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来源:中国科学报      作者:晋楠      时间:2017-05-03 18:57      关键词:
摘要:科学家用地下激光测量地球旋转和地震

最终目标是在全球有更多类似的装置。

ROMY 图片来源:LMU

铝制舱门是地下隐藏物的唯一线索。在距离慕尼黑市20公里远的德国菲尔斯腾费尔德布鲁克县一处安静的修道院村落的玉米地和小麦地中间,埋藏着一座混凝土倒金字塔建筑以及钢铁管道、精密反应器,它们位于约3层楼深的地底深处。今年3月,当激光开始穿过名为“地震旋转运动”(ROMY)的四面体边缘时,世界上最复杂的唤醒激光器由此启动,它能够反映地球自身如何旋转运动。

“这是此前从未建造过的一种结构。”慕尼黑路德维希马克西米利安大学地震学家Heiner Igel说,“它是一种很特别的设施。”使它独一无二的特征包括保持激光稳定以及检测其波长的细微变化。

这样做使得ROMY能够测量地球旋转率和旋转轴的细微变化。测量相关速度和步伐有助于增加GPA导航的精度,ROMY甚至还可以预测由阿尔伯特·爱因斯坦预测的广义相对论:旋转行星对附近时空的推拽作用,就像一只勺子进了一锅蜂蜜那样。ROMY还对伴随地震的轻微旋转非常敏感,这一长期被忽视的运动含有关于地球内部结构的线索。通过展示记录相关运动的数值,ROMY可以为帮助石油和天然气勘探者乃至想要聆听月球和火星震颤的行星科学家的微型感应器铺平道路。

4个环

根据法国物理学家Georges Sagnac在1913年证实的一种效应,环形激光器是最精确的旋转感应器。他把光分成围绕一个旋转桌面的周边朝着相反方向运行的两束光。当他把两束光重新结合在一起时,他看到了相互干扰的“条纹”——暗色和亮色的光带表明两束光波的反相位。朝着旋转方向运动的光比相反方向运动的光移动得稍远,从而导致相移。

在接下来的几十年,科学家用Sagnac效应跟踪旋转。该原理支持用激光器和纤维光学陀螺仪替代上世纪70年代的精巧机械陀螺——现在这是导航的标准。他们测量的旋转像一架喷气战机的转弯和下潜一样又快又大。建立更大、更精确的大地测量学(测量地球自身)环形激光器的想法直到上世纪90年代才出现,当时近乎完美的反光镜刚开始出现。

首批类似激光器之一是C-II,这是一种拥有1米长方形手臂的环形激光器,于上世纪90年代中期在新西兰建造,位于二战后一个废弃的燃料库中,那里的温度比较稳定。C-II还推动了慕尼黑理工大学激光物理学家Ulrich Schreiber的职业发展。

在获得欧盟研究委员会的资助之后,Igel向Schreiber说出了他的最大挑战:设计ROMY。ROMY臂长12米,比此前的环形激光器更加精确,能够以十亿分之一的精度感应地球的旋转。它的设计不只是一个方形环,而是有4个三角形的环。其中3个被用来约束朝着任何方向进行的旋转,第4个环可以增加吞吐率。建造工作在2016年3月开始,并在6个月后竣工。

今年3月,工程师在4个环中同时实现了第一束光,这表明这个几何四面体非常精确,足以保证所有激光正常产生共鸣。“它看似无足轻重实则不可或缺。”Igel说,“每次当红色激光可见时,人们都会非常激动地尖叫出声。”该团队正在研究对这些激光进行干涉以测量Sagnac效应。他们将于近日在奥地利维也纳召开的欧洲地球物理科学联合会的一次会议上报告首次测量结果。

实时测量

最终,ROMY科学家将监测白昼时长和地球轴心位置的变化。它们都不像人们想象的那样固定不变,而是每天进行着毫秒或厘米级的变化。太阳和月亮都在用引力拖拽着地球,同时大陆漂移、洋流变化以及冰河时代冰川退化改变周围质量造成的地壳反弹,都会改变地球的惯性运动,从而改变它的旋转。即便是飓风和地震也会对其产生微小的推力。

目前,这些变化的最佳测量结果来自于一个叫作甚长基线干涉测量(VLBI)的系统,该系统利用地球上以一定距离排列的射电抛物面天线观察类星体——遥远宇宙中不时闪烁的明亮“灯塔”。通过计算广泛分布的抛物面天线何时会记录到一个亮度变化,大地测量工作者能够计算出地球的旋转速率及轴的坐标。但这一系统需要数十个天文观测台放弃宝贵的天文观测时间,为了达到最佳时序比较,硬盘需要连夜从遥远的地方运输到超级计算机中心。将观测结果转化为发表的测量结果也要花费数日。

ROMY将设法匹敌VLBI的精度,同时将会超过它的速度。澳大利亚塔斯马尼亚大学大地测量专家、帮助提供VLBI服务的Lucia Plank说,在理论上,ROMY能够连续监测地球的旋转速率和地轴,实时更新测量结果。“ROMY的优势是,你可以立刻获得观测结果。”Plank说。不过她同时表示,VLBI技术则更加稳定,因此不可能很快被弃之不用。

实验漂移

由于ROMY非常新,它仍被实验漂移困扰。该结构位于菲尔斯腾费尔德布鲁克县的松软沉积物中,与其他固定在微晶玻璃(一种可抵抗温度变化的陶瓷制品)模块上的环形激光器不同,ROMY的钢管会随着昼夜温差的变化而变化。它还会在雨水浸透地面后产生变化。最终,Igel希望通过在ROMY的每个镜片后面放置小发动机消除这些漂移,从而实时对激光环做出微调。但他很欢迎其中一种快动“漂移”:地震。

过去,地震学家仅仅测量了平移——沿着三个主轴的任何地面位移,但地震波还会推动倾角移动,其中旋转点不会改变位置。传统地震仪不能测量倾角移动,但理论表明,它们非常小,可以忽略不计。比如提出著名地震等级的地震学家Charles Richter在1958年写道:“这样的旋转可以忽略。”

“但它们毕竟存在。”Igel说。ROMY将帮助科学家探索这一新地震学前沿,展示它的确存在。在该团队打开首个三角环形激光器之后,它感应到了去年10月意大利6.6级诺尔恰地震的旋转。

最终,科学家希望能够接近地震源头。“ROMY不能移动。”法国巴黎导航感应器公司iXBlue地震旋转感应器负责人Frédéric Guattari说。iXBlue的解决办法是一个由长达5公里的纤维光环构成的紧凑传感器,那些纤维光环被缠绕为一个直径仅有20厘米的线圈。该装置可将光子通过线圈发射到相反方向,干扰它们,并跟踪相移探测旋转。Guattari已经把该设备原型放置在斯特龙博利火山两边和佛罗伦萨大教堂中。

但Igel和Schreiber 希望这样的小事,他们还希望ROMY能够孵化出后代。通过布置在全球各地的多个大型环形激光器,大地测量可以通过协调、校准和验证各个激光器的测量结果,形成更加丰富、更加精确的地球旋转和移动的结果。尽管Plank非常认可VLBI,她同样希望德国大型环形激光器不会单兵作战。“最终目标是在全球有更多类似的装置。”

(晋楠编译)


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