:

## Computer Algebra in Theoretical Physics: the IOP webinar

Maple

Below is the worksheet with the whole material presented yesterday in the webinar, “Applying the power of computer algebra to theoretical physics”, broadcasted by the “Institute of Physics” (IOP, England). The material was very well received, rated 4.5 out of 5 (around 30 voters among the more than 300 attendants), and generated a lot of feedback. The webinar was recorded so that it is possible to watch it (for free, of course, click the link above, it will ask you for registration, though, that’s how IOP works).

Anyway, you can reproduce the presentation with the worksheet below (mw file linked at the end, or the corresponding pdf also linked with all the input lines executed). As usual, to reproduce the input/output you need to have installed the latest version of Physics, available in the Maplesoft R&D Physics webpage.

 Why computer algebra? ... and why computer algebra? We can concentrate more on the ideas instead of on the algebraic manipulations   We can extend results with ease   We can explore the mathematics surrounding a problem   We can share results in a reproducible way
 Representation issues that were preventing the use of computer algebra in Physics Notation and related mathematical methods that were missing: coordinate free representations for vectors and vectorial differential operators, covariant tensors distinguished from contravariant tensors, functional differentiation, relativity differential operators and sum rule for tensor contracted (repeated) indices Bras, Kets, projectors and all related to Dirac's notation in Quantum Mechanics   Inert representations of operations, mathematical functions, and related typesetting were missing:   inert versus active representations for mathematical operations ability to move from inert to active representations of computations and viceversa as necessary hand-like style for entering computations and textbook-like notation for displaying results   Key elements of the computational domain of theoretical physics were missing:   ability to handle products and derivatives involving commutative, anticommutative and noncommutative variables and functions ability to perform computations taking into account custom-defined algebra rules of different kinds (commutator, anticommutator and bracket rules, etc.)

Examples

 The Maple computer algebra environment

Classical Mechanics

 Inertia tensor for a triatomic molecule

Classical Field Theory

 *The field equations for the  model
 *Maxwell equations departing from the 4-dimensional Action for Electrodynamics
 *The Gross-Pitaevskii field equations for a quantum system of identical particles

Quantum mechanics

 *The quantum operator components of   satisfy
 Quantization of the energy of a particle in a magnetic field

Unitary Operators in Quantum Mechanics

 *Eigenvalues of an unitary operator and exponential of Hermitian operators

Properties of unitary operators

Consider two set of kets  and , each of them constituting a complete orthonormal basis of the same space.

One can always build an unitary operator  that maps one basis to the other, i.e.:

 *Verify that  implies on
 *Show that is unitary
 *Show that the matrix elements of  in the  and   basis are equal
 Show that  and have the same spectrum

Schrödinger equation and unitary transform

Consider a ket  that solves the time-dependant Schrödinger equation:

and consider

,

where  is a unitary operator.

Does  evolves according a Schrödinger equation

and if yes, which is the expression of ?

 Solution

Translation operators using Dirac notation

In this section, we focus on the operator

 Settings
 The Action (translation) of the operator  on a ket
 Action of  on an operator

General Relativity

 *Exact Solutions to Einstein's Equations

*"Physical Review D" 87, 044053 (2013)

Given the spacetime metric,

a) Compute the Ricci and Weyl scalars

b) Compute the trace of

where  is some function of the radial coordinate,  is the Ricci tensor,  is the covariant derivative operator and  is the stress-energy tensor

c) Compute the components of the traceless part of   of item b)

d) Compute an exact solution to the nonlinear system of differential equations conformed by the components of   obtained in c)

Background: paper from February/2013, "Withholding Potentials, Absence of Ghosts and Relationship between Minimal Dilatonic Gravity and f(R) Theories", by P. Fiziev.

 a) The Ricci and Weyl scalars
 b) The trace of
 b) The components of the traceless part of
 c) An exact solution for the nonlinear system of differential equations conformed by the components of

*The Equivalence problem between two metrics

From the "What is new in Physics in Maple 2016" page:

 In the Maple PDEtools package, you have the mathematical tools - including a complete symmetry approach - to work with the underlying [Einstein’s] partial differential equations. [By combining that functionality with the one in the Physics and Physics:-Tetrads package] you can also formulate and, depending on the metrics also resolve, the equivalence problem; that is: to answer whether or not, given two metrics, they can be obtained from each other by a transformation of coordinates, as well as compute the transformation.
 Example from: A. Karlhede, "A Review of the Geometrical Equivalence of Metrics in General Relativity", General Relativity and Gravitation, Vol. 12, No. 9, 1980
 *Equivalence for Schwarzschild metric (spherical and Krustal coordinates)

Tetrads and Weyl scalars in canonical form

Generally speaking a canonical form is obtained using transformations that leave invariant the tetrad metric in a tetrad system of references, so that theWeyl scalars are fixed as much as possible (conventionally, either equal to 0 or to 1).

Bringing a tetrad in canonical form is a relevant step in the tackling of the equivalence problem between two spacetime metrics.

The implementation is as in "General Relativity, an Einstein century survey", edited by S.W. Hawking (Cambridge) and W. Israel (U. Alberta, Canada), specifically Chapter 7 written by S. Chandrasekhar, page 388:

 Residual invariance Petrov type I 0 1 0 none Petrov type II 0 0 1 0 none Petrov type III 0 0 0 1 0 none Petrov type D 0 0 0 0 remains invariant under rotations of Class III Petrov type N 0 0 0 0 1 remains invariant under rotations of Class II

The transformations (rotations of the tetrad system of references) used are of Class I, II and III as defined in Chandrasekar's chapter - equations (7.79) in page 384, (7.83) and (7.84) in page 385. Transformations of Class I can be performed with the command Physics:-Tetrads:-TransformTetrad using the optional argument nullrotationwithfixedl_, of Class II using nullrotationwithfixedn_ and of Class III by calling TransformTetrad(spatialrotationsm_mb_plan, boostsn_l_plane), so with the two optional arguments simultaneously.

The determination of appropriate transformation parameters to be used in these rotations, as well as the sequence of transformations happens all automatically by using the optional argument, canonicalform of TransformTetrad .

 >
 (7.4.1)
 Petrov type I
 Petrov type II
 Petrov type III
 Petrov type N
 Petrov type D

Edgardo S. Cheb-Terrab
Physics, Differential Equations and Mathematical Functions, Maplesoft

﻿