I would argue that it is actually the mathematicians who make the better theoretical physicists. A physics degree rarely prepares you for the modern mathematical constructions used in theoretical models.
.
Of course, you'll need to study physics along with your fibre bundles. :)
What courses do you think would be needed for the physics people?
I'll be honest, the mathematics I've studied has been self taught for the explicit purpose of understanding curvature and covariant derivatives for GR. What mathematics would be required for loop quantum gravity, I really don't know.
.
But, at a minimum, I would say that courses in topology, group theory, and differential geometry are a requirement for a foundation.
this are kind of math classes i'd do  to add to the physics
anything with a [x] would be normally taken for a physics degree
............
Mathematical Physics

..
Calculus

[x] Math 151 Calculus I
[x] Math 152 Calculus II
[x] Math 251 Calculus III
[x] Math 252 Vector Calculus I
Math 313 Vector Calculus II / Differential Geometry
Math 466 Tensor Analysis [needs Differential Geometry]
Math 471 Special Relativity [needs Differential Geometry and Butkov] [Butkov needs Diff Eqs and Griffith EM]
..
Analysis and Topology

Math 242 Intro to Analysis
Math 320 Theory of Convergence [aka Advanced Calculus of One Variable]
Math 425 Introduction to Metric Spaces
Math 426 Introduction to Lebesque Theory
Math 444 Topology
..
Differential Equations

[x] Math 310 Introduction to Ordinary Differential Equations
[x  maybe honours] Math 314 Boundary Value Problems
Math 415 Ordinary Differential Equations [needs Complex Analysis]
Math 418 Partial Differential Equations [needs Differential Geometry]
Math 419 Linear Analysis [needs Theory of Convergence]
Math 467 Vibrations [needs Symon]
Math 470 Variational Calculus [needs Symon and Differential Geometry]
..
Complex Analysis

[x  maybe honors] Math 322 Complex Analysis
Math 424 Applications of Complex Analysis
..
Linear Algebra

[x] Math 232 Elementary Linear Algebra
Math 438 Linear Algebra
Math 439 Introduction to Algebraic Systems [aka Abstract Algebra]
..
minor stuff
..
Fluid Mechanics [fluid motion/air motion/turbulence]

Math 362 Fluid Mechanics I [needs Vector Calculus and Symon]
Math 462 Fluid Mechanics II [needs Boundary Value Problems]
..
Continuum Mechanics [aka deformation/stress/elasticity]

Math 361 Mechanics of Deformable Media [needs Vector Calculus and Engineering Dynamics]
Math 468 Continuum Mechanics [needs Differential Geometry and Boundary Value Problems]
..
Probability and Statistics

Math 272 Introduction to Probability and Statistics
Math 387 Introduction to Stochastic Processes
..
Numerical Analysis

Math 316 Numerical Analysis I [needs Fortran or PL/I]
Math 416 Numerical Analysis II [needs Differential Equations]
yeah, its possible. Go research, nothings stopping you. You dont even need an education, just an imagination, and a firm grip on reality at the same time ha.
Yes of course , but You simply can't do truely interesting stuff unless you know Quantum field theory and the gravitational force if you want to do string theory . You have to be comfortable with handwaving not rigorously defining and proving every thing .
you're assuming everyone wants to do string theory.
QFT?
okay
[we're already off to a BRUTAL start if you need QFT]
.......
[solid background in QM and Special Relativity  yow]
[maybe adding  Tensors  Group Theory  Differential Geometry  Lie Theory]
.......
a. one Quantum Text [Eisberg  Liboff  Griffith]
b. one Grad Level QM Text [like Sakurai]
.....
group theory like books
a. Tinkham
b. Group Theory in Quantum Mechanics  Volker Heine  Dover 1993
.....
Baby Books on QFT
a. A First Book of QFT  Lahiri  CRC Press
b. A Modern Introduction to Quantum Field Theory  Michele Maggiore  Oxford
c. Quantum Field Theory for the Gifted Amateur  Tom Lancaster  Oxford 2014
d. From Special Relativity to Feynman Diagrams: A Course of Theoretical Particle Physics for Beginners  D'Auria & Trigiante
[gives all the prerequisites for QFT  Lie groups, relativistic electrodynamics, Lagrange & Hamiltonian mechanics]
e. Problem book in QFT  Radovanovic
f. Quantum Field Theory and the Standard Model  Matthew D. Schwartz  Cambridge 2013
g. Student Friendly Quantum Field Theory  Robert D. Klauber
h. Mandl and Shaw  Quantum Field Theory
..............................
i. Gauge Theories in Particle Physics  Two Volume  IJR Aitchison
[Volume I: From Relativistic Quantum Mechanics to QED]
[Volume II: A Practical Introduction : NonAbelian Gauge Theories : Qcd and the Electoweak Theory]
[Once you've mastered QM, you might want to dip a toe into the Quantum Field Theory waters.Â This is a gentle introduction.]
..............................
j. Introduction to Quantum Field Theory  V.G. Kiselev  CRC Press 2000
...............................
k. Gauge Theory and Variational Principles  David Bleecker  Dover
[This text serves as an introduction to the QFT and guage theories recast in the 'modern' mathematical setting of differential geometry. This book is only 167 pages long. Although selfcontained I highly recommend the reader have a working knowledge of QFT and at least an introductory course in GR. The mathematical tools of the reader should include a course in analysis on manifolds at the Spivak level or higher, acquintance with fibre bundles and basic lie groups.]
........
[i said it was gonna be BRUTAL]
......
old style people got QFT from Bjorken and Drell
new style poeple got QFT from Peskin
....
Classic Texts
a. Itzykson and Zuber  Quantum Field Theory  1980
b. Peskin & Schroder  the standard QFT textbook
c. Ryder  Quantum Field Theory  Cambridge 1984
d. An Interpretive Introduction to Quantum Field Theory  Paul Teller  Princeton 1995
d. Local Quantum Physics: Fields, Particles, Algebras  R. Haag
e. Anomalies in Quantum Field Theory  Reinhold A. Bertlmann  Oxford 2001
g. Quantum Field Theory in a NutshellÂ  A. Zee  Princeton 2000s?
h.Tom Banks  [very concise and inaccessible to beginners]
....................
i. Mark Srednicki  Quantum Field Theory  Cambridge 2007
[a higher level of abstraction and is great for a second book]
[This accessible and conceptually structured introduction to quantum field theory will be of value not only to beginning students but also to practicing physicists interested in learning or reviewing specific topics. The book is organized in a modular fashion, which makes it easy to extract the basic information relevant to the reader's area(s) of interest. The material is presented in an intuitively clear and informal style.]
.....
j. Weinberg's 3 volumes  Cambridge 1995
[notoriously difficult to learn from, but still the reference for certain topics]
[Read Ryder/Peskin/Zee first]
...................
k. Supersymmetric Gauge Field Theory and String Theory  Bailin and Love
l. Wess and Bagger  Supersymmetry and Supergravity  Princeton
m. Field Theory: A Modern Primer  Pierre Raymond  Westview Press
................
n. Relativistic Quantum Mechanics/Relativistic Quantum Fields  Two Volumes  Bjorken and Drell  McGrawHill 1964/1965
Book 1  Relativistic Quantum Mechanics  McGrawHill 1964
Book 2  Relativistic Quantum Fields  McGrawHill 1965
[Still used at Berkerley 15 years ago for QFT]
..................
........
........
........
For Stupid Strings
...............................
a. Superstring Theory  volume I and II  M.B. Green, J.H. Schwarz, E. Witten  Cambridge 1988
[Although these two volumes do not touch the important new developments in string theories they are still the best texts for the basics]
[GSW is slightly old now; it was written in 88 and doesn't contains a lot of the recent developments like duality and Mtheory. It's still quite interesting to read; you can learn a lot about field theory by studying this book. And if you want to do string theory, you'll probably have to read it anyways.]
[considered the readable text on superstrings where polchinski isnt readable]
........
b. tring Theory and MTheory: A Modern Introduction  Katrin Becker  Cambridge 2006
c. Michio Kaku  Strings, Conformal Fields and Topology  Springer 2000
....................
Lastly
Berkeley  probably did
a. Peskin & Schroeder
b.Wess and Bagger
c. Weinberg
d. Bialin and Love
e. Ramond
f. Bjorken & Drell
g. Ryder
THEN then go nuts with silly string, though i'd waste it more with
ADM Formalism or KaluzaKlein
since Strings are nutty like the Stock Market crazies who do  Technical Analysis  and lots of hocus pocus
.........................
.........................
oh yeah none of these books are recommendations, just my taste for books
almost makes ya wanna swear off physics, i tell ya
and i thought oh man, do i really wanna add to the clutter of textbooks way too hard for 97% of humans who took 3 physics courses or less
but i thought i would say
a. QFT, oh man talk about a scary cliff for some people to jump off into, very tackle it as an undergrad
b. Superstrings with a background in QFT, usually done if you wish to take the slow route, but still brutal for what might be a theoretical waste of time. What happens if ADM Formalism opens up in 30 years because String Theory was just abstract weirdness that didnt do much?
oops.....
...
QFT, oh man talk about a scary cliff for some people to jump off into, very few tackle it as an undergrad....
.....
But at least from the above rant, you can say, oh i heard of Steve Weinberg with quarks and things like the electroweak theory and the unification of the forces and the Big Bang Theory stuff, when he was all famous in the early 1970s, and on NOVA endlessly....
Theoretical Physics? What is that?<br />
<br />
I'm sorry. I'll probably know someday, but for now, this goes beyond my freshmaninhighschool level education.
You could think of it this way....
Theoretical Physics is everything in physics if you take away the experiments.
The guy with the atom smasher is playing with toys...
the theory guy like Einstein, is playing with chalk and a blackboard.
what have you taken with math, and what have you taken with physics, and where would you like to go with 'more courses'?
some people get a Bachelors in one and then hop to the other for grad school and fill in the gaps for a few semesters..
and others take an extra year or two, and aim for a mathematical physics degree where it's pretty easy to shift to one or the other, depending where you wanna specialize.

Basically some people dont wanna do real analysis, and other people arent too keen on a lot of intermediate electromagnetism.

I like to say that if you get a physics degree and you take analysis and you take differential geometry, fairly early on, you can wander anywhere in both subjects.

When you have
a. vector calculus, and
b.a whole textbook on differential equations
most everything hard is pushed out of your way, and
c. with differential geometry,
you can tackle most any course.....
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