# Why isn't gravity the result of a variable speed of light?

1. Jun 7, 2009

### marlowgs

Einstein first suggested that gravity was the result of a slowing of the speed of light in the proximity to mass. Why was this hypothesis abandoned?

2. Jun 7, 2009

### Staff: Mentor

Do you have a reference for that? It is new to me.

3. Jun 7, 2009

### HallsofIvy

Staff Emeritus
I'm with DaleSpam. Please give a reference for your assertion that Einstein ever suggested that the speed of light slowed in the proximithy of mass.

4. Jun 7, 2009

### Ich

"Über den Einfluß der Schwerkraft auf die Ausbreitung des Lichtes", 1911.
That's only a matter of coordinates. Einstein always explicitly stated that the speed of light, as measured by a local observer, is always the same. The varying speed of light ansatz is sometimes useful, Einstein used it in his 1915 paper to derive light deflection. But it may be confusing, so I guess that's why it is not often taught.
But I also never heard that gravity is caused by a varying light speed; one could say that gravity is the same as a varying light speed under some circumstances.

5. Jun 7, 2009

"On the influence of gravitation on the propogation of light" by A Einstein

I quote :"If we call the velocity of light at the origin of co-ordinates co,then the velocity of light c at a place wiith the gravitational potential P will be given by the relation:

c=co(1 + P/c^2)

The principle of the constancy of light holds good according to this theory in a different form from that which usually underlies the ordinary theory of relativity".

Don't ask me,I just looked it up.

6. Jun 7, 2009

### DrGreg

Note, the title is not "On the influence of the propagation of light on gravitation", which was what the original question was implying.

7. Jun 7, 2009

### HallsofIvy

Staff Emeritus
Changing the velocity of light does NOT imply changing the speed of light!

And, as Dr. Greg said, if gravity changes the velocity of light, it does not follow that the velocity of light causes gravity.

8. Jun 7, 2009

### DrGreg

In general relativity, any observer measures the local speed of light to be the same regardless of the motion of the observer or the source, just as in special relativity. But the speed of light some distance from the observer can take a different value.

An observer hovering a long way from a black hole will calculate that the outward speed of light near the event horizon drops to zero, but anyone falling into the black hole would still measure the usual constant speed of light there.

But this effect is a consequence of gravity, not the cause of it.

9. Jun 7, 2009