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The gravity of light

  1. Aug 10, 2005 #1
    If I shine my new yottawatt laser onto a black hole (yotta = 10^24), I expect that the mass of the black hole will increase, along with it's gravity. I know that 'massless' photons can also be converted into electron/positron pairs, (and vice versa). But can the light itself generate it's own gravitational field? If so, I assume that it's own gravity can't affect itself, since gravity can't propagate faster than light, and I expect the beam won't 'gravity focus' itself (not sure though), but how about an observer who is close to the beam? I had planned to try these experiments myself, but found the electricity costs to run the laser are a bit on the high side.
  2. jcsd
  3. Aug 11, 2005 #2
    Darn ... I calculate that my yottawatt laser beam only contains a mass-energy equivalent of about .037kg/meter of beam. 'Light-Gravity', (if any) would be small for a yottawatt laser.

    1 yottawatt ~= 1,000,000,000,000,000,000,000,000 watts (joules/sec)
    1kg mass ~= 90,000,000,000,000,000 joules
    1 yottawatt ~= 11,111,111 kg/sec mass-energy
    length of 1 second laser beam ~= 300,000km
    beam density ~= (11,111,111 kg/300,000km) = 37kg/km = .037kg/meter (mass equivalent)

    Time to upgrade to the googlewatt model! (Muahahaha)
    Last edited: Aug 11, 2005
  4. Aug 11, 2005 #3
    what if you try mirrors a la Fabry-Perot to obtain reinforcement and localization?

    I found your post very interesting in deed. I am also concerned with the gravitational effects light can have on light itself.

    Is it possible to have an estimative on the power existing inside an star like the Sun, just where particles are formed ? Through your calculations we could get the notion of how big is the gravitational effect of this amount of radiation.

    Best Regards,

  5. Aug 11, 2005 #4
    The Equivalence:http://relativity.livingreviews.org/open?pubNo=lrr-2001-4&page=node3.html [Broken]

    has to be taken into considerations?

    What is local and what is non-local? In an Einstein Thought experiment, a light beam enters a spacecraft, and "bends" downwards, before leaving the window on the other side of the craft, according to the local spacemen onboard, the acceleration and gravitation are equivalent, so the local cause is the fact the craft is moving in a certain direction.

    If one performs experiments onboard the craft, then there will be other factors that have to be considered, for instance, what if there are no windows(they are blocked out), and the light source from the thought experiment is replaced with an apparatus that shines a light form one side of the craft to the other?..will the onboard light source follow the geodesics of the external lightsource?

    One has to apply certain factors to certain events, in "your" thought experiment, the 'windows' are the Event Horizons, which does not allow light to re-emerge from another side. Now one can also conclude that the lightbeam itself does not "Enter" the Blackhole, but collects around the Mass source. Think of a water hose that sends a finite jet of water at a target, make the target a basketball, the water hits the ball, and dissapates over the SURFACE, it is 'bent' locally around the surface-horizon.

    Locally, one can not confirm that water penetrates the surface horizon, but far away one can conclude that there exists a "ball-of-fluid", that is behaving with a Mass different to that of just Water?..one can conclude that there must be some sort of 'basketball', with a Mass that is preventing the water from splashing back to source.

    If the water generated its own 'gravitational field', then it would not reach the surface-horizon, it would be comparable to a person taking aim with a water-pistol, at a student friend, pulling the trigger..the water comes out so far, then turns around like a 'WATER-FOUNTAIN', and falls backwards and drenches the person pulling the trigger!
    Last edited by a moderator: May 2, 2017
  6. Aug 11, 2005 #5


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    If you take the limit of the gravitational field of a bunch of moving particles, as the energy is kept constant and v->c, you arrive at the conclusion that there will not be any self focusing effects.

    Googling, I find this old usenet post by Steve Carlip which makes a similar point. Carlip is a good source, with numerous published papers in GR and quantum gravity.

    I stumbled across a useful link to the literature while looking at these archived usenet posts:

    I don't have it personally, but it looks like it's $11.16 from amazon.com

    Last edited by a moderator: May 2, 2017
  7. Aug 11, 2005 #6
    Thanks for a good post, pervect. After thinking about this last night I realized that based on a simple thought experiment, light must indeed gravitate:

    I have a box containing hydrogen ... the walls of the box are 'perfect' mirrors (they even reflect neutrons!). I make the hydrogen undergo fusion, and now a percentage of the mass has been converted to light that is bouncing around inside. It would not make sense for an outside observer to sense a sudden large change in the gravity from the box.

    As an interesting side-note, the measured gravity from the box might be allowed to change slightly. As far as I know, we still have not experimentally confirmed that matter and antimatter have identical gravitational characteristics. Some physicists think that there may be slightly higher attraction between two objects if one is made of matter and the other of antimatter, compared to the attraction resulting if both objects are made of matter (or both of anti-matter). Thus if some of the light was converted to anti-matter (by converting the light to electron/positron pairs), the measured gravity could increase (although most likely by a very small amount). Many experiments have been proposed to look for such a difference, but as far as I can tell, they still have not been able to settle this one way or the other.
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