6 Easy Pieces by Richard Feynman

Very easy to read and isn't long so is a great introductory book to physics. Doesn't go into detail about the concepts and doesn't use words that are difficult to understand. Discusses energy, atoms, gravity, Kepler's laws, quantum mechanics, biology and other concepts. 

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Conservation of energy – you can move energy around and change its form but you can’t create or destroy it.

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

Mass increases with velocity but noticeable increases require velocities near that of light.

Everything is made of atoms.

Air consists almost entirely of N, O, some water vapor and lessor amounts of carbon dioxide, argon and other things.

An ion is an atom which either has a few extra electrons or has lost a few electrons.

Everything is made of atoms. That is the key hypothesis.  The most important hypothesis in all of biology, for example, is that everything that animals do, atoms do.  In others words, there are nothing that living things do that cannot be understood from the point of view that they are made of atoms acting according to the laws of physics.  This was not known from the beginning: it took some experimenting and theorizing to suggest this hypothesis, but now it is accepted, and it is the most useful theory for producing new ideas in the field of biology.

That is the way it is in physics.  For a long time we will have a rule that works excellently in an overall way, even when we cannot follow the details, and then sometime we may discover a new rule.

Before 1920, our world picture was something like this: 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 elements on the stage are particles, for example the atoms, which have some properties.  First, the property of inertia: if a particle is moving it keeps on going in the same direction unless forces act upon it. 

The second element, then, is forces, which were then thought to be of two varieties: First, an enormously complicated, detailed kind of interaction force which held the various atoms in different combinations in a complicated way, which determined whether salt, would dissolve faster or slower when we raise the temperature.  The other force that was known was a long-range interaction – a smooth and quiet attraction – which varied inversely as the square of the distance, and was called gravitation.

Atom number eight is called oxygen, etc., because the chemical properties depend upon the electrons on the outside, and in fact only upon how many electrons there are.  So the chemical properties of a substance depend only on a number, the number of electrons.

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.  So the “stage” is changed into space-time, and gravitation is presumably a modification of space-time.

An atom has a diameter of about 10^-8 cm.  The nucleus has a diameter of about 10^-13 cm.  If we had an atom and wished to see the nucleus, we would have to magnify it until the whole atom was the size of a large room, and then the nucleus would be a bare speck which you could just about make out with the eye, but very nearly all the weight of the atom is in that infinitesimal nucleus.

Another most interesting change in the ideas and philosophy of science brought about by quantum mechanics is this: it is not possible to predict exactly what will happen in any circumstance.

Nature, as we understand it today, behaves in such a way that it is fundamentally impossible to make a precise prediction of exactly what will happen in a given experiment.  This is a horrible thing; in fact, philosophers have said before that one of the fundamental requisites of science is that whenever you set up the same conditions, the same thing must happen.  This is simply not true; it not a fundamental condition of science.  The fact is that the same thing does not happen, that we can find only an average, statistically, as to what happens…. For example, some philosopher or other said it fundamental to the scientific effort that if an experiment is performed in, say, Stockholm, and then the same experiment is done in, say, Quito, the same results must occur.  That is quite false.

What is the fundamental hypothesis of science, the fundamental philosophy? We state it in the first chapter: the sole test of the validity of any idea is experiment.

This fundamental theory of the interaction of light and matter, or electric field and charges, is our greatest success so far in physics.  In this one theory we have the basic rules for all ordinary phenomena except for gravitation and nuclear processes.  For example, out of quantum electrodynamics come all known electrical, mechanical, and chemical laws: the laws for the collision of billiard balls, the motions of wires in magnetic fields, the specific heat of carbon monoxide, the color of neon signs, the density of salt, and the reactions of hydrogen and oxygen to make water are all consequences of this one law.

We know what is happening when we step on a sharp stone, and that somehow or other the information goes from the leg up.  It is interesting how that happens.  In their study of nerves, the biologists have come to the conclusion that nerves are very fine tubes with a complex wall which is very thin; through this wall the cell pumps ions, so that there are positive ions on the outside and negative ions on the inside, like a capacitor.  Now this membrane has an interesting property; if it “discharges” in one place, i.e., if some of the ions were able to move through one place, so that the electric voltage is reduced there, that electrical influence makes itself felt on the ions in the neighborhood, and it affects the membrane in such a way that it lets the ions through at neighboring points also.  This in turn affects it farther along, etc., and so there is a wave of “penetrability” of the membrane which runs down the fiber when it is “excited” at one end by stepping on the sharp stone.  This wave is somewhat analogous to a long sequence of vertical dominoes; if the end one is pushed over, that one pushes the next, etc.

We discover that all living things have a great many characteristics in common.  The most common feature is that they are made of cells, within each of which is complex machinery for doing things chemically.

A red-eyed fly makes a red-eyed fly baby, and so the information for the whole pattern of enzymes to make red pigment must be passed from one fly to the next.  This is done by a substance in the nucleus of the cell, not a protein, called DNA (short for deoxyribonucleic acid).  This is the key substance which is passed from one cell to another (for instance, sperm cells consist mostly of DNA) and carries the information as to how to make the enzymes.  DNA is the “blue print.” What does the blueprint look like and how does it work? First, the blueprint must be able to reproduce itself.  Secondly, it must be able to instruct the protein.

There is a fact, or if you wish, a law, governing all natural phenomena that are known to date.  There is no known exception to this law – it is exact so far as we know.  The law is called the conservation of energy.  It states that there is a certain quantity, which we call energy that does not change in the manifold changes which nature undergoes.  That is a most abstract idea, because it is a mathematical principle; it says that there is a numerical quantity which does not change when something happens.  It is not a description of a mechanism, or anything concrete; it is just a strange fact that we can calculate some number and when we finish watching nature go through her tricks and calculate the number again, it is the same.

The energy has a large number of different forms, and there is a formula for each one.  These are gravitational energy, kinetic energy, heat energy, elastic energy, electrical energy, chemical energy, radiant energy, nuclear energy, mass energy.

Electrical energy has to do with pushing and pulling by electric charges.

There is radiant energy, the energy of light, which we know is a form of electrical energy because light can be represented as wigglings in the electromagnetic field.

There is chemical energy, the energy which is released in chemical reactions.

Elastic energy is, to a certain extent, like chemical energy, because chemical energy is the energy of the attraction of the atoms, one for the other, and so is elastic energy.

Next we come to nuclear energy, the energy which is involved with the arrangement of particles inside the nucleus, and we have formulas for that, but we do not have the fundamental laws.

In quantum mechanics it turns out that the conservation of energy is very closely related to another important property of the world, things do not depend on the absolute time.  We can set up an experiment at a given moment and try it out, and then do the same experiment at a later moment, and it will behave in exactly the same way.  Whether this is strictly true or not, we do not know.

The laws which govern how much energy is available are called the laws of thermodynamics and involves a concept called entropy for irreversible thermodynamic processes.

What is [the] 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.

Kepler’s Laws – First of all, 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.

A third law was discovered by Kepler. This law says that when the orbital period and orbit size of any two planets are compared, the periods are proportional to the 3/2 power of the orbit size.

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.  (Why does it keep on coasting? We do not know, but that is the way it is) Newton modified the principle of inertia by saying that the only way to change the motion of a body is to use force.  If the body speeds up, a force has been applied in the direction of motion.  On the other hand, if its motion is changed to a new direction, a force has been applied sideways.  Newton thus added the idea that a force is needed to change the speed or the direction of motion of a body.

Everyone knows the earth is round.  Why is the earth round? That is easy; it is due to gravitation.  The earth can be understood to be round merely because everything attracts everything else and so it has attracted itself together as far as it can!

It takes a little while to see the moons of Jupiter because of the time it takes light to travel from Jupiter to the earth. When Jupiter is closer to the earth the time is a little less, and when it is farther from the earth, the time is more. This is why moons appear to be, on the average, a little ahead or a little behind, depending on whether they are closer to or farther from the earth.

The earth’s distance from the sun is 8 1/3 light-minutes.

*[Here is how you can interpret the quote above.  The distance between the Earth and the sun, also known as an AU or astronomical unit, is 92.96 million miles.  Light travels at a speed of 670,616,629 miles per hour and there are 60 minutes in an hour.  So if we divide 670,616,629 miles per hour by 60 we get miles per minute, so in other words, light also travels at a speed of 11,176,944 miles per minute.

 Lastly, we divide the distance between the earth and the sun of 92.96 million miles by 11.176944 million miles per minute and we get 8.32 minutes.  In other words, if we rode a beam of light, it would take us 8 minutes and 19 seconds to reach the sun from earth provided that we could survive the atmosphere of outer space.]

In the Einstein relativity theory, anything which has energy has mass – mass in the sense that it is attracted gravitationally.  Even light, which has an energy, has a “mass”. When a light beam, which has energy in it, comes past the sun there is an attraction on it by the sun.  Thus the light does not go straight, but is deflected.  During the eclipse of the sun, for example, the stars which are around the sun should appear displaced from when they would be if the sun were not there, and this has been observed.

“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 Quantum behavior of atomic objects (electrons, protons, neutrons, photons, and so on) is the same for all; they are all “particle waves,” or whatever you want to call them.

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