If you're standing in a dark night-time place on the Earth and look up at the sky, you see stars. (About 6000, if you're in a sufficiently dark place). You're seeing photons that have passed though thousands of lightyears of space, and have passed through the ~5mm diameter hole that is your pupil.

(One calculation I came across estimated that 6th magnitude stars, the dimmest visible to human perception, send about 2,000 photons per second through the pupil).

If you move your head fractionally to the left, you see the same stars. This isn't surprising: stars are huge and blast out photons in all directions, so for any given star that's close enough to be visible from Earth, every point on the Earth's surface will be struck by some photons from that star.

If you use some kind of aid (like a telescope), you can see many more stars. Some photons from these more distant stars strike every point on the Earth's surface, though not enough to be visible to the naked eye.

Likewise, the Hubble Space Telescope can pick up light from distant galaxies. If you had a similar telescope located deep in interstellar space, it would also be able to pick up light from remote stars and galaxies in all directions.

Does this imply that at any point in the universe, there are trillions of photons passing in all directions? How does this work? Is it that photons are so small that they don't interfere with one another, or is it that the wave-like nature of light permits trillions of EM waves to intersect at the same point without interference?

  • $\begingroup$ Never mind stars. Think about two radio transmitters a few 100 meters apart. Not only are they closer than two stars, the photons associated with radio are much lower in energy, so the number of photons present around these towers is vastly higher than the number reaching one star from another. $\endgroup$ – The Photon Feb 16 '18 at 0:16

because photons are spin-1, they can pass through one another in different directions without interference. So yes, there are countless ones passing through one another everywhere in the universe, all the time.

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