# Thread: Black holes and the speed of light

1. ## Black holes and the speed of light

Hello all,

I am not educated in the mathematics and computations required to calculate orbits and material phases, but I have a very good ability to grasp and understand the concepts of astro physics explained. My favorite scientists to hear speak are Neil Tyson, William Nye, and Stephen Hawking(not really speak, but maybe read). When I see these men speaking before audiences, and allow questions at the end of their presentations, they allow the audience to ask several questions. The audience nearly always ask rhetorical or philosophical questions rather than detailed technical questions, which of course I am most interested in. I have a few questions that I believe can be answered, but I have yet to hear asked:

If light behaves as both a particle, and a wave, and light can be affected by gravity, such as being unable to escape an event horizon of a black hole, then what happens when light passes near a black hole, but stays outside the event horizon? Does light possibly enter orbit, or simply change direction and speed? If light changes speed, does it speed up going towards the black hole, and slow down leaving the black hole? If light changes velocity based on gravity, then how can the speed of light be a constant? Is the constant of the speed of light simply a maximum attainable speed, leaving light able to slow down to an extent before becoming a form of matter?

I would like to hear anyone's thoughts on this subject, or even better, I would like to see links to sources that can answer my questions. These can either be reading material, or videos. In the off chance that any of you regulars of MMO-Champion are astrophysicists or theoretical physicists, would you please shed some light on my questions?

2. If I'm not mistaken, the light "bends" around the gravity of a black hole, without getting sucked in (provided, as you say, it stays clear of the event horizon).

About the speed of light: it's only a constant in optimal situation. The absolute speed of light as we know it (for example) is only true in a vacuum. [url=http://www.cracked.com/article_19978_5-mind-blowing-ways-that-science-has-done-impossible.html]With the right medium, you can slow light down so it's slower than a car.

Edit: did a quick google search.

3. If you want real answers from real physicists, then try here: http://www.physicsforums.com/

The short answer is that light always travels at a constant speed in a vacuum (or near vacuum like space). Light rays get "bent" by gravity not just from black holes, but by any massive object (look up "gravitational lensing")

4. Originally Posted by Demoness
Does light possibly enter orbit, or simply change direction and speed? If light changes speed, does it speed up going towards the black hole, and slow down leaving the black hole? If light changes velocity based on gravity, then how can the speed of light be a constant? Is the constant of the speed of light simply a maximum attainable speed, leaving light able to slow down to an extent before becoming a form of matter?
Gravity doesn't change the speed of light, which is a constant when in a vacuum. Light can be bent by gravity though.

It seems the short answer is light will always travel at a constant speed and from our perspective time moves more slowly around a black hole. So it may seem light is moving slower around the black hole.

6. These phenomena are in the field of cosmology which is an application of general relativity in the universe. In general relativity, there is no gravity as a force the way classical mechanics sees it. Although the light may behave as particles, photons, it is not affected by gravity in the same way as an apple falls on your head. It is the existence of matter that alters the space around it, it in someway "shrinks" the space. So when a ray of light passes near an object with enough mass, it still continues to move in a straight line, but through more "dense" space, and that line now appears to us like curved through gravity cause the universe is rather Riemannian than Euclidean. The speed of light can not be affected by anything but it is not the same in various optical mediums.

7. Originally Posted by semaphore
Gravity doesn't change the speed of light, which is a constant when in a vacuum. Light can be bent by gravity though.
The speed of light is constant. There is no condition in which the speed of light changes.

---------- Post added 2012-10-03 at 01:40 PM ----------

Originally Posted by algoferon
<snip>
Photons are particle-waves. However, if you study elementary particles a photon has no mass. And to a more fun extension. Scientists recently found the Higgs boson which is the reason why all particles have mass. Lovely! When I used particle in the last sentence I meant elementary particles. Not atoms, or humans.

The speed of light is constant. There is no condition in which the speed of light changes.
Sure there is. Light slows down when it passes into, say, water. That's a change.

9. Originally Posted by Demoness
If light behaves as both a particle, and a wave, and light can be affected by gravity, such as being unable to escape an event horizon of a black hole, then what happens when light passes near a black hole, but stays outside the event horizon? Does light possibly enter orbit, or simply change direction and speed? If light changes speed, does it speed up going towards the black hole, and slow down leaving the black hole? If light changes velocity based on gravity, then how can the speed of light be a constant? Is the constant of the speed of light simply a maximum attainable speed, leaving light able to slow down to an extent before becoming a form of matter?
The reason why this isn't asked is that light has a constant velocity. Velocity is displacement over time and Speed is total distance traveled over time. However since light travels in a ray, these are equal.

You have two cases to this question:
A) If light enters the even horizon then it gets absorbed by the black hole.
B) If light comes near the event horizon, it travels past it. Although lights path is change by gravity, but the lights velocity does not. Once the light escapes the gravitation field it regains its original path. Think of it as a form of refraction. Light's path changes as it travels through the glass, but its speed does not change. Once it exits the glass it returns to its original path.

10. Originally Posted by semaphore
Sure there is. Light slows down when it passes into, say, water. That's a change.
Indeed. In the link I posted higher up, they've managed to slow down the speed of light even more (Yeah, it's a Cracked article, but there are sources in that article).

11. Originally Posted by semaphore
Sure there is. Light slows down when it passes into, say, water. That's a change.
Refraction is a change in direction, and not a change in magnitude.

12. Originally Posted by semaphore
Sure there is. Light slows down when it passes into, say, water. That's a change.
The speed in a vacuum is constant.

13. Originally Posted by Rukentuts
The speed in a vacuum is constant.
He knew. He was responding to someone who claimed there was no condition in which the speed of light changed.

14. That cracked article is interesting however it is irrelevant to the OP. It mentions that light can be "slowed" by pointing a laser into a block of sodium. Which is basically a lens. Meaning the light doesn't "slow" down. It has to travel a different path to exit the other side of the sodium block. Which is the definition of refraction.

However as Rukentuts mentioned, we are talking about it in a vacuum and thus refraction is irrelevant.

---------- Post added 2012-10-03 at 01:52 PM ----------

Originally Posted by Ynna
He knew. He was responding to someone who claimed there was no condition in which the speed of light changed.
It doesn't. If Light's speed could be changed we would live in an entirely different universe.

15. The speed of light is constant but the effective speed is slowed by propagation through a medium. It's not dissimilar to a car moving through traffic. Imagine a car that could only move at one speed and had instantaneous acceleration and deceleration. As it moved through traffic it would still move at the same speed, but the time it takes to get from point A to point B varies with the density of traffic (the refractive index of the medium of propagation.)

16. And I'm a Senior in College for both Mathematics and Phyics.

17. Light has speed, not velocity. If velocity of the light was constant and unaffected by optical mediums then we would not have rainbows or shiny, precious diamonds /sigh

And I'm a Senior in College for both Mathematics and Phyics.
By the way, I wasn't trying to argue with you. I should've quoted one of the other two guys, just for clarity.

19. Originally Posted by Beavis
The speed of light is constant but the effective speed is slowed by propagation through a medium. It's not dissimilar to a car moving through traffic. Imagine a car that could only move at one speed and had instantaneous acceleration and deceleration. As it moved through traffic it would still move at the same speed, but the time it takes to get from point A to point B varies with the density of traffic (the medium of propagation.)
Exactly. And to OP I think what you want to read or watch videos on in Relativity. All this talk about the speed of light and gravity, General Relativity will be the topic you would want to study. And since you like Hawking, you probably know what happens to information when anything travels across the even horizon. That's a really interesting area.

---------- Post added 2012-10-03 at 01:59 PM ----------

Originally Posted by Beavis
By the way, I wasn't trying to argue with you. I should've quoted one of the other two guys, just for clarity.
I don't argue. I just regurgitate the same information over again.

20. Originally Posted by Ynna
He knew. He was responding to someone who claimed there was no condition in which the speed of light changed.