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How to Teach Quantum Mechanics

David Z. Albert

Published Jan 7, 2019• 1 citation
OpenAlex

Abstract

I distinguish between two conceptually different kinds of physical space: a space of ordinary material bodies, which is the space of points at which I could imaginably place (say) the tip of my finger, or the center of a billiard-ball, and a space of elementary physical determinables, which is the smallest space of points such that stipulating what is happening at each one of those points, at every time, amounts to an exhaustive physical history of the universe. In all classical physical theories, these two spaces happen to coincide – and what we mean by calling a theory “classical”, and all we mean by calling a theory “classical”, is (I will argue) precisely that these two spaces coincide. But once the distinction between these two spaces in on the table, it becomes clear that there is no logical or conceptual reason why they must coincide – and it turns out (and this is the main topic of the present paper) that a very simple way of pulling them apart from one another gives us quantum mechanics.

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Scott Aaronson

There are two ways to teach quantum mechanics. The first way – which for most physicists today is still the only way – follows the historical order in which the ideas were discovered. So, you start with classical mechanics and electrodynamics, solving lots of grueling differential equations at every step. Then, you learn about the “blackbody paradox” and various strange experimental results, and the great crisis these things posed for physics. Next, you learn a complicated patchwork of ideas that physicists invented between 1900 and 1926 to try to make the crisis go away. Then, if you're lucky, after years of study, you finally get around to the central conceptual point: that nature is described not by probabilities (which are always nonnegative), but by numbers called amplitudes that can be positive, negative, or even complex. Look, obviously the physicists had their reasons for teaching quantum mechanics that way, and it works great for a certain kind of student. But the “historical” approach also has disadvantages, which in the quantum information age are becoming increasingly apparent. For example, I’ve had experts in quantum field theory – people who've spent years calculating path integrals of mind-boggling complexity – ask me to explain the Bell inequality to them , or other simple conceptual things like Grover's algorithm. I felt as if Andrew Wiles had asked me to explain the Pythagorean Theorem.

Cambridge University Press eBooks•Mar 14, 2013•1 citation

Classical and quantum framing of the Now

N. David Mermin

Mermin replies: Understanding the Now was not the main purpose of my March 2014 commentary. My primary point was that a new way of resolving the puzzles and paradoxes of quantum mechanics, called QBism by Christopher Fuchs and Rüdiger Schack, was also able to settle that longstanding problem in classical physics. After my earlier commentary on QBism (Physics Today, July 2012, page 8), I was concerned that many of the letters about it (December 2012, page 8) had sounded the same theme: that the letter writer’s own way of looking at quantum mechanics was already perfectly satisfactory. I hoped that by applying QBist thinking to a strictly classical problem, I could disengage those readers from their favorite interpretation of quantum mechanics and help them think about QBism on its own merits. I recently emphasized in Nature11. N. D. Mermin, Nature 507, 421 (2014). https://doi.org/10.1038/507421a that QBism sheds light on classical physics, too, but there I discussed quantum applications of QBism as well as classical applications (“CBism”).The letters here all address only the problem of the Now but not the fact that I deal with it through a classical application of QBism. While I’m disappointed that they say nothing about QBism or CBism, I’m pleased that they all agree that the problem of the Now is indeed a problem. Not everybody does.Berge Tatian and B. K. Ridley both criticize me for taking the Now to be a point. But I don’t. I call the Now an event “whose duration and location are restricted enough that it can usefully be represented as a point in space and time.” I say that “the events we experience are complex, extended entities” and that “to represent our actual experiences as a collection of mathematical points … is a brilliant strategic simplification, but we ought not to confuse a cartoon … with the experience itself.”Ridley’s comment “Nor could one person’s Now be exactly the same as another’s” suggests that I say it could be. What I do say is “The commonality of my Now and your Now whenever we are together requires that our Nows must coincide at each of two consecutive meetings.” “Commonality” or “coincide” mean only that our two private Now experiences happen at a common place and time, not that they are identical. Indeed, the personal experiences of different people are incomparable, except through the imperfect medium of language.I agree with James Hartle on much in the paper he cites, but we have important differences. He takes spacetime to be objective and fundamental; I take it to be an abstract tool used by an agent to organize her experience. He uses the notion of a point in spacetime uncritically; I regard it as an approximate representation of an agent’s spatially and temporally extended experience. He takes an agent’s experience to be an objective property of the agent, like the contents of a register. I take an agent’s experience to be private and self-evident to that agent and to be the fundamental basis for her inference of an external world; the experience of each agent plays a special role for that—and only that—agent, analogous to the special role played by “the classical domain” in the quantum mechanics of Lev Landau and Evgeny Lifshitz.Rudolf Peierls wrote to John Bell in 1980, “In my view, a description of the laws of physics consists in giving us a set of correlations between successive observations. By observations I mean … what our senses can experience. That we have senses and can experience such sensations is an empirical fact, which has not been deduced (and in my opinion cannot be deduced) from current physics.”22. S. Lee, ed., Sir Rudolf Peierls: Selected Private and Scientific Correspondence, vol. 2, World Scientific, River Edge, NJ (2009), p. 807. If “us” is expanded to “each of us,” then nobody has ever put QBism and CBism more concisely than that.REFERENCESSection:ChooseTop of pageREFERENCES <<CITING ARTICLES1. N. D. Mermin, Nature 507, 421 (2014). https://doi.org/10.1038/507421a, Google ScholarCrossref, ISI2. S. Lee, ed., Sir Rudolf Peierls: Selected Private and Scientific Correspondence, vol. 2, World Scientific, River Edge, NJ (2009), p. 807. Google ScholarCrossref© 2014 American Institute of Physics.

Physics Today•Sep 1, 2014•2 citations

Is there a mass above which quantum superposition stops? Registered readings of mass and distance, the 2026 interference record and a prediction dated to 2035

Ugo Lanciano

This record. Whether a mass exists above which quantum superposition stops: registered readings confronted with the 2026 interference record, and a prediction dated to 2035. Reading path: Quantum mechanics, 2 of 2 · before: Born geometry, the Tsirelson bound and the singlet · all themes. Common introduction to all the records of this series. The content of this record follows below. Mathematical Materiality (MM) in brief. Why are the laws of physics the same everywhere and at every instant, even though nothing forces matter to obey them? MM proposes an answer: the laws are not rules imposed on matter from the outside; rather, matter itself is made of mathematical relations. And mathematics cannot change: a relation binds two distinct terms everywhere and always. In logic, nothing exists on its own: to exist means to be in relation to another logical entity, and outside this web of relations nothing can exist, not even other universes. This reading already meets experimental measurements at two points. First, the ratio of dark to ordinary matter is read as 3√3 = 5.196; when left free in the fits, Planck and DESI DR2 data give 5.21–5.22 (as registered in record 2.44, https://doi.org/10.5281/zenodo.23329080). Second, the acceleration scale of galaxies is read as a₀ = cH₀/6, the same scale that appears in Verlinde's emergent gravity: the computed value is 1.09–1.18 × 10⁻¹⁰ m/s² against the measured value of 1.20 (record 2.46.1, https://doi.org/10.5281/zenodo.23506655). Both readings were formulated with the measurements already known; the tests to check them against future data are registered in the same records. In the first logical nexus between space and time — which became homogeneous with Einstein — there is a non-local phase, made only of a pure logical distinction: space is not time. Between the two there is no relation of quantitative calculation, only the logical entity they have in common, which is needed to make them exist with respect to each other. This level is not distant: it is present in every phase of reality, in every place and at every instant. It shows in entanglement, where two distant systems remain correlated more strongly than any explanation based on separate properties and free of faster-than-light influences allows, without anything physically travelling between them. In this non-local phase the two distinct terms, space and time, share only one logical element, which we shall call their greatest common divisor (GCD), just as the numbers 3 and 5 have only 1 in common. The local world arises when a relation is counted through this GCD: its multiples and its fractions form a matrix. Seen by an observer inside the system like us, the multiples appear as time and the fractions as space. When that same multiple is directly contained in the matrix of the point of observation, it turns into mass — that is, into concrete matter as we usually understand it. There is therefore a close link between time and mass, in full agreement with Einstein's relativity, which ties mass to spacetime. The observed objects, while being logically autonomous entities in their local relations, enter into relation with the observer through a logical diffraction — that is, a decomposition into fractions — of the observer's matrix. This reading also gives a reason for the limit of the speed of light, which in Einstein's relativity is instead taken as a starting principle. The speed of light is the fixed exchange rate between the multiples and the fractions of the same GCD, meaning how much space a single step of time is worth: exceeding it would mean, for some observer, covering space without counting time, and thus leaving the local world. Logic always starts from a logical relative, and this exchange rate has nothing absolute in itself either: it is a necessary relative which, inside the system, becomes an absolute hypothesis, the same for everyone and impossible to exceed. Outside the system, by contrast, it has no justification at all, because outside there is only non-existence: justifying something requires another term to relate it to, and out there there is none. Asking for a reason from outside is like asking what lies north of the North Pole. The same theoretical framework that reads space, time, mass and the observed objects also admits quantum mechanics. The non-local phase and the mirror-like logical reciprocity between the two ends of the nexus give a reason for the probabilistic behaviour of matter. When the observed object is large enough, its mass fixes it in the matrix of the point of observation: for this reason we see it in one precise place and at one precise instant. When instead the object falls below a certain mass threshold, the principal GCD (that of the point of observation) and the GCD of the diffraction that generates the object are forced to become equal, reflecting each other as in a mirror. At that point the object no longer occupies a single point of the spacetime generated by the observer, but spreads over several possible positions, and we observe it as probability. It is like having to describe the number 10 using only the number 1: the only way is to distribute the 1 over ten positions, one for each unit of the 10. Today we know that even objects made of thousands of atoms behave in this way, and no experiment has yet found the boundary. The idea that the passage from quantum to classical behaviour depends on mass is close to that of Penrose and Diósi; in MM the threshold is the point where the two GCDs, the principal one and that of the local diffraction, become equal. For all these reasons the universe behaves like a simulation without really being one. It follows exact rules, made of counts, everywhere and at every instant, but no machine runs them and no programmer wrote them: a structure of mathematical relations does not need to be computed in order to exist. Tegmark (2008) had already noted that such a simulation need not be run, while Vazza (2025) estimated that running it physically on a machine would require resources that the very laws of the universe do not allow. This record develops one aspect of the theoretical framework. The work is shared openly so that it can be judged by as many readers as possible: anyone who knows someone interested can pass on this single link, valid for the whole framework (with an overview in English and Italian): https://doi.org/10.5281/zenodo.23535916. The entry page, with the complete list of all records, is https://doi.org/10.5281/zenodo.22667516. To cite the framework: Lanciano, U. (2026). The number 1 in front of the mirror: Mathematical Materiality from the nexus to the cosmos. Zenodo. https://doi.org/10.5281/zenodo.23535916. The records have been deposited since 5 August 2026, each with its own date, and are released under CC BY 4.0: anyone may use them by citing the source. Objections and errors found are welcome at riverantares@gmail.com, since Zenodo has no space for comments. This record. Standard quantum mechanics has no mass at which a superposition stops: large objects look classical because of decoherence, the environment that keeps recording where they are. Collapse models (Ghirardi–Rimini–Weber, CSL, Diósi–Penrose) add such a limit by hand. The framework says that a threshold exists: below a certain mass level the principal GCD of the point of observation and the GCD of the diffraction that generates the object become equal, and the object spreads over several positions (see the box above). The box gives no number. This deposit tries to give the threshold a number, or to say clearly why it cannot. A second part tests two later remarks of the author: the limit cannot be fixed as a mass, because a calculation of the «fractional distance» is missing; and large distant bodies stay fixed because their size makes up for the distance. It was announced in deposit 2.64 (DOI 10.5281/zenodo.23610071). The main result: for distance, the limit is not a mass alone. In the two readings of the fractional distance that hold, it is a ratio between the size that has to be told apart and the cell of the copies that do the telling, together with the number of copies; the readings of part one with a mass threshold at the Planck scale remain possible, untested, and that the two readings are one rule of the framework is a proposal. For the state of an object the copies are the photons and molecules the environment carries away, and one copy cannot tell apart two positions closer than about half a wavelength: a 16-microgram crystal stays quantum over 2 × 10⁻¹⁸ m because about 10³⁴ thermal copies would be needed to fix it. For the way a distant observer reads an object, the cell is the diffraction cell of the observer's aperture at the object's distance: two stars of almost the same mass fall on opposite sides (the disc of Sirius B, 1.02 solar masses at 2.64 pc, inferred from its radius, is 20–60 times smaller than the finest cell of today's interferometers; τ Ceti, 0.78 at 3.65 pc, and 61 Cyg A, 0.67–0.74 at 3.49 pc, are resolved): with this instrumental classification the sample does not support a mass-only limit growing with distance. Both rules coincide with known physics (decoherence and wave optics): the framework admits them; experiments put them to the test but cannot tell them apart from standard physics. Prior work, first. Decoherence: Joos and Zeh (1985), Zurek (2003), Schlosshauer (2019); measured against separation and number of photons: Chapman et al. (1995), Kokorowski et al. (2001), Hornberger et al. (2003), Hackermüller et al. (2004). Collapse models: Károlyházy (1966), Ghirardi, Rimini and Weber (1986), Diósi (1987, 1989), Pearle (1989), Ghirardi, Pearle and Rimini (1990), Penrose (1996), the review of Bassi et al. (2013), Howl, Penrose and Fuentes (2019). Tests and limits: Nimmrichter and Hornberger (2013), Romero-Isart (2011), Donadi et al. (2021), the Majorana Demonstrator (2022), Carlesso et al. (2022), Gasbarri et

•Oct 25, 2026•2 citations

Fundamental considerations inspired by quantum physics

Anton Zeilinger

I have been interested in quantum mechanics or, synonymously, quantum physics since the very days when I first learned about them as a student. The initial impression was that of an immense mathematical beauty of the formalism of quantum mechanics. The second impression was the fantastic agreement of the theoretical predictions of quantum physics for experiments, and the third and somewhat surprising impression was that there was widespread disagreement about the interpretation of the new theory.

Libreria Editrice Vaticana eBooks•Dec 3, 2025•0 citations

Does Newtonian space provide identity to quantum systems?

Décio Krause

Physics is not just mathematics. This seems trivial, but poses difficult and interesting questions. In this paper we analyse a particular discrepancy between non-relativistic quantum mechanics (QM) and `classical' (Newtonian) space and time (NST). We also suggest, but not discuss, the case of the relativistic QM. In this work, we are more concerned with the notion of space and its mathematical representation. The mathematics entails that any two spatially separated objects are necessarily \ita{different}, which implies that they are \ita{discernible} (in classical logic, identity is defined by means of indiscernibility) --- we say that the space is $T_2$, or "Hausdorff". But when enters QM, sometimes the systems need to be taken as \ita{completely indistinguishable}, so that there is no way to tell which system is which, and this holds even in the case of fermions. But in the NST setting, it seems that we can always give an \ita{identity} to them by means of their individuation, which seems to be contra the physical situation, where individuation (isolation) does not entail identity (as we argue in this paper). Here we discuss this topic by considering a case study (that of two potentially infinite wells) and conclude that, taking into account the quantum case, that is, when physics enter the discussion, even NST cannot be used to say that the systems do have identity. This case study seems to be relevant for a more detailed discussion on the interplay between physical theories (such as quantum theory) and their underlying mathematics (and logic), in a simple way apparently never realized before.

arXiv:1703.05998•Mar 17, 2017•0 citations

Quantum Mechanics

Eduardo R. Mucciolo

Now that we went through the math and the notation, we can introduce the fundamentals of quantum mechanics in a formal and precise way.

CRC Press eBooks•Aug 6, 2025•0 citations
I distinguish between two conceptually different kinds of physical space: a space of ordinary material bodies, which is the space of points at which I could imaginably place (say) the tip of my finger, or the center of a billiard-ball, and a space of elementary physical determinables, which is the smallest space of points such that stipulating what is happening at each one of those points, at every time, amounts to an exhaustive physical history of the universe. In all classical physical theories, these two spaces happen to coincide – and what we mean by calling a theory “classical”, and all we mean by calling a theory “classical”, is (I will argue) precisely that these two spaces coincide. But once the distinction between these two spaces in on the table, it becomes clear that there is no logical or conceptual reason why they must coincide – and it turns out (and this is the main topic of the present paper) that a very simple way of pulling them apart from one another gives us quantum mechanics.
There are two ways to teach quantum mechanics. The first way – which for most physicists today is still the only way – follows the historical order in which the ideas were discovered. So, you start with classical mechanics and electrodynamics, solving lots of grueling differential equations at every step. Then, you learn about the “blackbody paradox” and various strange experimental results, and the great crisis these things posed for physics. Next, you learn a complicated patchwork of ideas that physicists invented between 1900 and 1926 to try to make the crisis go away. Then, if you're lucky, after years of study, you finally get around to the central conceptual point: that nature is described not by probabilities (which are always nonnegative), but by numbers called amplitudes that can be positive, negative, or even complex. Look, obviously the physicists had their reasons for teaching quantum mechanics that way, and it works great for a certain kind of student. But the “historical” approach also has disadvantages, which in the quantum information age are becoming increasingly apparent. For example, I’ve had experts in quantum field theory – people who've spent years calculating path integrals of mind-boggling complexity – ask me to explain the Bell inequality to them , or other simple conceptual things like Grover's algorithm. I felt as if Andrew Wiles had asked me to explain the Pythagorean Theorem.
Mermin replies: Understanding the Now was not the main purpose of my March 2014 commentary. My primary point was that a new way of resolving the puzzles and paradoxes of quantum mechanics, called QBism by Christopher Fuchs and Rüdiger Schack, was also able to settle that longstanding problem in classical physics. After my earlier commentary on QBism (Physics Today, July 2012, page 8), I was concerned that many of the letters about it (December 2012, page 8) had sounded the same theme: that the letter writer’s own way of looking at quantum mechanics was already perfectly satisfactory. I hoped that by applying QBist thinking to a strictly classical problem, I could disengage those readers from their favorite interpretation of quantum mechanics and help them think about QBism on its own merits. I recently emphasized in Nature11. N. D. Mermin, Nature 507, 421 (2014). https://doi.org/10.1038/507421a that QBism sheds light on classical physics, too, but there I discussed quantum applications of QBism as well as classical applications (“CBism”).The letters here all address only the problem of the Now but not the fact that I deal with it through a classical application of QBism. While I’m disappointed that they say nothing about QBism or CBism, I’m pleased that they all agree that the problem of the Now is indeed a problem. Not everybody does.Berge Tatian and B. K. Ridley both criticize me for taking the Now to be a point. But I don’t. I call the Now an event “whose duration and location are restricted enough that it can usefully be represented as a point in space and time.” I say that “the events we experience are complex, extended entities” and that “to represent our actual experiences as a collection of mathematical points … is a brilliant strategic simplification, but we ought not to confuse a cartoon … with the experience itself.”Ridley’s comment “Nor could one person’s Now be exactly the same as another’s” suggests that I say it could be. What I do say is “The commonality of my Now and your Now whenever we are together requires that our Nows must coincide at each of two consecutive meetings.” “Commonality” or “coincide” mean only that our two private Now experiences happen at a common place and time, not that they are identical. Indeed, the personal experiences of different people are incomparable, except through the imperfect medium of language.I agree with James Hartle on much in the paper he cites, but we have important differences. He takes spacetime to be objective and fundamental; I take it to be an abstract tool used by an agent to organize her experience. He uses the notion of a point in spacetime uncritically; I regard it as an approximate representation of an agent’s spatially and temporally extended experience. He takes an agent’s experience to be an objective property of the agent, like the contents of a register. I take an agent’s experience to be private and self-evident to that agent and to be the fundamental basis for her inference of an external world; the experience of each agent plays a special role for that—and only that—agent, analogous to the special role played by “the classical domain” in the quantum mechanics of Lev Landau and Evgeny Lifshitz.Rudolf Peierls wrote to John Bell in 1980, “In my view, a description of the laws of physics consists in giving us a set of correlations between successive observations. By observations I mean … what our senses can experience. That we have senses and can experience such sensations is an empirical fact, which has not been deduced (and in my opinion cannot be deduced) from current physics.”22. S. Lee, ed., Sir Rudolf Peierls: Selected Private and Scientific Correspondence, vol. 2, World Scientific, River Edge, NJ (2009), p. 807. If “us” is expanded to “each of us,” then nobody has ever put QBism and CBism more concisely than that.REFERENCESSection:ChooseTop of pageREFERENCES <<CITING ARTICLES1. N. D. Mermin, Nature 507, 421 (2014). https://doi.org/10.1038/507421a, Google ScholarCrossref, ISI2. S. Lee, ed., Sir Rudolf Peierls: Selected Private and Scientific Correspondence, vol. 2, World Scientific, River Edge, NJ (2009), p. 807. Google ScholarCrossref© 2014 American Institute of Physics.
This record. Whether a mass exists above which quantum superposition stops: registered readings confronted with the 2026 interference record, and a prediction dated to 2035. Reading path: Quantum mechanics, 2 of 2 · before: Born geometry, the Tsirelson bound and the singlet · all themes. Common introduction to all the records of this series. The content of this record follows below. Mathematical Materiality (MM) in brief. Why are the laws of physics the same everywhere and at every instant, even though nothing forces matter to obey them? MM proposes an answer: the laws are not rules imposed on matter from the outside; rather, matter itself is made of mathematical relations. And mathematics cannot change: a relation binds two distinct terms everywhere and always. In logic, nothing exists on its own: to exist means to be in relation to another logical entity, and outside this web of relations nothing can exist, not even other universes. This reading already meets experimental measurements at two points. First, the ratio of dark to ordinary matter is read as 3√3 = 5.196; when left free in the fits, Planck and DESI DR2 data give 5.21–5.22 (as registered in record 2.44, https://doi.org/10.5281/zenodo.23329080). Second, the acceleration scale of galaxies is read as a₀ = cH₀/6, the same scale that appears in Verlinde's emergent gravity: the computed value is 1.09–1.18 × 10⁻¹⁰ m/s² against the measured value of 1.20 (record 2.46.1, https://doi.org/10.5281/zenodo.23506655). Both readings were formulated with the measurements already known; the tests to check them against future data are registered in the same records. In the first logical nexus between space and time — which became homogeneous with Einstein — there is a non-local phase, made only of a pure logical distinction: space is not time. Between the two there is no relation of quantitative calculation, only the logical entity they have in common, which is needed to make them exist with respect to each other. This level is not distant: it is present in every phase of reality, in every place and at every instant. It shows in entanglement, where two distant systems remain correlated more strongly than any explanation based on separate properties and free of faster-than-light influences allows, without anything physically travelling between them. In this non-local phase the two distinct terms, space and time, share only one logical element, which we shall call their greatest common divisor (GCD), just as the numbers 3 and 5 have only 1 in common. The local world arises when a relation is counted through this GCD: its multiples and its fractions form a matrix. Seen by an observer inside the system like us, the multiples appear as time and the fractions as space. When that same multiple is directly contained in the matrix of the point of observation, it turns into mass — that is, into concrete matter as we usually understand it. There is therefore a close link between time and mass, in full agreement with Einstein's relativity, which ties mass to spacetime. The observed objects, while being logically autonomous entities in their local relations, enter into relation with the observer through a logical diffraction — that is, a decomposition into fractions — of the observer's matrix. This reading also gives a reason for the limit of the speed of light, which in Einstein's relativity is instead taken as a starting principle. The speed of light is the fixed exchange rate between the multiples and the fractions of the same GCD, meaning how much space a single step of time is worth: exceeding it would mean, for some observer, covering space without counting time, and thus leaving the local world. Logic always starts from a logical relative, and this exchange rate has nothing absolute in itself either: it is a necessary relative which, inside the system, becomes an absolute hypothesis, the same for everyone and impossible to exceed. Outside the system, by contrast, it has no justification at all, because outside there is only non-existence: justifying something requires another term to relate it to, and out there there is none. Asking for a reason from outside is like asking what lies north of the North Pole. The same theoretical framework that reads space, time, mass and the observed objects also admits quantum mechanics. The non-local phase and the mirror-like logical reciprocity between the two ends of the nexus give a reason for the probabilistic behaviour of matter. When the observed object is large enough, its mass fixes it in the matrix of the point of observation: for this reason we see it in one precise place and at one precise instant. When instead the object falls below a certain mass threshold, the principal GCD (that of the point of observation) and the GCD of the diffraction that generates the object are forced to become equal, reflecting each other as in a mirror. At that point the object no longer occupies a single point of the spacetime generated by the observer, but spreads over several possible positions, and we observe it as probability. It is like having to describe the number 10 using only the number 1: the only way is to distribute the 1 over ten positions, one for each unit of the 10. Today we know that even objects made of thousands of atoms behave in this way, and no experiment has yet found the boundary. The idea that the passage from quantum to classical behaviour depends on mass is close to that of Penrose and Diósi; in MM the threshold is the point where the two GCDs, the principal one and that of the local diffraction, become equal. For all these reasons the universe behaves like a simulation without really being one. It follows exact rules, made of counts, everywhere and at every instant, but no machine runs them and no programmer wrote them: a structure of mathematical relations does not need to be computed in order to exist. Tegmark (2008) had already noted that such a simulation need not be run, while Vazza (2025) estimated that running it physically on a machine would require resources that the very laws of the universe do not allow. This record develops one aspect of the theoretical framework. The work is shared openly so that it can be judged by as many readers as possible: anyone who knows someone interested can pass on this single link, valid for the whole framework (with an overview in English and Italian): https://doi.org/10.5281/zenodo.23535916. The entry page, with the complete list of all records, is https://doi.org/10.5281/zenodo.22667516. To cite the framework: Lanciano, U. (2026). The number 1 in front of the mirror: Mathematical Materiality from the nexus to the cosmos. Zenodo. https://doi.org/10.5281/zenodo.23535916. The records have been deposited since 5 August 2026, each with its own date, and are released under CC BY 4.0: anyone may use them by citing the source. Objections and errors found are welcome at riverantares@gmail.com, since Zenodo has no space for comments. This record. Standard quantum mechanics has no mass at which a superposition stops: large objects look classical because of decoherence, the environment that keeps recording where they are. Collapse models (Ghirardi–Rimini–Weber, CSL, Diósi–Penrose) add such a limit by hand. The framework says that a threshold exists: below a certain mass level the principal GCD of the point of observation and the GCD of the diffraction that generates the object become equal, and the object spreads over several positions (see the box above). The box gives no number. This deposit tries to give the threshold a number, or to say clearly why it cannot. A second part tests two later remarks of the author: the limit cannot be fixed as a mass, because a calculation of the «fractional distance» is missing; and large distant bodies stay fixed because their size makes up for the distance. It was announced in deposit 2.64 (DOI 10.5281/zenodo.23610071). The main result: for distance, the limit is not a mass alone. In the two readings of the fractional distance that hold, it is a ratio between the size that has to be told apart and the cell of the copies that do the telling, together with the number of copies; the readings of part one with a mass threshold at the Planck scale remain possible, untested, and that the two readings are one rule of the framework is a proposal. For the state of an object the copies are the photons and molecules the environment carries away, and one copy cannot tell apart two positions closer than about half a wavelength: a 16-microgram crystal stays quantum over 2 × 10⁻¹⁸ m because about 10³⁴ thermal copies would be needed to fix it. For the way a distant observer reads an object, the cell is the diffraction cell of the observer's aperture at the object's distance: two stars of almost the same mass fall on opposite sides (the disc of Sirius B, 1.02 solar masses at 2.64 pc, inferred from its radius, is 20–60 times smaller than the finest cell of today's interferometers; τ Ceti, 0.78 at 3.65 pc, and 61 Cyg A, 0.67–0.74 at 3.49 pc, are resolved): with this instrumental classification the sample does not support a mass-only limit growing with distance. Both rules coincide with known physics (decoherence and wave optics): the framework admits them; experiments put them to the test but cannot tell them apart from standard physics. Prior work, first. Decoherence: Joos and Zeh (1985), Zurek (2003), Schlosshauer (2019); measured against separation and number of photons: Chapman et al. (1995), Kokorowski et al. (2001), Hornberger et al. (2003), Hackermüller et al. (2004). Collapse models: Károlyházy (1966), Ghirardi, Rimini and Weber (1986), Diósi (1987, 1989), Pearle (1989), Ghirardi, Pearle and Rimini (1990), Penrose (1996), the review of Bassi et al. (2013), Howl, Penrose and Fuentes (2019). Tests and limits: Nimmrichter and Hornberger (2013), Romero-Isart (2011), Donadi et al. (2021), the Majorana Demonstrator (2022), Carlesso et al. (2022), Gasbarri et
I have been interested in quantum mechanics or, synonymously, quantum physics since the very days when I first learned about them as a student. The initial impression was that of an immense mathematical beauty of the formalism of quantum mechanics. The second impression was the fantastic agreement of the theoretical predictions of quantum physics for experiments, and the third and somewhat surprising impression was that there was widespread disagreement about the interpretation of the new theory.
Physics is not just mathematics. This seems trivial, but poses difficult and interesting questions. In this paper we analyse a particular discrepancy between non-relativistic quantum mechanics (QM) and `classical' (Newtonian) space and time (NST). We also suggest, but not discuss, the case of the relativistic QM. In this work, we are more concerned with the notion of space and its mathematical representation. The mathematics entails that any two spatially separated objects are necessarily \ita{different}, which implies that they are \ita{discernible} (in classical logic, identity is defined by means of indiscernibility) --- we say that the space is T2T_2T2​, or "Hausdorff". But when enters QM, sometimes the systems need to be taken as \ita{completely indistinguishable}, so that there is no way to tell which system is which, and this holds even in the case of fermions. But in the NST setting, it seems that we can always give an \ita{identity} to them by means of their individuation, which seems to be contra the physical situation, where individuation (isolation) does not entail identity (as we argue in this paper). Here we discuss this topic by considering a case study (that of two potentially infinite wells) and conclude that, taking into account the quantum case, that is, when physics enter the discussion, even NST cannot be used to say that the systems do have identity. This case study seems to be relevant for a more detailed discussion on the interplay between physical theories (such as quantum theory) and their underlying mathematics (and logic), in a simple way apparently never realized before.