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I Light sails for starships

  1. Jun 3, 2016 #1
    I have been told in this forum, I believe, that photons do not have mass. Which brings up the question of how can light be bent around a gravitational field...but I digress.

    If photons do not have mass then how can light be used in a light sail to propel a ship through space? How will the light have any effect on the sail if photons have no mass.

    If they do have mass how will the sail get more light from one direction than the other in order to have a pull in the right direction, especially in deep space where light is pretty much equally arriving from all directions?

    And...why not use a laser or plasma engine to direct concentrated amounts of light energy in a steerable direction? The requirement for an energy source is probably a factor but solar panels could provide that I guess.

    Isn't the sail idea a little far fetched?

  2. jcsd
  3. Jun 3, 2016 #2


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    Because space-time itself is curving.

    They don't have mass, but they do have momentum. That's all that is needed!

    The push you get out of shooting out photons is very small, so if the spaceship is going to carry its own energy source, then its not an efficient solution. If you are going to use solar panels, then I guess that making the photons bounce off the solar sail is again more efficient.

    There has already been some successful prototypes, so it does work.
  4. Jun 3, 2016 #3


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    If the energy source is on the spaceship it probably does make more sense to generate the thrust directly. Light sails really come into their own when there is an external energy source that already happens to be pushing in the desired direction. For example.... a spaceship may not be able to carry a multi-megawatt power source, but it can carry a sail being driven by an earthbound laser powered by a multi-megawatt power plant.
  5. Jun 3, 2016 #4


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    That's no REST mass, the ##m## that appears in ##E^2=(mc^2)^2+(pc)^2##. Rest mass is required neither to follow a curved path through spacetime nor to carry momentum to push on a light sail.

    (As an aside, if you're working with a light sails you needn't pay much attention to the photon concept - photons won't appear in any of your calculations, just electromagnetic waves).
  6. Jun 3, 2016 #5


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    You are referring to an Ion Drive (?). These have tremendous advantages as an alternative form of rocket engine. They eject a very small propellant mass at extremely high speed. A normal rocket has to carry vast amounts of propellant mass as a proportion of its original mass so its range can be much increased by ejecting ions. Of course, the craft needs to carry a lot of energy in some form or another - but that's another matter. Using 'free' energy in the form of light pressure from the Sun or another star is good when you are relatively close to it (not many astronomical Units). You can't expect to be getting anywhere fast in Space, however you do it.
  7. Jun 3, 2016 #6


    Staff: Mentor

    There is a more down to earth example. Since the 1950s, we have had the hydrogen bomb. It wouldn't work without the intense pressure caused by light. Like a light sail, but very much stronger.

  8. Jun 3, 2016 #7
    The acceleration produced by using a sail is very small, but it is continuous as long as the light has an effect on it. Thus, fairly high velocities can be reached. Of course, if you want to decelerate at the other end of the journey, you would need a suitable light source at the destination also.
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