Video
The Speed of Light — The Universe's Ultimate Limit
Presented by Prof. Eric Chamchoum, PhD
Published 2026-08-19Page updated 2026-08-23
In short
Light travels through a vacuum at about 299,792 kilometres per second. Physics treats that figure as a universal limit: no observed signal or object carrying information has ever exceeded it, which is why the speed of light also sets the scale of distance and time across the universe.
About this video
The speed of light in a vacuum is a measured constant, written as c, of approximately 299,792,458 metres per second. Because it is constant for every observer, it functions less like an ordinary speed and more like a structural feature of space and time.
That constancy is why astronomical distances are described in light-years: the distance light covers in one year. Observing a distant object therefore means observing its past, since the light reaching an observer left it long before.
This video sets out the measurement, the reasoning behind treating it as a limit, and the consequences for how large and how old the observable universe appears from where we stand.
Key ideas and takeaways
- The speed of light in a vacuum is approximately 299,792 kilometres per second.
- It is the same for every observer, which makes it a limit rather than an ordinary velocity.
- Distances in astronomy are measured in light-years, so looking outward is also looking back in time.
People, places and concepts
- Speed of light
- Light-year
- Observable universe
Frequently asked questions
Where to go next
- Knowledge entryWhy is the speed of light a universal limit?The speed of light is not simply fast. It is constant for every observer, which is what turns it into a structural limit.
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