Physics in a minute: The double slit experiment

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One of the most famous experiments in physics is the double slit experiment. It demonstrates, with unparalleled strangeness, that little particles of matter have something of a wave about them, and suggests that the very act of observing a particle has a dramatic effect on its behaviour.

To start off, imagine a wall with two slits in it. Imagine throwing tennis balls at the wall. Some will bounce off the wall, but some will travel through the slits. If there's another wall behind the first, the tennis balls that have travelled through the slits will hit it. If you mark all the spots where a ball has hit the second wall, what do you expect to see? That's right. Two strips of marks roughly the same shape as the slits.

In the image below, the first wall is shown from the top, and the second wall is shown from the front.

Double slit

The pattern you get from particles.

Now imagine shining a light (of a single colour, that is, of a single wavelength) at a wall with two slits (where the distance between the slits is roughly the same as the light's wavelength). In the image below, we show the light wave and the wall from the top. The blue lines represent the peaks of the wave. As the wave passes though both slits, it essentially splits into two new waves, each spreading out from one of the slits. These two waves then interfere with each other. At some points, where a peak meets a trough, they will cancel each other out. And at others, where peak meets peak (that's where the blue curves cross in the diagram), they will reinforce each other. Places where the waves reinforce each other give the brightest light. When the light meets a second wall placed behind the first, you will see a stripy pattern, called an interference pattern. The bright stripes come from the waves reinforcing each other.

Double slit

An interference pattern.

Here is a picture of a real interference pattern. There are more stripes because the picture captures more detail than our diagram. (For the sake of correctness, we should say that the image also shows a diffraction pattern, which you would get from a single slit, but we won't go into this here, and you don't need to think about it.)

Now let's go into the quantum realm. Imagine firing electrons at our wall with the two slits, but block one of those slits off for the moment. You'll find that some of the electrons will pass through the open slit and strike the second wall just as tennis balls would: the spots they arrive at form a strip roughly the same shape as the slit.

Now open the second slit. You'd expect two rectangular strips on the second wall, as with the tennis balls, but what you actually see is very different: the spots where electrons hit build up to replicate the interference pattern from a wave.

Double slit

Here is an image of a real double slit experiment with electrons. The individual pictures show the pattern you get on the second wall as more and more electrons are fired. The result is a stripy interference pattern.

How can this be?

One possibility might be that the electrons somehow interfere with each other, so they don't arrive in the same places they would if they were alone. However, the interference pattern remains even when you fire the electrons one by one, so that they have no chance of interfering. Strangely, each individual electron contributes one dot to an overall pattern that looks like the interference pattern of a wave.

Could it be that each electrons somehow splits, passes through both slits at once, interferes with itself, and then recombines to meet the second screen as a single, localised particle?

To find out, you might place a detector by the slits, to see which slit an electron passes through. And that's the really weird bit. If you do that, then the pattern on the detector screen turns into the particle pattern of two strips, as seen in the first picture above! The interference pattern disappears. Somehow, the very act of looking makes sure that the electrons travel like well-behaved little tennis balls. It's as if they knew they were being spied on and decided not to be caught in the act of performing weird quantum shenanigans.

What does the experiment tell us? It suggests that what we call "particles", such as electrons, somehow combine characteristics of particles and characteristics of waves. That's the famous wave particle duality of quantum mechanics. It also suggests that the act of observing, of measuring, a quantum system has a profound effect on the system. The question of exactly how that happens constitutes the measurement problem of quantum mechanics.

Further reading



If people are matter, consisting of both particle and wave impulses, would using the mind, as the observation device mentioned in the experiment with electrons, to focus the mind on the intent of a thought or external thing effect a predictable response.?
If you are able to expose people to your tool of observation (focus) - media, song, subliminal messaging, - that identifies a trough of human experience, could you manipulate thoughts and beliefs en masse?


Was the experiment conducted in a vacuum? It would seem to me that in order for this experiment to be conclusive it would require that ALL possibility of outside variables be eliminated. Since atmosphere would interact with any particles in motion, it would inevitably taint the results. I also suspect that the nature of atomic structure would introduce interference related to the positive and negative charges individual atoms possess. In addition, I believe that gravity could also create variables that are detrimental to the conclusions drawn from the experiment. I'm not sure, given the current state of technology, that the double slit experiment was able to sufficiently eliminate outside interference.
It is an interesting experiment. However, I'm not convinced that sufficient efforts were made or could even be made to produce conclusive results. That is precisely why what we believe to be scientific fact is always changing. As we gain understanding we inevitably find ourselves rewriting what was previously thought to be infallible. It is also why our imaginations constantly ponder the "what if" that has created so many science fiction authors.


Some other comment said that the electron does not interfere with itself in single slit. Is this true? why does the probabilistic nature of the electron only begin once there are multiple slits? what if you have an infinite set of slits, does the Electron interact with all of them? If so what if the slits are angled or have different sized gaps? what if there are curved slits? do the electrons still behave in a wave pattern or does it behave in a particle pattern? what if the half the probe wall is brought forward and the other half backwards and then a another double slit experiment is placed at a 90 degree angle there and the electrons are observed? does the electron being observed (by a small light wave) still behave as a particle and have changed angles by 90 degrees after being observed? is it possible then to turn the electron 180 degrees and have it interfere with electron pattern from the original double slits? what happens if the wave pattern meets particle behaviour now that they are at the same frequency. Does what happens with light and light destructively interfering with it self also happen here. Is the momentum equally split between slits or is it based on which slit the electron went through? is the momentum split probabilistically? is it then possible to have momentum split so small that the velocity results in a slit having to cross a Planck length in a magnitude of time greater than a couple of seconds? If so could we observe that slit seemingly teleport across Plancks length?


But Why does it happen? Why do the particles behave differently when they are being detected ? I'm getting the distinct impression that physicists don't know why this happens because I can't find an answer to this question. If it's not other particles neccessary for measurement, such as photons or even smaller low energy paticles, hitting the subatomic particles and interfering with them why are they behaving differently? Does anyone know?


Out of all the talk on double slit experiments, I have never heard anything about how electrons or light interact with the material of the slits. I am use to using RHEED/LEED (electron diffraction) in semiconductor manufacturing, so could the patterns just be electron diffraction off the atomic lattice of the edges of the slit material? Then you get an interference envelope from the 2 electron diffraction patterns from 2 slits. If it is just electron diffraction off the atomic lattice of the slit material, then even if one electron at a time, you naturally get a gradual build-up of a diffraction pattern off the atomic lattice. Nothing mysterious. Maybe no need for some 'spooky' quantum explanation?
Also, I think it has been shown (by an Italian group) that when an atom/electron detector is there, usually in front of the slit, it of course disturbs such electron scattering, so the pattern changes. Again, seems nothing 'spooky'.

Permalink In reply to by WILLIAM HOLLEY (not verified)

What retroactive pathway?
As that Italian group showed, a detector disturbs the electon trajectory. It seems if electron has inelastic scattering with detector, interference pattern changes. If elastic scattering with detector, pattern doesn’t change. Should be the same with detector on either side of the slits.


Wave interference is a such misnomer. There is no such thing as wave interference. On the contrary, the waves are passing each other without any distortion ( providing the medium through which the waves are traveling is elastic enough).
What we are calling interference is the wave traveling through medium which has been distorted by for example another wave.
Double slit experiment may indicate that there is a medium which is being distorted by photon or electron traveling through the other slit.
It would be very interesting to check the time distance between electron passing through one slit and electron through the other slit within which so called interference is still occuring.


Is the double-slit experiment and its bizarre observable effects the reason cerns large hadron collider exists? Do, they collide particles similar to what was used in the double-slit exp. And taking the tests to the extremes of what is possible. It just seems the 2 would be directly related, but I have not seen anyone mention this in these comments. Are we on the brink of inventing time travel just by being aware of this quantum phenomenon, or are our brains cramping for no purpose at all?


To make it short,maybe the observation bends gravity and the fabric of space and that's why electrons condense. Light just follows bent space. Not observing allows it be to a wave.


What exactly is the detector? If it’s an electronic, then when it’s on it surely produces it’s own set of waves. Waves that could affect the particles shooting through the slits. There are too many variables here to just determine that the atoms “know they are being observed.” To me this just seems egocentric. And like a comment said down below, bouncing atoms can cause a greater affect on a micro scale that’s being overlooked. There is definitely something weird going on, but we fail to realize that we could be jumping to conclusions. Sabine Hossenfelder even debunks this quantum eraser theory in one of her videos that’s worth a watch.


Honestly after I heard about this from a video I watch on YouTube, I have no explanation for how it is even possible for the results to change depending on wether or not an observer is in use or not, it’s almost like the observer alerts something i.e a program and it corrects its mistake as if it was never made in the first place. I feel like this experiment solidified my reasoning on how I think this existence we are living as a civilisation is a simulation. First of all, if we were in a simulation we would never truly find out that we were in one because that is the way that it has been designed. Now I am just a sceptic, obviously I have no hard proof or facts, but all’s I’m gonna say is, “keep an open mind to any possibility as we may just be a battery charging someone’s spacecar” - Rick and morty reference iykyk😉

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