 Fact

There are four fundamental forces (interactions) exhibited by elementary particles.

## What are the Four Forces (Interactions)?

There are four fundamental forces (interactions) in the universegravitational, electromagnetic, strong nuclear, and weak nuclear. Each force has a corresponding carrier particle called a gauge boson.

TIP: The term forces used to be in common use, now the term “interactions” is preferred. Both terms are describing the same thing, and both are separate concepts from the gauge bosons that carry the force.

### The Force Carrier Particles: the Gauge Bosons

The gauge bosons (the particles that carry the four forces and mediate exchanges, AKA interactions, of the forces) are the graviton (gravitational force), the photon (electromagnetic force), the gluon (strong force), and the W+Z bosons (weak force). There is another boson (that isn’t a gauge boson) called “the Higgs boson.”

### How Force Carrier Particles Mediate the Four Interactions

As implied above, the “forces” (or the “interactions”) are terms used to describe the “forces” (or “interactions”) carried by and exchanged between the gauge bosons (the “force carrier” particles).

Consider the following additional points to better understand the forces and the force carrier particles:

• Bosons mediate interactions between fermions (quarks and leptons, the particles that make up matter).
• Together bosons and fermions make up all known elementary particles as represented by the standard model (of particle physics).
• Photons, gluons, and W+Z bosons are each understood as a quantum field with a measurable unit called a particle. Gravity is understood as a classical field with no known measurable particle, as the graviton is theoretical.
• The interactions of the elementary particles, their fields, and the systems that they make up, account for everything that isn’t true empty space.

The Four Fundamental Forces Of Physics Explained. A good introduction video to the four forces.

### Elementary Particle Overview – How Fermions and Bosons Interact

When bosons mediate an interaction between fermions, they can change each of the affected fermion’s types (mass, spin, flavor, color, charge, etc.). When bosons travel unimpeded, they simply exhibit their corresponding force in its natural state.

For instance, the photon will always travel light speed in a constant direction in a true vacuum (this is why it’s called “light speed“). The photon, the massless energy particle is both a particle and wave existing in a state of superposition that quantizes to energy states at the Planck length. It is a quantum field responsible for all electromagnetic energy including visible light. The photon is the only mediator of the electromagnetic force. Any quark, electron, or system that has electromagnetic energy has photons essentially “bound” in the system as potential energy. When bound photons are exchanged they are called “virtual photons.”

Now that you understand the gist of how particles interact let’s take a look at each force and its corresponding boson. Also called the four interactions.

## The Four Forces (Interactions)

There are four fundamental interactions. Below we explain each interaction, it’s corresponding boson, and how it relates to fermions. The explainers are accompanied by explainer videos from the awesome SciShow (which you should subscribe to on YouTube).

The Four Interactions of particles are:

Strong Nuclear Force  (Boson = Gluon): The force that binds together quarks in protons and an atom’s nucleus. This force when bound accounts for most of an object’s mass. It is the strongest force, about 100 times stronger than electromagnetism, a million times stronger than weak interaction, and 1038 times stronger than gravitation at that range. Strong force binds composite particles together (like a mini gravity).

Strong Interaction: The Four Fundamental Forces of Physics #1a.

Strong Interaction: The Four Fundamental Forces of Physics #1b.

Weak Nuclear Force (Boson = W+Z, theoretically X+Y): The force responsible for atomic decay and entropy of an atom’s nucleus via radiation. Weak force unbinds that which is bound by a strong force.

Weak Interaction: The Four Fundamental Forces of Physics #2.

Gravitational Force (Boson = theoretically the Graviton): What we call gravity is  mass-energy curving spacetime (general relativity). It can warp spacetime toward a system and emit gravitational waves from the system. It’s the only one of the four forces that does not have a significant effect at the quantum level. Gravity binds large systems like people, planets, stars, and satellites.

Gravitation: The Four Fundamental Forces of Physics #3.

Electromagnetic Force  (Boson = Photon): A combination of the forces of electricity and magnetism. This force, when not bound to a system adds to relativistic mass, but not the intrinsic mass of the object. Einstein’s mass-energy equivalence shows that kinetic energy adds mass to a system. Electromagnetic force gives particles their “charge”.

Electromagnetism – Electrostatic Force: The Four Fundamental Forces of Physics #4a.

Electromagnetism – Electrostatic Force: The Four Fundamental Forces of Physics #4b.

### How to Determine the Energy Content of Fields

The energy content of a photon field is E = hf. That is a photon’s energy = the Planck constant X it’s oscillating frequency. The total energy in a given system can be figured out by E=mc2. That is total energy in a system = mass times the constant speed of light squared. The Planck constant is the shortest length in the universe that things “quantize” to, and light speed is the longest length (and max speed of the universe.)

The energy content of other particle fields can be figured out using the same equations and universal limits.

TIP: Frequency and spin are very important properties of quanta and relate back to their energy content and what they can interact with. You can learn more about the elementary particles here.

### Gravity and Electromagnetism Have Infinite Ranges

Gravity and electromagnetism have a theoretically infinite range.  Strong and weak forces don’t. Gravitational waves and light waves both travel at light speed forever if unimpeded. Despite this, EM waves can be canceled out and amplified. Given the only recent findings on gravitational waves, it’s unclear if they can cancel and amplify as well. Both types of waves have a “spectrum”. Interestingly, gravity takes more time to propagate, so just like thunder and lightning, you would see the light (the visible part of the electromagnetic spectrum) from a distant supernova before you’ve “felt” its gravitational waves.

## Unifying the Forces

As you can see from the above, if we accept that strong and weak nuclear forces are holding atoms together and taking them apart then we only need to understand the general concepts of gravity and electromagnetism. Or rather, to be clear, all forces except gravity can be unified at the start of the big bang, and we call this Grand Unified Theory. This only leaves us with a few things to consider such as gravity, the Higgs BosonDark Energy, and other more wild theories like string theory. Of course, there are things we still don’t understand, and this is why we have the standard model instead of “a theory of everything.”

The Standard Model. A look at the standard model in simple terms.

TIP: A system can only be defined by its properties. Once we get down to fundamental properties like charge, spin, the forces, and mass-energy it’s important to keep each concept separate. All these properties act on each other in particle interactions.  There isn’t really one core property (although electromagnetic force is a strong candidate), and arguably there is no tangible “widget” to pin the properties on (although we call the “widget” a “particle” in particle physics, or just “quanta” which describes the field and particle).

### The Force is Strong With This Chart

The chart below shows the four forces (interactions), then it shows what they act on (what properties or systems are affected by it), then what particles experience it (what particles are affected by it), then what the mediating particle is (the particle that carries that force between fermions), and then the strength of the interaction at quark scale, and lastly at the proton scale (notice gravity is the weakest by far).

If you want to learn more about the elementary particle, check out our page dedicated to the elementary particles and the standard model.

The four fundamental interactions of nature (source)
Property/Forces (Interaction) Gravitation Weak Electromagnetic Strong
(Electroweak) Fundamental Residual
Acts on: Mass – Energy Flavor Electric charge Color charge Atomic nuclei
Particles experiencing: All Quarks, leptons Electrically charged Quarks, Gluons Hadrons
Particles mediating: Not yet observed
(Graviton hypothesized)
W+ W Z0 γ (photon) Gluons Mesons
Strength at the scale of quarks: 10−41 10−4 1 60 Not applicable
to quarks
Strength at the scale of
protons/neutrons:
10−36 10−7 1 Not applicable