1. May 13, 2006

### jay_598

I have a question or few. If the speed of light is constant in a vacuum, is it subject to gravity? I 'know' that the gravity of a black hole is so strong that not even light can escape it, so does that mean that a blackhole could STOP light? It is true that light travels at 186,282 mps, but does it's speed vary from enormous object's gravity in space? I mean, I have read somewhere that when we look into deepspace that the images of certainstars or planets seem to "bend" the light around it. Does this mean that the gravity is actually slowing down or speeding up light?
Also, this next question is hypothetical.
Say that I am in a spaceship traveling at a constant 186,281.95 mps around a 6 trillion mile elliptical track. Now there is a startline that I am going to pass momentarily. At the start line, there is a stationary quantum super computer that activates a floodlight(that in turn, fires a beam of light that travels along the side of the path of my ship parallel to me) as soon as a spaceship flies over the line. Now here is my question. To my perspective, would I be seeing a beam of light fired from the floodlight moving approx. .05mps? My next question is, If i fired a beam of light from the floodlamp on my ship that is still traveling at that constant speed, would that beam of light, that i fired, be traveling at nearly twice the speed of light in perspective of the stationary quantum super computer? Thanx for reading........
jay

2. May 13, 2006

### scott1

Yes it does. It wouldn't be called a blackhhole. Time slows down as you go into the blakhole untill time and light stop.

3. May 13, 2006

### jay_598

If photons have mass, then that means that gravity can effect them, right?

4. May 13, 2006

### Danger

Photons do not have mass, which is why they travel at light speed. Gravity doesn't affect the speed of light, only the path and wavelength. Light trying to escape a black hole still travels at c, but it's redshifted flat.
c is constant regardless of your perspective. You would see a light shone ahead of your ship as travelling at 300,000kps, and so would someone on the sidelines watching you.

5. May 13, 2006

### pervect

Staff Emeritus
The speed of light is always constant when measured by local clocks and local rulers, regardless of gravity.

An outgoing photon, right at the event horizon, would appear to maintain a constant Schwarzschild 'r' coordinate (distance from the black hole).

However, it is impossible for any physical observer to remain stationary at the event horizon, so no physical observer could stop and admire the stationary photon. Any physical observer falling through the event horizon would still measure the speed of light as being 'c'.

Probably the best way of answering this question is to say that the speed of light doesn't vary - instead, the clocks and rulers vary.

If you think of light as travelling at its own constant speed, 'c', but realize that all clocks slow down near the event horizon, you'll probably have a reasonably accurate picture.

The problem here is that it's hard to untangle "speed' from some specific coordinate system.

6. May 15, 2006

### Farsight

Jay: "The speed of light" is a bit of a problem actually. It's perhaps better to think of light as an instant spacetime effect. It doesn't get bent, because a star bends the spacetime that the light is in. And it kinda doesn't travel, because there's nothing to travel. It's not some bead falling down a string, it's more like a tug on the string. It moves as fast as the string can be tugged, or as fast as spacetime, or better still, as fast as time. What you're really talking about is the "speed at which things happen". It doesn't matter how fast you're going, things happen. If you were going real fast and time slowed down by a significant factor, you wouldn't be walking in slow motion or talking real slow. And in the same vein you wouldn't notice much about them light beams. Something like that.

7. May 15, 2006

### jay_598

Thank you everybdoy for helping me out there.......I have many more questions about everything and I am so glad I found someone to ask!
jay

8. May 20, 2006

### thoms2543

one question.does the speed of light vary in different medium?

9. May 20, 2006

### Strategiest

Yes It does. The speed is slower in water and it also depends on the density of the medium.
I think it can not go faster than c though. not in any medium.

10. May 20, 2006

### ZapperZ

Staff Emeritus
Keep in mind that when people talked about the "speed of light", one typically measures the group velocity. This is the speed that appears to change as one goes from one ordinary dispersive medium to another.

For anomolous medium, such as the gain-assisted Cs gas used in the NEC experiment, such a thing may no longer be true even if no part of the light wave is moving faster than c.

Zz.

11. May 20, 2006

### thoms2543

if the light is slower in other medium, does photons appear to have mass?

12. May 20, 2006

### ZapperZ

Staff Emeritus
No, it does not. Please read our FAQ in the General Physics forum for one mechanism that explains this.

Zz.

13. May 21, 2006

### Ich

I think they could at least appear to have mass. I remember vaguely things like polaritons and whatever. Unfortunately, I immediately forgot everything I learned about it.

14. May 21, 2006

### lazycai

I thought photons in other medium do not really travel through that medium, but are repeatedly absorbed and emitted by the medium during their travel? That is why they get slowed down and have their wavelengths changed in the medium. Hopefully I did not remember wrongly.

15. May 22, 2006

### Ich

ThatÂ´s still ok. But under special circumstances a photon can be mixed with the quanta of lattice vibrations. Or so.