Light travels at a finite speed.
In vacuum, light travels at exactly 299,792,458 meters per second under the modern SI definition.
Show me why
Light takes time to travel. The SI meter is defined using the fixed vacuum speed of light and the second. Earlier experiments measured that speed; the exact numerical value today is part of how the units are defined.
A common misconception
“Very fast” does not mean instantaneous.
Where the limits are
Propagation through a material differs from propagation in vacuum. This entry concerns vacuum speed.
How we know
NIST distinguishes the measured history from the present definition. This prevents a common confusion: an exact defined value in SI does not make the observation of finite travel time a matter of convention.
What would change this conclusion?
Reproducible evidence inconsistent with finite propagation would require a new physical explanation; changing units changes the numeral, not the phenomenon.
Sources you can inspect
- NISTSI Units – Length
Gives the SI value for the speed of light.
Publication date not stated · Checked in initial research 2026-10-08 - NISTMeter
Describes experiments and the modern definition.
Publication date not stated · Checked in initial research 2026-10-08
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