# Velocity of light remain costant in a gravitational field?

1. Jun 23, 2008

### Ignition

Does the velocity of light remain costant in a gravitational field?
In my calculation it is less than c.

2. Jun 23, 2008

### shamrock5585

velocity is a vector quantity... it depends on its direction in relation to the field

3. Jun 23, 2008

### Ignition

So c in a gravitational field is bigger than c?

4. Jun 23, 2008

### tiny-tim

Hi Ignition!

(You mean speed)

In general coordinates, the speed of light depends on the metric you have chosen.

But in local inertial coordinates, the speed of light is always c.

5. Jun 23, 2008

### shamrock5585

well gravity can bend the path of light, and a black hole can stop light from escaping because gravity is so strong... im not sure if light slows down and turns back around when trying to escape a black hole or if it just cant be bounced back out in the first place or if it goes c out and then is reversed to c in the opposite direction from the gravity... i dont think light accelerates it just goes c... I'm not sure ive never "seen" a black hole haha

6. Jun 23, 2008

### peter0302

The *magnitude* is always 'c'. The direction can change, and therefore it can "accelerate" in a given direction.

7. Jun 23, 2008

### MeJennifer

The speed of light in vacuum remains locally the same in a gravitational field. Observers may measure a different speed due to the curvature of spacetime.

The light is not slowing down, its wordline is curved back into the black hole, so it has nowhere to go but back.

Light in vacuum does not accelerate since its wordline always follows a geodesic of spacetime.

8. Jun 23, 2008

### Unkraut

I think you meant "worldline".
Unfortunately that is all I can contribute to this topic.

9. Jun 24, 2008

### mtworkowski@o

Re: Light

10. Jun 25, 2008

### HallsofIvy

Staff Emeritus
Re: Light

11. Jun 25, 2008

### mtworkowski@o

Re: Light

12. Jun 25, 2008

### matheinste

Re: Light

Quote:--

--You'll remember that history is full of dicarded axioms that were once believed in religiously.---

That's the easy answer as it will always be true of any theory.

Matheinste.

13. Jun 25, 2008

### mtworkowski@o

Re: Light

True enough.

14. Jun 25, 2008

### Chrisc

Re: Light

Relativity is a convention. As it works, it is not wrong, but incomplete. It is a convention that is self sustaining because it is self consistent and therefore inescapable.
All relativity theory must concede, accept or agree on the first instance of measure which is
usually referred to as "proper" time, mass or length. This first "quantification" sets all subsequent
quantification of dimension simply because space, time and mass cannot be defined
uniquely from each other.
Without this first premise of the constancy of dimension set by choice and sustained by
the convention of the relativity of "proper" dimension, the equations of mechanics become
meaningless, arbitrary expressions.

15. Jun 25, 2008

### pmb_phy

Re: Light

Why can't I PM shamrock5585??

Pete

16. Jun 26, 2008

### mtworkowski@o

Re: Light

I think you said it well. I like the convention thing. What is reality after all. I've been in God's pocket and I can't even find his watch, no less take it apart.