• Question: Whats the speed of light?

    Asked by emilyclifton to Natalia, Andrew, Daniel, Peta on 24 Nov 2011. This question was also asked by bgibson.
    • Photo: Andrew Cairns

      Andrew Cairns answered on 24 Nov 2011:

      299 792 458 m/s

      I used to know this off the top of my head, but had to check it just now! At this speed it takes light about 8 minutes to get from the sun to the earth.

      As you have probably read in the papers recently, it is a law of nature that nothing can travel faster than this…. until we think neutrinos did….

    • Photo: Daniel Scully

      Daniel Scully answered on 24 Nov 2011:

      The speed of light (in a vacuum) is exactly: 299 792 458 metres per second.

      How do we know it is that exactly? Because we define it to be that speed.

      A second is defined by the time taken for a certain radiation of a Caesium133 atom.

      Then we define the metre to be 1 / 299792458 of the distance travelled by light in 1 second.

      Because the speed of light is believed to be a fundamental constant of the Universe, we use it to define other things: like the metre.

      This is a good way to define units, because a fundamental property of the Universe should be the same for everyone everywhere.

    • Photo: Peta Foster

      Peta Foster answered on 24 Nov 2011:

      The speed of light is the speed at which packets of light travel… this will change depending on what the light is travelling through. The fastest light can ever go is in vaccum, which as Andrew said is about 300 million metres every second. To put this in perspective, it takes 1second for light to travel between the earth and the moon… and the moon is a pretty long way away 🙂

      Great question!

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