Heisenberg’s uncertainty principle is the foundational concept of quantum theory. It’s also commonly misunderstood, which leads to a great deal of confusion about what quantum theory really says about the universe.
The uncertainty principle is often presented in terms of an observer effect. Suppose you want to measure the position of an electron. One way to do this is to shine light in the electron’s direction. When a photon scatters off the electron, you can measure how it scatters and determine where the electron is. But of course when the photon scatters off the electron it would cause the electron to scatter off in some direction. Measuring the position thus makes the electron’s motion (momentum) somewhat uncertain. Since any measurement of an electron’s position or momentum would make one or the other uncertain, there is a limit to what can be measured about the electron.
While that makes a nice intuitive picture, it’s completely wrong. The uncertainty principle isn’t a limit on what you can measure, but an inherent property of quantum objects. The reason you can’t precisely measure the position and momentum of an electron is not because you’re experiment is sloppy, but because electrons don’t have a precise simultaneous position and momentum.
Heisenberg’s uncertainty is what leads to all the strange aspects of quantum objects, such as particle-wave duality and quantum tunneling. Unfortunately, quantum systems are often portrayed as weird things that keep changing the rules to keep you in the dark, or only become real when you look at them. Such descriptions assume that quantum systems should behave like everyday objects. But the universe is far more subtle. The everyday, common sense ideas we have about the world are often only rough approximations that human-sized objects seem to follow.
To really understand the universe, we sometimes have to view things through Heisenberg’s mirror.