Speed of light is no barrier - if a black hole sucks light back faster than it can escape - then...?

Question

Light moving toward it is surely faster than the constant we use for lightspeed? So.. Darkspeed?

Answer 1

Not correct. Light always moves at the same speed when approaching, or even in the black hole. It cannot escape the black hole because the black hole's gravity well has distorted space-time in such a fashion that it cannot get out. Time slows down, as well inside.

However, you touch on a great point: what is the speed of gravity, does gravity travel faster than light. It is such a tiny force on our everyday scale that we have a tough time measuring it at all. And nobody has yet found any evidence of carriers of gravitational force, such as the graviton.

Answer 2

No, Einstein proved that you can't add velocity to light. If you're moving at 99.9% of the speed of light, then shine a flashlight towards the direction of motion, it doesn't go at lightspeed plus 99.9%. It still goes at lightspeed.

Light travelling towards a black hole still goes no faster than c (the speed of light).

Answer 3

Your argument sounds logical, but it just doesn't work that way. The curvature of spacetime is a tough thing to get a grip on. I took a short online course on black holes a while back and part of the issue is that curved spacetime can sort of cram in more space into a place than what's normally there.

I know I'm saying this in a clumsy way, but there really is "more distance" to travel in a weird sense as you go radially in toward a black hole. It's all related mathematically by the spacetime "metric." But it's one thing to do the math and another to try to get a physical grip -- that's hard.

Maybe this spacetime "stretching" is one way to think about light not being able to get out. Another might be to think that the light's energy gets "sucked out" rather than it getting pulled back in a "speed" sort of sense.

It's easy to get tripped up by our everyday instincts when thinking about relativity. I'm still struggling with it...

Answer 4

Newtons laws of motion to not apply to light or objects moving at speeds near light.

If you are moving 0.6c and you toss a rock forward at 0.6c the rock does not move 1.2c for an outside observer. When adding velocities, you use the following equation.

v_result = (v1 + v2) / ( 1 + ((v1*v2)/(c*c)))

v_result = (0.6c + 0.6c) / (1 + (0.6c*0.6c)/(c*c))
v_result = 1.2c / (1 + 0.36 )
v_result = 1.2c / (1.36)
v_result = 0.75c

Add normal velocities which we typically experience -- speeds of sound, jets, cars, falling objects on earth, etc. -- the result is almost the same as v_result = v1 + v2. However, if you look at the equation for speeds near the speed of light, you can't make any combination of velocities result in greater than the speed of light. Even 1c + 1c still equals 1c.

Answer 5

it's not so much that the black hole sucks the light toward it, the mass of the black hole is so great and curves space-time so much that light can not escape. the light is still travelling at the same speed. it's just following the curved path around this great mass.

Answer 6

Not so, the speed of light is constant under any circumstances when it moves through a vacuum, if you don't accept this fact you are sure to make errors in your predictions.

Answer 7

that is an interesting thought. i agree with Steve H though.

Answer 8

i agree with Steve H.

i found this page to be very helpful

http://science.howstuffworks.com/relativity.htm

Answer 9

Good point.
Excellent observation.
Go get 'em.

Answer 10

all that proves is that light has mass and is subject to gravity.