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 scientist at Washington University found that quasimeasurements cause the zeno effect possibly explaining why the particles do not form a interference pattern if one detects which slit they pass through.

Seem to be leaving out the fact that the difference occurs when being actively observed.

Everything we see is our brain "interpreting" the photons of light reflected off a object. Just like our brains turns 30 FPS and up into a smooth video image. Any experiment that has the word "Observation" in it is flawed. A human used as test equipment for the observation part of a experiment can never be accurate.

It has nothing to do with a human observing anything. It has to do with how one observes things at the atomic and quantum scale. We make these observations by bouncing other particles off of the particles we're interested in examining. At the macro-scale this is not a problem as the particles were bounce off of things are much smaller and have little no affects at the macro.

But at the atomic and smaller scales, the particles we bounce off of things to observe them are similar in "size" (this is a stand in for mass, charge, etc.) to the particles we are trying to observe.

You can think of it like trying to figure out where a billiard ball is by bouncing a golf ball it. That will change the position, spin, etc. of the billiard ball.

I agree with you. Yet, seemingly, the rest of the world either believes - when they do not believe what you've just said - humans have a psychic grip on subatomic particles, or this proves a God exists.

I believe both, and agree with both of you. The two dont have to be mutually exclusive... It actually makes for a more narrow view that way.

I agree. What we don't know is much greater than what we do and much of what we think we know will change. I think it's best to leave room for many possibilities. Magic is just science undiscovered. If we keep placing boundaries on what's possible and teach others to ignore something for lack of explanations, scientific discovery suffers.

This isn't actually true either. Experiments have shown that even if the photon used to make an observation is of low enough energy that it doesn't alter a large molecules trajectory much, the interference pattern still disappears.

Also, null interaction experiments have been done in which there is no contact between particles at all. But if info about which slit can be found, the interference pattern disappears.

Hey, do you have a source on finding information about which slit with out active observation causing the interference pattern to not appear?

Try QED: The Strange Theory of Light and Matter by Richard P. Feynman. It's been a while since I've enjoyed this book, but my recollection is that it covered the topic well.

Molecules are much larger than photons, yet you get the same result.

Placing only 1 detector in front of one of the double slits ALSO collapses the wave function of both slits. This unequivocally proves that it isn’t the measurement method, but the ACT of measurement itself.

For example, we get the wave pattern. We place a detector in front of both holes we then get 2 bands., if it were the detector interfering as you mentioned, we will take 1 detector away and leave the other. This way only 1 slit has a detector that interacts with the electron as it passes through.

This would mean the slit with the detector produces a band while the slit without the detector produces a partial striped pattern of the wave.

This is not what we observe however. Measuring just 1 slit still causes 2 bands.

This is because even measuring just 1 slit gives us information on the other. It’s the information that is seemingly the cause of the collapse of the wave function. That is definitely not what happens. If so, there would be no conceptual problem. But don't take my word for it. Here's what Richard Feynman said: “I think I can safely say that nobody really understands quantum mechanics." That's because QM seems to suggest that there's a real connection between the mind of the observer and the results obtained. Also, you're not taking into account the results of the quantum eraser phenomenon, which is another aspect of this experiment that suggests the trajectory of an electron in the past can be altered by an experimenter's actions in the present. You'll have to look it up as it's rather lengthy.

Thank you so much for this explanation! We have been fretting about this for quite a while. I wish this physical interaction were more clearly included in other explanations of the slit experiment or just quantum mechanics and general

Objective observation, unlike subjective observation, is very much allowed in conducting data in experiments.

Plus, unless you have robots conducting the experiment ans/or collecting the data, humans are going to be involved.

A houseplant would work too. Humans cannot see at this level in any case. Machinery is used, and humans don't have to be in the room for the effects to continue. Observation just means measurement, and Wheeler's Delayed Choice and the Quantum Eraser experiments showed the measurement can occur after the photon, electron, or molecule has hit the wall...and it will still change.

This word is used for convenience, but no conscious observer is required. You can also say “detected”. And by detected what is meant is that information exists that is, in principal, detectable even if not yet technically feasible. Look up “The World’s Smallest Double-Slit Experiment” (2007) and you will find that a single low-energy electron can be an “observer” and collapse the quantum interference pattern of a high-energy electron exiting a single hydrogen molecule.

This is BS. When being observed by a sensor, electrons behave as particles. You lack sufficient understanding of the double slit experiment.

A summarily dismissal of a complex phenomenon that has world class physicists, accompanied by a " You don't understand" line illustrates a simple mind


"Seem to be leaving out the fact that the difference occurs when being actively observed" EXACTLY!! This experiment shows that matter is not what we think it is. Scientists have known this for a century yet scientific materialism for some reason still prevails. Matter is a product of Mind. NOT the other way around. For more information read "Ontological Mathematics"

That is not what comes out this. "Observer" is a misleading term. It does not specifically refer to humans, nor even conscious creatures, although they can be.

I do agree with you

What if the light is reacting to the material the slits were cut out from. Maybe electromagnetism causing the light particles to bend and change their direction just like how planets and comets change their orbits when passing near something with mass.

The double slits, are on the both sides of the direction of the flow of light. Also, the slit will be more massive on the two outer edges of the double slits. If it were to be hindered due to presence of slits, wouldn't the effect be more on the outer edges and not on the inner edges, i.e. in the middle.

Such is the problem. Particles don't bend when acted upon by electromagnetic fields. They simply form a trajectory. Bending or warping is the property of a wave. Their trajectory could be altered but I'm sure the material is neutral in all aspects to avoid interference.

Electrons have almost no mass and therefore almost no gravity. Atoms of the slit have a huge mass compared to the electrons. As the electrons passes the slit the gravity of the atoms cause some of the closest electrons to start to spin, the same way water spins when shot through a slit. This spin then sends some of the electrons out of their normal straight line trajectory which causes the apparent wave effect.

If that were the case it would happen if there was only one slit. But it doesn't.

Electrons always have a spin of 1/2. This is a fundemental property of electrons and all fermions.
Even if electrons were pushed off their trajectory, how do electrons shot one at a time form an interference pattern?

Because it behaves like wave regardless by itself or by groups

I also thought the materials used may have properties that interfere, and the detector might too.

How does the detector itself work? Does it not rely on an intrinsic property of electrons to function? Connecting the detector to that property then necessarily interferes with the experiment. That interference is then to be expected and can be explained rationally rather than through a spooky effect, quantum effect.

Agree with you. Details of double/single slit experiments should give more information on the slits material, their dimensions (gap width and distance between slits) and the probability of these materials influencing the path/direction of WAVICLES (waves-cum-particles!). This topic needs to discussed and debated.

Have you found that discussion anywhere?

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

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

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

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.

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?

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.

Thanks. Makes perfect sense.

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!

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

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.

You can know a precise definition of a word, or you can know the context of a word but you can not know both the definition and the context at the same time.

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.

Could you please share a link or two of your youtube videos and books?