Season 17, Episode 60: Next Stop, Alpha Centauri! How Light Sails Could Allow Interstellar Travel

Hey StarTalkians! Episode 60 of season 17 was a basic, Neil-and-Chuck Cosmic Queries edition, with topics ranging from fractals to the boundaries between universes. One listener asked about how we could actually make it to Alpha Centauri:

Cosmic Queries – Light Sails & Quantum Scales - StarTalk Radio

(from 17:45)

Neil brings up light sails, which is one of the most promising approaches we’ve come up with so far, and one specific project that intended to use them. Here’s how it would work.

The Physics of Light Sails

The core concept underpinning light sails is that light has momentum. The momentum (p) of a photon can be simply calculated based on its energy divided by the speed of light (p = E / c), or as Planck’s constant divided by the wavelength (p = h / λ). This is what pushes the tails of comets away from the sun, for example.

A 1971 NASA paper goes into detail about the force you’d expect from solar radiation acting on a flat surface. While there’s a lot more detail, basically the momentum comes from photons which reflect off the surface, and is strongest if the light hits perpendicular to the surface (dropping off with the square of cosine of the angle).

The Challenges of Making a Working Light Sail

Many of the key challenges were outlined a paper by Philip Lubin. In particular – and as you might guess – the actual amount of momentum from light is not huge. This means that you need two things right off the bat: as big a sail as possible, and as lightweight a craft as possible.

Lubin proposes a “spacecraft on a wafer,” and suggests boosting the power to the sail by using lasers rather than sunlight, but both aspects of this are challenging. As discussed on the podcast, he calculates that this could achieve a quarter of the speed of light in a matter of minutes.

Other serious issues include the problem of decelerating when the craft arrives at its destination.

The Failure of Breakthrough Starshot

The project Neil mentioned, though, ultimately did not achieve these goals. As detailed by Scientific American, the project intended to use a fleet of tiny spacecraft powered by laser-light to make a relatively quick 20-year journey to Alpha Centauri. The article has all the details, but the short version is that the billionaire funder seemed to lose interest.