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Why does Gravity affects Light?

  1. Mar 27, 2009 #1
    Hi all , its my first post here in PF.

    Well basically , I have this really simple question (To you guys , but its hard for me).

    Why does Gravity affects light?

    In school , we are taught that gravity affects things that has mass. Now since light is composed of photons and since they are massless , how does gravity affects them?

    Can anyone explains this to me? Try to give me a really simple answer , cuz Im not really a physics whizz ;)
  2. jcsd
  3. Mar 27, 2009 #2


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    Welcome at PF.

    The simplest way of looking at it, is that light does not have mass, but it does have energy. For example, a photon with frequency f, has energy E = h f (with h Planck's constant, up to possible factors of 2 pi). Now according to special relativity, energy is equivalent with mass, so although photons do not have a rest mass, they have energy which gives them some - if you want - "effective" mass which can be affected by gravity.

    If you want the more complicated answer, you need to invoke the analogy of gravity "bending" space while light always goes "straight", which makes it look to us like it's not. The famous (flawed, but for this purpose sufficient) analogy is of space like a rubber sheet and heavy objects making a dent in it. So in this image a photon would go "straight" along one of the black lines, for example - but to us it would appear as if it is being curved.

    If you want the technically correct answer (which I suppose you don't, given your last sentence, but I think it's good to state it): a photon always follows a geodesic. The geodesics are defined by the metric on the space, which in turn depends on the mass content of the universe. If the metric is not flat but we are pretending it is, the geodesic will look curved.
    Basically this the same as the "more complicated" answer above but without involving things like rubber sheets.
    Last edited: Mar 27, 2009
  4. Mar 27, 2009 #3

    @ Your first answer , what is the relationship between frequency and energy? Pi is that 3.142 thing.. ?

    Does that means that light is going straight , but the path it is moving is not straight , but bent? So to us it appears to be curved? And was the path that the 'ball' take spacetime?

    Sorry for asking these questions.. They dont teach these stuff in school.
  5. Mar 27, 2009 #4


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    Please read an entry in the FAQ thread. This has been addressed.

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