- #1
K space
- 5
- 0
I'm new to the concepts of quanum mechanics and the bra-ket representation in general.
I've seen in the textbook that the compleatness relation is used all the time when working with the bra and kets. I'm a bit confused about how this relation is being used when applied more than once in a single expression, and why it's always applied as summations/inegrals over different eigenvalues then.
For example, consider:
[itex]<ψ|\hat X|ψ>[/itex]
Expanding ψ in a basis:
[itex]|ψ>=\sum_{a}c_a|a>[/itex] and [itex]<ψ|=\sum_{a}c_a<a|[/itex]
which gives
[itex]<ψ|\hat X|ψ>=\sum_{a}c_ac^*_a<a|\hat X|a>=\sum_{a}c_ac^*_aa<a|a>[/itex]
In the book they would insted have two different sums for the bra and the ket:
[itex]<ψ|\hat X|ψ>=\sum_{a}\sum_{b}c_ac^*_b<b|\hat X|a>=\sum_{a}\sum_{b}c_ac^*_ba<b|a>[/itex]
Will these two expressions be equal each other? If that's the case, why is it always introduced summations over different eigenvales as in the latter case? It feels like I'm missing something fundamental here.
I've seen in the textbook that the compleatness relation is used all the time when working with the bra and kets. I'm a bit confused about how this relation is being used when applied more than once in a single expression, and why it's always applied as summations/inegrals over different eigenvalues then.
For example, consider:
[itex]<ψ|\hat X|ψ>[/itex]
Expanding ψ in a basis:
[itex]|ψ>=\sum_{a}c_a|a>[/itex] and [itex]<ψ|=\sum_{a}c_a<a|[/itex]
which gives
[itex]<ψ|\hat X|ψ>=\sum_{a}c_ac^*_a<a|\hat X|a>=\sum_{a}c_ac^*_aa<a|a>[/itex]
In the book they would insted have two different sums for the bra and the ket:
[itex]<ψ|\hat X|ψ>=\sum_{a}\sum_{b}c_ac^*_b<b|\hat X|a>=\sum_{a}\sum_{b}c_ac^*_ba<b|a>[/itex]
Will these two expressions be equal each other? If that's the case, why is it always introduced summations over different eigenvales as in the latter case? It feels like I'm missing something fundamental here.