[Physics FAQ] - [Copyright]

Original by Philip Gibbs, 1996, 1997.

Notation used in the Physics FAQ and in the sci.physics newsgroups

Mathematical equations

In this FAQ and in the sci.physics newsgroups, we usually manage to avoid getting too deeply into detailed mathematics; but physics is a very mathematical subject, and it is impossible to avoid a few equations from time to time.  At present there is no universally accepted mechanism for writing equations in the newsgroups.  Indeed, Usenet is one of the last bastions of pure ASCII text, and many would like to see it stay that way.  So when it is not possible to write equations using simple computer characters on one line, most people resort to using either ASCII art or TeX/LaTeX.

ASCII art means constructing symbols using the ASCII characters suitably placed on consecutive lines.  With some care it is possible to do integral and summations signs in this way.  Here are some examples.

    oo                2
   ---    1        __
   \     ---   =   ||
   /       2       ---
   ---    n         6
           /   1
          |    -  dx    = log(x) + C
          /    x

Diagrams can be produced in a similar way.  With skill you really can be artistic!

TeX/LaTeX is a system used to produce mathematical documents with equations and diagrams.  It is not really very readable in ASCII.  If you see things like. . .

$ \int{e^{\mu x} dx} $

. . . then it is TeX.

The only part of TeX that we use in the FAQ is the notation for subscripts and superscripts.  x_1 means x1 (x subscript 1), and x^2 means x2 (x squared).  This is often preferred to ASCII art when you don't want to break the flow of text.

Sometimes people try to use special characters that extend the ASCII character set.  Sadly, these give different characters on different systems, and so they should be avoided.

By the way, it is possible to write some mathematical equations correctly in HTML web pages, but traditionally we have stuck to ASCII art and TeX so that extracts can be conveniently cut and pasted from the web browser straight into sci.physics when someone asks a question.  Many of the pages now use proper subscripts and superscripts to improve readability.  Images will also be used where ASCII art becomes impossible for figures and equations in the FAQ.


There are a lot of unexplained acronyms used as abbreviations in Usenet.  They are not a terribly good thing because it means that people who are not familiar with the territory don't understand what is being said, and all for the sake of a little brevity.

Here are a few of the more common ones used in the physics groups and throughout Usenet.  These are listed to help you understand them, not to encourage you to over-use them.  If you do want to use them, think about including a key to those you use in your signature.  OTOH the ones I use in this FAQ are OK and need no explanation :-)


AE    Albert Einstein
BB    Big Bang
BH    Black Hole
c     speed of light
CBR   Cosmic Background Radiation
CMB   Cosmic Microwave Background
E-M   Electromagnetic
EP    Equivalence Principle
EPR   Einstein-Podolsky-Rosen paradox
FOR   Frame Of Reference
FTL   Faster Than Light
GR    General Relativity
GUT   Grand Unified Theory
HST   Hubble Space Telescope
HUP   Heisenberg Uncertainty Principle
LET   Lorentz Ether Theory
LNH   Dirac's Large Number Hypothesis
M&M   Michelson–Morley experiment
MMX   Michelson–Morley experiment
MTW   Misner, Thorn, Wheeler, ("gravitation" a common reference)
MWI   Many Worlds Interpretation
NM    Newtonian Mechanics
PEP   Pauli Exclusion Principle
PMT   Photon Multiplier Tube
PR    Principle of Relativity
QCD   Quantum Chromo Dynamics
QED   Quantum Electrodynamics
QFT   Quantum Field Theory
QM    Quantum Mechanics
SM    Standard Model
SOL   Speed Of Light
SR    Special Relativity
TP    Twin Paradox
TT    Time Travel


AFAIK As Far As I Know
BTW   By The Way
DYOH  Do Your Own Homework
FAQ   Frequently Asked Question
FWIW  For What It's Worth
FYI   For your Information
IIRC  If I Remember Correctly
IMHO  In My Humble Opinion
IOW   In Other Words
OTOH  On The Other Hand
ROTFL Rolls On The Floor Laughing
TIA   Thanks In Advance
WRT   With Respect To
:-)   happy face
:-(   sad face
:-|   straight face

e-print citations

Sometimes people cite papers which are available as electronic pre-prints (e-prints).  You may come across something like. . .

. . . see Dr Underwood in hep-th/9501078 . . .

This is a reference to an e-print available at https://arxiv.org/abs/hep-th/9501078.  The hep-th is the name of an e-print archive for papers in high energy physics - theory, while the 9501078 is the number of the paper submitted to the archive in January (the 01) of 1995.

There are other e-print archives apart from hep-th. Most of those covering areas of physics can be found at https://arxiv.org.  If you want to read the papers you will need to install software on your system to read TeX, Postscript or PDF.

Large and small numbers and SI prefixes

Large and small numbers and quantities are often described by using scientific notation.  For example, 3*10^20 eV means 3 times 10 raised to 20 electron volts.  We may also say 300 EeV.  E stands for exa, which is the Systeme International (SI) prefix for 10^18.

Prefix     value    symbol   date     example        etymology

yotta      10^24    Y        1991     yottajoule     from otto, eight Italian
zetta      10^21    Z        1991     zettamole      from sette, seven Italian
exa        10^18    E        1975     exaweber       from hex, six in Greek
peta       10^15    P        1975     petahertz      from pente, five in Greek
tera       10^12    T                 teracandela    teras, monster in Greek
giga       10^9     G                 gigawatt       gigas, giant in Greek
mega       10^6     M                 megakelvin     megas, huge in Greek
kilo       10^3     k                 kilovolt       khilioi, thousand in Greek
hecto      10^2     h                 hectoradian    hekaton, hundred in Greek
deca/deka  10       D (or da)         dekapascal     deka, ten in Greek
deci       10^-1    d                 decisievert    decimus, tenth in Latin
centi      10^-2    c                 centimetre     centum, hundred in Latin
milli      10^-3    m                 milliampere    mille, thousand in Latin
micro      10^-6    μ (Greek mu)      microohm       mikros, small in Greek
nano       10^-9    n                 nanosecond     nanos, dwarf in Greek
pico       10^-12   p                 picofarad      pico, little bit in Spanish
femto      10^-15   f        1964     femtonewton    femten, 15 in Danish or Norwegian
atto       10^-18   a        1964     attogram       atten, 18 in Danish or Norwegian
zepto      10^-21   z        1991     zeptohenri     from sept, seven Greek 
yocto      10^-24   y        1991     yoctolitre     from okto, eight Greek     

The etymology of the SI prefixes is quite interesting in itself.  Many came from Greek and Latin via French, but a few are from other European languages.  The choice of femto aligns with the fact that "fm" for 10−15 metres is often called a "fermi", in honour of Enrico Fermi, a giant of early nuclear theory.  Peta and exa were apparently modelled on tera by imagining that it came from tetra (four in Greek) with the r dropped.  Hence peta is penta with the n dropped and exa is hexa with the h dropped.  In the cases of yotta, zetta, zepto and yocto, the initial letters were obviously required to avoid confusion with other symbols.  Other than that, they are modelled roughly on previous prefixes.