太空旅途中的解闷音乐电台- 遥远巨人的脚步声 The Radio During Space Travel

随着人类最近对着我们的邻居行星发射了各种各样的仪器，许多研究组织也在考虑在未来10年里向我们太阳家族中的那些巨型冰球们——天王星和海王星——发射些东西。

With scientists sending all sorts of things to our neighbouring planets recently, numerous organisations have also been considering missions in the coming decade to those ice giants in our Solar family - Uranus and Neptune.

但，想象一下，你是一个科学家，刚刚向太空发射了一个任务是到达那些最遥远的行星的探测器。在所有成功发射的兴奋之后，在探测器到达那里的10年里，你能做些什么呢? 不会是干等吧 

However, image you are a scientist who just sent a probe into space with the mission of arriving at those furthest planets. After all that excitement of a successful launch, what do you do for the next decade before the probe gets there? Just wait there?

好吧，我有个主意：

在收音机里听太空音乐 - 引力波！

Well, here is an idea: listen to music of space on the radio! The gravitational waves!

那这用娱乐方式是否可行呢？

让我们一起来探索一下咯!

Whether this might be a potential entertainment method, let’s find out!

图为太阳系的一个艺术表达

An artist concept of the solar system

巨型收音机的诞生

Creation of the Giant Radio

引力波是宇宙中那些最剧烈和最具能量的‘物种’产生的，例如移动的黑洞，它们会在空间中产生小小的涟漪。

Gravitational waves are the tiny ripples in space caused by some of the most violent and energetic processes in the Universe, for example, the motion black holes.

当引力波经过时，两个自由物体之间的距离会有非常小的波动。因此，利用遥远的探测器来探测这种微小波动的想法从1977年就开始了，当时NASA发射了“旅行者”号宇宙飞船，目标木星、土星、天王星和海王星。

As a gravitational wave pass by, there would extremely small fluctuations of the distance between two free bodies. Therefore, idea of using faraway probes to detect this tiny fluctuation has begun since 1977, when the Voyager spacecrafts by NASA were launched, targeted at Jupiter, Saturn, Uranus and Neptune.

太空中的所有探测器都会与地球保持着不间断的联系。对于NASA的探测仪来说，接听这些长途电话的便是他们的深空网络(Deep Space Network - DSN)。那是一系列遍布世界各地的巨型无线电天线，不断观察着太空任何的新变化。

All probes in space are in constant contact with the Earth, for NASA, it would be the Deep Space Network (DSN) that’s taking the long-distance calls. That is a group of giant radio antennas around the world, constantly looking up into space any new updates.

由于深空网络与那些外太空探测器不断联系的距离常年超过10个天文单位（15亿公里），它们的系统可以被看作是一个地面干涉仪的巨大单臂，就像LIGO和Virgo 天文台一样，可以探测引力波。

With the DSN and outer space probes constantly contacting over 10AU (1.5 billion km), their system can be considered as the giant single arm of a ground-based interferometer, like LIGO and Virgo, detecting for gravitational waves.

图为来自GW190521的数值相对论模拟，显示了黑洞合并后的引力波，刚好可以在最中心看到最初黑洞的轨迹和最后黑洞的视界

An image from a numerical relativity simulation for GW190521 showing the gravitational waves just after a black hole merger, with the trajectories of the initial black holes and the horizon of the final black hole just visible in the center

呲啦 呲啦

Bzzz Bzzz

干涉仪一般是用来测量两个路径长度之间的相对位移的，这里就可以利用多普勒效应来跟踪每个探测器与地球之间距离的变化。
Interferometer are used by measuring the relative displacement between two path lengths, tracking the distance changes of the single probe can be done by applying the doppler effects.

深空网络和探测器之间会通过一个单一的无线电频率进行对话，当探测器在靠近或离开地球时，如果遇到引力波的碰撞，无线电频率会轻微的升高或降低。

Between the DSN and the probe, they talk with a single radio frequency that shifts slightly to higher or lower values when the probe experiences a bump while moving towards or away from Earth.

经过的引力波会在不同的时间使探测器和地球产生颤动，从而使深空网络接收到的信号会因为多普勒频移产生相应的波动。但不仅仅是这两个颤动，深空网络还会收到第三个信号变化，那便是来自于地球颤动从探测器反射回来的信号。

A passing gravitational wave would jitter both the probe and Earth at different times, resulting corresponding pulses on the signals received by the DSN via doppler shift. There would also be a third pulse on the signal originated from the Earth jitter and reflected back from the probe.

通常，第一个被探测到的信号波动是来自地球自己的，随后，来自探测器的信号才会到餐，但到达时间取决于引力波传播的角度，以及地球看探测仪的视角。被反射的信号是最后出现的，而反射回来的时间恰好是光在地球和探测器之间飞行的时间的两倍。

While the shake on the Earth is first detected, the change in signal from the probe arrives at a range of times, depending on the angle of the gravitational wave propagation and the Earth view on the probe. The reflection comes last but is back exactly at twice the light travel-time between the Earth and probe.

图为三个颤动带来的多普勒频移，它们的振幅和相对位置取决于引力波的到达方向、双向光时间、以及波的应变振幅和极化状态。到达顺序从左到右，依次是地球，航天器和由于地球抖振而反射回来的信号

The three pulses expected, their amplitudes and relative locations depending on the gravitational wave’s arrival direction, the two-way light time, and the wave’s strain amplitude and polarization state.  The order of arrival is the Earth, the spacecraft and he signal due to the Earth buffeting, reflected back respectively

调频好难呀

Turning for the Right Channel

尽管产生引力波的那些巨兽们都体型庞大，破坏力极强，但当引力波到达地球时，它们已经被岁月磨平了棱角，比当初弱了几千亿倍。

Although the beasts that generate these gravitational waves are humongous and very destructive, by the time the waves arrive at Earth, they are already thousands of billions weaker than first started.

比如当LIGO首次探测到的引力波到达地球时，它们产生的时空晃动量比原子里的原子核还要小上千倍!（LIGO是2002年发射的一项专门用于引力波研究的任务。）

In particular, by the time gravitational waves from LIGO's first detection reached us, the amount of space-time wobbling they generated was 1000 times smaller than the nucleus of an atom! (LIGO is a mission launched in 2002 dedicated for gravitational wave research.)

除了因为信号非常微弱而导致在实践中很难观察到到这三个信号波动之外。用于通信的无线电频率也不一定稳定。因此，现实中还必须采取一系列的程序来过滤掉所有的背景噪声，只挑选出引力波的多普勒位移。

Apart from the signal being tiny and therefore extremely difficult to see the three pulses in practice. The radio frequencies used to communicate is not always stable. Hence a series of procedures have to be taken to filter out all the background noise and pick out only gravitational wave doppler shifts.

为了尽可能地避免任何偏差，收集信号时必须仔细考虑探测仪相对于地球的位置，并从信号中减去其产生的噪声信号。即便如此，地球上的天线和太空中探测器的机械振动还是会引起多普勒频移。

To avoid offsets as much as possible, the spacecraft’s trajectory relative to the Earth must be carefully accounted for and subtracted from the signal. Even then, antennas on Earth and the space probes can have mechanical vibrations that cause doppler shifts.

通常在实验中，增加测试次数可以减少偶然误差对总体结果的影响。所以这里我们可以增加观测时间，这样就能探测到更多的引力波，得到更好的数据。但还有一个问题：地球是绕着太阳转的。

Often with experiments, increasing the number of tests can reduce the effect of random errors on the overall result. So here we can increase the time of observation so more gravitational waves can be detected and have better data. But here comes another problem: Earth is orbiting the Sun.

当太阳加入这个游戏时，要有高质量地追踪多普勒位移信号就不是件容易的事了。由于来自太阳风和地球电离层的不规则等离子体闪烁噪声，理想的‘通话’位置是地-日-探测器的角度大于150度，也就是说太阳位于地球和探测器之间。

Tracking the doppler shifts on the signal with enough sensitivity is not always possible when the Sun joins the game. Due to plasma scintillation noise from solar winds and irregularities in Earth’s ionosphere, the ideal position would be the Earth-Sun-Probe angle greater than 150 degrees, so the Sun between the Earth and probe.

这样，每年只有40天的观测时间，在这是长达10年的旅行中仅有10次机会。想象一下，在10个小时的车程中，只能听1个小时的音乐…

This leaves only a window of 40 days per year to observe, 10 chances for a decade-long journey. Imagen only being able to listen to music for 1 hour in total for a 10-hour drive…

图为与LISA和aLIGO(底部灰色线)与类似卡西尼(Cassini)的太空探测器的其他引力波探测器的低频引力波(y轴)敏感度(y轴)比较。(卡西尼是1997年，NASA、欧洲航天局和意大利航天局联合发射了一枚研究土星及其系统的太空探测器。)

The sensitivity (y-axis) to low-frequency (x-axis) gravitational waves of a space probe of a Cassini-like mission, compared to other gravitational wave detectors like LISA and aLIGO. (Cassini mission involving NASA, the European Space Agency, and the Italian Space Agency to send a space probe to study the planet Saturn and its system, launched in 1997.)

🎸摇滚还是古典🎻

Rock or Classical

尽管享受音乐的机会不多，但至少让我们看看在这段旅程中，我们能听到什么样的音乐吧。

Despite the little amounts of opportunities to enjoy the music, let us at least find out what sort of the music be possible for this journey.

由于探测器距离地球至少有几个天文单位(AU, 1AU ~ 0.15亿公里)，所以这种探测引力波的方法只适用于波长较长的引力波，或频率在毫赫兹范围内的低频引力波。这些隆隆声通常来自于超大质量黑洞双星（SMBH）或极端质量比例旋（EMRI）的合并，就是恒星质量的黑洞坍缩成超大质量的黑洞。

Since the probes are at least a few astronomical units (AU, 1AU ~ 0.15 billion km) away, our method of detecting gravitational wave is only suitable for those with long wavelengths, or low frequencies in the millihertz range. These rumbles are generally coming from mergers of Supermassive Black Hole binaries (SMBH) or Extreme Mass Ratio Inspirals (EMRIs), where a stellar mass black hole collapses into a supermassive black hole.

相比之下，专为引力波探测而设计的 LIGO 和 Virgo 正在寻找的频率就高的多了，从数十赫兹到数千赫兹的引力波都在它们接收范围内。这些高能量的引力波通常来自于恒星质量的黑洞和中子星的双星合并。

In comparison, LIGO and Virgo, which are observatories specially designed for gravitational waves, are in search for the waves with frequency from tens to thousands for hertz. These higher energy waves are generally from binary mergers of stellar mass black holes and neutron stars.

视频为Virgo探测到的GW190814过程模拟，是一个有23个太阳质量的黑洞和一个神秘的较轻物体的合并过程

An simulation of GW190814 observed by Virgo - a process of a merger of a 23-solar-mass black-hole and an enigmatic lighter object

随着引力波探测成为一个越来越重要的研究领域，ESA 和 NASA 现在正共同致力于一个新的太空引力波探测器任务 - LISA。计划于2034年发射的它，会专注于搜索低频引力波，并尽可能克服各种噪音，以实现更好的无线电通信。

With gravitational wave detection becoming more and more of an important research field, ESA and NASA are now also working on the mission of LISA - a space-based gravitational wave detector. Planning to be launched in 2034, it will be dedicated for the searched of low frequency gravitational waves and overcoming various kinds of noise as much as possible for the better radio communications.

图为欧洲航天局的LISA天文台，是一个研究引力波的多航天器任务，预计将于2034年发射。它将由三个部分组成一个跨越数百万公里的三角形编队，通过激光连接并探测通过的引力波。

The European Space Agency's LISA observatory, which is a multi-spacecraft mission to study gravitational waves expected to launch in 2034. It will be consisting three spacecraft in a triangular formation spanning millions of km, linked together by lasers to detect passing gravitational waves.

自1977年来，人类在物理学和科技上都有了巨大的突破，所以也许未来十年正是一个我们继续探索那些巨冰行星的好时机。

Since 1977, there has huge breakthroughs in physics and technology, so now could be a good time to continue our expeditions to those ice giants in the next decade.

尽管那会是个漫长的旅程，

但途中仍有免费享受

独一无二的引力音乐时刻。

何乐而不为呢?

Though it might a long journey, 

there would still be moments to enjoy 

the unique gravitational music for free. 

Why not try?

图为海王星的一张真实的图像，是在对ESO甚大望远镜上的MUSE/GALACSI仪器的窄场自适应光学模式进行测试时获得的。

The image of planet Neptune, obtained during the testing of the Narrow-Field adaptive optics mode of the MUSE/GALACSI instrument on ESO’s Very Large Telescope.

图片和视频来自 NASA, LIGO, Virgo 官网以及以下论文

文中部分英文信息参考来自Armstrong, J.W.的 ‘Low-Frequency Gravitational Wave Searches Using Spacecraft Doppler Tracking.‘ 论文 和 苏黎世大学的D Soyuer, L Zwick, D J. D’Orazio, P Saha 的 ‘Searching for gravitational waves via Doppler tracking by future missions to Uranus and Neptune’ 论文

其余中英文内容为原创

Images from official website of NASA, LIGO, Virgo and the below articles

Parts are sited from ‘Low-Frequency Gravitational Wave Searches Using Spacecraft Doppler Tracking.‘  from Armstrong, J.W. and ‘Searching for gravitational waves via Doppler tracking by future missions to Uranus and Neptune’ from D Soyuer, L Zwick, D J. D’Orazio, P Saha from University of Zurich

The rest of the Chinese and English content are original