What is the speed of light? Why the speed of light is constant on your fingers™

> Speed ​​of light

Find out which speed of light in a vacuum is a fundamental constant in physics. Read what the speed of light propagation m/s is equal to, the law, the measurement formula.

Speed ​​of light in vacuum– one of the fundamental constants in physics.

Learning Objective

• Compare the speed of light with the refractive index of the medium.

Main points

• The maximum possible indicator of the speed of light is light in a vacuum (unchanged).
• C is the symbol for the speed of light in a vacuum. Reaches 299,792,458 m/s.
• When light enters a medium, its speed slows down due to refraction. Calculated using the formula v = c/n.

Terms

• Special speed of light: reconciling the principle of relativity and the constancy of light speed.
• Refractive index is the ratio of the speed of light in air/vacuum to another medium.

Speed ​​of light

The speed of light acts as a point of comparison to define something as extremely fast. But what is it?

The light beam moves from the Earth to the Moon in the time period required for the passage of a light pulse - 1.255 s at the average orbital distance

The answer is simple: we are talking about the speed of photons and light particles. What is the speed of light? The speed of light in a vacuum reaches 299,792,458 m/s. This is a universal constant applicable in various fields of physics.

Let's take the equation E = mc 2 (E is energy and m is mass). It is the mass-energy equivalent, using the speed of light to bind space and time. Here you can find not only an explanation for energy, but also identify obstacles to speed.

The wave speed of light in a vacuum is actively used for various purposes. For example, the special theory of relativity states that this is a natural speed limit. But we know that speed depends on the medium and refraction:

v = c/n (v is the actual speed of light passing through the medium, c is the speed of light in a vacuum and n is the refractive index). The refractive index of air is 1.0003, and the speed of visible light is 90 km/s slower than s.

Lorentz coefficient

Rapidly moving objects show certain characteristics that conflict with the position of classical mechanics. For example, long contacts and time are expanding. Usually these effects are minimal, but are more visible at such high speeds. The Lorentz coefficient (γ) is the factor where time expansion and length contraction occur:

γ = (1 - v 2 /c 2) -1/2 γ = (1 - v 2 /c 2) -1/2 γ = (1 - v 2 /c 2) -1/2.

At low speeds v 2 /c 2 approaches 0, and γ approximately = 1. However, when the speed approaches c, γ increases to infinity.

epigraph
The teacher asks: Children, what is the fastest thing in the world?
Tanechka says: The fastest word. I just said, you won’t come back.
Vanechka says: No, light is the fastest.
As soon as I pressed the switch, the room immediately became light.
And Vovochka objects: The fastest thing in the world is diarrhea.
I was once so impatient that I didn’t say a word
I didn’t have time to say anything or turn on the light.

Have you ever wondered why the speed of light is maximum, finite and constant in our Universe? This is a very interesting question, and right away, as a spoiler, I’ll give away the terrible secret of the answer to it - no one knows exactly why. The speed of light is taken, i.e. mentally accepted for a constant, and on this postulate, as well as on the idea that all inertial frames of reference are equal, Albert Einstein built his special theory of relativity, which has been pissing scientists off for a hundred years, allowing Einstein to stick his tongue out at the world with impunity and grin in his grave over the dimensions the pig that he planted on all of humanity.

But why, in fact, is it so constant, so maximum and so final, there is no answer, this is just an axiom, i.e. a statement taken on faith, confirmed by observations and common sense, but not logically or mathematically deducible from anywhere. And it is quite likely that it is not so true, but no one has yet been able to refute it with any experience.

I have my own thoughts on this matter, more on them later, but for now, let’s keep it simple, on your fingers™ I’ll try to answer at least one part - what does the speed of light mean “constant”.

No, I won’t bore you with thought experiments about what would happen if you turn on the headlights in a rocket flying at the speed of light, etc., that’s a little off topic now.

If you look in a reference book or Wikipedia, the speed of light in a vacuum is defined as a fundamental physical constant that exactly equal to 299,792,458 m/s. Well, that is, roughly speaking, it will be about 300,000 km/s, but if exactly right- 299,792,458 meters per second.

It would seem, where does such accuracy come from? Any mathematical or physical constant, whatever, even Pi, even the base of the natural logarithm e, even the gravitational constant G, or Planck’s constant h, always contain some numbers after the decimal point. In Pi, about 5 trillion of these decimal places are currently known (although only the first 39 digits have any physical meaning), the gravitational constant is today defined as G ~ 6.67384(80)x10 -11, and the constant Plank h~ 6.62606957(29)x10 -34 .

The speed of light in vacuum is smooth 299,792,458 m/s, not a centimeter more, not a nanosecond less. Want to know where this accuracy comes from?

It all started as usual with the ancient Greeks. Science, as such, in the modern sense of the word, did not exist among them. The philosophers of ancient Greece were called philosophers because they first invented some crap in their heads, and then, using logical conclusions (and sometimes real physical experiments), they tried to prove or disprove it. However, the use of real-life physical measurements and phenomena was considered by them to be “second-class” evidence, which cannot be compared with first-class logical conclusions obtained directly from the head.

The first person to think about the existence of light's own speed is considered to be the philosopher Empidocles, who stated that light is movement, and movement must have speed. He was objected to by Aristotle, who argued that light is simply the presence of something in nature, and that’s all. And nothing is moving anywhere. But that's something else! Euclid and Ptolemy generally believed that light is emitted from our eyes, and then falls on objects, and therefore we see them. In short, the ancient Greeks were as stupid as they could until they were conquered by the same ancient Romans.

In the Middle Ages, most scientists continued to believe that the speed of propagation of light was infinite, among them were, say, Descartes, Kepler and Fermat.

But some, like Galileo, believed that light had speed and therefore could be measured. The experiment of Galileo, who lit a lamp and gave light to an assistant located several kilometers from Galileo, is widely known. Having seen the light, the assistant lit his lamp, and Galileo tried to measure the delay between these moments. Naturally, he didn’t succeed, and in the end he was forced to write in his writings that if light has a speed, then it is extremely high and cannot be measured by human effort, and therefore can be considered infinite.

The first documented measurement of the speed of light is attributed to the Danish astronomer Olaf Roemer in 1676. By this year, astronomers, armed with the telescopes of that same Galileo, were actively observing the satellites of Jupiter and even calculated their rotation periods. Scientists have determined that the closest moon to Jupiter, Io, has a rotation period of approximately 42 hours. However, Roemer noticed that sometimes Io appears from behind Jupiter 11 minutes earlier than expected, and sometimes 11 minutes later. As it turned out, Io appears earlier in those periods when the Earth, rotating around the Sun, approaches Jupiter at a minimum distance, and lags behind by 11 minutes when the Earth is in the opposite place of the orbit, and therefore is further from Jupiter.

Stupidly dividing the diameter of the earth's orbit (and it was already more or less known in those days) by 22 minutes, Roemer received the speed of light 220,000 km/s, missing the true value by about a third.

In 1729, the English astronomer James Bradley, observing parallax(by a slight deviation in location) the star Etamin (Gamma Draconis) discovered the effect aberrations of light, i.e. a change in the position of the stars closest to us in the sky due to the movement of the Earth around the Sun.

From the effect of light aberration, discovered by Bradley, it can also be concluded that light has a finite speed of propagation, which Bradley seized upon, calculating it to be approximately 301,000 km/s, which is already within an accuracy of 1% of the value known today.

This was followed by all the clarifying measurements by other scientists, but since it was believed that light is a wave, and a wave cannot propagate on its own, something needs to be “excited,” the idea of ​​the existence of a “luminiferous ether” arose, the discovery of which the American failed miserably physicist Albert Michelson. He did not discover any luminiferous ether, but in 1879 he clarified the speed of light to 299,910±50 km/s.

Around the same time, Maxwell published his theory of electromagnetism, which means that the speed of light became possible not only to directly measure, but also to derive from the values ​​of electrical and magnetic permeability, which was done by clarifying the value of the speed of light to 299,788 km/s in 1907.

Finally, Einstein declared that the speed of light in a vacuum is a constant and does not depend on anything at all. On the contrary, everything else - adding velocities and finding the correct reference systems, the effects of time dilation and changes in distances when moving at high speeds and many other relativistic effects depend on the speed of light (because it is included in all formulas as a constant). In short, everything in the world is relative, and the speed of light is the quantity relative to which all other things in our world are relative. Here, perhaps, we should give the palm to Lorentz, but let’s not be mercantile, Einstein is Einstein.

The exact determination of the value of this constant continued throughout the 20th century, with each decade scientists found more and more numbers after decimal point at the speed of light, until vague suspicions began to arise in their heads.

Determining more and more accurately how many meters light travels in a vacuum per second, scientists began to wonder what it is we are measuring in meters? After all, in the end, a meter is just the length of some platinum-iridium stick that someone forgot in some museum near Paris!

And at first the idea of ​​​​introducing a standard meter seemed great. In order not to suffer with yards, feet and other oblique fathoms, the French in 1791 decided to take as a standard measure of length one ten-millionth of the distance from the North Pole to the equator along the meridian passing through Paris. They measured this distance with the accuracy available at that time, cast a stick from a platinum-iridium (more precisely, first brass, then platinum, and then platinum-iridium) alloy and put it in this very Parisian Chamber of Weights and Measures as a sample. The further we go, the more it turns out that the earth's surface is changing, the continents are deforming, the meridians are shifting, and by one ten-millionth part they have forgotten, and began to count as a meter the length of the stick that lies in the crystal coffin of the Parisian "mausoleum."

Such idolatry does not suit a real scientist, this is not Red Square (!), and in 1960 it was decided to simplify the concept of the meter to a completely obvious definition - the meter is exactly equal to 1,650,763.73 wavelengths emitted by the transition of electrons between the energy levels 2p10 and 5d5 of the unexcited isotope of the element Krypton-86 in a vacuum. Well, how much more clear?

This went on for 23 years, while the speed of light in a vacuum was measured with increasing accuracy, until in 1983, finally, even the most stubborn retrogrades realized that the speed of light is the most accurate and ideal constant, and not some kind of isotope of krypton. And it was decided to turn everything upside down (more precisely, if you think about it, it was decided to turn everything back upside down), now the speed of light With is a true constant, and a meter is the distance that light travels in a vacuum in (1/299,792,458) seconds.

The real value of the speed of light continues to be clarified today, but what is interesting is that with each new experiment, scientists do not clarify the speed of light, but the true length of the meter. And the more accurately the speed of light is found in the coming decades, the more accurate the meter we will eventually get.

And not vice versa.

Well, now let's get back to our sheep. Why is the speed of light in the vacuum of our Universe maximum, finite and constant? This is how I understand it.

Everyone knows that the speed of sound in metal, and in almost any solid body, is much higher than the speed of sound in air. This is very easy to check; just put your ear to the rail, and you will be able to hear the sounds of an approaching train much earlier than through the air. Why is that? It is obvious that the sound is essentially the same, and the speed of its propagation depends on the medium, on the configuration of the molecules from which this medium consists, on its density, on the parameters of its crystal lattice - in short, on the current state of the medium through which the sound transmitted.

And although the idea of ​​luminiferous ether has long been abandoned, the vacuum through which electromagnetic waves propagate is not absolutely absolute nothing, no matter how empty it may seem to us.

I understand that the analogy is somewhat far-fetched, but that’s true on your fingers™ same! Precisely as an accessible analogy, and in no way as a direct transition from one set of physical laws to others, I only ask you to imagine that the speed of propagation of electromagnetic (and in general, any, including gluon and gravitational) vibrations, just as the speed of sound in steel is “sewn into” the rail. From here we dance.

UPD: By the way, I invite “readers with an asterisk” to imagine whether the speed of light remains constant in a “difficult vacuum.” For example, it is believed that at energies of the order of temperature 10–30 K, the vacuum stops simply boiling with virtual particles, and begins to “boil away,” i.e. the fabric of space falls to pieces, Planck quantities blur and lose their physical meaning, etc. Would the speed of light in such a vacuum still be equal to c, or will this mark the beginning of a new theory of “relativistic vacuum” with corrections like Lorentz coefficients at extreme speeds? I don't know, I don't know, time will tell...

The speed of light in different media varies significantly. The difficulty is that the human eye does not see it in the entire spectral range. The nature of the origin of light rays has interested scientists since ancient times. The first attempts to calculate the speed of light were made as early as 300 BC. At that time, scientists determined that the wave propagated in a straight line.

Quick response

They managed to describe with mathematical formulas the properties of light and the trajectory of its movement. became known 2 thousand years after the first research.

What is luminous flux?

A light beam is an electromagnetic wave combined with photons. Photons are understood as the simplest elements, which are also called quanta of electromagnetic radiation. The luminous flux in all spectra is invisible. It does not move in space in the traditional sense of the word. To describe the state of an electromagnetic wave with quantum particles, the concept of the refractive index of an optical medium is introduced.

The light flux is transferred in space in the form of a beam with a small cross section. The method of movement in space is derived by geometric methods. This is a rectilinear beam, which, at the border with various media, begins to refract, forming a curvilinear trajectory. Scientists have proven that the maximum speed is created in a vacuum; in other environments, the speed of movement can vary significantly. Scientists have developed a system in which a light beam and a derived value are the basis for the derivation and reading of certain SI units.

Some historical facts

About 900 years ago, Avicena suggested that, regardless of the nominal value, the speed of light has a finite value. Galileo Galilei tried to experimentally calculate the speed of light. Using two flashlights, the experimenters tried to measure the time during which a light beam from one object would be visible to another. But such an experiment turned out to be unsuccessful. The speed was so high that they were unable to detect the delay time.

Galileo Galilei noticed that Jupiter had an interval between eclipses of its four satellites of 1320 seconds. Based on these discoveries, in 1676, Danish astronomer Ole Roemer calculated the speed of propagation of a light beam as 222 thousand km/sec. At that time, this measurement was the most accurate, but it could not be verified by earthly standards.

After 200 years, Louise Fizeau was able to calculate the speed of a light beam experimentally. He created a special installation with a mirror and a gear mechanism that rotated at high speed. The light flux was reflected from the mirror and returned back after 8 km. As the wheel speed increased, a moment arose when the gear mechanism blocked the beam. Thus, the speed of the beam was set at 312 thousand kilometers per second.

Foucault improved this equipment, reducing the parameters by replacing the gear mechanism with a flat mirror. His measurement accuracy turned out to be closest to the modern standard and amounted to 288 thousand meters per second. Foucault made attempts to calculate the speed of light in a foreign medium, using water as a basis. The physicist was able to conclude that this value is not constant and depends on the characteristics of refraction in a given medium.

A vacuum is a space free of matter. The speed of light in vacuum in the C system is designated by the Latin letter C. It is unattainable. No item can be overclocked to such a value. Physicists can only imagine what might happen to objects if they accelerate to such an extent. The speed of propagation of a light beam has constant characteristics, it is:

• constant and final;
• unattainable and unchangeable.

Knowing this constant allows us to calculate the maximum speed at which objects can move in space. The amount of propagation of a light beam is recognized as a fundamental constant. It is used to characterize space-time. This is the maximum permissible value for moving particles. What is the speed of light in a vacuum? The current value was obtained through laboratory measurements and mathematical calculations. She equal to 299.792.458 meters per second with an accuracy of ± 1.2 m/s. In many disciplines, including school ones, approximate calculations are used to solve problems. An indicator equal to 3,108 m/s is taken.

Light waves in the human visible spectrum and X-ray waves can be accelerated to readings approaching the speed of light. They cannot equal this constant, nor exceed its value. The constant was derived based on tracking the behavior of cosmic rays at the moment of their acceleration in special accelerators. It depends on the inertial medium in which the beam propagates. In water, the transmission of light is 25% lower, and in air it will depend on temperature and pressure at the time of calculations.

All calculations were carried out using the theory of relativity and the law of causality derived by Einstein. The physicist believes that if objects reach a speed of 1,079,252,848.8 kilometers/hour and exceed it, then irreversible changes will occur in the structure of our world and the system will break down. Time will begin to count down, disrupting the order of events.

The definition of meter is derived from the speed of a light ray. It is understood as the area that a light beam manages to travel in 1/299792458 of a second. This concept should not be confused with the standard. The meter standard is a special cadmium-based technical device with shading that allows you to physically see a given distance.

Artist's representation of a 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.

In doing so, they gained a better understanding of the mechanics of light, and how it plays an important role 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 travel at the speed of light, you could circle the globe about seven and a half times per second. Meanwhile, it would take a person flying at an average speed of 800 km/h 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 reported on Roemer's calculations in his seminal 1706 work Optics. 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, 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.

Using this statement and Galileo's principle of relativity as a basis, Einstein derived the special theory of relativity, in which the speed of light in a 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 than the speed of light, which could put a limit 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.

In addition, 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 an 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 relativity, the modern value for the speed of light in a vacuum has evolved from cosmology, quantum mechanics, and the Standard Model of particle physics. 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. While we have no practical ideas about how this might happen, it appears the "smart money" in technology will allow us to circumvent 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 limit on most highways is between 90 and 110 kilometers. Although there are no road signs in the vacuum of outer space, there is a speed limit there too - this is 1080000000 kilometers per hour.

The highest speed in nature

This is the fastest speed of light in nature. Scientists usually give the speed of light in kilometers per second - 300,000 kilometers per second. Light consists of photons. They are the ones who can fly at such crazy speeds.

Peculiar particles - photons

Scientists call photons particles. But these are very peculiar particles. They have no rest mass, that is, in the usual sense they have no weight. It is difficult to imagine something so real that would be pure energy and would not contain a single grain of matter. Photons are such a reality. compare the maximum speed of photons with those speeds that we are accustomed to consider high.

A spaceship flying at the speed of light would not have linear dimensions for an outside observer. Take, for example, the Pioneer rocket, built to fly beyond the solar system. So, leaving the solar system, the Pioneer had a speed of 60 kilometers per second. Not bad! He could cover the distance from New York to San Francisco in one and a half minutes. But compared to the speed of a photon at 300,000 kilometers per second, the speed of the Pioneer looks like a snail's pace. Or let's see how fast the Sun moves through space.

Related materials:

Why do the stars shine?

But in the time that you are reading this sentence, the Sun, Earth and the other eight planets of our solar system are rushing around the Milky Way, like carousel horses, at a speed of 230 kilometers per second (at the same time, we ourselves do not even notice that we are flying at such an incredible speed). But this enormous speed is very small compared to the speed of light and amounts to about one percent.

Speed ​​of light and objects

If you accelerate an ordinary object to about the speed of light, extraordinary adventures will begin to happen to it. When the body reaches such speeds, the observer will notice a change in the linear dimensions and mass of the object. Even time will begin to change. A spacecraft traveling at 90 percent of the speed of light will shrink in size by about half. As the speed increases, it will decrease more and more until, when it reaches the speed of light, it completely loses its linear dimensions.