Why do ket vectors not have magnitudes?

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Discussion Overview

The discussion revolves around the concept of ket vectors in quantum mechanics, specifically addressing why their magnitudes are considered irrelevant. Participants explore the implications of this idea for representing states of particles and the interpretation of these vectors within Hilbert space.

Discussion Character

  • Conceptual clarification
  • Exploratory
  • Technical explanation

Main Points Raised

  • One participant questions why the magnitude of ket vectors should not represent the density or average number of particles, suggesting a potential role for magnitude in describing physical systems.
  • Another participant explains that kets represent the state of the entire system as rays in Hilbert space, indicating that kets of different magnitudes correspond to the same physical state, with a common convention of using unit vectors for probability normalization.
  • A participant expresses gratitude for the explanation but admits to uncertainty about the term "ray," prompting further clarification about the concept of rays as vectors extended in both directions.
  • Clarification is provided that multiplying a ket vector by any complex number results in a different vector that still represents the same state, emphasizing the nature of rays in this context.
  • Some participants recommend external resources, such as Leonard Susskind's lectures, for better understanding the mathematical foundations and interpretations of bra and ket vectors.

Areas of Agreement / Disagreement

Participants express varying levels of understanding regarding the implications of ket vector magnitudes, with some agreeing on the conventional use of unit vectors while others question this convention. The discussion remains unresolved regarding the potential physical significance of ket magnitudes.

Contextual Notes

The discussion highlights a lack of consensus on the interpretation of ket magnitudes and their relevance to physical representations, as well as varying levels of familiarity with the underlying mathematics among participants.

entropy1
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Why does the magnitude of a ket vector not matter?

The motivation appears to be that a state vector only can decribe a particle, or no particle.

But why shouldn't the magnitude of ket vectors not be used to represent the density of the particles, the average number of particles?

I'm am fairly new on the mathematics, in fact just starting again. ;)
 
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The ket is the state of the entire system - so it always represents all particles.

The state of the system is a ray in Hilbert space, so kets of different magnitudes represent the same state. The most usual convention is to take all kets to be unit vectors, basically so that the probabilities sum to one.
 
Thank you for your reply! :smile: I'm glad you understand my question. I'm not sure if I understand your answer, but I will dive into it! Thanks!

Oh, a question though: what do you mean with 'a ray'? :smile: Thanks again!
 
entropy1 said:
Oh, a question though: what do you mean with 'a ray'? :smile: Thanks again!

If one pictures each vector as an arrow of a certain length sticking out from the origin, then a ray is just that vector extended in both directions. It's just another way of saying that multiplying the vector by any complex number gives a different vector, but it is the same state.

http://www.theory.caltech.edu/people/preskill/ph229/notes/chap2.pdf (talks about rays)
 
Entropy: Have a look at Leonard Susskind's quantum mechanics lectures at http://theoreticalminimum.com/courses/quantum-mechanics/2012/winter. He doesn't assume much prior mathematical knowledge and spends a lot of time in the first few lectures explaining the linear algebra and interpretation of bra and ket vectors. I probably wouldn't have gotten past square one without his lectures.
 
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Adrian B said:
Entropy: Have a look at Leonard Susskind's quantum mechanics lectures at http://theoreticalminimum.com/courses/quantum-mechanics/2012/winter. He doesn't assume much prior mathematical knowledge and spends a lot of time in the first few lectures explaining the linear algebra and interpretation of bra and ket vectors. I probably wouldn't have gotten past square one without his lectures.

What is the plural of "apparatus"? :smile: http://theoreticalminimum.com/courses/quantum-mechanics/2012/winter/lecture-2
 

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