Season 17, Episode 31: The Casimir Effect (and Wormhole Maintenance)

Hey StarTalkers! Season 17, episode 31 was another grab-bag Cosmic Queries edition, with Neil and Chuck going through a bunch of reader questions about everything “from wine to wormholes.” The second-to-last question was a really wild one:

Cosmic Queries – From Wine to Wormholes - StarTalk Radio

(from 48:00)

Neil and Chuck are floored, and I was too. So what is this person even talking about? Is Neil right about the Casimir effect only being attractive?

The Casimir Effect Explained

Hendrik Casimir’s original paper explains the effect fully in mathematical terms, but you can get a good idea of what’s going on without the math.

Casimir was studying van der Waals forces – an attractive force between neutral molecules – and was struck by a question. What if there were two mirrors facing each other in a vacuum instead of molecules?

In the same way quantum fluctuations create virtual particles in seemingly empty space, electromagnetic vacuum fluctuations would happen between these mirrors (and outside of them, too).

In free space, the whole spectrum of wavelengths can exist. But if the mirrors were close enough together, specific waves would be amplified while others were cancelled out. This would leave only waves where a whole number of half-wavelengths fit in the gap (see below).

These fields contain energy, but there is an imbalance. Outside, we have all wavelengths, but inside, only selected ones. This creates a difference in radiation pressure, pushing the plates microscopically closer together. This is the Casimir effect.

Neil’s Slight Mistake: Casimir Forces Can Separate Plates

Neil makes a slight error when he said that it’s a purely attractive force. Under some circumstances, with carefully-chosen materials, you can also make repulsive Casimir forces. It’s not normal but it is possible.

Where the Question Comes From

The mind-bending question likely has its origins in a 1988 paper co-authored by Kip Thorne, which has an interesting section on “wormhole maintenance.” In it, the authors wonder how a highly advanced civilization might be able to keep a wormhole they created open, and suggest that they may use perfectly conducting spherical plates to produce a repulsive Casimir effect to kind of pry it open.  

Is it possible? Even the authors of the paper didn’t know. Given the challenge in even testing Casimir’s idea on Earth, it seems practical concerns would stop them even if theory doesn’t.