why is the conclusion of the double slit experiment that particles have different behavior depending on whether they are being observed and what does this mean?
In: Physics
You have an electron gun, and you repeatedly fire it at a wall. Each time a single electron shoots out and hits the wall. If you put a hole in the wall, then the electrons that pass through the hole hit the wall behind it.
Make two different holes, and you’d expect an electron that passed through either hole to land behind that hole. Electrons will diffract a bit, so instead of directly behind the hole you get a spread of them landing vaguely behind the hole.
But if you open two holes at once, you’ll actually notice that even single electrons will land in places that don’t make sense for one hole or the other. The “shadow” created by the two holes won’t be the same as the shadow for each hole added together. The two holes seem to be both interacting with the electron somehow.
So you set up a device that measures which hole an electron goes through. The shadow changes – now the holes don’t seem to both interact at all. The shadow of this looks just like the shadow of two holes added together.
The fact that we observe which hole the electron goes through changes how it acts.
“Observed” means “measured”.
To measure something you need to do something to it.
Doing something to the particles changes the result.
It’s less mysterious than it sounds.
Perhaps a crude analogy will help: you want to know if a ball goes through hole A or hole B but you’re in the dark so you have to use some object for that, relying on touch instead of sight.
A very light object, a piece of yarn or a strip of paper, won’t deflect the ball very much but will give poor position information. Something more sturdy like a stick will definitely locate the ball but will also send it in an unknown direction.
Same goes for the particle and the slits. To know what it does there you have to shine some kind of light on it. Low-energy, reddish, soft light won’t perturb the particle much but will have poor resolution. Making the light bluer, more energetic, you’ll get better position information but the particle will be sent on a new trajectory and you’ll loose the interferences behind the slits.
Reframing the touch analogy, the light particles (photons) behave either like big, fluffy cotton balls when low-energy or hard heavy marbles when high-energy. In the first case you get low resolution (the image is blurry), in the second a sharp image but you knock the particle you want to observe into an unknown state.
So it’s not a matter of being observed / not being observed, it’s a matter of how precisely you want to observe.
Don’t think of Observation (or measurement) with its normal meaning, think of it as a technical term. Yeah it’s confusing that it has a familiar meaning that’s subtly wrong.
A particle is “observed” when it interacts with a sufficiently large (“macroscopically behaving”) object.
So if you leave a small thing alone for long enough it starts to behave more like a wave. (How long is long enough depends on how small it is, essentially.)
As for what it means, that’s more nuanced, but my simple answer is that the wave function is what’s physical (what’s “real”), and not something more intuitive.
The intuitive perspective only starts to emerge on “large” scales, and by that definition of large everything in our day to day life is truly gigantic.
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