Quantum Mechanics: Friend Or Foe Of Physics Laws?

does quantum mechanics break any of the laws of physics

Quantum mechanics is a fundamental theory that describes the behaviour of nature at and below the scale of atoms. It is the foundation of all quantum physics, which includes quantum chemistry, quantum field theory, quantum technology, and quantum information science. Quantum mechanics can describe many systems that classical physics cannot. Classical physics can describe many aspects of nature at an ordinary (macroscopic and (optical) microscopic) scale, but is not sufficient for describing them at very small submicroscopic (atomic and subatomic) scales. Quantum mechanics does not violate the laws of classical physics; rather, it is a more accurate model that can explain observations that could not be reconciled with classical physics.

Characteristics Values
Quantum mechanics violates laws of physics No
Quantum mechanics violates classical physics Yes
Quantum mechanics is a theory Yes
Quantum mechanics is a hypothesis No
Quantum mechanics can describe many systems that classical physics cannot Yes
Quantum mechanics is the foundation of Quantum physics
Quantum mechanics is typically applied to Microscopic systems

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Quantum mechanics and classical physics

Quantum mechanics is a fundamental theory that describes the behaviour of nature at and below the scale of atoms. It is the foundation of all quantum physics, which includes quantum chemistry, quantum field theory, quantum technology, and quantum information science. Quantum mechanics can describe many systems that classical physics cannot. Classical physics can describe many aspects of nature at an ordinary (macroscopic and (optical) microscopic) scale, but is not sufficient for describing them at very small submicroscopic (atomic and subatomic) scales.

Most theories in classical physics can be derived from quantum mechanics as an approximation, valid at large (macroscopic/microscopic) scale. Quantum mechanics arose gradually from theories to explain observations that could not be reconciled with classical physics.

Quantum mechanics allows the calculation of properties and behaviour of physical systems. It is typically applied to microscopic systems: molecules, atoms and sub-atomic particles. It has been demonstrated to hold for complex molecules with thousands of atoms, but its application to human beings raises philosophical problems.

One of the most important features of quantum mechanics is that it usually cannot predict with certainty what will happen, but only give probabilities. For example, a quantum particle like an electron can be described by a wave function, which associates to each point in space a probability amplitude. Applying the Born rule to these amplitudes gives a probability density function for the position that the electron will be found to have when an experiment is performed to measure it. This is the best the theory can do; it cannot say for certain where the electron will be found.

Classical physics, on the other hand, is a theory that deals with matter and energy on a macroscopic level. It is a branch of physics that studies the behaviour of systems of intermediate size, which are too large for quantum mechanics to be applicable and too small to be treated as a collection of individual molecules. Classical physics includes mechanics, optics, acoustics, thermodynamics, and electromagnetism.

Classical physics is based on Newton's laws of motion, which describe the motion of objects under the influence of forces. It also includes the laws of thermodynamics, which describe the behaviour of heat and energy in systems. Classical physics provides a good approximation to the behaviour of macroscopic objects, such as baseballs, planets, and stars. It also provides a good description of many microscopic systems, such as the motion of molecules in a gas.

While classical physics and quantum mechanics are two different frameworks, they are not mutually exclusive. In fact, classical physics can be derived from quantum mechanics as an approximation valid at large scales. Quantum mechanics, on the other hand, can be seen as a more fundamental theory that reduces to classical physics in certain limits.

In conclusion, quantum mechanics and classical physics are two different but complementary frameworks for understanding the world. Classical physics works well for large systems, while quantum mechanics is needed to describe the behaviour of very small particles. Together, they provide a powerful set of tools for understanding the universe, from the smallest particles to the largest galaxies.

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Quantum mechanics and thermodynamics

Quantum thermodynamics is the study of the relationship between thermodynamics and quantum mechanics. The two theories address the physical phenomena of light and matter.

The two theories don't get along well when it comes to the second law of thermodynamics. This states that a closed system will move towards maximum entropy (a measure of the system's disorder) in an irreversible way. However, the theory of quantum mechanics allows previous states of particles to be calculated back, meaning that the flow of information and time are both reversible.

In recent years, there have been several attempts to explore this conflict using entangled quantum systems such as ultracold atoms or superconducting qubits. The problem is that quantum systems are very sensitive to interactions with their environment, and they are also prone to losing their quantum nature, a process known as decoherence, which makes time reversal difficult to implement.

However, a team of researchers from the University of Twente and the Freie Universität Berlin have shown that the theories of thermodynamics and quantum mechanics are both valid ways of describing the behaviour of photons in a quantum processor. They studied thermalization and equilibration in systems of entangled photons. Photons have several advantages over quantum systems composed of atoms. Their intrinsically quantum nature means they do not suffer from decoherence. They can be studied at room temperature, in contrast to the ultralow temperatures necessary for atoms, and are easy to manipulate with interference. Most importantly, they allow for time reversibility: any mixing of the photons can be reversed by performing the inverse operation, meaning that entangled photons can, in effect, be “disentangled”.

The researchers' measurements showed that photon numbers could no longer be precisely defined, as the photons were in an entangled state together and no longer individually localized in a single channel as they were at the input. However, the photon statistics they measured in each channel did show that entropy increased locally in all the channels, consistent with the second law of thermodynamics. At the same time, the entanglement that built up between photons was not visible in the individual channels: only when considering the entire system did it become clear that the overall quantum state is in a pure form, consistent with quantum mechanics.

Hence, the experiment showed that thermodynamics and quantum mechanics can both be true at the same time.

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Quantum mechanics and gravity

Three of the four fundamental forces of nature are described within the framework of quantum mechanics and quantum field theory: the electromagnetic interaction, the strong force, and the weak force. This leaves gravity as the only interaction that has not been fully accommodated. The current understanding of gravity is based on Albert Einstein's general theory of relativity, which incorporates his theory of special relativity and deeply modifies the understanding of concepts like time and space. Although general relativity is highly regarded for its elegance and accuracy, it has limitations, including the gravitational singularities inside black holes and the ad hoc postulation of dark matter. These limitations signal the collapse of the general theory of relativity at different scales and highlight the need for a gravitational theory that goes into the quantum realm.

The field of quantum gravity is actively developing, and theorists are exploring a variety of approaches to the problem, the most popular being M-theory and loop quantum gravity. All of these approaches aim to describe the quantum behaviour of the gravitational field, which does not necessarily include unifying all fundamental interactions into a single mathematical framework. However, many approaches to quantum gravity, such as string theory, try to develop a framework that describes all fundamental forces. Such a theory is often referred to as a "theory of everything".

One of the difficulties of formulating a quantum gravity theory is that direct observation of quantum gravitational effects is thought to only appear at length scales near the Planck scale, around 10−35 meters, a scale far smaller, and hence only accessible with far higher energies, than those currently available in high-energy particle accelerators. Therefore, physicists lack experimental data that could distinguish between the competing theories that have been proposed.

Quantum mechanics suggests that everything is made of quanta or packets of energy that can behave like both a particle and a wave. For example, quanta of light are called photons. Detecting gravitons, the hypothetical quanta of gravity, would prove that gravity is quantum. However, gravity is extraordinarily weak, and to directly observe the minuscule effects a graviton would have on matter, a graviton detector would have to be so massive that it would collapse on itself to form a black hole.

Despite these challenges, scientists are actively pursuing a variety of approaches to find gravitational quantum effects. For example, in 2017, two independent studies suggested that if gravity is quantum, it could generate a link known as "entanglement" between particles, so that one particle instantaneously influences another regardless of their location in the cosmos. Another strategy to find evidence for quantum gravity is to look at the cosmic microwave background radiation, the faint afterglow of the Big Bang. Quanta such as gravitons fluctuate like waves, and the shortest wavelengths would have the most intense fluctuations. When the cosmos expanded staggeringly in size within a sliver of a second after the Big Bang, according to the widely supported cosmological model known as inflation, these short wavelengths would have stretched to longer scales across the universe. This evidence of quantum gravity could be visible as swirls in the polarization, or alignment, of photons from the cosmic microwave background radiation.

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Quantum mechanics and relativity

Quantum mechanics and general relativity are two separate rulebooks explaining how nature works. They are fundamentally different theories with incompatible descriptions of reality. While quantum mechanics handles the three forces of electromagnetism and the two nuclear forces, relativity beautifully accounts for gravity and all the things it dominates, such as orbiting planets and the dynamics of the expanding universe.

The conflict between the two halves of physics has been brewing for more than a century, sparked by a pair of 1905 papers by Einstein, one outlining relativity and the other introducing quantum mechanics. The main issue is that relativity and quantum mechanics are "smooth" versus "chunky". In relativity, events are continuous and deterministic, meaning that every cause matches up to a specific, local effect. In quantum mechanics, events produced by the interaction of subatomic particles happen in jumps with probabilistic rather than definite outcomes.

When you try to interpret smooth relativistic laws in a chunky quantum style, or vice versa, things go wrong. Relativity gives nonsensical answers when scaled down to quantum size, eventually descending to infinite values in its description of gravity. Likewise, quantum mechanics runs into serious trouble when expanded to cosmic dimensions. Quantum fields carry a certain amount of energy, and as the fields get bigger, so does the amount of energy. According to Einstein, energy and mass are equivalent, so piling up energy is like piling up mass. Go big enough, and the amount of energy in the quantum fields becomes so great that it creates a black hole that causes the universe to fold in on itself.

The problem of time is a conceptual conflict between quantum mechanics and general relativity. Quantum mechanics regards the flow of time as universal and absolute, whereas general relativity regards it as malleable and relative. This raises the question of what time really is in a physical sense and whether it is a real, distinct phenomenon.

Quantum mechanics is "digital" and comes in discrete quantities, whereas general relativity is "analogue" and deals with things that are continuous. When you try to "quantise" general relativity, you get mathematical nonsense. It's like dividing by zero.

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Quantum mechanics and classical electrodynamics

Quantum mechanics does not break the laws of physics, but it does expose some of classical physics' limitations. Quantum mechanics is a statistical description of an ontology described by classical electrodynamics. In essence, it describes how light and matter interact and is the first theory where full agreement between quantum mechanics and special relativity is achieved.

In particle physics, quantum electrodynamics (QED) is the relativistic quantum field theory of electrodynamics. QED mathematically describes all phenomena involving electrically charged particles interacting by means of the exchange of photons. It is the quantum counterpart of classical electromagnetism, giving a complete account of the interaction between matter and light.

QED can be described as a very accurate way to calculate the probability of the position and movement of particles, even those without mass, such as photons, and the quantity depending on the position (field) of those particles. It also describes light and matter beyond the wave-particle duality proposed by Albert Einstein in 1905.

However, QED has its limitations. The wave functions quickly become too complex to solve, and approximations are needed to make them more manageable. These approximations introduce errors that become more significant as the time and spatial scales of the system under study increase.

Additionally, the Hilbert space of quantum states grows exponentially with the number of particles, making it challenging to simulate large systems accurately. This is known as the "curse of dimensionality". While Monte Carlo methods can be used to mitigate this issue, the convergence is slow.

In conclusion, while quantum mechanics does not break the laws of physics, it does expose the limitations of classical physics and provides a more accurate framework for understanding the behaviour of light and matter at the quantum level.

Frequently asked questions

Quantum mechanics is a fundamental theory that describes the behaviour of nature at and below the scale of atoms. It does not break the laws of physics but rather provides a more accurate description of the behaviour of particles at very small, submicroscopic (atomic and subatomic) scales, where classical physics falls short.

Quantum mechanics allows for the calculation of properties and behaviours of physical systems at these small scales. It describes many systems that classical physics cannot, such as the behaviour of particles that exhibit both particle and wave characteristics (wave-particle duality) and the limits to how accurately the value of a physical quantity can be predicted (the uncertainty principle).

Quantum mechanics explains phenomena such as radioactive decay, nuclear fusion in stars, and applications such as scanning tunnelling microscopy, tunnel diode and tunnel field-effect transistor. It is also the basis for technologies such as lasers, LEDs, and the microprocessor.

The many counter-intuitive aspects and results of quantum mechanics have provoked strong philosophical debates and many interpretations. The arguments centre on the probabilistic nature of quantum mechanics, the measurement problem, and quantum nonlocality.

Some of the interpretations include the Copenhagen interpretation, Bohmian mechanics, Everett's many-worlds interpretation, relational quantum mechanics, and QBism.

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