In the Star Wars films, Obi-Wan Kenobi describes ‘The Force’ as “an energy field created by all living things. It surrounds us and penetrates us; it binds the galaxy together.” If that sounds exotic, wait until you hear how fundamental physics portrays the concept of ‘force’.
In high school, we learn that force is an agent that induces an object to change its velocity, according to Newton’s second law. Then our textbooks casually mention the astonishing fact that all the forces we have encountered – tension, friction, van der Waals force, weight, etc. – come from just four fundamental types of interactions between elementary particles.
Is this really so? Why, of all numbers, four? Are there more? Are there fewer?
The four interactions
Gravity is the force that everybody knows about, yet the one that most eludes modern physics. In Newtonian physics, gravity is the attraction between any two objects in the universe. The force’s strength increases with the objects’ mass and diminishes with the distance separating them.
Gravity keeps you from floating away, and brings galaxies together. It is the weakest of the fundamental forces. That may come as a surprise to some. Isn’t it gravity that bends one’s spine with age or keeps planets and stars from breaking apart? The net effect of gravity is powerful only because it has a very long reach without causing any repulsion. The feeble gravitational forces exerted by every small piece in a body all add up.
The electromagnetic force, like gravity, has infinite range but is a lot stronger. However, its net effects are often not felt because it can be both attractive and repulsive, which tend to cancel. This is the force, through interaction between electric charges, that makes television work and magnets stick, causes friction between bodies and tension in strings, and all of chemistry.
The weak force operates only up to distances of 10-18 m – about one-thousandth the size of a proton. For reasons we don’t understand, it acts only on particles that, if they are moving near the speed of light, spin counter-clockwise with respect to the direction of their momentum. As a result, this force would vanish in a universe in the mirror dimension: there, the same particles would spin clockwise relative to their momentum.
The weak interaction is responsible for producing the radiation used in nuclear medicine.
The strong force ranges over somewhat longer distances, around 10-15 m. It keeps the nucleus of an atom bound together, rather than flying apart, and sustains the nuclear fusion that powers the sun.
Carriers vs. feelers
A key concept that describes all these interactions is the force-carrier. A force-carrier is any species of particle that mediates the interactions between particles that experience that force. We can call these particles force-feelers. The force-carrier of gravity is the graviton, and that of electromagnetism is the photon. The weak force is carried by W bosons and the Z boson. The strong force is carried by gluons.
Should we then count the number of fundamental forces by the number of force-carrier species? That would be a problem for two reasons.
First, there is no shortage of force-carriers in nature. For example, the Higgs boson, discovered in 2012, mediates interactions between several known particle species and often one hears of a “Higgs force”. Similarly, the pion is a species treated as the carrier of the strong force between protons. But nobody really regards these as ‘fundamental’ forces.
The second reason is that, sometimes, two or more force-feelers can unite to become a force-carrier. This happens as an inevitable effect in theories describing particle interactions. For example, the two-neutrino exchange force is a known phenomenon whereas neutrinos in isolation feel nothing but the weak force.
So if it isn’t force-carriers, what separates the fundamental interactions from the rest? It’s the fact that the fundamental force-carriers exist purely due to mathematical regularities in the equations describing the interactions. That is, at a fundamental level, these forces are nature’s symmetries.
Once together, now separate
But even this classification scheme runs into trouble. The programme of identifying these symmetries led to an incredible discovery in the 1960s. Scientists found that right after the Big Bang, electromagnetism and the weak force were fused into the electroweak force. Then, as the universe cooled, the electroweak force broke apart into two different-looking forces: weak and electromagnetic.
Does that mean only three interactions are fundamental, since the universe was created with just those? Now, some physicists have theorised that even the electroweak and strong forces were once part of a single entity. Would that mean there are fundamentally only two forces?
The black sheep
Things get even more unclear if we consider gravity. It is unlike any of the other forces. Theoretical physics explains non-gravitational interactions by embedding particles in energy fields. For example, the photons that mediate electromagnetic interactions operate in an energy field described by Maxwell’s equations.
But for gravity, the field is spacetime itself – which is the basic insight of Albert Einstein’s theory of general relativity. This is peculiar. The mathematical symmetry governing Einstein’s equations can’t be easily put in the same class as the symmetries of the other fundamental forces.
Einstein’s insight also predicts such fantastical phenomena as gravitational waves, black holes, and an expanding universe – all of which have been verified, meaning that they can’t be done away with. Yet these phenomena are not the images one associates with ‘force’ in the Newtonian sense, or the results of interactions between particles as in the other fundamental forces.
So taken together, it is all right to say there are four fundamental forces in everyday speech. But remember that the true picture is often richer than widespread notions, for it is the equations that best capture the tapestry of nature.
Nirmal Raj is an assistant professor of theoretical physics at the Centre for High Energy Physics in the Indian Institute of Science, Bengaluru, and tweets at @PhysicsNirmal.
