The human species is soon to see the first picture of that mysterious, iconic, invisible object Princeton physicist John Archibald Wheeler dubbed a black hole that lurks at the center of our Milky Way galaxy. On Wednesday, astronomers across the globe will hold “six major press conferences” simultaneously to announce the first results of the Event Horizon Telescope (EHT), a virtual telescope with an effective diameter of the Earth—that has been pointing at the Milky Way’s central supermassive black hole for the last several years.
What will we actually “see”? Astronomers began speculating about these omnivorous “dark stars” in the 1700s, and since then indirect evidence has slowly accumulated. Scientific speculation about these enigmatic objects has ranged from their giving the birth to our cosmos to being portals to another universe.
Indeed, researchers at Canada’s Perimeter Institute for Theoretical Physics propose that the Big Bang could be the three-dimensional “mirage” of a black hole, collapsing star in a universe profoundly different than our own.
One eminent skeptic, physicist George Chapline at Lawrence Berkeley National Laboratory predicted that we’ll soon find that it does not exist, while acknowledging there are objects that general relativity would predict are black holes at the centers of galaxies. “Ironically, Einstein also didn’t believe in black holes even though he created general relativity.”
From the point of view of quantum mechanics, Chapline wrote in 2014 in Physics Today, black holes cannot exist because quantum mechanical evolution does not allow for the destruction of information and because black hole spacetimes do not provide a universal time. At the present time there is no astrophysical evidence that event horizons exist in nature, says Chapline.
“It is, of course, incontrovertible that compact objects exist whose size approximates the event-horizon radius predicted by classical general relativity. The pregnant issue is whether matter falling onto the surface of such an object encounters an event horizon where nothing remarkable occurs or whether it encounters a real surface. One might consider distinguishing an event horizon from a real surface by observing whether matter falling onto a compact object produces x rays.”
The best opportunity for seeing an asymmetric gamma-ray spectrum, a distinct signature for the absence of an event horizon, suggests Chapline, may arise from matter falling onto Sagittarius A*. Although the background of other gamma-ray sources in the central region of our galaxy makes it difficult to see this signature under ordinary circumstances, the EHT may be able to detect the feature because of the infall of a gas cloud that is now approaching SgA*.
“More than 50 years ago, scientists saw that there was something very bright at the center of our galaxy,” Paul McNamara, an astrophysicist at the European Space Agency and an expert on black holes, AFP’s Marlowe Hood. It has a gravitational pull strong enough to make stars orbit around it very quickly—as fast as 20 years, compared to our Solar System’s journey, which takes about 230 million years to circle the center of the Milky Way.
“The event horizon”—a.k.a. the point-of-no-return—”is not a physical barrier, you couldn’t stand on it,” McNamara explained. “If you’re on the inside of it, you can’t escape because you would need infinite energy. And if you are on the other side, you can—in principle.”
At its center, the mass of a black hole is compressed into a single, zero-dimensional point. The distance between this so-called “singularity” and the event horizon is the radius, or half the width, of a black hole.
The EHT that collected the data for the first-ever image is unlike any ever devised. “Instead of constructing a giant telescope—which would collapse under its own weight—we combined several observatories as if they were fragments of a giant mirror,” Michael Bremer, an astronomer at the Institute for Millimetric Radio Astronomy in Grenoble, told AFP.
At its center, the mass of a black hole is compressed into a single, zero-dimensional point. The distance between this so-called “singularity” and the event horizon is the radius, or half the width, of the black hole.
In April 2017, eight such radio telescopes scattered across the globe—in Hawaii, Arizona, Spain, Mexico, Chile, and the South Pole—were trained on two black holes in very different corners of the universe to collect data. Studies that could be unveiled next week are likely to zoom in on one or the other. Oddsmakers favor Sagittarius A*, the black hole at the center of our own galaxy that first caught the eye of astronomers.
Sag A* has four million times the mass of our sun, which means that the black hole is generates is about 44 million kilometers across. That may sound like a big target, but for the telescope array on Earth some 26,000 light-years (or 245 trillion kilometers) away, it’s like trying to photograph a golf ball on the Moon.
The other candidate is a monster black hole—1,500 times more massive even than Sag A*—in an elliptical galaxy known as M87. It’s also a lot farther from Earth, but distance and size balance out, making it roughly as easy (or difficult) to pinpoint. One reason this dark horse might be the one revealed next week is light smog within the Milky Way.
“We are sitting in the plain of our galaxy—you have to look through all the stars and dust to get to the center,” said McNamara.
The data collected by the far-flung telescope array still had to be collected and collated. “The imaging algorithms we developed fill the gaps of data we are missing in order to reconstruct a picture of a black hole,” the team said on their website.
Astrophysicists not involved in the project, including McNamara, are eagerly—perhaps anxiously—waiting to see if the findings challenge Einstein’s theory of general relativity, which has never been tested on this scale.
Breakthrough observations in 2015 that earned the scientists involved a Nobel Prize used gravitational wave detectors to track two black holes smashing together. As they merged, ripples in the curvatures of time-space creating a unique, and detectable, signature. “Einstein’s theory of general relativity says that this is exactly what should happen,” said McNamara.
But those were tiny black holes—only 60 times more massive than the Sun—compared to either of the ones under the gaze of the EHT. “Maybe the ones that are millions of times more massive are different—we just don’t know yet.”
The images at the top of the page shows some examples of the simulated shadow of the event horizon of a black hole. The images from this simulation demonstrate what we expect to see in 1.3-mm emission in eventual images from the Event Horizon Telescope. (Adapted from Medeiros et al. 2018)