What is Quantum?

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A friend of mine recently suggested a documentary to me about the mind’s healing power. Terrible pseudoscience all around, but it landed some nice points about stress. Nevertheless, I was horrified at how easy the word quantum was dropped, without any context, and any reason. And this is not an isolated event, I have seen this trend for some time. And I think that is not only a trend in the circles of pseudoscience. In science-fiction and superhero movies for instance, quantum jargon is used as mumbojumbo and plot devices, admittedly a whole more harmless way. However, with buzzwords like quantum healing or quantum consciousness on the rise, we physicists must take a stand and clarify what does the word quantum really mean.

Quantum mysticism and media

I think it is best (and easiest) to start by stating what quantum is not. First of all, quantum is always a verb or and adjective. If you hear someone saying something like the quantum, they probably don’t understand it or you could even be facing a snake oil vendor.

But most importantly, quantum mechanics, the proper name of the area in physics, is not a mystical mystery. Quantum does not stand as a prefix to some occult, arcane, spiritual unknown, but for a well-established field of human knowledge which has produced some of the most thoroughly tested predictions in the whole of science! I have heard a lot of people, like Deepak Chopra using it as a way to connect to consciousness, to heal yourself. Normally such people use these words in a surrealisticly incoherent word salad. You think what Tony Stark says in any of the Avengers movies is weird science-y mumbo-jumbo? Well it is much worse. Since there is just no evidence that links directly the microscopic quantum world and consciousness, there is no need to start to even wonder whether these people are onto something. Additionally, with this kind of discourse comes no explained mechanism, and blatant disregard for the scientific method in general. For example, it is mostly established that neurons are macroscopic non-quantum entities. So the point of my post is not just to diss Chopra and others like him. The problem is that this technobabble is actually hurting science. Mixing in good science with pseudo-science tends to damage the reputation of the former, and create a general distrust for, say, news on quantum results.

Yes, I understand the word sometimes stands, in the media that deals with communication of science, for some otherwordly results. Even in more reputable newspapers and journals it is often somewhat fictionalized as a transcendent new area, which is too much for our minds to fathom. Why? Well, clickbait of course! Don’t get me wrong, I believe it is breathtakingly interesting, sometimes confusing, and definitely worth exploring. However, in my opinion, this approach tends to shut people out of the beautiful ideas in Quantum Mechanics. It unjustly transforms it into a hermetic secret, only available to a few enlightened. And this can’t be more wrong. For example, I see many concepts from quantum mechanics in articles in the news and even in memes which are used out of context, at best, and straight up misrepresented, at worst. The first one that comes to my mind, and probably your’s too, is the famous Schrödinger’s cat. To try to disentangle all this, I want to explain a couple of things first.

Just a tiny bit of History (I promise)

The word Quantum comes from the -let’s be honest here- original guesswork from Planck. He wanted to explain (calculate) the radiation emission spectrum of a black body. Let’s dissect that. A black body is an object that – as in fact every object- radiates light because of their temperature. However, contrary to a real material it does not reflect light, only absorbs it. In case you are wondering, the best fitting object to this idealization is the Vanta Black color (see it here), but even the Sun does a good job faking it. Such a body will radiate light, for sure, but which colors are radiated will depend on the temperature of the object. Think of glowing hot metal, which starts out dark red and slowly progresses to orange, yellow and maybe even white-bluish when it’s the hottest. This is what Planck was trying to calculate, the frequency (i.e. color) dependence of the light’s brightness emitted by these bodies.

He used classical theories, which were state-of-the-art around 1890, and failed badly at describing observations. With these theories, the energy or frequency of the light was taken to be continous. That is, that between any two allowed frequencies, there are always a lot more in between. When he decided to use some finite energy-spacing instead (disallowing most values of energies except some integer multiples of a fixed number) as a guess, suddenly his prediction was quite good. He didn’t believe it at all, but fortunately published his results, and called these packages quanta [1].

After this, the idea of quanta was further popularized, by successful results like Einstein’s quantum description of the photoelectric effect, the underlying principle of solar cells. Nevertheless, if I may choose one result which I think was the most world-shattering for the scientists of the time it would be the double-slit experiment. Now, I know a lot is said about this, about the observers and the wavefunctions, and what-nots. I think, to some level, this is part of the mystification that I mentioned before. Instead, I want to focus on something else. In this experiment, light is shone through two settings: a small slit, and two close small slits. When shone upon the single slit, you can observe that the light reproduces the shape of the single slit on the target panel. However, when using the double-slit panels, you can observe an interference pattern instead, a pattern of many dark and bright stripes instead of a picture of the two slits. In the same way that waves cancel each other in a pond, it seems to be that light does the same. Depending of the setting, we can describe light as a wave and as particles.

What does this mean?

Without interpreting too much (which is what I think people normally try to do), it means that the fundamental properties of these “quantum” particles are neither particle-like, nor wave-like, but both. And this, at its very core, what quantum mechanics is.

So what is quantum mechanics?

At it simplest, and its most mundane, I will simply summarize that quantum mechanics is just a set of rules. These rules apply in different scenarios than the ones that we have in our every day life, also called classical or Newtonian mechanics. In fact, quantum mechanics contains classical mechanics as well, in the limit in which one basically sees . This is why it seems to be so strange. Such rules are in extremely good agreement with experimental data, at a lot of fascinating levels. It is thanks to quantum mechanics, that we know the energy levels of atoms, and the basic idea of how chemistry then develops from there. It also describes materials, such as metals, semiconductors and superconductors. It even describes well the fluctuations of density at the very beginning of our universe. Some of the numbers predicted by quantum mechanics have been tested to the 10th digit after the comma!

With those rules, comes a small conceptual separation which did not exist before. We now have observables and non-observables. First of all, observables are, of course, quantities you can measure. A particle on a detector, the alignment of an atom with respect to a magnetic field, or the magnetization of a magnet are all observables. On the other hand, and because of the mathematics that are used in the area, some quantities are needed to calculate observables, but they cannot be measured themselves. The famous wave-function of the Schrödinger equation, or quantum fields are cases of non-observable entities.

What are these non-observables and what happens to them before and after measurement are subject to the interpretations of quantum mechanics. This is a very popular topic for the communication of science. The orthodox, established interpretation of most quantum physicists is: the universe is fundamentally random (Copenhagen interpretation). Alternatively, some people believe that there is a collection of all possibilities, and every time a quantum measurement is performed, a new “universe” is created (Many-Worlds interpretation). There are also, alternatives in which there is no randomness, but allow for particles to slide deterministically on top of some ethereal wave (Bohmian Mechanics). All of these interpretations may be enticing and mindblowing, however it is of paramount importance to have always in mind that quantum mechanics does not need an interpretation. You do not need it to calculate and explain experimental data. This has been often humorously referred to the Shut-up-and-calculate interpretation of quantum mechanics. The point of this is that apart for some interesting and important subtleties we will write about in the future, interpretations are mostly views of a meaning asigned to these rules beyond the scope of the measurable. Some people therefore believe that the interpretation of quantum mechanics is not really science, as science has as one of its basic principles that theories need to produce observable and measurable predictions in order to be useful.

So what are the rules?

First of all, the basic rules of quantum mechanics are empirical, i.e. deduced from experiment. The rules were found (among others) by Heisenberg (Werner, not Walter) by interpreting the spectra -distribution of emitted light- from hydrogen atoms. He formalised the mathematical aspects [2], and found using this, his famous relationship, the Heisenberg uncertainty principle. It states that one cannot exactly know at any moment the position and momentum (speed) of a particle. The better you know the position, the less you know its momentum, and vice versa. This is clashing, of course, with Newtonian Mechanics, where for one can know all the details for a falling ball, or the Earth, revolving around the sun. Fancy as it sounds, this implies one thing: the particle is now delocalized, that is, it is not a point in space, but something blurrier. You know what else is not localized? Waves.

A is a classical particle: its momentum and position can always be known with arbitrary precision. B behaves as quantum particle, because it is fundamentally not possible to know its momentum and position with arbitrary precision, the particle becomes “blurry” or wave-like.

This rule can be stated more formally and leads to all the interesting results discovered afterwards, and is called quantization. Well… duh-doy, you may rightfully retort, but let me explain. The trick lies on finding the appropriate quantities that describe your system (for example, position, x, and momentum, p) and changing the relationships for these canonical variables, from the normal classical mechanics relations to special new ones. Basically this means that position and momentum (velocity/speed) are not numbers anymore, and they don’t commute. This means x*p is not p*x. In fact, for such variables we use xp -px =ih/2π, where i is the imaginary number, and h= 6.626×10^{-34} J⋅ s is a universal constant, which is a tiny, tiny number in the SI unit system. This means it’s not the same to measure first momentum and then position and first position and then momentum, which also implies Heisenberg’s uncertainty principle – that you cannot know both position and momentum to arbitrary precision.

It all comes full circle. This is what it means when we talk about the particle-wave duality. Quantum objects are behaving both as waves, and as particles as much as we don’t know both their momentum and their position. But then we have to ask ourselves one more question.

Which things are quantum?

Actually, it may come as a surprise, by the fact of how quantum mechanics is protrayed normally in media, but this question is in itself, nonsensical. Quantumness is a behavior, not a property. Therefore, nothing is quantum, but everything is determined by the rules of quantum mechanics. There are scenarios in which things, even macroscopic, can became more quantum, or less quantum.

How is this possible? Well, any object, be it a jet plane, or a mosquito, or a proton, can be characterized as waves as much as they can be described as particles. These waves have a typical size, or wavelength, which is called the de Broglie wavelength. This size decreases with the mass and speed of the object. Heavier and faster things will exhibit smaller sizes. Now think that you are measuring properties of these systems. If you decide to measure things which are inherently larger than the de Broglie wavelength, then you will see that this particle does not exhibit quantum properties. For, say, a tennis ball, measuring such wavy patterns, is basically impossible. But, if you are seeing something smaller, and with less speed or energy, then measuring these wavelengths is possible, and we can do it since a century or so [3].

Now, the traditional notion is that quantum mechanics rules systems which are tiny and with little masses. As the mass decreases, the waviness appears. However, another way of making objects quantum again is to decrease their speeds. And this is possible by decreasing the temperature, which is just a measure of the energy of the individual particles of a medium. If the medium is colder, they have less energy to move around. So, less speed. That is why there are so many interesting quantum experiments using, for example, bunches of ultra-cold atoms. Here, ultra-cold means that they are very close to the minimum temperature of the Universe: 0 Kelvin. And this means that these objects can be very large but still quantum – as large as millimeters, visible with the bare eye! I also would like to shout out to my alma-mater, which hosts a very big array of such interesting experiments. In the future we may cover more of this.

So… What now?

With all these articles going around about quantum that and quantum this, I felt like there are too many of them trying to explain the this and thats and  not enough explaining what we mean with quantum. I just wanted to explain a little bit my take on this area, and therefore, you have now this long rant. Why did I want to do this? Quantum mechanics is one of the coolest, most exciting areas in physics. It not only challenges us to rethink the fundamental behaviors of the Universe, but also promises a lot on potentially incredible discoveries. From the fact that the mobile phone, tablet or computer that you are using basically runs thanks to quantum mechanics, to more medical uses such as MRIs, knowing all of this has tremendous real life consequences for all of us. And then, there is also quantum computation, the idea that we can create a computer in which the logical mechanisms obey quantum laws. Such a computer will have incredible capabilities, beyond what your laptop can do. But let’s not rush on that one, as we need to sort it out first.

Quantum mechanics is a not only a set of mind-bending thought experiments. It is a reality for all society. Conferring it with an undeserved aura of mysticism and arcaneness, tarnishes one of the brightest, most amazing discoveries of humanity.

Footnotes and References

[1] Quanta is the plural of Quantum, which comes from Latin, meaning “How big” or “How great”.
[2] Heisenberg actually created a clunky mathematical apparatus out of this, which he called “rule of multiplication”. Max Born realized this mathematics were already known as linear algebra (or matrix algebra) since a long time. However, it was at that point not really a part of the normal phsyicist’s curriculum.
[3] Although Young performed the first experiment on the early 19th century, the first low-intensity version was done in 1909 by G.I. Taylor. Low intensitylight was needed to see the individual photons in the experiment. Since 1961, it has been possible to do it with other objects, like electrons and even very big molecules called Buckminsterfüllerenes, or buckyballs for short.

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