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This question already has an answer here:

As far as I know photons don't have mass but they do have momentum ($p=mv$). Scientists say that if we put a shiny (reflective) shield of large radius in the vacuum of space, then light from sun will will start moving the shield. How can these two facts be commensurate?

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marked as duplicate by Kyle Kanos, John Rennie, Willie Wong, dmckee Feb 19 '14 at 16:02

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