Conservation of energy and light

[Nelourir]

Hello everyone,

I'm not sure that this fits in the general physics topic but here it is anyway:
If you have a certain mass on a planet that you convert to energy, specifically photons (just because it seems to make sense to me) and 'send' those photons to another planet where they will be converted back to mass, where is the energy needed to 'move' the mass 'gone'?
This is all assuming that the energy conversions are extremely efficient and that no photon is lost.

I'm guessing there must be some sort of potential energy hidden somewhere... But I just can't see it.

Thanks for your help.

 

[netheril96]

Photon's energy is affected by gravitational field, which is called gravitational redshift.

 

[Nelourir]

What happens if the photon goes 'up hill' out of the gravity well but comes down the other planet's well which is, let's say, roughly equal to the source planet? The light blueshifts doesn't it? And how does that explain the apparent gain of energy?

 

[Dale]

If you go up and down the same potential then there is no change in energy.

 

[Nelourir]

Then how can you effectively move mass by converting it into energy then converting it back?

 

[Dale]

It doesn't use energy to move something. You gain energy by moving down a potential or you require energy to move up a potential, but moving to the same potential doesn't change energy. That comes from Newtons first law.

 

[Nelourir]

Does that mean that moving a mass does not necessarily involve some energy being converted (and effectively lost because it is unusable) in the process?

 

[Dale]

Correct. Have you heard of Newtons first law?

 

[Nelourir]

Haha, how didn't I think of that?