How do we know the idea that particles don’t exist in one definitive spot until we measure them, isn’t just our lack of knowledge due to not measuring them yet?

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How do we know the idea that particles don’t exist in one definitive spot until we measure them, isn’t just our lack of knowledge due to not measuring them yet?

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I recommend looking up the double slit experiment. It’s usually most newcomer’s first exposure to just how weird quantum physics can be. The full implications of the results of this experiment are beyond the scope of ELI5, but the general principle and observation are.

Basic facts in classical physics: if you shoot balls randomly at a board with a single slit big enough for balls to pass through, and measure where they land on another backboard (imagine paintballs leaving a splat). You will see a single bar matching the slit in the first board. Sure, some may hit the corner and fly off at an angle meaning you will get specklings elsewhere, but there will be a big, clear bar. Also, if you increase the number of slits to 2, you will get 2 bars on the backboard. For waves, pretend you lower the boards into water so the slit is half submerged. Then drop a pebble in the water, watch the waves hit the board. The wave on the other side of the board looks like you dropped the pebble right at the slit, moving radially outward. To measure it is a bit trickier, but imagine the backboard is color changing when it gets wet. The higher the wave laps against it, the higher the second color will show. You would expect to see the tallest spot in the middle and it tapers off on either side since the wave had to travel further due to hypotenuse stuff). If you increase the slits to 2, you end up with two different waves as if you’d dropped two pebbles at the slits. These waves collide. If a peak happens to line up with a peak, they add to each other, making the splash on the backboard higher. If a peak and a trough line up, they cancel out and there is no change in the height of the color on the backboard. This ends up with some wavy patterns showing up called “interference patterns” as the two waves interfere with each other.

Ok, now for quantum stuff. Instead of paintballs, let’s fire electrons at the slits. (And the slits are a lot smaller now). We think of electrons as little particles. Little spheres that we can shoot like paintballs out of an electron gun. And if we fire them at one slit, we see what we would expect from particles like paintballs out the other side: a single bar. However, if we shoot them through 2 slits, we don’t see 2 bars like we did for paintballs, we see an interference pattern like we did with waves.

“Well” think the physicists, “maybe we sent them through too quickly and they bumped into each other causing the patterns.” So they send them through one electron at a time. Still, they see wave-like patterns. Almost like the electron splits in two, goes through both slits, then interferes with itself. “No way!” Shout the physicists, “Let’s put a special detector next to one of the slits to see which one the electron went through, surely, it can’t be both.” But when they measure which slit it went through, the interference pattern goes away and out come the two bars like the paintballs.

You see, when we say an electron is both a particle and a wave, one simplified interpretation is that it’s a particle, but we don’t know where it is exactly. We can only predict a wave of probabilities where it could be, sort of like the individual molecules of water in the wave of classical mechanics… Sort of. Except it’s all of the molecules at once in all possible positions. See, when we set the electrons through one slit, any marks on the backboard has to come through that one slit. If you were to freeze the image with an electron on the other side of the slit and measure the probability wave (well, that’s impossible, but…) you would see a 100% chance that it came through the one slit (because all the ones that didn’t have already beed cut out) and it would look like a superposition of every possible trajectory a particle could have taken.

If you did the same freeze frame with the double slit, you would see that there’s a 50% chance it came from either slit. It would start out as a superposition of every possible trajectory of a particle through both slits, but when those probability waves meet each other, the trajectories can interfere and collide with each other. So the electron is in a superposition of having gone through both slits at the same time and interfering with itself. Until we measure it, and the superposition collapses.

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