Knowledge entry
Why is the speed of light a universal limit?
By Prof. Eric Chamchoum, PhD
Published 2026-08-23Updated 2026-08-234 min read
Why is the speed of light a universal limit?
Light travels at about 299,792 kilometres per second in a vacuum, and that value is the same for every observer regardless of their own motion. Because of that constancy, it functions as a limit on how fast information can travel, not merely as a very high speed.
Most speeds are relative: they depend on who is measuring. The speed of light behaves differently, and that difference is the whole point.
A measured constant
The speed of light in a vacuum, denoted c, is approximately 299,792,458 metres per second. The metre is now defined in terms of it, which makes the figure exact by definition.
Why constancy implies a limit
Because every observer measures the same value for c, space and time must adjust between observers instead. That relationship is what forbids information-carrying signals from exceeding c.
Consequences for observation
Astronomical distances are quoted in light-years, the distance light travels in a year. Observing a distant galaxy therefore means observing it as it was when the light left it.
What the evidence supports
- c is approximately 299,792,458 metres per second in a vacuum.
- Its value is identical for all observers.
- One light-year is the distance light travels in one year.
People, places and concepts
- Speed of light
- Light-year
- Vacuum
Frequently asked questions
Sources
- SI definition of the metre in terms of the speed of light
Continue from here
Every entry connects to the video, book or topic it belongs to.
- Watch the videoThe Speed of Light — The Universe's Ultimate LimitA video essay on why light travels at roughly 299,792 kilometres per second, why nothing observed can exceed it, and what that limit means for space, time and our picture of the universe.
- Knowledge topicScienceMethod, evidence and discovery, and where they meet belief and history.
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