B Why are gravitons needed to explain gravitational attraction?

[lukegregor]

Hi! Isn't gravity just a smaller object moving toward the lower energy state created by a larger object (time slows down the closer you are to a massive object)? Why do we need a force carrying particle for gravity?

 

[DaveC426913]

Hi! Isn't gravity just a smaller object moving toward the lower energy state created by a larger object (time slows down the closer you are to a massive object)? Why do we need a force carrying particle for gravity?

Because what you have written there is not an explanation; it is merely an observation.
What we want to know is the mechanism by which the above happens.

 

[lukegregor]

Isn't the mechanism that any object settles/moves toward the lowest available energy state?

 

[DaveC426913]

Isn't the mechanism that any object settles/moves toward the lowest available energy state?

Thats not a mechanism, that's an observation. It doesn't explain how it works.

 

[Melbourne Guy]

Why do we need a force carrying particle for gravity?

As you may know, @lukegregor, general relativity (GR) describes how mass effects spacetime, but as @DaveC426913 has noted, that does not explain how gravity works. In particular, GR does not reconcile with quantum mechanics (QM) which means there are situations where our understanding of physical processes is incomplete..and even seemingly impossible. Black holes with infinite density are an example, as is the firewall paradox.

Physicists expect that to reconcile GR and QM requires a theory of quantum gravity, and the graviton is the hypothetical elementary particle that mediates the force of gravitational interaction. Whether we need the graviton is not yet settled because nobody has quantised gravity.

Perhaps the reconciliation of GR and QM will upend QM and some other mechanism will be shown to be driving the universe, but at the moment, the graviton is the best idea we have.

 

[Ibix]

Hi! Isn't gravity just a smaller object moving toward the lower energy state created by a larger object (time slows down the closer you are to a massive object)?

This is a very inaccurate picture of gravity. A simple question: what happens if there are two equal mass objects?

In fact, the "time slows down near a massive object" claim, although often stated, is only true in a very restricted class of spacetimes called "stationary spacetimes". Most spacetimes - including all realistic ones - are not stationary and there isn't even a way to define "time" in the sense it's being used in that phrase. However, in most every day circumstances you can get away with pretending that realistic spacetimes (e.g. the one in the region we inhabit near Earth) are stationary. But it's only an approximation.

(Note that the above is the answer to my first question. A spacetime containing two objects of equal non-negligible masses is not remotely stationary, and trying to define a potential energy and/or a time dilation factor does not work.)

Futhermore, there are serious problems with general relativity. An incredible range of spacetimes can be shown to have singularities, which are regions where the mathematical machinery of general relativity breaks down. Notable singularities are somewhere inside a black hole and about 14 billion years ago in the Big Bang model. So we know we need a better theory, one which would be able to explain what happens in these regions. We expect this better theory to be a quantum theory of gravity (roughly speaking, because everything that generates gravity can do quantum stuff like exist in superpositions, so we need a way to make their gravitational fields superpose, and you can't do that in GR), which would include some kind of force carrier particle that we've christened the graviton.

Isn't the mechanism that any object settles/moves toward the lowest available energy state?

As I said above, that isn't a generally applicable description in GR. Even if it were, it doesn't provide a mechanism to calculate the energy states near singularities our only model of gravity fails.

 

[lukegregor]

Thank you for your replies! If you have an object traveling through "normal" space/time and it comes across the lower energy space/time distortion field (i.e. "gravity well") created by a massive object, wouldn't the object naturally move toward the lower energy distortion field due to the conservation of energy? Seems like this already defines the mechanism for "gravitational attraction" and there's no need for a graviton...?

 

[Melbourne Guy]

Thank you for your replies! If you have an object traveling through "normal" space/time and it comes across the lower energy space/time distortion field (i.e. "gravity well") created by a massive object, wouldn't the object naturally move toward the lower energy distortion field due to the conservation of energy? Seems like this already defines the mechanism for "gravitational attraction" and there's no need for a graviton...?

It may be useful to research gravity to improve your understanding, @lukegregor, because you seem to have just asked the same question you have already asked in this thread. Gravity is not based on 'wells' or 'bendable sheets', which are commonly used to convey a simplistic view of GR but they won't take you anywhere near far enough to usefully discuss gravitons...or not gravitons.

 

[Ibix]

wouldn't the object naturally move toward the lower energy distortion field due to the conservation of energy?

As I already said, this is not a general explanation of gravity. You cannot apply conservation of energy when you cannot define gravitational potential energy. Also, we need a better theory of gravity to cover extreme circumstances where GR fails, and we have reason to expect this to be a quantum theory.