Illustrated Physics
physics, drawn
Concept

Matter waves — where it appears

The waves that describe the motion of massive particles, with a wavelength equal to Planck's constant divided by the momentum. They make electrons, neutrons, atoms and molecules diffract and interfere, and their short wavelengths are what electron microscopes and atom interferometers use.

Named by 2 essays across 2 fields — each of them below, with the objects they name alongside it.

The ripple that sits on top of an absorption edge. The Cu K absorption of copper foil, 293 K through its edge and for seven hundred electronvolts above it, with the smooth atomic background it would have if the absorbing atom were alone drawn beneath it. The difference between the two is the fine structure: a modulation reaching 11 per cent, dying away as the photon energy rises, and entirely absent from a free atom. It is there because the ejected electron is a wave that the neighbouring atoms scatter back onto the atom that emitted it, so the absorption depends on whether the returning wave arrives in step with the outgoing one — which depends on the distance to the neighbour and on nothing else about the sample.

The ripple that counts the neighbours

An absorption edge is drawn as a step and it is a step with a ripple on it — a modulation of eleven per cent in copper, five in a zinc site buried in a protein. The ripple is the ejected electron's own wave, scattered back onto the atom that emitted it, so its period is a distance. It is the only way of measuring where an atom's neighbours are that does not need a crystal.

waves · Attenuation The same fringes from cooler and hotter molecules. The rate at which molecules arrive across a few periods of the interference pattern behind the last grating, relative to its mean, for molecules at 1000, 2500, 3000 K, each with the visibility the emission model gives: 46 per cent, 14 per cent, 4 per cent. The cooler molecules draw fringes nearly as strong as cold ones; at 3000 K the pattern is a faint ripple on a flat background. The average number arriving is the same in every case: the heated molecules are not lost, and none of them has been deflected out of the beam. They arrive in the same places on average and no longer interfere.

The molecule that interferes until it glows

A molecule of seventy carbon atoms, sent through a matter-wave interferometer, travels both arms at once and lands in fringes, as an electron or a neutron does. Heat it, and the fringes fade. Nothing has touched it from outside: a hot molecule glows, and each photon it gives off is a messenger that may carry away the information of which arm it was in. Whether it does depends on one comparison — the photon's wavelength against the separation of the arms. Long photons leave the fringes alone; short ones erase them. The loss of interference that is usually described as the onset of the classical world is here a measurement made by the molecule's own light, and its threshold can be computed.

quantum · Matter waves

Named alongside it

The objects these essays reach for when they reach for this one.

InterferenceAbsorptionAttenuationBinding energyCoordination numberDebye waller factorDecoherenceFourier transformFringe visibilityHeisenberg microscopeLocal structureMean free path

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