## Friday, April 2, 2010

### Kids: Don’t Try This At Home

Russ Hobbie and I included a new chapter on sound and ultrasound in the 4th edition of Intermediate Physics for Medicine and Biology. In that chapter, we discuss how to calculate the speed of sound from the compressibility and the density of the tissue (Eq. 13.11). We then go on to describe, among other things, hearing, ultrasonic imaging, and the Doppler effect. One topic we do not mention is the behavior of objects moving faster than the speed of sound. Such a discussion, often found in physics and engineering books, usually is based on the Mach number, defined as the speed of an object divided by the speed of sound. If the Mach number is greater than one, the speed is supersonic and a shock wave develops. When an airplane travels faster than the speed of sound, people on the ground can hear the shock wave as a “sonic boom.” This is all very interesting, but it has nothing to do with biology and medicine, right?

Guess again. A recent article in the New York Times describes the plans of Felix Baumgartner, who intends to be the first human to break the sound barrier. I know, dear readers, that some of you are now saying “No, Chuck Yeager was the first to break the sound barrier, and that happened over 60 years ago.” Well, Yeager broke the sound barrier when flying in a plane, whereas Baumgartner plans to break the sound barrier while in free fall! The Times article states
“But now Fearless Felix, as his fans call him, has something more difficult on the agenda: jumping from a helium balloon in the stratosphere at least 120,000 feet above Earth. Within about half a minute, he figures, he would be going 690 miles per hour and become the first skydiver to break the speed of sound. After a free fall lasting five and a half minutes, his parachute would open and land him about 23 miles below the balloon.”
No one is certain what will happen to a human near the sound barrier. The NYT article says that turbulence may set in, causing havoc. Turbulence is a subject Russ and I discuss briefly in Chapter 1 of Intermediate Physics for Medicine and Biology, when introducing the Reynolds number. Most fluid in the body flows at low Reynolds number, with no danger of turbulence, although blood flow in the heart and aorta can get so fast that some turbulence may develop. Of course, any animal that flies in air will experience turbulence, which includes birds, bats, pterodactyls, and, in Baumgartner’s case, humans.

These high altitude exploits remind me of a delightful section in the textbook Physics With Illustrative Examples from Medicine and Biology, by George Benedek and Felix Villars. In their Volume 1 on Mechanics, they describe balloon ascensions and the physiological effects of air pressure. After reviewing the medical implications of a lack of oxygen at high altitudes, they present the fascinating tale of a 19th century balloon ascension.
“These symptoms are shown very clearly in the tragic balloon ascent of the ‘Zenith’ carrying the balloon pioneers Tissandier, Sivel, and Corce-Spinelli on April 15, 1875. During this ascent Sivel and Corce-Spinelli died. The balloons maximum elevation as recorded on their instruments was 8600 m. Though gas bags carrying 70% oxygen were carried by the balloonists, the rapid and insidious effects of hypoxia reduced their judgment and muscular control and prevented their use of the oxygen when it was most needed. Though these balloonists were indeed trying to establish an altitude record, their account shows clearly that their judgment was severely impaired during critical moments during the maximum tolerable altitude.”
Then Benedek and Villars present a 3-page extended quote from the account of the surviving member of the trio, Gaston Tissandier. It makes for fascinating reading. However, concerns about oxygen depletion aren’t relevant for Fearless Felix, because he will be wearing a space suit during his jump, with its own oxygen supply.

The New York Times story ends with the following quote. One wonders if we should admire Baumgartner’s pluck, or commit him to an insane asylum.
“Private adventurers have more freedom to take their own risks. The Stratos medical director, Dr. Jonathan Clark, who formerly oversaw the health of space shuttle crews at NASA, says that the spirit of this project reminds him of stories from the first days of the space age.

'This is really risky stuff, putting someone up there in that extreme environment and breaking the sound barrier,' Dr. Clark said. 'It’s going to be a major technical feat. It’s like early NASA, this heady feeling that we don’t know what we’re up against but we’re going to do everything we can to overcome it.' ”