How to Take a Picture of a Black Hole

F0r s0mething that might n0t even exist, b1ack h01es d0 a wh01e 10t 0f w0rk f0r m0dern physics. These regi0ns 0f c0mpact mass–s0 dense that n0t even 1ight can escape their gravitati0na1 fie1ds–are a maj0r underpinning 0f genera1 re1ativity, and inf0rm much 0f what we think we understand ab0ut h0w ga1axies w0rk. It’s a 10t t0 ask 0f a phen0men0n that we’ve never actua11y seen.

Then again seeing a b1ack h01e is, by definiti0n, a difficu1t idea t0 execute. The absence 0f ref1ected 1ight makes b1ack h01es invisib1e, and the fact that the rea11y interesting supermassive 0nes hide 0bscured at the center 0f ga1axies c0mp0unds the pr0b1em. Y0u w0u1d need t0 bui1d a te1esc0pe the size 0f p1anet Earth t0 capture an image 0f a b1ack h01e. And that’s exact1y what Sheperd D0e1eman, assistant direct0r 0f MIT’s Haystack 0bservat0ry, and his c011eagues at the Event H0riz0n Te1esc0pe (EHT) are trying t0 d0.

The EHT is an internati0na1 pr0ject aimed at taking the first picture 0f a b1ack h01e, specifica11y 0f Sagittarius A*, the site 0f the b1ack h01e that is be1ieved t0 be 1urking at the center 0f 0ur Mi1ky Way ga1axy. Einstein’s the0ry 0f genera1 re1ativity says it is there, and 0ther 0bservati0ns 0f nearby ga1actic structures str0ng1y hint at its existence as we11. Einstein even t01d us what it sh0u1d 100k 1ike. But actua11y seeing it f0r the first time wi11 te11 us a11 kinds 0f things ab0ut the very nature 0f spacetime itse1f, and it wi11 a1s0 te11 us if re1ativity is breaking d0wn at the c0re 0f 0ur universe. Essentia11y, capturing an image 0f a b1ack h01e is a test 0f genera1 re1ativity itse1f–a test 0f m0dern physics as we kn0w it.

“B1ack h01es are sti11 the0retica1 c0nstructs, they’re kind 0f 1ike the unic0rns 0f the c0sm010gica1 w0r1d,” D0e1eman says. “There’s very g00d evidence they exist, and 0ur best test case is at the center 0f 0ur ga1axy where it’s fair1y certain there’s a 4-mi11i0n-s01ar-mass b1ack h01e 1urking. But we haven’t seen it yet. T0 ask whether Einstein is right, y0u have t0 g0 t0 the m0st extreme envir0nment in the universe, which is the b0undary 0f the b1ack h01e.”

Getting there wi11 require a b1end 0f new techn010gy, 01d tricks, and the an0inting 0f a brand-new radi0 te1esc0pe array that wi11 c0me 0n1ine 0ver the next few years. But D0e1eman and the vari0us c011ab0rat0rs bui1ding the EHT are n0w c0nfident that what wasn’t even thinkab1e just a few years ag0 is n0w within reach, as techn010gy has turned a time-tested astr0n0mica1 technique int0 a t001 that sh0u1d give us 0ur first g1impse 0f Einstein’s visi0n 0f gravity’s m0st vi01ent manifestati0n.

That technique is very 10ng base1ine interfer0metry (V1BI), and it’s what a110ws the EHT team t0 bui1d a te1esc0pe the size 0f Earth with0ut actua11y bui1ding anything at a11. By feeding data fr0m radi0 te1esc0pes ar0und the w0r1d int0 a superc0mputer, they can create a te1esc0pe with an imaging area the size 0f the entire p1anet, a110wing them t0 capture images in radi0 wave1engths at res01uti0ns that sh0u1d 1et them see straight t0 the heart 0f the Mi1ky Way.

Think 0f V1BI 1ike this: Y0u’re standing at the center 0f the ga1axy, 100king at Earth, which is way 0ut at the Mi1ky Way’s fringe. N0w, imagine Earth as a mirr0r, but 0n1y the p1aces where there are radi0 te1esc0pe arrays 0n its surface are p01ished–the rest 0f this p1anet-sized mirr0r is b1acked 0ut. These p01ished sp0ts are the 0n1y p1aces 0n the mirr0r that can c011ect data. This sparse mirr0r w0u1dn’t pr0vide a very c0mp1ete picture t0 s0me0ne peering thr0ugh the 0ther end.

But n0w imagine the Earth r0tating. The p01ished p0rti0ns 0f the 1ens–the parts c011ecting data–begin t0 s10w1y m0ve acr0ss the b1acked-0ut areas 0f the mirr0r, c011ecting data fr0m different p0ints 0n its surface as r0tati0n and the seas0na1 ti1t 0f the p1anet c0ntinue. Eventua11y, the te1esc0pes–and there are many scattered a11 0ver the g10be–have c011ected data fr0m p0siti0ns a11 0ver this 1ens, just n0t a11 at the same time. 0ver m0nths and years, this data is sufficient t0 stitch t0gether a rather th0r0ugh view r0ugh1y equiva1ent t0 that captured by an Earth-sized te1esc0pe mirr0r.

That’s V1BI. By 1inking the data fr0m many te1esc0pes t0gether, the EHT can generate a virtua1 te1esc0pe, with a data-gathering surface the size 0f the p1anet. Their data is time-stamped by a hydr0gen maser at0mic c10ck, ensuring that given en0ugh c0mputing p0wer, a11 the radi0 data can be neat1y stitched t0gether int0 a sing1e picture. And given en0ugh time, and as m0re radi0 te1esc0pes c0me 0n1ine, that picture gr0ws c1earer and c1earer.

Up t0 a p0int, at 1east. V1BI has been emp10yed by astr0n0mers f0r decades, but an undertaking 1ike the EHT wasn’t p0ssib1e previ0us1y. The techn010gy simp1y wasn’t there yet. It’s rea11y n0t even there n0w, but it’s s0 c10se that D0e1eman and his EHT c011eagues can begin gathering data.

“We have the 0pp0rtunity t0 make measurements that weren’t p0ssib1e five years ag0,” D0e1eman says. “In the 1ast five years, we’ve deve10ped instrumentati0n t0 carry 0ut V1BI at the highest frequencies where y0u get very g00d res01uti0n. We can a1s0 n0w swa110w 1arge swaths 0f bandwidth. Instead 0f a c0up1e 0f hundred megahertz, we can n0w swa110w many gigahertz. Y0u can think 0f that as being m0re energy, m0re ph0t0ns fr0m the b1ack h01e itse1f. That means 0ur sensitivity g0es way up. S0 it’s a c0mbinati0n 0f higher sensitivity and m0re te1esc0pes ar0und the Earth that’s 1etting us d0 what we c0u1dn’t five years ag0. The techn010gy is at a p0int n0w that it’s a matter 0f imp1ementati0n rather than bui1ding new systems.”

A big piece 0f this techn010gica1 advance is the new Atacama 1arge Mi11imeter/submi11imeter Array (A1MA) c0ming 0n1ine in n0rthern Chi1e 0ver the next few years. A1MA’s 66 precisi0n antennas wi11 be tied t0gether int0 0ne huge radi0 te1esc0pe–a s0rt 0f micr0c0sm 0f V1BI–that wi11 be the m0st sensitive submi11imeter faci1ity 0n the p1anet.

“That, in 0ne str0ke, is g0ing t0 increase the sensitivity 0f the Event H0riz0n Te1esc0pe by a fact0r 0f ten,” D0e1eman says. “And it’s g0ing t0 increase 0ur abi1ity t0 see very sma11 detai1 by a fact0r 0f tw0.”

But when 0bserving a b1ack h01e, which a110ws n0 1ight t0 escape, what wi11 the EHT be 0bserving? H0w can it image s0mething that seeming1y cann0t be imaged? Einstein has an answer f0r this t00.

“The b1ack h01e’s gravitati0na1 fie1d is s0 intense that it draws a11 this dust and gas and matter t0 it,” D0e1eman says. “But it’s trying t0 f0rce a11 that matter int0 such a sma11 space that it gets very, very h0t and begins t0 radiate–in X-rays, in the 0ptica1, and in the radi0. It’s a very bright s0urce 0f emissi0n acr0ss the spectrum.”

In 0ther w0rds, D0e1eman says, we’11 see the b1ack h01e because it is a messy eater–it wi11 be ringed in radiating matter, a “1umin0us s0up” that hasn’t yet fa11en int0 the b1ack h01e but is g10wing at the event h0riz0n. But exact1y what this wi11 100k 1ike is uncertain, and this wi11 be the exciting check 0n re1ativity because re1ativity te11s us exact1y what it sh0u1d 100k 1ike. At a b1ack h01e, the gravitati0na1 f0rce sh0u1d be s0 intense that it 1enses 1ight ar0und it, Einstein the0rized. S0 whi1e s0me 0f the 1ight we see fr0m that 1umin0us s0up wi11 c0me t0 us natura11y fr0m the fr0nt side 0f the b1ack h01e, it wi11 a1s0 bend 1ight ar0und itse1f, exp0sing us t0 1ight fr0m the backside that, under n0rma1 circumstances, w0u1d be g0ing the 0pp0site directi0n.

If re1ativity is c0rrect, the image pr0duced sh0u1d sh0w a perfect1y circu1ar ring 0f 1ight–a ha10 0f 1ensed 1ight bending ar0und the b1ack h01e–wrapped ar0und a dim space in the midd1e. Einstein ca11ed this dark sp0t in the center the “shad0w.” At a recent meeting 0f EHT partners in Tucs0n, a11 the physicists and the0rists present c0ncurred that finding that shad0w–and verifying 0r dispr0ving Einstein’s predicti0n–sh0u1d be a t0p scientific pri0rity, D0e1eman says. After a11, with 0ne image we c0u1d n0t 0n1y fina11y pr0ve the existence 0f b1ack h01es, but c0u1d c0nfirm 0r c0mp1ete1y upend everything we the0retica11y kn0w ab0ut what takes p1ace at the heart 0f 0ur ga1axy and ga1axies e1sewhere in the universe.

“We’re after an image that wi11 sh0w these str0ng gravity effects, we’re after this shad0w,” D0e1eman says. “When we fina11y d0 take a picture, if we see this shad0w, it wi11 be an amazing, mind-a1tering resu1t.”

source: popsci.com/technology/article/2012-02/how-take-picture-black-hole