What are the ultimate constituents of the universe?


One of the more intriguing questions posed by modern physics is what are the ultimate constituents of the universe. Most of the lay public, and many professional scientists for that matter, believe that the universe consists of fundamental particles (electrons, protons, neutrons, etc.), together with force fields (gravity, electromagnetism, etc.) that govern how they move.

But recent research in physics makes it clear that things aren’t really this simple. In fact, the situation is significantly more indistinct and unsettled than most realize. The world may consist of “bundles” of properties and relations, in ways that we can only dimly envision at the present time.

The catalogue of elementary particles

The notion that the world consists of particles goes back to the ancient Greeks, who proposed that the world consists of “atoms.” Here is a list a taxonomy of elementary particles, ranging from molecules down to what are currently considered to be the most fundamental constituents of matter:

  • Molecules. These particles are more in keeping with what the Greeks thought as “atoms,” in that they are fundamental entities of chemically-combined matter that possess the essential characteristics of large-scale matter.
  • Atoms. Molecules, in turn, consist of atoms, which are the building blocks of molecules. There are now over 100 known atomic species, ranging from hydrogen, the lightest and simplest of all, to ununoctium (element 118). By the time you read this, one or more even heavier elements may have been discovered [Periodic2013]. If we distinguish different nuclides (i.e., atomic species with different numbers of neutrons), then the total exceeds 3000 [Nuclides2012].
  • Electrons, protons and neutrons. Atoms, in turn, consist of a nucleus containing protons and neutrons, encircled by a “cloud” of electrons. The nucleus (protons and neutrons) contains almost all the mass of the atom, while the electrons are responsible for the chemical properties of the atom.
  • Fermions and bosons. Protons and neutrons, in turn, consist of fermions, which are the basic constituents of matter. Bosons are the elementary force-carrying particles.
  • Quarks, leptons and bosons. Fermions are categorized into quarks (up, down, charm, strange, top, bottom) and leptons (electron, electron neutrino, muon, muon neutrino, tau, tau neutrino). A proton, for instance, is composed of two up quarks and one down quark, whereas a neutron is composed of two down and one up quark. Bosons are categorized into the “gauge bosons” (gluon, W and Z bosons, photon), and also the recently discovered Higgs boson. The gauge bosons mediate forces (e.g., the photon mediates the electromagnetic force), while the Higgs boson is thought to be responsible for particles exhibiting mass.

In addition, it is widely believed that there is a “graviton” that mediates the force of gravitation, but it is currently beyond the “standard model” of physics.

There are thought to be 1080 atoms in the visible universe, mostly hydrogen. However, the total number of elementary particles is at least 1086, mostly neutrinos and photons. For additional details on these fundamental particles, see [Elementary2013].

What, really, are particles?

Interestingly, however, the entire notion of a particle appears hazy when we consider the most fundamental physical laws.

The “standard model” of physics is more than a collection of particles and forces. As it is formulated in quantum field theory terms, the notion of a particle is not clear-cut. To begin with, a particle does not have a well-defined, precise location. Instead there the probability of finding a given particle in a given position is given by a field, which in many cases extends far from the particle. This is most clearly seen in the “cloud” of electrons surrounding an atom — we can only specify the probability of finding the electron in a certain position or at a certain level.

A related phenomenon is to consider a region that is a true vacuum, devoid of particles. But here again, quantum field theory predicts, and experiments confirm, that even a “vacuum” may result in clicks in a Geiger counter or other measurement device, as particles briefly flit in and out of existence. What’s more, an observer at rest with the “vacuum” may see no particles, but for an observer accelerating past the chamber, he/she observes it bathed in a warm gas of particles. Does that “vacuum” have particles in it, or not?

Particles presumably have well-defined properties, such as energy, charge, spin, etc. But experiments show that two particles can be “entangled,” so that, for instance, if one has an up spin, then the other must have a down spin, even though the particles are separated by many meters in distance. Here it is best to consider the system of the two particles as a unified system, not as two independent particles. See [Kuhlmann2013] for additional details and discussion.

What are fields?

Such difficulties have led some researchers to postulate that fundamentally speaking, the universe does not consist of particles, but instead is best thought of as a vast set of “fields.” For example, a field of electromagnetism surrounds any wire conducting an electric current.

But there are difficulties here as well. Quantum field theory does not describe the universe as permeated with discrete and/or independent fields. A point in space does not possess a physical quantity corresponding to a field. Instead, the field must be applied to a “state vector,” which is not assigned to any specific location; it permeates all of the universe. See [Kuhlmann2013] for additional details and discussion.

Does it matter?

These developments have led some observers of science to gleefully declare that scientists are fundamentally ignorant regarding the most basic properties of the universe. And it certainly is true that these difficulties are (or should be) cause for considerable humility on the part of physicists in particular.

Many physicist simply refuse to be sidetracked by these issues, and adopt what is often called an “instrumentalist” approach. They point out, quite correctly, that in the current “standard model” we have a theory that can predict, with breathtaking precision, essentially every experiment involving particles and forces.

In any event, it is clear that these difficulties are hardly cause for any of the traditional opponents of modern science to rejoice. Yes, there are unanswered questions in modern physics, even at the most fundamental levels. But there are also unanswered questions even in areas of science that one would think are extremely well established, such as gravitational physics [Grossman2012a] and reproductive biology [Ridley1995]. Thus claims by creationists, for instance, that unknowns in the fundamental physics arena “prove” that scientists do not have all the answers are only met with puzzled stares by real research scientists. Of course scientists do not have all the answers — exploring unknown, unanswered questions is what science is all about.

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