Archive for October, 2010


The Great Filter

With no evidence of intelligent life other than ourselves, it appears that the process of starting with a star and ending with “advanced explosive lasting life” must be unlikely. This implies that at least one step in this process must be improbable. Hanson’s list, while incomplete, describes the following nine steps in an “evolutionary path” that results in the colonization of the observable universe:

1. The right star system (including organics & potentially habitable planets)
2. Reproductive molecules (e.g. RNA)
3. Simple (prokaryotic) single-cell life
4. Complex (archaeatic & eukaryotic) single-cell life
5. Sexual reproduction
6. Multi-cell life
7. Tool-using animals with big brains
8. Where we are now
9. Colonization explosion

According to the Great Filter hypothesis at least one of these steps – if the list were complete – must be improbable. If it’s not an early step (i.e. in our past), then the implication is that the improbable step lies in our future and our prospects of reaching step 9 (interstellar colonization) are still bleak. If the past steps are likely, then many civilizations would have developed to the current level of the human race. However, none appear to have made it to step 9, or the Milky Way would be full of colonies. So perhaps step 9 is the unlikely one, and the only thing that appears likely to keep us from step 9 is some sort of catastrophe. So by this argument, finding multicellular life on Mars (provided it evolved independently) would be bad news, since it would imply steps 2–6 are easy, and hence only 1, 7, 8 or 9 (or some unknown step) could be the big problem.

Although steps 1–7 have occurred on Earth any one of these may be unlikely. If the first seven steps are necessary preconditions to calculating the likelihood (using the local environment) then an anthropically biased observer can infer nothing about the general probabilities from its (pre-determined) surroundings.

尽管宇宙中的绝大部分星系都在逐渐远离银河系,但离我们最近的仙女系,正以120km/s的速度接近银河系。

在接下来的20亿年,仙女系将和银河系碰撞,在潮汐力的作用下,2个星系的外缘将会变形,成为巨大的潮汐尾。

如果最初的变形确实发生,天文学家估计,太阳系有12%的几率被吸入银河系的潮汐尾,3%的几率逐渐地因为万有引力被仙女系吸引,从而成为仙女座的一部分。

在接下来的一系列碰撞中,太阳系从银河系脱离的几率提高到30%,最终在70亿年,2个星系中部的超级黑洞将融合,银河系和仙女座将融合成为一个新的巨型椭圆星系。

在这个融合过程中,如果有足够的气态物质存在,逐渐增强的引力将迫使气态物质向星系中部集中,这将导致一个短期并密集的新星体诞生期,被称为星爆。此时中部的气态物质将会填充刚刚融合的黑洞,将其变成一个活动星系核。这个互动过程可能将整个太阳系推到新星系的外缘光环区,太阳系相对来说会避开星系碰撞时的辐射。

很多人认为,这种大规模的星系碰撞会破坏太阳系内部星体的轨道。确实会有这种情况发生,例如一个大质量星体会导致小型星系内的星体脱离轨道,逃逸到外太空。但星体之间的距离是如此之远,银河系与仙女系的碰撞造成太阳系星体脱离轨道的概率可以忽略不计。也许整个太阳系会被影响到,但星系内部的星体是不会被干扰到的。

尽管如此,经过时间以及概率的累积,新的星系内产生越来越多的星体,太阳系内部星体被干扰的可能性越来越来,直到不可避免。太阳系中的太阳也许能幸存,但最终,在10 x10的17次方年,已经死去的太阳及其他星体,将被附近的星体撕裂。整个太阳系,从任何角度来说,都将不复存在。

-Wikipedia The Free Encyclopedia