# Schrödinger's Cat: Could someone explain what this is supposed to mean? **URL:** \ **Category:** Factual Questions\ **Created:** [May 8, 2005, 10:43pm UTC](https://boards.straightdope.com/t/schrodingers-cat-could-someone-explain-what-this-is-supposed-to-mean/302976 "2005-05-08T22:43:09Z")\ **Posts on this page:** 20\ **Page:** 1
**Author:** ![HPL](https://avatars.discourse-cdn.com/v4/letter/h/3da27b/32.png) [@HPL](https://boards.straightdope.com/u/HPL)\ **Post date:** [May 8, 2005, 10:43pm UTC](https://boards.straightdope.com/t/schrodingers-cat-could-someone-explain-what-this-is-supposed-to-mean/302976/1 "2005-05-08T22:43:09Z")
Okay, I understand the experiement. There’s a cat in a box. Inside the box is a posion gas canister, triggered by the decay of a certain particle that has a 50% chance of decaying on any particular day. Until the box is open, there is no way of telling if the particle decayed thus Dead cat or didn’t thus live cat. My question is, exactly what is this supposed to prove? That you won’t know the cat is alive or dead until the box is open? That you won’t know if the particle is decayed until the box is open? For some reason the hesienberg uncertainty principle comes to mind, that there’s some information that cannot be known, but I suspect I’m on the wrong track there. ---
**Author:** ![Khadaji](https://avatars.discourse-cdn.com/v4/letter/k/9e8a1a/32.png) [@Khadaji](https://boards.straightdope.com/u/Khadaji)\ **Post date:** [May 8, 2005, 11:19pm UTC](https://boards.straightdope.com/t/schrodingers-cat-could-someone-explain-what-this-is-supposed-to-mean/302976/2 "2005-05-08T23:19:13Z")
I don’t claim to understand it myself, but here is a link that tries to explain it: [Cat](http://tf.nist.gov/ion/qucomp/cat.htm) > [@](#): > > In 1935, Erwin Schrödinger proposed a “thought experiment” to highlight one of the ways in which quantum mechanics contradicts our experiences of reality (E. Schrödinger, Naturwissenschaften 23, 807 (1935)). His proposal involved placing a cat (a macroscopic object) inside a closed box with a vial of cyanide and a radioactive atom initially prepared in the metastable state (a microscopic object). The radioactive atom has a probability of ½ of decaying in one hour. If it decays, then the cyanide is released and the cat dies; if it does not decay, then the cyanide is not released and the cat remains unharmed. > > The paradox arises because the atom, being a microscopic object, must be described by quantum mechanics. After one hour, and before it is observed, the atom is in an equal superposition of being decayed and undecayed. However, if quantum mechanics is a universal and complete theory, it must describe the whole system. And, since the state of the cat is correlated with the state of the atom, the cat must also be in a superposition of being dead and alive. This clearly contradicts our everyday experience of cats! > > This sort of apparent contradiction arises whenever the state of macroscopic objects is correlated with that of microscopic objects. It comes about because in our experience of our macroscopic surroundings, objects are either in one state or another, but never in a superposition of several states at the same time. The link continues, but I hesitate to quote too much, due to copyright issues. ---
**Author:** ![ticker](https://avatars.discourse-cdn.com/v4/letter/t/d07c76/32.png) [@ticker](https://boards.straightdope.com/u/ticker)\ **Post date:** [May 8, 2005, 11:44pm UTC](https://boards.straightdope.com/t/schrodingers-cat-could-someone-explain-what-this-is-supposed-to-mean/302976/3 "2005-05-08T23:44:24Z")
It wasn’t an ordinary cat, it was a _quantum mechanical_ cat. As such it was not the case that, until we opened the box, we did not _know_ if the cat was alive or dead, but that the cat was _neither alive nor dead_ (or both alive and dead, whicever you prefer). The cat only settled on being alive or dead when the box was opened and the very act of observing it made the superposition of alive/dead states collapse into one or other, with a 50% probability for each. The motivation was not to explain anything about the nature of cats. It was to illustrate the nature of quantum mechanics and show just how odd, compared to our everyday experiences, quantum mechanics is. ---
**Author:** ![Lumpy](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/lumpy/32/446_2.png) [@Lumpy](https://boards.straightdope.com/u/Lumpy)\ **Post date:** [May 8, 2005, 11:54pm UTC](https://boards.straightdope.com/t/schrodingers-cat-could-someone-explain-what-this-is-supposed-to-mean/302976/4 "2005-05-08T23:54:36Z")
More specifically, it was meant to illustrate the big unanswered question in Quantum Mechanics: if atomic and subatomic particles exist in superposition, then why doesn’t the macroscopic world? This question is the subject of an article in the latest Disocover magazine. Mathematician Roger Penrose proposes that the answer is that gravity imposes a time limit for how long an object can be superimposed. He postulates an experiment in which a macroscopically observable piece of matter ( a small piece of very light gold foil) could in his theory remain in superposition for about a second. By using this piece of foil in a beam-splitter experiment, you could demonstrate (if not directly observe) that it had in fact been superimposed. His version of the thought experiment isn’t currently workable, but physicists are trying to modify it into something that could be performed in a lab. ---
**Author:** ![smiling\_bandit](https://avatars.discourse-cdn.com/v4/letter/s/e9a140/32.png) [@smiling\_bandit](https://boards.straightdope.com/u/smiling_bandit)\ **Post date:** [May 8, 2005, 11:55pm UTC](https://boards.straightdope.com/t/schrodingers-cat-could-someone-explain-what-this-is-supposed-to-mean/302976/5 "2005-05-08T23:55:49Z")
You wanns know the worst/best part? That’s about as basic as physics gets after Newton. Schroedingers cat is arguably the least complicated idea in physics (of the commonly known problems, anyway). ---
**Author:** ![hawkeyejo](https://avatars.discourse-cdn.com/v4/letter/h/e19b73/32.png) [@hawkeyejo](https://boards.straightdope.com/u/hawkeyejo)\ **Post date:** [May 9, 2005, 12:17am UTC](https://boards.straightdope.com/t/schrodingers-cat-could-someone-explain-what-this-is-supposed-to-mean/302976/6 "2005-05-09T00:17:31Z")
So why is this different than saying the probability of the cat being alive is 1/2? ---
**Author:** ![laughingboy](https://avatars.discourse-cdn.com/v4/letter/l/9f8e36/32.png) [@laughingboy](https://boards.straightdope.com/u/laughingboy)\ **Post date:** [May 9, 2005, 12:54am UTC](https://boards.straightdope.com/t/schrodingers-cat-could-someone-explain-what-this-is-supposed-to-mean/302976/7 "2005-05-09T00:54:21Z")
The Master speaks, [in verse](http://www.straightdope.com/classics/a1_122.html). ---
**Author:** ![Muffin](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/muffin/32/4341_2.png) [@Muffin](https://boards.straightdope.com/u/Muffin)\ **Post date:** [May 9, 2005, 3:31am UTC](https://boards.straightdope.com/t/schrodingers-cat-could-someone-explain-what-this-is-supposed-to-mean/302976/8 "2005-05-09T03:31:51Z")
What is PETA’s position? ---
**Author:** ![ultrafilter](https://avatars.discourse-cdn.com/v4/letter/u/3d9bf3/32.png) [@ultrafilter](https://boards.straightdope.com/u/ultrafilter)\ **Post date:** [May 9, 2005, 3:32am UTC](https://boards.straightdope.com/t/schrodingers-cat-could-someone-explain-what-this-is-supposed-to-mean/302976/9 "2005-05-09T03:32:23Z")
> [@hawkeyejo](#): > > So why is this different than saying the probability of the cat being alive is 1/2? What’s been said implies that, but the converse is not true. ---
**Author:** ![Princhester](https://avatars.discourse-cdn.com/v4/letter/p/3e96dc/32.png) [@Princhester](https://boards.straightdope.com/u/Princhester)\ **Post date:** [May 9, 2005, 3:46am UTC](https://boards.straightdope.com/t/schrodingers-cat-could-someone-explain-what-this-is-supposed-to-mean/302976/10 "2005-05-09T03:46:47Z")
> [@hawkeyejo](#): > > So why is this different than saying the probability of the cat being alive is 1/2? The probability of the cat being alive when you open the box is 1/2. But the thought experiment is going further. It is speculating as to whether, if atomic and subatomic particles exist in superposition (that is, exist in two states at the same time till it is observed which state they are in), and if the macroscopic world is dependent on atomic and subatomic particles, the cat is both dead and alive until you observe which state it is in. ---
**Author:** ![HPL](https://avatars.discourse-cdn.com/v4/letter/h/3da27b/32.png) [@HPL](https://boards.straightdope.com/u/HPL)\ **Post date:** [May 9, 2005, 5:31am UTC](https://boards.straightdope.com/t/schrodingers-cat-could-someone-explain-what-this-is-supposed-to-mean/302976/11 "2005-05-09T05:31:49Z")
> [@Princhester](#): > > The probability of the cat being alive when you open the box is 1/2. But the thought experiment is going further. It is speculating as to whether, if atomic and subatomic particles exist in superposition (that is, exist in two states at the same time till it is observed which state they are in), and if the macroscopic world is dependent on atomic and subatomic particles, the cat is both dead and alive until you observe which state it is in. Alive and dead at the same time? Would that make it a zombie cat? I’m starting to get the idea, but my mind is still working it’s way around the concept. So in it’s essence, it’s sort of a microcosmic attempt to develop a grand unified field theory? ---
**Author:** ![Marley23](https://avatars.discourse-cdn.com/v4/letter/m/45deac/32.png) [@Marley23](https://boards.straightdope.com/u/Marley23)\ **Post date:** [May 9, 2005, 5:35am UTC](https://boards.straightdope.com/t/schrodingers-cat-could-someone-explain-what-this-is-supposed-to-mean/302976/12 "2005-05-09T05:35:46Z")
No, it’s Schrodinger trying to illustrate a problem with Quantum Mechanics. In the everyday world, a cat is either alive or dead, period. But in this experiment, it’s alive AND dead at the same time until you open the box. So Quantum Mechanics isn’t adequately describing reality. ---
**Author:** ![Mathochist](https://avatars.discourse-cdn.com/v4/letter/m/c89c15/32.png) [@Mathochist](https://boards.straightdope.com/u/Mathochist)\ **Post date:** [May 9, 2005, 5:39am UTC](https://boards.straightdope.com/t/schrodingers-cat-could-someone-explain-what-this-is-supposed-to-mean/302976/13 "2005-05-09T05:39:40Z")
> [@HPL](#): > > Okay, I understand the experiement. There’s a cat in a box. Inside the box is a posion gas canister, triggered by the decay of a certain particle that has a 50% chance of decaying on any particular day. Until the box is open, there is no way of telling if the particle decayed thus Dead cat or didn’t thus live cat. > > My question is, exactly what is this supposed to prove? That you won’t know the cat is alive or dead until the box is open? That you won’t know if the particle is decayed until the box is open? > > For some reason the hesienberg uncertainty principle comes to mind, that there’s some information that cannot be known, but I suspect I’m on the wrong track there. Don’t worry about the uncertainty principle in this one. Basically, Schrödinger had figured out something that the equations said particles should do, but when you blow it up to the everyday world it was nutty. He picked this concrete example to link the two levels in an attempt to show that _something_ must be wrong with the way everyone was going with quantum mechanics. Basically, we have our little ideas about atomic particles, but we’ve got real experience with things like cats. Something weird at the atomic scale doesn’t trigger our “hey, that’s not right” reaction as much as something weird on the classical scale. So using the decay of an atom (an event at the quantum scale) to trigger the death of a cat (an event at the classical scale) turns the superposition of decays/doesn’t decay – which we might chalk up to “hey, particles are weird” – into a superposition of cat dead/cat alive – which is simply ludicrous. Obviously, then, quantum mechanics must have made a mistake somewhere. That’s Schrödinger’s point. Except he was wrong. Quantum mechanics was dead-on, and particles _do_ end up in superpositions. I don’t recall a link offhand, but I think someone’s even taken a picture of it (in some sense). ---
**Author:** ![Mathochist](https://avatars.discourse-cdn.com/v4/letter/m/c89c15/32.png) [@Mathochist](https://boards.straightdope.com/u/Mathochist)\ **Post date:** [May 9, 2005, 5:41am UTC](https://boards.straightdope.com/t/schrodingers-cat-could-someone-explain-what-this-is-supposed-to-mean/302976/14 "2005-05-09T05:41:38Z")
> [@Lumpy](#): > > This question is the subject of an article in the latest Disocover magazine. Mathematician Roger Penrose proposes that the answer is that gravity imposes a time limit for how long an object can be superimposed. [nitpick]“superposed”[/nitpick] And is he _still_ talking about that? God, it’s been on his tour for a decade now. I mean, I think he may well be right, but c’mon… ---
**Author:** ![HMS\_Irruncible](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/hms_irruncible/32/7394_2.png) [@HMS\_Irruncible](https://boards.straightdope.com/u/HMS_Irruncible)\ **Post date:** [May 9, 2005, 6:06am UTC](https://boards.straightdope.com/t/schrodingers-cat-could-someone-explain-what-this-is-supposed-to-mean/302976/15 "2005-05-09T06:06:47Z")
> [@HPL](#): > > Alive and dead at the same time? Would that make it a zombie cat? > > I’m starting to get the idea, but my mind is still working it’s way around the concept. > > So in it’s essence, it’s sort of a microcosmic attempt to develop a grand unified field theory? Sort of, but it’s also attempt that acknowledges its own failure. The assumption is that cats, at least as we know them, cannot be alive and dead at the same time. Thus the point is made that, quantum physics, at least as we understand it, cannot describe macroscopic objects such as cats. Thus, in order to find unifying theories, we must gain more understanding about either quantum physics or cats. It has also been theorized that quantum physics actually describes the system accurately because cats can be superimposed into no less than 9 possible states. 🙂 ---
**Author:** ![Alan\_Smithee](https://avatars.discourse-cdn.com/v4/letter/a/2acd7d/32.png) [@Alan\_Smithee](https://boards.straightdope.com/u/Alan_Smithee)\ **Post date:** [May 9, 2005, 6:21am UTC](https://boards.straightdope.com/t/schrodingers-cat-could-someone-explain-what-this-is-supposed-to-mean/302976/16 "2005-05-09T06:21:24Z")
Just in case the OP still doesn’t get it and because no one’s yet made it pervectly clear: According to Quantum Mechanics, the particle **does not** have a 50% chance of decaying or not. The particle actually exists in a superimposed state of both having and not having decayed. Upon detecting the decay (by opening the box) the particle becomes fixed in one state or the other, ans so we have a 50% chance of finding a decayed or undecayed particle, but up to that point it was both. Schroedinger was showing how this could have problems at the macro scale. What I don’t get is how you mathematcally define observation. Why doesn’t the gas canister “observe” the particle by either being triggered or not, thereby collapsing the waveform. What is it about human eyes (or a human brain) that affects uantum reality like nothing else does? ---
**Author:** ![David\_Simmons](https://avatars.discourse-cdn.com/v4/letter/d/9de053/32.png) [@David\_Simmons](https://boards.straightdope.com/u/David_Simmons)\ **Post date:** [May 9, 2005, 6:29am UTC](https://boards.straightdope.com/t/schrodingers-cat-could-someone-explain-what-this-is-supposed-to-mean/302976/17 "2005-05-09T06:29:56Z")
> [@Alan Smithee](#): > > Just in case the OP still doesn’t get it and because no one’s yet made it pervectly clear: > According to Quantum Mechanics, the particle **does not** have a 50% chance of decaying or not. The particle actually exists in a superimposed state of both having and not having decayed. Upon detecting the decay (by opening the box) the particle becomes fixed in one state or the other, ans so we have a 50% chance of finding a decayed or undecayed particle, but up to that point it was both. Schroedinger was showing how this could have problems at the macro scale. > > What I don’t get is how you mathematcally define observation. Why doesn’t the gas canister “observe” the particle by either being triggered or not, thereby collapsing the waveform. What is it about human eyes (or a human brain) that affects uantum reality like nothing else does? The way it was explained to me by ex Univ. of Calif. physics professor Howard Wilcox is that an interaction of a quantum entity with anything is an “observation.” Macro entities are always existing because their quantum particles are always interacting with each other and are therefore “observed” and so are in some defined state or other. In the case of the gas canister, as I understand the experiment the quantum entity has not interacted with the canister until some kind of observation is made. ---
**Author:** ![HPL](https://avatars.discourse-cdn.com/v4/letter/h/3da27b/32.png) [@HPL](https://boards.straightdope.com/u/HPL)\ **Post date:** [May 9, 2005, 6:38am UTC](https://boards.straightdope.com/t/schrodingers-cat-could-someone-explain-what-this-is-supposed-to-mean/302976/18 "2005-05-09T06:38:33Z")
I think I understand it now. Thanks to everyone who contributed. I think quatum/relativitic physics are facinating, but sometimes it’s a bit tough to fully understand. I guess I’ll have to take another crack at reading **The Elegant Universe** , which I liked but had to quit because it simply became too difficult to comprehend after a certain point. ---
**Author:** ![Lord\_Mondegreen](https://avatars.discourse-cdn.com/v4/letter/l/91b2a8/32.png) [@Lord\_Mondegreen](https://boards.straightdope.com/u/Lord_Mondegreen)\ **Post date:** [May 9, 2005, 6:39am UTC](https://boards.straightdope.com/t/schrodingers-cat-could-someone-explain-what-this-is-supposed-to-mean/302976/19 "2005-05-09T06:39:28Z")
> [@David Simmons](#): > > In the case of the gas canister, as I understand the experiment the quantum entity has not interacted with the canister until some kind of observation is made. If nothing else, the cat has certainly made an observation. I don’t think quantum mechanics insists on human observation! Schrodinger’s Cat is a good thought experiment. It’s not perfect, but it doesn’t have to be. Its point is to make you think about the implications of QM theory (and illustrate some of its potential problems), and it does that quite well. ---
**Author:** ![HPL](https://avatars.discourse-cdn.com/v4/letter/h/3da27b/32.png) [@HPL](https://boards.straightdope.com/u/HPL)\ **Post date:** [May 9, 2005, 6:53am UTC](https://boards.straightdope.com/t/schrodingers-cat-could-someone-explain-what-this-is-supposed-to-mean/302976/20 "2005-05-09T06:53:07Z")
As a bit of a small hijack, is it just me, or do certain elements of quatum physics bear a striking similarity to philosophy? [Next page](https://boards.straightdope.com/t/schrodingers-cat-could-someone-explain-what-this-is-supposed-to-mean/302976.md?page=2)