http://physicshelpforum.com/showthread.php?t=11542Another thing about this that students will need to learn is what happens when a photon whose momentum has some uncertainty to it.

Point for discussion.

What does this mean?

Does it mean that the (quantum) object does not have an exactly defined momentum?

Or does it mean that there is an exact number, just that we can never know it?

The UP arises naturally in the pure mathematics of the Schrodinger Wave equation or Dirac's relativistic version, as a result of non commutativity of operators.

So mathematically the quoted uncertainty means that the calculated momentum doesn't satisfy the equation perfectly. i.e. the equation is not a perfect model.

Physically we can interpret this as noting that we get slightly different results (solutions) depending upon whether we calculate the momentum first or the the position first. This slight difference is, of course, the uncertaintly.