• Question: how fast is the speed of light

    Asked by scientist124 to Daniel, Jon, Louise, Sharon, Zoe on 22 Jun 2010 in Categories: . This question was also asked by hell.
    • Photo: Louise Dash

      Louise Dash answered on 21 Jun 2010:

      In a vacuum, light travels at 299,792,458 metres per second. That’s really fast!!! If light is travelling through some substance though, such as air, it slows down slightly. Light is actually electromagnetic radiation, so other forms, like radio waves or X-rays, also travel at the speed of light.

      There are several methods for measuring the speed of light, such as by comparing the time it takes for radio signals to get to different satellites, or by a very clever method called laser interferometry, which splits the beam of light from a laser to travel along two different paths of different lengths, and then using the special interference pattern they make when they recombine to work out the speed of light!

    • Photo: Sharon Sneddon

      Sharon Sneddon answered on 21 Jun 2010:

      I like your name!!

      The speed of light is 299,792,458 metres per second! Pretty fast!

    • Photo: Jon Copley

      Jon Copley answered on 21 Jun 2010:

      In a vacuum: 299792458 metres per second, which is approximately 186000 miles per second. Or 1 Planck unit.

      But light travels slower through most things that are not a vacuum, such as air. Light travels through the air of our atmosphere about 50 miles per second slower than through the vacuum of space. Physicists have even made light travel at just 38 miles per *hour* (slower than a sports cyclist!) through a substance called a Bose-Einstein condensate, which is made by cooling certain gases very close to absolute zero (minus 273.15 degrees C).

      So to answer the question “how fast is the speed of light?”, we also need to think about “through what?”.

      And it’s worth remembering that from light’s own point of view, it has no speed. Under Einstein’s Special Relativity, there is no such thing as time or distance for a massless particle (i.e. a photon). And because speed = distance / time, then there is no such thing as speed for photons either.

      Now clearly that’s just crazy talk, so physicists avoid it by saying “a photon does not have a frame of reference”. And that’s why, after doing my undergraduate dissertation on relativity, quantum mechanics and chaos theory (an unusual choice in a zoology degree, but that’s a long story), I became a deep-sea biologist…

    • Photo: Zoe Duck

      Zoe Duck answered on 22 Jun 2010:

      299 792 458 m / s

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