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Event Horizon Telescope

Announcement 10th April


Event Horizon Telescope Q & A


What is the Event Horizon Telescope?

The EHT is a global array of radio telescopes which uses a technique called Very Long Baseline Interferometry (VLBI) to create a virtual telescope the size of the Earth.


Which telescopes comprise the EHT?


Image result for eht telescopes

What is the EHT for?

The goal of the EHT is to take an image of the event horizon of Sagittarius A*, the supermassive black hole at the centre of our galaxy.

How is that even possible? Aren't black holes invisible?

Yes, black holes are usually invisible. The EHT plans to take an image of the event horizon silhouetted against the background of energetic radiation it is believed surrounds the black hole.

But aren't black holes too small and far away to be seen?

So-called “stellar mass” black holes are indeed small on a cosmic scale, perhaps 30-40km in diameter. But supermassive black holes, which exist at the heart of most galaxies, are a lot larger. The diameter of Sagittarius A* (often abbreviated to Sag A*) is thought to be 27 million miles. The largest black hole known has a diameter of 14 billion miles!

But still, it's a long way away....

Yes. Sag A*, at the galactic centre, is 26,000 light years away. To capture an image of the event horizon, the EHT must attain a resolving power the same as being able to see a coin in New York from London. And then being able to read the writing on the coin!

What do scientists hope to learn from all this?

Many things. If all goes to plan, we will see our first direct image of a black hole. Einstein's Theory of Relativity makes several predictions about what that should look like and how it should behave. So the EHT is primarily a test of Relativity. But we will also learn very useful information about the environment surrounding black holes.

Will Relativity pass the test?

The truth is that nobody knows what the image will reveal. While it's a reasonable assumption that Einstein was right - because he's never been proven wrong - it's certainly not a given. And that's the exciting possibility - that the image may reveal totally new physics, things we've never even thought about. However, there are probably not many scientists who believe we'll see new physics. The idea cannot be completely discounted, however.

So we may be wrong about black holes?

Maybe. The behaviour of black holes we have observed until now, even though we haven't been able to see them directly, is in keeping with how gravity behaves in Special Relativity terms. But that does not rule out surprises. In fact, it would be surprising if we'd got everything totally right about black holes.

Why are the EHT findings being announced now?

The EHT carried out its first observing run, in its current configuration, in 2017. It has taken since then to correlate, analyse and process the data. That is now complete.

Where can I learn more?

Event Horizon Telescope:




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