The term Quantum Physics is generally just intimidating. It’s sort of odd, and it might seem counter-intuitive, even for the physicists who deal with it every day. But this isn’t nonsensical. If you’re reading something about quantum physics, there are actually six main concepts that you should bear in mind. Do that, and you’ll find quantum mechanics much easier to understand.

**All is made of waves; particles, too.**

There are a lot of ways to start this kind of conversation, and this is as good as any: everything in the universe has both a particle and a wave existence at the same time. There is a line in Greg Bear’s fantasy ideology (The Infinity Concerto and The Serpent Mage) where a character explaining the fundamentals of magic says, “Everything is waves, nothing waves, no distance at all.” I’ve always liked that as a poetic definition of quantum physics—deep down, everything in the universe has wave nature.

All in the universe, of course, also has a particle nature. This sounds totally insane, but it’s an experimental reality that has been carried out by a remarkably familiar process:

Of course, defining actual objects as both particles and waves is inherently a little imprecise. Properly speaking, the phenomena mentioned in quantum physics are not particles or waves, but a third category that shares some of the properties of waves (a characteristic frequency and wavelength, some distributed over space) and some of the properties of particles (they are usually countable and can be localised to some degree). This leads to some vigorous discussion within the physics education community as to whether it is really acceptable to speak about light as a particle in intro physics courses; not because there is some dispute as to whether light is of a particle type, but because calling photons “particles” rather than “quantum field excitement” could lead to some student misconceptions. I tend not to agree with this, since many of the same questions may be posed about naming electrons ‘particles,’ but it is a credible source of blog conversations.

This “door number three” nature of quantum objects is expressed in the often confusing vocabulary that physicists use to speak about quantum phenomena. The Higgs boson was discovered as a particle in the Large Hadron Collider, but you’ll also hear physicists talking about the “Higgs field” as a delocalized thing that fills all space. This occurs because in certain cases, such as collider experiments, it is more convenient to address Higgs field excitement in a way that emphasises particle-like characteristics, whereas in other circumstances, such as general discussion of why those particles have mass, it is more convenient to discuss physics in terms of interactions with a universe-filled quantum field. It’s just a different language that describes the same mathematical object.

## Discrete Quantum Physics

The word “quantum” comes from the Latin for “how many,” and it refers to the fact that quantum models often contain something that comes in discrete quantities. Quantum fields contain energy that is integer multiples of some fundamental energy. This is related to the frequency and wavelength of light, with high-frequency, short-wavelength light having a broad characteristic energy and low-frequency, long-wavelength light having a small characteristic energy.

However, in all cases, the total energy present in a light field is an integer multiple of that energy— 1, 2, 14, 137 times— never a strange fraction like one-and-a-half or the square root of two. This property can also be seen in atoms’ distinct energy levels and solids’ energy bands, where some energy values are permitted while others are not. Because of the discrete existence of quantum physics, atomic clocks operate by using the frequency of light associated with a transition between two permitted states in cesium to hold time at a level that does not necessitate the much-discussed “leap second” introduced last week.

Ultra-precise spectroscopy can also be used to search for dark matter, which is one of the reasons for the establishment of a low-energy fundamental physics institute.

Also fundamentally quantum phenomena like black-body radiation tend to include continuous distributions, which isn’t always clear. However, digging into the mathematics shows a granularity to the underlying truth, which is a large part of what contributes to the theory’s strangeness.

## Quantum mechanics is a probabilistic science

One of the most shocking and (at least historically) contentious aspects of quantum physics is that the result of a single experiment on a quantum system cannot be predicted with certainty. When physicists predict the outcome of an experiment, they always give a probability for discovering each of the specific possible outcomes, and comparisons between theory and experiment always include inferring probability distributions from a large number of repeated experiments.

A “wavefunction” is a mathematical representation of a quantum system, which is usually expressed in equations by the Greek letter psi:. There’s a lot of debate on what this wavefunction represents, and it’s divided into two camps: those who believe the wavefunction is a real physical thing (jargon term: “ontic” theories), and those who believe the wavefunction is merely an expression of our knowledge (or lack thereof) about the universe (“epistemic” theories).

The probability of finding an outcome is not explicitly supplied by the wavefunction in either class of fundamental model, but by the square of the wavefunction (loosely speaking; the wavefunction is a complex mathematical entity (meaning it includes imaginary numbers like the square root of negative one), and the operation to get probability is slightly more involved, but “square of the wavefunction”). The “Born Law,” named after German physicist Max Born, who proposed it (in a footnote to a paper in 1926), is seen by some as an unattractive afterthought. Some sections of the quantum foundations group are working hard to find a way to derive the Born rule from a more fundamental principle; so far, none of them have been completely effective, but it’s generating a lot of interesting research.

This is also the function of the theory that allows particles to live in several states at the same time. Only probability can be predicted, and before a calculation that decides a specific outcome, the system being evaluated is in an indeterminate state that mathematically maps to a superposition of all possible outcomes of varying probabilities. If you think of this as the system being in all of the states at once or only one unknown state depends a lot on how you feel about ontic versus epistemic models, but all are constrained by the next point on the list:

## Quantum physics is a nonlocal technology

The last major contribution Einstein made to physics was underappreciated, largely because he was incorrect. Einstein made a simple mathematical statement of something that had been troubling him for some time, a concept that we now call “entanglement,” in a 1935 paper with his younger colleagues Boris Podolsky and Nathan Rosen (the “EPR paper”).

According to the EPR paper, quantum physics permitted the existence of systems in which measurements taken at widely separated locations could be correlated in such a way that the outcome of one was decided by the outcome of the other. They claimed that this meant that the measurement results had to be predetermined by some common factor, since the alternative would entail sending the result of one measurement to the position of the other at speeds greater than the speed of light. Thus, quantum mechanics must be incomplete, a mere approximation to a deeper theory (a “local hidden variable” theory, in which the results of a particular measurement are determined by a factor common to both systems in an entangled pair (the “hidden variable”), rather than something further away from the measurement location than a signal might travel at the speed of light (“local”).

For around thirty years, this was thought to be a curious footnote since there appeared to be no way to test it, but in the mid-1960s, Irish physicist John Bell carried out the implications of the EPR paper in greater detail. Bell demonstrated that quantum mechanics can predict stronger correlations between distant measurements than any other conceivable theory of the kind favoured by E, P, and R. This was tested experimentally by John Clauser in the mid-1970s, and Alain Aspect’s early 1980s experiments are generally regarded as conclusively demonstrating that these intertwined structures cannot be explained by any local hidden variable theory.

The most common explanation for this finding is that quantum mechanics is non-local, meaning that the results of measurements taken at a specific location can be influenced by the properties of distant objects in ways that cannot be clarified using light-speed signals. This does not, however, allow for the transmission of information at speeds greater than the speed of light, despite several attempts to do so using quantum non-locality. Refuting these has proven to be a remarkably fruitful endeavour; for more information, see David Kaiser’s How the Hippies Saved Physics. Quantum non-locality is also at the core of the information issue in evaporating black holes, as well as the “firewall” debate that has ignited a lot of recent debate. There are even some radical ideas involving a mathematical relation between entangled particles and wormholes, as defined in the EPR paper.

## The Scale of Quantum Physics Is (Mostly) Miniscule

Quantum physics has a reputation for being odd because its predictions, at least for humans, are radically different from our daily experience.

This happens because the effects involved get smaller as the objects get bigger—if you want to see unmistakably quantum action, you essentially want the particles to behave like waves, and the wavelength decreases as the momentum rises. The wavelength of a macroscopic object like a dog walking around the room is so ridiculously small that if you were to expand it so that a single atom in the room would be the size of the entire Solar System, the dog’s wavelength would be around the size of a single atom inside that solar system.

This implies that, for the most part, quantum phenomena are limited to the size of atoms and fundamental particles, where the masses and velocity are small enough for the wavelengths to be large enough to be studied directly. There is an ongoing initiative in a bunch of places, however, to push the scale of systems exhibiting quantum effects to larger sizes. I’ve blogged a bunch of studies by Markus Arndt’s group showing wave-like activity in larger and larger molecules, and there are a bunch of groups in “cavity optomechanics” trying to use light to slow down the motion of chunks of silicon to the point that the discreet quantum nature of the motion will become apparent. There are also rumours that it may be possible to do this with suspended mirrors having masses of a few grammes, which would be wonderfully awesome.

## Quantum Physics Is Not Magical

The previous argument leads very naturally to this one: as strange as it might seem, quantum physics is not magic at all. Stuff that he predicts is strange to the norms of ordinary physics, but they are rigorously limited by well-understood mathematical rules and principles.

So, if anyone comes up with a “quantum” concept that sounds too good to be true—free energy, magical healing abilities, unimaginable space drives—almost it’s definitely true. That doesn’t mean that we can’t use quantum physics to do incredible stuff—you can find some pretty interesting physics in worldly technology—but those things remain well within the limits of the thermodynamics laws and just simple common sense.

So there you have it: the basics of quantum physics. I have probably left a few items out or made some comments that are not sufficiently correct to please everybody, but this should at least serve as a valuable starting point for further discussion.