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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.


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Comments

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Where's the triple, quadruple slit experiments?? What if there's more than one observer? 4 observers and 5 slits? What if the observer is an animal, that's totally ignorant to the experiment or a blind person who's not really observing anything? What if you could delay, or increase the time which observers see the experiment. Will results change over time? What if past observer time travels to future observer and they observe together? Would that cause changes in the past or in the future?

What kind of detector is used? I never see it mentioned. That is key since it is doing something to the system. I really doubt looking at it changes it. But when it is looked at something is effecting it.

Permalink In reply to by JohnM (not verified)

I thought that too, but they will change even if the detector is on the other far side of the slit after the “waveicles” or whatever have gone through. It’s like they knew going through the slit before they even came to a detector that they were going to be observed. It’s like us using the Wayz app to see where cops are before we get to them on a highway.

Indeed, this is a very interesting question and requires a profound sequence of thoughts that indefinitely compound onto one another. Yes, what if the spectating conscious organism is merely a feline unbeknownst to the whereabouts of quantum mechanics and the experiment? I believe the electrons would still act accordingly, due to the fact that an organism is still prompting a flow of information through the process of observation. In terms of the blind persona, vision goes far deeper than just the eyes. It is perception and the manifestation of the mind that creates our discernation of this world. Ah, I agree, it is very broad to sort this experiment into binary terms and incredulous claims. There is much to this world we do not know, but the only thing left to do is constantly steering the wagon of your intelligence towards the feast of knowledge that awaits.

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Thanks for this article! Been looking for something like this for a while and this finally cleared the fog in my mind.

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The mystery of this experiment is revealed by the inverse relationship of time and matter noticed by the fact that when a "particle" leaves a mark in matter like a wave it acts in time like a particle when it leaves a mark in matter like a particle it acts in time like a wave. This was discovered when analysing experiment videos recorded by Nebraska Lincoln Univerisity's physics department in 2013 the experiment described by the British Royal Institution in the youtube lecture and by Akira Tonomura and his team at Hitachi in 1989. The way to make sense of this inverse relationship is the realisation that time while appearing to move forward in one dimension is only measured along one dimension but it is in fact 3 dimensional. I provide 3 new experiment variations to prove it and a unified field theory/theory of everything with robust logic and new mathematical formula (that describe exactly what mass is and gives the calculations for almost all standard model particles) on https://doubleslitsolution.weebly.com

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I suppose there is a rational explanation behind this experiment, but we must maintain focal point on the objective linked to quantum mechanics. To elaborate further, electrons bear no mass, so they create a seemingly enunciated wave effect when passing through these slits. However, each particle, no matter its position, is entangled with one that reaches an identical subatomic level, I believe. It would not be ludicrous to entertain the thought that these very particles branch off into separate forms as they traverse through the empty space, then reunite as a sole electron at the other side. Information is triggered inevitably by observation, by the linking and co-authoring of time, and by the energy relations of every conscious organism inhabiting the planet we know to be indigenous to. We cannot constantly impose these boundaries upon ourselves; these limitations of fleecing ourselves into the perception of only certain quintessences. If we keep doing that, we shall never go beyond. In order to develop a larger sense of thinking, we must first discern the world in segments prone to be debunked. Humanity generates existing energy and information through the inevitable connection between time; time, the non-linear essence whose properties we do not pay as much attention to as we should. After all, solving the mystery of time can lead to answers for a raft of questions concerning topics far broader than quantum mechanics.

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More of a question. What is the effect of relativity on the electrons if any? If the electrons are traveling at c (are they?) Then for an electron "no time passes" and any entity traveling at c has no dimension in the direction of motion. There always seems to be more surprise when the experimenters say "one at a time" but what does "one at a time mean" for a particle traveling at the speed of light. "Nothing happens" to a photon until it interacts with something. Is this significant or just a misunderstanding of physics. Has anyone done the experiment where they fired the electrons through the slit over a very long and extended duration. I would imagine it would make little difference. But firing the electrons one at a time a week apart gives an interesting look into the nature of that thing we call time.

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What happens if collapsed wave form single photons then go through a double slit. Do you still get interference patterns?

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I am no expert in physics but intuitive.

I think we are looking at the wrong thing to explain the interference pattern.

It is not the electrons that are of interest to study.

It is the matter/perhaps yet unknown particles the electrons travel trough.

The electrons creates a wave pattern in the matter around it. With each particle shot a new wave is created. These waves created roads where the waves take eachother out. The electrons slide trough these roads.

Perhaps when the electron hits the wall on which the pattern is detected creates some kind of push back to the media trough which it travels which leads to i sideway push to left and right scattering the incoming electrons sideways as well.

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Instead of slits cut into a solid structure. Use 3 particle beams spaced equivalent to the normal set up to recreate the 2 slits. I know what you're going to say, the beams will cause the single wave/particle to collapse/cancel and travel through one or the other slit and we won't see the interference pattern. But it might be worth a shot.

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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?

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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.

Fascinating and apparently replicating results for a century! I agree and to understand the experiment's intractability, I need to know more about the conditions under which the results were measured without "observation". Were electrons, et al, fired at a wall with slits in a room devoid of observers, utilizing some kind of delayed turn-on switch? And the results reviewed after the fact? How can you possibly control the experiment to the extent that the only difference leading to this "weird" result is the presence of an observer? Is the equipment used in measuring the same as when observing and measuring? (Has anyone observed from an adjacent room through a window?) My scientific background is non-extant and I'm striving to understand what an objective experiment in this case would look like.

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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?

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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?

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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.

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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.

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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?

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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.

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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.

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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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We dont just possess a consciousness, We Are Consciousness Itself. Every "thing" is made of Consciousness, including of course, our brain and body. When Consciousness observes any modification of its own inherent radiance, it creates a "brain fart" and makes scientific experiments go wonky.
When you Realize the Enlightened State, you are resonating at the speed of light. You Realize you are not a mortal body-mind, rather you Are Consciousness Itself, but not as an ego personality who says to itself, "I am consciousness" There is no duality left. Only Consciousness Itself, blissfully and eternally enjoying its own Play and Self-Radiance. Consciousness is not a creator, yet all manifestation occurs within Consciousness Itself. Consciousness is Acausal, Self-Existing prior to cause and effect, prior to time and space, yet includes time and space. And dont forget, Im not talking about an abstract "thing" Im talking about the royal "we", our True Self nature. We Are truly One on the most intimate level possible, to the degree of it being absolutely impossible for there to be any kind of separation In Reality, there are only modifications of the Light of Consciousness.

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Particle and wave concepts are complementary aspects, in need of unification via channels transmitting packets of quantum information (qubits). The Network Model resolves all other mysteries: beam splitter delayed choices, entanglement, quantum tunneling etc. It builds at foundations of physics, unifying fermions and bosons as excitations transmitted through fermionic channels ...

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Right. There is no way to discern which previous comment is being skewered, bent, corrected, deflected or obviated, and chronological order has been discarded as too Aristotelian, I reckon, so I'll just note that some of these comments reflect silliness and plain old not reading the material carefully, and are therefore kinda funny. Some are on point but when out of order, which point they're on is a tricky detail. Still, somebody named John laid down the law when he said, "Atoms of the Slit" because YES, obviously. Gotta go write a short story, kids. Thanks for the article , Marianne. Keep 'em flying! -Spacebeagle

Just by observing something, sapiens have the ability to influence it's behavior. If there is confusion or if this 'makes no sense' that is the answer. If it makes sense, extrapolate the data and apply it at a macro level. We just like to dismiss things we cannot see or explain, because it reinforces the primal brain's need for safety. We are not as evolved as we think we are...

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There is no need for the ai program that we call "our lives" to write our story, based on the laws of physics, unless we are observing it. We are not all actually observing this phenomenon. We are simply sharing the story line written by the program that writes "our lives".

Permalink In reply to by Paul (not verified)

I think many of you fools would understand when I say that this debate is pointless over text or comment. I think you should all be debating face-to-face with each other to determine each other's points properly.

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My thought is that where an electron is physically located is actually a chance, or probability, of where it could be. Once the electron is observed, we know for certain where on the "wave" of physical space the electron actually is. The observation of where the electron actually is removes all other location possibilities of that electron. In the "larger" world we exist in, I think of it like a range of choices a person could make. There is a range of probabilities what choice that person will make, but we only know for sure what choice the person will make when they actually make it. Like the Quantum Realm version of Free Will.

This is easily explained, I cannot understand the best minds not working this out.
There is an underlying 'vibration' in the universe.
(Probably not, technically, a vibration, something more complex, but in this physical plane gives the effect of vibration so works for this example).
This vibration makes quantum's appear in two places at one time, the opposite is in reality true, it's everything else - us, our 'measuring', all our physical existence that is in fact 'buzzing'.
This vibration is only observable at the quantum scale, as we are vibrating at the same frequency, but photons travelling at light speed are affected differently - maybe less, more, or not at all, but anyway, in some way, out of 'phase'.
Observing or measuring will always for 'us' (everything not travelling at light speed) align along the same 'level' of vibration, what scientists would call 'collapsing the wave function'.
You are not collapsing the wave function, all parts of this experiment are vibrating in phase EXCEPT the photons, observing the 'path' of each photon just cancels any possible differential in vibration for us each time, ie the two aspects - observation at slit and observation at screen of individual photons - will always by definition be at the same 'point' of vibration.
The photons are behaving as particles, the slits are behaving as waves and observations at the slit are behaving as 'in phase' waves.
We cannot measure the 'wavelength' of the apparatus as we and all ways of measuring are in phase with the vibration.
This 'vibration' is the unknown, but one simpler explanation could be, say, a general instability, reverberation or tension due to the rate of expansion of universe being faster than light speed?
Come on science get a grip......

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Could it possible that the light from the detector or the electric current inside of it is affecting the energy state of the particle/wave, changing it's total energy/momentum and thus can cause a difference in outcome? I believe the same could be said about the case of human observation affecting the outcome. Observing the experiment causes some of the light/energy to escape into the eye of the observer, thus changing the energy total and behavior of the particle/wave. On that note, I like to imagine that light goes into one eye (observing the world) and antilight/light waves made up of dark matter goes out the other (affecting the world).

I have many neat theories about stuff like this, well, pretty much everything. I have constructed somewhat of a holistic world view.

Cheerio!

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What if you put a mirror on the side and the wall behind the light emitting device has a clear transparent window built in it and behind it there would be a camera that would be focused on the mirror. Would the light particals still know there being observed.

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Light, photons, electrons can surely only work in this physical universe as particles. Think about a star, if every photon had to spread across the universe as a wave we wouldn't be able to see anything 100 miles away let alone 13 billion light years.
There are many incongruities in science that prove nothing. Think about this - two cars travelling on parallel roads. Car A is moving at 10m/sec and is 10m ahead of car B which is travelling at twice the speed. Half a second later car B reaches where car A was originally, car A has moved 5m, 1/4 second later when car B reaches this point car A has moved 2.5m and so on into infinity. Despite travelling twice as fast car B NEVER overtakes car A as far as this experiment proves. Obviously this is not what happens in reality. Something about conservation of momentum?
The wave form in this experiment means little to nothing, the photons are only ever actually detected as particles.
It makes more sense for everything in the universe (except light speed particles), including us and all our apparatus and ways of measuring etc to be in some form of duality, than photons.
Or perhaps this experiment is an explainable anomoly, but photons can only be and only behave as - particles. NOT WAVES!
Even, say, if a photon was just a transfer of information through a medium at light speed (like a relay), it is feasible that the medium could have some type of wave form, but the photon itself must, in essence, be as a particle.