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Interference and the production of wave packets require the principle of linear superposition. Quantum theory is concerned with interference at the sub-microscopic level -- the level of interaction of the quantum disturbances themselves (including measuring device quanta). There is some relation to the physical reality of this level in QM's wave equation and wave functions wrt phases, phase relations, and amplitudes. It seems pretty certain that the details aren't in one to one correspondence with the physical reality of the sub-microscopic phenomena. Anyway, in order to say anything unambiguous about the quantum realm it's necessary to have these phenomena interact with macroscopic instruments.vanesch said:Well, the problem is that if you take quantum theory seriously, that's exactly what happens: your detector IS in two "mutually exclusive states" at the same time. That's what unitary evolution dictates, and it is the very founding principle of quantum mechanics.

This is called the superposition principle, and it is exactly the same principle that says that an electron in a hydrogen atom is both above and below the nucleus, and to the left and to the right of it, which are also "classically mutually exclusive states". This is exactly what the wavefunction is supposed to mean: the electron is in the state ABOVE the nucleus, is ALSO to the left of it, is ALSO to the right of it, and is ALSO below it, with the amplitudes given by the value of the wavefunction.

A quantum particle that impinges on a screen with several holes goes through the first hole, and ALSO goes through the second hole, and ALSO goes through the third hole.

And if you take this principle seriously all the way (that's what MWI does) then your particle detector SAW the particle, and DIDN'T see the particle. So on the display of the detector it is written "CLICK" AND it is written also "NO CLICK". And if you look at it, your eyes will BOTH see "click" and "no click". And your brain will BOTH register the information of the fact that your eyes saw "click" and that your eyes DIDN'T see click.

Only... you are only consciously aware of ONE of these possibilities.

The recorded (at a certain time) position of a particle at some location, or that a cat is alive (or dead) is unambiguous (and necessarily thermodynamically irreversible for the consistency of quantum theory). Afaik, quantum theory doesn't say that a detecting screen will detect an individual quantum in two different locations, or that a cat will be found to be both alive and dead. Measurement results are well defined values. Of course, in any set of many measurements of an identically prepared system, a detecting screen will have detected in many different locations, and the cat(s) will sometimes be alive and sometimes dead after a certain delta t from the opening of the radioactive material's enclosure.

I prefer number 5. The physics of the measurement process depends in part on the hardware that's doing the measuring, doesn't it? The wave equation for a free particle is different than for one that is interacting with some measuring device.vanesch said:*IF* quantum theory as we know it applies to all the particles and interactions in this scheme (the atoms of the detector, of your eyes, of your brain etc...) then there is no escaping this conclusion. This is due to the fact that *ALL* interactions we know (electroweak, strong, except for gravity), are, as far as we know in current quantum theory, described by a UNITARY EVOLUTION OPERATOR.

So what are the ways out of this riddle ?

1) this is indeed what happens, and for some strange (?) reason, we are only aware of one of the states. This is the picture I'm advocating - unless we've good indications of the other possibilities.

2) this unitary evolution is a very good approximation which is in fact, slightly non-linear. this can be a minor modification to QM, or this can be just an indication that QM is a good effective theory for something totally different.

3) we've not yet included gravity. Maybe gravity will NOT be described by a unitary evolution operator.

4) there's maybe another interaction that spoils the strictly unitary evolution

5) somehow the act of observation (what's that ?) is a physical process that acts upon the wavefunction (that's the von Neumann view: but WHAT PHYSICS is this act of observation then ?) and reduces the state of whatever you're "observing".

In the S-cat scenario, the measuring device includes whatever an emitted quantum disturbance interacts with that eventually amplifies the quantum disturbance and frees the poisonous gas, the poisonous gas itself, and the cat. The cat is the "pointer" or "clicker" of the device.

There is a problem in that quantum measurement processes are essentially uncontrollable and unpredictable. In the process of measuring the quantum disturbance, definite phase relations are destroyed, and the wavelike object that has been evolving unitarily is transformed into a particle-like object which eventually manifests macroscopically as a well defined value.

The problem doesn't really have to do with why we don't see the S-cat alive and dead, or a quantum particle here and there as a singular outcome of an individual measurement. It has to do with the fact that we can't see what's happening at the sub-microscopic level of the quantum disturbance itself.