Who is generally regarded as the founder of quantum mechanics

At the scale of atoms and electrons, many of the equations of classical mechanics , which describe how things move at everyday sizes and speeds, cease to be useful. In classical mechanics, objects exist in a specific place at a specific time. However, in quantum mechanics, objects instead exist in a haze of probability; they have a certain chance of being at point A, another chance of being at point B and so on.

Quantum mechanics QM developed over many decades, beginning as a set of controversial mathematical explanations of experiments that the math of classical mechanics could not explain.

It began at the turn of the 20th century, around the same time that Albert Einstein published his theory of relativity , a separate mathematical revolution in physics that describes the motion of things at high speeds. Unlike relativity, however, the origins of QM cannot be attributed to any one scientist. Rather, multiple scientists contributed to a foundation of three revolutionary principles that gradually gained acceptance and experimental verification between and They are:.

Quantized properties : Certain properties, such as position, speed and color, can sometimes only occur in specific, set amounts, much like a dial that "clicks" from number to number. This challenged a fundamental assumption of classical mechanics, which said that such properties should exist on a smooth, continuous spectrum. To describe the idea that some properties "clicked" like a dial with specific settings, scientists coined the word "quantized. Particles of light : Light can sometimes behave as a particle.

This was initially met with harsh criticism, as it ran contrary to years of experiments showing that light behaved as a wave; much like ripples on the surface of a calm lake. Light behaves similarly in that it bounces off walls and bends around corners, and that the crests and troughs of the wave can add up or cancel out.

Added wave crests result in brighter light, while waves that cancel out produce darkness. A light source can be thought of as a ball on a stick being rhythmically dipped in the center of a lake. The color emitted corresponds to the distance between the crests, which is determined by the speed of the ball's rhythm.

Waves of matter : Matter can also behave as a wave. This ran counter to the roughly 30 years of experiments showing that matter such as electrons exists as particles. In , German physicist Max Planck sought to explain the distribution of colors emitted over the spectrum in the glow of red-hot and white-hot objects, such as light-bulb filaments.

When making physical sense of the equation he had derived to describe this distribution, Planck realized it implied that combinations of only certain colors albeit a great number of them were emitted, specifically those that were whole-number multiples of some base value.

Somehow, colors were quantized! This was unexpected because light was understood to act as a wave, meaning that values of color should be a continuous spectrum. What could be forbidding atoms from producing the colors between these whole-number multiples? This seemed so strange that Planck regarded quantization as nothing more than a mathematical trick. This insight has begun to emerge among historians and philosophers of science over the last ten to fifteen years.

Don Howard , p. However, it should also be mentioned that in later work, Feyerabend , was one of the first philosophers who gave a painstaking analysis of complementarity in order to clear up the myth of it being unintelligible. Feyerabend urged philosophers and physicists to go back to Bohr and read him carefully. So the formulation of complementarity was restricted to the concept of stationary states because only there does the system have a well-defined energy state independent of any measurement.

This observation deserves general recognition. But when Bohr rather soon thereafter began analysing the double slit experiment in his discussion with Einstein , he had to extend his interpretation to cover the electron in interaction with the measuring apparatus.

As Heisenberg understood complementarity between the space-time description and causal description, it holds between the classical description of experimental phenomena and the description of the state of the system in terms of the wave function. A quotation from Heisenberg , p. In other words, Heisenberg, in contrast to Bohr, believed that the wave equation gave a causal, albeit probabilistic description of the free electron in configuration space.

It also explains why so many philosophers and physicists have identified the Copenhagen interpretation with the mysterious collapse of the wave packet. According to Heisenberg, these two modes of description are complementary. More recently, Henderson has come to a similar conclusion.

He makes a distinction between different versions of Copenhagen interpretations based on statements from some of the main characters.

Apparently, we are living in a quantum world since everything is constituted by atomic and subatomic particles. Hence classical physics seems merely to be a useful approximation to a world which is quantum mechanical on all scales.

Such a view, which many modern physicists support, can be called quantum fundamentalism Zinkernagel , It can be defined as a position containing two components: 1 everything in the Universe is fundamentally of quantum nature the ontological component ; and 2 everything in the Universe is ultimately describable in quantum mechanical terms the epistemological component.

Thus, we may define quantum fundamentalism to be the position holding that everything in the world is essentially quantized and that the quantum theory gives us a literal description of this nature. The basic assumption behind quantum fundamentalism is that the structure of the formalism, in this case the wave function, corresponds to how the world is structured.

For instance, according to the wave function description every quantum system may be in a superposition of different states because a combination of state vectors is also a state vector. Now, assuming that both the quantum object and the measuring apparatus are quantum systems that each can be described by a wave function, it follows that their entangled state would likewise be represented by a state vector.

Then the challenge is, of course, how we can explain why the pointer of a measuring instrument enters a definite and not a superposition position, as experience tells us, whenever the apparatus interacts with the object. In a nutshell this is the measurement problem.

In [], von Neumann suggested that the entangled state of the object and the instrument collapses to a determinate state whenever a measurement takes place. This measurement process a type 1-process as he called it could not be described by quantum mechanics; quantum mechanics can only described type-2 processes i. In his discussion of the measurement problem, von Neumann then distinguished between i the system actually observed; ii the measuring instrument; and iii the actual observer.

He argues that during a measurement the actual observer gets a subjective perception of what is going on that has a non-physical nature, which distinguishes it from the observed object and the measuring instrument. However, he holds on to psycho-physical parallelism as a scientific principle, which he interprets such that there exists a physical correlate to any extra-physical process of the subjective experience. So in every case where we have a subjective perception we must divide the world into the observed system and the observer.

But where the division takes place is partly arbitrary. In other words, von Neumann argues that the observer can never be included in a type 2-process description, but the measuring instrument may sometime be part of a type 2-process, although it gives the same result with respect to the observed object i.

Therefore, the mind seems to play an active role in forming a type 1-process, which would be incompatible with psycho-physical parallelism. Indeed, within philosophy of mind one cannot consistently maintain both psycho-physical parallelism and the existence of an interaction between the brain and the mind.

But Wigner never explained how it was possible for something mental to produce a material effect like the collapse of a quantum system.

Quantum fundamentalists must indeed be ready to explain why the macroscopic world appears classical. But there are ontological cost, which is significant to some. In one interpretation the world divides into as many worlds as there are possible measurement outcomes each time a system is observed or interacts with another system.

Other fundamentalists had hoped that the decoherence program might come up with an appropriate explanation. The decoherence theory sees entanglement to exist not only between object and the measurement but also as something which includes the environment. However, it is generally agreed that decoherence does not solve the measurement problem Bacciagaluppi ; Zinkernagel We just have to interpret the formulas correctly.

Time and again Bohr emphasized that the epistemological distinction between the instrument and the object is necessary because this is the only way one can functionally make sense of a measurement. The epistemic purpose of a measuring instrument is to yield information about an object separated from the instrument itself.

He sometimes included parts of the measuring instrument to which the quantum mechanical description should be applied. Don Howard therefore concludes that Bohr was not only an ontological quantum fundamentalist but in fact also a sort of an epistemological one. According to him, Bohr never considered the measuring instrument as a classical object. Moreover, he thinks that this implies that Bohr had to understand the use of classical concepts differently from what scholars usually think.

The consequence would be that the instrument and the object exist in a definite quantum state since such a state could be represented as a product of the wave function for the instrument and for the object.

But, as Maximilian Schlosshauer and Kristian Camilleri Other Internet Resources , have pointed out, this does not solve the measurement problem. Howard does not explain under which circumstances one can move from a quantum system-cum-measuring apparatus being in a non-separable state to a mixture of separated states.

Therefore one cannot be sure that the measuring apparatus is in a definite state and its pointer in a definite place. Some philosophers seem to argue that Bohr was an ontological but not an epistemological quantum fundamentalist. Landsman argues that Bohr held that the measuring instrument should be described in classical terms since the results of any measurement like in classical physics would always have a definite value.

However, Landsman agrees that Bohr understood all objects as essentially quantum mechanical objects. Bohr mentioned more than once that physics was not about finding the essence of nature but about describing the phenomena in an unambiguous way.

This is definitely a non-classical feature which is described by quantum mechanics alone. In his response to the EPR-paper, Bohr strongly rejected that this form of interaction could be regarded as a mechanical influence. The influence was on the conditions of description, i. But during a measurement we need to separate the system from the measuring instrument and the environment for pragmatic reasons.

The pragmatic reasons seem to be reasonably clear. The outcomes of whatever experiment always yield a definite value, so the entanglement of object and the measurement instrument described by the quantum formalism only lasts until the interaction between object and instrument stops.

The quantum formalism can predict the statistical outcome of these interactions but it cannot say anything about the trajectory of objects. This problem arises from the fact that quantum mechanics itself cannot account for why experiments on objects in a state of superposition always produce a definite outcome.

Hence if one does not argue for spontaneous collapse of the wave function, hidden variables, or many worlds, one needs to supplement quantum mechanics with a classical description of measuring instruments in terms of clocks and rods.

According to his interpretation, Bohr believed in a quantum world, but only relative to a particular classical description and a certain classical world. The distinction between classical and quantum both ontic and epistemic is contextual. He thinks that the measurement problem is ultimately a consequence of ontological quantum fundamentalism that everything is quantum. Because if everything is quantum — and correctly described by quantum formalism what else would it mean to call everything quantum?

One could say with Zinkernagel that Bohr believed all objects can be treated as quantum objects, but they cannot all be treated as quantum objects at the same time. Borrowing a conception from the two Russian physicists, Landau and Lifshitz, Zinkernagel claims that only some parts of the measuring device are entangled with the object in question, but those parts which are not entangled exists as a classical object.

Depending on the context, objects cannot be treated as quantum objects in those situations in which they acts as measuring apparatuses. In these situations the classical treatment of the measuring device provides us with a frame of reference of space and time with respect to which an atomic object has a position, and, mutatis mutandis, with respect to which it has energy and momentum.

Such a frame of reference is necessary for our ability to define and measure a particular property. What characterizes a frame of reference is that it establishes the conditions for the ascription of a well-defined position or a well-defined momentum, and treated classically measuring instruments act exactly as frames of reference.

The implication is that Bohr did not exclude the application of quantum theory to any system. Every system can in principle be treated quantum mechanically, but since we always need a frame of reference to describe experimental outcomes, not all systems can be treated quantum mechanically at once. Despite this position Dorato argues that in order to justify his entity realism and anti-instrumentalist interpretations, Bohr also needed to postulate something ontologically distinct from the realm of quantum mechanics, a claim that creates the well-known problem of defining in a non-ambiguous and exact way the cut between the classical and the quantum realm.

Nonetheless, the question is to what extent Bohr really believed that the classical world is not only epistemically but also ontologically different from the quantum world? If he did not make an ontological distinction, there would be no contradiction between his epistemic view that the outcome of measurement needs to be described classically but that the apparatus ontologically is just as much a quantum object as the object under investigation.

So when Bohr regarded quantum mechanics as a rational generalization of classical physics, he always thought of it as a way to secure the epistemic validity of quantum mechanics and not a way to save a classical ontology.

Classical mechanics is a mathematical approximation. Moreover, Bohr believed for epistemic reasons that we had to use classical language because this language is a refinement of our everyday language, which is adapted to describe our sensory experience and therefore the only language that can endow the quantum formalism with an empirical content.

Measuring devices are not classical objects even though we need classical concepts to describe our general physical experiences and the outcome of quantum experiments.

So Dieks concludes that the interaction between the measuring device and the quantum object determines, in the classical textbook examples, whether position or momentum talk can be carried over to the quantum object that is measured.

The measuring device itself, if macroscopic and under ordinary circumstances so that it really is a measuring device that can be used by us allows both position and momentum talk in its own description. The measurement interaction determines which correlations are forged with the micro-world.

Around the millennium a new recognition of the Copenhagen interpretation has emerged. It turns out that either position or momentum are dynamically significant, but it is not permissible to assume that position and momentum are both dynamically significant in any single context. Rather, Clifton and Halvorson and Halvorson believe that complementarity may give us a realist interpretation of quantum field theory.

Although Bohr assumed that the measuring apparatus is altogether a quantum mechanical system, he nevertheless believed that the instrument could be approximately described by classical theory. The Background 2. Classical Physics 3.

The Correspondence Rule 4. Complementarity 5. The Use of Classical Concepts 6. The Interpretation of the Quantum Formalism 7.

Misunderstandings of Complementarity 8. The Divergent Views 9. The theory was based on two postulates: An atomic system is only stable in a certain set of states, called stationary states, each state being associated with a discrete energy, and every change of energy corresponds to a complete transition from one state to another.

It introduced an element of discontinuity and indeterminism foreign to classical mechanics: Apparently not every point in space was accessible to an electron moving around a hydrogen nucleus. An electron moved in classical orbits, but during its transition from one orbit to another it was at no definite place between these orbits. Thus, an electron could only be in its ground state the orbit of lowest energy or an excited state if an impact of another particle had forced it to leave its ground state.

It was impossible to predict when the transition would take place and how it would take place. Any excited electron might in principle move spontaneously to either a lower state or down to the ground state. Einstein made another strange observation. He was curious to know in which direction the photon decided to move off from the electron. Classical Physics Bohr saw quantum mechanics as a generalization of classical physics although it violates some of the basic ontological principles on which classical physics rests.

Some of these principles are: The principles of physical objects and their identity : Physical objects systems of objects exist in space and time and physical processes take place in space and time, i. Far from being a temporary compromise in this dilemma, the recourse to essentially statistical considerations is our only conceivable means of arriving at a generalization of the customary way of description sufficiently wide to account for the features of individuality expressed by the quantum postulates and reducing to classical theory in the limiting case where all actions involved in the analysis of the phenomena are large compared with a single quantum.

In the search for the formulation of such a generalization, our only guide has just been the so called correspondence argument, which gives expression for the exigency of upholding the use of classical concepts to the largest possible extent compatible with the quantum postulates. He directly mentioned the relationship between the use of classical concepts and the correspondence principle in when he wrote in the Introduction to Atomic Theory and the Description of Nature : [T]he necessity of making an extensive use … of the classical concepts, upon which depends ultimately the interpretation of all experience, gave rise to the formulation of the so-called correspondence principle which expresses our endeavours to utilize all the classical concepts by giving them a suitable quantum-theoretical re-interpretation ATDN , p.

Complementarity After Heisenberg had managed to formulate a consistent quantum mechanics in , both he and Bohr began their struggle to find a coherent interpretation for the mathematical formalism. The experimental practice presupposes a certain pre-scientific practice of description, which establishes the norm for experimental measurement apparatus, and consequently what counts as scientific experience.

Our pre-scientific practice of understanding our environment is an adaptation to the sense experience of separation, orientation, identification and reidentification over time of physical objects. These common categories yield the preconditions for objective knowledge, and any description of nature has to use these concepts to be objective. The concepts of classical physics are merely exact specifications of the above categories. Their use is well defined only if they apply to experimental interactions in which the quantization of action can be regarded as negligible.

In experimental cases where the quantization of action plays a significant role, the application of a classical concept does not refer to independent properties of the object; rather the ascription of either kinematic or dynamic properties to the object as it exists independently of a specific experimental interaction is ill-defined. The quantization of action demands a limitation of the use of classical concepts so that these concepts apply only to a phenomenon, which Bohr understood as the macroscopic manifestation of a measurement on the object, i.

The quantum mechanical description of the object differs from the classical description of the measuring apparatus, and this requires that the object and the measuring device should be separated in the description, but the line of separation is not the one between macroscopic instruments and microscopic objects. It has been argued in detail Howard that Bohr pointed out that parts of the measuring device may sometimes be treated as parts of the object in the quantum mechanical description.

Due to the fact that the wave equation involves an imaginary quantity this equation can have only a symbolic character, but the formalism may be used to predict the outcome of a measurement that establishes the conditions under which concepts like position, momentum, time and energy apply to the phenomena. The ascription of these classical concepts to the phenomena of measurements rely on the experimental context of the phenomena, so that the entire setup provides us with the defining conditions for the application of kinematic and dynamic concepts in the domain of quantum physics.

Such phenomena are complementary in the sense that their manifestations depend on mutually exclusive measurements, but that the information gained through these various experiments exhausts all possible objective knowledge of the object.

Here is a quotation from No more is it likely that the fundamental concepts of the classical theories will ever become superfluous for the description of physical experience. Later he expressed the same view in an often quoted passage: It is decisive to recognize that, however far the phenomena transcend the scope of classical physical explanation, the account of all evidence must be expressed in classical terms.

The Interpretation of the Quantum Formalism Classical concepts serve the important function of connecting the quantum mechanical symbolism with experimental observations. Misunderstandings of Complementarity Complementarity has been commonly misunderstood in several ways, some of which shall be outlined in this section. The Divergent Views The Copenhagen interpretation is not a homogenous view.

The probability function obeys an equation of motion as did the co-ordinates in Newtonian mechanics; its change in the course of time is completely determined by the quantum mechanical equation; it does not allow a description in space and time but breaks the determined continuity of the probability function by changing our knowledge of the system.

The Measurement Problem and the Classical-Quantum Distinction Apparently, we are living in a quantum world since everything is constituted by atomic and subatomic particles.

However, within less than a year, the investigation of these problems revealed an almost complete failure of Bohr's atomic theory. The matrix formulation of quantum mechanics As more and more situations were encountered, more and more recipes for allowed values were required.

The art of guessing correct formulas. He began to develop systematic tables of allowed physical quantities, be they energies, or positions, or speeds. Born looked at these tables and saw that they could be interpreted as mathematical matrices. Fifty years later matrix mathematics would be taught even in high schools. But in it was an advanced and abstract technique, and Heisenberg struggled with it. His work was cut short in June He asked his research director, Max Born, for a vacation, and spent it on the rocky North Sea island of Helgoland.

At first he was so ill that could only stay in his rented room and admire the view of the sea. As his condition improved he began to take walks and to swim. With further improvement he began also to read Goethe and to work on physics.

Heisenberg reproduced his earlier work, cleaning up the mathematics and simplifying the formulation. He worried that the mathematical scheme he invented might prove to be inconsistent, and in particular that it might violate the principle of the conservation of energy. In Heisenberg's own words:. One evening I reached the point where I was ready to determine the individual terms in the energy table, or, as we put it today, in the energy matrix, by what would now be considered an extremely clumsy series of calculations.

When the first terms seemed to accord with the energy principle, I became rather excited, and I began to make countless arithmetical errors.

As a result, it was almost three o'clock in the morning before the final result of my computations lay before me. The energy principle had held for all the terms, and I could no longer doubt the mathematical consistency and coherence of the kind of quantum mechanics to which my calculations pointed. At first, I was deeply alarmed. I had the feeling that, through the surface of atomic phenomena, I was looking at a strangely beautiful interior, and felt almost giddy at the thought that I now had to probe this wealth of mathematical structures nature had so generously spread out before me.

I was far too excited to sleep, and so, as a new day dawned, I made for the southern tip of the island, where I had been longing to climb a rock jutting out into the sea. I now did so without too much trouble, and waited for the sun to rise. By the end of the summer Heisenberg, Born, and Pascual Jordan age 22 had developed a complete and consistent theory of quantum mechanics. Jordan had entered the collaboration when he overheard Born discussing quantum mechanics with a colleague on a train.

This theory, called "matrix mechanics" or "the matrix formulation of quantum mechanics", is not the theory I have presented in this book. It is extremely and intrinsically mathematical, and even for master mathematicians it was difficult to work with.

Although we now know it to be complete and consistent, this wasn't clear until much later. Heisenberg had been keeping Wolfgang Pauli apprised of his progress. Pauli, age 25, was Heisenberg's friend from graduate student days, when they studied together under Sommerfeld. On 12 October Heisenberg could stand Pauli's biting criticism no longer. He wrote to Pauli:. With respect to both of your last letters I must preach you a sermon, and beg your pardon for proceeding in Bavarian: It is really a pigsty that you cannot stop indulging in a slanging match.

You will have to allow that, in any case, we are not seeking to ruin physics out of malicious intent. When you reproach us that we are such big donkeys that we have never produced anything new in physics, it may well be true.

But then, you are also an equally big jackass because you have not accomplished it either. The dots denote a curse of about two-minute duration! Do not think badly of me and many greetings.

In Louis de Broglie age 31 , associated an "internal periodic phenomenon" -- a wave -- with a particle. He was never very precise about just what that meant. De Broglie is sometimes called "Prince de Broglie" because his family descended from the French nobility.