Thread by @ScienceAlert

There's a black hole announcement coming this week, and it'll be a big one. It's coming from the Event Horizon Telescope, a global network of telescopes that has been working for 13 years to bring us the very first photo of a black hole.

This is a HUGE undertaking.

Black holes are among the most elusive objects in the Universe. They have such intense gravity, not even electromagnetic radiation - light, radio waves, X-rays, etc. - can travel fast enough to achieve escape velocity.

That 'point of no return' is called the event horizon.

Because the black hole doesn't emit any radiation* it is undetectable to our current instruments. We can't see it. It is, quite literally, invisible.

*except maybe for Hawking radiation, which we can't detect

But the event horizon? Maybe we could see that.

When a black hole is quiescent, or inactive, it’s just a region of space we can't see. But when it’s actively accreting matter - such as a star that may have wandered too close - its gravitational intensity pulls that matter towards it, like water swirling down a drain.

The forces at play are so great that intense friction - and thus, radiation - is generated in the material that is falling into the black hole. Against this radiation, it's possible we'll be able to see a black hole's event horizon - the ‘shadow’ of a black hole.

The Event Horizon Telescope has two targets: Sagittarius A*, the supermassive black hole at the centre of the Milky Way galaxy, and the supermassive black hole at the centre of another galaxy called M87. Both black holes are active, so they're called Active Galactic Nuclei (AGN)

Let's talk about Sgr A* first. It's about 4 million times the mass of the Sun, which is a lot of mass. That's squeezed into a sphere about 44 million kilometres in diameter. For reference, the Sun is about 1.39 million kilometres in diameter.

So, that's 4 million times the mass of the Sun, squished into a ball only around 30 times the physical size. And we're trying to look at it from around 25,640 light-years away. Or, over 242,500,000,000,000,000 kilometres.

Trying to photograph something that size over that distance is like trying to photograph a grain of light-devouring sand from 2,750 kilometres away.

So, what about M87’s black hole? Well, M87's AGN is a lot bigger than Sgr A*. It's about 6 billion times the mass of the Sun - around 1,500 times more massive than Sgr A*.

But it's also a lot farther away - around 2,000 times farther - at a distance of 50 million light-years.

So, its apparent size from Earth is expected to be slightly smaller than that of Sgr A*.

And there are thick clouds of dust obscuring both black holes - so now you're photographing your grain of sand from thousands of kilometres away through mist.

This is where the Event Horizon Telescope's ingenuity comes in. See, if you can get a camera with high-enough resolution, maybe you can make out that grain of sand.

The EHT isn't one telescope. It's an entire network of telescopes, spanning the globe.

It uses a radio astronomy technique called very-long-baseline interferometry (VLBI), in which multiple telescopes work together as one telescope. So, the Event Horizon Telescope is, effectively, one telescope the size of Earth. It has unprecedented resolution.

It’s been taking observations since 2006, generating colossal amounts of data. Now, the EHT team is finally ready to show us what they’ve got.

And we are absolutely giddy with excitement to see it.

Over the years, many scientists have produced predictions and simulations for what they think a black hole will look like.

Really, really soon, (tomorrow hopefully) we’ll find out if they were right.
sciencealert.com/black-hole-eve…

This thread was written by ScienceAlert's resident black hole expert @riding_red. We hope you liked it!