I've read and heard multiple times that gravity can't be explained by the current understandings of quantum physics. Wikipedia's Gravity entry briefly explains the incompatibility between general relativity and quantum mechanics on the subject of gravity as follows:

In the decades after the discovery of general relativity, it was realized that general relativity is incompatible with quantum mechanics. It is possible to describe gravity in the framework of quantum field theory like the other fundamental forces, such that the attractive force of gravity arises due to exchange of virtual gravitons, in the same way as the electromagnetic force arises from exchange of virtual photons. This reproduces general relativity in the classical limit. However, this approach fails at short distances of the order of the Planck length, where a more complete theory of quantum gravity (or a new approach to quantum mechanics) is required.

Photons, as I understand, are the force carriers of the electromagnetic field which 'transfer' quantized 'bulks' of energy and, as far as I know, they are not needed for a negative particle (let's say an electron) and a positive particle (let's say a positron) to attract.

So, if two positively and negatively charged particles can experience attraction without the need of photons in the process, why are gravitons (or another explanation) needed for gravitational attraction to exist between two masses?

Edit: I'll explain why my question is not a duplicate. These other questions have good, related answers which I actually learned a lot from. I understand that just as the other fundamental forces have force carrier particles described by QM, gravity still has to be explained in the quantum level. However this is specifically what I don't get (and I could be totally wrong): Photons, as I've understood, do not play a role in the attraction between oppositely charged particles. They have other functions like the transfer of energy. So, why does gravitational attraction between two masses need to be explained by the exchange of gravitons?

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    $\begingroup$ Possible duplicate of :physics.stackexchange.com/q/427 $\endgroup$ – user108787 Oct 22 '16 at 1:23
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    $\begingroup$ "Although photons are the particles who mediate the interactions in the electromagnetic field, they are not needed for a negative particle (let's say an electron) and a positive particle (let's say a positron) to attract." - ??? As written, this seems to imply that attraction (repulsion) is not an interaction. $\endgroup$ – Alfred Centauri Oct 22 '16 at 2:10
  • $\begingroup$ @AlfredCentauri You're right. I just edited, hope it articulates better my point. $\endgroup$ – Gabe12 Oct 22 '16 at 2:27
  • $\begingroup$ I'm very confused. The premise of the question is false; who told you this? $\endgroup$ – knzhou Oct 22 '16 at 2:47
  • $\begingroup$ Possibly related question here. $\endgroup$ – knzhou Oct 22 '16 at 2:47

If a universe has any quantization of energy at all (as our universe does), then everything must be quantized and nothing continuous. Otherwise, if there are both quantized and continuous forms of energy, where is the extra energy stored as it moves from one form to the other? If we have quantized energy flowing into a continuous form, then it must either flow continuously out of the quantized form (and therefore that isn't quantized), or enter the continuous form as a packet (and therefore is quantized). If energy moves from a continuous form to a quantized form, then it must be temporarily stored somewhere as it builds up enough to be moved as a packet to the quantized form--and where is it stored? Energy can certainly be held in a gravitational system, therefore it must be quantized, and that's a graviton.

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  • $\begingroup$ great argument. $\endgroup$ – Isobutane Oct 22 '16 at 3:57
  • $\begingroup$ While I agree with this answer in general, it is probably worth mentioning that the requirement of quantization of gravitational field is not equivalent to the requirement of the existence of gravitons, since not all quantum-mechanical theories admit particle interpretation. $\endgroup$ – Prof. Legolasov Oct 22 '16 at 6:13
  • $\begingroup$ I didn't think that energy is quantized, is it? Angular momentum, charge, parity, etc. are quantized. But energy, momentum, etc. are not. So position * momentum and energy * time are quantized (because they have the same dimensions as planck's constant or angular momentum), but neither position, momentum, energy nor time alone are quantized. Any of them alone can potentially have any value on a continuous range. At least that's always been my understanding. $\endgroup$ – RBarryYoung Aug 1 '19 at 17:46

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