The laws of light speed still apply.

Last June, researchers from the Hong Kong University of Science and Technology published the results of an experiment that proved that light does not move faster than light—specifically, that single photons can’t move faster than the official speed of light under certain conditions.
Today, Skulls in the Stars—the nom de Internet of a UNC Charlotte physics professor—hasa really great blog post up about this paper. It’s very much worth a read. After all, this was basically a test to double check something we were already pretty sure was true. And what’s the benefit to proving something you already knew?

A big part of why I’m recommending this post is because Skulls in the Stars does a good job of explaining some tangly optical physics in a way that is quite clear and should make good sense even if you don’t have a deep background in this stuff. If you follow along, you’ll come away with a good idea of why this particular study matters, and with a deeper understanding of the speed of light itself.

Let’s talk about how we measure the speed of an object first. If we’re looking at the motion of a rigid object, like a speeding car or a thrown baseball, the speed can be determined simply by measuring how much time it takes for an object to travel a distance. The speed is simply the distance divided by the time

There’s a small subtlety to this definition: cars and baseballs are extended objects! To accurately measure an object’s speed, we have to be consistent in how we define its position. For a car driving down the road, for instance, we should do all measurements of its position from a fixed position, such as the front bumper, to measure the speed.

But what do we do when the object doesn’t have a fixed position on it? For example, what is the best way to measure the speed of a hurled bucketful of water?


Image: Tap’s eye view of water falling into bucket., a Creative Commons Attribution Share-Alike (2.0) image from kamathln’s photostream