If you shone two lasers directly into each other, what would happen?

We put this question to Dr Graeme Hirst, from the STFC's Central Laser Facility...

Graeme - Well that's a good question. If you shone them directly into one another, then you're quite likely to break the lasers unfortunately. But if you imagine you're going to misalign them slightly so that the beams crossed, but they don't go down the throat of the opposing laser, then if you were to do that in a vacuum, if you just have one light beam crossing another then to all intents and purposes, nothing happens. Once you get to extreme physics conditions where the laser intensity does become spectacular - and in some experiments we do, we're getting there - you might imagine that you can perturb the vacuum. You can change the vacuum with the intensity of light you need. But mostly, this is interesting if you cross the two beams over in some kind of material, and if you do that, you can achieve all sorts of interesting effects in the material by combining two laser beams, through the properties of the material.