Month: January 2023

Yesterday I asked why there's no name for a unit of momentum. Today I have answers. Plus, if you read all the way to the end, I have a genuinely constructive suggestion.

First things first, in case you have no idea what I'm talking about. In the metric system—officially known as SI—there are three basic quantities: the meter, the kilogram, and the second.¹ Everything else is derived from those three. For example, force = mass * acceleration, so:

F = ma

a = distance / seconds²

Therefore, F = mass * distance / seconds²

One unit of force = 1 kg * 1 meter / 1 second²

This quantity is called a newton, named after Isaac Newton. Lots of other things have names too: ohm, watt, lumen, joule, and so forth. Click here for a list.

Momentum is a critically important quantity, equal to mass * velocity. So why wasn't it ever given a name? I did several minutes of research on this question, and the most authoritative sounding answer came from a commenter at Stack Exchange called Conifold. He or she explains that there were two waves of standardization and naming:

The second wave, started in the 1860s and formalized by the 1880s in both SI and its competitor CGS, was meant to catch up with developments in thermodynamics and electromagnetism, and gave us ohms, volts, farads, watts, etc. Kilograve was renamed into kilogram and became the unit of mass. The unit of force was named dyne in CGS (from Greek dynamis — force) and newton in SI.

....The unit for power, watt, was suggested even before joule, by Siemens in 1882, to replace Watt's own horsepower used to measure the output of steam engines. Siemens was an electric engineer. Joule himself was honored by a unit name for determining the mechanical equivalent of heat. Momentum was out of luck.

In other words, momentum has no name because no one ever bothered to give it one. However, another commenter, jkien, tells us that it was given a name in the CGS system

In 1887 a committee of the British Association was appointed for the purpose of "considering the desirability of introducing uniform nomenclature for the fundamental units of mechanics of co-operating with other bodies engaged in similar work.” The committee issued a series of questions to members, and collected their replies. The result was that in 1888, when the committee met at Bath, they were able, amid much difference of opinion, to agree as to the desirability of introducing names for the C. G. S. units of velocity, momentum, and pressure; the names suggested being, kine, bole, and barad respectively.

So in the CGS system, a single unit of momentum is the bole. It never caught on and goes entirely unused today.

Long story short, there's no special reason that there's no name for a unit of momentum. There just isn't. But this prompts an obvious question: What should it be, and how do we get it adopted?

Let's take the second question first. The problem is that traditionally these units are named after people who had something to do with discovering or theorizing about them. But momentum goes back a long way, and everyone associated with it is a dead white man. This is boring and will get no one excited.

So what if we proposed naming it after a woman? That would not only be well deserved, but it would get the attention and support of lots of people. But who should it be?

By chance there's an excellent candidate. The importance of momentum in physics is not so much in the quantity per se, but in the fact that it's always conserved. When a rocket throws off propellant in one direction (down), the rocket goes in the other direction (up) because the total momentum of the system must be conserved.²

But why must it be conserved? In 1915, a brilliant German mathematician named Emmy Noether proved that all conservation laws can be expressed as symmetries. In particular, conservation of momentum is a consequence of the symmetry of space. This was a critical contribution to physics.

So I propose that the unit of linear momentum be named a noether. Who's with me?³

¹There's a competing system called CGS, in which the three basic units are the centimeter, the gram, and the second. It is little used.

²In case you're curious, this is also part of the explanation for how airplanes work. The shape of the wing forces air downward at considerable speed, which means the airplane is forced upward. When these are balanced, the plane travels at a steady altitude.

³We can argue about angular momentum some other time.

I haven't paid any attention to the demolition of Confederate statues on Monument Avenue in Richmond, so I didn't know it had all been done by a single person, Devon Henry:

The name carries weight in Richmond these days. Over the past three years, as the former capital of the Confederacy has taken down more than a dozen monuments to the Lost Cause, Henry — who is Black — has overseen all the work.

These aren't huge jobs, but they pay well and plenty of contractors would like a shot at them. So why did Henry get every single contract?

City and state officials said they turned to Team Henry Enterprises after a long list of bigger contractors — all White-owned — said they wanted no part of taking down Confederate statues.

For a Black man to step in carried enormous risk. Henry concealed the name of his company for a time and long shunned media interviews. He has endured death threats, seen employees walk away and been told by others in the industry that his future is ruined. He started wearing a bulletproof vest on job sites and got a permit to carry a concealed firearm for protection.

....The call that changed Henry’s life came in the middle of a business meeting in early June 2020....On the line was Clark Mercer, the chief of staff for then-Gov. Ralph Northam....“I was pretty forthcoming that we hadn’t been able to find anybody to take on the job,” Mercer said in an interview. In fact, the responses from other contractors were “pretty overtly racist,” he said, including language that he found threatening.

I can't even think of anything to say about this. It's just so goddamn depressing.

This is a fountain at the Place de la Concorde at dawn.

Tyler Cowen explains today why he's skeptical of the lead-crime hypothesis. It turns out that it's because of a simple bit of confusion.

You'll have to click the link and read Tyler's post to understand my reply—so don't blame me if you're too lazy to do it—but the key thing is that the LCH is not a general explanation for crime. It is solely a theory for the unusually large rise and fall of violent crime in postwar America through about 2010:

As it happens, the LCH also turns out to explain the rise and fall of violent crime in some other places during roughly the same period. Since the lead in question derives from the ethyl additive in gasoline, it all depends on how much car ownership rose (a lot in rich countries, not so much in poor ones) and when leaded gasoline was phased out (the mid-70s in the US, generally later in other countries).

And two other quick things. First, neither left nor right is all that friendly toward the LCH. The right doesn't like it because it diminishes the role of favored theories like moral decay, the breakdown of families, broken windows, etc. Likewise, lefties dislike it because it diminishes the role of favored theories like poverty, structural racism, guns, and the supremacy of culture over "essentialism." I think lefties have come around a little faster, but that's probably just because it was a lefty who first popularized it.¹

Second, I too assume publication bias is everywhere. I just think that it's probably a little low in LCH studies relative to some other areas of study.

¹Me!

Why is there no name for a unit of momentum? Or for angular momentum? Did we run out of famous people to name things after?

How about one sina or one descartes for linear momentum and one foucault for angular momentum?

UPDATE: More here.