[6gvONxR4sf7o]

Angles aren't dimensionless any more than lengths are dimensionless (feet per second makes just as much sense as rpm). It's just that angles have symmetries that lengths don't, which is where 2 pi comes in. Do you want units where your symmetries are expressed in multiples of 1, 2, or 2 pi (for turns, half-turns, and radians, respectively)?

[eigenket]

Angles are absolutely more dimensionless than lengths are. For an easy check you can't add quantities where the dimension differs, which means it doesn't make sense to add a length to its cube. On the other hand it does make sense to add an angle to its cube - this is a necessary component of computing sin(angle) by the power series sin(angle) = angle - (angle^3)/6 + ...

[adrian_b]

Your argument is wrong.

Any physical quantity, for instance length, can appear as an argument of a nonlinear function that can be developed in a Taylor series. So your example would be identical for any other quantity not only for angle. I can make an analog computing element where a voltage is equal to the sinus of another voltage, so after your theory, voltage is dimensionless.

The reason why this is possible is that the arguments of such nonlinear functions are either explicitly or implicitly not the physical quantities, but their numeric values, i.e. the ratios between those quantities and their units, which are dimensionless.

In the case of the nonlinear sinus function, what is usually written as sin(x) is just one member of a family of functions where the arguments are angles implicitly divided by units of plane angle:

sin(x) is the sinus function with the angle implicitly divided by 1 radian

sin(x * Pi/2) is the sinus function with the angle implicitly divided by 1 right angle

sin(x * Pi*2) is the sinus function with the angle implicitly divided by 1 cycle a.k.a. turn

sin(x * Pi/180) is the sinus function with the angle implicitly divided by 1 sexagesimal degree

It is very sad that the logical thinking about angles of most people has been perverted by what they have been taught in school, which is just a bunch of nonsense copied again and again from one textbook to another.

[mgunyho]

> sin(x) is the sinus function with the angle implicitly divided by 1 radian

This to me sounds like the most natural explanation. For example, in a sibling comment someone mentioned that "you can calculate e^(-t)", but I disagree: in physics it's always e^(-t / T), where T is some time constant, so that the argument of the exponential is dimensionless. Same applies to sin(x): usually we write something like sin(2pi f t), where the units of f and t cancel out, and the 2pi is there to cancel out the invisible implicit 1 radian. sin(ft) would be wrong, at t = 1 / f you wouldn't have advanced by a full cycle.

[6gvONxR4sf7o]

How can we compute angle - (angle^3)/6?

    360 - (360^3)/6 = -7M degrees

or is it this?

    2*pi - (2 * pi)^3 / 6 = -35 radians = -2k degrees

Or maybe this?

    1 - (1^3)/6 = 0.8 turns = 300 degrees

They're wildly inconsistent because I'm not taking the units into account and we have to take the units into account.

[eigenket]

Units are not the same as dimensions, something can have a dimension of 1 (which is what we usually mean by "dimensionless") and still have different units, just as something can have a dimension of length but still be measured in meters or feet.

As far as you three examples go, which is "correct" depends on what you are trying to calculate - if you want this to approximate the power series for sin close to 0 you should use radians. Otherwise you use something else.

[adrian_b]

Actually the dimensions are by definition the same as the fundamental units, i.e. the units that are chosen freely, independently of all other units.

A dimensional formula of a quantity just writes its unit as a function of the fundamental units.

In any equality of physical quantities, in the two sides not only the dimensionless numeric values must be equal, but also the units must be equal, which is usually expressed by saying that the dimensions must be the same, and it is verified by writing in both sides the dimensional formulae, i.e. the units of both sides as functions of the fundamental units.

A dimensionless quantity is a ratio of two quantities that are measured by the same unit, so that the units simplify during the division.

There may be different but related dimensionless quantities, which are differentiated by different definitions of those quantities, but a dimensionless quantity cannot have different units.

This is just meaningless mumbo-jumbo that has been sadly introduced in the documents of the International System of Units, in 1995, after a shameful vote of the delegates, who have voted automatically, without thinking or discussing, a vote equivalent with establishing by vote that 2 + 2 = 5.

[6gvONxR4sf7o]

Sure, but that's orthogonal to the "angles don't really have units" assertion and the "it does make sense to add an angle to its cube" assertion, which are the ones I'm responding to.

As another example for the second assertion, you can compute e(-t) via power series too, adding seconds to seconds squared and seconds cubed, etc, which comes up all the time. But that doesn't mean `dimensionless + seconds + seconds^2` implies seconds are dimensionless any more than sin's series with `angle + angle^3` implies that angles are dimensionless.

[orangecat]

you can compute e(-t) via power series too, adding seconds to seconds squared and seconds cubed

The argument of exp does have to be dimensionless, exactly because adding seconds to seconds squared doesn't work. If t has units of time, there has to be another factor with units of inverse time, for example continuous compound interest is exp(rate*time).

[eigenket]

If you say "angles have units" I agree with you - obviously you can measure them in degrees or radians or whatever you want. I was responding to the claim

> Angles aren't dimensionless any more than lengths are dimensionless

They are dimensionless, but they still have units. The concepts are orthogonal.

As for the question about adding an angle to its cube, I would say the enormous usefulness of computing trig functions by power series suggests strongly that this is meaningful.

[adrian_b]

> They are dimensionless, but they still have units. The concepts are orthogonal.

The concepts are not orthogonal, they are incompatible.

A dimensionless quantity (which angles are not) is by definition the ratio of two quantities that are measured with the same unit.

When you compute the ratio by division, the two identical units disappear from the result, therefore the result is indeed dimensionless.

There is no way to choose a unit for a dimensionless quantity in the usual sense.

At most you could define a new different dimensionless quantity, as the ratio of two dimensionless quantities, i.e. as a ratio of ratios, but because it needs a different definition this should better be viewed as a different quantity, not as the same quantity with a different unit.

[6gvONxR4sf7o]

> I would say the enormous usefulness of computing trig functions by power series suggests strongly that this is meaningful.

That argument holds just as true for dimensional quantities frequently computed by power series though, which means it can't be valid.

[Someone]

I’m not sure it will work everywhere, but for sin(x), one can write

  sin(x)
  = x/(1 radian)! - x³/(3 radians)! + …
  = x/(1 radian) - x³/(1 radian × 2 radians × 3 radians) + …

That makes the ‘radians’ units cancel out.

[linuxdude314]

If you mean units instead of dimensions I agree.

There is a natural reason for pi occurring in physics that makes little sense to ignore.

Treating it as something to be dealt with misses the forest for the trees.

Radians are the naturally occurring Euclidean unit of angular measurement.