I'm a little bit confused over the essence of thermal radiation.

The thermal radiation is electromagnetic waves, and EM waves are generated from the acceleration of charged particles. So I feel that the thermal radiation from a material originates from the random motion of charged particles. Charged particles accelerate and change direction randomly, which could result in EM waves.

However, neutrons are charge neutral, so the acceleration of neutrons shouldn't generate EM waves. Then how to understand that neutron stars have thermal radiation?

  • 7
    $\begingroup$ Neutron stars are not made of neutrons only. Also, neutrons do have electric dipole moment, so they can interact electromagnetically. $\endgroup$ Dec 14, 2023 at 1:51
  • $\begingroup$ Some info about neutron star cooling: physics.stackexchange.com/a/312830/123208 Also see physics.stackexchange.com/q/63383/123208 $\endgroup$
    – PM 2Ring
    Dec 14, 2023 at 3:50
  • $\begingroup$ Is this of any help? How do neutron stars emit black body radiation? $\endgroup$
    – Quillo
    Dec 14, 2023 at 6:07
  • $\begingroup$ I don't think your understanding is right. EM is generated due to electron transition to lower energy level either spontaneously or not. This doesn't happen due to simple acceleration of electron, because technically it accelerates all the time around nuclei, and if it's the case,- electron would loose it's energy almost instantly. Hence classical physics fails describing EM radiation. Now what makes electron to go to the upper energy levels,- is the vibration of lattice, so called electron-phonon iteraction. Phonon(s) are basically responsible to EM thermal radiation. $\endgroup$ Dec 14, 2023 at 10:04

2 Answers 2


There are no free neutrons anywhere near the emitting surface of a neutron star. The surface layers (a few cm thick) consist of ionised gases that may contain a mixture of many chemical elements. There are plenty of charged particles.

Free neutrons do not appear in the neutron star "crust" until densities exceed $\sim 4\times 10^{14}$ kg/m$^3$; they would simply decay into protons and electrons otherwise. These densities are not reached until you are kilometres into the neutron star, whereas the optical depth into a neutron star is more likely to be centimetres, even for X-rays.

  • 1
    $\begingroup$ And, theoretically, even deep inside the star, there are plenty of electrons, number density a few % of the neutrons. That's a very high density, much more than the electron density in ordinary matter. Electron pressure is what stabilizes the neutrons against beta decay. $\endgroup$
    – John Doty
    Dec 14, 2023 at 15:50

A concept that envisages EM radiation only from charges is fundamentally wrong. It is easy to prove that neutrons also absorb and emit photons. Take a laser beam and use it to accelerate a free neutron. Due to the impossibility of "generating" energy from nothing, there must be an explanation for the increase in the kinetic energy of the neutron. Part of the energy of the laser beam must be transferred to the neutron. This is only possible through the absorption of photons. If you now set up your experiment in such a way that you hit a moving neutron against the direction of movement with the laser beam, the neutron is slowed down. This means that more photons are emitted than absorbed. Or the wavelengths of the incoming and outgoing radiation differ. Or both.

Now, of course, someone will point out that neutrons are subatomic particles that consist of more fundamental particles. Incidentally, this also applies to the positive electric charge, the proton. But that doesn't detract from the fact that I manage to elicit EM radiation from a free neutron before it decays.

What does this have to do with your question about the thermal radiation of a neutron star? In a neutron star, according to the mechanism described above, there should be an exchange of energy between the neutrons. And this is an exchange of EM radiation.

  • $\begingroup$ Is there any actual demonstration of this experiment? $\endgroup$ Dec 15, 2023 at 7:50

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