Fundamental Forces




The are Four Fundamental Forces:
Gravity (the weakest), Electromagnetic, and Nuclear both weak and strong.
Einstein’s General Theory of Relativity explains gravity.
Quantum Mechanics explains the other three.


The force that keeps us all glued to the surface of Earth, gravity, dominates any discussion of the evolution and fate of the universe. Although it ranks as the weakest of the Four Fundamental forces, the others pale when you talk about the universe as a whole because the two nuclear forces act only over very short distances, while most large objects are electrically neutral and therefore unaffected by the electromagnetic force.

Isaac Newton first described gravity and had the insight to realize that the force that holds us to Earth (and makes apples fall) is the same one that keeps the planets in their orbits around the Sun. He deduced the mathematical nature of the mutual force and correctly hypothesized that gravity acts across the entire universe.

Albert Einstein modified this view of gravity by arguing that the gravitational force is a manifestation of the curvature of space-time. Although Einstein’s idea is necessary for describing the evolution of the universe as a whole, Newton’s theory works well enough when gravitational forces are not extremely strong.


Einstein's Gravity

Einstein envisioned gravity as a curvature of space-time caused by the matter in it, as opposed to Newton’s idea of a force acting at a distance. Although objects try to move through space-time in straight lines, this warpage makes their paths appear bent .

Quantum Mechanics

The Uncertainty Principle

At the heart of quantum mechanics—the mathematical theory of the structure and behavior of atoms—lies a certain degree of unpredictability. As first stated by the German physicist Werner Heisenberg, the uncertainty principle says you can't simultaneously measure both the position and velocity of a particle with perfect accuracy. This means that no one can ever predict precisely the future behavior of a particle because it’s impossible to measure the particle’s current state exactly.

The uncertainty principle does not simply state that scientists don’t yet have the proper equipment to measure positions and velocities: instead, the very process of performing the measurement changes those quantities. The uncertainty principle implies that space can never truly be empty. In reality, the quantum vacuum is filled with particles and antiparticles that briefly appear and then disappear just as quickly.

Schrodinger’s Cat

Quantum mechanics, the best theory we have for describing the atomic and subatomic world, comes up with some extraordinary portraits of nature. One example is the uncertainty principle, which says that the position and velocity of a particle (its state, as a physicist would say) can’t both be measured with unlimited accuracy. A related idea is that the state of a particle cannot be known precisely until the particle is observed; in other words, each and every particle has a probability of being in any state. It does not exist in a particular state until an experimenter observes it.

The Austrian physicist Erwin Schrödinger, one of the founders of quantum mechanics, thought of a paradox to show that quantum mechanics doesn’t apply to larger, tangible things. He envisioned a cat locked in a steel chamber with a tiny amount of radioactive material, a Geiger counter, and a diabolical device designed so that if the Geiger counter detects a radioactive decay, it activates a hammer that breaks a flask of acid and poisons the cat. It takes only one decaying atom to kill the cat, but whether an atom decays or not is governed by probability. Applying the rules of quantum mechanics to this system would mean that the cat is neither alive nor dead until a human observer actually looks in the chamber. Schrödinger argued that this was nonsense and merely an example of applying quantum mechanics to situations in which it doesn’t apply. Still, a few physicists defend the idea that the cat doesn’t take on an existence (or lack thereof!) until it’s observed.