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发表于 2005-11-22 16:30:32
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http://boinc.equn.com/einstein/ask/archive/andrew-spain.html
In terms of interferometry, is the baseline between LIGO and the German detector long enough to be able to pinpoint the source of the waves? The Einstein@home screensaver displays the location on the sky of where it is searching, so how is this directionality achieved? This seems to imply that it is an active system rather than just a passive one of detecting waves and then trying to pinpoint them. Or have I misunderstood?
Submitted by Andrew from Spain
用干涉测量的术语来说，LIGO和德国探测器间的基线长度是否足够长，从而能精确定位引力波源的位置？Einstein@home的屏保中显示了一个天空中的当前探索位置，这个位置是怎么来的？这让人感觉它是一个主动的观测系统而不是一个被动的、先探测到波然后试图定位波源的系统。是我理解错了吗？
－－由西班牙的 Andrew 提交
Physically, LIGO is passive. It sits where it is, and that's it. The “pointing” is done in the processing of the data.
物理上来讲，LIGO是被动的。造在哪就在哪了。所谓的“指向”仅存在于数据处理的过程中。
How that's done is different for different types of sources. You might guess that with two LIGO sites, something like triangulation could be used to pick up a direction. That's basically true for short-lived signals, although the directionality even then is not too great. LIGO is more like an ear than an eye, since the wavelengths are long compared the size of the detector, and if you try with your eyes closed you'll find that you can't localize sounds nearly as well as sights.
具体的过程依赖于波源的类型。你可能会猜测：造两个LIGO，然后使用类似三角测量的方法来确定一个方向。对于短期的信号来说，基本可以这样认为，虽然这样得到的方向性并不太好。LIGO更象是耳朵而不是眼睛，因为引力波的波长要比探测器的尺寸来得大，如果你试图闭上眼睛，除了看不见，你也会发现没法定位声音的来源。（？）
But long-lived periodic signals - the ones Einstein@Home is searching for - are another matter. Even if you start with something (like a bump on a rotating neutron star) that gives off a perfect sine wave signal, it won't stay that way by the time it gets into the data stream. The detectors are stuck to the Earth, which spins in little circles every day and big circles every year. That motion changes (Doppler shifts) the frequency of the signal in a complicated way that is a function of time and also of position on the sky. For example, a source located over the North Pole will not be Doppler shifted by the daily rotation but will be affected by the Earth's orbital motion. And this doesn't depend on having multiple detectors with a long baseline between them, although that's good for other things; it just depends on the Earth moving.
但是对于 Einsein@home 正在搜寻的长期信号来说就不一样了。即使你开始时探测到一个完美的正弦波信号，等到记录它的时候可就是另外一个样子了。探测器固定在地球上，而地球除了每天绕着自身转一圈，每年还要绕着太阳转一个大圈。这种运动使得信号的频率变得很复杂（多普勒偏移），依赖于时间和当时在天空中的位置。举例来说，在北极上空的波源不会因为每日的自旋而产生多普勒偏移，但却会被地球绕太阳运行所影响。另外，它也并不依赖于在探测器间是否有足够长的基线，虽然后者在其它方面有好处，它仅仅依赖于地球的运动。
Those complicated Doppler shifts are where the angular resolution comes from. The data analysis has to compensate for the Doppler shift to make any signal as sinusoidal as possible, which helps pull it out of the noise (with something based on a Fourier transform). It has to do one Doppler shift for one sky location, then Fourier transform to see if there's something there; another Doppler shift for another sky location, then Fourier transform; and so on. For an in-depth search, even a small change in sky location makes the Doppler shift different enough to completely wash out any signal if done wrong. The net result is a lot of sky positions to search.
有了这些复杂的多普勒偏移，就有了角分辨率的问题。数据分析中比较对多普勒偏移进行补偿以使信号尽可能地接近正弦曲线，以方便从噪音中提取真正的信号（通过傅立叶变换）。对于任一天空位置，都必须做一次多普勒偏移补偿，再进行傅立叶变换才能知道是否有我们需要的信号；对于另一个位置，同样也得先进行多普勒偏移补偿再进行傅立叶变换，等等。对于深入的研究，即使是天空中位置的小小变换，相应的多普勒偏移也是不同的，如果处理不对，什么有用的信号都观测不到。最后的结果就是我们有大量的天空位置需要进行搜寻。
So while the raw data contains (presumably) signals from all over the sky, the processing code picks a single point on the sphere, corrects for the Doppler effect at that point, and looks for periodic signals. The way the correction is done washes out anything that is not coming from very near that point, so then the code repeats for another nearby point, and another, and another....
因此，原始的数据里面包含了全天空的信号，分析代码在天球上选择一个点，对其进行多普勒偏移补偿，然后寻找周期性的信号。除了该点附近相当小的区域，其它部分的信号在这个校正过程都被去除掉了，然后代码再挑选另外一个点进行分析，再另外一个，再...
There's the rub: A more sensitive search needs more sky positions, which means more CPU cycles. Thus Einstein@Home.
然后问题来了，更灵敏的搜寻需要更多的天空位置，也就是需要更多的计算能力，于是就有了 Einstein@Home 项目 ;)
中间有（？）那段的最后一名话没理解他的意思。 |
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发表于 2006-1-13 21:36:20