Four fundamental forces appear to be holding the universe together: the electromagnetic force, two nuclear forces, and gravity. They're all caused by matter's absorption or emission of energy-carrying particles called bosons. For this reason, all might be more aptly called interactions.

Most physicists today believe the four forces are actually just aspects of one Force of the universe. That's why many are working on a Unified Theory to explain how they all relate. In fact, three of the force types have already been linked with experiments. Gravity — the renegade of the bunch — is the only one that remains unlinked.

Gravitational force

Direction: Always attractive

Affects: Gravity affects anything with mass and/or energy — in other words, all particles.

Carried by: Gravitons — not yet observed in a lab, but generally believed to exist.

Claim to fame: Alternately considered the curvature of space-time, a force field, or a graviton exchange, gravity is by far the weakest of all the forces, and yet (because it acts over infinite distance) it's the easiest to macroscopically observe.

Relative strength: 6×10-39

Electromagnetic force

Direction: Attractive or repulsive

Affects: Electrically charged particles

Carried by: Virtual photons — called virtual not because they're imaginary, but because they're exchanged between charged particles (like electrons) so quickly.

Claim to fame: Macroscopically useful, this force is the basis of all electrical designs today. The photons released when electrons change shells also allow human sight.

Relative strength: 7×10-3

Weak nuclear force

Direction: Though not directional in the usual sense, the force affects matter or antimatter in two distinct, nonsymmetrical ways. This is why the universe contains more matter than antimatter.

Affects: Particles with a spin of ½ or some multiple thereof — in other words, matter particles only.

Carried by: Very heavy W+, W-, and Z bosons (which act just like photons at high energy).

Claim to fame: Unlike the other forces, the weak nuclear force actually changes matter particles into completely different particles by allowing all leptons and quarks and antiparticles to interchange energy, mass, electric charge, and flavor.

Relative strength: 10-5

Strong nuclear force

Direction: Always attractive

Affects: Quarks, binding them into 1) unstable pairings of quarks and antiquarks to form mesons or more commonly 2) stable quark triplets called baryons. Protons and neutrons are two kinds of baryons.

Carried by: Gluons

Claim to fame: This force groups quarks only into neutrally “colored” particles. In addition, the gluons (through and on which this force acts) also stay grouped in these special arrangements. Atomic bombs and nuclear reactors cash in on the strong nuclear force stored in atomic nuclei by knocking them apart.

Relative strength: 1