# The Naked Scientists Forum

### Author Topic: Can light from a torch move an astronaut?  (Read 3658 times)

#### Jesse

• Guest
##### Can light from a torch move an astronaut?
« on: 28/09/2009 09:30:04 »
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?

#### graham.d

• Neilep Level Member
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##### Can light from a torch move an astronaut?
« Reply #1 on: 28/09/2009 10:23:05 »
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.

#### lightarrow

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##### Can light from a torch move an astronaut?
« Reply #2 on: 28/09/2009 17:55:52 »
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!)
« Last Edit: 29/09/2009 15:53:39 by lightarrow »

#### thedoc

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##### Can light from a torch move an astronaut?
« Reply #3 on: 13/10/2009 19:41:37 »

#### Mr. Scientist

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##### Can light from a torch move an astronaut?
« Reply #4 on: 14/10/2009 23:28:52 »
The momentum from light is what causes the push. The push however is miniscule compared to what was in example.

#### The Naked Scientists Forum

##### Can light from a torch move an astronaut?
« Reply #4 on: 14/10/2009 23:28:52 »