Nobody in the world understands quantum mechanics - this is the main thing you need to know about it. Yes, many physicists have learned to use its laws and even predict phenomena using quantum calculations. But it is still not clear why the presence of an observer determines the fate of the system and forces it to make a choice in favor of one state. “Theories and Practices” selected examples of experiments, the outcome of which is inevitably influenced by the observer, and tried to figure out what quantum mechanics is going to do with such interference of consciousness in material reality.

Shroedinger `s cat

Today there are many interpretations of quantum mechanics, the most popular of which remains the Copenhagen one. Its main principles were formulated in the 1920s by Niels Bohr and Werner Heisenberg. And the central term of the Copenhagen interpretation was the wave function - a mathematical function that contains information about all possible states of a quantum system in which it simultaneously resides.

According to the Copenhagen interpretation, only observation can reliably determine the state of a system and distinguish it from the rest (the wave function only helps to mathematically calculate the probability of detecting a system in a particular state). We can say that after observation, a quantum system becomes classical: it instantly ceases to coexist in many states at once in favor of one of them.

This approach has always had its opponents (remember, for example, “God doesn’t play dice” by Albert Einstein), but the accuracy of calculations and predictions has taken its toll. However, recently there have been fewer and fewer supporters of the Copenhagen interpretation, and not the least reason for this is the very mysterious instantaneous collapse of the wave function during measurement. Erwin Schrödinger's famous thought experiment with the poor cat was precisely intended to show the absurdity of this phenomenon.

So, let us recall the contents of the experiment. A live cat, an ampoule with poison and a certain mechanism that can at random put the poison into action are placed in a black box. For example, one radioactive atom, the decay of which will break the ampoule. The exact time of atomic decay is unknown. Only the half-life is known: the time during which decay will occur with a 50% probability.

It turns out that for an external observer, the cat inside the box exists in two states at once: it is either alive, if everything goes fine, or dead, if decay has occurred and the ampoule has broken. Both of these states are described by the cat's wave function, which changes over time: the further away, the greater the likelihood that radioactive decay has already occurred. But as soon as the box is opened, the wave function collapses and we immediately see the outcome of the knacker’s experiment.

It turns out that until the observer opens the box, the cat will forever balance on the border between life and death, and only the action of the observer will determine its fate. This is the absurdity that Schrödinger pointed out.

Electron diffraction

According to a survey of leading physicists conducted by The New York Times, the experiment with electron diffraction, carried out in 1961 by Klaus Jenson, became one of the most beautiful in the history of science. What is its essence?

There is a source emitting a stream of electrons towards a photographic plate screen. And there is an obstacle in the way of these electrons - a copper plate with two slits. What kind of picture can you expect on the screen if you think of electrons as just small charged balls? Two illuminated stripes opposite the slits.

In reality, a much more complex pattern of alternating black and white stripes appears on the screen. The fact is that when passing through the slits, electrons begin to behave not like particles, but like waves (just as photons, particles of light, can simultaneously be waves). Then these waves interact in space, weakening and strengthening each other in some places, and as a result a complex picture of alternating light and dark stripes appears on the screen.

In this case, the result of the experiment does not change, and if electrons are sent through the slit not in a continuous flow, but individually, even one particle can simultaneously be a wave. Even one electron can simultaneously pass through two slits (and this is another important position of the Copenhagen interpretation of quantum mechanics - objects can simultaneously exhibit their “usual” material properties and exotic wave properties).

But what does the observer have to do with it? Despite the fact that his already complicated story became even more complicated. When, in similar experiments, physicists tried to detect with the help of instruments which slit the electron actually passed through, the picture on the screen changed dramatically and became “classical”: two illuminated areas opposite the slits and no alternating stripes.

It was as if the electrons did not want to show their wave nature under the watchful gaze of the observer. We adjusted to his instinctive desire to see a simple and understandable picture. Mystic? There is a much simpler explanation: no observation of the system can be carried out without physical influence on it. But we’ll come back to this a little later.

Heated fullerene

Experiments on particle diffraction were carried out not only on electrons, but also on much larger objects. For example, fullerenes are large, closed molecules made up of dozens of carbon atoms (for example, a fullerene of sixty carbon atoms is very similar in shape to a soccer ball: a hollow sphere stitched together from pentagons and hexagons).

Recently, a group from the University of Vienna, led by Professor Zeilinger, tried to introduce an element of observation into such experiments. To do this, they irradiated moving fullerene molecules with a laser beam. Afterwards, heated by external influence, the molecules began to glow and thereby inevitably revealed to the observer their place in space.

Along with this innovation, the behavior of molecules also changed. Before the start of total surveillance, fullerenes quite successfully skirted obstacles (exhibited wave properties) like electrons from the previous example passing through an opaque screen. But later, with the appearance of an observer, fullerenes calmed down and began to behave like completely law-abiding particles of matter.

Cooling dimension

One of the most famous laws of the quantum world is Heisenberg's uncertainty principle: it is impossible to simultaneously determine the position and speed of a quantum object. The more accurately we measure the momentum of a particle, the less accurately its position can be measured. But the effects of quantum laws operating at the level of tiny particles are usually unnoticeable in our world of large macro objects.

Therefore, the more valuable are the recent experiments of Professor Schwab’s group from the USA, in which quantum effects were demonstrated not at the level of the same electrons or fullerene molecules (their characteristic diameter is about 1 nm), but on a slightly more tangible object - a tiny aluminum strip.

This strip was secured on both sides so that its middle was suspended and could vibrate under external influence. In addition, next to the strip there was a device capable of recording its position with high accuracy.

As a result, the experimenters discovered two interesting effects. Firstly, any measurement of the object’s position or observation of the strip did not pass without leaving a trace for her - after each measurement the position of the strip changed. Roughly speaking, experimenters determined the coordinates of the strip with great accuracy and thereby, according to the Heisenberg principle, changed its speed, and therefore its subsequent position.

Secondly, and quite unexpectedly, some measurements also led to cooling of the strip. It turns out that an observer can change the physical characteristics of objects just by his presence. It sounds completely incredible, but to the credit of physicists, let’s say that they were not at a loss - now Professor Schwab’s group is thinking about how to apply the discovered effect to cool electronic chips.

Freezing particles

As you know, unstable radioactive particles decay in the world not only for the sake of experiments on cats, but also completely on their own. Moreover, each particle is characterized by an average lifetime, which, it turns out, can increase under the watchful gaze of the observer.

This quantum effect was first predicted back in the 1960s, and its brilliant experimental confirmation appeared in a paper published in 2006 by the group of Nobel laureate physicist Wolfgang Ketterle at the Massachusetts Institute of Technology.

In this work, we studied the decay of unstable excited rubidium atoms (decay into rubidium atoms in the ground state and photons). Immediately after the system was prepared and the atoms were excited, they began to be observed - they were illuminated with a laser beam. In this case, the observation was carried out in two modes: continuous (small light pulses are constantly supplied to the system) and pulsed (the system is irradiated from time to time with more powerful pulses).

The results obtained were in excellent agreement with theoretical predictions. External light influences actually slow down the decay of particles, as if returning them to their original state, far from decay. Moreover, the magnitude of the effect for the two regimes studied also coincides with predictions. And the maximum life of unstable excited rubidium atoms was extended by 30 times.

Quantum mechanics and consciousness

Electrons and fullerenes cease to exhibit their wave properties, aluminum plates cool, and unstable particles freeze in their decay: under the omnipotent gaze of the observer, the world is changing. What is not evidence of the involvement of our mind in the work of the world around us? So maybe Carl Jung and Wolfgang Pauli (Austrian physicist, Nobel Prize laureate, one of the pioneers of quantum mechanics) were right when they said that the laws of physics and consciousness should be considered complementary?

But this is only one step away from the routine recognition: the whole world around us is the essence of our mind. Creepy? (“Do you really think that the Moon exists only when you look at it?” Einstein commented on the principles of quantum mechanics). Then let's try to turn to physicists again. Moreover, in recent years they have become less and less fond of the Copenhagen interpretation of quantum mechanics with its mysterious collapse of a function wave, which is being replaced by another, quite down-to-earth and reliable term - decoherence.

The point is this: in all the observational experiments described, the experimenters inevitably influenced the system. They illuminated it with a laser and installed measuring instruments. And this is a general, very important principle: you cannot observe a system, measure its properties without interacting with it. And where there is interaction, there is a change in properties. Moreover, when the colossus of quantum objects interacts with a tiny quantum system. So eternal, Buddhist neutrality of the observer is impossible.

This is precisely what explains the term “decoherence” - an irreversible process of violation of the quantum properties of a system during its interaction with another, larger system. During such interaction, the quantum system loses its original features and becomes classical, “submitting” to the large system. This explains the paradox with Schrödinger's cat: the cat is such a large system that it simply cannot be isolated from the world. The thought experiment itself is not entirely correct.

In any case, compared to reality as an act of creation of consciousness, decoherence sounds much calmer. Maybe even too calm. After all, with this approach, the entire classical world becomes one big decoherence effect. And according to the authors of one of the most serious books in this field, statements like “there are no particles in the world” or “there is no time at a fundamental level” also logically follow from such approaches.

Creative observer or all-powerful decoherence? You have to choose between two evils. But remember - now scientists are increasingly convinced that the basis of our thought processes are those same notorious quantum effects. So where observation ends and reality begins - each of us has to choose.

- 97.50 Kb

Ministry of Education and Science of the Russian Federation

Federal State Educational Institution of Secondary Professional Education "Alekseevsky College of Economics and Information Technologies"

"The emergence and development of quantum physics"

Completed by: student of group 22

specialties: 080110

Economics and Accounting

(by industry)

Rysikov Artem

Checked by: general education teacher

Koryaka Lyudmila Mikhailovna

Alekseevka 2010

Introduction..…………………………………………………… …………………3

Chapter I The emergence and development of quantum physics………………………4

1.1 Quantum hypothesis……………………………………………………... 8

1.2 The theory of the atom by I. Bohr. Principle of correspondence………………………...11

Chapter II Problems of quantum mechanics…………………………………….13

1.4 The problem of interpretation of quantum mechanics.............. .16

Conclusion………………………………………………………………19

List of references……………………………………………………………...2 0

Introduction

According to the electromagnetic picture of the world, the world around a person is a continuous medium - a field that can have different temperatures at different points, concentrate different energy potentials, move differently, etc. A continuous medium can occupy large areas of space, its properties change continuously, and it has no sharp boundaries. These properties distinguish the field from physical bodies that have definite and clear boundaries. The division of the world into bodies and field particles, into field and space is evidence of the existence of two extreme properties of the world - discreteness and continuity. Discreteness (discontinuity) of the world means the final divisibility of the entire space-time structure into separate limited objects, properties and forms of movement, while continuity (continuity) expresses the unity, integrity and indivisibility of the object.

Within the framework of classical physics, discreteness and continuity of the world initially appear as opposite to each other, separate and independent, although in general complementary properties. In modern physics, this unity of opposites, discrete and continuous, has found its justification in the concept of wave-particle duality.

The modern quantum field picture of the world is based on a new physical theory - quantum mechanics, which describes the state and movement of micro-objects of the material world.

Chapter I. The emergence and development of quantum physics

Quantum mechanics is a theory that establishes the method of description and laws of motion of microparticles (elementary particles, atoms, molecules, atomic nuclei) and their systems, as well as the connection between quantities characterizing particles and systems with physical quantities directly measured experimentally.

The laws of quantum mechanics form the basis for the study of the structure of matter. They make it possible to clarify the structure of atoms, establish the nature of chemical bonds, explain the periodic system of elements, and study the properties of elementary particles.

Since the properties of macroscopic bodies are determined by the movement and interaction of the particles of which they are composed, the laws of quantum mechanics underlie the understanding of most macroscopic phenomena. For example, quantum mechanics made it possible to determine the structure and understand many properties of solids, to consistently explain the phenomena of ferromagnetism, superfluidity, superconductivity, to understand the nature of astrophysical objects - white dwarfs, neutron stars, and to clarify the mechanism of thermonuclear reactions in the Sun and stars.

The development of quantum mechanics dates back to the beginning of the 20th century, when physical phenomena were discovered indicating the inapplicability of Newtonian mechanics and classical electrodynamics to the processes of interaction of light with matter and processes occurring in the atom. The establishment of connections between these groups of phenomena and attempts to explain them on the basis of theory led to the discovery of the laws of quantum mechanics.

For the first time in science, ideas about quantum were expressed in 1900 by M. Planck in the process of studying the thermal radiation of bodies. Through his research, he demonstrated that energy emission occurs discretely, in certain portions - quanta, the energy of which depends on the frequency of the light wave. Planck's experiments led to the recognition of the dual nature of light, which has both corpuscular and wave properties, thus representing a dialectical unity of these opposites. Dialectics, in particular, is expressed in the fact that the shorter the wavelength of radiation, the more clearly quantum properties appear; The longer the wavelength, the brighter the wave properties of light appear.

In 1924, the French physicist L. de Broglie put forward the hypothesis that wave-particle duality is universal in nature, i.e. All particles of matter have wave properties. Later, this idea was confirmed experimentally, and the principle of wave-particle duality was extended to all processes of motion and interaction in the microworld.

In particular, N. Bohr applied the idea of ​​energy quantization to the theory of atomic structure. According to his ideas, at the center of the atom there is a positively charged nucleus, in which almost the entire mass of the atom is concentrated, and negatively charged electrons rotate in orbits around the nucleus. Rotating electrons must lose part of their energy, which entails the unstable existence of atoms. However, in practice, atoms not only exist, but are also very stable. Explaining this issue, Bohr suggested that an electron, moving along its orbit, does not emit quanta. Radiation occurs only when an electron moves from one orbit to another, i.e. from one energy level to another, with less energy. At the moment of transition, a radiation quantum is born.

In accordance with the quantum field picture of the world, any micro-object, having wave and corpuscular properties, does not have a specific trajectory of movement and cannot have certain coordinates and speed (momentum). This can only be done by determining the wave function at a given moment, and then finding its wave function at any other moment. The square of the modulus gives the probability of finding a particle at a given point in space.

In addition, the relativity of space-time in this picture of the world leads to uncertainty of coordinates and speed at a given moment, to the absence of a trajectory of movement of a micro-object. And if in classical physics the behavior of a large number of particles was subject to probabilistic laws, then in quantum mechanics the behavior of each microparticle is subject not to dynamic, but to statistical laws.

Thus, matter is two-faced: it has both corpuscular and wave properties, which manifest themselves depending on conditions. Hence, the general picture of reality in the quantum field picture of the world becomes, as it were, two-dimensional: on the one hand, it includes the characteristics of the object under study, and on the other, the observation conditions on which the certainty of these characteristics depends. This means that the picture of reality in modern physics is not only a picture of an object, but also a picture of the process of its cognition.

The idea of ​​motion changes radically, which becomes only a special case of fundamental physical interactions. There are four types of fundamental physical interactions: gravitational, electromagnetic, strong and weak. All of them are described on the basis of the modern principle of short-range action. In accordance with it, the interaction of each type is transmitted by the corresponding field from point to point. In this case, the speed of interaction transmission is always finite and cannot exceed the speed of light in a vacuum (300,000 km/s).

The specificity of quantum field concepts of regularity and causality is that they always appear in a probabilistic form, in the form of so-called statistical laws. They correspond to a deeper level of knowledge of natural laws. Thus, it turned out that our world is based on chance, probability.

Also, the new picture of the world for the first time included an observer, on whose presence the obtained research results depended. Moreover, the so-called anthropic principle was formulated, which states that our world is what it is only thanks to the existence of man. From now on, the emergence of man is considered a natural result of the evolution of the Universe.

THE EMERGENCE AND DEVELOPMENT OF QUANTUM PHYSICS

1.1 Quantum hypothesis

The origins of quantum physics can be found in studies of the processes of radiation of bodies. Back in 1809, P. Prevost concluded that every body radiates regardless of its environment. Development of spectroscopy in the 19th century. led to the fact that when studying emission spectra, attention is also beginning to be paid to absorption spectra. It turns out that there is a simple connection between the radiation and absorption of a body: in the absorption spectra, those parts of the spectrum that are emitted by a given body are absent or weakened. This law was explained only in quantum theory.

G. Kirchhoff in 1860 formulated a new law, which states that for radiation of the same wavelength at the same temperature, the ratio of emissivity and absorption abilities is the same for all bodies. In other words, if EλT and AλT are the emissive and absorption abilities of a body, respectively, depending on the wavelength λ and temperature T, then

where φ(λ, T) is some universal function of λ and T, the same for all bodies.

Kirchhoff introduced the concept of an absolutely black body as a body that absorbs all rays falling on it. For such a body, obviously, AλT = 1; then the universal function φ(λ, T) is equal to the emissivity of an absolutely black body. Kirchhoff himself did not determine the form of the function φ(λ, T), but only noted some of its properties.

When determining the form of the universal function φ(λ, T), it was natural to assume that one could use theoretical considerations, primarily the basic laws of thermodynamics. L. Boltzmann showed that the total radiation energy of a completely black body is proportional to the fourth power of its temperature. However, the task of specifically determining the form of the Kirchhoff function turned out to be very difficult, and research in this direction, based on thermodynamics and optics, did not lead to success.

The experiment gave a picture that cannot be explained from the point of view of classical concepts: in thermodynamic equilibrium between the oscillating atoms of matter and electromagnetic radiation, almost all the energy is concentrated in the oscillating atoms and only an insignificant part of it accounts for the radiation, whereas according to the classical theory, almost all the energy should go to the electromagnetic field.

In the 80s XIX century Empirical studies of the patterns of distribution of spectral lines and the study of the function φ(λ, T) have become more intensive and systematic. Experimental equipment has been improved. For the radiation energy of a completely black body, V. Wien in 1896 and J. Rayleigh and J. Jeans in 1900 proposed two different formulas. As experimental results have shown, the Wien formula is asymptotically correct in the region of short waves and gives sharp discrepancies with experiment in the region of long waves, and the Rayleigh-Jeans formula is asymptotically correct for long waves, but is not applicable for short waves.

In 1900, at a meeting of the Berlin Physical Society, M. Planck proposed a new formula for the distribution of energy in the spectrum of a sulfur body. This formula gave full agreement with experiment, but its physical meaning was not entirely clear. Additional analysis showed that it makes sense only if we omit that the radiation of energy does not occur continuously, but in limited portions - quanta (ε). Moreover, ε is not any quantity, namely, ε = hν, where h is a certain constant and v is the frequency of light. This led to the recognition, along with the atomism of matter, of the atomism of energy or action, the discrete, quantum nature of radiation, which did not fit into the framework of the concepts of classical physics.

The formulation of the energy quanta hypothesis was the beginning of a new era in the development of theoretical physics. With great success, this hypothesis began to be used to explain other phenomena that could not be described on the basis of the concepts of classical physics.

An essentially new step in the development of the quantum hypothesis was the introduction of the concept of light quanta. This idea was developed in 1905 by Einstein and used by him to explain the photoelectric effect. A number of studies have provided evidence of the truth of this idea. In 1909, Einstein, continuing his research into the laws of radiation, showed that light has both wave and corpuscular properties. It became increasingly obvious that the wave-particle duality of light radiation cannot be explained from the standpoint of classical physics. In 1912, A. Poincaré finally proved the incompatibility of Planck’s formula and classical mechanics. New concepts, new ideas and a new scientific language were required so that physicists could comprehend these unusual phenomena. All this appeared later - along with the creation and development of quantum mechanics.

Chapter II Problems of quantum mechanics…………………………………….13
1.3 Creation of non-relativistic quantum mechanics………………...13
1.4 The problem of interpretation of quantum mechanics............16
Conclusion………………………………………………………………………………19
List of references……………………………………………………………...20

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, formulate 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 London newspapers wrote on the eve of 1900: “When the streets of London were lit up with festive lights made of bright light bulbs instead of dim oil bowls, 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 radiation flux, 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: carry out 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.”

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 appear 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 increasingly 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