The Four Fundamental Interactions

Interaction	Relative Strength	Range	Particles Influenced
Strong	1	~ 10-15 m	Quarks
Electromagnetic	10-2	Infinite	Charged particles
Weak	10-9	~ 10-17 m	Fermions
Gravitational	10-38	Infinite	Massive particles

Surely, however, the odds are against us. A comparison of these interactions reveals differences (e.g., mass vs. color, infinite vs. subnuclei range) so profound that there can’t be much hope of unification. Could these four interactions really be but manifestations of something more fundamental?

ATOM TRAP

We have focused on the types of particles that are influenced by the four basic interactions (like quarks or fermions). We will learn that there is another type of particle very important to these interactions—the so-called gauge bosons—which act as mediators during the interaction between matter particles.

Believe it or not, there have been some hints that unification may not be just a pipe dream. There has even been one very notable success. The 1979 Nobel Prize in physics was awarded to Sheldon Glashow, Abdus Salam, and Steven Weinberg for the theoretical unification of the electromagnetic and weak interactions.

The nature of this unification has resemblances to the way that electricity and magnetism turned out to be the same phenomenon. There is a significant difference, though. We can witness the common nature of electricity and magnetism under the most usual of circumstances. In the case of the electroweak interaction, however, the electromagnetic and weak interaction only join up at particle energies significantly above those encountered on Earth today.

Particles with sufficient energy to demonstrate electroweak unification are provided by nature only in the form of cosmic rays—particles from space that actually originated at much early stages of the universe when energies were much higher. This indicates that at an earlier era of the universe, these two interactions were combined. (If we want to study the unified electroweak interaction, we must resort to high-energy experiments conducted at large particle accelerators.)

In the 1970s, a governing theory for the strong interaction was developed that shared certain similarities to the electroweak theory. This has given rise to a belief that if we went back even further in time to a point where the particles in the universe were even more energetic, the strong interaction could be united with the electromagnetic and weak interactions. Research in this direction continues today.

QUANTUM QUOTE

Everything should be made as simple as possible, but not simpler.

—Albert Einstein

Most physicists would agree that the key remaining challenge is therefore the unification of gravity with the other interactions. Today, there exist quantum theories for the electromagnetic, strong, and weak interactions. Gravity sticks out, however, in its stubbornness to bend to “quantization.” The gravitational interaction is still generally regarded as a classical (or “continuous”) theory that doesn’t break down into discrete levels when examined closely.

Unlike the Newtonian gravitational theory, Einstein’s general theory of relativity predicts that accelerating masses might radiate gravitational waves similarly to accelerating electric charges radiating electromagnetic waves. However, attempts to directly observe gravitational waves have been inconclusive because they are so darn weak.

There has been extraordinary progress to explain a very complex universe with only a handful of interactions. But there are still many questions out there to motivate physicists in this quest. Some current avenues, such as superstring theory, speculate that the present mix of elementary particles may actually be composites of something even more fundamental. Although these theories are speculative, and there is no experimental evidence yet for substructures, there are many true believers in the physics community. The only real certainty is that the jury is still out, and that we must patiently await the outcome of this important fundamental research.