Starts With A Bang

Ask Ethan: Does Light Always Move At The Same Speed? (Synopsis)

A multiwavelength view of the galactic center shows stars, gas, radiation and black holes, among other sources. But the light coming from all of these sources, from gamma rays to visible to radio light, always moves at the same speed through empty space: the speed of light in a vacuum. Image credit: NASA/ESA/SSC/CXC/STScI.

“There was a young lady named Bright,
Whose speed was far faster than light;
She set out one day
In a relative way,
And returned on the previous night.” -A.H. Reginald Butler

Throughout the entire Universe, there’s a fundamental law that governs the motions of all particles: Einstein’s relativity. It states that all particles with mass can never attain the speed of light, no matter how much energy you put into it. Additionally, all massless particles only move at the speed of light, no matter what you do to either them or to the device/person observing them. No matter what reference frame you’re in, the speed of light in a vacuum is a constant.

The size, wavelength and temperature/energy scales that correspond to various parts of the electromagnetic spectrum. Image credit: NASA and Wikimedia Commons user Inductiveload.

But light isn’t always in a vacuum!  From air to quartz to acrylic to glass to many other media, light can pass through transparent material, and when it does, it slows down. Not only that, but light of different energy slows down by different amounts. In what ways is the speed of light always the same, and in what ways can it change?

The behavior of white light as it passes through a prism demonstrates how light of different energies move at different speeds through a medium, but not through a vacuum. Image credit: University of Iowa.

And most importantly, what do the known properties of light mean for the rest of the Universe? Find out on this edition of Ask Ethan!