On the Existence of Unobservables in Modern Physics
"I'm astounded by people who want to know the universe when it's hard enough to find your way around Chinatown." --Woody Allen
Humans have long felt an innate need to explain the world around them. Throughout history, explanations have ranged from the highly mystical to the concrete and pragmatic. Modern society--having inherited millennia worth of stories and tales and explanations--has by and large settled on science as the best and most useful way to describe our universe. Science, after all, has led to technology. Technologies that work. An airplane based on Bernouli's principle will fly you from New York to San Francisco; a nuclear power plant based on Einstein's famous E=mc2 will provide energy. Consistently, no less. No amount of prayer to gods, goddesses or amorphous life forces has ever done the same. Few can deny that modern science has given us a reliable way to describe and manipulate nature.
But describing the world does not neccessarily equate to explaining it. One can imagine that the world behaves as if there are nuclei to give off energy without the necessity of their actually being any nuclei. At the extreme, G-d could simply be running the equivalent of a complex computer program that makes matter behave as if atoms contain nuclei. Or, much less extreme, our theories could simply be wrong.
Scientific realism asserts that, by and large, science does not just offer a description but a correct explanation of nature. This can be divided up into at least two theses: 1) scientific theories are not to be interpreted figuratively but literally (I.e. the world works as if there are nuclei, quite simply because there are nuclei) 2) in general, scientific theories get more and more accurate with time, and therefore we can justly consider current theories to be--at least very close to--a valid explanation of the world. (1) One can however, accept both of these theses about observable objects, without being willing to accept them for unobservable ones. I may, for example, be perfectly willing to believe that two magnets will time and time again be attracted to each other, without committing to the belief that there exists atoms with individual magnetic spins that contribute to that attraction.
It is the the reality of these unobservables that I will debate here. I will discuss them in the context of the two theses I have outlined above. First, I will tackle whether unobservables should be understood as literal entities. This discussion will explain why there is a distinction between observables and unobservables but that, in general, these distinctions can be overcome. I will show that unobservables should be interpreted as literal entities. Second, I will turn my attention to whether or not the specific unobservables in today's theories should be understood as literal entities. Specifically, the question is whether believing in the reality of unobservables in general requires one to believe in the unobservables presented in the Standard Model (I.e. neutrinos, quarks, mesons, etc.) I will argue that many of today's theories are not as sound as some would believe and that we should not be wholeheartedly committed to the unobservables posited by modern physics. This argument will not just admit that science isn't always right--a trivial truism--but that the edges of science are always based on a complex series of assumptions that should leave us skeptical.
I will begin with the assumption that observables do exist and that we as humans can interact in a predictable way with those observables (2) .We are aware of the existence of observables, quite simply, because we can detect them with our five senses. In general, we trust our senses. But inherent in the assertion that "seeing is believing" is the equally oft-heard claim "I need to see it to believe it." We are wary of trusting in the existence of things we cannot observe. No philosopher has done more to highlight this distinction than Bas van Fraasen. Van Fraasen pointed out that the minute we jump from describing the observable to the unobservable we've introduced theories, theories which are always questionable. For example, positing the existence of an electron involves a series of steps that requires a belief in atoms, charge, energy, etc. The moment we can't observe something directly, we must depend on layers of theories to believe it exists. Van Fraasen admits that unobservables come in different categories, and that which we can see through an optical microscope (since it is so close an analogue to our own vision) has less uncertainty than that which we see through an electron microscope. Regardless of where we draw the line between that which is observable and unobservable, we do have an intuitive sense that objects exist in each category. It is certainly reasonable to recognize a distinction between the observable and the unobservable.
But the mere lack of direct observation does not prove that unobservables don't exist. Let's take a very simple example. While we talk of observables being that which we can experience with our senses, in real life humans rely on their vision more than anything else. Yet, by virtue of experience, we know that the smell of an apple pie goes with the reality of seeing one in the oven. A person sitting in the living room smelling that pie believes in its existence as strongly as if she'd seen it. So, too, experience can teach us additional ways of "observing" things in nature. One could argue that the tiny stumble from smelling to seeing is a far different thing than the great leap from directly sensing an object to measuring a proton's movements based on theory-laden accelerators. Yes, the latter requires many more steps to get from the observable quantity to a belief about an unobservable one--but if each step makes sense, why should the mere quantity of steps be problematic? In a crude sense, if we believe our eyes work then we should believe that optical microscopes work and then we should believe that cells exist. Similar steps will carry us to believing that accelerators show us real particles not artifacts. The idea that we can methodically add layer upon layer of information to prove the existence of unobservables is known as abduction and it seems a very powerful one to me. Why should there be some magical line at which the types of "observation" we devise should no longer be valid? As Churchland pointed out--there is no reason to think that some alien being could not be born with electron microscopes for eyes. Would we really accept a dichotomous world where they could believe in atoms because they could sense them directly, while we could not? Do blind people deny the existence of colors because they cannot see them when others can?
Abduction is by no means the final word on why we should believe unobservables exist in the same way observables do. In fact, both Observationists and Instrumentalists accept that modern theories make useful predictions about the world but insist that the unobservable entities posited can be understood figuratively instead of literally. For example, there exists something which consistently behaves in such-and-such a way and we name that an electron because it is useful to do so, not because it is in fact a small, negatively-charged particle. Devitt has stated that this leaves one, in the end, without any valid explanations (Devitt 128).
Having laid down, albeit briefly, a reason to believe in the existence of unobservables, in general, I turn to the question of whether or not we should believe in the specific unobservables posited by modern physics.
To deny all the unobservables of modern science, leaves us with nothing. I am hard-pressed to come up with any single example of a description of the world that does not in the end rely on some of those unobservables. The sun and earth seem like possible candidates--big and very observable. Yet to know anything about them beyond the most simplistic--the earth has some soil on the outermost layer and the sun seems pretty hot--is to believe in gravity. The earth spins. It travels around the sun. To turn our back on very basic theories such as the theory of gravity is to leave oneself with a pretty poor scaffolding for knowledge indeed. We must introduce the Newtonian concept of forces at the least, from there it's a short step to Einstein's gravitational fields, and, then scientists will want to add the yet-to-be-observed gravitons as well. To know anything about the earth is to accept that gravity exists. But how much of the theories about gravity must we accept?
No one has observed a graviton. The theory of the graviton's existence falls neatly out of the equations that describe gravity. Even beyond the math, there is a seductive symmetry: particles govern the three other major forces: electromagnetic, weak, and strong--why shouldn't gravity have such a particle as well? But math and symmetry do not a particle make. While I am content to believe in particles that are observed indirectly--they still must be observed.
So I have defined a line already. Hypothesized unobservables that have not been observed even indirectly are not to be considered real. I will move that line up even further.
I have stated that abduction is a reasonable way to learn about unobservables--but this does not mean that it should be trusted as surely as we trust our own senses. Yes, we have worked out properties of optics and so we believe we see a blood cell through a microscope and so on . . but with each step it cannot be denied that we are introducing some uncertainty. And one can come up with seemingly correct answers for quite some time after even great mistakes have been incorporated. (Anyone who's filled in an incorrect word in a crossword puzzle and has continued to add numerous words that seem correct only to be left with an impossible "WQX" somewhere knows this.) I envision science as a high narrow tower of boxes built up upon boxes that inherently gets wobbly at the top. It's easy to see how that tower can be severely shaken and much of the top will fall. The bottom, the foundation, is stable and may grow in size over time. I offer this metaphor in contrast to Neurath's metaphor that one creates their explanation of the world much as one would rebuild a boat while staying afloat. The boat metaphor implies that every bit of knowledge is about the same distance from being accurate--any of it can be reexamined at any time. I deny that the newest theories at the top of the tower have the same, or ar even anywhere near the same, accuracy as the ones at the bottom. I am not merely saying that science is sometimes wrong--but that there is a good chance that at any given time, a large portion of modern theory may come tumbling down. And yet, I am also not willing to embrace Kuhn whole-heartedly and say that the whole tower will crash. A foundation will remain.
Which leaves us with the question: what makes for the top of the tower and what makes for the foundation? What unobservables shall we believe in? To some degree, these questions must be answered on a case by case bases, but some broad outlines have been given us by some of the great philosophers of science. To paraphrase Popper, for example, one can never be 100% sure, but the more experiments one does proving something exists the more reason to believe it does in fact exist. Nothing like centuries of uncontradicted evidence to build up the foundation of my tower.
Hacking adds an additional criteria that makes a great deal of sense and also becomes important in the current context of modern day physics theories: "Experimenting on an entity does not commit you to believing that it exists. Only manipulating an entity, in order to experiment on something else need do that," (Hacking 263). Once one begins to use one's unobservables in the next round of experiments, in the attempt to probe the next layer of knowledge, does it make sense to truly believe such particles exist. Hacking also adds another reason to believe in an unobservable is if it can be perceived through a number of different indirect observation methods, (Hacking 201).
All of which is to say that the Standard Model sits squarely in that very wobbly bit at the top of the tower. The big particles, protons and neutrons and electrons, match Hacking's criteria--but no one has ever made a neutrino do anything. Moreover the particles of the Standard Model are observed in one way and one way only--the detectors at the end of accelerators. Scientists may insist as loudly as they want that no one has been able to contradict the Standard Model in experiment--but an experiment that observes only, and one type of experiment at that, is simply not enough evidence.
In summary, there is no good reason to deny the existence of unobservables altogether, but we must acknowledge how theory-laden is the framework we build to prove their existence. In the meantime, we need be committed to the reality only of the very simplest of particles. The Standard Model, sitting at the top of our knowledge tower, may presently be a useful tool to describe the universe, but cannot yet be believed to be an accurate explanation.
1) I take these two broad theses from maxims that have been laid out in numerous configurations throughout scientific realism literature. In "The Current Status of Scientific Realism," for example, Boyd offers these with two additional assertions, namely that scientific theories are confirmable using standard scientific methodology and that scientific reality is independent of our thoughts or beliefs. These offer additional important fodder for thought, but not necessarily useful in the current discussion.
2) I am aware that this is a big assumption, but to argue for common-sense realism and to prove the existence of cats and trees is beyond the scope of this essay. For my purposes here, bear with me and assume that the world exists in some kind of concrete way independently of our own thoughts and beliefs.
Boyd, Richard N. "The Current Status of Scientific Realism"
in Scientific Realism. Ed. Leplin.
Devitt, Michael. Realism and Truth. 2nd Ed. Princeton: Princeton University Press, 1997.
Hacking, Ian. Representing and Intervening:introductory Topics in the Philosophy of Natural Science. Cambridge: Cambridge University Press, 1983.
Redhead, Michael. From Physics to Metaphysics. Cambridge, Cambridge University Press, 1995
Van Fraasen, Bas C. The Scientific Image. Clarendon Library of Logic and Philosophy series. Oxford: Oxford University Press, 1980.