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About gravity and distance

  1. Mar 23, 2013 #1

    I don't really understand this formula but if light has no mass, then how comes a blackhole can pull it in?

    [itex]F=G\frac{MassLight\times MassWormhole}{WormholeRadius^2} = G\frac{0}{WormholeRadius^2}= 0[/itex]

    My question is if light experiences no gravitational force wherever it is in the universe, why can a wormhole pull it in?

    I know you guys probably get stupid questions like this all the time but my mind often wonders into things I don't understand.

    Hope someone can clear up my ill thinking.
  2. jcsd
  3. Mar 23, 2013 #2


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    Last edited by a moderator: May 6, 2017
  4. Mar 23, 2013 #3
    Nevermind, I just found on the forums it's due to GR and the fact that the space-time itself is bent so light isn't actually being pulled due to the sheer gravitational force of the wormhole but because space-time is curved.

    Pretty neat really
  5. Mar 23, 2013 #4
    There is a gravitational deflection of light in classical mechanics:

    [itex]F = G \cdot \frac{{M \cdot m }}{{r^2 }} = m \cdot a[/itex]

    [itex]a = G \cdot \frac{M}{{r^2 }}[/itex]

    As light is not massless in classical mechanics this works for photons without problems and due to

    [itex]\mathop {\lim }\limits_{m \to 0} \frac{m}{m} = 1[/itex]

    it could also be used for massless objects.

    However, the results does not fit to reality. (e.g. the deflection of light in the gravitational field of the Sun is double as high) You can't use Newton's law of gravity for light or black holes. General relativity must be used to get the correct results.
    Last edited: Mar 23, 2013
  6. Mar 23, 2013 #5
    How is it possible that in one field of physics light is massless but in another it isn't? How can you guys just chop and change things like that?
  7. Mar 23, 2013 #6
    1. There are different theories for light.
    2. There are different definitions of mass.
  8. Mar 23, 2013 #7


    Staff: Mentor

    According to Newton's second law, how much force is required to accelerate a massless object?

    Of course, the real answer requires relativity. Newtonian physics doesn't treat massless particles correctly. But the point is that you need to think about your premise a bit and see if it makes sense.
  9. Mar 25, 2013 #8
    I think the answer is that one theory is correct in all instances that we're discussing (Relativity), while another is correct in only some instances (Newton). Ideally we would just use Relativity for everything, but Newton's theory is much simpler and easier to use...so we only use Relativity when we really really have to. The key is knowing when that is.

    As an example, we all know that the earth is round. Nevertheless, for everyday basic tasks such as walking around, throwing a ball, etc, thinking of the earth as being flat is good enough, because accounting for the curvature of the earth will give you practically the same result, but with a much larger headache.
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