# Sciency Words: Gravity Waves vs. Gravitational Waves

A few years back, I made a bit of a fool of myself in front of a professional physicist from LIGO.  You see, I kind of have a reputation, both online and in real life.  I’m the Sciency Words guy.  I’m the guy who knows stuff about scientific terminology.

So it’s pretty embarrassing when I get my scientific terms mixed up!  For today’s episode of Sciency Words, I’d like to share with you the two terms I got confused about so that the next time you meet a physicist from LIGO, you won’t make my mistake.

## GRAVITY WAVES

Gravity waves have to do with fluid dynamics: the movement of liquids and gases.  As an example, imagine an air mass being blown up and over a mountain range. Once over the mountains, that air mass will start to fall downwards again due to the force of gravity.

But of course air masses don’t sink straight down like lead weights.  Air has a lot of buoyancy, so that air mass will bob up and down for a while until it settles into a stable equilibrium.  This bobbing up and down motion will produce ripples in the atmosphere, and those ripples are called gravity waves.

Gravity waves have been observed both in Earth’s atmosphere and Earth’s oceans.  They’ve been observed on other planets as well.  Basically any time part of a liquid or gaseous medium is forced upwards, you can expect gravity to pull it back down again, producing gravity waves.

## GRAVITATIONAL WAVES

Gravitational waves have to do with Einstein’s theory of general relativity.  As an example, imagine two black holes spinning rapidly around each other. Even if you’re watching this from a safe distance, you might notice the combined gravitational attraction of those black holes grows stronger and weaker in a regular, oscillating pattern.

Well, actually you probably won’t notice that.  Even in the most extreme circumstances, those oscillations in gravity are barely detectable.  But they do happen.  The LIGO Project confirmed that in 2015 (the news wasn’t announced until 2016).

French theoretical physicist Henri Poincaré gets credit for coining the term gravitational waves (ondes gravifiques in French).  He first wrote about them in 1905, around the same time Einstein was formulating his theory of special relativity.  I’m not sure who coined the term gravity wave, but English mathematician George Biddle Airy was the first to mathematically describe gravity waves in 1841.

My mistake was asking a physicist who studies gravitational waves for LIGO a question about gravity waves in the atmosphere of Titan.  I mean, it’s an understandable mistake, getting these two terms confused—unless you’ve been introduced as an expert on scientific terminology!!!  Then it’s super embarrassing!!!

P.S.: As it so happens, I got the chance to meet up with that same LIGO physicist once again this week.  She was giving a presentation at Princeton University.  Don’t worry.  I didn’t embarrass myself too much this time.  I’ll tell you more in Monday’s post!

## 7 thoughts on “Sciency Words: Gravity Waves vs. Gravitational Waves”

1. Fran says:

Well I did not know they were different things. Don’t fret! Mixing up very similar terms isn’t a bad mistake at all. LIGO scientists probably have all done this at least once!

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1. You’re right! And now that I think about it, she seemed to know right away where I’d gone wrong. Surely she’s heard people make that sort of mistake many times before.

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2. I wonder if the butterfly effect is sufficiently strong so that a gravitational wave could have an impact on molecules in the atmosphere of Titan?

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1. I guess it’s possible, but based on what my LIGO scientist friend has told me about her work, I’d say that’s unlikely. The flapping of a butterfly wing puts exponentially more energy into the atmosphere than a gravitational wave would. Any effects gravitational waves might have on Titan’s atmosphere would probably be drowned out by the effects of other things.

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3. I think the waves you see when river rafting – standing waves – are a form of gravity wave. Water is lifted a rock and drops down the other side, looking like a hole in the water. Gravity waves occur in any fluid; for dynamic meteorologists, air is a (very thin) fluid.

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1. Yeah, that sounds right to me. That’s probably a better example than the one I used.

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