How do we know for certain all the "facts" about black holes (singularity, time stops, light can't escape) if we can't go in one and the closest one is so far away? originally appeared on Quora, the place to gain and share knowledge, empowering people to learn from others and better understand the world. You can follow Quora on Twitter, Facebook, and Google Plus.

Actually, we know for certain everything about black holes, except for one (somewhat important) thing.

You see, black holes are theoretical constructs. They are mathematical entities that arise in certain solutions of the field equations of general relativity. Because they are mathematical constructs, we can study their properties in detail. We can make exact statements about them, which can be validated, proven using mathematical techniques.

So what is it, you ask, that we don’t know about them? Well, one little detail, that is, do they actually exist? Are there actual objects in this universe of ours that behave in accordance with the mathematical model?

The jury is still out on this one. Sure, we have many black hole candidates. But are they really black holes? For that, we’d have to establish for sure that they are confined within their respective Schwarzschild horizons and that gravity behaves in their immediate vicinity as the theory predicts.

This is why The Event Horizon Telescope: an on-going campaign to obtain a radio telescope “image” of the “shadow” cast by the Milky Way’s supermassive black hole, Sgr A*, is so important. If this effort is successful, and we find that the object indeed occupies a very compact region of space, that would go a long way towards demonstrating that objects in this universe, such as Sgr A*, do indeed behave like the black holes of the mathematical theory.

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