When we imagine space, we usually think of it as cold and empty. However, in quantum field theory, space is not really empty. Even in a vacuum, there are quantum fluctuations and virtual particle pairs, which are created and annihilated in a very short time. These pairs of virtual particles are usually invisible to inertial observers because they don't have enough energy to separate them. But what if there is an observer or detector in accelerated motion? What will they see?
In 1976, physicist William Unruh discovered a surprising result: from the perspective of an accelerating observer or detector, there is a hot bath in space whose temperature is proportional to the acceleration. This phenomenon is known as the Unruh effect, also known as the Fulling-Davies-Unruh effect (FDU effect), because it was also independently discovered by two other physicists, Stephen Fulling and Paul Davies.
Physicist William Anrou
How does the Anru effect work? Simply put, when a detector moves at constant acceleration, it senses an effective horizon caused by Hawking radiation. Hawking radiation is thermal radiation emitted from the surface of a black hole. When a pair of virtual particles approaches a black hole, one of the particles may be sucked into the black hole and disappear, while the other escapes and becomes a real particle. This is equivalent to the black hole radiating energy outward and having a certain temperature.
Similarly, there are two horizons in accelerating motion: the Rindler horizon and the past horizon. The front horizon refers to the position that the detector can never reach or exceed, while the rear horizon refers to the position that the detector can never return to or pass through. When pairs of virtual particles approach these event horizons, one of the particles may be blocked by the event horizon and disappear, while the other escapes and becomes a real particle. This is equivalent to accelerating the observer to feel an outward radiated energy and have a certain temperature.
This temperature is given by the Unruh formula:
$ T = \frac{\hbar a}{2 \pi k_B c} $
where $T$ is temperature, $\hbar$ is the reduced Planck constant, $a$ is the acceleration, $k_B$ is the Boltzmann constant, and $c$ is the speed of light.
What is the application of the Anru effect?
First, it reveals an important principle in quantum field theory: the observation of physical phenomena depends on the state of motion of the observer. Different observers may have different descriptions and interpretations of the same event. For example, a vacuum seen in an inertial reference frame may see thermal radiation in an accelerated reference frame; A single particle seen in an inertial reference frame may see two entangled particles in an accelerated reference frame. This observer dependence is also linked to the equivalence principle in general relativity, which states that gravitational fields cannot be distinguished from accelerated motion on a local scale.
Second, the Anruh effect provides a powerful tool for understanding black hole physics. Since there is an analogy between Hawking radiation and the Anru effect, we can simulate the phenomena that occur near black holes by studying the accelerating motion. For example, the thermal radiation generated during accelerated motion can be used to explain black hole evaporation. The information loss problem generated in accelerated motion can be used to explore the black hole information paradox; The entanglement entropy generated in accelerated motion can be used to measure black hole entropy.
Finally, the Anru effect provides an interesting direction for us to explore quantum gravity. Quantum gravity refers to the theory that unifies quantum mechanics and general relativity, which is not yet fully established. The Anru effect involves two basic concepts of theory: acceleration and temperature. Acceleration is an important factor in general relativity, which determines how much space-time is curved. Temperature is an important factor in quantum mechanics, which determines the probability distribution of a system at different energy levels. The Anru effect shows that there is a simple but profound link between these two factors2.
Can the Anru effect be experimentally verified?
For now, this is a challenge. Since the required acceleration is so large that it cannot be achieved in the laboratory, we can only look for indirect or analogous ways to detect this effect. Some possible candidates include: Bose-Einstein condensate in atomic physics, fiber or medium in optics, superconductor or superfluid in condensed matter physics, and so on.
Nevertheless, the Anru effect is still a theoretical effect that deserves attention and study. It not only reveals some interesting connections between quantum field theory and general relativity, but also gives us some ideas and methods to explore new physics.