Why Aren’t All Scientists Physicists?

Alexander Franklin on why it’s not all about particles

Some people study traffic jams: it’s their job to think about the flow of traffic along particular roads, and how traffic jams can best be avoided by appropriately distributing traffic lights and roundabouts.

Surprisingly it turns out that, in general, the people who study traffic jams don’t need to know anything about cars: the predictions as to when traffic jams will stop and start can be formulated in terms of traffic flow, density, and average speed without talking about the positions and speeds of individual cars, much less the make of the car or the aggressiveness of the drivers.^[1]

In fact, traffic scientists can be good at their job despite being ignorant of a great deal: not only don’t they need to know about cars, but they can be completely ignorant about the atoms that make up the cars and the sub-atomic particles that make up the atoms. So, how come those people who study traffic don’t need to know about the things that make up traffic?

To answer that question, we should note that the theories that describe traffic and traffic jams can be called ‘effective’ theories: they are effective at describing traffic despite not talking at all about the cars or the particles that make up the cars. Effective theories are everywhere in science: almost all scientific theories don’t end up talking about particles; and, even within particle physics, there are effective theories that leave out sub-atomic particles.

So how come everything seems, ultimately, to be made out of particles, and yet almost all of science can concern itself with effective theories that don’t talk about those particles? Why aren’t all scientists particle physicists?

By thinking about traffic jams, we can get some insight into what makes effective theories work in general. We can then relate this discussion to particle physics, and see how this bears on some broader philosophical debates.

Consider a theory about the flow of traffic along a country road. Given the average speed and traffic density along each part of the road, one can predict fairly accurately where and when traffic jams will arise. However, if we had no rules—if, for example, cars were allowed just to travel across fields and drive across the road at any point—then no such accurate predictions could be made.

An effective traffic theory is possible only because we have structures in place such as roads and traffic laws. These structures allow the traffic theory to be an effective theory—one that doesn’t talk about the positions and speeds of individual cars, or the aggressiveness of the drivers. Of course, there are complicating factors; but the basic idea is that the existence of these structures are what allow traffic scientists not to talk about individual cars when making predictions about traffic jams.

Given that science is full of effective theories, similar questions to those posed in the traffic case may be posed more generally: how is it that a theory about such-and-such a subject matter may be formulated without talking about the stuff that makes up that subject matter? My claim is that analogous answers to these questions can be given—that we can detail what makes just about any theory effective.

So, let’s see how this goes in the case of particle physics. Take any theory in particle physics and let’s consider how it allows for extremely accurate predictions and explanations despite not talking about some more zoomed-in set of particles. In order to understand the structures that make particle physics theories effective, we need some preliminaries in place.

Particles interact with other particles, and there is a quantity called ‘interaction strength’ that tells us how strongly they attract or repel one another.^[2] A crucial and surprising discovery of modern physics is that interaction strengths change with length scale. So, if you zoom in further, two particles that stay the same distance away from each other may interact more strongly than they would interact at more zoomed-out length scales.

Particle physics theories are effective if their interaction strengths are such that they diminish at increasing length scales. That is, if their interactions get weaker as you zoom out, then effective theories will be available at zoomed-out length scales. The reason this works is that once we’ve zoomed out far enough, we don’t need to talk about the particles that were relevant at more zoomed-in scales, because their interactions are now so weak that they don’t make any difference.

Now that we have two explanations of effectiveness in place, I can spell out the explanatory project more abstractly. As a general recipe: (1) we find an effective theory; (2) we identify the potentially relevant details that this theory doesn’t talk about; (3) we identify structures that are responsible for the irrelevance of the details in step (2). This is summarized in the table.

Step (3) answers the questions we started with: there are scientists who aren’t physicists because many scientists don’t need to know anything about physics. Physics isn’t relevant to the work of many scientists because there are structures in place that guarantee the irrelevance of physics to the subject matters of the rest of science.

My claim is that such structures may be identified across science. When an ecologist describes how lions hunt gazelles, they are only legitimated in leaving out the details of cell biology and particle physics because structures are in place to ensure that those details don’t matter for the lion–gazelle predator–prey relationship. The search for such structures is part of the job of science, but also of philosophy of science. In the remainder of this essay, I’ll explain how this search fits into an existing debate: the ‘reduction–emergence’ debate in the philosophy of science.

Some philosophers—let’s call these the ‘reductionists’—hold the view that all that there is can be reduced to fundamental physics. According to these philosophers, given a large enough computer and knowledge of the exact state of the fundamental physical world, all the facts about biology, chemistry, psychology, and so on can be recovered. On the other hand, a group of philosophers—the ‘anti-reductionists’  or ‘strong emergentists’—claim that even with a large enough computer and knowledge of the exact state of the fundamental physical world, there are parts of the subject matters of chemistry, biology, and psychology that just couldn’t be recovered.

This debate has lasted many decades and, given that it’s a philosophical debate, there’s a huge amount of wrangling over the precise meanings of the terms. It’s sufficient for our purposes that there’s a debate with two sides, and that I mean to chart a middle course.

It seems to me that the reductionist position is roughly right; however, there’s also something right about the anti-reductionist position. The anti-reductionist correctly argues that sciences other than physics shouldn’t be eliminated, and that not all scientists should be physicists.^[3]

The reason all scientists shouldn’t be physicists is that there are many extremely successful effective theories. At first glance, the existence of such effective theories may tell against reductionism. It is, thus, incumbent on the reductionist to explain the success of the effective theories. If they can do so, then they will be able to account for the motivation behind the anti-reductionist position, without denying reductionism.

In other words, if a reductionist can explain why effective theories work as well as they do, the reductionist can show that such theories shouldn’t be eliminated, and that chemists and biologists are safe in their jobs. The recipe discussed above can help us out. Reductionists can explain the success of effective theories by identifying the structures in virtue of which the theories are effective. This may lead to a milder version of reductionism: one which grants that, in some sense, all the other sciences reduce to physics, but holds that effective theories are nonetheless not to be eliminated.

Let’s conclude by coming back to particle physics. Particle physicists have used effective theories to come up with the most accurate and successful predictions ever made.^[4] Anti-reductionists have appealed to such effective theories in defence of their position: they claim that surely these effective theories are worth keeping! An important way of responding to their arguments is to explain how it is that such theories are successful, notwithstanding that they don’t talk about all the more zoomed-in particles. This is done by identifying the structure of scaling interaction strengths, and relating that structure to the effectiveness of such theories.

So, a middle course can be charted between the reductionist and anti-reductionist positions. This involves accepting reductionism, but also recognizing that successful effective theories ought not to be eliminated. This middle course is defensible if the structures that underwrite effectiveness can be found. Such structures help explain why all scientists aren’t physicists!

Image credit: I went bowling—everyone was impressed, Bill Harrison

^[1] See Chapter 5 of Nicholas J. Garber and Lester A. Hoel Garber’s Traffic and Highway Engineering (1988, West Publishing Company).

^[2] It would be more accurate to talk about fields as well as particles, but the difference shouldn’t matter for our purposes.

^[3] This view corresponds to weak emergence and is defended in a paper I co-authored with Eleanor Know, ‘Emergence without Limits: The Case of Phonons‘.

^[4] The agreement between theory and data for the electron’s magnetic moment is better than one part in one trillion!