Last spring, scientific and popular science magazines around the world reported sensational news. American physicists conducted a unique experiment: they managed to reduce the speed of light to 17 meters per second.
Everyone knows that light travels at enormous speed - almost 300 thousand kilometers per second. The exact value of its value in vacuum = 299792458 m/s is a fundamental physical constant. According to the theory of relativity, this is the maximum possible signal transmission speed.
In any transparent medium, light travels more slowly. Its speed v depends on the refractive index of the medium n: v = c/n. The refractive index of air is 1.0003, of water - 1.33, of various types of glass - from 1.5 to 1.8. Diamond has one of the highest refractive index values - 2.42. Thus, the speed of light in ordinary substances will decrease by no more than 2.5 times.
In early 1999, a group of physicists from the Rowland Institute for Scientific Research at Harvard University (Massachusetts, USA) and Stanford University (California) studied the macroscopic quantum effect - the so-called self-induced transparency, passing laser pulses through a medium that is normally opaque. This medium was sodium atoms in a special state called the Bose-Einstein condensate. When irradiated with a laser pulse, it acquires optical properties that reduce the group velocity of the pulse by 20 million times compared to the speed in vacuum. Experimenters managed to increase the speed of light to 17 m/s!
Before describing the essence of this unique experiment, let us recall the meaning of some physical concepts.
Group speed. When light propagates through a medium, two velocities are distinguished: phase and group. Phase velocity vf characterizes the movement of the phase of an ideal monochromatic wave - an infinite sine wave of strictly one frequency and determines the direction of light propagation. The phase velocity in the medium corresponds to the phase refractive index - the same one whose values are measured for various substances. The phase refractive index, and therefore the phase velocity, depends on the wavelength. This dependence is called dispersion; it leads, in particular, to the decomposition of white light passing through a prism into a spectrum.
But a real light wave consists of a set of waves of different frequencies, grouped in a certain spectral interval. Such a set is called a group of waves, a wave packet or a light pulse. These waves propagate through the medium at different phase velocities due to dispersion. In this case, the impulse is stretched and its shape changes. Therefore, to describe the movement of an impulse, a group of waves as a whole, the concept of group velocity is introduced. It makes sense only in the case of a narrow spectrum and in a medium with weak dispersion, when the difference in the phase velocities of the individual components is small. To better understand the situation, we can give a clear analogy.
Let's imagine that seven athletes lined up on the starting line, dressed in different colored jerseys according to the colors of the spectrum: red, orange, yellow, etc. At the signal of the starting pistol, they simultaneously start running, but the “red” athlete runs faster than the “orange” one. , "orange" is faster than "yellow", etc., so that they stretch into a chain, the length of which continuously increases. Now imagine that we are looking at them from above from such a height that we cannot distinguish individual runners, but just see a motley spot. Is it possible to talk about the speed of movement of this spot as a whole? It is possible, but only if it is not very blurry, when the difference in the speeds of different colored runners is small. Otherwise, the spot may stretch over the entire length of the route, and the question of its speed will lose meaning. This corresponds to strong dispersion - a large spread of speeds. If runners are dressed in jerseys of almost the same color, differing only in shades (say, from dark red to light red), this becomes consistent with the case of a narrow spectrum. Then the speeds of the runners will not differ much; the group will remain quite compact when moving and can be characterized by a very definite value of speed, which is called group speed.
Bose-Einstein statistics. This is one of the types of so-called quantum statistics - a theory that describes the state of systems containing a very large number of particles that obey the laws of quantum mechanics.
All particles - both those contained in an atom and free ones - are divided into two classes. For one of them, the Pauli exclusion principle is valid, according to which there cannot be more than one particle at each energy level. Particles of this class are called fermions (these are electrons, protons and neutrons; the same class includes particles consisting of an odd number of fermions), and the law of their distribution is called Fermi-Dirac statistics. Particles of another class are called bosons and do not obey the Pauli principle: an unlimited number of bosons can accumulate at one energy level. In this case we talk about Bose-Einstein statistics. Bosons include photons, some short-lived elementary particles (for example, pi-mesons), as well as atoms consisting of an even number of fermions. At very low temperatures, bosons congregate at their lowest—basic—energy level; then they say that Bose-Einstein condensation occurs. The condensate atoms lose their individual properties, and several millions of them begin to behave as one, their wave functions merge, and their behavior is described by a single equation. This makes it possible to say that the atoms of the condensate have become coherent, like photons in laser radiation. Researchers from the American National Institute of Standards and Technology used this property of the Bose-Einstein condensate to create an “atomic laser” (see Science and Life No. 10, 1997).
Self-induced transparency. This is one of the effects of nonlinear optics - the optics of powerful light fields. It consists in the fact that a very short and powerful light pulse passes without attenuation through a medium that absorbs continuous radiation or long pulses: an opaque medium becomes transparent to it. Self-induced transparency is observed in rarefied gases with a pulse duration of the order of 10-7 - 10-8 s and in condensed media - less than 10-11 s. In this case, a delay of the pulse occurs - its group velocity decreases greatly. This effect was first demonstrated by McCall and Khan in 1967 on ruby at a temperature of 4 K. In 1970, delays corresponding to pulse velocities three orders of magnitude (1000 times) less than the speed of light in vacuum were obtained in rubidium vapor.
Let us now turn to the unique experiment of 1999. It was carried out by Len Westergaard Howe, Zachary Dutton, Cyrus Berusi (Rowland Institute) and Steve Harris (Stanford University). They cooled a dense, magnetically held cloud of sodium atoms until they returned to the ground state, the lowest energy level. In this case, only those atoms were isolated whose magnetic dipole moment was directed opposite to the direction of the magnetic field. The researchers then cooled the cloud to less than 435 nK (nanokelvins, or 0.000000435 K, almost absolute zero).
After this, the condensate was illuminated with a “coupling beam” of linearly polarized laser light with a frequency corresponding to its weak excitation energy. The atoms moved to a higher energy level and stopped absorbing light. As a result, the condensate became transparent to the following laser radiation. And here very strange and unusual effects appeared. The measurements showed that, under certain conditions, a pulse passing through a Bose-Einstein condensate experiences a delay corresponding to the slowing of light by more than seven orders of magnitude - a factor of 20 million. The speed of the light pulse slowed down to 17 m/s, and its length decreased several times - to 43 micrometers.
The researchers believe that by avoiding laser heating of the condensate, they will be able to slow down the light even further - perhaps to a speed of several centimeters per second.
A system with such unusual characteristics will make it possible to study the quantum optical properties of matter, as well as create various devices for quantum computers of the future, for example, single-photon switches.
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?".
- What is the speed of light in a vacuum?
It is believed that the speed of light is(most accurate measurement) 299,792,458 m/s = 299,792.458 km/s. Counts as one Planck unit. Often these numbers are rounded (for example, in school physics problems) to 300,000,000 m/s = 300,000 km/s.
A very interesting article (more precisely, a chapter from a 9th grade physics textbook) telling how a Danish scientist O. Rmer measured the approximate speed of light for the first time in 1676. And here's another article.
- What is the speed of light propagation in various transparent media??
The speed of light in various transparent media is always less than the speed of light in a vacuum, since in order to obtain the speed of light in any transparent medium, we divide the speed of light in a vacuum by the refractive index of this medium. The refractive index of vacuum is equal to unity.
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 in air is, we have already figured out the speed of light in a vacuum, which we divided by refractive index of air, which is denoted as n. And this same coefficient depends on the wavelength, pressure, and 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 refractive index of glass is not specified, an alternative is to take the average value (n = 1.75).
- What is the speed of light in water?
Water has a refractive index(n) is equal to 1.33. Then:
v = c: n = 299,792,458 m/s: 1.33,225,407,863 m/s - the speed of light in water.
To all of the above, I would like to add that if you want to more clearly understand what the speed of light is, then you can note that light from the Moon to the Earth travels a distance in 1.255 s, and sunlight travels a distance of 150 million km (!) in 8 min 19 sec.
Not only light propagates at the speed of light, but also other types of electromagnetic radiation (radio waves (from ultra-long), infrared, ultraviolet, terahertz and x-ray radiation, as well as gamma radiation).
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 light cannot develop speed for more than 1/299792458 second, because... light is an ordinary electromagnetic wave (the same as x-rays or heat and radio waves), only the wavelength and frequency differ, then in the modern view it is a wave in stratified space-time, and when we quantize this wave we get a photon (quantum of light). This is a massless particle, therefore 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 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 from this that the speed of the massless faton is 1/299792458 seconds (the speed of light) is the maximum speed in our visible universe.
Speed of light a-priory equal to 299,792,458 m/s.
The modern trend is to determine standards of physical units based on fundamental physical constants and highly stable natural processes. Therefore, the main physical quantity is time (defined through frequency), because technically maximum stability (and therefore accuracy) is achieved precisely in the frequency standard. Therefore, they try to reduce other units of measurement to frequency and fundamental constants. And therefore, the meter, as a unit of dyne, was defined through frequency, as the most accurately recorded value, and a fundamental constant - the speed of light.
Minor note: the definition of a meter and the standard of a meter are two different things. Definition A meter is the distance that light travels in 1/299,792,458 of a second. A reference a meter is a 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, more than 7 times. This is a very huge speed. People have achieved a maximum speed of 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, use the figure 300,000,000 m/s. The speed of light in a 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 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 is open information and this value is as follows:
299,792,458 meters per second.
Having learned such data, you can obviously be a little shocked, because this is indeed a huge speed that has no equal yet, and it is unlikely that it will be possible to surpass it.
Here is another interesting plate with interesting data:
In 1975, the greatest discovery was made, namely, the speed of light was measured, which is:
For a clearer understanding, I suggest you look at the drawing.
Sunlight takes about 8 minutes 19 seconds to reach Earth.
In the video below, we tried to explain such a quantity as the speed of light in a more accessible language in order to imagine how fast it is in human understanding and inaccessible for reproduction.
The speed of light is currently believed to be 299,792,458 meters per second.
But if you do not need this value with scientific accuracy, for example in school problems, it is customary to round this value 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 the bounds, now hypersonic fighters are already being built, which should enter service by 2030.
The speed of light is 299,792,458 meters per second, or 1,079,252,848.8 kilometers per hour, which was first determined in 1676 by the Dane O. C. Rmer.
The fundamental physical constant - the speed of light in 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 is used to solve problems.
Even in ancient times, they tried to figure out this value, but many scientists believed that the speed of light was constant. And only in 1676, the Danish astronomer Olaf Roemer 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 that can reflect this light.
Example: We look at a star in the dark sky (which is important) and if suddenly, for example, a cloud appears between our eye and the direction towards the star, then it will reflect this invisible light.
This is the first.
Light is a standing wave.
The light isn't going anywhere. Light is carried by a luminous object that reflects this light, for example, a torchbearer with a torch, and we see it as a reflection from the torch, on which reactions occur.
A torch is not a source of light!
The torch only reflects the light that appeared on the surface of the torch due to a chemical reaction.
The same goes for the filament.
We take a flashlight and remove the reflector from it, and in a dark room, just 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 remain in one place forever, or until the filament, heating up, is able to reflect light (glow)! But, if we place a reflector, we will see that the light was localized into a beam and was able to penetrate further without any increase in the luminous power; 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 spot of light. We close our eyes and see nothing; we open them and immediately see a bright spot from a flashlight on a dark background.
What speed of light can we talk about?
Light has no speed. Light is a standing wave. A standing light wave has the ability, while its volume remains unchanged, due to the power of the 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 light spot on a dark background and not moreover.
Since you did not specify in what environments 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 a vacuum. But the speed of light in different media is not constant and is 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, or rather taken into account in optics. In optics, the concept of refractive index of an optical medium was 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 a vacuum. 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 significantly 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.
The speed of light is the most unusual measurement quantity known to date. The first person who tried to explain the phenomenon of light propagation was Albert Einstein. It was he who came up with the well-known formula E = mc² , Where E is the total energy of the body, m- mass, and c— speed of light in vacuum.
The formula was first published in the journal 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 quantity, he came to the conclusion that space and time must change.
Thus, at the speed of light, an object will shrink endlessly, its mass will increase endlessly, and time will practically stop.
In 1977, it was possible to calculate the speed of light; a figure was given as 299,792,458 ± 1.2 meters per second. For rougher calculations, a value of 300,000 km/s is always assumed. It is from this value that all other cosmic dimensions are based. This is how the concept of “light year” and “parsec” (3.26 light years) appeared.
It is impossible to move at the speed of light, much less overcome it. At least at this stage of human development. On the other hand, science fiction writers have been trying to solve this problem on the pages of their novels for about 100 years. Perhaps one day science fiction will become a reality, because back in the 19th century, Jules Verne predicted the appearance of a helicopter, an airplane and the electric chair, and then it was pure science fiction!
Speed of light in vacuum- absolute value of the speed of propagation of electromagnetic waves in a vacuum. In physics it is denoted by the Latin letter c.
The speed of light in a vacuum is a fundamental constant, independent of the choice of inertial reference frame.
By definition, it is exactly 299,792,458 m/s (approximate value 300 thousand km/s).
According to the special theory of relativity, is maximum speed for the propagation of any physical interactions that transmit energy and information.
How was the speed of light determined?
For the first time the speed of light was determined in 1676 O. K. Roemer by changes in time intervals between eclipses of Jupiter's satellites.
In 1728 it was installed by J. Bradley, based on his observations of starlight aberrations.
In 1849 A. I. L. Fizeau was the first to measure the speed of light by the time it takes light to travel a precisely known distance (base); Since the refractive index of air differs very little from 1, ground-based measurements give a value very close to c.
In Fizeau's experiment, a beam of light from a source S, reflected by a translucent mirror N, was periodically interrupted by a rotating toothed disk W, passed the base MN (about 8 km) and, reflected from the mirror M, returned to the disk. When the light hit the tooth, it did not reach the observer, and the light that fell into the gap between the teeth could be observed through eyepiece E. Based on the known speeds of rotation of the disk, the time it took the light to travel through the base was determined. Fizeau obtained the value c = 313300 km/s.
In 1862 J. B. L. Foucault implemented the idea expressed in 1838 by D. Arago, using a rapidly rotating (512 r/s) mirror instead of a toothed disk. Reflecting from the mirror, the beam of light was directed to the base and upon returning again fell on the same mirror, which had time to rotate through a certain small angle. With a base of only 20 m, Foucault found that the speed light is equal to 29800080 ± 500 km/s. The schemes and main ideas of the experiments of Fizeau and Foucault were repeatedly used in subsequent works on the definition of s.
