# Gravity-energy question

1. Mar 19, 2009

### cam875

I was thinking about a theoretical situation where you have two particles with some mass inside a vacuum and you were trying to model gravity between them. If they were both at rest, therefore 0 kinetic energy and then gravity acted upon them whether its through tensors in GR or gravitons in QFT they would have to increase the kinetic energy of the particles since once gravity acted upon them they would start to move a bit. But if there was no kinetic energy before, where did the energy come from that caused the particles to start moving? was it from their rest energy(mass)? and wouldn't that imply that the particles would be getting lighter over time? thanks in advance for any helpful replies.

2. Mar 20, 2009

### DaleSwanson

The kinetic energy comes from the gravitational potential energy. However many joules they gain in kinetic energy they lose in GPE. However they were initially separated is how they gain the GPE in the first place. As for weight I'm think it would remain constant, and that both GPE and kinetic energy would add (equal) weight. Rest mass (also called invariant mass) remains constant.

3. Mar 20, 2009

### Staff: Mentor

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4. Mar 20, 2009

### cam875

ok so if QFT works out than the graviton would have to carry some kind of kinetic energy in order to transfer it into the particles that are being accelerated by gravity, right?

5. Mar 20, 2009

### cam875

so does anyone have an answer to this? It still doesn't make sense how two particles with some value for rest energy at rest are able to generate kinetic energy for eachother while being alone in a vacuum. Unless there replacing rest energy for kinetic energy than I do not see how this can happen without defying the law of conservation of energy.

6. Mar 21, 2009

Staff Emeritus

7. Mar 21, 2009

### cam875

Ok so as the rest energy of a particle increases so does the energy stored in its gravitational field that it uses on the other particle to give it kinetic energy. Hmm i dunno it seems weird in my head how does the gravitational field just magically have energy depending on the particle's rest energy, it seems like its just energy that doesn't yet exist until its converted to kinetic energy on the other particle which seems fishy lol like invisible energy or something. So where does that energy actually come from that gets stored in the gravitational field until being converted to kinetic.

8. Mar 21, 2009

### Staff: Mentor

Potential energy does exist. It is real. A coiled spring is another example. The wiki on potential energy describes it well: it isn't the potential to be energy, it is the potential to be other forms of energy.

9. Mar 21, 2009

### cam875

so if QFT works out then the graviton will be the carrier of the potential energy for gravity that will turn into kinetic?

10. Mar 21, 2009

Staff Emeritus
Cam, I think you should make sure you understand kinetic and potential energy before launching into gravitons. You have to walk before you can run. Besides, the answer to your question doesn't involve gravitons: just potential energy.

11. Mar 21, 2009

### Gear300

Gravity provides the energy as potential energy.

12. Mar 21, 2009

### cam875

i was just trying to apply what I read about potential energy(which I do understand now) and ive understood kinetic for some time now, Im not gunna try to understand GR or QFT yet just trying to get a feel for what they mean when it comes to energy stored in a gravitational field in a sense.

So even in classical newtonian gravity which is what were studying now, would it be safe to say that the energy stored in the gravitational field that traverses between the two particles is potential until it gets to the other particle, than it is transformed to kinetic hence the increase in kinetic energy due to gravity (attraction)?

13. Mar 22, 2009