You know that if you hear an ambulance siren pass you on the road. Bzzzz as it goes past. Or a train whistle, when it’s blowing. That is the Doppler effect, because when a sound emitter, in this case, is moving towards you, the sound waves are compressed, so that the pitch of the tone rises. When it’s disappearing away from you, it is attenuated, and the note drops in tone.

The gamma ray is also electromagnetic radiation, like light, but of a very much shorter wavelength, very much more energetic, in fact. And so you can get devices which move backwards and forwards with particular wave shapes, and they will be a Doppler velocity which gives more and less energy, just around the absorption energy. So one gets a curve, or perhaps I’ll start this way… there is the baseline, comes down like that.

And magically, that works… well, Mössbauer found it first in iridium. It turns out that iron is by far the easiest, the 57 isotope of iron is the pre-eminent Mössbauer absorber, and that’s of course the one that we started on as well, when I’d heard about this.