The two-slit experiment – and Bolognese Perfecto!

Richard Feynman in his element. Among the best-ever theoretical physicists.

The two-slit experiment: “In reality, it contains the only mystery [of quantum mechanics].” – Richard Feynman, physics Nobelist, safecracker, bongo player

If you really want to get to the heart of quantum weirdness – which says, yes, this is what happens, but, no, it’s impossible – you need look no further than the two-slit (or double-slit) experiment. It’s easy to describe, but impossible to explain.

  • Start with an electron gun. It fires electrons in a steady stream, or can be adjusted to fire them one at a time. The electrons are not perfectly aimed – they come out in a kind of narrow spray.
  • Several feet away, place a screen that has two narrow parallel slits cut into it.
  • In back of the slit screen, place a receptor screen that will visually record the arrival of electrons through the slits – in essence, turning the electrons into photons.

Now we start to play:

  • Cover either of the slits and fire up the electron gun. On the receptor screen, we’ll find a single band of light made up of many tiny photons.
  • Leave both slits open, and we get an interference pattern, like this:

With one slit open, the electrons behave like particles. With both slits open, they behave like a wave, with the peaks and the troughs of the wave either cancelling each other out or enhancing their intensity, spreading out on either side with diminishing brightness, like this:

And the weird part is…

  • With one slit open, electrons start as particles, go through the slit as particles, and end on the receptor screen as particles.
  • With two slits open, electrons start as particles, go through both slits as a wave, and end up on the receptor screen as particles.
  • So: how does an electron “know” when both slits are open? Fire the electron gun very slowly, say one electron every second or so, and the results are the same. That single particle is able to go through both slits simultaneously and interfere with itself – just like a wave.

Even weirder…

Now we’re going to place a detector, a measuring device, aimed at one of the two slits. This device will tell us when an electron passes through that slit. (The electron gun is designed so that a single electron hitting the receptor screen has a 50% chance of going through one slit or another. Of course, many will hit the two-slit screen and just bounce off).

We’re also going to fire the electron gun slowly – one electron at a time.

  • Leave the detector off, fire the electrons at the two slits, and over time we get the wave-like interference pattern as shown above.
  • Turn the detector on, fire the electrons, and the interference pattern disappears! Instead, we’ll get two narrow bands of light on the recording panel. The wave behavior vanishes when the particles are being observed.

Somehow, the electrons seem to “know” when they’re being observed by the detector. But this is absurd. And it is bedrock truth in quantum theory: tiny particles act differently, depending on whether or not they’re being observed.This experiment has been done many times with different particles – electrons, photons, whole atoms, and even large molecules containing up to 810 atoms. The larger the particle, the more difficult the experiment is to conduct, but the results are the same: these tiny things have wave-particle duality, which changes when they’re being observed or measured.

Main idea? It’s all a system.

The system includes the experimental apparatus, the detecting device, and the human observer. It is the core principle of the so-called Copenhagen Interpretation of quantum mechanics. If you watch it, the tiny thing stops its wave-like behavior and becomes a “real” thing.

This is as good a presentation by a physicist as you’ll ever see. About 9 minutes, and worth it:

Okay, all done, time to eat.


Bolognese Perfecto!

My favorite.

I wrote about this a long time ago, and maybe didn’t do it justice. This is a pasta meat sauce with some tomato in it, but no tomato sauce. And it’s really very simple to make.

For four people, you’ll need:

  • 1 large carrot, 1 celery stalk, 5-6 garlic cloves – all of them minced or chopped fine in a food processor
  • 2 oz. olive oil
  • 1/2 cup white wine
  • 1 (14 oz.) can diced tomatoes, mostly drained.
  • 1/2 lb. each of ground veal, beef, and pork
  • 1/4 lb. diced pancetta
  • 2-3 oz. evaporated milk
  • herbs of your choice: oregano, pepper, cayenne, etc.

Now do this:

  • In a large skillet, get the olive oil hot.
  • Add minced carrot, celery, and garlic. Saute till soft, about 5 minutes, over medium heat.
  • Add the white wine. Lower the heat and cook uncovered for about ten minutes, or until most of the wine has been cooked off.
  • Add the ground meats (including pancetta), raise the heat a bit, and cook until they’re well-browned. Drain off most of the fat (keep a little in the pan).
  • Add the diced tomatoes, evaporated milk, and the herbs/spices. Cover and cook gently for about 45 minutes, stirring occasionally.

Spoon out over pasta. Have plenty of Romano and/or Parmesan on hand!


Ned White

About Ned White

Ned White is a writer, novelist, crossword puzzle constructor, traveler through 49 states, and at times a danger in the kitchen. He lives with his wife in South Thomaston.