This article says that the water drop is added to the back of the paper “as a wall against which the metal plume could push.” It seems more likely that the heat of the laser would even more violently vaporize the water, creating a small explosion that would drive the plane forward via the relatively unaffected metal plate.

David Bump
Flushing, Mich.

The sentence about the wall reminds me of a similar gaffe that The New York Times made early last century: It criticized Robert Goddard’s claim that a rocket could travel in a vacuum because, the reporter said, there was no air for the rocket to push against. Similarly, the force needed to make the laser plane fly needs only to act on the plane itself.

Douglas Warshow
Ann Arbor, Mich.

According to Takashi Yabe of the Tokyo Institute of Technology, the laser vaporizes some of the aluminum and some of the water, creating an explosion and forward thrust. But that’s not the whole story. In other experiments, the scientists used a layer of transparent acryl plastic instead of a water droplet. The plane took off fastest when the acryl layer was very thick. In that instance, the explosion sent the plane flying, but the massive plastic block remained stationary. Says Yabe, that finding indicates that propulsion of the plane also results from the overlying substance (either acryl or water) acting as a wall that contains the explosion. –P. Weiss