angledimensions

Can Angles be Assigned a Dimension?

[Total: 3    Average: 3.7/5]


1. Some Background on Dimensional Analysis

… if you are not already familiar with it.

1.1 Dimensions

Dimensional Analysis is a way of analysing physics equations that considers only the qualitative dimensions – mass, length, time, charge .. – of the quantities involved, not the values that they take in the given problem.  This is not to be confused with Euclidean dimensions.

The fundamental rule is that you can only add, subtract or equate terms that have exactly the same constituent dimensions, and each to the same degree.  An acceleration cannot be a force, or be added to a force, because the latter includes a mass dimension while the former does not.  An area cannot be compared to a distance because it is length squared.

For a detailed discussion see e.g.  http://web.mit.edu/2.25/www/pdf/DA_unified.pdf, or any text on the Buckingham Pi theorem.   (There are also Wikipedia and Khan academy links, but these unfortunately confuse DA with managing units as variables, which is a separate topic.)

1.2 Notation

The standard notation in DA is that if x is a variable in an equation then [x] extracts its dimensionality.  The dimensions themselves are labelled M for mass, L for length, T for time, Q for charge, θ for temperature …

Thus, if F is a force then ##[F]=MLT^{-2}##.  The well-known equation F=ma would be analysed as ##MLT^{-2} = (M)(LT^{-2})##, which is clearly true.

1.3 Uses

1.3.1 Predicting the form of a relationship

DA can be a useful shortcut to establishing the general form of how one quantity depends on others.

Example: We presume that the pressure difference, ##\Delta P##, between the inside and outside of a bubble depends on the radius, ##r## and the surface tension ##S##.

## [ \Delta P]=ML^{-1}T^{-2}##

##[r]=L##

##[S]=MT^{-2}##

The only way to combine the pressure difference and surface tension to obtain a length is

## (MT^{-2})/(ML^{-1}T^{-2})=L##

Hence we can say

##r=kS/\Delta P##

for some constant k.

1.3.2 Error checking

Many algebraic errors can be caught by checking dimensional consistency.

2. Angles

Angles have never been considered to have dimension.  Consider, for example, the formula for arc length ##s##:

##s = r\theta##

Since ##s## and ##r## each have dimension ##L##, the angle cannot have a dimension, or so it seems.

2.1 Units Matter

Dimensionless combinations, like ##force/(mass \times acceleration)##, generally have the useful feature that they are invariant to the units used.  As long as the units used for the variables are consistent, the same number results whether you use SI or Imperial, or Babylonian.

This is not true of angles; many problems posted on the Homework forums are resolved when the student remembers to plug in the number of radians as argument to the sine function instead of degrees.

2.2 Distinct Entities with Same Dimensionality

It is somewhat unsatisfactory that, when reduced to mere dimensions, some pairs of quite different entities appear to be the same.

Torque and energy are both force times distance.  In terms of vectors, the first is the vector product, the second the scalar.

Angular momentum and action are both ##ML^2T^{-1}##.  Again, a vector and a scalar.

3 Axioms for an “Imaginary” Dimension

Consider assigning angles the dimension ##\Theta##, with some unusual properties:

  • ##\Theta^2=1##
  • The cross product operator itself has dimension ##\Theta##
  • ##i##, the square root of -1, has dimension ##\Theta##

3.1 Vectors and Angles

The following table illustrates the use of the dimensionality of angles with cross and dot products.

Entity Sample Equation Dimension
 Arc length element  ##\vec{ds}=\vec r\times\vec{d\theta}## ## L=(L)(\Theta)(\Theta)##
 Torque  ##\vec\tau=\vec r\times\vec F## ##ML^2T^{-2}\Theta=(L)(\Theta)(MLT^{-2})##
 Work   ##E=\vec r.\vec F##  ##ML^2T^{-2}=(L)(MLT^{-2})##
 Angular momentum  ##\vec L=\vec r\times\vec p##  ##ML^2T^{-1}\Theta=(L)(\Theta)(MLT^{-1})##
 Gyroscopic precession  ##\vec \tau=\vec \Omega_p\times\vec L##  ##ML^2T^{-2}\Theta=(\Theta T^{-1})(\Theta)(ML^2T^{-1}\Theta)##
 Velocity  ##\vec v=\vec r\times\vec{\omega}##  ##LT^{-1}=(L)(\Theta)(\Theta T^{-1})##

E.g. to resolve the ##s=r\theta## case for arc length, we can argue that this should really be expressed as the integral of the magnitude of a vector:

##s=\int|\vec{dr}|=\int |\vec r\times\vec{d\theta}|##.  The ##\Theta## dimension of the angle is neutralised by that of the cross-product operator.

3.2 Functions of Angles

Raising a dimensioned entity to a power is fine, because we can still express the dimensions of the result.  For other functions, such as exp, log and trig functions, it is more problematic.  If you ever find you have an equation of the form ##e^x##, where ##x## has dimension, you can be pretty sure you have erred.

For trig functions, it would be reasonable to require the argument to be an angle, but ##e^{i\theta}## would appear to create a difficulty for assigning angles a dimension.

This can be resolved by giving ##i## the ##\Theta## dimension also.  Based on the power series expansions, we see that the odd trig functions (those for which f(-x)=-f(x)) necessarily return the ##\Theta## dimension but the even functions, such as cosine, return a dimensionless value.

Thus, ##e^{i\theta}=\cos(\theta)+i\sin(\theta)## is entirely dimensionally consistent.

3.3 Areas and Volumes

Areas are naturally generated as cross products of vectors, imbuing them with the dimension ##L^2\Theta##.  Since the vector is normal to the surface, this is analogous to rotations as vectors.

Since volumes arise from the triple scalar product, it would seem that these should also have the ##\Theta## dimension.  This is more surprising.

The solid angle element subtended at the origin by a surface element ##\vec {dS}## at position ##\vec r## is given by ##d\Omega=\frac{\vec r.\vec{dS}}{|r|^3}##.  Or, if we wish to make this a vector in the direction ##\vec r##, ##\vec{d\Omega}=\vec r\frac{\vec r.\vec{dS}}{|r|^4}##.

Alternatively, in polar coordinates, ##d\Omega=\sin(\theta).d\theta d\phi##

Whichever way, the dimension is again ##\Theta##, which feels consistent with the result for volumes.

3.4 Complex Arithmetic

If ##i## is to be given dimension, what are we to make of ##1+i##?

Despite appearances, there is no difficulty.  ##1+i## is a convenient notation, but it is not addition in the same sense as in ##1+1##.  The 1 and the ##i## retain their separate existences.  We might just as easily, if less conveniently, have chosen to write complex numbers as an ordered pair, like <x, y>.  The real and imaginary parts never get crunched together in the same way as in normal addition, so they can have different dimensions without creating any inconsistencies.

3.5 Frequency and Angular Frequency

In wave expressions, frequency, ##f##, is the number of cycles per unit time, while angular frequency, ##\omega##, is radians per unit time.  ##\omega=2\pi f##.

Clearly ##\omega## should have dimension ##\Theta T^{-1}##.  The dimension for ##f## depends on whether the factor ##\pi## is to be taken as an angle or as a dimensionless number performing a conversion of units.  Taking ##f## as having dimension ##T^{-1}## appears to be best.

3.6 Planck’s Constants

Consider the equations

##E=hf##

##E=\hbar \omega##

Since ##h## has dimension of action, ##ML^2T^{-1}##, it has no angular dimension.  That is fine for the first equation since frequency is just ##T^{-1}##.

In the second equation, ##[\omega]=\Theta T^{-1}##.  ##\hbar## is defined as ##\frac h{2\pi}##.  Since ##\pi## is an angle here, that has dimension ##ML^2T^{-1}\Theta##, cancelling the ##\Theta## from ##\omega## and achieving dimensional consistency.

But note that this gives ##\hbar## the units of angular momentum, not action. An implication is that ##\hbar## should perhaps be considered a vector, and we should write the photon energy as

##E=\vec{\hbar}.\vec{ \omega}##

though how one is to justify that ##\vec {\hbar}## is necessarily in the direction of the velocity is unclear.

Likewise for momentum

##\vec p = k\vec{\hbar}##

where k is the wavenumber.  Note that this provides momentum as the vector it should be, rather than just defining its magnitude.

The Heisenberg Uncertainty relations, such as

##\frac 12\hbar\leqslant \Delta \vec p.\Delta \vec x##

would appear to violate both the dimensionality and the notion of making ##\hbar## a vector.  But if we follow the steps in the proof of the uncertainty relation we come to this penultimate statement:

##\frac 14\hbar^2\leqslant (\Delta \vec p.\Delta \vec x)^2##

At this point, giving ##\hbar## an angular component of dimensionality creates no problem.   Neither is there an issue with thinking of it as a vector.  These problems only appear when we overlook the ambiguities that so commonly arise when taking square roots.

4. Postscript

Subsequent to penning the original article, I have become aware of numerous prior attempts, dating back as far as 1936.  An excellent summary is by Quincey and Brown at https://arxiv.org/ftp/arxiv/papers/1604/1604.02373.pdf.

But their list misses a key one:

C. H . Page, J. Research National Bureau of Standards 65 B (Math. and Math. Phys.) No. 4, 227-235; (1961). http://nvlpubs.nist.gov/nistpubs/jres/65B/jresv65Bn4p227_A1b.pdf

It appears that most of my work above is a rediscovery of Page’s:

Result Page in “Page”
Cross product has angular dimension, dot product does not  231
##\Theta^2=1## Appendix 2
sine has angular dimension, cosine does not Appendix 2
solid angles have angular dimension (i.e. not squared) Appendix 2
whole cycles, as a unit, are dimensionless Appendix 3

Curiously, Page drew attention to the difficulty posed by ##e^{i\theta}=\cos(\theta)+i\sin(\theta)##, but overlooked the remedy of assigning angular dimension to ##i##.

He did not consider Planck’s constants.

 

Masters in Mathematics. Interests: climate change & renewable energy; travel; cycling, bushwalking; mathematical puzzles and paradoxes, Azed crosswords, bridge
103 replies
Newer Comments »
  1. Arnold Neumaier
    Arnold Neumaier says:

    Angles have the dimension of 1. That this is a true dimension can be seen from the fact that one measures angles in different units , namely either degrees or radians, and they convert into each other just like units for other dimensional quantities. Your ##\Theta## doesn't make sense unless it equals ##1## since for ##s=\sin\theta## to make sense, the dimension of ##\theta## must be ##1##.

  2. haruspex
    haruspex says:

    What is the justification of the claim: "The cross product operator also has dimension Θ"?

    That is not a claim, it is part of the definition of Θ.

    If i carries units, is there any meaning to (say) 1+i?

    I'm not especially attached to the part relating to i.  It is independent of the rest and probably needs more thought.  There might be a way around the 1+i problem similar to how I resolved s = rθ, i.e. one would have to agree to treating complex algebra in a slightly different way.

    Note: in exp (x), the x must be dimensionless

    That's why I assigned i the dimension Θ, to make iθ dimensionless.

    You might be interested in this old article from the American Journal of Physics.

    http://scitation.aip.org/content/aapt/journal/ajp/65/7/10.1119/1.18616

    "Angles—Let’s treat them squarely" by K. R. Brownstein

    That sounds very much as though it is not a new idea, which is at once heartening and disappointing.  Thanks for the reference.

    Angles have the dimension of 1. That this is a true dimension

    By definition, real numbers are dimensionless, so I do not understand what you mean by saying it is a true dimension.  Indeed, the fact that angles have units but not dimension is somewhat awkward, as I mentioned in the article.

    for s=sinθ to make sense, the dimension of θ must be 1.

    Not if you redefine trig functions as taking arguments of dimension Θ, as I did.

  3. haruspex
    haruspex says:

    Angles can be defined as dimensionless quantities if one thinks of them as fractions of a circle (multiplied by the constant 2π).

    I don't understand your point.  They are normally considered dimensionless anyway; I'm looking for a way to give them dimension. 

    Would thinking of all masses as fractions of 1kg make mass dimensionless?

  4. Ygggdrasil
    Ygggdrasil says:

    I don't understand your point.  They are normally considered dimensionless anyway; I'm looking for a way to give them dimension.

    Would thinking of all masses as fractions of 1kg make mass dimensionless?

    I guess I don't get the point of trying to give angles a dimension.  Angles are defined as a ratio (arc length : circumference) which is a dimensionless quantity and seems fundamentally different than something like mass.

  5. haruspex
    haruspex says:

    I guess I don't get the point of trying to give angles a dimension.  Angles are defined as a ratio (arc length : circumference) which is a dimensionless quantity and seems fundamentally different than something like mass.

    Fair question.

    I have always found it a bit unsatisfactory that some quite different pairs of physical entity are dimensionally indistinguishable.  I mentioned some in the post.  There seemed to be something distinct about rotational entities, such as angular momentum, that was not captured by DA.

    As regards utility, as I showed in the table, it can be added to normal DA for an extra bit (literally) of information.

  6. Stephen Tashi
    Stephen Tashi says:

    It would be nice to have a list of what axioms a "dimension" must satisfy.  Until that is available, arguments about what is or isn't a dimension are going to be personal opinions.

    A convenient online reference for dimensional analysis is http://web.mit.edu/2.25/www/pdf/DA_unified.pdf.   However, I don't know whether the axioms stated in that work are standard – or appropriate.

    For sake of discussion, consider what p10-11 of that book says about dimensions.  It says that there must be an operation of comparison and an operation of addition defined on the dimension.    All sorts of familiar ambiguities and confusions arise with angles because their "wrap-around" feature – e.g. is  360 degrees "equal" to zero degrees?  Does the sum ##pi## radians + ##pi## radians have a unique answer ?  – i.e. are ##2pi## radians and zero radians physically distinct ?    If we assume there is an "addition" is defined for dimensions, are we assuming (as we do in mathematics) that a sum is unique ?

    Page 11 postulates that  physical dimension must admit an ordering:

    the concept of larger and smaller for like quantities (if there exists a finite B such that A+B=C then C > A)

    To me, this is troublesome assumption for any dimension (including mass, length, time) where we wish to give a negative quantity a physical interpretation.    For example, is " – 10 meters"  a physical quantity ?  If we consider "-10 meters" a finite physical quantity  then the above assumption lets us conclude things like  15 meters + (-10 meters) = 5 meters,  so  15 meters < 5 meters.

    The topics of "changing units" and "changing coordinates" are usually treated as different subjects.  Are they really different concepts ?  Sometime as  "change of units" can involve more than mutiplying by a factor (e.g. converting degrees centigrade to degrees Kelvin).     In a  coordinate system for a "thing" it is permissible for the same thing to have several sets of different coordinates.   Does saying something is a "dimension" imply that there exists a coordinate system for measuring it where each distinct description in coordinates represent a different physical situation ?

  7. Baluncore
    Baluncore says:

    Angles are ratios, parts of a circle. Neither angles nor ratios have a dimension.

    Polynomials and dimensions are incompatible. Transcendental functions that are approximated by polynomials must have dimensionless inputs and outputs.

Newer Comments »

Leave a Reply

Want to join the discussion?
Feel free to contribute!

Leave a Reply