Eureka: Bridge to Dark Matter

The first time you hear about dark matter, it sounds kind of crazy– asserting that we’re surrounded by tons of invisible stuff is usually a good way to get locked up. But the process of its discovery is surprisingly ordinary: it’s just what you do when you play cards.

Here’s the second green-screen video I’ve done to promote Eureka: Discovering Your Inner Scientist, which comes out three weeks from tomorrow (but can be pre-ordered today!). This one is about card games, modern astrophysics, and why you probbaly shouldn’t play bridge against Vera Rubin:

For those who dislike video, I’ll put the approximate text at the end of this post. This is, of course, a shortened version of a longer argument from the book. This benefits enormously from images and data provided by Becky Koopmann, one of my astronomer colleagues at Union.

Because I get to use these to cross-promote stuff that I like, I threw in a great Surviving the World Comic, and by a weird coincidence, ended up wearing a T-shirt with that same slogan on it (the first time I recorded video, I had an astronomy-themed shirt, but the camera glitched and didn’t record any audio, so we had to re-shoot). And, of course, there are cute-kid photos…

There’s a third video in the works, but probably not next week because of work and Thanksgiving. Unless I get really inspired and it comes together awfully quickly. And, of course, if you like this please consider buying the book, which has more on this story, and many others like it….

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If you follow science news at all, it seems like every week astronomers announce the discovery of some amazing new thing: unusual stars, planets around those stars, giant black holes, even whole new galaxies. But here’s the most amazing part: for every new bit of matter astronomers see through their telescopes, there’s five times as much stuff out there that we can’t see. It’s called “dark matter,” and we have no idea what it is.

That might seem incredible—after all, insisting that you’re surrounded by lots of invisible stuff is a great we to get yourself locked up. So how can this be science? What could make astronomers think there’s all this invisible “dark matter” running around?
The answer is surprisingly mundane. Astronomers discovered that dark matter exists by using the same skills you use to play cards.

I don’t mean that they’re lucky, or the whole thing is a big bluff. I’m talking about the process a good card player uses to figure out the cards held by the other players, particularly in a game like bridge.

Bridge is one of the world’s most popular card games, and what sets it apart from other games is the “bidding” process. A game opens with the players taking turns offering a number, and a suit—two clubs, three hearts, four spades. The suit is what they want to be trump, and the number is how many tricks they think they can win, together with their partner.

Now, predicting how many tricks two of you can win together would be a lot easier if you know your partner’s cards. But you can’t just look at your partner’s hand, or even ask them what they have. Instead, you have to figure it out from the bidding, using a complicated set of “conventions” about what you should bid given what cards you have. It’s a tricky and indirect process, but it works. A good bridge player who knows the conventions will start the game knowing almost exactly which cards each other player is holding, just from the pattern of bids.

Astronomy is a lot like bridge. Astronomers want to know what’s going on in distant parts of the universe, but as much as they might like to, they can’t go there to investigate. All they can do is look at the light that reaches Earth. Astronomers can ask two questions—“How much light?” and “What color is the light?”— so they’re a little better off than bridge players, but only a little bit.

But like bridge players, they have conventions they can use—in this case, the laws of physics. Knowing the laws of physics lets them get an incredible amount of information out of those two questions. Because they know the physics of how stars operate, they can use the total amount of light to determine the distance to certain types of stars. More distant stars appear fainter, and if you know how much light it emits, and how much you detect, you can figure out how far away it is. This lets astronomers map the universe, all the way out to distant galaxies billions of light-years away.

The real goldmine, though, is asking what colors of light we see coming from a distant object—its spectrum. Quantum physics tells us that atoms emit light only at certain very special frequencies, and each element in the periodic table has its own unique pattern of frequencies. So looking at the spectrum of a distant star can tell you exactly what it’s made of.

More than that, though, the spectrum of a distant object tells you how it’s moving. This is because of the Doppler effect: waves emitted by a moving source shift their frequency depending on their velocity. That’s responsible for the “eeeeee-owwwww” sound of a fast moving car going past. As it approaches, the car catches up to the sound waves it already emitted, and the sound shifts to shorter wavelengths, and higher frequency. As it recedes, it “runs away” from the earlier waves, and shifts to longer wavelengths and lower frequencies. The same thing happens with light: if an object is moving toward us, we see the light shifted toward the blue, and if it’s moving away, we see the light shifted toward the red.

The Doppler shift is what tells us that the universe is expanding. It’s als one of the keys to dark matter. Some of the best and most important evidence was first measured by the astronomer Vera Rubin, who has spent about sixty years using the Doppler effect to measure the motion of galaxies.
She started in the 1950’s, looking at the motion of whole galaxies, and groups of galaxies, but in the late 1960’s, she turned to looking at motion within galaxies. When we look at a rotating spiral galaxy, the stars on one side are moving toward us, and have their light shifted to the blue, while the stars on the other side are moving away and shifted to the red. The difference tells us how fast the galaxy is rotating, at different distances from the center.

This is a very tricky measurement, because the light from a galaxy is very faint, but one of Rubin’s colleagues, Ken Ford, had invented a new type of spectrometer that made it much easier. So Rubin and Ford started looking at rotating galaxies, and they found something amazing. They saw the rotation they expected, with stars on opposite sides of the center of the disk moving in opposite directions, but as they looked farther out from the center, the speed stayed the same.

This was shocking, because they expected the rotation speed to decrease farther out, for the same reason that the outer planets in our solar system move slower than the inner ones. The farther you go from the Sun, the weaker its gravity. The force just isn’t strong enough to hold a fast-moving planet in a big orbit. The same thing should happen with galaxies: at the outer edge, where you run out of stars, there shouldn’t be enough gravity to keep fast-moving stars in orbit. But Rubin saw fast-moving stars as far out as there was light to see. The only way this can happen is if there’s a huge amount of other matter there, five times as much as the stars that we see, providing the extra gravity.

When Rubin first reported this, everybody thought it must’ve been a mistake. So she measured another galaxy, and saw the same thing. And another. And another. Other people measured more galaxies, and saw the same thing. Other bits of evidence started to accumulate, as well, all of it pointing in the same direction: for every bit of matter we see through a telescope, there’s five times as much dark matter. It doesn’t produce light, but we know it must be there because its gravity leaves an imprint on the light we do see. Combining the spectrum we see with knowledge of the laws of physics leads inevitably to dark matter in the same way that combining the bidding history with the conventions tells a good bridge player what’s in their partner’s hand.

I tell this story not just because dark matter is weird and cool—which it is—but because it tells us something important about the nature of science. Ideas like dark matter are so strange and improbable that it may seem like scientists must be making them up just to mess with your head.

But science isn’t weird and arbitrary like that. At its core, science is a surprisingly ordinary process: you look at the world, you think about why it might work that way, you test your theory with further observations, and you tell everyone about it. This process is as old as humanity itself, and something we all use, all the time. It’s what you do any time you decide to relax by playing a few hands of bridge: you look at the bidding history, you think about what it tells you about the other players’ cards, and you test your theory by playing the game. And, if you win, you brag about it to all your card-playing friends.

Modern science is full of amazing things that we can’t see directly, but know have to be true. From the outside, these might seem like they arrive out of nowhere, as if by magic. But then again, so does bridge, if you don’t know the process. When you put it all together, discovering dark matter is no more magical than figuring out who’s holding the ace of spades.