Christopher G said:
gamma5772 said:
It is experimental fact, as has been pointed out, that for microscopic objects, the Schrodinger cat experiment indicates that the cat (photon, electron, what have you) actually is in a superposition.
The Schrodinger cat experiment does not indicate that the cat is in superposition. Is is certainly not experimental fact that the cat is in superposition.
People seem to have a very difficult time understanding what the point of the experiment is. (Don't take this as offencive!) It simply serves to illustrate (as I have said too many times in this thread) the difference between the quantum world and the macroscopic world;
When does a measurement take place? If you don't look at a far away planet is it's weather/clouds in a massive superposition of all possibilities until a human comes by to look at it? Unlikely (what's so special about a human?); so when does it collapse?
You seem to have misunderstood my point, both in the sentence you quoted, and in the my entire post. In the sentence quoted, the "cat" is not a cat at all -- it is a photon or electron -- experiments have been performed which indicate that the photon or electron is in a superposition (humor me when I say superposition since strictly speaking we don't know if the particle
is in a superposition, we only know it behaves
as if it were). The argument I presented IS the idea you have articulated so many times, that there is a difference between microscopic and macroscopic. That is premise 1, reproduced here for your reading convenience:
gamma5772 said:
P1. It violates experience and intuition to have a macroscopic object in a quantum superposition. ("I've never seen a cat in a superposition before!").
I, and the MWI, reject P1. We will discuss this in a second.
Christopher G said:
Decoherence (in the way that you mean it with 'many worlds') is an attempt to allow objects to remain in superposition 'post measurement' while still seemingly collapsing. This may be true, and the whole universe may be in superposition, but it would still have to behave effectively as if it wasn't. This is because we don't see objects in superposition. So if they are, it must be suppressed immensely (we don't ever see two cats).
More importantly, Discoherence has nothing to say about WHEN a measurement takes place. (when do the worlds split?)
I think you are misunderstanding decoherence and the MWI.
You state that decoherence and the MWI does not say anything about when a measurement takes place and hence when the world splits. Of course it doesn't and why should it? You ask, what is so special about humans; I ask, what is so special about measurements? What is a "measurement" if it's not an interaction we can model with quantum mechanics? If you want the theoretical outcome of a measurement, then model the interaction between your measuring device and system!
You also talk about worlds splitting as some kind of physical event. If it were a physical event, you're right, we'd want to know exactly when it occurs. It is not a physical event, though. It is a layer of interpretation to make things easier to talk about. I could talk about the cat experiment (as MWI would predict -- no collapse) without ever saying that the world splits. In fact, I will do just that.
gamma5772 said:
How would the cat experiment work? The cat would be in a quantum superposition of alive and dead (let's assume it is perfectly isolated from the outside world). When the scientist observes the cat, he would be in a superposition of observing an alive cat and observing a dead cat. When the scientist is recounting his experiment, he is in a superposition of publishing a paper saying he observed an alive cat, and of publishing a paper saying he observed a dead cat.
Now, here's a point of confusion which you bring up ("we don't ever see two cats") -- the cat is in a superposition, so why doesn't anyone see the cat as in a superposition? Well, the scientist saw the cat in a superposition, and look what happened to him! Now he's in a superposition too.
Keep in mind that although everything is in a superposition, it is entangled. Precisely, the state of the universe is
| \text{cat dead} \rangle | \text{scientist sees dead cat} \rangle + | \text{cat alive} \rangle | \text{scientist sees live cat} \rangle.
Does the scientist see a cat in a superposition now? Does he see 2 cats? No! The scientist that sees a dead cat sees a dead cat, and the scientist that sees a live cat sees a live cat. Now, you ask the scientist who sees a dead cat, "did you see the wavefunction collapse?" The scientist who sees a dead cat says, "yes." You ask the same question to the scientist who sees a live cat -- "yes" again. So although collapse has not occurred, experiments will look as if collapse has. It's certainly strange that the scientist is in a superposition ("I've never been in a superposition!"), but that's just argument ("I've never seen a cat in a superposition") jazzed up a little.
