“Anyone who wasn’t shocked when they first encountered quantum theory probably just didn’t understand.” Niels Bohr

The premises of quantum theory are so stunning that it looks more like science fiction.

A particle of the microworld can be in two or more places at the same time!

(One very recent experiment showed that one of these particles can be in 3000 places at the same time!)

The same “object” can be both a localized particle and an energy wave propagating in space.

Einstein postulated that nothing can travel faster than the speed of light. But quantum physics has proven: subatomic particles can exchange information instantly - located at any distance from each other.

Classical physics was deterministic: given initial conditions, such as the location and speed of an object, we can calculate where it will go. Quantum physics is probabilistic: we can never say with absolute certainty how the object under study will behave.

Classical physics was mechanistic. It is based on the premise that only by knowing the individual parts of an object can we ultimately understand what it is.

Quantum physics is holistic: it paints a picture of the Universe as a single whole, the parts of which are interconnected and influence each other.

And perhaps most importantly, quantum physics destroyed the idea of ​​a fundamental difference between subject or object, observer and observed - which had dominated scientific minds for 400 years!

In quart physics, the observer influences the observed object. There are no isolated observers of the mechanical Universe - everything takes part in its existence.

SHOCK #1 - EMPTY SPACE

One of the first cracks in the solid structure of Newtonian physics was made by the following discovery: atoms are the solid building blocks of the physical Universe! - consist mainly of empty space. How empty? If you enlarge the nucleus of a hydrogen atom to the size of a basketball, the only electron orbiting it would be thirty kilometers away, with nothing between the nucleus and the electron. So, as you look around, remember: reality is the smallest points of matter surrounded by emptiness.

However, this is not entirely true. This supposed "emptiness" is not actually empty: it contains a colossal amount of incredibly powerful energy. We know that energy becomes more dense as it moves to a lower level of matter (for example, nuclear energy is a million times more powerful than chemical energy). Scientists now say that there is more energy in one cubic centimeter of empty space than in all the matter in the known universe. Although scientists have not been able to measure it, they are seeing the results of this sea of ​​energy.

SHOCK #2 - PARTICLE, WAVE OR WAVEPARTICLE?

Not only is the atom almost entirely made up of “space,” but when scientists explored it more deeply, they discovered that the subatomic (constituting the atom) particles are not solid either. And they seem to have a dual nature. Depending on how we observe them, they can behave either like solid microbodies or like waves.

Particles are individual solid objects that occupy a certain position in space. But waves do not have a “body”; they are not localized and propagate in space.

As a wave, an electron or photon (particle of light) does not have a precise location, but exists as a “field of probabilities.” In the particle state, the probability field “collapses” (collapses) into a solid object. Its coordinates in four-dimensional space-time can already be determined.

This is surprising, but the state of a particle (wave or solid object) is determined by acts of observation and measurement. Unmeasured and unobservable electrons behave like waves. As soon as we subject them to observation during the experiment, they “collapse” into solid particles and can be recorded in space.

But how can something be both a solid particle and a fluid wave at the same time? Perhaps the paradox will be resolved if we remember what we said recently: particles behave like waves or like solid objects. But the concepts of "wave" and "particle" are just analogies taken from our everyday world. The concept of a wave was introduced into quantum theory by Erwin Schrödinger. He is the author of the famous “wave equation,” which mathematically substantiates the existence of wave properties in a solid particle before the act of observation. Some physicists - in an attempt to explain something they have never encountered and cannot fully understand - call subatomic particles "wave particles."

SHOCK #3 - QUANTUM LEAPS AND PROBABILITY

While studying the atom, scientists discovered that when electrons, rotating around the nucleus, move from orbit to orbit, they do not move through space like ordinary objects. No, they cover the distance instantly. That is, they disappear in one place and appear in another. This phenomenon was called a quantum leap.

Moreover, scientists realized that they could not determine exactly where in the new orbit the missing electron would appear or at what moment it would make a jump. The most they could do was calculate the probability (based on the Schrödinger wave equation) of the electron's new location.

“Reality, as we experience it, is created at each moment in the totality of countless possibilities,” says Dr. Satinover. “But the real secret is that there is nothing in the physical Universe that determines which possibility from this totality will come true. There is no process that establishes that.”

Thus, quantum leaps are the only truly random events in the Universe.

SHOCK #4 - THE PRINCIPLE OF UNCERTAINTY

In classical physics, all parameters of an object, including its spatial coordinates and speed, can be measured with an accuracy limited only by the capabilities of experimental technologies. But at the quantum level, whenever you determine one quantitative characteristic of an object, such as speed, you cannot obtain precise values ​​for its other parameters, such as coordinates. In other words: if you know how fast an object is moving, you cannot know where it is. And vice versa: if you know where it is, you cannot know how fast it is moving.

No matter how sophisticated the experimenters are, no matter how advanced measurement technologies they use, they are unable to look behind this veil.

Werner Heisenberg, one of the pioneers of quantum physics, formulated the uncertainty principle. Its essence is as follows: no matter how hard you try, it is simultaneously impossible to obtain exact values ​​of the coordinates and speed of a quantum object. The more precision we achieve in measuring one parameter, the more uncertain the other becomes.

SHOCK #5 - NONLOCALITY, EPR PARADOX AND BELL'S THEOREM

Albert Einstein did not like quantum physics. Assessing the probabilistic nature of subatomic processes outlined in quantum physics, he said: “God does not play dice with the Universe.” But Niels Bohr answered him: “Stop teaching God what to do!”

In 1935, Einstein and his colleagues Podolsky and Rosen (EPR) attempted to defeat quantum theory. Scientists, based on the principles of quantum mechanics, conducted a thought experiment and came to a paradoxical conclusion. (He was supposed to show the inferiority of quantum theory). The essence of their thoughts is this. If we have two simultaneously arising particles, this means that they are interconnected or are in a state of superposition. Let's send them to different ends of the Universe. Then we change the state of one of the particles. Then, according to quantum theory, another particle instantly comes to the same state. Instantly! On the other edge of the universe!

Such an idea was so ridiculous that Einstein sarcastically referred to it as “supernatural action at a distance.” According to his theory of relativity, nothing can travel faster than light. And in the EPR experiment it turned out that the speed of information exchange between particles is infinite! In addition, the very idea that an electron could “track” the state of another electron on the opposite edge of the Universe completely contradicted generally accepted ideas about reality, and indeed common sense in general.

But in 1964, the Irish theoretical physicist John Bell formulated and proved a theorem from which it followed: the “ridiculous” conclusions from the EPR thought experiment are true!

Particles are intimately connected on a level that transcends time and space. Therefore, they are able to instantly exchange information.

The idea that any object in the Universe is local - i.e. exists in one place (point) in space - not true. Everything in this world is non-local.

Nevertheless, this phenomenon is a valid law of the Universe. Schrödinger said that the relationship between objects is not the only interesting aspect of quantum theory, but it is the most important. In 1975, theoretical physicist Henry Stapp called Bell's theorem "the most significant discovery of science." Note that he was talking about science, not just physics.

(The article was prepared based on the materials of the book by W. Arntz, B. Chace, M. Vicente “The Rabbit Hole, or what do we know about ourselves and the Universe?”, chapter “Quantum Physics”.)

In 1935, when quantum mechanics and Einstein's general theory of relativity were very young, the not-so-famous Soviet physicist Matvei Bronstein, at the age of 28, made the first detailed study of the reconciliation of these two theories in the quantum theory of gravity. This “perhaps a theory of the whole world,” as Bronstein wrote, could supplant Einstein’s classical description of gravity, in which it is seen as curves in the space-time continuum, and rewrite it in quantum language, like the rest of physics.

Bronstein figured out how to describe gravity in terms of quantized particles, now called gravitons, but only when the force of gravity is weak—that is, (in general relativity) when spacetime is so slightly curved that it is essentially flat. When gravity is strong, “the situation is completely different,” the scientist wrote. “Without a deep revision of classical concepts, it seems almost impossible to imagine a quantum theory of gravity in this area.”

His words were prophetic. Eighty-three years later, physicists are still trying to understand how spacetime curvature manifests itself on macroscopic scales, arising from a more fundamental and presumably quantum picture of gravity; This is perhaps the deepest question in physics. Perhaps, if there was a chance, Bronstein's bright mind would speed up the process of this search. In addition to quantum gravity, he also made contributions to astrophysics and cosmology, semiconductor theory, quantum electrodynamics, and wrote several books for children. In 1938, he fell under Stalin's repressions and was executed at the age of 31.

The search for a complete theory of quantum gravity is complicated by the fact that the quantum properties of gravity never manifest themselves in real experience. Physicists do not see how Einstein’s description of a smooth space-time continuum, or Bronstein’s quantum approximation of it in a slightly curved state, is violated.

The problem is the extreme weakness of the gravitational force. While quantized particles that transmit strong, weak and electromagnetic forces are so strong that they tightly bind matter into atoms and can be examined literally under a magnifying glass, individual gravitons are so weak that laboratories have no chance of detecting them. To have a high probability of catching a graviton, the particle detector would have to be so large and massive that it collapses into a black hole. This weakness explains why astronomical accumulations of mass are needed to influence other massive bodies through gravity, and why we see gravitational effects on enormous scales.

That's not all. The universe appears to be subject to some kind of cosmic censorship: regions of strong gravity—where spacetime curves are so sharp that Einstein's equations break down and the quantum nature of gravity and spacetime must be revealed—always lurk behind the horizons of black holes.

“Even a few years ago, there was a general consensus that it was most likely impossible to measure the quantization of the gravitational field in any way,” says Igor Pikovsky, a theoretical physicist at Harvard University.

Now, several recent papers published in Physical Review Letters have changed that. These papers make the claim that it may be possible to get to quantum gravity—even without knowing anything about it. The papers, written by Sugato Bose of University College London and Chiara Marletto and Vlatko Vedral of the University of Oxford, propose a technically challenging but feasible experiment that could confirm that gravity is a quantum force like all others, without requiring the detection of a graviton. Miles Blencowe, a quantum physicist at Dartmouth College who was not involved in this work, says such an experiment could reveal a clear signature of invisible quantum gravity - "the smile of the Cheshire Cat."

The proposed experiment will determine whether two objects—Bose's group plans to use a pair of microdiamonds—can become quantum mechanically entangled with each other through mutual gravitational attraction. Entanglement is a quantum phenomenon in which particles become inseparably intertwined, sharing a single physical description that defines their possible combined states. (The coexistence of different possible states is called "superposition" and defines a quantum system.) For example, a pair of entangled particles can exist in a superposition in which particle A has a 50% probability of spinning from bottom to top, and particle B will spin from top to bottom, and with a 50% probability vice versa. No one knows in advance what result you will get when measuring the direction of the spin of particles, but you can be sure that it will be the same for them.

The authors argue that the two objects in the proposed experiment can only become entangled in this way if the force acting between them - in this case gravity - is a quantum interaction mediated by gravitons, which can support quantum superpositions. "If the experiment is carried out and entanglement is obtained, according to the work, we can conclude that gravity is quantized," Blencowe explained.

Confuse the diamond

Quantum gravity is so subtle that some scientists have doubted its existence. Renowned mathematician and physicist Freeman Dyson, 94, has argued since 2001 that the universe could support a kind of “dualistic” description in which “the gravitational field described by Einstein’s general theory of relativity would be a purely classical field without any quantum behavior.” , while all matter in this smooth space-time continuum will be quantized by particles that obey the rules of probability.

Dyson, who helped develop quantum electrodynamics (the theory of interactions between matter and light) and is a professor emeritus at the Institute for Advanced Study in Princeton, New Jersey, does not believe that quantum gravity is necessary to describe the unreachable interiors of black holes. And he also believes that detecting a hypothetical graviton may be impossible in principle. In that case, he says, quantum gravity would be metaphysical, not physical.

He's not the only skeptic. The famous English physicist Sir Roger Penrose and the Hungarian scientist Lajos Diosi independently proposed that spacetime cannot support superpositions. They believe that its smooth, rigid, fundamentally classical nature prevents it from bending into two possible paths at once - and it is this rigidity that leads to the collapse of superpositions of quantum systems like electrons and photons. “Gravitational decoherence,” in their opinion, allows for a single, solid, classical reality to occur that can be felt on a macroscopic scale.

The ability to find the “smile” of quantum gravity would seem to refute Dyson's argument. It also kills the theory of gravitational decoherence by showing that gravity and spacetime actually support quantum superpositions.

The proposals of Bose and Marletto appeared simultaneously and completely by accident, although experts note that they reflect the spirit of the times. Experimental quantum physics laboratories around the world are putting ever larger microscopic objects into quantum superpositions and optimizing protocols for testing the entanglement of two quantum systems. The proposed experiment would need to combine these procedures, while requiring further improvements in scale and sensitivity; perhaps it will take ten years. "But there is no physical dead end," says Pikovsky, who is also exploring how laboratory experiments could probe gravitational phenomena. “I think it’s difficult, but not impossible.”

This plan is outlined in more detail in the work of Bose et al - Ocean's Eleven Experts for Different Stages of the Proposal. For example, in his laboratory at the University of Warwick, co-author Gavin Morley is working on the first step, trying to put a microdiamond into a quantum superposition in two places. To do this, he will confine a nitrogen atom in the microdiamond, next to a vacancy in the diamond structure (the so-called NV center, or nitrogen-substituted vacancy in diamond), and charge it with a microwave pulse. An electron rotating around the NV center simultaneously absorbs light and does not, and the system goes into a quantum superposition of two spin directions - up and down - like a top that rotates clockwise with a certain probability and counterclockwise with a certain probability. A microdiamond loaded with this superposition spin is subjected to a magnetic field that causes the top spin to move to the left and the bottom spin to move to the right. The diamond itself splits into a superposition of two trajectories.

In a full experiment, scientists would do all this with two diamonds - red and blue, for example - placed side by side in an ultra-cold vacuum. When the trap holding them is turned off, the two microdiamonds, each in a superposition of two positions, will fall vertically in a vacuum. As the diamonds fall, they will feel the gravity of each of them. How strong will their gravitational pull be?

If gravity is a quantum force, the answer is: it depends. Each component of the blue diamond's superposition will experience a stronger or weaker attraction towards the red diamond, depending on whether the latter is in a branch of the superposition that is closer or further away. And the gravity that each component of the red diamond's superposition will feel depends in the same way on the state of the blue diamond.

In each case, varying degrees of gravitational attraction act on the evolving components of the diamond superpositions. The two diamonds become interdependent because their states can only be determined in combination—if this means that—so eventually the spin directions of the two systems of NV centers will correlate.

After the microdiamonds fall side by side for three seconds—long enough to become entangled in gravity—they will pass through another magnetic field, which will bring the branches of each superposition back together. The final step of the experiment is the entanglement witness protocol developed by Danish physicist Barbara Theral and others: blue and red diamonds enter different devices that measure the spin directions of NV center systems. (Measurement causes superpositions to collapse into certain states.) The two results are then compared. By performing the experiment over and over again and comparing many pairs of spin measurements, scientists can determine whether the spins of two quantum systems actually correlated more often than the upper limit for objects that are not quantum mechanically entangled. If so, gravity actually entangles diamonds and may support superpositions.

"What's interesting about this experiment is that you don't need to know what quantum theory is," says Blencowe. “All that is needed is to say that there is some quantum aspect to this region that is mediated by the force between two particles.”

There are a lot of technical difficulties. The largest object that had been placed in superposition in two places before was an 800-atom molecule. Each microdiamond contains more than 100 billion carbon atoms - enough to accumulate a noticeable gravitational force. Unpacking its quantum mechanical nature will require low temperatures, deep vacuums and precise control. "It's a lot of work getting the initial superposition up and running," says Peter Barker, part of the experimental team that is refining laser cooling and microdiamond trapping techniques. If this could be done with one diamond, Bose adds, “a second one wouldn’t be a problem.”

What is unique about gravity?

Quantum gravity researchers have no doubt that gravity is a quantum interaction that can cause entanglement. Of course, gravity is somewhat unique, and there is still much to be learned about the origins of space and time, but quantum mechanics should definitely be involved, scientists say. "Really, what's the point of a theory in which most of the physics is quantum and gravity is classical," says Daniel Harlow, a quantum gravity researcher at MIT. The theoretical arguments against mixed quantum-classical models are very strong (though not conclusive).

On the other hand, theorists have been wrong before. “If you can check it, why not? If this shuts up these people who question the quantum nature of gravity, that would be great,” says Harlow.

After reading the papers, Dyson wrote: “The proposed experiment is certainly of great interest and requires carrying out under the conditions of a real quantum system.” However, he notes that the authors' lines of thought about quantum fields differ from his. “It is not clear to me whether this experiment can resolve the question of the existence of quantum gravity. The question I was asking—whether a single graviton is observed—is a different question and may have a different answer.”

The line of thought of Bose, Marletto and their colleagues on quantized gravity stems from the work of Bronstein as early as 1935. (Dyson called Bronstein's work "a beautiful piece of work" that he had not seen before). In particular, Bronstein showed that weak gravity generated by small mass can be approximated by Newton's law of gravitation. (This is the force that acts between superpositions of microdiamonds). According to Blencowe, calculations of weak quantized gravity have not been particularly carried out, although they are certainly more relevant than the physics of black holes or the Big Bang. He hopes the new experimental proposal will encourage theorists to seek subtle refinements to Newton's approximation, which future tabletop experiments could try to test.

Leonard Susskind, a renowned quantum gravity and string theorist at Stanford University, saw the value of the proposed experiment because "it provides observations of gravity in a new range of masses and distances." But he and other researchers stressed that microdiamonds cannot reveal anything about the full theory of quantum gravity or space-time. He and his colleagues would like to understand what happens at the center of a black hole and at the moment of the Big Bang.

Perhaps one clue to why quantizing gravity is so much harder than anything else is that other forces of nature have what is called “locality”: quantum particles in one region of the field (photons in an electromagnetic field, for example) are “independent of other physical entities in another region of space," says Mark van Raamsdonk, a quantum gravity theorist at the University of British Columbia. "But there is a lot of theoretical evidence that gravity doesn't work that way."

In the best sandbox models of quantum gravity (with simplified space-time geometries), it is impossible to assume that the ribbon of space-time fabric is divided into independent three-dimensional pieces, says van Raamsdonk. Instead, modern theory suggests that the underlying, fundamental components of space are "organized rather in a two-dimensional manner." The fabric of spacetime could be like a hologram or a video game. “Although the picture is three-dimensional, the information is stored on a two-dimensional computer chip.” In this case, the three-dimensional world would be an illusion in the sense that its different parts are not so independent. In a video game analogy, a few bits on a two-dimensional chip can encode global functions of the entire game universe.

And this difference matters when you are trying to create a quantum theory of gravity. The usual approach to quantizing something is to identify its independent parts—particles, for example—and then apply quantum mechanics to them. But if you don't define the right components, you end up with the wrong equations. The direct quantization of three-dimensional space that Bronstein wanted to do works to some extent with weak gravity, but turns out to be useless when spacetime is highly curved.

Some experts say that witnessing the “smile” of quantum gravity could lead to motivation for this kind of abstract reasoning. After all, even the loudest theoretical arguments about the existence of quantum gravity are not supported by experimental facts. When van Raamsdonk explains his research at a scientific colloquium, he says, it usually starts with a story about how gravity needs to be rethought with quantum mechanics because the classical description of spacetime breaks down with black holes and the Big Bang.

“But if you do this simple experiment and show that the gravitational field was in superposition, the failure of the classical description becomes obvious. Because there will be an experiment that implies that gravity is quantum.”

Based on materials from Quanta Magazine

E.S.,
, Municipal educational institution secondary school No. 16 with UIOP, Lysva, Perm region.

The Birth of Quantum Physics

Find the beginning of everything, and you will understand a lot!
Kozma Prutkov

Educational objective of the lesson: introduce the concept of discreteness of matter, form the concept of quantum-wave dualism of matter, justify the introduction of Planck’s formulas and de Broglie wavelength.

Developmental objective of the lesson: develop logical thinking, the ability to compare and analyze situations, and see interdisciplinary connections.

Educational objective of the lesson: to form dialectical-materialistic thinking.

Physics as a science has universal human values ​​and enormous humanitarian potential. During its study, the basic scientific methods are revealed (scientific experiment, modeling, thought experiment, creation and structure of scientific theory). Students must be given the opportunity to look at the world through the eyes of a physicist in order to understand the eternity and constant change of the world - a world in which there is so much that is huge and insignificantly small, very fast and unusually slow, simple and difficult to understand - to feel the constant desire of man for knowledge that delivers the deepest satisfaction, to get acquainted with examples of deep experience of “scientific doubts” and courageous movement along an unfamiliar path in search of elegance, brevity and clarity.

I. Teacher. When we started studying optics, I asked the question: “What is light?” How would you answer it now? Try to formulate your thought in one sentence. Start with the words “light is...” From F.I. Tyutchev has the following lines: “Again with greedy eyes // I drink the life-giving Light.” Please try to comment on these lines from a physics point of view. In poetry - from Homer to the present day - sensations generated by light have always been given a special place. Most often, poets perceived light as a special luminous, shining liquid.

To make today’s conversation about light complete, I would like to read the words of S.I. Vavilova: “The continuous, victorious war for truth, never ending in final victory, has, however, its indisputable justification. On the path to understanding the nature of light, man received microscopes, telescopes, range finders, radios, and X-rays; this research helped to master the energy of the atomic nucleus. In search of truth, man limitlessly expands the areas of his mastery of nature. Isn’t this the real task of science? (emphasis mine. – E.U.)»

II. Teacher. In the process of studying physics, we became acquainted with many theories, for example, MCT, thermodynamics, Maxwell's theory of electromagnetic field, etc. Today we are completing the study of wave optics. We must summarize the study of the topic and, perhaps, put a final point on the question: “What is light?” Could you use examples from wave optics to show the role of theory in the process of understanding nature?

Let us remember that the significance of the theory lies not only in the fact that it allows one to explain many phenomena, but also in the fact that it makes it possible to predict new, not yet known physical phenomena, properties of bodies and patterns. Thus, the wave theory explained the phenomena of interference, diffraction, polarization, refraction, dispersion of light and made it possible to make a “discovery at the tip of a pen” - a prediction. In 1815, an unknown retired engineer, Augustin Fresnel, presented a paper explaining the phenomenon of diffraction to the Paris Academy of Sciences. The analysis of the work was entrusted to famous scientists - physicist D. Arago and mathematician S. Poisson. Poisson, reading this work with passion, discovered a blatant absurdity in Fresnel’s conclusions: if a small round target is placed in a stream of light, then a light spot should appear in the center of the shadow! What do you think happened next? A few days later, Arago experimented and discovered that Fresnel was right! So, the 19th century is the century of the triumph of wave optics.

What is light? Light is an electromagnetic transverse wave.

Finishing the study of a large section of physics related to the nature of light and electromagnetic waves, I propose to independently complete the test task “Electromagnetic waves” (see Appendix 1). We check execution frontally.

III. Teacher. And here’s what the London newspapers wrote on the eve of 1900: “When the festive illumination of bright light bulbs instead of dim oil bowls was lit on the streets of London, cabs drove up to the ancient building on Fleet Street one after another. Respectable gentlemen dressed in robes ascended the wide, brightly lit staircase into the hall. Then members of the Royal Society of London gathered for their next meeting. Tall, gray-haired, with a thick beard, Sir William Thomson (do you know about his achievements in the field of physics? - E.U.), eight years ago granted from the hands of Queen Victoria the title of peer and Lord Kelvin (is this name familiar to you? - E.U.), and now the president of the society, began his New Year's speech. The great physicist of the 19th century noted the successes achieved over the past century, listed the merits of those present...

Those gathered nodded their heads approvingly. To be modest, they did a good job. And Sir William was right when he said that the grand edifice of physics had been built, that only small finishing touches remained.

True (Lord Kelvin interrupted his speech for a moment), in the cloudless horizon of physics there are two small clouds, two problems that have not yet found an explanation from the standpoint of classical physics... But these phenomena are temporary and fleeting. Calmly settled in antique chairs with high backs, the gentlemen smiled. Everyone knew what we were talking about:

1) classical physics could not explain Michelson’s experiments, which did not determine the influence of the Earth’s movement on the speed of light. In all reference systems (both moving and at rest relative to the Earth), the speed of light is the same - 300,000 km/s;

2) classical physics could not explain the graph of black body radiation obtained experimentally.”

Sir William could not even imagine what kind of lightning would soon strike from these clouds! Looking ahead, I will say: the solution to the first problem will lead to a revision of classical ideas about space and time, to the creation of the theory of relativity; the solution to the second problem will lead to the creation of a new theory - quantum. This is the solution to the second problem that will be discussed in today’s lesson!

IV. (Students make notes in their notebooks: Date Lesson No. Lesson topic: “The Origin of Quantum Physics.”) At the turn of the 19th and 20th centuries. A problem arose in physics that urgently needed to be solved: a theoretical explanation of the radiation graph of an absolutely black body. What is a perfect black body? ( Students' hypotheses. Demonstration of the video clip “Thermal Radiation” .)

Teacher. Write down: “A completely black body is a body capable of absorbing without reflection the entire incident flux of radiation, all electromagnetic waves of any wavelength (any frequency).”

But absolutely black bodies have one more feature. Remember why people with black skin live in the equatorial territories? “Black bodies, if heated, will glow brighter than any other body, that is, they emit energy in all frequency ranges,” write this down in your notebooks.

Scientists have experimentally determined the radiation spectrum of a completely black body. ( Draws a graph.) Rν – spectral density of energetic luminosity – the energy of electromagnetic radiation emitted per unit time from a unit surface area of ​​a body in a unit frequency interval ν. Maxwell's electromagnetic field theory predicted the existence of electromagnetic waves, but the theoretical black body radiation curve constructed on the basis of this theory had a discrepancy with the experimental curve in the high frequency region. The best minds of that time worked on the problem: the English Lord Rayleigh and J. Jeans, the Germans P. Kirchhoff and V. Wien, Moscow professor V.A. Mikhelson. Nothing worked!

Offer a way out of the current situation. The theoretical curve differs from the experimental one. How to be and what to do? ( Students express hypotheses: conduct experiments more carefully - they did, the result is the same; change the theory - but this is a disaster, the entire foundation of classical physics, which was created over thousands of years, collapses!) The created situation in physics was called ultraviolet disaster.

Write down: “The methods of classical physics turned out to be insufficient to explain the radiation of a completely black body in the high frequency region - it was an “ultraviolet catastrophe.”

Who can guess why this crisis was named ultraviolet catastrophe, and not infrared or violet? A crisis has broken out in physics! The Greek word κρίση [ a crisis] denote a difficult transition from one stable state to another. The problem had to be solved, and solved urgently!

V.Teacher. And so on October 19, 1900, at a meeting of the Physical Society, the German scientist M. Planck proposed using the formula to calculate the radiation of an absolutely black body E = hν. Planck's friend and colleague Heinrich Rubens sat at his desk all night, comparing his measurements with the results given by Planck's formula, and was amazed: his friend's formula described the radiation spectrum of an absolutely black body to the smallest detail! So, Planck’s formula eliminated the “ultraviolet catastrophe,” but at what cost! Planck proposed, contrary to established views, to consider that the emission of radiant energy by atoms of matter occurs discretely, that is, in portions, quanta. "Quantum" ( quant) translated from Latin simply means quantity .

What does "discrete" mean? Let's conduct a thought experiment. Imagine that you have a jar full of water in your hands. Is it possible to cast half? How about taking a sip? And even less? In principle, it is possible to reduce or increase the mass of water by an arbitrarily small amount. Now let’s imagine that we have in our hands a box of children’s cubes of 100 g each. Is it possible to reduce, for example, 370 g? No! You can't break the cubes! Therefore, the mass of the box can change discretely, only in portions that are multiples of 100 g! The smallest amount by which the mass of the box can be changed can be called portion, or quantum of mass.

Thus, a continuous flow of energy from a heated black body turned into a “machine gun burst” of separate portions - energy quanta. It would seem nothing special. But in fact, this meant the destruction of the entire excellently constructed edifice of classical physics, since instead of the basic fundamental laws built on the principle of continuity, Planck proposed the principle of discreteness. Planck himself did not like the idea of ​​discreteness. He sought to formulate the theory so that it would fit entirely within the framework of classical physics.

But there was a person who, on the contrary, went even more decisively beyond the boundaries of classical ideas. This man was A. Einstein. So that you understand the revolutionary nature of Einstein’s views, I will only say that, using Planck’s idea, he laid the foundations for the theory of lasers (quantum generators) and the principle of using atomic energy.

Academician S.I. For a very long time, Vavilov could not get used to the idea of ​​light as a substance of quanta, but he became an ardent admirer of this hypothesis and even came up with a way to observe quanta. He calculated that the eye is able to discern the illumination created by 52 quanta of green light.

So, according to Planck, light is... ( student statements).

VI. Teacher. Doesn't Planck's hypothesis remind you of the already known hypothesis about the nature of light? Sir Isaac Newton proposed to consider light as consisting of tiny particles - corpuscles. Any luminous body emits them in all directions. They fly in straight lines and if they hit our eyes, we see their source. Each color corresponds to its own corpuscles and they differ, most likely, in that they have different masses. The combined flow of corpuscles creates white light.

In the time of Sir Isaac Newton, physics was called natural philosophy. Why? Read (see Appendix 2) one of the basic laws of dialectics - the law of negation of negation. Try applying it to the question of the nature of light. ( Students' reasoning.)

So, according to M. Planck’s hypothesis, light is a stream of particles, corpuscles, quanta, each of which has energy E = hν. Please analyze this formula: what is ν? what's happened h (one of the students will definitely suggest that this is some kind of constant, named after Planck)? What is the unit of Planck's constant? what is the value of the constant ( working with the table of physical constants)? What is the name of Planck's constant? What is the physical meaning of Planck's constant?

To appreciate the beauty of Planck's formula, let's turn to problems... biology. I invite students to answer questions from the field of biology (Appendix 3).

Mechanism of vision. Through vision we receive about 90% of information about the world. Therefore, the question of the mechanism of vision has always interested people. Why does the human eye, and indeed most of the inhabitants of the Earth, perceive only a small range of waves from the spectrum of electromagnetic radiation existing in nature? What if a person had infrared vision, for example, like pit snakes?

At night we would see, as during the day, all organic bodies, because their temperature differs from the temperature of inanimate bodies. But the most powerful source of such rays for us would be our own body. If the eye is sensitive to infrared radiation, the light of the Sun would simply fade away for us against the background of its own radiation. We wouldn't see anything, our eyes would be useless.

Why don't our eyes react to infrared light? Let us calculate the energy of quanta of infrared and visible light using the formula:

The energy of IR quanta is less than the energy of visible light quanta. Several quanta cannot “get together” to cause an action that is beyond the power of one quantum - in the microworld there is a one-on-one interaction between a quantum and a particle. Only a quantum of visible light, which has an energy greater than that of infrared light, can cause a reaction in the rhodopsin molecule, i.e., the retinal rod. The effect of a visible light quantum on the retina can be compared to the impact of a tennis ball, which moved... a multi-story building. (The sensitivity of the retina is so high!)

Why does the eye not react to ultraviolet radiation? UV radiation is also invisible to the eye, although the energy of UV quanta is much greater than that of visible light quanta. The retina is sensitive to UV rays, but they are absorbed by the lens, otherwise they would have a destructive effect.

In the process of evolution, the eyes of living organisms have adapted to perceive the energy of radiation from the most powerful source on Earth - the Sun - and precisely those waves that account for the maximum energy of solar radiation incident on the Earth.

Photosynthesis. In green plants, the process through which all living things receive oxygen for breathing and food does not stop for a single second. This is photosynthesis. The leaf has a green color due to the presence of chlorophyll in its cells. Photosynthesis reactions occur under the influence of radiation in the red-violet part of the spectrum, and waves with a frequency corresponding to the green part of the spectrum are reflected, so the leaves have a green color.

Chlorophyll molecules are “responsible” for the unique process of converting light energy into the energy of organic substances. It begins with the absorption of a quantum of light by a chlorophyll molecule. Absorption of a quantum of light leads to chemical reactions of photosynthesis, which include many units.

All day long, chlorophyll molecules “are busy” with the fact that, having received a quantum, they use its energy, converting it into the potential energy of an electron. Their action can be compared to the action of a mechanism that lifts a ball up a staircase. Rolling down the steps, the ball loses its energy, but it does not disappear, but turns into the internal energy of substances formed during photosynthesis.

Chlorophyll molecules “work” only during daylight hours, when visible light hits them. At night they “rest”, despite the fact that there is no shortage of electromagnetic radiation: the earth and plants emit infrared light, but the energy of the quanta in this range is less than that required for photosynthesis. In the process of evolution, plants have adapted to accumulate the energy of the most powerful source of energy on Earth - the Sun.

Heredity.(Students answer questions 1–3 from Appendix 3, card “Heredity”). The hereditary characteristics of organisms are encoded in DNA molecules and are transmitted from generation to generation in a matrix way. How to cause a mutation? Under the influence of what radiation does the process of mutation occur?

To cause a single mutation, it is necessary to impart energy to the DNA molecule sufficient to change the structure of some part of the DNA gene. It is known that γ-quanta and X-rays, as biologists put it, highly mutagenic– their quanta carry energy sufficient to change the structure of a section of DNA. IR radiation, and apparently, cannot do such an action; their frequency, and therefore their energy, is too low. Now, if the energy of the electromagnetic field were absorbed not in portions, but continuously, then these radiations would be able to influence DNA, because in relation to its reproductive cells, the organism itself is the closest and most powerful, constantly operating source of radiation.

By the beginning of the 30s. XX century Thanks to the successes of quantum mechanics, physicists had a feeling of such power that they turned to life itself. There were many similarities in genetics. Biologists have discovered a discrete indivisible particle - a gene - that can move from one state to another. Changes in the configuration of genes are associated with changes in chromosomes, which causes mutations, and this turned out to be possible to explain on the basis of quantum concepts. One of the founders of molecular biology, who received the Nobel Prize for research in the field of mutation processes in bacteria and bacteriophages, was the German theoretical physicist M. Delbrück. In 1944, a short book by physicist E. Schrödinger, “What is Life?” was published. It gave a clear and concise presentation of the fundamentals of genetics and revealed the connection between genetics and quantum mechanics. The book gave impetus to the physicists' assault on the gene. Thanks to the work of American physicists J. Watson, F. Crick, M. Wilkins, biologists learned how the most basic “living” molecule, DNA, is “structured.” X-ray diffraction analysis made it possible to see it.

VII. Teacher. I return to the question: what is light? ( Student answers.) It turns out that physics returned to the Newtonian particle of light - the corpuscle - rejecting the idea of ​​light as a wave? No! It is impossible to cross out the entire legacy of the wave theory of light! After all, diffraction, interference and many other phenomena have long been known, which experimentally confirm that light is a wave. What should I do? ( Students' hypotheses.)

There is only one thing left: to somehow combine waves with particles. Recognize that there is one circle of phenomena where light exhibits wave properties, and there is another circle in which the corpuscular essence of light comes first. In other words – write it down! – light has quantum wave duality! This is the dual nature of light. It was very difficult for physicists to combine two hitherto incompatible ideas into one. A particle is something solid, unchanging, having a certain size, limited in space. A wave is something fluid, unsteady, without clear boundaries. More or less clearly, these ideas were connected using the concept of a wave packet. This is something like a wave “cut off” at both ends, or rather, a bunch of waves traveling through space as a single whole. The clot can shrink or stretch depending on the environment it enters. It resembles a flying spring.

What characteristic of the wave packet changes when light passes from one medium to another? ( Student answers.)

In 1927, the American physicist Lewis proposed calling this wave packet photon(from Greek φωτóς [phos, photos] – ) . What is a photon? ( Students work with the textbook and draw conclusions.)

Conclusions. A photon is: a quantum of electromagnetic radiation; a massless particle; a photon at rest does not exist; a particle moving in a vacuum at the speed of light c= 3 10 8 m/s is a single whole and indivisible, the existence of a fractional part of a photon is impossible; a particle with energy E = hν, where h= 6.63 · 10 -34 J · s; ν is the frequency of light; a particle with momentum is an electrically neutral particle.

The world is structured in such a way that light most often shows us a wave nature, until we consider its interaction with matter. And matter appears before us in corpuscular form, until we begin to consider the nature of interatomic bonds, transfer processes, electrical resistance, etc. But regardless of our position at each moment, a microparticle has both properties.

The process of creating quantum theory and, in particular, quantum theory of light is deeply dialectical. The ideas and images of old, classical mechanics and optics, enriched with new ideas, creatively applied to physical reality, ultimately gave rise to a fundamentally new physical theory.

Exercise: Read the philosophical law of unity and struggle of opposites and draw a conclusion regarding two theories of light: wave and quantum theories of light.

VIII. Teacher. In 1924, the French physicist Louis de Broglie (a former military radiotelegraph operator) expressed completely paradoxical, even for the brave physicists of that time, thoughts about the nature of the movement of atomic particles. De Broglie suggested that the properties of electrons and other particles are, in principle, no different from the properties of quanta! It followed from this that electrons and other particles should also exhibit wave properties, that, for example, electron diffraction should be observed. And it was indeed discovered in experiments that in 1927, independently of each other, were carried out by American physicists K.-J. Davisson and L. Germer, Soviet physicist P.S. Tartakovsky and English physicist J.-P. Thomson. The de Broglie wavelength is calculated using the formula:

Let's solve problems for calculating the de Broglie wavelength (Appendix 4).

As calculations show, a valence electron moving inside an atom at a speed of 0.01 With, diffracts on an ionic crystal lattice as a wave with a wavelength of ~10 -10 m, and the wavelength of a bullet flying at a speed of about 500 m/s is about 10 -34 m. Such a small wavelength cannot be registered in any way, and therefore the bullet behaves like a real particle.

The struggle between the ideas of discreteness and continuity of matter, which was waged from the very beginning of science, ended with the merging of both ideas in the idea of ​​​​the dual properties of elementary particles. The use of the wave properties of electrons has made it possible to significantly increase the resolution of microscopes. The wavelength of the electron depends on the speed, and therefore on the voltage accelerating the electrons (see problem 5 in Appendix 4). In most electron microscopes, the de Broglie wavelength is hundreds of times smaller than the wavelength of light. It became possible to see even smaller objects, down to single molecules.

Wave mechanics was born, the basis of the great edifice of quantum physics. De Broglie laid the foundations for the theory of interference and diffraction of light, gave a new derivation of Planck's formula, and established a deep correspondence between the motion of particles and the waves associated with them.

When studying any theory, we always noted the limits of applicability of this theory. The limits of applicability of quantum theory have not yet been established, but its laws should be applied to describe the movement of microparticles in small regions of space and at high frequencies of electromagnetic waves, when measuring instruments make it possible to register individual quanta (energy ~10 -16 J). Thus, to describe the interaction of matter and X-ray radiation, the energy of the quanta of which is two orders of magnitude greater than the limit established above, it is necessary to apply the laws of quantum physics, and to describe the properties of radio waves, the laws of classical electrodynamics are quite sufficient. It should be remembered that the main “testing ground” for quantum theory is the physics of the atom and the atomic nucleus.

Concluding today's lesson, I once again ask you the question: what is light? ( Student answers.)

Literature

1. Myakishev G.Ya., Bukhovtsev B.B. Physics. 11th grade: educational. for general education institutions: basic and professional. levels. M.: Education, 2009.
2. Video encyclopedia for public education. Lennauchfilm. Video studio "Kvart". [Electronic resource] Cassette No. 2 “Thermal radiation”.
3. Tomilin A.N. In search of origins: scientific-pop. edition. L.: Det. literature, 1990.
4. Quantum mechanics. Quantum electrodynamics // Encycl. sl. young physicist / Comp. V.A. Chuyanov. M.: Pedagogy, 1984.
5. Koltun M. World of Physics. M.: Det. literature, 1984.
6. Solopov E.F. Philosophy: textbook. aid for students higher textbook establishments. M.: Vlados, 2003.
7. Ilchenko V.R. Crossroads of physics, chemistry, biology: book. for students. M.: Education, 1986.
8. Katz Ts.B. Biophysics in physics lessons: book. for the teacher. M.: Education, 1988.

Elena Stepanovna Uvitskaya– physics teacher of the highest qualification category, graduated from the Tula State Pedagogical Institute named after. L.N. Tolstoy in 1977 and was assigned to the Urals, to the small industrial town of Lysva, where she still works. Honorary worker of general education of the Russian Federation, winner of the All-Russian competition for teachers of physics and mathematics (Dynasty Foundation). Graduates have been successfully passing the Unified State Exam for many years and entering universities in Moscow, St. Petersburg, Yekaterinburg, and Perm. Once, after reading about the Emerald Tablet, I was struck by the current relevance of the idea of ​​​​the legendary Hermes: every thing, object, process in our Universe carries the features of each other and of a single whole. Since then, he has been paying great attention to interdisciplinary connections and analogies: physics and biology, physics and mathematics, physics and literature, and now physics and the English language. He is engaged in scientific work with students, especially in elementary school: where does electricity live? Why is ordinary water so unusual? What is it like, the mysterious world of stars? The family has two sons, both graduated from Perm State Technical University. The junior is an engineer, the senior is a karate-do teacher, has a black belt, second dan, multiple champion of Russia, participant in the World Championship in Japan. The teacher’s success would have been impossible without the help of her husband, an electrical engineer by training: developing and conducting experiments, creating new devices, and simply support and advice that help in various life situations.

All applications are given in . – Ed.

The role of Maxwell’s theory was best expressed by the famous physicist Robert Feynman: “In the history of mankind (if we look at it, say, 10,000 years from now), the most significant event of the 19th century will undoubtedly be Maxwell’s discovery of the laws of electrodynamics. Against the backdrop of this important scientific discovery, the American Civil War in the same decade will look like a minor provincial incident.”

Planck hesitated for a long time whether to choose the humanities or physics. All of Planck's works are distinguished by grace and beauty. A. Einstein wrote about them: “When studying his works, one gets the impression that the requirement of artistry is one of the main springs of his creativity.”

Lesson objectives:

Educational: to form in students an idea of ​​the photoelectric effect and to study its laws to which it obeys; test the laws of the photoelectric effect using a virtual experiment.

Developmental: develop logical thinking.

Educational: fostering sociability (the ability to communicate), attention, activity, a sense of responsibility, instilling interest in the subject.

During the classes

I. Organizational moment.

– The topic of today’s lesson is “Photo effect”.

When considering this interesting topic, we continue to study the section “Quantum Physics”, we will try to find out what effect light has on matter and what this effect depends on. But first, we will review the material covered in the last lesson, without which it will be difficult to understand the intricacies of the photo effect. In the last lesson we looked at Planck's hypothesis.

What is the minimum amount of energy that a system can emit and absorb? (quantum)

Who first introduced the concept of “energy quantum” into science? (M. Planck)

An explanation of what experimental dependence contributed to the emergence of quantum physics? (law of radiation of heated solids)

What color do we see in a completely black body? (any color depending on temperature)

III. Learning new material

At the beginning of the 20th century, quantum theory was born - the theory of movement and interaction of elementary particles and systems consisting of them.

To explain the laws of thermal radiation, M. Planck suggested that atoms emit electromagnetic energy not continuously, but in separate portions - quanta. The energy of each such portion is determined by the formula E = h, Where
-Planck's constant; v is the frequency of the light wave.

Another confirmation of the correctness of quantum theory was the explanation by Albert Einstein in 1905. phenomenon photoelectric effect

Photo effect– the phenomenon of electrons being ejected from solid and liquid substances under the influence of light.

Types of PHOTO EFFECT:

1. The external photoelectric effect is the emission of electrons by a substance under the influence of electromagnetic radiation. The external photoelectric effect is observed in solids and also in gases.

2. Internal photoelectric effect is the electromagnetic radiation causing the transition of electrons inside a conductor or dielectric from bound states to free ones without escaping outside.

3. Valve photoelectric effect - the appearance of photo - emf. when illuminating the contact of two different semiconductors or a semiconductor and a metal.

Photoelectric effect was discovered in 1887 by a German physicist G. Hertz and in 1888–1890 it was experimentally studied by A.G. Stoletov. The most complete study of the phenomenon of the photoelectric effect was carried out by F. Lenard in 1900. By this time the electron had already been discovered (1897, J. Thomson), and it became clear that the photoelectric effect (or more precisely, the external photoelectric effect) consists of the ejection of electrons from a substance under the influence of light incident on it.

Study of the photoelectric effect.

The first experiments on the photoelectric effect were started by Stoletov already in February 1888.

The experiments used a glass vacuum bottle with two metal electrodes, the surface of which was thoroughly cleaned. Some voltage was applied to the electrodes U, the polarity of which could be changed using a double key. One of the electrodes (cathode K) was illuminated through a quartz window with monochromatic light of a certain wavelength. At a constant luminous flux, the dependence of the photocurrent strength was taken I from the applied voltage.

Laws of the photoelectric effect

The saturation photocurrent is directly proportional to the incident light flux.

the maximum kinetic energy of photoelectrons increases linearly with the frequency of light and does not depend on its intensity.

For each substance there is a minimum set frequency, called the red limit of the photoelectric effect, below which the photoelectric effect is impossible.

According to M. Planck's hypothesis, an electromagnetic wave consists of individual photons and radiation occurs discontinuously - in quanta, photons. Thus, the absorption of light must also occur discontinuously - photons transfer their energy to the atoms and molecules of the entire substance.

– Einstein’s equation for the photoelectric effect

mv 2 /2 = eU 0 – maximum value of the kinetic energy of the photoelectron;

– the minimum frequency of light at which the photoelectric effect is possible;

V max = hc/ Aout – maximum light frequency at which the photoelectric effect is possible

- red photo effect border

- photon momentum

Conversation with clarification of terms and concepts.

The phenomenon of a substance emitting electrons under the influence of light is called...

The number of electrons emitted by light from the surface of a substance in 1 s is directly proportional to...

The kinetic energy of photoelectrons increases linearly with ... and does not depend on ...

For each substance there is a minimum frequency of light at which the photoelectric effect is still possible. This frequency is called...

The work that needs to be done to remove electrons from the surface of a substance is called...

Einstein's equation for the photoelectric effect (formulation)…

IV. Consolidation and generalization of knowledge.

Problem 1. What is the lowest frequency of light at which the photoelectric effect is still observed if the work function of an electron from the metal is 3.3 * 10 -19 J?

Task 2. Determine the energy, mass and momentum of the photon corresponding to the longest and shortest waves of the visible spectrum?

Solution:

Problem 3. Find the photoelectric effect threshold for potassium if work function A = 1.32 EV?

Solution:

In Einstein's equation

Using the formulas you wrote down, solve the following problems on one's own.

The work function for the plate material is 4 eV. The plate is illuminated with monochromatic light. What is the energy of the photons of the incident light if the maximum kinetic energy of the photoelectrons is 2.5 eV?

A nickel plate is exposed to electromagnetic radiation with a photon energy of 8 eV. In this case, as a result of the photoelectric effect, electrons with a maximum energy of 3 eV are emitted from the plate. What is the work function of electrons from nickel?

A stream of photons with an energy of 12 eV knocks out photoelectrons from the metal, the maximum kinetic energy of which is 2 times less than the work function. Determine the work function for the given metal.

Work function of an electron leaving a metal. Find the maximum wavelength of radiation that can knock out electrons.

Determine the work function of electrons from the metal if the red limit of the photoelectric effect is 0.255 µm.

For some metal, the red limit of the photoelectric effect is light with a frequency . Determine the kinetic energy that electrons will acquire under the influence of radiation with a wavelength

Prepare a presentation on the topic “Application of the photoelectric effect”