Science Questions

Could I use a torch to push me in space?

Sun, 11th Oct 2009

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Jesse, Ithaca asked:

I’ve read that photons can push things and people have considered space ships propelled by large sheets or mirrors to catch sunlight. So, if I were in space and I had a flashlight powerful enough to push me, would it push me? ?And should I aim it towards myself or away from myself?


Chris -   Yeah and the answer is, it does work.  In fact, there’s something called the YORP effect, Yarkovsky–O'Keefe–Radzievskii–Paddack effect in honour of the four scientists who described this first of all.  If you look at asteroids and they are spinning in space and they're irregularly shaped, when they have one surface facing at the sun, which is a different size, say to the other face, then that face gets a disproportionately big push compared with when the object then turns on its side for example.  And what this does is to create a push or a torque effect, which tends to steer the asteroid and change its path through space.  And this is one effect which is thought to have unleashed this barrage of asteroids on earth that destroyed the dinosaurs 65 million years ago because out in vestiges of the early solar system near mars are whole remnants of bits of planet that failed to form.  And there are these fairly big objects out there and they are subject to the YORP effect.  So light can definitely push things along, and that could go for a person too.  We know we can push spacecrafts via the same thing. 

This is the whole concept of solar sailing.  You have a very big collector.  And when photons are incident on it, then they can give it a bit of a nudge and we can work out how much of a nudge a photon, when it arrives, gives something.  We know from the Planck constant, if you times that by the frequency of the light, you can work out how much energy is imparted by a photon, a particle of light hitting something, and I have to say very big thank you to Light Arrow on our forum who suggested a very neat solution to this problem.  So basically, yes.  Light can give things a push.  If our astronaut who’s drifting around in space were armed with a laser beam and he were armed with a 3 kilowatt laser beam, then the energy, the nudge that he would get would be given by the equation F=w/c, where F is the force and it’s equal to W, the wattage of the beam divided by the speed of light.  And if we put the numbers in, 3 kilowatts is 3 x103 that’s power of a laser.  You divide that by the speed of light, 3 x 108.  That would tell you that the force that the astronaut would feel through firing the laser would be about 10-5 newtons. On Earth, that would be the equivalent of holding up 1 milligram.  In other words, about 1,000th of a gram.  So, very small push but nonetheless, over enough time, given enough time, it would push the astronaut through space.  You would have to point the laser in the direction opposite to which he wanted to travel. 

Dave -   This is actually the ultimate rocket.  You get the maximum push for every kilogram of stuff you throw out  the back.  So, if you want to travel a very, very long way, this is the way to do it.


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Jesse asked the Naked Scientists: Dear Naked Scientists, I have a physics question. I've read that photons can push things, and people have considered spaceships propelled by large sheets or mirrors to catch sunlight. So my question is, if I were in space and I had a flashlight (torch) powerful enough to push me, should I aim it toward myself or away from myself? Thanks, Jesse Ithaca, NY, USA What do you think? Jesse , Mon, 28th Sep 2009

Most efficiently you would aim it away from yourself as though it were the output of a jet engine. Aiming it towards yourself would have a complex effect - if you were wearing a black, light absorbing suit, there would be no net force, but if you were wearing something that scattered the light, there would be some net force but a bit unpredictable. With all such propulsion systems creating rotation is a problem too.

However, the force produced by a torch (even a very powerful torch) is extremely small and would not be a practical means of propulsion. graham.d, Mon, 28th Sep 2009

As graham.d wrote, it would be more efficient to aim the beam away from yourself.
You can easily compute the push received back from the beam if you know the beam's power:
F = W/c
F = force; W = beam's power; c = light speed.
Example: with a 3 kilowatt laser you receive a push of 3*103/3*108 = 10-5N ~ 1 milligram.

Remember however that we don'y need to talk about photons in this case: light's push exist in the classical theory too = it's possible to prove from Maxwell's equations that electromagnetic radiation has momentum (and can have angular momentum too!) lightarrow, Mon, 28th Sep 2009

The momentum from light is what causes the push. The push however is miniscule compared to what was in example. Mr. Scientist, Wed, 14th Oct 2009

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