[Ben]

Ben asked the Naked Scientists:
   
Hi if I had a torch in a vacuum or space and I turned it on then I would have stream of photons coming out from the light source in the torch. 

Seeing as these photons are moving at light speed does this mean that the torch will start moving and eventually achieve c?  Assuming that the batteries would last. 

So why is light alone not used as a propulsion method when in a vacuum when the light emitting from it is moving at c?

What do you think?

[graham.d]

The torch would be propelled backards as you suggest, though not a lot unless the torch was extremely powerful. It can never achieve the velocity of light, however, as this would need an infinite amount of energy.

Light could be used as propulsion, in theory, but to exert any reasonable force (via conservation of momentum) the energy in the light has to be enormously high compared with what you may consider typical. In many ways it could be efficient in that you, theoretically, have the minimum mass possible as fuel compared with a conventional rocket. However, there would be practical difficulties in producing light (or em radiation in general) with sufficient energy to be useful.

[syhprum]

You would require a power of 300MW to produce a thrust of one Newton (tough on batteries)