IPMB100

I’m a regular reader of TIME magazine. Every year they publish an issue devoted to the TIME100: the hundred most influential people of that year. I thought I would do the same, except I’d focus on Intermediate Physics for Medicine and Biology. So, below is a list of the one hundred scientists, physicians, engineers, and mathematicians who most influenced IPMB. I list them by impact, with the most influential first.

Like for the TIME100, selecting the list is not an exact science. It’s based on mentions in IPMB, numbers of citations, and my own personal opinions. I’m sure your list would be different, and that’s okay.

There are many brilliant scientists who didn’t make the list (for example: Newton, Faraday, Maxwell, Rutherford, and Einstein). I tried to focus on people who had a direct impact on IPMB, rather than fundamental but not biomedical contributions to physics, so these and other luminaries were left off. 

The oldest scientists are Brown (born 1773) and Poiseuille (1797). The youngest are Basser, Goodsell, Hämäläinen, LeBihon, MacKinnon, Mattiello, Strogatz, and Xia (all more or less my age). I’m embarrassed to say there are only three women (Curie, Eleanor Adair, and Mielczarek; four if you count Abramowitz’s coauthor Stegun). Thirty three are alive today. Twenty are Nobel Prize winners (marked with an asterisk). I know ten personally (marked with a §). When I wasn’t sure about the year a scientist was born or died, I guessed and marked it with a question mark. There are many more I would like to honor, but I decided to—like TIME—stop at 100.

Enjoy!

1. Alan Hodgkin* (1914–1998) English physiologist who discovered how nerve action potentials work and developed the Hodgkin-Huxley model. 
2. Andrew Huxley* (1917–2012) English physiologist and computational biologist who discovered how nerve action potentials work and developed the Hodgkin-Huxley model. 
3. Godfrey Hounsfield* (1919–2004) British electrical engineer who invented the first clinical computed tomography scanner. 
4. Paul Lauterbur* (1929–2007) American chemist who developed a method to do magnetic resonance imaging using magnetic field gradients. 
5. Edward Purcell* (1912–1997) American physicist who co-discovered nuclear magnetic resonance, was author of the article “Life at Low Reynolds Number,” and wrote volume 2 of the Berkeley Physics Course titled Electricity and Magnetism. 
6. Allan Cormack* (1924–1998) South African physicist who developed much of the mathematical theory behind computed tomography. 
7. Hermann von Helmholtz (1821–1894) German physicist and physician; First to measure the propagation velocity of a nerve action potential. 
8. Adolf Fick (1829–1901) German physician and physiologist who derived the laws of diffusion (Fick’s laws). 
9. Willem Einthoven* (1860–1927) Dutch medical doctor and physiologist who was the first to accurately measure the electrocardiogram. 
10. Marie Curie** (1867-1934) Polish-French physicist and chemist who discovered the elements radium and polonium; The unit of the curie is named after her. 
11. Jean Léonard Marie Poiseuille (1797–1869) French physicist and physiologist who determined the law governing the flow of blood in small vessels. 
12. Max Kleiber (1893–1976) Swiss biologist who established a ¾ power law relating metabolic rate to mass. 
13. Felix Bloch* (1905–1983) Swiss-American physicist who co-discovered nuclear magnetic resonance and derived the Bloch equations. 
14. Peter Mansfield* (1933–2017) English physicist who developed techniques used in magnetic resonance imaging, including echo planar imaging. 
15. Roderick MacKinnon* (1956) American biophysicist who determined the structure of the potassium ion channel. 
16. Erwin Neher* (1944) German biophysicist who co-invented the patch clamp method to record from single ion channels. 
17. Bert Sakmann* (1942) German physiologist who co-invented the patch clamp method to record from single ion channels. 
18. Tony Barker (1950) English engineer who invented transcranial magnetic stimulation. 
19. Robert Plonsey§ (1924–2015) American engineer who contributed to theoretical bioelectricity and wrote Bioelectric Phenomena and other books. 
20. Peter Basser§ (1959?) American engineer who invented the magnetic resonance imaging technique of diffusion tensor imaging. 
21. William Oldendorf (1925-1992) American medical doctor who first designed a computed tomography device. 
22. J. B. S. Haldane (1892–1964) British evolutionary biologist who published “On Being the Right Size,” an essay about scaling. 
23. Geoffrey West (1940) British theoretical physicist who derived a model to explain the ¾ power law of metabolism. 
24. George Ralph Mines (1886–1914) English cardiac electrophysiologist who demonstrated reentry in cardiac tissue. 
25. Bernard Cohen (1924–2012) American physicist who opposed the linear no-threshold model of radiation risk. 
26. John Wikswo§ (1949) American physicist who measured the magnetic field of a nerve. 
27. Arthur Winfree§ (1942–2002) American mathematical biologist who studied cardiac arrhythmias and wrote When Time Breaks Down: The Three Dimensional Dynamics of Electrochemical Waves and Cardiac Arrhythmias. 
28. Richard Blakemore (1950?) Researcher who discovered magnetotactic bacteria. 
29. John Moulder (1945–2022) Radiation biologist who debunked suggestions that radiofrequency electromagnetic fields are dangerous. 
30. Kenneth Foster§ (1945) American bioengineer and expert on the biological effects of electromagnetic fields. 
31. Paul Callaghan (1947–2012) New Zealand physicist who wrote Principles of Nuclear Magnetic Resonance Microscopy. 
32. Paul Nunez (1950?) Analyzed electroencephalography using mathematics, and author of Electric Fields of the Brain. 
33. Charles Bean (1923–1996) American physicist who studied porous membranes and reverse osmosis. 
34. John Hubbell (1925–2007) American radiation physicist who measured and tabulated x-ray cross sections. 
35. Arthur Compton* (1892–1962) American physicist who analyzed Compton scattering of x-rays. 
36. William Bragg* (1862–1942) English physicist who discovered the Bragg peak of energy deposition from charged particles in tissue. 
37. Mark Hallett§ (1943) National Institutes of Health neurophysiologist who helped develop transcranial magnetic stimulation and wrote, with Leo Cohen, the article “Magnetism: A New Method for Stimulation of Nerve and Brain.” 
38. Selig Hecht (1892–1947) American physiologist who performed a classic experiment on scotopic vision. 
39. Milton Abramowitz (1915–1958) American mathematician who, with Irene Stegun, coauthored the Handbook of Mathematical Functions with Formulas, Graphs, and Mathematical Tables. 
40. Knut Schmidt-Nielsen (1915–2007) Norwegian-American comparative physiologist and author of How Animals Work and Scaling: Why Is Animal Size So Important? 
41. Steven Vogel (1940–2015) American biomechanics researcher and author of Life in Moving Fluids: The Physical Biology of Flow and other books. 
42. Mark Denny (1951) American physiologist and author of Air and Water: The Biology and Physics of Life’s Media. 
43. Howard Berg (1934–2021) American biophysicist who studied the motility of E. coli and wrote Random Walks in Biology. 
44. Frank Herbert Attix (1925) Radiologist who is the author of Introduction to Radiological Physics and Radiation Dosimetry. 
45. Steven Strogatz (1959) American mathematician and author of Nonlinear Dynamics and Chaos and other books. 
46. Frederick Donnan (1870–1956) British chemist who analyzed Donnan equilibrium. 
47. Gustav Bucky (1880–1963) German-American radiologist who invented the Bucky grid used in x-ray imaging. 
48. Gopalasamudram Narayanan Ramachandran (1922–2001) Indian physicist who, with A. V. Lakshminarayanan, developed mathematical methods used in computed tomography. 
49. Peter Agre* (1949) American molecular biologist who discovered membrane water channels called aquaporins. 
50. Eleanor Adair (1926–2013) American physiologist who studied the health risks of microwave radiation. 
51. Robert Adair (1924–2020) American physicist who studied the biological effects of weak, extremely-low-frequency electromagnetic fields. 
52. Yuan-Cheng Fung (1919–2019) Chinese-American biomedical engineer, and author of Biomechanics. 
53. Herman Carr (1924–2008) American physicist and pioneer in magnetic resonance imaging. 
54. Matti Hämäläinen (1958) Finnish physicist who was lead author on the article “Magnetoencephalography—theory, instrumentation, and applications to noninvasive studies of the working human brain”. 
55. Saul Meiboom (1916–1998) Israeli researcher who, with David Gill, co-invented of the Carr-Purcell-Meiboom-Gill pulse sequence used in magnetic resonance imaging. 
56. Oskar Klein (1894–1977) Swedish physicist who, with Japanese physicist Yoshio Nishina, developed the Klein-Nishina formula for Compton scattering of x-rays. 
57. Chad Calland (1934–1972) Medical doctor, kidney transplant patient, and author of the paper “Iatrogenic Problems in End-Stage Renal Failure.” 
58. Walter Blount (1900–1992) American orthopedic surgeon who advocated for the use of a cane. 
59. Albert Bartlett (1923–2013) American physicist and author of The Essential Exponential! For the Future of Our Planet. 
60. Pierre Auger (1899–1993) French physicist who studied the emission of Auger electrons. 
61. Rolf Sievert (1896–1966) Swedish medical physicist who studied the biological effects of ionizing radiation; The unit of the sievert is named after him. 
62. Louis Gray (1905–1965) English physicist who worked on the effects of radiation on biological systems. The unit of the gray is named after him.
63. Richard Frankel (1943?) American researcher who studied magnetotactic bacteria. 
64. Frederick Reif (1927–2019) Austrian-American physicist who wrote volume 5 of the Berkeley Physics Course, titled Statistical Physics. 
65. Richard FitzHugh (1922–2007) Co-inventor, with Jinichi Nagumo, of the FitzHugh-Nagumo model of a neuron. 
66. Arthur Guyton (1919–2003) American physiologist and author of the Textbook of Medical Physiology. 
67. Leon Glass (1943) American researcher and co-author, with Michael Mackey, of From Clocks to Chaos: The Rhythms of Life. 
68. Ken Kwong (1948) Chinese-American nuclear physicist who studied functional magnetic resonance imaging. 
69. Seiji Ogawa (1934) Japanese biophysicist who studied functional magnetic resonance imaging. 
70. Jay Lubin (1947) National Cancer Institute epidemiologist who battled with Bernard Cohen over the linear no-threshold model and the risk of radon. 
71. Eugenie Mielczarek (1931-2017) American physicist and author of Iron: Nature’s Universal Element: Why People Need Iron and Animals Make Magnets. 
72. David Goodsell (1960?) American structural biologist and science illustrator who wrote the book The Machinery of Life. 
73. Philip Morrison (1915–2005) American physicist who was lead author on Powers of Ten. 
74. Henri Becquerel* (1852–1908) French physicist who discovered radioactivity; the unit of the becquerel is named after him. 
75. Wilhem Roentgen* (1845–1923) German physicist who discovered x rays; The unit of the roentgen is named after him. 
76. Bertil Hille (1940) American biologist and author of Ion Channels of Excitable Membranes. 
77. George Benedek (1928) American physicist who co-authored, with Felix Villars, the three-volume Physics with Illustrative Examples from Medicine and Biology. 
78. William Hendee (1938) Coauthor, with E. Russell Ritenour, of Medical Imaging Physics. 
79. John Cameron (1922–2005) Medical physicist and coauthor of Physics of the Body. 
80. Lawrence Stark (1926–2004) American neurologist and expert on the feedback system controlling the size of the pupil in the eye. 
81. Ernst Ruska* (1906–1988) German physicist who invented the electron microscope. 
82. Britton Chance (1913–2010) American physicist who developed biomedical photonics. 
83. Johann Radon (1887–1956) Austrian mathematician who developed the radon transform used in computed tomography. 
84. Alan Garfinkel (1944?) American researcher who analyzed cardiac restitution for controlling heart arrhythmias. 
85. Eric Hall (1950?) Author of Radiobiology for the Radiologist. 
86. Osborne Reynolds (1842–1912) British engineer who studied fluid mechanics; the Reynolds number is named after him. 
87. Bernard Katz* (1911–2003) German-British biophysicist and author of Nerve, Muscle, and Synapse. 
88. William Rushton (1901–1980) British physiologist who worked with Alan Hodgkin studying nerve conduction. 
89. Robert Eisberg (1928) Coauthor with Robert Resnick of Quantum Physics of Atoms, Molecules, Solids, Nuclei, and Particles. 
90. John Clark (1936–2017) American bioengineer who worked with Robert Plonsey. 
91. Raymond Ideker§ (1942) American physiologist and medical doctor who studied the electrical activity of the heart. 
92. Denis LeBihan§ (1957) French physicist and medical doctor who developed diffusion magnetic resonance imaging, and worked with Peter Basser on diffusion tensor imaging. 
93. Ronald Bracewell (1921–2007) Author of Fourier Transforms and Their Applications. 
94. Robert Brown (1773–1858) Scottish botanist who discovered Brownian motion. 
95. Louis DeFelice (1940?–2016) Wrote Introduction to Membrane Noise. 
96. H. M. Schey (1930?) Author of Div, Grad, Curl, and All That. 
97. Warren Weaver (1894–1978) American mathematician and science administrator who wrote Lady Luck: The Theory of Probability. 
98. Peter Atkins (1940) English chemist and author of The Second Law. 
99. Yang Xia§ (1955) Oakland University physicist who studied the magic angle in magnetic resonance imaging. 
100. James Mattiello§ (1958-2017) Oakland University alumnus and American physicist who worked with Peter Basser and Denis LeBihan to developed diffusion tensor imaging.

Where Have You Gone, Physicist Bob Park? Our Nation Turns Its Lonely Eyes to You. Woo, Woo, Woo.

Voodoo Science, by Bob Park, superimposed on
Intermediate Physics for Medicine and Biology.

Bob Park died five years ago this week. He had been in poor health since suffering a stroke in 2013. Park was a physicist and the director of public information at the Washington office of the American Physical Society. He was a leading voice against pseudoscience, both in his weekly column What’s New (which, when in graduate school, I used to look forward to seeing in my email every Friday) and in his books such as Voodoo Science.

I wonder what Park would say if he were alive today? I suspect he would be horrified. But I doubt he would have said that. He was not a whine-and-fuss sort of guy. His tools were humor, irony, and sarcasm. Here is what I imagine What’s New would have looked like this week.

What’s New, by Bob Park

Friday, April 25, 2025

1. VITAMIN A FOR THE MEASLES

The Texas measles outbreak continues. Over 600 cases have now been reported, which is more than for the entire year in 2024. Health and Human Services Secretary Robert F. Kennedy, Jr. encouraged parents to treat their children suffering from measles with vitamin A, and now children are suffering from liver disease because of vitamin A overdosing. Why don’t parents simply ask their pediatrician what to do? Because pediatricians are part of the conspiracy, of course!

2. IF WE IGNORE IT, IT WILL GO AWAY

The Trump administration is trying to undo all the progress fighting climate change that has accumulated over the last few decades. His thinking is: if you ignore climate change, the problem goes away. Besides, it’s all a HOAX! King Canute tried this. He commanded the tide to stop coming in. How do you think that turned out? Physics has a way of winning in the end, whether or not it’s politically popular.  

3. LAB LEAK

The Trump administration has rewritten the covid.gov website to advocate for the lab leak hypothesis for the source of covid-19. Don't worry that the evidence is flimsy! If covid resulted from a lab leak, then it’s the scientists fault. Blame those arrogant liberal elitists like Fauci. But watch out for the next spillover event! (Can I interest anyone in some bird flu?)

4. LYSENKO

Back in the USSR, when Stalin was in charge, a crackpot named Lysenko took control of Russian science. He didn’t believe in modern genetics, regardless of the evidence. Russian agriculture collapsed and millions died. Here in the United States, we have our own version of the Lysenko affair. Trump is Stalin, RFK Jr is Lysenko, and vaccine hesitancy and climate change are genetics. I fear the outcome will be the same, which is bad for science and worse for humanity.

5. HOORAY FOR HARVARD

The NIH (remember that place that used to be the greatest biomedical research institution anywhere, ever?) has stopped funding grants to several universities, including Harvard. HARVARD! Apparently these universities will not cave in to Trump's ideological agenda. What will happen next? Who knows. Maybe Trump will be stopped by the Supreme Court. Maybe the House and Senate will decide they’ve had enough. And maybe, just maybe, it will be the end of American science.

Asimov’s Corollary

Regular readers of this blog know that I am a huge fan of Isaac Asimov. I decided on a career in science in large part from reading Asimov’s books. As a teenager I particularly enjoyed his collections of essays from The Magazine of Fantasy and Science Fiction. He wrote an essay there each month about science: astronomy, physics, chemistry, biology, geology, medicine, and even mathematics. Every time he collected seventeen essays, he would publish them in a book. It would not be an exaggeration to say that I came to be a coauthor on Intermediate Physics for Medicine and Biology largely because of the influence those essay collections had on me when I was young.

Quasar, Quasar, Burning Bright,
by Isaac Asimov.

This week I want to look at one of those essays that is especially germane today. It appears as the final chapter in the book Quasar, Quasar, Burning Bright. The essay is titled “Asimov’s Corollary,” and was first published in the February, 1977 issue of The Magazine of Fantasy and Science Fiction. Now, almost fifty years later, it seems more relevant than ever.  I urge you to get a copy and read it in its entirety. I will quote parts that I think are especially important. 

To help set the stage, let me note a few things.

• When Asimov mentions “Arthur” he is talking about Arthur C. Clarke, his fellow science fiction writer and good buddy. Along with Robert Heinlein, Asimov and Clarke are considered the “Big Three” in science fiction.
• Asimov loved to talk about himself. You might at first think he’s egotistical, but once you’ve read enough of his works you will realize it’s all a big act…sort of. It is one of the reasons I loved to read his essays.
• By today’s standards Asimov and Clarke sound a bit sexist, assuming all scientists are men. This is, in part, a sign of the times when they lived. I won’t defend their sexism, but I’ll forgive them because of all the good they did and all they taught me.

Asimov writes:

In Arthur’s book Profiles of the Future (Harper & Row, 1962) he advances what he himself calls “Clarke’s Law.” It goes as follows:

When a distinguished but elderly scientist states that something is possible, he is almost certainly right. When he states that something is impossible, he is very probably wrong…

…Naturally when I read a paragraph like that, knowing Arthur as I do, I begin to wonder if, among all the others, he is thinking of me…

Asimov was an elderly scientist at that time, and was fond of making all sorts of predictions, many of which claimed something was impossible.

Doesn’t Clarke’s Law make me uneasy, then? Don’t I feel as though I am sure to be quoted extensively, and with derision, in some book written a century hence by some successor to Arthur?

No, I don’t. Although I accept Clarke’s Law and think Arthur is right in his suspicion that the forward-looking pioneers of today are the backward-yearning conservatives of tomorrow, I have no worries about myself. I am very selective about the scientific heresies I denounce, for I am guided by what I call Asimov’s Corollary to Clarke’s Law. Here is Asimov’s Corollary:

When, however, the lay public rallies around an idea that is denounced by elderly but distinguished scientists and supports that idea with great fervor and emotion—the distinguished but elderly scientists are then, after all, probably right.

But why should this be?… Human beings have the habit (a bad one, perhaps, but an unavoidable one) of being human; which is to say that they believe in that which comforts them…

Asimov then examines a few cases of people believing things without evidence. He concludes

Then why do people believe? Because they want to. Because the mass desire to believe creates a social pressure that is difficult (and, in most times and places, dangerous) to face down. Because few people have had the chance of being educated into the understanding of what is meant by evidence or into the techniques of arguing rationally.

But mostly because they want to...

When I read this, I think of people claiming (falsely, we know from the evidence) that vaccines cause autism; I think of people claiming (again, falsely) that cell phone radiation causes cancer; and I think of people claiming (still again, falsely) that climate change is a hoax. When I hear these assertions, made passionately and vehemently but with no evidence provided, I think that the elderly scientists (what I would call “the scientific consensus”) is right after all. And while Asimov writes “probably,” I would write “almost certainly.”

I miss you, Isaac Asimov. We need you now more than ever.

 

There is a cult of ignorance. 

https://www.youtube.com/watch?v=oTV1iQyjFFU

 

 Isaac Asimov predicts the future.

https://www.youtube.com/watch?v=cIB1b_8hqB0

Oops!

Finding a mistake in something you wrote is always annoying. When revising the chapter on Atoms and Light for the sixth edition of Intermediate Physics for Medicine and Biology, I found a whopper. It’s on page 402 of the 5th edition, in the section on Blue and Ultraviolet Radiation. Here is the offending sentence:

The minimum erythemal dose at 254 nm is about 6 × 10⁷ J m^-2.

I was trying to add a homework problem to the sixth edition in which I would ask the student to calculate how long it would take to get sunburn for some typical ultraviolet light intensity and I kept getting a ridiculously long time (years) because our value of 6 × 10⁷ is way, way too big. The error goes back to the 3rd edition of IPMB, where you find a reference for that value:

Diffey, B. L. and Farr, P. M. (1991) Quantitative aspects of ultraviolet erythema. Clin Phys Physiol Meas 12:311-325.

The 3rd edition is even more specific, saying the value is in Table 2 in that paper. So, I obtained the article interlibrary loan (kudos to the Oakland University interlibrary loan office, who got the paper for me in about an hour on a Sunday evening). Here is Diffey and Farr’s Table 2. 

The value for minimal erythema at 254 nm is 6 mJ cm^-2, which is equivalent to 60 J m^-2. I think the incorrect value in IPMB arose because of a unit conversion error. There are 1000 millijoules in a joule, not 1000 joules in a millijoule. Such a mistake would cause a factor of one million error, which would result in an erroneous value of 60 × 10⁶ J m^-2, or 6 × 10⁷.

You may have some questions.

• Who did it? Although Russ Hobbie made the initial mistake (he was sole author on the 3rd edition), I read this number when teaching from our book, and when preparing the 4th edition and then again when preparing the 5th edition and never batted an eye. Apparently 60 MJ of UV light causing a person to have only a mild reddening of the skin didn’t bother me at all. I always tell my students to “THINK BEFORE YOU CALCULATE!” but I didn’t. 
• If it was in the book, how could it be wrong? Don’t believe everything you read. Just because something is written in a textbook doesn’t make it true. Authors try their best to get everything right, but sometimes they make mistakes. Read critically and thoughtfully. (I’m giving this advice to myself here, more than to you, dear reader). 
• What’s erythema? Erythema is redness of the skin. In our context, it is a the initial stages of a sunburn.
  
• What is the “minimum erythemal dose”? Here’s what Diffey and Farr say: “The erythemal response of the skin to ultraviolet radiation is usually inferred from the minimal erythemal dose (MED). This value is determined by exposing adjacent areas of skin to increasing doses of radiation (usually employing a geometrical series of dose increments) and recording the lowest dose of radiation to achieve erythema at a specified time, usually 24 hours, after irradiation. The visual detection of erythema is subjective and is affected by unrelated factors such as viewing geometry, intensity and spectral composition of ambient illumination, colour of unexposed surrounding skin… and the experience and visual acuity of the observer.” So, it’s the dose where you say “Gosh, my skin is slightly red, I must have gotten a little too much sun today,” and then go about your business with hardly another thought. 
• Why did Russ and I give the value for 254 nm? We mean that the ultraviolet radiation has a wavelength of 254 nm, which puts it in the UVC range (100–280 nm). UVC light can certainly cause damage and sunburn, but almost no UVC gets through the earth’s atmosphere to reach our bodies. Most sun tans and sunburns are caused by UVB, which is in a narrow band of wavelengths from 280–315 nm. Wavelengths much shorter are removed by the atmosphere, and the photons for wavelengths much longer do not have enough energy to do significant damage. The wavelength in the above Table 2 that’s most appropriate for this discussion is 300 nm. So, looking at Table 2, perhaps a better value for the minimum erythemal dose would be 24 mJ cm^-2 or 240 J m^-2. In the sixth edition of IPMB, we will use 200 J m^-2 as our typical value (unless we change our minds…when it comes to revising a textbook, it ain’t over till it’s over). Warning: this value depends on factors such as your complexion, so don’t take it too seriously. It’s a ballpark estimate. Everyone is a bit different. 
• Well, just how much UVB are we exposed to? We can estimate that from Figure 14.28 in IPMB. In the range from about 295 to 315 nm, the average value of the spectral dose is about 10 mW m^-2 nm^-1. If we multiply by a 20 nm range, we get 200 mW m^-2, or about 0.2 W m^-2. That value is for the sun straight overhead (noon near the equator with a clear sky). It’s consistent with other values I have found. 
• How is all this related to the “UV index” that the weather forecaster talks about? The UV index is a linear scale (not logarithmic like the decibel scale for hearing), and to calculate it you multiply the intensity in W m^-2 by 40. So, the value of 0.2 W m^-2 that I quoted earlier corresponds to a UV index of 8. Here in southeast Michigan we can reach a UV index of 8 at noon on a cloudless summer day. In January we are at a UV index of about 2. Latitude and time of the year make a big difference, as does time of the day (in the morning and evening, sunlight comes in at an angle and must therefore pass through more atmosphere than at noon). 
• So, how long can I stay in the sun before getting sunburn? On the beach in Hawaii during the summer at noon you can reach a UV index of about 12, so the intensity is 0.3 W m^-2, which means 0.3 joules per second per square meter. If your minimum erythemal dose is 200 J m^-2, then (0.3 J m^-2 s^-1) t = 200 J m^-2, so t = 667 seconds or 11 minutes. That’s the minimum dose. I bet you could go a half hour before suffering from something you would call a serious sunburn. But if you stay out all afternoon surfing at Waikiki, it could be a problem. 
• Can’t I protect myself with sunscreen? Yes, the sun protection factor (SPF) is the factor by which the intensity actually reaching your skin is reduced by the sunscreen. If you put on SPF 30 sunscreen there in Hawaii, your time for a minimum erythemal dose goes up from 11 minutes to five and a half hours. Maybe you can get away with all day, since the UV index will be lower in the morning and evening, extending your time. Just make sure it doesn’t get washed off in the water. You may have to reapply it often.
  

Let me apologize one more time for the bogus value of minimal erythemal dose in the 5th edition of IPMB. I feel bad about it. I sure hope no one used it to justify spending lots of time in a tanning booth. I call those things “cancer booths.” Stay away from them. 

I'll be proofreading the 6th edition of IPMB extra carefully. My motto will be: THINK BEFORE YOU WRITE!

Tomie De Paola

Sound,
written by Lisa Miller,
illustrated by Tomie De Paola

Children’s book author Tomie De Paola died five years ago last Sunday. I fondly recall reading De Paola’s books to my daughters Stephanie and Kathy when they were growing up. But how could Tomie De Paola possibly intersect with Intermediate Physics for Medicine and Biology? Well, you might be surprised! The first book that De Paola illustrated was Sound, written by Lisa Miller. It was part of the “Science is What and Why” series published by Coward–McCann, Inc.

Each book in the Science is What and Why series introduces fundamentals of physical science using a simple, attractive approach specifically designed for young boys and girls. Straightforward, lively language and distinguished illustrations which are a practical extension of the text present scientific facts as fascinating and exciting as the realm of the imagination.

As Gene Surdutovich and I work on the 6th edition of IPMB, I think we should strive for “straightforward, lively language and distinguished illustrations.”

De Paola’s drawings in Sound have much more charm than the figures in Chapter 13 of IPMB, which is about Sound and Ultrasound. Yet, his book covers topics that Russ Hobbie and I also discuss, such as the wavelength, frequency, and amplitude of a sound wave, and echos. I can’t help but think of De Paola as a kindred soul.

Sound appeared early in De Paola’s career; it was published in 1965. He continued illustrating books about science (I need to read The Popcorn Book), but he is best known for his children’s stories. Many of his books were autobiographical. I loved reading The Art Lesson and Tom with my girls. Although I’m not particularly religious, I thought his best work was The Clown of God.

Now, with my first grandchild due this summer, I’m looking forward to rereading many of De Paola’s books. I can’t wait.

 

Meet Tomie dePaola

https://www.youtube.com/watch?v=3_XINGTzl5U

Tomie De Paola on the television show Barney, another favorite of my daughters. 

https://www.youtube.com/watch?v=s01_ikK_SrQ

 


Tomie De Paola: Why Reading is Important 

https://www.youtube.com/watch?v=7epT0qUaaX4&t=16s

 

 

The Art Lesson 

https://www.youtube.com/watch?v=9TUQ4F27HMo 

 

Tom by Tomie de Paola 

https://www.youtube.com/watch?v=doFJAxHX5yw

 

The Clown Of God by Tomie De Paola 

https://www.youtube.com/watch?v=Gnwlzj1xdmM

The Rest of the Story 5

Bill grew up in Schenectady, New York, the youngest of four children. While a child he became interested in science because of his fascination with telescopes. He was smart; he graduated from high school at the age of 15, and then attended Schenectady’s Union College, finishing in just three years. By the age of 22 he had graduated with an M.D. from the Albany Medical College. Many of his friends didn’t realize how bright Bill was, because he was so modest and friendly, and had such a wonderful sense of humor.

Bill became an active duty medical officer posted at the U.S. Naval Hospital in Newport, Rhode Island. After a fellowship in neurology at the University of Minnesota, he joined the new medical school at UCLA. He was much loved as a medical doctor and a mentor, but he disliked many of the invasive procedures that he had to perform as a clinical neurologist.

In 1959, Bill had an idea how to noninvasively image the brain using multiple x-ray beams in different directions. After two years of effort he had a working prototype, applied for a patent, and published an article about this work. But when he approached a leading x-ray manufacturer, the company president couldn’t image there would ever be a market for such a device. Frustrated, Bill turned his attention to other things.

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Bill’s idea for how to image the brain did not go away. Other scientists took up the challenge. Physicist Allan Cormack and engineer Ronald Bracewell each developed detailed mathematical techniques for obtaining an image from beams in different directions. Engineer Godfrey Hounsfield built the first brain scanner in 1971. And the rest is history. Bill’s invention is now known as Computed Tomography (originally called a CAT scan and now referred to as CT for short). It has revolutionized medicine. In 1979, Cormack and Hounsfield won the Nobel Prize in Physiology or Medicine for their contributions to CT. William (“Bill”) Oldendorf did not share the prize, but he shared in the discovery.

William Oldendorf.

And now you know the rest of the story.

Good day! 

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This blog post was written in the style of Paul Harvey’s The Rest of the Story radio program. You can find four other of my The Rest of the Story posts here, here, here, and here. 

You can learn more about Computed Tomography in Chapter 16 of Intermediate Physics for Medicine and Biology.

William Oldendorf was born March 27, 1925, one hundred years ago yesterday. Happy birthday Bill!

Dipole-Dipole Interaction

One strength of Intermediate Physics for Medicine and Biology is its many homework problems. The problems stress (but perhaps not enough) the ability to make general arguments about how some quantity will depend on a variable. Often getting a calculation exactly right is not as important as just knowing how something varies with something else. For instance, you could spend all day learning how to compute the volume and surface area of complicated objects, but it’s still useful simply to know that volume goes as size cubed and surface area as size squared. Below is a new homework problem that emphasizes the ability to determine a functional form.

Section 6.7

Problem 20½. Consider an electric dipole p a distance r from a small dielectric object. Calculate how the energy of interaction between the dipole and the induced dipole in the dielectric varies with r. Will the dipole be attracted to or repelled from the dielectric? Use the following facts:

1. The energy U of a dipole in an electric field E is U = – p · E,

2. The net dipole induced in a dielectric, p', is proportional to the electric field the dielectric experiences,

3. The electric potential produced by a dipole is given by Eq. 7.30.

Let’s take a closer look at these three facts.

1. When discussing magnetic resonance imaging in Chapter 18 of IPMB, we give the energy U of a magnetic dipole μ in a magnetic field B as U = – μ · B (Eq. 18.3). An analogous relationship holds for an electric dipole in an electric field. The energy is lowest when the dipole and the electric field are in the same direction, and varies as the cosine of the angle between them. I suggest treating the original dipole p as producing the electric field E, and the induced dipole p' as interacting it. 

2. Section 6.7 of IPMB discusses how an electric field polarizes a dielectric. The net dipole p' induced in the dielectric object will depend on the electric field and the objects shape and volume. I don’t want you to have to worry about the details, so the problem simply says that the net dipole is proportional to the electric field. You might get worried and say “wait, the electric field in the dielectric is not uniform!” That’s why I said the dielectric object is small. Assume that it’s small enough compared to the distance to the dipole that the electric field is approximately uniform over the volume of the dielectric. 

3. What is the electric field produced by a dipole? Russ Hobbie and I don’t actually calculate that, but we do give an equation for a dipole’s electrical potential, which falls off as one over the square of the distance. (It may look like the cube of the distance in Eq. 7.13, but there’s a factor of distance in the numerator that cancels one factor of distance cubed in the denominator, so it’s an inverse square falloff.) The electric field is the negative gradient of the potential. Calculating the electric field can be complicated in the general case. I suggest you assume the dipole p points toward the dielectric. Fortunately, the functional dependence of the energy on the distance r does not depend on the dipole direction.

I won’t work out all theentire solution here. When all is said and done, the energy falls off as 1/r⁶, and the dipole is attracted to the dielectric. It doesn’t matter if the dipole originally pointed toward the dielectric or away from it, the force is always attractive.

This result is significant for a couple reasons. First, van der Waals interactions are important in biology. Two dielectrics attract each other with an energy that falls as 1/r⁶. Why is there any interaction at all between two dielectrics? Because random thermal motion can create a fluctuating dipole in one dielectric, which then induces a dipole in a nearby dielectric, causing them to be attracted. These van der Waals forces play a role in how biomolecules interact, such as during protein folding.

From Photon to Neuron:
Light, Imaging, Vision.

Second, there is a technique to determine the separation between two molecules called fluorescence resonance energy transfer (FRET). The fluorescence of two molecules, the donor and the acceptor, is affected by their dipole-dipole interaction. Because this energy falls off as the sixth power of the distance between them, FRET is very sensitive to distance. You can use this technique as a spectroscopic ruler. It’s not exactly the same as in the problem above, because both the donor and acceptor have permanent dipole moments, instead of one being a dielectric in which a dipole moment is induced. But nevertheless, the 1/r⁶ argument still holds, as long as the dipoles aren’t too close together. You can learn more about FRET in Philip Nelson’s book From Photon to Neuron: Light, Imaging, Vision.

The First Measurement of the Magnetocardiogram

Biomagnetism: The First Sixty Years.

A couple years ago, I published a review article titled “Biomagnetism: The First Sixty Years.” I wrote about that article before in this blog, but I thought it was time for an update. The paper is popular: according to Google Scholar it has been cited 28 times in two years, which is more citations than any other of my publications in the last decade. I remember working on this paper because it was my Covid project. That year I got Covid for the first—and, so far, only—time. I quarantined myself in our upstairs bedroom, wore a mask, and somehow avoided infecting my wife. I remember having little to do except work on my biomagnetism review.

As a treat, I thought I would reproduce one of the initial sections of the article (references removed) about the first measurement of the magnetocardiogram. Russ Hobbie and I talk about the MCG in Chapter 8 of Intermediate Physics for Medicine and Biology. This excerpt goes into more detail about how MCG measurements began. Enjoy!

2.1. The First Measurement of the Magnetocardiogram

In 1963, Gerhard Baule and Richard McFee first measured the magnetic field generated by the human body. Working in a field in Syracuse, New York, they recorded the magnetic field of the heart: the magnetocardiogram (MCG). To sense the signal, they wound two million turns of wire around a dumbbell-shaped ferrite core that responded to the changing magnetic field by electromagnetic induction. The induced voltage in the pickup coil was detected with a low-noise amplifier.

The ferrite core was about one-third of a meter long, so the magnetic field was not measured at a single point above the chest, but instead was averaged over the entire coil. One question repeatedly examined in this review is spatial resolution. Small detectors are often noisy and large detectors integrate over the area, creating a trade-off between spatial resolution and the signal-to-noise ratio.

The heart’s magnetic field is tiny, on the order of 50–100 pT (Figure 1). A picotesla (pT) is less than a millionth of a millionth as strong as the magnetic field in a magnetic resonance imaging machine. The magnetic field of the earth is about 30,000,000 pT (Figure 1), and the only reason it does not obscure the heart’s field is that the earth’s field is static. That is not strictly true. The earth’s field varies slightly over time, which causes geomagnetic noise that tends to mask the magnetocardiogram (Figure 1). Moreover, even a perfectly static geomagnetic field would influence the MCG if the pickup coil slightly vibrated. A key challenge in biomagnetic recordings, and a major theme in this review, is the battle to lower the noise enough so the signal is detectable

Figure 1. Noise sources in biomagnetism.

Most laboratories contain stray magnetic fields from sources such as electronic equipment, elevators, or passing cars (Figure 1). Baule and McFee avoided much of this noise by performing their experiments at a remote location. Even so, they had to filter out the ubiquitous 60 Hz magnetic field arising from electrical power distribution. A magnetic field changing at 60 Hz is a particular nuisance for biomagnetism because the magnetic field typically exists in a frequency band extending from 1 Hz (1 s between heartbeats) to 1000 Hz (1 ms rise time of a nerve or muscle action potential).

One limitation of a metal pickup coil is the thermal currents in the winding due to the random motion of electrons, creating extraneous magnetic fields caused by the measuring device itself. The ultimate source of noise is thermal currents in the body, but fortunately, their magnetic field is minuscule (Figure 1).

Baule and McFee suppressed background noise by subtracting the output of two pickup coils. A distant source of noise gave the same signal in both coils and did not contribute to their difference. One coil was placed over the heart, and the magnetocardiogram was larger there and did not cancel out. The two coils formed a rudimentary type of gradiometer (Figure 2).

The magnetocardiogram resembled the electrocardiogram (ECG) sensed by electrodes attached to the skin. Baule and McFee speculated that the MCG might contain different information than the ECG, another idea that reappears throughout this review. In a followup article, they theoretically calculated the magnetic field produced by the heart. The interplay between theory and experiments is yet one more subject that frequently arises in this article.

Figure 2. Types of gradiometers.