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曲率引擎的一项突破使我们距离星际迷航更近了一步

Paul Sutter 中科院物理所
2024-08-25

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物理学家们指出了曲率引擎可能的工作模式——相对于星际旅行来说,这对于我们理解引力更有帮助。
一个物理学家团队发现,可能能建造一个真正的,不会违反任何已知的物理规律的曲率引擎。但有一点需要注意:进行曲率航行的飞船不能超过光速,所以你不能随时到达你想去的地方。但是这仍然表明我们对引力的认识前进了一大步。

 不动之动 


爱因斯坦的广义相对论是解决引力问题的一个工具,它将质量、能量与时空弯曲联系在了一起。反过来,时空弯曲又告诉物质和能量该如何运动。在几乎所有的情况下,物理学家都是用相对论方程来计算一组物体如何运动。他们有一些物理情景,比如一颗行星围绕着一颗恒星运行或者两颗黑洞相撞,物理学家们会思考这些物体会如何改变时空结构,以及系统的后续演化应该是什么样子。

曲速飞行过程的想象图
不过反过来,利用爱因斯坦的数学,也是可行的。例如想象某种期望的运动形式,然后思考什么样的时空结构可以实现它。墨西哥物理学家米给尔·阿库别瑞(Miguel Alcubierre)就是这样发现曲率引擎背后的物理基础的——长久以来,这都是《星际迷航》的核心内容。
曲速飞行是通过超光速的方式使我们能够在一段时间内从A点位置移动到B点。然而狭义相对论明确禁止了超光速,这虽然从来没有困扰过《星际迷航》的编剧,但是这个矛盾确实困扰了阿库别瑞许久。他发现,通过巧妙操纵时空结构,可以构建一个曲率引擎,使得飞船前方的空间被压缩,后方的空间被拉伸。严格意义上来说,这确实会产生运动,但是却并不需要移动。
这听起来像是一个悖论,但它只是广义相对论诸多奇妙的方面之一。阿库别瑞的曲率引擎避免了超光速的问题,因为它并不是在空间中运动,而是操纵了空间本身,本质上是将目的地带到了飞船的附近。
虽然阿库别瑞的设计很诱人,但它有一个致命的缺陷。为了提供必要的时空扭曲,宇宙飞船必须包含某种特殊的物质,这些物质通常被需要具有负的质量。负质量带来了一些概念上的问题,它们似乎违背了我们对于物理学的理解。比如,如果你踢一个负五公斤的球,它会向后飞出去。此外,从来没有人在现实中见过负质量的物体。

电影《星际迷航》剧照
这些与负质量有关的问题促使物理学家们提出各种版本的“能量条件”来作为广义相对论的补充。这些条件并不直接嵌入到相对论本身,因为广义相对论的框架下允许存在一些实际宇宙中并不存在的东西,例如负质量,所以需要这些额外的条件来将它们排除在相对论方程之外。这是科学家对这样一个令人不安的事实做出的回应:标准的广义相对论允许超光速运动,但是事实似乎并不认同这一点。

 曲速0级 


(注:此处曲速级别 (Warp Factor) 是《星际迷航》系列中假设的FTL技术水平,一般曲速1级为光速移动)
这些能量条件并没有通过实验或者观测得到证实,但它们与我们观察到的宇宙相符,所以大多数物理学家对此非常重视。直到最近,物理学家们一直认为这些能量条件明确无误地表明,即使你非常想要,你也无法真的制造一个曲率引擎
但阿拉巴马大学亨茨维尔分校的贾里德·福克斯(Jared Fuchs)领导的国际物理学家团队发现了一种解决办法。(该团队还隶属于应用物理应用推进实验室,这是一个虚拟智库,致力于研究曲率引擎等多个领域。)发表在《经典和量子引力》(Classical and Quantum Gravity)期刊上的一篇论文中,研究者们深入探讨了相对论,探索是否可能存在任意形式的曲率引擎
广义相对论方程组以难以求解著称,尤其是在曲率引擎等复杂情形下。因此研究团队转向了软件算法;他们不再尝试手动求解这些方程组,而是通过数值计算探索可行的解,并验证数值解是否满足各种能量条件。
科研团队并没有真的尝试建造一个推进装置。相反,他们研究了广义相对论各种满足条件的解,这些解的共同特征是允许飞船在不经历任何加速或任何引力潮汐的情况下,从一点旅行到另一点。这对任何假想的乘客来说都是极大的舒适。随后,物理学家们检查了这些解是否符合防止使用奇异物质的能量条件。
星际迷航》中曲速飞行的示意图
研究者们确实发现了一个曲率引擎解:一种通过操纵空间使得旅行者们无需加速即可移动的方法。然而,任何事物都存在一定的代价。这种曲率引擎的物理特性有一个重要的注意事项:飞船和乘客们永远不能以超光速飞行。同样令人失望的是,新研究背后的研究人员似乎并不关心找出允许空间翘曲发生的物质配置。

 引力的未来 


一方面,这非常令人失望。我们已经有了很多比光速慢得多的旅行方式(例如,步行,火箭,等等),所以再增加一种方式并没有那么令人激动。另一方面,即使我们想要建造这种曲率引擎,这种超理论工作与实际上的物理推进机制之间的差异,也相当于写下牛顿定律与建造猎鹰九号之间的差距。
但这并不意味着这一新的进展没有趣。我们并不完全理解引力,而且我们知道爱因斯坦的理论是不完备的。基于广义相对论允许有趣的、奇特的解存在这一事实——例如曲率引擎存在,我们可以对引力有更进一步的理解。
物理学家希望所有的理论都能被统一起来,并且与宇宙的本质相一致。因此,如果能量条件确实对物理学设定了真正的限制,要求负质量之类的事物不仅不存在,而且不可能存在,那么我们希望物理理论可以从一开始就明确指出这一点,而不是依赖类似于能量条件的附加条件。
探索曲率引擎如何工作,在什么条件和限制下工作,或者探索它为何不能存在,就是向着这个方向迈出的一步。多年来,物理学家们认为能量条件不允许任何类型的曲率引擎存在,而新的研究却展示了一个与现有理论兼容的例子。无论我们能否得到一个不错的超光速引擎,迎接我们的都将是一场胜利。因为无论未来沿着这些方向的探索得出什么结论,我们对引力的理解都将更进一步,而且有机会革命性地改变我们对于引力的理解。


谁知道一旦我们更好地理解引力,我们将会得到什么呢?

作者:Paul Sutter

翻译:K.Collider(

校:小聪

原文链接:A Warp Drive Breakthrough Inches a Tiny Bit Closer to Star Trek

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