*Question of the Week, 20.12.2010*

Richard Feynman once called the fine structure constant α “one of the greatest damn mysteries of physics: a magic number that comes to us with no understanding by man”.

Given by:

*e*^{2}/ℏ*c*

in cgs units, it contains ℏ as the reduced Planck constant, *e* as the elementary charge and *c* as the speed of light in vacuum and is thus composed of other physical constants that you can find in any physics textbook. The fine structure constant can nowadays be measured with very high accuracy. But although we know its value so precisely, no physicist knows why it takes exactly this special value. If it were just a little smaller or bigger the universe would look a lot different, e. g. stars would have never produced oxygen and carbon.

String theorists have found out that many constants of nature could take several different values without any contradiction to theory. Since string theory works with many more than our three dimensions, they look upon our measurements of the fine structure constant as just measuring the “shadow” of it in lower dimensions. And there seems to be no reason why it really be a constant in lower dimensions.

One way of measuring if the fine structure constant is really a constant is looking at quasars. Since they are so far away and the light that reaches us from a quasar has travelled for billions of years, looking at a quasar is like looking into the distant past. Comparing the light close to quasars to the light on earth might give insight into the time evolution of the fine structure constant. But so far scientists still disagree on its constancy.

References:

http://plus.maths.org/issue51/outerspace/index.html

Search for Time Variation of the Fine Structure Constant: John K. Webb et al, *Phys. Rev. Lett* **82**, 884-887 (1999) (http://prl.aps.org/abstract/PRL/v82/i5/p884_1)

Leonie Mueck