# Einstein's Theory of Relativity

## Einstein's Law of Relativity’ is one of the most beautiful laws invented in the history of physics.

Einstein's Theory of Relativity

Einstein's Law of Relativity’ is one of the most beautiful laws invented in the history of physics. This ideal research by him proved the Newtonian laws of motion wrong. Besides, Einstein established many new ideas with this law. His findings on relativity were not only accepted widely but also became a subject of further study. This law opened a new era in the space journey. He spent his life’s precious years to find his masterpiece, the ‘Law of Relativity’. It is between the two pillars of modern physics accompanied by quantum mechanics.

E = mc^2 is the mathematical equation of his mass-energy equivalence formula. It is the most famous equation in the world. For this great discovery, Einstein was awarded the prestigious Nobel Prize in physics in 1921.

Starting his career as a scientist, he at first thought that Newton’s law of motion is incorrect. The laws by Newton refers to an “Inertial frame of reference.” He stated the laws assuming ‘an object at rest.’ But in reality, there is no such object like Newton said. Every object in the universe is moving in comparison to another object. He also found the laws of motion were inactive when he tried to reconcile the laws of classical mechanics with the laws of the electromagnetic field.

It was the primary step for Einstein to research on the subject. He had a deeper look at many former scientists works. At first, he monitored the Newtonian quantum mechanics. Before Einstein, it was Isaac Newton who worked on the physics of motion. In his ‘Philosophiæ Naturalis Principia Mathematica’, he described the three laws of motion in 1687. This book depicts that everything in the universe is affected by gravity. That is why an apple fall on the ground and the earth moves around the sun. But what is the source of gravity? Newton himself had no clue about that.

As Einstein was thinking about space and time, he got influenced by David Hume. Einstein considered Hume’s book “A Treatise of Human Nature” to be fruitful in his research. The book was published in 1738. As a skeptic and empiricist, Hume believed that scientific concepts should not be developed based on reason only. Experience and evidence are also needed.

He also noticed the movement of objects and decided that time did not exist separately.

Einstein remarked on the works on Hume-

“It is very well possible that without these philosophical studies I would not                 have arrived at the solution”

American scientist, Albert A. Michelson is well known for his measuring the speed of light. He also made an experiment with Edward Williams Morley. Today, we know their experiment as the Michelson–Morley interferometry experiment. They did this experiment to measure the motion of the earth relative to the aether. Einstein agreed that the experiment was true and it helped him in developing the theory of special relativity.

Hendrik Lorentz was also an influential physicist in his time. He worked on hydrodynamics to general relativity. While working on electromagnetic fields, Lorentz used reference frames. The frames were relative to the postulated luminiferous aether. According to him, the transition of the reference frames can be simplified by using a new time variable. This experiment leads him to the way to explain the aberration of light. It also explains the result of the Fizeau experiment. Lorentz discussed many concepts, ideas, and results of his findings with Einstein. Then Einstein wrote his book “On the Electrodynamics of Moving Bodies" which is known as the ‘theory of special relativity’. As it is a joint collaboration, it was originally known as the ‘Lorentz-Einstein theory.’

French mathematician, Henri Poincaré was a contemporary of Einstein. He and Einstein both researched relativity in the same period. Henri got many results in his experiments while working on local time, the principle of relativity and Lorentz transformations, mass-energy relation, gravitational waves. He published a short paper prior to Einstein’s publication of his first paper on relativity. Later, he published a longer version of his research. Though they both worked on the same Poincaré never gives any recognition to Einstein for his efforts in special relativity. But Einstein gave him due respect before his death. He said, “Lorentz had already recognized that the transformation named after him is essential for the analysis of Maxwell's equations, and Poincaré deepened this insight still further…”

After the publication of Einstein’s work, it was Max Plank who immediately realized the importance of his work. He took initiatives on behalf of the theory by Einstein. For this reason, the theory was widely accepted in Germany and worldwide afterward.

The theory of Special Relativity

The scientists who were developing the theory of relativity influenced Einstein in many ways. He first published his work paper in 1905. The name of the work is "On the Electrodynamics of Moving Bodies." This paper includes two postulates and they contradict with the classical mechanics. One of these is physics’ laws are the same for all observers in a definite inertial frame of reference relative to one another. The other one is that the speed of light in a vacuum is the same for all observers, regardless of their relative motion or of the motion of the light source. Based on these two premises, he decided that space and time are interwoven. He illustrated both of them in a single continuum spacetime. He also stated that time span may differ for events occurring in different observers.

The postulates lead to experiments that are better than those of classical mechanics. Earlier we have heard about the Michelson–Morley experiment. The later premise explains the results of the experiment more accurately. Some of the surprising consequences of this theory are-

Relativity of simultaneity: If two observers are in relative motion, events that are simultaneous for one observer, may not be simultaneous for another.

Time dilation: A clock moves faster in a less motioned observer. But the clock moves slowly when it is in motion.

Length contraction: When an object is moving fast, it seems shorter in the direction they are moving. Of course, the observer has an influence on it. The same object in less motion in another observer may look bigger.

Maximum speed is finite: This theory depicts that nothing can move faster than the speed of light in a specific vacuum. No matter what field line, physical object or message it is. They cannot beat the speed of light.

Mass–energy equivalence: E = mc^2, energy and mass are equivalent and metamorphose.

The theory of General Relativity

It took nearly ten years for Einstein to establish the theory of general relativity. The earlier one did not focus on gravity. But this theory does. He worked from 1907 to 1915 to develop a theory of gravitation. The theory begins with the equivalence principle. Under this principle, we can find physical identities of any object’s two types of states - 1) Its accelerated motion and 2) Its state when is at rest in a gravitational field. Einstein states that free fall is inertial motion. In classical mechanics, the reason behind the free fall is gravity. But Einstein disagreed with this. As there is no force exerted on the falling object, it will naturally fall down. He further describes in his theories that inertially moving objects cannot accelerate with respect to each other, but objects in free fall can.

To explain his theory clearly, he interwoven space and time in one specific continuum; spacetime. Besides, he proposed that spacetime is curved. He devised the Einstein field equations in 1915. They relate the curvature of spacetime with the mass, energy, and any momentum within it. As spacetime is curved, it causes distortion.  You can set a large body in the center of a trampoline to understand this. The large body goes a little bit deeper into the fabric and makes a dimple. If you roll a smaller object like a golf ball you will see a slight inward movement when it passes through the dimpled area in the trampoline. This pull is gravity. For this, a planet pulls at rocks in space.

Gravitational time dilation, precession, light deflection, frame-dragging, and metric expansion of space are some of the mentionable consequences of the theory of general relativity.

Examples of the laws in our life

In our life, we find many instances where the theories of relativity are clearly apparent. Some of the examples are-

• Electromagnets: We all know that magnetism is a relativistic effect. The same thing applies in terms of electromagnets.  Faraday's law is “A changing magnetic field creates an electric current.” This law has been proven true by Thomas Moore, a professor of physics. He used the theory of relativity to do so.

• GPS: Global Positioning System is performed by the satellites above our head in the space. Moving at a great speed, they not only take pictures but also send them to the stations on the earth. Both the satellites and the earth are moving. So, the term relativity is of great importance here. Without considering the laws of relativity and maintaining it carefully, the system should not be useful at all.

• Gold’s color and corrosion: Gold is a heavy metal with fast-moving inner electrons. The fast-moving inner electrons move faster in small orbitals because of the law of relativity. This movement creates enough energy and is able to absorb longer light wavelengths. This is the main reason that gold appears to us as a yellowish object. It is the same reason that the metal does not want to corrode or react with other things easily.

• Mercury, a liquid metal: Mercury is a heavy metal too. Like gold, its electrons spin near the nucleus with great speed and increase mass. For this reason, the bond of atoms is weak in mercury and we see this as a liquid in real life.

• Lights bending: Due to gravity, lights bend. We know this term as gravitational lensing. A light ray between the earth and a distant galaxy seems bent when we see them with our telescopes. We also get multiple images of the galaxy in the telescope. This happens for the law of general relativity.