MadSci Network: Physics

Re: Light speed in a black hole

Date: Sat Sep 18 21:59:44 2004
Posted By: Layne Johnson, Undergraduate
Area of science: Physics
ID: 1095537678.Ph

Hello Vitalii.

Gravity, according to Einstein's principle of relativity, does more than
just attract objects toward one another. It curves the very fabric of space
and time.

To an observer inside a black hole, light is moving at light speed. This is
because a really massive gravity well makes space-time warp so much that a
clock inside a black hole moves very slow and a measuring stick becomes
very small. But since our observer is moving slow himself, and has shrunk
to the same dimension as the measuring stick, he would still say light
moves at 300,000,000 kps.

Now for the complicated part of the question - we need to imagine a ship
sailing the oceans of Earth. It can sail in two dimensions, toward the
horizon ahead or behind, or to the horizon on the left or on the right. It
can sail all around the three dimensional world. But it never leaves sea
level, which is two dimentional. With that image in mind, let's consider
the event horizon of a black hole.

Light can travel along the curve of an event horizion just as a
sailing ship can travel along the curve of the Earth. But just as
gravity keeps our ship from flying away from the Earth, gravity keeps light
from escaping the borders of an event horizon. This does not limit the
speed of light, just the available directions it can go. Due to the
relativistic nature of the universe, the light inside a black hole appears
to always travel in a straight line, just as a ship sailing west appears to
always travel in a straight line. But from a sufficently distant
observation point, it can be seen that both the light within an event
horizion and a ship sailing west are both circling their own worlds.

In other words, it's all relative. Where you are when you're watching makes
a world of difference in what you see.

Layne Johnson

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