What is the name of the theory that observation alone affects the outcome of an experiment? Did this idea stem from Schrodinger’s Cat paradox, or vice versa? …Or, not even close?
Wiki seems to say it is, but I thought Schrodinger’s Cat paradox was simply an analogy for particles that physicists claim sometimes have mass, and sometimes do not. Perhaps, I understood the point behind Schrodinger’s cat? 
Not sure what you are asking here.
FWIW, and to stir the pot, here is an article where scientists have now witnessed a macro quantum effect. Normally the cat bit is deemed an analogy because quantum weirdness supposedly only existed in the super tiny world. With this experiment apparently it can be seen in the “big” world too. Basically, they have seen the cat alive and dead at the same time with this experiment and clearly this thing has mass.
Copenhagen interpretation is what you’re thinking of. Measurement “collapses” the various probabilities to the measured value.
(Never mind)
Is Schroedinger’s Cat really a paradox? Well, maybe.
But anyway, Schroedinger’s Cat is an example of the postulate of quantum mechanics you’re talking about – that only observation determines the actual state of a probabilistic event. It’s not “part” of Schroedinger’s Cat any more than the Theory of Gravity is “part” of that story where Galileo dropped the one cannonball and shot the other. But they’re both experiments (thought experiments, if not actual) used to illustrate and understand beliefs about natural laws.
–Cliffy
It would make me happier if instead of saying “the cat is alive and dead at the same time”, people would say something like “the cat is in a state where it is not meaningful to ask if it is alive or dead… however after we attempt to measure whether the cat is alive he will either be in a state where he is definitely alive or will be in a state where he is definitely dead, and both those possibilities are equally likely.”
However, it would make me happier still if people gave up the cat metaphor altogether in favor of something less confusing. How about musical notes? A C-major cord is in a state where it is not meaningful to ask if the cord is a C or an E or a G… it’s a mix of the three. But the weird thing about quantum mechanics is that when you listen to a C-major cord it becomes a C or an E or a G, and the other notes go away.
In other words, the weirdness of quantum mechanics (much of it anyway) is in the way states collapse, not in the fact that states can be superpositions. Superposition is a feature of plain old classical wave mechanics as well. The Schrodinger’s Cat metaphor obscures this point, in addition to its other flaws (such as the fact that |living cat> and |dead cat> aren’t pure quantum states.)
in a rather longwinded nutshell:
there’s this cat. he’s in a box. the box is complete sealed off from the outside world. also in the box is a vial of poisonous gas, hammer, geiger counter, and radioactive source. now, this radioactive source has a half-life where in the next hour there’s a 50/50 chance that it will decay, setting off the geiger counter. if the geiger counter clicks, the hammer will smash the vial and kill the cat.
the copenhagen interpretation (important to specify. there are other interpretations eg. many worlds interpretation) deals with quantum superposition. the psi-function of the system (roughly where and when things are) gives us the square of that function psi-squared (probability of something being somewhere at some time). the psi-squared equation says that the atom has an equal chance of staying intact and setting off the geiger counter, and according to the principles of quantum superposition, the particle isn’t either or but rather in both places at the same time. this is to say the vial is both shattered and not, and the cat is both alive and dead. there is no way to determine which, unless you open the box and look, in which the psi function “collapses” and becomes null and void.
the connection with particle-wave duality might have something to do with the psi function. it is often used to describe photons, which is a massless particle.
it could be further confused with the heisenberg uncertainty principle. that says that you cannot know for certain both the position and momentum of a particle. So, when you shine a beam of light into a wall with two holes in it very closely spaced together, the light actually “goes through both holes” because of the aforementioned psi-squared. you can’t tell unless you look at it. however you can’t look at something unless photons hit it and bounce back into your eye. thus, you can’t know which hole the photon is at without hitting it with a photon - thus changing its momentum. however if you don’t hit it with a photon and change its momentum, you won’t know which hole it goes through.
i don’t know how much of this is helpful, but at least it’s interesting?
Unfortunately quantum weirdness is not so polite about this.
I understand your discomfort. Most people coming to grips with QM the first time find it disturbing and not sure that ever completely goes away.
As it happens QM has it that a superposition is real and not semantics. The cat truly is dead and alive at the same time. Not that we just do not know; it is actually both.
In reality I would like to think the cat counts as an observer but this highlights perhaps the most difficult part of QM and the observer effect. Which is to say what constitutes an observer? Afterall a photon will hit the cat and bounce off yet the cat remains in a superposed state till it enters your eye? How does the cat (or rather its QM state) know the photon was “perceived”? Does a camera count? Or only count if a consciousness looks at it? Otherwise the camera is an inert object like any other.
Philosophy meets physics here. Literally the act of observation changes the results. What defines “observation” though?
Yeah, this should blow your mind (why it is fun to ponder).
tim314 wasn’t making the usual lay mistake, which you have attempted to address. On the contrary, his point is completely correct. It isn’t both dead and alive at the same time. It is neither dead nor alive. It is something else that this sort of language is not equipped to handle.
A system that is in the state |0> + |1> (properly normalized) is not in the state |0> and the state |1> at the same time. It is, in fact, in the state |0> + |1>, which is a perfectly valid state. One can attempt to classical-ize the situation with language by saying that it’s “in both state |0> and state |1> at the same time”, but that is simply incorrect verbiage… a holdover from classical thinking.
Tim,
i think shroedinger’s cat has unnecessarily confused a lot of laymen and physics students. it’s actually a very poor analogy, possibly as bad as analogies get while still conveying the general gist of the idea. the chord analogy is a lot more elegant, though it does come up lacking when explaining collapse. however, the cat doesn’t explain either well, so it really should just be stricken from the record all together.
maybe replace it with one about the 3 card monty. before you look, the probability of the red queen is equal for all 3 cards, which is the crux of quantum anyway - probability. and if you locate the queen, you know where it is, and the probability collapses since you know where the queen is.
Isn’t it true that originally Schrödinger was trying to *disprove *the theory and his cat, as a thought experiment, was meant to show this? Only later did he come to accept the ideas of QM?
I always liked Pratchett’s Addendum to Shroedinger’s Cat. The third state – the cat is royally pissed off for being locked into a box.
I think we need to create the Schoedinger’s Cat Box and sell it on late night TV.
You don’t know if your cat has pooped until you open the box.
This is incorrect and provably so.
We have a particle traveling from A → B:
The double slit experiment shows this clearly. The particle travels through both slits at the same time. It is here AND there in a very real sense. Quantum computers exploit this feature to work. All possible paths between A and B are traversed at the same time till someone observes it.
So, the cat is dead AND alive at the same time.
I’m sorry, professor, but last night I accidently determinied the location and speed of my term paper so precisely that it ceased to exist.
That’s location and momentum. Sorry, you flunk.
If it’s both dead and alive at the same time, you could make the case that there was more than one cat. In which case you would have two cats, one box.
In some interpretations this would be correct. The Many Worlds interpretation has it that there is a universe with a dead cat superposed with a universe with a live cat. So, two cats.
I would have said:
In the double slit experiment, I think it is dangerous to say that a photon had two definite positions, here and there. On the contrary, the photon has an extended spatial wavefunction, but it is only one photon in one state. Only when you attempt to describe it with purely local language do you need to say something like “it is in two places at once”.
A (loose) analogy would be saying that Manhattan is located at 35th and Broadway and 125th and Park at the same time. You could bend the language to make that okay to say, but it actually doesn’t make any sense, because Manhattan is non-local.
In non-spatial cases, the intuition about locality doesn’t come into it, which is why I stuck with a generalized system:
Assume a system is in state |0>+|1>.
Those are all the answers. The question “Is it in both |0> and |1>?” is undefined (which I previously and sloppily cast as meaning its answer is “no”, but really it’s neither yes or no.)
Indeed, the phrase “The state is in both states |0> and |1> at the same time” would imply that you could ask the system both “Are you in state |0>?” and “Are you in state |1>?” and get positive answers, but you can’t, which implies that that is an imperfect way of describing the system.
I guess if you wish to define the phrase “is in both states at the same time” as a shorthand for “is in a quantum mechanical state which, when measured, can produce either answer in a probabilistic fashion”, then your statement is correct, but I don’t think that’s what people are hearing when someone says that the cat is both alive and dead.
It actually is in two places at once.
You are assuming the photon is sort of smeared out. Thing is we never see half a photon such that half the photon went through one slit and the other half went through the other slit.
It went through both slits at the same time. All of it.
The probabilistic language is our attempt to say, “If I look, where will the photon be?” Since we cannot know that the best we can do is assign probabilities to where we might expect to see it.