Thread by @asipoec

The Event Horizon Telescope will be announcing its results at 6.30 pm IST on 10th April. The @ehtelescope is trying to image the shadow of the supermassive black hole at the centre for our Milky Way, so this is going to be big news indeed! This is a thread on the #EHTBlackHole

You can watch the press conference live at nsf.gov/news/special_r… and
. Press note on #EHTBlackHole at nsf.gov/news/news_summ… & eso.org/public/announc…. Why are we all excited about this? What will the @ehtelescope announce? How can we image a black hole?

Will they show us a blurry image of #EHTBlackHole where we can't make out anything, like xkcd.com/2133/, or something that will make us gasp in awe, like bbc.com/news/science-e…? Follow this thread to figure out the why, how and what ...

#Blackholes (BH) are strange, whose gravity is so strong that light can't escape from it. Nothing travels faster than light, so nothing can escape from BH. 1st solution to @AlbertEinstein eqns of General Relativity was for BHs. We now know they exist. Fat stars end as BHs with ..

mass few times our Sun. @LIGO has seen merger of BHs with few tens of solar masses. ligo.caltech.edu/image/ligo2017…
See a lovely site hubble.stsci.edu/explore_astron… from @stsci. On to #EHTBlackHole now ...

Almost all galaxies have a supermassive #blackhole at their centres, and these can be millions to billions of solar masses. Our own #milkyway has a modest BH right at its very centre, which is only 4.3 million times the mass of our Sun!

Our BH is not very hungry and not much material falls into it. It was first noticed in the radio since all the dust between us and the BH hide it in optical. It causes close-by stars to move around it at few million km/hr which can be seen in infrared!
e-education.psu.edu/astro801/conte…

Two groups have followed many stars go around the #EHTBlackHole for many years at this breakneck speed. Using Kepler's laws, they have finally proved that the object in the centre is a BH and is about 4 million solar masses! See astro.ucla.edu/~ghezgroup/gc/ for some amazing info.

They have a cool movie based on their years of tracking these stars around the BH. They did this using techniques that effectively 'cancelled' the twinkling due to our atmosphere on the largest near-infrared telescopes we have (@keckobservatory & @eso VLT) astro.ucla.edu/~ghezgroup/gc/…

But we can't 'see' the BH itself, right? The BH swallows the surrounding gas, which swirls around the BH in a flattened disk, spiralling into it at speeds close to light! The radiation from this hot maelstrom feeding the beast can be seen by us!

But can we IMAGE this radiation? Yes, we can, and we expect to see the shadow of the #EHTBlackHole against this radiation and hence image its outer most edge, the event horizon. Hence the name of the @ehtelescope. eso.org/public/usa/out…

The @ehtelescope works at 230 GHz. At lower frequencies, we can't see through the hot gas right up to the BH. And the galactic gas between us and #EHTBlackHole makes the image more blurred as well. At higher freq, Earths atmosphere gets more opaque. Hence 230 GHz or 1.3 mm.

But observing at 230 GHz is hard! The size of the shadow of #EHTBlackHole is expected to be around 0.05 milli arc second. To image this at 230 GHz, we need a Earth-sized telescope! And so @ehtelescope went ahead and got one! blackholecentral.com/eht_project_pa…

Martin Ryle got the 1974 @NobelPrize for 'Aperture Synthesis' (radio interferometry) where we build many small telescopes, put them far apart, & combine their data in special ways. The image we make is as if we had a single scope as big as the area over which the smaller ones are

This technique has been used to make radio images for many decades. Eg the #GMRT near Pune, run by NCRA (Pic credit NCRA) has 30 antennas of 45 m dis, over 25 km. Its image has as much detail as a 25 km sized telescope would show!
ncra.tifr.res.in/ncra/main

#EHTBlackHole is opaque at radio, so we can't use telescopes like these. So, we need to combine up many sub-mm telescopes, at 230 GHz, to image the shadow. This is hard technologically. @ehtelescope used 8 telescopes, one of which itself was made of 66 smaller telescopes!

These are @almaobs & APEX in Chile, IRAM in Spain, LMT in Mexico, SMT in Arizona, JCMT & SMA in Hawaii, and SPT in the South Pole! @ehtelescope team had to build and install new hardware and software at some of them so that they could work together. eso.org/public/images/…

Each @ehtelescope site had an atomic clock, accurate to 1 in a million billion! 8 datasets had to b combined with precise time-stamps. Error of few 1000th of 1 microsecond wud make imaging impossible! Data from all 66 ALMA antennas had to be joined as one eventhorizontelescope.org/technology

The @ehtelescope observations were done over a week in July 2017 when the weather over all 8 sites was good. Data was recorded and shipped to two locations for processing. They had to wait for 8 months more to get the South Pole data when winter ended! eso.org/public/usa/out…

The teams took many months to make an image; they had to correct for the weather over each @ehtelescope before combining them in powerful computers. They had 22 working groups from 62 institutes worldwide. medium.com/starts-with-a-…

They are now ready to announce their result and we hope it would be a spectacular images. Ah, we forgot to tell you that they also observed the supermassive #EHTBlackHole in M87, a giant elliptical galaxy and the nearest such BH to our galaxy en.wikipedia.org/wiki/Messier_87

The BH in M87's centre is almost 2000 times more massive and hence about 2000 times larger than our own. But it is also farther away from us by about the same factor compared to the measly 26000 light years of our Milky Way BH. So it ends up being about half its size on the sky.

Einsteins General Relativity tells us how motion of objects and spacetime curvature r linked & has passed all tests so far. From our GPS to the orbit of Mercury, to pulsars, lensing etc, all tests have been in moderately weak gravity limit, where curvature is not that large.

LIGO detections of merging black holes was a really good test of GR, and the @ehtelescope will be another strong gravity test of the theory. The shape of the shadow, determined by gravitational lensing by the BH will tell us a lot. sciencenews.org/article/event-…

@threadreaderapp unroll

Sorry - that should be - error of a few millionth of 1 microsecond - an even more amazing number!