In 1919 when the gravitational defl ection of light was measured, why did the scientists travel to Africa and South America? How likely is it for a black hole to collide with Earth? Would we have much...

In 1919 when the gravitational defl ection of light was measured, why did the scientists travel to Africa and South America?

How likely is it for a black hole to collide with Earth? Would we have much warning?

Why is it diffi cult to test models of general relativity experimentally?

 We mention in the text that gravitational redshifts can be observed and measured during the collapse of a star into a black hole. When might the redshifts cease?