It’s not the observation itself, it’s how you do the observing. You need to bounce light off of something to see it. While photons usually don’t impart much energy, it’s a different story for single particles. You are interacting with it by measuring it, that changes the state of the system and this is unavoidable.
Observation means a detection method of the testing apparatus. Not an actual observer looking at it with eyes. So for example in the double slit experiment, if you pass molecules, atoms, electrons, or photons through the two slits you get an interference pattern on the other end which points to these things acting like waves. If you put a detector to determine which slit each molecule, atom, electron or photon goes through, the pattern just projects 2 rows which points to particles. The action of ‘observing’ which slit the stuff goes through changes the result.
It doesn’t know you’re looking. Particles aren’t sentient.
Looking is not a passive action. For you to look at the moon. Photons from the sun have to first hit the moon. Some reflect off the surface and then travel to earth. Some find your eyes and the back of your retina and then you “see” the moon. Particles, photons in this case had to hit both an atom on the moons surface and an atom on your retina for you to see anything.
So now you want to see a single particle pass through a double slit. Well in order to see anything, a moon or a single particle, you have to bounce something off of it.
What we think happens is quantum particles exist as a wave. But when they interact with another particle, say a photon you shot at it to “look” at which door it will take, the wave collapses and the particle becomes, well a particle. A fixed point in space.
Left without being bounced on, particles aren’t points in space but a collection of probabilities of points in space we describe as a wave. And this wave can interact with itself.
Thus when we look, by beaming particles with other particles. A double slit experiment produces a double slit. But when we don’t look, and thus don’t interact with the particle, a double slip experiment produced an interference pattern.
But all you have to remember is the particle doesn’t know it’s being watched. It just interacts with space differently when something is bouncing off of it. What we call “looking” is just our brain interpretation of particles bouncing off of each other.
Looking is not passive. If you want to see your reflection, the process involves lots of photons coming from a light source, bouncing off you, bouncing off a mirror perfectly, and hitting your eyes. In day to day interactions (and in many experiments), photons bouncing off of things doesn’t change the outcome a whole lot, if any.
For observing extremely small particles, we bounce a photon or an electron off just like we do looking at ourselves in the mirror. The difference is the small particle were are sending a photon at is so small that the photon sends it shooting somewhere else.
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