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Can we use powerful lights to propel spacecraft at the speed of light?

Your space questions, answered.
October 28, 2019
Traveling through space at the speed of light requires more than just Newton's Third Law.
Traveling through space at the speed of light requires more than just Newton's Third Law.
Traveling through space at the speed of light requires more than just Newton's Third Law.Pixabay

Editor's note 10/28: The original answer to the question posed in this story was incorrect. We regret the error, and we've updated the answer accordingly.

Every week, the readers of our space newsletter, The Airlock, send in their questions for space reporter Neel V. Patel to answer. This week: light-speed travel.

Reader question

Here is a question I’ve had since childhood. Why don't we use Newton's Third Law of Motion (every action has an equal and opposite reaction) to help us travel through space at the speed of light? Simply use powerful lights as our propulsion system for our rockets––since photons are moving at light speed we must be pushed in the opposite direction at the same speed. Why doesn’t this work? –– Kiran

Neel’s answer

To answer this question, let’s take a step back. You correctly say that Newton’s Third Law states that every action has an equal and opposite reaction. Another way of expressing this is to say that momentum is conserved. In Newton’s mechanics, momentum is the product of mass and velocity. A heavy object moving slowly has a lot of momentum, and so does a lightweight object moving quickly. This doesn’t mean though, that all the speeds involved cancel out. When a gun fires, for instance, there is a recoil, but because the gun is more massive than the bullet, the recoil is much slower than the speed of the bullet.

Photons, the particles of light, have no mass, but paradoxically, they do still have momentum. The momentum of a photon is inversely proportional to its wavelength. Longer-wavelength photons, like radio waves, have less momentum, while shorter wavelength photons, like visible light or x-rays, have more momentum. 

So yes, in principle, you can propel a spacecraft by shooting a powerful light out the back.

But the momentum of each photon is very, very small. (It’s a number called Planck’s constant, which is tiny, divided by the photon’s wavelength.) You would need a whole lot of photons. 

This hasn’t stopped physicists and engineers from mulling about the design of such a “photon rocket”. This 1960 paper is the oldest summary we could find. Paul Glister gives a good overview of this history here. The idea would be to carry anti-matter on your spaceship. When matter and anti-matter combine, an enormous amount of energy is released in the form of photons. If you could guide these photons in one direction, you’d have a photon rocket, which would indeed be a terrific way to get around in space. You wouldn’t travel at the speed of light, but you could get up to high speeds very quickly.

But there are a few problems with this idea: first off, it’s very hard to create and store antimatter. Only miniscule quantities have been created, and there’s no practical way to store it onboard a spacecraft at present. And even if you could come up with a bunch of antimatter, with present-day technology, it would still be just about impossible to steer the photons that are created in a particular direction. You’d likely just kill yourself in a blaze of radiation.

That doesn’t mean using light for propulsion in space is out of the question. But taking advantage of the momentum produced by light requires rethinking the design of your spacecraft. Rather than create the light yourself, instead reflect light that is shone at your spacecraft, whether from the sun or from an artificial source, like a laser pointed at your spacecraft from Earth. 

This is the principle behind solar sails, which rely on reflected sunlight. The trick is to use a material that is extremely lightweight and reflective. When light hits and bounces off this material, it imparts momentum to it. If the light is not very bright, this only produces a tiny amount of force. But because you don’t have to carry fuel with you, the acceleration caused by that force can accumulate over time. This makes solar sails (and laser or maser sails) extremely appealing.

A handful of solar sails have been tested in space, by NASA, the Japanese Space Agency, and the Planetary Society, a private group. If you can create highly reflective materials, then you can subject your sails to higher energies, either by deploying them close to the sun or shining very powerful lasers at them. (If the material isn’t very reflective, bright light, whether natural or artificial, would just melt it.) That’s the idea behind ambitious projects like Starshot that seek to use a laser on Earth to accelerate tiny, lightweight, reflective spacecraft to as much as ten percent of the speed of light.

You’re not entirely wrong, then, for thinking of light as a potential form of propulsion. It's just that you probably wouldn't generate the light on the spaceship itself.

To see our reader questions first, make sure you sign up for The Airlock here. It’s free—and it’s sent out every Wednesday.

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