By now you might be scratching your head in frustration. That’s normal. The universe is easy to explain the less you know. The more you know the weirder things become. Let’s introduce two more central concepts, both of which add immeasurably to the weirdness.
The first is complementarity. Recall that there are bosons for carrying forces and fermions that make up matter. Photons carry electromagnetic waves like radio waves, light waves, microwaves, x-rays, gamma rays, etc. Complementarity says that anything can act like a particle or a wave depending on the circumstances. In one classic experiment, light shines through two narrow slits onto a screen. Interference patters appear on the screen as if the waves of light are interfering like waves on a pond of water. So far so good because electromagnetic radiation is thought of as a wave and that behavior is expected in waves. But if the light source is turned way down so that the light escapes one photon at a time the interference patterns still show up, as if the photon passes through both slits and interferes with itself. Richard Feynman, one of the 20th century’s leaders in quantum theory, said if you can explain this, you understand quantum mechanics. Then he added, “no one understands quantum mechanics.”
A second concept is the uncertainty principle. This was developed by Werner Heisenberg and bears his name as the Heisenberg Uncertainty Principle. It states that the mere act of observing something at the subatomic level changes its properties.
Think of what happens when we see something. Light reflects off whatever we are looking at, hits the retina in our eyes and is transmitted as an image by our brain. Similarly, when we observe something at the subatomic level, a wave of some sort reflects off the thing and makes an image that can be detected by highly sensitive equipment.
In order for something to reflect back an electromagnetic wave so that it can be detected the wavelength has to be much smaller than the object. That’s why police “radar guns” don’t really use radio waves; they use microwaves. Radio waves are much too long to give a good image of a car.
Electromagnetic waves of whatever wavelength carry energy. The longer the wavelength the less energy the wave has; the shorter the wavelength the more energy it has. So a radio wave, with a wavelength of one to several meters, has less energy than a light wave, which in turn has less energy than x-rays or gamma rays.
When a light wave hits a car some of its energy is transmitted to the car. Because the car is so massive and the energy of the light so small in comparison, there is no discernible effect on the car. But when we start trying to “see” subatomic particles using very short wavelength electromagnetic waves the energy of the wave is enough to affect the particle. As soon as the wave hits the particle, it is no longer in the same position but has moved, simply because we “looked” at it. Thus we can never be certain of a particle’s position. All we can do is express its position in terms of probabilities as to where it might be.
Complementarity and the uncertainty principle are linked. Particles have a definite position while waves have a definite direction and momentum. Since anything can exhibit both “particleness” and “waviness” at the same time, trying to observe anything changes one or the other of these properties. When we determine the position, it comes at the expense of knowing its direction and momentum and vice versa.
Einstein was frustrated by this lack of certainty, which led to his famous statement that God does not play dice with the universe. However, quantum mechanics is nearly a century old and has stood the test of time. The universe is governed by probabilities, like it or not. Nothing is real until it is observed. An electron is a wave of probability until it is observed, at which time it collapses into a finite reality.