类比引力有关问题研究进展

Recent progress on analog gravity and related issues

尽管引力是人类最先认识的自然界基本相互作用力,但人们对引力本质的认识还相当不够.至今我们仍然没有一个完全自洽的量子引力理论.近年来研究表明,类比引力为我们深入理解引力的性质提供了一个新的窗口.类比引力的研究一方面为Hawking辐射等弯曲时空量子效应的理论研究和实验验证提供了平台,另一方面也加深了人们对类比系统本身的认识.本文简要介绍类比引力的基本思想以及近年来理论和实验方面的进展,并对未来的研究进行展望.

Of the four kinds of fundamental interactions in nature, gravity is the first known to humankind, but at the same time, it seems to be the least understood. A self-consistent theory of quantum gravity is still elusive. Even though, the efforts to unify quantum theory and general relativity continue, and recent researches show that analog gravitational systems open up new possibilities for our understanding of the nature of gravity. The analog gravity provides us concrete non-gravitational physical systems to study the quantum effects in curved spacetime such as the Hawking radiation both theoretically and experimentally on the one hand, while on the other hand, the techniques dealing with issues in curved spacetime can also be applied to the analog systems themselves and shed light on them in return. In this paper, we review various analog models that capture different aspects of the behaviors of gravitational systems. First, we introduce the analog of the bending of light in curved spacetime with artificial metamaterials based on transformation optics, which is an analog of a classical general relativistic effect. We then review Unruh＇s sonic black hole model. When the speed of the background fluid is faster than the speed of sound in the fluid, one creates a sonic analog of a gravitational black hole. If the flow of an ideal fluid is irrotational, then the equation of motion describing the acoustic disturbance is identical to the d＇Alembertian equation of a minimally coupled massless scalar filed in a curved spacetime, which forms the foundation of the phonon Hawking radiation in analog systems. Unruh＇s sonic black hole model has become a paradigm for experimental verification of quantum effects in curved spacetime, and there have been, in recent years, many attempts to realize it in analogue systems such as the Bose-Einstein condensate, ion traps, optical fibers, and etc. Other related issues, such as the detection of modified vacuum fluctuations, which are intrinsically related to quantum effects in curved space, by means of superconducting circuit devices, and the metamaterial models of cosmological evolutions are also reviewed. Finally, we give an outlook for the future research.