Built on Facts

The Speed of Light

May 8th, 2008 · No Comments

The speed of light is both one of the most famous and one of the most fundamental quantities in physics. Even people who don’t really follow physics can often tell you how fast it is. 186,000 miles per second is the figure usually quoted. We know that the speed of light is the same in any reference frame (a story for another day!), and that fact has been used to actually define the meter as a unit of distance. It works like this:

The speed of light is exactly 299,792,458 meters per second. We know this because we’ve defined the meter as whatever distance light can travel 299,792,458 of in one second.

That’s pretty fast. But of course the universe is a big place, so even light can take a long time to travel between planets, stars, and galaxies. What’s just as interesting but less popularly known is that there are also things that happen so fast that light can’t travel very far during such a short time. How about the computer you’re reading this on? It probably has a processor with a clock rate on the order of 1GHz. That means each cycle takes a billionth of a second. In that time, light will only travel slightly less than a foot. Looked at another way, the speed of light is very close to 1 foot per nanosecond.

This puts an upper limit on how fast computers can process information, by the way. If your RAM chips are 6 inches from your CPU, if things go much above 2GHz there’s no way even in theory that those chips can communicate with each other in real time compared to how fast they process information internally. Now in practice this is not nearly the limiting factor in computing speed. There’s plenty of other obstacles which are much more relevant to why a computer is as fast (or slow) as it is. It is an interesting way to demonstrate that light - fast as it is - is still not necessarily as fast as we might like in certain applications that don’t nearly involve astronomical distances.

A nanosecond is itself practically an eternity compared to many processes physicists study every day. Right now we can create pulses of laser light which are only a few hundred attoseconds in duration. An attosecond is a billionth of a nanosecond, so it’s really staggeringly quick. If you slowed down a movie so that an attosecond went by on screen for every second in real time, a nanosecond would take more than 30 years to play in its entirely. In one attosecond, light will only travel the width of a couple of atoms. Steady progress is being made toward even shorter laser pulses. We live in very cool times.

Tags: Physical Concepts

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