8 - Optical Interference

Summary

Overview

We have uncovered the first central principle of quantum mechanics, which is that the outcome of an experiment cannot, in general, be predicted exactly; only the probabilities of the various outcomes can be found. In particular, for the magnetic arrow of a silver atom, we know:

If m_z has a definite value, then m_x doesn't have a value. If you measure m_x, then of course you find some value, but no one (not even the atom itself!) can say with certainty what that value will be — only the probabilities of measuring the various values can be calculated.

How do you like it? Do you feel liberated from the shackles of classical determinism? Or do you feel like Matthew Arnold, who wrote in Dover Beach that

… the world, which seems

To lie before us like a land of dreams,

So various, so beautiful, so new,

Hath really neither joy, nor love, nor light,

Nor certitude, nor peace, nor help from pain;

And we are here as on a darkling plain

Swept with confused alarms of struggle and flight,

Where ignorant armies clash by night.

Regardless of your personal reaction, it is our job as scientists to describe nature, not to dictate to it!

In particular, we know that the model of a magnetic needle as an arrow, so carefully developed in chapter 2 and so correct within the domain of classical mechanics, must be wrong.