> Young's double slit experiment

Explore Young's experiment with cracks. Read what is the distance between slits in Young's experiment, the width of the strip and two holes, the characteristics of light as a wave, experiment.

In his experiment, Thomas Young showed that matter and energy are capable of exhibiting the characteristics of waves and particles.

Learning Objective

• Understand why Jung's experiment seems more plausible than Huygens's expressions.

Main points

• The wave characteristics cause the light passing through the slit to interfere with itself, forming light and dark areas.
• If the waves interfere at the crests, but converge in phase, then we encounter constructive interference. If the waves do not completely coincide, then this is destructive interference.
• Each point on the wall has a different distance to the slot. These paths correspond to different numbers of waves.

Terms

• Destructive interference - waves interfere and do not correspond to each other.
• Constructive interference - waves interfere at the crests, but are in phase.

The double slit experiment shows that matter and energy can behave like waves or particles. In 1628, Christian Huygengs proved that light appears as a wave. But some people disagreed, especially Isaac Newton. He believed that an explanation would require color interference and diffraction effects. Until 1801, no one believed that light was a wave, until Thomas Young came along with his double slit experiment - Young's experiment. He made two closely spaced vertical slits (the approximate distance between the slits in Jung's experiment can be seen in the lower diagram) and shined light through them, observing the pattern created on the wall.

Light passes through two vertical slits and is diffracted into two vertical lines arranged horizontally. If it were not for diffraction and interference, then the light would simply create two lines

Duality of wave particles

Due to wave characteristics, light passes through the slits and collides, forming light and dark regions on the wall. It is scattered and absorbed by the wall, acquiring the characteristics of particles.

Jung's experiment

Why did Young's experiment with two slits convince everyone? Huygens was initially right, but he was unable to demonstrate his conclusions in practice. Light has relatively short wavelengths, so for demonstration it must come into contact with something small.

The example uses two coherent light sources with the same monochromatic wavelength (in phase). That is, the two sources will create constructive or destructive interference.

Constructive and destructive interference

Constructive interference occurs when waves interfere along the crests but are in phase. This will amplify the resulting wave. The destructive ones completely interfere with each other and do not coincide, which cancels the wave.

The two slits form two coherent wave sources that interfere with each other. (a) – Light is scattered from each slit due to their narrowness. The waves overlap and interfere constructively (bright lines) and destructively (dark areas). (b) – The double slit pattern for water waves practically coincides with light waves. The greatest activity is noticeable in areas with destructive interference. (c) – When light hits the screen, we encounter a similar pattern

The wave amplitudes add up. (a) – Pure constructive interference is possible if identical waves are out of phase. (b) – Pure destructive interference – identical waves are not exactly in phase

The created pattern will not be random. Each slot is located at a certain distance. All waves start out with the same phase, but the distance from a point on the wall to the crack creates a type of interference.

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 stream, 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 the 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.

The essence of the experiment is that a beam of light is directed onto an opaque screen screen with two parallel slits, behind which another projection screen is installed. The peculiarity of the slits is that their width is approximately equal to the wavelength of the emitted light. It would be logical to assume that photons should pass through the slits, creating two parallel stripes of light on the back screen. But instead, light travels in stripes that alternate between areas of light and darkness, meaning light behaves like a wave. This phenomenon is called "interference", and it was its demonstration by Thomas Young that proved the validity of the wave theory. Rethinking this experiment could combine quantum mechanics with another mainstay of theoretical physics, Einstein's general theory of relativity, a challenge that remains elusive in practice.

In order to calculate the probability of a photon appearing at a particular location on a screen, physicists use a principle called the Born rule. However, there is no reason for this - the experiment always goes the same way, but no one knows why. Some enthusiasts have tried to explain this phenomenon by interpreting the quantum mechanical "many worlds" theory, which suggests that all possible states of a quantum system can exist in parallel universes, but these efforts have come to nothing.

This circumstance allows us to use the Born rule as proof of the presence of inconsistencies in quantum theory. In order to unify quantum mechanics, which operates on the universe on narrow time scales, and general relativity, which operates on vast time scales, one of the theories must give way. If Born's rule is incorrect, then this will be the first step towards studying quantum gravity. “If Born's rule is broken, then the fundamental axiom of quantum mechanics will be broken, and we will know where to look for the answer to theories about quantum gravity,” says James Quatsch of the Institute of Science and Technology in Spain.

Kwatch proposed a new way to test Born's rule. He started from the idea of ​​the physicist Feynman: in order to calculate the probability of a particle appearing at a particular point on the screen, you must consider all the possible ways in which this could happen, even if they seem ridiculous. “Even the probability that the particle will fly to the Moon and return back is taken into account,” says Quatsch. Almost none of the paths will affect the final location of the photon, but some, quite unusual ones, may end up changing its coordinates. For example, suppose we have three ways for a particle to fly through a screen, instead of the obvious two (i.e., instead of one slit or another). The Born rule in this case allows us to consider interference that may arise between two obvious options, but not between all three.

James showed that if all possible deviations are taken into account, the final probability of a photon hitting point X will be different from the result assumed by Born's rule. He proposed using a wandering zigzag as a third path: thus, the particle passes first through the left hole, then through the right, and only then goes to the screen. If the third path interferes with the first two, the result of the calculations will also change. Quatch's work has generated a lot of interest, and Aninda Sinha at the Indian Institute of Science in Bangalore, a member of the team that first proposed using tortuous, "unconventional" routes to disprove the Born rule, fully agrees. However, the scientist also points out that there are too many unaccounted probabilities for us to now be able to talk about the purity of the experiment. Be that as it may, the results of this work will open the door for humanity to a deeper understanding of reality.

According to a survey of famous physicists conducted by The New York Times, the electron diffraction experiment is one of the most amazing studies in the history of science. What is its nature? There is a source that emits a beam of electrons onto a light-sensitive screen. And there is an obstacle in the way of these electrons, a copper plate with two slits.

What kind of picture can we expect on the screen if electrons usually appear to us as small charged balls? Two stripes opposite the slots in the copper plate. But in fact, a much more complex pattern of alternating white and black stripes appears on the screen. This is due to the fact that when passing through a slit, electrons begin to behave not only as particles, but also as waves (photons or other light particles that can be a wave at the same time behave in the same way).

These waves interact in space, colliding and reinforcing each other, and as a result, a complex pattern of alternating light and dark stripes is displayed on the screen. At the same time, the result of this experiment does not change even if the electrons pass one after another - even one particle can be a wave and pass through two slits simultaneously. This postulate was one of the main ones in the Copenhagen interpretation of quantum mechanics, where particles can simultaneously exhibit their “ordinary” physical properties and exotic properties as a wave.

But what about the observer? It is he who makes this confusing story even more confusing. When physicists, during similar experiments, tried to determine with the help of instruments which slit the electron actually passed through, the picture on the screen changed dramatically and became “classical”: with two illuminated sections exactly opposite the slits, without any alternating stripes.

The electrons seemed reluctant to reveal their wave nature to the watchful eye of observers. It looks like a mystery shrouded in darkness. But there is a simpler explanation: observation of the system cannot be carried out without physical influence on it. We will discuss this later.

2. Heated fullerenes

Experiments on particle diffraction were carried out not only with electrons, but also with other, much larger objects. For example, fullerenes, large and closed molecules consisting of several dozen carbon atoms, were used. Recently, a group of scientists from the University of Vienna, led by Professor Zeilinger, tried to incorporate an element of observation into these experiments. To do this, they irradiated moving fullerene molecules with laser beams. Then, heated by an external source, the molecules began to glow and inevitably display their presence to the observer.

Along with this innovation, the behavior of molecules also changed. Before such comprehensive observations began, fullerenes were quite successful in avoiding obstacles (exhibiting wave properties), similar to the previous example with electrons hitting the screen. But with the presence of an observer, fullerenes began to behave like completely law-abiding physical particles.

3. Cooling dimension

One of the most famous laws in the world of quantum physics is that it is impossible to determine the speed and position of a quantum object at the same time. The more accurately we measure a particle's momentum, the less accurately we can measure its position. However, in our macroscopic real world, the validity of quantum laws acting on tiny particles usually goes unnoticed.

The recent experiments of Professor Schwab from the USA make a very valuable contribution to this field. Quantum effects in these experiments were demonstrated not at the level of electrons or fullerene molecules (the approximate diameter of which is 1 nm), but on larger objects, a tiny aluminum strip. This tape was fixed on both sides so that its middle was suspended and could vibrate under external influence. In addition, a device was placed nearby that could accurately record the position of the tape. The experiment revealed several interesting things. First, any measurement related to the position of the object and observation of the tape influenced it; after each measurement, the position of the tape changed.

The experimenters determined the coordinates of the tape with high accuracy, and thus, in accordance with the Heisenberg principle, changed its speed, and therefore its subsequent position. Secondly, and quite unexpectedly, some measurements led to cooling of the tape. Thus, an observer can change the physical characteristics of objects simply by his presence.

4. Freezing particles

As is known, unstable radioactive particles decay not only in experiments with cats, but also on their own. Each particle has an average lifespan, which, as it turns out, can increase under the watchful eye of an observer. This quantum effect was predicted back in the 60s, and its brilliant experimental proof appeared in a paper published by a team led by Nobel laureate physicist Wolfgang Ketterle from the Massachusetts Institute of Technology.

In this work, the decay of unstable excited rubidium atoms was studied. Immediately after preparing the system, the atoms were excited using a laser beam. The observation took place in two modes: continuous (the system was constantly exposed to small light pulses) and pulsed (the system was irradiated from time to time with more powerful pulses).

The results obtained were fully consistent with theoretical predictions. External light effects slow down the decay of particles, returning them to their original state, which is far from the state of decay. The magnitude of this effect was also consistent with predictions. The maximum lifetime of unstable excited rubidium atoms increased by 30 times.

5. Quantum mechanics and consciousness

Electrons and fullerenes cease to show their wave properties, aluminum plates cool down, and unstable particles slow down their decay. The watchful eye of the observer literally changes the world. Why can't this be proof of the involvement of our minds in the workings of the world? Perhaps Carl Jung and Wolfgang Pauli (Austrian physicist, Nobel Prize winner, pioneer of quantum mechanics) were right, after all, when they said that the laws of physics and consciousness should be seen as complementary to each other?

We are one step away from recognizing that the world around us is... The idea is scary and tempting. Let's try to turn to physicists again. Especially in recent years, when fewer and fewer people believe the Copenhagen interpretation of quantum mechanics with its mysterious wave function collapses, turning to the more mundane and reliable decoherence.

The point is that in all these observational experiments, the experimenters inevitably influenced the system. They lit it with a laser and installed measuring instruments. They shared an important principle: you cannot observe a system or measure its properties without interacting with it. Any interaction is a process of modification of properties. Especially when a tiny quantum system is exposed to colossal quantum objects. Some eternally neutral Buddhist observer is impossible in principle. This is where the term “decoherence” comes into play, which is irreversible from a thermodynamic point of view: the quantum properties of a system change when it interacts with another large system.

During this interaction, the quantum system loses its original properties and becomes classical, as if “submitting” to the larger system. This also explains the paradox of Schrödinger's cat: a cat is too large a system, so it cannot be isolated from the rest of the world. The very design of this thought experiment is not entirely correct.

In any case, if we assume the reality of the act of creation by consciousness, decoherence seems to be a much more convenient approach. Perhaps even too convenient. With this approach, the entire classical world becomes one big consequence of decoherence. And as the author of one of the most famous books in this field stated, this approach logically leads to statements like “there are no particles in the world” or “there is no time at a fundamental level.”

What is the truth: the creator-observer or powerful decoherence? We need to choose between two evils. Nevertheless, scientists are increasingly convinced that quantum effects are a manifestation of our mental processes. And where observation ends and reality begins depends on each of us.

Based on materials from topinfopost.com

The King's New Mind [On Computers, Thinking and the Laws of Physics] Roger Penrose

Double slit experiment

Double slit experiment

Consider an “archetypal” quantum mechanical experiment in which a beam of electrons, light, or any other “wave-particle” is directed through two narrow slits onto a screen located behind them (Figure 6.3).

Rice. 6.Z. Experiment with two slits and monochromatic light (Notations in the figure: S (English) source) - source, t (English) top) - upper [slit], b (English) bottom) - lower [gap]. - Note edit.)

For greater specificity, let's choose light and we agree to call a quantum of light a “photon” according to the accepted terminology. The most obvious manifestation of light as a flow particles(photons) is observed on the screen. Light reaches the screen in the form of discrete point portions of energy, which are always related to the frequency of light by Planck's formula: E = hv . Energy is never transferred as a “half” (or other fraction) of a photon. Photon detection is an all-or-nothing phenomenon. Only an integer number of photons is ever observed.

But when photons pass through two slits, they detect wave behavior. Let us assume that at first only one slot is open (and the second is tightly closed). Having passed through this slit, the light beam is “scattered” (this phenomenon is called diffraction and is characteristic of wave propagation). For now, we can still adhere to the corpuscular point of view and assume that the expansion of the beam is due to the influence of the edges of the slit, causing the photons to deviate by a random amount in both directions. When the light passing through the slit is of sufficient intensity (the number of photons is large), the illumination of the screen appears uniform. But if the light intensity is reduced, then we can say with confidence that the illumination of the screen will break up into separate spots - in accordance with the corpuscular theory. Bright spots are located where individual photons reach the screen. The seemingly uniform distribution of illumination is a statistical effect due to the very large number of photons involved in the phenomenon (Fig. 6.4).

Rice. 6.4. A picture of the intensity distribution on the screen when only one slit is open: a distribution of discrete tiny specks is observed

(For comparison, a 60-watt light bulb emits about 100,000,000,000,000,000,000 photons per second!) Photons do indeed deflect randomly as they pass through the slit. Moreover, deviations at different angles have different probabilities, which gives rise to the observed distribution of illumination on the screen.

But the main difficulty for the corpuscular picture arises when we open the second slit! Suppose the light is emitted by a yellow sodium lamp, this means that it has a pure color without impurities, or, to use a physics term, light monochromatic, i.e., it has one specific frequency, or, in the language of the corpuscular picture, all photons have the same energy. The wavelength in this case is about 5 x 10 -7 m. Let us assume that the slits are about 0.001 mm wide and spaced from each other at a distance of about 0.15 mm, and the screen is located at a distance of about 1 m from them. With sufficient high light intensity, the illumination distribution still appears uniform, but now there is some semblance of undulation, called interference pattern - stripes are observed on the screen approximately 3 mm from the center (Fig. 6.5).

Rice. 6.5. Pattern of intensity distribution when both slits are open: a wave-like distribution of discrete spots is observed

By opening the second slit, we hoped to see twice the illumination of the screen (and this, indeed, would be true if we consider full screen illumination). But it turned out that now detailed painting illumination is completely different from that which occurred with one open slit. At those points of the screen where illumination is maximum, its intensity is not in two, and in four times more than what it was before. At other points, where illumination is minimal, the intensity drops to zero. Points with zero intensity perhaps pose the greatest mystery to the corpuscular point of view. These are the points that a photon could safely reach if only one slit were open. Now, when we opened the second crack, it unexpectedly turned out that something interfered the photon to get to where it could have gotten before. How could it happen that by giving the photon alternative route, we actually prevented its passage along any of the routes?

If we take its wavelength as the “size” of a photon, then on the photon scale the second slit is located from the first at a distance of about 300 “photon sizes” (and the width of each slit is about two photon wavelengths) (Fig. 6.6).

Rice. 6.6. Slits “from the point of view” of the photon! Can it really matter to a photon whether the second slit, located at a distance of about 300 “photon sizes,” is open or closed?

How does a photon passing through one of the slits “know” whether the other slit is open or closed? In fact, there is, in principle, no limit to the distance that slits can be spaced apart for the "quenching or amplification" phenomenon to occur.

It seems that when light passes through one or two slits, it behaves like wave , and not as a corpuscle (particle)! Such suppression - destructive interference - a well-known property of ordinary waves. If each of the two routes can be traversed separately by a wave, then when are they open to it? both route, it may turn out that they cancel each other out. In Fig. Figure 6.7 shows how this happens.

Rice. 6.7. The pure wave picture allows us to conceptualize the distribution of light and dark stripes on the screen (but not discreteness) in terms of wave interference

When some part of the wave, having passed through one of the slits, meets part of the wave that has passed through the other slit, they reinforce each other if they are “in phase” (i.e., if two crests or two troughs meet), or cancel each other each other if they are “out of phase” (i.e. the crest of one part meets the trough of the other). In the double-slit experiment, bright spots on the screen appear where the distances to the slits differ by whole the number of wavelengths so that the crests meet the crests, and the troughs - the troughs, and dark places appear where the difference of these distances is equal to a half-integer number of wavelengths so that the crests meet the troughs, and the troughs - with the crests.

There is nothing mysterious in the behavior of an ordinary macroscopic classical wave passing simultaneously through two slits. A wave is ultimately just a “perturbation” of either some continuous medium (field) or some substance consisting of myriads of tiny point particles. The disturbance can partially pass through one slit and partially through another slit. But in the corpuscular picture the situation is different: each individual photon itself behaves like a wave! In a sense, every particle passes through both slits at once and interferes with myself ! For, if you significantly reduce the total intensity of light, you can guarantee that no more than one photon will be located near the slits at a time. The phenomenon of destructive interference, where two alternative routes somehow “manage” to exclude each other from being realized, is something that applies to alone photon. If only one of two routes is open to a photon, then the photon can travel along it. If another route is open, then the photon can take the second route instead of the first route. But if in front of the photon are open both route, then these two possibilities miraculously exclude each other, and it turns out that the photon cannot travel along either route!

I strongly advise the reader to stop and think about the meaning of this unusual fact. The point is not that light behaves like waves in some cases and like particles in others. Each particle separately itself behaves like a wave; And the various alternative possibilities that open up to a particle can sometimes completely destroy each other!

Does a photon actually split into two and partly pass through one slit and partly through the other? Most physicists will object to this formulation of the question. In their opinion, both routes open to the particle should contribute to the final result; they are just additional modes of motion, and one should not think that a particle must split into two in order to pass through the slits. To confirm the point of view that a particle does not pass partly through one slit and partly through another, we can consider a modified situation in which a particle detector. In this case, the photon (or any other particle) always appears as a single whole, and not as some fraction of the whole: after all, our detector registers either a whole photon or a complete absence of photons. However, if the detector is located close enough to one of the slits that the observer can distinguish, through which of them the photon passed, then the interference pattern on the screen disappears. For interference to occur there appears to be a "lack of knowledge" as to which of the slits the particle "really" passed through.

To get interference, both the alternatives must contribute, sometimes “adding”, reinforcing each other twice as much as might be expected, and sometimes “subtracting” to mysteriously to repay each other. In fact, according to the rules of quantum mechanics, something even more mysterious is actually happening! Of course, alternatives can add up (the brightest dots on the screen), alternatives can subtract (dark dots), but they can also form such strange combinations as:

alternative A + i x alternative IN ,

Where i - “square root of minus one” ( i = ? -1 ), which we already met in Chapter 3 (at points on the screen with intermediate light intensity). In fact any complex the number can play the role of a coefficient in a “combination of alternatives”!

The reader may already recall my warning in Chapter 3 that complex numbers play “an absolutely fundamental role in the structure of quantum mechanics.” Complex numbers are not just mathematical curiosities. Physicists were forced to pay attention to them by convincing and unexpected experimental facts. To understand quantum mechanics, we must become more familiar with the language of complex-valued weighting coefficients. Let's look at what consequences this leads to.

III. EXCHANGE BETWEEN TWO DIVISIONS: I (v+ t)ON II c We start with a major exchange between two divisions. (1,000v +1,000m.) I - these values, which in the hands of their producers exist in the natural form of means of production, are exchanged for 2,000 IIc, for values,

CHOICE BETWEEN TWO WORLDS During the day, our awareness rushes between two worlds, and only one of them is a reliable reality. The first world can be called objective; it includes what exists or happens - but nothing beyond that. For example, I

III. Exchange between two divisions: I (v+ t) to II c We start with a large exchange between two divisions. (1,000v +1,000m.) I - these values, which in the hands of their producers exist in the natural form of means of production, are exchanged for 2,000 IIc, for values,