Physics in a minute: The double slit experiment

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


Assume that each particle is actually an object of unlimited size, with a gradient per property. Also assume that a slit allows part of the object (again per property) to pass faster than a part thats get hindered by the wall. The object reforms (back to an efficient form) after passing the slit, before/while it hits the observant.

The whole universe being changed by the act of observation and forced to split into multiple world lines is ridiculous. It seems evident to me that the universe already exists across all those world lines, as a wave, and the only thing that is being divided into multiple world lines is the observer's state of awareness (what we sometimes refer to as its state of consciousness or, simply, consciousness).

But only the state of awareness is split. The observer still encompasses all states of awareness in those world-lines where it still extends (exists). Further, if any two of those states of awareness should ever become identical, then they are once again the SAME state of awareness. This is why the wave behavior reasserts itself as soon as the observer is not focused precisely on where the "particle" is (it's not really a particle at all, if anything the observer "particalized" their state of awareness).

You may get some understanding with a very rough metaphor by looking out at a vast forest of identical trees, then focusing on a specific tree, then looking back at the forest. Chances are when you again focus on a specific, random selected tree, it won't be the same one you had previously focused on. Ooh, quantum uncertainty!

The basis gist of all this is the observer is not changing the universe by the act of observation, the observer is changing the observer's state of awareness of the universe, and in a way relative only to the observer. This seems to explain all of it much more reasonably than anything I have ever read, and my attempts at the math (alas, I studied the wrong subjects in college) appear to support me thus far.

The reason there are multiple interpretations of the double slit experiment is because there are pros and cons in all of them. You seem to be describing a version of the Many Worlds Interpretation similar to the arguments of Sean Carroll.

The idea that the state of awareness of the observer collapses, but not the waveform, fails because the which-path information can be recorded but not yet observed by anyone. In this case nobody's state of awareness has changed in the least, yet the wave has collapsed or appears to have collapsed.

If the waveform is really multiple full universes, that raises more questions than it answers. What creates universes? Why don't these other full universes impact us in any way as they affect unobserved particles? Just how many of these alleged universes are there?

So, at a theme park there are 15 lasers each running through a diffraction grating, displaying a clear interference pattern for the entertainment of the audience. Are you claiming that each photon within one laser beam is in the same universe (not ours?) as one photon on another and so forth?

All MWI versions fail to explain how mere information existing can lock us in to one of those alleged universes. To be fair, though, all explanations are counterintuitive or seemingly impossible.

This is probably why some people remember Saturday morning cartoons as being 'Looney Toons" whilst the accepted way according to Wikipedia and that the other half remembers it as is "Looney Tunes"....

I haven't a clue about any of this but what I caught was the electrons ,which are particles, act like waves when not observed. I seen your responce about human observation and to my tiny brain it seems you are saying it's the mind that's altering the image but in the expierment cameras were used so doesn't that take the human observation element out of it? Also when the humans were watching did the waves..only when there was a device with the ability to observe did it change to particles ...or That's how my little uneducated brain understood it. Please elaborate further. I really want to try to understand this as best as an average Jane can.

What happens if the detectors are there but aren't turned on?

What would happen if you electrostatically charged the slit material to a high negative potential and increased the electron velocity? In my mind, you could decrease the effective slit diameter to an almost one dimensional gap removing the chance we are looking at secondary emissions from the gap material (and maybe a kind of newtons cradle effect).

Why if the particles used are sand rather than atoms do you end up with a uniform and identical result regardless of whether individually observed or not? Seems the accuracy and impact of the selected mediums and observing equipment influence the result. I don’t know anything about the equipment used or physics of atoms but for this to be valid it would need to be replicable for infinite different materials and equipment with different states of vacuum, no vacuum, gravity, electromagnetivity etc. and the observor(observing equipment) to be physically separated as well as all forces ( friction, gravity, electomagneticivity etc.) to be measured/monitored.

Does polarization of the light have any affect on the slit experiment.

In May this year a philosopher proposed that 3 dimensional time at the quantum level is a law of nature and responsible for the double slit experiment phenomenon. In October he found the formula that explains it and on December 4th found the evidence that matches his prediction in a 2013 recording of the experiment by Nebraska Lincoln University's physics department. The video is on Alf Eaton's youtube channel and the theoretical paper is available online at doubleslitsolution dot weebly dot com. Free for anyone to view and review and help send viral.

how is the change in the slit pattern observed as we increase the slit size.

how is the single slit pattern change as we increase the slit size?

Double slit experiment showed properties of electrons capable of outsmart us by not showing how they achieve a wave like pattern. They act as if they know the future. Have they tried to observe by adding a time delayed observation? Such as using thick mirrors that can capture electrons movement and deliver its data 4/5 seconds after the experiment been conducted?

They have tried this. I believe it is the delayed choice experiment.

They have tried to cheat. Check out:

And watch the video. It's really only about the first half of the video, but it's extraordinary. They did some nifty attempts to cheat only to discover that they couldn't get around it, and the results just got weirder -- as if effect preceded cause. They first split photons into entangled pairs to be able to measure which slit each came through, but it didn't work. Then they delayed the results of the split photos on the right-side slit so that the left-side slit photons would arrive first. But even when they did that, it was as if the photons on the left knew what was up and adjusted how they behaved. Very bizarre!

When you say, "the interference pattern disappears" do you mean that the pattern made before they were observed is no longer there?

Who was the scientist doing this the experiment (douple slit )

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.

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.

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

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

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.

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.

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

What is the material they have used in the second wall? Please answer me ASAP

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.

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.

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.