There has been no greater period of rethinking in science than the 20th century. It brought a new paradigm of understanding the universe, followed by the invention of shockingly-complex technologies. They have allowed us to extract energy from the atom, and by the second half of the century, to overcome Earth’s gravity and usher in the era of space exploration. This series of articles on the development of science will talk about the most significant achievements that have shaped modern science and technology. In our first installation on the history of the 20th century, we will discuss the birth of quantum mechanics and Einstein’s theory of relativity.
Quanta of the universe: Planck’s new physics
At the end of the 19th century, Newton’s theory of classical mechanics completely dominated contemporary physics. Contemporary science tried to explain everything with it, from the mechanism of wall clocks to the movement of cosmic bodies. Nevertheless, it was obvious even to the most faithful of Newton’s adherents that his physics contained a number of inaccuracies. Explanations of the motion of planets and satellites using Newtonian mechanics contained unexplainable errors, and it also failed to accurately make sense of the physics of processes occurring at the atomic level.
In 1900, the German physicist Max Planck tried to expand Newtonian knowledge of the universe. He formulated the foundations of quantum mechanics by exploring the idea of the blackbody – an idealized physical entity that absorbs all radiation that enters it, and then is able to radiate it back at any frequency. Gustav Kirchhoff (who first formulated the idea of a blackbody in 1860) assumed that the spectrum of electromagnetic radiation of a blackbody could only be observed when in thermodynamic equilibrium with its environment. Upon reaching this temperature equilibrium, a total blackbody should radiate the large amounts of thermal energy it had absorbed earlier back out into outer space.
When thermodynamic equilibrium is reached, the blackbody would begin to continuously radiate energy out with an intensity that depended only on the temperature of the surface of the body itself (but not on its chemical and physical properties, which was typical of all real objects observed in nature). Moreover, experimental observations of the radiation of gray bodies (solid objects with properties similar to black bodies) were completely different from the models predicted by classical physics. This quandary became a fundamental question of physics and one of the most mysterious of the unsolved problems in natural science by the end of the 19th century.
Attempts to mathematically describe the process of thermal radiation from a blackbody were made by a number of physicists, among whom the British Lord Rayleigh (John William Strutt) came close to solving the problem. The Rayleigh-Jeans law was well suited for describing the distribution of energy at long wavelengths, but it was completely inapplicable for short wavelength radiation.
This was a real paradox of classical physics, which postulated that the distribution of energy in space is continuous and has a strictly wavelike character. The paradox was called the “ultraviolet catastrophe,” and by the end of the 19th century, it was ready to put in doubt all of classical physics. But Max Planck had begun work on the issue.
To solve the problem of blackbody energy radiation, Planck applied an innovative approach which at that time completely contradicted classical Newtonian physics. He suggested that the energy that atoms of a total blackbody can emit is not released continuously, but discretely, with the help of small and indivisible portions, which Planck calls “energy quanta.” His theory was fully confirmed when he introduced his elementary quantum of action (or Planck’s constant, h = 6.626 ×10-34J) into his equations, compared them with experimental data, and made sure that his calculations perfectly matched the data obtained in laboratories. This is how the famous Planck’s Law was born, which caused a real revolution in understanding the physics that takes place at the atomic level of the universe.
The quantum nature of radiation, once understood by Max Planck and written down in his simple, iconic formula, allowed scientists to take a fresh look at the behavior of matter at the micro level, the chemical elements of Mendeleev’s periodic table, and also expand their knowledge about the age and origins of our Universe. The principles of quantum mechanics serve as the basis of all of today’s electrical appliances. This new physics completely undermined the main narrative of classical Newtonian physics, which claimed that our reality is determined (predetermined) and obeys the same fundamental laws at all its levels.
Having solved the problem of blackbody radiation, Planck, unwittingly, introduced even more uncertainty into science than all his colleagues in scientific activity before him. It is this uncertainty that would form the basis of the work of another great physicist of the 20th century – Albert Einstein.
Nonlinear time: Einstein and relativity
In addition to the physical processes occurring on the Earth, Newtonian physics also tried to describe the motion of cosmic bodies, particularly the planets of the solar system. To do this, Newton rethought the laws of planetary motion previously formulated by Kepler, applying to them his universal law of universal gravitation, according to which the mass of a body generated a force field (gravity) around it, which interacted with the same force field of another object, causing them to attract each other. .
For Newton, the force of gravitational interaction instantly arising between two objects was determined only by the distance between them – the farther a more massive object was located from a less massive one, the weaker its gravitational force manifested itself. In Newton’s cosmological system, mass, velocity, and distance for all objects in the solar system were absolute constants that did not change with time. Newton believed that the world around him ran like clockwork, once wound up by God and now working according to the same immutable laws.
The cosmological model proposed by Newton worked, and its predictions were able to explain the Earth’s tides caused by the gravitational influence of the Moon. It also led to the discovery of the eighth planet of the solar system, Neptune, whose existence was indirectly evidenced by the displacement of the orbit of Uranus, discovered a century earlier. Despite all this, Newton’s theory of gravity still contained a number of inaccuracies, the largest of which was the anomalous shift of Mercury’s perihelion. First noticed in 1859, it called into question the mathematical calculations of Newton’s law of gravity as applied to cosmos.
The scientific community could not accept the fact that Newton’s theory was erroneous, because earlier it had already successfully predicted the presence of other planets in the solar system which astronomers would indeed, later discover. The deviations of Mercury’s perihelion from where it was calculated to be were first explained by the presence of another planet near the Sun – Vulcan, the mass of which was supposed to deflect orbit of Mercury, while remaining invisible from the Earth due to the very bright glow of the Sun. However, for more than 50 years, Vulcan could never be found, and a young German physicist, Albert Einstein, took up the solution of the orbital riddle of Mercury.
Einstein posited a structure of the world which was unthinkable for Newtonian physics – he assumes that space and time are inextricably linked with each other and can change depending on the reference frame in which they are observed. Taking the constant speed of light (c ≈ 300 km / s), Einstein was able to mathematically prove that for fast moving objects, time begins to flow more slowly.
In addition to the inseparable connection of space with time, Einstein also proved the direct dependence of energy on the mass of an object. Einstein’s theory of relativity postulated that the energy of any object can be calculated by the formula (simplified below):
E=mc2 (where E is energy, m is the mass of an object, and c is the speed of light, equal to ≈ 300 km/s)
It was this simple and concise formula that gave the scientific community the understanding that even the smallest objects in the universe contained colossal amounts of energy. Subsequently, it is this concept that would lay the foundation for experiments on the splitting of the atom, which will allow people to produce unprecedented amounts of energy at nuclear power plants, and by the middle of the century to create the most destructive weapon in the world: the atomic bomb.
Einstein published his special theory of relativity (SRT) in 1905, and after a long 10 years of trial and error, he presented its augmented version – the general theory of relativity (GTR) published in 1915. General relativity tried to explain the mechanisms of gravitational influence and their influence on the curvature of the fabric of space-time.
With his GTR, Einstein rejected the Newtonian idea of the instantaneous propagation of gravity in space. To do this, he drew analogies with Maxwell’s electromagnetic field and suggested that gravitational perturbations should also propagate in space-time in a manner analogous to electromagnetic waves, which Einstein called gravitational waves. At the same time, the instantaneous propagation of such waves was impossible due to the fact that, according to general relativity, no object in the Universe could move faster than the speed of light.
He also rethought the concept of gravity itself: with his GTR, Einstein would prove that gravity was not a separate and omnipresent force, but only a consequence of the curvature of space-time. In other words, it was mass enclosed in matter that provoked the curvature of space-time, and the larger this mass was, the greater the curvature of space-time observed. Massive bodies (for example, stars) created a funnel in the fabric of space-time, forcing the surrounding bodies (planets) to seem to “fall” into it, making their orbital motion.
The scientific community was able to see firsthand how the gravity of massive objects contributes to the curvature of space-time four years after the GTR was published. In 1919, during a solar eclipse, Einstein, based on his theory of relativity, predicted the bending of light coming from the bright star cluster Hyades as it passed near the Sun.
During the eclipse, a group of astronomers led by Arthur Eddington and Andrew Crommelin made observations from several observatories located in Brazil and Africa, and saw for themselves how Einstein’s predictive model worked – the light was indeed bent by 1.61 inches ± 0.30 when passing close to the Sun, which was strikingly close to Einstein’s predicted curvature of 1.75 inches.
This proved that Newton’s theory, which had dominated science for more than 200 years, did indeed contain a number of fundamental inaccuracies. Light really did have (which Newton categorically denied), and the next morning, Einstein found himself to be a massive figure in the scientific community: he was placed on all the leading pages of scientific publications as the man who made a fundamental breakthrough in the study of natural science.
Once general relativity received its empirical evidence, it was also able to explain the anomalous curvature observed at Mercury’s perihelion – since the planet was closest to the Sun, the star’s gravity caused an additional precession of Mercury, equal to the deviation of its orbit by 43 inches every century. Einstein won a smashing victory, and the founding father of relativism himself became the main scientific figure of the 20th century, paving the way for the amazing technological innovations of our time.
GPS, MRIs, and lasers: everyday proof of GTR
With the start of the space age in the second half of the 20th century, the basic principles of general relativity and quantum mechanics formed the basis of a number of technologies with which we interact almost every day.
The principles of GTR underlie the functioning of the American satellites of the global positioning system (GPS), as well as the European satellite navigation system Galileo. In order to accurately transmit and locate a GPS receiver on Earth, satellites in orbit must be perfectly synchronized with each other in time, which they are able to do thanks to their atomic cesium clocks, capable of determining time with amazing accuracy.
However, in orbit, time must not only be accurately determined, but also corrected, since general relativity indicates that the speed with which satellites move in orbit and their distance from the center of mass of the Earth will steadily lead to the appearance of time errors.
That is why, at the time of its launch in the 90s, each GPS satellite had a special synchronizer that was set to turn on if the prediction of GTR that time passes more slowly on Earth than in orbit was confirmed. After running for only 20 days without the synchronizer turned on, the cesium clock on the positioning satellite recorded a time deviation of 442.5 parts in 1012, which, without the introduction of time correction, could lead to an error of 38,000 nanoseconds per day. There could no longer be any question that GPS devices could only function accurately by adding for the corrections foreseen by Einstein’s GTR..
Quantum mechanics, in turn, gave rise to the widespread use of technologies such as lasers, solar panels, electron microscopes, and MRI machines, whose scanners are used for medical imaging and diagnosing diseases.
Today, quantum mechanics is on the cusp of new technological breakthroughs, including quantum computers capable of simultaneously processing numerous quantities of computational tasks. The principles of quantum teleportation will be used in the future to organize new encrypted communication systems, but today this technology remains in the initial stage of development.