The population of stellar mass black holes
GW170104 is the third confirmed direct detection of gravitational waves, and the fourth member (including the candidate event LVT151012) of our growing family ‘portrait’ of heavy black hole binary systems with directly measured masses. Figure 2 also shows how the estimated masses for the GW170104 black holes compare with those of the other three events; we see that GW170104 sits neatly in the gap between GW150914 and LVT151012. Moreover, its detection has improved our estimate of the global rate at which black hole mergers occur. Although this rate is still quite uncertain, it already appears mildly inconsistent with some astrophysical models for how black holes might form and merge.
图 2 还显⽰了 GW170104 对应双⿊洞系统的估计质量与其它三个事件对应源的质量相⽐;我们看到 GW170104 刚好介于 GW150914 和 LVT151012 之间。此外，对它的探测改善了我们对⿊洞并合事件发⽣率的全球估计。虽然这个发⽣率还是很不确定的，但如何形成⿊洞并进⾏合并这⼀过程似乎与⼀些天体物理学模型略有不同。
A further handle on these formation models can be obtained from measurements of black hole spin – since, for example, they make different predictions about how well aligned with each other the
black holes’ axes of rotation should be. Although these spin
parameters are not yet very well constrained, our observations do already hint towards a possible tendency for the spin axes in a
merging binary system to be misaligned.
Testing General Relativity
The addition of a third, confirmed gravitational-wave detection has also improved our ability to test some fundamental aspects of general relativity (GR). Combining our new observations with those
of GW150914 and GW151226, we compared them with some
specific GR predictions for how the waveforms should evolve, and searched for any systematic departures from those GR predictions in the data. The results were consistent with our previous findings (see here and here), meaning that Einstein’s theory once again passed the test with flying colours!
第三次成功检测到引⼒波也为检验⼴义相对论（GR）提供了更多实验依据。将我们的新观测结果与 GW150914 和 GW151226 的观测结合起来，我们将其与 GR 预测的波形演化进⾏了⽐较，并搜索数据中是否存在对 GR预测的偏离。所得结果与我们之前的研究结果⼀致（见这⾥和这⾥），这
The large distance of GW170104 also allowed us to test another prediction of GR: that gravitational waves travel at the speed of light and are non-dispersive. In some situations a wave can be dispersed as it travels through a material, meaning that the wave becomes ‘spread out’ because components with different frequencies travel at different speeds. (Everyday examples of this phenomenon include
the spreading out of white light into a rainbow, or the distortion of e.g. sounds heard underwater in a swimming pool. On the other
hand, to a good approximation sound waves are not dispersed very much as they travel through the air in a concert hall; if they were, then the audience at an orchestral concert would hear the notes
from the piccolo and the double bass arrive out of step with each other).
GW170104 的⼤距离传播还给我们提供了测试另⼀个 GR 预测的机会：引⼒波以光速传播，⽆⾊散。在某些情况下，当波穿过材料时，波会发⽣⾊散，这意味着由于具有不同频率的单⾊波以不同的速度传播，波会“分散展开”。 （这种现象的⽇常⽣活中很常见，例如：将⽩光发散成彩虹，在游泳池⽔中听到失真的声⾳，另⼀⽅⾯，为了更好地接近原声，声波在传播时不会分散的很厉害⽐如⾳乐在⾳乐厅空⽓中的传播; 否则管弦乐演奏会上的观众会听到短笛声和双重低⾳的相互⼲扰）。
According to GR, however, the gravitational waves from GW170104 should not have been dispersed as they travelled across billion of light years to reach us. To test this we considered a simple model for
the dispersion, motivated by some alternative theories to GR in which the phenomenon is predicted to occur, and compared these predictions with our GW170104 observations, again combining with those of GW150914 and GW151226.
根据⼴义相对论，GW170104 的引⼒波经过⼗⼏亿光年的传播到达地球时不应该有⾊散。为了测试这⼀点，我们考虑了⼀个简单的⾊散模型，选择⼀个⾮ GR 的引⼒理论，这种理论预测引⼒波会有⾊散发⽣，并将这些预测与 GW170104 以及之前的GW150914 和 GW151226 观测进⾏⽐较。