# Where does the loss of gravitational potential energy goes?

1. Aug 26, 2008

### J. Richter

Hi.

Imagine a closed physical system (a universe) with only one star like our Sun, and one planet like our Earth, but so far away from each other, that the gravity from both matters only has a very small effect on each other.

There is a (almost maximum) gravitational potential energy between the star and the planet, even that they are far away from each other.

The star converts mass into energy as time goes by, and the gravitational potential energy between the star and the planet gets smaller because of that.

This loss of potential energy doesn’t seem to be included in the physical processes around the star, because how is the star to know the mass of the planet so far away, and the amount of the potential energy?

My question is:

Where does the loss of gravitational potential energy between the star and the planet goes?

Regards from J. Richter

2. Aug 26, 2008

### D H

Staff Emeritus
The potential is negative definite, with a maximum of zero at infinite acceleration.

The potential energy gets smaller in magnitude but larger in value. However, at infinite separation, the potential energy is still zero. The star's changing mass results in essentially no change in potential energy for a planet and a star separated by a very, very large distance.

How does the star "know" the mass of a planet that is fairly close to the star? How is a separation of 1AU any different than a separation of billions of AUs?

3. Aug 26, 2008

### J. Richter

Since the star and the planet has a very small effect on each other after all, during very long time, they would be drawn towards each other because of gravity, stronger and stronger, resulting in a big collision, that would result in more heat in that universe.
But for now, this has not happened yet therefore there must be a positive (not yet released) energy between the star and the planet.

The star loses energy, and the eventual collision in the future between the star and the planet would result in a smaller and smaller crash.

Where does this potential energy go?
Your right, that was a clumsy way of trying to say what I meant:
It’s hard to imagine that this “lost” potential energy mentioned above, is converted into another kind of energy together with the converted mass from the sun.

4. Aug 27, 2008

### gel

the star is constantly radiating energy in the form of light and cosmic radiation, which should make up for any 'lost' gravitational potential.