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
I think your confusion stems from the word “observe”. On the quantum level, it’s impossible to peek at a particle and know its state without also interacting with it. Imagine if you were blind, and the only way you could see the world was to throw a tennis ball straight forward and measure the time until it came back to you. You might get some information about the world, but every time you threw that tennis ball, the object that it struck would change state. It might be pushed back a bit, it might get dented, or it might be completely destroyed. That’s also how we observe quantum particles.
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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Let’s start with the classical double slit experiment.
Light is a wave. It goes up and down with a certain frequency and wavelength. If two waves overlap such that they peak and fall in the same places, we get constructive interference, so it looks like we just have one big wave. If the peaks of one line up with the falls of another, we get destructive interference, and it looks like there’s no wave there.
When we send light through the double slits, it’s like we have two light sources. When they hit the screen,the light has to travel some distance from each slit. If the difference between those two distances is a multiple of the wavelength of the light, then the peaks line up, and we get a bright spot on the screen. If the difference in distance is a multiple of the wavelength, plus an additional half wavelength, then a peak and fall will line up with each other, causing destructive interference and a dark spot.
Now, once we do the single photon double slit experiment, we need to know that a photon is both a particle and a wave. When the photon acts as a wave, we describe its location as a probability curve. When it passes through the slit, we don’t know which one it goes through, so we have two different probability curves that we have to add together to describe the photon’s position. This is essentially schrodinger’s cat, we don’t know if it’s alive or dead, so it’s in a superposition of both states until we observe it, and it must collapse into one of the valid states. While the photon is traveling to the screen, it’s in this superposition, the two halves of the probability curve (left slit and right slit) are interfering with each other. This makes the probability curve look just like the diffraction pattern we saw in the classical double slit experiment, and once the photon hits the screen, it collapses that probability function into a single point. We still don’t know which slit the photon went through, but we do know where it is now.
If you were to put something that could detect a photon passing through it on just one of the slits, you would no longer get the diffraction pattern. Instead, you would just get two images of the slits on your screen. That’s because that superposition is no longer occurring because you will always know which slit the photon went through depending on whether or not it set off the detector. No interference occurs.
Finding a photon in the dark spot of the single photon double slit experiment would be like doing Schrodingher’s cat, but once you open the box at the end, you find an eggplant rather than a cat.
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