I don’t care to wed my arguments to any specific rendition of fundamental physics, but all of my arguments’ illustrations will be of things in our Universe painted by such physics, and we can better appreciate the illustrations if we know something about the style. There are four fundamental interactions that the physical primitives engage in: the strong nuclear force, the weak nuclear force, the electromagnetic force, and gravity. The exact profile of interactions reflects the type of primitive. Quarks interact through all of them, for instance, and electrons through all but the strong force. All of the forces but the weak force participate in bonding, one of the most important kinds of interactions. The weak force mostly functions as an intermediary between the strong force and the electromagnetic force, supporting the interface between the intranuclear and extranuclear environments during radioactive decay. The bonding forces vary drastically in their bonding strength, with the strong force by far the strongest and gravity by far the weakest, with electromagnetism in between.
Just as relevant as the relative strengths of the bonding forces is their valency and their relationship with distance. The strong force and gravity are only attractive, and thus we can call them monovalent, and no two primitives will be repelled through these forces. Electromagnetism, on the other hand, is both attractive and repulsive and thus divalent. If two primitives have the same electromagnetic charge, they are repelled, and if they have the opposite charge, they are attracted. A consequence of this is that if you have three primitives together, they cannot all be attracted to each other electromagnetically. With many more than three, what eventually happens is that the primitives distribute themselves evenly such that no large region has more positive charge than negative charge and thus most objects do not attract or repel each other through the electromagnetic force. When two primitives are attracted, they move towards each other until they no longer can, either because they latch into quantum mechanical structures (like electrons and protons into hydrogen atoms) or because they meet and transform into other particles (like electrons and protons into neutrons and neutrinos, with help from the weak force).
The electromagnetic force and gravity get weaker with distance. Since gravity is monovalent, larger and larger objects only attract each other gravitationally even more as the force cannot cancel locally like electromagnetism. At very large scales like the solar system, we then see that gravity ends up doing all the work of keeping everything orbiting together while the strong force and electromagnetism do none of it. The strong nuclear force, despite being monovalently attractive, comes with three types of charge, known as “color charge,” each of which can be positive or negative. Unlike gravity and electromagnetism, it does not get weaker with distance.1 Just like electromagnetism and unlike gravity, charges end up cancelling almost entirely at small scales, now within protons and neutrons, with some slight leakage beyond them that reaches the scale of an atomic nucleus known as the residual strong force, which does get weaker with distance. Protons and neutrons are the quantum mechanical structures that quarks latch into because they’re prevented from collapsing all the way by the Pauli exclusion principle, which also gives atoms larger than hydrogen their layered electronic structure.
The valency and distance-dependence of the three bonding forces combine so as to produce various domains of material density separated by quantifiable boundaries. Immediately after the start of the Big Bang, the Dust was distributed almost entirely homogeneously throughout the Universe in a very dense quark-gluon plasma. After several minutes of explosive expansion, the strong force acted to preserve tiny pockets of high quark-gluon density in the face of the expansion as protons and neutrons and nuclei like those of hydrogen and helium. Then about 380,000 years later, still a cosmological eye blink, the nuclei wrapped themselves in as many electrons as they had protons, locally cancelling electromagnetic charges and making the Universe transparent to photons, the first of which are visible as the cosmic microwave background radiation, the quintessential piece of evidence for the Big Bang. The atoms were still distributed homogeneously, however, and since the strong force and the electromagnetic force had already created their bonds, the only force left to do anything was gravity.
And gravity was ready. Since gravity wants to pull everything together, the state of the Universe where gravity has the most work left to do is the state where everything is apart, diffuse, and homogeneous. Unfortunately, a homogeneous Universe has no preferred locations where matter can gravitationally coalesce.2 Fortunately, the Universe was only almost homogeneous, however, with shallow ripples created by the spontaneous meows of probabilistic interactions dead and alive.3 The ripples in the Dust created regions in the primordial gas that were slightly denser than others, regions where gravity could take hold and pull things together. Without them, the Dust would have remained smooth, and no gravitational coalescence would occur, nor would any further bonding be produced among the atoms, and nature would have produced no more structure.4 Gravity, though not yet in conflict with either the strong force nor the electromagnetic force, was already in conflict with something else: heat. Local spontaneously-produced high-density regions in the gas could be disbanded by heatwaves pulsing through them. Ultimately, the interference of heat merely postponed the further coalescence of matter because local regions of sufficient density and gravitation to overcome the outward pressure of heat would eventually materialize in the vastness of space. This instability of gas to gravity is known as the Jeans instability, and it is the first limit between the homogeneous cosmic gas and its opposite: the black hole.
Once matter has coalesced enough, further action by gravity cannot be blocked by heat, and the gas collapses in free fall. The collapse stops when gravity comes into conflict with the electromagnetic force, when electrons can get no closer together due to Pauli exclusion, creating an outward pressure known as electron degeneracy pressure, which is much stronger than the pressure from heat that gravity has previously conquered. This is the density regime where most stars and all planets and moons lie, squeezed together by gravity but upheld by electromagnetism. If even more matter has collapsed together, then gravity is strong enough to transform the primitives, each proton vacuuming up an electron with the weak force, flipping one of its up-quarks into a down-quark and making it a neutron, while ejecting a neutrino. With the electrons and their degeneracy pressure out of the way, the matter once again collapses in free fall until the neutrons can get no closer together due to their own Pauli exclusion.
This is the density regime of neutron stars, with the amount of mass required to overcome the electron degeneracy pressure known as the Chandrasekhar limit, the second limit between cosmic gas and black holes. Our sun’s mass is only 72% of the way to this limit, so it will never become a neutron star or a black hole. Heat again acts to postpone collapse to the neutron star regime; a star above the Chandrasekhar limit will only become a neutron star once it has cooled sufficiently. With even more mass, the neutron degeneracy pressure can also be overcome, melting the neutrons together into a quark-gluon plasma. The amount of mass required for that is known as the Tolman–Oppenheimer–Volkoff limit. Finally, with even more mass, the quark degeneracy pressure is also overcome, but the limit for that has not been named because its details are even murkier than those for the Tolman–Oppenheimer–Volkoff limit. Heat, electromagnetism, and the strong force all vanquished by gravity, the sufficiently massive object implodes into a black hole, which is beyond the description of our current physics.
It’s interesting to summarize how the relative strengths of the forces map themselves to events in Universal history and in the development of local pockets. The strong force is stronger than electromagnetism which is stronger than gravity. The first bonds that were formed in the Universe were strong bonds followed by electromagnetic bonds and then gravitational bonds. When gas clouds collapse to a black hole, the first limit lies at the frontier of heat and gravity, the next at the frontier of electromagnetism and gravity, where every human has lived and died, and the last at the staggered frontier of the strong force and gravity. Just like a humid atmosphere filled with water vapor spontaneously develops flakes of snow and droplets of water, so did the Universe filled with Dust spontaneously develop flakes of planet and droplets of star in swirling galactic storms.
1. The strong force is very strange for many reasons. Physicist still haven’t characterized it completely.
2. See Buridan’s ass.
3. See Schrödinger’s cat.
4. See “clinamen.”