The word ‘quantum’ tends to evoke a lot of mystery and mysticism and it tends to be misused by mostly charlatans and New Age spirituals to describe supernatural phenomena. Quantum Physics, however, can not be any more closer to reality than anything else there exist out there. If you search for ‘quantum’ on Google’s Ngram Viewer, you’ll notice that the word existed at least as far back as the 1600s, perhaps with a meaning quite different from today’s. Regardless, it’s not a new word sprouting out of physics’s cauldron. Also, if you search for both ‘quantum’ and ‘physics’ within Ngram, you’ll see a pattern whereby those two words would have quite opposite trends up to until 1900 and then move in synchrony until merging into one single line around 2005. This goes to show that the number of times ‘quantum’ and ‘physics’ get quoted in books gradually tend to the same value over the decades.
In 1900 ‘quantum’ took a whole new meaning when the physicist, Max Planck, suggested that energy can be emitted or radiated from a hot body only in a given amount, only as a predetermined quantity based on the frequency of the radiation being emitted. In other words, energy can only radiate in well-defined packets as opposed to a continuous stream. This idea that energy is quantised was what triggered the Quantum revolution in physics. Up until then, the universe was a well understood and predictable world. With the advent of quantum theory, this image of the clockwork, Newtonian universe would become blurred and upset quite a few eminent physicists as a result. What is it about quantum theory that is so, well, upsetting and baffling? And this is one of the reasons why it’s been hijacked and used as a ‘wildcard’ by those who attempt to explain the supernatural.
Ever since the concept of quantised energy was introduced, a whole host of new, unintuitive and world-toppling concepts followed. You see, prior to quantum physics, everyone was happy with our understanding of how things work, in general. Galileo and Newton had set the foundations for a mechanical description of how the world ticks. From their point of view everything ran according to clockwork. If we were given a set of conditions to begin with, we would be able to tell in advance how things would change over time and space. If you drop a pencil then the laws of gravity and motion demands that it moves towards the centre of the Earth and, as a result, hits the ground where its downwards journey ends. If you measure the movement of stars and planets and comets you would be able to tell days, months or even years in advance where they would end up in the sky. Edmond Halley was able to predict in 1705 when a comet would appear in the night sky based on his friend’s then recently published work, the Philosophiæ Naturalis Principia Mathematica. He concluded that the comet should make a comeback every 76 years or so. His friend was none other than Sir Isaac Newton and the comet is, of course, known as Halley’s Comet.
I think ‘clockwork’ is a good description of how the mechanics of the world was understood to be. Like a clock, or watch, you could set the time and be certain that the hands would be where they’re supposed to be an hour later, a day later and so forth. We can count on that mechanism to tell the time and to continue telling the time once we’ve set the clock in motion. In reality some clocks slow down while others speed up. If a clock is set or synchronised at 12:00 then, in an hour’s time, the time shown on the clock face could be 1:02 instead of 1:00. But that is due to the mechanical inaccuracy of the clock, the shortcomings of the watchmaker, the in-built defects in the clock. Errors which we could take into account and rectify to come up with the right time that should be shown on the clock.
If something is not exactly as what we had calculated it to be, then it must be down to our inaccuracy in measurement and the defects in our instruments and our inability to calculate with high degree of precision. Regardless, we could in theory correct those errors and have a clearer picture of how things should really be. In that sense, everything is known, everything is well-defined, we can figure out how things should be, we are comfortable with predicting how things should change; in short, we live in a predictable universe.
This was the view of the universe, this exquisite painting, so to speak, of the world we live in where everything is neatly set on canvas, in predefined order and pattern, with vibrant colours and clear lines. Then, quantum physics showed up at the beginning of the 20th century and smudged this beautiful painting and turned the canvas upside down and completely messed up our depiction of reality. It did so with the following arguments.
One of the main tenets of quantum physics is that nothing is or can be predetermined when it comes down to a certain level. For everyday objects like shoes, pens, wallets, buses et cetera, things are pretty clear. We are almost certain that, if we leave our pen on the desk and look away then that pen will stay there even when we’re not looking at it. We can expect it to be resting on the desk as long as something or someone doesn’t move it. We won’t expect the pen to be moving about on its own accord and jiggling over the desk or dashing to the bottom of the Mariana trench or simultaneously be inside a whale’s stomach and perched at the top of Notre Dame cathedral in Paris. Such ludicrous behaviour is not expected from an ordinary pen. Ordinary objects and everyday experiences seem to follow ordinary mechanisms and seem to suit our intuition. This is not the case for ‘quantum’ entities.
What do we mean by ‘quantum’ entities? What differentiates a ‘quantum’ entity from a ‘non-quantum’ one? Strictly speaking there is no difference. Everything in the Universe is made up of ‘quantum’ stuff. However, when we consider relatively smaller entities, such as subatomic particles, that is when their quantum nature is more pronounced. Even though all of matter in the universe is composed of those quantum or fundamental particles (see Blocks), when go beyond the realms of those particles, the collective nature of these particles tend to be less ‘quantum-like’ than the individual particles. The key point here is that there is an order of magnitude in size and mass beyond which matter behaves in a certain way. If an entity’s mass and/or size is comparable to that order of magnitude then it will display its ‘quantum-like’ nature. Otherwise, we tend not to observe that behaviour. What order of magnitude are we talking about here? How small should those entities be for them to display their ‘quantum’ nature?
There’s this constant of nature called the Planck’s constant and is denoted by the letter ‘h‘. It is named after Max Planck, the forefather of quantum theory. Its value is about 6.6 × 10-34. This is an extremely small number; it’s something like 0.00000000000000000000000000000000066. It’s so small that, if we were to compare the value of the mass of an electron, it would be about 1 400 times that. An electron, by the way, is one of the tiniest particles that exists. As you can imagine, Planck’s constant is a ridiculously tiny number. This is the benchmark, if you will, against which you can compare the ‘quantum’ nature of physical entities. An electron’s mass is close enough, in comparison, to Planck’s constant. Therefore, we can expect that an electron will display its quantum-like nature. An apple that has a mass of 100 grams, say, will be too large, relatively speaking, compared to h so we won’t expect an apple to display its quantum-like nature. So we have this physical limit or threshold which separates those which display observable ‘quantum-like’ behaviour from those which do not appear to behave in that peculiar manner. This threshold is not a strict, clear-cut, dividing line but is only an indication, a fairly lenient benchmark.
Fundamental particles have properties which are close to that benchmark and are therefore more likely to exhibit ‘quantum’ behaviours. To come back to our pen-on-a-desk example, we don’t expect such an object to behave like fundamental particles – even though that very object is made up of billions of such particles.
You must have noticed that I have been speaking a lot about expected behaviours. This is because, in quantum physics, what we deal with are probabilities as opposed to certainties. Again, this contrasting nature between classical physics, where everything moves like clockwork, and the seemingly fuzzy nature of the quantum world. This is an important concept to grasp when it comes to quantum physics: that nothing is fully determined, nothing is a certainty, everything is dictated upon by probabilities. A pen left to rest on a desk in Tokyo is not expected to disappear and reappear in Park Güell, Barcelona. On the other hand, an individual electron can be expected to jump from one location to another or be in multiple locations at the same time. Such irrational behaviour is the very nature of the quantum world. It doesn’t mean that an electron will always behave in the same random way but that there is a chance it will be in several locations simultaneously.
Another pillar of quantum physics is that the act of observing determines the outcome of the experiment. Just as we mentioned that an electron can be in several places at the same time, as soon as it is observed, its location is completely determined. Of all places it can possibly be, only one of them is made apparent upon the act of observing it. So, in principle, that pen you left behind on your desk could very well be orbiting Jupiter until you turn around and look at it again. The pen could also be on the floor as another possible location. It is more likely to be on the desk than on the floor and even far less likely to be orbiting Jupiter. Which possible location will it be found to be in? Well, until we actually look at it, until we actually make an observation, we cannot tell for sure. It’s all a set of probabilities until we make our observation. But once we do, then its one definite location is set once and for all. The same reasoning applies to anything else, even elephants and needles. Large or small, massive or light, round or oblong, squishy or volatile, dead or alive, invisible or bright red, whatever it is, the same principle applies to it. And again, the closer it is to that magic number h, the more pronounced its quantum behaviour would be.
Radioactivity is the decomposition of heavy, unstable elements into lighter, more stable ones. This process is random and spontaneous in nature. It involves the emission of fundamental particles from the nucleus of such unstable atom. As such, it is a quantum process. One cannot predict which particle will decompose or decay inside the atom nor would one be able to predict when it will happen. Now imagine a little box containing some radioactive element. This little box is placed next to a detector which will be activated if it senses any radioactivity. As soon as the detector is activated, it will trigger a mechanism whereby it switches on an oven. So far, so good. Radioactive element decays, sets of detector which switches on the oven. Radioactive element doesn’t decay, doesn’t trigger detector, oven stays cold. Simple logic. We’re happy. In that oven, there sits a large pepperoni pizza. The fate of the pizza, as you can imagine, is completely determined by the radioactive element. If it decays, you end up with a piping hot pepperoni pizza. If it doesn’t decay, you’ll have a cold, placid pizza. Since we cannot tell beforehand if the radioactive element will decay or not, we won’t know for sure whether we have a cooked or uncooked pizza unless we actually open the oven and peek inside. Let’s say the radioactive element has equal chances, equal probability of decaying or not then, the pizza has equal chances of being cooked or not. And because quantum physics tells us that a quantum process can be in several states simultaneously then the radioactive element can be both in a state of decay and non-decay. There is no problem with that. It can easily be in both states at the same time simply because it behaves according to its quantum nature. And by this logic, our pizza can be in both cook and uncook states at the same time! We won’t be able to tell which state it is in until we open the oven. Just as I have mentioned earlier, it is our act of observation that determines the outcome of some process. Its our peeking inside the oven that determines whether the pizza will be cooked or not. Until then or unless we make the observation, the pizza will be in both states simultaneously. And, of course, by determining the state of the pizza, we would also determine the state of the radioactive element and, consequently, the state in which the particles are inside that radioactive atoms. Cook or uncook, which will it be? We won’t know until we look. That might sound obvious. But what is it before we actually look? Quantum theory tells us that it is both! No wonder that with such baffling logic, some physicists found it hard to come to terms with quantum theory when it emerged in the early twentieth century.
There’s another, more famous, thought experiment just like the one I’ve described above. It involves a cat in a box. Through a similar mechanism triggered by the radioactive decay of an element, the fate of the cat can only be determined when we open the box and look at the cat. Until then, the cat is both dead and alive! This experiment is known as the Schrödinger’s cat thought experiment and is used over and over again to illustrate the quaint nature of quantum physics.
Another idea which is often quoted as being core to quantum physics is Heisenberg’s Uncertainty Principle. We can say, for definite, that there is nothing uncertain about this principle. In fact, to name it as the ‘uncertainty’ principle is quite misleading. It has more to do with indeterminacy than uncertainty. Now, you can say that I’m being pedantic and that a rose is a rose is a rose so that it doesn’t matter what we call it, it is what it is. Heisenberg’s definition for that principle was in German, to begin with. So there is this loss in translation when trying to convey the meaning behind his principle. From something that was intended as being ‘indeterminate’, we’ve come to define it as ‘uncertain’. It is true that Werner Heisenberg himself did at the end of this thesis refer to that principle as the uncertainty principle but I believe it is more in line with his concept to refer to it as the indeterminacy principle. Like I said, it doesn’t quite matter how you call it as long as you know what it is.
Heisenberg tells us that there are certain physical quantities that are linked and can only change in conjunction with the other. For example, the position of a particle and its momentum are two such quantities. In short what the principle tells us is this: we cannot know for sure where a particle is (position) and where it’s going (momentum). The emphasis on the ‘and’ is what matters it. We can either know exactly where it is but have no clue as to where it’s heading or we know precisely where it’s going but cannot fix its position at that instant in time. This pair of quantities is linked in this peculiar way. Let me give you an example to illustrate this point.
Typically when we try and locate our position on Google Maps, we have this type of picture: a blue dot and larger circle around the dot.
The blue dot shows us where we are most likely to be but because our instruments and measurements are not perfect, we have an error in determining our exact position. This error is represented by the larger blue circle. This tell us that we are likely to be anywhere within that blue circle but most likely to be where the blue dot is. This is how we read it. Had there been no error at all in our measurements, had there been perfect instruments to take those measurements, then we would see the blue dot only and no other circle for there would have been no error in locating our true position. Reality is not perfect and therefore trying to determine anything with infinite precision is physically impossible.
What quantum physics tells us is that, even if we had the extraordinary means to construct and build the perfect instruments, even if we were perfect in taking measurements so that not a single atom of error would crawl in our data, even if our calculations were absolutely flawless and precise up to an infinite number of decimal places, even if we were totally and utterly error-free in observation, there would still be a degree of ‘uncertainty’, a small blue circle in our observation. Because the reality of the physical world is such that this indeterminacy is in-built in those physical quantities, not in our lack of perfect instruments and measurements. Because the reality of things are such that the absolute does not exist. Because everything has a degree of probability attached to it. Position cannot be exactly determined; we would always have an amount of indeterminacy around our observation. This is the intrinsic nature of the world.
We can definitely try and make that error smaller but by so doing, we would lose precision on the other linked quantity. We can try and focus our position and make that circle of uncertainty smaller and smaller but by so doing, we would be less and less determined about our momentum. Think of it that way: you’re trying to pin down exactly where something is. But by being able to put your finger on it, by fixing its location, you lose all information about where it’s trying to go. Conversely, by allowing it to move about so that you can determine its direction and speed and, hence, momentum, you are less able to pinpoint its exact location. What quantum physics tells you is that you can’t know both quantities for certain. It’s not exactly “one or the other” but more like “one at the expense of the other”. By losing precision on one you gain it on the other. This is what Heisenberg tells us.
Again, you might argue that we do not observe this behaviour in our daily experiences. Take a car, for instance. We know how fast and in which direction its going (momentum) and we also know where it is at any one moment (position), even if we take into consideration our (small) error in measurement. How does that comply to what Heisenberg says? Well, it does. The more we try and pinpoint the car’s location the less precise we are about its momentum. It’s not very apparent, however, because of that magic number again: h. Because the scale of the physical measurements about the car’s mass, speed and direction and distance it’s travelling and its position et cetera, compared to the tiny, tiny number h, then the uncertainty in position or momentum really is too small to bother us. It is almost completely insignificant. That is why we do not usually come across Heisenberg’s Uncertainty Principle as an everyday observable and measurable phenomena. This does not mean that it is not true or that it only applies to tiny particles.
Like momentum and position, there is another pair which are linked in a similar way. Time and energy is such a pair. I’m not going to go too deeply into this but, briefly, what this tells us is the following: A certain amount of energy can appear out of nothing, of absolutely nothing and then disappear out of existence again. Yes, such, spooky behaviour is completely permissible according to the laws of physics. Energy can pop out nothing and hang about for some time then disappear, just like that. No qualms about that, no vexing anyone, nothing illegal here. How long can this amount of energy stay around for, now that is the question. The more there is of that energy that came into existence, the less time it has to hang around. The smaller the energy, the longer it has to remain in existence. Because, once again, the quantities are related by Planck’s constant h, this typically has to do with very, very small amount of energy or, very, very small duration of existence. A millisecond might be too small a duration for us to take notice but to an electron, for example, it means a hell lot of a time. It’s enough for an electron to go round the nucleus in a hydrogen atom more than 3 trillion times! In that fraction of an instant, during that millisecond, you wouldn’t have had the time to blink yet the electron would have completed at least 3 trillion revolutions as it goes round and round that nucleus in a hydrogen atom! Truly incredible. So, on such incredibly tiny scale, time is experienced completely differently from our human-centric experiences. It would take the Earth 3 trillion years (or what I call an eternity) to go round the Sun 3 trillion times. Yet, tiny electrons do 3 trillion revolutions in less than a millisecond. In what is an eternity for an electron, many a strange thing can happen in the quantum world. In that time, energy can appear from nothing and disappear again, multiple times. And because energy is linked to mass and mass to matter, particles can pop in and out of existence, just like that, in such tiny durations.
A concept I had already explained in this blog (Spectrum) but worth recalling is the wave-particle duality. This is another central idea in quantum physics. What is tell us is the following: physical entities such as electrons, photons, cars, us, planets and so on, have characteristics which we typically associate to particles or waves. Sometimes, the particle nature is more dominant and sometimes, the wave nature is more prominent. But at all times, the wave-particle nature of such quantities are there. It’s only when we observe them that either one or the other nature becomes more apparent. What they really are, we don’t have a name for that yet but to call them a ‘wavicle’ somewhat captures their true essence. Why do we not have more of this wave-particle duality experience with everyday stuff, well, it’s down to that quirky alphabet again: h. A car does not, generally, display its wave-like nature. That is, it does not make sense to measure its frequency or amplitude or it does not make sense to expect two cars in a collision path to interfere just like water waves would interfere on the surface of a lake. It does not make sense because their wave nature is dwarfed by their particle nature. Therefore, things like momentum, mass, collisions and elasticity make more sense and are more likely to be observed than their wave-like characteristics. Two colliding cars behave like, well, two colliding cars and not like two waves interfering and passing through each other. Electrons and photons and other fundamental particles, on the other hand, do exhibit their particle and wave nature more often compared to much bigger and more massive stuff like cars and planets. Electrons have been observed to interact with each other as if they were waves interfering. Focused beams of light, or lasers, have been observed to behave and interact with matter as if light was composed of particles. Yet, on other occasions, the very same light or laser would behave as if it were a wave. In reality, however, they are neither wave nor particle but this other thing we might call a ‘wavicle’. This concept was brought forward by Louis de Broglie as an offshoot of the idea that if electrons can display wave-like behaviours and be diffracted and cause interference patterns then, perhaps, everything else can display such wave-like nature and, perhaps, waves can also, in return, display particle-like nature.
With such mind-boggling concepts and observations, no wonder that eminent physicists, such as Albert Einstein, found it hard to accept quantum physics as a rigorous description of reality. It was hard for them to accept that everything is down to probabilities and uncertainties and dualities. This clear, well-defined, predictable and clockwork nature of the world was gradually smudged as quantum physics developed through the first half of the twentieth century.
Quantum physics doesn’t end here. There are many more concepts and mind-bending notions that sprout from quantum theory but we won’t have time to cover all of it. But the key points about it are that it deals with the fundamental particles or those entities which have physical quantities comparable to h in magnitude. It is all about probabilities. It helps explain the macroscopic world in terms on the microscopic, so to speak. It is not purely about electrons and photons but it has implications which impact us and the whole universe directly. To go into such detail would require much more space and time but it is important to realise that quantum physics has more to do with explaining how the world is and why it is the way it is, than relying solely on our understanding of classical physics.
The greatest challenge facing physicists since the twentieth century is the merging of quantum theory and the theory of relativity. If we can think of the four fundamental forces as being what define our universe, then quantum theory is at the base of three of them and relativity holds the key to the fourth one. Unifying our understanding of the two fundamental theories would be, perhaps, the most comprehensive explanation of everything there ever was, is and will be. Now, wouldn’t that be grand?