Questions on potential gravitational energy

[Xilor]

I think you are making far too many classical assumptions about a relativistic model to be able to draw any valid conclusions. How do you envisage gravitons actually interacting with a mass?
You can be pretty sure that it's not as simple as you are implying.

Fair enough. There isn't anything that tells how gravitons would interact right? Pushing mass back to where the graviton came from is as far as I got without any idea how this might occur.
Anyway am I completely imagining a problem here then, or is there still a small amount of validity to my questions?

I interpreted the OP's question in that way: assuming that the total system mass incl. radiation must be conserved, the same atoms must have reduced mass when at rest at reduced potential. Consequently the inverse should also be true: if we lifted an object up from the earth, that object should indeed have increased mass. Note: I think that that is an SR argument.

Yes that was sort of my line of thinking, well, I didn't really think of that myself I was confused for a bit when I read that potential gravitational energy increased mass but it made sense. Apparently it seems it was wrong.

 

[Simon Bridge]

I think the best that can be drawn is that classical gravity does not work under the kinds of conditions in which you need relativity. Which would be a bit of a no-brainer.

Perhaps the question that is wanted is:
"How does General Relativity handle the kinds of situations where we would normally use gravitational potentials?"

 

[harrylin]

[..] Yes that was sort of my line of thinking, well, I didn't really think of that myself I was confused for a bit when I read that potential gravitational energy increased mass but it made sense. Apparently it seems it was wrong.

I don't know why what you read now seems wrong to you. It is not contained in classical mechanics while in the context of GR there is an issue with definitions. However, it appears to be correct in the context of SR, just as D_H illustrated.

 

[Xilor]

I think the best that can be drawn is that classical gravity does not work under the kinds of conditions in which you need relativity. Which would be a bit of a no-brainer.

Perhaps the question that is wanted is:
"How does General Relativity handle the kinds of situations where we would normally use gravitational potentials?"

That sounds good, what is the answer?

I don't know why what you read now seems wrong to you. It is not contained in classical mechanics while in the context of GR there is an issue with definitions. However, it appears to be correct in the context of SR, just as D_H illustrated.

I see, so what exactly is gaining mass then within SR? The system or the particles themselves? And has any mass gain ever been measured?

 

[sophiecentaur]

afaic, SR is totally irrelevant to all of this. It just doesn't apply where there's acceleration or gravity so it would be better not to introduce it into the conversation.
Either stick to classical ideas or go the whole hog and that means GR and beyond - in which case, there can be no valid conclusions based on classical ideas.

 

[harrylin]

[...] I see, so what exactly is gaining mass then within SR? The system or the particles themselves? And has any mass gain ever been measured?

The point of the argument was the assumption that of a closed system, total system energy must be conserved. So any changes can only be in the particles (and/or in the fields - this is where things get fuzzy!).

Now, your questions slowly drift more and more into the field of GR in which commonly slightly different terms and definitions are used. What has been directly measured (and is thus independent of definitions), is the GR prediction that light from the sun is red-shifted compared to light from a similar source on earth.

Note also: SR applies in good approximation to many problems with gravitation (happily so for people at CERN!) and it certainly applies where there is acceleration. The Lorentz equations just don't refer to accelerating frames.

 

[sophiecentaur]

It seems to me that the thread is looking at very small effects that can only be explained in terms of relativity. Whereas a difference in mass may be looked at in terms of SR, the actual process of change needs to take in GR. I just think that, once you've gone that far, there is absolutely no mileage in trying to go further with simplistic classical ideas - at least, not without a lot of knowledge of GR which could, possibly tell you when and where you are 'permitted' to take such liberties.

I reckon you are right down towards the 'what is mass and how does gravity actually work' regions. How can one justify discussing the way a small mass and a big mass will behave in isolation with each other when, at that level of discussion, it's the whole mass of the Universe that determines what goes on and what determines the value of G, for instance. It is just so much pushing your luck in the dark and a guarantee of tears before bedtime. It's almost trivialising the whole business - as if there's a simple answer just round the corner. `Some hopes'.

 

[Xilor]

Well I still have my hopes up that physics will eventually be able to solve everything as having very simple rules, with very simple explanations for those rules. But I understand that that's not where physics is at yet.
So is the eventual answer then: it's unexplainable now or we can't explain it on a board to someone who is missing essential knowledge or that different theories say different things and that they can't be combined too well?

 

[sophiecentaur]

I have only one response to that. Why should it be 'simple'?

 

[Xilor]

Because the concept of a universe makes far more sense if everything that happens can derived from just a few simple rules, rules which may combine in complex ways to create other rules. So far it seems that science has done a great job at doing so, finding complex system, figuring out what are the essential elements that combine to form these systems and using them to be able to predict these systems. Rules and laws which we have found seem to be mostly just complex systems again which we don't fully understand yet. I can't really think of any rules we have found that couldn't possibly be the result of another system.

 

[sophiecentaur]

"makes far more sense" is making a few demands of reality. Our experience, so far is that an initially simple rule - like, say, Newton's Second Law of motion - turns out to be more complicated at even very slow speeds and then when there a slight touch of gravity about, it gets even more complicated.
I know that our minds really love the reductionist principle in life in general but pretty much everything does seem to be more and more complicated once you get down to it.

 

[Xilor]

Fair enough, I still do hope it is simple though.

But let me restate my question:
So is the eventual answer then: it's unexplainable now or we can't explain it on a board to someone who is missing essential knowledge or that different theories say different things and that they can't be combined too well?

 

[sophiecentaur]

My personal answer is that the question is unanswerable because it seems to demand answers which are too much inside the set of familiar concepts.

 

[harrylin]

[..] So is the eventual answer then: it's unexplainable now or we can't explain it on a board to someone who is missing essential knowledge or that different theories say different things and that they can't be combined too well?

- it's explained (up to a certain level, or in a certain sense) by relativity theory

- there is a good reason to think that the above-mentioned SR argument by D_H is quite good for its purpose and compatible with the GR jargon: Einstein developed GR based on exactly that kind of SR arguments (I recall that it's in one of his 1911 papers).

Harald