Gauss and von Humboldt

The Age of Napoleon,
by Will and Ariel Durant,
Volume 11 of The Story of Civilization.

Regular readers of this blog may recall that over the last few years I’ve been reading Will and Ariel Durant’s magnificent The Story of Civilization. I’m almost done.  I’m currently finishing the final chapters of the last volume: The Age of Napoleon. In the chapter about the German people is a section on science. It states

Two men especially brought scientific honors to Germany in this age—Karl Friedrich Gauss (1777–1855) and Alexander von Humboldt (1769–1859).

Humboldt is never mentioned in Intermediate Physics for Medicine and Biology, but Gauss is everywhere. When speaking of Gauss, the Durants write

We shall not pretend to understand, much less to expound, the discoveries—in number theory, imaginary numbers, quadratic residues, the method of least squares, the infinitesimal calculus—by which Gauss transformed mathematics from what it had been in Newton’s time into an almost new science, which became a tool of the scientific miracles of our time. His observations of the orbit of Ceres (the first planetoid, discovered on January 1, 1801) led him to formulate a new and expeditious method of determining planetary orbits [least squares is discussed in Chapter 11 of IPMB]. He made researches which placed the theory of magnetism and electricity upon a mathematical basis [Gauss’s law for calculating the electric field is discussed in Chapter 6 of IPMB; the now somewhat obsolete unit of magnetic field strength is the gauss]. He was a burden and blessing [definitely a blessing] to all scientists, who believe that nothing is science until it can be stated in mathematical terms. [He also invented the Gaussian probability distribution, which plays a major role in diffusion, discussed in Chapter 4 of IPMB]…. He is now ranked with Archimedes and Newton.

Humboldt was more of a naturalist, and his name never appears in IPMB. But the Durants devoted even more space in their history to him than to Gauss.

The other hero of German Science in this age was Wilhelm von Humboldt’s younger brother Alexander…. In 1796 he began, by accident, the long tour of scientific discovery (rivaling Darwin’s on the Beagle) whose results made him, according to a contemporary quip, “the most famous man in Europe, next to Napoleon.”

Humboldt is particularly famous for his work in geography and geology. I become familiar with him when I taught earth science. I was a new, untenured faculty member at Oakland University when the physics department needed someone to teach our earth science class. OU does not have a geology department, but some students do need a course in earth science, so the physics department was in charge of it. When the faculty member who traditionally taught it retired, I was asked to take it over. I knew nothing about earth science, but neither did anyone else in the department, and being the newest member of the department I didn’t feel that I could say no. I taught the class for about five years, and found that I enjoyed it. Most students in the course were elementary education majors. They weren’t the strongest science students I ever taught, but they were some of the nicest.

Here is what the Durants had to say about Humboldt.

He discovered (1804) that the earth’s magnetic force decreases in intensity from the poles to the equator. He enriched geology with his studies of the igneous origin of certain rocks, the formation of mountains, the geographical distribution of volcanoes. He provided the earliest clues to the laws governing atmospheric disturbances, and thereby shed light on the origin and direction of tropical storms. He made classic studies of air and ocean currents…. His Essai sur la geographie des plantes began the science of biogeography—the study of plant distribution as affected by the physical conditions of the terrain. These and a hundred other contributions, modest in appearance but of wide and lasting influence, were published in thirty volumes from 1805 to 1834 as Voyages de Humboldt et Bonpland aux regions equinoxiales du nouveau continent.

Humboldt is particularly relevant these days as one of the first environmentalists and discoverer of the concept of human-induced climate change. The closest he came to IPMB may be his work on muscle excitation and bioelectricity. In “Alexander von Humboldt and the Concept of Animal Electricity” (Trends in Neurosciences, Volume 20, Pages 239–242, 1997), Helmut Kettenmann wrote

More than two hundred years ago, Alexander von Humboldt helped to establish Galvani's view that muscle and nerve tissue are electrically excitable. His 1797 publication was a landmark for establishing the concept of animal electricity. Almost half a century later, von Humboldt became the mentor of the young du Bois-Reymond. With the help of von Humboldt's promotion, du Bois-Reymond demonstrated convincingly that animal tissue has the intrinsic capacity to generate electrical activity, and thus laid the ground for modern electrophysiology. 

Gauss and Humboldt; what a pair. Put them together with Goethe and Beethoven and Germany around 1800 becomes a pretty interesting place.

Oh, what will I do with myself now that my reading of The Story of Civilization is complete? I guess I will have to focus on the 6th edition of IPMB.

 

My favorite Gauss story, about how as a child he added all the numbers from 1 to 100.

https://www.youtube.com/watch?v=cD9rI4wSc7o

Ken Jennings narrates this video about Alexander von Humboldt.

https://www.youtube.com/watch?v=fj7tRMdmOgs

Alexander von Humboldt and the discovery of climate change.

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

The Partition Function

Any good undergrad statistical mechanics class analyzes the partition function. However, Russ Hobbie and I don’t introduce the partition function in Intermediate Physics for Medicine and Biology. Why not? It’s a long story.

Russ and I do discuss the Boltzmann factor. Suppose you have a system that is in thermal equilibrium with a reservoir at absolute temperature T. Furthermore, suppose your system has discrete energy levels with energy E_i, where i is just an integer labeling the levels. The probability P_i of the system being in level i, is proportional to the Boltzmann factor, exp(–E_i/k_BT),

P_i = C exp(–E_i/k_BT),

where exp is the exponential function, k_B is the Boltzmann constant and C is a constant of proportionality. How do you find C? Any probability must be normalized: the sum of the probabilities must equal one,

Σ P_i = 1 ,

where Σ indicates a summation over all values of i. This means

Σ C exp(–E_i/k_BT) = 1,

or

C = 1/[Σ exp(–E_i/k_BT)] .

The sum in the denominator is the partition function, and is usually given the symbol Z,

Z = Σ exp(–E_i/k_BT) .

In terms of the partition function, the probability of being in state P_i is simply

P_i = (1/Z) exp(–E_i/k_BT) .

An Introduction to Thermal Physics,
by Daniel Schroeder.

Here is what Daniel Schroeder writes in his excellent textbook An Introduction to Thermal Physics,

The quantity Z is called the partition function, and turns out to be far more useful than I would have suspected. It is a “constant” in that it does not depend on any particular state s [he uses “s” rather than “i” to count states], but it does depend on temperature. To interpret it further, suppose once again that the ground state has energy zero. Then the Boltzmann factor for the ground state is 1, and the rest of the Boltzmann factors are less than 1, by a little or a lot, in proportion to the probabilities of the associated states. Thus, the partition function essentially counts how many states are accessible to the system, weighting each one in proportion to its probability.

To see why it’s so useful, let’s define β as 1/k_BT. The Boltzmann factor is then

exp(–βE_i)

and the partition function is

Z = Σ exp(–βE_i) .

The average energy, <E>, is

<E> = [Σ E_i exp(–βE_i)]/[Σ exp(–βE_i)] .

The denominator is just Z. The numerator can be written as the negative of the derivative of Z with respect to β, dZ/dβ (try it and see). So, the average energy is

<E> = – (1/Z) dZ/dβ .

I won’t go on, but there are other quantities that are similarly related to the partition function. It’s surprisingly useful.

Is the partition function hidden in IPMB? You might recognize it in Eq. 3.37, which determines the average kinetic energy of a particle at temperature T (the equipartition of energy theorem) It looks a little different, because there’s a continuous range of energy levels, so the sum is disguised as an integral. You can see it again in Eq. 18.8, when evaluating the average value of the magnetic moment during magnetic resonance imaging. The partition function’s there, but it’s nameless.

Why didn’t Russ and I introduce the partition function? In the Introduction of IPMB Russ wrote: “Each subject is approached in as simple a fashion as possible. I feel that sophisticated mathematics, such as vector analysis or complex exponential notation, often hides physical reality from the student.” Like Russ, I think that the partition function is a trick that makes some equations more compact, but hides the essential physics. So we didn’t use it.

LaTeX and Mathematica

Gene Surdutovich and I are hard at work on the 6th edition of Intermediate Physics for Medicine and Biology. So far, the main thrust of our work involves LaTeX and Mathematica.

Russ Hobbie and I wrote the 5th edition of IPMB using LaTeX, a computer program that is particularly useful for typesetting equations. Russ was our LaTeX guru. I merely read pdf documents that he created and sent him my suggested changes, and then he implemented those changes into the book. With Russ gone, I can no longer escape dealing with LaTeX commands. LaTeX is an extremely powerful piece of software, but mastering it requires a long learning curve. Fortunately, Gene has extensive experience with it.

Let me give you a little peek behind the curtain at typesetting an equation with LaTeX. Equation 4.74 in the 5th edition is the definition of the error function,

In LaTeX it looks like this:

\begin{equation}
\operatorname{erf}(z)=\frac{2}{\sqrt{\uppi}}\int_{0}^{z}e^{-t^{2}}dt.
\label{4.74}%
\end{equation}

Kind of complicated, isn’t it? Sometimes I find myself getting LaTeX and html mixed up. 

LaTeX numbers the equations automatically. They each get a label, such as “\label{4.74}” but this label does not specify the equation number, it’s just a pointer. If I want to refer to this equation later I can write “see Eq.~\ref{4.74}”. If I decide I want to add an equation before Eq. 4.74, I can just give it any label I want—say, “\label{4.73b}”— and then LaTeX will renumber all the equations properly. For a book like IPMB, which has hundreds of equations, this automatic numbering is wonderful.

The index is also created automatically. Whenever I use a term such as “error function” that I want included in the index, I add “\index{Error function}”. LaTeX will keep track of the page number where that code is placed and then include that term with the correct page number in the index. This same sort of internal labeling can be used to create the list of symbols at the end of each chapter,  the list of homework problems, and the section and subsection numbering. In fact, I didn’t have to renumber anything when I added an entire new chapter about... more on that later. LaTeX is amazing. How did I write my PhD dissertation without it?

Also, LaTeX can number and label figures and illustrations, but you have to create the figures using another program. We’ve started using Mathematica for that job. (I’m ashamed to say, I’m not sure what software Russ used.) Mathematica, produced by Wolfram Research, is very powerful, and can do all sorts of symbolic computations. We don’t take advantage of those features, but mainly use the program to make beautiful plots. Fortunately, Gene is even better at Mathematica than at LaTeX, and he helps me a lot. IPMB’s publisher, Springer, says we can use as much color as we want for the 6th edition. Think of the 5th edition of IPMB as like when Dorothy is in Kansas. Publication of the 6th edition will correspond to that memorable scene when she opens the door of the farmhouse and finds herself in the colorful Land of Oz.

Preparing the 6th edition is going to be a long-term project, so don’t expect it anytime soon. Maybe it’ll be ready by the end of 2024, but maybe not. Thanks to all of you who responded to our recent survey. If you have further suggestions, there is still lots of time and we would appreciate hearing any ideas.

And now, back to work!

Dorothy enters the Land of Oz.

https://www.youtube.com/watch?v=F4eQmTizTSo

“If I Only Had a Brain”, from the Wizard of Oz. The song has nothing to do with LaTeX or Mathematica or IPMB, but it’s such a great number that you just have to watch.

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

Is Quantum Mechanics Necessary for Understanding Magnetic Resonance?

Hanson, L.,
“Is Quantum Mechanics Necessary for
Understanding Magnetic Resonance?”
Concepts Magn.Reson., 32:329–340, 2008

In Chapter 18 of Intermediate Physics for Medicine and Biology, Russ Hobbie and I discuss magnetic resonance imaging. Like many authors, we derive an expression for the magnetization of the tissue using quantum mechanics. Must we use quantum theory? In an article published in Concepts in Magnetic Resonance, Lars Hanson asks the same question: “Is Quantum Mechanics Necessary for Understanding Magnetic Resonance?” (Volume 32, Pages 329–340, 2008). The abstract is given below.

Educational material introducing magnetic resonance typically contains sections on the underlying principles. Unfortunately the explanations given are often unnecessarily complicated or even wrong. Magnetic resonance is often presented as a phenomenon that necessitates a quantum mechanical explanation whereas it really is a classical effect, i.e. a consequence of the common sense expressed in classical mechanics. This insight is not new, but there have been few attempts to challenge common misleading explanations, so authors and educators are inadvertently keeping myths alive. As a result, new students’ first encounters with magnetic resonance are often obscured by explanations that make the subject difficult to understand. Typical problems are addressed and alternative intuitive explanations are provided.

How would IPMB have to be changed to remove quantum mechanics from the analysis of MRI? Quantum ideas first appear in the last paragraph of Section 18.2 about the source of the magnetic moment, where we introduce the idea that the z-component of the nuclear spin in a magnetic field is quantized and can take on values that are integral multiples of the reduced Planck’s constant, ℏ. Delete that paragraph.

Section 18.3 is entirely based on quantum mechanics. To find the average value of the z-component of the spin, we sum over all quantum states, weighted by the Boltzmann factor. The end result is an expression for the magnetization as a function of the magnetic field. We could, alternatively, do this calculation classically. Below is a revised Section 18.3 that uses only classical mechanics and classical thermodynamics.

18.3 The Magnetization

The MR [magnetic resonance] image depends on the magnetization of the tissue. The magnetization of a sample, M, is the average magnetic moment per unit volume. In the absence of an external magnetic field to align the nuclear spins, the magnetization is zero. As an external magnetic field B is applied, the spins tend to align in spite of their thermal motion, and the magnetization increases, proportional at first to the external field. If the external field is strong enough, all of the nuclear magnetic moments are aligned, and the magnetization reaches its saturation value.

We can calculate how the magnetization depends on B. Consider a collection of spins of a nuclear species in an external magnetic field. This might be the hydrogen nuclei (protons) in a sample. The spins do not interact with each other but are in thermal equilibrium with the surroundings, which are at temperature T. We do not consider the mechanism by which they reach thermal equilibrium. Since the magnetization is the average magnetic moment per unit volume, it is the number of spins per unit volume, N, times the average magnetic moment of each spin: M=N<μ>, where μ is the magnetic moment of a single spin.

To obtain the average value of the z component of the magnetic moment, we must average over all spin directions, weighted by the probability that the z component of the magnetic moment is in that direction. Since the spins are in thermal equilibrium with the surroundings, the probability is proportional to the Boltzmann factor of Chap. 3, e^–(U/k_BT) = e^{μBcosθ/k_BT}, where k_B is the Boltzmann constant. The denominator in Eq. 18.8 normalizes the probability:

The factor of sinθ arises when calculating the solid angle in spherical coordinates (see Appendix L). At room temperature μB/(k_BT) ≪ 1 (see Problem 4), and it is possible to make the approximation e^x ≈ 1 + x. The integral in the numerator then has two terms:

The first integral vanishes. The second is equal to 2/3 (hint: use the substitution u = cosθ). The denominator is

The first integral is 2; the second vanishes. Therefore we obtain

The z component of M is

which is proportional to the applied field.

The last place quantum mechanics is mentioned is in Section 18.6 about relaxation times. The second paragraph, starting “One way to analyze the effect…”, can be deleted with little loss of meaning; it is almost an aside.

So, to summarize, if you want to modify Chapter 18 of IPMB to eliminate any reference to quantum mechanics, then 1) delete the last paragraph of Section 18.2, 2) replace Section 18.3 with the modified text given above, and 3) delete the second paragraph in Section 18.6. Then, no quantum mechanics appears, and Planck’s constant is absent. Everything is classical, just the way I like it.

Calculus Made Easy

Intermediate Physics for Medicine and Biology assumes the reader knows calculus. Most medical doctors and biologists have studied some calculus, but I’m not sure they remember much of it. And most high school students, and even college freshman, have yet to take their first calculus course. What should these readers of IPMB do if they don’t know any calculus?  

Calculus Made Easy,
by Silvanus Thompson.

What these readers need is a quick and easy way to learn calculus without delving into all the subtle and complicated details. How can they do that? Read the delightful old book Calculus Made Easy, by Silvanus Thompson. Here’s the prologue:

Considering how many fools can calculate, it is surprising that it should be thought either a difficult or a tedious task for any other fool to learn how to master the same tricks. 

Some calculus-tricks are quite easy. Some are enormously difficult. The fools who write the textbooks of advanced mathematics — and they are mostly clever fools — seldom take the trouble to show you how easy the easy calculations are. On the contrary, they seem to desire to impress you with their tremendous cleverness by going about it in the most difficult way. 

Being myself a remarkably stupid fellow, I have had to unteach myself the difficulties, and now beg to present to my fellow fools the parts that are not hard. Master these thoroughly, and the rest will follow. What one fool can do, another can.

I know what you’re thinking: “That sounds like just what I need, but how much is it going to cost me?” The good news is that you can access the book for free online, at http://calculusmadeeasy.org.

Silvanus P. Thompson

The author, Silvanus Phillips Thompson (1851–1916), was an English physicist and a fellow of the Royal Society. I have a particular fondness for physicists from the Victorian era, especially one such as Thompson who was interested in science education and whose strength was his ability to explain difficult concepts clearly.

For those of you turned off by the dated style of Calculus Made Easy, written in 1910, I suggest Quick Calculus or Used Math instead. For those who, like me, love the Victorian style, I recommend Flatland by Edwin Abbott.

Enjoy!

Calculus Made Easy, by Silvanus P. Thompson, Part 1/2. A LibriVox audiobook. 

https://www.youtube.com/watch?v=hF2FLi5UnlE

 

Calculus Made Easy, by Silvanus P. Thompson, Part 2/2. A LibriVox audiobook.

https://www.youtube.com/watch?v=uqQtQNTKo-A