Curving Spacetime: Mind-Boggling Facts about Black Holes

Daniel Hautzinger
An illustration of a black hole. Image: Courtesy WGBH
An illustration of a black hole. Image: Courtesy WGBH

NOVA: Black Hole Apocalypse airs Wednesday, January 10 at 9:00 pm and is available to stream.

The two-hour NOVA special Black Hole Apocalypse, hosted by the multi-talented astrophysicist and novelist Janna Levin, explores one of the most confounding, unbelievable objects known to humans: black holes. In case two hours isn’t enough gravitational curving of spacetime for you, here’s some more mind-boggling information about black holes.

Black holes are a curvature in spacetime. They are an object where gravity is so powerful that spacetime – a fabric of the three dimensions of space plus the fourth dimension of time shown to be linked by Einstein’s theory of relativity – is bent so far that it becomes a hole. Not even light can escape the gravitational pull of a black hole.

The existence of astronomical objects that even light couldn’t escape was first proposed in the 18th century. In 1783, the English clergyman and scientist John Michell posited the existence of an object whose gravity was so strong that even light couldn’t escape, imagining these as massive stars that he called “dark stars.” 13 years later, the French scientist and mathematician Pierre-Simon Laplace came up with a similar idea.

An illustration of a black hole curving spacetime. Image: Courtesy WGBHAn illustration of a black hole curving spacetime. Image: Courtesy WGBH

Black holes were first proposed in 1915, but were considered only a mathematical oddity until the 1960s. Einstein’s 1915 theory of general relativity implied the existence of black holes, but he himself doubted that they could exist. Several solutions of general relativity in the following decades showed the possibility of black holes, but it wasn’t until the 1960s, with the discovery of such astronomical curiosities as neutron stars and their subcategory pulsars, that black holes were believed to be a real physical object. The first use of the name “black hole” in print was by journalist Ann Ewing in 1964.

An object pulled into a black hole is “spaghettified.” Yes, that’s a technical term. “Spaghettification,” also known as the “noodle effect,” is a description of the stretching caused by the intense gravity of a black hole. Because the gravitational pull is stronger the closer an object gets to the center, the pull would be more powerful on the bottom of an object than on the top, thus stretching it vertically and squeezing it horizontally – just like a long, thin piece of spaghetti.

A “gravitational singularity” sits at the center of a black hole. This is a point where the curvature of spacetime is infinite (I know, it’s hard to comprehend). In a specific type of theoretical black hole, an object will be crushed into infinite density and added to the black hole when it reaches the gravitational singularity.

There is a supermassive black hole at the center of the Milky Way. 

No one know what black holes look like. Because they are relatively small, despite their power, and because they are generally surrounded by bright objects drawn by their gravitational pull, black holes are difficult to image. They are usually found by looking for their effects: clusters of stars with unusual orbits around one region. But in April 2017, eight telescopes around the world all pointed at the same spot – the supermassive black hole at the center of the Milky Way – in effect creating a telescope the size of Earth, to attempt to capture an image of a black hole. The data is still being stitched together – each telescope generated so much data that it was more efficient to fly it by plane than to transfer it over the Internet – but is expected to be published in 2018.

But we can “hear” them. On September 14, 2015, the Laser Interferometer Gravitational-Wave Observatory (LIGO) detected a “cosmic chirp,” a gravitational wave created by the collision of two black holes around 1.3 billion years ago. Gravitational waves were, like black holes, first proposed by Einstein’s theory of relativity; they are vibrations in spacetime caused by the acceleration of mass, but only massive cosmic events such as the collision of black holes generate a wave powerful enough to travel far. Scientists have long tried to prove the existence of these waves, but it took decades for the technology to be refined. While gravitational waves are not sound waves, their frequency is equivalent; hence the “chirp.” The detection of one by LIGO, the most expensive project ever funded by the National Science Foundation, was a revolutionary discovery that won the Nobel Prize in Physics in 2017. It could open up new worlds of inquiry and an entirely novel way of learning about the universe, which humans have previously only explored through light.

One arm of the LIGO detector in Hanford, Washington. Photo: Courtesy WGBHOne arm of the LIGO detector in Hanford, Washington. Photo: Courtesy WGBH