Eli5: Does a photon really act different by looking at it? How does it know it’s being observed?


Eli5: Does a photon really act different by looking at it? How does it know it’s being observed?

In: 11

Observation in this case doesn’t really mean “A person perceives it.” Really it means: “The quantum system is perturbed by interaction with some other system.” When you think about it, what is observing something? Generally it means that we bounce light off of it, or use a magnetic field to interact with it, or use some sort of detector to interact with it.

When a quantum system interacts with another system there’s a tendency for the state of both systems to change. That’s the “observation causes a change” we’re really talking about.

It knows that it’s being observed because it has to interact with the detector. So, when photon interacts with the detector they change their behavior.

Imagine a billiard table is a field, and billiard balls are photons. The only manner you have of detecting photons is by hitting the cue ball. If you hear the strike of cue ball on billiard ball, you have detected a photon. And the photon’s trajectory has been affected.

The only photons you “see” are the ones that hit your retina and have their energy absorbed. All those photons zipping by in front of you, but missing your eyes, you don’t even know about.

Yes, the act of “observing” a photon does change its behavior. We discovered this by way of an experiment known as the “double slit experiment.”

It’s a bit difficult to explain without visual aids, but I’ll try and will also include a video detailing it at the end.

So you take a barrier and put two parallel slits in it, wide enough to allow something to pass through. Depending on what this something is (particle or wave), you will end up with two different patterns on the other side of the barrier. For a particle, you will have two lines of particles which neatly correspond to the slit locations with little to nothing beyond where the slits are. However if you have waves (like water, for instance) you end up with the one wave being split in half and then interfering with itself, giving you a wide pattern that repeats beyond the “boundaries” of the original slits. This is an interference pattern, and is the hallmark of waves interacting with one another.

So as an experiment, scientists fired photons at the two slits. Believing photons to be particles, they were surprised to find an interference pattern on the other side. That makes light a wave. But photons are particles, right? So how did it produce a wave pattern? So they set up equipment to record the path of the photons to see how the wave pattern was produced by particles. And in watching it happen, the pattern on the other side was changed to a particle pattern as each photon was observed passing through each slit individually.

Like I said, difficult to explain without visual aid, so [here’s a link for you.](https://youtu.be/Iuv6hY6zsd0)

For what it’s worth, I have not actually watched this video in particular. However, I like Veritasium and know he’s very good at explaining things in an easy-to-digest manner. I hope this helped!