Into the Unknown, page 26
There are several ways one might use the word “law” in the context of physics (we won’t even touch the legal system). For our purposes here, it is important to clearly distinguish between a “scientific law” and a “law of nature.” A scientific law refers to the use of the word “law” in the scientific method, as you read in the chapter on epistemology. In this case, we mean a prescription that has been well tested but could potentially be falsified. As a reminder, even within this context, science does not use the word “law” self-consistently: there are things that have “law” in their name that are not actually laws (e.g., Newton’s law of gravity), and there are things that are not called laws that probably should be (e.g., the theory of general relativity). We generally have high confidence in this proximate type of “law,” but we also hold our positions to be defeasible—if there were sufficient evidence to the contrary, we can and should change our minds (which is, in fact, what caused Newton’s “law” of gravity to be superseded).
Ultimately, the laws we infer could just be a projection or manifestation of a deeper law. In this scenario, a positivist might argue that, if a proposed law perfectly describes the observations, the model is interchangeable with reality. I would argue that the positivist position has some practical value, but we risk misleading ourselves and missing out on deeper truths. This deeper truth is what I am calling a true law of nature.
The true underlying laws of nature are what we really want to know, but they may be beyond our ability to grasp or test. This version of a “law” has a bit of a Platonic flavor—it is the sort of ideal law that might reside in Plato’s realm of forms, in which the very essence of a thing is said to exist. Sometimes when I am teaching this subject material, I will have my students read Plato’s Allegory of the Cave (which you are, of course, welcome to do if you want extra credit). The laws we infer could be only a shadow of the true underlying form of the law.
The point of dragging you through these two types of laws is to appreciate the difference between what we can do and understand as mere mortals and what may be a more fundamental reality we are trying to uncover. The laws of the second category, the true underlying fundamental laws of nature, are what we are after in this chapter.
These fundamental laws necessarily have a number of characteristics: they are omnipotent, meaning everything in the universe (or multiverse) is subject to them; they are universal (or multiversal), meaning they apply everywhere in all dimensionality that exists; they are atemporal, meaning they are true at all times (if they were not true at all times, that would suggest the existence of a more fundamental law that dictates how they can change); and they are absolute, meaning they are not contingent on anything else (if they depended on something else, that would suggest a more fundamental underlying law).
The situation gets metaphysically muddy at this point. In fact, there is a philosophical debate as to whether the laws of nature are “necessary,” which in the philosophical usage of the word means “the laws could not have been otherwise.” If these laws were contingent on some other factor, we are left wondering what that other factor is, which must be a still deeper underlying requirement. To be clear about my own bias, I mostly fall into the metaphysical philosophy camp3 of “necessitarians,” which means I lean toward a belief that there are “necessary” laws (or perhaps even a single law) that govern all of reality. This is a core axiom I take as given, at least in part because I simply can’t fathom a logical scheme in which there are not fundamental laws from which behaviors in the cosmos arise. That being said, just because something doesn’t make logical sense to me, doesn’t mean it might not be true. In fact, in debating whether the laws of nature are “necessary,” we run smack into both issues of infinite regression and causality yet again. I’m sure you’re shocked.
In the case of laws, we could imagine that there are some set of necessary laws from which the rest of the universe (or multiverse) takes its instructions. If these laws are philosophically necessary, they could not have been and could not be otherwise. The first question that might surface in your mind might be along the lines of “Well, where did those laws come from, and why those particular laws and not some other laws?” Good questions. That is the problem with being “necessary”—it is equivalent to saying, “That is just the way things are,” which is deeply unsatisfying. A second issue with there being necessary laws—that could not have been otherwise—is the challenge in figuring out how anything contingent could ever happen without being necessary itself: if the laws are necessary and could not be otherwise, then what arises from those laws could also not be otherwise. In the end, we end up with a universe (or multiverse) that appears to be entirely “necessary,” which—for example—leaves no room for beloved notions like free will. On the other hand, if the laws are contingent, what are they contingent on? And around and around we go. There is clearly more to the story.
It is not hard to understand why countless theologians and philosophers have turned toward these ontological arguments as proof of God, including a formal logical argument by famed logician and mathematician Kurt Gödel (whom we will see again later in this chapter).4 My own thinking on this is that our apparent inability to untangle the philosophical knot of whether the laws of nature are necessary does not prove the existence of a higher power or intelligence—to my mind, this is just a “God of the gaps” fallacy (see Chapter 2) wearing a high-end cloak of philosophy. However, I think this philosophical conundrum does prove there is an aspect of the cosmos (which may or may not include a higher power) that defies current human logic and comprehension. This should not really be a surprise if we have an ounce of humility about our own intellectual ability—just because we might be the most intelligent species on this planet does not imply we represent the pinnacle of intelligence, nor that we are able to understand everything about reality, try as we might.
This is one of those times when the most productive path forward might be to acknowledge that there is a mystery lurking about and start by taking stock of what we think we know about how the laws of nature are manifest in our universe and see if it gives us any insight.
The Four Known Forces
We tend to think of the laws of nature and the forces that act in the universe as synonymous, but this need not be the case. Rather, the forces influence how things behave through interactions. In turn, the characteristics of these interactions are manifestations of the laws of nature made visible to us through the properties of the universe we happen to live in. This may sound like a picky semantics point, but distinguishing between the observable forces and the underlying laws is important when we consider why we have the forces that we do (and whether they could have been otherwise).
That being said, the known forces in the universe are the only handle we currently have on grasping any underlying laws, so trying to understand these forces and their properties is essential in getting to the bottom of the mystery at hand. We currently have four known forces that operate in our universe (if you want to sound more physics-y, you can refer to these as “fundamental interactions”): the gravitational force, the electromagnetic force, the weak force, and the strong force. You might have noted the word “currently” in the previous sentence—we have only known about the weak and strong interactions for less than a century, so I think we would be remiss to exclude the possibility that we might discover something new.5 These four forces appear to be radically different, which doesn’t sit well in modern physics because physicists really like symmetry. Many of us believe, with almost religious fervor, that elegant symmetries must underlie the architecture of the universe.
There are many reasons for this conviction, some of which lean into aesthetics—nonsymmetries are messy, and messy requires more ad hoc explanations, and ad hoc explanations feel philosophically uncomfortable. Beyond the aesthetics of symmetry, history has also been a guide; in building our understanding of the universe, we have been shown over and over again that, all things being equal, the default of nature is symmetry. Generally, physicists don’t need an explanation when things are symmetric, but when things are not symmetric there darn well better be a reason.
To illustrate one aspect of the apparent asymmetry, I want you to imagine a little demonstration, which you can easily carry out on your own (if you can’t, that’s OK—I am confident that you will be able to follow along). When I do this demonstration in class, I tell my students this is the most mind-blowing demonstration I can do the entire semester. Here is what you need to do: find any old magnet—this could just be a little refrigerator magnet, or something like the miniature magnetic calendars I get from my insurance salesman every year (does anyone use these as calendars?). In fact, the smaller the magnet, the better, because that will make this demonstration all the more impressive. Next, find something small and magnetic, like a paper clip. Now carefully hold the magnet and attach the paper clip to dangle from underneath it. Ta-da!
Do you see how extraordinary this is? If you are like my students, at this point there is usually some polite or nervous laughter and odd looks suggesting they are questioning their curricular choices. The entire mass of the Earth is pulling down on that paper clip (or whatever little object you used), but this tiny little throwaway magnet has enough force to counteract the entire planet. If you have gone your whole life without appreciating how ridiculous this imbalance is, I promise you are not alone. Yet, this simple do-it-yourself experiment reveals one of the great mysteries of modern physics—the fact that gravity is unfathomably weak compared to the other forces. Even the weak force (which comes by its name honestly) is roughly 1033 times stronger than gravity.
I hope you are curious to know a little more about the forces now. The four forces each have their own personality (envision an online quiz entitled “Which of the Four Forces Are You?”). The key characteristics of the forces that distinguish them from one another are their range (i.e., how far away they can affect things); their strength (in basic physics we parametrize these strengths as things called “coupling constants”); the characteristics the forces couple to (I think of this as each force having its own kind of Velcro, which I realize makes no sense at all, but nevertheless that is the analogy I’ve had in my mind since I was in college); and the type of particle the force uses to interact (these are “virtual” particles, also known as “exchange” particles). I am sure that a clever person can map those onto human characteristics, and possibly even put them into a dating profile.
Gravity has an outsized role in our lives, at least in part because we are aware of our continuous interactions with it. I will admit that I go through my day and mostly don’t even think about gravity; it is just there, completely in the background, which just doesn’t do justice to how weird it is. Gravity comes from mass, but have you ever thought about what mass even is? This is one of those multitude of things I think most of us go through life taking for granted because it is so utterly familiar. Many of my students understandably have trouble getting their minds around how a particle—or any thing—could not have mass. In our normal lives we are duped into thinking that “thingness” goes hand in hand with having mass. But in the world of particle physics, this is not so. Just like a particle may or may not have charge, or a pizza may or may not have pepperoni, a particle may or may not have mass.
In fact, in our current understanding of mass, particles that interact with gravity don’t inherently have mass at all—rather mass is only acquired by certain types of particles when they interact with a field called the “Higgs field” (the discovery of which won the Nobel Prize in 2013). If we really want to stretch the pizza analogy where it was never intended to go, pizzas do not inherently have pepperoni, but some pizzas acquire pepperoni in the assembly line. Vegetarian pizzas, as a rule, do not. So, in this pizza-particle world, particles like photons would be pepperoni-free, but they are still pizzas.
If I anthropomorphize the forces, I think of gravity as the little force with a chip on its shoulder and something to prove. It probably drives a pickup around town with more horsepower than it will ever need. If you list the forces in order of strength, gravity is at the bottom—as we’ve already illustrated with a handy magnet and a paper clip, gravity is mind-blowingly weak.
The two reasons gravity takes on this outsized role in our lives are (1) that it has an infinite range, and (2) that it doesn’t get canceled out by a negative gravity (at least not apparently in our normal daily lives, but remember dark energy). The reason gravity has an infinite range is that the hypothetical exchange particle that carries it, the graviton, doesn’t have any mass, so it gets to travel off into the universe as far as it wants to. The more massive the carrier particle, the shorter the range. There is this tiny issue that we haven’t detected a graviton yet, but let’s just sweep that under the rug for now.
In contrast to gravity (which is easy to take for granted), when I play with magnets it seems like sorcery, with forces that can’t be seen or touched. But at least we can interact with electromagnetism in our daily lives, which gives us some sense of its reality and behavior. As we saw with the paper clip and magnet above, electromagnetism is way stronger than gravity, by a factor of around 1033. True to its name, this force comes about from interactions between electromagnetic fields and particles with charge. Electricity and magnetism were originally thought to be two separate forces but were merged into a single force in 1873 by James Clerk Maxwell (after whom Maxwell’s equations are appropriately named).6 If you are wondering what electrically charged particles and magnetism have in common, when electric charges move, they generate a magnetic field, so the two concepts are tightly entangled.
You may be wondering how the magnet you used (or envisioned) above has a magnetic field if there are no charges moving around, which is a great question; the answer comes down to the minuscule little electrons that are in the atoms that make up the magnet—these little fellows are spinning, and because they spin, they have something we call a magnetic dipole, which is where the magnetic field comes from (once again, I want to pause and thank whoever called it a “magnetic dipole” for giving it a name that actually makes sense). The reason a magnet is magnetized (as opposed to, say, an apple) is that in a permanent magnet like my miniature insurance calendar, the magnetic dipoles more or less line up and face the same direction. The electrons in an apple have magnetic dipoles as well, but they are oriented every which way and mostly just cancel each other out. Most everyday objects have a bit of a net magnetic field—a disturbing example of which can be seen in a physics experiment that levitated a frog using magnetic fields7 (which you can easily find by a quick search of “levitating frog”). I don’t think anyone has tried this with humans yet, and that is almost certainly a good thing.
Like the gravitational force, the electromagnetic force has an infinite range, in this case due to its carrier particle, the photon, not having any mass. In contrast to the graviton, we are pretty sure that photons exist. Given that both gravity and electromagnetism have infinite ranges, electromagnetism would overwhelmingly dominate our lives if not for the simple fact that positive and negative charges can cancel each other out. The fact that there are both positive and negative charges and that they usually almost completely balance each other is an astounding fact hiding in plain sight, especially in contrast with gravity (which always seems to be the odd duck among the forces).
The strong and weak forces are another matter entirely.8 If you routinely perceive interactions with the weak or strong force as part of your normal life, probably you should seek professional help. The exact behavior of the strong force is important when we talk about fine-tuning in the next chapter, so I want you to make a mental note of all the different knobs and dials that could be tweaked if you were an omniscient being designing a force. If you were trying to write a novel with quarks and gluons as characters, it would be very hard to keep track of who was doing what, and your editor would probably tell you to ease off on the number of characters. For better or worse, the universe did not have an editor with this requirement, which makes for a very thick plot. If your eyes glaze over the next couple paragraphs, that is OK, but know that you can come back to them later if you want to. The point is to illustrate how many different parameters are at play.
The strong force is more complicated than the gravitational and electromagnetic forces, and also a fair bit less intuitive, so I’m going to skip over a lot of the physics for the sake of not completely changing the title of this book to Introduction to Nuclear Physics. The strong interaction works to keep subatomic particles called “quarks” close together (usually in pairs or triplets), using particles that carry the force called “gluons.” This is what keeps nuclei of atoms together; without the strong force, the mutual electromagnetic repulsion of protons would keep them apart and we wouldn’t have any atoms other than hydrogen.
Both gluons and quarks have their own characteristics, which include a “color”; you can kind of think of “color” in this context as being like an electric charge in the sense that it is a property of the subatomic particle and determines who it is allowed to be friends with in the particle-physics novel. But you should not think of them as actually having a color, although this whimsical notion can help to visualize them and keep them straight. When you mix three “primary”-colored quarks together, the resulting particle becomes “colorless” (i.e., it has a net color of 0), which means it is allowed to exist by itself, like an adult child who has their own bank account, insurance, and place to live. In case that isn’t complicated enough, in addition to the color charges, there are six separate types of quarks, which are each said to have a “flavor” (please grant physicists some poetic license here for their use of the word “flavor,” and maybe also don’t let them season your food): up, down, charm, strange, top, and bottom.
