Quantum mechanics was the most important scientific discovery of the twentieth century. It correctly describes everything we see around us with astonishing accuracy. It explains how all the atoms in your body behave, how molecules form and why things have colour. And it is needed to create the electronics that power your computer.
Quantum mechanics was formulated in the 1920s and first used to describe atoms such as hydrogen. Energy exists in tiny packets, known as quanta. Particles only have energy in discrete numbers of quanta, and nothing in between, occupying an energy ‘ladder’. We don’t experience this in everyday life because one quantum of energy is incredibly small. Indeed the quanta are the set by the size of Planck’s constant ħ (‘h-bar’) , approximately 0.0000000000000000000000000000000001 Joule seconds.
According to the uncertainty principle we can never determine the exact position and speed of a particle at the same time. Even with the best conceivable equipment, either the measurement of speed or the measurement of position would be imprecise. This has a profound effect on our understanding of energy.
The uncertainty principle forces everything to have a small amount of energy, even at zero temperature. We call this zero-point energy. Even the seemingly empty vacuum of space is teeming with energy, which causes particles to pop in and out of existence.
Quantum mechanics also shows there is a fundamental bond between waves and particles. That is, sometimes things can behave like both! Physicists refer to this as wave-particle duality.
Unlike general relativity, quantum mechanics is a probabilistic theory. This means it only tells us the chance that a measurement has a certain outcome. Making an observation forces a system to choose one possible value. Physicists have confirmed this behaviour experimentally. The values we obtain are just throws of the quantum dice.
This has some interesting philosophical implications: before a measurement is taken, can we truly say that a particle exists? This is still one of the great mysteries in modern physics. We don’t know what the quantum world looks like when we have our backs turned.
We know that quantum mechanics accurately describes the world we see, so we want to make all of our other physical theories compatible with it. We call these new theories, quantum theories, whilst theories which do not incorporate quantum mechanics are classical theories.
Over the past 50 years physicists have developed quantum theories of three of the four fundamental forces. But so far gravity has resisted. A new theory may reshape our view of the quantum world, allowing us to include classical gravity. And perhaps it will answer some of the deep unresolved questions at the heart of quantum mechanics.