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

2 questions

  1. Jun 17, 2004 #1


    User Avatar
    Science Advisor

    here are two kinda simple questions that I wanted to ask, just to clairify something for me.

    1. at the event horizon of a black hole, the gravitational force pulling you towards the center, should be the speed of light correct?, and/or inside the event horizon aswell, equal to (or greater than) the speed of light?

    2.how does a theory become a law, for example the theory of relativity, which has been show to be correct under many circumstances, and the 3 laws of motion.
    I know it takes along time of extensive testing to make sure it is correct, but is there anything that makes it official or pushes it over the edge of being a law.
  2. jcsd
  3. Jun 17, 2004 #2
    Well, since you seem to be talking of relativity, you should ask yourself what a black hole is. Yes, it is a severe curviture in space (time) do to a very large concentrated mass.But then again, what is the curviture of spacetime due to relativity? Yes, it is gravity.

    Say you have a trampoline. This is spacetime. You put a concentrated mass in the center. (say a marble that weighs as much as a bowling ball) Now there is no doubt that if you roll a marble that weighs normal to the event horizon (or where spacetime starts to curve), it will not all of a sudden drop at the speed of light, but it will travel very fast.

    I hope that answers one of your questions at least.

    Paden Roder
  4. Jun 18, 2004 #3


    User Avatar
    Science Advisor
    Gold Member

    Speed is a relative concept, so you must specify the reference frame. I'll assume you want the frame of the black hole. If you are falling into a black hole, you can be at any speed. As you accelerate, you will increase in mass as well as speed, but never get to the speed of light. Only photons go at the speed of light.
  5. Jun 18, 2004 #4


    User Avatar
    Science Advisor
    Gold Member

    1. A force is not a speed, however at the event horizon you need a local co-ordinate velocity of c in order to remain sationery, as viewed by a remote observer, wrt to the black hole within the event horizon you need a velcoity greater than c.

    2. no real difference between an accepted theory and a law. but a law can usually be put into a concise statement.
  6. Jun 18, 2004 #5


    User Avatar

    Force is not speed.

    Theories do not become laws. The axioms and implications of a theory are the laws of the theory. For example that force is proportional to acceleration or f = ma is a law of Newtonian mechanics. Also for example, that four-force is proportional to four-acceleration or F = mA, is a law of general relativity. That charge is conserved is a law of modern electrodynamics. All of these are examples of our representations of laws of physics.
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook

Have something to add?

Similar Discussions: 2 questions
  1. 2 Questions (Replies: 2)

  2. 2 Questions (Replies: 3)