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Can someone explain please? We had this in school, but my friend here forgot what this was about.
It's the Young's double slit experiment. It proves that light (or electrons, or even small bacteria) is both a particle and a wave.
There is a quirk of quantum mechanics. When you observe a system, you fundamentally change it. In scientific terms "observe" has a very different meaning to layman usage. This leads to a lot of woo around the topic. In practice, observing is measuring. In quantum mechanics, the measurement system is of the same scale as the system being measured.
Imagine observing a good train, by bouncing BB bullets off it with a gun. That is classical measurement. You can assume the BBs had no effect on the train.
Now imagine the same measurement. However you are measuring how a bunch of glass playing cards are balanced in a house of cards. You can tell a lot still, but the BBs will smash it up doing so. This is quantum measurements.
In the first, the observer is independent of the system. In the second, the observer is a fundamental part of the system, and so can change its way of functioning.
Another classic case of "Scientists are bad at naming things"
Some people will spend their entire lives thinking math is stupid because of imaginary numbers.
Thinking that electrons behave differently when you "look" at them.
Think that radio towers and microwaves cause cancer because they emit radiation
Many of these are failures of the education system and to be fair scientists don't have the power of hindsight. Still it annoys me how inefficient it is having these names
The problem with how you are describing it is that it's not that the physical mechanics of measurement are necessarily causing collapse as if you end up erasing the persistent information about the measurement it reverses the collapse, such as if you add a polarizer to the other slit as well or add a polarizer downstream that untags the initial measurement.
So in your example, if you simultaneously shoot a bunch of BBs at empty space next to the pile of glass cards where they could have been, or discard the BBs which reflected measuring the cards in the first place, suddenly the pile of glass cards reassemble themselves.
Attempts to try and dismiss the 'weirdness' of the measurement problem or QM behavior IMO ultimately do the reader more of a disservice than a service.
I'm massively simplifying, and a lot of the interesting stuff gets lost with that. Unfortunately, when you try and maintain that, the analogy gets so convoluted that it's useless.
The actual answer for understanding quantum mechanics is to chunk the maths, again, and again... and again. It also involves working almost entirely in the wave dominant domains. Trying to simplify that down to a quick comment is basically impossible.
Particles act different when you aren't looking.
But Why? What should I Google to find Infos about it?
You can't Google it because that changes the definition.
Double slit experiment
it's really annoying how bad this experiment is explained to the general public. the wording generally used is so poor it implies there's something supernatural about the phenomenon
I'm not a physicist, but as far as I understand the principle, any human actively looking at the experiment changes absolutely nothing. what it really postulates is that light behaves as a wave until it is interacted with. at that point, the wave "collapses" and it starts to behave as a particle, positioned somewhere within the probability zone described by the wave initially. when you measure it in any way, using some measuring tool, it inevitably interacts with it