The Speed of Light Rock

I spent a few hours on the interstate this weekend, and I heard a Kid Rock song on the radio called "All Summer Long". If your tastes are anything like mine you'd probably rather not hear it. The song describes Kid's life as an 18-year-old, when "it was summertime in northern Michigan". Basically he does several irresponsible things in the chorus, topping the list off with the assertion that these things were done while "singing Sweet Home Alabama all summer long".

Come on, Kid. You were closer to thirteen-letter-expletive Quebec than you were to Alabama. Your lyrical choice lacks a certain... authenticity. When I was 18, I lived in Baton Rouge, Louisiana. We were probably a third the distance to Alabama but we never felt the need to import some other state's song. If anything we felt sorry for those poor people with the singular misfortune to live in a state containing both Auburn and the Crimson Tide. (kidding!)

Sadly, physics can't stop weird bits of culture traveling the miles between Alabama and the mind of an impressionable young rocker. But it does limit the speed of movement to the speed of light.

Normally this limit of influences traveling between points isn't terribly significant in daily life. Conscious human perception is hard pressed to detect processes which happen on timescales much under a few dozen milliseconds, and even in one millisecond light manages to travel quite a long way. 186 miles, or 300 kilometers if you prefer.

On the other hand, the processor in your computer is probably flipping along a few billion times per second. A 3 GHz processor executes each clock cycle in an average of 0.3 nanoseconds, and light doesn't get very far in that time even by human standards. It travels about 4 inches. The inside of a CPU is a lot smaller than that, but you can see how real-time communication between (say) the RAM and the processor is already precluded by fundamental physics.

Even nanoseconds are pretty long by physics standards. Chemical reactions can occur on timescales of a few hundred femtoseconds. In one femtosecond light travels roughly the width of a few thousand atoms. Therefore the process of two molecules snapping together to form a single molecules usually takes a lot longer than it does for light to carry information from one side of the molecule to the other. But physicists have been only comparatively recently able to directly produce light on such short timescales in order to "look" at those reactions in progress using techniques like pump-probe spectroscopy.

But that's virtually slow motion compared to events in particle physics. The lifetime of the W and Z bosons are on the order of 10^-25 seconds. In that time, light travels only about a 10th the diameter of a proton. Anything that occurs so quickly can't possibly take place over a larger distance. As a result, those particles pretty much blow themselves apart as soon as they know they exist.

We'll probably someday be able to study even shorter timescales. The record for short light pulses is constantly shrinking, and the LHC could very possibly generate particles with lifetimes shorter than the W and Z bosons.

Preventing questionable music is unfortunately somewhat beyond our present capabilities. But we're working on it.