Date		Conflict			Type of Conflict			Description
1900		Ultraviolet Catastrophe: Classical theory fails to account for blackbody radiation at high frequencies ( Planck, 1900 )			Theoretical			Classical physics predicted that the energy emitted by a blackbody would increase without bounds as the frequency of the radiation increases, known as the ultraviolet catastrophe. Planck introduced the idea of energy quantization to explain the observed spectrum of blackbody radiation. Discrete emission and absorption is in conflict with the principles of classical physics
1905		Photoelectric effect: Classical theory cannot explain the energy dependence of electron emission from metal surfaces ( Einstein, 1905a , 1905c ; Arons & Preppard, 1965 )			Experimental			The photoelectric effect is the phenomenon where electrons are emitted from a metal surface when exposed to light. Classical theory predicted that the energy of the emitted electrons would increase with the intensity of the incident light, but not with its frequency. Einstein proposed that light behaves as discrete packets of energy (photons) whose energy is proportional to their frequency, which successfully explained the observed energy dependence of the photoelectric effect.
1913		Bohr Model of the Atom: Classical mechanics cannot explain the stability of atoms and atomic spectra ( Bohr, 1913 )			Theoretical			Classical mechanics failed to explain the stability of atoms and the spectral lines they produced. Bohr developed a model of the atom based on the quantization of atomic energy levels, which explained the discrete spectra of elements and introduced the idea of quantum jumps between energy levels.
1925		Pauli’s Exclusion Principle: Two electrons with the same quantum number cannot be in the same state ( Pauli, 1925 )			Theoretical			Pauli’s Exclusion Principle has no basis in Classical Physics and is also in conflict with Bohr’s correspondence rule
1923		Wave-Particle Duality: Classical mechanics cannot explain the behavior of particles as both waves and particles ( Davisson & Germer, 1927 )			Experimental			The wave-particle duality is the concept that particles, such as electrons, can exhibit both wave-like and particle-like behavior. Classical mechanics was unable to explain the diffraction and interference patterns observed with electrons in the Davisson-Germer experiment.
1926		Schrodinger’s Equation: Probability Wave Function			Theoretical			A matter wave function, which is not possible in classical mechanics
1927		Uncertainty Principle: Classical mechanics cannot simultaneously measure the position and momentum of a particle with arbitrary precision ( Heisenberg, 1927 )			Theoretical			Classical mechanics assumed that it was possible to measure both the position and momentum of a particle with arbitrary precision. Heisenberg’s uncertainty principle states that the more precisely the position of a particle is known, the less precisely its momentum can be known, and vice versa.
1928		Dirac Equation: Classical mechanics cannot describe relativistic quantum mechanics ( Dirac, 1928 )			Theoretical			Classical mechanics is non-relativistic and cannot describe the behavior of particles traveling at high speeds. Dirac’s equation describes the behavior of relativistic particles in a quantum mechanical framework..
1935		EPR Paradox: Classical mechanics cannot explain quantum entanglement ( Crull & Bacciagaluppi, 2022 )			Theoretical			The EPR paradox is a thought experiment that highlights the apparent contradiction between quantum mechanics and classical mechanics. It involves the entanglement of two particles, where a measurement on one particle seems to instantaneously affect the other particle, even if they are separated by large distances. This led to the development of the concept of quantum entanglement, which is a fundamental concept in quantum mechanics.
1947		Lamb Shift: Classical mechanics cannot explain the energy shift in the hydrogen spectrum ( Lamb & Retherford, 1947 )		Experimental			The Lamb shift is the small energy shift observed in the hydrogen spectrum that could not be explained by classical mechanics. It was a significant confirmation of the predictions of quantum electrodynamics, which is the relativistic quantum field theory that describes the interactions between charged particles and
1955		Double-Slit Experiment with Electrons: Classical mechanics cannot explain the interference pattern of electrons ( Davisson, 1955 )		Theoretical/ Experimental			The double-slit experiment with electrons demonstrated the wave-like nature of particles and the interference pattern they produce, which could not be explained by classical mechanics. The experiment provided further evidence for the wave-particle duality of matter and established the foundation for quantum mechanics.
1964		Bell’s Theorem: Classical mechanics cannot explain the correlations between entangled particles ( Bell, 1964 )		Theoretical			Bell’s theorem is a mathematical proof that shows that classical mechanics cannot explain the correlations between entangled particles. It states that if particles have definite properties before being measured, as classical mechanics assumes, then the results of a series of measurements must satisfy certain mathematical inequalities. However, these inequalities are violated by the predictions of quantum mechanics, indicating that particles do not have definite properties before being measured. This led to the development of quantum information theory and the study of quantum entanglement.
1972		Aharonov-Bohm Effect: Classical mechanics cannot explain the behavior of particles in the presence of electromagnetic fields ( Aharonov & Bohm, 1959 )		Experimental			The Aharonov-Bohm effect is a phenomenon where the behavior of a particle is affected by the presence of an electromagnetic field, even when the particle does not directly interact with the field. This effect cannot be explained by classical mechanics, which assumes that the behavior of a particle is determined solely by the forces acting on it. The effect is a consequence of the non-locality of the electromagnetic field, which is described by the electromagnetic potential, and is a fundamental concept in quantum mechanics.
1982		Quantum Teleportation: Classical mechanics cannot explain the transfer of quantum information ( Bouwmeester, 1997 )		Experimental			Quantum teleportation is a process where the quantum state of one particle is transferred to another particle due to quantum entanglement, without involving any travel.
1986		Quantum Hall Effect: Classical mechanics cannot explain the quantization of electrical conductance ( Klitzing et al., 1980 )		Experimental			The quantum Hall effect is a phenomenon where the electrical conductance of a two-dimensional electron gas is quantized, meaning that it can only take on discrete values. This effect cannot be explained by classical mechanics, which assumes that the behavior of electrons can be described using classical equations of motion. Instead, the effect is a consequence of the quantum mechanical properties of electrons, such as their wave-like nature and quantization of energy levels. The quantum Hall effect has important applications in metrology and the determination of fundamental physical constants.
2001		Bose-Einstein Condensation: Classical mechanics cannot explain the behavior of ultra-cold atoms ( Anderson et al., 1995 )		Experimental			Bose-Einstein condensation is a phenomenon where a gas of ultra-cold atoms collapses into a single quantum state, forming a macroscopic quantum object. This phenomenon cannot be explained by classical mechanics, which assumes that the behavior of particles is described using classical equations of motion. Instead, Bose-Einstein condensation is a consequence of the wave-like nature of particles and their quantum mechanical properties, such as indistinguishability and coherence. The observation of Bose-Einstein condensation was a major breakthrough in the field of atomic physics and has led to the development of new technologies, such as atom lasers and ultra-cold atom interferometry.
	2010		Quantum Computing: Classical mechanics cannot explain the speedup achieved by quantum algorithms ( Shor, 1994 )			Theoretical			Quantum computing is a type of computing that uses quantum mechanics to perform certain types of calculations much faster than classical computers. This speedup cannot be explained by classical mechanics, which assumes that the behavior of particles is described using classical equations of motion. Instead, quantum computing relies on the principles of quantum mechanics, such as superposition and entanglement, to perform multiple calculations simultaneously. The speedup achieved by quantum algorithms, such as Shor’s algorithm for factoring large numbers, has important applications in cryptography and other fields.
	2015		Quantum Biology: Classical mechanics cannot explain the role of quantum mechanics in biological systems ( Lambert et al., 2013 )			Theoretical			Quantum biology is an emerging field that explores the role of quantum mechanics in biological systems, such as photosynthesis, neurology etc.
	2018		Quantum Mechanics vs. Realism: Quantum mechanics violates the principle of local realism			Theoretical			The principle of local realism states that the properties of a particle are predetermined and independent of any measurement or observation. This principle is violated by quantum mechanics, which predicts that the properties of a particle can only be determined through measurement and that the act of measurement can affect the state of the particle. This conflict was first described in the famous EPR paradox paper and was later formalized by John Bell in his inequalities. Experimental tests of Bell’s inequalities have consistently shown that quantum mechanics violates the principle of local realism, confirming the predictions of quantum mechanics and ruling out any local hidden variable theory. This conflict has important implications for our understanding of the nature of reality and the fundamental laws of physics.
	2021		Quantum Mechanics vs. Relativity: Quantum mechanics does not account for gravity			Theoretical			Quantum mechanics and general relativity are the two most successful theories in physics, but they are incompatible with each other. While general relativity describes the behavior of gravity on a large scale, quantum mechanics describes the behavior of particles on a small scale. The problem is that quantum mechanics does not account for the force of gravity, making it impossible to describe the behavior of particles in a gravitational field. This conflict is known as the problem of quantum gravity, and it is one of the biggest unsolved problems in physics. In 2021, a team of researchers demonstrated a way to simulate quantum gravity in the lab using quantum teleportation, which could lead to new insights into this problem.
	2021		Quantum Mechanics vs. Reality: The measurement problem ( Schlosshauer, 2004 )			Theoretical			The measurement problem is one of the most famous and controversial conflicts in quantum mechanics. It refers to the paradoxical nature of quantum mechanics, where the act of measurement can collapse the wave function of a particle and determine its state. This conflict arises because the wave function describes the probabilities of different states, but measurement seems to force the particle to take on a definite state. There are many interpretations of quantum mechanics that attempt to resolve this conflict, including the Copenhagen interpretation, the many-worlds interpretation, and the pilot wave theory. However, the measurement problem remains an open question and a subject of intense debate among physicists.