Quantum Tea

Thoughts

Quantum Mechanics

Part One

If a tree falls in the forest and there's no-one there to observe it, is it real? George Berkeley (March 12 1685 to January 14 1753) said "Esse est percipi" ("to be is to be perceived") which fits nicely with quantum mechanics, where observing an experiment can change the outcome. Ronald Knox wrote this pair of limericks to sum up Berkeley's view:

There was a young man who said "God,
I find it exceedingly odd,
That the willow oak tree
Continues to be
When there's no one about in the Quad."

"Dear Sir, your astonishment's odd,
For I'm always about in the Quad;
And that's why the tree
Continues to be.
Signed Yours faithfully, GOD."

Quantum Mechanics was one of my favourite subjects at university, partly because of the cool lecturer, Jim Al-Khalili.

Part Two

The double slit experiment is the classic example of light behaving like a wave. You have a light source at one end of the table. The light source is pointed at a narrow slit in a piece of plastic. Only the light going through that slit enters the rest of the experiment. Beyond that slit, you have another piece of plastic with parallel slits. Beyond that, you have a screen (the blue line) where the light pattern is projected.

Double slit experiment.

The pattern you get is a series of dark and light bands, not two patches of light. You get those bands because light has wave-like behaviour, it's the same thing that would happen if you were in a big swimming pool and had a wave generator instead of a light source. There would be alternating peaks and troughs at the far end of the pool as the waves bounce off each other, sometimes they would cancel each other out, sometimes they would reinforce each other. It's called an interference pattern because the waves affect each other. The process of the waves going through the slits and spreading out is called diffraction. So light is a wave?

You can do the same experiment, and get the same results, with electrons, though you need much smaller slots. This is a problem. If particles can behave like waves, how do we know they're really particles? And if waves can behave like particles, are they really waves? Light comes in discrete chunks called photons, the smallest unit of light energy you can get, and you can think of it like a stream of massless particles that behave like waves. Light is wave and particle at the same time, you can prove experimentally that both states are true. The odd thing is that you can fine-tune your light source so that only one photon goes through the system at a time, and you still get the interference pattern. Even when there's logically nothing to cause the interference. Welcome to wave-particle duality.

Part Three

Going back to the double slit experiment post, suppose you run the experiment with a light source letting only one photon through the system at a time. If the light is a particle, there are no other particles in the system for it to interact with. You still get the same interference pattern, though it takes a long time to build up as each photon lands on the screen. Physics World has more details on the experiment but it's more technical.

Suppose you could set up a system that would prove which slit your individual particle went through. Turn on that system, and the interference pattern vanishes. Take it away, and the pattern returns. Werner Heisenberg said, "The path [of the electron] comes into existence only when we observe it." Heisenberg is remembered for the Heisenberg Uncertainty Principle, which limits how accurately we can measure things: "The more precisely the position is determined, the less precisely the momentum is known in this instant, and vice versa." You can't get both measurements with total accuracy, and Heisenberg found an equation governing how accurate you can be. This doesn't come into play in everyday life because the precision you use when you're looking at subatomic particles is so much greater than precision in knowing where your car is and how fast it's going.

Observing the experiment changes the result. How does the experiment "know" I'm watching it? What if the light came from a star light years away, diffracted around a black hole instead of through a slit. Does determining which side the light came from alter its path in the past? We don't know for sure, and that's why I love quantum mechanics.