Conservation of energy and light

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Discussion Overview

The discussion revolves around the conservation of energy in the context of converting mass to energy in the form of photons and then back to mass on another planet. Participants explore the implications of gravitational effects on photon energy and the energy dynamics involved in moving mass through gravitational fields.

Discussion Character

  • Exploratory
  • Debate/contested
  • Conceptual clarification

Main Points Raised

  • One participant questions where the energy needed to 'move' mass is 'gone' when converting mass to photons and sending them to another planet.
  • Another participant introduces the concept of gravitational redshift, suggesting that a photon's energy is affected by gravitational fields.
  • A participant raises a scenario where a photon moves 'uphill' out of a gravity well and then 'downhill' into another, questioning how this affects energy gain.
  • It is noted that moving up and down the same potential does not change energy, implying that energy is conserved in this process.
  • Questions arise about whether moving mass necessarily involves energy conversion and potential energy loss.
  • One participant asserts that moving a mass does not require energy conversion unless moving against a potential gradient, referencing Newton's first law.

Areas of Agreement / Disagreement

Participants express differing views on the implications of energy conservation when converting mass to energy and back, particularly in relation to gravitational effects. The discussion remains unresolved regarding the specifics of energy dynamics in this context.

Contextual Notes

Participants reference Newton's first law and gravitational effects, but there are unresolved assumptions regarding the efficiency of energy conversions and the nature of potential energy in this scenario.

Who May Find This Useful

This discussion may be of interest to those exploring concepts in physics related to energy conservation, gravitational effects on light, and the interplay between mass and energy.

Nelourir
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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.
 
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Photon's energy is affected by gravitational field, which is called gravitational redshift.
 
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?
 
If you go up and down the same potential then there is no change in energy.
 
Then how can you effectively move mass by converting it into energy then converting it back?
 
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.
 
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?
 
Correct. Have you heard of Newtons first law?
 
Haha, how didn't I think of that?
 

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