Clear Science!

We talked about frets on a guitar and how when you push the string into the fret board it raises the pitch on the note by changing the length of the standing wave on the string. Another interesting thing you can do is this: after plucking the open string, just lightly touch the string with your finger at the 12th fret. A sound like a bell or flute will result, as you create the second harmonic of the original standing wave.

Here’s what happens: by touching the string lightly while it’s vibrating, you cause it to come to rest at that point you’re touching. This results in another node, as shown above with the second harmonic. Touching the string at the 7th or 5th fret can make the third and fourth harmonics, with higher numbers of nodes.

But if you touch the string someplace that does not result in equal segments between nodes, no harmonic is produced and the string stops. This is simply math. Guitar players can tell you all the spots where harmonics can be produced, even if they don’t care too much about the math and physics behind it. (Some of them care though!)

So we’re wondering if you could hear sounds in a nebula. We’ve figured out that sounds, which are waves of pressure, can be detected by humans if they are larger than 20 micro-pascals (μPa).

So what is the pressure in a nebula? The Clear Science staff looked it up, and found that a cold, dark nebula (like the Horsehead Nebula) will have at most at its core 100,000 particles per cubic centimeter, which is a box about the size of the end of your pinkie finger. Also, the temperature will be about 10 kelvins, or -263 °C.

Using a little math, we can figure out about what pressure this would mean. Don’t panic! This is using the ideal gas equation, PV=nRT, and the level of difficulty is about the same as in a high school Chemistry I class.

1. The quantity n/V is a concentration of particles, so we plug in the 100,000 part. per cm^3 we looked up
2. We use Avogadro’s number to convert the particles to moles, because problems are easier to do in moles
3. R is the gas constant, which we look up: 8.314 J/mol/K
4. T is the temp, 10 kelvins
5. And the last two terms we add are unit conversions: 1: a joule is a newton meter, and 2: we convert to make sure all lengths are in meters and not centimeters

The answer we get is 14 pico pascals or pPa. This is much lower than 20 μPa, so no, there is not enough gas density in nebulae to support sound waves! (At least not the kind of waves we call “sound.”)

Sound is technically an oscillation of pressure, i.e. waves, through a material. We generally experience sound as waves in our atmosphere, which is a gas, air. Gases are fairly separated particles moving rapidly and not well-connected to each other. There is also gas in space. Nebulae are interstellar clouds of dust and gas, often very pretty seen through a telescope.

So is there sound in nebulae? Luckily one member of the Clear Science staff moonlights at JPL when not clarifying science, and knew exactly how to approach this question. Let’s use the Horsehead Nebula as an example, which is what’s called a dark nebula. The Horsehead Nebula is in Orion, and was discovered in 1888 by Williamina Fleming at Harvard College. (You heard that right, Williamina was a woman, although science was pretty male back then.)

By the way, the visualization of sound waves shown above is from Bell Labs, and the Clear Science staff got the image from Modern Mechanix, one of the best websites on the internet. They love black and white science just like the Clear Science staff does.

So “light” is electromagnetic radiation, or EMR. And the different kinds of light seem different because of their wavelengths (λ). Okay great. But what exactly is waving? And going back to the question, how does it travel through the vacuum of space?

It’s a simultaneous waving of the electric and magnetic fields, at a 90 degree angle to each other. (Hence the name: electro-magentic radiation.) The blue wave (electric field) is going up and down, and the red wave (magnetic field) is going left and right.

These fields exist in the vacuum of space, and require no medium to propagate through. So, light can go anywhere. A very famous experiment called the Michelson-Morley Experiment was conducted at Case Western Reserve University in Ohio in 1887, and proved that light doesn’t propagate through any “stuff.”

Light is a wave (for one), but let’s back up a second and think about what light really is. The “light” we see is a very small slice of something know generally as electromagnetic radiation (EMR). Different kinds of EMR exist because of their wavelength. We’ve talked about wavelength before, and how it’s similar to frequency. Both things mean: how rapidly is the wave waving?

The entire range of possible wavelengths of EMR is called the “spectrum.” The plot above looks complicated, but it’s not. All that’s changing is that waves are rapid at the top and slow at the bottom.

The slice we call “visible light” is shown. The difference between colors is the wavelength of the wave. Radio and TV waves are the exact same thing as visible light, except much longer wavelength. (You could say they are redder than red.) Microwaves, X-rays, gamma-rays … it’s all EMR.