Every time I think I understand something about QED some very basic things just trip me up again. So, I realise a photon can be thought of as an observation that carries two parameters, its wave length and its direction of propagation. Also, I realise that if an observer observes a photon because of some interaction with matter (meaning electrons) - the classic example being a mirror, - the observation can be thought of to be built of all possible paths of the photon cancelling out each other and averaging out. I also appreciate you should not overthink this and it’s just what happens. I also understand the dual split + detector observation that illustrates a collapsing of the wavelike nature of photon observations into a particle like behaviour when it hits a detector at a split.

However, I just can’t understand anymore how multiple observers can all look at a small source of EM, a light source, and all see the same at the same time. It’s driving me insane all of a sudden. Suppose billions of observations are made at the same time. All of humanity is looking at a small led in the vacuum of space. Why don’t they collapse the photons into only a couple of them reaching a couple of observers? Am I getting a traditional confusion of Copenhagen interpretation or just not seeing something silly?

Also, if we consider matter being observed through its interaction with/by photons (which is maybe the only way to observe matter aside of gravity?), why do the photons not emerge as somewhat like pin needles sticking out and why do faraway observations not experience even more “holes” because of collapsed observations..

Sorry if this is trivial Copenhagen confusion. I’m just really confused and suddenly can no longer explain anything about the world.

  • $\begingroup$ Why do you say all will see the photon? It’s analogous to saying that the photon reaches multiple points on the detector simultaneously. $\endgroup$ Feb 29, 2020 at 4:29
  • $\begingroup$ But then how are we all able to observe the same star together? What is the size of a star before some people claim to see it and some do not or before it starts flickering? It doesn’t make sense, everyone will see it no matter how small $\endgroup$
    – buddhabrot
    Feb 29, 2020 at 4:31
  • $\begingroup$ Ok so it’s just a matter of scale then? Why not answer that? It’s an acceptable answer although it still trips my mind. How small can we make a light source so 10 humans observing it see different things regardless of their position. I am entering a rabbit hole.. $\endgroup$
    – buddhabrot
    Feb 29, 2020 at 4:36
  • $\begingroup$ @buddhabrot there’s a very easy way to not get ratholed here — just replace “humans” with “photon detectors” in your question. In fact, such experiments are done all the time, and they clearly show that only one detector can detect an individual photon. But when there’s many photons, each will detect a portion of them $\endgroup$ Mar 1, 2020 at 23:28
  • $\begingroup$ The reason I use humans is because it allows me to say: “adding more humans observing the star, changes the experience (more flickering observed by some)”. Is that correct? $\endgroup$
    – buddhabrot
    Mar 3, 2020 at 4:35

1 Answer 1


The EM wave of light is made up of gazillions of photons. This is why multiple observers can see the same event. But they aren’t detecting the same photons, rather they are detecting photons from the same source.

However, if our optic system was capable of very high frame rates, high enough to resolve between photons in an EM wave, then we’d see in flashes. And this now will be unique for every observer.

Take this experiment for example. Photons are sent one at a time through a double slit and captured on a screen. Each time, the photon only blips at one point on the screen. And many many photons later what we see is an interference pattern.

enter image description here

Now, if you repeat the experiment, you won’t get the same sequence of detections but finally the pattern will still be that of interference. If our detectors aren’t fast enough to refresh, we won’t see the building up of the pattern but the pattern directly.

  • $\begingroup$ Thanks. But does this mean if you have enough people looking at a star, or camera’s recording it, it would disappear for some of them? $\endgroup$
    – buddhabrot
    Feb 29, 2020 at 8:23
  • $\begingroup$ Stars are fairly bright, you would need to find a very dim one ... You could get yourself and 10 friends in a dark room and use a dim LED with some dense filters ... in theory people would see flashes at different times. $\endgroup$ Mar 1, 2020 at 17:51
  • $\begingroup$ @buddhabrot the density of photons in a unit of space is constant and doesn’t depend on the density of humans. What you’re saying is — what if we block all of the photons by detecting them with something, eg a Dyson sphere. Then clearly the answer is — yes, it will disappear for outside observers. $\endgroup$ Mar 1, 2020 at 23:32

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