Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Is there a prediction for the frequency of a gravity wave?

  1. Apr 14, 2003 #1


    User Avatar
    Gold Member

    is it known how much gravitational force is needed to deflect a photon from its true path?
    is there a prediction for the frequency of a gravity wave?
    apart from the graviton are there other in vogue thories for the propogation of gravity?
    can gravity be deflected?
    does the speed of gravity change when traveling different mediums?
    when light is deflected around the gravity of a planet,does the light follow the contours of the planet
    a lot of questions, but searching the net has drawn a blank so any info on relevant sites would be appreciated......
    Last edited by a moderator: Feb 4, 2013
  2. jcsd
  3. Apr 14, 2003 #2


    User Avatar
    Science Advisor
    Gold Member
    Dearly Missed

    Re: gravity

    When light passes a pointmass it is bent by an angle
    which (expressed in radians) can be calculated from the mass M
    and the distance R of closest approach.

    The formula for the angle of deflection is

    4GM/c^2 divided by R

    For the sun, GM/c^2 is a bit less than a mile (around 1.5 km).

    So 4GM/c^2 is something under 4 miles (around 6 km)

    So if a ray of light passes within 600,000 km of the center of the sun, then it gets bent by an angle of E-5 radians----ten microradians.

    This is just 6 km divided by 600,000 km.

    BTW this 4GM/c^2 length is interesting, it is the diameter of an equal mass generic (non-rotating) black hole. So a black hole with same mass as the sun would have diameter of 6 km. That is, twice the Schw. radius, the diameter of the event horizon, which from an outsider's viewpoint is a good index of the diameter of the thing.
  4. Apr 14, 2003 #3


    User Avatar
    Science Advisor
    Gold Member
    Dearly Missed

    webreference for that light-bending formula


    This webpage is about the formula 4GM/c^2 divided by R

    the light-bending formula is historical, in 1913 Einstein came up with the first version which had a 2 in place of the 4, and in 1914 some people went to a place in Russia to observe an eclipse and check it but war broke out and they were prevented

    then in 1916 Einstein completed Gen Rel and got the right formula for light-bending, with the 4, and as soon as the war was over in 1919 another team, led by Eddington, observed an eclipse and checked it to within a 20 percent margin of error.

    this was big news. it was the second confirmation of Gen Rel and made a big splash in the media

    BTW you say "how much gravitational force" but the usual
    idea is that the light is not feeling any force but is just going along a ("straight" = geodesic) line. So I should imagine that anything that goes in straight lines at lightspeed would be deflected the same way as light, following the curvature in the neighborhood of a massive object. So a planar gravity wave, if it's propagating at the same speed as light, would be deflected the same amount. Maybe someone else wants to correct or confirm this.
  5. Apr 14, 2003 #4


    User Avatar
    Science Advisor

    The frequency of the gravity waves would depend entirely upon the phenomenon that produced them. One of the Current "Great Challenges" facing modern computational science is the prediction of what types of gravity waves (wave signatures) would be produced by different cosmic events. I seem to recall reading that there is a global team led by a doctor Suen (sp?) to predict these wave signatures, so that LIGO can look for them.

    As I understand it, then main events for which they are trying to predict wave signatures are the collision of black holes, and the final spiral of a neutron star into a black hole.

    Of course, I read that article about a year ago (I think), and these wave signatures may already be known now.

    As for gravity waves being slowed by the medium through which they pass, it is currently theorized that no medium impedes their progress. This is one of the great proposed advantages to gravity wave astronomy; a source of radiation that cannot be "blocked" by intervening dust or gas or other interference.
  6. Apr 14, 2003 #5
    Good writing, marcus. Is it the event horizon or the Schwartzschild radius where photons may be bent into a shell around a black hole? (I believe that shell marks a symmetry between the spacetimes within and without.)

    That gravity propagates at the speed of light was only just recently demonstrated.

    "Branes" were hypothesized to conduct gravity directly (short-circuit dimensions) compared to the actual spacetime travel of light waves. (Conflict with previous mentioned measurement?)
  7. Apr 15, 2003 #6


    User Avatar
    Science Advisor
    Gold Member
    Dearly Missed

    The Schwartzschild radius is 2GM/c^2.

    The radius of the "photon sphere" is 3GM/c^2.

    In theory it is at the latter distance that light can circle round the black hole. Nice thought, light in orbit.

    I think these formulas for the radius of the event horizon and the photon sphere apply only in the "vanilla" case of a non-rotating black hole. I'll see what there is on the web about it, but there is probably someone here who knows and can speak with more assurance.
  8. Apr 15, 2003 #7


    User Avatar
    Gold Member


    thanks for interesting replies, im realy into gravity, but its hard to come up with info that you guys find easily ,where do you get it from? who is or are the fore most publishers on gravity related papers
  9. Apr 15, 2003 #8


    User Avatar
    Science Advisor
    Gold Member
    Dearly Missed

    Re: gravity

    Ned Wright's cosmology site at UCLA

    John Baez physics site at UCR (UC Riverside)

    You asked "where do you get..." so anybody can answer and
    everybody's answers are probably different.

    Ned Wright's site is incredible. I had better get you the url




    Nature magazine has "Science News Update"
    with latest research of all kinds and a keyword SEARCH box.

    Also a kind of strange site I just came across on google


    This is Andrew Hamilton's homepage.
    He likes pictures, gravity, color-vision, relativity, computer animations. Oxford maths undergrad, University of Virginia
    astrophysics, a special talent for simpatico explanation
    (or is that merely my impression?)
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook