X-Message-Number: 14170 From: "John Clark" <> Subject: The Identity Of Indiscernibles Date: Tue, 25 Jul 2000 12:15:23 -0400 Yvan Bozzonetti in #14147 Wrote: >If (say) two atoms are widely separated in space, then they > ARE distinguishable both in principle and in practice. How? If we were talking about apples and oranges it would be easy to tell if I switched them around, but I claim I instantly exchanged two carbon atoms. Prove I didn't. >If I recall well, space coordinates are included in the quantum state >definition How could it be, the more you know about the coordinates of a particle the less you know about its velocity. If you know exactly where something is right now you'll have absolutely no idea where it will be an instant from now. What quantum mechanics describes is the wave function, and that's much more abstract than just coordinates, it's not even a probability, it's the square root of a probability. >so that two objects at two different places can't be in the >same quantum state. Actually, two fermions at the same place can't be in the same quantum state. It's called The Pauli Exclusion Principle and it tells us that 2 identical electrons can not be in the same orbit in an atom. It's the reason chemistry is the way it is and it's the reason matter is not infinitely compressible, it's why everything doesn't collapse into a point. >Only bosons are able to pile up at the same place True. >and so may be indistinguishable Fermions are indistinguishable too. If two things are distinguishable that means if I exchange their positions a detectable change will happen in the system. There is no way you could tell if I exchanged the position of two electrons so they're indistinguishable. In essence this means that electrons don't have scratches on them to tell one from another. John K Clark Rate This Message: http://www.cryonet.org/cgi-bin/rate.cgi?msg=14170