Eli5: Does a photon really act different by looking at it? How does it know it’s being observed?
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It doesn’t “know” its being looked at. The fact is that to observe anything, we have to bounce stuff off of it, and measure what comes back (most of the time we bounce electrons or photons off of things).
Now most things are very heavy, and electrons/photons are very light. So chucking, for example, a photon at a blue wall isn’t going to do anything to the blue wall. But when you chuck a photon at another photon, or at an electron/proton/neutron, is going to affect those tiny particles.
“Looking is touching” (dunno who said that).
Bounce a photon of an electron, and now measure the photon. You have effectively “looked at” the electron. Now do the same, but don’t measure the photon. The effect on the electron will still be the same (we can check this by bouncing a second electron off of both, and measuring both). The measuring itself isn’t really what changes things, it’s the tools we use for measuring.
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