# B Could light be slowed down and be observed

1. Jul 17, 2016

### RandyD123

Would it be possible for me to be anywhere in the universe and have a flashlight in hand and watch the beam of light move from point A to point B?

If I was on a planet with extreme gravity (and could live) would I be able to watch light move?
What if I was on the edge....not quite over the event horizon of a black hole could I see it then?

2. Jul 17, 2016

### Ibix

How are you going to see it? You can't observe it directly. It is light, it doesn't emit light. You could install partial reflectors along the beam path and "tap" a bit of the light pulse so that you can see when it passes the reflectors, I suppose.

If you set up such a system about a light second long about a light second away it would fit in your visual field and you'd be able to see the reflectors light up in sequence over a perceptible period.

3. Jul 17, 2016

### PAllen

... And it would alway show light moving at c, no matter where or when in the universe you were, as long as the light is propagating through vacuum or a medium with sufficiently low refactive index. This principle is known as local Lorentz invariance, and it has no predicted exceptions per current theory. If, instead, you want to consider optically exotic media, look no further than earth, at the work of Lene Hau (google it).

My answer assumes you want to be right next to the flashlight. You can arrange to 'see' the beam with dust that only minimally affects refractive index. If instead, you watch from far away, then a super high energy (frequency) gamma ray burst emitted tangentially near a BH horizon could, in principle be seen as visible light moving slowly.

Last edited: Jul 17, 2016
4. Jul 17, 2016

### RandyD123

Does light really ALWAYS move at c in a vacuum? Can't gravity affect the speed of light? If light can't crawl out of a black hole is there a point near the event horizon that it could "bleed out" little by little. Is the event horizon an all or nothing place?

I'm not assuming any type of medium, only space.

I know what you are saying about actually "seeing" the light but a beam should be able to be observed, like a laser pointer beam.

The reason I ask all this is because I saw a video on youtube that showed light propagating outwards using a high speed camera. I was just wondering if this could happen in real time because of gravity, thinking of black holes.

5. Jul 17, 2016

### Staff: Mentor

Yes.
No.
1. You can see a laser pointer beam because dust in the air scatters some of the light. Again: you can only see light that hits you in the eye.
2. Since you can already see a beam of light traveling at (about) C, why would slowing it down change anything useful?
Can you post a link to the video?

6. Jul 17, 2016

### RandyD123

7. Jul 17, 2016

### RandyD123

I don't get how gravity does't affect the speed of light? If light goes into a black hole it can't come back out because of gravity....right?

8. Jul 17, 2016

### Staff: Mentor

That is not right.
Light in a vacuum always travels in a straight line through space at the speed $c$. However inside the event horizon of a black hole spacetime is so curved that no matter what direction the light is moving, its straight-line path leads to the central singularity.

If we start outside the event horizon, or if the spacetime curvature is less extreme (as it is outside an object that is not a black hole) the light can travel outwards against the gravitational influence. However, it still travels at $c$ - it is redshifted instead of slowing down.

9. Jul 17, 2016

### RandyD123

I guess I'm getting confuse using the old waterfall scenario trying to show how a black hole works. There is a point that a person in a boat can paddle hard and fast enough to stop from getting any closer to the waterfalls edge.

I just assume that light works the same way just before it goes over the horizon.

10. Jul 17, 2016

### PAllen

Please read my prior post carefully. It fully answers all your questions. The key difference is local versus observed at distance. Locally, physics is always, everywhere, every when, exactly like special relativity. This is built in to the mathematical structure of a pseudo-Riemannian manifold, so there can be no exceptions at all, even in the the most extreme conditions in GR.

11. Jul 17, 2016

### Staff: Mentor

That is an "it's kinda sorta a bit like this...." analogy. Like any analogy, there's only so far it can pushed before it becomes misleading.

12. Jul 17, 2016

### ZapperZ

Staff Emeritus
I will also suggest that you spend some time reading the Relativity FAQ entries. It might answer a few of the misconception that you have here.

Zz.

13. Jul 18, 2016

### A.T.

If you could observe something similar happening deep in a gravity well, watching from far away, then yes. But when you watch from far away, even light at c looks slow (small angular speed), so the whole gravity thing is superfluous.

14. Jul 18, 2016

### David Lewis

"In 1999, Danish physicist Lene Hau led a team from Harvard University which slowed a beam of light to about 17 meters per second."
-Wikipedia: Bose-Einstein condensate

15. Jul 18, 2016

### Staff: Mentor

This thread is about the speed of light in a vacuum, while that experiment is describing a different phenomenon - light in a superfluid.

16. Jul 18, 2016

### RandyD123

Maybe I am misunderstanding how the event horizon works then. Is it or is it not like a waterfall? Or is it something you cross and you are instantly at the speed of light?

17. Jul 19, 2016

### A.T.

Maybe. But as mentioned above you don't need a BH to observe propagation of light similar to the HFR-videos you posted. The flash from a star explosion can be seen propagating through nebula over many years:

http://www.nasa.gov/multimedia/imagegallery/image_feature_2472.html

18. Jul 19, 2016

### Staff: Mentor

Neither. Locally, there's nothing special about the event horizon. If we were to pack an entire physics lab into a shipping container and drop it into a black hole, no experiments or instruments would allow the scientists inside to know when they were passing through the horizon - it would just be free fall, and the same above the horizon and below. (They would be able to detect tidal effects, but these are present around any mass, whether black hole or not, and show up outside the event horizon).

19. Jul 19, 2016

### RandyD123

Dam I hate being such a novice at this stuff. So let me ask, at what point does light not have a chance of escaping a black hole? If it's not at the horizon, then when is it?

20. Jul 19, 2016

### Staff: Mentor

At the horizon. But that does not mean the horizon is "like a waterfall", nor does it mean that as soon as you reach the horizon you are moving at the speed of light.

A key concept you might be missing here is that "escape" from the black hole is not a local concept; it's a global concept. Even if you see a light beam in your local vicinity that appears to be moving outward and "escaping", you can't tell, locally, whether it actually is going to escape or not. To know that, you would have to know the entire future of the spacetime. It's not even enough to extrapolate from your local knowledge of the light ray, because additional mass could fall into the hole in the future and cause that light ray to be trapped, even though it looked like it was going to escape when you measured it.

21. Jul 19, 2016

### RandyD123

mind blown~~~

22. Jul 20, 2016

### I Like Science

For God's sake. Light always travels at c no matter where the hell it is or what the surrounding gravity is. And my understanding is that light will get drawn into a black hole but will still be moving at c when it enters it.

23. Jul 20, 2016

### jbriggs444

The event horizon is not a place in space that you can pass at a speed of c relative to that place. It is a surface that moves outward at c relative to any local inertial frame. Since it moves at c, it does not itself define a local inertial rest frame.

Both the outward-moving horizon and any inbound light crossing the horizon move at c relative to any local free falling observer.

Edit: Note that the horizon only moves at c relative to a local inertial frame. A distant observer will not see the horizon growing as a result.