First ever picture of a black hole may be revealed this week

A simulated view of a black hole
A simulated view of a black hole

Hotaka Shiokawa

What is it like to stare directly into the dark heart of our galaxy? We’re about to find out. The team at the Event Horizon Telescope (EHT) – a network of telescopes around the globe working together to make an image of a black hole – is going to release its first results on 10 April.

Taking a picture of a black hole is difficult because they don’t emit or reflect any light. So, what are we going to see? “They’re trying to get an image of the black hole’s shadow,” says Avi Loeb at Harvard University. Black holes are surrounded by bright material glowing as it falls in to their maw, and part of this material should be obscured by the black hole itself.

“It’s very different from the shadow cast by an opaque object because a black hole isn’t opaque, it’s absorbing light,” says Loeb. “As a result, we should see a dark inner region surrounded by a sliver of light that looks like a crescent moon.”

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That crescent shape is predicted by Albert Einstein’s theory of special relativity, which says that matter moving towards us will look brighter and anything moving away would be dimmed. We may also see the effects of the immense gravity of a black hole, says Loeb, in the form of gravitational lensing which can bend light as it passes nearby.

EHT is targeting two black holes, the biggest in the sky from our point of view. The first is Sagittarius A*, the supermassive black hole at the centre of the Milky Way, while the second is an even larger black hole at the centre of the Messier 87 galaxy, found in the constellation Virgo.

Despite this, the EHT pictures will be extremely small. Heino Falcke, an astronomer who works on the EHT, has said that the Sagittarius A* shadow is predicted to be about 50 microarcseconds wide. One microarcsecond is about the size of a period at the end of a sentence, if it were viewed from as far away as the moon.

The resolution of the EHT is 20 microarcseconds at best, he said. That means we will only see a very fuzzy picture of the two black holes – it won’t be anything like the artist renderings seen in films like Interstellar, according to Falcke, or even like the simulation above.

“It turns out that as big as we think a black hole would be, especially one we call supermassive, they’re actually very small on the sky,” says Dan Marrone, another astrophysicist who works on EHT. “We need the telescope to be Earth sized.”

That is why the EHT is not just one telescope, but is made up of telescopes all over the world synchronised to take data at the same time and in the same wavelengths. The data being released on 10 April will come from the 2017 operations, which included radio telescopes in the US, Chile, Spain, Mexico and the South Pole.

The images and data this concerted effort will produce could help us answer some of the biggest questions in physics. For one thing, we’ll have the first image of the environment around a black hole, which can help determine if our theories about their structure is correct.

“Since the early 1970s, people have tried to model how gas would accrete onto a black hole and there are a lot of uncertainties,” says Loeb. Those include the magnetic fields around a black hole, which may play a role in forming jets like the ones emanating from M87. These streams of radiation and charged particles form out of the disc of material around the black hole and travel close to the speed of light, but we’re not sure just how they come to be.

We’ll also get a better idea of whether black holes behave as Einstein’s general theory of relativity suggests. “If I was a betting person, I would say the results won’t contradict general relativity,” says Samir Mathur at Ohio State University.

The event horizon of a black hole is the front line of the battle between quantum mechanics and general relativity – our theories don’t agree on what really happens there. Mathur says this new data isn’t likely to clear things up, because the light EHT will see isn’t coming from the event horizon, but slightly further out – light truly at the horizon would be swallowed by the black hole.

Still, the image will be a great achievement, even if it isn’t really a surprise. “We have tested general relativity in these environments with gravitational waves, and the results pretty much agreed with what we expected. But in this case we will be seeing it, and seeing is believing,” says Loeb.

More on these topics:

https://www.newscientist.com/article/2198937-first-ever-picture-of-a-black-hole-may-be-revealed-this-week/?utm_campaign=RSS%7CNSNS&utm_source=NSNS&utm_medium=RSS&utm_content=home

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