Understanding Hilbert Space in Quantum Mechanics: A Beginner's Guide

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Discussion Overview

The discussion revolves around the concept of Hilbert space in quantum mechanics, addressing various foundational questions and calculations related to its properties and applications. Participants explore the mathematical structure of Hilbert spaces, the significance of bra and ket vectors, and the implications of wave functions and operators within this framework.

Discussion Character

  • Exploratory
  • Technical explanation
  • Debate/contested
  • Mathematical reasoning

Main Points Raised

  • Some participants question the difference between bra vectors, with one asserting that they are elements of the dual space and others suggesting they are simply complex conjugates.
  • The inner product is described as involving the integration of the product of p and the complex conjugate of b over all space.
  • There is confusion regarding whether Hilbert space is a function space or a vector space, with some asserting it is both, while others emphasize its nature as a complete metric space.
  • Normalization of wave functions is discussed, with some participants explaining that it relates to the probability interpretation in quantum mechanics.
  • Participants confirm the definition of the commutator [A,B]=AB-BA and discuss its implications.
  • One participant suggests that the integral of the product of a wave function and another function could relate to Fourier transforms.
  • The expectation value is defined, with some participants clarifying the notation and its relation to probability theory.
  • There is a debate over the nature of the dual space of Hilbert spaces, with conflicting views on whether it includes only bounded linear functionals or more general ones.
  • Arguments are made regarding the application of the Riesz representation theorem, with some participants asserting that it applies only to bounded functionals, while others challenge this interpretation.

Areas of Agreement / Disagreement

Participants express differing views on several key points, particularly regarding the nature of the dual space of Hilbert spaces and the application of the Riesz representation theorem. There is no consensus on these issues, and the discussion remains unresolved.

Contextual Notes

Some claims depend on specific definitions and assumptions, particularly regarding the continuity and boundedness of functionals in the context of Hilbert spaces. The discussion also highlights the complexity of mathematical formalism in quantum mechanics.

  • #31
It cannot be normalized,simply because it's not a state vector.It's not an eigenvector of momentum operator,because the latter does not have eigenvectors,as it doesn't have discrete spectrum...

Pay attention with such details.Mathematical details mean very much in QM.

Daniel.
 
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  • #32
dextercioby said:
I'm sorry,but your "proof" is useless.In the previous post you stated that for a topological metric space everywhere continuity is completely equivalent to boundness.
The Theorem of Riesz applies ONLY TO LINEAR FUNCTIONAL EVERYWHERE CONTINUOS IN H,thind which,by virtues of topology,means that the functionals need to be bounded on H.So assuming them unbounded and applying the Riesz Representation Theorem makes no sense,okay...??

So yeah,i was wrong,\tilde{H} comprises only bounded (hence everywhere continuous) linear functionals on H.

Daniel.

Riesz thm indeed makes great sense. But I guess a more insightful version would be using the GNS construction and Gelfand-Naimark theorem where Hilbert space is constructed through non-commutative structure rather than "postulated" as in Dirac-Neumann axiomatic system.
In this sense, I prefer QM be essentially an ALGEBRAIC theory... prob. somehow becomes a byproduct.
 

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