Commutators, operators and eigenvalues

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SUMMARY

The discussion centers on the commutation of eigenvalues derived from quantum mechanical operators, specifically position (x) and momentum (px) operators. It is established that while the operators do not share eigenstates, the eigenvalues themselves commute as they are treated as real numbers. The conversation references the path integral formulation of quantum mechanics, specifically citing "QFT for the Gifted Amateur" by Lancaster & Blundell as a resource for further understanding. The conclusion is that eigenvalues derived from operators in quantum mechanics can be treated as commuting numbers.

PREREQUISITES
  • Understanding of quantum mechanics operators, specifically position and momentum operators.
  • Familiarity with eigenvalues and eigenstates in quantum mechanics.
  • Knowledge of the path integral formulation of quantum mechanics.
  • Basic concepts of commutation relations in quantum mechanics.
NEXT STEPS
  • Study the path integral formulation of quantum mechanics in detail, particularly in "QFT for the Gifted Amateur" by Lancaster & Blundell.
  • Learn about the implications of non-commuting operators in quantum mechanics.
  • Explore the mathematical framework of eigenvalues and eigenstates in quantum mechanics.
  • Investigate the physical significance of measuring position and momentum in quantum systems.
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Students and researchers in quantum mechanics, physicists studying quantum field theory, and anyone interested in the mathematical foundations of quantum operators and their properties.

dyn
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Hi
I just wanted to check my understanding of something which has come up when first studying path integrals in QM. If x and px are operators then [ x , px ] = iħ but if x and px operate on states to produce eigenvalues then the eigenvalues x and px commute because they are just numbers. Is that correct ?
Thanks
 
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Operators ##x## and ##p_x## don't have eigenstates in common.
 
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Yes . i know that but x can act on a position eigenstate and p can act on a momentum eigenstate. I just want to check that once the operators act on states to produce eigenvalues , that eigenvalues commute because they are just numbers ?
 
dyn said:
the eigenvalues x and px commute because they are just numbers. Is that correct ?
No, it makes no sense, because you don't multiply eigenvalues in QM. You could of course measure the position of one quantum system and the momentum of another and then multiply the results you get, but what sense would it make?
 
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dyn said:
something which has come up when first studying path integrals in QM
I'm struggling to see how the question you are asking could arise in that context. Can you give a reference?
 
dyn said:
. I just want to check that once the operators act on states to produce eigenvalues , that eigenvalues commute because they are just numbers ?
Numbers and operators commute. It does not matter from where the numbers come. 2024 commutes with operators. We do not care whether it is a measured value of position of some state or new calendar year. Happy new year!
 
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PeterDonis said:
I'm struggling to see how the question you are asking could arise in that context. Can you give a reference?
The derivation of the path integral in QM on P211-212 of "QFT for the gifted amateur" by Lancaster & Blundell
 
dyn said:
The derivation of the path integral in QM on P211-212 of "QFT for the gifted amateur" by Lancaster & Blundell
Yes, that is correct. All the ##p##'s and ##q_n##'s here,

1704165225828.png


are just real numbers and of course commute.
 
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