What the heck is a boson?

Particles are strange. In more than one way: for example they behave as if they were spinning, like a top. But they aren’t moving at all. Even if they were, trying to imagine how some of them “spin” would probably melt your brain. The property that measures this “spinning” (though nothing is moving at all here!) is, creatively, called spin.

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Circles, social circles and Pi Day

March 14 (or 3/14) is Pi Day. During this somewhat whimsical holiday, science nerds around the globe eat pies and perform needlessly complicated operations to celebrate the fact that the ratio between a circle’s circumference and diameter is 3.14152653… It may be a little confusing from the outside, but that’s sort of the point.

There are several reason behind Pi day, I think: Pi is a good symbol for science, it’s a fantastically inclusive one, and it’s the perfect thing to turn into a nerdy holiday.

Let’s start from its symbolic value. Pi is very recognizable, because most people have run into it at some point in their education. That holds also for a lot of important physical and mathematical constants. Physical constants, however, are not really absolutely constant (their value depending units of measure), plus they are often unsavorily large or very small.

Mathematical constants, instead, are just numbers, like 0 and 1. So why not celebrate excellent numbers like those? Well, Pi has more depth. Nobody knows all of Pi because it’s an infinite, ever-changing sequence of digits. Irrational numbers like Pi (or the golden ratio, e, square root of 2) are elusive and fascinating, but none makes as good a holiday as Pi.

Few (if any) of them can be as easily turned into a date. Then, none is as well-known as Pi. This number is freakin’ everywhere: from school geometry to quantum mechanics, from pendulums to number theory and probability.

Its ubiquity is a testament of how circles enter everywhere in science: whether something involves actual circles (or spheres) or trigonometry (which is just badly disguised circles), Pi is bound to pop up. Any oscillation, from a pendulum to the waves in the sea, to the wave function of quantum mechanics, calls for some trigonometry, and its Pi. Actually it shows up so much in quantum mechanics that scientists found ways to avoid having to write it.

In statistics and mathematics, Pi often comes out through calculations that involve the famous Gaussian probability distribution. This amazing function describes an unbelievable number of phenomena, from the result of rolling many many dice to the distribution of people’s height.

Students organized by height in an old experiment: they follow the characteristic bell shape of a Gaussian distribution.

The Gaussian is circles’ ninja way to come back in the picture (because of details in the math: won’t bore you with that). And one can tell they came through, you guessed it, from Pi.

So mathematician, physicists, engineers and all scientists alike are familiar with this fantastic number and use it practically every day. At the same time, Pi appears almost only in scientific contexts. As a symbol, it includes every branch of science, nothing more and nothing less.

This is also why it’s a great nerdy holiday. One of my favorite definition (-ish) of nerd comes from John Green:

What is nerdier, then, than celebrate the fact that a date looks like the ratio of a circle’s circumference to its radius? In other words, it’s not really about Pi: it’s about meeting and eating pies and finding creative new ways to calculate the ineffable number.

As Christmas is actually a day about love and family, Pi day is actually about community, nerd identity, and being unironically enthusiastic about science and math. There aren’t many such days, let’s cherish this one.

Cover photo: CC-BY Bill Ward/flickr

How to make something clean itself

The secret of perpetually stainless stuff hid for centuries in plain sight—at least for those of us living around lotus leaves. Now physics can help you never to clean again, no matter what you spill.

It all comes down to how water sticks to stuff, in other words, how stuff gets wet. Continue reading

Jazz and the atmosphere of exoplanets

The atmosphere of a planet holds the keys to make it habitable, so we need to look at them to figure if exoplanets are habitable. They are too far to send probes to measure them directly like we do with Mars or Jupiter’s moons, but scientists can study them from right here, looking at how they block light.

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What makes a planet habitable

A couple of days ago, NASA announced the discovery of seven-count-them-seven rocky planets orbiting a small star called TRAPPIST1. Three of them seem to even be in the “habitable zone”. So, have we found the aliens’ home?

Nope. However, words are a bit confusing, so let’s review what it takes for a planet to be habitable.

On planets too close to the star it’s too hot and water evaporates, on the far ones it freezes. Only those in the sweet spot stay just the right amount of wet. credit: NASA/JPL/Caltech

The key is liquid water. All life we know—from bacteria to cats, from carrots to Lionel Messi—hinges on chemical reaction that only take place in water. No water, no life.

The habitable zone of a star is the space around it where planets can have sustained liquid water on their surface. Closer, and the heat from the star will fry the planet with all the water (think Mercury), farther and the surface will freeze (think Pluto).

Now we need a surface to collect this water on. That’s why rocky planets are interesting: no rocky surface, no place for water. And again, no water, no life.

Location isn’t everything, though: the atmosphere is key too. Earth’s atmosphere keeps water on the surface, and temperatures friendly (for planetary standards). Less of it and we’d risk turning into Mars, which is in the habitable zone, but is a frozen desert, where water is more like a killer sludge. More atmosphere, and we might become Venus (also in the habitable zone), which is effectively hell, molten lead lakes and deadly acid rains included. Winds in the atmosphere also favor habitability on some exoplanets.

Then you must hold on to your atmosphere. Earth has a cozy magnetic field that deflects part of the Sun’s radiation. Mars probably had an atmosphere, when it also had oceans, but has no magnetic field. Atom by atom, the constant barraging of energy and particles from the Sun eroded it. With the atmosphere gone, so was the water. No water, no life.

How Google’s doodle portrayed the discovery

The planet discovered around TRAPPIST1 are important not because they might be habitable (though some are promising), but because TRAPPIST1 is the first star of its kind we study, and it immediately delivered several promising planets. That means it should be relatively easy to find interesting planets around that kind of star, which multiplies the chances of finding actually habitable ones.

Moreover, they are close to Earth (“just” 40 light years), so we can study their atmospheres with existing telescopes, or with the James Webb space telescope slated to launch next year. We’ll train on these for the multitude of planets we are about to find.

For now, as usual, no aliens.

If you want more
  • Do you remember what a fuss was made that time NASA found the first Earth-sized planet in a habitable zone? This time they were three at once.
  • NASA put together an impressive amount of information, graphics, and even an app. You can find everything here
  • The real star of the bunch is this phenomenal 360 video (fullscreen highly recommended!) of the view standing on TRAPPIST1d

Cover photo: CC0 David Mark/pixabay.com

The thirsty dragon and other capillarity magic

The mighty thorny dragon. Credit: wikimedia

There once was a dragon, who lived in the desert and loved to eat ants. He liked it so much that he gave up the ability to drink for that: he made his mouth perfect for eating, but useless when it came to sipping.

Instead, he learned something way cooler: how to conjure water from sand itself, summoning a force even stronger than gravity.

It’s actually a true story: the thorny dragon is a majestic, 20cm long Australian lizard. A recent study revealed that, to drink, it uses a complicated network of narrow channels that start from its feet, suck water from the ground. Then, snaking between its scales, they deliver the water directly into the lizard’s throat.

We bottled that force into wondrous technologies such as… er… paper towels.

Seriously, though, paper towels do suck water against gravity. Since nothing is pushing the water up, that’s more or less sorcery. However, scientists prefer to call it capillarity, and it’s fascinating.

You probably noticed that water sticks to stuff. It’s scientifically called—I kid you not—wetting. But water also sticks to itself, and these two things combined make capillarity work.

In a way it’s like keeping up a hammock. The net is made to stay together, it’s suspended from the poles, and it sags in between under its own weight. In a tube with water it’s much the same: the tube walls act as many tiny poles around the water surface hammock.


Water in a capillary tube is like a hammock, but the poles around it grow a little on their own. credit: Stefanie Laubscher/pixabay

Water has an important difference from the hammock net: it self-sticks to the tube walls. That means it can climb (a little) up the wall. That’s why capillarity works so well in narrow channels, with walls close to each other. Basically, the distance between the “hammock poles” is small, so there’s less space for the net to sag.

A paper towel and its maze of narrow tubes under the microscope. credit: Mattheyses Lab/Emory University

Though it seems a little more mundane, paper towels work the same way. Inside their mesh hides an intricate network of tiny tubes through which water can climb. A little dragon magic in our kitchens.

If you want more
  • Water sticks to stuff because of van der Waals forces. They’re very cool, but they deserve their own thing.
  • Now that we know how to get stuff wet, can we do the opposite? Well, yes… stay tuned!!

Cover photo: CC0 Tom Mathews/pixabay

2 deep questions on gravitational waves (with puppies!)

It seems like yesterday, but it’s already been a year since LIGO announced they found the elusive gravitational waves, that stretch and squeeze space (a tiny bit) in their wake.

But what does it mean “they stretch and squeeze space”? and how do you actually measure something like this? Let’s answer these important, very deep questions!

What does it even mean to “stretch and squeeze space”?

That is kind of a hard question, and I also struggle a bit with it. I’ll try my best, and please do correct me if I get it wrong.

Let’s consider an adorable universe where all of space consists of the picture of this puppy:

credit: Torsten Dettlaff/pexels.com

So cute. If we squeeze space in one direction, we make all distances smaller, we literally squeeze points together. Like this:Notice that we are scaling down distances, not chopping space off. The puppy is still all there, its ears got closer together. All the universe we started with is still there. Gravitational waves squeeze bits of space, while stretching others. Kind of like this¹:

Basically, stretching a portion of space means pulling everything in that portion of space away from everything else, by the same proportion.

Needless to say, spacetime is more complicated than puppies, but this should give an idea.

How do you measure a change of space itself?

If every length changes the same way, you can’t use rulers to measure changes in space, because they change too. To explain how it’s done, I might need some math. I promise it won’t hurt and I’ll use puppies to make it better.

Let’s say we have two perfectly identical puppies², Brian and Stella.

credit: Chiemsee2016/pixabay

They run to fetch a ball 50m away from us. They always run at 10 meters per second (freakin fast puppies), so it takes them 10 seconds to fetch the ball: 5 seconds to cross the 50m to the ball, and another 5 on the way back.


How Stella and Brian breathe as they run to fetch the ball. Since their paths are the same length, and the puppies start identically, they come back perfectly in sync.

These puppies also breathe very regularly, taking a breath every two seconds (one second breathe in, one breathe out). So, if they both start running with a breath in, by the time they come back with the ball, each of them will have taken exactly 5 breaths (one every two seconds), and they both arrive breathing out.

Good girl! Good boy!

Ok, now we throw the ball again, but an amazingly strong gravitational wave passes through, stretching Brian’s way path to 100m, and squeezing Stella’s to 25m. If they leave at the exact same time, breathing in, and running at the same speed, they will not come back together anymore: Stella arrives first, after 5 seconds (half as much as before, because she has half the distance), breathing in, whereas Brian arrives after 20 seconds breathing out.


What changed now that a gravitational wave stretched Brian’s path and squeezed Stella’s. Stella has less distance to cover and breathes fewer times than Brian: now they come back out of sync.

And that is how we measure if space itself changed. We can tell the two paths are not the same length anymore, because identical puppies come back out of sync.

Real dogs, of course, don’t breathe that regularly, or change speed instantly, regardless of how much you train them. You know what behaves like that, though? Light. Light travels at the same speed no matter what, and each “color” has its constant frequency (it breathes regularly in time).

LIGO sends trains of perfectly identical “laser puppies” on perfectly identical paths and measures any teeny tiny difference in their “breathing” (their wave phase) when they come back.

Bonus (without puppies): doesn’t the laser get blue- or red-shifted?

It should indeed. Like the stretching of space between us and distant galaxies shifts their light towards longer wavelengths, so the passing of gravitational waves shifts the light of the laser inside LIGO.

However, despite the scientists’ best efforts, lasers at LIGO can only produce a range of wavelength. They’re extremely close to each other, like between 1063.99999999999999999nm and 1064.00000000000000001nm. Yet, gravitational waves change them by ten times less than that, so it’s impossible to distinguish the effect from the instrument uncertainty.

Also, the effect the time it takes light to cross space is waaaaaay larger, so it’s “easier” to measure.

A huge thank you to my friend Leila, who patiently and graciously explained all this stuff to me

If you want more

… then you must really be into gravitational waves! I got a few posts for you, and this great video on how freakin hard it is to make the measurements.


¹Gravitational waves don’t quite work like that. It’s just an example, please don’t take it literally.

²These are not real puppies, these are example puppies. Please, for goodness’ sake, PLEASE don’t take this literally.

Cover photo: CC0 Chiemsee2016/pixabay