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Why do all electromagnetic waves travel at the speed of light in a vacuum?

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All electromagnetic waves travel at the same speed in a vacuum because this velocity is a fundamental constant of nature, denoted as c.12

In classical physics, James Clerk Maxwell’s equations for electromagnetic fields demonstrated that changing electric fields produce magnetic fields, and vice versa, creating self-propagating waves. Maxwell’s wave equation showed that the speed of these waves is determined by the permittivity and magnetic permeability of free space. Because these properties are constant in a vacuum, the resulting speed of propagation—approximately 299,792,458 metres per second—is also constant for all electromagnetic radiation, regardless of its frequency or wavelength.345

In modern quantum theory, this radiation is described as a flow of photons, which are massless packets of energy that always travel at this universal speed limit. This constant c serves as the upper bound for the propagation of all signals and material particles in the universe.15