It seriously makes my mind hurt.
Can the double split experiment be explained in an eli5 way or is it just too complex for the layman with an average IQ to understand?
How does observing a photon crate a wave pattern *AND* change events that have already happened?
In: 5
Observation doesn’t create a wave pattern, and it doesn’t change events that have already happened.
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When you fire a photon at the two slits, the photon – like everything in the Universe – is in some particular state.
In classical physics, we think of this state as being the position and velocity of some tiny ball, and that model is just fundamentally wrong in a way that cannot explain the Universe in which we live. Instead, the state is a more complicated thing called a *wavefunction*. Exactly how this wavefunction corresponds to our intuitions about the way the Universe behaves is a matter of considerable argument, but everyone agrees on the math: the wavefunction takes all of the possible “ball in a position going in a direction” states* and assigns a number (which turns out to correspond to a probability) to each of them. For example, it might assign one number to the state “is at position (2,1) with velocity <3,4>” and another number to “is at position (7,9) with velocity <-4, 6>”, and so on.
As the photon moves, the wavefunction is changing. The numbers it assigns to each of the classical states (each possible position and velocity) evolve over time, and this evolution is nice and predictable.
When the photon reaches the slits, there are non-zero numbers (and thus, non-zero probabilities) assigned to positions passing through each slit. In this sense, it “goes through both slits”. More properly, the notion that the photon was “a thing” with a defined position was simply a bad model in the first place.
As the photon keeps traveling, its wavefunction keeps changing. Again, this change is nice and predictable, and in this case, it happens to create an interference pattern, in the sense that if you looked at the numbers the wavefunction assigns to various positions, you’d see the size of those numbers go up and down like the bright and dark patterns that we’ll ultimately observe.
Now, here’s the tricky bit. When the photon strikes the screen, its wavefunction *collapses* – it suddenly “jumps” from having different numbers assigned to different states to assigning all of its probability to *one* state. And that state, which is chosen randomly depending on the numbers assigned to different states just before the wavefunction collapses, is the point where we ultimately observe the photon. Wavefunction collapse is, in some sense, “the weird thing” that is going on in quantum mechanics, and exactly what the hell is happening here is a source of considerable debate, although everyone agrees on what observers will see in the experiment.**
Since the position the photon’s wavefunction collapses into depends on the numbers it was assigning to different states before the collapse, and since those numbers encoded an interference pattern, the photon’s eventual position is more likely in the “up” parts of the interference pattern and less likely in the “down” parts. And if you observe *many* photons, these probabilities will generate your nice interference bands.
To sum up:
* The interference pattern comes from the way the photon’s own state changes over time. It “interferes with itself” in the same way that a single wave in a pool can ripple around in ways that create interference patterns, even if no other waves are present.
* The single points at which a photon is observed come from wavefunction collapse, a fundamentally Weird Quantum Thing that causes the photon to pick a single state (rather than the weird smeared-out quantum wave) when it interacts with something else.
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* Strictly speaking it encodes some other information too, but we’ll keep it simple here.
** Not every way of thinking about quantum mechanics involves wavefunction collapse, but they all produce the same observations we’re talking about here.
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