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Explain Gravitational Waves to a Five-Year-Old · 7:03pm Feb 12th, 2016

The Guardian posted an article today in which they had four physicists try to explain the idea of (newly-discovered) gravitational waves to a five year old. My feeling is that this met with varying levels of success. (These are professional physicists, after all.)

The Guardian also asked for commenters to try their hand at explaining the discovery as well. I thought that might be a bit of fun, so I gave it a shot. I know a bunch of you guys like science topics, so I figured y'all might enjoy if I shared what I wrote. Feel free to: provide your own kindergarten explanations of high-level physics phenomena, call me out on how bad mine is, or generally geek out about gravitational m*****f***ing waves!

Two friends named Anne and Brian want to play a game. They drive out to a lake at night—Anne goes to one side, and Brian goes to the other. Here's the game: Anne will drop a rock into the lake on her side, and Brian will try to guess how big Anne's rock was—even though he never gets to see it.

How can Brian guess this? When Anne drops her rock in the lake, it will make a splash. The water underneath the rock will try to move away—who wants to get squished by a big rock, anyway? But the lake is full of water, so it's hard for the water underneath the rock to move. It pushes up against other water and that water tries to move.

Think of a queue[1] for a roller coaster. Sometimes everyone pushes together because they want to get on, but they have nowhere to go until there's a new roller coaster car to take people. Sometimes the queue stretches out as some people get on and others hurry forward to fill the space in the queue they just left. This is how the water moves—in waves.

Again, how can Brian guess the size of Anne's rock? He can do it with maths, if he can see the size of the waves that get made when Anne drops her rock in. Those waves get smaller and smaller as they move away from Anne—which is why Brian also needs to know how big the lake is. But if he knows those two things, he can figure out the size of Anne's rock, even though he never saw it.

A hundred years ago, a very smart scientist said that the space all around us acts like water—it can bunch up or stretch out in waves. You can't drop a rock into space, though, so what causes those waves? The waves in space are made by very big things (even bigger than the sun) trying to move very, very fast. We've been trying to find these waves in space ever since the scientist predicted them, but it's hard. The waves are very, very small when they get to us—even smaller than a strand of your hair. (This sounds bad, but it's actually good. For the waves to be bigger, we'd have to be much closer to the things causing them. I won't explain why, but being much closer to those things would be very, very bad.)

When Anne and Brian go to the lake to play their game, Brian can learn the size of Anne's rock even though he never gets to see it. The cool thing about these waves in space is that they let us learn about things we'd never be able to see, too. There are things in space called black holes. Black holes are like giant rocks you can't look at—because if you shine a light at them, the light just goes away. But thanks to the waves this scientist predicted, now we have a way to learn more about black holes. And not just black holes. There are other things in space that we can't see—very, very old things—but now we might be able to learn more about them. All because of these waves in space.

[1] You know the Guardian is a British newspaper, right? Just be glad my examples didn't involve crumpets and lorries.

Comments ( 12 )

I happen to be the parent of a precocious almost-five year old (who has explained things like symmetry to me)... I should give this explanation thing a try

Randall Munroe, creator of the webcomic XKCD, recently wrote a really nice piece in the New Yorker in which he describes special and general relativity using only the 1,000 most used words in the English language. It doesn't touch on gravity waves directly, but it does a really nice job explaining these concepts in a simple manner:

I quite like that! And I have to admit that Randall Munroe was very much in the forefront of my thinking when I tried writing the above.

If you do, let me know how it goes. :ajsmug:

I might tackle this when I am not reeling from my sixth infusion of ketamine. :applejackconfused:

The theory of general relativity explains the force of gravity by supposing it is very, very different from other forces. Unlike magnets, gravity doesn't hold stuff down by pulling on it. It holds stuff down by changing the direction that stuff can move: it bends the space we live in so that straight lines are no longer completely straight. When you're near something heavy, straight lines curve toward the heavy thing. Even if you try to move away from the heavy thing, going away in a straight line can still bring you back to it.

How much straight lines bend toward a heavy thing depends on how heavy the thing is, how close you are to it, and how fast you're moving when you draw them. You can't fly away from the ground unless you move very fast, because all slow lines bend right back down to the ground. If a thing is heavy enough and you're close enough to it, all lines head toward the thing, no matter how fast you draw them. That's what a black hole is, and it's why not even light can escape: all straight lines head directly back inside.

General relativity is interesting because it's very simple, it makes almost no sense, and it's correct. It's also frustrating because it doesn't work with our theories about matter and energy, and nopony has figured how to fix that. The discovery of gravitational waves is very important because it's the final prediction that hadn't been confirmed yet.

Gravitational waves are a temporary stretching of space. They're caused by the fact that there's a limit to how quickly space can be stretched from one shape to another, so when very heavy things move around they can send off ripples of stretchy space. At first, scientists thought that the ripples didn't matter, because if you're just stretching space a little one way and then another, it would be stretching everything the same way so you couldn't possibly see an effect. For example, if space stretched, this would mean your tape measure stretched too, and you wouldn't be able to measure it. But a very clever man named Richard pointed out that physical effects would happen because objects themselves can be less or more stretchy. Objects that are less stretchy would be less affected by the waves than objects that are more stretchy. This means that gravitational waves can cause physical changes, like producing heat, which means they have to carry actual energy.

Unfortunately, gravity is a very, very weak thing. It takes a very large amount of stuff to see gravity work at all, and the only things that can cause gravitational waves are really big things that live very, very far away from us. This is why it took a very long time to build things that could detect gravitational waves. But we recently did build something that works, and we were able to detect the gravitational waves made by two very large black holes merging, even though it happened very far away. Even then, the stretchiness that we had to detect was much smaller than the width of an atom.

You know the Guardian is a British newspaper, right? Just be glad my examples didn't involve crumpets and lorries.

"Imagine if Geoff's double-decker bus stalls in middle of the M25. All the motorcars back up behind him and his bus up and over the hill on the verge. Now imagine Graeme driving his delivery lorry whilst eating a crumpet for breakfast. Graeme drops his crumpet, and as he is distracted reaching down to retrieve it, rams into the back of one of the queued motorcars. That motorcar hits the back of another motorcar, which hits the back of another motorcar, which hits the back of another motorcar,..up and over the hill until the last motorcar hits Geoff's bus. Officer Henry is helping Geoff with his bus with the last motorcar hits. Officer Henry knows how heavy the bus is (he just radioed for a breakdown lorry) and he knows how long the motorcars are backed up on the verge over the hill (his partner in the police car heard it on their radio). By measuring how far the last motorcar pushed Geoff's bus, Officer Henry can tell how big Graeme's lorry was, even though Officer Henry cannot see Graeme over the hill."


When you hop onto your bed, you bounce a little, don't you? But when daddy jumps into bed, he has a huge bounce and the whole thing really shakes, and can send you flying! You don't have to see to know daddy jumped on the bed, you can feel it. Well, in outer space, when things move, they shake space. We can't see them, but we can tell by the shakes what those things are like, just like you can tell if its me or your baby sister jumping on the bed.

Think of a queue[1] for a roller coaster.

[1] You know the Guardian is a British newspaper, right?


He can do it with math, if he can see the size of the waves...

I feel so conflicted.

(But speaking as an undergraduate who is learning general relativity right now, I have to say I quite liked this explanation. If only it were that easy to do tensor calculus!)



Match all the upper indices with the lower indices, brush the dangling one you can't find a partner for under the carpet and hope it wasn't important, and Bob's your uncle? :derpyderp1:

Thank you for that link. I had no idea Munroe had published it, and I quite enjoyed it -- both for the science and for the ending bit about how not to feel dumb, which I think is an important part of science outreach, perhaps most of all to people young enough that they couldn't understand more than the ten hundred most common English words.

Needs more misspellings of words like "utilize" and "color".

It wasn't quite as groundbreaking to her as to the rest of us. I explained how objects create gravity waves, just like how she can create waves in the tub, but these kinds of waves are so weak that it takes highly specialized instruments and the movement of planet-sized objects-- and that's about as far as I got before we were off on a new topic. Sorry to disappoint...

Instead, here's some of her own astute observations (when she was three-to-four if memory serves):
-nobody has powers in their hands, only bones
-I'm made of skin
-(while looking at a Halloween decoration): why doesn't that boy statue have a face?

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