While rockets and sailboats are both means of transportation to get from one location to another, they have a key difference in propulsion: rockets carry fuel to accelerate while sailboats don’t. When a rocket’s fuel runs out, it can’t accelerate anymore. However, sailboats have a constant source of acceleration from the wind without having to carry any fuel. Looking at these differences, it seems that sailing is a more efficient method of travel than rockets!
How can we create a constant source of fuel for a spacecraft? It turns out, we can propel light sails (thin and lightweight spacecraft) with photons. These light sails can reach speeds up to a significant fraction of the speed of light – which is incredibly fast! In fact, a gram-scale, photon-propelled spacecraft could reach Alpha Centauri (the closest star in our solar system) in merely ten years!
Well, how does photon propulsion work? Photons, massless particles of light, bump into and transfer their momentum to the light sails, pushing the sails forward.
Momentum is mass multiplied by velocity. But how can photons have momentum without having mass? Photons have something called wave-particle duality, meaning that they are waves and particles at the same time. Waves carry energy, but not mass. Since photons have energy, it is possible to rearrange Einstein’s equation, E=mc^2, isolate mass, and plug the result into our previous momentum equation as shown in the figure above.
It is also known that E=hf (energy equals Plank’s constant multiplied by frequency), and c=fλ (the speed of light is frequency times wavelength). Substitution brings us to the final equation for the momentum of a photon: p=h/λ.
At this point, you might be thinking, “Why can’t I just turn on a flashlight and push objects?” Well, the force and momentum generated by photons are extremely small – not enough to overcome other forces. Whereas, light sails have a low mass and large surface area, so they can catch the momentum of a great number of photons. The momentum of the photons adds up to make a difference in the vacuum of space.
This is possible because solar/light sails have been launched into space before, and they work! If we increase the surface area of the light sails and the power of the photon propulsion system, perhaps light sails could carry humans! With advances in this technology, we could achieve interstellar exploration.