29.09.2019

How was the speed of light measured and what is its real value. What is the speed of light

The speed of light is the absolute value of the propagation speed of electromagnetic waves in vacuum. In physics, it is traditionally denoted by the Latin letter "c" (pronounced as [tse]). The speed of light in vacuum is a fundamental constant, independent of the choice of inertial reference frame (ISR). It refers to the fundamental physical constants that characterize not just individual bodies, but the properties of space-time as a whole. According to modern concepts, the speed of light in vacuum is the limiting speed of particles and propagation of interactions. Also important is the fact that this value is absolute. This is one of the postulates of SRT.

In a vacuum (emptiness)

In 1977, it was possible to calculate the approximate speed of light, equal to 299,792,458 ± 1.2 m / s, calculated on the basis of a 1960 reference meter. On this moment consider that the speed of light in vacuum is a fundamental physical constant, by definition exactly equal to 299,792,458 m/s, or approximately 1,079,252,848.8 km/h. The exact value is due to the fact that since 1983 the standard of the meter has been the distance traveled by light in a vacuum in a time interval equal to 1/299,792,458 seconds. The speed of light is denoted by the letter c.

Michelson's fundamental experience for SRT showed that the speed of light in vacuum does not depend on the speed of the light source, nor on the speed of the observer. In nature, the speed of light propagates:

actual visible light

other types electromagnetic radiation(radio waves, x-rays, etc.)

From special theory relativity, it follows that the acceleration of particles having a rest mass to the speed of light is impossible, since this event would violate fundamental principle causation. That is, the excess of the speed of light by the signal, or the movement of mass at such a speed, is excluded. However, the theory does not exclude the motion of particles in space-time with superluminal speed. Hypothetical particles moving at superluminal speeds are called tachyons. Mathematically, tachyons easily fit into the Lorentz transformation - these are particles with an imaginary mass. The higher the speed of these particles, the less energy they carry, and vice versa, the closer their speed is to the speed of light, the greater their energy - just like the energy of ordinary particles, the energy of tachyons tends to infinity when approaching the speed of light. This is the most obvious consequence of the Lorentz transformation, which does not allow the particle to accelerate to the speed of light - it is simply impossible to give the particle an infinite amount of energy. It should be understood that, firstly, tachyons are a class of particles, and not just one kind of particles, and, secondly, no physical interaction can propagate faster speed Sveta. It follows from this that tachyons do not violate the principle of causality - they do not interact with ordinary particles in any way, and the difference between their velocities also does not exist. equal speed Sveta.

Ordinary particles moving slower than light are called tardyons. Tardions cannot reach the speed of light, but can only approach it as close as they like, since in this case their energy becomes infinitely large. All tardions have a rest mass, unlike massless photons and gravitons, which always move at the speed of light.

In Planck units, the speed of light in vacuum is 1, that is, light travels 1 Planck unit of length per unit of Planck time.

In a transparent environment

The speed of light in a transparent medium is the speed at which light travels in a medium other than vacuum. In a medium with dispersion, phase and group velocity are distinguished.

The phase velocity relates the frequency and wavelength of monochromatic light in a medium (λ=c/ν). This speed is usually (but not necessarily) less than c. The ratio of the phase speed of light in vacuum to the speed of light in a medium is called the refractive index of the medium. The group speed of light in an equilibrium medium is always less than c. However, in nonequilibrium media it can exceed c. In this case, however, the leading edge of the pulse still moves at a speed not exceeding the speed of light in vacuum.

Armand Hippolyte Louis Fizeau proved by experience that the movement of a medium relative to a light beam can also affect the speed of light propagation in this medium.

Denial of the postulate about the maximum speed of light

IN last years often there are reports that in the so-called quantum teleportation, the interaction propagates faster than the speed of light. For example, August 15, 2008 research group Dr. Nicolas Gisin of the University of Geneva, examining bound photon states separated by 18 km in space, allegedly showed that "the interaction between particles is carried out at a speed of about one hundred thousand times the speed of light." The so-called Hartmann's paradox - superluminal speed in the tunnel effect - was also discussed earlier.

Scientific analysis of the significance of these and similar results shows that in principle they cannot be used for superluminal transmission of any signal or movement of matter.

History of measurements of the speed of light

Ancient scientists, with rare exceptions, considered the speed of light to be infinite. In modern times, this issue became the subject of discussion. Galileo and Hooke assumed that it was finite, although very large, while Kepler, Descartes and Fermat still defended the infinity of the speed of light.

The first estimate of the speed of light was given by Olaf Römer (1676). He noticed that when the Earth and Jupiter are on opposite sides of the Sun, the eclipses of Jupiter's moon Io are delayed by 22 minutes compared to the calculations. From this he obtained a value for the speed of light of about 220,000 km/sec - inaccurate, but close to the true value. Half a century later, the discovery of aberration made it possible to confirm the finiteness of the speed of light and to refine its estimate.

The topic of how to measure, as well as what the speed of light is, has been of interest to scientists since antiquity. This is very fascinating topic, which from time immemorial has been the object of scientific disputes. It is believed that such a speed is finite, unattainable and constant. It is unattainable and constant, like infinity. However, it is finite. It turns out an interesting physical and mathematical puzzle. There is one solution to this problem. After all, the speed of light still managed to be measured.

In ancient times, thinkers believed that speed of light is an infinite quantity. The first estimate of this indicator was given in 1676. Olaf Remer. According to his calculations, the speed of light was approximately 220,000 km/s. It was not quite the exact value, but close to the true.

The finiteness and estimate of the speed of light were confirmed after half a century.

In the future, the scientist fizo It was possible to determine the speed of light from the time it takes the beam to travel the exact distance.

He set up an experiment (see figure), during which a beam of light departed from the source S, reflected by mirror 3, interrupted by toothed disk 2, and passed through the base (8 km). Then it was reflected by mirror 1 and returned to the disk. The light fell into the gap between the teeth and could be observed through eyepiece 4. The time it took for the beam to pass through the base was determined depending on the speed of rotation of the disk. The value obtained by Fizeau was: c = 313,300 km/s.

The speed of propagation of a beam in any particular medium is less than this speed in a vacuum. In addition, for different substances, this indicator takes various meanings. After few years Foucault replaced the disk with a rapidly rotating mirror. The followers of these scientists repeatedly used their methods and research schemes.

Lenses are the basis of optical devices. Do you know how it is calculated? You can find out by reading one of our articles.

And you can find information about how to set up an optical sight consisting of such lenses. Read our material and you will not have questions on the topic.

What is the speed of light in vacuum?

Most precise measurement the speed of light shows the figure 1,079,252,848.8 kilometers per hour or 299 792 458 m/s. This figure is valid only for conditions created in a vacuum.

But to solve problems, the indicator is usually used 300,000,000 m/s. In a vacuum, the speed of light in Planck units is 1. Thus, the energy of light travels 1 Planck unit of length in 1 unit of Planck time. If a vacuum is created in natural conditions, then X-rays, light waves of the visible spectrum and gravitational waves can move at such a speed.

There is an unequivocal opinion of scientists that particles that have mass can take a speed that is as close as possible to the speed of light. But they are not able to reach and exceed the indicator. The most high speed, close to the speed of light, was recorded in the study of cosmic rays and in the acceleration of certain particles in accelerators.

The value of the speed of light in any medium depends on the refractive index of this medium.

This indicator may be different for different frequencies. Precise measurement of the quantity is important for the calculation of other physical parameters. For example, to determine the distance during the passage of light or radio signals in optical location, radar, light ranging and other areas.

Modern scientists use different methods to determine the speed of light. Some experts use astronomical methods, as well as measurement methods using experimental techniques. An improved Fizeau method is often used. In this case, the gear wheel is replaced by a light modulator, which weakens or interrupts the light beam. The receiver here is a photoelectric multiplier or photocell. The light source can be a laser, which helps to reduce the measurement error. Determination of the speed of light the time base can be passed by direct or indirect methods, which also allow you to get accurate results.

What formulas are used to calculate the speed of light

The speed of light in a vacuum is an absolute value. Physicists designate it with the letter "c". This is fundamental and constant, which does not depend on the choice of reporting system and characterizes time and space as a whole. Scientists suggest that this speed is the limiting speed of particles.
Formula for the speed of light in vacuum:

c = 3 * 10^8 = 299792458 m/s

here c is the speed of light in vacuum.
Scientists have proven that speed of light in air almost equals the speed of light in vacuum. It can be calculated using the formula:

Light at all times occupied an important place in the survival of people and the creation of an advanced civilization that we see today. The speed of light throughout the history of human development has excited the minds of first philosophers and naturalists, and then scientists and physicists. This is the fundamental constant of the existence of our universe.

Many scientists in different times sought to find out what the propagation of light in various media is. Highest value for science was the calculation of the value that has the speed of light in a vacuum. This article will help you understand this issue and learn a lot of interesting things about how light behaves in a vacuum.

Light and the question of speed

light in modern physics plays a key role, because, as it turned out, it is impossible to overcome the value of its speed at this stage in the development of our civilization. It took many years to measure what the speed of light is. Before that, scientists have done a lot of research, trying to answer the most important question What is the speed of light in a vacuum?
At this point in time, scientists have proven that the speed of light (CPC) has the following characteristics:

she is constant;
she is unchanging;
she is unattainable;
she is finite.

Note! The speed of light at the current moment in the development of science is an absolutely unattainable value. Physicists have only some assumptions about what happens to an object that hypothetically reaches the value of the speed of propagation of a light flux in a vacuum.

Light speed

Why is it so important how fast light travels in a vacuum? The answer is simple. After all, the vacuum is in space. Therefore, having learned what digital indicator has the speed of light in a vacuum, we will be able to understand with what maximum possible speed it is possible to move through the expanses solar system and beyond.
The elementary particles that carry light in our universe are photons. And the speed with which light moves in a vacuum is considered an absolute value.

Note! SRS refers to the speed at which electromagnetic waves move. Interestingly, light simultaneously represents elementary particles (photons) and a wave. This follows from the corpuscular-wave theory. According to it, in certain situations, light behaves like a particle, and in others, like a wave.

At this point in time, the propagation of light in space (vacuum) is considered to be a fundamental constant, which does not depend on the choice of the used inertial frame of reference. This value refers to physical fundamental constants. In this case, the value of CPC characterizes in general the basic properties of the space-time geometry.
Modern ideas characterize CPC as a constant, which is the maximum allowable value for the movement of particles, as well as the propagation of their interaction. In physics, this quantity is denoted by the Latin letter "c".

History of the study of the issue

In ancient times, surprisingly, even ancient thinkers wondered about the propagation of light in our universe. Then it was believed that this is an infinite value. The first estimate of the physical phenomenon of the speed of light was given by Olaf Remer only in 1676. According to his calculations, the propagation of light was approximately 220 thousand km / s.

Note! Olaf Remer gave an approximate value, but, as it turned out later, not very far from the real one.

The correct value for the speed at which light travels in a vacuum was only determined half a century after Olaf Roemer. Could do it French physicist A.I.L. Fizeau by conducting a special experiment.

Fizeau experiment

He was able to measure it physical phenomenon by measuring the time it took the beam to travel through a defined and precisely measured area.
The experience looked like this:

the source S emitted a luminous flux;
it was reflected from the mirror (3);
after that, the luminous flux was interrupted by means of a toothed disk (2);
then it passed the base, the distance of which was 8 km;
after that, the light flux was reflected by the mirror (1) and went on its way back to the disk.

During the experiment, the light flux fell into the gaps between the teeth of the disk, and it could be observed through the eyepiece (4). Fizeau determined the time of passage of the beam from the speed of rotation of the disk. As a result of this experiment, he obtained the value c = 313,300 km/s.
But this is not the end of the research that has been devoted to this issue. The ultimate formula for calculating a physical constant came about thanks to many scientists, including Albert Einstein.

Einstein and vacuum: final results of the calculation

Today, every person on Earth knows that the maximum permissible value for the movement of material objects, as well as any signals, is considered to be the speed of light in vacuum. The exact value of this indicator is almost 300 thousand km / s. To be precise, the speed of light in vacuum is 299,792,458 m/s.
The theory that it is impossible to exceed given value, put forward by the famous physicist of the past Albert Einstein in his special theory of relativity or SRT.

Note! Einstein's theory of relativity is considered unshakable until the advent of real evidence the fact that signal transmission is possible at speeds exceeding the CPC in a vacuum.

Einstein's theory of relativity

But today, some researchers have discovered phenomena that may serve as a prerequisite for the fact that Einstein's SRT can be changed. With some specially given conditions it is possible to track the appearance of superluminal velocities. It is interesting that in this case the violation of the theory of relativity does not occur.

Why can't you move faster than light?

To date, there are some "pitfalls" in this issue. For example, why when normal conditions CPC constant cannot be overcome? According to the accepted theory, in this situation the fundamental principle of the structure of our world, namely, the law of causality, will be violated. He argues that the effect, by definition, is not able to outstrip its cause. Figuratively speaking, it cannot be that at first the bear falls dead, and only then the shot of the hunter who shot him will be heard. But if the CPC is exceeded, then events should begin to occur in the reverse order. As a result, time will begin its reverse run.

So what is the speed of propagation of a light beam?

After numerous studies, which were given in order to determine the exact value of what the CPC is, specific numbers were obtained. Today c = 1,079,252,848.8 km/h or 299,792,458 m/s. and in Planck units, this parameter is defined as one. This means that the energy of light travels 1 Planck unit of length in 1 unit of Planck time.

Note! These figures are valid only for the conditions that exist in a vacuum.

Constant value formula

But in physics for more easy way solving problems, a rounded value is used - 300,000,000 m / s.
This is the rule in normal conditions concerns all objects, as well as x-rays, gravitational and light waves of the spectrum visible to us. In addition, scientists have proven that particles with mass can approach the speed of a light beam. But they are unable to reach or exceed it.

Note! The maximum speed, close to the speed of light, was obtained in the study of cosmic rays accelerated in special accelerators.

It is worth noting that this physical constant depends on the medium in which it is measured, namely the refractive index. Therefore, its actual rate may vary depending on the frequencies.

How to calculate the value of a fundamental constant

To date, there are various methods for determining the SRS. It can be:

astronomical methods;
improved Fizeau method. Here, the gear wheel is replaced with a modern modulator.

Note! Scientists have proven that the CPC indicators in air and in vacuum are almost the same. And it is less than about 25% water.

The following formula is used to calculate the amount of propagation of a light beam.

Formula for calculating the speed of light

This formula is suitable for vacuum calculations.

Conclusion

Light in our world is very important and the moment when scientists can prove the possibility of the existence of superluminal speeds can completely change our familiar world. What this discovery will mean for people is even difficult to assess. But it will definitely be an incredible breakthrough!

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The speed of light is the most unusual measurement known to date. The first person who tried to explain the phenomenon of light propagation was Albert Einstein. It was he who deduced the well-known formula E = mc² , Where E is the total energy of the body, m is the mass, and c is the speed of light in vacuum.

The formula was first published in Annalen der Physik in 1905. Around the same time, Einstein put forward a theory about what would happen to a body moving at absolute speed. Based on the fact that the speed of light is a constant value, he came to the conclusion that space and time must change.

Thus, at the speed of light, an object will shrink indefinitely, its mass will increase indefinitely, and time will practically stop.

In 1977, it was possible to calculate the speed of light, a figure of 299,792,458 ± 1.2 meters per second was named. For more rough calculations, a value of 300,000 km/s is always taken. It is from this value that all the rest are repelled. space measurements. This is how the concept of "light year" and "parsec" (3.26 light years) appeared.

Neither to move at the speed of light, nor, moreover, to overcome it is impossible. At least at this stage of human development. On the other hand, science fiction writers have been trying to solve this problem in the pages of their novels for about 100 years. Perhaps one day fantasy will become a reality, because back in the 19th century, Jules Verne predicted the appearance of a helicopter, an airplane and an electric chair, and then it was pure fantasy!

So by the way. The speed of light in a vacuum and the speed of light in another medium can differ dramatically. For example, in America (unfortunately I don’t remember in which laboratory) they were able to slow down light almost to a complete stop.

But more than 1/299792458 of a second, light cannot develop speed, because. light is normal electromagnetic wave(same as X-ray or heat and radio waves), only the wavelength, frequency differs, then in the modern view it is a wave in a stratified space-time, and when this wave is quantized, we get a photon (quantum of light). This is a massless particle, so there is no time for a photon. This means that for a photon that was born billions of years ago (relative to today's observer) no time has passed at all. According to the formula E = MC2 (mass is equivalent to energy), the speed of light can be considered as a postulate, it turns out that if you accelerate a particle with a non-zero mass (for example, an Electron) to the speed of light, then an infinite amount of energy must be pumped into it, which is physically impossible. it follows that the speed of a massless phaton of 1/299792458 second (the speed of light) is the maximum speed in our visible universe.

speed of light a-priory is equal to 299 792 458 m/s.

The current trend is the definition of standards of physical units based on fundamental physical constants and highly stable natural processes. Imenoo therefore basic physical quantity- time (defined in terms of frequency), because technically the maximum stability (and therefore accuracy) is achieved precisely in the frequency standard. Therefore, other units of measurement are also trying to reduce to frequency and fundamental constants. And therefore, the meter, as a unit of dyne, was determined through frequency, as the most accurately fixed value, and the fundamental constant - the speed of light.

A small note: the definition of a meter and the standard of a meter are two different things. Definition meters is the distance that light travels in 1/299792458 of a second. A reference meters is some technical device, the design of which can be based on other things.

For a simpler understanding, the speed of light can be considered 300,000 km per second. For comparison: The length of the earth's equator is 40,000 km, that is, in a second, light can fly around the earth, even along the equator line, more than 7 times. This is a very high speed. People have achieved top speed speeds are only 2-3 times the speed of sound, that is, about 3-4 thousand kilometers per hour, or about 1 km per second. This is what the speed of light is compared to the existing technologies of mankind.

The most accurate speed of light in a vacuum is 299,792,458 m/s or 1,079,252,848.8 kilometers per hour. Based on a reference meter, it was carried out in 1975.

According to Wikipedia, the speed of light is

299,792,458 m/s is the speed of light in a vacuum. For convenience, in solving problems, the figure 300,000,000 m / s is used. The speed of light in vacuum is determined by the formula:

If we talk about the speed of light in any medium, then

The speed of light in air is almost equal to the speed of light in vacuum.

But already in water it is about 25% less than in air.

Now, in our time, having a computer and the Internet at hand, it is not a problem to find out what the speed of light is, since this open information and this value is as follows:

299,792,458 meters per second.

Having learned such data, one can obviously be a little shocked, because indeed this is a huge speed, which so far has no equal, and it is unlikely to be surpassed.

Here is another interesting table with interesting data:

In 1975 it was produced greatest discovery, namely, the speed of light is measured, which is:

For a better understanding, I suggest looking at the figure.

Sunlight takes about 8 minutes and 19 seconds to reach Earth.

In the video clip presented below, they tried to explain such a value as the speed of light in a more accessible language in order to imagine how fast it is in human understanding and unattainable for reproduction.

At the moment, it is believed that the speed of light is 299,792,458 meters per second.

But if you do not need this value with scientific precision, for example in school problems, it is customary to round this value up to 300,000,000 meters per second, or 300,000 kilometers per second, as they say more often.

If earlier the concept of the speed of light meant something beyond, now they are already building hypersonic fighters, which should enter service by 2030.

The speed of light is 299,792,458 meters per second, or if we translate 1,079,252,848.8 km per hour, which was first determined in 1676 by the Dane O. K. Rmer.

- What is the speed of light in vacuum?
It is believed that the speed of light is(most accurate measurement) 299792458 m/s = 299,792.458 km/s. Counts as one Planck unit. Often these figures are rounded (for example, in school problems in physics) to 300,000,000 m/s = 300,000 km/s.
A very interesting article (more precisely, a chapter from a 9th grade physics textbook) tells how a Danish scientist O. Rmer in 1676 for the first time measured the approximate speed of light. And here is another article.
- What is the speed of propagation of light in various transparent media?
The speed of light in various transparent media is always less than the speed of light in vacuum, since to get the speed of light in any transparent medium, we divide the speed of light in vacuum by the refractive index of this medium. The vacuum refractive index is equal to one.

To get v (the speed of light in a particular medium), you need to divide c (the speed of light in a vacuum) by n. Therefore, the propagation of light in any transparent medium is determined by the formula:
- What is the speed of light in air?
The speed of light propagation in air is, we have already figured out, the speed of light in vacuum, which we divided by coefficient (index) of refraction of air, which is denoted as n. And already this very coefficient depends on the wavelength, and on pressure, and on temperature. That is, for different n, the speed of light in air will be different, but definitely less than the speed of light in vacuum.
- What is the speed of light in glass?
All the same formula, as you understand, and n will be equal to from 1.47 to 2.04. If the glass refractive index is not specified, as an option, take the average value (n = 1.75).
- What is the speed of light in water?
Water has a refractive index(n) is 1.33. Then:
v \u003d c: n \u003d 299 792 458 m / s: 1.33 225 407 863 m / s - the speed of light in water.

The fundamental physical constant - the speed of light in a vacuum is 299,792,458 m/s, this measurement of the speed of light was made in 1975. At school, this value is usually written as 300,000,000 m / s and used to solve problems.

Even in ancient times, they tried to find out this value, but many scientists believed that the speed of light was a constant value. And only in 1676, the Danish astronomer Olaf Remer was the first to measure the speed of light and, according to his calculations, it was equal to 220 thousand kilometers per second.

The speed of light is zero!

Well, let's start with the fact that light in all its spectra is invisible.

We don't see the light!

We see only objects capable of reflecting this light.

Example: We look at a star in the dark sky (which is important) and if suddenly a cloud appears between our eye and the direction to the star, for example, then it will reflect this invisible light.

This is the first.

Light is a standing wave.

Light doesn't go anywhere. Light is carried by a luminous object that reflects this light, for example, a torchman with a torch, and we see it as a reflection from a torch, on which reactions occur.

The torch is not a source of light!

The torch only reflects the light that appeared on the surface of the torch due to chemical reaction.

Same with filament.

We take a flashlight and remove the reflector from it and in a dark room only one light bulb will illuminate evenly (which is important), only a fairly small space. And no matter how much time we spend waiting, the light will still not reach anywhere else. The light will stay in one place forever, or until the filament, when heated, will be able to reflect light (glow)! But, if we put a reflector, we will see that the light was localized into a beam and could penetrate further without any increase in the power of the glow; if we change the focus without any increase in power, then the light will penetrate even further, but is localized even more in a limited beam.

But, even at a great distance and even away from the direction of the beam, we, being in complete darkness, will still see a light spot. We close our eyes and see nothing, open them and immediately see a bright spot from a flashlight on a dark background.

What is the speed of light we are talking about?

Light has no speed. Light is a standing wave. A standing light wave has the ability, with its volume unchanged, due to the power of a chemical reaction, to change its configuration and a standing wave can be visible only when illuminating objects that reflect the standing wave, and we see it as a bright spot on a dark background and not moreover.

Since you did not specify in what media you are interested in the speed of light, you will have to give a detailed answer. Anasteisha Ana accurately told about the speed of light in vacuum. But the speed of light various environments is not constant and necessarily less than in vacuum. Moreover, in the same medium, the speed of light of different wavelengths is different. And this property of light is very widely used, more precisely, it is taken into account in optics. In optics, the concept of the refractive index of an optical medium has been introduced. This parameter shows how many times the speed of light of a certain wavelength in a given medium is less than the speed of light in vacuum. So, for example, in optical glass LK8, the speed of propagation of red light with a wavelength of 706.52 nanometers is 1.46751 times less than in vacuum. Those. the speed of red light in LK8 glass is approximately 299 792 458/1.46751 = 204286484 m/s, and the speed of blue light with a wavelength of 479.99 nanometers is 203113916 m/s. There are optical media in which the speed of light is much lower. In laser crystals, for some wavelengths, the refractive index is close to 2.8. Thus, the speed of light in these crystals is almost three times less than the speed of light in vacuum.

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