Quantum Physics

Chad Orzel, author of How to Teach Physics to Your Dog, gives an introduction to Seven principles of Quantum Physics.

Here's a list of essential elements of quantum physics that everyone ought to know, at least in broad outlines:

Particles are waves, and vice versa.

Quantum physics tells us that every object in the universe has both particle-like and wave-like properties....

Quantum states are discrete.

The "quantum" in quantum physics refers to the fact that everything in quantum physics comes in discrete amounts. A beam of light can only contain integer numbers of photons-- 1, 2, 3, 137, but never 1.5 or 22.7. An electron in an atom can only have certain discrete energy values-- -13.6 electron volts, or -3.4 electron volts in hydrogen, but never -7.5 electron volts.

Probability is all we ever know.

When physicists use quantum mechanics to predict the results of an experiment, the only thing they can predict is the probability of detecting each of the possible outcomes. Given an experiment in which an electron will end up in one of two places, we can say that there is a 17% probability of finding it at point A and an 83% probability of finding it at point B, but we can never say for sure that a single given electron will definitely end up at A or definitely end up at B.

Measurement determines reality.

Until the moment that the exact state of a quantum particle is measured, that state is indeterminate, and in fact can be thought of as spread out over all the possible outcomes. After a measurement is made, the state of the particle is absolutely determined, and all subsequent measurements on that particle will return produce exactly the same outcome.

Quantum correlations are non-local.

One of the strangest and most important consequences of quantum mechanics is the idea of "entanglement." When two quantum particles interact in the right way, their states will depend on one another, no matter how far apart they are.

Everything not forbidden is mandatory.

A quantum particle moving from point A to point B will take absolutely every possible path from A to B, at the same time. This includes paths that involve highly improbable events like electron-positron pairs appearing out of nowhere, and disappearing again. The full theory of quantum electro-dynamics (QED) involves contributions from every possible process, even the ridiculously unlikely ones.

Quantum physics is not magic.

Yeah, this was on the other list as well, but it's so important that it needs repeating. As strange as quantum physics is-- and don't get me wrong, it's plenty weird-- it does not suspend all the rules of common sense. The bedrock principles of physics are still intact: energy is still conserved, entropy still increases, nothing can move faster than the speed of light. You cannot exploit quantum effects to build a perpetual motion machine, or to create telepathy or clairvoyance.