In the famous thought experiment, a quantum event (the decay of a radioactive atom) is linked to a macroscopic event in such a way as to present the apparent requirement of having a macroscopic object (a cat) exist in a state of superposition.
My question is with the radioactive atom. The thought experiment seems to imply that after a half-life period of the atom has passed, it is indeterminate whether it has actually decayed or not, until an observation is made. I was unaware that radioactive decay was subject to this sort of indeterminancy- I thought that at any given moment an unstable atom has either decayed or not. In fact, it seems to me that radiocative decay is as completely independant of external influence as an event can be; the atom “volunteers” the information that it has decayed to the universe. Or am I mistaken?
I might have misinterpreted your question, but I think it means at the exact moment the half-life has passed, because there is a level of uncertainty. However, if you waited 12 hours on an atom with a half-life of 3 minutes, I have a feeling you could be sure the poor cat was dead.
Just thought I’d toss in Cecil’s wonderfully entertaining column on the subject.
I don’t know where you heard that the indeterminacy begins at the half-life, but you intuition is correct. The atom could decay at any time. Right after the experiment is closed or a billion years into the future. So, the indeterminacy starts right at the beginning.
Perhaps the emphasis on the half-life is that at that moment the cat is exactly half-alive and half-dead, because because the odds that the atom has decayed by the time you reach the half-life are fifty-fifty.
Odds do not specify truth. The cat is either alive or dead; your uncertainty does not change if the atom has decayed or not. Suppose a tree might or might not have fallen in the forest (50/50 chance). The tree is not half-fallen, it either fell or it did not.
Half-dead wasn’t the exact word, but you’re also right. The point in question is that it’s impossible to confirm experimentally whether the cat is alive or dead.
What Marley said I can go with, ie I’ll buy that you can’t know whether kitty is breathing.
But it’s been stated (elsewhere) that the cat is definitely not dead and definitely not alive, that we know for a fact that it is in a third state never directly observed by man.
How can that be justified? (if it is indeed stated that way by those in the know)
Because the cat is now part of a larger system, which includes the apparatus whose functioning is determined by the state of the radioactive atom. It is this entire system which is quantumly linked and so in an indeterminate state until an observation has been made.
This, as I understand it, is a consequence of the standard or Copenhagen interpretation of quantum theory. There are other interpretations - mathematically equivalent - that have been developed to avoid the oddities raised by requiring an observer - but these have different oddities buried in them.
This appears to be a basic property of quantum theories. They all confound our “common sense” view of how the universe works.
Actually, what I was getting at is questioning whether an unstable atom can be in a superposition of being decayed and not decayed. I can accept superposition for things like particle spin and so forth, but whether an atom or particle has decayed or not seems too classical. My point is that decoherence ordinarily happens when a superposed state interacts with the rest of the universe in such a way as to transmit information; whereas radioactive decay seems to be an example of “spontaneous” decoherence.
no pun intended- and I previewed that too. ARGH!
[bump]
No need to worry, Lumpy. Radioactive decay is a fundamentally quantum process, and is thus subject to all the standard quantum weirdness. We normally treat it as a classical, stochastic process because the decays of separate particles are not strongly correlated–that means that the decays of different atoms don’t interfere. Thus, we don’t lose much predictive power by thinking about it as a classical, random process, for each atom. But rest assured, it is quantum mechanical. One way to think of a decay quantum mechanically is to think of the decay product (e.g., an alpha particle) as having “tunnelled” out of the nucleus.
Ok, but at some point the superposition breaks down- you have to say “yes the atom decayed”, not “I haven’t looked at the atom to see if it’s decayed yet”.
If you put an unstable atom in a box and surround the box with gamma detectors, sooner or later the detectors will announce that a gamma ray photon of characteristic energy struck them. I don’t see how this can be considered a measurement of the atom; the reception of the gamma ray photon was completely unasked for. Isn’t the atom spontaneously “forcing” the universe to recognize that it has decayed?
The alternative would seem to be that if an unstable atom were isolated sufficiently, it would persist forever in a state in which you could only say that it very probably has decayed by now.
Lumpy, the point of all this is it is the actual act of observation istelf that collapses the probability function into a state.
No matter how subtle or ingenious the act of measurement, it is the act of observation itself that does it, nothing to do with mans fat-fingered attempts to measure a precious little quantum object.
You seem to have a pretty clear understanding of what’s going on, so I’ll just make this comment–the cat gedankenexperiment ignores the possibility of decoherence. If you start worrying about decoherence (as the real universe does), then there never is any superposition of “live” and “dead”.
…some time passes…
I think I now understand your question. Basically, I believe your last statement is correct. If the atomic nucleus were the only thing in the universe, it could persist in a state which is a superposition of “decayed” and “not decayed”.
Where we actually draw the line between unitary evolution and measurement (that is, actually quantifying decoherence) is the subject of much current research, especially among quantum information guys.
This is similar to some of the questions I have about this whole business. I have been told (and this might be what Exapno Mapcase was talking about) that there many interpretations of how all this stuff works. Some of them say that the superposition breaks down only when a conscious human makes an observation. Others feel that when the cat sees the poison gas being released, that also counts as an observation, so that we can legitimately say that the cat is either definitely alive or definitely dead.
Yet another version (which might be what Lumpy is saying here) is that the gamma detectors – inorganic though they be – are also capable of making this observation and collapsing the superposition. This is my favorite, because if one maintains that the gamma detectors are not capable of making the observation, then how would they ever release the poison gas?
Keeve,
The experiment as normally described ignores decoherence–it thus treats the detectors as macroscopic quantum objects. The detectors can then be in superpositions themselves–this means the air in the box can be in a superposition of poisoned and not poisoned, and the cat is correspondingly both dead and alive.
None of this is to take away from your point, however–I agree with you totally. Real gamma detectors are NOT quantum objects, and they do in fact measure the decay. The cat is never in any superposition.
This, I believe, is separate from Lumpy’s confusion; the nucleus IS a quantum object, and probably can be in a superposition of decayed and not decayed, because the decay is a quantum process. Of course, it’s tough to say for sure, because nuclear physics has a formidable reputation for a reason: it’s hard.