QFT unites the classical field theory, special relativity and quantum mechanics into a single framework. It provides explanations for various phenomenons, such as the Casimir effect.

According to QFT, space-time is filled with quantum fields corresponding to every particle and boson. These fields oscillate with various frequencies and permeate all of space even though they are invisible to us.

Virtual Particles

Quantum fields are not nothing: even in their lowest energy state, they are full of virtual particle-antiparticle pairs that appear and annihilate in an incredibly brief period. 

For instance, the up-quark field perpetually generates pairs of up-quarks and anti-up-quarks. 

They cannot be directly observed, but nature allows the creation of those virtual particles by „borrowing“ energy from the vacuum and returning it quickly, as permitted by the Heisenberg uncertainty principle.

The particle-antiparticle pairs are not physical objects but mathematical tools that help calculate the infinite ways a quantum field can fluctuate. 

In a vacuum state, these fluctuations maintain a balance between positive and negative frequencies, meaning the contributions of virtual matter and antimatter cancel each other our, leaving no real particles. However, this balance can be disrupted by the curvature of spacetime, as seen near a black hole's event horizon. Such disturbances allow for measurable effects like hawking radiation, where virtual particles become real.

Quantum Fields

Every particle and force of the standard model of particle physics has a corresponding quantum field that exists everywhere in spacetime. For example, there are fields for fermions like the up and down quarks and fields for bosons like the W boson. These fields interact with each other to give rise to observable phenomena.

Consider the beta decay: the down quark field transfers its energy to the up-quark field and the W boson field. The W boson field transfers its energy to the electron field and anti-neutrino field.

Quantum field can be thought of as invisible, ever-present seas with constant ripples and fluctuations. Real particles- those we can measure- are the peaks of these ripples, created when energy is introduced into a field. 

For instance, in the up-quark field, an up quark can materialize if there is at least 2.3 MeV of energy. An antiparticle can be visualized as a wave that moves in the opposite direction within the field.

In QFT, forces arise from the exchange of energy between fields, mediated by boson fields. For example: the electromagnetic force arises from the photon field.