Black holes

Black hole Black hole and the Wolf–Rayet star, spiral galaxy NGC 300 (artist’s impression) - for more details click here. © ESO/L. Calçada.

By Brian Greene

Professor of Physics, Columbia University

What is a black hole?

Black holes are one of nature's most extreme creations. They are regions of space endowed with such a powerful gravitational pull that anything which ventures too close, including light itself, is unable to escape. The possibility of such objects was contemplated as far back as the 18th century, but the modern study of black holes was initiated by Einstein's insights into the gravitational force, embodied in his General Theory of Relativity.

What does it take to create a black hole?

As stars burn up their nuclear fuel, they may collapse under their own weight—creating black holes. To create one out of the Sun, you’d need to squeeze the Sun to less than 2 miles across; for the earth, you’d need to squeeze it to half an inch across!

How do we now know they exist?

Because they’re black, you can’t generally see black holes directly, but their existence can be affirmed based on how they affect their surroundings. For instance, as hot gas streams toward a black hole, it can shoot out x-ray radiation, some of which may be observable here on Earth.

What do you think is the most interesting thing about black holes?

Black holes dramatically warp and curve space and time in their vicinity. Near the edge of a black hole—near what we call its horizon—time passes ever more slowly compared to how it elapses far from the black hole. And, in a manner we are still trying to figure out, time may come to an end at the central point of a black hole. When we understand this later point fully, I believe our understanding of the cosmos will take a profound leap forward.