Quantum Mechanics: Challenging Newton's First Law

Newton's first law of motion states that objects naturally move at constant speeds and along straight lines unless they encounter a force that pushes them to do otherwise. However, this law appears to break down in the quantum realm. An experiment with light shows that the quantum version of that assumption fails for quantum particles of light. Scientists at Innsbruck, in collaboration with theorists from Munich, Paris, and Cambridge, studied a marble falling through water oscillating up and down rather than just moving straight downwards. They discovered a quantum particle exhibiting what scientists call quantum interference, which is the fact that quantum mechanics allows particles to behave like waves, which can add up or cancel each other. This suggests that the correlations in combination with the wave nature of matter determine the motion of particles in the quantum world.

Quantum particles of light do not move in straight lines

Quantum particles of light, or photons, do not always move in straight lines. In fact, they can manifest different properties depending on the situation. While they can sometimes act as point-like particles that travel in straight lines, they can also behave as waves.

In the context of optics, it is often assumed that photons travel in straight lines. However, this interpretation contradicts the principles of pure quantum physics, which suggests that photons can evolve over multiple paths simultaneously. This idea of photons travelling along multiple paths is known as the "all possible paths" interpretation.

According to the "all possible paths" interpretation, a photon released by an atom in a vacuum will propagate outwards as an expanding sphere at the speed of light. However, upon measurement, the photon's wavefunction will collapse, resulting in a definite position and detection of the photon particle. This collapse of the wavefunction occurs regardless of whether the measurement is observed or not; it is the resulting information that determines the collapse.

The concept of photons travelling in straight lines is a consequence of classical electromagnetism and general relativity, while the idea of photons interfering with themselves and travelling along multiple paths is derived from quantum mechanics. Classical theories do not always align with quantum theories because massive objects (such as beams of light) behave differently than individual photons.

It is worth noting that mirrors and lenses also contradict the idea of photons travelling in straight lines, as these optical devices rely on the ability to alter the path of light.

Quantum particles can be influenced by forces other than those described by Newton's laws

Newton's laws of motion describe the relationship between a physical object and the forces acting upon it, providing the basis for modern physics. His first law, also known as the law of inertia, states that an object at rest will remain at rest, and an object in motion will continue moving at a constant speed and in a straight line unless acted upon by an external force.

However, at the quantum level, particles can be influenced by forces beyond those described by Newton's laws. For example, the gravitational and electromagnetic interactions produce long-range forces that can be observed in everyday life, while the strong and weak interactions produce forces at the subatomic level and govern nuclear interactions within atoms.

The Standard Model of particle physics describes the electromagnetic, strong, and weak interactions that occur between elementary particles, and their behaviours are modelled using quantum mechanics. These fundamental interactions are mediated by force-carrying particles known as gauge bosons, which interact with matter particles called fermions. For example, photons mediate the interaction between electric charges, while gluons mediate the interaction between colour charges, binding quarks together to form protons and neutrons.

While Newton's laws provide a solid foundation for understanding motion and forces at the macroscopic level, they do not fully capture the complexities of quantum particles and the forces that influence them. Thus, quantum mechanics provides a more nuanced understanding of the forces at play in the microscopic realm.

Quantum particles can be influenced by forces with no physical contact

Newton's first law of motion states that objects naturally move at constant speeds and along straight lines unless they encounter a force that pushes them to do otherwise. This law, however, appears to break down in the quantum realm.

In the quantum world, the motion of objects is strongly challenged and may sometimes even completely fail. An experiment conducted by scientists in Innsbruck, in collaboration with theorists from Munich, Paris, and Cambridge, observed a quantum particle oscillating back and forth in a one-dimensional atomic gas, rather than moving in a straight line. This phenomenon, known as quantum interference, is a result of quantum particles behaving like waves, which can add up or cancel each other out.

The experiment involved cooling down a gas of Cesium atoms and confining them to an arrangement of very thin tubes using high-power laser beams. The atoms, under such extreme conditions, formed a quantum fluid whose motion was restricted to the direction of the tubes. By accelerating an impurity atom, an atom in a different spin state, through the gas, the scientists observed the quantum particles being scattered off the gas particles and reflected backward, an effect termed 'Bragg scattering'.

The study suggests that the correlations between quantum particles, in combination with their wave-like nature, determine their motion in the quantum world. This could lead to novel phenomena that contradict our everyday experiences and challenge classical mechanics.

One of the limitations of Newton's laws is that new theories are necessary when dealing with very small objects, as in the case of quantum mechanics. While Newton's laws provide a good approximation for larger objects, they may not accurately describe the motion of extremely small entities, such as quantum particles.

Quantum particles can behave like waves

In the late 17th century, Sir Isaac Newton advocated that light was corpuscular (particulate), while Christiaan Huygens took an opposing wave description. However, it was later discovered that light exhibited both properties. During the 19th and early 20th centuries, experiments showed that electrons, which were previously believed to be particles, also displayed wavelike behaviour.

In 1924, Louis de Broglie introduced his theory of electron waves, suggesting that an electron could be thought of as a standing wave. He proposed that particles are bundles of waves (wave packets) that move with a group velocity and have an effective mass. Erwin Schrödinger further developed this idea in 1925-1926 with the wave equation of motion for electrons, now known as the Schrödinger equation.

The wave nature of electrons was empirically confirmed by two experiments in 1927: the Davisson-Germer experiment at Bell Labs, and George Paget Thomson and Alexander Reid's experiment at Cambridge University. These experiments demonstrated that electrons scattered from metal surfaces or passed through thin metal films displayed wave-like interference patterns.

The concept of wave-particle duality challenges classical concepts of particles and waves, as it shows that quantum objects cannot be fully described by these traditional models. Instead, they exhibit both particle-like and wave-like properties depending on the context of the experiment.

Even something as massive as a human being should, in theory, have wave properties, although measuring them would be extremely difficult. The wavelength of a human being moving at 1 millimetre per second would be minuscule, far smaller than the scales probed at the Large Hadron Collider.

The ability of quantum particles to behave like waves has opened up new areas of technology, such as atomic optics, where slow-moving atomic beams are used for imaging and studying nanoscale structures without disrupting them as high-energy photons would.

Quantum particles can be influenced by forces that are extremely small

Newton's first law of motion states that objects naturally move at constant speeds and along straight lines unless they encounter a force that pushes them to do otherwise. However, this assumption fails for quantum particles of light, as they can be influenced by extremely small forces.

An experiment with light showed that one of the fundamental laws of motion may not always hold in the quantum realm. This was demonstrated by scientists in Innsbruck, in collaboration with theorists from Munich, Paris, and Cambridge, who studied a marble falling through water and oscillating up and down rather than just moving in a straight line downwards.

To observe this phenomenon, scientists cooled down a gas of Cesium atoms and confined it to an arrangement of very thin tubes created by high-power laser beams. They then accelerated an impurity atom, an atom in a different spin state, through the gas. When the quantum particles started moving, they were scattered off the gas particles and reflected backward, causing an oscillatory motion.

The study suggests that the correlations between particles, in combination with the wave nature of matter, determine the motion of particles in the quantum world. This could lead to novel phenomena that contradict our everyday experiences.

In summary, while Newton's first law holds for most objects in classical mechanics, it breaks down in the quantum realm, where particles can be influenced by extremely small forces, leading to unexpected behaviours.

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