Suzanne McEndoo answered on 22 Mar 2012:
Big question in four words!
Quantum mechanics or quantum physics is the physics of very small things. This might be very small in size, very small in temperature, very small in energy, etc (often all of the above). It tells us that light, which we usually think of as a wave, can also be made to behave light a particle (called a photon). This is where the word quantum comes from, quantum basically means bit or discrete thing, so scientists described these light particles as quanta of light.
Similarly, if a wave can be a particle, a particle can also be a wave. So electrons, protons, even atoms can be treated as if they’re waves.
Quantum physics talks a lot about what we can know about something (or what information we can get from something) and probabilities.
1) Heisenberg’s uncertainty principle: In short, the more we know about the position of a particle, the less we know about it’s speed, and visa versa. This isn’t just that we can’t measure it well enough yet, it’s really a basic property, a physical limit that we can’t beat.
2) Superpositions: If something is in a superposition, we’re saying that it’s in two “states” at once. These “states” can mean a lot of different things. An atom in two places at once, or travelling two directions at once. Or a spin that’s up and down at the same time. This doesn’t make sense in our everyday world, so it’s hard to think about. Once someone looks at (or measures) the atom, it makes the atom be in one place or the other, and it’s no longer in both (we say that the waveform collapses, in technical speak).
3) Entanglement: If two particles are entangled, that means that they’re linked, even if there’s no way for them to communicate. So if we do something to one particle, something happens to the other particle at the same time, even if one was on earth, and the other was in another galaxy. This is really important for things like quantum codes, quantum teleportation, and quantum computers.
If none of that makes sense, you’re in good company. Nobody truly understands quantum physics, not even us quantum physicists. It’s all based on logical steps from some rather simple maths. As one scientist (Richard Feynmann) said “shut up and calculate”, which means that we just have to follow where the maths takes us, even if we don’t understand how it works physically.
Despite all that, quantum physics is the most successful theory scientists have ever come up with. No experiment has ever contradicted it, and it’s been really good at predicting things. For example, Einstein and Bose predicted what would happen when certain atoms were cooled to extremely low temperatures in 1920. It was only in 1995 that we had the experiments to do that cooling, and Einstien and Bose were proven right.
Robert Thompson answered on 22 Mar 2012:
Wow I really can’t add anything to this.
That’s a video with me discussing quantum mechanics and quantum computing
Martin Zaltz Austwick answered on 23 Mar 2012:
The physics of the very small: the atom, the photon, the electron. At these sizes the world behaves very differently from how you might expect. Quantum Mechanics rules all our lives, but in most situations it’s not obvious – it just looks like the normal world!