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I guess a related question is "why does a flashlight in space not propel itself". And I don't know the answer to that. Edit: Ok. That makes sense. I guess it's

• oneandoneis2 12y ago
  It does. But the propulsion is really, really tiny.
• leishulang 12y ago
  https://answers.yahoo.com/question/index?qid=20111214233110AAicrQm https://answers.yahoo.com/question/index?qid=20111214233110A...
• IanCal 12y ago
  A flashlight in space would propel itself, since photons have momentum.
• dragonwriter 12y ago
  A flashlight in space does propel itself, but that's not really a related question. A more related question is why does a flashlight in a fixed mount inside of a windowless spacecraft not propel the spacecraft (or, similarly, why does a fan inside of a sealed, ventless housing attached to an airplane wing not function as an engine.) Of course, if there's some way this does work despite what we think we know about physics, its a pretty gigantic breakthrough (well, at least, understanding and practically applying it would be -- otherwise its just an anomaly that provides a pretty gigantic question mark...)