I consider myself at least… Minorly able to read a newspaper digest about physics. And I’ve read about the double slit experiment. You have two slits in a piece of paper, fire electrons at them and they form wave patterns. “Observe” them and they act like particles and form particle patterns.
Here’s the the thing. Every single class, teacher, physicist I have known has said the same word. “Observe”. But…. What does that *mean*? If I look at it? If I have a detector? What does the detector do? How do we know that isn’t interfering with the particle? Why does this never seem to be extrapolated on and just that one fucking word pops up everywhere? Is it just a thought experiment? This had been driving me nuts, can someone explain?
In: Physics
Observe means interaction with the system that the observer is a part of. The observer can be anything.
Also there is a myth going around that observation requires disturbing or touching the particle and that’s why it collapses. The quantum eraser experiment showed this to be false.
> How do we know that isn’t interfering with the particle?
You answered your own question. It is interfering with the particle, that’s why it changed. It’s impossible to observe a particle without interfering with it.
On a macroscopic scale “observe” could be go up and touch it, which will interfere. Or stand back and watch, which will not. However, this luxury doesn’t exist if you want to be really accurate at the small scale. “sight” is bouncing photons off something. Observering a monkey in biology, sight is just fine. Observering an electron, bouncing a single photon off it is going to be an issue.
With the double split experiment, you are always observing the electron at some point. You have two options though. Observe it at the screen, and see a nice interference pattern. Or observe it at the slit, and ruin the pattern later at the screen. It’s not some voodoo magic about human consciousness and the electrons knowing if we’re looking at then or not. It’s just by checking where the electron was as the slit (observing) is synonymous with altering the electron. You tighten it’s wave function down, to the point it’s not going to give you the pattern.
When they say “observing,” That’s really what they mean. It doesn’t matter how you do it. In this case you can’t just look at it because that’s not possible the way the experiment is set up (the light source, slits, detection screen are all in an enclosed box or tube because outside light ruins the experiment). But by setting up some way to detect which slit it goes through, the interference pattern will disappear. One way people have tried was to use polarizers before each slit, with their axes 90 degrees apart. That means that at the detector you can measure what polarization the light had, which will tell you which slit it went through. Doing this results in the loss of the interference pattern.
Something interesting about that setup is that if you “erase” the information about which slit it went through – in this case, putting a third polarizer at the detector, 45 degrees between the axes of the two in front of the slits – then the interference pattern comes back.
‘Observe’ means having the particle interact with something (and hopefully detecting the interaction). In the double slit experiment, the detector may be as simple as a screen that the particle hits, leaving a mark. When the particle interacts with (hits) the screen, the wavefunction collapses, and the particle “chooses” a spot to hit.
Tl;dr: observation = interaction with something external
The act of observation is *anything* that turns “possible” into “actual”.
Any comparison between quantum scale and common human scales is littered with absurdity, but here goes.
Say I’ve got one perfectly weighted dice in a black box, that I vigorously shake the box and wonder what number faces up on the dice. *Anything* that turns possibilities (a number from 1 to 6) into something actual (a 3 and nothing else) *is* an observation. Could be someone peeking inside the box. Could be a camera inside the box. Could be X-rays. From the moment it happens, it’s not a possibility anymore, it’s actual; even if it’s someone else looking, and I don’t know the value yet, it’s a definite 3.
As mentioned, this sounds like a trivial and pointless philosophical discussion (just like “If a tree falls in the forest and no one’s around, does it make a sound?”), but in quantum physics (such as the double-slit experiment) this has important implications; the [Quantum Zeno effect](https://en.wikipedia.org/wiki/Quantum_Zeno_effect) is a good illustration of observation greatly influencing an experiment.
You can’t observe something without interfering with it. You have to bounce something off it and measure the thing you bounced. Most commonly the thing you’re bouncing is photons and you’re measuring it by catching the bounced photons with your eyeballs, the ultra-technical scientific term for that is “seeing”. But any method of measuring something is going to involve something similar.
Observation doesn’t require a conscious observer or anything magical like that, just interference.
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