• Question: Can light be accelerated by a black hole to a point where it has weight or is its mass tottally zero?

    Asked by E=MC@lex to Alison, Hannah, Jonny, MarthaNari, Paul on 23 Jun 2015.
    • Photo: Paul Brack

      Paul Brack answered on 23 Jun 2015:

      Light has no mass, so it can’t be directly affected by gravity. However, really big masses have the effect of distorting space and time such that light is caused to bend – it still thinks it’s traveling on a straight path, but it’s actually following this bent path that comes from the space-time distortion. Black holes distort space-time so much that the ‘straight path’ for light can never get out, so it appears that the black holes ‘attract’ light.

    • Photo: Jonny Brooks-Bartlett

      Jonny Brooks-Bartlett answered on 23 Jun 2015:

      I’m afraid I don’t know enough crazy physics to be certain of a correct answer to this question.

      Acceleration is a change of speed or direction. Light can be bent by black holes so in that sense they do accelerate light. However, locally (following the light closely) the speed of light doesn’t change and it’s here at which I’m stumped.
      It’s possible that the mass of the light (a light photon) is always zero but there may be an explanation that I don’t know of that could say that light can gain mass.

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