Assume we had a particle which initially was at position ๐ ๐โR iโ โ . Now, after time ๐กt has passed, what is the probability of finding that particle in a position ๐ ๐โR fโ โ ?โจ๐ ๐โโฃ๐(๐ก)โฃ๐ ๐โโฉโจ R iโ โ โฃU(t)โฃ R iโ โ โฉโจ๐ ๐โโฃ๐ ๐โโฉโจ๐ ๐โโฃ๐ ๐โโฉโจ R fโ โ โฃ R iโ โ โฉโจ R iโ โ โฃ R fโ โ โฉโจ๐ ๐โโฃ๐(๐ก)โฃ๐ ๐โโฉโจ R fโ โ โฃU(t)โฃ R iโ โ โฉโจ๐ ๐โโฃ๐(๐ก)โฃ๐ ๐โโฉโจ๐ ๐โโฃ๐(๐ก)โฃ๐ ๐โโฉโจ R fโ โ โฃU(t)โฃ R iโ โ โฉโจ R iโ โ โฃU(t)โฃ R fโ โ โฉโจ๐ ๐โโฃ๐ ๐โโฉโจ R fโ โ โฃ R iโ โ โฉ
Question
Assume we had a particle which initially was at position ๐ ๐โR iโ โ . Now, after time ๐กt has passed, what is the probability of finding that particle in a position ๐ ๐โR fโ โ ?โจ๐ ๐โโฃ๐(๐ก)โฃ๐ ๐โโฉโจ R iโ โ โฃU(t)โฃ R iโ โ โฉโจ๐ ๐โโฃ๐ ๐โโฉโจ๐ ๐โโฃ๐ ๐โโฉโจ R fโ โ โฃ R iโ โ โฉโจ R iโ โ โฃ R fโ โ โฉโจ๐ ๐โโฃ๐(๐ก)โฃ๐ ๐โโฉโจ R fโ โ โฃU(t)โฃ R iโ โ โฉโจ๐ ๐โโฃ๐(๐ก)โฃ๐ ๐โโฉโจ๐ ๐โโฃ๐(๐ก)โฃ๐ ๐โโฉโจ R fโ โ โฃU(t)โฃ R iโ โ โฉโจ R iโ โ โฃU(t)โฃ R fโ โ โฉโจ๐ ๐โโฃ๐ ๐โโฉโจ R fโ โ โฃ R iโ โ โฉ
Solution
The question seems to be asking for the probability of finding a particle at a certain position after a certain time has passed, given its initial position. This is a common question in quantum mechanics, and the answer involves the time evolution operator U(t).
The probability amplitude of finding the particle at position ๐ ๐โ after time t, given that it started at position ๐ ๐โ, is given by the matrix element โจ๐ ๐โโฃ๐(๐ก)โฃ๐ ๐โโฉ. This is the overlap of the state |๐ ๐โโฉ evolved forward in time by U(t) with the state |๐ ๐โโฉ.
The probability is then given by the absolute square of this amplitude, i.e., |โจ๐ ๐โโฃ๐(๐ก)โฃ๐ ๐โโฉ|^2.
Note that โจ๐ ๐โโฃ๐(๐ก)โฃ๐ ๐โโฉ is the probability of the particle remaining at the initial position after time t, and โจ๐ ๐โโฃ๐ ๐โโฉ and โจ๐ ๐โโฃ๐ ๐โโฉ are the overlaps of the initial and final states, which are generally not relevant for this calculation unless the states are not normalized.
Also, โจ๐ ๐โโฃ๐(๐ก)โฃ๐ ๐โโฉ and โจ๐ ๐โโฃ๐(๐ก)โฃ๐ ๐โโฉ are generally not equal because U(t) is not necessarily a Hermitian operator, so the order of the states matters.
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