Does light ever decay?

21 December 2008


Does light ever decay? Is there a limit to how bright light can be?


Dave - These two are actually slightly related. Light will carry on going for an awful long time. As far as the photons which were created at the end of the Big Bang that's when there was a big soup of protons and electrons. As soon as the electrons got trapped by the nuclei of the atoms the universe went transparent. The photons have been travelling since then which is about 13.7 billion years ago. They reach us as part of the cosmic background radiation. They've carried on going. What they have done because the universe is expanding - one way to think about it - they've been stretched so their wavelength has got longer. They've moved further red. It's called red shift which is the reason why the sky is black. If there was none of this effect then you'd see the effect of the Big Bang as incredibly bright gamma rays all around us. The sky would be bright white, bright x-ray really. Because the universe has expanded so much these photons have been stretched so they're actually in the microwave region. The photons can go on forever but they can change their wavelength. Is there a limit to how bright light can be? The only one I've found possibly is you can get incredibly bright lights photons will start to scatter each other. People have suggested than in order to be able to see this effect you'd need an exowatt laser. That's a billion watts of energy - a billion nuclear power stations being funnelled into the size of something a cubic millimetre. Then you'd need photons, instead of just going past each other, bouncing off each other at which point you won't be able to get much brighter than that. There would be a limit somewhere with that.

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