• Question: How can light travel in space if it needs a medium to travel as a way?

    Asked by lizzie3006 to Ben, Jony, Katharine, Mark, Peter on 19 Nov 2011.
    • Photo: Mark Basham

      Mark Basham answered on 18 Nov 2011:

      Hi lizzie3006,

      I think we were discussing this in the live chat, but light does not need a medium in which to travel it travels on its own. It was used to be thought that it must travel through some sort of medium and this was called the Aether. around 1900 a pair of scientists worked out how they could see if the aether existed, and preformed an experiment. The result that they came up with was that there was no Aether. The experiment is explained quite well here.

      http://en.wikipedia.org/wiki/Michelson%E2%80%93Morley_experiment

      Hope this helps

    • Photo: Peter Williams

      Peter Williams answered on 19 Nov 2011:

      This is quite a deep one i think.
      Light doesn’t need an external medium – it is able to travel across a vacuum, Michelson-Morley showed that.
      However, the particles constituting light do need to have a particular property. They need to be able to interact using the electromagnetic force. This is technically called a “gauge degree of freedom”. They can then be governed by the rules of that force – one of which is that, as a 1/r^2 force, it has infinite reach. If we were to replace photons with something that doesn’t have that degree of freedom, but some other one – for instance a gluon, we would see that it does not propogate over infinite distances. That’s because it is governed by a force with different rules.

      So – no external medium, but the particles themselves need to have the correct degree of freedom.

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