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  #1  
Old 11-03-2010, 09:05 PM
Jim B. Jim B. is online now
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Schroedinger's Cat and Clocks.

I think we all know the story of the Schroedinger's Cat experiment (here is Cecil's take on it). IANAQuatum physicist. So my explanation and take on it might not be that good. But basically (as I understand it in my "limited" way) it says that reality only collapses into one form when we perceive it. Schroedinger's experiment gives the example of a cat, and a device that kills it at random in a closed box (why the cat's perception is not relevant also puzzles me--but I digress).

Anyways, forgive me, but I don't believe this assertion at all. It goes totally against common sense, I think. For example, now I am at my computer in one room in my house. Just because I am not in another room, for example, doesn't mean there is anything unusual happening in the other room/s. Anyways, I have an example which I believe demonstrates what I am talking about: clocks. The clocks in my house continue telling time, even when I am not directly perceiving them. Now, it is claimed the cat isn't dead until perceived by a human. But if this was true of clocks, wouldn't they show a different time or stop(-?) completely if I exited a room? Then when I come back to the room, I would notice this had happened--no?

Anyways, as I've said, IANAScientist. So please someone tell me what I am missing here (including as I said above, why the cat's perception isn't important BTW).

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  #2  
Old 11-03-2010, 10:07 PM
Cheshire Human Cheshire Human is offline
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Actually, this is based on a misunderstanding of the Schroedinger's Cat thought-experiment. His idea was actually to show why certain interpretations of quantum mechanics were ridiculous. He never believed that's what actually happened. He agreed with you.

Last edited by Cheshire Human; 11-03-2010 at 10:08 PM.
  #3  
Old 11-03-2010, 10:10 PM
Keeve Keeve is offline
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These same questions puzzle me too.
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Originally Posted by Jim B. View Post
why the cat's perception is not relevant also puzzles me--but I digress.
Rather than being a digression, I have long suspected that the answer to this one is the key to understanding all the others.
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Old 11-03-2010, 10:11 PM
Inner Stickler Inner Stickler is offline
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Right, Schroedinger's intent was for you to get to the end of the thought experiment and say, "Wait, the cat is both dead and alive?! BULLSHIT."
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Old 11-03-2010, 10:14 PM
Keeve Keeve is offline
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Originally Posted by Cheshire Human View Post
Actually, this is based on a misunderstanding of the Schroedinger's Cat thought-experiment. His idea was actually to show why certain interpretations of quantum mechanics were ridiculous. He never believed that's what actually happened. He agreed with you.
My understanding was that although Schroedinger himself proposed it as you say, it was nevertheless accepted by other quantum scientists then and now, with the reaction of "Yes, you've illustrated it quite well. And it does seem ridiculous. But it is nevertheless true and accurate. If only we could understand it!"

Am I mistaken?
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Old 11-03-2010, 10:27 PM
Exapno Mapcase Exapno Mapcase is offline
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As soon as anyone says "It goes totally against common sense" about any aspect of QM, the battle is already lost.

Nothing about QM has any relation to common sense. In the first place, there's not the slightest reason to think that the behavior of atoms at the most basic level of existence should have any relationship to what we see in the everyday world. Common sense tells us an amazing series of wrong things, like the obvious common sense fact that the world must be flat because people on the other side would fall off.

QM works as math. When people try to express it in words they need to use analogies, which are more or less bad but never can express things that otherwise have no correspondence to anything in the macro world. (Understanding exactly why there is a macro world is a great question, but not the one asked by people wanting common sense answers.)

But of course we continue to insist on "common sense" because that's the human way. And we continue to insist on words because few of us want to study for the years needed to understand the math.

I just wish that people would stop blaming the analogies for not making sense. It's very much like zen. You comprehend the answer by letting the truth emerge. It's very much not like a textbook in which the answers are in back.

Still, the physics pedants on the Board are often fairly adequate at giving the subject some slight comprehensibility. Which in QM terms is high praise.
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Old 11-03-2010, 10:29 PM
Peremensoe Peremensoe is offline
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Of course, Schrodinger's own opinion isn't determinative. The illustration may be useful even if he was wrong.

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Originally Posted by Jim B. View Post
Anyways, I have an example which I believe demonstrates what I am talking about: clocks. The clocks in my house continue telling time, even when I am not directly perceiving them. Now, it is claimed the cat isn't dead until perceived by a human. But if this was true of clocks, wouldn't they show a different time or stop(-?) completely if I exited a room? Then when I come back to the room, I would notice this had happened--no?
There's nothing special about clocks.

The walls in your house continue being the same color in the same sense (whatever that may, in fact, be) that the clocks continue telling time. You'd notice if the wall was suddenly a different color when you came back, too--but there's no reason for such an inexplicable reality to collapse out of the quantum possibilities.

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Originally Posted by Jim B. View Post
(why the cat's perception is not relevant also puzzles me--but I digress).
Think of it this way. If there is an observer effect, there must be some characteristic or quality which separates observers from mere objects in the universe. What is that characteristic? Does it imply or require consciousness? How much consciousness? Do all humans do it all the time, necessarily? (While asleep?) Conceiving of this characteristic as a functional boundary, actual cats might or might not be on the same side of it as we are. (If they are, substitute something else for the cat to keep using the illustration.)
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Old 11-03-2010, 10:44 PM
Markxxx Markxxx is offline
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As usual the Family Circus has already covered this
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Old 11-03-2010, 10:56 PM
CookingWithGas CookingWithGas is offline
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There is a pretty accessible explanation of at least part of this issue in From Eternity to Here by Sean Carroll. (The book has received generally positive reviews though is not without its critics.) It is primarily about the nature of time, but does have a few things to say about QM. Most interestingly, Carroll explains one model used to describe this type of phenomenon, the "many worlds" interpretation.
Quote:
Originally Posted by Sean Carroll on p. 249
"Many worlds" is a scary and misleading term for what is really a very straightforward idea. That idea is this: There is no such thing as "collapse of the wave function."....The response of the many-worlds advocate is simply that your are thinking about it wrong. In particular, you have misidentified yourself in the wave function of the universe....It's not right to set yourself off as some objective classical observing apparatus; we need to take your own state into account in the wave function.
He goes on to say that, in the many worlds model, after you observe an outcome, your consciousness splits off into two branches of the wave function; one, for example, who saw a dead cat and one who saw a live one. But that's not how the experience seems to us, so it's difficult to accept. Further, to clarify that "many worlds" is really a misnomer:
Quote:
...the universe is described by a single wave function, which assigns a particular amplitude to every possible observational outcome. Before or after, the wave function of the universe is just particular point in the space of states describing the universe, and that space of states didn't get any bigger or smaller. No new "worlds" have really been created.
He also describes the Copenhagen interpretation that describes wave function collapse. He seems partial to "many worlds."
  #10  
Old 11-03-2010, 11:02 PM
Exapno Mapcase Exapno Mapcase is offline
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Originally Posted by CookingWithGas View Post
He also describes the Copenhagen interpretation that describes wave function collapse. He seems partial to "many worlds."
It's important to remember that both interpretations of QM are mathematically equivalent, even though the way the math is interpreted seems contradictory.

Again, set "common sense" aside.
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Old 11-03-2010, 11:06 PM
njtt njtt is offline
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The point of the Schroedinger's Cat thought experiment is that the situation is set up in such a way that a macroscopic event (the killing or otherwise, of the cat) is dependent on the occurrence of events (radioactive decay of atoms) that occur at the quantum level. This is a very special and unusual circumstance. Very few macroscopic events are subject to quantum indeterminacy of this sort. Although everything is technically subject to some quantum indeterminacy, for most ordinary sized things, things observable at the human scale, it is vanishingly small, and can be ignored for all practical (and most impractical) purposes. Unless your clocks are somehow set up to be under the control of radioactive decays or some similar quantum level process, there is no reason to expect them to behave in an indeterministic manner, and whether you observe them or not will have no impact on how they do behave (just as common sense tells us).

Cheshire Human (who should know, if anyone does), is, of course, right to point out that the cat thought experiment was intended to show that something is wrong, or incomplete, about quantum mechanics (as interpreted by Bohr and Heisenberg). The story is meant to be absurd, but most physicists seem to have preferred to embrace the absurdity rather than reject the theory (because it works so well in so many other respects). However, be that as it may, quantum mechanics does not, in any meaningful way, predict anything absurd about your unobserved clocks.
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Old 11-03-2010, 11:40 PM
TriPolar TriPolar is offline
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I don't know what Scroedinger intended, but it's not absurd to have a mathematical model that defines multiple possibilities when there is an a unknown factor. njtt points out that the absurdity comes from the way the situation is posed. The system inside the box can't have the cat alternate between life and death as random events occur, the first random event kills the cat. The model can't determine whether the event occur or not at any specific time, and neither can you unless you open the box.

I don't understand what happens in quantum mechanics, but if it exhibits the behavior of a mathematical model like that, there's nothing absurd about it.

Exapno, people can't fall off the other side of the world because no one could be there in the first place. If you make it past the monsters, mountains, and waterfalls that encircle the world, you'd just fall off and land on the back of a turtle.
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Old 11-03-2010, 11:41 PM
Peremensoe Peremensoe is offline
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Quote:
Originally Posted by Sean Carroll
No new "worlds" have really been created.
By which he means that the possibilities--the "paths," if you will, of the split consciousness in that interpretation--continue to coexist, right? No "worlds" have been added or removed; they're all "there" all along, threads unwinding out of the same skein of possibility. But any one observer's perception will seem, internally, to just follow one thread. (Correct me if I have misstated Carroll.)

This interpretation lends itself to many kinds of further speculation, like this.

Quote:
Originally Posted by Wikipedia
In quantum mechanics, quantum suicide is a thought experiment.
...
It attempts to distinguish between the Copenhagen interpretation of quantum mechanics and the Everett many-worlds interpretation by means of a variation of the Schrödinger's cat thought experiment, from the cat's point of view.
And finally, if you like what reading this stuff does to your brain, be sure to check out the writings of Greg Egan, particularly the novel Quarantine (Copenhagen wavefunction collapse) and the story "The Infinite Assassin" (Everett-DeWitt many-worlds).

These are fun fiction based on ideas about quantum physics, not actual physics: see Egan's essay on the distinctions.

Quote:
Originally Posted by Greg Egan
So, what happened to the other possibilities — the other “versions” of the particle? How does the co-existence of many different values for a particle's location get to be replaced by a state with a single location? When, how, and why does the wave function collapse? Here are some of the answers that have been given to this question:

1. Don't ask stupid questions. Quantum mechanics lets you calculate the probabilities for the various outcomes of any experiment you can describe. What more do you want from a scientific theory? Just shut up and calculate!
2. The wave function is not a real thing, the way an ocean wave or a wave on a string is a thing. Rather, it describes what we know about a quantum system. When we make a measurement, we get to know something new, so the wave function has to be changed to reflect that. But there is no “objective collapse” — any more than a pea beneath one of three thimbles undergoes some kind of physical change when we see where it is, and we change our probability for its presence there from one in three to 100%.
3. When the particle interacts with the measuring device (or any other macroscopic object on which it has a significant effect) there is an objective collapse of the particle's wave function.
4. When the measuring device interacts with an observer, there is an objective collapse of the combined wave function of the particle and the measuring device.
5. The wave function doesn't collapse, and all its possibilities continue to exist; we are simply unable to detect them because of the effects of decoherence.
6. Conventional quantum mechanics is incomplete. We need a better theory to truly understand what's going on.

As readers of Quarantine will know, the novel posits a variation on position 4: human beings alone have a special structure in their brains that actively causes the collapse of the wave function. Only once a human has interacted with the quantum system and perceived a definite outcome are all the other possibilities obliterated.

As I've already said, though this hypothesis leads to some entertaining consequences, I don't actually believe it for a moment.
  #14  
Old 11-03-2010, 11:49 PM
Cheshire Human Cheshire Human is offline
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Originally Posted by Keeve View Post
My understanding was that although Schroedinger himself proposed it as you say, it was nevertheless accepted by other quantum scientists then and now, with the reaction of "Yes, you've illustrated it quite well. And it does seem ridiculous. But it is nevertheless true and accurate. If only we could understand it!"

Am I mistaken?
Nope. He thought "Bullshit", others though "How Profound". I'm not a physicist, so I can't say with any authority, but I agree with "Bullshit".
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Old 11-04-2010, 05:31 AM
Keeve Keeve is offline
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You'd notice if the wall was suddenly a different color when you came back, too--but there's no reason for such an inexplicable reality to collapse out of the quantum possibilities.
On the contrary, the whole point is that this could indeed happen. The probability is absurdly small, but that's not the same as impossible.

All of the subatomic particles in that wall are very busy doing whatever it is that subatomic particles do, and one of the great things about being a subatomic particle is that you don't have to do anything in particular. Of course, you'll probably do this, and probably not do that, and the probabilities are such that on a macro level, the local humans will never notice the occasional particles that did the unexpected. But the fact is that the laws of probabilities do allow the unexpected to occur, provided that it doesn't happen too often.

But once in a very great while -- and for our purposes, this could take a googolplex years or more -- all the subatomic particles in that wall could transmute their atoms into pure gold.

I will concede that for practical purposes this will never actually happen, but for theoretical purposes it certainly can.
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Old 11-05-2010, 12:00 PM
Reno Nevada Reno Nevada is offline
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No discussion of Schroedinger's Cat is complete without a link to my personal favorite Cecil column.
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Old 11-05-2010, 12:12 PM
Kobal2 Kobal2 is offline
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Nope. He thought "Bullshit", others though "How Profound". I'm not a physicist, so I can't say with any authority, but I agree with "Bullshit".
Maybe it's both profound and bullshit at the same time, and we don't know which until we've opened Schrödinger.
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Old 11-05-2010, 12:36 PM
Peremensoe Peremensoe is offline
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On the contrary, the whole point is that this could indeed happen. The probability is absurdly small, but that's not the same as impossible.
...
I will concede that for practical purposes this will never actually happen, but for theoretical purposes it certainly can.
You're right. At least to the limits of my understanding.

I think the illustration still correctly suggests why the evidence of clocks seeming to keep time in accord with a pre-quantum understanding of reality is no more meaningful than the evidence of other parts of our physical surroundings seeming to keep existing as they are.

Last edited by Peremensoe; 11-05-2010 at 12:38 PM.
  #19  
Old 11-05-2010, 12:54 PM
Cayuga Cayuga is offline
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Hey, all you experts:

Is this a valid interpretation?

Schroedinger made up the story of the cat to say, "If what you're saying about quantum mechanics is true, then the cat is both dead and alive. That's obviously ridiculous, so what you're saying about quantum mechanics must not be true."

And the quantum greasemonkeys (what do you call comeone who works with quantum mechanics, anyway?) replied. "You're right. In the large-scale, everyday world it would be ridiculous. But it's a fact of life in the sub-atomic quantum world. Which is why quantum mechanics is such a bear to understand."

Or have all of you said this already?
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Old 11-05-2010, 01:15 PM
Exapno Mapcase Exapno Mapcase is offline
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Originally Posted by Reno Nevada View Post
No discussion of Schroedinger's Cat is complete without a link to my personal favorite Cecil column.
You mean the one linked to in the first sentence of the OP?
  #21  
Old 11-05-2010, 01:29 PM
slitterst slitterst is offline
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Now, it is claimed the cat isn't dead until perceived by a human. But if this was true of clocks, wouldn't they show a different time or stop(-?) completely if I exited a room? Then when I come back to the room, I would notice this had happened--no?
The difference between the cat and the clocks is the poison gas in the box. As soon as you lock the box, you have no idea if the gas has been triggered or not. So you have no rational way to know if the cat is alive or dead.

With a clock, there's no reason to expect the possibility of them stopping when you leave the room. It is entirely reasonable to expect that they will continue keeping time while they are unobserved. Now, if your clock should happen to break while you were out of the room, you would not know about it until it was observed. Then you'd be into Schrodinger territory.
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Old 11-05-2010, 01:52 PM
SmithCommaJohn SmithCommaJohn is offline
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This stuff is very interesting and I desperately want to have even an elementary grasp of what the hell is going on.

I feel that this may be futile, at least for me.
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Old 11-05-2010, 02:25 PM
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The problem I have with Schrodinger's Cat is the mechanism by which the decay is detected and smashes the vial. I don't think such an item can exist and if it did exist then how does that qualify as keeping the particle "unobserved." So while the physicist can stay uninformed, that doesn't mean the rest of the universe is uninformed.

I don't pretend to understand QM either though.
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Old 11-05-2010, 04:29 PM
Stranger On A Train Stranger On A Train is offline
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Originally Posted by njtt View Post
Very few macroscopic events are subject to quantum indeterminacy of this sort. Although everything is technically subject to some quantum indeterminacy, for most ordinary sized things, things observable at the human scale, it is vanishingly small, and can be ignored for all practical (and most impractical) purposes.
Not only can it be ignored, but for any system in which multiple particles are in superposition (i.e. their states are not just complementary, as with entangled particles or a Bose condensate) the probabilities all mush out to a mean, such that the uncertainty is less than your ability to measure its variation, and for all intents and purposes, the object is in a single pseudo-quantum state. This isn't just for everyday-sized objects; for the most part, even heavy composite particles and molecules have a quantum wavelength much smaller than the actual size of the particle, and thus, your ability to detect any spontaneous variations in position or momentum. In theory, it is infinitesimally possible that all of the fundamental particles that make up a cheerleader's skirt could suddenly jump five feet to the right; in reality, the probability of any detectable amount of particles suddenly jumping in any direction by a measurable amount is so tiny that it literally just can't be observed for any real timeframe; even if her skirt did suddenly disappear in a mass state change, it would return to the original decoherent state in a shorter interval than could be measured by any classical observer. For all intents and purposes, even if it happened, it never happened.

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Originally Posted by sachertorte View Post
The problem I have with Schrodinger's Cat is the mechanism by which the decay is detected and smashes the vial. I don't think such an item can exist and if it did exist then how does that qualify as keeping the particle "unobserved." So while the physicist can stay uninformed, that doesn't mean the rest of the universe is uninformed.

I don't pretend to understand QM either though.
No, that's a very salient point. The Schrödinger's Cat gedankenexperiment is an attempt to conceive of a macroscale event that is directly influenced by a single quantum state change. The single radioactive particle and the detector make an isolated quantum system that triggers a macroscale event. However, in reality, the detector, the cat, the box, and the observer are all part of an interlinked quantum system in which there are a huge number of possible superimposed states; just as the cat may be both alive and dead, the observer may have both looked and not looked in the box, and so forth. Schrödinger obviously intended this as an absurdity; that the boundaries between quantum mechanics and everyday experience are not merely arbitrary states of relative perception, but actually form a fundamental break between isolated quantum systems and real world collections of objects.

The problem of Wigner's friend extends this; he is outside the room and doesn't know if the friend has looked in the box yet or not, so not only does the cat exist in superposition, but so does the friend, until the Wigner goes into the room and observes. This leads to two possible conclusions; either Wigner is a solipsist and no definite state exists until he personally observes it (absurd), or real collections of quantum systems are more complex than an assay of the individual systems themselves. Waveform collapse (or whatever other interpretation you invoke) is nothing more than a mathematical formality, a way of setting the initial condition for solving a defined set of interactions, with no physically real and discrete condition.

The scenario of the clock as posited by the o.p. is no different than the cat; the clock, while not resolved, is in a similar superposition of states, which in summed average equal the "real" time as measured by the clock when observed. Under any interpretation the clock never disappears or disintegrates into a quantum mist or stops working or anything like that; it just exists in an array of states that are mostly really, really close to telling the actual time. How you interpret "being in an array of states" is really up to you; you can take it as a mathematical formality, or dispense with local realism and assume that it doesn't really exist unless you are looking at it, or whatever, but the incontrovertible fact remains that unless someone unplugs it or the power goes out, it'll keep the same interval of time whether you are observing it or not within the tolerances of its mechanism.

Stranger

Last edited by Stranger On A Train; 11-05-2010 at 04:29 PM.
  #25  
Old 11-05-2010, 05:12 PM
Quercus Quercus is offline
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Quote:
Originally Posted by Cayuga View Post
Hey, all you experts:

Is this a valid interpretation?

Schroedinger made up the story of the cat to say, "If what you're saying about quantum mechanics is true, then the cat is both dead and alive. That's obviously ridiculous, so what you're saying about quantum mechanics must not be true."

And the quantum greasemonkeys (what do you call comeone who works with quantum mechanics, anyway?) replied. "You're right. In the large-scale, everyday world it would be ridiculous. But it's a fact of life in the sub-atomic quantum world. Which is why quantum mechanics is such a bear to understand."

Or have all of you said this already?
That's one of the best summaries of the responses to the cat that I've seen.

The only thing I'd add is that smart quantum greasemonkeys might point out that our common sense is often a limited version of what's really going on. For instance it seems ridiculous that time goes slower when you're moving quickly. But it does (as proven every day by your GPS); it's just that we move so slowly that there's not enough effect for our common sense to notice.
  #26  
Old 11-05-2010, 05:22 PM
Cheshire Human Cheshire Human is offline
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Originally Posted by Kobal2 View Post
Maybe it's both profound and bullshit at the same time, and we don't know which until we've opened Schrödinger.
No, that's what you find out by opening Schrödinger's Bull. That was actually the true scientific basis of the ancient Greeks' practice of reading animal entrails....

Last edited by Cheshire Human; 11-05-2010 at 05:23 PM.
  #27  
Old 11-05-2010, 06:05 PM
dracoi dracoi is offline
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I thin it's essential to understand that quantum mechanics is built out of observation, not understanding. ("Common sense" being a type of understanding).

At the quantum level, we observe all kinds of things happening that simply don't make sense. They shouldn't be happening. When someone says it is happening, we want to call him a total idiot, except that we can replicate what he said happened. We just know that it does happen.

Furthermore, it happens predictably and reliably so that we can fit it into a mathematical model and math can be used as a substitute for true understanding.

So, the bottom line for the cat is this: we can't ever observe the state of the cat without observing it; and we know that observing it changes the state. We know what the cat was like at some previous point in time, and we know what it was like after the observation; for the entire time in between, we simply do not know. That's not so far off from common sense, except that observations and match say that it really appears to exist in a dual state of maybe dead/maybe alive up until the point we make the observation.
  #28  
Old 11-05-2010, 07:04 PM
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Anytime I see the words “Schrodinger’s Cat” I feel like I’ve been gagged by a spoon.

1. All the various interpretations of QM are mathematically equivalent. Pick the one you like.
2. Quantum Mechanics is absurd.
3. Why shouldn’t the macro world also be absurd?
4. There is absolutely nothing you can say that is objectively true about a micro or macro system unless you observe it.
5. If you’re a solipsist you can’t even truly observe anything in the first place.
6. If Erwin were alive today I’d punch his lights out for even proposing this damn pain in the ass crapola
  #29  
Old 11-05-2010, 07:08 PM
SmithCommaJohn SmithCommaJohn is offline
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How do we know that observing something changes its state?

One of the big things I've never been able to wrap my head around is this significance of observation in quantum mechanics.

1. Why does simply observing something necessarily change it, and;

2. How do we know this if we can't observe something without observing it?
  #30  
Old 11-05-2010, 07:24 PM
Johnny Q Johnny Q is offline
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Shrodinger wasn't the only one who had problems with quantum mechanics. Still, however much they hoped to disprove it, they continued to use it.
  #31  
Old 11-06-2010, 12:10 PM
Peremensoe Peremensoe is offline
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Originally Posted by Stranger On A Train View Post
The scenario of the clock as posited by the o.p. is no different than the cat; the clock, while not resolved, is in a similar superposition of states, which in summed average equal the "real" time as measured by the clock when observed. Under any interpretation the clock never disappears or disintegrates into a quantum mist or stops working or anything like that; it just exists in an array of states that are mostly really, really close to telling the actual time. How you interpret "being in an array of states" is really up to you; you can take it as a mathematical formality, or dispense with local realism and assume that it doesn't really exist unless you are looking at it, or whatever, but the incontrovertible fact remains that unless someone unplugs it or the power goes out, it'll keep the same interval of time whether you are observing it or not within the tolerances of its mechanism.
This is a curious paragraph. It asserts that the clock never "disappears or disintegrates into a quantum mist," then explains exactly how, in my understanding, it does just that. When I've read phrases like "quantum mist" before, I've understood that to be a direct reference to the array of states--"mist" indicating fuzziness, uncertainty, the "smeared" particle.
  #32  
Old 11-06-2010, 02:17 PM
Stranger On A Train Stranger On A Train is offline
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Originally Posted by spark240 View Post
This is a curious paragraph. It asserts that the clock never "disappears or disintegrates into a quantum mist," then explains exactly how, in my understanding, it does just that. When I've read phrases like "quantum mist" before, I've understood that to be a direct reference to the array of states--"mist" indicating fuzziness, uncertainty, the "smeared" particle.
"Uncertainty" in quantum mechanics has a very specific meaning, to wit the limit of an inequality of a combination of two complementary properties of a single element (usually taken to be position and momentum). The composition of all possible states is not uncertain (for a finite system) and in fact is very precisely known. This leads to the principle of quantum indeterminacy: that this uncertainty isn't just our inability to measure the state of the system, or some kind of filter between quantum states and the macroscale world, but an actual behavior of quantum particles, that they don't exist in a precise location and momentum state. Quantum particles, despite coming in discrete packets, are actually spread out little blobs of conditional probability.

However, this doesn't mean that quantum systems stop functioning on the macroscale in absence of an observer. The correspondence principle allows that while quantum mechanics describes bizarre and sometimes "spooky" behavior on the level of fundamental particles, when you get to a system of large quantum numbers (a system with a lot of particles or very high energies) the behavior of the system is fundamentally indistinguishable from classical mechanics. The system posed in the o.p.--the clock in a room--can be modeled as a superposition of all possible states of the particles that make up the system, but that doesn't mean it literally disappears into a "quantum mist" (whatever that is) or stops functioning in an essentially classical fashion, or whatever. All of the possible states still sum to conditions for the overall system that are approximately classical, with a limit of the differences between states that is less than can possibly be observed.

Quantum mechanics applies to particles, or the rare coherent system in which probabilistic behavior is seen on a macroscale. For normal decohered systems, doing all the work to resolve the superposition of quantum states gives you something that converges to the same result you would obtain via an application of classical mechanics.

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  #33  
Old 11-06-2010, 02:40 PM
Peremensoe Peremensoe is offline
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Originally Posted by Stranger On A Train View Post
The system posed in the o.p.--the clock in a room--can be modeled as a superposition of all possible states of the particles that make up the system, but that doesn't mean it literally disappears into a "quantum mist" (whatever that is) or stops functioning in an essentially classical fashion, or whatever.
If you don't know what you, or anyone, means by a phrase, you can't possibly say whether it is literally true.
  #34  
Old 11-06-2010, 03:57 PM
Stranger On A Train Stranger On A Train is offline
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Originally Posted by spark240 View Post
If you don't know what you, or anyone, means by a phrase, you can't possibly say whether it is literally true.
So, pray tell, what is a "quantum mist", besides an odd cocktail of Irish Mist and Quantum Vodka?

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