No reasonable definition of reality could be expected to permit this

A thousand years from now, the twentieth century will be remembered as the time when we discovered quantum mechanics. Forget wars, computers, bombs, cars and airplanes: quantum mechanics is a deep truth that will continue to be a part of our understanding of the universe into the foreseeable future.

Schrodinger's Cat So it’s kind of embarassing that we still don’t understand it. Unlike relativity, which seems complicated but is actually quite crystal clear when you get to know it, quantum mechanics remains somewhat mysterious despite its many empirical successes, as Dennis Overbye writes in today’s New York Times.

Don’t get me wrong: we can use quantum mechanics quite fearlessly, making predictions that are tested to the twelfth decimal place. And we even understand the deep difference between quantum mechanics and its predecessor, classical (Newtonian) mechanics. In classical mechanics, any system is described by some set of quantities (such as the position and velocity), and we can imagine careful experiments that measure these quantities with arbitrary precision. The fundamentally new idea in quantum mechanics is that what we can observe is only a small fraction of what really exists. We think there is an electron with a position and a velocity, because that’s what we can observe; but what exists is a wavefunction that tells us the probability of various outcomes when we make such a measurement. There is no such thing as “where the electron really is,” there is only a wavefunction that tells us the relative likelihood of observing it to be in different places.

What we don’t understand is what that word “observing” really means. What happens when we observe something? I don’t claim to have the answer; I have my half-baked ideas, but I’m still working through David Albert’s book and my ideas are not yet firm convictions. It’s interesting to note that some very smart people (like Tony Leggett) are sufficiently troubled by the implications of conventional quantum mechanics that they are willing to contemplate dramatic changes in the basic framework of our current picture. The real trouble is that you can’t address the measurement problem without talking about what constitutes an “observer,” and then you get into all these problematic notions of consciousness and other issues that physicists would just as soon try to avoid whenever possible.

I feel strongly that every educated person should understand the basic outline of quantum mechanics. That is, anyone with a college degree should, when asked “what’s the difference between classical mechanics and quantum mechanics?”, be able to say “in classical mechanics we can observe the state of the system to arbitrary accuracy, whereas in quantum mechanics we can only observe certain limited properties of the wave function.” It’s not too much to ask, I think. It would also be great if everyone could explain the distinction between bosons and fermions. Someday I will write a very short book that explains the major laws of modern physics — special relativity, general relativity, quantum mechanics, and the Standard Model of particle physics — in bite-sized pieces that anyone can understand. If it sells as many copies as On Bullshit, I’ll be quite happy.

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101 Responses to No reasonable definition of reality could be expected to permit this

  1. Plato says:

    Mathematics is always a continuum, linked to its history, the past – nothing comes out of zeroAtiyah

    While mine is a layman perspective, it is evolving, as I learn.

    The very nature of geometrical propensities are hard thing to resolve in my mind if they come from the quantum regimes, found as quantum geometries?

    Yet, such inclinations were understood by Einstein with the help of Grossman in a classical sense? Dirac, at a quantum level, understood as well. Feynman developed from him? While these were matrices to consider, it still held geometrical insights by Dirac?

    Am I missing something here that might have safely said that the discretium will prevail, and continuity slowly falls by the wayside?

    Your points need to be constantly gone over and considered.

  2. Suz says:

    I like the cute Schrödinger Cat.

  3. Plato says:

    I enjoyed your plates

    How are you going to introduce “superfluid” from early universe perspective?

    “Symmetry breaking” would have to break from something? 🙂

  4. anon. says:

    This is rather off-topic, but is there any chance you could give some opinions on the new Mansouri paper (astro-ph/0512605), which claims to explain away dark energy using a model that takes into account inhomogeneities and nonperturbative effects? (Note that this seems to be qualitatively different from e.g. the Kolb claims about perturbations causing the effect.) Especially in light of the recent Shapiro/Turner paper (astro-ph/0512586) that claims to find 5 sigma model-independent evidence for acceleration. I wonder if these two things can be reconciled? (By comparing assumptions about coordinate systems, …?) I’ve barely begun to read through the Mansouri paper but already I’m wondering what a specialist thinks. I always get confused about precisely which quantities correspond to experimental observations….

  5. Dissident says:

    Sean, don’t forget another smart Nobel laureate with doubts about QM: ‘t Hooft has been pounding the table about the possibility of a deterministic substructure for years (and yes, he does know about Bell’s inequalities). Speaking of the devil, he seems to have just submitted this reminder

    to Physics World. Anyone with an interest in these questions could do worse than browse his list of publications at

    and read

  6. Cat says:

    Cute cat…
    I wonder that small amount of quantum mechanically acting atoms of radioactive materal could kill this big cute cat.
    Is this possible because it’s just over simplified thought experiment?

    I guess killing a cat needs quite amount of radioactive material and long time exposure, which in turn would not act quantum mechanically.
    A system of (large particle number and long period of time) would not act QM’lly, so can not be described as superposition state of wave function. This is just my guess.
    That radioactive half-life can not be seen as a sign of QM? Or can it be?

    What Schroedinger wanted was to couple a QM system and a CM system, right?

    Just layman’s curiousity…

  7. Christine says:

    This was posted on my blog and thought I would pass it along ..

    December 25, 2005


    It was Christmas eve day and all through the house Two little children ran around like a mouse. The excitement was building. The moment was near. It would not be long be fore St. Nick would be here.

    Christmas is shared due to divorce, this was dads year, it sadly turned into remorse.

    Granted they were being fresh, as most kids are, but as I see it dad took it a little to far.

    They were told they were bad & the gifts would go back, if they didn’t behave & stop the crap. As children this is what we’ve all been told, but to actually have it happen is out of control.

    Christmas for these kids is the same for you & me, they woke with excitement to see what’s under the tree.
    Except for these kids who were told they were bad, awoke on Christmas morn & instead being happy, they were very , very sad.

    These children awoke from their beds with excitement & glee, to find NOTHING at all, under the tree.

    They were sad & disappointed as most of us would be, Imagine how you would have felt at age 9 & 13, to find a bare tree.

    Ya see, to them, Santa was a very nice man who lived long ago, that’s the story that is said, but as far as this house believes , he is now dead. Presents come from me & what I say goes, if I say NO gifts then that’s the way it goes.

    Later that day when they returned home, their was toys for the boy & a bible book for the girl, sadly but true, she was told, that if you didn’t open your mouth to the courts & start your stuff, you would have received more, I have to pay for an attorney now & he walked away in a huff.

    PLEASE E- Post your comments @ or E- MAIL me at My goal is to have as many emails & or letters to show this Judge the mental abuse that is going on and must be stopped!!!!!!!

    PLEASE e mail everyone you know & ask them to do the same ( a chain if you will ) I know I will receive a lot & hope this will help.

    Sincerely yours, MOTHER.

  8. Paul Valletta says:

    Having read D Z Alberts book:QME, some years ago, I found myself even more troubled by experience!

    Then something triggered me to inqure further into QM and HUP, questions raised called for a re-read of Alberts book, and thus more of the same, it really is an amazing book.

    Some basic problems I have formulated into:
    1)Observation in three dimensions, does not equal measure in 2-dimensions.

    2)A far-off measure translates to one-way observation.

    3A local measure can only occur at a specific scale.

    4)From a classical location of 3-D, 2-D objects being investigated do not return any precise values for measure or observation.

    5)From a QM location to a Classical Location, accuracy prevails, Macro domains of 3-D, are easy, big ‘targets’ for Quantums to locate, and thus any 2-D Quantum can and will have absolute determination over a higher dimensional location.

    6)Macro observers do not perform measure by default, you can ‘observe’ a finite speck of dust without altering it in any way.

    7)Observation can operate in linear directions, one-way, but for measure, you need the information to be returned to the observer, and upon its return, this information must not interject with any other quantity, else it be not “true”.

    8)Measure can NOT translate across different Dimensions ie (3-D to 2-D/2-D to 3-D) without a change in its value

    My confusion is further confirmed by my understanding of current theories of Q fields?

    It seems to me that the interplay of (2-D) E-M waves and the Photon,(3-D), are really the same as stating that there is inter-dimensional correspondence occuring, over a number of obvious dimensional parimiters, and make the H.U.P work or not work?..because of the change in dimensions?, rather than the change in position?

    So my really fundemental question is:Does the H.U.P really occur without a change in position, but with a change in dimensions?,,Thus does a change in dimension equal a change in position?

    Sorry if I confuse the issue more than needed!

  9. Dissident says:

    Cat: Schrödinger-style feline-killing contraptions typically feature a radiation detector set up to trigger a macroscopic and lethal process (e.g. breaking a container full of poison gas, or firing a revolver at the darn animal). If an atom in the small radioactive sample decays, the detector triggers the cat-killer, and the cat dies… or does it? 😉

  10. Andrew Jaffe says:


    This may turn into a blog post of my own, but for now, I’ll just point out that there are people who firmly believe in Quantum Mechanics, but take issue with your statement that

    what we can observe is only a small fraction of what really exists.

    Chris Fuchs, in particular, has been espousing a “Bayesian” interpretation of Quantum Mechanics (check out e.g., quant-ph/0205039 ) — the wavefunction isn’t real; it’s all about information. This is uncomfortable, of course, for us as physicists: we want (need?) to be realists. There has to be a something out there for us to observe. But the wavefunction itself pretty much can’t be that beast, since it depends too much on what observations have happened (i.e., on what information you have).

  11. Helge says:

    Hey Sean. Would you call it an “understanding” of quantum mechanics, if you can name the experiments, that makes the difference between QM and classical mechanics.
    The high school education here kinda teaches that … so you kinda know what a double slit experiment is, or the photoelectric effect, but don’t really hear about a wavefunction.

  12. Cat says:

    I still guess that “to trigger a macroscopic and lethal process” needs huge amount of radioactive material. Life do not die so easily…or do they?

    “If an atom in the small radioactive sample decays, the detector triggers the cat-killer”
    ———but if this is how it works,
    Does the property of QM disappear when the detector detects the decay of QM’al atom?
    Or the system isn’t in the superposition state any more.
    As soon as the decay is detected, the system is either in |live> or in |dead>. Not both.
    The trigger executes only one state, not superposition state.

    How about this? So the cat just becomes a measurement device in this case.
    It’s the same if I remove the triggering detector in the killing process.

    QM system–>detection—>CM system.
    i.e., Act of detection is boundary between QM system and CM system. Without detection these two can never be coupled. In other words, if a QM’al system ends up in a CM system after some amount of time, there was always measurement process between them.

    Or the decaying itself is measurement, if the photon is not absorbed back in a given time after, or if the cute cat eats the photon, or if the detector eats it.


  13. Dissident says:

    No dear Cat, I know this is frightful to you, but it’s quite enough for a single radioactive atom to decay for the detector to be triggered and have you poisoned, or shot, or whatever, by a thoroughly ordinary electromechanical Cat-killer.

    Which brings us to the chain “QM -> detection -> CM” and the followup question: what exactly qualifies as “detection”? Is an inanimate Geiger counter hooked up to a Cat-killer all it takes to collapse the wave function to either |alive> or |dead>? If so, why? After all, the detector is also ultimately made of quantum mechanical entities, so why should it be special? Just because it’s larger? Then how large does it have to be, exactly? Or is observation by a conscious observer needed? If so, why? The observer too is also ultimately made of quantum mechanical entities, so this amounts to giving consciousness a special role transcending that of ordinary physical phenomena… and this is the point where most physicists give up and exclaim “shut up and calculate!”, leaving such questions for the philosophers of science to ponder.

  14. rof says:

    In quantum mechanics, the wavefunction is not real. Read what Heisenberg, Bohr and Von Neumann have to say about this.

    The fundamentally new idea in quantum mechanics is that what we can observe is only a small fraction of what really exists. We think there is an electron with a position and a velocity, because that’s what we can observe; but what exists is a wavefunction that tells us the probability of various outcomes when we make such a measurement.

    This is not correct.

    What we don’t understand is what that word “observing” really means. What happens when we observe something?

    This is not correct either. You all know very well what it means to observe something.

    The real trouble is that you can’t address the measurement problem without talking about what constitutes an “observer,” and then you get into all these problematic notions of consciousness and other issues that physicists would just as soon try to avoid whenever possible.

    Yes. Physicists need to get over their mental problems and start thinking seriously about consciousness. The current dogma in physics, materialism, or physicalism, as some call it, is in fact a form of religion, which asserts that the physical world is all there is. It is not. There is subjective experience. Subjective experience is not made of physical material. Materialists who get as far as thinking about this often respond by saying that “in a sense”, subjective experience is made of physical material, based on the observation that the brain is a physical object. But the brain and the mind are not the same thing, so in a more accurate sense than the aforementioned one, subjective experience is not a physical thing.

    I should mention that there are ways that physicists deter one another from serious thought about consciousness. Accuse anyone who mentions it of believing in spirits. Suggest that thinking about such things constitutes philosophy and is therefore a waste of time. Suggest that in philosophy, everything is opinion and nothing can ever be known with certainty, and that therefore no rigour need be applied in the course of investigations – merely adopt an opinion and forget about it, because it doesn’t matter. These arguments are nothing more than immature schoolyard taunts. However, they have successfully persuaded several generations of physicists that ignorance is strength.

    What you are asking for (which is the wrong thing to ask for), is that everything mentioned in the method of predicting experimental results should be a mathematical object which is defined and is a part of the theory. This is not possible. Quantum mechanics is little more than induction – the probability of getting a particular experimental result is nothing other than the fraction of times that that result has been obtained before under similar circumstances.

    The desire to remove references to “measurement”, “experiment” and an observer from the formulation of the inductive method, or to define them in terms of more elementary mathematical notions cannot be accomplished. You can only define things which are arbitrary, for example, I can define a vector space and then I know all about vector spaces because they have exactly the properties that I gave to them when I defined them. But I cannot define a duck. A duck is an empirical object, discovered in experience (yes, subjective experience), and the properties of ducks must be discovered by examination, not by definition.

    Physics is sufficiently distinguished from mathematics already by the fact that it takes account of the results of experiments.

    There are therefore two branches of theoretical physics:

    1. Applied mathematics, which considers the application of mathematical formulae to the aggregated results of experiments for the purposes of organising them into a system, rather than leaving them as a mere aggregate.

    2. Metaphysics, which considers the conditions of the possibility of an experimental result. All experimental results can be encountered only in experience, therefore metaphysics is the study of the conditions of the possibility of experience, which include *time*, and *causation* (because the inference from an observed effect to a putative cause is necessary for all interpretation (including interpretation of empirical data), and interpretation (of empirical data) is necessary for experience).

    Physicists today know nothing about metaphysics, and use it merely as a label to insult their enemies. So theoretical physics today has only one branch, applied mathematics, and the process of doing theoretical physics consists in the practice of producing mathematical formulae and then checking to see if they produce the right predictions. Where do the formulae come from? They come from the minds of great men, from insight and from principles. But what is the name of the systematic investigation of these principles, since the insight of a great man is merely a dogmatic assertion of authority, and can never be acceptable as a foundation for a science?

    The investigation is called metaphysics, and metaphysics was studied by the founders of quantum mechanics, but is not studied any more, and that is why nobody today has a clue what they were talking about. Instead today there is just materialism, which asserts that physical matter is “fundamental” (the people are fundamentalists, you see), and have not given enough attention to subjective experience, and the conditions of its possibility (including space and time) are not material.

  15. rof says:

    The desire to remove references to “measurement”, “experiment” and an observer from the formulation of the inductive method, or to define them in terms of more elementary mathematical notions cannot be accomplished.

    My apologies. Of course, the desire can be accomplished, but what is desired cannot.

  16. Amara says:

    #10 Andrew Jaffe and Sean
    As I understand this topic (not well, but learning) quantum physics from a Bayesian point of view probably goes back to Bohr, or at least to Edwin Jaynes’. Einstein was looking at the realities of nature, describing at an *ontological* level. And Bohr was thinking on the *epistemological* level, not describing reality but, instead, information about reality. It’s about the difference between our knowledge of reality, and reality itself.

    Writing about quantum physics from a Bayesian point of view:

    Edwin Jaynes Probability:

    For example the papers: “Probability in Quantum Theory”, “Clearing up Mysteries- the Original Goal”.

    Plus this one, “Role and Meaning of Subjective Probability: Some Comments on Common Misconceptions.”

    from here:
    Giulio D’Agostini – Probability and Statistics

    (and this one too.. some parts in Italian:)
    Quantum Mechanics and Probability (+Confidence Intervals)
    G. D’Agostini — 18/01/2000:

    Bohr said: “There is no quantum world. There is only an abstract quantum physical description. It is wrong to think that the task of physics is to find out how nature _is_. Physics concerns what we can say about nature.” In his statement, he is stressing the information processing aspect of science. Although Einstein and many physicists would disagree with him and say that science is about learning what is reality, and what are its ‘laws’, Bohr is pointing out that any theory about reality can have no consequences testable by us, unless that theory can also describe what humans can see and know. If one incorporates human information into science, i.e. the original “logical inference” as described long ago by Bernoulli and Laplace, the quantum mechanical mud becomes clearer.

  17. Count Iblis says:

    If, somehow, you could measure observables with eigenstates like |dead> + |alive> you could kill just by observing 🙂

  18. Plato says:

    Aa layman, history, is very important to me.

    It is interesting that calorimetric design might of helped solve some of the issues with “spooky action at a distance?” While this created some consternation, one where we might never have understood these connections as with Einstein, today, it is much different?

    Entanglement and it’s history in development help to reveal some of these interactive features?

    So we set up the “circumstances” for early universe “information” to interact(glast), or, we “measure” the effect of colllisions in Onion skins?

    But somethings go beyond this. :)Are reduced from a fifth dimensional perspective, to two? Bekenstein bound and horizon mapping help to keep the blackhole interior in perspective?

  19. Ponderer says:

    I don’t think it’s ever realistic to expect every “educated” college-degree person to understand what even “wavefunction” means. I would draw a line at Heisenberg uncertainty, stated in a non-mathematical language.

    We spend too much time surrounded by physicists to think that everyone must know what we are talking about – however many PhD/Master students in biology will have trouble doing simple calculus assignments, never mind humanity majors in college!

    Remember that most educated people out there think that Einstein’s theory of relativity has proven that “everything is relative”, and some will give you an example of how time passes by faster when you are doing something fun, as opposed to slowly killing time when you are bored as Einstein’s “time” relativity.

    I wish someone did Jay-walking show, aka Jay Leno going to the mall to ask average Joe basic physics questions – 0.00001% of population (professionally trained physicists) would find this type of show hillarious!

    A short book describing everything in layman’s term would be great, but using words like “wavefunction” assuming everyone understands what it means will make it sound like gibberish to non-physicists and trivial “dumbing physics down” book to physicists.

  20. Count Iblis says:

    Ponderer, perhaps these things should be taught in secondary school. Just like art and literature, physical theories like Quantum mechanics, relativity etc. are also part of our culture. So, some popular books about these subjects should be compulsory reading.

  21. Plato says:

    JUst for some clarity then?

  22. George Musser says:

    The fundamentally new idea in quantum mechanics is that what we can observe is only a small fraction of what really exists.

    Isn’t it even worse than that? I thought certain properties did not even exist until measured. Otherwise you’re describing a hidden variables theory.


  23. Moshe says:

    I had some problem with the phrasing of that sentence, since I am not sure what “really exists” really means. One viewpoint I am sympathetic to is that physics is all about results of measurments, and other things may or may not “exist”, but if they do not make any measurable difference maybe they can be left out of the discussion. In that spirit I never understood what is the measurment “problem”, is it just the lack of a convenient mental image or is there something more concrete?
    (candidate for the latter is the emergence of classical physics out of the quantum world).