Question: Lasers. If lasers have the energy to cut through objects due to their intensity, then technically shouldn't a larger source of light be able to - at the very least - move things around a little if it's powerful enough? I just don't understand the forces behind these things.

Andrew Maynard answered on 15 Jun 2010:

Hi blubber,

You’re quite right, although the way lasers cut through stuff and the way light can move things around is a little different.

Lasers can cut through some materials by the light being absorbed and heating the material up. This doesn’t give the material the “push” needed to move it. Rather, the high temperatures melt the stuff.

But… the individual photons (a bit like particles of light) making up laser beams also have a really small amount of mass, and this can be used to move objects around if the photons are transmitted through the material or bounce off it, rather than being absorbed. Even in an intense laser beam, the force from the photons hitting a surface is very small indeed. But under the right conditions, it can be used to push small particles around.

If the laser is focused just right, it can be used as an optical tweezer for picking up small particles and moving them around – because of the way in which the photons hit and bounce off the particle.