Nothing Travels Faster Than Light, But… #physics #science

I like to think I can have an amateur’s understanding of physics. For things like projectile motion, I feel pretty good. But when it comes to space-time waves:

CF researchers have developed a way to control the speed of pulses of light. Not only can they speed up a pulse of light and slow it down, they can also make it travel backward. UFC

What? I found this analogy from Brian Dunning, who puts out the excellent podcast Skeptoid (listen or read transcripts.)

[image error] Event A precedes B in the red frame, is simultaneous with B in the green frame, and follows B in the blue frame… which isn’t the same thing at all, but also weird.

Phase velocities are free to go faster than the speed of light, c, because they are conceptual, like a moire pattern from parallel fences you pass by.

Imagine you have a laser pointer that can paint a dot on the Moon from your back yard. (Caution: Do not look into that laser with your remaining eye.) If you sweep it quickly across the lunar surface the dot can appear to move across the regolith at greater than the speed of light, but the dot isn’t a thing so much as a concept, our name for what the light looks like. The actual photons that are flung to the Moon are moving at good old-fashioned c. Brian Dunning

Huh. If you have the chops for physics, check out “Optical space-time wave packets having arbitrary group velocities in free space” by Ayman F. Abouraddy in Nature Communications 10, Article number: 929 (2019). Open access (yay!) Yes, it’s taken me over a year to trip over the study. When you’ve absorbed the article, come back and explain it to me.