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Why do electromagnetic, strong, and weak forces have spin-1 mediators, while gravity's hypothesized mediator has spin 2?

This question aims to explore the fundamental differences between gravity and other fundamental forces, focusing on why their force-carrying particles have different spins. I am not asking about the technical derivation of the graviton's spin from the Einstein-Hilbert action, but rather seeking to understand the physical or theoretical reasons behind this spin difference across forces.

Neutralino
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2 Answers2

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There's different levels of sophistication you could give to this question, but at the most basic level: empirically, general relativity is an extremely successful theory of gravity and agrees with observations every time we can test it. So, we tend to trust it's predictions as a reasonable, default hypothesis even when we cannot directly verify them.

General relativity predicts gravitational waves have a spin-2 character. While we've never directly measured the polarization (beyond ruling out some extreme, strawman possibilities), many other predictions of GR with respect to gravitational waves have turned out to be correct (such as the spin-down rate of the Hulse-Taylor pulsar, the direct detection of gravitational waves by LIGO/Virgo from compact binary systems, the speed of gravitational waves, etc). According to the normal rules of quantization, classical gravitational waves become gravitons. So we expect spin-2 polarized classical gravitational waves to correspond to a spin-2 graviton after quantization.

Andrew
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From a theoretical point of view you can see this as the fact that the 3 standard model interactions are forces (they generate 1D displacements), while in general relativity gravity exerts deformations* (which are described roughly as 2D displacements).

1D displacements are described by vectors (i.e. spin-1 objects) while deformations are described by particular rank-2 tensors (i.e. spin-2 objects). This is the reason.

Why gravity does exerts those deformations instead of forces and is so different is yet to discover.

*Deformations are intended, roughly, as the application of force in 2 different directions, usually not collinear, so that it deforms the path and shape of objects immersed in the gravitational field

LolloBoldo
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