Books I've read

Six Easy Pieces by Richard P. Feynman

Date read: 2024-01-04. How strongly I recommend it: 9/10

Go to the Amazon page for details and reviews.

A very good introduction to physics for beginners. Most examples in the book are easy to understand and applicable to everyday phenomena. My favorite chapters: Chapter 1 (Atoms in Motion), is the most relatable in everyday life; Chapter 5 (Theory of Gravitation), is the most fun to imagine; and Chapter 6 (Quantum Behavior), is the most 'what the heck' yet intriguing. However, visualizing some concepts can be challenging. I highly recommend reading along with other resources, such as YouTube videos.

My Notes

#physic #science #teaching #chemistry #space

First figure out why you want the students to learn the subject and what you want them to know, and the method will result more or less by common sense.

Chapter 1: Atoms In Motion

Develop the right mindset and attitude for the subject before learning.

Everything we know is only some kind of approximation to the complete truth. Therefore, things must be learned only to be unlearned again or, more likely, to be corrected.

The principle of science, the definition, is the following: The test of all knowledge is experiment. Experiment is the sole judge of scientific “truth.”

Theories explain things. Laws describe things.

A very small effect - a minor deviation or anomaly - requires profound changes in our fundamental ideas.

If all of scientific knowledge were to be destroyed, and only one sentence passed on to the next generations of creatures, what statement would contain the most information in the fewest words? (What are the most fundamental element of nature?)

Atomic Hypothesis: All things are made of atoms — little particles that move around in perpetual motion, attracting each other when they are a little distance apart, but repelling upon being squeezed into one another.

If an apple is magnified to the size of the earth, then the atoms in the apple are approximately the size of the original apple.

The jiggling motion is what we represent as heat: when we increase the temperature, we increase the motion. If we heat the water, the jiggling increases and the volume between the atoms increases, and if the heating continues there comes a time when the pull between the molecules is not enough to hold them together and they do fly apart and become separated from one another.

If we decrease the temperature of a drop of water. The jiggling decreasing. We know that there are forces of attraction between the atoms, so that after a while they will not be able to jiggle so well. The molecules lock into a new pattern which is ice.

Ice: In its solid form, water molecules arrange themselves in a crystalline structure that occupies more space than in the liquid form. This is why ice floats on water - it's less dense than liquid water.

The shape of snowflakes is determined by the arrangement of hydrogen and oxygen atoms in water molecules.

The molecules in the water are always jiggling around (See: Brownian motion). From time to time, one on the surface happens to be hit a little harder than usual, and gets knocked away. Thus, molecule by molecule, the water disappears — it evaporates.

Those that leave have more energy than the average, the ones that are left have less average motion than they had before. So when they leave they take away heat; when they come back they generate heat. If we blow on the water so as to maintain a continuous preponderance in the number evaporating, then the water is cooled. Hence, blow on soup to cool it!

How Water Dissolves Salt.

There are circumstances in which the atoms do change combinations, forming new molecules. A process in which the rearrangement of the atomic partners occurs is what we call a chemical reaction.

One of the oxygen molecules can come over to the carbon, and each atom can pick up a carbon atom and go flying off in a new combination — “carbon-oxygen” — which is a molecule of the gas called carbon monoxide (CO). But carbon attracts oxygen much more than oxygen attracts oxygen or carbon attracts carbon. Therefore in this process the oxygen may arrive with only a little energy, but the oxygen and carbon will snap together with a tremendous vengeance and commotion, and everything near them will pick up the energy. This of course is burning; we are getting heat from the combination of oxygen and carbon. The heat is ordinarily in the form of the molecular motion of the hot gas, but in certain circumstances it can be so enormous that it generates light. That is how one gets flames.

One oxygen atom could attach itself to the CO and ultimately form a molecule, composed of one carbon and two oxygens, which is designated CO2 and called carbon dioxide. If we burn the carbon with very little oxygen in a very rapid reaction (for example, in an automobile engine, where the explosion is so fast that there is not time for it to make carbon dioxide) a considerable amount of carbon monoxide is formed.

Chapter 2: Basic Physics

Generalization: We put together things which at first sight look different, with the hope that we may be able to reduce the number of different things and thereby understand them better.

Is the sand other than the rocks? Is the sand perhaps nothing but a great number of very tiny stones? Is the moon a great rock? If we understood rocks, would we also understand the sand and the moon? What are the common between them?

Scientific method = Observation + Reason + Experiment.

The world is something like a chess game being played by the gods, and we are observers of the game. We don't know what the rules of the game are; all we are allowed to do is to watch the playing. The rules of the game are what we mean by fundamental physics. Even if we knew every rule, we might not be able to understand why a particular move is made in the game, merely because it is too complicated and our minds are limited.

How can we determine if the rules we 'guess' are correct when we cannot thoroughly analyze the game? Three methods: predicting the future (visualizing the simple-puzzle board, and being able to predict precisely what will happen next); observing less specific rules derived from the main rule (for instance, a bishop moves diagonally, so it must stay on either black or light squares); and approximation (making sense of the opponent's intentions from the overall situation without knowing the reasons for each specific move).

Physic before 1920: The “stage” on which the universe goes is the three-dimensional space of geometry, as described by Euclid, and things change in a medium called time. The element of this stage are particles, e.g. atom, these particles have certain properties: inertia and forces.

Inertia: the tendency for matter to remain in its current stage of motion.

Atom: “nucleus” at the center, which is positively electrically charged and very massive, and surrounded by an “electrons” which are very light and negatively charged.

Nucleus have protons and neutrons, almost of the same weight and very heavy. The protons are electrically charged and the neutrons are neutral. If we have an atom with six protons and surrounded by six electrons, this would be atom number six in the chemical table, and it is called carbon.

The ultimate basis of an interaction between the atoms is electrical.

The chemical properties of a substance depend only on a number, the number of electrons.

The natural interpretation of electrical interaction is that two objects simply attract each other: plus against minus. However, this was discovered to be an inadequate idea to represent it. A more adequate representation of the situation is to say that the existence of the positive charge, in some sense, distorts, or creates a “condition” in space, so that when we put the negative charge in, it feels a force. This potentiality for producing a force is called an electric field.

Magnetic influences have to do with charges in relative motion, so magnetic forces and electric forces can really be attributed to one field, as two different aspects of exactly the same thing.

The electromagnetic field can carry waves; some of these waves are light, others are used in radio broadcasts, but the general name is electromagnetic waves.

The only thing that is really different from one wave to another is the frequency of oscillation. If we increase the frequency to 500 or 1000 kilocycles per second, for this is the frequency range which is used for radio broadcasts. (see: The Electromagnetic Spectrum, Synchrotron)

In the years before 1920, the picture of space as a three-dimensional space, and of time as a separate thing, was changed by Einstein, first into a combination which we call space-time, and then still further into a curved space-time to represent gravitation. Then it was also found that the rules for the motions of particles were incorrect. The mechanical rules of “inertia” and “forces” are wrong — Newton’s laws are wrong — in the world of atoms. Instead, it was discovered that things on a small scale behave nothing like things on a large scale.

Heisenberg's uncertainty principle: one cannot know both where something is and how fast it is moving.

Things which we used to consider as waves also behave like particles, and particles behave like waves; in fact everything behaves the same way. There is no distinction between a wave and a particle. So quantum mechanics unifies the idea of the field and its waves, and the particles, all into one.

Particle and wave behaviors are terms devised by humans to describe phenomena based on our experiences, but nature is nature; it just acts as it does.

The new view of the interaction of electrons and photons that is electromagnetic theory, but with everything quantum-mechanically correct, is called quantum electrodynamics.

Furthermore, the same quantum electrodynamics, predicts a lot of new things. In the first place, it tells the properties of very high-energy photons, gamma rays, etc. It predicted another very remarkable thing: besides the electron, there should be another particle of the same mass, but of opposite charge, called a positron, and these two, coming together, could annihilate each other with the emission of light or gamma rays.

To summarize: outside the nucleus, we seem to know all; inside it, quantum mechanics is valid — the principles of quantum mechanics have not been found to fail. The stage on which we put all of our knowledge, we would say, is relativistic space-time; perhaps gravity is involved in space-time. We do not know how the universe got started, and we have never made experiments which check our ideas of space and time accurately, below some tiny distance, so we only know that our ideas work above that distance. We should also add that the rules of the game are the quantum-mechanical principles, and those principles apply, so far as we can tell, to the new particles as well as to the old. The origin of the forces in nuclei leads us to new particles, but unfortunately they appear in great profusion and we lack a complete understanding of their interrelationship, although we already know that there are some very surprising relationships among them.

Chapter 3: The Relation Of Physics To Other Sciences

Physics is the present-day equivalent of what used to be called natural philosophy, from which most of our modern sciences arose.

The early days of chemistry dealt almost entirely with what we now call inorganic chemistry, the chemistry of substances which are not associated with living things.

The theory of chemistry was summarized to a large extent in the periodic chart of Mendeléev, which brings out many strange relationships among the various elements, and it was the collection of rules as to which substance is combined with which, and how, that constituted inorganic chemistry. All these rules were ultimately explained in principle by quantum mechanics, so that theoretical chemistry is in fact physics.

Inorganic chemistry is reduced essentially to what are called physical chemistry and quantum chemistry: physical chemistry to study the rates at which reactions occur and what is happening in detail (How do the molecules hit? Which pieces fly off first?, etc.), and quantum chemistry to help us understand what happens in terms of the physical laws.

The other branch of chemistry is organic chemistry, the chemistry of the substances which are associated with living things. Which closely related to biochemistry, biology, and molecular biology.

Biology: the study of living things. Categorizing, observing, and understanding machinery inside the living bodies.

We see many physical phenomena if we look at the processes of biology of living animals more closely: the circulation of blood, pressure, nerves impulse (electrical effects), enzymes, etc.

Enzymes were first found in the process of fermenting sugar, which is why they were originally called ferments.

With the help of enzymes, reactions that wouldn't normally happen, or would require a lot of energy (activation energy), can occur easily.

An enzyme is made of another substance called protein. Enzymes are very big and complicated, and each one is different, each being built to control a certain special reaction.

Enzymes themselves are not involved in the reaction directly. They do not change; they let an atom go from one place to another. Having done so, the enzyme is ready to do it to the next molecule, like a machine in a factory.

All proteins are not enzymes, but all enzymes are proteins. Proteins are a series, or chain, of different amino acids.

DNA is the key substance which is passed from one cell to another and carries the information as to how to make the enzymes.

The DNA molecule is a pair of chains, twisted upon each other. The backbone of each of these chains is a series of sugar and phosphate groups. The specific instructions for the manufacture of proteins are contained in the specific series of the DNA.

DNA replication: the chain splits down the middle during cell division, one half ultimately to go with one cell, the other half to end up in the other cell; when separated, a new complementary chain is made by each half-chain.

All things are made of atoms, and that everything that living things do can be understood in terms of the jigglings and wigglings of atoms.

The stars are made of atoms of the same kind as those on the earth.

We understand the distribution of matter in the interior of the sun far better than we understand the interior of the earth. Because we can calculate what the atoms in the stars should do in most circumstances.

A poet once said, “The whole universe is in a glass of wine.” We will probably never know in what sense he meant that. But it is true that if we look at a glass of wine closely enough we see the entire universe. There are the things of physics: the twisting liquid which evaporates depending on the wind and weather, the reflections in the glass, and our imagination adds the atoms. The glass is a distillation of the earth’s rocks, and in its composition we see the secrets of the universe’s age, and the evolution of stars. What strange array of chemicals are in the wine? How did they come to be? There are the ferments, the enzymes, the substrates, and the products. There in wine is found the great generalization: all life is fermentation. Nobody can discover the chemistry of wine without discovering, as did Louis Pasteur, the cause of much disease. How vivid is the claret, pressing its existence into the consciousness that watches it!

If our small minds, for some convenience, divide this glass of wine, this universe, into parts — physics, biology, geology, astronomy, psychology, and so on — remember that nature does not know it! So let us put it all back together, not forgetting ultimately what it is for. Let it give us one more final pleasure: drink it and forget it all!

Chapter 4: Conservation Of Energy

The conservation of energy law: the total amount of energy in an isolated system remains constant, even though it may change forms.

Gravitational Potential Energy is the energy that an object possesses because of its position relative to a gravitational field.

Kinetic energy: As a pendulum swings, it alternates between gravitational potential energy (GPE) and kinetic energy. At its highest points, it has maximum GPE. When swinging downwards, this GPE is converted into kinetic energy, which is highest at the bottom where the pendulum moves fastest. As it swings back up, kinetic energy decreases and turns back into GPE.

What causes a pendulum to eventually slow down and stop swinging?

The kinetic energy at the bottom equals the weight times the height that it could go, corresponding to its velocity: K.E. = WH.

Elastic energy is another form of energy, related to the stretching or compressing of objects like springs. When you stretch a spring, you do work on it, storing energy. This stored energy, known as elastic energy, can do work, like lifting weights. If you release the stretched spring, the elastic energy converts into kinetic energy, causing the spring to move. Some of the energy is converted into heat.

Other forms of energy: heat, electric, radiant, light, chemical, nuclear, mass energy.

Our supplies of energy are from the sun, rain, coal, uranium, and hydrogen. The sun makes the rain, and the coal also, so that all these are from the sun. Although energy is conserved, nature does not seem to be interested in it; she liberates a lot of energy from the sun, but only one part in two billion falls on the earth. Nature has conservation of energy, but does not really care; she spends a lot of it in all directions. We have already obtained energy from uranium; we can also get energy from hydrogen, but at present only in an explosive and dangerous condition. If it can be controlled in thermonuclear reactions, it turns out that the energy that can be obtained from 10 quarts of water per second is equal to all of the electrical power generated in the United States. With 150 gallons of running water a minute, you have enough fuel to supply all the energy which is used in the United States today! Therefore it is up to the physicist to figure out how to liberate us from the need for having energy. It can be done.

Chapter 5: The Theory Of Gravitation

What is this law of gravitation? It is that every object in the universe attracts every other object with a force which for any two bodies is proportional to the mass of each and varies inversely as the square of the distance between them.

The story begins with the ancients observing the motions of planets among the stars, and finally deducing that they went around the sun, a fact that was rediscovered later by Copernicus.

In the early 15th century, there were great debates as to whether they really went around the sun or not. Tycho Brahe had an idea that was different from anything proposed by the ancients: his idea was that these debates about the nature of the motions of the planets would best be resolved if the actual positions of the planets in the sky were measured sufficiently accurately. If measurement showed exactly how the planets moved, then perhaps it would be possible to establish one or another viewpoint.

To find something out, it is better to perform some careful experiments and measurement than to carry on deep philosophical arguments.

Pursuing this idea, Tycho Brahe studied the positions of the planets for many years in his observatory on the island of Hven, near Copenhagen. He made voluminous tables, which were then studied by Kepler, after Tycho’s death. Kepler discovered from the data some very beautiful and remarkable, but simple, laws regarding planetary motion.

Kepler found that each planet goes around the sun in a curve called an ellipse, with the sun at a focus of the ellipse.

Kepler’s second observation was that the planets do not go around the sun at a uniform speed, but move faster when they are nearer the sun and more slowly when they are farther from the sun. Make equal areas in equal time.

Third observation: When the orbital period and orbit size (length of greatest diameter) of any two planets are compared, the periods are proportional to the 3/2 power of the orbit size.

While Kepler was discovering these laws, Galileo was studying the laws of motion. The problem was, what makes the planets go around?

In those days, one of the theories proposed was that the planets went around because behind them were invisible angels, beating their wings and driving the planets forward.

Galileo discovered a remarkable fact about motion. That is the principle of inertia — if something is moving, with nothing touching it and completely undisturbed, it will go on forever, coasting at a uniform speed in a straight line. Newton modified this idea, saying that the only way to change the motion of a body is to use force.

Newton appreciated that the sun could be the seat or organization of forces that govern the motion of the planets. Newton proved to himself that the very fact that equal areas are swept out in equal times is a precise signpost of the proposition that all deviations are precisely radial — that the law of areas is a direct consequence of the idea that all of the forces are directed exactly toward the sun.

By analyzing Kepler’s third law it is possible to show that the farther away the planet, the weaker the forces.

Newton supposed that this relationship applied more generally than just to the sun holding the planets. It was already known, for example, that the planet Jupiter had moons going around it as the moon of the earth goes around the earth, and Newton felt certain that each planet held its moons with a force. He already knew of the force holding us on the earth, so he proposed that this was a universal force — that everything pulls everything else.

The next problem was whether the pull of the earth on its people was the “same” as its pull on the moon. If an object on the surface of the earth falls 16 feet in the first second after it is released, how far does the moon fall in the same time? We might say that the moon does not fall at all. But if there were no force on the moon, it would go off in a straight line, whereas it goes in a circle instead. We can calculate from the radius of the moon’s orbit (which is about 240,000 miles) and how long it takes to go around the earth (approximately 29 days) how far the moon moves in its orbit in one second, and can then calculate how far it falls in one second. This distance turns out to be roughly 1/20 of an inch in a second.

This idea that the moon “falls” is somewhat confusing, because, as you see, it does not come any closer. The idea is sufficiently interesting to merit further explanation: the moon falls in the sense that it falls away from the straight line that it would pursue if there were no forces. (See: Newton's Cannon)

The law of gravitation reveals that the Moon's pull on the Earth causes tides. Initially, people believed there should be only one high and one low tide each day, thinking the Moon's gravity pulls water up beneath it, and Earth's rotation results in daily tide changes at a given location.

Actually the tide goes up and down in 12 hours. Tides are caused by the Moon's gravitational pull on Earth and its oceans. The pull is strongest on the water closest to the Moon, causing a high tide. On the opposite side of Earth, another high tide occurs because the Moon's pull is weaker there, creating an imbalance. Additionally, the Earth and the Moon orbit a common center, not each other directly.

The earth is not exactly a sphere because it is rotating, and this brings in centrifugal effects which tend to oppose gravity near the equator.

How Jupiter's Moons Showed Us the Speed of Light.

Cavendish's experiment demonstrated the direct gravitation force between two large, fixed balls of lead and two smaller balls of lead on the ends of an arm supported by a very fine fiber, called a torsion fiber.

What is the machinery of gravity? All we have done is to describe how the earth moves around the sun, but we have not said what makes it go. It is characteristic of the physical laws that they have this abstract character.

Many mechanisms for gravitation have been suggested. It is interesting to consider one of these. At first, one is quite excited and happy when he “discovers” it, but he soon finds that it is not correct. Suppose there were many small particles moving in space at a very high speed in all directions and being only slightly absorbed in going through matter. When they are absorbed, they give an impulse to the earth. However, since there are as many going one way as another, the impulses all balance. But when the sun is nearby, the particles coming toward the earth through the sun are partially absorbed, so fewer of them are coming from the sun than are coming from the other side. Therefore, the earth feels a net impulse toward the sun. (See: Le Sage's theory of gravitation)

What is wrong with that machinery? It involves some new consequences which are not true. The earth, in moving around the sun, would impinge on more particles which are coming from its forward side than from its hind side (when you run in the rain, the rain in your face is stronger than that on the back of your head!). Therefore there would be more impulse given the earth from the front, and the earth would feel a resistance to motion and would be slowing up in its orbit.

No machinery has ever been invented that “explains” gravity without also predicting some other phenomenon that does not exist.

The so-called unified field theory is only a very elegant attempt to combine electricity and gravitation; but, in comparing gravitation and electricity, the most interesting thing is the relative strengths of the forces.

If we take, in some natural units, the repulsion of two electrons due to electricity, and the attraction of two electrons due to their masses, we can measure the ratio of electrical repulsion to the gravitational attraction. The ratio is independent of the distance and is a fundamental constant of nature. The gravitational attraction relative to the electrical repulsion between two electrons is 1 divided by 4.17 × 10^42!

This fantastic number is a natural constant, so it involves something deep in nature. One of the possibilities have been thought of; one is to relate it to the age of the universe. Clearly, we have to find another large number somewhere. But do we mean the age of the universe in years? No, because years are not “natural”; they were devised by men. As an example of something natural, let us consider the time it takes light to go across a proton, 10^(—24) second. If we compare this time with the age of the universe, 2 × 10^10 years, the answer is 10^(-42). It has about the same number of zeros going off it, so it has been proposed that the gravitational constant is related to the age of the universe. If that were the case, the gravitational constant would change with time, because as the universe got older the ratio of the age of the universe to the time which it takes for light to go across a proton would be gradually increasing.

Mass versus weight: Mass is the amount of "matter" in an object (kg), but weight is the force (newton) exerted on an object's matter by gravity.

According to Newton, the gravitational effect is instantaneous, that is, if we were to move a mass, we would at once feel a new force because of the new position of that mass; by such means we could send signals at infinite speed. Einstein advanced arguments which suggest that we cannot send signals faster than the speed of light, so the law of gravitation must be wrong.

In recent years we have discovered that all mass is made of tiny particles and that there are several kinds of interactions, such as nuclear forces, etc. None of these nuclear or electrical forces has yet been found to explain gravitation. The quantum-mechanical aspects of nature have not yet been carried over to gravitation. When the scale is so small that we need the quantum effects, the gravitational effects are so weak that the need for a quantum theory of gravitation has not yet developed. On the other hand, for consistency in our physical theories it would be important to see whether Newton’s law modified to Einstein’s law can be further modified to be consistent with the uncertainty principle. This last modification has not yet been completed.

Chapter 6: Quantum Behavior

“Quantum mechanics” is the description of the behavior of matter in all its details and, in particular, of the happenings on an atomic scale.

Things on a very small scale behave like nothing that you have any direct experience about. They do not behave like waves, they do not behave like particles, they do not behave like clouds, or billiard balls, or weights on springs, or like anything that you have ever seen.

The Double-Slit Experiment (firing electrons): The first thing we would say is that since electrons come in lumps, each lump has come either through hole 1 or through hole 2. Let us write this in the form of a “Proposition”: Each electron either goes through hole 1 or it goes through hole 2 (Proposition A).

Assuming Proposition A, all electrons that arrive at the backstop can be divided into two classes: (1) those that come through hole 1, and (2) those that come through hole 2. So our observed curve must be the sum of the effects of the electrons which come through hole 1 and the electrons which come through hole 2. Let us check this idea by experiment. First, we will make a measurement for those electrons that come through hole 1. We block off hole 2 and make our counts of the clicks from the detector. The result seems quite reasonable (particle-liked result). In a similar way, we measure P2, the probability distribution for the electrons that come through hole 2. The result P12 obtained with both holes open is clearly not the sum of P1 and P2, the probabilities for each hole alone. In analogy with our water-wave experiment, we say: “There is interference!” (P1 + P2 ≠ P12)

We conclude the following: The electrons arrive in lumps, like particles, and the probability of arrival of these lumps is distributed like the distribution of intensity of a wave. It is in this sense that an electron behaves “sometimes like a particle and sometimes like a wave.”

Watching the electrons experiment: To our electron apparatus we add a very strong light source, placed behind the wall and between the two holes. We know that electric charges scatter light. So when an electron passes on its way to the detector, it will scatter some light to our eye, and we can see where the electron goes. If an electron passes through hole 1 we would expect to see a flash from the vicinity of the upper hole. If it should happen that we get light from both places at the same time, because the electron divides in half.

Here is what we see: every time that we hear a “click” from our electron detector, we also see a flash of light either near hole 1 or near hole 2, but never both at once! And we observe the same result no matter where we put the detector. From this observation we conclude that when we look at the electrons we find that the electrons go either through one hole or the other. Experimentally, Proposition A is necessarily true.

What's wrong with our argument against Proposition A? Why isn’t P12 just equal to P1 + P2? Back to experiment! We can keep track of things this way: Every electron which arrives is recorded in one of two classes: those which come through 1 and those which come through 2. From the number recorded in Column 1 we get the probability P‘1 that an electron will arrive at the detector via hole 1; and from the number recorded in Column 2 we get P’2. For the probability that an electron will arrive at the backstop by passing through either hole, we do find P‘12 = P’1 + P‘2. That is, although we succeeded in watching which hole our electrons come through, we no longer get the old interference curve P12, but a new one, P’12, showing no interference! If we turn out the light P12 is restored.

We must conclude that when we look at the electrons the distribution of them on the screen is different than when we do not look. Perhaps it is turning on our light source that disturbs things?

You may be thinking: “Don’t use such a bright source! Turn the brightness down! The light waves will then be weaker and will not disturb the electrons so much.” OK. Let’s try it. The first thing we observe is that the flash of light scattered from the electrons as they pass by does not get weaker. It is always the same-sized flash. The only thing that happens as the light is made dimmer is that sometimes we hear a “click” from the detector but see no flash at all. The electron has gone by without being “seen.” What we are observing is that light also acts like electrons; we knew that it was “wavy,” but now we find that it is also “lumpy.” It always arrives in lumps that we call “photons.” As we turn down the intensity of the light source we do not change the size of the photons, only the rate at which they are emitted. That explains why, when our source is dim, some electrons get by without being seen. There did not happen to be a photon around at the time the electron went through.

Let try the experiment with a dim light anyway. Now whenever we hear a click in the detector we will keep a count in three columns: in Column 1 those electrons seen by hole 1, in Column 2 for hole 2, and in Column 3 those electrons not seen at all. When we work up our data (computing the probabilities) we find these results: Those “seen by hole 1” have a distribution like P‘1; those ”seen by hole 2” have a distribution like P’2 (so that those “seen by either hole 1 or 2” have a distribution like P‘12); and those not seen at all have a wavy distribution just like P12. If the electrons are not seen, we have interference! When we do not see the electron, no photon disturbs it, and when we do see it, a photon has disturbed it.

The complete theory of quantum mechanics which we now use to describe atoms and, in fact, all matter depends on the correctness of the uncertainty principle. Since quantum mechanics is such a successful theory, our belief in the uncertainty principle is reinforced. But if a way to “beat” the uncertainty principle were ever discovered, quantum mechanics would give inconsistent results and would have to be discarded as a valid theory of nature.

If the motion of all matter — as well as electrons — must be described in terms of waves, what about the bullets in our first experiment? Why didn’t we see an interference pattern there? It turns out that for the bullets the wavelengths were so tiny that the interference patterns became very fine. So fine, in fact, that with any detector of finite size one could not distinguish the separate maxima and minima.

the uncertainty principle: If you make the measurement on any object, and you can determine the x-component of its momentum with an uncertainty Δp, you cannot, at the same time, know its x-position more accurately than Δx ≥ ħ/2Δp.

The uncertainty principle “protects” quantum mechanics. Heisenberg recognized that if it were possible to measure the momentum and the position simultaneously with a greater accuracy, the quantum mechanics would collapse. So he proposed that it must be impossible. Then people sat down and tried to figure out ways of doing it, and nobody could figure out a way to measure the position and the momentum of anything — a screen, an electron, a billiard ball, anything — with any greater accuracy. Quantum mechanics maintains its perilous but accurate existence.