Several scholars, theories, and equations have become household names in the history of science and physics. Notable examples of scientists include Pythagoras and Galileo. In terms of theories, there are Archimedes, Eureka, andNewton. Albert Einstein is arguably the most famous and renowned. Few people understand the concept of Relativity, but it is one of the most well-known scientific concepts.
Einstein's Theory of Relativity is divided into two parts, the Special Theory of Relativity and the General Theory of Relativity. Galileo Galilee had an explanation for why motion and velocity are relative to the observer. Explaining how Einstein's theory works requires a deep dive into the history of physics, some advanced concepts, and how it all came together for one of the greatest minds of all time.
Einstein proposed SR in 1905 to resolve light and classical physics experiments. Over the next ten years, Einstien would attempt to generalize the theory to explain how electromagnetism and classic mechanics could be solved with gravity. Einstein's insights would be confirmed within a few years, but they still have to be tested and verified.
If you can't explain it to a six-year-old, you don't understand it yourself.
The work of Galileo Galilee, an astronomer and polymath, goes back to the 17th century. Dialogue Concerning the Two Chief World Systems was published in 1632 by Galileo. In this work, Galileo explained how the Heliocentric Model of the Universe solved issues that the other model couldn't explain. Galileo explained why the Earth's motion was not obvious to people on its surface.
Galileo used the metaphor of a ship at sea to show how he could convey complex ideas with simple and erudite logic. Galileo said that if a person stood on the deck and dropped a ball of wax into a vase of water, the ball would descend to the bottom. This would apply even if the ship was not moving. He said that the ball and everything aboard the ship is part of the ship's reference frame.
He argued that the same holds for a person standing on the surface of Earth.
“Now these things take place in motion which is not natural, and in materials with which we can experiment also in a state of rest or moving in the opposite direction, yet we can discover no difference in the appearances, and it seems that our senses are deceived.
“Then what can we be expected to detect as to the earth, which, whether it is in motion or at rest, has always been in the same state? And when is it that we are supposed to test by experiment whether there is any difference to be discovered among these events of local motion in their different states of motion and of rest, if the earth remains forever in one or the other of these two states?”
Galileo claimed that things would look different to an observer on the shore. If the person standing on the deck dropped the ball over the side, it would look like it fell straight down. It would look like it was following a path. The ball's motion would be the result of the ship's motion with the Earth's gravity. The motion would be relative to the observer.
This came to be known as Galilean Invariance, which was based on the idea that two observers moving at constant speed and direction will get the same results for all mechanical experiments. The mechanics will change if either of these parameters change.
The explanation would be used to defend the model. The motions of the planets, the Sun, the Moon, and the stars were relative to the observer. When one cataloged the motions of these objects in the night sky over time, they would see how these observations could only be explained by the motion of the Earth around the Sun.
By 1687, Sir IsaacNewton would change our understanding of physics with his work, Natural Mathematicais Principia. The Three Laws of Motion were summarized in this tome byNewton. These were included.
Intertia, which states that bodies will remain in a state of motion unless an external force speeds them up or slows them down, is one of the three physical constants that remain central to modern physics.
This is where the groundwork was laid for the Inverse Square Law, which states that the force of gravity is dependent on the mass of both objects. The same force that caused the apple to fall from a tree is what causes the planets to circle the Sun, the Moon and Earth.
Space and time were fixed and separate as reference frames because ofNewton's Universality. An object's position and motion can be described in terms of three dimensions in space. The framework for understanding the Universe will become a canon over the next two hundred years. The terms Classical Physics andNewtonian Physics would be used interchangeably.
By the late 19th century, new discoveries in astronomy, electromagnetism, and particle theory would change the way people think. The Universe used to be an orderly one with space and time, matter and energy, and universal reference frames.
By the mid-19th century, scientists had made many discoveries in the study of light and colors. Light behaves like a wave and is similar to the propagation of electrical current. Experiments performed by this time yielded highly- accurate estimates in the speed of light.
The theoretical work of James Clerk Maxwell and Hendrik Lorentz showed that magnetic and electric fields exert force on point charges. TheLorentz Force Law (1895) describes how electric and magnetic fields are generated by charges, currents and changes of the fields. Classical electromagnetism, optics, and electric circuits are based on these principles.
The estimates for the speed of light were highly accurate. These experiments presented theoretical problems for Classical Physics. The measured speed of light was constant regardless of whether the source was moving relative to the observer or not. The basic tenets of Classical Mechanics and Galilean Relativity were not in agreement with this.
Earth's rotation on its axis means that it is moving towards the Sun. When the Sun is in the east, the light reaching an observer will have a greater measured velocity than light from any other direction. Augustin Fresnel performed experiments in the 19th century that showed no change in the speed of light.
By the early 19th century, scientists began to argue that space must be filled with something invisible to allow light to travel through it. The partial aether-drag hypothesis states that the ether is partially carried along by the earth and light waves inside the optical field.
This is similar to how sound travels in the air or water. Experiments conducted throughout the 19th century indicated that the speed of light was constant. Scientists needed to measure the effects of this aether on its properties to resolve the theoretical issues with the experimental results. Scientists needed to show that the measured speed of the light was a simple sum of its speed through the medium and the speed of the medium.
Hippolyte Fizeau tried to prove this with a water tube experiment. After measuring the speed of light in moving water through tubes, Fizeau's results indicated that light was being dragged along by the water. Augustin Fresnel and Sir George Strokes had conducted experimental results. The magnitude of the effect that Fizeau observed was lower than expected.
The experiment was conducted by Albert A.Michelson and Edward W. Morley. They used a series of mirrors and a chamber to measure the speed of light from different angles. The Earth's movement through it would result in a noticeable difference with the horizontal beam.
The experiment yielded no results since there was no difference between the measured speeds of the light beams. At this point in the game, Einstein would offer a great insight, analysis, and synthesis of the theoretical and experimental data. Einstein's Theory of Special Relativity was revealed in 1905.
In 1905, Einstein published his thesis and four other papers that would bring him to the attention of the international scientific community. Einstein proposed his Theory of Special Relativity, which is now known as the "Einstein Theory", on the electrical behavior of moving bodies. The theory solved the equations of motion and the force law with the Laws of Motion.
Einstein's breakthrough was the creation of theLorentz Transformations, which was derived from the experiments concerning the behavior of light. To explain why light did not conform to the laws of physics, Lorentz believed that things become distorted along the path of travel. objects approaching the speed of light will not observe a change in c coming from external sources, but they will notice time is moving slower for them than it is for external sources.
Einstein used a metaphor to explain the mechanics of the concept like Galileo did. Galileo said that a ball will fall straight to the floor if it is thrown on a train. An observer beside the tracks would see that the sameBoll dropped over the side of the train. The ball should be replaced with a series of mirrors.
A person is holding a hand in their hand while another person is underneath the train. A person holding a mirror would see a beam of light bouncing up and down. Imagine a mirror on the wall at the head of the car. A beam of light would appear as if it were bouncing back and forth across the train car if the person reoriented the mirror in their hand to face it. The light would appear to be traveling at a constant speed.
In the first scenario, the light would zig-zagging along, trying to catch up with the moving mirrors, while the person stood beside the tracks. As the light moved from the handheld mirror to the one in the front of the car, it appeared as if it was moving slower. They would record a constant speed if they could time it. It would make little sense to the two observers until they looked at their watches.
Time would have moved slower for the person riding in the train cart. If the reference frame were like a spaceship that could travel at a fraction of the speed of light, the difference would be impossible to miss. As objects get closer to the speed of light, the person in the moving reference frame will experience time at a slower rate.
Einstein and his peers held onto the idea of theConservation of Energy Law, which was first proposed and tested in the 18th century. The law states that the total energy of an isolated system is constant. The equation E is the total amount of energy in a system, m is the system's mass, and c is the system.
The law states that objects that accelerate towards the speed will have an increase in their mass. The speed of light is absolute and more energy is required to maintain the object's acceleration over time. An object would need an infinite amount of energy to reach the speed of light, but its mass would also become infinite in the process. Mass and energy are not the same in this equation.
The outcome remains the same if mass and energy are switched around. The principle of Mass-Energy Equivalence states that energy and mass are both sides of the same coin. Space and time are both expressions of the same reality. Scientists looked at the geometry of the Universe in terms of three dimensions: height, length, and width.
Space and time were seen as separate and fixed. Einstein presented a four-dimensional geometry consisting of three dimensions of space and one dimensions of time, by showing how time was relative to the observer in an accelerated reference frame. Time! Einstein's SR was adopted by scientists because of how it solved electromagnetism and how it did away with the need for an aether.
Einstein wanted to account for gravity between 1905 and 1915. The problems were caused by the theory of Universal Gravitation. Astronomers found that the equations ofNewton could account for the orbits of most of the Solar bodies. The long-term peculiarity that Mercury presented was impossible to account for. Mercury's perihelion moves around the Sun over time.
This is known as a precession of perihelion, where the farthest point in a planet's path moves around the parent body over time. There was a way in which gravity was seen as an attraction between points. The force of attraction would be something that happened instantly between objects, even if it was weak over long distances. Einstein showed that information is not communicated instantly across time.
There were several outstanding issues regarding how SR applied to the Universe. The idea of instantaneous communication was the first issue. Information is only communicated at the speed of light. A supernova that takes place 1 billion light-years away will look like it exploded in the night sky, but it actually took place 1 billion years ago.
Einstein believed that gravity acted as a field rather than an instantaneous pull. The bigger the mass, the more powerful the field. Einstein illustrated a passenger on an elevator as a metaphor for the issue of acceleration. The elevator would fall at a rate of 9.8 m/s 2 if someone were to cut the cable.
The passenger would feel the sensation of weightlessness until the elevator crashed. The same holds for all objects that experience acceleration. In the absence of external reference points, people traveling within an insturment frame would not be aware that they were moving. If the spaceship were at rest or moving at a constant speed, the passenger or crew would feel lighter.
If the reference frame accelerated, anyone inside would be thrown in the opposite direction of travel. The crew would feel the sensation of Earth-normal gravity if the acceleration were equal to 9.8 m/s 2. The crew's feet would be firmly planted on the floor if the spaceship were oriented with its vertical axis pointed in the direction of travel. The same principle applies to pinwheel stations or rotating cylinders in space, where the rotation creates a centripetal force that pulls objects out of the air.
The force creates a sensation of gravity for people on the station. The artificial gravity of the station can be equal to Earth-normal gravity. The key to exploring and settling the Solar System has been proposed by many scientists since the late 20th century. The bottom line is that gravity and acceleration are not the same thing.
There was an issue of time dilation raised by the authors. This means that gravity has an effect on spacetime. Einstein's General Relativity was born from this. Einstein said that gravity is not a force of attraction between points, but a consequence of spacetime being altered by a massive object. When objects are in the same spacetime, they are not pulling each other, but tracing the spacetime.
Einstein presented his Field Equations to the Academy of Science in Berlin in 1915. The four-dimensional geometry of spacetime is influenced by two things. John Wheeler said that spacetime tells matter how to move and matter tells spacetime how to curve.
Einstein's theory of Relativity would have consequences. If Einstein was correct, the resulting curvature of spacetime would affect everything, including light. The first opportunity to test GR came in 1919. Frank Dyson, Arthur Eddington, and a team of astrophysicists conducted an experiment during a solar eclipse.
Einstein's theory has been repeatedly tested andvalidated in the century since. Some of the tests involved small-scale experiments, while others were conducted in the most extreme conditions. In the case of the Eddington Experiment, the test consisted of observations made during a solar eclipse from two equatorial observatories, one located on the northeast coast of Brazil, the other on the island of Sao Tome and Principe off the coast of West Africa.
The team was looking for stars to pass behind the Sun during the eclipse. The light coming from these stars would show the spacetime curve caused by the Sun's gravity. The effect would make it look like the stars were next to the Sun. The light would be visible to their instruments if the Sun was blocked by the Moon.
The teams at both observatories saw these stars, but their positions in the night sky were where Einstein predicted they would be. The story was picked up by newspapers all over the world and posted on their front pages. This was one of the many tests and predictions that proved Einstein's theories to be correct.
In time, GR would be incorporated into all areas of modern physics, from particle physics to quantum mechanics. Some of the theories that would arise from Einstein's breakthrough wouldn't sit well with the astrophysicist. He would consider some of them to be heretical.
Einstein tried to use GR to create a model of the Universe in 1917. His Field Equations predicted that the Universe was either expansion or contraction, and he was not happy about it. It was necessary to counteract gravity on the largest scales in order to prevent the collapse of the large-scale structure of the Universe. Einstein introduced a new concept to the idea of a constant and unchanging Universe.
This was represented by a mathematical character in his Field Equations. The force was responsible for keeping the matter-energy density of the universe the same over time. Einstein was caught up in the debate between the proponents of the Steady State Hypothesis and the Big Bang Theory of cosmology, which would eventually be resolved in favor of the Big Bang model.
Some of Einstein's peers viewed his new theory as an unstable fix to the problems presented by GR. Alexander Friedmann showed how Einstein's Field Equations were consistent with a dynamic Universe in 1922. In 1927, Belgian astronomer Georges Lema showed that the expanding Universe was in line with the observations of American astronomer Edwin Hubble.
Einstein visited Hubble at the Mount Wilson Observatory in 1931, where he saw how the stars were moving. Einstein said that he was dropping his theories because it was the biggest mistake of his career. The Hubble Space Telescope showed that the rate of expansion increased with time.
astrophysicists theorize that there was a force counteracting gravity. The force was driving the Universe apart rather than preventing it from collapsing. Dark Energy is the force we know today. The Cold Dark Matter model is a key ingredient to the Dark Matter model.
German physicist and astronomer Karl Schwarzschild found a solution to Einstein's Field Equations that predicted the existence of black holes in 1915. The solution states that the mass of a sphere can be compressed so that the escape velocity from the surface is equal to the speed of light. The minimum dimensions of a spherical mass must collapse to form a black hole.
Einstein's theory allowed astronomy to rule out the existence of visible stars with large densities. The spacetime metric would close up around the star and leave us outside.
Subrahmanyan Chandrasekhar offered a solution to the problem of how a sufficient mass of electron-degenerate matter would collapse in on itself. This was known as the Chandrasekhar Limit. astrophysicists now have estimates on the mass and radius limits of black holes.
The scientists agreed with Chandrasekhar's analysis that stars above a prescribed limit would collapse into black holes. The edge of a singularity was defined as the outer boundary of the Schwarzschild radius. An infalling observer would have a different view of a black hole than an external observer.
The effect predicted by GR is how light from distant sources can be focused on. A massive object acts as a lens to amplify light forces beyond it. This method has been used to test Einstein's GR under extreme conditions, such as observations of Sagittarius A*, the black hole at the center of the Milky Way. A modified version of this technique can detect exoplanets around distant stars.
The rippling effect that can be caused by the forces of gravity was predicted by GR. This occurs when two massive objects merge and release a huge amount of energy in the form of waves. The first confirmed detection of these waves was made by the Laser Interferometer Gravitational-Wave Observatory in 2016 around a century after Einstein first predicted them.
Einstein's Theory of Relativity would have an influence on the field of quantum mechanics. The discoveries he would make here were a source of concern for him. The principle of quantumentanglement, which he would describe as spooky action at a distance, and that the Universe was characterized by the semi-chaotic nature of the equation of quantum wave function.
The role Einstein played in revolutionizing modern physics cannot be denied, even though he would resist some of the breakthrough he helped inspire. None of his contributions approach the significance or consequence of Relativity. After a century, advanced experiments still show how correct he was. It is part of the foundation upon which modern physics, quantum physics, astrophysics, and cosmology rest.
Here at Universe Today, we have many articles on Einstein's theories. Einstein still rules, but who was Albert Einstein? Astronomy Jargon 101: Gravity, and What are Gravitational Waves?
Two Astronomy Cast episodes are worth a listen.
Albert Einstein and the Theory of Relativity can be found at the University of Tennessee.
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