Thursday Throughput: Units of Measure Edition
[ThTh1] This article is a few years old, but was brought to my attention this week. And since it concerns a subject dear to me, I thought I’d give it a go. It’s one of the more recent dispatches from the annals of Physicists telling Astronomers How We Can Do Our Jobs Better:
In a paper published April 1 in Astronomy & Geophysics, Keith Atkin, a retired associate lecturer in physics at the University of Sheffield, UK, argues that while the professional field of astronomy has moved away from the imperial units of miles, pounds, and degrees Fahrenheit, “this transition has not been complete,” according to the abstract of his paper. The use of units of measurement such as light-years (the distance light covers in a year: 5.88 trillion miles [9.5 trillion kilometers]) and astronomical units (abbreviated AU, the average Earth-Sun distance: 93 million miles [150 million km]) persists, he says, when “simpler logical units would help both within the subject and in multidisciplinary research.”
In the paper, Atkin first focuses on units of distance measurement, noting not only the seemingly random and often redundant nature of units of measurement unique to the astronomical field, but sometimes their completely strange construction as well. “My bête noire is the megaparsec — a clumsy and ugly fusion of an SI prefix and a non-SI unit,” he writes.
His solution? “To encourage the use of SI units of length in all astronomical work: all distances and lengths should be based on, and simply related to, the metre. The metre is defined as the length of the path travelled by light in vacuum during a time interval of 1/299 792 458 of a second.”
Thus, he argues, all units of astronomical distance, from the AU to the parsec (equivalent to 3.26 light-years, and derived from the apparent motion of nearer stars against the background that occurs as Earth orbits the Sun), can really be expressed in meters, with the appropriate SI prefix attached.
Atkin expands his argument to other units of measurement — why use kilograms or solar masses (the Sun’s mass, 1.98 × 1030 kg, is equal to 1 solar mass, and the masses of other stars can be measured on this scale, such that Betelgeuse is almost 8 solar masses) when SI prefixes and grams will do? He suggests that the unofficial prefix besa (1033) be used, such that the Sun is 1.98 Bg and Betelgeuse is 15 Bg. Similarly, the electron volt (eV), which measures the energy of particles in both astronomy and physics, could instead be expressed in terms of attojoules (aJ, 10−18 joules).
He also argues that units such as arcminute and arcsecond should be done away with, though admits that “the degree is presumably here to stay.” Atkin proposes measuring all angles in terms of either decimal degrees or radians, rather than using degrees, arcminutes, and arcseconds, as astronomers do today.
My response to this can be most easily summarized by Dr. Evil:
The editors, unfortunately, insisted that merely posting a YouTube clip did not, contractually, count as a Throughput. So I’ll expand on this a bit. Because this is a question I get asked sometimes and it’s worth going over why astronomers measure things the way we do (with the caveat that sophisticated calculations actually do use SI units).
The universe exists on unimaginably vast scales; scales that dwarf human comprehension. Hell, just our solar system exists on an unimaginably vast scale. In describing the grandeur and scope of the universe, units intended for the human experience — meters, kilograms, liters — are simply not up to the challenge. And so we use units of measurement that, while not as “scientific”, make the universe comprehensible.
If I tell someone that the Andromeda Galaxy is 20 zeta-meters away, that means nothing to them. It’s just a lot of zeros. But if I tell them that it takes light nine minutes to reach from the Sun and two million years to get to Andromeda … that can give them a glimpse of just how vast the universe is. If I tell someone that Betelegeuse weighs 15 “besagrams” that means nothing. If I tell them it’s 15-20 times the mass of the Sun, that means something. “Astronomer units” are designed to make the universe at least scaleable if not comprehensible.
Are these unit arbitrary? Perhaps, but SI units are arbitrary too. The meter was originally based on the swing of a pendulum in Paris and later tied to the size of the Earth and then a platinum bar and eventually light. The kilogram is based on the mass of a liter of water but the liter is based on the centimeter. In other words, the entire metric system is based on a few French toffs farting around in the 18th century. They didn’t descend from the heavens in a golden chariot. We just set these units of measurement to what they were because they were useful — for Earthbound endeavors.
But there’s another and perhaps more important reason why we use these units. One of the defining elements of astronomy is that we can’t touch the things we study. We can’t interact with them. All of our measurements are indirect. We can’t actually get a tape measure and plumb the distance to Andromeda. We can’t get Betelgeuse on a scale.
What astronomer units do is allow us to measure the universe in a sense relative to things we can measure. The standard unit of distance is the parsec. What is the parsec? Is it the distance at which an object has a parallax angle (a shift in its position as the Earth orbits the Sun) of one arc-second. Which means it is exactly 206,265 times the distance between the Earth and the Sun. Every distance in astronomy is therefore linked to the Earth-Sun distance, something we can measure in, literally, 20 minutes. Many of the distances in astronomy are uncertain because most of the universe is too far away for us to triangulate using the Earth’s orbit as a baseline. We have to make comparisons to things we can triangulate. And that all falls back to the parsec. It is the entire basis of the astronomical distance scale. Parsecs are meaningful to astronomy in a very real and tangible sense. Meters are not.
The same goes for other measurements. We can measure the mass, radius and brightness of the Sun directly. Therefore we scale everything to that standard. Isn’t scaling things to the Sun more meaningful and sensible than scaling things to some dusty old bar in Paris and a big gulp of water?
I certainly think so.
[ThTh2] The story of how women came to be excluded from the space program is an interesting one. In the early days of the space program, 19 women volunteered to be part of the astronaut corps and 13 of them passed the qualification tests. NASA, however, went with an all-male astronaut corps. Sexism no doubt played a role but the biggest reason was that, in the 1950’s and 1960’s, the astronaut corps was seen as an extension of the military. All seven of the Mercury astronauts were military pilots. All ten of the Gemini pilots were military. Neil Armstrong was technically a civilian, but was a military veteran. The first non-military astronauts was not selected until 1965 (and four of those were veterans). Harrison Schmidt became the first true civilian in space when he flew on Apollo 17 in 1972. It would be a decade later that Sally Ride would become the first American woman in space.
Which makes this aspect of last week’s Blue Origin flight all the more satisfying. Wally Funk was part of the “Mercury 13”, the women who passed the Phase 1 testing but were excluded for the astronaut corps. At age 82, she was Jeff Bezos’ guest on the flight and finally got into space.
[ThTh3] And speaking of space flights, there’s been some sparing over where exactly the boundary is that defines “space”. The difference between the two is … not really meaningful.
[ThTh4] Nice demonstration of how we’ve gotten to now our most distant planet.
Views of Pluto through the years! pic.twitter.com/mxbrx039Cz
— Universal-Sci (@universal_sci) July 17, 2021
Yeah, I said it.
[ThTh5] Why does it seem like the vaccines create better immunity than COVID itself? This Twitter thread digs into it.
[ThTh6] An interesting look at how a grad student prank became a conspiracy theory.
[ThTh7] Ivermectin is the latest “miracle cure” being touted for COVID-19. It now looks like the study supporting its uses was a fraud
[ThTh8] A couple more fun Twitter threads for you. The first is on why the claims that the vaccines have graphene is hot nonesense. The second is something for our West Virginia peeps: the paleontology of the Appalachian mountains.
[ThTh9] The headlines scream that the moon is going to cause massive flooding. What’s the reality?
I’ve written before about the tides and how they can vary depending on the alignment of the Sun and the Moon and Earth. Well, it can be a bit more complicated than that. The moon has a long precessional cycle around the Earth of about 19 years. The changing alignment of the Moon and Sun means that the size of the tides varies by about 3 or 4 inches over the course of that cycle, with low tides being lower and high tides being higher. Right now, we are at a low point. But in a decade that will be a highpoint. When you combined this with the inch-per-decade sea level rise due to global warming, that might result in unprecedented flooding in low-lying coastal areas. It’s hard to know exactly how bad it’s going to be. But it will be a peek at the world might look like by mid-century if we don’t get a handle on global warming.
[ThTh10] And speaking of global warming, shutting down nuclear power plants doesn’t get us closer to zero carbon. Just the opposite.
[ThTh11] Jane Coaston’s podcast had a debate on the wisdom or folly of attempting to contact alien civilizations. My view?
I find Dr. Kaku’s argument that broadcasting our existence to the universe risks annihilation to be extremely shaky. He falls back on history and sci-fi tropes a lot. But history tells us little about what such an unprecedented encounter would look like and sci-fi is usually just a way of retelling history in a different setting. If there are aliens out there, they are so far away, the resources they would have to expend just to reach Earth vastly outstrip whatever they would hope to gain by conquering us. His contention that Darwinian notions apply to interstellar contact crosses me as specious. A civilization that can travel between planets is definitionally a post-scarcity society, with energy and resources that are effectively unlimited.
This taps into one of those sci-fi tropes I despise: where the aliens invade us for our resources. Why on Earth would they do that? Earth is tiny; the universe vast. Even if they came here, why would they want, for example, our water? Jupiter’s moon Ganymede alone has more water than the entirety of Earth’s surface! Energy? The energy output of the entire globe would pale in comparison to an interstellar ship would use.
The only reason any alien would want to come to Earth is because of us. That would mean they are either so xenophobic, they’ll burn vast resources to just wipe out another species; or they are benign enough to just pop in and say hi. Either way, we have little choice in the matter. Dr. Vakoch may be overly optimistic about the potential benefits of contacting aliens. Again, the vast distances makes it unlikely to be worth their while. But he does make a good point: if they’re looking for us, they’ll find us no matter what we do. Even if we shut off our radio signals … they’ll know what to look for. We know what to look for and we aren’t super-advanced aliens. So actively trying to contact them poses no downside, even if the upside is also minimal.
Of course, having typed that, I’ll probably wake up tomorrow to find the aliens have arrived and blown up Congress. Of course … that may not necessarily indicate hostility …