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We have previously heard about Newton's three laws regarding concepts such as inertia, force, and the reaction force corresponding to every action. The aim of relativity is to connect such physical laws to different frameworks.
When we think of relativity, Albert Einstein immediately comes to mind. However, many scientists and philosophers attempted to explain relativity even before him. Nevertheless, it was Einstein who clarified the confusions and fully explained the concept. In this discussion, we will provide an overview of relativity, exploring the theory that starts with Galileo and extends to the special theory of relativity completed by Einstein.
Galileo's Theory of Relativity:
Even before Einstein, Galileo had somewhat explained the theory of relativity. Imagine you are standing still in one place while your friend passes by in a vehicle at a speed of 50 kilometers per hour.
As he drives by, he drops a ball from the vehicle. From your perspective, it appears that the ball is coming toward you. However, from your friend’s perspective, it seems that the ball is moving away from him. In this scenario, none of the laws of physics are violated. Instead, Galileo suggested that the direction the ball moves depends on the angle of the observer’s viewpoint. This idea became a starting point for the theory of relativity.
Then, in the year 1687, Newton's laws of motion were published. These laws provided an explanation for the motion of an object. However, we cannot apply these laws to objects that move at speeds greater than 0.1% of the speed of light. This is because, at such speeds, properties like mass, length, and time vary significantly. Explaining these variations is what the special theory of relativity is about.
In 1865, the physicist James Clerk Maxwell proved through equations that light incorporates both electric and magnetic fields. Later, this property of light was also confirmed through experiments."
Relation of ethers to speed of light:
Now, let’s consider a star emitting light while moving in a specific direction. According to Newton's laws of motion, the speed of the light emitted by that star should be equal to the sum of the speed of light in a vacuum (3×10⁸ m/s) and the speed of the star. However, the speed of light in a vacuum is constant in this universe. Its speed never increases or decreases.
To resolve this contradiction, scientists believed in a medium called ether, which was thought to assist in the propagation of light, and that the universe was filled with ether. They assumed that the speed of light in this medium would always be constant. However, in 1887, the results of an experiment conducted by Michelson and Morley proved that there is no such medium as ether in the universe and that the speed of light remains constant.
Therefore, scientists became confused again when measuring the speed of light emitted from moving systems.
(To learn more about the Michelson-Morley experiment, please read the article titled "The Ether-Filled Universe" available on this site.)
Einstein's Thoughts on Light:
Then, in 1896, at the age of 16, Einstein imagined an experiment related to light. He began to think about how light would appear if he were moving at the speed of light and looked back at the light beam in the direction it was traveling.
Now, when two trains are moving close to each other at the same speed, a person in one train would appear to be standing still when viewed from the other train, right? Similarly, if one were to move at the speed of light and look back at that light, it should also appear to be standing still. However, Maxwell had already proven that light travels in the form of waves, which consist of electric and magnetic fields, and that it does not remain stationary in any medium.
Therefore, Einstein concluded that the speed of light does not change in any medium. He also concluded that there is no relationship between the speed of light and the speed of the emitting source.If a headlight is attached to his head, the speed of light emitted from it will also be 3×10⁸ m/s.
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Let’s consider a scenario where someone is shooting a gun forward from a vehicle moving at 100 km/hr. For that person, let’s assume the speed of the bullet emitted from the gun is 900 km/hr. If someone measures the speed of the bullet while standing still, it would appear that the bullet is traveling at 1000 km/hr. However, if instead of firing a gun, the person turns on a light, both to his own eyes and to the eyes of an outside observer, the light will always propagate at the same speed.
Relation of Laws of Physics to Velocity of Systems:
When a ball is thrown upward from the ground, it eventually comes back down to our hands. Similarly, if a ball is thrown upward inside a flying airplane, it will also return to our hands. Therefore, the principle of physics confirms that the speed of the system we are in does not change.
Based on what he had seen so far, Einstein put forward two important hypotheses. These hypotheses are common to all topics in relativity. His hypotheses are as follows.
- The laws of physics are the same in all systems, whether that system is at rest in one place or moving at a constant velocity in a specific direction.
- The speed of light does not change in any system. Whether measured from a moving system or a stationary system, the speed of light will be measured as the same in both cases.
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Imagine you are standing on the roof of a train moving at 100 kilometers per hour. You can see both the front and the back of the train at the same time. From your position, two lightning strikes occur at equal distances, one in front of you and one behind you, simultaneously. Which lightning strike will you see first?
You will definitely see the lightning strike at the front first. Later, you will see the lightning that struck behind you. This is because the train is moving forward at 100 km/hr. However, if the train were stationary, you would see both lightning strikes at the same time.
Thus, we can perceive that time is relative here. After seeing the lightning in front, a little time passes before the lightning behind becomes visible to your eyes.
Therefore, Einstein demonstrated that, during high-speed motion, time moves faster in front and slower behind us. He concluded that time is not a constant in this universe, but varies depending on the motion of each system. Based on such conclusions, he discovered the equation E=MC², which relates energy and mass. This equation expresses the fact that a small amount of mass can be converted into a significant amount of energy."
The theories of light and time proven by Einstein, including the changes in mass and length of objects in high-speed motion, the relationship between time and speed, and other concepts, were compiled and published in 1905 under the title 'The special theory of relativity.'
However, there is no gravitational force in any part of the special theory of relativity. The explanation of the relationship between time, speed, and distance with gravitational force is what is referred to as 'The general theory of relativity.'
We will explore this in more detail in the future.