In quantum field theory in curved spacetime, the prediction of particle creation by time-dependent gravitational fields is known as the process of particle creation or vacuum fluctuations. To understand this concept in layman's terms, let's break it down into simpler terms:
Vacuum and Quantum Fields: In quantum physics, the vacuum is not an empty void but rather a state filled with quantum fields. These fields exist everywhere in space, even in seemingly empty regions. Quantum fields are like invisible fields permeating the entire universe.
Fluctuations and Particle-Antiparticle Pairs: Quantum fields are not static but constantly fluctuating. These fluctuations can sometimes be significant enough to produce particle-antiparticle pairs. In simple terms, particles and their corresponding antiparticles spontaneously appear from the vacuum, exist for a brief moment, and then annihilate each other, returning their energy back to the vacuum.
Time-Dependent Gravitational Fields: When we introduce the concept of curved spacetime from general relativity, the curvature of spacetime can change with time, leading to time-dependent gravitational fields. These changing gravitational fields interact with the quantum fields, influencing their behavior.
Particle Creation: In the presence of a time-dependent gravitational field, the fluctuating quantum fields can be affected. The changing curvature of spacetime can provide the necessary energy for the creation of particle-antiparticle pairs from the vacuum fluctuations. Essentially, the gravitational field can extract energy from itself and convert it into particles.
Hawking Radiation as an Example: One of the most famous examples of particle creation by a gravitational field is Hawking radiation. According to physicist Stephen Hawking's work, near the event horizon of a black hole, the strong gravitational field causes particle-antiparticle pairs to form. In this case, one particle may fall into the black hole while the other escapes, appearing as radiation emitted by the black hole. This process is attributed to the time-dependent nature of the gravitational field near the event horizon.
In summary, quantum field theory in curved spacetime predicts particle creation by time-dependent gravitational fields through the mechanism of vacuum fluctuations. The changing curvature of spacetime can provide the necessary energy to create particle-antiparticle pairs from the quantum fields that fill the vacuum. This phenomenon is a remarkable consequence of the interplay between quantum mechanics and general relativity.