Why do we need a graviton?

[DannyWalter]

As one who meditates on cosmology, relativity, and quantum (you know, why are we here), as a layperson, I don't know that I've seen an answer to this question.

If gravity is the curvature of spacetime by mass, why is the standard model "incomplete" without a graviton and a quantum theory of gravity.

Why wouldn't we postulate that gravity doesn't apply in quantum because there is effectively no mass, just energy, that only becomes a particle when somehow the wave function collapses?

Why does there need to be any relationship between quantum and spacetime? Can't it just be said that one exists inside the other?

Apologies in advance if this is naïve and not well stated, but it bothers me; hoping someone can set me straight. Thanks.

[swansont]

In quantum models you need an exchange particle, and GR has singularity issues, so we know it’s not a complete theory.

[Genady]

1 hour ago, DannyWalter said:

Why does there need to be any relationship between quantum and spacetime? Can't it just be said that one exists inside the other?

This relationship is needed because fields and particles don't only exist inside a curved spacetime, but they tell spacetime how to curve.

[beecee]

1 hour ago, swansont said:

In quantum models you need an exchange particle, and GR has singularity issues, so we know it’s not a complete theory.

 While GR certainly fails us at the quantum/Planck level, (just as Newtonian fails us at other levels covered by GR) it is generally accepted that the infinite qualities of space/time curvature and density do not eventuate, which leaves us to rationally consider a  "surface of sorts" at or below that level, where GR isn't accurate enough to ascertain. Could this possibly be the quantum model of gravity?

Will we ever have a validated QTG, considering that we can never see beyond the EH? And the loop QTG and String theory and other derivitives are still speculative in nature.

Just some passing thoughts.

[MigL]

Current theories of gravity don't need a graviton as they are classical theories.
However we know these theories are not applicable at high energies and small separations.
Much like electromagnetic and chromdynamic theories, what seems to work out better at high energies and small separations are quantized field theories, which, because of quantum uncertainty, can produce excitations of those fields. These excitations can be virtual, if below a certain threshold, or real, if above that threshold. What we call 'particles' are manifestations of these vrtual or real excitations, and the graviton is essentially an excitation of the gravity field.
Gravitons are simply a requirement of a quantized field theory.
( which we hope to eventually have )

Edited April 23, 2022 by MigL

[beecee]

4 hours ago, DannyWalter said:

Apologies in advance if this is naïve and not well stated, but it bothers me; hoping someone can set me straight. Thanks.

Actually a very good question, and no apologies needed. It's only on the occasions that some ask questions like this, with a hidden agenda and with no intention of accepting or learning from the replies.  

[Markus Hanke]

5 hours ago, DannyWalter said:

If gravity is the curvature of spacetime by mass

It’s not that simple, I’m afraid. The source of gravity isn’t just mass, but energy-momentum; things like massless particles, electromagnetic fields, stress, pressure etc etc all have a gravitational effect too. What’s more, gravity also couples to itself, making it non-linear.

5 hours ago, DannyWalter said:

Why does there need to be any relationship between quantum and spacetime?

Well, in principle there’s no guarantee that there is such a thing as quantum gravity - at present there is no experimental evidence for its existence. The issue however is that if gravity is always classical, then we run into all manner of inconsistencies that are difficult to resolve - for example the formation of different kinds of singularities, or violations of unitarity at event horizons. It’s not dissimilar to the kind of problems that historically lead to the development of quantum mechanics.

Thus, most physicists assume that gravity can be quantised somehow.

One important point though - quantisation of gravity does not necessarily imply the existence of gravitons. Gravitons arise if we attempt to quantise GR in the same way as the other fundamental interactions, namely by using the framework of quantum field theory. We pretty much know by now that this approach doesn’t really work out very well, so current research is looking at other options. Depending on how things turn out in the end, the graviton may or may not be a part of the solution.

[Tutoroot]

Thanks a lot. This is a helpful information about the graviton.