Over the past several decades, researchers have achieved a greater understanding of how quantum mechanics inevitably becomes classical mechanics through an interaction between a particle or other microscopic system and its surrounding environment. This means that many independent observers can make measurements of a quantum system and agree on the outcome — a hallmark of classical behavior.

This idea, called quantum Darwinism QD , explains a lot about why we experience the world the way we do rather than in the peculiar way it manifests at the scale of atoms and fundamental particles. Although aspects of the puzzle remain unresolved, QD helps heal the apparent rift between quantum and classical physics. Only recently, however, has quantum Darwinism been put to the experimental test.

Three research groups, working independently in Italy, China and Germany, have looked for the telltale signature of the natural selection process by which information about a quantum system gets repeatedly imprinted on various controlled environments. Yet so far, the theory checks out. At the heart of quantum Darwinism is the slippery notion of measurement — the process of making an observation. In classical physics, what you see is simply how things are. What more is there to say? Before the measurement, the various superposed states interfere with one another in a wavelike manner, producing outcomes with higher or lower probabilities.

The answer often given is that superpositions are fragile, easily disrupted when a delicate quantum system is buffeted by its noisy environment. The two are now in a joint superposition. If the atom is spin-up, then the air molecule might be pushed one way, while, if the atom is spin-down, the air molecule goes another way — and these two possibilities coexist. As the particles experience yet more collisions with other air molecules, the entanglement spreads, and the superposition initially specific to the atom becomes ever more diffuse.

The process happens extremely fast. Surprisingly, although decoherence is a straightforward consequence of quantum mechanics, it was only identified in the s, by the late German physicist Heinz-Dieter Zeh. The Polish-American physicist Wojciech Zurek further developed the idea in the early s and made it better known, and there is now good experimental support for it.

## The Universe Has A Speed Limit, And It Isn't The Speed Of Light

First, quantum systems must have states that are especially robust in the face of disruptive decoherence by the environment. A particular location of a particle, for instance, or its speed, the value of its quantum spin, or its polarization direction can be registered as the position of a pointer on a measuring device. Zurek argues that classical behavior — the existence of well-defined, stable, objective properties — is possible only because pointer states of quantum objects exist. Superpositions of different locations, however, are not pointer states: Interactions with the environment decohere them into localized pointer states, so that only one can be observed.

Although immunity to interaction with the environment assures the stability of a pointer state, we still have to get at the information about it somehow. When you see an object, for example, that information is delivered to your retina by the photons scattering off it. They carry information to you in the form of a partial replica of certain aspects of the object, saying something about its position, shape and color.

Lots of replicas are needed if many observers are to agree on a measured value — a hallmark of classicality. Thus, as Zurek argued in the s, our ability to observe some property depends not only on whether it is selected as a pointer state, but also on how substantial a footprint it makes in the environment.

It turns out that the same stability property that promotes environment-induced superselection of pointer states also promotes quantum Darwinian fitness, or the capacity to generate replicas. Just how perfectly remains contentious, however. Some researchers think decoherence and QD provide a complete account of the quantum-classical transition. When a particular location of a particle is selected, what happens to the other possibilities inherent in its quantum description?

Were they ever in any sense real?

Researchers are compelled to adopt philosophical interpretations of quantum mechanics precisely because no one can figure out a way to answer that question experimentally. Starting from only one simple assumption about the rule that the orbits must obey, the Bohr model was able to relate the observed spectral lines in the emission spectrum of hydrogen to previously known constants.

In Bohr's model the electron was not allowed to emit energy continuously and crash into the nucleus: once it was in the closest permitted orbit, it was stable forever. Bohr's model didn't explain why the orbits should be quantized in that way, nor was it able to make accurate predictions for atoms with more than one electron, or to explain why some spectral lines are brighter than others. Some fundamental assumptions of the Bohr model were soon proven wrong—but the key result that the discrete lines in emission spectra are due to some property of the electrons in atoms being quantized is correct.

The way that the electrons actually behave is strikingly different from Bohr's atom, and from what we see in the world of our everyday experience; this modern quantum mechanical model of the atom is discussed below. Bohr theorized that the angular momentum , L , of an electron is quantized:. Starting from this assumption, Coulomb's law and the equations of circular motion show that an electron with n units of angular momentum orbit a proton at a distance r given by.

For simplicity this is written as. The Bohr radius is the radius of the smallest allowed orbit.

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The energy of the electron [note 6] can also be calculated, and is given by. Thus Bohr's assumption that angular momentum is quantized means that an electron can inhabit only certain orbits around the nucleus, and that it can have only certain energies. A consequence of these constraints is that the electron does not crash into the nucleus: it cannot continuously emit energy, and it cannot come closer to the nucleus than a 0 the Bohr radius.

An electron loses energy by jumping instantaneously from its original orbit to a lower orbit; the extra energy is emitted in the form of a photon.

Conversely, an electron that absorbs a photon gains energy, hence it jumps to an orbit that is farther from the nucleus. Each photon from glowing atomic hydrogen is due to an electron moving from a higher orbit, with radius r n , to a lower orbit, r m. This equation has the same form as the Rydberg formula, and predicts that the constant R should be given by.

Therefore, the Bohr model of the atom can predict the emission spectrum of hydrogen in terms of fundamental constants. Just as light has both wave-like and particle-like properties, matter also has wave-like properties. Matter behaving as a wave was first demonstrated experimentally for electrons: a beam of electrons can exhibit diffraction , just like a beam of light or a water wave. The relationship, called the de Broglie hypothesis, holds for all types of matter: all matter exhibits properties of both particles and waves.

The concept of wave—particle duality says that neither the classical concept of "particle" nor of "wave" can fully describe the behavior of quantum-scale objects, either photons or matter. Wave—particle duality is an example of the principle of complementarity in quantum physics. In the double-slit experiment, as originally performed by Thomas Young and Augustin Fresnel in , a beam of light is directed through two narrow, closely spaced slits, producing an interference pattern of light and dark bands on a screen.

In fact, a much simpler pattern is seen, a simple diffraction pattern. Closing one slit results in a much simpler pattern diametrically opposite the open slit. Exactly the same behavior can be demonstrated in water waves, and so the double-slit experiment was seen as a demonstration of the wave nature of light. Variations of the double-slit experiment have been performed using electrons, atoms, and even large molecules, [29] [30] and the same type of interference pattern is seen.

Thus it has been demonstrated that all matter possesses both particle and wave characteristics.

Even if the source intensity is turned down, so that only one particle e. The quantum particle acts as a wave when passing through the double slits, but as a particle when it is detected. This is a typical feature of quantum complementarity: a quantum particle acts as a wave in an experiment to measure its wave-like properties, and like a particle in an experiment to measure its particle-like properties.

The point on the detector screen where any individual particle shows up is the result of a random process. However, the distribution pattern of many individual particles mimics the diffraction pattern produced by waves. De Broglie expanded the Bohr model of the atom by showing that an electron in orbit around a nucleus could be thought of as having wave-like properties. In particular, an electron is observed only in situations that permit a standing wave around a nucleus.

An example of a standing wave is a violin string, which is fixed at both ends and can be made to vibrate. The waves created by a stringed instrument appear to oscillate in place, moving from crest to trough in an up-and-down motion. The wavelength of a standing wave is related to the length of the vibrating object and the boundary conditions. De Broglie suggested that the allowed electron orbits were those for which the circumference of the orbit would be an integer number of wavelengths.

The electron's wavelength therefore determines that only Bohr orbits of certain distances from the nucleus are possible. In turn, at any distance from the nucleus smaller than a certain value it would be impossible to establish an orbit. The minimum possible distance from the nucleus is called the Bohr radius.

## Nuclear Physics

In , Otto Stern and Walther Gerlach shot silver atoms through an inhomogeneous magnetic field. In classical mechanics, a magnet thrown through a magnetic field may be, depending on its orientation if it is pointing with its northern pole upwards or down, or somewhere in between , deflected a small or large distance upwards or downwards.

The atoms that Stern and Gerlach shot through the magnetic field acted in a similar way. However, while the magnets could be deflected variable distances, the atoms would always be deflected a constant distance either up or down. This implied that the property of the atom that corresponds to the magnet's orientation must be quantized, taking one of two values either up or down , as opposed to being chosen freely from any angle. Ralph Kronig originated the theory that particles such as atoms or electrons behave as if they rotate, or "spin", about an axis.

Spin would account for the missing magnetic moment , [ clarification needed ] and allow two electrons in the same orbital to occupy distinct quantum states if they "spun" in opposite directions, thus satisfying the exclusion principle. The quantum number represented the sense positive or negative of spin.

The choice of orientation of the magnetic field used in the Stern—Gerlach experiment is arbitrary. In the animation shown here, the field is vertical and so the atoms are deflected either up or down. If the magnet is rotated a quarter turn, the atoms are deflected either left or right.

Using a vertical field shows that the spin along the vertical axis is quantized, and using a horizontal field shows that the spin along the horizontal axis is quantized. If, instead of hitting a detector screen, one of the beams of atoms coming out of the Stern—Gerlach apparatus is passed into another inhomogeneous magnetic field oriented in the same direction, all of the atoms are deflected the same way in this second field.

However, if one of these beams e. The action of measuring the atoms' spin with respect to a horizontal field has changed their spin with respect to a vertical field. In , Werner Heisenberg attempted to solve one of the problems that the Bohr model left unanswered, explaining the intensities of the different lines in the hydrogen emission spectrum. Through a series of mathematical analogies, he wrote out the quantum-mechanical analog for the classical computation of intensities. This calculation accurately reproduced the energy levels of the Bohr model.

Yet the two men disagreed on the interpretation of their mutual theory. Bohr, Heisenberg, and others tried to explain what these experimental results and mathematical models really mean. Their description, known as the Copenhagen interpretation of quantum mechanics, aimed to describe the nature of reality that was being probed by the measurements and described by the mathematical formulations of quantum mechanics.

Various consequences of these principles are discussed in more detail in the following subsections.

- Quantum Physics.
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It can be assumed that the car has a definite position and speed at a particular moment in time. How accurately these values can be measured depends on the quality of the measuring equipment. If the precision of the measuring equipment is improved, it provides a result closer to the true value.