Photon versus particle density

eg., for 3K m-wave background photons -

Corresponding to about 10 photons / m

9

3

No of nuclei in space is about 10 / m

6

3

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So comparing the microwave background photon density with the ISM proton density, we see that photons are about 1000 times more numerous than particles, thus photon-photon collisions are far more important.