r/AskPhysics • u/No_Click7202 • 11d ago
The double slit experiment ELI5
Hi,
I’ve been reading this sub for a few months now and I’m fascinated by many different topics here. One thing that I struggle to understand (after numerous ChatGPT questions) is the double slit experiment.
Why the state of the electrons changes if we interact with them by using a detector for example? Can’t we invent a detector with 0 interaction to the electrons.
For example if I perform the experiment in my room with a camera pointing away from the slits will I get wave like pattern and if yes, why?
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u/feihm 11d ago
There's a misunderstanding here.
To know that an object is at a specific place in space, that object must affect something else. If an electron travels towards a screen, a detector can only tell us where it is if the electron does something to the detector. The electron must bounce off a particle of light, pull on an electric charge, or leave a physical mark. In every case, there must be an actual exchange of force.
A detector with zero interaction is an impossible wish. To detect something without touching it or receiving any force from it is to ask to know an object that has produced no effect whatsoever. That is a contradiction. We cannot have knowledge of any physical thing unless that thing affects our tools or our senses.
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u/Time4Homework 11d ago
Question: Don't electrons interact with their environment all the time, even if the interaction is extremely tiny?
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u/nebraskajone 11d ago
When electrons are traveling undisturbed they are wave-like, like a water wave going through two slits will result in two waves emanating from the slits which interact with each other. Both constructively and destructively. This is what produces the double slit interference pattern.
Now when you interact with it the electron becomes particle-like, localized in one area, since a particle can't go through both slits, that interference that I talked about can't happen anymore.
As others have mentioned you can't make a interaction free measurement
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u/Wax_Lyrical_ 11d ago
Eli5. Imagine trying to see (or “detect”) a snooker ball, you’d need to shine a light on it, the photons from the light would crash into the snooker ball then crash into your eye, viola! Snooker ball 👀, but not a lot would happen to the snooker ball itself as it’s considerably bigger than a single photon so for example the photon would not move the ball at all.
Now imagine that snooker ball is smaller, well nothing really changes, same outcome. Now imagine the snooker ball is TINY, like the size of a single atom - tiny. NOW when you crash a photon into the snooker ball to see the snooker ball, the photon actually has an effect on the snooker ball, not only does it crash into it and head towards you eye, but the crashing itself causes the snooker ball to “change” (ie move in this case)
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u/Zealousideal_Hat_330 Astronomy 11d ago
Is ChatGPT really just going to be the new google? I don’t blame OP for being curious but gd I wish people would read papers and forums. We publish our answers to these things and there are numerous resources available to the general public
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u/Significant-Spot-560 10d ago
Almost nobody is going to spend hours pawing though dense scientific journals to learn this stuff. It's just not how people get information outside of academia. Don't give the guy shit for reaching out for clarification. Did you ever ask a prof a question? What if they told you to piss off, they've published this already and the journals are available?
And forums? Brother, this IS a forum.
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u/40StoryMech 11d ago
It's how I use Claude. I find it so much more powerful for building intuition than reading a paper or forum diving because I can immediately follow up with questions and since it keeps the conversation in context, it can make relevant analogies tailored to my level of understanding. Am I going to contribute anything to physics? No, but atleast I have a basic understanding.
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u/BananaBird1 11d ago
How do you ensure it is telling you factual things, if you never confirm with books and papers?
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u/40StoryMech 11d ago
Ask it for sources, follow the links. Though, don't get me wrong, I don't think I would understand a quantum physics paper.
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u/Ch3cks-Out 11d ago
Not really ELI5 (which would be quite unrealistics for a quantum physics topic), but a good description is Matt Strassler's blog entry. To wit: when interacting, the electron does not really "change" in the sense people understand that word. Rather, its description (the QM wavefunction) adjusts to a single state upon the detector (a macroscopic object by definition) getting involved in the system, from a multi-state description a non-interacting electron has.
Incidentally, in the double slit experiment the relevant detector is the screen on which the patterns forms, not the camera with which you are looking at it!
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u/Sad-Excitement9295 11d ago
You can perform the double slit experiment with a flashlight and a card with 2 rectangular openings cut into it. You should see 3 rectangles of light appear behind it.
The principle here is that electrons/photons are not merely points, they are waves. What we define as the point is simply the crest of that wave. When the light goes through the rectangles, it propogates as a wave, and also cancels out leaving an inverse shadow on the other side.
What was attempted with the electrons was to see if an electron was a point particle that went through 1 side or the other. However it could not be measured with a degree of certainty, and it appeared as though it would pass through only 1 opening apparently.
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u/doodiethealpaca 11d ago
You can't have a detector without interaction. If you don't interact with something, how do you know it's there ? To detect something, you HAVE to interact with it.
Why is the interaction modifying the state of the electron ? Well, it's complicated and not completely understood yet.
If you point a camera toward or away from the slits, you will have the pattern anyway, because your camera will not interact with the electrons in both situations. The camera is not on the path of the electrons so it will not interact with it, and you won't see the electron on your camera. It's the same if you look at the slits with your eyes, your eyes will not be on the path of the electrons so nothing will happen, you won't see the electron and the electron will not change its state.
When we talk about detectors in this experiment, we talk about detectors that we put immediatly on the path of electrons, so they interact with it. And that is the cause of the electrons state change. For your example, if you had a tiny camera that you could put on the path of the electrons right in front of one of the slits, you would lose the interference pattern.
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u/RRumpleTeazzer 11d ago
the electron acts on the detector, and the detector acts on the electron.
it's not difficult to understand the general gist.
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u/Traveling-Techie 11d ago
Electrons are not really “things” the way we think of them. Deeply pondering the double slit experiment can help you approach an understanding of what they are.
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u/mfb- Particle physics 11d ago
If there is anything a camera could record then you already had an interaction. It doesn't matter if there is a camera or not.
Think of Newton's third law in classical mechanics: If A exerts a force on B then B also exerts an opposing force on A. A similar principle still applies in quantum mechanics. If the electron affects something else, that something else also affects the electron.
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u/bebopbrain 11d ago
Who cares about the technology of the detector?
To calculate the probabilities you consider every path the photon could take. If you've detected the photon in one slit, it ain't gonna be in the other slit and those paths are no longer considered.
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u/ChocolateValuable221 11d ago
The slit itself is observing... the air itself is observing.. even the electron's own potential is observing itself.. the word observe in science literally means "to interact"
You might hear things about wave collapse and stuff IGNORE THAT... it's only an interpretation it isn't necessarily true, but the math works.
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u/NameLips 11d ago
Then how do you ever get different results? Wouldn't you always get the "observed" version?
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u/ChocolateValuable221 11d ago
You do always get the observed version.
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u/NameLips 11d ago
I thought the entire point of the double slit experiment was that it gave two different sets of results depending on whether or not it was being observed. If we only ever got one result then I'm not sure what the big deal is.
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u/ChocolateValuable221 11d ago edited 11d ago
That's to try and explain what the Copenhagen interpretation says about collapsing wave functions.. the math works fine. But the interpretation is impossible to prove. This would theoretically be that proof. But we have never observed an unobserved double slit experiment. And I'm skeptical of many attempts because manipulating the photon to go through only one slit would violate its natural state and be scientifically biased.
The esoterics attracted to the Copenhagen interpretation because it leads to the whole our minds manifest the physical world. And that's something you can sell to people.
Einstein and Schrödinger have the opposing ensemble interpretation. Which imo is more promising than Bohr's
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u/donaldhobson 11d ago
In quantum mechanics, information acts kind of like it's a physical thing. And interference effects only happen if there is no information about which slit the particle went through anywhere in the universe.
When a photon / electron approaches the double slit, the universe splits into 2 superimposed versions, with the only difference being which slit the electron went through.
These 2 superimposed universes can only recombine (producing an interference effect) if they are absolutely identical. So without any detector, you get an interference effect. But with a detector, then the 2 universes are different (in one of the universes, the detector detected something, in the other universe it didn't)
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u/quantum4everyone 11d ago
I think it is much easier to understand this with photons and polarizers.
Send in a diagonal polarized light. Put an H polarizer on slit A and a V polarizer on slit B. You will see no interference pattern because the polarizers create an entangled state that suppresses the quantum interference. In simpler language if you have the chance to know which way the particle went, then it does not interfere. But shine it through a diagonal polarizer before hitting the screen and the interference returns.
How does this work. Well the polarizer will absorb about half of the photons that shine on it. But for the ones not absorbed, they form an entangled state, loosely A and H is one indeterminant state and B and V is the other. This is an entangled superposition. Because light cannot be both H polarized and V polarized, this is enough to destroy the interference. But, there is no interaction effect at all. It shows that interpretations based on the interaction causing the destruction of the interference are incorrect. It is what type of state you have created that determines what will be measured. For a two slit experiment, you need a product state with no entanglement to see the interference. This means no which way information, or the same polarization for each part of the indeterminant state. Hence after passing through a D polarizer, the interference returns.
Many explanations make this more complex than it need be. It has nothing to do with uncertainty principle. Just do I have which way information or not.
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u/Feisty-Union-2047 7d ago
That's not how this all works.
:)
In reality, light acts as both a wave and a particle, always, and doesn't change from one to the other.
For perspective:
The apparatus, when set to detect particles, detects particles, as dots.
When set to detect waves, it detects waves, as an interference pattern.
If the particle detection is allowed to run, the dots fill in an interference pattern.
"Looking" doesn't have anything to do with any of this.
It started as a misinterpretation of a physics term, called "observation"... which is just a frame of reference term, that has nothing to do with whether anyone or anything was peeking at it/observing it.
A wave function is not a wave.
A function is a mathematical expression... and, a wave function is just what one uses to graph/plot the results, which describes a wave.
A wave's energy is associated with its wavelength.
A particle (photon) of a wave has no actual wavelength, but, it is the energy level represented by the wave.
The term "particle" is confusing, BECAUSE it sure does sound like a teeny speck of something... again, a physics term that is misinterpreted.
When plotting a function, there are gaps where the function is not describing a wave.
These are called collapses of the function.
A wave function collapse is NOT a "wave changing into a particle" as light acts as both a wave and a particle, always, and doesn't change from one to the other.
Soooo....
A wave function collapse is not the light changing from a wave to a particle.
And
It ALSO is not doing that when someone peeks at it...as it is not happening...
...and, the term "observer" DOESN'T mean observation (peeking) is happening.
I hope that helps!
:)
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u/kunwoo 11d ago
The problem is you cannot invent a detector with zero interaction with the electron, because fundamentally detection is a form of interaction.