Gravitas of Prefixes

By The Metric Maven

Recently I read the book Gravitational Waves by Brian Clegg in conjunction with attending a talk on the subject. Both were quite interesting and had their method of numerical presentation in common. During the presentation it was revealed that the distance of the source of the first gravitational wave detected was 1.8 Billion light years. “Is this a lot?”—as my friend Dr. Sunshine likes to ask when putting numbers in context. I immediately wanted to know the distance with a metric prefix. If it is in Exameters, then it would be inside of our galaxy. Our galaxy is about 1000 Exameters or a Zettameter. I did not stop to estimate the values as I wanted to listen to the presentation.

First we have an Olde English prefix with a ersatz “unit” called the light year. 1.8 billion of them is 1.8 Giga units, and the light year unit is 9.4607 Petameters. We end up with  1.8 * 9.4 x 109 * 1015 = 16.92 x 1024  or about 17 Yottameters. Wow! the observable universe is about 880 Yottameters, can this possibly be right? It seems very large, just based on the metric prefix. I go to Wikipedia to see if I can verify this number. They currently quote it as 1.4 +/- 0.6 billion light years. It’s a bit less, but same magnitude. They also state it is 440 Megaparsecs. A parsec is about 31 Petameters, so we have 440*31 x 106 * 1015  or 13.64 Yottameters! I’m immediately able to  grasp the size of this number in metric, and it seems astonishing.

Assuming I haven’t made a mistake, what are the detection distances in ascending order of the gravitational wave observations to date?

GW170817 2017-08-17         1.24 Ym

GW170608 2017-06-08       10.54 Ym

GW150914 2015-09-14       13.64 Ym

GW151226 2015-12-26       13.64 Ym

GW170814 2017-08-14       16.74 Ym

GW170104 2017-01-04        27.28 Ym

This is a rather amazing list to me. They are all further out than I would have expected gravitational waves to be detected. There is an unconfirmed observation that occurred at 31 Ym. This gives me some idea of the approximate detection limit for the current version of LIGO. This list gives you metric units that allow you to compare the distances to the size of the observable universe. As our Milky Way Galaxy is about 1 Zettameter across, we could write the list in a way that allows us to use our galaxy as a measurement touchstone:

GW170817 2017-08-17        1 240 Zm

GW170608 2017-06-08       10 540 Zm

GW150914 2015-09-14       13 640 Zm

GW151226 2015-12-26       13 640 Zm

GW170814 2017-08-14       16 740 Zm

GW170104 2017-01-04       27 280 Zm

That is a lot of galactic lengths from us. According to Brian Clegg, it is expected that around 2020 a LIGO upgrade has the potential to increase the detection distance by about a factor of three. If my estimate is right, this will be about 75 Yottameters. The detection volume will increase by 30 %. A set of enhancements scheduled for implementation from now to 2026 (LIGO A+) are expected to double the sensitivity distance again. So if my estimate is good, it would be out to 150 Yottameters! With this sensitivity, several black hole mergers per hour are expected to be detected.

There are discussions of a 40 Kilometer long LIGO receiver in space called the Cosmic Explorer. This is expected to increase the volume of sensitivity to black hole merger detection to cover the entire 880 Yottameter extent of the visible Universe. That would be amazing.

Why stop there? Brian Clegg discusses a concept known as LISA (Laser Interferometer Space Antenna). The arms of the interferometer would be formed between three satellites in a triangular configuration with 2.5 Gigameter sides!  LISA would orbit the Sun following along Earth’s orbit at a distance of about 50 to 65 Gigameters! Wow that seems just really big. Below is an animated GIF of the LISA satellite array orbit.

LISA Motion — Wikimedia Commons

In Brian Clegg’s words:

Unlike a ground-based observatory such as LIGO, LISA would have the chance to take in the whole of the sky. Rather than orbit the Earth as most satellites do, LISA is planned to be  in an orbit around the Sun, following the Earth’s path at a distance of between 50 and 65 million kilometres, about a quarter again the distance at which the Moon orbits. (pg 142)

Did I compute this distance wrong? 65 * 106 * 103 meters = 65 Gigameters. The distance from the Earth to Venus is about 42 Gm unless I’m mistaken. The length of the arc the Earth travels around the Sun is about 940 Gm. This is about one-fifteenth the distance arc length of the orbit. The animated gif above seems consistent with this value.

The distance from the Earth to the Moon is 384 402 Km or 384 Megameters. 1.25 multiplied by this number is 480 Megameters. The number is not even in the right metric prefix “area code.” The Olde English prefixes when used with metric are a pigfish disaster. They provide no real magnitude distinction when concatenated with metric prefixes. I’m still concerned I’ve made a conversion error or misinterpreted Glegg’s prose.  He seems to be conflating a distance in Gigameters with one in Megameters. Perhaps the Megameter distance is the closest approach of each satellite.

Clegg discusses the history of LISA on Page 142-143:

LISA was originally a joint venture between the European Space Agency (ESA) and NASA, but in 2011, suffering severe funding restrictions, NASA pulled out. Initially, ESA looked likely to go for a scaled-down version, known as the New Gravitational Wave Observatory, but with a renewed interest in gravitational waves after the LIGO discoveries, in early 2017 a revamped version of LISA, now featuring 2.5-million-kilometre beams, was proposed at the time, was proposed and at the time of writing has just been accepted for funding. This followed the test launch in 2015 of the LISA Pathfinder, as single satellite with tiny 38-centimetre (15 inch) interferometer arms……

He uses the pseudo-inch known as the centimeter with conversion to barleycorn inches next to it to express the tiny arm length. Would writing 380 mm arms killed him?

I don’t want my readers to get the wrong impression. I like Brian Clegg’s book. It is well worth reading if you are interested in gravitational waves. (I recommended it to the audience at the talk I attended) Its pigfish metric usage is common in science writing. He is doing what essentially all other contemporary science writers do. Astronomers only offer the same manner of visceral push-back at using metric units that citizens of the US exhibit. For those of you who might be interested in metric astronomy, I recommend my essay Long Distance Voyager.

On page 58-59 Clegg explains the density of a neutron star thus:

But a neutron star consists only of neutrons. With no electrical charge to repel each other, these particles can be pulled closer and closer by gravity until the exclusion principle kicks in when they’re practically on top of one another, enabling that great mass to be squeezed into a ridiculously small space. The result is that a teaspoonful of neutron star material would weigh about 100 million tonnes.

Once again an Olde English prefix (million) and a retro Olde English “metric” value tonne serve to obscure as much as impress. When the Olde English prefix is converted to metric and the tonne converted to metric we have a MegaMegagram or Teragram! Wow 100 Teragrams! The total mass of humanity is about 423 Teragrams, so about 65 mL of neutron star would contain the mass of all the humans on Earth. If you cup both of your hands together side-by-side, they would easily contain all of humanity at this density.

The future of gravitational wave astronomy is bright, it would be brighter if it was expressed exclusively with the metric system.

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Mnemonic Metric Prefixes

By The Metric Maven

Integer Solar Orbit Day

Engraving of Kilroy on the WWII Memorial in Washington DC

Years ago, my friend Ty took an interest in how to remember information. He pointed out that often you will think of something you want to do or retrieve, leave the room where you made the decision, and by the time you arrive in the next room, have forgotten. Often you return to the the original room, and then suddenly can recall what you meant to retrieve or view. Ty asserted it was because you had associated the decision with the original room, and when you returned, the two things were attached in your mind and you immediately recalled why you left in the first place. Years ago, when I was a young boy, people would tie a string around their finger to remind them to remember an important piece of information.

When I was taking trigonometry in high school, the teacher indicated we should remember words and phrases to recall the definitions of the sine, cosine, and tangent of a right triangle. He offered:

The adjacent side of the triangle was closest to the angle, the opposite side was well, opposite of the angle, and the hypotenuse was the long side that was not the others. Silly Cold Tigers? and Oscar Had A Happy Old Aunt?—how ridiculous!—but decades later, I still remember this method of recalling the definitions of the basic trigonometric functions of a right triangle. He encouraged his students to make up their own, and indeed they came up with more memorable phrases that were the sort that teenage boys were more likely to remember.

A number of “metric advocates” have ridiculed my assertion that grade school children, middle school students, and high school pupils, should be instructed in the use of all the metric prefixes. In my view, all the prefixes means the eight magnifying and eight reducing prefixes separated by 1000. One of the most effective instructive methods for recalling information is the use of a mnemonic device. Here I propose a pair of these, one for the magnifying prefixes, and one for the reducing prefixes. The first mnemonic is presented in the table below for the magnifying prefixes:

The mnemonic phrase for the magnifying prefixes is: “Kilroy Might Get To Paris Escorting Zombies Yonder.” The first letter of each word corresponds to the prefix symbol. The first prefix is Kilo is suggested by the name Kilroy, but the rest of the prefixes all end with an “a.” This can be thought of as the prefixes “above” unity.

The second table for the reducing prefixes is:

The mnemonic phrase for the reducing prefixes is “Millie might not protest fetching another zesty yeti.” Again the first letter of each word corresponds to the prefix symbols except for micro. The student would have to spell out micro and then recall the μ symbol is used, rather than another m. The first word is again a name, Millie, which in this case contains the spelled out prefix. Again means we need to forget it, but realize the reducing prefixes all end with “o” and are “below” unity.

In both cases the phrase begins with a name, and involves that person compelling mythical creatures.

If students were taught these mnemonics from perhaps grade 6 or 7 onward, with metric prefix examples, like those found in The Dimensions of the Cosmos, by the time they graduated from high school, they could have the tools needed to recall the metric prefixes without a textbook, and be reminded to use them in their work.

I would be interested in any comments or suggestions readers might have about these proposed mnemonic devices that might improve them. The best way to promote their use would be for the US to become a mandatory metric nation, but as this country celebrates its reactionary nature with religious fervor, I’ll have to settle for whatever good these mnemonics might do without a change.

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