Double Slit Experiment Explained Step-By-Step 

So what do you think of this Double Slit Experiment? Were you able to guess the patterns we would see on the end screen in the different scenarios and variations? Also, is there another experiment you like that you would want to have covered in a video?

Can relate to the photon. I too behave differently when watched.

I've answered correctly both all and none of your questions

How exactly does the detector make an observation of where the particle is? Is it an electron microscope that records visual data, is that even possible? This was the best explanation I've seen for the double split experiment.

I have been a fan of the double slit experiment for years. I have a pretty good post-secondary education, but beyond that, I am about as ordinary a Joe Six-Pack as you will ever meet. Now having said, I must say that your video was very well produced, edited and presented. And it's one of best (out of many) videos on this topic I have yet seen. Your page name was good choice because, despite your obvious acumen, it never feels like you're talking down to the viewer. Well done!

I’ve always struggled with the “observer” portion of this experiment. The fact that the observer is a device with electrical, magnetic properties etc…may in fact be the issue here…right? I’d like to see this experiment done with greater variations regarding the observer…I want to believe consciousness plays apart but I’m not there yet….help me!!!!!

It’s amazing how one of the simplest experiments brings about some of the most complex questions, quantum physics is as mind boggling as the vastness of the universe. Ok now on to quantum eraser! Thanks for explaining some of the details that are very hard for most of us to understand. Keep up the good work

This is the best double slit experiment lecture that I have heard! 👏

So here is a question. If a electron propagates out as a wave, and you have two detectors at random points throughout the wave, one that is easier to trigger, but further away from the source, then another detector that is closer but more difficult to trigger. Which detector will observe the electron with a higher probability? If the one that's easier but further away from the source detects the electron the electron more of the time, at what distance does the other detector (closer but more difficult) start to become more common to detect the electron? If that were to be a scenario, what is the relationship between distance and detection

Dude you rock. I had to go into a class of 9th graders and explain it to them. Harder then it sounds. Especially when they ask questions you have to tell them about the probability factors and how theyre measured and that gets an instant 'Eyes Glaze Over' effect. You know you achieved your goal when no one even gets it. Stay in physics kids

Probably the best explanation I've heard so far. However, I'm still a bit dim on the difference between a particle and a wave. I'm assuming that a particle is a piece of matter (electron, etc.), but what is a wave? Pure energy? What, specifically, is "waving"?

One of the most under-appreciated channels on RU-vid. I regret not to have discovered it earlier.

Your comparison of programming and quantum mechanics makes me wonder if everything's a simulation (o.0) mind blown. Great vid

Thank you very much - this is the best explanation i’ve seen so far, of the double-slit experiment. I love that you give us time (at appropriate moments) to pause the video, so we have time to think about what is happening in the experiment. I found this so helpful. For the first time, I actually understood what happens. Brilliant, Thanks again.

Does the position of the detector/observer influence the double slit experiment? At what distance is the result an interference pattern again? I’m asking this because the detector/observer influences the result of the double slit experiment.

I first learned about this 25 years ago in high school physics and it still blows my mind to this day, it’s something I have real trouble wrapping my simple mind around!

Like everyone else you fail to mention what a 'detector' really is. I think you will find that a detector is some sort of instrument that can detect where a particle is located in space and time. It does this by firing photons which collide with the particle and return to the detection instrument providing information about the impact. I have no idea what 'collapsing' the wave function actually means, but it is clear that any measuring instrument must somehow interact with the particle it is measuring - otherwise it will not be measuring anything.

I watched many double slit experiment videos. Do you know what is common? No one mentions about how to detect the photon or observe the photon. How much energy is required to detect the photon, electron or atom.

Hey Jon, I answered all correctly. The consciousness requirement at the end of your video raises a question: It seems that people have no problem accepting that post-slit wave collapse can determine events in the past. Somehow, recording a collapse event without consciousness is enough. However, at some point after that recording event, someone (a consciousness) will read the results of that original unconscious recording, right? otherwise, how would we know about the result? Could it be that that knowledge derived from the second recording (the conscious one) is the one responsible for collapsing the wave retroactively? (and not the original one) Thanks!

I remember doing this in Physics in College. Interesting thing is, my son did this in a summer Physics Camp (before he was a Junior in high school).

I have read several books about quantum physics that visit this topic, this is the best explanation I have seen. Thanks.

This almost seems to suggest that reality only exists when being observed, much like how when moving in a video game you only generate the sections of the world that are within your viewpoint. The "wave" function of the particle is just a function of "computer code" that defines where the particle COULD BE when observed and it is forced to choose a location when observed. And the reason consciousness is not required to collapse it is because even simply requesting data on particle locations for a non-conscious source triggers this "function" to choose/calculate a position from it's probabilities.

head hurts... i get it and don't get it all at the same time. call it the 'double-slit thought process' :)

Could reflactions of light caused by the light passing through those slits also cause the same wave pattern we see? Even light pointed at a wall can still cause other areas to reflect light right?

Nicely done. You make a good point in the opening. I find people have no idea how much the quantum world contributes to modern western life. They take transistors for granted. But my parents remembered taking the first transistor radios to picnics and the beach.

Thx for posting Jon, I already knew the double slit experiment and it's features, but you managed to bring it clear and easy to understand. Well done my friend.

One of the interesting experiments we do in first year physics. It teaches the students to really think about constructing good experiments. And also blows their minds,

Best explanation of the Double Slit Experiment I have ever seen.

Excellent work. I've seen many experiment on the double-slit, always left in a state awe. Looking forward to your delayed choice quantum eraser video. (Resonant inelastic x-ray) Is a new version of the experiment, didn't have any time yet to read on it.

Shouldn't the experiment WITH an observer show two thick bands for the impact screen, and some scatter around? The way you have it now it looks like a purely random scatter of particles with no slits involved. There will be particles that bounce off the sides of the slits and hit the screen at an angle, but there must be two thick bands corresponding to the slits, otherwise the slits mean nothing and they should not even be there (7:32 t0 7:35). Just my two cents...

Why does the detection of the photon collapse the wave function, when the detector is placed after the slits, but the interaction with the screen does not? Is the screen not a part of the real world just as the detector is?

It's quite simple to explain: They only used ONE recording device, but actually there are TWO devices needed. It's like recording a 3D scene with only one camera, and wondering, why the 3D scene is different from the 2D recording.

7:20 - 7:36 schematic was wildly misleading. You’d see 2 lines where the photons align as particles passing through the slit, not a wave of photons moving through the slits.

This really blew my mind. Thanks for expanding my consciousness

This may have been asked...here is my question. What would happen if the detector is NOT turned off, but the question of which slit it went through was not made known? So for example... If the photon went through slit #1 the detector would turn on a green light. If the photon went through slit #2 the detector would turn on a red light. NOW....let's NOT turn off the detector, let's turn off the green and red lights. So the detector is still "detecting", it's just not telling you which slot the photon went through. What would the result be? No interference pattern? Or an interference pattern?

Good stuff, although I was hoping you were going to show a live demo you did yourself.

In regards to consciousness not being "needed": If the data was stored, it can be retrieved. If it can be retrieved, it can be observed by a conscious observer. So the mere possibility of the observation by a consciousness is enough to collapse the waveform and so consciousness is still a factor. Is it not?

Very cool experiment, kinda crazy how it proves just by observing or measuring something can change the outcome. If quantum computing eventually takes over for binary logic at a consumer level I can't wait to see where this goes. If a flat earther observes the earth maybe it does become flat... mind blown lol

Water is a solid thing and when a cave from it goes through a gap or slit as you want to call it the waters.miving effects the calm water in the other side of slit. This does not happen with light. Hence shadows

I read through that paper and I’m not convinced it rules out consciousness.

This is the best presentation of the double slit experiment I've seen. Great work!

I’m guessing the weirdness arises in using flawed metaphors and ambiguous terminology of ‘particle’ ‘wave’ and ‘collapse’ - this causes a dissonance, eg like trying to fit a square peg into a round hole.

Double slit best explanation ever! Congrats!!

I've always felt like this sounds a lot like the way a computer simulation might conserve performance by making every particle have a wide range of theoretical states, represented perhaps by a quick-to-run global algorithm, before it needing to be narrowed down to a performance intensive specific state when interacting with another object that makes this necessary... Simulation Theory doesn't sound so whack sometimes... It's still probably not true but who knows...

Brilliant explanation of the double-slit experiment. Thanks for posting!

I’m amazed. After 5 minutes of watching this everything went dark. Because I fell asleep.

This video is among the best explanations of the experiment.

I'll be satisfied when someone can explain the results without anthropomorphizing photons.

If you send only a single stream of protons then you could only send it through one slit. ????

11:00 Hold up, isn't the act of storing information into atoms still counts as a which-way detection initiated by a conscious mind?

This is the best video on the double slit experiment that I have seen and I have seen a lot. Really thorough, great I'm graphics, answered lots of questions. Thanks.

Efectul dublei fante se datoreaza interactiunii fotonilor cu materialul din care sunt facute fantele. Interactiunea este posibila datorita frecventei de pulsatie a materialului fantelor, comparabila cu frecventa fotonilor optici. Din interactiunea fotonilor cu materialul fantei, apare la marginile fantelor efectul de difractie a luminii, de ocolire aparenta a obstacolelor. Inteferenta undelor care au suferit difractie la marginile primei fante, creaza figura de interferenta din fata celor doua fante. Dar fenomenul se observa doar la fotonii optici. Fotonii grei, ics si gama, nu se reflecta, nu se refracta si nu se difracta si nu produc efectul dublei fante. Efect care vrea sa fie o dovada indubitabila pentru natura ondulatorie a luminii. The effect of the double slit is due to the interaction of photons with the material from which the slits are made. The interaction is possible due to the pulsation frequency of the slit material, comparable to the frequency of the optical photons. From the interaction of the photons with the material of the slit, appears at the edges of the slits the effect of light diffraction, of apparent circumvention of obstacles. The interference of the waves that underwent diffraction at the edges of the first slot, creates the figure of interference in front of the two slots. But the phenomenon is observed only in optical photons. Heavy photons, ics and gamma, do not reflect, do not refract and do not diffract and do not produce the effect of the double slit. Effect that wants to be an undoubted proof for the undulating nature of light.

What on earth were they trying to achieve when they performed this experiment? What was the aim of shining light through slits?

Holy fuck finally someone with a proper explanation, this man has single handedly saved my chances of my dream uni program

“It’s as if the universe is trying to save resources by reducing computations until it is absolutely necessary to do so”. This blew my mind,.

This lecture is an excillent example of excillencein of performance,thanks.wish you good luck.

Sorry for a silly question from a man, who have secondary school level knowledge about physic: what is the working principle of the detector, which 'observes' double slit experiment?

Best explanation on the web.

This is one of the best explanations I’ve watched! I actually understood it all the way to this end. This may explain a puzzling phenomena that has happened to me repeatedly. I film long abandoned properties because I find the decay interesting, how nature retakes its space. In many of them a mirror survived and I would film them with a cinema camera carefully observing with my naked eye. I like to try to imagine what the people were like who chose that house , that mirror. I have never seen anything unusual with my naked eye but upon developing the digital negatives from the camera’s recording device, an SSD (hard drive) I sometimes find amazing images that simply were not there before. I’ve captured photo-realistic images of people, and objects from buildings to cars to an old fashioned wall phone from the 20s. It’s very bizarre and has baffled me. Maybe the act of recording the light pushed it back in time changing what it recorded. I don’t know. Thanks for this video! Well done!

My head hurts. So if the detector is moved to a position after the slits then the wave is still retrospectively affected? So does the particle still arrive at screen in the same time frame as a wave that is collapsed prior to the slits? If it does then what has happened to that proportion of time? As presumably it’s takes an infinitesimally longer time for the wave that’s detected later to amend itself compared to a wave that heads directly to the screen already collapsed into a particle. If they do both arrive at the same time, doesn’t that prove that at quantum level that speed/light/time isn’t in fact constant? I guess what I’m trying to say is that if two identical photon waves are fired at the speed of light, one collapses and proceeds directly to the detector, but one has to retrospectively change itself as it’s been “tricked” by the detector position. How can their journeys to the detector take the same time?

This is why they say be careful what you wish for. What you observe collapses the other possibilities and becomes your reality.

This is the best explanation of the DSE I have seen. I know I’m very late to the introduction to this video (three years according to RU-vid). Nonetheless, I have a question. You mentioned that even when single photons are sent through the double slits one at a time an interference pattern emerges-I assumed that that is what would happen because when single electrons are sent through the same interference pattern emerges. My question is: how is it possible to measure out a single photon and then send it through one at a time. I am amazed that this is possible. Or did you mean to say a single electron was sent through the double slits, not a single photon? I look forward to some kind of explanation.

That's great, but why doesn't anyone say what the detector is and how it interacts with the light in order to measure it. Maybe it's just me, but I feel like as one that likes to think about these things, that is the question I end up at.

Best explanation for double slit I have ever seen. Amazing job 👍

Hey I have read the paper you prvide in the description about the fact that counscious observer wasn't needed to collapse the wave function. So if the set up allow us to know the wich-way data of the photon but we haven't turned on the detector then the interference pattern is observed right ? I'm in a discussion with a tenant of idealism and he provided me with a video about IONS experiment about the influence of consciousness on the interference pattern, do you have any feedback on IONS and their credibility ?

Best explanation yet!

Is it true the the detector has to be on AND RECORDING or storing the measurement in order to collapse the wave function? And as a corollary the wave function does NOT collapse if the detector is on with a indicator (such as a meter) detecting the light WITHOUT recording (storing) the measurement?

Admirably clear - thanks!

Sorry, just a question, I didn’t study physic and I learn English by myself so I don’t understand: when you are NOT observing it, the light acts like a wave, but then… how do you know that ? I mean, if you observe it, it will collapse. So how do you know that the photons act like a wave ? Can someone help me ?

Excellent explanation to the core.

But if you unplug the detector - 8:18 - you will be no longer interacting with the wave, right? so it it not mind-blowing :) to "measure" something you NEED TO interact with it, so you're disrupting what you're trying to measure. Is it possible to build "passive" detector ?

Question: Did I understand that a "consciousness" is needed to "interpret" the results of the experiment, i.e., a wave or particle? If so, what would happen if a particle detector were turned on, the observer would leave the room, and the light (or electron) source would be activated via remote control? Since the conscious observer would have no idea of the result, having left the room, would a wave function appear on the screen? Or would the photon or electron "know" that a detector is focused on it and hit the screen as a particle? Great video! Thank you.

I find this superinteresting like all others but one aspect I really don't understand. These individual photons have a direction when they leave their device exit. And that exact direction steers them either in the direction of slit 1 or slit 2. There is no choice here, as they are small enough to get through the opening. Or do I make a thinking mistake and should I consider the individual photon that leaves the device (that makes individual photons) as a wave rather than a particle? This part I don't get, the individual photon is "fired" with a direction, so that is why in my thoughts there is no "choice" involved at all. Give me your comments and be rude enough so I understand it : ) I have no science background

The fact that a particle/waveform can change retrospectively once detected is contrary to everything we think we know. Going from a wave of probability to a particle is fine but retrospectively changing from source when detected once it has gone through the slit is truly mind bending.

Single slit already shows a mild diffraction pattern, but you hand _wave_ it like it doesn't matter? Seems fishy. This confusion, to me, seems to go away when you consider that there's no such thing as particles, there are just wave-like excitations of a field and the energy threshold and the oscillation frequency makes it so that observable events are quantized. "simply adding an observation" is not simple nor an observation, nor is it adding. You're interacting directly with the wave in order to observe it! The observer is not off to the side as everyone always indicates, you're passing it through something to detect the harmonic peak and that suppresses the interference a bit. You're complexly subtracting from the wave. It doesn't matter where you interfere with the wave, do it before/after the slit or during the beam splitter within the eraser; you're always interfering with the wave. The delayed choice quantum eraser is someone looking for the interference pattern after the fact by looking at a fully scattered spread of nonsense and the ones they find are the ones that weren't interfered with before landing, or which were interfered with in exactly the same way so that they are still in sync when they reach the end. It's always just waves interfering with themselves because someone put something with two slits in it, or it's a wave interfering with whatever was inserted in the path to measure the part of the wave that was impeded.

Hey! This is great experiment especially with the detector on case!! How is the detector defined? I mean what is the interaction between the detector and the photon? How does the photon recognize that it is being detected or monitored? Is the human eye a detector? If some one sees the light will it collapse?

Outstanding explanation!

This was amazing!!So much exciting information and so well explained

How can anyone downvote such a good explanation? They must be reality deniers.

What if we put mirror as screen 1 such that it detects the single particle as well as reflects it (light). Then we put another double slit opposite to screen 1 and another normal screen 2 behind that double slit. And then test the results. Does the wave function again keeps changing itself?

i would like to know more about how the detector works. I suspect that the electric device has something to do

I got all of the questions correct except the delayed choice one. That delayed choice experiment is amazing.

Surely if you are observing then you are absorbing some of the light which is what affects the wave pattern?

Ohhh yeah this is extreemly interesting and well explained the best i have came across ;)

What if I send one photon at the time and the CMOS sensor that detects the photon keeps a time lapse recording of the progress of the buildup of the interference pattern, and then I start monitoring one of the slits but not the other to be able to have the CMOS sensor to only store the results of the photons that were sent out and that never got detected and thereby never got measured, would that still collapse its waveform? If I do all above but are duplicating it into a double, double slit experiment using a polarizer to split and create entangled photons, then any detection and measurement to the slits in one of the setups should collapse the waveform in both of the recordings of interference patterns that we make. What would happen if I bounce one of the polarized and entangled single photon beams against a mirror on the moon to create a time delay between the detection of the patterns that are being stored by the different CMOS sensors and the one with the shortest route immediately would report if it has received an interference pattern or not, and then a second later (that should be about 1,5 s before the entangled twin of the already detected photon here on earth would have bounced back from the moon and reached its double slit) we suddenly decide to use the exclusion method from above to check for which way the entangled photons that were sent out to bounce back from the moon and that never got detected actually took. What will happen? Will the first detector have a change of heart or will the first slit collapse the beam without any measurements are being done yet because they are still in some kind of local causality because no time has passed to an entangled photons that are traveling at the speed of light. Will a measurement of the state of one of two photons that have been entangled break the waveform for both of them? Why can’t I read anything about this? We have mirrors on the moon! Are people who are setting up the experiment that I am suggesting being deleted from the simulation?! 🙂

Best explanation I have seen! Thank you :)

Great video. Thank you so much.

Love the videos Jon, I have seen many videos on this experiment years ago, but I did learn about the tests done with matter beyond the electron from you. many thanks as always. -Tom

Is it possible that the detector is putting out a small magnetic field that is having an impact on the experiment? And, when the detector is unplugged, the magnetic field alternation goes back to normal?

Wow, that was excellent, you explained that so well, can't wait for your next video, thanks Tony

I haven't yet found anyone to explain the exact set-up and what they mean. All everyone does is to explain at the surface, and that's frustrating. You get my curosity so high... but you don't give me the basic -- yet essential -- information I need to try to understand what could happen.

How can a single wayve go through two slits. ????

I just have a question and nobody answered me this: lets say we have the interference pattern now if we start to measure do we see with our own eyes the change to particles (two lines) or is this only theory... Plz answer

Doesn't measuring equipment have to receive photons in order to get information on whether the particle went through either slit? Wouldnt that affect it? I.e. when you look at an object your eyes use photons to receive information. Wouldnt these photons interact with the particles?

This video came up in my feed. I've seen quite a few videos about the slit experiment and I have to say yours was by far the easiest to follow. Well done. Subscribed and looking forward to checking out the quantam eraser video

The photon is a small magnet with a magnetic field. Some of the magnetic field interferes with the edge of the slits making it appear one object is in two places at the same time, when its just one large object going through both slits at the same time.

Nice video, this experiment is so simple and shows that the world is not what we think it is. I would say it is one of the most, if not the most, important experiment in history and spawned Quantum Mechanics, which make many many things possible in our modern lives.

Hi great video. Just wondering how the detectors gather the information?

Very well explained. Thank you for this video !!

Why jumping from 5 to 7 slits? Just curious. 3:50 Why 1, 2, 3, 4, 5 and 7 instead of 6?