How Does Gravity Affect Particle Kinetic Energy in a Vacuum?

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Discussion Overview

The discussion revolves around the effects of gravity on the kinetic energy of two particles in a vacuum. Participants explore theoretical models involving gravitational interactions, potential energy, and the nature of energy transformation between rest energy and kinetic energy.

Discussion Character

  • Exploratory
  • Technical explanation
  • Conceptual clarification
  • Debate/contested

Main Points Raised

  • One participant posits that gravity acting on two initially at-rest particles would increase their kinetic energy, questioning the source of this energy and whether it implies a change in rest mass over time.
  • Another participant asserts that the kinetic energy gained comes from gravitational potential energy (GPE), suggesting that GPE and kinetic energy are equal and that rest mass remains constant.
  • A participant raises the idea that if gravitons are involved, they would need to carry kinetic energy to transfer it to the particles.
  • Concerns are expressed about how particles can generate kinetic energy from rest energy while alone in a vacuum without violating conservation of energy.
  • Some participants clarify that potential energy is a real form of energy, using analogies like a coiled spring to illustrate its existence and transformation into kinetic energy.
  • There is a discussion about whether the energy in the gravitational field is dependent on the rest energy of the particles and how this energy is perceived before it is converted to kinetic energy.
  • One participant emphasizes that potential energy is not a physical substance that moves but rather a property of the configuration of objects.

Areas of Agreement / Disagreement

Participants express differing views on the nature of energy transformation and the role of potential energy in gravitational interactions. There is no consensus on how energy is sourced or transformed in this context, indicating ongoing debate and uncertainty.

Contextual Notes

Participants reference concepts from both classical Newtonian gravity and quantum field theory (QFT), indicating a mix of theoretical frameworks. The discussion includes unresolved questions about the nature of energy in gravitational fields and the implications for rest mass.

cam875
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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.
 
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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.
 
You may also want to read about http://en.wikipedia.org/wiki/Binding_energy" .
 
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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?
 
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 each other 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.
 
cam875 said:
so does anyone have an answer to this?

DaleSwanson said:
The kinetic energy comes from the gravitational potential energy.

Is your answer. Is there something you don't like about it? If so, please be specific.
 
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 don't know 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.
 
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.
 
so if QFT works out then the graviton will be the carrier of the potential energy for gravity that will turn into kinetic?
 
  • #10
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
Gravity provides the energy as potential energy.
 
  • #12
i was just trying to apply what I read about potential energy(which I do understand now) and I've understood kinetic for some time now, I am 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
Potential energy is not "stuff" that moves around. Potential energy is a property of a configuration of objects, not some sort of pseudo-material.
 

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