The ten greatest discoveries in physics and astronomy

The ten discoveries that advance us to the answer to the big questions

Curiosity is the fuel that drives the wheels of physics, and physics is not satisfied with trivial questions, but tries to explain essential questions: What are the building blocks of matter? How is the universe built? How did it all start? And how will it end? A modest attempt to answer these questions is given in the five discoveries that seal this list. The scientist, for his part, raises additional questions and tries to understand the processes themselves, develop theories and, just as importantly, test them experimentally. The first five discoveries deal with such questions, and together they complete a complex picture of physics and astronomy, where experiment and theory are intertwined. The ten discoveries presented below are in my opinion important points in time and understanding, and each of them opens a window to a wonderful and diverse world of knowledge, courage and most of all - curiosity.

1. Newton's laws
We will begin our journey towards the great discoveries of physics with Galileo Galilei, one of the greatest scientists of all time. Although Galileo was not the first to criticize Aristotle's view, according to which a body can only move under the condition that a force acts on it, nevertheless he was the first to deeply understand the concept of persistence (inertia) and formulate it in the form known today as Newton's first law: a body will maintain its velocity and in the direction of its movement, unless an external force acts on it.

Newton's second law, the fundamental law of mechanics, defines the concept of mass. Newton claimed that this property, which is unique to every body, determines its ability to resist a change in speed under the influence of an external force. According to the third law, the law of action and reaction, the application of a force on another body will always be accompanied by a force opposite in direction and equal in magnitude. Newton's laws, published in 1687, provide not only an intuitive understanding of basic physical quantities, but also constitute a first-class didactic tool, and moreover, they allow for the first time to carry out precise mechanical calculations.

It is appropriate to end this chapter with a sentence by Isaac Newton, which in my opinion is the essence of science: "I see myself as a child playing on the beach, and having fun, sometimes with a smoother stone and sometimes with a more beautiful shell, while the huge ocean of truth stretches before my eyes and conceals the His secrets". Galileo and Newton will also star in the second discovery, which will deal with the theory of gravity, but this time they will be joined by none other than Albert Einstein.

2. General relativity
Galileo was the first to understand the "principle of equivalence", according to which gravitational mass, which causes attraction between two bodies, is the same as inertial mass, defined by Newton's second law. The famous experiment in which Galileo drops two objects from the Tower of Pisa, one heavier than the other, and both reach the ground together, was probably only a thought experiment, because Galileo realized that air resistance would prevent him from conducting the experiment correctly. Nevertheless, this idea is the first expression of the principle of equivalence, and its importance in that it contradicted the Aristotelian approach, according to which a heavy body falls faster.

The leaning bell tower of Pisa, from which, according to the story, Galileo Galilei (1642-1564) threw various bodies from its top in order to prove the principle of equivalence. From Wikipedia

Let's jump three hundred years ahead. In 1907, while sitting in the patent office in Bern, a thought occurred to Einstein, which he later described as "the happiest thought of my life". "A person falling from the roof of a building does not feel his own weight", Einstein concluded from the principle of equivalence, and with the help of another idea, called the Mach principle, according to which every body in the universe is in free fall relative to the other bodies, within eight years Einstein was able to formulate the theory general relativity. On the one hand, this theory says that space-time determines how the mass will move, while mass determines how the space-time will be curved, and on the other hand, complete nonsense...

Significant differences between Newton's gravity and the theory of general relativity are manifested only for strong gravity, such as black holes, those bodies that even light cannot escape from their gravitational field, and whose existence is explained only by Einstein's theory. However, when high precision is required, then even under conditions of weak gravity, the general relationship must be taken into account, which is why it is taken into account when planning spacecraft trajectories and even in the GPS system. From here we turn to another basic force, which, like gravity, has an infinite range of action - the electromagnetic force.

3. Maxwell's equations
Most of the inventions of the last two hundred years are related in one way or another to electricity and magnetism. The scientist who more than anyone else provided the experimental basis for these inventions was undoubtedly the British Michael Faraday. Faraday, lacking a formal education, acquired knowledge by reading the books he bound for a living. After being accepted to the position of scientific assistant, Faraday managed, in the work of countless experiments, to register to his credit a considerable number of discoveries in chemistry and physics. Among them, the law that bears his name deserves special mention, according to which a changing magnetic field induces an electric voltage. Based on this discovery, Faraday built the first dynamo in 1831.

James Maxwell had the ability to see the overall picture. When he noticed the symmetry between electricity and magnetism, Maxwell decided to act as a kind of "chief editor", and to unite electricity and magnetism into one set of equations. First he assumed, as an analogy to Faraday's law, that a changing electric field would create a magnetic field, and in the next step he was able to concentrate all the accumulated electromagnetic knowledge into only four equations. In addition to Faraday's law, the equations contain Gauss's electric law, Gauss's magnetic law and Ampere's law with Maxwell's expansion. Electromagnetic radiation is one of the solutions to these equations, and Maxwell  who himself realized that light is a type of such radiation  was able to calculate the speed of light from his equations for the first time. As a curiosity, it can be noted that color television owes its existence to Maxwell, starting with the television broadcasts as electromagnetic radiation, through the transformers and other electrical components within it, and ending with the principle of combining colors by three primary colors, which he discovered.

Another theoretical development in the field of electricity and magnetism only occurred with the introduction of quantum theory. According to quantum theory, light is a wave that sometimes behaves like a particle, and on the other hand matter is composed of particles that sometimes behave like waves. On quantum theory and its implications in the next discovery.

4. Quantum theory
In his views, Max Planck was a conservative person. In 1900, when he claimed that light consists of quanta of energy (photons) and thus explained the blackbody radiation formula, the last thing he wanted was to cause a physical revolution. Five years later, Planck opposed Einstein's quantum explanation of the photoelectric effect, because he wanted to "save Maxwell's equations", which describe light as a wave. But Planck created a revolution, and entered history as the father of quantum theory, which is today the most useful physical model; So much so, that an accurate microscopic calculation is not possible which does not take it into account.

Many have contributed to quantum theory, and many have interpreted it, and it is not possible to list them all here. Nevertheless, we will try to bring the essence of quantum theory, or in other words - we will tell about the Schrödinger equation. In 1926, Erwin Schrödinger (Schrödinger) was impressed by the doctoral thesis of Louis de Broglie, and especially by the fact that he presented particles as waves. Schrödinger proposed to describe some quantum system with the help of a wave function, which is the solution of the equation that bears his name. The function The wave can be presented as a superposition (overlap) of states, and this way of recording simply allows one to find the allowed values ​​that will be obtained in an experiment and to calculate the probabilities of measuring these values. It can be said that he proposed a simple solution method for some quantum problem, although in practice one usually has to settle for a solution It should be emphasized that from the philosophical aspect, quantum theory is both probabilistic and deterministic, since the probabilities of receiving certain measurements are calculated deterministically.

Erwin Schrödinger, formulates the fundamental equation of quantum theory
Erwin Schrödinger, formulates the fundamental equation of quantum theory
One of the fields that received a renewed refinement with the introduction of quantum theory is the field of thermodynamics, and their successful combination led to the development of new materials and a better understanding of their properties. On a decisive moment in the development of thermodynamics in the following discovery.

5. The second law of thermodynamics
The industrial revolution, which began at the end of the 18th century, is closely related to the field of thermodynamics and its development. The second law of thermodynamics, which can be formulated in three ways, is the one that determines the limits of the use of heat energy, and to a certain extent places a reservation on industrial capacity. According to the first version, the efficiency of a heat engine is limited, and it will never exceed the efficiency of an engine based on the Carnot cycle. The second version states that for the purpose of cooling energy must be invested, and the third version, the most peppered of them, claims that the entropy of a closed system will never decrease.

We owe the understanding of the concept of entropy, the degree of disorder of a system, to Ludwig Boltzmann, who defined it using the number of possible arrangements of the molecules, thereby laying down one of the foundations of statistical mechanics, which provides the physical explanation for thermodynamics. The philosophical significance of the second law is that it gives a definite direction to the pressure of time, and distinguishes between the past and the future. It can be said about Boltzmann himself that he was ahead of his time, that his ideas were not always accepted, and that the endless debates dampened his spirit. He also apparently suffered from manic-depression, and while on vacation in Italy in 1906, he lost consciousness. Years later, the concept of entropy served as inspiration for a new mathematical field, called information theory.

Ludwig Boltzmann, who defined the concept of entropy
Ludwig Boltzmann, who defined the concept of entropy
Another achievement of Boltzmann was in formulating an equation that is now named after him, and describes the change in particle density over time, under the influence of external factors. This equation is widely used in understanding processes in galaxies, and in understanding processes that happened in the young universe after the big bang. These two topics will occupy us in the following discoveries.

6. The discovery of the galaxies
Two major astronomical debates took place in the 20th century. One dealt with the structure of the universe and the other with its beginning. The debate about the existence of galaxies other than the Milky Way, also called "The Great Debate", was a real public debate in the Renaissance tradition, and it took place on April 26, 1920, at the Smithsonian Museum in Washington. It is worth noting that more than 150 years earlier, Immanuel Kant, the famous philosopher, hypothesized that the Milky Way is not the entire universe, but that there are other "island worlds", which today are called galaxies. In the great debate itself, Heber Curtis (Curtis) took the same position as Kant, and argued that the Andromeda Nebula is a galaxy in itself outside the Milky Way. He based his claim on observations according to which the number of nova events in this nebula, during which a star shines brightly for a short time, is comparable to the number of nova events in all other directions. On the other hand, Herlow Shapley relied on the report that the Andromeda Nebula was observed rotating, therefore it cannot be too far from us, that is, it is inside the Milky Way.

The decision was made a few years later, when Edwin Hubble (Hubble) observed the Andromeda Nebula and established beyond any doubt that it is a separate and distant galaxy from us. Today, the number of galaxies in the universe is estimated to be in the hundreds of billions. The solution of the riddle of the structure of the universe was also the signal for the opening of another long-standing astronomical debate about the origin of the universe. The next discovery will deal with the Big Bang.

7. the big Bang
The debate between a static universe and an expanding universe originating from a single point was only resolved in the XNUMXs. Here we will present three key pieces of evidence for the Big Bang model, or in other words - the Big Bang in three systems. In the first act we will return to Edwin Hubble, and this time we will tell about the law that bears his name. Hubble discovered that the farther a galaxy is from us, the faster it moves away. Hubble's law tipped the scales in favor of an expanding universe, but the question remains open as to whether it originates from a single point or whether new matter is constantly being created in it.

The second piece of evidence was discovered entirely by accident, and it tipped the scales in favor of the Big Bang. In 1964, isotropic radio radiation was measured, which later turned out to be an exact match for the cosmic background radiation, which can only come from a hot and small universe. The origin of these photons is in a young universe approximately 400 thousand years old, where the temperature dropped enough and reached 3000 degrees Kelvin, so that the electrons could create the first atoms together with the nuclei. Before that, photons were constantly colliding with electrons and nuclei, and the universe was impervious to radiation.

The antenna used by Arno Penzias and Robert Wilson in 1964, during the accidental discovery of the cosmic background radiation, which was predicted by the Big Bang model
The antenna used by Arno Penzias and Robert Wilson in 1964, during the accidental discovery of the cosmic background radiation, which was predicted by the Big Bang model
The third and final act takes us even further back in time, to the first three minutes of the universe. It turns out that the prevalence of the light elements today corresponds exactly to the prediction of the Big Bang. In the first three minutes, the temperature was high enough for nuclear fusion, and in this short period of time, for example, most of the helium in the universe was created, which makes up a quarter of the mass of the measured material.

It is worth mentioning that the understanding that the earth is not a special place in the universe, also called the "Copernican principle", is what allowed the development of today's cosmological theories. More on Copernicus in the next discovery, where we will once again have to correct a misconception of Aristotle.

8. The earth revolves around the sun
Nicolaus Copernicus was not the first to claim that the Earth revolved around the Sun. He was preceded by Indian and Greek scientists, who lived thousands of years before him. The Greek Aristarchus, who was born on the island of Samos, and lived in the third century BC deserves a special mention. Apart from the book, which has not survived, in which Aristarchus developed the heliocentric model, we have another book by him in which he makes brave attempts to measure the size of the moon and the sun and the distances to them. The heliocentric idea is not mentioned in this book, and according to evidence from that time, his attempt to freeze the Aristotelian geocentric approach was not accepted, and his teachings were forgotten.

The heliocentric model of Copernicus, as it appears in his book De revolutionibus orbium coelestium
The heliocentric model of Copernicus, as it appears in his book De revolutionibus orbium coelestium
The observations made over the years did not support the geocentric model, and in order to save it they added over time to the movement of the sun and the planets around the earth another circular movement with a much smaller radius. These circles, which from a historical point of view constitute sin upon sin, are called epicycles. Copernicus, who dared to publish his great book on the heliocentric method only on his deathbed, assumed that the planets revolved around the sun in circles, and therefore he too had to add epicycles to his picture of the solar system.

Later in the 16th century, Tycho Brahe made precise measurements regarding the movement of the planets, but did not interpret them correctly. He thought that the sun revolved around the earth while the other planets revolved around the sun. It was Johannes Kepler, the first to understand the elliptical motion of the planets around the sun, and this understanding is one of the three famous laws that bear his name, and summarize our knowledge of planetary systems.

Today we know very well not only our solar system, but we have information about other planetary systems. The next discovery will also deal with a quasi-planetary model, but it will be a completely different model, dozens of orders of magnitude smaller than our solar system - the atom.

9. The structure of the atom
In the first decade of the 20th century, Joseph Thomson's model of the atom, the discoverer of the electron, prevailed, according to which the atom is made up of negatively charged electrons scattered inside a sphere charged with a uniform positive electric charge. It is less known that even then there was another, less accepted model, the brainchild of the Japanese Nagaoka, who claimed that the electrons in an atom revolve around a positive and massive nucleus.

In 1911, Ernest Rutherford, a recent Nobel laureate, decided to use his knowledge of alpha particles and conduct an experiment in his laboratory that would later go down in history. He bombarded gold leaf with alpha particles and examined the scattering angles. To his surprise, his assistants discovered that the alpha particles scatter at large angles and sometimes even go back. "It's as if you were to shoot a shell at a piece of paper, and it would bounce back and hit you", as Rutherford himself put it. Rutherford's experiment confirmed the planetary model of the atom, but it had one problem - such an atom was unstable. Here Niels Bohr came to his aid, and while incorporating the new principles of quantum theory, he limited the allowed paths for electrons moving around the nucleus. Bohr later discovered that the assumption that electrons passing from one orbit to another emit or absorb photons explains the emission frequencies of the hydrogen atom, which were measured in previous experiments. He published the result in 1913, thereby opening the branch of modern atomic physics, based on quantum theory.

Rutherford himself continued to investigate the structure of the atom and in 1920 came to the conclusion that the nucleus contains not only the protons, which have the positive charge, but also neutral particles, which were shortly named neutrons, and were discovered in an experiment 12 years later. Many more particles of subatomic size were discovered during the 20th century. The tenth and last discovery will deal with this topic.

10. The standard model
The first piece in the assembly of elementary particles, the building blocks of matter and radiation, was laid in 1897 with the discovery of the electron, and the last piece, for now, was put in place in 2000, when a particle called the tau-neutrino was discovered. The theory of elementary particles, known simply as the "standard model", is the most accurate model in science, and many physicists have contributed to it, both on the theoretical side and on the experimental side. The fact that its mathematical formulation, based on symmetry, is so simple that it is possible to write down the entire Torah in five lines is impressive.

According to the standard model, the origin of a force (interaction) between two particles is a continuous exchange of force-carrying particles between them. In the sixties, a number of physicists came to an important prediction, which put the entire theory to the test. According to them, there exist in nature three power-carrying particles, similar to a photon, which together with it are responsible for the action of the electro-weak force, a kind of union of the electromagnetic force and the weak nuclear force (which is responsible for beta radiation). They claimed that these particles, named W plus, W minus and Z zero, have a high mass, and therefore have not yet been detected in particle accelerators. This was the signal for the opening of a race between different research groups in the world to discover the new particles. The race reached its climax in December 1982, and was decided only by a photo-finish. The UA1 detector group (at CERN near Geneva) led by the Italian Carlo Rubbia, found six W particles and was thus a few days ahead of the competition.

The central part of the UA1 detector, where W particles were first discovered in December 1982, and the Z particle - half a year later
Source: CERN
The central part of the UA1 detector, where W particles were first discovered in December 1982, and the Z particle - half a year later
Source: CERN
Today we are at a crossroads again. Experiments from recent years have shown that neutrino particles have a mass greater than zero, contrary to the prediction of the standard model. Could it be that the standard model is part of a larger, more accurate aggregate? This important question, and many others, including those that have not yet been asked, will occupy us for many years to come, and guide the way towards knowledge and understanding.

Aryeh Melamed-Katz is about to finish his doctoral studies in the Department of Particle Physics at the Weizmann Institute. In his spare time, he tries to track down special scientific stories and uncover unknown details