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Spacedrive

 Imagine a space going bar magnet, with added lasers.
 
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The spacecraft is surrounded by a magnetic field. Whenever it encounters anything going in the direction it wants to go, it ionises it with a laser. The positively charged object has its forward motion converted into sideways motion (by the magnetic field). Due to conservation of momentum the spaceship will have gained some forwards speed.

Problems: *computer must be able to determine the direction of atoms. *laser must be able to target atoms. *must not be too many ions lying in the way of the craft. *might start drifting sideways.

Advantage: *No reaction mass needed.

—RobertKidney, Mar 03 2004

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       Not exactly. You see, what with all the virtual particles, some of them are bound to be going the right way.
—RobertKidney, Mar 04 2004
   

       It would work, as long as you didn't care how long it takes you to get anywhere.
—ato_de, Mar 04 2004
   

       Due to conservation of energy, you'll get no more forward momentum than the energy you've put in via the laser. Which you'll need that reaction mass to power.
—DrCurry, Mar 04 2004
   

       You also forgot the Uncertainty Principle. If you fire a laser at a small object you will affect its velocity, usually by deflecting its path, so it won't necessarily be travelling 'your way' any more.
—gnomethang, Mar 04 2004
   

       Ah, but if you do that then you gain some momentum. And only if it ends up going completely the wrong way do you lose momentum via the magnetic field.
—RobertKidney, Mar 04 2004
   

       [+] Sounds like Maxwell’s Demon put to good use.
—ldischler, Mar 04 2004
   

       Ah!.Probably!
—gnomethang, Mar 05 2004
   

       Ford Prefect's electronic thumb?
—unclepete, Mar 05 2004
   

       I could power the laser with a solar pannel.   

       Conservation of momentum is quite clear. If a particle moving towards you has its momentum changed in your direction by an amount, then your momentum will change by the same amount. (Provided there is nothing else nearby to absorb the momentum). Does seem to be a problem with conservation of energy. I'll think about it.   

       Maybe it would be best to just zap everything behind you with the laser, and forget the field. At least you still won't have to carry actual reaction mass.
—RobertKidney, Mar 05 2004
   
      
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