[alok99]

The editorial on Hz is particularly funny and insightful.

    // Beware the SI's broken definition
    // of Hz.  You should treat the radian as being correct, as a fundamental
    // dimensionless property of the universe that falls out of pure math like
    // the Taylor series for sin[x], and you should treat the Hz as being a 
    // fundamental property of incompetence by committee.
    
    ...
    
    // You are perfectly right.  You are perfectly wrong.  You look dumb and
    // unreasonable.  The person arguing the opposite looks dumb and unreasonable.
    //
    // Hz == YOU CANNOT WIN
    //
    // (Insert "IT'S A TRAP" image here.)

[fsh]

I don't find this rambling insightful at all. Here is what the BIPM actually says about the topic:

The SI unit of frequency is hertz, the SI unit of angular velocity and angular frequency is radian per second, and the SI unit of activity is becquerel, implying counts per second. Although it is formally correct to write all three of these units as the reciprocal second, the use of the different names emphasizes the different nature of the quantities concerned. It is especially important to carefully distinguish frequencies from angular frequencies, because by definition their numerical values differ by a factor1 of 2π. Ignoring this fact may cause an error of 2π. Note that in some countries, frequency values are conventionally expressed using “cycle/s” or “cps” instead of the SI unit Hz, although “cycle” and “cps” are not units in the SI. Note also that it is common, although not recommended, to use the term frequency for quantities expressed in rad/s. Because of this, it is recommended that quantities called “frequency”, “angular frequency”, and “angular velocity” always be given explicit units of Hz or rad/s and not 1/s.

I find this very concise and clear.

[heisenzombie]

Right, you can explain it but that doesn’t make it elegant. This is the equivalent of a function with a comment in it saying “Here be dragons!”.

One very desirable property of a units system is that incompatible quantities have incompatible units. So if I try to add 1 Joule (== 1 kg⋅m^2⋅s^−2) to 1 meter, I’m obviously doing something wrong and I just can’t continue.

But in SI I can take 1 Hertz (==1 s^-1) and add it to 1 radian/second (== 1 s^-1) and SI says sure, that’s 2 s^-1. But it’s not, that’s a totally unphysical thing to do and the answer is nonsense.

[fsh]

There is absolutely no guarantee that the sum of two quantities with compatible units makes any physical sense.

The issue here is that angles are inherently unitless geometrical quantities. "Radian" is merely a reminder that a number represents an angle. There is no way to make it a consistent physical unit.

[throw_a_grenade]

This is misunderstanding. There are many situations in SI where two units boil down to the same expression in terms on fundamental units, yet addition does not make physical sense.

More clear example: both Gy and Sv are J/kg (m^2 s^-2), but to add two values, one in each of those units, you need to multiply one of them by a coefficient (that coefficient depends on the kind of radiation and specific organ that absorbed this radiation). So somewhere in the math world lives a constant that is unitless 1, but x Sv/Gy, where x != 1 (again that x depends on circumstances).

Those situations just happen. It's how physics work.

[jonwiseman]

That sum to me is both physical and not nonsense, and I think your interpretation is wrong. 1 radian/second != 1 s^-1, but instead ≈ 0.159155 Hz. This is from interpreting a radian as a ratio, in which "doing" one radian in one second is only going over 1/2pi of a whole in a second. So 1 Hz + 1 radian/second, physically and sensibly to me, means ≈ 1.159155 Hz.

[whatshisface]

I am afraid you may have misunderstood them. It's not that you can't add a centimeter to an inch, it's that you have to convert units first. The unit symbols are intended to help indicate when a unit conversion would be necessary. What many find confusing is the fact that Hz and rad/s are both (in terms of basic physical units) advertised as s^-1.

To resolve the issue, you could define 1Hz as 2pi/s on the grounds that the Taylor series for sin(t) is in "units" of rad/s. You could also define one rad/s as 1/(2pi s), arguing that since the becquerel is 1/s and means "the number of times something happens per second," it would be inconsistent for frequencies to be denominated other ways, and that Hz is applicable to functions other than sin(t), including square and triangle waves, which have no relationship to angles.

[jonwiseman]

I might have misunderstood them, but I tried to argue that I was indeed converting units. Namely, 1 inch + 1 cm = 1.254 in (cm->in implicitly converted).

Also, defining a Hz as 2pi/s seems to be mixing units and quantities, like a Hz is 2 [pi/s] (where [] stores units), where pi is a unit equal to 6.28 [unitless]. Intead, defining a Hz as 2pi/s is to me changing the definition of Hz, unless it's like 2pi / 2pi [1 / s]. >You could also define one rad/s as 1/(2pi s) I was, specifically 1/2pi [1/s].

But as to your point that things like Hz are semi-meaningless unless attached to physical concepts, I'll agree with that (using a potentially different definition of semi-meaninglessness.)

My original statement that 1 Hz + 1 radian/second (i.e. 1 [1/s] + 1/2pi [1/s]) is both physical (semi-meaningless, meaning somewhat meaningful and fully meaningful when we flexibly attach it to a large range of physical concepts) and non nonsense (again, semi-meaningless until we attach to a physical concept) I think still hold.

The key to me is that we can attach physical concepts to units and measures flexibly (but not arbitarily). Hz is indeed applicable to non-sin functions, like square waves, but then we're all of the sudden talking about "the Hz of the nth harmonic of the square wave" (which has loads of relationship to angles).

[chrismorgan]

> 1 inch + 1 cm = 1.254 in

That should be 3.54cm.

[delta_p_delta_x]

> incompatible quantities have incompatible units

Okay; how do you add the unit of energy, one joule, to the unit of torque, one newton-metre?

Both have the same SI units: kilogram square metre per second per second. But one is a scalar quantity, whereas the other is a vector quantity.

[ars]

The Hubble Constant is also in HZ.

And its reciprocal looks like the age of the universe (apparently this is just a coincidence).

[fsh]

No, the Hubble constant has the unit 1/s. According to SI "the hertz shall only be used for periodic phenomena".

[cycomanic]

If complaining about Hz one should really be consistent and not use tau not radian as the fundamental property of the universe. Like that we could get rid of all those bloddy factor 2s