## Fundamentals of Quantum Physics

By the end of the 19th century, physicists almost universally accepted the wave theory of light. However, though the ideas of classical physics explain interference and diffraction phenomena relating to the propagation of light, they do not account for the absorption and emission of light. All bodies radiate electromagnetic energy as heat; in fact, a body emits radiation at all wavelengths.

The energy radiated at different wavelengths is a maximum at a wavelength that depends on the temperature of the body; the hotter the body, the shorter the wavelength for maximum radiation. Attempts to calculate the energy distribution for the radiation from a blackbody using classical ideas were unsuccessful. A blackbody is a hypothetical ideal body or surface that absorbs and reemits all radiant energy falling on it. One formula, proposed by Wilhelm Wien of Germany, did not agree with observations at long wavelengths, and another, proposed by Lord Rayleigh John William Strutt of England, disagreed with those at short wavelengths.

In the German theoretical physicist Max Planck made a bold suggestion. He assumed that the radiation energy is emitted, not continuously, but rather in discrete packets called quanta. Planck showed that the calculated energy spectrum then agreed with observation over the entire wavelength range.

You are using an outdated browser. Please upgrade your browser to improve your experience and security. Quantum mechanics. Article Media. Info Print Print. Table Of Contents. Submit Feedback. Thank you for your feedback. Hidden variables Paradox of Einstein, Podolsky, and Rosen Measurement in quantum mechanics Applications of quantum mechanics Decay of the kaon Cesium clock A quantum voltage standard. Written By: Gordon Leslie Squires.

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This seems completely crazy, but is an experimental fact, worked out by a surprisingly familiar process:. Of course, describing real objects as both particles and waves is necessarily somewhat imprecise. Properly speaking, the objects described by quantum physics are neither particles nor waves, but a third category that shares some properties of waves a characteristic frequency and wavelength, some spread over space and some properties of particles they're generally countable and can be localized to some degree.

This leads to some lively debate within the physics education community about whether it's really appropriate to talk about light as a particle in intro physics courses; not because there's any controversy about whether light has some particle nature, but because calling photons "particles" rather than "excitations of a quantum field" might lead to some student misconceptions. I tend not to agree with this, because many of the same concerns could be raised about calling electrons "particles," but it makes for a reliable source of blog conversations.

This "door number three" nature of quantum objects is reflected in the sometimes confusing language physicists use to talk about quantum phenomena.

The Higgs boson was discovered at the Large Hadron Collider as a particle, but you will also hear physicists talk about the "Higgs field" as a delocalized thing filling all of space. This happens because in some circumstances, such as collider experiments, it's more convenient to discuss excitations of the Higgs field in a way that emphasizes the particle-like characteristics, while in other circumstances, like general discussion of why certain particles have mass, it's more convenient to discuss the physics in terms of interactions with a universe-filling quantum field.

It's just different language describing the same mathematical object.

## Six Things Everyone Should Know About Quantum Physics

These oscillations created an image of "frozen" light. Credit: Princeton. It's right there in the name-- the word "quantum" comes from the Latin for "how much" and reflects the fact that quantum models always involve something coming in discrete amounts. The energy contained in a quantum field comes in integer multiples of some fundamental energy.

For light, this is associated with the frequency and wavelength of the light-- high-frequency, short-wavelength light has a large characteristic energy, which low-frequency, long-wavelength light has a small characteristic energy. This property is also seen in the discrete energy levels of atoms, and the energy bands of solids-- certain values of energy are allowed, others are not. Atomic clocks work because of the discreteness of quantum physics, using the frequency of light associated with a transition between two allowed states in cesium to keep time at a level requiring the much-discussed "leap second" added last week.

Ultra-precise spectroscopy can also be used to look for things like dark matter , and is part of the motivation for a low-energy fundamental physics institute.

## Fundamentals Of Quantum Mechanics

This isn't always obvious-- even some things that are fundamentally quantum, like black-body radiation , appear to involve continuous distributions. But there's always a kind of granularity to the underlying reality if you dig into the mathematics, and that's a large part of what leads to the weirdness of the theory.

One of the most surprising and historically, at least controversial aspects of quantum physics is that it's impossible to predict with certainty the outcome of a single experiment on a quantum system. When physicists predict the outcome of some experiment, the prediction always takes the form of a probability for finding each of the particular possible outcomes, and comparisons between theory and experiment always involve inferring probability distributions from many repeated experiments.

There's a lot of debate about what, exactly, this wavefunction represents, breaking down into two main camps: those who think of the wavefunction as a real physical thing the jargon term for these is "ontic" theories, leading some witty person to dub their proponents "psi-ontologists" and those who think of the wavefunction as merely an expression of our knowledge or lack thereof regarding the underlying state of a particular quantum object "epistemic" theories. In either class of foundational model, the probability of finding an outcome is not given directly by the wavefunction, but by the square of the wavefunction loosely speaking, anyway; the wavefunction is a complex mathematical object meaning it involves imaginary numbers like the square root of negative one , and the operation to get probability is slightly more involved, but "square of the wavefunction" is enough to get the basic idea.

This is known as the "Born Rule" after German physicist Max Born who first suggested this in a footnote to a paper in , and strikes some people as an ugly ad hoc addition.

There's an active effort in some parts of the quantum foundations community to find a way to derive the Born rule from a more fundamental principle; to date, none of these have been fully successful, but it generates a lot of interesting science. This is also the aspect of the theory that leads to things like particles being in multiple states at the same time.

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All we can predict is probability, and prior to a measurement that determines a particular outcome, the system being measured is in an indeterminate state that mathematically maps to a superposition of all possibilities with different probabilities. Whether you consider this as the system really being in all of the states at once, or just being in one unknown state depends largely on your feelings about ontic versus epistemic models, though these are both subject to constraints from the next item on the list:.

A quantum teleportation experiment in action.

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The last great contribution Einstein made to physics was not widely recognized as such, mostly because he was wrong. In a paper with his younger colleagues Boris Podolsky and Nathan Rosen the "EPR paper" , Einstein provided a clear mathematical statement of something that had been bothering him for some time, an idea that we now call "entanglement. The EPR paper argued that quantum physics allowed the existence of systems where measurements made at widely separated locations could be correlated in ways that suggested the outcome of one was determined by the other.

They argued that this meant the measurement outcomes must be determined in advance, by some common factor, because the alternative would require transmitting the result of one measurement to the location of the other at speeds faster than the speed of light. Thus, quantum mechanics must be incomplete, a mere approximation to some deeper theory a "local hidden variable" theory, one where the results of a particular measurement do not depend on anything farther away from the measurement location than a signal could travel at the speed of light "local" , but are determined by some factor common to both systems in an entangled pair the "hidden variable".

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This was regarded as an odd footnote for about thirty years, as there seemed to be no way to test it, but in the mid's the Irish physicist John Bell worked out the consequences of the EPR paper in greater detail.