The lights are accurate. What is the speed of light

Although in everyday life it is rare for anyone to directly calculate what the speed of light is, interest in this issue manifests itself in childhood. Surprisingly, we all encounter the sign of the speed constant of propagation of electromagnetic waves every day. The speed of light is a fundamental quantity due to which the entire Universe exists exactly as we know it.

Surely, everyone, watching in childhood a flash of lightning and the subsequent clap of thunder, tried to understand what caused the delay between the first and second phenomenon. Simple mental reasoning quickly led to a logical conclusion: the speed of light and sound are different. This is the first acquaintance with two important physical quantities. Subsequently someone received necessary knowledge and could easily explain what was happening. What causes the strange behavior of thunder? The answer is that the speed of light, which is about 300 thousand km/s, is almost a million times higher than the speed of propagation in air (330 m/s). Therefore, a person first sees from lightning and only after a while hears the roar of thunder. For example, if there is 1 km from the epicenter to the observer, then light will cover this distance in 3 microseconds, but sound will take as much as 3 s. Knowing the speed of light and the delay time between the flash and thunder, you can calculate the distance.

Attempts to measure it have been made for a long time. Now it’s quite funny to read about the experiments being carried out, however, in those distant times, before the advent of precision instruments, everything was more than serious. In attempts to find out what the speed of light is, one study was carried out interesting experience. At one end of the car of a fast-moving train there was a man with a precise chronometer, and on the opposite side his assistant on the team opened the lamp shutter. According to the idea, the chronometer was supposed to make it possible to determine the speed of propagation of photons of light. Moreover, by changing the positions of the lamp and the chronometer (while maintaining the direction of movement of the train), it would be possible to find out whether the speed of light is constant, or whether it can be increased/decreased (depending on the direction of the beam, theoretically, the speed of the train could influence the speed measured in the experiment ). Of course, the experiment was a failure, since the speed of light and registration by a chronometer are incomparable.

For the first time, the most accurate measurement was made in 1676 thanks to observations of Olaf Roemer noticed that the actual appearance of Io and the calculated data differed by 22 minutes. As the planets got closer, the delay decreased. Knowing the distance, it was possible to calculate the speed of light. It was about 215 thousand km/s. Then, in 1926, D. Bradley, while studying changes in the apparent positions of stars (aberration), drew attention to a pattern. The location of the star changed depending on the time of year. Consequently, the position of the planet relative to the Sun had an influence. An analogy can be given - raindrops. Without wind, they fly vertically downwards, but as soon as they run, their apparent trajectory changes. Knowing the speed of rotation of the planet around the Sun, it was possible to calculate the speed of light. It amounted to 301 thousand km/s.

In 1849, A. Fizeau conducted the following experiment: between a light source and a mirror, 8 km away, there was a rotating one. The speed of its rotation was increased until in the next gap the flow of reflected light turned into a constant (non-flicker). Calculations gave 315 thousand km/s. Three years later, L. Foucault used a rotating mirror and received 298 thousand km/s.

Subsequent experiments became more and more accurate, taking into account refraction in air, etc. Currently, data obtained using cesium clocks and laser beam. According to them, it is equal to 299 thousand km/s.

Really, how? How to measure the highest speed in Universe in our modest, Earthly conditions? We no longer need to rack our brains over this - after all, over several centuries, so many people have worked on this issue, developing methods for measuring the speed of light. Let's start the story in order.

Speed ​​of light– speed of propagation of electromagnetic waves in vacuum. It is denoted by the Latin letter c. The speed of light is approximately 300,000,000 m/s.

At first, no one thought about the issue of measuring the speed of light. There is light - that’s great. Then, in the era of antiquity, the prevailing opinion among scientific philosophers was that the speed of light is infinite, that is, instantaneous. Then it happened Middle Ages with the Inquisition, when the main question of thinking and progressive people was “How to avoid getting caught in the fire?” And only in epochs Renaissance And Enlightenment The opinions of scientists multiplied and, of course, were divided.

So, Descartes, Kepler And Farm were of the same opinion as the scientists of antiquity. But he believed that the speed of light is finite, although very high. In fact, he made the first measurement of the speed of light. More precisely, he made the first attempt to measure it.

Galileo's experiment

Experience Galileo Galilei was brilliant in its simplicity. The scientist conducted an experiment to measure the speed of light, armed with simple improvised means. At a large and well-known distance from each other, on different hills, Galileo and his assistant stood with lit lanterns. One of them opened the shutter on the lantern, and the second had to do the same when he saw the light of the first lantern. Knowing the distance and time (the delay before the assistant opens the lantern), Galileo expected to calculate the speed of light. Unfortunately, for this experiment to succeed, Galileo and his assistant had to choose hills that were several million kilometers apart. I would like to remind you that you can by filling out an application on the website.

Experiments by Roemer and Bradley

The first successful and surprisingly accurate experiment in determining the speed of light was that of a Danish astronomer Olaf Roemer. Roemer used the astronomical method of measuring the speed of light. In 1676, he observed Jupiter's satellite Io through a telescope, and discovered that the time of the eclipse of the satellite changes as the Earth moves away from Jupiter. The maximum delay time was 22 minutes. Having considered that the Earth is moving away from Jupiter by the distance of the diameter of the Earth's orbit, Roemer divided approximate value diameter by the delay time, and received a value of 214,000 kilometers per second. Of course, such a calculation was very rough, the distances between the planets were known only approximately, but the result turned out to be relatively close to the truth.

Bradley's experience. In 1728 James Bradley estimated the speed of light by observing the aberration of stars. Abberation is a change in the apparent position of a star caused by the movement of the earth in its orbit. Knowing the speed of the Earth and measuring the aberration angle, Bradley obtained a value of 301,000 kilometers per second.

Fizeau's experience

As a result of the experience of Roemer and Bradley, the then scientific world reacted with disbelief. However, Bradley's result was the most accurate for over a hundred years, right up until 1849. That year, a French scientist Armand Fizeau measured the speed of light using the rotating shutter method, without observing celestial bodies, but here on Earth. In fact, this was the first laboratory method for measuring the speed of light since Galileo. Below is a diagram of its laboratory setup.

The light, reflected from the mirror, passed through the teeth of the wheel and was reflected from another mirror, 8.6 kilometers away. The speed of the wheel was increased until the light became visible in the next gap. Fizeau's calculations gave a result of 313,000 kilometers per second. A year later, a similar experiment with a rotating mirror was carried out by Leon Foucault, who obtained a result of 298,000 kilometers per second.

With the advent of masers and lasers, people have new opportunities and ways to measure the speed of light, and the development of the theory also made it possible to calculate the speed of light indirectly, without making direct measurements.

The most accurate value of the speed of light

Humanity has accumulated vast experience in measuring the speed of light. By far the most exact value the speed of light is considered to be 299,792,458 meters per second, received in 1983. It is interesting that further, more accurate measurement of the speed of light turned out to be impossible due to errors in the measurement meters. Currently, the value of a meter is tied to the speed of light and is equal to the distance that light travels in 1/299,792,458 of a second.

Finally, as always, we suggest watching an educational video. Friends, even if you are faced with such a task as independently measuring the speed of light using improvised means, you can safely turn to our authors for help. You can fill out an application on the Correspondence Student website. We wish you a pleasant and easy study!

Artistic representation spaceship, making the jump to the "speed of light". Credit: NASA/Glenn Research Center.

Since ancient times, philosophers and scientists have sought to understand light. In addition to trying to determine its basic properties (i.e. whether it is a particle or a wave, etc.), they also sought to make finite measurements of how fast it moves. Since the late 17th century, scientists have been doing just that, and with increasing precision.

By doing this, they gained a better understanding of the mechanics of light, and how important role he plays in physics, astronomy and cosmology. Simply put, light travels at incredible speeds and is the fastest moving object in the universe. Its speed is a constant and impenetrable barrier and is used as a measure of distance. But how fast is it moving?

Speed ​​of light (s):

Light moves at a constant speed of 1,079,252,848.8 km/h (1.07 billion). Which turns out to be 299,792,458 m/s. Let's put everything in its place. If you could move at the speed of light, you could go around globe approximately seven and a half times per second. Meanwhile, a man flying from average speed 800 km/h, it would take more than 50 hours to circumnavigate the planet.

An illustration showing the distance light travels between the Earth and the Sun. Credit: LucasVB/Public Domain.

Let's look at this from an astronomical point of view, the average distance from to 384,398.25 km. Therefore, light travels this distance in about a second. Meanwhile, the average is 149,597,886 km, which means it only takes about 8 minutes for light to make this journey.

It's no wonder then why the speed of light is the metric used to determine astronomical distances. When we say that a star such as , is 4.25 light years away, we mean that traveling at a constant speed of 1.07 billion km/h would take about 4 years and 3 months to get there. But how did we arrive at this very specific value for the speed of light?

History of study:

Until the 17th century, scientists were confident that light traveled at a finite speed, or instantaneously. From the time of the ancient Greeks to medieval Islamic theologians and modern scholars, there has been debate. But until the work of the Danish astronomer Ole Roemer (1644-1710) appeared, in which the first quantitative measurements were carried out.

In 1676, Römer observed that the periods of Jupiter's innermost moon Io appeared shorter when the Earth was approaching Jupiter than when it was moving away. From this he concluded that light travels at a finite speed and is estimated to take about 22 minutes to cross the diameter of the Earth's orbit.

Professor Albert Einstein at the 11th Josiah Willard Gibbs Lecture at the Carnegie Institute of Technology on December 28, 1934, where he explains his theory that matter and energy are the same thing in different forms. Credit: AP Photo

Christiaan Huygens used this estimate and combined it with an estimate of the diameter of the Earth's orbit to arrive at an estimate of 220,000 km/s. Isaac Newton also spoke about Roemer's calculations in his seminal work"Optics" 1706. By adjusting for the distance between the Earth and the Sun, he calculated that light would take seven or eight minutes to travel from one to the other. In both cases there was a relatively small error.

Later measurements by French physicists Hippolyte Fizeau (1819-1896) and Léon Foucault (1819-1868) refined these figures, leading to a value of 315,000 km/s. And by the second half of the 19th century, scientists became aware of the connection between light and electromagnetism.

This was achieved by physicists by measuring electromagnetic and electrostatic charges. They then discovered that the numerical value was very close to the speed of light (as measured by Fizeau). Based on his own work, which showed that electromagnetic waves propagate in empty space, the German physicist Wilhelm Eduard Weber proposed that light was an electromagnetic wave.

The next big breakthrough came at the beginning of the 20th century. In his paper entitled “On the Electrodynamics of Moving Bodies,” Albert Einstein states that the speed of light in a vacuum, measured by an observer having constant speed, is the same in all inertial frames of reference and is independent of the motion of the source or the observer.

A laser beam shining through a glass of water shows how many changes it undergoes as it passes from air to glass to water and back to air. Credit: Bob King.

Taking this statement and Galileo's principle of relativity as a basis, Einstein derived special theory relativity, in which the speed of light in vacuum (c) is a fundamental constant. Prior to this, the agreement among scientists was that space was filled with a “luminiferous ether”, which was responsible for its propagation - i.e. light moving through a moving medium will trail in the tail of the medium.

This in turn means that the measured speed of light would be the simple sum of its speed through a medium plus the speed of that medium. However, Einstein's theory rendered the concept of a stationary ether useless and changed the concept of space and time.

Not only did it advance the idea that the speed of light is the same in all inertial frames, but it also suggested that major changes occur when things move close to the speed of light. These include the space-time frame of a moving body appearing to slow down, and the direction of motion when the measurement is from the observer's point of view (i.e., relativistic time dilation, where time slows down as it approaches the speed of light).

His observations also agree with Maxwell's equations for electricity and magnetism with the laws of mechanics, simplify mathematical calculations by avoiding the unrelated arguments of other scientists, and are consistent with direct observation of the speed of light.

How similar are matter and energy?

In the second half of the 20th century, increasingly precise measurements using laser interferometers and resonant cavities further refined estimates of the speed of light. By 1972, a group at the US National Bureau of Standards in Boulder, Colorado, used laser interferometry to arrive at the currently accepted value of 299,792,458 m/s.

Role in modern astrophysics:

Einstein's theory that the speed of light in a vacuum does not depend on the movement of the source and the inertial frame of reference of the observer has since been invariably confirmed by many experiments. It also sets an upper limit on the speed at which all massless particles and waves (including light) can travel in a vacuum.

One result of this is that cosmologies now view space and time as a single structure known as spacetime, in which the speed of light can be used to determine the value of both (i.e. light years, light minutes and light seconds). Measuring the speed of light can also be an important factor in determining the acceleration of the expansion of the Universe.

In the early 1920s, with the observations of Lemaître and Hubble, scientists and astronomers became aware that the Universe was expanding from its point of origin. Hubble also noticed that the further away a galaxy is, the faster it moves. What is now called the Hubble constant is the speed at which the Universe is expanding, it is equal to 68 km/s per megaparsec.

How fast is the Universe expanding?

This phenomenon, presented as a theory, means that some galaxies may actually be moving. faster speed light, which could place limits on what we observe in our Universe. Essentially, galaxies traveling faster than the speed of light would cross the "cosmological event horizon" where they are no longer visible to us.

Additionally, by the 1990s, measurements of the redshift of distant galaxies showed that the expansion of the Universe has been accelerating over the past few billion years. This led to the theory of "Dark Energy", where invisible force drives the expansion of space itself, rather than objects moving through it (without placing a limit on the speed of light or breaking relativity).

Along with special and general theory relativity modern meaning the speed of light in a vacuum was formed from cosmology, quantum mechanics and Standard model physicists elementary particles. It remains constant when it comes to the upper limit at which massless particles can move and remains an unattainable barrier for particles with mass.

We will probably someday find a way to exceed the speed of light. We don't have yet practical ideas on how this could happen, it looks like the "smart money" on technology will allow us to bypass the laws of spacetime, either by creating warp bubbles (aka. Alcubierre warp drive) or tunneling through it (aka. wormholes).

What are wormholes?

Until then, we will simply have to be content with the Universe we see, and stick to exploring the part that can be reached using conventional methods.

Title of the article you read "What is the speed of light?".

The speed of light is the distance that light travels per unit time. This value depends on the substance in which the light propagates.

In a vacuum, the speed of light is 299,792,458 m/s. This is the highest speed that can be achieved. When solving problems that do not require special accuracy, this value is taken equal to 300,000,000 m/s. It is assumed that all types of electromagnetic radiation propagate in a vacuum at the speed of light: radio waves, infrared radiation, visible light, ultraviolet radiation, x-rays, gamma radiation. It is designated by a letter With .

How was the speed of light determined?

In ancient times, scientists believed that the speed of light was infinite. Later, discussions on this issue began among scientists. Kepler, Descartes and Fermat agreed with the opinion of ancient scientists. And Galileo and Hooke believed that, although the speed of light is very high, it still has a finite value.

Galileo Galilei

One of the first to try to measure the speed of light was the Italian scientist Galileo Galilei. During the experiment, he and his assistant were on different hills. Galileo opened the shutter on his lantern. At the moment when the assistant saw this light, he had to do the same actions with his lantern. The time it took the light to travel from Galileo to the assistant and back turned out to be so short that Galileo realized that the speed of light is very high, and it is impossible to measure it at such a short distance, since light travels almost instantly. And the time he recorded only shows the speed of a person’s reaction.

The speed of light was first determined in 1676 by the Danish astronomer Olaf Roemer using astronomical distances. Using a telescope to observe the eclipse of Jupiter's moon Io, he discovered that as the Earth moves away from Jupiter, each subsequent eclipse occurs later than calculated. The maximum delay, when the Earth moves to the other side of the Sun and moves away from Jupiter at a distance equal to the diameter of the Earth's orbit, is 22 hours. Although the exact diameter of the Earth was not known at that time, the scientist divided its approximate value by 22 hours and obtained a value of about 220,000 km/s.

Olaf Roemer

The result obtained by Roemer caused distrust among scientists. But in 1849, the French physicist Armand Hippolyte Louis Fizeau measured the speed of light using the rotating shutter method. In his experiment, light from a source passed between the teeth of a rotating wheel and was directed onto a mirror. Reflected from him, he returned back. The speed of rotation of the wheel increased. When it reached a certain value, the beam reflected from the mirror was delayed by a moving tooth, and the observer did not see anything at that moment.

Fizeau's experience

Fizeau calculated the speed of light as follows. The light goes its way L from the wheel to the mirror in a time equal to t 1 = 2L/c . The time it takes for the wheel to turn ½ slot is t 2 = T/2N , Where T - period of wheel rotation, N - number of teeth. Rotational speed v = 1/T . The moment when the observer does not see light occurs when t 1 = t 2 . From here we get the formula for determining the speed of light:

c = 4LNv

Having carried out calculations using this formula, Fizeau determined that With = 313,000,000 m/s. This result was much more accurate.

Armand Hippolyte Louis Fizeau

In 1838, French physicist and astronomer Dominique François Jean Arago proposed using the rotating mirror method to calculate the speed of light. This idea was put into practice by the French physicist, mechanic and astronomer Jean Bernard Leon Foucault, who in 1862 obtained the value of the speed of light (298,000,000±500,000) m/s.

Dominique Francois Jean Arago

In 1891, the result of the American astronomer Simon Newcomb turned out to be an order of magnitude more accurate than Foucault's result. As a result of his calculations With = (99,810,000±50,000) m/s.

Research by the American physicist Albert Abraham Michelson, who used a setup with a rotating octagonal mirror, made it possible to determine the speed of light even more accurately. In 1926, the scientist measured the time it took light to travel the distance between the tops of two mountains, equal to 35.4 km, and obtained With = (299,796,000±4,000) m/s.

The most accurate measurement was carried out in 1975. In the same year, the General Conference on Weights and Measures recommended that the speed of light be considered equal to 299,792,458 ± 1.2 m/s.

What does the speed of light depend on?

The speed of light in a vacuum does not depend on the frame of reference or on the position of the observer. It remains constant, equal to 299,792,458 ± 1.2 m/s. But in various transparent media this speed will be lower than its speed in vacuum. Any transparent medium has an optical density. And the higher it is, the slower the speed of light propagates in it. For example, the speed of light in air is higher than its speed in water, and in pure optical glass it is less than in water.

If light moves from a less dense medium to a denser one, its speed decreases. And if the transition occurs from a more dense medium to a less dense one, then the speed, on the contrary, increases. This explains why the light beam is deflected at the transition boundary between two media.

Light is one of the key concepts of optical physics. Light represents electromagnetic radiation, accessible to the human eye.

For many decades, the best minds struggled with the problem of determining at what speed light moves and what it is equal to, as well as all the calculations that accompany it. In 1676, a revolution occurred among physicists. A Danish astronomer named Ole Roemer refuted the claim that light travels through the universe at unlimited speed.

In 1676, Ole Roemer determined that the speed of light in a vacuum is 299792458 m/s.

For convenience, this figure began to be rounded. The nominal value of 300,000 m/s is still used today.

This rule, under normal conditions for us, applies to all objects without exception, including x-rays, light and gravitational waves of the spectrum that is tangible to our eyes.

Modern physicists studying optics have proven that the speed of light has several characteristics:

• constancy;
• unattainability;
• limb.

Speed ​​of light in different media

It should be remembered that the physical constant directly depends on its environment, especially on the refractive index. In this regard, the exact value can change, because it is determined by frequencies.

The formula for calculating the speed of light is written as s = 3 * 10^8 m/s.

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The speed of light in water differs from the same speed in vacuum. To find out its value, you need to divide the number 299,792,458 by 1.33. The result will be a number 225407 km/s- this is the speed of light propagation in water.

The speed of light in air in km is 1,079,252,848.8 (or 299,700 km/sec). To find it, you need to divide the speed of light in a vacuum by the refractive index of air. The answer can be displayed in either kilometers per hour or meters per second.

Is the speed of light the maximum possible speed?

Many schoolchildren and students wonder: what speed is greater than the speed of light? Is there such a thing at all? The answer is clear: no!

The speed of light propagation in a vacuum is considered an unattainable value. Scientists have not come to a consensus on what can happen to atoms that reach this limit.

Among other things, the researchers found that a particle with mass can approach the speed of a light beam. But she cannot catch up with it, much less exceed it. The maximum speed of light remains unchanged for now.

The closest numerical indicator was achieved in the study cosmic rays. They were accelerated in specially equipped particle accelerators, taking into account the wavelength.

Why is this number so important? The fact is that vacuum envelops everything outer space. Knowing how light behaves in a vacuum, we can imagine what the maximum speed of travel in our Universe is.

Why is it impossible to travel faster than light?

So why is the CPC constant unable to be overcome under normal conditions? Based on the theory, we can safely say that in a situation of excess it will be violated fundamental law the construction of the world, to be specific - the law of causality. According to this law, the effect cannot get ahead of its cause.

Let's consider this paradox at specific example: It cannot happen that the deer first falls dead, and only then the hunter shoots, killing him. So, when the SRS increases, the unfolding actions should begin in the reverse order. As a result, time must go backwards, and this contradicts all established laws of physics.

Einstein and vacuum: final calculation results

Currently, most people on the planet know that the maximum permissible value for the movement of material objects and various signals is the speed of light in a vacuum. Who was the first to think of this?

The idea that it is impossible to exceed the speed of light was expressed by the great physicist Albert Einstein. He formalized his observations and called them the theory of relativity.

Einstein's greatest theory is still unshakable. It will remain so until real evidence is presented that it is possible to transmit a signal at a speed exceeding the SPC in a vacuum. This moment may never come.

However, several studies have already been conducted that foreshadow a disagreement with some points of Einstein's most famous theory. Measuring superluminal speeds is already possible under given conditions. It is noteworthy that the theory of relativity is not completely violated.