No Definite Properties Before Measurement

  • Thread starter daezy
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Main Question or Discussion Point

Mass, Charge, Spin has definite properties before measurement.


What else aside from position, momentum that have NO definite
properties before measurement. Is it only position and momentum?
 

Answers and Replies

  • #2
Fredrik
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The first quantities you mention are the type of numbers that define a particle species. If you change one of those numbers, we'd be dealing with a different type of particle. Every other quantity is of the "doesn't have a definite value before measurement" kind.

In the non-relativistic theory of a single spin-0 particle, every observable is a function of x and p (the expressions that define them may include other numbers, but no other operators), e.g. energy ([itex]\vec p\,^2/2m[/itex]) and angular momentum ([itex]\vec x\cdot\vec p[/itex]).
 
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  • #3
tom.stoer
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The charge is a bad example as it is not clear what concept of charge we are talking about. In QED this is somehow hidden, but in QCD it becomes explicit. In QED we have the "e" in the Lagrangian and we know that an electron has charge Q=-e.

In QCD we have a "g" in the Lagrangian which is NOT a charge but a coupling constant. That shows that the e in QED is not a charge either, that it is again a coupling constant, and that this coupling constant is by no means constant as it is affected by renormalization.

In QCD we do not have one charge, but we have an SU(3) color-charge algebra with 8 charges Qa, a=1..8. The same applies to SU(2) el.-weak theory and to a lot of other charges like SU(2) isospin or in general flavor.

The algebra of charges Qa looks similar to the familiar angular momentum algebra. And b/c of this algebra a certain particle or state cannot have definite values for all different charges a=1..8. If you look at angular momentum and label your states as |lm> with m=lz then in each such state lx and ly have no definite value.
 

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