Real and Unreal

In Physics by Brian Koberlein28 Comments

Quantum theory is strange, but very real. Through countless experiments we’ve found that quantum objects have both particle-like and wave-like properties. In some experiments the particle nature dominates, while in others the wave nature dominates. Some experiments can even show the effects of both properties. This duality between particles and waves in quantum theory is deeply counterintuitive, which means often the results of quantum experiments are interpreted incorrectly.

Take, for example, recent claims that reality doesn’t exist until we measure it. The claims arise from a recent experiment published in Nature that uses a single atom to perform what is known as the delayed-choice experiment. This experiment was first proposed as a thought experiment (gedanken experiment) by John Wheeler as a way of exploring the counterintuitive aspects of particle-wave duality.

Wheeler’s idea was to imagine a “cosmic interferometer.” Suppose light from a distant distant quasar were to be gravitationally lensed by closer galaxy. As a result, light from a single quasar would appear as coming from two slightly different locations. Wheeler then noted that this light could be observed in two different ways. The first would be to have a detector aimed at each lensed image, thus making a particle measurement. The second would be to combine light from these two images in an interferometer, thus making a wave measurement. According to quantum theory, the results of these two types of experiments (particle or wave) would be exactly as we’ve observed in their standard form. But the light began its journey billions of years ago, long before we decided on which experiment to perform. Through this “delayed choice” it would seem as if the quasar light “knew” whether it would be seen as a particle or wave billions of years before the experiment was devised.

Although the quasar experiment Wheeler proposed isn’t practical, modern experimental equipment allows us to perform a similar experiment in the lab, where the decision to measure a particle or wave is done at random after the quantum system is “committed.” For example, in 2007 a delayed-choice experiment was made using laser light to create a delayed-choice double slit experiment. In this new paper, the team used an ultracold helium atom to do a similar delayed-choice interference experiment. With both experiments the results were exactly as predicted by quantum theory. So both matter and light exhibit this strange quantum effect.

nYuzAo4While this is great work, the result isn’t unexpected. Quantum theory made a very clear prediction about this kind of experiment, and its prediction has been confirmed. Where things get fuzzy is in the interpretation. One popular way to interpret quantum theory is to presume quanta have a potential wavefunction, which then collapses into a definite state when observed. In this view the act of measurement gives reality to the quantum. In the delayed-choice experiment that would mean the quantum doesn’t become “real” until you measure it, which could be billions of years after its origin in the case of quasar light. But this is an overly simplistic take on things. Quantum objects are real, but simply have indefinite properties. These properties are defined by the experiments we do. What the delayed choice experiments really show is that quanta don’t exist as particles or waves, but are truly unique objects which can exhibit particle and wave properties in certain experiments.

While that might seem strange, it isn’t magical or mystical. The Moon wouldn’t vanish from existence if everyone closed their eyes, and reality isn’t dependent upon us observing it.

Paper: A. G. Manning, et al. Wheeler’s delayed-choice gedanken experiment with a single atom. Nature Physics, DOI: 10.1038/nphys3343 (2015)

Paper: Jacques, V. et al. Experimental realization of Wheeler’s delayed-choice gedanken experiment. Science 315, 966–968 (2007).


  1. If we use the word “quanta” when we write about non-collapsed entities, then it avoids the loaded terms of “wave” and “particle”, so the physics can be framed without misinforming readers.

  2. In most all aspects of our lives, we are far better served by treating the sun as rising and setting, and the world as being flat (when we need to use a Google maps), and species as being separate, even though we know each of these is not “really” the case. In some unusual contexts however, “real” ontology matters and then, we need to acknowledge the earth’s rotation, globe shape, and shared evolution.

    I propose we assume a consistent, similar view of particles, spacetime, etc. as fine to treat as real for most of our lives, but not ultimately real in special contexts where it can crucially matter, just as we would above. That is: as we treat our current object concepts in our physics paradigms as we do those of other, past scientific paradigms. This would give a view of particles, spacetime, etc. are not really “real”, but observational consequences of the way we observe and measure things.

    From such a perspective, detection of a particle or wave attribute is unproblematic. Under such a view, duality doesn’t violate the law of non-contradiction, realism is preserved, and the Copernican Principle is not violated. Such a view is also consistent with what we know about how scientific revolutions occur. It offers the kind of domain within which theoretical reduction can take place. These seem to comprise a non-trivial list of benefits.

    The article implies accuracy in predicting later observations and measurements is evidence that what we observe and measure are fundamental aspects of reality, and cannot be emergent from more basic structures.

    No matter the decimal place accuracy of its predictions, we would not regard accuracy as legitimately supporting geocentrism, although its followers would. Therefore, standard model claims of this form seem to be a kind of special pleading fallacy.

    Is there a principled case to be made for regarding these observations and measurements as demonstrating proof of ultimate reality that when applied with an even hand, support certainty of standard model interpretations and assumptions and rule out stuff we’ve abandoned long ago?

  3. I read an article about this experiment done in Australia that said due to the results we must entertain the idea that future events determine what happens in the past. Though feasible i would be more inclined to say that this experiment sheads light on the strength of quantum entanglement. Is it not likely that at the quantum level the particles “knew” that the second screen was going to be inserted therfore chose whether to be “wave” or “particle” form? I am no physicist or scholor of anykind, just curious about the world around me.

  4. Well this is all very confusing to the layman. I remember reading, a couple of years back, an article in “Scientific American” stating that ‘physicists now agree that the only reality is observations’ – which is much the same as saying that the only reality is consciousness.
    Then I read other articles saying that there IS a ‘real’ universe but it consists of ‘quantized fields’. Then I read yet other articles criticising the ‘quantized field’ idea as a sort of loss of nerve – an inability of certain physicists to cope with the fact that consciousness (“observations”) is all that exists, therefore these physicists must invent some sort of indefinable ‘material stuff’ (“quantized fields”) to reassure themselves that there realIy is a real reality ‘out there’ somewhere.
    And of course IF the universe DOES consist of ‘quantized fields’, then it would indeed continue to exist if all consciousness in the universe were extinguished – but it wouldn’t ‘look’ like anything at all because there would be no conscious minds to turn the ‘quantized fields’ into something resembling the reality which we experience – or even that which microbes experience. The universe would just exist as a mass of mathematical probability distributions, not light or sound or smell or texture or anything resembling ‘matter’. My brain hurts.

    1. LOL. A great summary. The way I’ve come to look at it is this: there is consciousness, which has experiences; and there is a set of rules governing those experiences, which science seeks to elucidate. Currently we conceptualize those rules, in part, as quantum objects described by a probability wave function. Maybe we could say that this ‘set of rules’ exists independently of the conscious experiences subject to those rules. But that won’t satisfy many people, who still want something ‘physical’ – a word which I think has steadily lost meaning.

  5. “Quantum objects are real, but simply have indefinite properties.”

    I don’t think that statement has any meaning. In it, the word “real” is simply being redefined, to support the view that some sort of realism must be true. Actually it might be correct to say that the word “real” lacks any non-circular definition in this context. It’s merely a placeholder; insert your own concept – or vague feeling – in it’s place.

    1. Author

      The term “real” is generally taken to mean that it actually exists (as opposed to imaginary). Quantum objects are real in that sense. Electrons exist. Protons exist. They also have indefinite properties.

      1. I feel the same way about the word “exists” – it has no meaningful non-circular definition. Basically “exists” means “is real” and vice versa.

        I invoke the conceivability argument: we can’t conceive of something having no definite properties. We have no way to think about it. But maybe your position is that quantum objects have some definite properties, and some indefinite ones?

        Yes, you can distinguish something as being “real” rather than a product of someone’s imagination – but I think that’s another context shift. It amounts to saying that an object is perceived by everyone – it’s part of our shared reality.

        1. Author

          Quantum particles do have definite and indefinite properties. Electrons have definite charge and rest mass, for example. Any word can seem to have a circular definition if you push it hard enough. Everything in the dictionary is defined by words which are themselves defined by other words. So that’s a weak argument. When we say that quanta are real, that is accurate in the common sense of the word. The precise description is mathematical in nature.

          1. I think the idea of quantum objects that have some intrinsic and well-defined properties, but some that take on values only as part of an observer’s experience, is already moving us away from realism, towards some form of idealism. But it’s too easy to get locked into 19th-century ideas of “realism” and “idealism”.

            Maybe we agree on this: when we consider things like these delayed-choice experiments, we realize that our concept of “real” has to evolve.

  6. Hi Brian, I know this post is old now but there was something I read about delayed choice experiments showing that the future can affect the past, however from what I’ve read apparently this is wrong and the future cannot affect the past, it just looks as if it does?

  7. “At the quantum level, reality does not exist if you are not looking at it,” said Associate Professor Andrew Truscott from the ANU Research School of Physics and Engineering. (Read more at:

    Brian, I take it that, in your opinion, Associate Professor Truscott’s view of the matter is “overly simplistic?” I am sure the ANU Research School of Physics and Engineering would beg to differ…

    1. Author

      Yes, the statement is overly simplistic. What defines the “one” looking for example? Truscott is talking about measurement in the lab, and that the measurement defines the system. What that means in the broad is still a matter of discussion. It would be ridiculous to claim, for example, that the Moon doesn’t exist unless humans are looking at it.

      1. I admire you for sticking to your guns – I suppose, in your article, I was looking for acknowledgement that there are competing interpretations, often contradictory (as with yourself and Truscott), which can make researching quantum physics rather confusing (as the some of the replies above demonstrate).

        I quite agree that it would be absurd to claim the moon doesn’t exist if we all closed our eyes, but would it be quite so ridiculous to question whether the moon would exist if the universe did not contain a conscious observer? This is more of a philosophical question, of course.

        I agree also that there is nothing ‘magical’ going on (in the literal sense of the word), indeed these are truly natural phenomena. However, as for ‘mystical’ – to quote your illustration, “I do not think it means what you think it means”. Whether the quantum world or, indeed, the world of everyday life are ‘mystical’ is entirely a subjective impression, not a scientifically-verifiable fact.

        1. Author

          Despite the quotes from Truscott, the actual scientific results have been “quantum mechanics works as we expect.” There isn’t the disagreement you think there is. The whole “conscious observer” idea is a misnomer, and is not how quantum theory works.

  8. “Quantum mechanics works as we expect” – indeed, I concur entirely, this is the accepted understanding of the theory in its practical application. However, I don’t agree that there is little or no disagreement about the wider interpretation of quantum theory.

    As theoretical physicist Professor Jim Khalili says: “Basically, while the mathematical construct that is quantum mechanics is not much in doubt these days, what it MEANS (the interpretation of the theory) is still up for grabs. There’s the Copenhagen view, Bohmian mechanics, the Multiverse interpretation, the Transactional theory, spontaneous wave function collapse theory…) We don’t know which of these is right.” (

    Whether or not our friend Truscott was guilty of over-simplicity in his interpretation of the results of his experiment, we can safely say that the discoveries of quantum physics have challenged our Classical notions of reality, at the level of the very small at least (e.g. entanglement, non-locality). Yes, it is “strange” – it is deeply mysterious and I don’t think this point should be underestimated.

    PS My point about the moon was more philosophical in nature – could the moon be said to exist in a universe devoid of consciousness, when any notion of ‘existence’ is defined by the very consciousness that would be lacking in such a universe?

    Thanks for the replies, by the way.

  9. Does this mean, all manners in which quanta characteristics are observable, simultaneously exist just like movie frames; and different perspective-points/observation-methods determine which characteristics we observe? Or other analogies such as viewing HTML-code versus the pixeled web-page and/or viewing individual pixels(particles-of) versus the full-screen (wave)?

  10. I am always confused as to the difference between an unobserved dual slit experiment, which results in an interference pattern and an observed dual slit experiment which results in what we would expect to see. As the very nature of either experiment requires us to eventually observe the results then how can either be considered unobserved?

    1. I think where i am headed is how can there be such a concept as before, now and later with an artifact that is moving at the speed of light…..doesn’t this mean that to the photon all concepts of time don’t exist, that every time is now ???

  11. Yeah, it does tell us what quantum mechanics predicts is true. And quantum formalism doesn’t include any physical particles. It only includes math that predicts a result.

    This experiment proves there is no objective reality, or reallly no particle before it’s measured. The only realist explanations are untenable. There is no physical particle existing with undefined properties before measurement. That’s a delusion. Experiment proves that if there are any physical objects in these experiments, then they are moving faster than light. Not possible. Anton Zeilinger and his colleagues have concluded in further experiment that there are no physical objects in these quantum experiments.. The logical result (not interpretation) of this experiment is that there is no objective reality before we measure it.

    Does this mean the moon doesn’t exist if it’s not being measured? No. The particles don’t exist even when they are measured. They just must be rendered to us when we look for them, but when we don’t look, the default answer is a probability distribution. The effects are still real enough even when we don’t look. The moon is also not there, and when nobody is looking it’s not being rendered, but the effects of a moon are still rendered.

  12. If our experiment is set up to measure particles, we get particle results. If we measure for waves, we get wave results. Just means that particles have some characteristics of both.I really can’t see any problem here at all (except limitations in human thought processes).

  13. I fail to see the dilemma in Wheeler’s Delayed Decision thought experiment, at least when talking about light. When we look at the experiment, when we change the design of the experiment to switch between measuring particles and waves, it appears to us that the packet of light energy must have made the decision about whether to behave as a particle or as a wave a billion years before we made the decision which form we were going to measure. However this paradox of changing history only occurs when we look at things from the point of view of the experimenter.
    When we view things from the perspective of the light, there is no dilemma. Light, I believe, travels at the speed of light. So with the effects of time dilation, from the light’s perspective, the rest of the universe is in suspended animation. The universe does not age at all during the billion years of time that the light took to travel from its source galaxy to the experiment’s detector. That must mean that from the light’s perspective, the universe, including the set up of the experiment, was exactly the same when it left the galaxy as when it hits the experiment’s detector. So yes, the light did know exactly how the experiment was set up when it left its galaxy.
    The odd thing is that we see the light taking a billion years to get here from its source, meaning that we think we have aged a billion years since it left, when from the light’s perspective the surrounding universe did not change at all. Even stranger is that if we could have observed that light throughout its billion years’ journey, we would see that it does not age at all throughout its journey while the light will have perceived that it has aged a billion years over the journey.
    So it appears to me that the odd thing going on is nothing to do with changing history but how we perceive time.
    The fact that the same effects are observed with massive particles (as opposed to the massless particles such as photons) is stranger as these particles won’t travel at the speed of light, so they will age and perceive the universe ageing around them. Yet the same delayed decision process occurs.
    This does beg the question “What is Time?”

  14. So let me get this straight…. your argument is if everyone closes their eyes the moon would still be there? If that’s correct try thinking of it like this…. closing your eyes is to consciousness is what putting a lens cap on is to a camera. Either situation the observer (consciousness) has simply put blinders on momentarily. Your moon experiment would work (nothing would exist) if everyones concisinous vanished forever. And since that’s a silly thing to do please tell me your article is not debunked if you believe that and I’ll try my best to explain this again

  15. Thank you for this, it explains what’s wrong with the coverage of that experiment and other quantum theory experiments that I couldn’t quite put my finger on!! Yes!

  16. So, does retrocausality really exist, or are they misinterpreting or misrepresenting the results of these delayed choice experiments? Does this prove that a future event can effect a past event?

  17. I like the way you have described this, but I wasn’t aware some people actually believed that things, such as the Moon, would actually cease to be (implying a “loss” of something “material”) if nobody was observing it! Some people look way too far into it, others it seems might not look deep enough because they will outright claim it is “silly” to include consciousness at all in an interpretation of quantum physics.
    The way I have interpreted these experiments, and I’m not a scientist at all so I could be wrong, is that: the act of perception by a “conscious” observer (anyone or even any device capable of “acknowledging” this information) “unfolds” the wave form information that describes the object and allows us to interact with it (and the rest of the world) If everyone is to stop looking at it, it would then not possess three dimensional, “physical” qualities as long as that remains the case. The “matter” would revert to wave form information.
    an analogy is how a DVD only displays the movie when the laser is “looking” at the waves on the disc and “unfolding” them into “reality” (tv screen). The only problem I Can see with this interpretation, is how do we know that something else entirely isn’t causing the three dimensional world to unfold, because of course we can’t obviously see it when we aren’t looking at it!

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