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Photon, Gravity, Mass and Black Holes.

  1. Sep 18, 2005 #1

    Here's the deal:

    As far as I understand gravity acts upon anything that have mass.
    Once a star is engulfed, no information is given out by the BH since nothing escapes its gravity. Not even light.

    Question 1: Why photons cannot escape from the BH since photons have no mass and they are not subjected to gravitational pull ?

    Question 2: How do we determine the the mass of a BH ? What indication is used to calculate its mass ?

  2. jcsd
  3. Sep 18, 2005 #2


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    1) photons are attracted by the gravitational pull according to General Relativity.

    2) While there are many ways to calculate the mass, one would be to measure its attraction; ie throw a ball towards the black hole and measure its acceleration.
  4. Sep 18, 2005 #3
    Because photons follow sapcetime curvature, so they go down the black hole and get lost in it. As you may know black hole means "infinite" spacetime curvature, so our photons get lost in their.

    I think we just look the effect it has on its surroundings...like we would do for a normal star.

    Last edited: Sep 18, 2005
  5. Sep 18, 2005 #4
    Welle actually you can also calculate a deviation for photons just using Newtonian Mechanics (you get a wrong result). Since gravity is not influenced by the mass of the deviated object, you just have to consider that photons have an extremly small mass (not zero). This was used in the 1919 eclipse to confirm GR against Newtonnian Mechanics.

    Last edited: Sep 18, 2005
  6. Sep 19, 2005 #5
    Thanks to all of you.
    Watch out for the Warnings.
    Don't get in trouble because of me.

    Once again thanks for the explanation. Sorry about my ignorace.

  7. Sep 20, 2005 #6


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    There are several entries in the sci.physics.faq

    which address your questions.
  8. Sep 20, 2005 #7


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    :confused: There seems to be no problem here. If you have a concern, use the feedback forum or send a PM to a forum mentor.

    Questions of all levels are welcome.
  9. Oct 1, 2005 #8
    my astronomy teacher said that black holes have zero volume and infinite density. Also he said they all have fixed mass. i realy dont get the first two and personally dont think this is true.
  10. Oct 1, 2005 #9


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    Black holes don't have fixed masses. They become more massive from accumulating matter, and less massive by 'evaporation'.
  11. Oct 2, 2005 #10
    They have mass, they just jhace zero proper mass (aka they have "rest" mass). Recall Feyman's take on this.
    From The Feynman Lectures on Physics V-II, Feynam, Leighton, Sands. Turn to page 7-11 where Feynman is speaking in the section labeled Gravity and relativity
    Any classical body can be a black hole if all of its matter is confined within a sphere which has a radius less than or equal to the Schwarzschild radius R defined by M = c2R/2G

  12. Oct 2, 2005 #11
    That is a descsription of the phenomena and then only when spacetime curvature is present.

    In flat spacetime in a uniform gravitational field there is no spacetime curvature. However a beam of light will be deflected iin such a field due to the gravitational pull on the light.

    Whoa there. If a beam of light passes by the earth then the gravitational field of the beam of light must exert a pull on the earth since otherwise there'd be a violation in the conservation of momentum.

  13. Oct 2, 2005 #12
    Well he was actually speaking about a blackhole...

    Ok i think i didn't express myself corecctly : earth will be influenced by the gravitational fiel generated by the photon, but that is not the point ! What i wanted to say in my message is that as in Newtonian mechanics gravity doesn't deped on the masse of the attracted object (since its gravitationnal field is far less powerful than earth), then you can calculate how much a light beam should be deviated only using newtonian mechanics.
    Since you consider that light is attracted (in NM) like any other massiveobjects, you can caclculate how much it should be deviated.

    To be more precise : if you calculate the deviation of a photon (coming for a far star) next to the sun, you find approximately alpha=0.875'', wheras the real value (given by GR) is alpha= 1.745"

    In my sentence i wasn't taking into account the grav. effect of the light beam on the sun because.... it is very near to 0 !.
    Last edited: Oct 2, 2005
  14. Nov 3, 2005 #13
    Your first answer is:Its right photon has no mass but the entire mass of photon come by the its energy and enourmous velocity by Einstein mass energy relation.E=mc square. so it has mass and gravity act on it.

    your second answer is:Mass of black hole is known by its radiation or temperatre. mass of black hole is inversly proportional to temperature. lower is temp higher will mass or vice-versa.
  15. Nov 3, 2005 #14


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    Hi suraj, and welcome to these Forums!

    Although that answer is correct for Hawking radiation you will be hard pushed to measure it! Unless the BH is a tiny primordial BH that is about to 'pop' the Hawking temperature of a BH is a fraction above absolute zero.

    A BH's mass is determined by the gravitational pull it makes on orbiting stars. The mass of Cygnus X-1 is about 6-8 M solar the mass of the BH at the centre of the Milky Way is about 106 Msolar. These masses are detrmined by the velocities of the orbiting companion to Cygnus X-1 a O9.7 Iab type supergiant HDE 226868, and the velocities of stars/nebulae around the galactic centre respectively.

  16. Nov 4, 2005 #15
    I am sorry if this is out of place but in curved space-time are all things affected by gravity, even with 0 mass.
  17. Nov 4, 2005 #16


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    Yes that is why light is 'bent' by the Sun, the first confirmed prediction of GR by Eddington in the 1919 eclipse.

    GR reinterprets gravitational force as the effect of space-time being curved. In free fall all objects travel along 'straight lines' called geodesics, but the geodesic itself is along a curved surface.

    Consider ants walking in a straight line over the dip near to the stalk of an apple. They will diverge and converge on each other and an observer not seeing the dip might conclude there are forces acting on them pushing and pulling them, whereas in fact they had been 'putting one foot in front of the other' and walking in a line that was locally straight for them.

    Newton's apple fell off the tree because when its stalk broke the apple and the Earth continued on straight lines through a space-time curved by the Earth's mass with the result that they converged.

    BTW I have visited Newton's home in Woolsthorpe, near Grantham in Lincolnshire. He retired there from Cambridge to escape the plague in 1665-6 and formulated his laws of motion and gravitation, eventually published in Philosophiae Naturalis Principia Mathematica (Mathematical Principles of Natural Philosophy) commonly known as the Principia, 1687.

    Guess what? His apple tree is still there - actually the original tree was struck by lightening in the 19th century but the stump sprouted shoots and they live on today. My prized possession was a tiny apple that fell from that tree until it went rotten! How do you preserve apples for posterity?

  18. Nov 4, 2005 #17
    thanks I was confused on how light was bent by the Sun until I asked that question and you cleared it up for me, thank you
  19. Nov 4, 2005 #18


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    The easiest way is probably to encase it in acrylic epoxy, such as you get for making paperweights and keychain fobs. It comes as a binary liquid that you pour into a mould.
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