# Thread: What happens to the Ek of light slowed down ?

1. Light is slowed down under increasing gravitational conditions, such as radially approaching a supposed massive planet.

So work has been done to slow it down : E = d x m x a

We have conservation of energy as a basic principle, so.

The kinetic energy of the light has been transferred :

-either onto something else > kinetic energy lives on in the movement of that something else

-either into potentional energy by means of absorption by something else.

-or a combination of partly both.

>What is that something else ?

>Doesn't the above also imply that the photon must have a mass, ever small as it may be ?

2.

3. Originally Posted by Noa Drake
Light is slowed down under increasing gravitational conditions, such as radially approaching a supposed massive planet.
I am not sure where you got this idea, but it is not really correct. The local speed of light is always exactly c, no matter what the gravitational conditions are; it is only its coordinate speed ( what a far-away reference observer measures ) that changes in the presence of gravity. However, remote coordinate speed measurements tell you nothing about what photons "actually" do in their own small local neighbourhood, just as such an observer's failure to observe an object cross the event horizon does not allow him to conclude that it never reaches the singularity.

Curved space-times are inherently tricky, so be careful.

So work has been done to slow it down
You can't accelerate photons.

The kinetic energy of the light has been transferred
Again, things are tricky in curved space-times - conservation of energy still holds locally ( energy-momentum tensor is conserved ), but there is no globally valid concept of energy or its conservation in curved space-times.

4. Is the bending of light passing a massive planet not caused by a difference in speed of light, determined by its place in the gravitational field then ? Causing a certain angle for the wavefront that is.

5. Originally Posted by Noa Drake
Is the bending of light passing a massive planet not caused by a difference in speed of light
The cause of the bending of light is the geometry of space-time, specifically the global geometry of null geodesics. Like I said, light cannot be accelerated; just view it as a particular solution to the geodesic equations.

6. I will check it this evening, but i did get that info from the explanation by Einstein in his paper on the bending of light.The difference in the geometry on the wavefront causes the lightspeed to be lower on the inside then on the outside, no ?Apart from that Newtonian bendingcomponent, there is the curvagecomponent providing the resulting angle to go times 2, resulting in the 1915 more correct result, no ?

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