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Physics      John Harland and I talk about physics. How might we think of it in terms of "ways of figuring things out" and my overview of that? John and I were graduate students at UCSD in the math department where we received our Ph.D's. I have a B.A. in Physics and I think John does, too, but he certainly knows and thinks a lot more about physics than I do. I share my notes based on my understanding of ideas that John sparked or stated and I tried to make sense of in my system. 849


Measurement      We can always start fresh with a new measurement. Each measurement assumes a partial view, an interest in some part of the system. We don't need a complete description, but rather we tease out whatever part of reality that we are interested, even though it is dubious in the big picture, yet our point of view (say, particle or wave) can be successful, even though incomplete. Yet therefore we need to keep working with independent measurements. Analogously, in math we can start fresh with a new piece of paper, or in life we can give a person a new chance. 850

        Particle point of view

Particle point of view      Our measurement can take place within the frame of measurement. We have a natural frame of reference, for example, the center of mass. That center of mass can then be considered as balancing different masses, and integrating a system of masses, and ultimately defining a vector space. This is a static, spatial, nontemporal point of view. Every state has a location. We can speak of the state of a system. Analogously, in math we have a blank sheet with a natural frame, a center, a balance around that center, a polynomial algebra of constructions, and ultimately, a vector space where a basis makes explicit that every point can be the center. And in life, we can discard the unessential, presume only God, allow for both self and others, find harmony amongst our interests, and create a space for good Spirit. 851

        Wave point of view

Wave point of view       Our measurement can take place outside of our frames of measurement and thus link several such frames. This is a dynamic point of view where there is no distinction between the future and the past so that all is reversible. We can think of the wave in terms of where it starts and where it ends. It includes all paths between these two points. This is a deterministic, nonspatial point of view, which establishes time, an ideal continuum that is beyond the frames but thus relevant for us. Analogously, in math we may have a sequence of sheets, as with mathematical induction, some of which may be of ultimate importance, as with the extreme principle, thus allowing for boxing in with greatest lower bounds and least upper bounds, leading to limits that may transcend, go beyond what we can account for. Or in life, we can be open to care about everything, then care about our minds by which we care, then come up against our personal limits, then allow for an ideal (such as Jesus) that transcends our limits. 848

        Variational principle

Variational principle      Wikipedia: A variational principle is a scientific principle used within the calculus of variations, which develops general methods for finding functions which minimize or maximize the value of quantities that depends upon those functions. For example, to answer this question: "What is the shape of a chain suspended at both ends?" we can use the variational principle that the shape must minimize the gravitational potential energy. According to Cornelius Lanczos, any physical law which can be expressed as a variational principle describes an expression which is self-adjoint. These expressions are also called Hermitian. Such an expression describes an invariant under a Hermitian transformation. Felix Klein's Erlangen program attempted to identify such invariants under a group of transformations. In what is referred to in physics as Noether's theorem, the Poincaré group of transformations (what is now called a gauge group) for general relativity defines symmetries under a group of transformations which depend on a variational principle, or action principle. See: History of variational principles in physics and Brachistochrone, the curve of fastest descent907

          Fermat's principle      Wikipedia: In optics, Fermat's principle or the principle of least time is the principle that the path taken between two points by a ray of light is the path that can be traversed in the least time. This principle is sometimes taken as the definition of a ray of light. However, this version of the principle is not general; a more modern statement of the principle is that rays of light traverse the path of stationary optical length. Fermat's principle can be used to describe the properties of light rays reflected off mirrors, refracted through different media, or undergoing total internal reflection. It follows mathematically from Huygens' principle (at the limit of small wavelength), and can be used to derive Snell's law of refraction and the law of reflection.908

          Gauss' principle of least constraint      Wikipedia: The principle of least constraint is a least squares principle stating that the true motion of a mechanical system of N masses is the minimum of the quantity above for all trajectories satisfying any imposed constraints, where m-k, r-k and F-k represent the mass, position and applied forces of the kth mass. Gauss' principle is equivalent to D'Alembert's principle. The principle of least constraint is qualitatively similar to Hamilton's principle, which states that the true path taken by a mechanical system is an extremum of the action. However, Gauss' principle is a true (local) minimal principle, whereas the other is an extremal principle.904

          Hertz's principle of least curvature      Wikipedia: Hertz's principle of least curvature is a special case of Gauss' principle, restricted by the two conditions that there be no applied forces and that all masses are identical.905

          Principle of least action      Wikipedia: In physics, the principle of least action - or, more accurately, the principle of stationary action - is a variational principle that, when applied to the action of a mechanical system, can be used to obtain the equations of motion for that system. The principle led to the development of the Lagrangian and Hamiltonian formulations of classical mechanics. ... Maupertuis felt that "Nature is thrifty in all its actions" ... the action principle is not localized to a point; rather, it involves integrals over an interval of time and (for fields) an extended region of space. Moreover, in the usual formulation of classical action principles, the initial and final states of the system are fixed, e.g., Given that the particle begins at position x1 at time t1 and ends at position x2 at time t2, the physical trajectory that connects these two endpoints is an extremum of the action integral. In particular, the fixing of the final state appears to give the action principle a teleological character which has been controversial historically.903

        Scientific method

Scientific method      We design experiments that link together, tangle together the two incomplete outlooks of space and time, single frame and multiple frames, particle and wave, static and dynamic, free and deterministic. This is because each experiment presumes an experimenter and thus takes place both within a frame of measurement and beyond it. Each experiment includes a hypothesis, an experimental test, and an appraisal of the results. Analogously, in math, given a constraint, we extend its domain to include a new domain, we stitch them together by presuming continuity, and we relate the two applications by superimposing them, yielding a more general constraint. In life, we take a stand, follow through and reflect. 852

     Physics experiments      Wikipedia documents more than 200 physics experiments. The experiment page gives examples of how the scientific method is applied. There is also a page listing key physics experiments.877


Hypothesis      Wikipedia: A hypothesis is a proposed explanation for a phenomenon. The term derives from the Greek, hypotithenai meaning "to put under" or "to suppose." For a hypothesis to be put forward as a scientific hypothesis, the scientific method requires that one can test it. Scientists generally base scientific hypotheses on previous observations that cannot satisfactorily be explained with the available scientific theories.913

          Mapping observables and observations      Edward Cherlin, 2011.04.24: I like your cycle of scientific method: take a stand (hypothesize), follow through (experiment), reflect (conclude), although I find that there is more to it. It has been pointed out that a hypothesis must include a model (usually mathematical) and a mapping between parts of the model (observables) and observations, including experiments. 911

        Experimental design

Experimental design      Wikipedia: In general usage, design of experiments (DOE) or experimental design is the design of any information-gathering exercises where variation is present, whether under the full control of the experimenter or not.914

          Design experiments to rule models in or out      Edward Cherlin, 2011.04.24: But that is not enough. We must also think of other possible models, and design experiments to rule them in or out, and we must think of every possible experiment that could invalidate our model. This is the great service that Einstein performed for Quantum Mechanics, because he disliked it so much. Every time he thought he had found a contradiction or something nonsensical in the math, the lab boys verified that it really worked that way in experiments. 912


Observe      915

          Extend our senses with improved scientific instruments      Edward Cherlin, 2011.04.24: We know that our models are reasonably complete and accurate at best in the areas we have been able to observe, and that every new addition to our senses in improved scientific instruments, going back to Galileo's first telescope, reveals surprises like the mountains of the moon, the constancy of the speed of light (interferometers) or neutrino oscillation (simple but quite large neutrino detectors).916

        Sets of objects exist

Sets of objects exist      We can fuse the particle and wave points of view to work with a partial reality. For example, we can talk about a banana, an apple or an orange as well defined objects that mean something more than a random assemblage of half of the atoms in a banana with half of the atoms in the apple. We are then no longer talking about the symmetry of the universe. John says: A symmetry group commutes with the underlying symmetry of a particular phenomenon, its spacial symmetry, as the set of possible transformations, possible futures. Previously, we worked with the entire universe, and if we translated it abstractly in a symmetry group and then ran things forward in the translated frame, it was exactly isomorphic to what it would have been if we had not translated it. But now, as we want to compartmentalize the universe, then the price to pay is that we are not translating by the full symmetry group, but only by some part of it. This is analogous to having a tensor product and considering only one component, so that we have partial symmetry. We are going to treat one part of the universe as a compartment. This gives the reality to the symmetry group because otherwise it couldn't be measured. Andrius: This compartmentalization is also what allows us to define entropy and the one-way direction of time, which says that states drift away from deliberateness, which is expressed by the compartmentalization. Compartmentalization also indicates the philosophical gaps or boundaries that allow for measurement to take place, allow for objectivity, the separation of the observer and the observed. Analogously, in math we have symmetry groups, and in life we have meanings, the essence of what we wish to say, which me take to be absolute, in cases where we have fundamental agreement. 847


Conservation      998

          Noether's Theorem      Wikipedia: Noether's (first) theorem states that any differentiable symmetry of the action of a physical system has a corresponding conservation law. ... The action of a physical system is the integral over time of a Lagrangian function (which may or may not be an integral over space of a Lagrangian density function), from which the system's behavior can be determined by the principle of least action. Noether's theorem has become a fundamental tool of modern theoretical physics and the calculus of variations. ... For illustration, if a physical system behaves the same regardless of how it is oriented in space, its Lagrangian is rotationally symmetric; from this symmetry, Noether's theorem shows the angular momentum of the system must be conserved. ... Noether's theorem is important, both because of the insight it gives into conservation laws, and also as a practical calculational tool. It allows researchers to determine the conserved quantities from the observed symmetries of a physical system. Conversely, it allows researchers to consider whole classes of hypothetical Lagrangians to describe a physical system. For illustration, suppose that a new field is discovered that conserves a quantity X. Using Noether's theorem, the types of Lagrangians that conserve X because of a continuous symmetry can be determined, and then their fitness judged by other criteria.981

     Conservation law      Wikipedia: Noether's (first) theorem states that any differentiable symmetry of the action of a physical system has a corresponding conservation law. ... The action of a physical system is the integral over time of a Lagrangian function (which may or may not be an integral over space of a Lagrangian density function), from which the system's behavior can be determined by the principle of least action. Noether's theorem has become a fundamental tool of modern theoretical physics and the calculus of variations. ... For illustration, if a physical system behaves the same regardless of how it is oriented in space, its Lagrangian is rotationally symmetric; from this symmetry, Noether's theorem shows the angular momentum of the system must be conserved. ... Noether's theorem is important, both because of the insight it gives into conservation laws, and also as a practical calculational tool. It allows researchers to determine the conserved quantities from the observed symmetries of a physical system. Conversely, it allows researchers to consider whole classes of hypothetical Lagrangians to describe a physical system. For illustration, suppose that a new field is discovered that conserves a quantity X. Using Noether's theorem, the types of Lagrangians that conserve X because of a continuous symmetry can be determined, and then their fitness judged by other criteria.982

          Conservation of angular momentum      Wikipedia: In a closed system angular momentum is constant. This conservation law mathematically follows from continuous directional symmetry of space (no direction in space is any different from any other direction). If a planet is found to rotate slower than expected, then astronomers suspect that the planet is accompanied by a satellite, because the total angular momentum is shared between the planet and its satellite in order to be conserved. The conservation of angular momentum is used extensively in analyzing what is called central force motion. If the net force on some body is directed always toward some fixed point, the center, then there is no torque on the body with respect to the center, and so the angular momentum of the body about the center is constant. 985

          Conservation of color charge      Wikipedia: In particle physics, color charge is a property of quarks and gluons that is related to the particles' strong interactions in the theory of quantum chromodynamics (QCD). ... Color charge is conserved, but the book-keeping involved in this is more complicated than just adding up the charges, as is done in quantum electrodynamics.987

          Conservation of electric charge      Wikipedia: In physics, charge conservation is the principle that electric charge can neither be created nor destroyed. The quantity of electric charge, the amount of positive charge minus the amount of negative charge in the universe, is always conserved. Charge conservation is a physical law that states that the change in the amount of electric charge in any volume of space is exactly equal to the amount of charge flowing into the volume minus the amount of charge flowing out of the volume. In essence, charge conservation is an accounting relationship between the amount of charge in a region and the flow of charge into and out of that region.986

          Conservation of linear momentum      Wikipedia: The law of conservation of linear momentum is a fundamental law of nature, and it states that if no external force acts on a closed system of objects the momentum of the closed system remains constant. One of the consequences of this is that the center of mass of any system of objects will always continue with the same velocity unless acted on by a force from outside the system. Conservation of momentum is a mathematical consequence of the homogeneity (shift symmetry) of space (position in space is the canonical conjugate quantity to momentum). So, momentum conservation can be philosophically stated as "nothing depends on location per se".984

          Conservation of mass-energy      Wikipedia: In physics, mass-energy equivalence is the concept that the mass of a body is a measure of its energy content. In this concept the total internal energy E of a body at rest is equal to the product of its rest mass m and a suitable conversion factor to transform from units of mass to units of energy. If the body is not stationary relative to the observer then account must be made for relativistic effects where m is given by the relativistic mass and E the relativistic energy of the body.983

          Conservation of probability density      Wikipedia: In quantum mechanics, the probability current (sometimes called probability flux) is a concept describing the flow of probability density. In particular, if one pictures the probability density as an inhomogeneous fluid, then the probability current is the rate of flow of this fluid (the density times the velocity). ... This is the conservation law for probability in quantum mechanics.... 989

          Conservation of weak isospin      Wikipedia: In particle physics, weak isospin is a quantum number relating to the weak interaction, and parallels the idea of isospin under the strong interaction. ... The weak isospin conservation law relates the conservation of T3; all weak interactions must preserve T3. It is also conserved by the other interactions and is therefore a conserved quantity in general.988

          CPT symmetry      Wikipedia: CPT symmetry is a fundamental symmetry of physical laws under transformations that involve the inversions of charge, parity, and time simultaneously. ... The CPT theorem requires the preservation of CPT symmetry by all physical phenomena. It assumes the correctness of quantum laws and Lorentz invariance. Specifically, the CPT theorem states that any Lorentz invariant local quantum field theory with a Hermitian Hamiltonian must have CPT symmetry.990

          Lorentz symmetry      Wikipedia: In standard physics, Lorentz symmetry is "the feature of nature that says experimental results are independent of the orientation or the boost velocity of the laboratory through space".991

        Experiments and Theory

Experiments and Theory      Experiments (specific instances) and theory (general laws) are related as level and metalevel. There is a dualism. But, actually, they are not qualitatively different. For an experiment is never a single instance, but always a set of instances, for it must be reproducible. In that sense, every experiment has a generality, just as a theory does. These two levels can be conflated, which is how we view Reality, where the facts and the laws coincide. Or the levels can be distinct to various degrees, and completely distinct when the facts are considered to be applications of the rules. Andrius: There are four possible levels (Whether, What, How, Why) for relating facts and rules, and there are six pairs of possible levels, with the wider level reserved for the rules (the imagined observer) and the narrower level reserved for the facts (the imagined observed). Analogously, in Math we have the mathematical structures that describe (on paper) our problem, and we have the mathematical structures that describe how our minds are solving the problem. The two are conflated as Truth. They are distinguished as Model, Implication and Variable. There are six kinds of variables. In life, we have four ways of distinguishing the truths of the heart and the world, given by Whether, What, How, Why we know what we know, and there are six ways that the two truths may be related. 853

        Symmetry breaking

Symmetry breaking      1029

          Explicit symmetry breaking      Wikipedia: Explicit symmetry breaking indicates a situation where the dynamical equations are not manifestly invariant under the symmetry group considered. This means, in the Lagrangian (Hamiltonian) formulation, that the Lagrangian (Hamiltonian) of the system contains one or more terms explicitly breaking the symmetry. Such terms can have different origins:

  • Symmetry-breaking terms may be introduced into the theory by hand on the basis of theoretical/experimental results, as in the case of the quantum field theory of the weak interactions, which is expressly constructed in a way that manifestly violates mirror symmetry or parity.
  • Symmetry-breaking terms may appear in the theory because of quantum-mechanical effects. One reason for the presence of such terms — known as "anomalies" — is that in passing from the classical to the quantum level, because of possible operator ordering ambiguities for composite quantities such as Noether charges and currents, it may be that the classical symmetry algebra (generated through the Poisson bracket structure) is no longer realized in terms of the commutation relations of the Noether charges. Moreover, the use of a "regulator" (or "cut-off") required in the renormalization procedure to achieve actual calculations may itself be a source of anomalies.
  • Finally, symmetry-breaking terms may appear because of non-renormalizable effects. Physicists now have good reasons for viewing current renormalizable field theories as effective field theories, that is low-energy approximations to a deeper theory (each effective theory explicitly referring only to those particles that are of importance at the range of energies considered). The effects of non-renormalizable interactions (due to the heavy particles not included in the theory) are small and can therefore be ignored at the low-energy regime. It may then happen that the coarse-grained description thus obtained possesses more symmetries than the deeper theory. That is, the effective Lagrangian obeys symmetries that are not symmetries of the underlying theory. These "accidental" symmetries, as Weinberg has called them, may then be violated by the non-renormalizable terms arising from higher mass scales and suppressed in the effective Lagrangian.

          Spontaneous symmetry breaking      Wikipedia: Spontaneous symmetry breaking is the process by which a system described in a theoretically symmetrical way ends up in an apparently asymmetric state. For spontaneous symmetry breaking to occur, there must be a system in which there are several equally likely outcomes. The system as a whole is therefore symmetric with respect to these outcomes (if we consider any two outcomes, the probability is the same). However, if the system is sampled (i.e. if the system is actually used or interacted with in any way), a specific outcome must occur. Though we know the system as a whole is symmetric, we also see that it is never encountered with this symmetry, only in one specific state. Because one of the outcomes is always found with probability 1, and the others with probability 0, they are no longer symmetric. Hence, the symmetry is said to be spontaneously broken in that theory. Nevertheless, the fact that each outcome is equally likely is a reflection of the underlying symmetry, which is thus often dubbed "hidden symmetry", and has crucial formal consequences, such as the presence of Nambu-Goldstone bosons. When a theory is symmetric with respect to a symmetry group, but requires that one element of the group is distinct, then spontaneous symmetry breaking has occurred. The theory must not dictate which member is distinct, only that one is. 1027

               A ball on top of a hill      Wikipedia: A common example to help explain this phenomenon is a ball sitting on top of a hill. This ball is in a completely symmetric state. However, its state is unstable: the slightest perturbing force will cause the ball to roll down the hill in some particular direction. At that point, symmetry has been broken, because the direction in which the ball rolled has a visible feature that distinguishes it from all other directions. The "choice" of direction is immaterial, however, as any other direction would do, i.e. the system is still bearing traces of the symmetry of the hill, albeit now somewhat less apparent.1028

          Symmetry breaking      Wikipedia: Symmetry breaking in physics describes a phenomenon where (infinitesimally) small fluctuations acting on a system which is crossing a critical point decide the system's fate, by determining which branch of a bifurcation is taken. To an outside observer unaware of the fluctuations (or "noise"), the choice will appear arbitrary. This process is called symmetry "breaking", because such transitions usually bring the system from a disorderly state into one of two definite states. Since disorder is more symmetric, in the sense that small variations to it don't change its overall appearance, the symmetry gets "broken". Symmetry breaking is supposed to play a major role in pattern formation.1025

               Phase transition      Wikipedia: A phase transition is the transformation of a thermodynamic system from one phase or state of matter to another. A phase of a thermodynamic system and the states of matter have uniform physical properties. During a phase transition of a given medium certain properties of the medium change, often discontinuously, as a result of some external condition, such as temperature, pressure, and others. For example, a liquid may become gas upon heating to the boiling point, resulting in an abrupt change in volume. The measurement of the external conditions at which the transformation occurs is termed the phase transition point. The term is most commonly used to describe transitions between solid, liquid and gaseous states of matter, in rare cases including plasma.1030


Forces      Forces are expressions of causality, of the relationship between before and after. They allow for a system to break down into subsystems. Rules are applied in six different ways to link states before and after an event. In math, these are the kinds of subsystems that Implication forms. In life, these are the ways that we visualize. 854


Identity      John says that the whole concept of identity is very flexible in physics, even shaky. What is a storm? Where does it go when it no longer exists? What is the reality of the kludge? You can't know where things come and go in physics. Once you write about it you are not talking about physics in the big picture. This is where there is slack and wiggle room. Identity is what allows for eternal nature. We acknowledge the storm and, though it comes and goes, yet its eternal nature jumps over into us. This is eternal life. In math, there is Context, which can change the meaning entirely, as when 10+4 turns out to be 2'o-clock on a clock. In life, if we obey God, then we can imagine God's point of view, and that opens up incredible freedom. 855

        Other ways: Physics

Other ways: Physics      862

          Classifying      1031

               Ehrenfest classification of phase transitions      Wikipedia: Paul Ehrenfest classified phase transitions based on the behavior of the thermodynamic free energy as a function of other thermodynamic variables. Under this scheme, phase transitions were labeled by the lowest derivative of the free energy that is discontinuous at the transition. First-order phase transitions exhibit a discontinuity in the first derivative of the free energy with respect to some thermodynamic variable.[1] The various solid/liquid/gas transitions are classified as first-order transitions because they involve a discontinuous change in density, which is the first derivative of the free energy with respect to chemical potential. Second-order phase transitions are continuous in the first derivative (the order parameter, which is the first derivative of the free energy with respect to the external field, is continuous across the transition) but exhibit discontinuity in a second derivative of the free energy.[1] These include the ferromagnetic phase transition in materials such as iron, where the magnetization, which is the first derivative of the free energy with the applied magnetic field strength, increases continuously from zero as the temperature is lowered below the Curie temperature. The magnetic susceptibility, the second derivative of the free energy with the field, changes discontinuously. Under the Ehrenfest classification scheme, there could in principle be third, fourth, and higher-order phase transitions.1032

               Modern classification of phase transitions      Wikipedia: In the modern classification scheme, phase transitions are divided into two broad categories, named similarly to the Ehrenfest classes:

  • First-order phase transitions are those that involve a latent heat. During such a transition, a system either absorbs or releases a fixed (and typically large) amount of energy. During this process, the temperature of the system will stay constant as heat is added: the system is in a "mixed-phase regime" in which some parts of the system have completed the transition and others have not. Familiar examples are the melting of ice or the boiling of water (the water does not instantly turn into vapor, but forms a turbulent mixture of water and vapor bubbles).
  • Second-order phase transitions are also called continuous phase transitions. They are characterized by a divergent susceptibility, an infinite correlation length, and a power-law decay of correlations near criticality. Examples of second-order phase transitions are the ferromagnetic transition, superconductor and the superfluid transition. Lev Landau gave a phenomenological theory of second order phase transitions.
  • Several transitions are known as the infinite-order phase transitions. They are continuous but break no symmetries. The most famous example is the Kosterlitz-Thouless transition in the two-dimensional XY model. Many quantum phase transitions in two-dimensional electron gases belong to this class.
  • The liquid-glass transition is observed in many polymers and other liquids that can be supercooled far below the melting point of the crystalline phase. This is atypical in several respects. It is not a transition between thermodynamic ground states: it is widely believed that the true ground state is always crystalline. Glass is a quenched disorder state, and its entropy, density, and so on, depend on the thermal history. Therefore, the glass transition is primarily a dynamic phenomenon: on cooling a liquid, internal degrees of freedom successively fall out of equilibrium. However, there is a longstanding debate whether there is an underlying second-order phase transition in the hypothetical limit of infinitely long relaxation times.

                Noting a case that a classification does not account for      Wikipedia: Though useful, Ehrenfest's classification has been found to be an inaccurate method of classifying phase transitions, for it does not take into account the case where a derivative of free energy diverges (which is only possible in the thermodynamic limit). For instance, in the ferromagnetic transition, the heat capacity diverges to infinity.1034

           Thought experiments in Physics      Wikipedia lists thought experiments in Physics: Galileo's ship (classical relativity principle) 1632, Galileo's Leaning Tower of Pisa experiment (rebuttal of Aristotelian Gravity), GHZ experiment (quantum mechanics), Heisenberg's microscope (quantum mechanics), Kepler's Dream (change of point of view as support for the Copernican hypothesis), Ladder paradox (special relativity), Laplace's demon, Maxwell's demon (thermodynamics) 1871, Monkey and the Hunter, The (gravitation), Moving magnet and conductor problem, Newton's cannonball (Newton's laws of motion), Popper's experiment (quantum mechanics), Quantum pseudo telepathy (quantum mechanics), Quantum suicide (quantum mechanics), Schrödinger's cat (quantum mechanics), Sticky bead argument (general relativity), Renninger negative-result experiment (quantum mechanics), Twin paradox (special relativity), Wheeler's delayed choice experiment (quantum mechanics), Wigner's friend (quantum mechanics)856

                     EPR paradox      Wikipedia: The original paper describes what happens to "two systems I and II, which we permit to interact ...", and, after some time, "we suppose that there is no longer any interaction between the two parts." In the words of Kumar (2009), it has "Two particles, A and B, [which] interact briefly and then move off in opposite directions." According to Heisenberg's uncertainty principle, it is impossible to measure both the momentum and the position of particle B, say, exactly. However, it is possible to measure the exact position of particle A and the exact momentum of particle B. By calculation, therefore, with the exact position of particle A known, the exact position of particle B can be known.844

                     Bell's spaceship paradox (special relativity)      Wikipedia: In Bell's version of the thought experiment, two spaceships, which are initially at rest in some common inertial reference frame, are connected by a taut string. At time zero in the common inertial frame, both spaceships start to accelerate, in such a way that they remain a fixed distance apart as viewed from the original rest frame. Question: Does the string break (i.e. does the distance between the two spaceships increase in the reference frame of either spaceship)?838

                     Brownian ratchet      Wikipedia: Richard Feynman's "perpetual motion" machine that does not violate the second law and does no work at thermal equilibrium839

                     Bucket argument      Wikipedia: Argues that space is absolute, not relational. Isaac Newton's rotating bucket argument (also known as "Newton's bucket") was designed to demonstrate that true rotational motion cannot be defined as the relative rotation of the body with respect to the immediately surrounding bodies. It is one of five arguments from the "properties, causes, and effects" of true motion and rest that support his contention that, in general, true motion and rest cannot be defined as special instances of motion or rest relative to other bodies, but instead can be defined only by reference to absolute space. Newton discusses a bucket filled with water hung by a cord. If the cord is twisted up tightly on itself and then the bucket is released, it begins to spin rapidly, not only with respect to the experimenter, but also in relation to the water it contains. Although the relative motion at this stage is the greatest, the surface of the water remains flat, indicating that the parts of the water have no tendency to recede from the axis of relative motion, despite proximity to the pail. Eventually, as the cord continues to unwind, the surface of the water assumes a concave shape as it acquires the motion of the bucket spinning relative to the experimenter. This concave shape shows that the water is rotating, despite the fact that the water is at rest relative to the pail. In other words, it is not the relative motion of the pail and water that causes concavity of the water, contrary to the idea that motions can only be relative, and that there is no absolute motion.840

                     Double-slit experiment      Wikipedia: The double-slit experiment or Thomas Young's experiment involves particle beams or coherent waves passing through two closely-spaced slits, after which in many circumstances they are found to interfere with each other. In quantum mechanics the double-slit experiment demonstrates the inseparability of the wave and particle natures of light and other quantum particles (wave–particle duality).841

                     Einstein's box      Wikipedia: Einstein considers a box (called Einstein's box; see figure) containing electromagnetic radiation and a clock which controls the opening of a shutter which covers a hole made in one of the walls of the box. The shutter uncovers the hole for a time Δt which can be chosen arbitrarily. During the opening, we are to suppose that a photon, from among those inside the box, escapes through the hole. In this way a wave of limited spatial extension has been created, following the explanation given above. In order to challenge the indeterminacy relation between time and energy, it is necessary to find a way to determine with adequate precision the energy that the photon has brought with it. At this point, Einstein turns to his celebrated relation between mass and energy of special relativity: E = mc2. From this it follows that knowledge of the mass of an object provides a precise indication about its energy. The argument is therefore very simple: if one weighs the box before and after the opening of the shutter and if a certain amount of energy has escaped from the box, the box will be lighter.... Bohr showed that, in order for Einstein's experiment to function, the box would have to be suspended on a spring in the middle of a gravitational field. ... 843

                     Elitzur-Vaidman bomb-tester      Wikipedia: Consider a collection of bombs, some of which are duds. Suppose these bombs carry a certain perfect property: usable bombs have a photon-triggered sensor which will absorb a photon and detonate. Dud bombs have a malfunctioning sensor which will not interfere with any photons.[4] The problem is how to separate at least some of the usable bombs from the duds. A solution is for the sorter to use a mode of observation known as counterfactual measurement, which relies on properties of quantum mechanics842

                     Feynman sprinkler      Wikipedia: The question of which way an inverse sprinkler would turn (so, with the sprinkler sucking the water in rather than pumping it out) was the subject of an intense and remarkably long-lived debate.845

                     Galileo: combine masses, large and small      Galileo describes in Discorsi e dimostrazioni matematiche (1628) (literally, 'Mathematical Discourses and Demonstrations') a thought experiment that refutes Aristotle's theory:

      • If then we take two bodies whose natural speeds are different, it is clear that on uniting the two, the more rapid one will be partly retarded by the slower, and the slower will be somewhat hastened by the swifter. Do you not agree with me in this opinion?
      • Simplicio. You are unquestionably right.
      • Salviati. But if this is true, and if a large stone moves with a speed of, say, eight while a smaller moves with a speed of four, then when they are united, the system will move with a speed less than eight; but the two stones when tied together make a stone larger than that which before moved with a speed of eight. Hence the heavier body moves with less speed than the lighter; an effect which is contrary to your supposition. Thus you see how, from your assumption that the heavier body moves more rapidly than ' the lighter one, I infer that the heavier body moves more slowly.