Question: How does the sounds created by the atoms tell you what they are doing?

Daniel Roach answered on 12 Mar 2014:

Really good question!

Imagine a crystal is a bell – if you hit it with a hammer, the sound it makes is determined by it’s shape and the kind of metal it’s made from. Let’s pretend the shape makes no difference, for now, and concentrate on the metal…

The atoms that make up the metal are all regularly spaced, in a repeating pattern called a crystal lattice. You could imagine a load of springs between the atoms, like those in a matress, joining these atoms. The atoms naturally jiggle about, but are kept together by the forces in these springs.

Now, if you ‘ring’ that bell with a neutron, a sound wave (we call these ‘phonons’) ripples through the crystal. If we keep ringing the bell with more neutrons, the atoms jiggle a bit more, but because we know the direction we fired the neutron, when we catch it again (in a detector), we know how much energy has gone into the crystal and so we also can work out (with a whole load of maths!) how strong the springs are between the atoms. We can then make a model in a computer of the atoms-jiggling-with-springs and see if this gives us the same as the experiment. If it does, great! We know how strong the springs are (the forces between the atoms are the springs). If the computer springs are different, then that’s ok – we just change them in the computer and try again.