My attempt at explaining double slit experiment

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Bust Nak
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My attempt at explaining double slit experiment

Post #1

Post by Bust Nak »

... Or at least the basics there of, in my own words. So we are on the same page, I will start with the macro world that we are familiar with, of water waves and solid pellets.

Two important wave behaviors for waves such as those on the surface of water:

When a wave hits a screen with a slot of a similar gap as its wavelength, it generating a circular wave on the other side of the screen. This is known as diffraction. If you were to capture the result on a graph at the far end of a water tank, you would get a general smudge - the energy is spread out in all direction this side of the screen.

Waves have trough and crest, when the trough of one wave coincide with the crest of another wave, they cancel out and we are left with a local spot of calm water. On the other hand, when the crest of one wave coincide with the crest of another, they add up and we get a localized high point. When two waves of the same wave length, originate from nearby points meets, the crests and troughs lines up and form an interference pattern. Which is easily visualized as light and dark patches when a lamp is shined on the surface of the water. Such synchronized waves can be easily generated by making use of diffraction effect as described above, by sending a wave towards a screen with two slots.

Particle behavior such as pellets fired from a gun:

Not much to explain, a pellet has a distinct location and interact with other solids when one hits the other. If you fire pellets at a screen you would see them land at distinct spots. Put a shield with a slit in the middle, some would hit the shield and be stopped. Some would pass through the slit and continue it's path to screen. Similarly, fire a bunch of pellets towards a shield with two slits, you would get a two distinct piles of pellets as the result.

Nothing out of the ordinary so far. Now on to the micro scale of the double slits experiment. Here I use photons, but they can be atoms, or sub other atomic entities such as electrons.

Quantum weirdness as demonstrated by a series of experiments

1) Fire streams of photons (beam of light) at a screen with one slit, the experiment shows one distinct light band and dark area elsewhere. This is as one would expect as if firing pellets through one slit.

2) Fire streams of photons at a screen with two slits, the experiment show light and dark bands of an interference pattern, as opposed to the two distinct light bands. This is as one would expect as if a water wave travel through two slits.

3) Cover up one slit of experiment 2) and you get the result of 1) again.

This much shows that photons (and atoms and electrons and so on) are both waves AND Particles. Weird but not crazy weird.

4) Now power the photon gun right down, so instead of streams of photons, it fires one individual photon, at one slit. You get one localize pin point of light on the screen. Fire the gun again with another photon, it goes through the slot and lands nears the first one. Fire them one at a time, over many shots, you get the same distinct band of light as experiment 1). Just as one would expect, it would not matter if the photons were fired one at a time or a whole bunch together.

5) But what if you fire photons one at a time at two slits? Common sense tells us that it would go through one slit or the other. This is where firing one at a time would make a difference, as you need two waves to generate a interference pattern, right? Run the experiment and one photon goes through a slit and land at a distinct spot on the screen. Now fire lots more, one at a time, and each photon lands individually, to build up the interference pattern as in experiment 2).

Now this is crazy weird, for the interference to happen, you need the troughs and crests of two wave. This implies that one individual photon traveled through both slits, then collapse back to a distinct particle as it hits the screen. You can describe this weirdness as the photon "existing at two places at once," or "everywhere at once," or perhaps "no where at all, until it hits the screen."

6) Bonus weirdness, if you place a detector at one of the slot to record the path of the photons, then sure enough 50% of the time, photons go through one slit and 50% of the time they go through the other. And you would get two distinct bands of light, not the interference pattern in 5).

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Post #2

Post by Bust Nak »

That was hard without pictures...

Now for the reason why I attempted this to begin with. In another thread other posters and I were discussing determinism. The weirdness seen in experiment 5) where an individual particle does not exist in one location, but as general field of probability, where it "exist" as 0.01% chance here, and 0.0005% chance over there, is what I would call genuine randomness. We don't know where it is not because we lack information, but because we can't know where it is because it's not really anywhere.

This phenomenon, I'd argue, is fatal to determinism in the purest sense of the term. There is genuine uncertainty in the universe.

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Post #3

Post by Miles »

Bust Nak wrote: That was hard without pictures...

Now for the reason why I attempted this to begin with. In another thread other posters and I were discussing determinism. The weirdness seen in experiment 5) where an individual particle does not exist in one location, but as general field of probability, where it "exist" as 0.01% chance here, and 0.0005% chance over there, is what I would call genuine randomness. We don't know where it is not because we lack information, but because we can't know where it is because it's not really anywhere.

This phenomenon, I'd argue, is fatal to determinism in the purest sense of the term. There is genuine uncertainty in the universe.
And determinism isn't saying there isn't, although at one time it did because science wasn't aware of quantum indeterminacy. In light of quantum uncertainty determinism is relegated to events that happen at the super-atomic level, those where all physical events occur, including all brain functions. So, as a practical matter, those matters that speak to human functioning and everything else we commonly perceive, determinism is very much alive and well. Cancel the funeral. :D Determinism "in the purest sense" has simply changed it's sphere of operation.

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Post #4

Post by Bust Nak »

Blastcat wrote:
Blastcat wrote: Thanks for that but I was hoping for some other source than yourself for your claims, honestly.
Bust Nak wrote:Okay, that's much easier.

Thanks for the very informative video. Alas! it explains in detail why you are wrong.

In this video, Jim Al-Khalili never talks about ONE particle or ONE photon becoming a wave.

Here are a few examples of how this source proves the exact opposite of what you have previously claimed:

1) At 0:33 Jim Al-Khalili says "Imagine you have a source of light...." this isn't about ONE particle of light.. but many particles of light.

...

12) At 3:25 he says "of course, a lot of the atoms will be blocked by the first screen..." Not one atom but a lot of them..
That would corespond with what I labelled experiment 1) and 2) above. "Fire streams of photons (beam of light)..."
13) At 5:12 he asks what would happen if we didn't send the atoms all at once, but sent them through "one at a time". Again, we are not sending one atom, but many atoms, one at a time. ( I presume he means one of many, at a time.. not two at a time, but only one at time, as there are many that could go together.. )
One at a time is the important bit.
14) At 5:32 he says "slowly we will see that the atoms...." Again, not one single atom, but enough atoms to make his use of the plural for "atom" appropriate.

And the most telling is at 5:47 when he states that:

15) "Each atom, by itself, is somehow contributing it's small part to the overall wave-like behavior that we see in the interference pattern. "
Exactly. One atom somehow contribute to the interference pattern.
I think it's safe to say that Jim Al-Khalili is telling us that an atom is a PART of a wave interference pattern.. it's not the wave, the two waves, or the interference pattern that the two waves generate.

Please provide some source for you claim that the double slit experiment only uses ONE particle to make a wave, and interference patterns of two or more waves. This source completely fails to demonstrate it. So, I still await a source for your claim. In the meantime, I contend that you are just wrong. One particle isn't a wave of particles, not photons, not atoms, not water droplets, not grains of sand.
But an atom or photon can only contribute to the interference pattern, if it is a wave. The experiment demonstrates this clearly.
Is it still your claim that a particle is a wave at the very same time?
I would say a photon (or atom) is both a particle and a wave.

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Post #5

Post by Bust Nak »

Miles wrote: And determinism isn't saying there isn't, although at one time it did because science wasn't aware of quantum indeterminacy. In light of quantum uncertainty determinism is relegated to events that happen at the super-atomic level, those where all physical events occur, including all brain functions. So, as a practical matter, those matters that speak to human functioning and everything else we commonly perceive, determinism is very much alive and well. Cancel the funeral. :D Determinism "in the purest sense" has simply changed it's sphere of operation.
I don't necessarily disagree with you overall, I just disagree with what "determinism is in the purest sense." Allowing quantum uncertainty as part of the equation in determining super-atomic events, isn't pure.

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Post #6

Post by Miles »

Bust Nak wrote:
Miles wrote: And determinism isn't saying there isn't, although at one time it did because science wasn't aware of quantum indeterminacy. In light of quantum uncertainty determinism is relegated to events that happen at the super-atomic level, those where all physical events occur, including all brain functions. So, as a practical matter, those matters that speak to human functioning and everything else we commonly perceive, determinism is very much alive and well. Cancel the funeral. :D Determinism "in the purest sense" has simply changed it's sphere of operation.
I don't necessarily disagree with you overall, I just disagree with what "determinism is in the purest sense." Allowing quantum uncertainty as part of the equation in determining super-atomic events, isn't pure.
And I agree. I don't believe quantum events have anything to do with determining super-atomic events. As Max Tegmark observed,

"The main argument against the quantum mind proposition is that quantum states in the brain would decohere before they reached a spatial or temporal scale, at which they could be useful for neural processing."

(Max Erik Tegmark
(born 5 May 1967) is a Swedish-American cosmologist. Tegmark is a professor at the Massachusetts Institute of Technology and is the scientific director of the Foundational Questions Institute. )

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Post #7

Post by Willum »

A need-to-know phenomenon is Hyugen's Wavelets

https://en.wikipedia.org/wiki/Huygens%E ... _principle

It is really one of the most important and neglected principles in physics.

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Re: My attempt at explaining double slit experiment

Post #8

Post by Talishi »

Bust Nak wrote: Now this is crazy weird, for the interference to happen, you need the troughs and crests of two wave. This implies that one individual photon traveled through both slits, then collapse back to a distinct particle as it hits the screen. You can describe this weirdness as the photon "existing at two places at once," or "everywhere at once," or perhaps "no where at all, until it hits the screen."
This is not weird at all. Photons are wavicles, they interfere with themselves (or at least, their probability waves do). Firing a bunch of them together makes no difference, it just builds up your image faster.
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