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Couple of random questions

  1. Jan 12, 2009 #1
    1. If gravity waves travel at the speed of light and it take about 8.5 minutes for to reach earth from the sun, If the sun suddenly collapsed into a black hole would it take 8.5 minutes for us to start being pulled in?

    2. If you could somehow set up 2 mirrors facing each other than reflected 100% of light. And if you could aim a laser pointer beam into one of the mirrors and take it away before the light bounced back to it,could the beam of light keep going back and forwards at the speed of light between the mirrors and maintain a beam of light in the air?

    just couple of random things i was thinking about :)
  2. jcsd
  3. Jan 12, 2009 #2
    1. If the sun suddenly collapsed into a black hole, nothing would happen except that we would stop seeing the sun. The mass of the sun does not change whether it is a black hole or not, so there is nothing that can pull us in more than the sun is doing right now already.
  4. Jan 12, 2009 #3
    i know its a bad example because extra matter cant just appear in the sun

    maybe if the sun was instantly replaced by a black hole of the same volume (with much larger mass)
    i know its impossible but just interested with the speed of gravity as such
  5. Jan 12, 2009 #4
    It's not gravity that travels at the speed of light (gravity is just there, it does not travel), but rather changes in gravity. So yes, if the sun would suddenly get twice as massive, it would take us 8 minutes to notice that. Likewise, if the sun would suddenly disappear, it would take us 8 minutes to realize it had gone (visually and 'gravitationally').
  6. Jan 12, 2009 #5
    kk yeah thats wat i meant lol
    thanks :)
  7. Jan 12, 2009 #6


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    2. Obviously impossible, but if you want to make the assumption that you have perfect mirrors, then yes. However, unless the laser beam interacts with the air (which invariably reduces the strength of the beam), it will remain invisible (unless you stick your eye in the reflecting path).
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