Question

Consider a star at rest in space. The star radiates isotropically, and the net
momentum flux of the radiation is therefore zero.
If the star is moving, the emitted radiation in the direction of motion is blue-
shifted, and therefore carries more momentum. The radiation is the oppo-
site direction is red-shifted, and therefore carries less momentum. Thus,
there is a net non-zero radiation of momentum.
Why doesn’t the star stop or slow down?

Answer 1

case first: When star if fix: -

                                                  In this case, the star radiates the radiation isotropically, which means that the net momentum flux of the radiation is zero. So, the star does not feel the net force (according to Newton’s second law).

Second Case: -Moving star

                       

    Redshift

(Higher wavelength)             STAR->                                         Blueshift,    More momentum(Pb)

Low momentum (Pr)                                                                         (low wavelength)

The blue and redshifts are due to the motion of the star, which is called the relativistic Doppler’s effect, and according to this effect the change in wavelength(red and blue shift) is for the frame of reference of the observer not for the star. i.e. we can say that in the frame of the star the radiation is also isotropically. Which states that the net momentum flux of the radiation is zero in the frame of the star. So the net force on the star is zero and according to Newton’s first law the star does not change its velocity. That is why the star does not stop or slow down.