Gravity in relation to mass & energy

In summary: I was thinking, a particle a certain distance from another would have a certain potential energy. Now the potential energy must be in the form of extra mass if I am not mistaken (as its not kinetic, however does kinetic energy increase mass?) so therefore an object is at its maximum mass at infinity and at its lowest at 0. If two particles approach themselves from infinity, the system of the two particles looses mass, in the form of energy radiated away. Where do this energy go? Of course it can't escape the universe, so the total energy of the universe doesn't change (at least, not for this reason). So, objects at rest in a gravitational field have less mass. Does this mean as something accelerates in a gravitational
  • #1
Denton
120
0
I was thinking, a particle a certain distance from another would have a certain potential energy. Now the potential energy must be in the form of extra mass if I am not mistaken (as its not kinetic, however does kinetic energy increase mass?) so therefore an object is at its maximum mass at infinity and at its lowest at 0.

I can then conclude that the at the pre universe, the 'singularity' or whatever you call it would have had less mass than the current universe. Does this mean that the early universe had less energy than the present day?
 
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  • #2
Was this too hard, or was i way off? My reasoning comes from the fact that you add energy to an atom in an excited state, its mass increases by E / c^2. Following this I am concluding that GPE is stored in mass also. (disregard my previous post asking if kinetic energy increases mass, I've blatantly overlooked special relativity :)

If this is correct then what i stated must be true, the pre big bang had less mass than it does today.
 
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  • #3
Denton said:
I was thinking, a particle a certain distance from another would have a certain potential energy. Now the potential energy must be in the form of extra mass if I am not mistaken (as its not kinetic, however does kinetic energy increase mass?) so therefore an object is at its maximum mass at infinity and at its lowest at 0.

I can then conclude that the at the pre universe, the 'singularity' or whatever you call it would have had less mass than the current universe. Does this mean that the early universe had less energy than the present day?
When two particles approach themselves from infinity, the system of the two particles looses mass, in the form of energy radiated away. Where do this energy go? Of course it can't escape the universe, so the total energy of the universe doesn't change (at least, not for this reason).
 
  • #4
As gravitational waves?
 
  • #5
GPE is negative stored mass. Kinetic Energy is positive. If the Objects are free floating, both parts change simultanely, conserving the total energy. If you bring the particles to rest wrt each other, you gain energy (from KE) in form of explosions, heat, or something usable. Radiate it away, and the mass of the System decreases. So, objects at rest in a gravitational field have less mass.
 
  • #6
Denton:
Now the potential energy must be in the form of extra mass if I'm not mistaken

I've never heard that before. Where did you get it ?

Ich:
So, objects at rest in a gravitational field have less mass.
Does this mean as something accelerates in a gravitational field it loses (inertial?) mass ?
 
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  • #7
Denton said:
As gravitational waves?
I don't see others possibilities.
 
  • #8
lightarrow said:
I don't see others possibilities.

Does it mean the planets lose continuously their mass because of orbital motion (acceleration)?
 
  • #9
Does it mean the planets lose continuously their mass because of orbital motion (acceleration)?

Well they're not changing their GPE.

GPE is negative stored mass.

Im not sure how to take this, are you saying this purely mathematical or realistic?
 
  • #10
Mentz114 said:
Does this mean as something accelerates in a gravitational field it loses (inertial?) mass ?
I'd say it loses rest mass and gains KE. Its total energy is conserved. This should be true at least in weak fields, when calculated in an inertial frame. Unfortunately, the people that would point out all the caveats are no longer around.


Denton said:
Ich said:
GPE is negative stored mass.

Im not sure how to take this, are you saying this purely mathematical or realistic?
Ok, if you bring two masses together, you gain energy. You can carry the energy away, and what's left ist a system with less energy (also less mass) than before. That deficit is called binding energy, which is negative. So I'm actually talking about mass defect here, not strictly potential energy.
 
  • #11
Ich said:
I'd say it loses rest mass and gains KE. Its total energy is conserved.
...

But what kind of mass is it? The number of particles does not change . Have particles less mass if they come nearer and why?
 
  • #12
I asked a question similar to this months ago.

https://www.physicsforums.com/showthread.php?t=226703

It went something like, if I lift an object does its mass increase due to increased potential energy? There was disagreement about whether a change in GPE was stored in the object, in both the object and the Earth, or "in the field", which puzzled me because I didn't know how an increase in energy -- which should also be an increase in mass -- could appear in a field and not concentrated in a particular location. Then someone posted a link to another thread

https://www.physicsforums.com/showthread.php?t=57502

which mentioned that in GR mass/energy is not even defined in a consistent way, or it depends on your state of motion, Killing vectors, etc...and it quickly went over my head.

You might find these threads helpful. My impression is that there is no simple answer to this kind of question.
 
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  • #13
cryptic said:
But what kind of mass is it? The number of particles does not change . Have particles less mass if they come nearer and why?
No, their mass don't change. The mass is not only in the particles but in the field too.
 
  • #14
lightarrow said:
No, their mass don't change. The mass is not only in the particles but in the field too.
I think I know what you are getting at but how do you rationalize that this position holds up to the fact that what you call 'the field' is on the other side of the Einstein field equations?
 
  • #15
lightarrow said:
No, their mass don't change. The mass is not only in the particles but in the field too.

This is true only in case of electromagnetic field, because em-field disappears completely if partikels move very close together whereas gravitational field does not change.
 
  • #16
MeJennifer said:
I think I know what you are getting at but how do you rationalize that this position holds up to the fact that what you call 'the field' is on the other side of the Einstein field equations?
Sorry, could you please explain better what you mean?
 
  • #17
snoopies622 said:
...

It went something like, if I lift an object does its mass increase due to increased potential energy?
...

We can use object's mass (heat) to increase its GPE. In this case system has less mass - mass is converted to GPE - but if object falls GPE is converted back to mass (heat).
 
  • #18
lightarrow said:
No, their mass don't change. The mass is not only in the particles but in the field too.

You are right, gravitational field has mass too. And the question remains, how is this mass induced in "empty" space.
 
  • #19
Wait what? A gravitational field has mass?
 
  • #20
This is a very entertaining thread. I'd like to throw in this - in GR, mass doesn't even couple to gravity. Mass is the source of the space-time configuration, so you could say that matter and energy couple to space-time, which acts like rails for the matter to move on. There is no interaction term between matter and the field in the Lagrangian or action of GR so no gravitons either.
 

1. How does mass affect gravity?

Mass is directly proportional to gravity. This means that the more mass an object has, the stronger its gravitational pull will be. This is why larger objects like planets have a stronger gravitational pull than smaller objects like asteroids.

2. How does energy affect gravity?

Energy is related to gravity through Einstein's famous equation, E=mc². This means that energy and mass are interchangeable and can affect gravity in the same way. For example, a massive object like a black hole has a tremendous amount of gravitational pull because it has a lot of mass, which is equivalent to a lot of energy.

3. Can gravity exist without mass?

No, gravity cannot exist without mass. Mass is necessary for gravity to exist because it is the source of the gravitational field. Without mass, there would be no gravitational pull between objects.

4. How does gravity affect the movement of objects?

Gravity affects the movement of objects by pulling them towards the center of mass of a larger object. This is why planets orbit around the sun and why objects fall towards the Earth. The strength of the gravitational pull depends on the mass of the objects and the distance between them.

5. Can mass and energy be created or destroyed by gravity?

Mass and energy cannot be created or destroyed by gravity. They can only be converted from one form to another, as seen in Einstein's equation. Gravity can cause a change in the energy of an object, but it cannot create or destroy mass or energy.

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