How does radiation (heat) take away momentum? In another post, I was taught that when we are moving (running, for example), radiation (in the form of heat, both from our muscles and friction with the ground) takes away from our momentum. That makes perfect sense and I understand.
I was also taught that heat/radiation is directional.
My question is: since radiation is directional, is that why it takes away from momentum (which is also directional)? Is it correct to say that? In other words, does radiation take away from momentum because they are both directional concepts?
Also, does radiation, since it is directional, dissipate in the same direction of momentum, in the opposite direction, or no particular direction?
I am not an expert, hoping just for simple clarification, if possible.
 A: Radiation will not "know" that it is being emitted by a uniformly moving body, so from the prespective of the body it will be emitted uniformly in all directions.  Momentum will be lost to the body to a very small degree, though, because the mass of the body will be reduced by a very small degree due to energy/mass equivalence.  However, the body will not change its velocity- only its mass and therefore its momentum.  In practice, the change is too small to observe.
A: Suppose that we have a lone body moving freely across space, his momentum then is conserved, it has to stay the same through time. This derives directly from the laws of dynamic. End of the story. But  if the body emits radiation then some of the momentum of the body can be stolen by the photons that it emits, in fact photons carry momentum. But as long as the radiation is emitted in the same way in every direction, the momentum has to be conserved. In fact if the radiation is emitted in the same way in every direction any momentum given in one direction will be compensated by the photon emitted in the opposite direction. If the radiation is not emitted equally then you can have a sort of jet effect and the momentum of the body could indeed vary.
It can be tempting to state: since the body emits photons and the photons carry momentum, then the body must lose some momentum, because it gave it to the photons. Unfortunately this intuitive line of reasoning is wrong: we have to remember that momentum is not a scalar quantity but a vectorial one, so two particle with momentum equal in module but opposite in direction amount to a total of zero momentum.
