Tag Archives: EventHorizonTelescope

In Our Galaxy’s Center, a Tiny Monster

It’s far from a perfect image. [Note added: if you need an introduction to what images like this actually represent (they aren’t photographs of black holes, which are, after all, black…), start with this.]

EHT’s blurry time-averaged image of the ring of material surrounding the black hole at the center of our galaxy

It’s blurred out in space by imperfections in the telescopic array that is the “Event Horizon Telescope” (EHT) and by dust between us and our galaxy’s center. It’s blurred out in time by the fact that the glowing material around the black hole changes appreciably by the hour, while the EHT’s effective exposure time is a day. There are bright spots in the image that may just be artifacts of exactly where the telescopes are located that are combined together to make up the EHT. The details of the reconstructed image depend on exactly what assumptions are made.

At best, it shows us just a thick ring of radio waves emitted over a day by an ever-changing thick disk of matter around a black hole.

But it’s our galaxy’s black hole. And it’s just the first image. There will be many more to come, sharper and more detailed. Movies will follow. A decade or two from now, what we have been shown today will look quaint.

We already knew the mass of this black hole from other measurements, so there was a prediction for the size of the ring to within twenty percent or so. The prediction was verified today, a basic test of Einstein’s gravity equations. Moreover, EHT’s results now provide some indications that the black hole spins (as expected). And (by pure luck) its spin axis points, very roughly, toward Earth (much like M87’s black hole, whose image was provided by EHT in 2019.)

We can explore these and other details in coming days, and there’s much more to learn in the coming years. But for now, let’s appreciate the picture for what it is. It is an achievement that history will always remember.