such as cats -- you can measure the location, or the velocity of those objects, but not both at the same time. This is just how it goes. So, if the position of the cat is known (in the box), then you don't know its velocity, and therefore you don't know if the cat is alive (moving) or dead (not moving). If you open the box and observe its velocity, you shortly won't know where the cat is. Unless you've placed some food nearby.

[Wigmeister General]

.... because observing the cat requires that you apply energy (e.g., light) to the system, whereby you have affected the system.

[Loner]

In donkey terms:
Your department kicked ass and doubled revenue, but the entire company's revenues only grew at 9.9%, not the goal of 10%, so nobody gets raises. You did well and did poorly at the same time.

[Nee-2-No-Bases]

Not gonna lie, there are a lot of good answers here. But they are all missing the concept of the cats probability wave. That's the Schrodinger Wave equation.

The cat has a probability wave. Sometimes the cats wave will be at a peak, and the water will be really deep, so the cat drowns and dies. BUT the cat's wave could be at it's low point, where the water is shallow. So it won't drown, but there is lots of water for the cat to drink to sustain the cat for a longer period of time.

You can't know without looking in the box whether the cat is alive, or had drown, so you just assume it's both.

[Jasper07734]

In donkey terms:
Your department kicked ass and doubled revenue, but the entire company's revenues only grew at 9.9%, not the goal of 10%, so nobody gets raises. You did well and did poorly at the same time.

http://aznbadger.files.wordpress.com/2010/06/scannersexplodinghead.gif?w=406

[Loner]

Not gonna lie, there are a lot of good answers here. But they are all missing the concept of the cats probability wave. That's the Schrodinger Wave equation.

The cat has a probability wave. Sometimes the cats wave will be at a peak, and the water will be really deep, so the cat drowns and dies. BUT the cat's wave could be at it's low point, where the water is shallow. So it won't drown, but there is lots of water for the cat to drink to sustain the cat for a longer period of time.

You can't know without looking in the box whether the cat is alive, or had drown, so you just assume it's both.

He said donkey terms, not geek terms. Pay attention!

[bokiz333]

Not gonna lie, there are a lot of good answers here. But they are all missing the concept of the cats probability wave. That's the Schrodinger Wave equation.

The cat has a probability wave. Sometimes the cats wave will be at a peak, and the water will be really deep, so the cat drowns and dies. BUT the cat's wave could be at it's low point, where the water is shallow. So it won't drown, but there is lots of water for the cat to drink to sustain the cat for a longer period of time.

You can't know without looking in the box whether the cat is alive, or had drown, so you just assume it's both.

I always hated this concept...I uderstand it but I think it's flawed. Why would you assume it's both? You just lack information, and all you know is that you don't know what state it's in. The one state it can't be in is both.

[Entropy]

The cat's probability wave consists of (simplified) two states. Either the radioactive element decayed and killed the cat or it didn't. Barring the confusing thought of the cat being it's own observer, outside of the closed system we have no way of telling which state the cat is in, so we give it nonzero probability of being in either state. We cannot say "the cat is dead" or "the cat is alive", so we must say "the cat is a little of both". Once outside observer (you) open the box and observe the system, that wave function collapses to one observable state.

[bokiz333]

The cat's probability wave consists of (simplified) two states. Either the radioactive element decayed and killed the cat or it didn't. Barring the confusing thought of the cat being it's own observer, outside of the closed system we have no way of telling which state the cat is in, so we give it nonzero probability of being in either state. We cannot say "the cat is dead" or "the cat is alive", so we must say "the cat is a little of both". Once outside observer (you) open the box and observe the system, that wave function collapses to one observable state.

That's my issues. We don't "have" to say it's a little of both. We can just say - we don't know what it is. Because it's not possible for it to actually be both.

[magillaG]

That's my issues. We don't "have" to say it's a little of both. We can just say - we don't know what it is. Because it's not possible for it to actually be both.

The issue is that the information, apparently, simply doesn't exist. It isn't knowable. The mathematics of, say, an electron can be understood as if it simultaneously is in two places at once.

Note that quantum measurement is much better understood (http://en.wikipedia.org/wiki/Decoherence) than when Schrodinger devised his cat paradox. I guess a lot of progress was made in the 1980s- I don't know that much about it, actually.

[Hydraskull]

That's my issues. We don't "have" to say it's a little of both. We can just say - we don't know what it is. Because it's not possible for it to actually be both.

Nobody actually says these things in practice, about a cat. In practice, it is used on a subatomic level to make measurements of quantum phenomena.

[oirg]

What if you build in a little window on the side of the box before putting the cat and the radiactive element in the box and sealing it?

[magillaG]

I don't think the cat example is supposed to make any sense. Schrodinger was using it to argue against a particular interpretation of quantum mechanics.

[bokiz333]

The issue is that the information, apparently, simply doesn't exist. It isn't knowable. The mathematics of, say, an electron can be understood as if it simultaneously is in two places at once.

Note that quantum measurement is much better understood (http://en.wikipedia.org/wiki/Decoherence) than when Schrodinger devised his cat paradox. I guess a lot of progress was made in the 1980s- I don't know that much about it, actually.

Sure in quantum mechanics and such (which I will not even pretend to understand) I understand the possibility of something being in two places, etc. What bugs me is that the commonly used example is flawed. It should be shrodingers electron or quantum particle or whatever the right term would be. Certainly not an adorable little feline that bast@rd is trying to drown.

[bokiz333]

I don't think the cat example is supposed to make any sense. Schrodinger was using it to argue against a particular interpretation of quantum mechanics.

Well I wish people would stop using it then. This is going to be my "non issue I'm going to make an issue" for 2012.

[Entropy]

I don't think the cat example is supposed to make any sense. Schrodinger was using it to argue against a particular interpretation of quantum mechanics.

And to bring up a lot of interesting questions. I agree that it makes more sense to start with the electron example and build up from there.

[Entropy]

Well I wish people would stop using it then. This is going to be my "non issue I'm going to make an issue" for 2012.

It's just a thought experiment. Maybe you'd feel better if you thought about it in terms of the many worlds interpretation.
http://en.wikipedia.org/wiki/Many-worlds_interpretation

[Will Durant]

But they are all missing the concept of the cats probability wave.
This.

You can't know without looking in the box whether the cat is alive, or had drown, so you just assume it's both.
But not this. It's not an assumption. It's a mathematical deduction from the axioms of the theory.

I always hated this concept...I uderstand it but I think it's flawed. Why would you assume it's both? You just lack information, and all you know is that you don't know what state it's in. The one state it can't be in is both.

That's my issues. We don't "have" to say it's a little of both. We can just say - we don't know what it is. Because it's not possible for it to actually be both.

These statements are not correct. This is essentially the "hidden variables" theory of Einstein-Podolsky-Rosen and other thinkers. This has been proven NOT to predict correctly the results of certain experiments (more on this to follow).

The issue is that the information, apparently, simply doesn't exist. It isn't knowable. The mathematics of, say, an electron can be understood as if it simultaneously is in two places at once.

Note that quantum measurement is much better understood (http://en.wikipedia.org/wiki/Decoherence) than when Schrodinger devised his cat paradox. I guess a lot of progress was made in the 1980s- I don't know that much about it, actually.

You're thinking about the Bell Inequality (1964) which proved that QM and hidden variables would make different predictions. Alain Aspect created just such experiments in the 1980s (others refined them in the 1990s) and definitively proved that the natural world obeys the predictions of QM but disagrees with the predictions of hidden variable theorems.

Therefore, the experimental conclusion is that (if QM is correct), the cat is "dead-alive". It's probably not correct to say both dead and alive; it's really a state that is neither dead nor alive; it's a third state.

Nobody actually says these things in practice, about a cat. In practice, it is used on a subatomic level to make measurements of quantum phenomena.

Correct. However, the axioms of the theory lead to the conclusion nonetheless.

Suppose you have a closed system consisting of a cat, a radioactive sample consisting of a single atom with a half-life of one hour, a very sensitive geiger counter, and a vial of poison attached in such a way that it will break when the the detection mechanism of said geiger counter is triggered. At the end of one hour, there's a 50% chance that the sample has decayed. With great simplification, the mathematics of QM tells us that

Wavefunction(atom) = 1/sqrt(2) * Wavefunction(non-decayed) + 1/sqrt(2) * Wavefunction(decayed)

Wavefunction(geiger counter) = 1/sqrt(2) * Wavefunction(non-triggered) + 1/sqrt(2) * Wavefunction(triggered)

Wavefunction(poison vial) = 1/sqrt(2) * Wavefunction(intact) + 1/sqrt(2) * Wavefunction(broken)

Wavefunction(cat) = 1/sqrt(2) * Wavefunction(alive) + 1/sqrt(2) * Wavefunction(dead)

The first equation is demonstrably true within the context of QM. If QM were universally true, then the last wavefunction would be true as well.

The unsolvable problem is how the overall closed system evolves from being described by the first equation to not being described by the last equation. This is know as the wavefunction collapse paradox.

I don't think any physicist believes that the cat is dead-alive. So when did QM stop applying? Is it because the cat is a macro system? If so, precisely what is the mechanism/reason of the collapse?

This is why macro-molecules that obey QM are so interesting. They blur the line between micro and macro.

Special relativity is well understood in terms of its limit - it applies less and less as speeds approach zero, but it never really stops applying. There's no clear boundary at which SR is wrong and classical mechanics is right.

Same thing with general relativity - it applies less and less as densities approach zero, but it never really stops applying. There's no clear boundary at which GR is wrong and Newtonian gravity is right.

But at present there's no such similar understanding of what happens to QM as systems get larger and larger. The cat is clearly not alive-dead, but by what mechanism (theoretical OR experimental) does QM "phase out" of application in favor of the classical mechanics we know and love.

It would appear to be a big gap in our understanding of the universe.

[Vorian Atreides]

You need to collect money--called premium--from the cat and it's your job to determine how much to collect in order to cover it's losses for the next year.


When you look at other cats, you see that they fall into two categories:

Some cats have had claims that were paid, and
a lot of cats that didn't have any claims at all.
The problem: all of these cats are in boxes, so you don't know if these other cats are identical (or even close to being similar) to the cat whose premium you are to determine.

So you deduce that your cat is a little of both.


Actually, there are three categories, the third being some cats that had claims that were investigated, but there were no payments.

The end result? No different, you deduce that your cat is a little of all three.

QED.

[Locrian]

I always hated this concept...I uderstand it but I think it's flawed. Why would you assume it's both? You just lack information, and all you know is that you don't know what state it's in. The one state it can't be in is both.

Maybe the cat can’t be both, but the particles in the thought experiment can be. That’s the question he’s addressing: a coherent system can be a mix of states that are mutually exclusive once observed. Decoherent systems (which include almost everything macroscopic) do not display this property. So how do you get from one to the other? It’s a deep, uncomfortable question, and Shroednger tries to get at it by linking them.

I agree with MagillaG that the study of quantum decoherence have largely rendered the paradox explainable, but it’s still a useful prop to start discussions, much like the EPR paradox.

[Nee-2-No-Bases]

The cat's probability wave consists of (simplified) two states. Either the radioactive element decayed and killed the cat or it didn't. Barring the confusing thought of the cat being it's own observer, outside of the closed system we have no way of telling which state the cat is in, so we give it nonzero probability of being in either state. We cannot say "the cat is dead" or "the cat is alive", so we must say "the cat is a little of both". Once outside observer (you) open the box and observe the system, that wave function collapses to one observable state.

So it's either dead, or it's alive.

So there's a 50/50 shot of either. Got it. 50% live cat. 50% dead cat.

Sure in quantum mechanics and such (which I will not even pretend to understand) I understand the possibility of something being in two places, etc. What bugs me is that the commonly used example is flawed. It should be shrodingers electron or quantum particle or whatever the right term would be. Certainly not an adorable little feline that bast@rd is trying to drown.

Serious response (mostly):

Well, I blieve in his example, he used radioactive decay, which is described via probability. Something like, "There's an atom holding a rope linked to a guillotine with a cat in the guillotine. If the atom decays, the cat's head gets chopped off. If it doesn't the cat lives."

So whether or not the cat is dead or alive is dependent on a quatum event that can only be predicted via probability. The issue isn't whether or not the cat is dead or alive, but whether or not the atom decayed.

[Will Durant]

What if you build in a little window on the side of the box before putting the cat and the radiactive element in the box and sealing it?
Then the system wouldn't be closed and the sequence of wavefunctions I described would not follow logically from each other. Most obviously, photons from outside the system would affect the system

I don't think the cat example is supposed to make any sense. Schrodinger was using it to argue against a particular interpretation of quantum mechanics.
However, he turned out to be wrong. The Aspect experiment proves that the interpretation was the best one among the known interpretations. Personally, I think this is still the wrong interpretation, but we know now that it's at least better than the one Schrodinger was putting forward.

Maybe you'd feel better if you thought about it in terms of the many worlds interpretation.
And DEFINITELY better than this one. Please, let's not go there.

To add to my previous post, in terms of the mathematics of QM I think the question boils down to what is an observable. Presumably, observations are taking place in some way within the system, even though it's closed. But the theory doesn't provide the mathematics to flesh this idea out to a valid conclusion.

I believe QM is incomplete, not in the way that E-P-R thought of hidden variables, but perhaps more closely analogous to the sense of Godel. The axioms of QM do not provide us a rich enough theory to explain everything it should, including Schrodinger's cat.

[Will Durant]

So it's either dead, or it's alive.

So there's a 50/50 shot of either. Got it. 50% live cat. 50% dead cat.
No. No. No.

[bokiz333]

I have it on good authority that Mr. Schrodinger never even HAD a cat!

[Will Durant]

I agree with MagillaG that the study of quantum decoherence have largely rendered the paradox explainable,
Has it? I was aware that there's been progress in quantum decoherence but did not think there had been any fundamental breakthrough. Can you point me to some recent references?

[Nee-2-No-Bases]

I have it on good authority that Mr. Schrodinger never even HAD a cat!

I heard the same thing. It was a dog. And the guillotine, being built for a cat, wasn't big enough for the dog. So the dog was sure to survive no matter what!!!

They have tried to prove this idea, but in doing so, nearly destroyed the planet.

[SirVLCIV]

http://sale.images.woot.com/Schr%C3%B6dinger_s_Equation_134Detail.png
http://shirt.woot.com/friends.aspx?k=22798

[LifeIsAPoissonProcess]

Here's an interesting tidbit to go along with the uncertainty principle:

http://en.wikipedia.org/wiki/Double-slit_experiment

[Hydraskull]

Thank you, Will, for your informative posts.

[Entropy]

Thank you, Will, for your informative posts.

I agree

[Locrian]

Has it? I was aware that there's been progress in quantum decoherence but did not think there had been any fundamental breakthrough. Can you point me to some recent references?

Depends on what you mean by breakthrough, I suppose. I’m not aware of any observation that is contrary to theory. Ultimately we are incapable of predicting almost any property of macroscopic matter (hardness, density, etc.) from first principles, and the fact that decoherent systems don’t exhibit entanglement is just part of the big list.

What you see as a big gap in our knowledge I see as a natural fact that we already know.

[magillaG]

You're thinking about the Bell Inequality (1964) which proved that QM and hidden variables would make different predictions. Alain Aspect created just such experiments in the 1980s (others refined them in the 1990s) and definitively proved that the natural world obeys the predictions of QM but disagrees with the predictions of hidden variable theorems.

I think there were really two different, related attacks on the Copenhagen interpretation of QM. One was that there was some sort of hidden variable- and as you point out, this was challenge was pretty convincingly answered by the Aspect experiments (at least if you want to preserve locality, which I think most physicists do.)

I think the other attack was on the notion of measurement. The claim was that measurement in quantum mechanics was not well defined. Related to that is how the quantum mechanical world eventually becomes the classical world, and all this quantum strangeness goes away. In other words, why do we think of the quantum process of the radioactive decay as having this quantum strangeness, but not the cat whose life depends on the radioactive decay.

[Will Durant]

Has it? I was aware that there's been progress in quantum decoherence but did not think there had been any fundamental breakthrough. Can you point me to some recent references?

I've started to answer my own question...

http://en.wikipedia.org/wiki/Quantum_decoherence

The collapse of a quantum superposition into a single definite state was quantitatively measured for the first time by Serge Haroche and his co-workers at the École Normale Supérieure in Paris in 1996. Their approach involved sending individual rubidium atoms, each in a superposition of two states, through a microwave-filled cavity. The two quantum states both cause shifts in the phase of the microwave field, but by different amounts, so that the field itself is also put into a superposition of two states. As the cavity field exchanges energy with its surroundings, however, its superposition appears to collapse into a single definite state. Haroche and his colleagues measured the resulting decoherence via correlations between the energy levels of pairs of atoms sent through the cavity with various time delays between the atoms.

The lay explanation of the paradox would seem to lie in the fact that there is no such thing as a perfectly closed system as is necessary for the cat to be alive-dead. The unavoidable interactions with the outside world move the wavefunction to a single state.

[magillaG]

Has it? I was aware that there's been progress in quantum decoherence but did not think there had been any fundamental breakthrough. Can you point me to some recent references?

This wiki page (http://en.wikipedia.org/wiki/Decoherence) has a lot of stuff on it that I don't remember seeing anywhere. You have a better background in theory- maybe you have seen it.

I don't think the idea of decoherence really existed in the 1930s and 1940s.

[Will Durant]

What you see as a big gap in our knowledge I see as a natural fact that we already know.
But to me it seems as a fact without an underlying explanation.

Why are we unable to predict properties of macroscopic matter? It's not a computational limitation; it seems to be an incompleteness of the theory. SR and GR clearly encompass classical mechanics as their low-velocity and low-density limits, respectively.

However, with QM it seems like we have...

QM <...> "here be dragons" <...> CM

To elaborate, quoting the abstract of a paper referenced in that wiki page.

A mesoscopic superposition of quantum states involving radiation fields with classically distinct phases was created and its progressive decoherence observed. The experiment involved Rydberg atoms interacting one at a time with a few photon coherent field trapped in a high Q microwave cavity. The mesoscopic superposition was the equivalent of an “ atom+measuring apparatus ” system in which the “meter” was pointing simultaneously towards two different directions—a “Schrödinger cat.” The decoherence phenomenon transforming this superposition into a statistical mixture was observed while it unfolded, providing a direct insight into a process at the heart of quantum measurement.

This to me seems to be an experimental result in search of a theory that explains it, much like the Michelson-Morley experiment or the blackbody radiation spectrum or the photoelectric effect.

[Locrian]

Why are we unable to predict properties of macroscopic matter? It's not a computational limitation; it seems to be an incompleteness of the theory.

Maybe, but some think otherwise. Some physicists believe spontaneous emergent phenomena simply prevent the calculation of properties of the macroscopic whole from its microscopic parts and further believe it is not a problem to be solved (as opposed as not being a problem that can be). Laughlin and Anderson come to mind, but I really think most condensed matter & materials science physicists fall into this camp, even if they don't know it.

[Listeria]

it's alive. it's dead. head assplode.


please explain in donkey terms. kthxbai

It's just a troll, undeserving of the time you're wasting on it.

[magillaG]

Maybe, but some think otherwise. Some physicists believe spontaneous emergent phenomena simply prevent the calculation of properties of the macroscopic whole from its microscopic parts and further believe it is not a problem to be solved (as opposed as not being a problem that can be). Laughlin and Anderson come to mind, but I really think most condensed matter & materials science physicists fall into this camp, even if they don't know it.

To that point, I think that even, say, Boltzmann's derivation of the behavior of an ideal class using classical mechanics doesn't really use only first principles. He has to make additional, statistical assumptions.

[Hydraskull]

It's just a troll, undeserving of the time you're wasting on it.

Well, I'm learning now and enjoying the discussion so please continue.

[Will Durant]

I really think most condensed matter & materials science physicists fall into this camp, even if they don't know it.
That's probably quite proper, given their emphasis. But what do the QFT physicists think of it. To me, this result could be considered an attack on the underpinnings of their theory.

I'd be interested in what Haag thinks. Actually, I think he's almost 90. I wonder who his intellectual descendants are?

[Will Durant]

To that point, I think that even, say, Boltzmann's derivation of the behavior of an ideal class using classical mechanics doesn't really use only first principles. He has to make additional, statistical assumptions.
But it's not a theoretical limitation. In principle, the interactions between gas molecules is understood and conceptually we could calculate the behavior. The ideal gas is a simplification made necessary only by the inability to calculate. The underlying physics doesn't stop being applicable, it simply stops being soluble.

This strikes me as different. Perhaps I am just not understanding the result, but it seems that there's no explanation as to why QM stops being applicable, just the experimental fact that it does.

[magillaG]

But it's not a theoretical limitation. In principle, the interactions between gas molecules is understood and conceptually we could calculate the behavior. The ideal gas is a simplification made necessary only by the inability to calculate. The underlying physics doesn't stop being applicable, it simply stops being soluble.

This strikes me as different. Perhaps I am just not understanding the result, but it seems that there's no explanation as to why QM stops being applicable, just the experimental fact that it does.

The only way I know to think about it is as you describe.

However, I do think there seems to be some important difference between simple, isolated systems, which can be described deterministically, and complex systems exposed to the environment, which must be explained statistically. I'm not sure we really understand that, or even have the philosophical tools to understand it. I'm certainly not clever enough to tell you what we are missing, though.

[ORLYLOL]

TTIA

[Keef]

Turtle shells all the way down.

[ORLYLOL]

http://verydemotivational.files.wordpress.com/2011/03/demotivational-posters-schroedinger.jpg

[NotUrAvgXBar]

If a tree falls on a cat and nobody is around to hear it, what is the tree doing in my kitchen?

[Hydraskull]

If a tree falls on a cat and nobody is around to hear it, what is the tree doing in my kitchen?

That's your wife! She's exceptionally thick-built.

[Entropy]

The lay explanation of the paradox would seem to lie in the fact that there is no such thing as a perfectly closed system as is necessary for the cat to be alive-dead. The unavoidable interactions with the outside world move the wavefunction to a single state.

This is how I've always understood it.

[Locrian]

But what do the QFT physicists think of it. To me, this result could be considered an attack on the underpinnings of their theory.

Maybe philosophically, but not scientifically. I’m not aware of any measurements that conflict with theory. There are a couple of reasons for that. Sometimes it’s just that you can’t get there until you’ve measured it; once you have information about the final state you can show that, obviously, that’s what was going to happen and you should have known all along. This happens often when physics is applied to chemistry. You can’t claim you can get there from first principles, but you can claim it doesn’t violate any known law. Laughlin likes the example of the various phases of ice as an example of this.

Other times it’s simply computationally out of the question to test theories like QM and QFT. Modeling even simple materials can quickly get out of hand using classical-quantum approximations that are orders of magnitude easier to calculate than QFT is. I didn’t do any theoretical modeling, but the question of processing time came up regularly at talks I attended, and sometimes the unit cells they were dealing with didn’t seem that complicated to me. While there are examples of QFT’s application in condensed matter physics, ultimately it is used very rarely. I skipped those classes and most people I knew who took them found no use for it at all. Nobody is out there arguing these tools are wrong, exactly, just largely irrelevant.

So I don’t think QFT theorists - assuming there is anyone who still calls themselves that - have any reason to see this philosophy as an attack on their theory. Their tool is useful for what it is useful for. But funding is another matter. If the universe includes numerous (maybe innumerable) rules that emerge in different regimes then even big discoveries in high energy physics may have little or no impact on the rest of physics, since those details are obscured by the system before they every matter. Reductionism is increasingly becoming a poor way to make a living (a continuing trend since the 70’s).

Every year that passes since I actually studied this stuff my soapbox gets a little shoddier. Sometime soon it’s going to buckle and I’m going to have a terrible intellectual accident.

[ElDucky]

My cat's breath smells like cat food.

[Gonzo]

It's just a troll, undeserving of the time you're wasting on it.

:troll:

[Gonzo]

if we could harness the alive-dead synergy, we could leverage it into a robust value proposition

[sweetiepie]

This to me seems to be an experimental result in search of a theory that explains it, much like the Michelson-Morley experiment or the blackbody radiation spectrum or the photoelectric effect.
It's the opposite of what you're saying. The uncertainty principle is a fairly straightforward result of Fourier analysis. It's just math. The experiments are attempts at making sense out of the theory.

From my last understanding, the issue is that you're asking the wrong question. "Where?" and "How fast?" are not fundamental attributes of the system.

[Will Durant]

It's the opposite of what you're saying. The uncertainty principle is a fairly straightforward result of Fourier analysis. It's just math. The experiments are attempts at making sense out of the theory.

From my last understanding, the issue is that you're asking the wrong question. "Where?" and "How fast?" are not fundamental attributes of the system.

I fail to see the relevance of this. The experiment in question is not in any way related to HUP. I haven't asked where or how fast. I asked about a wavefunction.

[oirg]

Then the system wouldn't be closed and the sequence of wavefunctions I described would not follow logically from each other. Most obviously, photons from outside the system would affect the system

Assuming that these photons do not affect the element's propensity to decay, nor the cat's propensity to die in case of decay, should I care that the system is not closed?

[Will Durant]

Yes. If the system is open, the wavefunction does not evolve as I outlined. It's only a paradox if the system is closed.

[Alpha12]

I didn't go through the tread, so forgive me if someone already posted this.

http://www.youtube.com/watch?v=IOYyCHGWJq4

It's a short animated and somewhat humorous description of the theory.

[sweetiepie]

I fail to see the relevance of this. The experiment in question is not in any way related to HUP. I haven't asked where or how fast. I asked about a wavefunction.

Oh, my bad for reading your post out of context. No idea what happens to wave-functions in the long run.

[Mark Cavazos]

I went to a house concert to see Deirdre Flint because of one line from a song.

She looks good in black. She looks good in white,
She looks good in Lycra or anything tight
She looks good in earth-tones, she looks good in red
I think she'd look really good with a spike through her head
She's my old boyfriend's new girlfriend, knows Tai Chi and her legs are waxed
She's my old boyfriend's new girlfriend. I've got a funny feeling he won't be coming back.

Well, when I first saw them together, I was more than surprised,
She stood at 5 foot 8, blond hair blue eyes
I said, "it figured that he'd rebound with a bimbo after me,"
They said, "She's Harvard educated in biotechnology."
"Well, okay so she can add and she looks good in a skirt,
Bet she's a selfish little princess who treats him like dirt."
They said, "She just bought him a Volvo, she cooks, she cleans
She spent three years digging ditches with the Peace Corps in Benin."
"Well, OK, I'll give her that but I bet their love life is bland."
They said, "She knows the Kama Sutra like the back of her hand."
I said, "Now playing the guitar that's something I know she can't do,"
I heard someone say, 'You're right, but frankly, either can you."

She's my old boyfriend's new girlfriend knows her way around the Red Sox stats
She's my old boyfriend's new girlfriend and female intuition tells me
He won't be coming back

When I dumped his butt a month ago he was supposed to pine away
Well, he's looking pretty healthy, in fact each time I see him
He gets better looking to me every day

She's my old boyfriend's new girlfriend. There's no hope with a girl like that
She's my old boyfriend's new girlfriend and would you like to bet that he won't be coming back.

She can parallel park without any hassle. She feng shuied the whole Hearst Castle
She runs a 10 K in 30.05 and all her Schrodinger's cats survive
She stopped a civil war in an Eastern Bloc nation
She found the fourth planet in the Virgo constellation
On her lunch break. With a magnifying glass.

She has very clever songs.

[Mark Cavazos]

It is easy to get seduce into using the reality of the macroscopic world to discuss quantum reality.

a radioactive sample consisting of a single atom with a half-life of one hour

This does not make any sense. "Half-life" is a property of a population of atoms. It is not a characteristic of a single atom.

If a population has a life expectancy of 74 years, it does not mean that any particular individual has a 50% chance of dying before 74.

[magillaG]

It is easy to get seduce into using the reality of the macroscopic world to discuss quantum reality.



This does not make any sense. "Half-life" is a property of a population of atoms. It is not a characteristic of a single atom.

If a population has a life expectancy of 74 years, it does not mean that any particular individual has a 50% chance of dying before 74.

Half life is also a property of an individual atom. The atom decays according to a poisson process, ie there is an exponentially distributed waiting period. If it has a half life of 74 years, and hasn't decayed yet, then it has a 50% chance of decaying in the next 74 years. See, for example, this wiki entry. (http://en.wikipedia.org/wiki/Half_life#Probabilistic_nature_of_half-life)

[Will Durant]

It is easy to get seduce into using the reality of the macroscopic world to discuss quantum reality.

This does not make any sense. "Half-life" is a property of a population of atoms. It is not a characteristic of a single atom.

If a population has a life expectancy of 74 years, it does not mean that any particular individual has a 50% chance of dying before 74.

:shake:

[Will Durant]

I've done a little reading and given this a lot of thought, and here's where I think I have a problem, again with the caveats that my concern may just be ignorance of the nuances involved and that my analogy is probably overly simplistic.

Decoherence seems akin to friction. We know that the classical mechanics construct of frictionless surface is flawed because there is in fact friction. There's no way to calculate friction from first principles, but we have a conceptual understanding of friction. So we introduce the coefficient of friction into classical mechanics and move on.

Decoherence might be considered friction against QM. We know that the QM construct of wavefunctions generally fails for macro objects because there is decoherence. However, the difference is that we don't seem to have a conceptual understanding of it. We can explain at a very high level that it's related to lack of perfect closure of systems, but we don't really understand what precisely happened (I may mean what collapses the wavefunction, but I'm actually not 100% sure this is what I mean).

Thoughts from our more recently informed physics friends?

[Bicycle Repair Man]

You can derive friction from first principles. It's just really complicated to do so.

[Will Durant]

That's what I said. We understand how it would be done if it could be done, but it can't be done.

I don't believe there's a similar understanding of how decoherence works.

[campbell]

It's only a model.

[Browncoat]

For a full explanation, read Dirk Gently, excerpted here:

"...what was all that rubbish about cats and quantum mechanics?"

With a sigh Dirk flipped up the lid of the pizza with a single Hick of practiced fingers. He surveyed the cold round thing with a kind of sadness and then tore off a hunk of it. Pieces of pepperoni and anchovy scattered over his desk.

"I am sure, Richard," he said, "that you are familiar with the notion of Schrödinger's Cat," and he stuffed the larger part of the hunk into his mouth.

"Of course," said Richard. "Well, reasonably familiar."

"What is it?" said Dirk through a mouthful.

Richard shifted irritably in his seat. "It's an illustration," he said, "of the principle that at a quantum level all events are governed by probabilities..."

"At a quantum level, and therefore at all levels," interrupted Dirk. "Though at any level higher than the subatomic the cumulative effect of those probabilities is, in the normal course of events, indistinguishable from the effect of hard and fast physical laws. Continue."

He put some more cold pizza into his face.

Richard reflected that Dirk's was a face into which too much had already been put. What with that and the amount he talked, the traffic through his mouth was almost incessant. His ears, on the other hand, remained almost totally unused in normal conversation.

It occurred to Richard that if Lamarck had been right and you were to take a line through this behavior for several generations, the chances were that some radical replumbing of the interior of the skull would eventually take place.

Richard continued, "Not only are quantum level events governed by probabilities, but those probabilities aren't even resolved into actual events until they are measured. Or to use a phrase that I just heard you use in a rather bizarre context, the act of measurement collapses the probability waveform. Up until that point all the possible courses of action open to, say, an electron, coexist as probability waveforms. Nothing is decided until it's measured."

Dirk nodded. "More or less," he said, taking another mouthful. "But what of the cat?"

Richard decided that there was only one way to avoid having to watch Dirk eat his way through all the rest of the pizza, and that was to eat the rest himself. He rolled it up and took a token nibble off the end. It was rather good. He took another bite.

Dirk watched this with startled dismay.

"So," said Richard, "the idea behind Schrödinger's Cat was to try and imagine a way in which the effects of probabilistic behavior at a quantum level could be considered at a macroscopic level. Or let's say an everyday level."

"Yes, let's," said Dirk, regarding the rest of the pizza with a stricken look. Richard took another bite and continued cheerfully.

"So you imagine that you take a cat and put it in a box that you can seal completely. Also in the box you put a small lump of radioactive material, and a phial of poison gas. You arrange it so that within a given period of time there is an exactly fifty-fifty chance that an atom in the radioactive lump will decay and emit an electron. If it does decay then it triggers the release of the gas and kills the cat. If it doesn't, the cat lives. Fifty-fifty. Depending on the fifty-fifty chance that a single atom does or does not decay.

"The point as I understand it is this: since the decay of a single atom is a quantum level event that wouldn't be resolved either way until it was observed, and since you don't make the observation until you open the box and see whether the cat is alive or dead, then there's a rather extraordinary consequence.

"Until you do open the box the cat itself exists in an indeterminate state. The possibilities that it is alive, and the possibility that it is dead, are two different waveforms superimposed on each other inside the box. Schrödinger put forward this idea to illustrate what he thought was absurd about quantum theory."

Dirk got up and padded over to the window, probably not so much for the meager view it afforded over an old warehouse on which an alternative comedian was lavishing his vast lager commercial fees developing into luxury apartments, as for the lack of view it afforded of the last piece of pizza disappearing.

"Exactly," said Dirk, "bravo!"

"But what's all that got to do with this--this Detective Agency?"

"Oh, that. Well, some researchers were once conducting such an experiment, but when they opened up the box, the cat was neither alive nor dead but was in fact completely missing, and they called me in to investigate. I was able to deduce that nothing very dramatic had happened. The cat had merely got fed up with being repeatedly locked up in a box and occasionally gassed and had taken the first opportunity to hoof it through the window. It was for me the work of a moment to set a saucer of milk by the window and call `Bernice' in an enticing voice--the cat's name was Bernice, you understand--”

"Now, wait a minute--" said Richard.

"--and the cat was soon restored. A simple enough matter, but it seemed to create quite an impression in certain circles, and soon one thing led to another as they do and it all culminated in the thriving career you see before you."

"Wait a minute, wait a minute," insisted Richard, slapping the table.

"Yes?" enquired Dirk innocently.

"Now, what are you talking about, Dirk?"

"You have a problem with what I have told you?"

"Well, I hardly know where to begin," protested Richard. "All right. You said that some people were performing the experiment. That's nonsense. Schrödinger's Cat isn't a real experiment. It's just an illustration for arguing about the idea. It's not something you'd actually do."

Dirk was watching him with odd attention.

"Oh, really?" he said at last. "And why not?"

"Well, there's nothing you can test. The whole point of the idea is to think about what happens before you make your observation. You can't know what's going on inside the box without looking, and the very instant you look the wave packet collapses and the probabilities resolve. It's self-defeating. It's completely purposeless."

"You are, of course, perfectly correct as far as you go," replied Dirk, returning to his seat. He drew a cigarette out of the packet, tapped it several times on the desk, and leant across the desk and pointed the filter at Richard.

"But think about this," he continued. "Supposing you were to introduce a psychic, someone with clairvoyant powers, into the experiment--someone who is able to divine what state of health the cat is in without opening the box. Someone who has, perhaps, a certain eerie sympathy with cats. What then? Might that furnish us with an additional insight into the problem of quantum physics?"

"Is that what they wanted to do?"

"It's what they did."

"Dirk, this is complete nonsense."

Dirk raised his eyebrows challengingly.

"All right, all right," said Richard, holding up his palms, "let's just follow it through. Even if I accepted--which I don't for one second--that there was any basis at all for clairvoyance, it wouldn't alter the fundamental undoableness of the experiment. As I said, the whole thing turns on what happens inside the box before it's observed. It doesn't matter how you observe it whether you look into the box with your eyes or--well, with your mind, if you insist. If clairvoyance works, then it's just another way of looking into the box, and if it doesn't then of course it's irrelevant."

"It might depend, of course, on the view you take of clairvoyance. . . "

[Incredible Hulctuary]

The big flaw is that the cat is an observer, even if it doesn't understand what's observing.

Or if you used an inanimate object instead of a cat, like a domino that would get knocked down, the domino is effectively a measuring device.

Either way, the quantum event gets measured or observed before you open the box.

[Vorian Atreides]

It's only a model a very silly place.
IFYP.

[whisper]

That's my issues. We don't "have" to say it's a little of both. We can just say - we don't know what it is. Because it's not possible for it to actually be both.

You're taking this a bit too literally.
Plus, if I understand it correctly - the wave form has both states at the same time. It's significantly more than just the cat is dead or alive, and we don't know it.