Adding the human touch

Subha Das Mollick

Many students do not like physics because it is too cold and objective, devoid of emotion. To get these students interested in physics, one may try reading out passages from popular science books. Good popular science books effectively bring out the lateral thinking that scientists are capable of as well as the intellectual discipline with which they negotiate the problems. These writings bring alive a problem in physics and give it a human dimension.

Below are excerpts from two popular science books and the exercises are based on these passages. Physics teachers may try them out in class.
Excerpt from Life in Science, by Michael White, Jojn Gribbin. (Chapter 9, page 136, 137)

It was, in fact, satellite borne instruments that identified, at about this time, the first really plausible black hole candidate in our Milky Way Galaxy. Just as great new discoveries in astronomy had come about in the 1960s through the investigation of the radio part of the spectrum, at wavelengths longer than those of light, so great new advances came in the 1970s through the investigation of the X ray part of the spectrum, at wavelengths much shorter than those of light. Unlike radio waves, however, X rays from space are blocked by the earth’s atmosphere, and do not reach the ground (which is just as well or we would be fried). So X ray astronomy came of age as a branch of science only when suitable detectors were placed in orbit around the earth. These unmanned satellites transformed astronomers’ view of the universe, showing it to be a much more violent and energetic place than they had thought. And at least some of that violence, they are now convinced, is associated with black holes.

1. Compare the frequency range of radio waves to that of visible light and to the frequency range of X rays.
2. Why do you think X rays, that can penetrate human flesh, are blocked by the earth’s atmosphere?
3. How do you think satellites with X ray detectors revealed the universe to be a much more violent place than it was earlier thought to be?
4. What kind of violent activities do you think goes on in the universe?
5. The Milky Way galaxy is our home galaxy. Name the galaxy that is our nearest neighbour.
6. What does the above paragraph tell you about challenges in observational astronomy?
7. Why are black holes black?

Note: This exercise may be given to students of classes IX and X.

Here is a longer excerpt from The Information: A History, A Theory, A Flood by James Gleick, Chapter 9, page 274, 275.

The improbability of heat passing from a colder to a warmer body (without help from elsewhere) is identical to the improbability of order arranging itself from disorder. Both, fundamentally, are due only to statistics. Counting all the possible ways a system can be arranged, the disorderly ones far outnumber the orderly ones. There are many arrangements or states in which molecules are all jumbled, and few in which they are neatly sorted. The orderly states have low probability and low entropy. For impressive degree of orderliness, the probabilities may be very low. Alan Turing once whimsically proposed a number N, defined as “the odds against a piece of chalk leaping across the room and writing a line of Shakespeare on the board.

Eventually physicists began speaking of microstates and macrostates. A macrostate might be all the gas in the top half of a box. The corresponding microstates would be all the possible arrangements of all particles – positions and velocities. Entropy thus became a physical equivalent of a probability: the entropy of a given microstate is the logarithm of its possible microstates.

It was still puzzling, though, to hang so much physics on a matter of mere probability. Can it be right to say that nothing in physics is stopping a gas from dividing itself into hot and cold – that it is only a matter of chance and statistics? Maxwell illustrated this conundrum with a thought experiment. “Imagine”, he suggested, “a finite being”, who stands watch over a tiny hole in the diaphragm dividing the box of gas. This creature can see molecules coming, can tell whether they are fast or slow, and can choose whether or not to let them pass. Thus he could tilt the odds. By sorting fast from slow, he could make side A hotter and side B colder – “and yet, no work has been done. Only the intelligence of a very observant and neat fingered being has been employed.” The being defies ordinary probabilities. The chances are, things get mixed together. To sort them out requires information.

Thompson loved the idea. He dubbed the notional creature a demon: “Maxwell’s intelligent demon”, “Maxwell’s sorting demon” and soon, just “Maxwell’s demon”.

1. In the very first sentence of the above passage, two laws of thermodynamics are implicitly implied. State these two laws.
2. If there is 0.001 gm of oxygen gas in a closed container, how many molecules of oxygen will be there?
3. If the temperature of the enclosed gas is 100° Kelvin, what will be the average kinetic energy of the molecules?
4. If instead of 0.001 gm of oxygen gas there are just 10 molecules of oxygen gas in the container, how will the temperature of the gas undergo a change? Do you think the temperature will still be a measure of the average kinetic energy of the gas molecules?
5. If the container is cubical, where each side of the cube is 10 cm, what will be the pressure of the enclosed gas in this container?
6. What will be the momentum transfer per second on any once face of the cube?
7. Given that the collisions are elastic, how many molecules will be impinging on any one of the walls in one second?
8. The above passage is about randomness and disorder in the equilibrium state of matter. There is no physical explanation as to why a system cannot go from a state of disorder to order when left to itself. And this is what has bothered scientists for a long time. The molecules of gas in a closed container move randomly and collide with each other. There is a tiny probability that at an instant of time all the molecules will be moving in the same direction. That is the instant of perfect order for the gas. It is one of the many many microstates the gas can achieve and the probability of attaining this microstate is infinitesimally small. Try to imagine what will happen if the gas by chance achieves this microstate of perfect order.
9. Can you design a thought experiment to record, say, 100 microstates of an enclosed mass of oxygen gas attains in one second? (Hint: In the 21^st Century you have access to many high speed digital imaging techniques. Some of these techniques may be handy in your thought experiment.)
10. Maxwell had conceived of a demon to separate molecules of higher temperature (speed) from those of a lower temperature. Can you conceive a mechanical system for doing this separation, that can actually work on principles of physics? How long do you think this system will take to achieve the separation?

Note: This exercise may be given to students of classes XI and XII. An exercise like this is likely to hone the student’s imagination as well as reasoning. Question number 10 above is an open ended question and can be viewed in many ways. The idea of Maxwell’s Demon had captured the imagination of scientists and they referred to the demon in different contexts in scientific presentations. In the 21^st Century context, the demon capable of separating molecules of different velocities is seen as a being possessing information. The entire thrust of information science is to decrease entropy. Maxwell’s generation did not consider information on par with matter and energy.

The author is the secretary of Bichitra Pathshala, an organization that promotes learning with moving images. She is also an associate director at iLEAD Institute, Kolkata. She can be reached at
subha.dasmollick@gmail.com.