Does a black hole cause light to move faster?

In summary: Outside the event horizon, you can calculate the speed of light to be less than c. But that's not a local measurement either.
  • #1
Gamerex
25
0
Say light was a stream of water. What would the black hole be? A bend in the river or a waterfall?
 
Physics news on Phys.org
  • #2
wonderful question! i guess a bend, the point is as i know light can go faster but i can not transform information faster than c...
 
  • #3
It would be a bend that bends on itself and nothing spills out of it (Except that backholes to radiate - Hawkin's radiation)
 
  • #4
Welcome to PF!

Gamerex said:
Say light was a stream of water. What would the black hole be? A bend in the river or a waterfall?

Hi Gamerex! Welcome to PF! :smile:

I think it would be a plughole!
 
  • #5
A whirlpool I suppose if we're going to stick with the river theme. :smile: Only this whirlpool leads to a singularity.
 
  • #6
Since light is not a stream of water, I have no answer to that.

However, in response to your title "Does a black hole cause light to move faster? "
the answer is "NO". Nothing can cause the speed of light to change. What happens to light as it tries to come out of a black hole is that its wavelength becomes longer and longer until, eventually, you no longer have a wave.
 
  • #7
Gamerex said:
Does a black hole cause light to move faster?
Shapiro delay shows that any gravity field actually slows down the speed of light.
http://en.wikipedia.org/wiki/Shapiro_effect
So presumably, if you could watch a photon fly into a black hole, you would observe its speed to decrease

Gamerex said:
Say light was a stream of water. What would the black hole be? A bend in the river or a waterfall?
I'd say it wouldn't behave like a plug hole at all. Not if the Shapiro delay is true. Seems to me the slowing of light would make each photon appear to be repulsed by the black hole rather than sucked into it.
 
  • #8
A physics professor at my university posed this same question to his students. He asked, "if a black hole sucks everything into it, does that mean light accelerates into a black hole?"
 
  • #9
Gamerex said:
Say light was a stream of water. What would the black hole be? A bend in the river or a waterfall?
This document might help you understand more about that:
http://arxiv.org/abs/gr-qc/0411060
 
  • #10
So it's a no. Or a yes. Unfortunatly, I know almost nothing about space-time(Nor the terminology), and like theory more than other things. I guess it's a no.
 
  • #11
Gamerex said:
So it's a no. Or a yes. Unfortunatly, I know almost nothing about space-time(Nor the terminology), and like theory more than other things. I guess it's a no.

Maybe this is completely wrong, but I think light will be bent toward the black hole for sure.

The only thing is, that if you could track the motion of each photon it wouldn't appear to speed up as it got closer, but in fact slow down. In fact I think it stops altogether at either the Swartzchild radius or 2xSwartzchild radius. So it would look like it was attracted to the outside circumference of the hole - rather than toward the centre of it. And it gets stuck there forever. If that's wrong some expert here will correct it I hope.
 
  • #12
what if the black hole was directly infront of it
 
  • #13
YellowTaxi said:
Shapiro delay shows that any gravity field actually slows down the speed of light.
http://en.wikipedia.org/wiki/Shapiro_effect
Not really, it just increases the distance over which the light has to travel. (Well... perhaps you could think about it as slowing light, but that interpretation doesn't really make sense to me)
 
  • #14
robokar said:
what if the black hole was directly infront of it
Still no.
 
  • #15
In a system of any observer - no. Locally, passing light always have speed=c

From perspective of a distant observer, light entering the black hole can have different speeds, based on the choice of the coordinate system. In different coordinate systems speed of light can be different, but this is pure mathematics, not physics. You can do the same by turning around: in coordinate system associated with you Adromeda galaxy made a full circle with speed >1000000c
 
  • #16
The photon's gravitational potential energy is converted into increased frequency?
 
  • #17
I hypothesis that because a photon has no mass gravity only affects it by adjusting it's trajectory.
 
  • #18
First, the whole "light is a stream of water" is a really poor similie. However, the more basic question about light moving faster or slower is actually fairly complicated.

In a curved spacetime there is simply no way to compare the velocity of two objects unless they are right next to each other. Also, in GR light always follows a null geodesic. Any observer at any event will always measure the relative speed of any local null geodesic to be c.

In GR you can assign any coordinate system you like to any spacetime. In these coordinate systems it is common to have light travel at speeds other than c. But there is no physical significance to the coordinate system.
 
  • #19
Stephen Hawking said light will travel faster than c in a black hole but I think he's lost the plot a bit these days, light will travel at c in the vacuum end of story. Gravitational lensing may make it look like it is going faster or slower but speed=d/t will never be more than c.
 
  • #20
Schrodinger's Dog said:
Stephen Hawking said light will travel faster than c in a black hole but I think he's lost the plot a bit these days, light will travel at c in the vacuum end of story. Gravitational lensing may make it look like it is going faster or slower but speed=d/t will never be more than c.
As others have already indicated above, there's more than one way to measure speed in general relativity. Any observer, anywhere in the Universe, making a local measurement of light passing right by, using local rulers and local clocks, measures light to travel at c in vacuum. But if you try to measure light some distance away from you, you may well get a different answer. So if you are hovering outside a black hole, you can calculate the speed of light inside the event horizon to be greater than c, but if you fell into the hole and made a local measurement you would get c.

It's a bit like the problem cartographers have mapping the world on a flat piece of paper. Small areas (say, less than 50 miles / 100 km across) can be mapped pretty accurately. But if you try to draw a map representing thousands of miles, although part of the map may be accurate, another part will inevitably be on the wrong scale or have the wrong angles.

Hawking hasn't lost the plot.
 
  • #21
DrGreg said:
As others have already indicated above, there's more than one way to measure speed in general relativity. Any observer, anywhere in the Universe, making a local measurement of light passing right by, using local rulers and local clocks, measures light to travel at c in vacuum. But if you try to measure light some distance away from you, you may well get a different answer. So if you are hovering outside a black hole, you can calculate the speed of light inside the event horizon to be greater than c, but if you fell into the hole and made a local measurement you would get c.

It's a bit like the problem cartographers have mapping the world on a flat piece of paper. Small areas (say, less than 50 miles / 100 km across) can be mapped pretty accurately. But if you try to draw a map representing thousands of miles, although part of the map may be accurate, another part will inevitably be on the wrong scale or have the wrong angles.

Hawking hasn't lost the plot.

Yes I'm aware of the Schwarzschild metric and the positional aspects not being quite the same as SR, what I'm referring to is c being inviolate in SR, nothing with mass can exceed .999...>c nothing massless can travel at less than c or propogate at less than c and presumably if such things exist nothing that travels faster than light can reach c, nor exceed it. That's what I was getting at.

Obviously it might seem as if light traveled faster than c if we put a massive gravitational field in front of a beam of light, that is gravitational lensing but if we look at distance and even adjusting for the bend in co-ordinates, or time dilation, nothing exceeds c at any point in the journey of the photon.

Well hawking does come out with some weird stuff lately.
 

1. What is a black hole?

A black hole is a region in space with a gravitational pull so strong that nothing, including light, can escape it.

2. Does a black hole cause light to move faster?

No, a black hole does not cause light to move faster. In fact, the strong gravitational pull of a black hole causes light to slow down as it approaches the event horizon.

3. How does a black hole affect the speed of light?

The speed of light is constant in a vacuum, but the gravitational pull of a black hole can bend the fabric of space-time, causing light to appear to slow down or speed up as it passes near the black hole's event horizon.

4. Can anything escape a black hole, including light?

No, nothing can escape a black hole, including light. Once light crosses the event horizon of a black hole, it can never escape.

5. Does the speed of light change inside a black hole?

The speed of light is constant in a vacuum, even inside a black hole. However, because of the extreme gravitational pull, the path of light inside a black hole may appear distorted or curved.

Similar threads

  • Special and General Relativity
Replies
4
Views
288
  • Special and General Relativity
2
Replies
62
Views
3K
  • Special and General Relativity
Replies
15
Views
1K
  • Special and General Relativity
2
Replies
35
Views
776
  • Special and General Relativity
2
Replies
67
Views
2K
Replies
13
Views
534
  • Special and General Relativity
Replies
22
Views
1K
  • Special and General Relativity
Replies
2
Views
566
  • Special and General Relativity
Replies
7
Views
147
  • Special and General Relativity
2
Replies
57
Views
1K
Back
Top