The theory of special and general relativity in simple language

There are few people in the world whose nameAlbert Einsteinhas not heard This outstanding physicist, who succeeded in presenting the theories of special and general relativity in the 20th century, caused a great change in our belief about the universe. Einstein also discovered the phenomenon Photoelectric Won the Nobel Prize in Physics. In this article, we are going to review the theory of special and general relativity in order to subvert the world around us with a new look.

Theory of relativity and classical mechanics

The emergence of the theory of relativity during the 20th century created a revolution in theoretical physics, especially in the field of astronomy. Before this classical mechanics, which was developed by the English physicist,Isaac Newton” was raised, it answered human questions in the world of physics.

The theory of relativity had a new look at the concept of space-time and discussed issues such as simultaneity, time dilation and length contraction. Such problems opened new doors to the world of astrophysics and cosmology, and thus various extraordinary phenomena were predicted, including the existence of neutron stars, black holes, and gravitational waves.

Classical mechanics, which itself is one of the fundamental revolutions in the world of science, studies the behavior of objects at the speed of a normal life. For example, you can consider a moving car and calculate its location in a few seconds based on its speed.

Classical mechanics

It is even possible to calculate the orbits of some planets and the angular speed of rotation of objects, but all these applications are true as long as the problems are within the usual velocities; When the speed approaches the speed of light, we have to go to relativity; Because classical mechanics is no longer able to provide correct answers.


The theories of special and general relativity, which were presented in 1905 and 1916 respectively, brought us into the world of modern physics. In other words, it can be said that modern physics is based on these two theories. As we said, when the speed of the object reaches the speed of light, we are no longer able to describe its behavior with classical physics, and this is where we have to go to the theory of relativity.

One of the basic and important foundations of the theory of relativity is the limited speed of light. In his studies, Einstein found that the speed of light is limited and no object in the universe can have a speed greater than the speed of light. The theory of relativity considers the upper limit of speed in the universe to be the speed of light in a vacuum (the speed of light in a vacuum is about three hundred thousand kilometers per second). Of course, it is important to mention that the concept of relativity was also quoted by other physicists before Einstein; However, the one who formulated these two theories precisely was Einstein.

Relativity simply describes how different observers interpret an event at the speed of light. Nuclear physics, astronomy and cosmology are fields in which these theories are widely used. Scientists active in these branches regularly deal with the principles and foundations of relativity.

The theory of special relativity

The theory of special relativity based on the previous studies of scientists such as “Albert Michaelson“, “Hendrik Lorentz“, “Henri Poincare” and so on was raised and by “Max Planck” And Herman MinkowskiIt became what we know today.

theory of relativity

In this theory, Albert Einstein stated that the laws of physics are the same for all observers who move without acceleration and showed that the speed of light in vacuum also has a constant value. He also understood that the concepts of space and time are not separate concepts, but space and time are intertwined as a unified texture, which is originally called space-time.

Another basic concept that he received was the concept of relativity. Einstein believed that in studying the position and movement of objects, what matters is the position and the way the observer moves; In other words, in the interpretation of the movement of objects, the movement of the observer must also be taken into account, and this is where “relativity” makes sense: the study of the movement of the object relative to the observer’s position.

Also, the theory of special relativity has other consequences that we will review together.

Theory of relativity and time dilation

The concept of time dilation in relativity is used to describe the time difference measured by two clocks. Suppose you are standing on Earth and your friend is going into space in a spaceship traveling at a speed close to the speed of light.

The theory of special relativity believes that time passes more slowly for your friend; Because it is moving at a speed close to the speed of light. In other words, every tick of your watch is not equivalent to a tick of your friend’s watch, time moves at a slower rate for him. Slowing down the rate of progress of time is related to getting closer to the speed of light; As the speed increases, time moves at a slower rate.

Length contraction

As the rate at which time advances slows down at speeds close to the speed of light, an object’s length also decreases—or rather, it contracts. Let’s say your friend who went on a space trip is holding a ruler. Since your astronaut friend is traveling at nearly the speed of light, the length of the ruler is said to be contracting and decreasing.

theory of relativity

It is another consequence of special relativity that the length contraction of an object at speeds close to the speed of light can be obtained using the mathematical relationships presented in this theory.

“Length contraction” with the names “Lorentz contraction” or “Lawrence-Fitzgerald contractionThey also know You should keep in mind that length contraction occurs only in the direction of movement of the object. In other words, if the said ruler is moving east to west, it will contract only in this direction, not in the north-south direction!

Mass-energy equivalence

Albert Einstein believed that when considering a physical system that is at rest, a simple relationship can be established between the mass and energy of the system, which is called the mass-energy equivalence relationship.

As you can see in the picture, the mass of an object, denoted by m, can be related to its energy, E. Meanwhile, c is the speed of light, which has a constant value in vacuum. This relationship holds when the system is at rest.

Given the numerical value of the speed of light, this formula describes how a small mass can correspond to a large amount of energy, regardless of the composition of matter.

time dilation

Rest mass and relativistic mass

Among other phenomena studied by special relativity, we can mention the concepts of rest mass and relativistic mass. Special relativity claims that the mass of an object when it is at rest is different from when it is moving at a speed close to the speed of light.

The rest mass of an object is the mass that the object has at rest, and this mass is the same for all observers standing in different frames. On the other hand, relativistic mass is the mass that changes with the velocity of the observer. According to the above-mentioned concept of mass-energy equivalence, constant mass is rest mass, while relativistic mass is equal to relativistic energy, which is Total energy is also known.

What has been stated so far was only the consequences of the theory of special relativity. But we said that Einstein’s theory of relativity also includes the theory of general relativity. In the following, we intend to get acquainted with general relativity and its interesting predictions.

The theory of general relativity

Despite all the valuable predictions this theory made, it was initially thought not to be as useful as special relativity. Since most of the claims were astronomical in scale, there seemed little potential to test them experimentally. On the other hand, general relativity also seemed mathematically difficult and only few people could understand it. But from 1960 onwards, general relativity became an integral part of physics.

The theory of general relativity, published in 1915, extended special relativity, which had been published about ten years earlier. As we said, special relativity studies the movement of objects at speeds close to the speed of light, but without considering acceleration. In fact, it assumes the speed of objects to be constant. What was investigated in general relativity was the accelerated motion of objects.

To better understand general relativity, let us first review the concept of gravity. Gravity is actually the force of attraction that two objects exert on each other. This force depends on the mass of each object and the distance between them. Newton, who proposed the laws related to gravity, believed that gravity is an inherent force of an object that can act even over long distances. Einstein’s understanding of gravity is the force caused by the mass of objects, and the heavier an object is, the more force it pulls the surrounding objects towards it.

In fact, Einstein spent a decade determining that massive objects – in the mass range of planets and celestial bodies – distort the fabric of space-time, a distortion that manifests as gravity.

theory of relativity

Imagine placing a large, heavy ball in the center of a trampoline. The object pushes against the fabric and causes a dent in the center of the trampoline. Now if we drop a small ball in one corner of the trampoline, this ball will roll on circular tracks around the bigger ball in the center until it finally falls into it; This is exactly what happens to objects in space. The heavier mass, which has distorted space-time, pulls the lighter objects towards it.

In addition, there are other phenomena that Einstein managed to describe in the theory of general relativity. Some of these phenomena are:

Gravitational lensing in the theory of relativity

Suppose we want to observe a mass in the universe that is billions of years away from us; So its light travels a long time in the universe to reach us. Sometimes it happens that in line with our view of that object, there is another huge object like a black hole. This is good news; Because the light of the object that is the target of our observation, on its way to reach us, it encounters the gravity of the black hole, it bends, and the black hole acts like a lens, exactly like a lens, and magnifies the objects.

For example, it has been seen in cases that the phenomenon of gravitational lensing has caused such a magnification that it seems as if the object is ten times closer to us.

Perigee of the planet Mercury

Mercury is the closest planet to the sun and is more affected by the sun’s gravity than other bodies. In his calculations in general relativity, Einstein showed that due to the curvature of space-time around the sun, the orbit of Mercury changes over time.

It should be noted that Newton also predicted the change of Mercury’s orbit, but the figure he obtained for the amount of change was not correct. Einstein corrected this figure and the figure 43 seconds of arc gained in each century (each second of arc is equal to 1/3600 degrees).

Gravitational redshift

To better understand this phenomenon, we can first “Doppler effectreviewed. Have you noticed how the sound of the fire engine sirens changes when it comes from a distance and then passes near you? As the fire engine approaches you, the sound waves become more compressed and you hear the sound louder, then as the engine moves away, the sound waves are stretched and so called redshifted.

This is exactly what happens to light; Because light is an electromagnetic radiation that spreads in the world as a wave. Red transition occurs for light waves at any wavelength.

Gravitational waves in the theory of relativity

Using the equations of general relativity, Einstein predicted that when two objects with a very large mass – such as two black holes – collide with each other, waves will be created in space-time, which are called “Gravitational wavesare known.

Gravitational waves

In 2016, the Laser Interferometer Gravitational Wave Observatory (LIGO) announced that he had detected such a signal for the first time. Thus, another prediction of the theory of general relativity was confirmed. Since then, scientists are recording other gravitational waves and studying them.

What is the difference between special and general relativity?

Special relativity studies the relationship between the observer and constant velocity and is related to the Earth and constant velocity, while general relativity describes the relationship between the observer and acceleration and is related to gravity and acceleration.
So, the behavior of an observer moving at a constant speed close to the speed of light is described by special relativity, and the behavior of an observer in accelerated motion is described by general relativity.

Is general relativity applicable in our daily life?

Yes. The theory of relativity is not only limited to the boundaries of theoretical physics, but is also used in everyday life. Its most important application is in the use of GPS or a locator. The satellites with the help of which we determine our position at any moment follow the principles of relativity. Keeping the speed of light constant and calculating how long it takes to reach the earth while our position is changing requires the principles of relativity.

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