Self-coupling, in the context of physics, specifically refers to the phenomenon where a fundamental field or particle interacts with itself. In the case of gravity, it means that gravity gravitates; gravitational fields themselves are sources of gravity.
Here's a breakdown of what that implies:
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Gravity Gravitates: This is the most fundamental aspect. Unlike, for example, electromagnetism where photons (which mediate the electromagnetic force) are not charged themselves, gravitons (the hypothetical particles mediating gravity) would interact with each other because they carry energy and momentum, which are sources of gravity. Think of it this way: electromagnetic waves don't produce more electromagnetic waves just by existing, but gravitational waves would produce more gravitational waves (or alter their own path) because they carry energy, and energy warps spacetime, creating more gravity.
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Gravitational Waves are Affected by Gravity: As gravitational waves travel through space, they are affected by the gravitational fields of massive objects they encounter. This bending and distortion of gravitational waves due to gravity is a direct consequence of self-coupling.
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Graviton Interactions: If gravitons exist (though they haven't been directly detected yet), they would interact with each other. This interaction is a result of them carrying gravitational "charge" (energy/momentum). This contrasts with photons, which do not interact with each other directly in the Standard Model.
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Non-linearity: Self-coupling makes the theory of gravity (General Relativity) inherently non-linear. This means that the gravitational effect of two masses is not simply the sum of their individual gravitational effects; there are additional interaction terms. This non-linearity is what makes solving Einstein's field equations notoriously difficult.
In summary, self-coupling in gravity means that the gravitational field itself is a source of gravity, leading to graviton interactions, the bending of gravitational waves by gravity, and the non-linear nature of General Relativity.
