about schroedinger's cat

If you aren’t familiar with the problem in Quantum Physics known as “Schroedinger’s Cat”, I hope someone will post some links here, because most of my (admittedly amateurish) knowledge on the subject has been gleaned from paperbacks and magazine articles.

“Schroedinger’s Cat” involves a box containing some radioactive material, and a sealed bottle of poison gas, and a live cat. It also has a device which will detect when an atom of that radioactive material decays (which occurs at a totally random time) and break open the bottle of poison gas to kill the cat. The idea is that we never really know whether or not the material has decayed, so we don’t know if the bottle was broken, and we don’t know if the cat is alive or not.

Schroedinger’s point, as I understand it, is that until a thinking being looks inside the box at the results of the experiment, the truth is that the cat is both alive and dead. Not alive or dead as ordinary logic would suggest, but both alive and dead.

Now, I’m not here to discuss this and/or business, but a slightly different aspect of the concept. Namely, why do we need an intelligent person to do the observing? It seems to me that a critical element to this experiment is that there is a mechanical device which detects the radioactive decay, and it observes the decay and breaks the bottle!

Why do people think that Schroedinger’s Cat proves the need for living, intelligent observers, when the inanimate device inside is doing the observing all along?

First, my favorite of all Cecil’s columns (even though it doesn’t address the question at hand):The story of Schroedinger’s cat (an epic poem)

Now, to the OP. Those that take this thought experiment to mean that intelligent observers are necessary to collapse the wave functions of quantum mechanics are reading it wrong. It is a favorite way for the new age crowd to claim that physics will eventually turn out to be exactly the same as Zen philosophy.

In fact, many believe that Schrodinger described the gedanken experiment specifically to show that it is absurd to apply quantum mechanics to the macroscopic world. The world collapses wave functions all by itself, without the need for any of us to be looking at the time.

A couple of things:

  1. As I understand it, Schroedinger made up this thought experiment to show how ridiculous quantum theory was making the world. He didn’t do it, as it is now used, to give a practical example of how quantum uncertain operates.

Here’s a quote from a conveniently recent NYT article:

  1. Some people do now believe that human observation is not needed–that other types of observation do occur. Information must merely be transmitted:

The “wave function” is the uncertain state between life and death (or, in real experiments, the fuzzy position/velocity of an electron or other particle before it is measured).

Personally, I don’t know if I agree w/ the decoherence theory–or your proposal that the device inside the box does the observing. Yes, the device does observe, but it sends no information outside the box–so to US, no observation has yet been made, and the cat is still on the critical list.

Well… sometimes the state vector can be collapsed by the absense of a measurement, depending on how the experiment is set up. But I think you are off base generally.

But, Keeve – try this link. It is a little wordy, but nothing you should not be able to get your head around.

The Transactional Interpretation of Quantum Mechanics

thanks, jmullaney, i’m reading it now.

You assume that your confusion stems from your failure to understand quantum uncertainty.

Actually your confusion stems from your failure to understand cats. Come hang out with mine for a couple days and you will accept that cats can be both alive and dead. I admit that it can be rather disconcerting at times, but they seem to like it that way.

Von Neumann proved that if a quantum system is present in some eigenstate of a measuring device, then the product of this eigenstate and the state vector of the measuring device remains a smoothly evolving quantum system. And that only when the superposition encounters a system (a conscious brain) which is not subject to the time symmetrical laws of QM does it collapse.

Assuming Von Neumann is correct the cat must be both dead and alive until a conscious observer makes a measurement.

Another implication here involves our own evolution I.E. that we ourselves continued to evolve as a quantum system until our wavefunction became conscious and at that point we teleologicaly collapsed our own wavefunction.:slight_smile: The ultimate question here is what is the universe like if there are no conscious beings? (us/me), And as far as I am concerned " WHO CARES!

This focuses my OP pretty well. Namely: In what way is a brain not subject to Quantum Mechanics?

(Personally, as a religious person, I feel that this aspect of QM does tend to support the concept of a free-willed soul which exists outside of the physical world but interfaces with the physical world via the brain. But this is exactly why I have asked this OP: I want to understand from a purely scientific point of view what the difference is between the conscious brain and the unthinking Geiger counter inside Schroedinger’s box.)

A conscious brain has nothing to do with quantum mechanics. And even if it did, quantum mechanics would have nothing to do with the cat, because the cat is a classical, macroscopic object.

I think the biggest misunderstanding of quantum mechanics comes from the idea of measurement and observation. Imagine you have a particle in a superposition of two possible quantum states. When we try to measure which state it is in, we collapse it into one of those two states. However, the reason for the collapse is not the fact that our brain notices the information the measurement yielded, it is because to perform the measurement, we had to interact with the particle. We interact with things by bouncing other particles off them. On a quantum scale, any interaction significant enough to yield information is going to collapse the wave function.

As for Schroedinger’s Cat, saying the cat is in a superposition of “dead” and “alive” states implies that dead cat and live cat are pure quantum states, which they aren’t. A cat is a big macroscopic superposition of 10^23 quantum states, and can be treated with statistical mechanics. At room temperature, you can use classical stat mech.

You can find his cat here

Tell me, Giraffe, if * “A conscious brain has nothing to do with quantum mechanics.”* then perhaps you could explain what does collapse the wavefunction? Also maybe you could explain why Bohm felt it necessary to develop his “Implicate Order” theory and for Everett to develop the “Many Worlds” interpretation?

A good book on all of this is “Where Does the Weirdness Go?” I’ve forgotten the author’s name.

John A. Wheeler, fed up with New Agers claiming that a human mind is necessary to perceive quantum states, wrote passionately about the collapsing of QM wavefunctions by non-sentient observers (such as cameras taking photographs).

The cat, according to Erwin, is either alive or dead, but it is not truly determined. NOT BOTH! This ties into Heisenberg’s uncertainty principal a bit, but is to demonstrate that the act of observation has an effect on the outcome. There is an analogy of HUP with a baseball trying to be caught by an outfielder. If the baseball were teensy-weensy (scientific term), when a photon hit it to illuminate it, the collision would cause the baseball to change its path, so the outfielder wouldn’t be able to catch it. The act of trying to measure has an effect on the outcome.(I shan’t go into more detail on this here.)

Back to the cat. Since most people have a difficult time envisioning QM stuff, this was Erwin’s macro example.

Von Neumann, as mentioned, interpreted this thought experiment a bit differently.

vN’s interpretation has been taken to more extremes with the “Many Universe” theory that basically states that there are two parallel universes, one in which the cat is alive, and one in which it is dead. Looking into the box causes the collapse of one of the universes. That’s a bit too sci-fi for me.

As I’ve suggested before, in other threads, Gary Zukav wrote a nice book “Dancing Wu Li Masters” which breaks QM down into semi-layman’s terms.

“The Dancing Wu Li Masters”, like “The Tao of Physics”, is a little too Wild and Wu-Li for me. Zukav and the other fellow have an agenda that I’m sure not all physicists would agree with. Take their interpretations with a grain of salt. Me, I’m sticking with “Where Does the Weirdness Go?”

Now, I’m not an expert on quantum mechanics, but my layman’s understanding of the Cat problem was that it had to do with unobservable states. Basically, if the only concern we have for the cat is whether it is alive or dead, until the box is opened the cat cannot be said to exist in either state.

I seem to recall something along these lines concerning the statement that (warning: simplistics ahead!) that stream of water coming out of the faucet has no temperature until you stick your hand in it.

Both the cat and the faucet seem to be heading toward the idea that “nothing exists until it is observed,” which could spell trouble for Objectivists :wink:

I prefer Sadie’s question in “The Cat who Walks Through Walls”: “Did anyone think to ask the cat?”

OK, here goes. A particle can be in a superposition of multiple quantum states (“pure states”). (A superposition of pure states is called a “mixed state”.) By taking a measurement, you are localizing the particle. If the particle is localized to one point in space or one specific momentum, then the probability for being at the other point, or having the other momentum goes to zero. Thus, the particle is forced into a single pure state. The interaction used to get information about the particle’s state localizes the particle and collapses the wavefunction.

Two examples:

  1. The two-slit experiment: an electron is shot at a screen with two small slits. When you send point particles through two slits, you get two diffraction patterns with no interference. When you send waves through (e.g. light), you get interference fringes. When there are no detectors over the slits, the electrons produce interference fringes, indicating that they are in a delocalized quantum state which can interfere with itself. If you put a loop detector over one of the slits, you just get diffraction. To interact with the loop detector, the electron has to be localized at that particular slit. The detector changes the electron from a big fuzzy wave-like object to a point particle-like object.

  2. Aspect experiment: an atom with zero angular momentum decays, producing two photons. By conservation of momentum, the photons have exactly opposite polarization. Before you make a measurement, the system is in a superposition of two states: the state where photon 1 is up and photon 2 is down and the state where photon 1 is down and photon 2 is up. Then you send one of the photons through a polarizer, to see if it was up or down. The polarizer localizes it into one of the two polarization states. If it is (e.g.) up, then it can go squeeze through. If it is down, it is absorbed.

Interestingly, when one photon is localized, they both collapse instantaneously into their pure states. This proved that quantum mechanics is a non-local theory, which was very non-intuitive. We still can’t get past the speed of light barrier, though, as information can’t be transmitted through a wavefunction collapse.

I’m not familiar with the “Implicate Order” theory (or at least, the name isn’t familiar). The “Many Worlds” theory is philosophy, IMO, and not physics. There is room for other interpretations of quantum mechanics, but so far I’ve never seen one that is as simple or logical as the current one. Many Worlds and the like always require unprovable assumptions which are interesting to think about, but which don’t add anything scientifically.

There is currently no evidence that human consciousness plays a role in quantum mechanics. It is of course impossible to prove/disprove, by definition. I think that most other proposed interpretations are attempts to find more intuitively satisfying (e.g. local) interpretations. So far, I don’t believe anyone has found a better interpretation than the current one.

Screwtape, I just want to point out that your argument is not related to quantum mechanics. Rather, it’s the standard “if a tree falls in the forest” question.

People who believe that nothing exists until observed often use quantum mechanics, especially Schroedinger’s Cat, as evidence. It isn’t.

"By taking a measurement"

You don’t seem to understand that the crux of the entire discussion is just who or what can take the measurement. Anything material that interacts with the particle will enter a superposition with the original particle, and thus cannot collapse the wavefunction.

This is why Von Neumann postulated and proved that only an entity that transcends normal macroscopic reality can collapse the WF. This is also why Bohm postulated his Implicit Order theory and why one the greatest physicist’s of all time proposed the seemingly ridiculous Many Worlds interpretation.

This is very possibly an unanswerable question for the human intellect.

Silly me, I thought the crux of the entire discussion was physics.

Speaking of physics, your above statement is, in fact, completely false. An electron interacting with a metal slit does not “enter a superposition” with the slit. That statement is meaningless. The electron is restricted by the presence of the other atoms – it can not go through them, so it either bounces of the screen or it passes through the slit. By passing through the slit, all of the uncertainty in its position which allowed interference vanishes. The other particles “take a measurement” by occupying space and repelling the electron, restricting the part of space it can occupy, and thereby localizing it if it is going to pass through the metal screen.

Please explain how an electron could “enter a superposition” with other atoms. Feel free to write down some wavefunctions if that would make things more clear.

Proved?? Interesting. Would you mind elaborating on how he did that?

Your rest of your argument is devoid of content. Citing the credentials of the person who proposed a theory doesn’t make the theory more right. Many Worlds is unprovable and, in my opinion, stupid. Just my opinon, there. It certainly adds nothing to the science of quantum mechanics.

Arguing that the universe behaves by different rules when not observed by a human is of course unanswerable, at least by us humans.

OK I give up. Arguing with you is like discussing Laplace transforms with a headstrong first grader. You have obviously read some pop science books but they must not have been very good ones. You really need to obtain some knowledge of what a sub atomic particle really is (isn’t?) between observations, and how particle interactions occur. You also need to gain an understanding of just what the indeterminacy of conjugate variables means.
"That statement is meaningless. The electron is restricted by the presence of the other atoms – it can not go through them, so it either ““bounces””"??? of the screen or it passes through the slit.

This would be laughable if it weren’t so sad.