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  5. What is the speed of light? All you need to know about the speed limit of the universe


26.07.2022 21:10

What is the speed of light? All you need to know about the speed limit of the universe


The speed of light is called the most important value in the universe for a reason. Although such a complex issue is difficult to grasp with the mind, it is definitely worth trying – it is the key to understanding many rules of the reality around us.

What is the speed of light and how fast is it?

The speed of light determines the speed of propagation of an electromagnetic wave in a vacuum or material medium. This value, measured in a vacuum, is the highest, maximum impact velocity known to man – it is 299 792 458 m/s. Under these conditions, it is also a physical constant (marked with the symbol c). This term means that the speed of light in a vacuum is constant regardless of changes in inertial frames of reference or, for example, the frequency of the wave.

speed of light

Albert Einstein made a major contribution to our modern perception of the speed of light. In one of the postulates, he noticed that the speed of light is constant and equals c. The state is to be true for any frame of reference. Despite many studies, to this day we cannot obtain any result that would challenge the theory of the genius scientist, so it seems that the speed of light as a physical constant will stay with us for much longer.

What determines the speed of light?

Groups of particles (photons) that carry electromagnetic waves always move at the same speed and it is impossible for them to travel faster or slower than 299,792,458 m / s. However, if we emit a beam of light in any material medium, the speed of the electromagnetic wave will always be less than that of the same beam traveling through a vacuum this time. Why is it like that?

It is enough for the wave to hit even the weakest obstacle on its path for its phase velocity to drop immediately, sometimes to relatively low levels. Light moving in material centers causes their electric charges to vibrate. When the oscillations of the charges of the medium return to their original state, they release waves lagging behind the waves that were delivered to the medium first. The perception of light slowing down is the result of this property of this environment – the breakthrough of electromagnetic waves through a specific particle filter in material media is a much more time-consuming process, even though the photons themselves always move at the speed of light.

What determines the speed of light?

The results of measuring the speed of light we get also largely depend on the method of this measurement used. Electromagnetic waves travel so fast that it is impossible to find any effective method based solely on the human senses. Therefore, various more advanced tools, such as a modulated light detector, are used to measure the speed of light in a vacuum and in a material medium.

Can the light be overtaken?

Authors of science-fiction literature have been eagerly using the motif of superlumination propulsion in their works, which, for example, used in space travel, allows astronauts to find themselves in a distant part of space in a very short time. However, the reality established by the laws of physics makes us include this element more in fiction than in science. So is it possible to outrun the light? Any attempt to answer this question should be preceded by recalling information about the elementary properties of the light-electromagnetic wave itself. It is carried by photons – unique particles that have zero rest mass. Among other things, it is because of this property and the wave-particle nature of light that it can reach speeds as dizzying as 299 792 458 m / s.

Can you get ahead of the light?

The problem arises at the point where we would like to give this pace to any object that has a non-zero rest mass. To accelerate an object to this speed (whether it be a tiny particle or a huge spaceship), it would take an infinite amount of energy. This is because an increase in speed is associated with an increase in the mass of the object, which in turn means that it moves with increasing resistance to further acceleration. Therefore, this condition leaves no doubt – in the case of the speed of light propagation in a vacuum, we have a value that cannot be exceeded for any object that has even the smallest mass. Propelling particles other than photons into it would break the laws of physics.

Can you imagine the speed of light in a vacuum?

The answer to that question is: it depends. Nowadays, we do not have to imagine it in any way, because we have proven, very accurate methods of measuring it and we know that it is 299 792 458 m / s. In order to grasp with the mind such a great value as the speed of light, it may be necessary to find a reference point that will help us visually imagine it. Here is an example of such a picture.

In the scale of the immensity of the universe, even a speed as great as that at which light travels seems small. Its beams constantly travel huge distances between cosmic objects and it is on their basis that astronomers are able to accurately estimate such parameters of celestial bodies as, for example, the direction and speed of their movement. The need to travel long distances by electromagnetic waves to reach an observer on Earth means that a long time can pass between the emission of the wave by an object and noticing light on our planet.


This phenomenon is noticeable even in the case of the Sun so close to us (on the cosmic scale). If the central star of our system suddenly exploded, people on Earth would not be able to observe it for… 8 minutes after the explosion. This proportion increases significantly for objects distant from our home planet by tens, thousands or more light years. When we look at the sky at night, we see the stars as they were extremely long ago.

History of the most important constant in the universe. How was the speed of light studied before the age of modern technology?

The exact speed of light remained a mystery for a long time. Until our times, no information about measuring it by people who lived before the 17th century has survived, so it is difficult to say whether such attempts were made at all in ancient times.

Rather, observing the world gave people reason to believe that the speed of light is infinite. Such assumptions would be hardly surprising – just such conclusions were drawn by our distant ancestors, for whom the universe was characterized by a much smaller possible scale of observations. The lack of perceptible delays between emitting light (for example, by lighting a torch) and noticing it, led ancient people to suppose that they were dealing with a value unlimited by any known rule or limit.

Galileo and the first step on the right path

The earliest known attempt to measure the speed of light was undertaken by the Italian physicist and astronomer Galileo, born in the 16th century. He decided to confront the reality of the universal theory of infinite value by means of an experiment. He and his companion climbed the peaks of two distant hills, lanterns in their hands. The unveiling of the lantern by one of the physicists was a signal for the other to do the same immediately. The researcher who was discovering the lighthouse at that time was to measure how long it took for him to uncover the light source until he got an answer from his colleague.

Galileo and the first step on the right path
Source: polimaty.pl

The conclusions that Galileo drew from his experiment were not at all incorrect, although very imprecise from today’s perspective. He managed to estimate the speed of the movement of light – according to the astronomer, it would be more than 30 kilometers per hour. Although we now know that this value, even in material centers, is many thousands of times greater, and that a thorough examination of the speed of light by any of the methods available to modern Galileo would be impossible, the astronomer’s pioneering efforts are definitely worth appreciating.

How Jupiter helped estimating the speed of light?

Another breakthrough in the world of physics in the measurement of this value came at the end of the 17th century. The Danish scientist Ole Rømer presented in 1676 evidence that the speed of light was actually finite. He was able to draw this conclusion from observations of one of Jupiter’s moons and the different frequencies of its eclipses. They occurred more frequently as Jupiter approached Earth and less frequently as Jupiter moved away from it. The scientist explained this phenomenon by the longer path that light from Jupiter had to travel before it was delivered to an observer on Earth. Based on Rømer’s observations, the Dutch physics researcher Christiaan Huygens then estimated the speed of light in vacuum and air at 214,300 km / s.

Development of mechanical methods of determining the speed of light

In the 18th century, there was a development of measuring the speed of light in laboratory conditions with the use of various devices. It is worth mentioning, for example, the rotating mirror method (used for the first time by Jean Foucault), which determines the speed of light based on the shift of the image falling on a rotating mirror.

Measurement errors are not the only problems that physics researchers have had to deal with over the years in order to get even closer to discovering the extraordinary properties of the speed of light. Today, taking advantage of the opportunities offered by the latest technologies, we skilfully evaluate and apply this unique value among physical constants. Can we learn more about the speed of light? It is hard to say, but we are eagerly awaiting the next breakthroughs in physical research on it.

Related: Flat Earth – pseudoscience in the offensive. What is the shape of our planet?



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