嘿，黑洞，你在那干嘛?Hey Blackhole, why are you there?

大多数人对黑洞的了解是来自于它们无法被逃脱的传说。

但是，既然在浩瀚的宇宙中有那么多我们不知道的东西，那么是否存在黑洞逃脱不了的地方呢?

Most people know blackholes the by the fact that they cannot be escaped from. 

But, since there are so many things in the big wide universe that we don’t know about, are there places that Black Holes can’t even escape from?

图为黑洞模拟图像
Model of black hole

虽然这听起来很个疯狂的想法，但这个问题竟然是另一个谜题的答案。而那个迷就是LIGO-Virgo的最新发现。按照《新轭时报》的说法，“这些黑洞不应该存在，但它们就在那里”，《洛杉矶时报》也说那是“不可能的黑洞”。

Though this is seeming a crazy though, the answer to this question may be a solution another mystery. The recent discovery of LIGO-Virgo. “These Black Holes Should Not Exist, but There They Are”, quoted from the New Yoke Times. Also called the “Impossible Black Hole” by the Log Angeles Times. 

那个新发现便是GW19052：两个质量为85太阳质量和66太阳质量的黑洞的合并。这是有史以来引力波天文台观测到的最剧烈的黑洞碰撞，也是迄今为止发现的最强能量产生。

它们的产物是一个太阳质量142倍的巨大黑洞，碰撞中原本质量相加剩下的9个太阳质量的能量都以引力波能量的形式释放了出来。

The event making new was GW19052: a merge of two black holes with masses of ~85 solar mass and ~66 solar mass. Making it the most dramatic ever collision between black holes witnessed by gravitational wave observatories and the most energetic event ever detected. 

Their product was a gargantuan black hole of 142 times the mass of the sun and the other 9 solar masses released as gravitational wave energy.

LIGO-Virgo与引力波的微妙关系

The subtle relationship between LIGO-Virgo and gravitational waves

引力波是时空中，以光速(3亿米/秒)前进的隐形波，它们身上携带着时空中发生的任何件事的起源信息以及有待我们继续研究的重力线索。

Gravitational waves are invisible ripples in space-time that travel at the speed of light (300 million m/s), carrying information of their origins and clues of gravity yet to be studied by us. 

1916年，爱因斯坦在他的相对论中首次预言了它们。他的数学证明，随着大质量物体(如中子星和黑洞)的加速，时空会受到扰动，时空波会像池塘里的涟漪一样从源头向四面八方传播。

They were first predicted by Einstein in 1916 in his theory of relativity. He’s mathematics showed that with the acceleration of massive objects (e.g. neutron stars and black holes), space-time would be disturbed, and waves of space-time would propagate in all directions from the source like ripples of water in a pond.  

LIGO和Virgo都是两个来自世界各地的科学家们组成的合作项目，专注于引力波的直接探测研究。LIGO用它们来探索引力的基本物理原理，研究将来利用引力波作为天文学发现工具的方法。

LIGO and Virgo are both collaborations between scientists from all around the world focusing on the direct detection gravitational waves. LIGO is using them to explore the fundamental physics of gravity, developing topic to one day be used as a tool for astronomical discoveries. 

而Virgo本身是一个干涉式的引力波天线，装置是两个3公里长的臂，其中包含有用来形成激光干涉仪的各种机械。Virgo合作项目的研究人员便是它的设计者、建设者和运营者。

While Virgo itself is an interferometric gravitational-wave antenna, consisting of two 3-km-long arms, containing various machinery used to form a laser interferometer. The Virgo collaboration is the designer, builder and operator for it.

图为Virgo探测仪内部
Inside the Virgo

GW19052事件在科学界如此震惊是因为，通常核心坍缩超新星是不可能形成这样大质量的黑洞的。一般，核坍缩超新星是在质量约为太阳质量8倍的巨大的恒星在燃料耗尽后，由于重力而引起的坍缩时发生的。大量的质量会以能量的形式释放出来，而其余的会内爆形成黑洞。黑洞的大小将取决于原始恒星的大小，而在已知的超新星模型中，我们可以很容易地预测质量在5到60个太阳质量之间的黑洞的形成。

The event GW19052 came as surprise to many scientists, because with the existence core-collapse supernovas, it is very unlikely for such massive black holes to be formed. Core-collapse supernovas normally happens when very massive star, with masses ~8 solar mass, collapse due to gravity after all the fussion fuel runs out. A large amount of the mass would be released as energy, while the rest implodes to form black holes. The size of the black hole formed would depend of the size of the original star and the already know supernova models can easily predict the formation of black holes with masses between 5 to 60 solar mass.

图为恒星内部爆炸模拟
Model of explosion in the stellar interior

要产生更大的黑洞，初始恒星的质量至少要达到太阳的130倍。对于这样大的恒星，它们的核心会达到非常高的能量，以至于内部的伽马射线开始与原子核碰撞，产生电子和正电子。这意味着当初防止恒星坍缩的伽马射线会逐渐消失，核心的压力因为引力会越来越大，直到承受不住了而爆炸。巨大的爆炸会形成成对不稳定超新星，只留下真空。这样质量在60到150个太阳质量范围内的黑洞就不可能形成了，这也被称为慢质量黑洞的“上质量间隙”。

To produce even larger black holes would need the initial stars to be at least 130 solar masses. With such large stars, there is a point where their cores would reach such high energy that the gamma rays inside start to collide with the atomic nuclei, producing electrons and positrons. This means the gamma rays originally supporting the star from collapsing would disappear, meaning the core having higher and higher pressure until the star over comes its own forces, it just can’t handle the pressures anymore. This results in a huge explosion - pair-instability supernova, leaving nothing behind but empty space, preventing the formation of black holes with mass range from 60 to 150 solar masses, called the ‘upper mass gap’ of staller-mass black holes.

黑洞界的奇葩

The Weird Ones

然而，形成“上质量间隙”的黑洞并不是绝对不可能。GW150914就是个例子，一个36倍太阳质量的黑洞和一个29倍太阳质量的黑洞合并后形成了一个质量62倍太阳质量的“第二代”(2g) 黑洞，达到了“上质量间隙”的下限。而且这不是一个个例，GW170729、GW170823和GW170818都被观察到产生在上质量间隙下限的2g黑洞。

However, forming black holes in the ‘upper mass gap’ is not absolutely impossible. GW150914 was the cases, where a 36 solar masses black hole merged with a 29 solar masses black hole, forming a ‘second generation’ (2g) black hole with mass 62 solar masses, reaching the lower end of the ‘upper mass gap’. And that was not an exception, events like GW170729, GW170823, and GW170818 have also been observed to leave behind 2g black holes touching the lower limits of the upper mass gap.

那么，有了以上这些案例，进一步的合并会不会产生更多在上质量间隙上限的黑洞呢?那在我们进一步探索之前，先让我们回到最初的问题:是否有黑洞无法逃脱的地方?

So, could further mergers of those black holes populate the upper end of the upper mass gap? Before we explore more into that, let us go back to our original question: are there places black holes cannot even escape?

双星黑洞的合并有一个特性，那就是引力辐射必须向不同的方向发射。也就在是这种不完美下，线性动量给了新形成的2g黑洞一剂能量，给了它一个完美的离开的机会。关于这个令人着迷的课题有很多研究，但最终影响巨型黑洞形成的是它们被抛出时的速度。

To have mergers of binary black holes, a special characteristic is needed, having the gravitational radiation emitting all in different directions. With this imperfection, the linear momentum gives the newly formed 2g black hole a ‘kick’, giving it the perfect chance to get away. There are many studies on this fascinating kick topic, but what matter for the forming huge black holes is the velocity they were flung off.

在火箭发射的时候，它们需要足够的能量才能达到脱离地球引力的速度——逃逸速度，那黑洞也一样。如果它们在爆炸时的反冲产生的能量足够高，它们便能够脱离身边的恒星环境而获得自由。这一切都取决于周围环境的密度。

With rockets, enough energy would be needed for it reach the speed to escape Earth’s gravity – escape velocity, and that’s the same with black holes. If its energy from the recoil is high enough, it will be able to escape its stellar environment and be free. And that all depends on the density of its local environment. 

通常情况下，密度相对较小的星团的逃逸速度约为10km/s，密度较大的区域的逃逸速度约为50km/s。然而，位于星系核心的核星团区域是恒星密度最高的地方，逃逸速度超过100千米/秒。那里也是黑洞通常诞生的地方。

Normally, a relatively less dense stellar cluster would have escape velocity of ~10km/s, while the denser globular areas would have ~50km/s. However, the areas of nuclear star clusters, at the cores of the galaxies, are the places with highest stellar densities. That’s where black holes are generally born, escape velocity of more than 100km/s.

图为NASA用红外线拍摄的星系核
Galactic core in infared taken by NASA

在伯明翰大学的Davide Gerosa教授和约翰霍普金斯大学的 Emanuele Berti教授，作为在这个领域先锋科研人员，他们的实验目标是研究在一个特定的区域中，有多少个通过过往合并产生的残骸而组成的2g黑洞。在一个设定的逃逸速度环境中，一组黑洞将尽力多次模拟合并或重组。那些模拟合并中用到残骸来自于以前爆炸时速度不足而没有成功逃脱的黑洞，也就是这样它们能够参与到未来合并，形成能被LIGO-Virgo观测到黑洞，填上上物质间隙的上限空白处。

With the aim of find how many 2g black holes could be formed with the remnants of previous mergers in that area; Davide Gerosa from University of Birmingham and Emanuele Berti from Johns Hopkins University have been modeling multiple mergers over time from a set of black holes in a environment with defined escape velocities. The remnants in this case are the black holes what didn’t escape successfully due the lack of velocity they had during their explosions. So, they were able to participate in future mergers datable by the LIGO-Virgo, hence forming black holes populating the upper mass gap.

两位教授的数据模型显示，在逃逸速度较高的环境中，2g黑洞更有可能被滞留，因此那些区域可能藏有大量的上质量间隙黑洞。

Their model showed that with environments with higher escape speeds are more likely to detain 2g black holes, therefore harboring a large number of black holes in the upper mass gap. 

图表显示在设定逃逸速度的区域中，双星黑洞合并后的重量分布*
Distribution of the black holes as a part of binary black hole mergers in a environment with give escape velocity*

模型还证实，任何被发现的上质量间隙黑洞都含有以前其他黑洞合并的残留物。虽然之前在理论模型的预测里，不可能形成上质量间隙的黑洞，但仅凭GW19052这个案例就能指出，有了这样的环境，巨型黑洞就可能存在，而且还有无穷无尽的可能性有待我们发现。

The models also confirmed that any detection of black holes in the upper mass gap contains remnants from previous mergers. Though theoretical models predicted that there cannot be any black holes formed within the upper mass gap, the single detection of GW19052 pointed out that with the existence of such environment, black holes can be there. There are still endless possibilities yet to be discovered.

图表显示有了二代或更高代黑洞才能产生在上质量间隙的下一代黑洞*
Mergers involving black holes in the upper mass gap are of the second-generation or higher*

展望未来

Looking forward

现在，为什么GW190521如此令人兴奋和着迷了吧。它不仅产生了有史以来通过引力波观测到的最重的黑洞，而且其组成部分的质量也都在上质量间隙的范围内。如上所述，仅仅达到上质量间隙下限的黑洞非常罕见，大多是在致密恒星环境中诞生的2g黑洞。那有它们碰撞产生的巨型更为稀有，而且可能是第三代甚至更高代的黑洞。

Now, it is clear why the GW190521 was such an exciting and fascinating event: not only it produced the heaviest black hole ever seen through gravitational waves, but the components were also massive enough to be in the upper mass gap. As explained above, black holes reaching the lower limit of the upper mass gap are very rare, and mostly likely were 2g black holes born in dense stellar environment. Hence the newly formed product could be a third generation or even higher generation black hole. 

虽然这些发现只是上质量间隙的冰山一角，

但随着LIGO-Virgo的进一步模拟和探测，

我们将为无数的惊喜做好准备。

This is discovery is only the 

tip of the upper mass gap iceberg, 

with further simulations of the LIGO-Virgo 

we will be ready for the countless surprise

图为黑洞相撞时的引力波模拟
Simulation of gravitational waves when black holes collide

图片来自LIGO, Virgo, Max Planck Institute for Astrophysics, NASA官网

*部分来自Davide Gerosa和 Emanuele Berti的“Escape speed of stellar clusters from multiple-generation black-hole mergers in the upper mass gap"论文

Pictures from official website of LIGO, Virgo, Max Planck Institute for Astrophysics, NASA.

*Parts are from “Escape speed of stellar clusters from multiple-generation black-hole mergers in the upper mass gap" by Davide Gerosa and Emanuele Berti