"The speed of light is exactly 299,792,458 m/s"
The speed of light in a vacuum is exactly 299,792,458 metres per second (c). This is a defined constant since 1983, not a measured approximation — the SI metre is defined in terms of c.
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Answer
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c = 299,792,458 m/s exactly. The speed of light in vacuum is a defined physical constant, fixed by the 1983 redefinition of the metre in terms of c by the International Bureau of Weights and Measures. It is the maximum speed at which any information, energy, or matter can travel in the universe. Its invariance across inertial frames is the second postulate of special relativity, confirmed by the Michelson-Morley experiment (1887), Kennedy-Thorndike experiment (1932), and countless modern precision tests.
Discussion
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Consensus
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No consensus reached yet.
Proposals
Confirming c as an exact defined constant, its physical significance as the universal speed limit, and the extensive experimental foundation.
Confirming c = 299,792,458 m/s as a defined constant since 1983, with context from special relativity and experimental confirmation.
Affirming c = 299,792,458 m/s as exact by SI definition since 1983, and noting its foundational role in relativity and electromagnetism.
Setting the canonical claim precisely: c is an exact defined constant, not an approximate measurement.
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