The Most Beautiful Experiment

Quantum physics informs us that a system exists in superposition — that is, in all possible states — until we observe that it is only in one specific state.

According to a 2002 poll of Physics World readers, the “most beautiful experiment” in physics is one that simply and elegantly demonstrates how observation affects quantum systems: The double slit experiment. The double slit sets aside causality, determinism, and the notion that reality is “out there” as it blurs the line between the observer and the system being observed.

In the double slit experiment, a series of single photons (light particles) are fired at a solid plate that has two slits. On the other side of the solid plate, a photographic plate is set up to record what comes through those slits.


The question: What will we see on the photographic plate?

The answer: If one neglects to observe which slit a photon passes through, it appears to interfere with itself, suggesting that it behaves as a wave by traveling through both slits at once. But, if one chooses to observe the slits, the interference pattern disappears, and each photon travels through only one of the slits.

The formation of the interference pattern requires the existence of two slits… But how can a single photon pass through two slits simultaneously? At that point, we are forced to consider each photon as a wave that travels through both slits… Or we have to think of the photon as splitting and going through each slit separately — and wondering how the photon “knows” a pair of slits is coming.

The only solution is to abandon the idea of a photon — or any other quantum system — as having a location in spacetime until it is observed.

56 Responses to “The Most Beautiful Experiment”

  1. I am trying to figure out the truth about the current understanding of quantum physics. From my understanding nothing can be measured without at some level interfering with what is being measured. Do you know the details of how they measured which slit the photon goes through and how it may interfere with the behavior of the photon? Thanks.

    • Re: Details: There have been numerous double-slit experiments carried out since the early nineteenth century — using light, neutrons, atoms, electrons, and even molecules as large as carbon-60 and carbon-70. Some were measured by human observers, and some recorded by detectors which were then observed by human observers — who, I might add, also conceived and built the detectors specifically for the purpose of later observation. So, while a real-time human observer is not needed for the experiment itself, it nevertheless holds that a detector’s results are observed, at some point, by a human. And so the effect occurs.

      In QT circles, this phenomenon is known as the “measurement problem.” It’s also an ontological dilemma for many people. To my thinking, it seems a simple enough fact that everything we know about anything in this universe is a direct result of observation, for how could we “know” anything otherwise? From that perspective, it also makes sense that an observer (like, for instance, a human) will have an effect on any observable with which it interacts (like, for instance, a photon).

      To quote one of my favorite physicists, Erwin Schrodinger, “Subject and object are only one. The barrier between them cannot be said to have broken down as a result of recent experience in the physical sciences, for this barrier does not exist.”

  2. If you’d like to conduct the original experiment yourself, check out

  3. what about the reductionist approach? suppose I eliminate one intelligent being after another…when I finally eliminate myself, will the universe ‘as we know it’ still exist? will the bullet which blew my brain, continue to follow a newtonian trajectory? I think yes.

  4. You’re using reductionism to argue in favor of pluralism — neither of which can be reconciled with quantum theory. Although the conclusions you have drawn from your model are (superficially) intuitive, it is through deeper analysis that principles such as entanglement and complementarity have been uncovered — and, thus, the case for emergence/synergy successfully established.

    Even macro-cosmically speaking (in this case, bullets and brain spatter), reductionism does not work within the ontological framework of QT, as uncertainties at the micro level are increased.

    To quote Ernest Gellner, “Reductionism is rooted not in the nature of things, but in our ideal of explanation.”

  5. I had trouble understanding this experiment at first, but on YouTube there is a cartoon like video from Fred A Wolf (Dr Quantum) than explained it perfectly. It was the understanding of this experiment and how consciousness plays such an active role in our realities that has me so turned on to QP. Love the site!

  6. Thanks for your comment. If you are interested, I have a YouTube channel dedicated to exploring the Observer Effect, featuring over 230 videos at the present time (and I am always adding more!).

    You can visit my channel at

    • Yes, thank you- I’m really getting wrapped up in QP and now I know how I’ll be spending a good portion of tomorrow.

      • Have fun, Ross! Prepare to be astounded, wow’d, and to stumble upon as many or more questions than answers! ;)

    • Thanks so much for providing this link. I am certain I have minimized my life experience by trying to live within the “laws of nature”. This quantum stuff is a blast.

  7. Has anyone that you know of conducted the experiment to alter the observed effect with intent, rather than observing, actually manipulating the outcome by thought?

    I was a chiropractor for horses, and on the journey of life figured out how to adjust, or heal by thought.

    I realized that some of the misalignments of certain joints, particularly the Jaw, were being caused by my own observations, which eventually turned into a very common malady.

    When I realized that I had created this outcome, I then changed my thought about this particular mis-alignment, afterwards, it has been a rare ocourence.

    I guess now I would be considered a non observer to this particular joint.

    We are given such a gift, so simple, that it seems phenomenally difficult.

    My little story here is what prompts my question about the experiment.

    Thank you, Love your work here and look forward to watching your youtube videos

  8. What could be the possible implication of this in ‘real life’ i.e in all practicality?

    • It throws materialism and determinism out of the “reality” question, because it demonstrates that “reality” exists solely as waves of information (possibilities) until we observe it in some way.

      In terms of “practicality,” this means that we create reality by observing it.

  9. It is hard for me to understand the implication of observation affecting reality. How is it that observation is defined? Is it to be able to perceive the outcome through our senses? What about when the subject in question (the photon) cannot be detected by our fundamental senses? Well, we can put a camera by the slit and record and data to extend our perception. But what if we put the camera there, but delete the data without looking at it? Have we still observed it?
    If we were to do the same experiment with small marbles, why wouldn’t we get the interference pattern? You could say that we are observing the marbles as we see each of them go through a slit. Well, what if we all turn our backs to it, and cover our ears, so there is no way to know what path the marble took?
    How do you define observation really?

    • The question you ask – “How do you define observation?” – is argued to this day among physicists and philosophers. It’s part of what makes this experiment so compelling. ;)

  10. I have been watching and thinking about many of the theories, The String Theory, M Theory, The Measurement Problem, etc. I like the idea about multi Universes and the Big Bang could strongly have come from two different universes membrains crossed paths or collided to create a new membrain/universe. This to me still makes me think that we’re part of something else even bigger.Outside the multiverses there is what I call a megaverse etc. This is what is very close to, what seems to me, that cells do. Is it speculated that when these two membrains collide, is it borrowing energy from each? kind like when a sperm meets an egg? or is it creating it’s own?

    • A few physicists believe that our universe was created by a “collision” type of event between two previously existent universes’ membranes, which disintegrated in the collision. They get a lot of press, because M-theory is sexy and new.

      M-theory itself, however, is currently untestable and in physics, a theory is defined by its testability. And so many physicists consider M-theory to be more of a philosophical perspective than a workable physical theory.

  11. What would happen if you cut the photographic plate in two, and moved one half closer to the slit directly in front of it? Wouldn’t that also constitute an observation (to see if the photon went through the corresponding slit) and ‘collapse the wavefunction’?

  12. I am following you so far but I would like to take it a little further.

    Question: Are they watching the screen as they perform this test, if so would that not be considered an observation and thus collapsing the wave function at that point in time rather that just at the detectors?

    Question: The Delayed Choice experiment reveals that it can be observed and still provide an interference pattern, it is only when the data is know to the tester that the wave function collapses. So it would seem that it is not mere observation that alters the state but rather it is from our knowledge of it that reduces it from a wave to a particle. This does not deflate the experiment because it is by our observation that we come to know which slit it go through but it certainly complicates the issue and demands that we take it further as to just how we create our reality, is it just by our observation or is it through some deeper conscious act such as knowing? And if so, what does that mean?

    Thanks in advance


  13. Howard,

    The double slit experiment is over 200 years old when Thomas Young first performed it in 1801 since then and in more recent times the experiment has been revised to your liking look up double slit delayed choice, it answers just exactly what you were talking about.

  14. Interesting. It appears that small subatomic particles move in synchronicity with one another due to their wave vibrations matching as a function of their nature. Our own subatomic particles must vibrate at a rate close to the emission source in order to be detected by us allowing for thought waves to form and resonate at the same rate. Such an event would cause a synchronicity between ourselves and the particles. This is the first evidence for the reality of thought waves in physical space. It appears that particles this small are subject to connection with us. Larger objects tend to follow the law of relativity. The missing link is how gravity and larger object tie back into the smaller world of quantum mechanics. One possible explanation is that space can only hold so much “matter” in any one point in time. If the density of matter exceeds the carrying capacity at any one point in time, no matter how tiny, we get a temporal and thus dimensional shift, of equally tiny magnitude but of enormous scope. It is possible that concentrated thought waves could create a super miniature black hole and shift time and space for those around it into a new dimension very much like the one they left. It is also evident that vibrational levels determine the nature of the shift. Apparently like vibrations are attracted to like vibrations. We can create miniature worm holes that move us through time and space. If enough people have concentrated thoughts about the same thing over time, the worm hole can become quite large, perhaps becoming large enough to actually see at some point. All conjecture on my part, but interesting.

  15. I’ve been reading about this for only a very short time, but it is probably the most interesting and beautiful thing I’ve ever come across. I been experiencing an appreciation for physics that I havn’t felt since before my teachers tried their very hardest to completely ruin it in school. I’ve been looking to do something more fulfilling, perhaps a physics degree wouldn’t be such a bad thing. This website is great, thankyou.

  16. […] all of life is connected by a common "frequency" that allows for a form of telepathy? The Most Beautiful Experiment the observer effect […]

  17. This relationship can only be described in a parable of the master and the slave.Every time the master looks away the slave goofs offs and does as he pleases,but when the master is looking the slave quickly realizes and becomes diligent again.This is great writing.nice pages:)

  18. […] on to the double slit experiment (worthy of a repeat here, and here) observing something materially affects it’s effect on the world.  I repeat, […]

  19. I think that QM doesn’t say that a system exists in all possible states. I think that it says it can exist in any possible state. that is, according to the original 1930s statements that seem to be terribly misinterpreted.

    then theres these baseless assumptions:
    #photons are a single particle – despite the fact that they behave as if they were a tiny electromagnetic dipole.
    #the photon must pass through both slits for interference to be generated – despite physicists having no idea how it happens.

    • This is an ontological question that has been debated since the 1930s, the answer to which entirely depends upon which QT interpretation you favor. The Copenhagen interpretation arose as a collection of varying and contradictory opinions among the top physicists at that time. Nevertheless, one opinion prevailed. Copenhagen holds that that any unobserved system exists in all possible states simultaneously – and is thereby existentially indeterminate.

      Should “all possible states” apply only to those possible states that are distinguishable by observables? Bohr didn’t think so. I don’t think so.

  20. Could one extrapolate from this that matter only exists when we are looking at it? Do we actually live in The Matrix? I actually get a little queasy thinking about this experiment and another one where a series of particles’ rotation rates are measured (after it has been proven that they slow down when observed but are the same normally) before a few are chosen for random observation in the second part of the experiment, the particles that WERE TO BE OBSERVED in the LATER STAGE of the experiment had ALREADY SLOWED DOWN before the first measurement! WHY?

  21. Good day, Fascinating site. If that which creates reality, being the atoms change from being waves the being particles depending on whether or not we are observing them. The question I wonder is if the observer is different is the atom different? I do work with the use of psychedelic drugs and reality. On a powerful psychedelic a person often “leaves” where they are and find themselves in a totally different place/environment, interacting with different beings. Reality changes. One could say that the psychedelic action on the brain changes what the atoms become thus creating an entirely new reality. So just like the electrons, photons are a many places at the same time, we could say so are we. Our bodies are here in this world when we sleep, but in a dream we are still with our bodies and when we are on psychedelics we are still with out bodies.

    • If I understand what you’re asking, the question presumes that the self or the body (that which serves as the point of observation) is somehow more real (objective) or static (fixed) than the phenomenon observed, and is separate from the phenomenon observed.

      Ultimately, all reality is ever-changing (the atoms in your body right now, for instance, were not there two years ago); reality (and what we perceive as its aspects) are ultimately part of a self-organizing interrelational information system; we are not apart from it, but a part of it. If we can get to the place where we understand that all of it – including the “self” as observer – is first and foremost information in constant flux within an interrelational context, the questions begin to answer themselves. ;)

  22. Matter exist in two condition.
    1)Manifest condition
    2)Un-manifest condition

    Manifest———Universe is created.
    Un-manifest—–Universe is annihilated.

    Matter tendency is to increase disorder without a conscious being.
    Conscious being activates matter by injecting consciousness into it.

    Right after the big-bang occurred,universe expanded from a point to infinity.(In no time )
    Physics says that nothing is faster than speed of light i.e 300 km/sec.
    So,it is consciousness which creates universe.

  23. […] thực nghiệm “thông điệp từ nước”, hay các thí nghiệm về lưỡng tính sóng hạt cùng sự ảnh hưởng của những người tham gia lên photon, cũng như hàng loạt những thực chứng […]

  24. i am impressed with the theory of the double slit..the observer theory is potent and carries weight ,on a philosphical note…”we are but the weight of our own thoughts’…weight here emphasises our “existence”… we exist as an affect of higher concious thought…since our existence is the affect…what about the cause…?

    • In everyday life and in classical physics, events appear to be ordered in time, resulting in our model of cause-and-effect; the laws of quantum mechanics, on the other hand, are non-deterministic and time symmetric – that is, causal order does not appear to be a mandatory property of nature at the most fundamental level of reality, and so cause and effect do not necessarily apply.

  25. Amazing!

  26. In the first condition an observer is present “behind” the experiment or after it is over (on the film) if preferred. Yet the result is a wave. In the second condition the observer (he /she/ it) is between the the gun and the record.

    Does it not follow that it is a positional (mechanical) effect? The observer is present in both conditions.

    Apparently the observer / measurement need not be conscious and could be completely removed from the situation by never looking at the results or observed / measurement. If the same results are obtained with no observer then here too it must be mechanical not a result of measuring or observing.

    These things seem so obvious to me that someone must have addressed them. Thanks for correcting my lack of knowledge.

  27. I need to correct my statement. In the second condition the mechanical observer could be present and recording but no one would review the results of this recording. The only thing to be seen would be the results as in condition one. In this way the “observer” is removed from the study. If the traditional results are obtained it is not due to an observer. It may be the act of “measuring” but not the act of “observing.”

    Something must be wrong with my thinking because I can find no reference to this type of study

    • And who is observing that it is a positional (mechanical) effect? It’s impossible to exclude consciousness even here, if only because any hypothesis or proposed explanation for an observable phenomenon is, again, based on our conscious observation of that phenomenon. At the end of the day, we are observing whatever-it-is in order to form the hypothesis, even if that hypothesis purports to remove us as observers from the equation.

      This, of course, doesn’t necessarily disprove objective existence, but it does point to the fact that we can know nothing objectively, but only subjectively.

  28. Apparently the study I suggested has been conducted as presented by Carl Shaw on TED conversations July 30, 2012 part 2. Here is the URL

    “The results clearly show that WHEN the ‘which-slit’ information is available to the experimental conductor – the (fig 5) photons exhibit particle like behaviour. When the information is collected BUT SUBSEQUENTLY destroyed (randomly by the clever use of 50% mirrors) (figs 3 & 4) then the photons exhibit wave-like interference patterns.

    Therefore I think that the experiment clearly shows that it is ONLY WHEN information is made available to a CONSCIOUS OBSERVER that the light becomes a particle.”

    A conscious observer is required…… Hmmm

  29. Let’s see, will this device work? “The someonelookedomatic”.

    The double slit device is setup so that whenever a particle is detected going through one of the slits a light bulb inside a large vault flashes. Another sensor detects weather or not the resulting particle is “out of bounds” on the target. When an out of bounds particle is not detected in 2 seconds then an alarm goes off. For arguments sake the particles are fired at a rate off about 3 per second. The safe door is then closed and the device is activated.

    – Is it correct to expect that the alarm will not often sound when the safe door is closed because it is likely that at least 1 particle in 6 will be out of bounds every two seconds since there is no permanent record or observation of the led light ocuring.

    – Is it correct to assume that once a robber opens the door and looks inside accidentally noticing the flashing light that the alarm will sound?

    – Does the robber have to know that the led light is connected in some way to the experiment or alarm for the function to collapse?

    – If the flashing led light is green versus red. Will the robbers assumption of what the light signifies cause the alarm to sound or not sound even though he does not know that a double slit experiment is involved?

    Can a regular person off the street easily test the observation phenomenon? I really want to play with such a device.

  30. is it possible that the observer’s mass is affecting the electron (via a gravity pull that all objects possess)? or perhaps if it’s a metal camera, could it be the material interfering?

    • Gravity does not apparently play a role in the double-slit experiment. It is not a fundamental force. (Then again, we don’t yet have a working theory of gravity in QT.)

      Regarding the possible interference of a metal object, keep in mind that the experiment has been performed with not only electrons, but photons (the very first double-slit experiment used photons, in fact), and, more recently, large aggregate molecules (including one with a mass of 1,298 AMU).

  31. So, it is possible the cows do stand up after the car goes by. Just a joke. But there are infinite possibilities of outcomes and realities being influenced by one person.

  32. Ok, so when the observer is not present we get an interference pattern, but when we have an observer the results are as expected.

    Does this mean that if the observer expects an interference pattern, perhaps because of different education, that it would have those results instead?

    In other words, is my understanding correct that the reason the particles behave this way is because that is how logical scientists think the particles should act?

    • Not sure where you get the idea that an observer’s “expectations” are inherent to the results of the double slit experiment; perhaps this is a semantics issue. As Bertrand Russell wrote, science can only explain one thing in terms of another thing. That is because we live in a relational universe, where the boundary between an objective, “out there” reality and our own experience of that reality (that which defined classical physics) all but disappears where quantum mechanics is concerned.

      In this case, the experiment’s results point to the inextricable relationship between the observer and the observed.

  33. Hi there, i am currently interested in the observer effect and i have some questions. First of all i would like to mention that my field has nothing to do with physics so excuse me if i make any ”dummie ” question. Yet i am in the beginning of my small research for quantum physics and it would be great if you could give me some answers. So as i was watching the observer experiment i was confused about the probabilities. If probabilities are all over the place why photons go to a specific direction (lines)? I understand that photons behave like waves but why don’t they go all over the place on the photographic plate?

    • First of all, there are no “dummie” questions here.

      And what we are seeing, at that point, is what were waves of light behaving instead like particles of light. It bears noting that matter also has this dual (wave/particle) nature, as interference of atoms has also been observed. The deeper “why” – interpretation of the phenomenon – is a subject still debated among physicists.

      • Is this has anything to do with Niels Bohr light theory? Also thank you very much for your answer.

      • If you are referring to the principle of complementarity that he conceived, yes.

  34. hi, can someone kindly answer my questions: in this experiment the photon acts like a wave without any observation, and a particle whilst observed, does this mean the primary behaviour of a photon as it leaves its source is a wave? and will this be true to all quantum matter?

    • What it means, according to the Copenhagen interpretation of QT, is that it exists as a wave function unless observed, as you stated. Period. Theoretically, were we to set up an apparatus that could detect the photon at its source, the wave function would similarly collapse. The collapse of the wave function by observation is the transition from possibility to actuality.

      And yes, this seems to be true of all matter.

  35. Yes, the implication then, is that an “0bserver” is responsible for all matter created from the Big Bang, the Bang itself, and the subsequent matter we are NOT directly observing in the universe today. Basically a universal consciousness; Rubbing up against theology in that case. Another school of thought holds that matter is naturally always in its wave state and only collapses into form/matter when a conscious entity looks at it. Meaning the universe disappears when we aren’t paying attention to it. This presupposes that matter in its wave form functions the same as in material form, I.e., a planet or star continues doing it’s thing regardless if it is being observed or not. Messy concept. KEY point to keep in mind: “wave” in this case doesn’t mean strictly like waves in water, but in frequency of vibration and possibility of position at any given moment.

  36. Time and space are intertwined. A probability wave occurs when matter goes unrecorded or unmonitored. Could the future be a probability wave? In the double slit experiment, when unmonitored, the particles hit the sheet at every possible location they potentially could. Their waves of potential showed both their ideal paths and the paths if they ran into trouble. If they bounced off of something. If something went wrong. But when monitored, the paths they took were always the paths of last resistance. The paths they SHOULD have taken if there was nothing to interfere with their trajectory. If our future is a probability wave, then our PRESENT is the effects of a monitored moment in time. We are all our own recording devices of each moment in time. So each of our realities that we experience are paths of least resistance. In my reality I’ve never been seriously injured. I’ve never even broken a bone. Tomorrow I could hear that you’ve been diagnosed with terminal cancer. That you’re going to die. That’s OK because my reality, my path of least resistance allows me to keep on going and experiencing or creating the next moment. However in your reality you never get that diagnosis. Because, where that potential existed in your future probability wave, your own recording device on your moment in the present is always taking the path of least resistance. So even if you responded to this very message and told me that I was wrong and that you had indeed been diagnosed with cancer and it was terminal it would not matter because it is still my reality and not yours.

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