Enceladus: a song of ice and tides

An artist impression of Cassini diving into Enceladus water plumes. credit: NASA/JPL

Cassini will terminate its 20-odd-years-long mission in September. But it’s determined to go out with a bang. In yesterday’s press conference, NASA announced that the probe, during a 2015 flyby of Saturn’s moon Enceladus, found clues that the ocean within the icy moon has almost all we think it needs to spark life. Continue reading

The sound of silence

Think about the most quiet place you’ve ever been to. Now imagine something even quieter. What does that sound like? If you can’t figure it out, physics can help: let’s start by looking at how sound works.

Loudspeakers, vocal folds and instruments all function by the same principle: rhythmically push and pull on air. The air molecules, in turn, push and pull on their neighbors, that push on their neighbors, and so on. Air thus stretches in some points, compresses in others, creating a wave of pressure. A sound wave is born.


Molecules of air “staying put” actually do a lot of moving. CC-BY-SA Greg L, via Commons

But what if there is nothing to move air and produce sound: what does silence sound like? Does it have one at all or is it like questioning the color of an invisible thing?

Air molecules bump into each other and vibrate all the time. Just by being there and having a temperature, air is bound to create microscopic changes in pressure here and there. Even the most barren, isolated, still place has a sound: the sound of silence.

Molecular collisions like these are pretty much random and independent. Their sound—silence—is then white noise, the stuff some people use to relax or focus.

The loudness of silence kind of depends on what frequency range you look at: the narrower the window, the fewer kinds of bumps you will find, and the quieter silence will appear.

According to some calculations, in the band of maximal sensitivity of humans (around the pitch of speech), air sitting there is about -20 decibels. That’s silent. Too much for us: it’s just audible for an owl, a super-specialized stealth predator with ears literally the sound of its face.

That big circle around an owl's face funnels sound: it's basically a giant ear. CC-BY-NC-ND Brian Scott/flickr

The owl’s face circle funnels sound from the environment: it’s basically a giant ear. CC-BY-NC-ND Brian Scott/flickr

However, in the full range of human hearing, silence is considerably louder: about 0 decibels, which is also about the faintest thing we can hear.

That means our hearing can handle anything barely louder than total silence just as well as a conversation—a thousand times louder. Sounds good enough.

If you want more
  • How is 60dB a thousand times more than 0? Decibels are weird: here’s a summary of this and other peculiarities.
  • What does it mean “white noise”? Can it be other colors too?
  • Some say total silence drives you crazy. Scientists don’t work on hear-say. They try.


Cover photo: CC0 Sam Halstead, via pixabay.

A star like any other?

Think about it: the idea that the Sun is essentially the same thing as any star doesn’t make any sense. I mean, just look at them, they could hardly be more different!

Seriamente. Vi sembrano la stessa cosa? credit: CC-BY-ND Jessie Hodge via flickr

Seriously. Do they look anything like the Sun? credit: CC-BY-ND Jessie Hodge via flickr

So how the heck do you go about proving such an outrageous idea?

Well, it’s been quite a long journey,  that started from a number of guesses by ancient “scientists”. Granted, some of them turned out relatively correct, but others involved fiery stones hanging from the sky.

The first actually scientific step came only in 1838, when we learned how freaking far way stars are. German astronomer (among other things) Friedrich Bessel measured the distance to a star—now known as 61 Cygni—for the first time, without assuming anything about what the star was like. It turned out to be thousands of time farther than the Sun. Hundreds of thousands of times.

If they’re so far away, people concluded, stars might actually be as big as the Sun, if not bigger (spoiler: they get a lot bigger). But are they the same thing?

Shortly thereafter, we learned to read the chemical composition of a star from its light. Each element inside it, we learned, absorb light in a specific way. So when we look at the light from the star through a prism, we can see the thin black bands the elements leave behind and reverse-engineer what they were.


Solar light, broken down in its component colors, with the black bands (also called “Fraunhofer lines”) left by elements inside it. Via wikimedia

Then, we figured how to relate the colors (more exactly, the wavelength) of starlight to their temperature. As the video below explains, everything with a temperature—aka everything—glows, and it does in a particular way, depending on how hot it is. Only quantum mechanics explained how and why, but it works: it’s how infrared thermometers operate.

Altogether the Sun turned out about average in size, temperature and composition. Yet, it was a special star: the only one we knew to have planets. That, too, changed. It took a while (until the 1980s), but nowadays we find exoplanets—planets orbiting other stars—by the thousands.

So the Sun is just a star. We’ve thought about the universe and literally just looked at it, and we understood something so violently counterintuitive. No star that we know is home to anything with such remarkable ability. And that, I think, is pretty special.


Cover photo: CC0 Mayur Gala, via unsplash.com

If you want more
  • Nobody measured the distance to a star before Bessel because, at the time, it was really hard. Here’s a wikipedia page on the ingenious method he used.
  • Actually, quantum mechanics was born trying to explain how things glow depending on their temperature. This video from PhysicsGirl explains how

Every snowflake is unique

No Christmas landscape is complete without snow. Lots of snow. And every little snowflake is unique, everyone knows that! How come, tho?

Snow is nothing else than teeny tiny ice crystals that form in the clouds and stay solid all the way down to the ground. Water crystallizes around microscopic imperfections, like dust particles floating in the clouds. Once the initial nucleus is formed, the microscopic droplets gather around it very rapidly.

Even though all snowflakes are somewhat hexagonal (due to the geometry of water molecules), each of them grows in slightly different conditions. Some had more water droplets close by, some other was in a portion of space a fraction of a degree warmer. Each and every factor counts: the final shape of the crystal is sensitive to exactly everything.


Snowflake Sculpture 3, CC-BY-NC Julie Falk, via Flickr.

So the shape of each snowflake is random, it’s like rolling a die with infinitely many faces. You never know what will come out and all outcomes are different. If you want to put it in more physics-pompous terms, the formation of snowflakes is a stochastic process.

In the end, each snowflake is a picture of the exact conditions in which it formed. And since it’s impossible to reproduce the exact same conditions twice, each of them paints a slightly different picture.

Like pictures, snowflakes too come out better if the scene doesn’t move too much. Indeed, if conditions don’t stay relatively constant around the budding crystals, anything can happen. Most of the times, several crystals aggregate in one big snowflake, sort of a little snowball, which look a lot more like each other.

Regardless of the conditions, it’s really hard to tell them apart anyway.

Happy holidays from amorefisico!

Cover photo: Snow leopards playing in the snow, CC-BY-ND Tambako The Jaguar, via Flickr. Some rights reserved.

Frozen bubbles

In the cold of Canada, a man blows a soap bubble, which immediately freezes. Since that’s awesome, the guy makes a nice video about it.

Who knows, maybe he even knew about all the physics that was going on in front of his camera.

Let’s start from the easiest thing: why does the bottom of the bubble freeze last? The answer is gravity: the outermost layers of the bubble slide down, so the base just has more water to freeze. And it takes longer.

But why does it freeze in spots, instead of just from the top down?
Because cold is not the whole story. Water needs a point to start building its crystals, if it can’t find one, it stays liquid well below zero degrees (Celsius), but is very unstable and freezes at the slightest disturbance.

In a stunning turn of events, a soap bubble has plenty of soap molecules floating around, and they are excellent starting points for the ice crystals we see growing on the surface.

Finally, why does the bubble pop instead of just staying there? In the comments to the video, the author says he popped it, but I do believe he could have just waited a little.

The air he blew in the bubble came from his lungs, so it was over 30 degrees. Pressure inside the bubble, initially, is the same as the atmospheric one, but it drops as the air cools down. This puts a lot of stress on that thin and stiff ice surface. Sooner rather than later it was bound to collapse anyway.

All this magnificent physics in less than 30 seconds. What a wonderful world…


Foto: Frozen, CC-BY-NC-ND Benjamin Lehman, via Flickr. Some rights reserved.