# Q & A: various odd creatures

Q:
1) Could you please post the pictures of Mike W. and Tom? Thanks. 2) What is the smallest frequency for a photon? Is it 1 Hz? What are the especial properties of such a photon? Is Plank’s constant the energy of a 1 Hz photon? 3) A rain cloud is made of water droplets. What is an electron (cloud) made of?
- Mehran (age 53)
Lisle, Illinois
A:
Mehran- Good to hear from you again.

(1) Our pictures (outdated, fortunately) are available from
http://www.physics.uiuc.edu/People/index.html.

(2) To the best of our knowledge, there is no smallest frequency for a photon. We work all the time with ones at much less than 1 Hz. Planck's constant times 1 Hz does give the energy of a 1 Hz photon. However, Planck's constant by itself does not have the same dimensions as energy and thus cannot give an energy until multiplied by a frequency.

(3) That's a really nice question. It has, to the best of our current knowledge, no answer. We know how to describe the electron mathematically and to say how it will behave in various circumstances. We're not sure that there's anything else to say about it.

We suspect that there may be more to say, but that if we end up saying something like "The electron cloud is made of strings" you'll come back with the same question about them, and we'll give the same blank answer we now give about electrons.

Mike W.

Well, as long a strings got mentioned, it's worth talking about the things we know about electrons at lower energy scales (larger distance scales). As far as we can measure in the lab, electrons have no discernible substructure (that's not to say there isn't any, it's just that our accelerators aren't powerful enough, or that we haven't been able to devise an experiment to answer that that doesn't involve a big accelerator). An electron may have some "pieces" inside that are so tightly stuck together we cannot even knock them up to the next energy level (like knocking an electron up to a higher-energy state in an atom), let alone shake an electron apart into its constituent pieces. We've tried, though.

String theory describes substructure on a much, much smaller scale than has been probed by our accelerators so far. So there could even be a few layers of structure to an electron before you get down to the strings if any of that actually exists.

More on our own energy scale, and those accessible at today's (and yesterday's) accelerators, each electron has a cloud of photons, more electrons, and more exotic stuff buzzing virtually around it. These spontaneously appear (the fermions, like electrons, always in pairs, such as e+e-), and the pairs annihilate quickly and the photons get reabsorbed, all obeying the laws of quantum mechanics and special relativity. If you hit an electron really really hard, you can shake a few of these extra particles loose and observe them in a detector. This cloud of particles surrounding an electron even has tiny effects on the energy levels in atoms -- the big contribution comes from the e+e- pairs preferentially aligning themselves to screen out the electron's charge nearby the electron. One observable consequence of this effect, named "vacuum polarization" is the Lamb shift, a tiny change in atomic energy levels.

Tom

(published on 10/22/2007)