Sciency Words: Gravity Waves vs. Gravitational Waves

Sciency Words MATH

Today’s post is part of a special series here on Planet Pailly called Sciency Words. Each week, we take a closer look at an interesting science or science-related term to help us all expand our scientific vocabularies together. Today, we’re looking at two terms that have almost nothing to do with each other:




What happens when you combine a 29 solar mass black hole with a 36 solar mass back hole?

Fb08 Black Hole

In this not-so-hypothetical scenario, 29 solar masses plus 36 solar masses equals 62 solar masses. The remaining 3 solar masses are converted into energy in the form of gravity waves. I mean gravitational waves.

I’ve been making this mistake a lot lately, ever since LIGO announced that it had detected gravitational waves for the first time. It’s just easier to say gravity waves. It’s two syllables shorter. Unfortunately, gravity waves and gravitational waves are completely different concepts.

What are Gravitational Waves?

Gravitational waves are part of relativistic physics. According to Einstein’s general theory of relativity, gravity bends space-time. Among other things, this bending causes everything from spaceships to planets to even light itself to follow curved trajectories in the presence of a gravitational field.

Extremely massive objects moving rapidly together, such as a pair of co-orbiting neutron stars or, in the case of the recent LIGO discovery, a pair of merging black holes, bend space one way then the other so violently that they produce a rippling effect in the fabric of space-time. We call these ripples gravitational waves.

What are Gravity Waves?

Gravity waves are part of a different field of physics called fluid dynamics. A few years ago, gravity waves were observed in the atmosphere of Venus, most likely due to air masses rising over mountainous terrain and falling down the other side. After these air masses return to their original altitude (return to a “state of equilibrium,” to use the technical lingo), they tend to bob up and down a bit, producing characteristic ripples in the atmosphere around them. We call these ripples gravity waves (specifically, they’re atmospheric gravity waves).

While this phenomenon has been observed on Venus and Titan, it is best understood here on Earth. Gravity waves are known to appear in Earth’s atmosphere, in lakes and oceans, at the interface between the atmosphere and the ocean… basically anywhere you find a fluid or fluid-like medium. Whenever a fluid returns to a state of equilibrium, either due to gravity or buoyancy, you can expect to see gravity waves.

One Wave is Not Like the Other…

Of course if you’re having a casual conversation about the LIGO experiment (who doesn’t have casual conversations about experiments in relativistic physics?) and you mistakenly say gravity wave instead of gravitational wave, I doubt anyone will be confused. Nine times out of ten, context will make it clear which kind of wave you meant. Just so long as somewhere in the back of your mind, you know there is a difference.

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