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One interesting thing about black holes is that there's a point at which photons will actually orbit, meaning that you could look at the back of your own head if you were there. Of course, you'd be stretched into spaghetti shortly afterwards.

However, it would be possible to bend light into a circle under Earthlike conditions. How, you ask? It's simple. Even Earth's weak gravity can bend light a little bit. So, if the world in question had a large enough radius, it could bend light into a circle.

How large? About 8.257e+15 meters, which is 0.8727 light-years. This world would mass 1.001e+43 kilograms, or about 5 trillion solar masses. Finally, the world would have a density of 4.245e-09 grams per cubic centimeter.

Of course, since this place would be so big, you'd be looking at yourself from 5 years, 5 months, and 24 days ago. So if you want to look at the back of your head, you'd be better off using a webcam.

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