Although in general I have tremendous faith in science, there are a few concepts I’ve always had some trouble grasping. For example, textbooks have told us for decades—with great certainty—details about the interior of the Earth. We know how thick the crust and mantle are, what the core is made of, and how hot it is (among many other facts), even though no one has managed to dig or drill even halfway through the crust—the thinnest and outermost layer of the planet. How do people figure this stuff out? Yes, I know it’s all about earthquakes. When the ground shakes, sensitive instruments all over the world make detailed measurements. By carefully analyzing the way vibrations move from one point to another, scientists are able to infer a great deal about the planet’s structure. Although I accept that this is true, the actual physics and mathematics involved are so far beyond me that I can’t help harboring a slight doubt. Maybe the core is really made of flubber or, say, chocolate pudding, rather than iron.

If I wonder at proclamations about the composition of our own planet, you can imagine how I felt when I read that astronomers are now making claims about the interior of the Sun. Not only that, these claims are based on measurements of sound, which I am reliably informed does not travel through space. After a few minutes of eye rubbing and brow furrowing, I began the long, slow process of trying to wrap my brain around the emerging science of helioseismology, the study of vibrations that occur within the Sun and what they tell us about its interior.

Light Music
Helioseismology is a branch of the more general science of asteroseismology, which is not limited to our Sun in particular. But as the Sun is conveniently located much closer to Earth than any other star, it gives us the best opportunity for detailed study. Observers have noticed periodic changes in the brightness of the Sun and other stars for centuries, but a more detailed study in 1960 showed that the Sun’s surface has a very complex pattern of oscillation. In 1970, astronomer Roger Ulrich theorized that the oscillations were due to sound waves traveling through the interior of the Sun. Further research in the decades that followed has confirmed that hypothesis.

How exactly does one observe oscillations on the surface of the Sun? By measuring the Doppler shift of light coming from a given location on the Sun’s surface using specially designed spectrometers, astronomers can tell whether that spot is moving inward or outward, and at what rate. This data can be used to create computer models that graphically depict the waves on the surface. And this information, in turn, reveals what’s going on under the surface of the Sun. Some people liken helioseismology to a sonogram: using sound waves to “see” inside a body—in this case, a celestial body. In the case of the Sun, the vibrations are affected by temperature, density, and convection, meaning that by studying the patterns of sound waves, astronomers can create surprisingly detailed, dynamic maps of the interior of the Sun—not just its surface.

Weather Forecast: Sunny
Helioseismology has produced some interesting results. For example, astronomers now know that the Sun contains plasma “rivers” or “jet streams”—massive convection currents that produce what you might think of as solar cyclones thousands of miles below the surface. In addition, the research has shown that the Sun has a number of layers that rotate at different speeds, contributing to the formation of powerful magnetic fields. And helioseismology can even detect sunspots on the far side of the Sun—quite a neat trick if you think about it.

All this information can help scientists evaluate theories of stellar evolution. Of more immediate practical concern, helioseismology is the best tool currently available for predicting solar weather patterns. The solar flares and coronal mass ejections that often accompany sunspots can disrupt satellites in Earth’s orbit and damage electrical grids. The more we know about the timing and location of these events, the better we can prepare and adapt. The moral of the story? Although you should never look at the Sun, you should always listen to it. —Joe Kissell

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More Information about Helioseismology...

This article was featured in the first edition of panta rei.

Sunquakes: Probing the Interior of the Sun by Jack B. Zirker is a good layperson’s introduction to helioseismology.

The cover story in the July, 2004 issue of National Geographic was called “The Sun: Living with a Stormy Star,” and included an extensive discussion of helioseismology.

For more details, see:

• Sounding the Sun: Helioseismology by P.B. Stark, Department of Statistics, University of California, Berkeley
• Surface Waves and Helioseismology at NASA’s Marshall Space Flight Center
• Helioseismology at Stanford University
• SOLAR MUSIC—HELIOSEISMOLOGY is a curriculum for grades K-3 developed by the National Optical Astronomy Observatories. Although written for six-year-olds, it’s actually not a bad introduction for adults, too.
• Helioseismology in the Wikipedia

Related Articles from Interesting Thing of the Day

• Mantle Convection
• The Kepler Mission
• Measuring the Speed of Light
• Spontaneous Human Combustion
• Tsunami Warning Systems
• The Coriolis Force
• Plate Clouds
• SETI
• Solar Sails

℗ & © 2004, alt concepts. All rights reserved.

Perched atop my computer is a shiny, high-tech video camera. Through the miracles of modern technology, I can have live video chats with friends or business associates on the other side of the country or the other side of the world, without even paying long-distance phone charges. Although I could opt for an audio-only conversation or even the text-only format of email or instant messaging, there’s something about seeing another person’s face that makes communication much richer and more satisfying. Using similar technology, I’ve participated in countless videoconferences involving multiple people in each of two or more locations, using cameras mounted on large video monitors and special microphones so that we can all see and hear each other. This is all good. But there’s one thing about the current state of the art in video communication that still bothers me greatly: the inability to make eye contact with the person or people on the other end. This was never a problem on Star Trek, which was of course the source of all my technological expectations.

Look at Me When You Say That
If you have ever tried video chats or videoconferencing yourself, you undoubtedly know what I mean. If not, let me describe what’s going on. The camera that’s pointing at your face is positioned above, below, or to the side of your display. This means the angle at which you’re viewing the screen is different from the angle at which the camera (and therefore the person on the other end) sees you—an effect known as parallax. Only if you were looking directly into the camera would the viewer have the impression you’re looking into his or her eyes. As a result, while you see your friend’s image on the screen, your friend appears to be looking down (or in some direction other than right at you), and you appear the same way on your friend’s screen. You could, of course, position the camera directly in front of your own screen, but then, the camera itself would block your view of the person on the other end.

Eye contact is extremely important for meaningful communication, and after all, seeing the person you’re talking to is the whole point of videoconferencing. But if you can’t look that person in the eye, this eliminates much of the advantage of video over a regular phone call. Guides to effective business videoconferencing usually say you should look at the camera when speaking, to give the people on the other side the sense that you’re speaking directly to them. But this is unnatural, and prevents you from seeing their reactions as you speak. What we really need is exactly what they have on the starship Enterprise: video displays that also somehow function as cameras, such that wherever you direct your gaze on the screen, that’s where your eyes will appear to be looking on the other end. Sure enough, engineers are trying to achieve this effect right now, working from several different angles (as it were).

It’s All Done with Mirrors
One fairly easy way to get eye-to-eye contact over a video link is to use technology borrowed from the television industry: the teleprompter. If you watch a news broadcast on TV, you’ll notice that the announcer is looking directly at the camera. TV news anchors don’t memorize their reports in advance; they read them from a special video screen that appears to be directly in front of the camera. In reality, the screen (an ordinary CRT or LCD) is positioned face-up just below and in front of the camera lens, with its text displayed as a mirror image. Above this display, and thus in front of the lens, is a partially silvered (or two-way) mirror positioned at a 45° angle. The announcer sees the text reflected onto it from below, while the camera sees only the announcer.

Teleprompters are a simple and tested technology; they’ve been around for more than 50 years. When similar designs are used for video communication, they’re sometimes referred to as video tunnels. They do, however, have some problems. One issue is size: the equipment is by nature quite bulky, because it requires that angled mirror in front of the camera as well as special shielding to protect the camera from glare. So even a design that uses an LCD panel will end up being at least as large as a CRT display. Teleprompters also tend to be heavy, fragile, and expensive—all factors that make them unattractive for ordinary consumers.

There’s yet another problem, which comes into play when more than two people are involved in a videoconference. If I look directly into a camera, all the people who see me on the screen will perceive that I’m making eye contact, even if they’re in different locations. So the participants will not have the impression that my gaze shifts as I turn my attention from one person to another—nor can I tell who is looking at me (or my image) at any given time.

Just Like Being There

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