In the last post I talked about a measurement we did in our lab to characterize some properties of two parallel laser beams. The theory, if you want to dignify that equation with such a title, gave the power of the two beams as a function of mow much of the beams we cut off with a moving opaque block at position x.

The equation had 6 constants – the power of each beam, the width of each beam, the separation of the beams, and background power with no beams. Compare this to Newton’s law of gravitation:

For two masses m1 and m2 separated by a distance r, this equation gives you the force. If you have two known masses and you know their separation and the force of their attraction, you can solve for the single constant G. That G is the only *free parameter* in the theory. Once G is known for one set of masses, it will be the same regardless of what other m’s and r’s you may plug into the equation. Every single instance of masses producing a gravitational force has to satisfy that equation or the theory is wrong. The fact that is does is impressive.

Our toy theory of blocked laser beams has to fit similar requirements: once those 6 free parameters are set by fitting methods, the equation has to give the right answer for however many data points we choose to measure. And it does, to reasonable accuracy.

The number of free parameters in a theory is something that physicists prefer to keep low. You could, for instance, write down a 365-term polynomial equation that perfectly described the last year worth of closing prices on the NASDAQ, but it would be worthless as a predictive theory because you have so many free parameters you could perfectly fit *any* 365 data points. A theory is more likely to be right (and less likely to be wishful thinking) if it fits large amounts of data with few free parameters.

The standard model in particle physics, for instance, has something like 19 free parameters. That’s very small compared to the torrents of experimental data that it manages to fit quite well, but it’s a much larger number than most successful physical theories. Many physicists spend a lot of time trying to think of deeper theories which would relate those parameters to each other such that in fact maybe there would be only a few (or one! or zero!) free parameters which still manage to fit all the data. So far, no dice.

*Bonus question: Maxwell’s equations in SI units appear to have two free parameters, μ _{0} and ε_{0}. In CGS units they appear to have just one – he vacuum speed of light c. In CGS with “natural” units (i.e., c = 1), they appear to have zero. How many free parameters do Maxwell’s equations actually have?*