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

Light cones and black holes

  1. Oct 11, 2014 #1
    Why we talk and discuss about effects on light cones by black holes though we know there is no light left after a star dies and become a black hole?
    there should be no light and so no light cones...... Black_Hole_Milkyway.jpg
     
  2. jcsd
  3. Oct 11, 2014 #2

    Nugatory

    User Avatar

    Staff: Mentor

    It's not true that "there is no light left" after a star dies and collapses into a black hole. There's still plenty of light, it just can't get out.

    The "light cone" is a geometrical structure in four-dimensional spacetime: the forward light cone at a point is defined as the surface in four-dimensional spacetime along which a flash of light emitted at that point would propagate. Inside the event horizon of a black hole the light cones are tipped in such a way that they never cross the event horizon, so light traveling along the surface of the light cone never gets out.
     
  4. Oct 12, 2014 #3
    its true that if there will be a light cone it will be affected like this due to very strong gravitational field.But how could be possible there is light and plenty of it.Because we know light emittted by a star is nothing but the energy released due to the fusion of H atoms to form He atom.and we know a star dies because of unavailability of H fuel.So by this there should be no light.
    High gravity makes the cloud of gas to colapse.,atoms collide more frequently and produce more heat. This heat pressure balances the gravity untill the fuel finishes up , no balancing force left due to which it turns into a Black hole.
     
  5. Oct 12, 2014 #4

    Matterwave

    User Avatar
    Science Advisor
    Gold Member

    A "light cone" is called that because it is an imaginary boundary between those (from a local point of view) points (events) in space-time between which we can send signals ("inside/on the light cone") and those points (events) in space-time between which we can not send signals ("outside the light cone"). This has nothing to do with whether any actual light is present or not.

    Just like one may say "point a and b are within a meter-stick's reach of each other" without actually possessing a physical meter-stick.
     
  6. Oct 12, 2014 #5

    phinds

    User Avatar
    Gold Member
    2016 Award

    And perhaps an even better analogy would be "point a and point b lie on the same line", without having to draw the line.
     
  7. Oct 12, 2014 #6
    alright but point A ,B can be actually present and then we assume a line on which both points lie and i s actually true for any two points.
    But light, it cannot be there ,associated with a black hole so why talking about light cone....
    You mean to say that even though there is no light ,but talking about light cones and its effects due to black hole will give a better picture of black hole and its consequences.
     
  8. Oct 12, 2014 #7
    At first, there are other processes in stars: H burns to He, then 3 He burns to Carbon, up to Fe.
    Even if there is no fuel left, object still radiate light because it takes a long time to cool down (check neutron stars, they at still very hot for million years even without any fuel)
    Near BH (above and below horizon), infalling matter radiates, because it is very hot because of friction.
    And finally, even for an old BH without any infalling matter, don't forget about the infalling light.
    So there is ALWAYS light inside BH.
     
  9. Oct 12, 2014 #8
    "Even if there is no fuel left, object still radiate light because it takes a long time to cool down"

    How long a dead star takes to cool down in a universe at a temperature slightly higher than absolute zero.....
     
  10. Oct 12, 2014 #9

    phinds

    User Avatar
    Gold Member
    2016 Award

    Yes, and we can assume that if light were present, it would follow a path dictated by / described by the light cone.

    Now you've got it.
     
  11. Oct 12, 2014 #10

    Nugatory

    User Avatar

    Staff: Mentor

    Much longer than you might expect - many billions of years. Google for "white dwarf lifetime"
     
  12. Oct 12, 2014 #11

    Dale

    Staff: Mentor

    You have a wrong idea here somewhere. You most certainly can have light which follows any light cone (at least any future-directed light cone) including those in a black hole. Imagine a flash-bulb falling into a black hole. At some given event along its worldline it flashes. The light from that flash follows the future directed light cone whose apex is the given event. It doesn't matter if the given event is inside or outside the event horizon.

    However, although light follows a light cone, the light cone is a geometric structure that exists regardless of the presence or absence of physical objects. A light cone is the set of all null geodesics through a given event, regardless of whether or not a flash of light is emitted at the given event.
     
  13. Oct 12, 2014 #12

    Matterwave

    User Avatar
    Science Advisor
    Gold Member

    I think a lot of people are focusing on whether there "is light" in a black hole or not, but that's really not the issue here. A "light cone" really has nothing to do with any actual light being present. Talking about the "light cones" in a particular space-time is analogous to talking about distances between two points, or talking about the curvature present at a point. A "light cone" is just a name for a specific feature of the space-time.
     
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook




Similar Discussions: Light cones and black holes
  1. Black holes and light (Replies: 4)

Loading...