Thursday, April 16, 2009

A small kindling of the vanities

Just completed the third computer assignment, iCMA53, for SM358 - Quantum Mechanics with the Open University.

I began yesterday and after a promising start got stuck on two questions. What does stuck mean? I calculated the answer, typed it in and got the response "You are incorrect". An unwanted hint followed, plus a deduction of three scoring points. I tried again. Still wrong. So I left the two problem questions, moved on, got some more right, and then abandoned the exercise for the day.

This morning I completed the rest of the assignment with no problems and returned with a fresh eye to my two problematic questions.

The first requested a numerical value which I had entered as 0.76 to 2 s.f. The correct computation is 0.75 to 2 s.f. There followed a small period of self-laceration as I called myself a complete, careless idiot for failing to accurately truncate what I had read off the calculator screen.

The second question was about the time evolution of a wavefunction in a harmonic well. Given a specific wavefunction (a superposition of the 2nd and third eigenfunctions) at t =0, what was the wavefunction half a period later (π/ω0)? Iworked this out with exquisite care and entered my response ... only to get "Your answer is incorrect" to the point where there were no more hints. I had flunked it! I was advised by the system that I needed to spend more time studying the principles of wave packet superposition. Yeah, right.

I looked at their model solution - how had my answer been out by a factor of i?

Ah, I see it now. In the question, one of the eigenfunctions had i as a coefficient, which in my careless way I had entirely missed. Standard bug: misread the question.

I was so irritated with my carelessness, and with the rigidity of the computer system's misdiagnosis of my problem that I hastened to vent my indignation here. Oh vanity.

Perhaps I should get out more.