Can two bits of light bounce off each other?

Neil asked this question on our forum, so physicist Jess Wade explained...

Jess - Yeah. This is a really cool question. I think when you just ask it, you think, yeah, of course light can do that. It reflects it, it refracts, it does all of those things. But then if we think about light in terms of its kind of quantum state, we think about these little packets of photons, which I guess Neil might've heard of, people listening to this, might've heard of these photons. They carry force and they like to interact with things that have a charge. So photons can collide with each other, pass through with each other. They won't interact with one another because they don't have a charge. The only way you can make a photon interact with another photon is by it spontaneously degenerating into different kinds of quantum particles. And this can happen. It's super, super rare, unsurprisingly, which is why we don't talk about it all the time, but a photon can exist and spontaneously become an electron and an antielectron. And then one of those antielectrons can interact with an electron from another photon and combine to give out light. And in that way, two photons can interact, but it's super rare.

Chris - Well If I shine two light waves towards each other, then they can meet and if you've got two waves and they line up, you'll get a bright spot. And if you have them going where one's going up at that moment and one's going down, they cancel each other out. So why are they adding together then under those circumstances to make that bright spot?

Jess - This is kind of the weird and beautiful thing and you have to do a course in quantum physics to get the full interest of it and extremes. But when we think of the wave theory of light and the way that the peak of one wave can meet the trough of another wave or two peaks can interact. Yeah, we can think about amplitudes coupling and increasing like that. But when we think about the particle nature of light, the photon, the quantum nature of light, it's really, really hard to imagine a situation where two photons could interact and add up in the way that we see it in the wave theory.

Chris - So in other words, when we see the light as a bit brighter, where two light waves have coincided, is that just because I'm seeing the effect of one and I'm seeing the effect of the other, they don't necessarily need to know each other exists. They're not knowingly adding each other together. And that's why they can then carry on on their merry way afterwards. Having not, in other words, changed each other at all.

Jess - Exactly that, they don't know that the other one is there.

Chris - So if I did have a very bright laser and I find it at you, and you had an equivalently bright laser with all the light waves lining up exactly, could I bounce my light laser off of yours? Would I even know that the two had bounced off each other?

Jess - I think it'd be incredibly hard to prove that it's happened. We have shown in some cases that it's happened, so it's happened in a few different experiments all around the world. Really, really hard to get it right. Really hard to align everything as anyone with an optics lab will tell you, but also really, really hard to measure it, to know that you've done it. But yes, if we did it, I'm sure it would happen a few times. We just haven't come up with sophisticated enough technology to be able to interpret that yet.