Carlo Rovelli's ``relational interpretation'' of quantum mechanics tells us that our understanding of quantum states is limited to their interactions with other quantum states. This implies that we have no understanding of the symmetry properties of a state vector except when considered in relation to at least one other state vector. The SU(3)xSU(2)xU(1) symmetry of the Standard Model is derived from their interactions and therefore cannot necessarily be applied to a single state vector. Steven Weinberg showed that mixed density matrices can have symmetries that are not available to state vectors. We explore the symmetry properties of finite symmetry groups for mixed density matrices. Just as mixed density matrices can define mixed states that require multiple state vectors, we find that the symmetries of mixed density matrices define ``mixed symmetries'' that are similar in structure to the symmetry of the Standard Model. We explore the point group symmetries and show one that gives the Standard Model symmetry. With this symmetry, we can generalize the Pauli spin matrices to a set that has irreducible representations matching the Standard Model plus a dark matter candidate.
Carlo Rovelli's relational interpretation'' of quantum mechanics tells us that our understanding of quantum states is limited to their interactions with other quantum states. This implies that we have no understanding of the symmetry properties of a state vector except when considered in relation to at least one other state vector. The SU(3)xSU(2)xU(1) symmetry of the Standard Model is derived from their interactions and therefore cannot necessarily be applied to a single state vector. Steven Weinberg showed that mixed density matrices can have symmetries that are not available to state vectors. We explore the symmetry properties of finite symmetry groups for mixed density matrices. Just as mixed density matrices can define mixed states that require multiple state vectors, we find that the symmetries of mixed density matrices define mixed symmetries'' that are similar in structure to the symmetry of the Standard Model. We explore the point group symmetries and show one that gives the Standard Model symmetry. With this symmetry, we can generalize the Pauli spin matrices to a set that has irreducible representations matching the Standard Model plus a dark matter candidate.
Density matrices can be used to describe ensembles of particles in thermodynamic equilibrium. We assume that the density matrices are a more natural description of quantum objects than state vectors. This suggests that we generalize density matrices from the usual operators on a Hilbert space to be elements of an algebra. Using density matrix renormalization to change the temperature of the ensembles, we show how the choice of algebra determines the symmetry and particle content of these generalized density matrices. The symmetries are of the form SU(N)xSU(M)x...U(1). We propose that the Standard Model of elementary particles should include a dark matter SU(2) doublet.
In 2014 Steven Weinberg noted that quantum mechanics can avoid various difficulties, such as the many worlds hypothesis, by taking the quantum states to be density matrices without reference to state vectors. An immediate consequence of Weinberg's idea is that electron spin can be taken to follow SO(3) instead of SU(2). This radical departure from present understanding motivates our exploration of density matrices as a method of going beyond the Standard Model. An important tool for Standard Model calculations is the Feynman path integral formulation of quantum field theory. When the path integral is Wick rotated from time to imaginary time or temperature it becomes a method of cooling down density matrices. While this does not show that one goes beyond the Standard Model by Wick rotation, it does show a close relation between this method of cooling density matrices and quantum field theory of the Standard Model. We explore these ideas and exhibit toy models with particle content and symmetry similar to the Standard Model.
It is usually taken for granted that the natural mathematical framework for quantum mechanics is the theory of Hilbert spaces, where pure states of a quantum system correspond to complex vectors of unit length. These vectors can be combined to create more general states expressed in terms of positive semidefinite matrices of unit trace called density operators. A density operator tells us everything we know about a quantum system. In particular, it specifies a unique probability for any measurement outcome. Thus, to fully appreciate quantum mechanics as a statistical model for physical phenomena, it is necessary to understand the basic properties of its set of states. Studying the convex geometry of quantum states provides important clues as to why the theory is expressed most naturally in terms of complex amplitudes. At the very least, it gives us a new perspective into thinking about structure of quantum mechanics. \n \n \nThis thesis is concerned with the structure of quantum state space obtained from the geometry of the convex set of probability distributions for a special class of measurements called symmetric informationally complete (SIC) measurements. In this context, quantum mechanics is seen as a particular restriction of a regular simplex, where the state space is postulated to carry a symmetric set of states called SICs, which are associated with equiangular lines in a complex vector space. The analysis applies specifically to 3-dimensional quantum systems or qutrits, which is the simplest nontrivial case to consider according to Gleason's theorem. It includes a full characterization of qutrit SICs and includes specific proposals for implementing them using linear optics. The infinitely many qutrit SICs are classified into inequivalent families according to the Clifford group, where equivalence is defined by geometrically invariant numbers called triple products. The multiplication of SIC projectors is also used to define structure coefficients, which are convenient for elucidating some additional structure possessed by SICs, such as the Lie algebra associated with the operator basis defined by SICs, and a linear dependency structure inherited from the Weyl-Heisenberg symmetry. After describing the general one-to-one correspondence between density operators and SIC probabilities, many interesting features of the set of qutrits are described, including an elegant formula for its pure states, which reveals a permutation symmetry related to the structure of a finite affine plane, the exact rotational equivalence of different SIC probability spaces, the shape of qutrit state space defined by the radial distance of the boundary from the maximally mixed state, and a comparison of the 2-dimensional cross-sections of SIC probabilities to known results. Towards the end, the representation of quantum states in terms of SICs is used to develop a method for reconstructing quantum theory from the postulate of maximal consistency, and a procedure for building up qutrit state space from a finite set of points corresponding to a Hesse configuration in Hilbert space is sketched briefly.
We have generalised the concept of graph states to what we have called mixed graph states, which we define in terms of mixed graphs, that is graphs with both directed and undirected edges, as the density matrix stabilized by the associated stabilizer matrix defined by the mixed graph. We can interpret this matrix as a quantum object by making it part of a larger fully commuting matrix, i.e. where we choose the environment appropriately, and this will imply that our quantum object is a mixed state. We prove that, in the same way as (pure) graph states, the density matrix of a parent of mixed graph state can be written as sum of a few Pauli matrices, well defined from the mixed graph. We have proven that the set of matrices that appear in this sum is fully pair-wise commuting, and form a multiplicative group up to global constants, which is always of maximum size. Furthermore, the cardinality of the set depends solely of the miminum possible number of extension columns/rows, and the number of nodes of the mixed graph. We prove a formula for this cardinality. Finally, in the case of purely undirected graphs, this corresponds to the usual pure graph state. Also, we have developed a way of finding maximal commutative group of such Pauli matrices as a linear subspace problem, for any given mixed graph. We also have proven how the structure of maximal commutative groups is independent of the direction of the arrows of the mixed graph, and also of the undirected edges; this allows the simplification of the problem of finding these groups in general to finding them for a much smaller set of graphs.
If mathematics is the queen of science, then physics might well be the king. It successes are obvious. However, as a science, physics may have failed in one regard, and that is to explain what its basic concepts – such as state vectors, wavefunctions, and transformation matrices – actually represent. When studying quantum mechanics, it is, effectively, hard to keep up the initial enthusiasm, and those who branch out to other fields – which is most of us – quickly end up going through the motions only: we regurgitate models and equations and know how to solve the standard problems, so as to pass the exam, but then forget about them as soon as possible. This paper explores a very intuitive sentiment about the issue: the wavefunction is a rather ‘flat’ mathematical object – it is two-dimensional, basically – so it can’t do the trick, perhaps. In contrast, Maxwell’s equations have real vectors in them, which is why a deeper or more intuitive understanding of electromagnetism comes relatively easily. Indeed, when everything is said and done, we are just human beings living in three-dimensional space, and that is why vector equations (or systems of vector equations), as a mathematical tool, make sense to us. This paper further explores this sentiment. It also offers a way out by, predictably, presenting yet another possible physical interpretation of the wavefunction. More importantly (for the reviewer of this paper, at least), this paper offers a sensible response to the mainstream view that three-dimensional physical interpretations of the wavefunction cannot make any sense because of the weird 720° symmetry of the wavefunction when describing spin-1/2 particles (fermions or – for all practical purposes – electrons). The author does so by analyzing (1) Dirac’s belt trick more in detail – and what it implies in terms of the interaction between the observer and the object – as well as (2) Feynman’s derivation of the transformation matrices for spin-1/2 two-state systems.
A version of quantum theory is derived from a set of plausible assumptions related to the following general setting: For a given system there is a set of experiments that can be performed, and for each such experiment an ordinary statistical model is defined. The parameters of the single experiments are functions of a hyperparameter, which defines the state of the system. There is a symmetry group acting on the hyperparameters, and for the induced action on the parameters of the single experiment a simple consistency property is assumed, called permissibility of the parametric function. The other assumptions needed are rather weak. The derivation relies partly on quantum logic, partly on a group representation of the hyperparameter group, where the invariant spaces are shown to be in 1-1 correspondence with the equivalence classes of permissible parametric functions. Planck's constant only plays a role connected to generators of unitary group representations.