Group Theory why transformations of Hamiltonian are unitary?

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SUMMARY

The discussion centers on the necessity of the matrix D being unitary in the context of quantum mechanics, specifically regarding the transformation of wavefunctions. The transformation is expressed as Tψ(r) = ψ(Ur) = ΣDij ψ(r), where Dij must maintain unitarity to preserve the lengths of vectors. The participants clarify that transformations do not inherently require unitarity unless they involve vector lengths, emphasizing the importance of this property in quantum state transformations.

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This is what I have so far:

part1.png
part2.png


I'm trying to show that the matrix D has to be unitary. It is the matrix that transforms the wavefunction.
 
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The matrix that transforms the wave function how? So that it preserves some property? Transformations do NOT have to be unitary unless you are trying to lengths of vectors.
 
HallsofIvy said:
The matrix that transforms the wave function how? So that it preserves some property? Transformations do NOT have to be unitary unless you are trying to lengths of vectors.

In lectures we were showing
Tψ(r) = ψ(Ur) = ΣDij ψ(r)

Dij has to be unitary and form a representation of T - I'm just trying to figure out the proof. Are you saying this is only try if you scale the position vector r?
 

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