Solar sails have been proposed as a means of propulsion for interstellar exploration. While acceleration would be very slow, top speed would be much more than that attainable from chemical propulsion. Maybe...

Will they work? Well, for the reasons stated above, certainly not like Count Dooku's sail (scroll most of the way down). But some contend that sails will not work at all. The disagreement is over what principles are in play. In theory, light photons (which are not matter, but do have momentum) would bombard the sail, and push it away from the light source (conservation of momentum). The sail would have a mirrored surface to reflect the light, and thus keep the sail from heating. This lack of heating is the very reason the nay-sayers believe the sail will not work. Since energy cannot be created or destroyed, if the sail doesn't heat, it doesn't move. If the sail does heat, it does move, until the temperature of the object reaches equilibrium with the radiation. The proponents of the sail say that thermodynamics are not involved, and that the sail depends in whole upon the conservation of momentum. But does conservation of momentum apply to light, or only to matter?

The answer seems easy enough to my unschooled mind. If the sail depends on conservation of momentum, momentum would have to be transfered from the light photons to the sail. Correct? If this is the case, the light photons would slow after reflecting off of the mirror. Is this the case? (Anyone, anyone?) I know that light appears to slow when passing through a prism, but then upon exiting returns to it's normal speed. Apparently this is not an actual slowing of light.

We will find out the true answer soon enough. A civilian team hopes to launch a solar sail for testing by the end of October. In the meantime, if anyone with a better background in physics than I can answer my question about light slowing after being reflected please do so.

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