Outreach

Quantum Reality or lack thereof / Do we really have a choice?

“If all this damned quantum jumping were really here to stay, I should be sorry I ever got involved with quantum theory.” - E. Schroedinger

[I can't accept quantum mechanics because] "I like to think the moon is there even if I am not looking at it." - A. Einstein

“[Quantum mechanics] describes nature as absurd from the point of view of common sense. And yet it fully agrees with experiment. So I hope you can accept nature as She is - absurd.” - R. Feynman

Introduction - The quotes above by various Nobel Laureates are but a few among the several expressions by the leading physicists who helped develop the "quantum theory". Soon after the success of the quantum theory in resolving several experimentally observed paradoxes in the early 20th century, the world of physics threw itself into a bilateral heated debate about what the theory meant for the way we think about the real world. The famous "cat experiment" by Schroedinger was just a thought experiment, a line of reasoning put forth to elucidate how ridiculous this view of quantum reality sounded, so ridiculous that it cannot possibly be correct. The quantum reality made further important philosophical implications about the idea of "free will", i.e. whether or not everything that is happening around us is predetermined. Several of the elements of this discussion, when quoted out of context, have caught the fancy of even the non-physicists. In this article, I attempt to give just a flavor of the confusion and excitement about these ideas. So if you came home one evening, opened the door to your room and found your cat dead inside, you can stop beating yourself up for it and go to a peaceful guilt-free sleep: your opening the door did not kill the cat, she was already dead.

Physics is the study of nature. The objective is to understand the world we live in, and hopefully use this understanding to shape the world to the best of our convenience. It is then natural that we expect the theories that model our world to be consistent with what we experience in our everyday life. If I were to tell you that when I drop a stone in the middle of a swimming pool, it creates a disturbance leading to the water level rising and dipping in different parts of the pool in such a way that we experience the waves. I will tell you that the whole pattern is difficult to calculate, but I have some mathematical equations the solution to which will tell me exactly how this works. You would probably believe me [may be not the part that I can actually solve those equations ;) ]. But if I were to tell you that I drop a Billiards ball on an empty table, it will form similar waves and go in all different directions at once, you will probably look at me weird and think that I have lost it after all these years of staying up late. And you will be right! The Billiards ball would not do that. This is just an exaggerated example to give a feeling for what quantum theory predicts about a particle being dropped in an empty space. Of course, the billiard ball is composed of millions and millions of microscopic particles and the example I gave you applies to a tiny microscopic particle, such as an electron.

So what does the quantum theory tell us about a single microscopic particle, let us say an electron, which is dropped on a table. Schroedinger's equation predicts that the probability of finding the electron will indeed spread out in all directions like a wave. Pay attention, it is not the electron itself, it is the probability of finding the electron. This is the quantum reality that many have found disturbing. It says that we can only talk about the probability of events. This is often stated in an oversimplified manner by saying that the electron is at two (actually infinitely many) locations at the same time. That is not quite correct. There is a non-zero probability to find the electron at many places. But whenever it is "found", it will be at one place. The idea of "finding the electron" is more precisely stated as "measuring its position". Thus the quantum theory tells us that we can only predict the probability of finding the electron at any location. When we make the measurement, in this case by setting up detectors at many places on the table, the electron will be found at one place. This finding the electron is called "wavefunction collapse". The wavefunction describes the probability of the electron being at different locations. When we measure and the electron is found at one place, the wavefunction has collapsed, i.e. at that moment, the probability of finding the electron at that location is 1 and zero everywhere else. Immediately after the measurement, the electron probabilities start to spread out again in accordance with the Schroedinger equation. This interplay is often termed as "wave-particle duality". The electron behaves like a wave and a particle at the same time. More precisely, the wavefunction describing the electron probabilities propagates like a wave (according to the Schroedinger equation) but whenever you measure the position of the electron, it behaves like a single particle.

This brings us to, in my opinion, the single most important idea of the quantum theory - "superposition". Superposition is the concept that the electron wavefunction is a sum of electron being at one location plus it being at another location and so on. In imprecise language, this is often stated as electron being at multiple places at the same time. The concept of superposition spans the entire quantum theory and it becomes even more counterintuitive in other cases.

We are now ready to tackle Schroedinger's cat experiment. Again, it is just a chain of reasoning designed to elucidate the ridiculousness of the quantum theory. No kitten were actually hurt in any experiment. Schroedinger argued let us imagine a box in which there is an atom, which being a microscopic entity, can exists in a superposition of two states. Let us call these states A and B. So there is a probability that the atom is in state A and a probability that it is in state B. Fair enough. Now let us incorporate a diabolical device in this box which releases poison if the atom is in state A and does nothing if the atom is in state B. Now we have a problem. Since our atom, being a tiny tiny particle, is in a superposition of the two states A and B, the poison should be in superposition of being released and not released at the same time, because the poison's state is directly linked to the atom's state. And hence the poor cat is in superposition of being dead (Atom in state A and poison released) and alive. Since none of us has ever seen a dead and alive, at the same time, cat like that, this entire idea of superposition should be ridiculous. So if one takes the quantum reality seriously, only when we open the box to "find out" if the cat is dead or alive, its wavefunction collapses and the cat assumes one of the two possible states. Before opening the box to look inside, the cat was in a superposition of being dead and alive.

The above reasoning was used to ridicule the reality as put forth by the quantum theory. But there are some flaws in the above line of thought, which were not very obvious at that time. The moment there is established a one to one relation between the atom being in state A or B and the poison being released or not released, this constitutes a measurement. One does not actually have to open the box to make a measurement. This leads us to the highly technical concept of "measurement" in the quantum theory. There are long detailed texts available on the theory of measurement and the extent to which these concepts have evolved over the years. For the moment, it suffices to point out that any object which consists of a large number of microscopic particles may typically be considered as a "classical" object incapable of quantum superposition. Such classical objects act as measurement apparatus and lead to wave function collapse when they come into contact with the quantum entity in a certain manner. Whether humans look at the "measurement" process or not is irrelevant. Thus a lot of confusion about the idea of "looking inside the box" is an artifact of imprecise language used to represent the act of measuring a quantum system.

Now there is something else which is pretty subtle and exciting about the probabilities that we have been talking about. For this, it is fruitful to discuss the concepts of epistemic and ontic indeterminism. You could ask me the question that how many oxygen molecules are presently residing in my room. I could do some calculations and tell you the probability that there are N number of oxygen molecules on an (time) average, for any number N that you may ask. Now this way of solving the problems also talks about probabilities. The need for me to talk about probabilities is rooted in my lack of exact knowledge. But it is, in principle, possible to figure out how many molecules there are at any time. In other words, a superhero with super-counting abilities could in principle actually count the molecules and give you an exact answer instead of the probabilities. The situation we have is called epistemic indeterminism, where the need to resort to probabilities stems from the lack of knowledge rather than any fundamental constraint. Quantum theory, on the other hand, embodies ontic indeterminism in which it is not, even in principle, possible to associate the system with an exact number. This means that the aforementioned atom being in superposition of states A and B does NOT mean that we just don't know which state it is in and hence can only talk about probabilities, but rather it means that the atom itself does not know which state it is in.

It is the ontic indeterminism postulated by the quantum theory that has bothered several physicists including Einstein, who formulated the famous EPR paradox to argue the absurdity of this idea. I will not get into the details of the EPR paradox, or rather how it is not a paradox and has been tested experimentally. I simply remark that the quantum reality, along with its ontic indeterminism, has passed all experimental tests to date. The experiments testing the EPR paradox gave evidence in favor of the quantum theory and against Einstein's expectations.

This ontic indeterminism has also filled many of us with hope, the hope that we have free will, the hope that the future is not predetermined and we still have a chance to shape it. All the physics theories before quantum mechanics were fully deterministic i.e. given a certain present, the future was already decided. Imagine that another superhero knew everything about the current world, the state of every single particle in the universe. Now all the earlier theories say that if we know the state of a system at a given time, we can predict its future by solving the appropriate equations. Of course, this task is not possible since it requires an infinitely fast computer and knowledge of the whole universe. But from a philosophical point of view, it is not forbidden, an imaginary superhero could do it. But quantum theory states that we can only predict the probabilities and hence the future is still not decided or predictable. The wavefunction collapse during the measurement is unpredictable. The quantum theory thus makes such an omnipotent superhero incapable of predicting the future (no offence meant to the Indian fortune tellers). All this discussion is purely philosophical since even if there was no ontic indeterminism, the vast number of possibilities in a fully deterministic world would still render the future fully unpredictable. Hence ontic indeterminism or not, there is no excuse for giving up trying what you are doing just because the future may already be decided. It is not!

The advent of quantum mechanics was motivated by experimental paradoxes that could not be resolved by any theories that existed at the time. Almost a century after the development of the quantum theory, we have not encountered any further experimental paradoxes that would challenge the validity of the quantum theory. Of course the whole idea of unifying quantum theory with gravity is considered a different topic altogether. I speak merely of the experiments that can be done in our labs on Earth. Despite the "absurd" view of the quantum theory, the experiments have always spoken in favor of this ontic indeterminism based view of reality. Several theories have been put forward which try to interpret things somewhat differently and allow for removing this indeterminism, at least partly, at the expense of some more complex and unconventional ideas. At this point, the experimentally verifiable predictions of most of those do not differ from that of our present understanding of the quantum reality. However, one cannot rule out that there may be a time when the level of our experiments is so sophisticated that we might hit another paradox not explained by the present understanding of the quantum theory, even though there is no good reason at the moment to expect that to happen in our labs. But with the rapid technical advancements, our labs are getting better and better, and continue to edge towards new challenges and paradoxes.