MadSci Network: Physics

Re: VRD - black holes, light trajectories

Date: Tue Oct 26 18:51:08 1999
Posted By: Layne Johnson, Undergraduate
Area of science: Physics
ID: 940951587.Ph

Hello, Lucasz.

Yes, light speed is constant.  Also, it's direction of travel is constant. 
Gravity really doesn't bend light, or slow it down.  What gravity does is 
bend spacetime.

Let's imagine that spacetime is a sheet of paper, and a ray of light is a 
straight line drawn on the paper.  Gravity curves the paper.  Really strong 
gravity, like black holes, curve the paper so much that the the paper is 
rolled up into a cylinder, and the straight line never escapes its 
confines.  The line is still strait.  It hasn't moved on the paper.  But 
the paper has been rolled up, unless you're inside the cylinder, you can't 
see the line.

Once the paper has been rolled into a cylinder, new lines drawn on the 
inside curve of the paper wouldn't be seen from the outside.  So a flash of 
light inside a black hole wouldn't be seen outside of it.  Actually, with 
the incredible amounts of heat and energy contained in a black hole, 
they're extremely bright stars.  They're probably the brightest objects in 
the universe, but their light will never be seen outside their event 
horizons.  Kind of a shame, isn't it?

Remember, it's the fabric of the universe, spacetime, that is curved by 
gravity.  Light doesn't alter its speed or direction as it travels through 
the universe, but it appears to do so near objects with lots of 
gravitational pull.  What's actually being altered is time and space, not 
speed and direction.

Thanks for the question,

Layne Johnson

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