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

Black hole

  1. Apr 29, 2012 #1
    black holes have many times bigger gravity force...and they are black.. and inside of them theres lots of light. Is this because they absorb the light? I mean if the gravity can attract the light. and other thing, isnt black hole something like mega sun or just an object with mega gravity force that attracts all the light and therefore its black?
  2. jcsd
  3. Apr 29, 2012 #2


    User Avatar
    Science Advisor
    Homework Helper

    Welcome to PF!

    Hi Marshallaw4! Welcome to PF! :smile:
    no, there's no light inside a black hole, there's nothing inside a black hole except the singularity

    any light that gets into a black hole heads straight for the singularity and disappears
    it can be "mega" (there seems to be a mega black hole at the centre of every galaxy), but an ordinary star can also become a black hole if it becomes dense enough

    it's black because nothing can get out of it
  4. Apr 29, 2012 #3
    A black hole is formed when a supermassive star uses up all of its 'fuel.' During a star's lifespan, countless fusion reactions take place in the core. These fusion reactions create an outward pressure. This pressure is balanced by the force of gravity that is pressing on the star. Once the star runs out of 'fuel,' it no longer has enough power to support it's own mass and it collapses in on itself due to gravity. The entire mass of the star is then concentrated on an incredibly small point. The radius of the star decreases to what is known as the "Schwarzschild Radius." The Schwarzschild Radius is the critical density in which an object will become a black hole.

    They are black because they absorb anything that comes within reach of their gravitational force, including light.

    Here are some links for further reading:

  5. Apr 29, 2012 #4
    Helpful mental image for you: pretend you're a fish swimming up hill.

    The steeper the hill, the more difficult it is to swim against the current, yes?

    What if you were washed back into a hole with a sheer drop?

    Could you swim back out?

    A black hole is an object with such a deep gravity well that it produces an "edge" we call an event horizon, and anything which crosses that edge, even light, can't "swim" back out.
  6. Apr 29, 2012 #5


    User Avatar
    Staff Emeritus
    Science Advisor

    Re: Welcome to PF!

    You sure about this tiny-tim?
  7. Apr 29, 2012 #6


    User Avatar
    Science Advisor

    Re: Welcome to PF!

    Well, I don't even know if it's clear to me that this is a meaningful question to ask. For example, let's say I throw a baseball into a black hole from an exterior position. We know that I'll see the ball get redshifted and redshifted but never cross the horizon. On the other hand, I could calculate, according to a watch that might be attached to the baseball, the proper time it would take for it to impact the singularity.

    These two descriptions of these events are mutually exclusive, since they cannot ever communicate with each other. It seems therefore ill posed to ask the question of what is 'really' going on, or by extension to the present subject, what is 'really' inside the black hole.
  8. Apr 29, 2012 #7


    User Avatar
    Science Advisor
    Gold Member

    At a distance of 1.5 times the schwarzschild radius there exists a region called the photon sphere. This is the distance at which a photon can orbit the singularity. It is, however, an unstable orbit. The least little perturbation [like infalling matter] would knock it out of orbit. Inside the photon sphere there are no possible orbits for anything, save perhaps the hypotheoretical tachyon.
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook