Feynman’s physics
Science is a people-driven activity like all human endeavor, and just as subject to fashion and whim.
Feynman made a name for himself with the topic known as quantum electrodynamic or QED.
In 1900, the German physicist Max Plank proposed that light and other electromagnetic radiation, which had hitherto been regarded as waves, paradoxically behaved like tiny packets of energy, or “quanta”, when interacting with matter. These particular quanta became known as photons.
To place QED on a sound basis it was necessary to make the theory consistent not only with the principles of quantum mechanics but with those of the special theory of relativity too.
A great unifying theme among particle physicists has been the role of symmetry and conservation laws in bringing order to the subatomic zoo.
An electron cannot have a position in space and a well-defined speed at the same moment. If you look for where an electron is located, you will find it at a place, and if you measure its speed, you will obtain a definite answer, but you cannot make both observations at once. This indeterminism in the very nature of atomic particles is encapsulated by Heisenberg’s celebrated uncertainty principle.
The Feynman method encapsulates the idea that the path of a particle through space is not generally well defined in quantum mechanics. Feynman invites us to imagine that somehow the electron exploits all possible routes, and in the absence of an observation about which path is taken we must suppose that all these alternative paths somehow contribute to the reality. Feynman so-called path-integral, or sum-over-histories approach to quantum mechanics, set this remarkable concept out as a mathematical procedure.
Feynman teaching was about reducing deep ideas to simple, understandable terms.
Atoms in motion
The test of all knowledge is experiment. Experiment is the sole judge of scientific “truth”.
The imagining process in physics is so difficult that there is a division of labor between theoretical physicists who imagine, deduce and guess at new laws, but do not experiment; and experimental physicists who experiment, imagine, deduce, and guess.
The law that mass is constant, independent of speed was invented and was wrong. Mass does increase with speed, but only near the light speed velocity.
Atomic hypotheses are that 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.
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 the do fly apart.
Molecules in the gas being separated from one another will bounce against the walls. These things are in perpetual motion in all directions. We shall have to hold the piston down by certain force, which we call the pressure. The force is proportional to the area.
When we compress a gas slowly the temperature of the gas increases.
If we lower the temperature, the atoms will not be able to jiggle so well. The molecules lock into a new pattern which is ice. The material has a definitive place for every atom.
The difference between solids and liquids is that in solid the atoms are arranged in some kind of an array, called a crystalling array.
Helium, even at absolute zero, does not freeze, unless the pressure is made so great as to make the atoms squash together. If we increase the pressure, we can make it solidify.
When molecules leave, the ones with more energy, so that they can break free, and since the ones that stay have little less energy, the liquid gradually cools if it evaporates.
Strictly speaking crystal is not made of atoms, but of what we call ions. An ion is an atom which either has a few extra electrons or has lost a few electrons.
A process in which the rearrangement of the atomic patterns occur is what we call a chemical reaction.
Carbon attracts oxygen much more, that oxygen attracts oxygen or carbon attracts carbon.
Smell is some kind of molecule, arrangement of atoms, that has worked its way into our noses. Chemists can take special molecules like the odor of violets, and analyze them and tell us the exact arrangement of atoms in space.
Precise arrangement of all the atoms is actually known in three dimensions. A chemical formula is written in two dimensions.
A perpetual jiggling of the particles, which is result of the bombarding of the atoms is called the Brownian motion.
Basic physics
The most fundamental ideas that we have about physics – the nature of things as we see them at the present time.
Observation, reason, and experiment make up what we call the scientific method.
We do not know what the rules of the game are, all we are allowed to do is to watch the playing. If we would watch long enough, we may catch some rules. The rules of the game are what we mean by fundamental physics.
How can we tell if the rules that we guess are really right. First, they may be some cases where nature will be simple enough, that we can predicts exactly what will happen, and thus we can check how our rules work. A second good way to check rules is in terms of less specific rules derived from them. The third way is by rough appropriation.
At first the phenomena of nature were roughly divided into classes, like heat, electricity, magnetism … However, the aim is to see complete nature as different aspects of one set of phenomena. First physics joined heat and mechanics. Another example was connection between electricity, magnetism and light. Another one was quantum mechanics of chemistry, that unified various properties of various substances with behavior of atomic particles.
If we look at the nature, the stage would be particles, with some properties. They are all moving, if they move in the same direction, then we have wind, if it is random internal motions we talk about heat and if we have wave of excess density this is sound. The ultimate basis of an interaction between the atoms is electrical.
The chemical properties depend upon the electrons on the outside, and in fact only upon how many electrons there are.
Potentiality for producing a force is called an electric field. Charges make a field and charges in the field have forces on them and move. Electric forces and magnetic 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. These oscillatory waves can have various frequencies.
At higher frequencies waves behave much more like particles. It is quantum mechanics.
Newton’s laws are wrong in the world of atoms.
An atom has a diameter of about 10-8, the nucleus has a diameter of 10-13.
When the frequency is low, the field aspect of the phenomenon is more evident. But as the frequency increases, the particle aspects of the phenomenon become more evident.
We have a new view of electromagnetic interaction. We have a new kind of particle to add to the electron, the proton, and the neutron. That new particle is called photon.
Besides the electron there should be another particle of the same mass, but of opposite charge. It is called a positron. These two, coming together, could annihilate each other with the emission of light or gamma rays.
For each particle there is an antiparticle, unless particle is its own antiparticle.
It is found that the nuclei are held together by enormous forces.
Today we have approximately thirty particles.
The photon is coupled to all charged particles. The detail law of this coupling sis known, that is quantum electrodynamics. Gravity is coupled to all energy, but its coupling is extremely weak, much weaker than that of electricity.
The relation of physics to other sciences
Mathematics is not a science from our point of view, in the sense that it is not a natural science. The test of its validity is not experiment. If a thing is not a science, it is not necessarily bad.
The science which is perhaps the most deeply affected by physics is chemistry. Theoretical chemistry is in fact physics. Physical chemistry studies the rates at which reactions occur and what is happening in detail. Quantum chemistry helps us understand what happens in terms of the physical laws.
In the cells of living systems there are many elaborate chemical reactions. One such process is called the Krebs cycle, the respiratory cycle.
There are some very large molecules, which in some complicated way hold the smaller molecules just right, so that reactions can occur easily. They are called enzymes. An enzyme is made of another substance called protein.
GDP to GTP transaction is important from energy perspective.
Proteins have a very interesting and simple structure. They are a series, or chain, of different amino acids.
It is the nuclear “burning” of hydrogen which supplies the energy of the sun: the hydrogen is converted into helium.
Conservation of energy
The law of conservation of energy states that there is a certain quantity, which we call energy, that does not change in the manifold changes which nature undergoes.
It is important to realize that in physics today, we have no knowledge of what energy is.
Conversation of energy can be understood only if we have the formula for all its forms.
The general name of energy which has to do with location relative to something else is called potential energy. The principle of the conservation of energy is very useful for deducing what will happen in a number of circumstances.
To illustrate another type of energy we consider a pendulum. The gravitational energy goes into kinetic. The kinetic energy at the bottom equals the weight times the height that it could go, corresponding to its velocity.
Heat energy is just kinetic energy – internal motion. Other forms are: electrical energy (pushing and pulling by electric charges), radiant energy or the energy of light is another form of electrical energy; chemical energy is energy released in chemical reactions, elastic energy is one form of chemical energy; nuclear energy, which is involved with the arrangements of particles inside the nucleus; mass energy, an object has energy from its sheer existence.
Chemical energy has two parts. Kinetic energy of the electrons inside the atoms and electrical energy of interaction of the electrons and the protons.
Energy of a photon is Planck’s constant times the frequency.
In quantum mechanics it turns out that the conversation of energy is very closely related to another important property of the world, things do not depend on the absolute time.
The laws which govern how much energy is available are called the laws of thermodynamics and involve a concept called entropy for irreversible thermodynamics processes.
Our supplies of energy are from the sun, rain, coal, uranium and hydrogen.
The theory of gravitation
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. An object responds to a force by accelerating in the direction of the force by an amount that is inversely proportional to the mass of the object.
Copernicus discovered that planets are moving around the sun. How they move was discovered by Tycho Brahe. He was studied by Kepler. He found out that each planet goes around the sun in a curve an ellipse, with the sun focus on the elipse.
Galileo was studying the laws of motion. He discovered inertia – if something is moving, with nothing touching it and completely undisturbed, it will go on forever at 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 knew that there is a force holding us on the earth. He proposed that this was a universal force.
The moon falls in the sense that it falls away from the straight line that it would pursue if there were not forces.
Newton used the second and third of Kepler’s laws to deduce his law of gravitation.
Earth is round because of gravitation.
Newton’s laws are absolutely right in the solar system, but do they extend beyond the relatively small distances of the nearest planets? The first test lies in the question, do stars attract each other as well as planets? We have definite evidence that they do in the double stars.
The law of gravitation is true at even bigger distances.
There is no explanation of gravitation in terms of other forces at the present time. The force of electricity between two charged objects looks just like the law of gravitation. The force of electricity is a constant, with a minus sign, times the product of the charges, and varies inversely as the square of the distance. Many attempts have been made to unify them. The so-called unified field theory is only a very elegant attempt to combine electricity and gravitation.
It is a fact that the force of gravitation is proportional to the mass, the quantity which is fundamentally a measure of inertia.
Einstein modified Newton by taking into account the theory of relativity. In the Einstein relativity theory, anything which has energy has mass – mass in the sense that it is attracted gravitationally.
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.
Quantum behavior
The “classical theory” of electricity waves turns out to be a completely adequate description of nature for a large number of effects.
“Quantum mechanics” is the description of the behavior of matter in all its details and, in particular, of the happening on an atomic scale.
The quantum behavior of atomic objects is the same for all; they are all “particle waves”, or whatever you want to call them. In 1926 and 1927 Schrodinger, Heisenberg, and Born. They finally obtained a consistent description of the behavior of matter on a small scale.
We choose to examine a phenomenon which is impossible, absolutely impossible to explain in any classical way, and which has in it the heart of quantum mechanics. In reality, it contains the only mystery. We cannot explain the mystery in the sense of “explaining” how it works. We will tell you how it works. In telling you how it works we will have told you about the basic peculiarities of all quantum mechanics.
We will talk about experiment with bullets, coming from two holes. The experiment is about probabilities. Bullets always arrive in identical lumps. What we measure with the detector on the wall is the probability of arrival of a lump.
The probabilities just add together. The effect with both holes open is the sum of the effects with each hole open alone. We shall call this result an observation of “no interference”, for a reason that you will set later.
Then we can do experiment with waves, again with two holes. We will measure the intensity of the wave motion. We would not say that there is any “lumpiness” in the wave intensity. If the wave interferes with each other in a way that they amplify, we can say they interfere constructively, if it is opposite, they interfere destructively.
If we do the same experiment by using imaginative gun for electrons, we can hear that each electron has the same loudness, the rate can go up or down, depending on our measurement, but the size (loudness) stays the same. The probability that lumps will arrive at a particular x is proportional to the average rate of clicks at that x. Each electron either goes through hole 1 or it goes through hole 2. The result obtained with both holes open is clearly not the sum of probabilities of hole 1 and hole 2. There is interference. Probability of both holes is more than twice as large as sum of probabilities of hole 1 and hole 2.
We can augment our experiment, by adding a light, so that we would see through which hole the electron will pass. The light exerts a big influence on the experiment. By trying to “watch” the electrons we have changed their motions.
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. The momentum carried by a “photon” is inversely proportional to its wavelength.
If the electrons are not seen, we have interference. If we want to disturb the electrons only slightly, we should not have lowered the intensity of the light; we should have lowered its frequency (the same as increasing its wavelength.
Due to the wave nature of the light, there is a limitation on how close two spots can be and still be seen as two separate spots. This distance is of the order of the wavelength of light.
When we make the wavelength longer than the distance between our holes, we see a big fuzzy flash when the light is scattered by the electrons. We can no longer tell which hole the electron went through.
In our experiment we find that it is impossible to arrange the light in such a way that one can tell which hole the electron went through, and at the same time not disturb the pattern. It was suggested by Heisenberg that the then-new laws of nature could only be consistent if there were some basic limitations on our experimental capabilities not previously recognized. He proposed, as a general principle, his uncertainty principle.
The complete theory of quantum mechanics depends on correctness of the uncertainty principle. If the 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.
We do not know how to predict what would happen. The only thing that can be predicted is the probability of different events.
The uncertainties in the position and momentum at any instant must have their product greater than half the reduced Plank constant. We cannot design equipment in any way to determine which of two alternatives is taken, without, at the same time, destroying the pattern of interference.
