Finding the Invisible

When was the first time you ever heard of a black hole? 

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​In this book, I will trace the history of the concept of black holes, exploring the evolution of ideas, misconceptions, and physics that led to the discovery of the most fantastic objects in the entire universe.

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You can find the first few chapters here.

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The passcode for the page is the answer (in all caps) to the question:

 

"Which astronomical objects are known as the lighthouse of the sky?"

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