Sunlight and Graphene Could One Day Power a Spaceship

Illustration for article titled Sunlight and Graphene Could One Day Power a Spaceship

Graphene, already a plenty weird wondermaterial, has an unexpected new property that could one day play a role in space exploration: When hit with light, it propels forward. Huh!


Scientists accidentally stumbled across this discovery when studying graphene sponges, crumpled up versions of the single-atom thick sheets of carbon. As the team used a laser beam to cut the graphene sponge, the beam itself seemed to inch the sponge forward. So they set up some controlled experiments, which New Scientist describes below:

The team placed pieces of graphene sponge in a vacuum and shot them with lasers of different wavelength and intensity. They were able to push sponge pieces upwards by as much as 40 centimetres. They even got the graphene to move by focusing ordinary sunlight on it with a lens.


So what’s going on? One obvious theory would be something similar to the idea behind solar sails. Photons of light have momentum, and they transfer it to whatever they’re hitting. But with the graphene sponge, it seemed to be moving too much to be momentum alone.

Here’s how New Scientist explained the team’s alternative theory:

Instead, they think the graphene absorbs laser energy and builds up a charge of electrons. Eventually it can’t hold any more, and extra electrons are released, pushing the sponge in the opposite direction. Although it’s not clear why the electrons don’t fly off randomly, the team was able to confirm a current flowing away from the graphene as it was exposed to a laser, suggesting this hypothesis is correct.

To be clear, this is a bizarre observation about graphene that still needs to be confirmed by other scientists. And while graphene is great, it’s hard to make on a commercial scale. It’s easy and fun to dream up uses for graphene, but we’ll have to wait and see whether it lives up our imaginations.

[New Scientist, ArXiv]

Top image: An artist depiction of a solar sail, which graphene could make obsolete. NASA


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They have just rediscovered something first published in 1879. A porous material (like, say, their graphene foam), suspended in a significant vacuum and able to maintain a difference in temperature (due to the incident light and good thermal insulating properties of a graphene foam), will experience a force to the remaining gas from the cold side “transpiring” through to the warm side and producing a force. This is partially the cause of the turning of those spinning (Crooke’s) radiometers you sometimes see in stores that sell science-related toys. I read their paper (link below) and they did not consider this, but instead just went straight to wild theories of propelling electrons.

Which, BTW, blasting away electrons doesn’t make much sense. Where did the extra electrons come from? Light doesn’t make them, it just moves them. If the foam was ejecting a “rocket exhaust” of electrons, the foam would have become positively charged. Either the electrons would be pulled back, resulting in no net force, or they’d escape. If they escaped, then the foam is strongly positively charged, very light weight and should be attracted toward any surface that could try to offer it some neutralizing electrons - just as a rubbed and charged balloon will stick to a wall for a while. The fact that the foam didn’t stick to the glass tube tells me the foam was essentially neutral and thus their electron-rocket theory is hard to understand.

Anyway, I dug up the original reference on the 1879 paper:…

In this paper they only tested to 10 -2 Torr and the vacuum in the Chinese paper was harder, but I looked at this years ago with aerogels and the effect exists several orders of magnitude below 10 -2 Torr - down about where the Chinese were testing according to their paper here:…

Sorry, if this turns out to be a double post, my first one seems to have vaporized.