What's in a unit?

Foundations of Amateur Radio In our hobby we use kilohertz and megahertz enthusiastically. Sometimes even gigahertz. The other day during a discussion the question arose, what comes after tera, as in terahertz? I couldn't remember, so I had to look it up, peta comes next, then exa, zetta and yotta, derived from the Greek word for eight. That in and of itself was interesting, but it turns out that Greek isn't the only language used in attributing SI metric prefixes, SI being the International System of Units. Of the 20 units, which I'll get to in a moment, there's 12 with Greek origins, five deriving from Latin, two from Danish and one from Spanish. The units are used to describe how many of a thing there are in base-10, so, a thousand of something is kilo, or ten to the power of three, which gives us kilohertz. A gigahertz is ten to the power of nine and so-on. Interestingly, kilo is derived from the Greek word thousand, but mega comes from the Greek for great. Both hecto, as in hectopascals and deca as in decathlon originate in the Greek words for hundred and ten. The prefix pico, as in picofarad comes from the Spanish word peak and femto as in femtowatt comes from the Danish for fifteen, as in ten to the power of minus 15. Apparently a zeptomole of a substance contains 602 particles, even NASA says so, let me know if you can find a source for that. I could devote my entire discussion on these 20 units, adding for example that their naming wasn't all done at the same time, the most recent additions are yotta and yocto, as I said, derived from the Greek for eight, being ten to the power of 24. How's that eight you ask? Well, three times eight is 24. I'm not saying it's intuitive, but there is logic. In looking at all these units, and specifically the smaller ones, milli, micro, nano, pico and the like, it occurred to me, is there a way to go below one Hertz, could you have half a Hertz? Hertz is the number of oscillations per second, a single Hertz being one per second. Half a Hertz would be one oscillation per two seconds. I started wondering what to look for in discovering if anyone has been playing with this. For the life of me, I couldn't think of what to search for and my experience tells me that if you cannot find the answer online, you're asking the wrong question. This morning, with a fresh cup of coffee in my hands, it occurred to me that anyone doing this kind of stuff would be using SI units, so they'd be using decihertz, centihertz, millihertz, microhertz and nanohertz, perhaps even picohertz. So I went searching. Turns out that this actually exists. After wading through endless results with conversion tools and dictionaries, there's plenty of research to find. The unit decihertz is being used in gravitational wave interferometry, specifically, there's a Japanese, space-based gravitational wave observatory in the works with hopes of launching their three space craft if they can find funding. It doesn't end there. There are experimental imaging studies being made on malignant and benign human cancer cells and tissues looking at decihertz all the way down to yoctohertz, that's ten to the minus 24. Inside Apple software development documentation, in addition to mega, giga and terahertz you can find links to milli, micro and nanohertz as predefined units. NANOGrav stands for North American Nanohertz Observatory for Gravitational Waves and it uses the Galaxy to detect them. It was founded in 2007 and is part of a global community of scientists in places like Australia, where the Parkes Pulsar Timing Array is located - yes, that Parkes - made famous from the film "the Dish" and Europe with the European Pulsar Timing Array, combining five separate radio-telescopes, all coming together under the banner of the IPTA or International Pulsar Timing Array. The point of my little exploration is that if you're curious about random things, you can often come across activities and ideas you know nothing about and learn something along the way. Today I learnt that there is such a thing as a sub-Hertz signal, it's being explored all over the globe with scientists in different fields and it's happening without much in the way of public awareness. What did you learn today and which SI prefix didn't I use? I'm Onno VK6FLAB