If you’re saying that spontaneous decoherence cannot happen, you’re wrong. It is a possibility of the wave function just like quantum tunneling so if you’re going to convince me, I’ll need to see some authority other than your pronouncements.
OK, we’re clearly at an impasse. You’ve decided quantum mechanics has magic features that are conveniently unmeasurable. Go forth and spread the word. Me, I’ll be in a superposition with my stapler.
The wave function of a closed system always evolves unitarily (i.e. it takes pure states to pure states), while decoherence is nonunitary (taking pure states to mixed states). Wiki says the following:[
](Quantum decoherence - Wikipedia)
Here, the ‘viewed in isolation’-bit refers to the forgetting about the degrees of freedom you’re not interested in that I mentioned.
Those features aren’t magic, but a straightforward consequence of the formalism, and perfectly well measurable, though so far only for relatively small systems. Neither theory nor observation gives any reason to disbelieve its applicability to more macroscopic objects; if you want to insist that for some reason, it still doesn’t apply, you’re free to do so, but only at the cost of changing the formulation of quantum mechanics in a way for which there is zero evidence, and which most people don’t think will ever work. I’m not saying anything controversial here: most researchers in the field believe that quantum mechanics is universally applicable.
Here is the full quote;
Also, we do not have a closed system:
Did you note the bit about ‘arbitrarily large number of particles’?
Yes. 
I mean, was there some kind of point you were trying to make? The wiki article about quantum field theory is completely irrelevant, since we’re doing simple quantum mechanics, and even going to a QFT treatment, what I said about decoherence was completely accurate. I can still define the notion of an isolated system in QFT, though it won’t be an eigenstate of the system Hamiltonian on its own, but rather, eigenstates of that Hamiltonian plus the interaction Hamiltonian (often called ‘dressed states’), and for which, again, there will be no ‘spontaneous decoherence’.
That falls apart once you have so many particles that it is no longer a closed system. Your comments rely on there being a limited number superposition states. That is not going to be the case with 10^25 particles at room temperature - I don’t care what kind of assumptions you want to make.
I’m not asserting that quantum phenomena can not exist at the macroscopic scale, such things are well documented (e.g. superfluidity, superconductivity, Bose-Einstein condensation). And they emerge naturally from a well-defined basis of atomic states and quantum statistical mechanics.
The problem I have is where you make assertions like a dead cat and live cat being able to exist in a superposition, because both objects are “simply collections of molecules in different arrangements”, or that “for a completely shielded system, its macroscopic nature wouldn’t do anything to wash out quantum effects—it’s solely uncontrollabe interactions with an environment that carry of quantum coherences”. That’s just made-up nonsense that you think sounds smart. The same statistical mechanics and quantum theory that elegantly demonstrates actual macroscopic quantum effects shows the impossibility of any such correlation and coherence in cats.
You can continue to assert that coherence exists wherever and however you like, but without math and experiment behind it, it’s just pointless philosophical masturbation.
Where and how?
OK, I’d like a cite for being wrong about that then, please. I mean, it’s clear that this is an idealized situation in a thought experiment unlikely to ever arise in the real world; but if it did arise, I’m just telling you what the rules of QM say.
Seriously? You’re throwing around terms like eigenstates and you’re asking me this? Uh, no. I don’t think so. :rolleyes:
Sure, but let’s be clear: what exactly do you believe the rules of QM say about Schrodinger’s Cat? Within said rules, how can a superposition of dead cat and live cat exist with a probability of greater than e.g. 10[sup]-100[/sup] (an arbitrarily small number chosen to make things quantitative)? Feel free to use a perfectly idealized box and cat, provided the cat is cat-sized (~5-10 lbs) and at room temperature.
When I first heard of the Many Worlds theory I thought much like the OP.
Now I’m not so sure.
However, I am getting old and more senile…As proof, I actually think there is a chance, and not an insignificant one, that we live in a simulated universe. The older I get, the more I up the odds.
Something about the MW theory just hits my guts as possible…but that deosn’t mean much as I am not a physicist.
That’s not true. First of all, it’s not “by definition untestable”, there’s been various scientific speculations about possible ways to test it. We just don’t have the technology yet for any of the proposed methods. And second, it does not violate the laws of physics, nor is it totally baseless like “the mind of God” is.
It isn’t, it’s in all of them. Our consciousness splits along with everything else.
OK, so you don’t actually have an argument. Noted.
If the system is perfectly shielded from environmental effects, then its evolution will be unitary, which preserves quantum coherence. So if that’s the case, and the unitary evolution is such that the cat ends up in a superposed state, then it’ll just stay that way. And even if there is an interaction with the environment—say, a stray electron entangles itself with the cat—then while the cat-and-box system may loose coherence, the cat-and-box-and-electron system will still be in superposition: in decoherence, the superposition doesn’t vanish for good, it’s just leaked into the environment.
Really, the key point is simply: the quantum evolution is unitary, which can’t destroy coherence (otherwise, there wouldn’t be any measurement problem). So, absent measurement (and then only on the right interpretation), there’s simply no mechanism to get rid of superpositions for good. Decoherence may solve this problem for all practical purposes, at least for some small system you are observing, but in theory, it’s always possible to undo it, to recohere the original state by undoing all environmental interactions, ‘deleting’ the information of the system from the environment. So the conceptual problem remains, because the superposition does.
Make some actual argument or don’t, but stop this coy song and dance number of posting vaguely related links and cryptic half-sentences. I know it’s your standard routine when your position is challenged, but it makes you look as if you just throw the top ten google results at people in the hopes something will stick.
What’s the matter. Have trouble with that link?
My answer was perfectly clear. Interactions collapse superpositions. Quantum mechanics 101.
Simplify it a bit for the rest of us. Do you mean a hydrogen molecule cannot exist in a superposition situation, because the two atoms interact? Superposition is only possible for elementary particles?
I know that most of the old original work involved photons or electrons – the original double-slit experiments – but what about a high-energy stream of hydrogen molecules going through a double-slit setup? What about a high-energy stream of – I dunno – salt crystals going through a double-slit setup?
Help out a little at the level of the typical reader of Science News…