分析以下文本并找出其中的主题或中心思想。


Humans have always been fascinated by the idea of traveling through time. Time travel has been explored in fiction for centuries, from H.G. Wells' "The Time Machine" to the television show "Doctor Who." However, time travel may not be as far-fetched as it once seemed. Some physicists believe that time travel is theoretically possible, although we don't yet have the technology to achieve it. Understanding the nature of time and how it relates to space is one of the biggest challenges currently facing physicists.
主题是关于时间旅行及在物理学中探索时间和空间关系的挑战。