What propels light particles travelling through space endlessly with the same speed?
Even though photons is very much a relativistic particle, one can use a Newtonian argument to argue for the first. As long as a particle moves along at constant speed, no force is acting on it. A photon doesn’t need a “power source” to propel it through the universe. And as for the second question, then a single photon doesn’t have all that much energy. The energy is given by Einsteins E=h*f, where h is Planck’s-constant (= 6.6*10^-37 J*s) and f is the frequency. For a typical photon coming from the sun, this means that it has the energy: 3,6*10^-22 J. It is only because of the huge number of photons hitting us, that we can feel the warmth of the sun on a bright summer day. As for the absorption of photons in intergalactic space, then yes, it is true that photons will get absorbed when it hits something. Something to think about when you hear that astronomers have found a new star billions of light-years away – that means that the photons from that object have traveled across most of t
