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Insights Can Angles be Assigned a Dimension? - Comments

  1. Nov 12, 2016 #1

    haruspex

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  2. jcsd
  3. Nov 12, 2016 #2

    robphy

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

    If ##i## carries units, is there any meaning to (say) 1+##i##?
    Note: in ##\exp (x)##, the ##x## must be dimensionless.
     
    Last edited: Nov 12, 2016
  4. Nov 12, 2016 #3

    robphy

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  5. Nov 12, 2016 #4

    A. Neumaier

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    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##.
     
  6. Nov 12, 2016 #5

    haruspex

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    That is not a claim, it is part of the definition of Θ.
    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.
    That's why I assigned i the dimension Θ, to make iθ dimensionless.
    That sounds very much as though it is not a new idea, which is at once heartening and disappointing. Thanks for the reference.
    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.
    Not if you redefine trig functions as taking arguments of dimension Θ, as I did.
     
  7. Nov 12, 2016 #6

    Ygggdrasil

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    Angles can be defined as dimensionless quantities if one thinks of them as fractions of a circle (multiplied by the constant 2π).
     
  8. Nov 12, 2016 #7

    haruspex

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    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?
     
  9. Nov 12, 2016 #8

    Ygggdrasil

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    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.
     
  10. Nov 12, 2016 #9

    haruspex

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    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.
     
  11. Nov 12, 2016 #10

    Stephen Tashi

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    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 ##2\pi## 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:
    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 represents a different physical situation ?
     
  12. Nov 14, 2016 #11

    Baluncore

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    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.
     
  13. Nov 14, 2016 #12

    haruspex

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    A ratio can have no dimension since it must be a ratio of two things of the same dimension. But at an angle is not a ratio. You can say it is a certain fraction of a complete circle, but whether that has dimension depends on whether you consider the complete circle as having a dimension. You are not used to thinking of it that way, but that does not mean it cannot be done.
    Not if the dimension has the unusual property that it becomes dimensionless when raised to some finite power. The ϑ2=1 axiom means that a polynomial function of an angle is fine if all the terms are even powers (dimensionless result) or all odd powers (result of dimension ϑ).
     
  14. Nov 14, 2016 #13

    robphy

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    Which circle are you referring to?
    The unit circle? Or maybe the circle of radius 7?
    One feature of the angle measure (defined as the ratio of circular-arc-length to radius) is that it is independent of the circle used to make that measurement.
    In this general discussion, one needs to distinguish an "angle" from an "angle measure".

    Certainly, you can try to make definitions... but they have to lead to a consistent system.
    At this stage, my question of the consistency of "1+i" in post 2 stands out as still unresolved, despite your reply in post 5.
     
  15. Nov 14, 2016 #14

    Stephen Tashi

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    I'm curious why you say that polynomials are incompatible with dimensions. Coefficients of different powers of x in a polynomial can be assigned different dimensions, so that each power of x is converted to the same dimension.

    If we have an equation that describes a dimensioned physical quantity as a power series, aren't we assigning different dimensions to each coefficient in the power series ?
     
  16. Nov 14, 2016 #15

    Stephen Tashi

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    You could also ask "The unit circle with center (0,0)? The unit circle with center (15,12)?"

    You have to use a circle with its center at the vertex of the angle, so the measurement process isn't really independent of which circle is used unless we think of "a circle" as a portable measuring instrument, just as we think of a meter stick as portable measuring instrument.

    If we have an object that moves along a path, to measure the property of the path called its "total length" with a meter stick, we have to move the meter stick to various locations on the path. If we are dealing with an object moving in a circular path and want to measure a property of the path called the "total angle swept out", we may need a measuring instrument that can produce results greater than 360 deg. Such a measuring instrument could involve a circle, but it would have to have the added feature of keeping track of arc lengths greater than ##2\pi##.

    I agree. It's the distinction between "a dimension" (e.g. length) and "a unit of measure" (e.g. meters).
     
  17. Nov 14, 2016 #16

    robphy

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    Yes, but I didn't think I had to make further clarification on this. Shall we bring up issues of parallel transport on a non-Euclidean space as well?
    I would hope that when one says "arc-length divided by radius" that the rest of this is assumed.

    My distinction is this... If two lines (or two segments) meet at a point, then one can talk about the angle [or an angle] at the location where the two lines meet, labeled by the vertex (call it) C or that vertex with a two points, one on each segment--like ACB. Before somehow specifying an angle-measure, one could talk about all sorts of properties of angles at this stage. Then, when introducing an angle-measure, it probably should be explicitly defined---maybe operationally.

    Given two lines (or line segments) meeting at a point, one could define an angle-measure the usual way (essentially with a circular protractor, appropriately calibrated in the likely possibility that protractors have different radii), or maybe in a different way (e.g. https://mathnow.wordpress.com/2009/11/06/a-rational-parameterization-of-the-unit-circle/ ) although it might not give your angle-measure the desired properties of additivity, or maybe using a hyperbolic-protractor (as one might use in special relativity).
    Issues of "units of angle-measure" come into play here.
    But all of this "angle-measure" discussion is distinct from the "angle" discussion in the previous paragraph.
     
  18. Nov 14, 2016 #17

    Stephen Tashi

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    Let me see if I understand you viewpoint.

    In the PDF linked in the Insight and post #10, the author, A. Sonin, makes a distinction among:
    1) A physical object or phenomena (e.g. a stick)
    2) A "dimension", which is a property of a physical object or phenomena (e.g. length)
    3) A "unit of measure", which is a way to quantify a dimension (e.g. meters)

    The author is careful to point out that a "dimension" is not a physical phenomena. It is a property of a physical phenomena.

    You describe "an angle" in mathematical terms, but since you say an "angle" can have various properties, I think you mean an "angle" to denote a physical phenomena, which is alternative 1)

    When you say "angle measure", I'm not sure whether you mean alternative 3) or alternative 2). But does alternative 3) (units of measure) make any sense without the existence of alternative 2) (dimension) ?

    As I mentioned previously, I haven't yet seen a precise statement of what mathematical or physical properties a "dimension" must have. I don't know whether other thread participants agree with those listed by A. Sonin.

    In regards to "dimensionless ratios", a dimensionless ratio can associated with a property of a physical object. Different dimensionless ratios can be associated with different properties (e.g. height of a person/ length of that persons right leg, weight of a person now / weight of that person at birth). "Dimensionless ratios" can obviously be quantified. So it is rather confusing to consider the question of whether a "dimensionless ratio" is (or isn't) associated with a "dimension".
     
  19. Nov 14, 2016 #18

    Stephen Tashi

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    It's interesting to consider the distinction between a mathematical definition of a function and a physical definition of a function. To define ##\sin(\theta)## mathematically (i.e. a mapping from real numbers to real numbers) one would have to unambiguously answer questions like "What is ##\sin(0.35)##?" without any discussion of "units of measure" - e.g. 0.35 deg vs 0.35 radians. From a mathematical point of view, ##\sin(\theta\ deg)## and ##\sin(\theta\ radians)## are different functions, even though we use the ambiguous notation ##\sin(\theta)## to denote both of them. Only the family of trig functions where ##\theta## is measured in radians satisfy mathematical laws like ##D sin(\theta) = cos(\theta)##.

    To give a physical law in the form of a function we may do it by assuming certain units of measure. Then it is assumed that changing the units of measure appropriately produces a new mathematical function which states the same physical law. So a physical definition of a function defines a set of different mathematical functions that are regarded as physically equivalent.

    The physical definition of ##\sin(\theta)## defines a set of different, but physically equivalent mathematical functions.
     
  20. Nov 14, 2016 #19

    robphy

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    I didn't read the PDF. So, I can't answer your questions using that author's distinctions.
    I think the bottom line here is: clearly define terms, especially when one is trying to change definitions.
     
  21. Nov 14, 2016 #20

    haruspex

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    Yes, I think that is why I have never been satisfied with the view that angles are utterly dimensionless.
    Whatever circle Baluncore had in mind.
    I haven't forgotten this. I want to take a look at the Brownstein article first.
     
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