Top Eleven: Rudolf Moessbauer

The penultimate experiment in the Top Eleven brings us up to the first nominee who's still with us..

Who: Rudolf Moessbauer (1929-present) (that's Mössbauer with a heavy-metal ö), a German physicist. (The Wikipedia link is for consistency with the other posts, but contains very little information. A better bio is available from the Nobel Prize site.)

When: 1957-58.

What: This one requires a bit of background, so there will be more below the fold, but basically, he's nominated for discovering an effect that makes it possible to do precision spectroscopy of nuclear transitions.

Spectroscopy is the chief experimental technique of atomic and molecular physics, and involves determining the energy states of atoms or molecules by looking at the light absorbed or emitted when they change states. The frequency of light absorbed or emitted is determined by the energy difference between the two states, and those differences are unique to each element (a fact that was recognized long before it was explained). Measuring the absorption and emission spectra of a given atom allows you to determine the energies of the allowed states in that atom, and can provide a sensitive probe of interactions which perturb those energy states.

The same basic physics applies to the states of atomic nuclei, but the energies involved are much higher (X-rays and gamma rays, rather than infrared and visible light), which creates a problem. The photons that are absorbed and emitted in these transitions carry momentum as well as energy. An atom absorbing or emitting a photon gets a "kick" in some direction, changing its velocity. And as part of this process, the light absorbed or emitted ends up being Doppler shifted away from the frequency you would expect without the recoil.

In the case of atoms absorbing and emitting visible light, the recoil shifts are completely insignificant-- a typical atom will absorb light whose frequency is within a few megahertz of the resonance frequency, and the recoil shifts are generally a few kilohertz. This isn't the case with nuclear transitions. The recoil shift involved in the emission or absorption of a gamma-ray photon is much larger than the width of the transition. This makes it extremely difficult to do gamma-ray spectroscopy, because the recoil shift on emission is large enough that another nucleus of the same type will not absorb the emitted photon, and the absorption shift is so big that you can't resolve the fine details of a spectrum.

What Mössbauer discovered is that, in certain situations, you can dramatically reduce the recoil shift. The shift in atomic transitions is very small because the atomic mass is very large, and the momentum involved is small. In nuclear transitions, the momentum involved is much larger, but if you embed your nuclei in a solid crystal, you can effectively increase the mass, as the whole crystal lattice recoils. Such "recoilless" emission allows you to produce photons that will be absorbed by other nuclei, and "recoilless" absorption allows you to see a very narrow transition width.

Why It's Important: The Mössbauer effect is important because it allows nuclear physicists to probe the states of nuclei and the interactions between nuclei and their environment in some of the same ways that atomic physicists probe the states and interactions of whole atoms. You can vary the frequency of a Mössbauer source by moving it back and forth in a controlled manner, and using the Doppler shift to produce a range of different frequencies, and then look for absorption by other nuclei, in the same way that atomic physicists will scan the frequency of a laser, and look for absorption by atoms. (Happily, I don't have to scan the frequency of my laser by running back and forth across the lab with it...)

This enables you to make extremely precise measurements of small effects that change the internal states of the nucleus. For example, you can look at the splitting of energy levels in iron nuclei due to the local magnetic field, an experiment that several of our students are going to get to know very well in the next couple of weeks.

On a more exotic level, the Mössbauer effect enabled a precision test of General Relativity. Physicists at Harvard sent gamma rays up and down a tower, and were able to measure the gravitational red shift by looking at the shift of the absorption lines for upward-going and downward-going photons. The shift is at the level of a few parts in 10¹⁵, so this is about as good as anything you can manage with atomic spectroscopy.

(There's an official Mössbauer Spectroscopy webpage, if you'd like to learn more.)

Reasons to Vote for Him:: The Mössbauer effect enables precision studies of a wide range of small effects through spectroscopy on nuclei. Testing General Relativity is way cool.

Reasons to Vote Against Him: Most of the really cool experiments were done by other people using his technique. A bone-deep hatred of spectroscopy in general.