# Does a black hole cause light to move faster?

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

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..........

It would be a bend that bends on itself and nothing spills out of it (Except that backholes to radiate - Hawkin's radiation)

tiny-tim
Homework Helper
Welcome to PF!

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!

I think it would be a plughole!

A whirlpool I suppose if we're going to stick with the river theme. Only this whirlpool leads to a singularity.

HallsofIvy
Homework Helper
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.

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

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.

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?"

Say light was a stream of water. What would the black hole be? A bend in the river or a waterfall?
http://arxiv.org/abs/gr-qc/0411060

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.

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.

what if the black hole was directly infront of it

diazona
Homework Helper
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)

russ_watters
Mentor
what if the black hole was directly infront of it
Still no.

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

The photon's gravitational potential energy is converted into increased frequency?

I hypothesis that because a photon has no mass gravity only affects it by adjusting it's trajectory.

Dale
Mentor
2020 Award
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.

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.

DrGreg
Gold Member
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.

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 travelled 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.