The sunlight, which is about 150 million kilometers away from Earth, travels this immense distance in just eight minutes, illuminating our path during morning walks. Therefore, light travels at a speed that surpasses human imagination. Today, this speed of light is known as the highest speed in nature according to Einstein's theory of relativity. Thus, no matter how fast a person runs or how quickly an airplane flies, it cannot match the speed of light. This speed of light is, therefore, a mystery that cannot be confirmed by the human eye. The ancient mathematician Hero also contemplated this mysterious speed of light, wondering if it was infinitely fast without limits. This represents a very reasonable understanding that ancient people could have about light at that time. This common understanding has influenced human thoughts about the speed of light.

A figure emerged who challenged the long-held belief that light speed is infinite and therefore measurable. Who could this person be? Today, we will begin by discussing this individual's experiment to measure the speed of light, and briefly review the efforts humanity has made to uncover the truth about light speed. We will also explore why, in Einstein's theory of special relativity, the speed of light is described as invariant—meaning it does not change and is absolute. Furthermore, we will explain why this understanding of light disrupts our common perceptions of space and time, making it accessible for everyone to understand. So, I hope you enjoy watching from now on. The speed of light, which has undergone precise verification by modern science, travels approximately 300,000 kilometers in a vacuum in one second, and the more accurate value is as follows. However, sound is fundamentally...

Sound travels through air at about 340 meters per second. Usain Bolt set a record by completing 100 meters in 9.58 seconds, but even this amazing achievement is slow compared to the speed of sound, which is also quite slow when compared to the speed of light. However, what we should focus on here is that even though sound is slower than light, it is still incredibly fast in the realm of human sensory experience. For instance, when you call someone's name from the upper floor to the lower floor, the sound reaches them immediately without any noticeable delay. Because sound is so quick, we can say that the speed of sound is remarkable in itself. However, there are moments when humans perceive even this speed as slow, particularly during thunderstorms. Thunder and lightning occur simultaneously, but when they reach us, light always arrives first.

The sound arrives late after a flash, creating this echoing effect. In this way, we come to realize that sound travels slower than light. Through this experience, we can cautiously consider that sound has its speed limits, and perhaps even light, which is relatively faster, also has its speed limits. The idea that the speed of light is finite could lead us to think that it can be measured. Of course, it's not easy for everyone to entertain such thoughts. However, throughout history, there have been individuals who engage in such unique thinking, and one of them is the Italian scientist Galileo. Based on the idea that the speed of light is finite and thus measurable, he conducted experiments. He had people climb to the peaks of two distant hills and signal between them.

When the light is shot, it will reach point two, and at that moment, the person immediately shoots the light again. If we divide the distance the light has traveled back and forth by the time it took for that movement, we can get the speed value. Galileo thought that this way, he could calculate the speed of light; however, unfortunately, this experiment failed. Why was that? As mentioned earlier, light travels at about 300,000 kilometers per second. If we divide this by the Earth's circumference of about 40,000 kilometers, we get 7.5. This means that light circles the Earth seven and a half times in one second. This speed is something that cannot be perceived by human senses. Therefore, the distance difference between those two peaks was not only too short to measure the speed of light but also could not be confirmed by the human eye. Thus, humanity's first experiment to determine the speed of light failed, but two years after Galileo passed away...

The Danish scientist Lehmann, surprisingly, observed the movements of Io, one of Jupiter's moons, rather than the Earth, and proved that the speed of light is finite, calculating its own speed value. So, what method did he use? The Danish scientist Lehmann observed Io among Jupiter's moons and discovered a strange phenomenon: the orbital period was not constant. Now, what exactly is an orbital period? Our Earth revolves around the Sun, which is called revolution, and the time it takes to complete this revolution is called the orbital period. The path that the Earth takes while orbiting the Sun is known as the orbital trajectory. But what does this have to do with the speed of light that we are trying to understand? Earlier, I mentioned that every time Lehmann observed Io, he...

I mentioned that I discovered a phenomenon where the orbital period changes. In other words, this means that the time it takes for Io to complete one orbit around Jupiter varies; sometimes it is faster, and sometimes it is slower. And right here lies the key to unraveling the mystery of the speed of light. Now, I understand that some of you might find this topic a bit difficult, especially since our channel mainly focuses on the humanities, and many of you might not be well-versed in science. Nevertheless, to accommodate those viewers, I will now explain this in a more calm and detailed manner, making it easier for you to understand. When celestial bodies orbit, the path they take is called the orbital path. But what if this path widens or narrows? Simply put, when the path widens, as you can see now, the circumference of the orbit...

If it got longer, it means it would take more time, and when it got shorter, the travel time would naturally decrease. Therefore, the fact that the orbital period of the second moon is not constant and changes indicates that its orbit is changing. However, the eccentricity of the orbit does not change instantly like a rubber band that stretches and contracts; this cannot happen in the blink of an eye. Therefore, such changes in the orbit cannot be the reason for the subsequent changes in the orbital period. With that said, there is one remaining variable. What could that be? It is, of course, the position of the observer, which is the position of the Earth. To help you understand, let me give you a simple example. If an observer is stationary and observes a car traveling at 30 km/h, this person would perceive the car's speed as 30 km/h. However, when that car is traveling at 30 km/h...

If an observer is driving in another car right next to it at the same speed, there would be no time difference between the two cars, and there would be no difference in speed either. Now, at this point, those who are a bit quick on the uptake might have an idea of what I am about to discuss. The question is, what happens if the Earth, which is stationary, is actually moving? As you can see, the Earth is indeed orbiting around the Sun. Furthermore, at a greater distance, Jupiter is also orbiting, and this Jupiter is in turn orbiting as well. Therefore, when the Earth is in this position, and if the other object is in that position, the distance between the two is at its longest. Conversely, when the Earth is here and the other object is there, the distance between them is at its shortest. This means that the orbital period of the other object has not been consistent.

The reason is that the Earth, which is observing the satellites, is also moving. Through this fact, Le Maan calculated the time difference between the fastest and slowest orbital periods of the Earth, discovering it to be about 22 minutes. Isn't that amazing? However, it's still too early to be surprised. Why? If the speed of light were limitless and infinite, there would be no reason for the orbital period to be inconsistent. But as the distance from the light to the Earth increased, the travel time took longer, and when the distance decreased, the travel time was shorter. Therefore, we can conclude that the speed of light is not infinite but rather finite. This scientific conclusion means that the traditional belief that the speed of light was infinite has been challenged since humans appeared on Earth.

I want to emphasize that the point at which you should be surprised is not earlier but right now, as this is something that breaks our understanding. After confirming that the speed of light is finite, Rømer then worked to determine what that value is, specifically how fast it is. The reason for this is that it takes about 42.5 minutes for Jupiter to complete one orbit, which is less than two days on Earth. During this time, Earth also moves slightly as it orbits, creating a distance. By using the differences in time and distance that arise in this manner, calculations can be made. However, at that time, there were no precise measuring tools like we have today, and the measurement results had significant errors. Therefore, with these various limitations, Rømer made his calculations in 1676, and as a result, the value he derived for the speed of light in a vacuum was approximately 24,000 kilometers per second, which is about 30% smaller than the actual value revealed today.

I can say that this was the value obtained for the speed of light by humans for the first time. It can be considered significant. After the experiment on the nature of light, as time passed into the 19th century, the French physicist Hippolyte Fizeau sought to measure the speed of light through a gear experiment rather than through direct observation. He set up a device to observe light and a device to reflect it, placed about 8.6 kilometers apart. He then positioned a rotating gear in the path of the light at the point where it would arrive, aiming to calculate the time it took for the light to travel back and forth. Now, just hearing this might make it hard to grasp how this relates to measuring the speed of light, especially for those who are hearing it for the first time. So, don’t think too hard about it. I will explain the details of the experiment, and as you listen, it will become easier to understand.

So please bear with me for a moment. First, he placed a gear with 720 teeth at the endpoint of the light's path and spun it rapidly. At this time, each tooth of the gear would either allow light to pass through or block it as it turned. By repeating this experiment countless times, he confirmed that when the gear's teeth were rotating at 12.6 kHz, the light reflected off the surface was completely blocked. How could this be verified? When the light from the teeth is completely obscured, the observed scene becomes perfectly dark. This allowed him to confirm this fact. Now, we need to determine the time it takes for one tooth to rotate. This can be calculated using the values of 1 second, 12.6, 720, and 2, resulting in approximately 0.005 seconds. And the round trip of light...

The distance is 8.66 km multiplied by 2, which makes a total of 17.2 km. If we divide this distance by the time, we can find the result. So, what was the value obtained from this calculation? Surprisingly, it is very close to the speed of light that we know today, approximately 31.3 km per second. Isn't that amazing? Human experiments to understand the speed of light continued, and thus the precision began to increase. This led to the implementation of the speed of light through theoretical calculations rather than measurement experiments. The person who accomplished this was Maxwell, known as the father of electromagnetic theory. Now, I will briefly discuss him and then talk about how Einstein, influenced by this electromagnetic theory, revealed the mysteries of light through his special theory of relativity.

He is the person who discovered that light is a type of electromagnetic wave. Essentially, electromagnetic waves refer to waves that are transmitted as electric and magnetic fields change periodically. In simpler terms, when the direction of an electric current changes, a magnetic field is created around it. This magnetic field generates an electric field, which in turn creates another magnetic field, resulting in waves that propagate perpendicularly. This is what electromagnetic waves are. 

In any case, Maxwell theoretically calculated the speed of these electromagnetic waves and found that, surprisingly, this value matches the speed of light. Thus, it was revealed that light is an electromagnetic wave, a wave of electricity and magnetism. This discovery had a significant impact on a 16-year-old student living in Switzerland in 1895. This student was curious about what would happen if he were to chase light at the same speed as it travels. For example, if an object...

If two objects depart from the same point at the same time and are traveling at 50 km/h, their relative speed is zero. In this case, from the perspective of object A, it appears to be stationary, and the same can be said for the other object. However, it is important to note that the speed of light, or the speed of photons, does not conform to this common sense. 

Simply put, if you are traveling at the speed of light and observing light, their relative speed would be zero, making it seem as if light is stationary to the observer. However, according to Maxwell's equations, the speed of light in a vacuum is derived as a constant. Therefore, it must still be observed at 300,000 km per second. This means that regardless of relative speed, the speed of light is always 300,000 km per second. 

In other words, even if you are traveling at the speed of light and observing this light, its speed remains 300,000 km per second and does not become zero. This presents a rather strange situation. For example, consider a box moving at 100,000 km per second. If you are inside this box...

Let's consider a person who turned on a flashlight. The speed of light should be added to the speed of the box, so for someone observing from a stationary position, it would be normal to see it moving at 400,000 kilometers per second. However, light does not behave this way; it consistently maintains a speed of 300,000 kilometers per second. This means that the speed of light remains constant at 300,000 kilometers per second regardless of any conditions or circumstances, indicating that it is absolute rather than relative. The principle that states the speed of light does not change is known as the principle of the invariance of the speed of light. Now, this situation conflicts with our common understanding. To resolve this issue, the young boy, Einstein, who pondered over it, conducted research and published the theory of special relativity in 1905. This lecture briefly introduces humanity's journey to understand the speed of light and the discoveries made regarding it.

I will summarize only the content regarding the immutable principles that beginners can easily understand, so unfortunately, I will not explain the entire theory of special relativity in detail. Therefore, I will address this topic separately later, so please keep that in mind. I would like you to think about the absolute space and absolute time presented by Isaac Newton in his Principia. Here, absolute time refers to a time that flows uniformly at the same rate in any location, unaffected by anything. Absolute space refers to a space that remains unchanged, regardless of any influences. In other words, the immutable stage on which all events of the universe unfold is absolute space, and the ticking clock, which ticks uniformly regardless of the Andromeda galaxy or Earth, represents absolute time. This concept has been an important standard in the scientific community for many years. In fact,

These concepts seem to align well with our intuitive understanding. For our family living in one house, time does not flow differently; my father is not in the future while I am in the past. The space of our home is also perceived as always being stationary. Therefore, if we understand the speed of light based on the premise of absolute space and absolute time, when Cheolsu chases light at a speed of 200,000 kilometers per second in the same direction, that light should appear to him at a speed of 100,000 kilometers per second, after subtracting his speed from the speed of light. This conclusion fits well with our common sense. However, in reality, Cheolsu, moving at a speed of 200,000 kilometers per second, still sees that light moving at 300,000 kilometers per second. Even if he increases his speed to 299,000 kilometers per second, the light still appears to him at 300,000 kilometers per second.

appeared to be invariant. Therefore, to understand this peculiar phenomenon, one must think differently, that is, in an innovative way. The classical mechanics world, which has understood the universe through absolute space and absolute time, cannot comprehend this. So, who was the protagonist that achieved this innovative thinking? Yes, it was Einstein. He rejected the concepts of absolute space and absolute time that had been used in classical physics for a long time. He realized that time and space could be connected using Minkowski's mathematical formulation. Thus, he asserted that time and space are one, or rather a spacetime continuum. He claimed that this spacetime possesses the property of being able to expand and contract, which truly shattered the existing common understanding of the subject. His assertions...

According to this, we can easily understand why the speed of light is constant, and this information is truly important. First, we should note that as an object's speed approaches the speed of light, its length in the direction of motion decreases, and the flow of time slows down. Therefore, as an object with mass approaches the speed of light, time for that object slows down, allowing it to still travel at the speed of light, which is 300,000 km/s. This is why the speed of light is constant, meaning it does not change. This fact also demonstrates that there is no such thing as absolute time, as Newton suggested. Isn't it interesting? We learn that time is relative, not absolute, through the constant and unchanging speed of light. Now, this raises the question: as we approach the speed of light, time flows more slowly. Can we actually confirm this?

The question is whether it is possible. A representative example of this is the phenomenon of the muon particle's extended lifespan. This particle was discovered through a series of processes, starting from the theory of the Japanese scholar Hideki Yukawa. It is known as a particle with mass next to that of an electron. These muon particles come from outer space, and experimental results have revealed that the lifespan of this particle is about 2.2 microseconds, which is very short. However, despite having such a brief lifespan, muon particles are scientifically confirmed to reach the Earth's surface before decaying. So how is this possible? When the muon particle travels to Earth, it moves at a speed close to the speed of light, which causes time for this particle to slow down. As a result, its lifespan appears to be relatively longer due to the effects of time dilation.

It's amazing that particles can come down from such a low altitude to the ground. It's a fascinating phenomenon, similar to how if time slows down, the lifespan of a person living within that time also increases. According to the theory of special relativity, as an object approaches the speed of light, its mass increases. Therefore, this fact serves as evidence that light is the ultimate speed limit in the universe. 

What this means is that let's imagine there is a super-fast spaceship created by the world's top scientists. Let's say it was designed to travel faster than the speed of light. When this spaceship flies through the vacuum of space and gradually increases its speed to approach the speed of light, it encounters issues that are independent of the systems designed for the spaceship. That is...

As an object with mass approaches the speed of light, its mass increases dramatically, and the same goes for time; the closer it gets to the speed of light, the more time slows down, and the mass increases as well. So, what happens when this occurs? Naturally, it becomes more difficult to move at the existing energy levels. Therefore, even if a spacecraft is designed to reach speeds exceeding 300,000 kilometers per second, in actual flight, due to the problem of increasing mass, it ultimately cannot overcome the speed of approximately 300,000 kilometers per second. This is why the claim that nothing in the universe can exceed the speed of light holds true. However, one must be cautious about the fact that an object with mass cannot exceed the speed of light. There should be no misunderstanding regarding this point, and as of today, no massive object has been observed to exceed the speed of light.

Particles have not been discovered. Nevertheless, the reason why the speed of light is absolute is that as an object with mass approaches the speed of light, its mass increases and time slows down, preventing it from exceeding 300,000 kilometers per second. Furthermore, when you understand the truth about the speed of light, the concepts of present and future become intertwined, and many fascinating phenomena occur. I will explain these in detail in a separate lecture on special relativity, where I will elaborate on why time slows down and provide explanations for why space contracts. So, with that said, I will conclude today’s lecture here. Einstein's remarkable assertion about the speed of light has had a profound impact not only on the scientific community but also on the field of modern philosophy.

In particular, there is a fascinating discussion by the French philosopher Henri Louis Bergson about duration and simultaneity in relation to the theory of special relativity. This is truly interesting. Therefore, on our channel, we will continuously delve into such topics. To start, we lightly addressed the measurement of the speed of light and the principle of invariance, so please keep this in mind. I will greet you again next time. Thank you.

빛의 속도는 왜 불변이며 절대적인가? - 광속 불변의 원리

🔔[지혜의 빛 : 인문학의 숲]

Philosophy는 "지혜를 사랑한다."는
뜻을  가지고 있습니다.  
그리고 이 어휘에  대한 동아시아의  
번역어는 "철학(哲學)"인데,  여기서
'철(哲)'은 기본적으로  '밝음'을 뜻함
과  동시에   '지혜로움'을   뜻하기도
합니다. 따라서 우리 채널 이름은 지
혜로  세상을  밝게  한다는  의미의  
“지혜의 빛"이며, 이와 관련된 인문 
콘텐츠들로 인문학의 숲을 이루고
자 합니다. 이러한 우리 채널에 대
해 많은 관심 부탁드릴게요.🖋

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