黑洞与奇点

Black Holes and Singularities

黑洞可以说是引力最极端的体现,其视界内是个连光也逃不出去的时空区域。近来黑洞在天文观测方面取得令人惊讶的发展,这其中包括:黑洞碰撞的引力波探测以及M87星系的超大质量黑洞的所谓第一张黑洞照片。但是在理论的层面上,黑洞物理尚有许多未解之谜。其中,信息遗失的悖论是最有名的。但是,有另一个问题至少和信息的丢失一样-甚至更加-令人费解的,就是黑洞内部的奇点性质。时空奇点是广义相对论本身无法描述的,在那里究竟发生什么事?黑洞内部的奇点和宇宙大爆炸时的奇点有何不同?奇点是否会裸露在黑洞外面?所谓“宇宙监督猜想”的假设目前有何进展?我们在这篇半科普的文章中简单的介绍这些课题,希望本文章对物理和数学的本科生有所帮助。

Black holes are arguably the most extreme manifestation of gravity,with horizons that mark the boundary of no return beyond which nothing,not even light,can escape.Recently,remarkable progress has been made on the observational fronts,with the detection of gravitational wave produced by colliding black holes,and“direct”imaging of the supermassive black holes in the galaxy M87.On the theoretical side however,there remains a lot of unsolved mysteries in black hole physics.Of these,the information paradox is the most well-known.Nevertheless,there is another equally puzzling–if not more so–issue,which concerns the very heart of black holes:their singularities,where general relativity breaks down.What happens at the singularities of black holes?Can quantum gravity really remove black hole singularities?Is there a difference between Big Bang singularity and those inside black holes?More crucially,can singularities become naked,i.e.no longer shrouded by black hole horizon and therefore visible to ordinary observers?What is the status of the so-called“cosmic censorship conjecture”?In this review we will go through this topic at a semi-technical level,which is suitable for an ambitious undergraduate students in physics or mathematics.