## Feynman Spirals Part 3 – FAQ

Here’s a brief F.A.Q. for things I’ve been asked about the spiral & the double slit experiment.

## Q: First off, what the actual directions of the plot correspond to?

Good question.  In the original 2D graph, one axis was distance along the screen, and the other was probability of detecting an electron.

The spiral is the same, but instead of a 1D probability, it’s a 2D probability amplitude.

Directions on the Feynman Amplitude Spiral (probabilities of the double slit experiment)

## Q: What’s so remarkable and ‘quantum’ about the double slit here? Doesn’t it just describe stuff which we’re familiar with like sound waves, or water waves?

A: When you’re dealing with a ton of particles (like from a bright laser) the experiment looks pretty standard. The results could be explained by some photons going through 1, and some photons going through 2, and when they meet up at the screen they interfere with each other.

The ‘quantum weirdness’ bit comes in when we adjust the intensity of the laser down the the point where there’s only a single photon in flight at any time. (This is actually fairly easy to do, making and detecting single photons takes only a few hundred dollars worth of equipment, and this experiment is done in university physics labs all the time)

When there’s only one particle, we might expect it to have to go via a definite route (through slit 1 only, or through slit 2 only), and the interference pattern to disappear. But it turns out that single photons (also electrons) still behave in ways that show it travels via both paths simultaneously.

This is also the reason why I made the very deliberate decision (like Feynman did) to stick to the terminology of ‘probability amplitude’ & ’probability’, which is fully general to all situations where complex amplitude flows between different configurations (e.g. electrons in the double slit experiment, photons location in an interferometer, etc. )  rather than terminology which is more specific and can give us license to ignore the weirdness (such as ‘electric field vector’ & ‘light intensity’).

Or to put it another way, I wanted to stick with the terminology that helps us remember that underneath it all, the universe itself runs on Complex numbers, not Real ones.

Also see here: https://www.smbc-comics.com/comic/the-talk

Edit: Also see part 4: Why the Bragg Condition sucks

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