I was looking at a wikipedia article (https://en.wikipedia.org/wiki/Pulsar#Formation,_mechanism,_turn_off) and I came across an interesting aspect that said pulsars turn off from rotation. What I don't understand is why emission of EM waves would slow a pulsar down?

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The first thing you need to recall is that electromagnetic waves do carry momentum as well as energy. This shows up in effects like light pressure. Specifically a photon of wavelength $\lambda$ carries momentum $h/\lambda$. In and of itself, that doesn't answer your question though, since you are asking about the rotation of the pulsar, and changed to its linear momentum don't affect that. However, there are two ways this can happen. First imagine a rotating sphere which is emitting radiation from its equator. Consider a point on that equator. The radiation it is emitting "forwards" (in the direction towards which it is rotating) will be blue shifted (as seen by a stationary distant observer) while the radiation emitted "backwards" will be correspondingly red-shifted. Since the blue-shifted radiation has shorter wavelength, it has more momentum, the net effect is a force on the source which tends to slow the rotation. (interestingly, the same effect is used at much lower energy levels to trap ultra-cold atoms in what is called "optical molasses").

The second effect is a bit more technical. The radiation which is emitted towards the poles of the pulsar has a property called "circular polarization" and a beam of circularly polarized light carries away angular momentum.

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