I remember reading on HN a article on small probes, that could be accelerated to near 0.25 c with lasers. My Question is- could one such probe, aim a laser on a probe behind it, and slow it down?
Laser light has very little mass but it does have some. The problem is the immense amounts of energy created to generate the laser beam.
The idea behind using a laser to push a probe is interesting because the power generation required can be kept on Earth.
On it's face, yes, a probe using a laser could fire it in the opposite direction in order to slow down... But the amount of power required would easily dwarf anything you could generate on a probe.
Yes, with a giant mirror. And that's the best way to do it. But note that all lasers spread out in a cone -- they aren't a perfect column. The angle of this cone can only be made smaller by making very large optical equipment (I'd guess in the form of a large parabolic mirror collimating an earlier stage as best it can) which means far away probes only get a fraction of the energy Earth is sending. This also means the reflection only has a small portion of the initial light, and then the reflected beam spreads out with its own cone as well. So the deceleration stage is vastly longer than the acceleration stage.
The idea behind using a laser to push a probe is interesting because the power generation required can be kept on Earth.
On it's face, yes, a probe using a laser could fire it in the opposite direction in order to slow down... But the amount of power required would easily dwarf anything you could generate on a probe.