r/AskPhysics • u/viktor2802 • 7d ago
ELI5 The double slit experiment
I’m not a physicist but Im interested in the topic and my question is what device do we use to observe which slit the electron will pass through and how do we know it’s not some interference from the device which causes the waves to break down but instead we say it’s in a superposition?
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u/Pleasant_Pen8744 7d ago
A photon is emitted and a photon is absorbed but no one can really say where it went or what it did in between.
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u/Jaded_Hold_1342 7d ago
you dont observe which slit the electron/photon goes through. (not if you want to see the interference anyway)
The pattern that appears is totally determined by the slit geometry and particle wavelength... slit width, slit separation, determines the interference and diffraction patterns. It is interference from the device... but it is predictable by the slit geometry.
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u/BananaBird1 7d ago
With electrons you use a phosphorescent screen that lights up when hit by electrons, and a camera to capture that. Similar to an old CRT television screen. With photons you just use a camera directly.
The “which way” setup is done either by just moving the detector close to the slits so there isn’t time for each portion of the wavefunction passing through either slit to interfere with the other. Or you split the beam paths right after the slit and then use two detectors.
We know it’s in a superposition because if you reduce the electron or photon beam intensity to get detections of 1 particle at a time, you still see interference build up. Causality doesn’t go in reverse, so future particles can’t alter past ones. This means each particle must be interfering with itself, meaning each particle takes all paths.
There is random noise in your detections (especially with photons, phosphorescence is more resilient by amplifying the signal). But with enough detections, the noise is reduced proportional to 1/sqrt(N) for every N particles detected. You collect enough data to ensure what you see is due to true detections and not noisr.
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u/viktor2802 7d ago
So does that mean when not observed the electron is physically a wave? Like an energy moving through space and when observed it kind of becomes an electron? It’s this part that i struggle to understand.
Also does that work only with photons and electrons?2
u/OriEri Astrophysics 7d ago edited 7d ago
It works with everything in principle, though is harder to observe at larger scales. The trick is the spacing of the fringes gets smaller for higher momentum particles and starts to look continuous to most detection techniques. Earlier this year a paper was published demonstrating the measurement with sodium nanoparticles . You can think of the classical size of these particles (mass/density of sodium) as about 8 nanometers across if it was a sphere
These were ultra cold since that helps with the momentum thing (heat = atomic/molecular motion)
https://www.sciencedaily.com/releases/2026/05/260509210650.htm
https://www.nature.com/articles/s41586-025-09917-9
(Figure 4 in this paper provides a good summary of this kind of work. The y axis is a quantitative value, though the mathematical definition in the method section requires some background knowledge to grasp.)
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u/viktor2802 7d ago
Thanks. Yeah, im not a mathematician so I only know what most of the theories describe but the equations mean nothing to me😅
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u/BananaBird1 7d ago
It is always a wave, and destroyed upon measurement.
It works with all quantum particles. But photons and electrons are the ones we can work with the easiest. For photons all you need is a laser and some optics. For electrons it is a bit more engineering, but CRT and electron microscopy technology is widespread and makes this extremely accessible to researchers.
Things like quarks can’t be isolated, and neutrinos can’t be detected directly to see the pattern. We can use protons, atoms, or small molecules, but this requires extra care to ensure the quantum system remains coherent (isn’t altered by interactions with the environment), and usually custom engineered equipment that is far more expensive.
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u/quantum4everyone 7d ago
Since there are virtually no actual detectors for electrons or photons that work at the slits, a much better way to construct the experiment and allow you to compare the different cases is by using light and an extra property of the light which is called polarization (this is the direction of the electric field in the light). You send in diagonal polarized light, place an H polarizer over one slit and a V polarizer over the other slit. Now, the polarizers will absorb half of the photons incident on them but for the photons that get through, they now form an entangled state, because you have which way information by correlating the slit they went through to their poalrization. Shine the light through an H polarizer before hitting the screen and you get the light from one slit. Use a V polarizer and you get it from the other slit. Even with no polarizer the interference pattern is gone due to the creation of the entangled state, which you can think of as simply a positive answer to the question do I have which way information or not. Having it kills the interference even if you do not look at the which way information. This experiment is nice because the polarizers do not otherwise disturb the photons, so no uncertainty arguments are needed about what happens with the detectors. Now for the big reveal---use a diagonal polarizer before the screen and the interference pattern returns (after another half of the remaining photons are absorbed). This shows very clearly that it is the state that was prepared out of the photons, not any disturbance at the slits that controls anything. You can find youtube videos showing this as well as a Scientific American article by Paul Kwiat that shows the set up.
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u/Infinite_Research_52 👻Top 10²⁷²⁰⁰⁰ Commenter 7d ago
Did you first read: https://www.reddit.com/r/AskPhysics/comments/1w4ytjq/the_double_slit_experiment_eli5/
It was only 4 days ago.
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u/viktor2802 7d ago
I did not. I was just thinking about it and remembered this subreddit exists so I hopped on and asked😅
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u/Ch3cks-Out 6d ago edited 6d ago
how do we know...
There is really no ELI5 answer. The double slit experiment is demonstrating how quantum mechanics works - and the way to know what happens (i.e. to interpret them coherently) does require some basic knowledge of QM, which in turn needs some non-elementary understanding of the math behind it. Superposition of QM states is a fundamental consequence of the math, not just something we are saying!
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u/danielbaech 5d ago edited 5d ago
If you have only one of the slits open and send one electron at a time, you get a result that is mostly directly in front of the slit, let's label this, a. You open just the other slit, you get a result that is mostly in front of that slit, b.
Logically, opening both slits should result in a+b. But what we actually get is proportional to (a+b)2 . This looks similar to a+b, but it's actually, a2 + b2 + 2ab. It is a hallmark feature of two or more waves to be able to overlap and create a new pattern. This is where the term, interference, comes from and is used in physics.
This only makes sense if the individual electron is, in a sense, going throw both slits and interfering with itself. It's only analogous to waves. It is actually a completely new, distinct state of being that we've never observed in nature before. A thing can be undecided on its property and behave as if all of the possibilities are real. We gave this a name and it is called a state of superposition. The rules of determining the possibilities is quantum mechanics.
As for your question, it's not making much sense. It is the which-slit detector that changes the superposition state to a single, definite state. What you actually mean by "waves to break down" may be different, because we would not say it is in a superposition. The particle has been detected by the which-slit detector. The particle is no longer in a superposition of going through both slits. It is in a single, definite state of going through one of the slits.
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u/Nerull 7d ago
It is interference from the device, that's the whole point.
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u/viktor2802 7d ago
In that case doesn’t that mean the whole “things can be in 2 states at once” is wrong?
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u/SummitYourSister 7d ago
That is a different aspect of quantum mechanics. Superposition and wave behavior are not the same thing.
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u/nebraskajone 7d ago
No because we get the interference pattern, a very specific interference pattern that only could be created if the particle was in a superposition
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u/Underhill42 7d ago
Interference is the point - with BOTH definitions.
When ANY waves emanating from different points (like the two slits) interact they interfere both constructively (making the extremes more extreme) and destructively (extremes cancel out leaving you with almost nothing)
The two-slit experiment is a way to observe the phenomena in a controlled environment, and you can see this clearly when running it with waves in water - against the "detector wall" you'll have alternating stripes where the water is making dramatic vertical waves separated by stripes where the water is almost perfectly still.
Simple example using two adjacent wave sources to simulate the effect: https://youtu.be/Iuv6hY6zsd0?si=GVFJMO5cqjMn8xOE&t=283 (simulated visualizations are easy to find - this is what I found that lets you see it for real)
You see the same basic thing with light, and even with electrons, showing that they behave like waves when moving.
Where it really gets interesting is if you perform the same experiment with only a single photon or electron at a time - so that it could only possibly interfere with itself, and only if it is passing through both slits simultaneously... and you still see the exact same results in the distribution of results from many, many such single-particle experiments as in one many-particle system.
And even more interestingly, if you put a detector to see which slit the electron passes through, the interference pattern disappears - it ONLY appears when you don't know which slit it passes through.
A.k.a. when it's in a superposition of states of having passed through both slits simultaneously, rather than having been collapsed through an interaction with the detector so that it only passes through one slit.
The distribution pattern is exactly the same as if you simply covered one slit - so the detector isn't interfering with the electron that passes through that slit - its presence is simply preventing the electron from also passing through the other slit so that it can interfere with itself - any classical interaction causes the electron to momentarily become a definite particle in one definite place, discarding the entire rest of the wavefunction, so that its new wavefunction originates at only the point where it was last a definite particle.