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当我们拍下黑洞时,我们实际上在拍摄什么?

Katie McCormick 中科院物理所 2023-10-01


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黑洞将它的秘密紧紧捂在怀里。他们把投入怀中的所有都牢牢禁锢住。即使是光,也无法逾越黑洞的引力牢笼。

这样来看,黑洞理应是看不见的——因此拍下它的照片也是不可能的。不过,在2019年,天文学家们发布第一张黑洞图像,并且大张旗鼓地进行了宣传。之后,在2022年春天,天文学家公布了另一张黑洞照片——这次是在我们银河系中心的黑洞。

这张照片展示了一个橙色的类似甜甜圈的斑点,看起来与早先在梅西耶87星系中心拍摄到的黑洞非常相似。但实际上,银河系的人马座A*中的黑洞比梅西耶87的黑洞要小得多,而且更难看到,因为它需要透过银河系的朦胧圆盘进行观测。因此,尽管天文学家们对人马座A*中的黑洞的观测是与对梅西耶87星系的黑洞是同一时间开始进行的,但是我们还是多花了三年的时间才绘制出这幅图像。这个项目需要数百名天文学家、工程师和计算机科学家的国际合作,并开发复杂的计算机算法,从原始数据中拼出图像。


黑洞人马座A*的新图像证实并完善了天文学家对其大小和取向的预测。黑洞的质量决定了它的大小,或者科学家所说的引力直径。没有光可以从黑洞逃逸的临界范围,称为事件视界,由黑洞的质量和自旋决定。热等离子体围绕吸积盘做高速旋转,发射无线电波。这些无线电波被重力扭曲(“引力透镜效应”),最终形成橙色外圈的样貌。这里显示的黑洞阴影和发射环分别是黑洞事件视界和吸积盘远端在引力透镜下的投影。 | 来源:Knowable Magazine,图片由作者K.McCormick提供


黑洞,被定义为一个被称为事件视界内的空间区域,而这些“照片”显然并没有直接展示出黑洞样貌。这些照片实际上记录了围绕黑洞高速旋转的扁平热等离子体,这一部分也被称为吸积盘。等离子体由高能带电粒子组成。当等离子体在黑洞周围旋转时,加速粒子发射出无线电波。图像中模糊的橙色环是由分散在地球各地的8台望远镜(统称为事件视界望远镜(Event Horizon Telescope, EHT))捕捉到的无线电波精心重建而成的。

这些黑洞的图像讲述了来自银河系中心的无线电波史诗般的旅程,提供了人马座A*中黑洞前所未有的细节。这幅图像也构成了“广义相对论最重要的视觉证明之一”,这也是我们目前最好的引力理论,阿姆斯特丹大学的天体物理学家、EHT合作项目的成员塞拉·马尔科夫(Sera Markoff)说。

对于人马座A*这样的超大质量黑洞的研究将帮助科学家们更全面地了解星系是如何随着时间的推移而进化的,以及这些星系是如何在宇宙中聚集成巨大的星团的。

从银河之眼眺望

人马座A*黑洞比2019年拍摄到的梅西耶87黑洞小1600倍,与地球之间的距离也近了2100倍左右。这意味着这两个黑洞在天空中看起来大小差不多。台北中央研究院天文与天体物理研究所EHT项目科学家杰弗里·鲍尔表示,从地球上观察人马座A*的黑洞所需的分辨率与在地球上拍摄月球表面的橙子所需的分辨率相同。



我们的星系中心离我们有26000光年远,所以收集来制作黑洞照片的无线电波是在已知最早的人类建造永久定居点之一的时候发射的。当无线电波第一次从黑洞吸积盘的粒子中发射出来时,它们的旅程就开始了。在向地球发射的各类电磁波中,波长约为1毫米的辐射相对来说没有受到星系气体和尘埃的干扰。对于波长短得多的电磁波,比如可见光,电磁波就会被尘埃散射。如果波长进一步增加,那么这些电磁波就会被带电的等离子体云弯曲,从而扭曲图像。

最后,经过26000年的长途跋涉,这些电磁辐射被EHT捕捉,并被分布在我们星球上的无线电观测站记录下来。天文台之间巨大的地理距离对收集并分析这些数据至关重要——这使得研究人员能够通过一种称为干涉测量法的过程检测到每个地点收集的无线电波之间极其细微的差异。这些微小的差异被用来推断每一束无线电波从源头至观察者之间传播距离的更加细微的差异。利用计算机算法,科学家们成功地解码了无线电波的路径长度差异,以重构发射它们的物体的形状。

研究人员再将所有这些收集到的数据上色,形成一张形象化的照片,其中橙色代表高强度电磁辐射信号色代表低强度电磁辐射信号。任教于亚利桑那大学的富尔维奥·梅利亚(Fulvio Melia)是一名研究银河系超大质量黑洞的天体物理学家,他解释说:“每台天文望远镜只能接收到黑洞发射的电磁信号的一小部分。由于我们无法收集到黑洞发射出来的所有电磁信号,因此在这张照片中,我们看到的不是一张清晰的照片,而是一些有点模糊的图像


天文学家通过一种称为干涉测量的技术拍下了人马座A*的黑洞图像。干涉测量的技术是采用位于世界各地的8台望远镜收集黑洞发出的电磁波,并对收集到的信号进行比较。如果两个地点收集到的波是“同相”的,意思是波的峰值相互对齐,那么这两个波就会加在一起,在图像上形成一个亮点。另一方面,如果波的相位不一致,即一个波的波峰与另一个波的波谷对齐,那么波就会相互抵消,在图像中产生一个黑点。这些望远镜一起工作,能够收集到比任何一台望远镜都要详细的数据。 | 来源:Knowable Magazine,图片由作者K.McCormick提供


这张照片有助于揭示更多关于黑洞事件视界的信息。事件视界是任何物体接近黑洞而不被吸入的最近点。在事件视界之内,连光都无法逃脱黑洞。

从人马座A*的黑洞图像中,科学家们已经能够更加精确地估计事件视界的大小,并推断出黑洞的吸积盘与银河系相比倾斜了40多度。所以,在“照片”中,我们看到的是黑洞吸积盘倾斜角度的投影,而不是它边缘。

但是,即使黑洞的吸积盘平行于银河系,也就是说我们能正对着观察到吸积盘的边缘,黑洞的引力也会扭曲它周围的空间,以至于从黑洞背面发射的光会弯曲着射向地球。因此,不管吸积盘相对于地球的取向是怎么样的,它在照片中都会形成一个环状图像。那么,科学家是如何判断黑洞吸积盘的取向的呢?因为实际上我们得到的照片中,黑洞发射环(emission ring)的大部分都是圆的。如果我们是正对着黑洞的边缘进行观察的话,那么这个发射环的照片会显得更扁,更长。

马尔科夫认为,这种观察银河系中心的新能力将有助于填补我们对星系演化和宇宙大尺度结构的理解空白。一个密度大、质量大的物体,比如星系中心的黑洞,会影响附近恒星和尘埃的运动,进而影响星系随时间的变化。黑洞的特性,比如它旋转的方向,取决于它与恒星或其他黑洞碰撞的历史。“很多人看着天空,认为天空是静止的,对吧?但事实并非如此。这是一个正在进化的巨大生态系统。”

到目前为止,这幅图像与科学家们的预期精准的吻合,这一事实使它成为对当前物理学理论的重要证实。鲍尔说:“我们二十年来的预测告诉我们将会看到这种规模的环状图片。但是,眼见为实。”

作者:Katie McCormick


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原文链接:What the image of the Milky Way’s black hole really shows

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