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引力波事件GW150914的发现经过 精选

已有 8078 次阅读 2016-2-23 20:54 |个人分类:大众物理学|系统分类:科普集锦

 

引力波事件GW150914的发现经过太富有戏剧性了。在最后测试完成了不到一个小时,GW150914引力波事件就到来了。《物理学评论快报》文章(Phys. Rev. Lett. 116, 061102说,第一个观测期结束于2016112,该文章分析的数据是从20159121020的数据。在此期间,最显著的就是GW150914引力波事件,PRL还说,另有一个很小的信号可能也是引力波事件。

关于引力波事件GW150914的发现经过,《纽约客》文章的说法是这样的:

9月13号是个星期天,Effler在华盛顿州的设施和另一位同事整整忙了一天,完成一些列的最后测试工作。“我们大喊大叫,我们摇晃东西,敲打东西,引入磁铁干扰,做了各种事情,”她回忆说。“不出所料,每件事都花费了比预期更多的时间。”终于在凌晨4点,只剩下一个测试要做——模拟附近的卡车司机踩刹车——他们终于决定收工。他们开车回家了,只留下设备静静的采集数据。引力波信号没等多久就到了,在凌晨4点50分,当地时间,相差只有7毫秒分别通过了两个探测器。距离高级LIGO的正式启动还有四天时间。

然后是各种检测,对信号可能来源的各种检测,最后认为它是个引力波事件。

我觉得,在关于引力波的各种报导中,这个发现经过与对引力波的科普混在一起,反而降低了其关注度。所以,我专门把这段描写摘录出来,而且是中英文对照的,以方便对此感兴趣的读者。至于说全文,可以在下面给出的网文链接里找到。

LIGO的运气太好了。

 

 

《纽约客》长文:发现引力波背后最完整的内幕故事

http://it.sohu.com/20160216/n437581290.shtml 

作者:NICOLA TWILLEY    机器之心编译出品    编译:赵巍

 

913号是个星期天,Effler在华盛顿州的设施和另一位同事整整忙了一天,完成一些列的最后测试工作。“我们大喊大叫,我们摇晃东西,敲打东西,引入磁铁干扰,做了各种事情,”她回忆说。“不出所料,每件事都花费了比预期更多的时间。”终于在凌晨4点,只剩下一个测试要做——模拟附近的卡车司机踩刹车——他们终于决定收工。他们开车回家了,只留下设备静静的采集数据。引力波信号没等多久就到了,在凌晨450分,当地时间,相差只有7毫秒分别通过了两个探测器。距离高级LIGO的正式启动还有四天时间。

引力波被如此早的探测到引来了很多迷惑和质疑。“我告诉所有人我们直到2017年获2018年才会探测到任何东西,”Reitze说。 Janna Levin,哥伦比亚大学的天体物理教授,没有参加LIGO合作研究,也感到非常吃惊。“当传言开始时,我的反应是:别逗了!”她说,“他们连锁才刚上好!”。再说这个信号实在是太完美了。“我们绝大多数人认为,当我们看到这个信号时,它将是从非常多的计算机和计算周期后中从噪音中拉出来的信号,”Weiss认为。大多数人认为这个信号是某种测试。

LIGO团队包括一小组人员,专门制造隐藏的信号注入——虚假的引力波证据——作为对科学家工作的监督。尽管每个人都认识这个四人小组的成员,“我们都不知道什么样的信号,在何时,以及是否被注入,”Gabriela Gonzalez合作研究的发言人说。当LIGO的最早的2010年的运行中,探测器捡到了好像是很强的信号。科学家们紧张的分析了六个月,最后认为是来自河外星系Canis Major的引力波。就在他们想把发现投到科技期刊之前,他们被告知这个信号是被注入的假信号。

  这一次,隐藏信号小组发誓他们和这个信号没有任何关系。Marco Drago认为他们的否认可能是测试的一部分,不过Reitze自己,作为四人小组的成员,有另外的担心。“我的担忧是——你可以理解为这是我们对做出任何虚假发现的本能畏惧——有没有其他的人恶意的在捣乱?”他说,“会不会是其他人在我们的探测器里伪造了一个信号,我们没有人知晓?”ReitzeWeissGonzalez还有其它几个人考虑了谁还出于对设备和系统算法的全面了解有可能制造这个假信号。他们只找到4个人选,其中没有一个人有任何动机去这样做。”我们深入盘问了这些人,”Weiss 说,“结论是,他们没有做这件事。”最后,他说,“我们接受了最经济直接的解释:这是一对黑洞造成的。”

LIGO合作研究机构的每一个分部门开始确认这一探测的有效性。他们检查每一件设备是怎么设定和校准的,每一行软件代码都分析,编辑了一个单子列出所有可能的环境干扰,从大气电离层的振荡到太平洋沿岸的地震。(“当时有一个很强大的发生在非洲的闪电,”Stan WhitcombLIGO首席科学家告诉我。“不过后来磁场测量仪显示它没有足够的干扰可以产生这个信号。”)最后,他们确认了这个探测结果满足统计学的5西格玛门槛,这是宣布任何物理学发现的黄金标准。换句话说,这意味着这个探测结果只有350万分之一的概率是由随机事件造成的。

914号的发现,现在正式命名为GW50914,已经附带来好几个非常重要的天体物理发现。举例说,它是第一个观测证据说明双黑洞对的存在。直到现在,这只是理论上的可能性,因为黑洞吞噬所有周围的光,让传统望远镜无法观测到。引力波是唯一可以逃出黑洞的压倒性的引力场的信息。

LIGO科学家已经从这个信号中提取出令人惊讶的信息,包括源头的黑洞的质量,轨道速度,它们边界接触的时刻。它们比想象的质量大很多,这个惊讶地发现,如果被以后的引一步观测印证,将会帮助解释各个银河中心的神秘的超级体量黑洞是如何形成的。研究团队还量化了黑洞的ringdown——三个能量脉冲在最终的合并后的新的更大的黑洞在球形化过程中释放出来——也就是说黑洞在合并后把自身的不完美部分通过引力波辐射出来。

  这次检测还证实了爱因斯坦关于这个物理宇宙的另一个特性的想法。尽管他的理论主要是关于引力,以前理论主要是在我们自己的太阳系进行检验,在这里引力的主导效应比较弱。“你仅在你爬楼梯的时候才会想起地球的引力,”Weiss 说,“但是,对于物理学来说,引力的效应相比之下只是个小角色,非常微弱,没有什么影响。”在黑洞附近就不同了,引力在那里是宇宙中最强的作用力,可以把原子撕碎。这些爱因斯坦在1916曾预言过,LIGO的实验结果显示爱因斯坦的方程和实际观测几乎是完美的一致。“他究竟怎么能知道这点?”Weiss 问道。”我多么希望在那个早晨可以把数据拿给他看,看看他脸上的反应。”

  自从914号的发现,LIGO继续观察到可以作为引力波候选的信号,尽管这些信号没有第一个信号那样具有戏剧性。”我们一开始这样折腾都是因为开始这个大信号,“Weiss 说道。”不过,我们非常高兴还有别的,强度小一些的信号,说明我们最开始的发现不是单一的,疯狂的,神经质的事件。”

 

GravitationalWaves Exist: The Inside Story of How Scientists Finally Found Them

BY NICOLA TWILLEY

http://www.newyorker.com/tech/elements/gravitational-waves-exist-heres-how-scientists-finally-found-them 

 

On Sunday, September 13th, Effler spent the day at the Livingston site with a colleague, finishing a battery of last-minute tests. “We yelled, we vibrated things with shakers, we tapped on things, we introduced magnetic radiation, we did all kinds of things,” she said. “And, of course, everything was taking longer than it was supposed to.” At four in the morning, with onetest still left to do — a simulation of a truck driver hitting his brakesnearby — they decided to pack it in. They drove home, leaving the instrument to gather data in peace. The signal arrived not long after, at 4:50 A.M. local time, passing through the two detectors within seven milliseconds of each other. It was four days before the start of Advanced LIGO’s first official run.

The fact that gravitational waves were detected so early prompted confusion and disbelief. “I had told everyone that we wouldn’t see anything until 2017 or 2018,” Reitze said. Janna Levin, a professor of astrophysics at Barnard College and Columbia University,who is not a member of the LIGO Scientific Collaboration, was equally surprised. “When the rumors started, I was like, Come on!” she said. “They only just got it locked!” The signal, moreover, was almost too perfect. “Most of us thought that, when we ever saw such a thing, it would be something that you would need many, many computers and calculations to drag out of the noise,”Weiss said. Many of his colleagues assumed that the signal was some kind of test.

The LIGO teamincludes a small group of people whose job is to create blind injections — bogus evidence of a gravitational wave — as a way of keeping the scientists on their toes. Although everyone knew who the four people in that group were, “we didn’t know what, when, or whether,” Gabriela González, the collaboration’s spokeswoman, said. During Initial LIGO’s final run, in 2010, the detectors picked up what appeared to be a strong signal. The scientists analyzed it intensively for six months, concluding that it was a gravitational wave from somewhere in the constellation of Canis Major. Just before they submitted their results for publication, however, they learned that the signal was a fake.

This time through,the blind-injection group swore that they had nothing to do with the signal. Marco Drago thought that their denials might also be part of the test, but Reitze, himself a member of the quartet, had a different concern. “My worry was—and you can file this under the fact that we are just paranoid cautious about making a false claim—could somebody have done this maliciously?” he said.“Could somebody have somehow faked a signal in our detector that we didn’t know about?” Reitze, Weiss, González, and a handful of others considered who, if anyone, was familiar enough with both the apparatus and the algorithms to have spoofed the system and covered his or her tracks. There were only four candidates, and none of them had a plausible motive. “We grilled those guys,”Weiss said. “And no, they didn’t do it.” Ultimately, he said, “We accepted that the most economical explanation was that it really is a black-hole pair.”

Subgroups within the LIGO Scientific Collaboration set about validating every aspect of the detection. They reviewed how the instruments had been calibrated, took their software code apart line by line, and compiled a list of possible environmentaldisturbances, from oscillations in the ionosphere to earthquakes in the Pacific Rim. (“There was a very large lightning strike inAfrica at about the same time,” Stan Whitcomb,LIGO’s chief scientist, told me. “But our magnetometers showed that it didn’tcreate enough of a disturbance to cause this event.”) Eventually, theyconfirmed that the detection met the statistical threshold of five sigma, thegold standard for declaring a discovery in physics. This meant that there was aprobability of only one in 3.5 million that the signal was spotted by chance.

The September 14thdetection, now officially known as GW150914, has already yielded a handful ofsignificant astrophysical findings. To begin with, it represents the firstobservational evidence that black-hole pairs exist. Until now, they had existedonly in theory, since by definition they swallow all light in their vicinity,rendering themselves invisible to conventional telescopes. Gravitational wavesare the only information known to be capable of escaping a black hole’scrushing gravity.

The LIGO scientistshave extracted an astonishing amount from the signal, including the masses ofthe black holes that produced it, their orbital speed, and the precise momentat which their surfaces touched. They are substantially heavier than expected,a surprise that, if confirmed by future observations, may help to explain howthe mysterious supermassive black holes at the heart of many galaxies areformed. The team has also been able to quantify what is known as theringdown—the three bursts of energy that the new, larger black hole gave off asit became spherical. “Seeing the ringdown is spectacular,” Levin said. Itoffers confirmation of one of relativity theory’s most important predictionsabout black holes—namely, that they radiate away imperfections in the form ofgravitational waves after they coalesce.

The detection alsoproves that Einstein was right about yet another aspect of the physicaluniverse. Although his theory deals with gravity, it has primarily been testedin our solar system, a place with a notably weak gravitational regime. “Youthink Earth’s gravity is really something when you’re climbing the stairs,”Weiss said. “But, as far as physics goes, it is a pipsqueak, infinitesimal,tiny little effect.” Near a black hole, however, gravity becomes the strongestforce in the universe, capable of tearing atoms apart. Einstein predicted asmuch in 1916, and the LIGO results suggest that his equations align almostperfectly with real-world observation. “How could he have ever known this?”Weiss asked. “I would love to present him with the data that I saw thatmorning, to see his face.”

Since theSeptember 14th detection, LIGO has continued to observe candidate signals,although none are quite as dramatic as the first event. “The reason we aremaking all this fuss is because of the big guy,” Weiss said. “But we’re veryhappy that there are other, smaller ones, because it says this is not someunique, crazy, cuckoo effect.”

 

 



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