Talking to the neighbours

Astrophysics and alien intelligence 天文物理学与外星智慧 
 
Talking to the neighbours
 与"邻居们"交谈

 

A modest proposal for an interstellar communications network
 对于星际通讯网络的小小建议

 

Apr 7th 2011 | from the print edition

 

 

 

 ALMOST as soon as radios were invented, people speculated about using them to listen to―and maybe even talk to―extraterrestrial civilisations. Since the 1960s attempts have been made to do so by sifting through signals from outer space in search of alien chit-chat. More recently, the use of lasers in telecommunications has suggested to some that they might be a better way to communicate across vast distances, so searching for telltale flashes from the sky is now in vogue.
 几乎是在无线电被发明的同时，人们就在思索如何用它们来倾听――甚至可能是对话――地外文明。而自上世纪六十年代起，人们就在不断尝试通过过滤来自外太空无线电信号来搜寻外星人的片言只语。近来，激光在电信上的应用已经显示出在某些方面它们可能是一种更合适的超远距离通信方式，这导致找寻天空的隐秘"闪光"成为一股时兴潮流。

 

 But techniques that work well on Earth are not necessarily ideal for talking across the vast chasms that separate stars. And for several years John Learned of the University of Hawaii and Anthony Zee of the University of California, Santa Barbara, have been promulgating what they believe is a better idea. They suggest that any alien civilisation worth its salt would alight not on the photons of the electromagnetic spectrum―whether optical or radio-frequency―to send messages to other solar systems. Rather, it would focus its attention on a different fundamental particle, one that is rather neglected by human technologists. That particle is the neutrino.
 但是在地球上行之有效的技术未必能在跨星际遥远距离的通话中理想运作。这些年来夏威夷大学的约翰・伦德（John Learned）和圣巴巴拉市加州大学的安东尼・徐（Anthony Zee）一直在宣传他们认为更加有效的方法。他们指出，任何有头脑的外星文明都不会发射电磁波谱中的光子――无论是可见光波段还是射频波段――向另一个太阳系传送信息。相反，其所关注的是另一种基本粒子，这种粒子被人类的技术专家长期忽视。它就是中微子。

 

 Neutrinos, it must be confessed, are neglected for a reason. Though abundant (the universe probably contains more of them than any other sort of particle except photons), they are fiendishly difficult to detect. That is because they interact only occasionally with other forms of matter. But that is precisely why Dr Learned and Dr Zee like the look of them. Light and radio waves are absorbed and scattered by interstellar gas and dust. Neutrinos would pass straight through such obstacles, and could easily be detected by neutrino telescopes on Earth (which typically consist of giant vats of water or, more recently, huge chunks of Antarctic ice).
 必须承认，中微子遭到忽视是有原因的。虽然含量丰富（除了光子以外，它在宇宙中的含量比所有其他类型的粒子都要多），但要探测到它们却难于登天。那是因为它们仅仅是在非常偶然的情况下才会与其他物质发生反应。而这恰恰是伦德博士和徐博士对它们如此看重的原因。光波和无线电波能被星际气体和尘埃吸收及散射。中微子却可以直接穿透这些障碍，使地球上的中微子望远镜（通常有巨大的水槽，近来也有采用体积庞大的南极洲冰块的）能方便地检测出来。

 

 The two researchers go further. They argue that powerful beams of neutrinos could be used to turn entire stars into flashing beacons, broadcasting information across the galaxy. Outlandish as this sounds, it is an idea that can easily be checked, for astronomers are already sitting on the data that might contain these extraterrestrial messages. They just need to analyse those data from a new perspective. Dr Learned and Dr Zee are therefore trying to persuade someone who studies the data in question to take their idea seriously and spend a little time having a look.
 而这两位研究者走得更远。他们指出，强大的中微子束能把整颗恒星变成闪亮的灯塔，向整个星系广播信息。听起来匪夷所思，但这个设想很容易得到验证，因为天文学家们早已拥有包含此类地外信息的数据。他们要做的仅仅是以一个全新的角度去分析这些数据。因此，伦德博士和徐博士试图说服研究这些可疑信息的研究人员认真对待他们的观点，花一点点时间去尝试分析。

 

Motes and beams
 星尘与粒子束

 

 To detect artificial neutrinos using existing telescopes means screening out the natural neutrino background. Fortunately, much of that is produced by nuclear reactions in stars, and such stellar neutrinos have relatively low energies. If the aliens made their beams out of neutrinos that were a billion times more energetic than the ones emanating from stars (something the researchers argue is not completely beyond the bounds of current technological imagination), the background noise would disappear. At high enough energies the rest of the galaxy is so quiet that if someone detected even a couple of energetic neutrinos arriving from the same direction, it would almost certainly mean they were artificial.
 采用现有的望远镜去检测人为的中微子意味着要将自然产生的中微子从背景中剔除出去。幸运的是，自然产生的中微子多半是由恒星核反应所产生的，而且此类恒星中微子能量相当低。如果外星人制造出比恒星所发射的中微子还要高十亿倍能量的中微子粒子束（研究人员指出，这并非完全超出现有技术所能想象的范畴之外），那么背景噪音将淡化至消失。以足够高的能量范畴，星系中的其余部分显得如此安静，以至于甚至只要检测出来自同样方向的数个高能中微子，就基本能断定它们是人为制造的。

 

 Moreover, Dr Learned proposes a specific energy that aliens might favour. The magic number is 6.3 quadrillion electron-volts. (An electron-volt is the energy with which a one-volt battery can accelerate an electron.) A neutrino with this energy has a good chance of producing a particle known as a W- when it passes through a detector. The W- particle will then decay in a characteristic way, leaving an unambiguous record of the neutrino's passage.
 而且，伦德博士提出，外星人可能对某个确定的能级情有独钟。这个神奇数字是6.3亿亿亿电子伏特（一个电子伏特是指一个电子在1伏特的电压下加速所获得的能量）。当这一能级的中微子通过检测器时有很大概率产生一种被称为W-的粒子。该W-粒子以一种独特的方式衰变，能留下明显的中微子轨道痕迹。

 

 This is not the first time that beams of neutrinos have been proposed as a means of interstellar communication. But past suggestions required enormous energies (of the order of the entire output of the sun) to create a sufficiently intense beam. Dr Learned and Dr Zee have come up with a design for a particle accelerator that would do the job a good deal more modestly, using another type of subatomic particle, the pion, as an intermediary. According to their estimates, this should require no more energy than a typical Earth-bound power station can provide. Aliens might therefore be willing to give it a go―as, indeed, might humanity, if that were thought wise.
 使用中微子束作为星际通信手段并非是首创。但过去的方法要求用巨大的能量（要求太阳整体能量输出的同等能级）去制造足够强度的粒子束。伦德博士和徐博士已经提出一种粒子加速器的设计，用小得多的适宜能量就能把此项工作做好，它采用的媒介是另一种亚原子粒子――介子。据他们测算，其需要的能量将不会超过一座常规地面发电站所能提供的能量。如果那是个好办法，外星人则可能愿意尝试一下――实际上，人类也不妨一试。

 

 Once a civilisation has mastered the trick of generating high-energy neutrinos, though, Dr Learned's imagination suggests it might signal its existence to the waiting universe another way―and this is where the existing astronomical archives come in. For 100 years astronomers have paid particular attention to a class of variable star known as the Cepheids. These are important because they produce regular pulses of light and the period of those pulses is precisely related to the luminosity of the star. A Cepheid's distance can thus be calculated with great accuracy. And, since individual Cepheids are bright enough to be seen in other galaxies, they were the first tool used to estimate the distances to such galaxies.
 然而在伦德博士的设想中，一旦某个文明精通了制造高能中微子的技巧，它可能成为在茫茫宇宙中标识自身存在的另一种手段――也就是现存的天文档案该发挥作用的时候了。一个世纪以来，天文学家对所谓的"造父变星"注【1】格外关注。其之所以重要是因为它们发出有规律的光脉冲，而且这些脉冲的周期与该星的亮度精确对应。因而一颗"造父变星"的距离能够由此以很大的精确度得以计算出来。且由于在另一个星系中的单颗"造父变星"的亮度就足于使其被观测到，所以它们是测算类似星系距离的首选工具。

 

 Dr Learned, however, reckons the pulse-period of a Cepheid could be modulated by a suitable beam of neutrinos. Such a beam would easily penetrate the star's outer layer, but the stellar core would be sufficiently dense to absorb part of it. That would be enough to heat the core slightly and this heating would, in turn, accelerate the Cepheid's pulse rate. Time the beam right and the star could be turned, in effect, into an FM transmitter―broadcasting to the universe on the underlying carrier-wave of the Cepheid's pulsation.
 而且，伦德博士测算出"造父变星"的脉动周期能够用适当的微中子束加以调整。这样的粒子束将轻而易举地穿透恒星外壳，但足够致密的星核能够将其部分吸收。这将足于稍微加热星核，而其温度上升将随之加速"造父变星"的脉动率。只要恰当地控制粒子束的照射时间，该恒星将有效地成为一台调频电台――以"造父变星"的脉冲为基础载波向宇宙广播。

 

Tickling a star
 "燎"动星辰

 

 Admittedly, the energy needed to produce a Cepheid-modulating neutrino beam really would be a sizeable fraction of the output of the sun. But putting aside such minor engineering challenges, the point Dr Learned is keen to make is that because Cepheids have been so thoroughly examined, looking for transmissions from the Betelgeuse Broadcasting Corporation might just be a matter of re-examining the archives. If an intergalactic version of "Yesterday in Parliament" showed up in such a trawl it might not demonstrate the existence of truly intelligent aliens. By contrast, coverage of the cricket on "Test Match Special" would surely be proof positive.
 应当承认，制造用来调整"造父变星"的中微子束所需要的能量确实将相当于太阳整体输出能量的大部分份额。但是且不管这个次要的能量障碍，伦德博士急于表述的重点是，由于"造父变星"已经被如此严密地审查过，搜寻来自"参宿四广播公司"的节目所要做的仅仅是要对档案进行重新审核而已。如果星际版"昨日议会"的节目出现在这样的拉网式搜索中，可能还不能证明存在真实智能的外星人。相反，板球的"预赛专题"新闻报道必将成为确凿的证据。

 

 

 

 注【1】：
 造父变星是一种周期性脉动的变星，它的光变周期与绝对星等的变化具有确定的关系，即周光关系。仙王座δ是这类变星中第一颗被证认出的，由于它的中文名是造父一，因此这类变星得名"造父变星"。造父变星利用周光关系可以测量恒星和星系的距离。