Photons are elementary massless particles, quanta of electromagnetic radiation that we perceive as light. Many photons fall on the objects around us and are reflected into the retina of our eyes, allowing us to see.
It would seem that counting the number of photons in the universe is an impossible task. But physicists at Clemson University, USA, don't think so. To understand at least an approximate number of photons in our world, it is necessary to take into account the whole time of existence of the Universe - about 13.7 billion years. After all, the light emitted by long-dead stars still flies through space.
For the basis of calculations, scientists have taken values of the so-called extragalactic background light (EBL), which accumulates in the infrared, visible and ultraviolet ranges after the formation of stars. Even with the most modern technology, EBL is very difficult to spot - it is easily damped out by other radiations that literally permeate our universe. But scientists have found an original way that still helps to analyze the extragalactic background radiation - blazars.
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