What holds it together?

The universe, which we know and love, exists because the fundamental particles interact. These interactions include attractive and repulsive forces, decay, and annihilation.

Forces vs. Interactions

A force is the effect on a particle due to the presence of other particles. The interactions of a particle include all the forces that affect it, but also include decays and annihilations that the particle might go through. For instance, the particles which carry the interactions are called force carrier particles.

It turns out that all interactions which affect matter particles are due to an exchange of force carrier particles, a different type of particle altogether. What we normally think of as "forces" are actually the effects of force carrier particles on matter particles. One important thing to know about force carriers is that a particular force carrier particle can only be absorbed or produced by a matter particle which is affected by that particular force.

Electromagnetism

The electromagnetic force causes like-charged things to repel and oppositely-charged things to attract. Many everyday forces, such as friction, and even magnetism, are caused by the electromagnetic, or E-M force.

The carrier particle of the electromagnetic force is the photon (gamma) which travels at the speed of light, 3.0*10^8 m/s.

The Nucleus

The nucleus is made up of protons (positive) and neutrons (negative). Because like-charged particles repel each other, the nucleus should come apart, but it doesn't because of the quarks that make up the protons and neutrons. Quarks have electromagnetic charge, and they also have an altogether different kind of charge called color charge. This color charge creates a force called "strong" because, as you may have expected, it is very strong.

The strong force holds quarks together to form hadrons, so its carrier particles are called gluons because they so tightly "glue" quarks together. Color charge behaves differently than electromagnetic charge. Gluons, themselves, have color charge, which is not at all like photons which do not have electromagnetic charge. And while quarks have color charge, composite particles made out of quarks have no net color charge (they are color neutral). For this reason, the strong force only takes place on the really small level of quark interactions.

When two quarks are close to one another, they exchange gluons and create a very strong color force field that binds the quarks together. The force field gets stronger as the quarks get further apart. Quarks constantly change their color charges as they exchange gluons with other quarks.

Color Charge (optional to learn but here just in case you were really interested)

First of all, just so you know, "color charge" has nothing to do with the visible colors, it is just a convenient naming convention for a mathematical system physicists developed to explain their observations about quarks in hadrons.

There are three color charges and three corresponding anticolor (complementary color) charges. Each quark has one of the three color charges and each antiquark has one of the three anticolor charges. For example, in a baryon a combination of "red," "green," and "blue" color charges is color neutral, and in an antibaryon "antired," "antigreen," and "antiblue" is also color neutral. Mesons are color neutral because they carry combinations such as "red" and "antired."

If one of the quarks in a given hadron is pulled away from its neighbors, the color-force field "stretches" between that quark and its neighbors. In so doing, more and more energy is added to the color-force field as the quarks are pulled apart. At some point, it is energetically cheaper for the color-force field to "snap" into a new quark-antiquark pair. In so doing, energy is conserved because the energy of the color-force field is converted into the mass of the new quarks, and the color-force field can "relax" back to an unstretched state.

Important Note: color must change to conserve color charge

Residual Strong Force

Here's what holds the nucleus together: The strong force between the quarks in one proton and the quarks in another proton is strong enough to overwhelm the repulsive electromagnetic force. This is called the residual strong interaction, and it is what "glues" the nucleus together.

Weak and Electroweak Interactions

Weak interactions are responsible for the decay of massive quarks and leptons into lighter quarks and leptons. We observe the particle vanishing and being replaced by two or more different particles. Although the total of mass and energy is conserved, some of the original particle's mass is converted into kinetic energy, and the resulting particles always have less mass than the original particle that decayed.

The Standard Model has united electromagnetic interactions and weak interactions into one unified interaction called electroweak. Physicists concluded that, in fact, the weak and electromagnetic forces have essentially equal strengths. This is because the strength of the interaction depends strongly on both the mass of the force carrier and the distance of the interaction.

Gravity and Summary

Gravity has such little effect on particles, it is almost always ignored when performing calculations. No discovery has been made of a gravity force carrier particle but it has been named: the graviton.

Here is a summary of forces:

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