by chance – developed traits that allowed them to survive. The idea that all of human behavior was completely explicable in physical terms gave rise to the school of thought known as behaviorism, of which B.F. Skinner was the major proponent.
So it was probably a bit of a relief to religious folks and others concerned about free will when Heisenberg and friends came along and said, “Hey, you know how we were trying to figure out deterministic principles to explain the entire physical world? Yeah, that’s not going to work.” For one thing, it turns out that at a subatomic level, events can happen without a cause. This is kind of a freaky notion, if you think about it: the whole universe is made up of subatomic particles, and these subatomic particles can apparently do things without any reason. And if the universe is made up of particles that can do things for no reason, what’s to keep the whole universe from suddenly disappearing or, say, turning into a giant purple chicken? An article I find online provides this answer:
Fortunately there are still a lot of limitations on these particles. They can act randomly within certain strict parameters, but they can’t just do whatever they want. A particle can’t just disappear out of existence, for example, because that would violate the principle of the conservation of matter and energy. What a particle can do, though, is appear first in one spot and then in another, for no apparent reason – and without passing through the space between these two locations. This is known as a “quantum jump” or “quantum leap.” The scale on which a particle can do this is so incredibly small, of course, that you would have no way of knowing that it’s happening without an extremely powerful microscope.
Even weirder is that, as I mentioned earlier, until you observe the particle at one location or another, it actually exists in multiple places at once . It has no “definite location”; it can only be said to have a certain probability of being found at one of several locations. Another article explains it by describing what is called a “double slit experiment.” (If you’re willing to trust me that objects can have no definite location, feel free to skip this part.)
In the experiment, a beam of light illuminates a plate pierced by two parallel slits. Initially, one of the slits is covered, so light can only pass through the top slit. The light passing through the slit is observed on a screen behind the plate.
If light consisted strictly of particles, and these particles were fired in a straight line through a slit and allowed to strike a screen on the other side, we would expect to see a pattern corresponding to the size and shape of the slit. However, when this “single-slit experiment” is actually performed, the pattern on the screen is a diffraction pattern in which the light is spread out. Despite this fuzzy behavior, however, the light is always found to be absorbed by the screen as though it were composed of discrete particles or photons. So the light appears to act like a wave, spread over an area, and like a particle occupying a specific point in space.
Now the cover is removed, so light can penetrate both slits. If light consisted strictly of particles, the expected pattern on the screen would simply be the sum of the two single-slit patterns. In actuality, however, the pattern changes to one with a series of light and dark bands. We can only explain this by looking at the light as if it were made up of waves: a wave is emitted from each slit, and when the two waves come into contact, they sometimes cancel each other out and sometimes amplify each other, producing what is known as an interference pattern.
Recall, though, that each photon hits the screen as a single, discrete particle, not as a spread-out wave. How is it possible for discrete particles to produce an interference pattern? One explanation is that part of the photon