Gravity is not considered part of the electromagnetic spectrum because it is fundamentally different from electromagnetic waves in several ways. While gravity does propagate at the speed of light, it is not an electromagnetic force. Here are some key distinctions:
Nature of the Force: Electromagnetism is a fundamental force of nature that arises from the interaction between electrically charged particles. It is mediated by photons, which are particles of light and electromagnetic radiation. Gravity, on the other hand, is a fundamental force that arises due to the presence of mass and energy. It is mediated by hypothetical particles called gravitons (though they have not been directly observed yet).
Interaction with Charged Particles: Electromagnetic forces act on charged particles (electrons, protons, etc.) and are responsible for phenomena such as electric fields, magnetic fields, and electromagnetic radiation. In contrast, gravity affects all forms of matter and energy, regardless of their charge or electromagnetic properties. It acts on mass and energy, including neutral particles such as neutrons and neutrinos.
Quantum Field Theory: Electromagnetism is described by quantum electrodynamics (QED), which combines quantum mechanics with special relativity. Gravity is described by the theory of general relativity, which provides a classical description of gravity but is not yet reconciled with quantum mechanics. Efforts to develop a quantum theory of gravity are still ongoing.
Different Mathematical Framework: Electromagnetic forces are described by Maxwell's equations, which are a set of differential equations that govern the behavior of electric and magnetic fields. Gravity, on the other hand, is described by Einstein's field equations, which relate the curvature of spacetime to the distribution of matter and energy.
While both gravity and electromagnetic waves propagate at the speed of light, their underlying principles, interactions, and mathematical descriptions are distinct. Hence, gravity is considered a separate force from electromagnetism and not part of the electromagnetic spectrum.