It was a cute and humorous article in FermiNews written by Sharon Butler and Arnold Pompos, about Santa at nearly the speed of light (Volume 21, No. 23, Dec. 11, 98, available on the Fermilab web site at http://www.fnal.gov/directorate/public_affairs/ferminews/santa/). I believe, however, that the physics is all wrong.

The article claims that light travels slower in transparent media than it does i n free space. In my view, light doesn't slow down at all: Light actually zigzags from atom to atom as it traverses a transparent medium. Hence it travels a longer distance compared to going straight, leading to an apparent decrease in speed.

The story also states that astronomers take advantage of the Doppler effect to figure out the speeds with which stars and galaxies in our expanding universe are moving with respect to us. Yet the expansion of the universe causes a redshift, not a Doppler effect.

Last but not least, the article talks about the length (Lorentz) contraction, the apparent foreshortening of a body traveling at high speed. I think that the Lorentz contraction is misnamed and is rather a rotation than a contraction: If a cube passes you at nearly the speed of light you would see some amount of the leading face (depending on exactly how fast it's moving) and the square side would appear foreshortened.

Sincerely,

Douglas George

Dear Douglas:

Thank you for bringing up the topic. Events at or near the speed of light are really some of the most fun and surprising of all physics phenomena. They are quite different from our day-to-day experience, and our intuition often fails us. So we'll try to give some additional insight into Santa's relativistic world.

Zigzagging light?

It is tempting to think of light traveling through a medium as light particles (photons), bouncing back and forth between atoms. This way of thinking, however, does not always lead to an easy understanding of all experimentally observed phenomena related to light. For example, a material with high density (heavy and/or closely packed atoms) does not necessarily slow down light more than a less dense medium, despite the fact that one would expect more scattering to take place inside the denser medium.

To easily understand the reduction of the speed of light inside matter, physicists take advantage of light's ìsplit personality.î Light can behave like a particle and like a wave.

Traveling through matter, light waves interact with the electrons of the material. The theoretical description is based on the famous Maxwell's equations. (See Fermilab's popular science descriptions at http://www.fnal.gov/pub/light/ for more details on topics related to light.) Knowing the electromagnetic properties of a material, such as the dielectric constant, scientists can compute the reduced speed of light, the speed at which light waves travel through matter.

Doppler effect vs. redshift

The Doppler effect, originally a theoretical prediction, was first observed for sound waves, in 1845, long before the sirens on police cars made this a publicly known phenomenon. It took another 60 years until the Doppler effect was also observed for light, in experiments with beams of light- emitting ions. The spectral lines measured in such experiments are shifted towards the blue end of the spectrum if the ions are moving toward the receiver, and to the red end if the ions are moving away from the receiver. The results are in agreement with the special theory of relativity.

When looking at light emitted by a star, for example, things are more complicated. In addition to a Doppler shift, which is present if a star moves through space, astrophysicists also observe gravitational and cosmological redshifts in the star's spectral lines. These can only be explained in the framework of the general theory of relativity.

The gravitational redshift refers to the shift in color that occurs when light is emitted by a massive object, say a star. It is due to the energy loss that light suffers when it travels against the star's strong gravitational field.

The cosmological redshift, which you mention, is due to the expansion of space itself. It plays a major role in the search for galaxies and quasars deeply hidden in our universe. (See related story on page 12.)

The world at near the speed of light

The Lorentz contraction cannot be explained by a simple rotation in three-dimensional space. Calculations must be based on the theory of relativity, and must be carried out in four-dimensional space-time.

Nevertheless, you brought up a very interesting question: How does Santa see the world when he travels near the speed of light?

Research groups at the universities of Stuttgart and Tuebingen, Germany, work on the visualization of effects, taking into account the finite speed of light. They created animations showing various objects when observers pass by at near the speed of light. Check out the movie gallery at http://www.tat.physik.uni-tuebingen.de/~weiskopf/sr/; we also offer some examples at right.

Sincerely,

Arnold Pompos and Kurt Riesselmann