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> I'm also OOTL on the boson part The idea behind that is that, for all other interactions (electromagnetic, weak, strong), our Standard Model of particle phys
by pdonis 3y ago
> I'm also OOTL on the boson part
The idea behind that is that, for all other interactions (electromagnetic, weak, strong), our Standard Model of particle physics tells us that there are "gauge bosons" (for electromagnetism it's the photon, for weak it's the W+, W-, and Z bosons, and for strong it's the eight gluons) that mediate the interaction. The full details of how this works involve quantum field theory and are way too long to go into here; all we need for this discussion is the basic idea that any interaction in quantum field theory has one or more "gauge bosons".
So if we are going to try to quantize gravity, the obvious way to do it--at least if you are a Standard Model particle physicist--is to treat it the same way and have a "gauge boson" for it. This has actually been done: the quantum field theory of a massless spin-2 field, which is what a "gauge boson" for gravity would look like, was developed in the 1960s and early 1970s. Furthermore, the classical limit of this quantum field theory is known to be General Relativity, i.e., the classical theory of gravity that we already have and that we already know works. So if we want to treat gravity as if it were a Standard Model interaction and give it a "gauge boson", this would be one obvious way to do it.
The reddit questioner was asking the more basic question of why we would want to do that at all--why do we need to quantize gravity. If we don't need to, the fact that we could do it using this known mathematical model (there are other issues that come into play with that, which one of the reddit responses mentions, but leave that aside) is irrelevant. Not all mathematical models have physical realizations.
- Zambyte 3y agoThank you for going deeper. I'm going to give this a more thorough look and investigate deeper when I have more time in a few hours.
- AnimalMuppet 3y agoProbably a dumb question, but... why massless? If I understand GR correctly, the gravitational field itself (especially in the form of a gravitational wave) has energy, and therefore contributes to the mass-energy tensor, further bending spacetime. Would that correspond to a gauge particle with mass?
- pdonis 3y ago> why massless? Meaning, why is the spin-2 field massless? The easiest short answer is, because gravity is a long range interaction, and only massless gauge bosons can give you a long range interaction. That's why electromagnetism is also long range (the photon is massless) but the weak interaction is not (W+, W-, and Z are massive). The strong interaction, unfortunately, breaks this simple heuristic (the gluon is massless but the strong interaction is short range), because it has other factors involved which aren't as easy to explain. But we know that those other factors don't come into play with the spin-2 field model of gravity, so we can still safely say that the graviton should be massless. > If I understand GR correctly, the gravitational field itself (especially in the form of a gravitational wave) has energy Only in a certain sense, which is... > and therefore contributes to the mass-energy tensor, further bending spacetime. ...not this sense. The stress-energy tensor does not contain any "gravity" contribution. In the Einstein Field Equation, "gravity" is on the LHS, represented by the Einstein tensor, and "mass-energy" is on the RHS, represented by the stress-energy tensor. That has to be the case in order for local conservation laws to hold. What all that means is that there is no valid local concept of "energy stored in the gravitational field" (because there is no tensor quantity that corresponds to this). But globally, you can still look at a system that emits gravitational waves and come up with a meaningful concept of "total energy" that decreases with time as gravitational waves are emitted. (This concept is called the "Bondi energy".) So we can say that "gravitational waves carry energy" in this sense. We just can't localize where that energy is.
- AnimalMuppet 3y agoThat's a very good explanation. Thank you. One of the things I love about HN: You don't just get "you're stupid not to know that" (though you sometimes get that too). You also get real explanations from people that actually know things and take the time to explain it well.
- pdonis 3y ago> Thank you. You're welcome! Thanks for the kudos.