INTRODUCTION
It seems as though time in this universe never stops or decelerates for anyone. It continuously moves forward without pausing for anyone or anything. However, is time truly like this? According to the special theory of relativity, time does not behave in this manner. Time is not uniform for everyone throughout the entire universe. Various observers experience different time intervals at different locations. So, why does this happen? How is this achievable? Let’s find out!
BASIC IDEA
Einstein’s special theory of relativity begins with two fundamental concepts. The first concept states that the laws of physics are consistent in all frames of reference. Consider a scenario where your friend is standing on the platform of a train station, and you are inside a train traveling at a steady speed. A constant velocity denotes that an object is moving at a fixed speed and in a fixed direction, without any acceleration, deceleration, or change in direction. From your friend’s perspective on the platform, they are stationary while you are moving forward. However, from your perspective inside the moving train, where you do not feel any motion due to the constant speed, you can also claim to be at rest, and the rest of the world may appear to be moving towards you.
If the train’s windows are shut, there is no external reference point, leading you to believe that you are at rest. Since the laws of physics apply to both you and your friend, both of you can be considered to be at rest. When you are inside a moving train and your friend is on the platform, whatever your friend is doing on the platform, you can also do inside the moving train. However, the speed is not the same for all observers. For example, if the train is moving at a constant speed of 100 km/h and you throw a ball at 20 km/h, from your perspective inside the train, the ball is traveling at 20 km/h. But from your friend’s perspective on the platform, observing the train, they perceive the ball to be traveling at a remarkable 120 km/h. If you have another friend who is running on the track towards your train at 10 km/h, if they measured the ball’s speed, they would determine it to be 130 km/h. The same moving object can display different speeds because speed is relative and can vary from one observer to another. Different observers can measure different speeds for the same moving object.
SPECIAL THEORY OF RELATIVITY
The special theory of relativity’s other postulate asserts that the speed of light remains constant for all observers. Regardless of the frame of reference, the speed of light is consistent and maintains the same value. Consider a simple scenario: you and your friend stand facing each other, with your friend holding a flashlight. If you were to measure the speed of light quickly enough, you would find it to be approximately 300000 km/s. This is not surprising, as both of you are at rest. However, if you were to run towards your friend at the extraordinary speed of 10000 km/s and he flashed the light, you would still measure it showing the same value of 300000 km/s, rather than 310000 km/s. This is due to the constant nature of the speed of light, which remains the same for all observers, whether at rest or in motion. To uphold the constant speed of light, space and time exhibit peculiar characteristics. So, what exactly is time? Even today, we do not have a precise understanding of what time is, but we do possess some basic comprehension. We universally agree that all clocks in our world tick at the same rate, and we have a shared understanding of the duration of events and simultaneous occurrences. These fundamental aspects of time are part of our everyday experience. However, the constancy of light reveals that all of this is Incorrect. How?
EXPLAIN WITH AN EXAMPLE
To better understand about special theory of relativity, let’s take an example, Nation A and Nation B are in a situation where they both want to sign an agreement at the same time. They have devised a procedure for this. The procedure involves the leaders of both nations sitting across from each other at a table with a light bulb positioned in the middle. Once the light bulb is turned on and the light reaches both leaders’ eyes, they can sign the agreement simultaneously. Both nations’ leaders decide to sit at exactly the same distance from the bulb as they know that light always travels at the same speed. As the event commences, they turn on the light, and it reaches both leaders at the same moment. Consequently, the leaders sign the agreement simultaneously, and everyone is pleased with the outcome. After a few months, they decide to sign another agreement. However, this time, both leaders are interested in doing things differently. Despite having different opinions, they share a strong passion for trains. They decide to recreate the same scenario on a moving train. As the train moves, the light flashes and reaches both leaders simultaneously. They both sign, and everyone on the train is pleased with the outcome. However, onlookers on the platform begin to argue because the people from Nation B claim that the leader of Nation A signed the agreement first. How did they reach this conclusion? Let’s reconsider the event. The train is in motion, the light flashes. Leader A is moving towards the flash, while leader B is moving away from it. Observers notice that the flash reached leader A first because he was moving towards it. They are aware that light always travels at a constant speed. Since leader B was moving away from the flash, it took longer to reach him compared to leader A. From the perspective of those watching from the platform, it appeared that both leaders did not sign at the same moment. The leaders on the moving train, who were at a constant velocity, claim that they both signed at the same time. It’s well-established that an object traveling at a constant speed in a particular frame of reference can consider itself to be at rest. Now, let’s consider the perspective from the platform. To illustrate this, let’s create a simple scenario. We will mark the moment the flash occurs and then mark when it reaches leader A and leader B. By doing this, we can clearly observe that the flash has to travel a greater distance to reach leader B compared to leader A. It’s important to note that light always travels at a constant speed. The leaders inside the train and the people watching from outside both have valid perspectives. The constant speed of light means that events occur at the same time, despite appearing differently to different observers due to relative motion. One group may not perceive any time difference, while the other group witnesses a time difference for the same event. This is not an optical illusion, but rather a reality where events occur at different moments from different perspectives.
TIME DILATION
Moving on to the next stage, let’s consider a clock. Despite the various types of clocks available to demonstrate time dilation, we will use a light clock for easier understanding. The light clock’s simplicity in mechanism is what makes it special, as it functions like any other clock. This type of clock consists of two mirrors with a light ball bouncing between them, and each bounce is equivalent to 1 second. Now, we introduce another light clock. One clock will remain stationary, while we set the other in motion. As the moving clock travels, we observe the light ball following a diagonal path, and notice that the counts on both clocks differ. Why is this the case? If we examine the trajectory of the moving clock, we see that it follows a longer diagonal path. This results in a longer duration for a full up-and-down movement compared to the stationary clock. The reason for this is that light must cover a greater distance, but its speed remains constant and the same for all observers. Therefore, in the moving clock, each tick-tock occurs at a slower rate. A slower tick-tock indicates that time passes more slowly. In the moving clock, 1 second takes longer than in the stationary clock. Time appears to pass more slowly in the moving clock from the viewpoint of an outside observer. Thus, according to our perspective, time runs slower in the moving clock. However, for anyone inside the clock, there is no perceived difference in time as they move along with the light clock and cannot sense the light ball following the diagonal path. Time doesn’t feel like it’s slowing down for moving objects. We know that when an object moves at a constant velocity, it can be considered at rest. Therefore, when an object is in motion, its time slows down. However, why don’t we observe time dilation in our daily lives? Let’s consider the time dilation formula:
Imagine two clocks: one on Earth and another on a rocket in space. If the rocket moves at 10 percent of the speed of light, the time difference is difficult to perceive, although it exists. Now, if we increase the rocket’s velocity to 70 percent of the speed of light, we can clearly observe the time difference. The time elapsed on the rocket ship is less than from the perspective of us on Earth. Moving the rocket at 98 percent of the speed of light allows us to experience a significant difference between the two clocks. From our viewpoint on Earth, the clock in motion ticks very slowly. Therefore, time dilation is a universal phenomenon, but its effects are only significantly noticeable when moving at speeds close to that of light. At everyday speeds, time dilation does occur, but its impact is extremely small. To delve deeper, let’s consider another example: the light clock placed on a moving train. At our current train velocity, we are traveling at 10 percent of the speed of light, and there is a noticeable but not significant time difference. It seems to closely follow the straight up and down path of the light ball. Upon increasing the velocity to 70 percent of the speed of light, we can clearly observe the diagonal path of the light ball. As we approach speeds extremely close to the speed of light, such as 99.99 percent, there is a substantial difference in the diagonal path. At this point, the light ball appears to almost stand still while the stationary clock continues to tick at its regular rate. While everything discussed so far is based on theory and formulas, it is difficult to definitively conclude that time dilation occurs when an object is in motion. However, there are numerous experiments that have demonstrated the existence of time dilation in real life. Let’s consider two simple experiments as examples. The first is a direct experiment conducted in 1971 by scientists Hafele and Keating, who used two synchronized atomic clocks. One clock remained stationary while the other was placed on an airplane that traveled around the world several times. Upon the plane’s return, the two clocks were compared. The clocks showed that different amounts of time had passed on each one, with a 60 nanoseconds difference. This difference precisely matched Einstein’s prediction, demonstrating that time on moving clocks ticks at a different rate from stationary ones. Another experiment involved the lifetime of a muon, a particle similar to an electron but with a lifespan of only 2.2 X 10-6 seconds before it decays. Scientists observed that muons sometimes take longer to decay when in motion, despite always decaying at the same rate. This observation confirmed that time slows down for objects in motion. Imagine this scenario: you and your friend are the same age, and both unaware of the special theory of relativity theory. If you leave your friend on Earth and travel in a rocket near the speed of light for 6 months, upon returning, you would find that only 6 months have passed for you, while your friend would tell you that 40 years have passed on Earth. This is how time dilation operates, showcasing the remarkable aspects of the special theory of relativity theory. It’s truly astonishing how Einstein deduced this phenomenon while considering the constant speed of light. He was indeed a brilliant mind.
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