Physics in a minute: The double slit experiment

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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



The Buddha mentioned the observer effect 2,600 years ago.

"Mind is the forerunner of all things....."

Permalink In reply to by Luke McKean (not verified)

Could you please submit a detailed testable theory, with an internally consistent mathematical formulation, of how the Buddha arrived at this conclusion.

Permalink In reply to by Skeptic (not verified)

Didnt the above experiment just provide a testable theory, with internal consistent mathematical formulation? Budha may not have had the math, but perhaps the insight. Or, he got it right for the wrong reason.

Hypothesis: Human consciousness in the form of measurement alters the behavior of photons. Hmm, seems like that has been already published.

Permalink In reply to by DML (not verified)

So are you then hypothesizing that the quantum realm is interactive with consciousness? What if two separate, distinct observers isolated from one another choose each option. Will they observe different results?

Permalink In reply to by Clint Woodward (not verified)

But if a particle knows it is being observed and that means the particle is observing its surroundings to see if there's an observer present making the particle itself is an observer, and If particles themselves are observers, then there would always be an observer present (particles) regardless of our recording device or not. And if there is always an observer it should always act as is being observed due to particles existing all around.

What you said makes sense. This seems to suggest that we effect the outcome by our presence. Do we get what we are looking for? Everything is controlled by energy. Constant moving particles. Dimensions beyond what we can truly see. Often what we believe to be true and what "is" are two different things. The slit theory just shows how little we really know, likely goes without saying. Keep an open mind. Pursue the rabbit hole. Take the red pill.

Permalink In reply to by Skeptic (not verified)

Im no larry bird but it's intresting to read the comments here each one has a reply stating how the previous comment is incorrect. Now im starting to wonder is it physics thats hard to understand or is it more like physics is hard to explain maybe we need to assign words more meanings then we can just make everything make sense a great example is the dollar - we all know the dollar isnt worth shit or hasnt when our genious money people decided to stop backing it with gold so in essense they said the dollar is worth this much because we said and no matter how many equations and big words you want to use to try and prove me wrong it dont matter because when it comes down to it paper is paper. I had a pretty good generic understanding of the double slit thingamabob until i decided to read the comments here now i cant hit a three pointer unless nobody is watching.... I never miss when nobody is watching....... Here is something you all can discuss - why is it that the english language has one word that means two different things? For example the word watch why not take the definition that came second and give it its own word? Seriously

I love your conclusion here. That is thinking out of the box. People like you are the ones that come up with new and brilliant ideas. I understand you well. Keep it up, and thank you.

While I too enjoyed the amiable discussions above and yet they did have an undercurrent of physical harm should the point be pursued any further... Your delivery above while lacking the weight of more technical reference points, was completed, in my eyes perfectly. Your points, while detailed with some much needed jocularity raised some interesting questions.

The only frustration for me, is that the answer to your as well as the general clarity of this, to be frank, completely bonkers conundrum seem to get further away, the closer we attempt to get to the truth.
What i can say with uncredited certainty is that the world/universe /dimension we live in and the laws that we futilely attempt to pin down to reassure our collective selves that we have at least some basic understanding of how it all works is likely to be only 50% of what we need to understand. And that my learned friends, is how in all of the multitudes of preexisting theories on "how it all works" is WHY is it like this. Like megalithic construction (apologies for the severe deviation off topic for the anology) it seems to me that the underlying rules that govern this quantum level were all devised on the most difficult of methods, when easier methods must surly have been available. Or one can only postulate I concede.
I fear too that a great many of us shall pass on from this mortal coil with very little more clarity than our generation has now. And that kinda sucks.

Love this. Sometimes common sense is too easy for people. It's a mind blowing experiment. Humans are never going to understand the universe and how it works. What is a fact is that this universe is an amazing beautiful place that was not created by us. It IS being ruined by us. It is fascinating to see experiments.. and watch people try to understand how it works, down to the smallest details. I don't even know why they teach things anymore because there's always another view, opinion or idea. We spend our life being told what to believe, and many take it too far and refuse to believe anything else...even when it's proven wrong. So yeah all the comments and arguments just prove that we are never going to know or understand. However, that is job security for many people so what's the rush? I love watching these experiments!

Permalink In reply to by LBD (not verified)

Would it somehow spoil the fun if we were authors of this universe?

Permalink In reply to by Herbert Wraczlavski (not verified)

I am 11 and understand this perfectly well, Good job!

Permalink In reply to by LBD (not verified)

Personally I don't think these things can be understood from a materialistic view point. But it all starts to fit together once mind and potentiality become the key players. Approach it from the fact that nothing can exist that was not created from an idea first. Then look at what the potentiality of the double slot is showing, bring the Higgs bosun into the equation and you have creation, which is all the Universe is.

Permalink In reply to by Luke McKean (not verified)

It is not Buddha. It is Vedanta -end part of the Vedas - that define - consciousness -as Consciousness is infinite - prajnaanam brahma. the mind reflects the all-pervading consciousness and by reflecting it acts as though a local conscious entity -just like moon by reflecting sunlight appears as though a luminous entity in the sky. Any observation involves this activated mind. Hence the observer is the local mind + the reflected consciousness that makes the mind as though a conscious entity. Any objective knowledge involves mind interacting with the information provides by sense input -here measurement itself. Interference happens due to Hysenburg uncertainty at these small scales. Hene observation can affect the observed at such a small scale. For more info listen to my talks on youtube or I have written few books on it available on Amazon. I am a material scientist.


So how can you pass an electron through the slit? We all know that electrons constantly revolve around the nucleus. They can get excited and change their orbits but still they rotate around the nucleus, right? We all know that they don't walk in a straight line. So how can you collect these electrons and send them through the slits in such a straight line?

Permalink In reply to by Sagar Parajuli (not verified)

Of course you can separate an electron from an atomic nucleus. That’s how cathode ray tubes work on the old TVs. A series of magnets propel the electron on to a phosphorus screen. Even static electricity is the release of free electrons that dart about the air without a nucleus until they find a positive charged partial to cling to.

Permalink In reply to by Don (not verified)

Actually, the positive Anode attracts the negative electrons that are boiled off of the heated Cathode. The magnetic coils above and below the stream of electrons are used to deflect the electrons horizontally at a rate of about 16KHz, while the magnetic coils to the right and left of the stream of electrons are used to deflect the electrons vertically at a rate of about 30Hz.

Permalink In reply to by Sagar Parajuli (not verified)

Electrons can be "ripped" away from their atom with the application of energy in one of many forms. One method that was common in every home for years, was in vacuum tubes. Also called electronic valves in some countries. Also, in CRT's, like television picture tubes. In this application, heat was applied to a little piece of tungsten, containing a bit of thorium. Electrons jump off of their atoms, many into the vacuum in the tube, where they are used to amplify signals, or used to bombard the phosphorus screen on a television tube, forming the TV image.

Permalink In reply to by Sagar Parajuli (not verified)

By seeing the demonstration in your mind.


Electrons are not tennis balls, but what if their spin causes them to travel in a helical pattern like a spun ball; like a bottom-right spun tennis ball or ping-pong ball, a curve-ball or a sliced golf ball? Would this not give their trajectory path a particular wavelength and amplitude? Would that not produce the pattern?


I think all y'all skeptics are forgetting that this test, while performed with particles, demonstrates the properties of a WAVE. The spin of the electron, amplitude or lambda of any ridiculousness some of you are suggesting, or anything else is affecting the pattern created when the experiment was left unobserved. What we see is the product of wave-interference - which is a physically observable pattern that you can create in a home environment by bouncing two balls next to each other on the surface of water. And yes, the very act of observation IS what changes the results; Erwin Schrodinger was able to demonstrate this with his thought experiment. ALL quantum particles behave the way that the electrons did, which is the basis of qunatum uncertainty and wave-particle duality.

A helical path viewed from the side IS a waveform. Such a trajectory is a three-dimensional wave. (Unlike the two-dimensional ripples in a pond). It could therefore be expected to produce the interference pattern. No duality required. Unlike EMR, electrons are fermions, of known mass.

What it does not explain is why the effect disappears when measured or, similarly, the distribution pattern through a single slit presented here.

But then again, neither does particle-wave duality, which really tells us nothing of substance. (Joke). It merely describes observed phenomena; it doesn't explain the cause or the principles at work or describe anything about what is actually happening. It merely describes what we have reason to believe isn't happening.

Permalink In reply to by Steve Moore (not verified)

I've thought exactly the same as you that a helical spin would explain this phenomena. Now if the spins were randomly gaussian distributed you would see constructive and destructive interference but the net result would be zero (which is what the waveform summation is). Having bother positive helical and negative helical spins would explain the double slit experiment phenomena. Observation may remove the helicality (X/Y components but not Z) but the spin is maintained. Since the quantum particle is ballistic it would still pass through the slots or bounce back and show 2 amplitude peaks without the interference pattern. ie particle mode vs wave mode.

The quantum uncertainty just introduces conjecture into science and is the closest thing to metaphysics that one can postulate while looking scientific. It just affirms to me that we are incapable of fathoming our universe at a minute level even as much as on a scale of galaxies. Yet somehow we applaud when someone says we emanated from a random inexplicable event and evolved from a single celled micro-organism into the the complex life forms we are. A cycle of silliness, if we are loathe to believe in a divine creator not subject to these sacred laws of physics that the electrons seem to violate willy-nilly in the double slit expperiment.


If you haven't read about this you should.
In an interview with Nautilus's Steve Paulson, Penrose uses an example from quantum computing to explain that qubits of information remain in multiple states until coming together into an instantaneous calculation, called “quantum coherence," making a large number of things act together in one quantum state.

Here's where Penrose's theory draws upon the work of Hameroff by saying that this quantum coherence takes place in protein structures called “microtubules". These microtubules reside inside the neurons in our brains and can store and process information and memory. Penrose and Hameroff think that microtubules are quantum devices that are orchestrating our conscious awareness.

This theory is not appreciated by everyone in the scientific community, with many critics saying the brain is too “warm, wet, and noisy" and cannot sustain a quantum process. Another physicist, Max Tegmark, even calculated that the brain cannot possibly think as fast as this idea requires. Hawking is also not on board, suggesting Penrose should stick with his field of expertise.

Yet, a 2013 study by Japanese scientists added some proof to the theory by Penrose and Hameroff as researchers detected vibrations in the microtubules. Penrose and Hameroff then proposed that by focusing brain stimulation on these vibrations one could conceivably “benefit a host of mental, neurological, and cognitive conditions."


This effect is happening because particles are segmented aspects of waves. If you shoot one particle or a few particles and you only see a portion of that particular wave. A wave in nature is not on an 2d x y axis but a 3d cyclone spiral. So you should think of particles on invisible 3d wave tracks or "guard rails". This is also why you look at a "wave" it turns into a particle that retroactivity forms back to the source. You see the particle tracks of that particular wave.


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.

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 ..it 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.

Permalink In reply to by Dumbdumb (not verified)

I may be wrong but forget the light or ping pong balls just consider potentiality.
Creation is the change from potentiality to reality. With energy healing they don't use observation they use intention and expectation. This just like the way the placebo effect can change a thought (potentiality) into a reality. Potentiality is the wave or trellis condition, reality is the particle condition. Mind is the activating factor

“They” behave like waves still. It’s like they know if it’s on or not. From what I understand is if information of where they go can be looked up, ever, they will act like particles. This is killing me and all I can think of, there has to be an explanation, or maybe not.. this whole thing is like a merry go round and I end up where I started


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 )