问HN：黑洞合并中的质量损失？这是怎么回事？
假设我们有两个黑洞，一个质量为20个太阳质量，另一个为50个太阳质量，它们靠得足够近并合并。在这个过程中，它们将一到两个太阳质量的物质转化为引力波，这些引力波向远处辐射出去。
让我重申一下，以强调我认为非常奇怪的地方：一到两个太阳质量的物质是在事件视界内的，却被转化为能量并辐射到事件视界外。
E=mc²。这部分我并不觉得困扰。让我困扰的是“在事件视界内”到“在事件视界外”的这一部分。有人能用简单的方式解释一下这是如何可能的吗？（或者更深入一些，适合有物理学本科背景的人。）
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Let's say we have two black holes, one 20 solar masses and the other 50 solar masses, and they get close enough and merge. In the process, they convert a solar mass or two into gravitational waves, which radiate off into the distance.<p>Let me restate that, to emphasize what I think is really weird: A solar mass or two <i>that is inside the event horizon</i> got converted to energy <i>and radiated outside the event horizon</i>.<p>E=mc^2. That part doesn't bother me. It's the "inside the event horizon" to "outside the event horizon" part that bothers me. Can someone ELI5 how that is possible? (Or even ELI20 with an undergrad in physics.)