A theoretical physics FAQ

From: Arnold Neumaier (Arnold.Neumaier_at_univie.ac.at)
Date: 01/09/05 

Date: Sun, 9 Jan 2005 13:46:49 +0000 (UTC)

                     A theoretical physics FAQ

         http://www.mat.univie.ac.at/~neum/physics-faq.txt

This site contains answers to some more or less frequently asked
questions from theoretical physics. They were edited from my answers
to postings to the moderated newsgroup sci.physics.research.

The 21 topics in the initial version, posted there on April 28, 2004,
have grown to 62 topics by October 18, 2004, to 88 by January 1, 2005,
and are likely to grow further. The answers to the various topics can
usually be read independently of each other.

Of course, the FAQ refers only to a tiny part of theoretical physics,
and the answers are only as good as my understanding of the matter.
This doesn't mean that they are poor but probably that they are
not perfect, and sometimes controversial.

Happy Reading!

                                      Arnold Neumaier
                                      University of Vienna
                                      http://www.mat.univie.ac.at/~neum/
                                      I like to see people grow

Table of Contents

  S1a. What are bras and kets?
  S1b. Projective geometry and quantum mechanics
  S1c. What is the meaning of the entries of a density matrix?
  S1d. How much information is in a particle?
  S1e. Can all quantum states be realized in nature?
  S1f. Are electrons pointlike/structureless?

  S2a. Lie groups and Lie algebras
  S2b. The Galilei group as contraction of the Poincare group
  S2c. Representations of the Poincare group, spin and gauge invariance
  S2d. Forms of relativistic dynamics
  S2e. Is there a multiparticle relativistic quantum mechanics?
  S2f. Localization and position operators

  S3a. What are 'bare' and 'dressed' particles?
  S3b. How meaningful are single Feynman diagrams?
  S3c. How real are 'virtual particles'?
  S3d. What is the meaning of 'on-shell' and 'off-shell'?
  S3e. Virtual particles and Coulomb interaction
  S3f. Are virtual particles and decaying particles the same?

  S4a. Can particles go backward in time?
  S4b. What about particles faster than light
  S4c. How real is the wave function?
  S4d. How real are Feynman's paths?

  S5a. QM pictures and representations
  S5b. Inequivalent representations of CCR/CAR
  S5c. Why does QFT look so different from QM?
  S5d. Why is QFT based on a classical action?
  S5e. Why does the action only contain first derivatives?
  S5f. Why normal ordering?

  S6a. Nonperturbative computations in QFT
  S6b. The formal functional integral approach to QFT
  S6c. Functional integrals, Wightman functions, and rigorous QFT
  S6d. Is there a rigorous interacting QFT in 4 dimensions?
  S6e. The classical limit of relativistic QFT

  S7a. Bound states in relativistic QFT
  S7b. Why can a bound state of massless quarks be heavy?
  S7c. What is the mass gap?

  S8a. Why renormalization?
  S8b. Renormalization without infinities I
  S8c. Renormalization without infinities II
  S8d. Renormalization and coarse graining
  S8e. What about infrared divergences?

  S9a. Summing divergent series
  S9b. Is QED consistent?
  S9c. What about relativistic QFT at finite times?

S10a. Is quantum mechanics compatible with general relativity?
S10b. Difficulties in quantizing gravity
S10c. Renormalization in quantum gravity
S10d. Why do gravitons have spin 2?
S10e. What is the tetrad formalism?
S10f. Energy in general relativity

S11a. A concise formulation of the measurement problem of QM
S11b. The double slit experiment
S11c. The Stern-Gerlach experiment
S11d. The minimal interpretation
S11e. The preferred basis problem
S11f. Master equation and pointer variables
S11g. Does decoherence solve the measurement problem?

S12a. Stochastic quantum mechanics
S12b. Which interpretation of quantum mechanics is most consistent?
S12c. Is there a relativistic measurement theory?

S13a. Random numbers and other random objects
S13b. What is the meaning of probabilities?
S13c. What about the subjective interpretation of probabilities?
S13d. Are probabilities limits of relative frequencies?
S13e. How meaningful are probabilities of single events?
S13f. How probable are realizations of stochastic processes?
S13g. How do probabilities apply in practice?
S13h. Incomplete knowledge and statistics
S13i. Priors and entropy in probability theory

S14a. Theoretical challenges close to experimental data
S14b. Does the standard model predict chemistry?
S14c. Is the result of a measurement a real number?
S14d. Why use complex numbers in physics?

S15a. How precise can physical language be?
S15b. Why bother about rigor in physics?
S15c. Justifying the foundations of a theory
S15d. Foundations, theory and experiment

S16a. On progress in science
S16b. How different are physical sciences and social sciences
S16c. Can good theories be falsified?
S16d. What, then, distinguishes a good theory?
S16e. When is a theory preferred to another one?

S17a. How to get information from sci.physics.research
S17b. How to sell your revolutionary idea
S17c. Useful background for theoretical physics
S17d. Stories about physicists
S17e. Other physics FAQs

S18a. Acknowledgments

Relevant Pages

  • Re: Is the speed of light really constant ?
    ... > acoustic signals could only travel at S, ... > inside the horizon-bounded region because virtual particles appearing ... >>assumed limits of technology not fundamental theory in physics. ... > without being special relativity. ...
    (sci.physics.relativity)
  • A theoretical physics FAQ
    ... (This lack of growth is due to the fact that I currently have almost no free time for discussing physics. ... Postulates for the formal core of quantum mechanics ... Why does QFT look so different from QM? ... Is quantum mechanics compatible with general relativity? ...
    (sci.physics.research)
  • Re: Some questions about gravity
    ... > modern physics and I have nobody to discuss the subjects with. ... Is the amount of deflection predicted by general relativity ... > with it's mass would fit into the Newtonian picture of gravity. ... > for particles susceptible to electromagnetic forces? ...
    (sci.physics)
  • Some questions about gravity
    ... I'm just a layman in physics. ... Is the amount of deflection predicted by general relativity ... with it's mass would fit into the Newtonian picture of gravity. ... for particles susceptible to electromagnetic forces? ...
    (sci.physics)
  • A theoretical physics FAQ
    ... questions from theoretical physics. ... Postulates for the formal core of quantum mechanics ... Why does QFT look so different from QM? ... Is quantum mechanics compatible with general relativity? ...
    (sci.physics.research)
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