## Tuesday, August 19, 2008

### Of waves, sounds, and solar wind

I've got a bunch of links here: sounds generated from scientific data from space! I have a long explanation afterwards, but first take a glance at some of these links so you know which direction I'm going.

Selected Sounds of Space
The Lion Roars
The Music of the Spheres
Sounds of the Magnetosphere

One of the important parts in thinking like a physicist is understanding waves. Waves appear everywhere. The music I'm currently listening to is a wave. The light coming from my computer screen is a wave. And thanks to quantum theory, we can consider the entire universe to be a many-dimensional wave of sorts.

Making a wave is really not all that difficult. If you have a system that is stable, like a pendulum, for instance, that means that a small push and it will just come swing back towards equilibrium. In some systems, it won't just go straight to equilibrium, but will swing back forth for a little while first. We call this an oscillator. If we have a bunch of oscillators linked together, then one oscillator will cause the next oscillator to move, which causes the next one to move, and so forth. Thus we have a traveling wave.

Nearly anything can be an oscillator. Each individual particle is an oscillator. If you knock a bunch of particles forward, they'll hit another bunch of particles, which will hit another bunch of particles, and so forth. These are sound waves. If you drop a pebble into some water, it will cause a small excess of water in that location, and this will push up the water in the surrounding area, which will push up even more water. This is a ripple, a water wave. If you run a bit of electricity through a wire, it creates electric and magnetic fields. Because of the peculiarities of Maxwell's equations, which govern electric and magnetic fields, the fields will spread outwards. This is an electromagnetic wave (aka light, though not necessarily visible light).

A more mundane example is traffic. Let's say we have a freeway full of cars, and one of them slows down to get a better look at an accident. The car behind it has to slow down too, and then the one behind that one, and then the next car. This wave can propagate backwards through the traffic a long way. This just goes to show, even when we look at non-physical laws (ie traffic laws), waves still appear as an emergent pattern.

In my research, I study yet another kind of wave that travels through something called a plasma. A plasma is basically ionized gas. By ionized, we mean that the gas is energetic enough that most of the atoms have been separated from one of their electrons. As a result, most of the particles in a plasma either have a positive charge (the ions) or a negative charge (the electrons). This gives plasma many electromagnetic properties, since electricity is basically the movement of charges. And recall that sound waves are basically made of moving particles. Thus, you might expect a combination of sound waves and electromagnetic waves to travel through plasma. These plasma waves come in all sorts, depending on the pressure of the plasma, the direction of the fields, the frequency of the wave, and so forth.

Pretty much all of interplanetary space is filled with plasma. I'm talking extremely low density, better than the artificial vacuums we can create down here on earth, but the plasma is still there. I often joke that I study invisible fields and fluids that exist within vacuums (and I must say, the invisible and the non-existent do not look alike). Most of this plasma comes from the sun. The sun ejects a constant stream of particles called the solar wind. The solar wind would be quite deadly for life on earth if it hit us, but luckily it is deflected around the Earth by Earth's magnetic field. It's sort of like a river being deflected by a rock. But let's not take the analogy too far--Earth's magnetic field is a bit squishier than a rock; it is shaped by the solar wind. And this wind is not entirely constant. You can see the temperature, speed, density, and magnetic field change in these graphs of live data. These changes interact with the Earth's field, and create all sorts of plasma waves.

Since plasma waves and sound waves are both waves, it is possible to convert between the two. You can't hear plasma waves, but you can use a special device or some simple software. A radio, for instance, converts radio waves (light waves) into sound waves. And your speaker converts electrical signals (also a wave) into sound. You can do the same with plasma waves, usually by converting it to data, and then into an electrical signal that goes through your speakers.

And these sounds aren't meaningless. If the pitch is higher, that means the wave has a higher frequency. If it's lower, the wave has a lower frequency. If the pitch is descending or ascending, that means the frequency is slowing down or speeding up. If we hear a sound that is pitchless, that means that there are waves of all different frequencies happening all at once. All of these indicate different kinds of important events in the magnetic field.

There are also plenty of waves that are so low frequency that the human ear cannot detect them. Luckily, there are ways to artificially speed up the waves. Some of the links at the top use waves that we can hear; others use slow waves that have been artificially sped up. And there's no reason to confine ourselves to plasma waves near Earth. Some of the links have sounds from Jupiter too.

One of the characteristics of this field is that we have a ton of data. Tens of thousands of numbers every day, from each of the many devices and satellites we have out there all over and above the world. But as anyone intuitively knows, having ten thousand numbers is useless because it's just an information overload. We need to find overall patterns so we can better understand the physics behind it, or even make predictions. Computers tend to be very inefficient at finding patterns, whereas the human mind is hardwired to find patterns (often even when there aren't any). In order to take advantage of this pattern-seeking superpower, we just need to reduce the data to something we can wrap our minds around. One way to do this is with a graph or a visual. Another way is by converting the data to sound. So if you listened to any of the space sounds, you may congratulate yourself for exercising one of the few scientific powers that is inborn.