Pro­fes­sor ex­plains mys­ter­ies of black holes at large in our uni­verse

The National - News - The Review - - Cover - Jonathan Gor­nall

The odds of de­tect­ing the grav­i­ta­tional waves cre­ated by two col­lid­ing black holes, ex­actly 100 years af­ter Ein­stein first pro­posed the pos­si­bil­ity, are prob­a­bly in­cal­cu­la­ble.

But un­til just a few decades ago the very con­cept of black holes it­self teetered pre­car­i­ously on the di­vid­ing line be­tween sci­ence fact and fic­tion, says Rob Fender, pro­fes­sor of astro­physics at Brasenose Col­lege, Ox­ford.

“For a long time, de­bate raged about black holes,” says Prof Fender, who vis­ited the UAE this week to talk about the sub­ject, in a pub­lic lecture ti­tled Black Holes at Large, at New York Univer­sity Abu Dhabi In­sti­tute.

But thanks to great leaps in tech­nol­ogy and in un­der­stand­ing, that de­bate – in as far as any the­o­ret­i­cal dis­cus­sion can be – it is all but over, he says.

“We are rather con­vinced that black holes are out there in very large num­bers in the uni­verse,” says Prof Fender. In fact, there are “prob­a­bly 100 mil­lion within our own galaxy, and of course we be­lieve there are thou­sands of mil­lions of gal­ax­ies out there”.

The the­ory of black holes was first pro­posed not by Ein­stein or any of the other big names in physics, but by a long-for­got­ten English cler­gy­man, in an ex­tra­or­di­nary pa­per writ­ten more than 220 years ago.

John Michell sug­gested the ex­is­tence of what he called “dark stars” in 1783, al­though it was 1967 be­fore the term “black hole” was fi­nally coined (by US physi­cist John Wheeler).

Nam­ing black holes and spot­ting them, how­ever, has proven to be two en­tirely dif­fer­ent things.

The prob­lem has al­ways been that, by def­i­ni­tion, black holes can­not be seen. They are formed by the cat­a­strophic col­lapse of a star, an event that com­presses mat­ter into such a rel­a­tively small space that noth­ing, not even light, can es­cape its mas­sive grav­i­ta­tional pull.

What can be seen, how­ever, is the ce­les­tial dance of vis­i­ble ob­jects or­bit­ing ghostly part­ners and bursts of X-rays emit­ting from th­ese so-called bi­nary sys­tems. Th­ese are two ob­jects, usu­ally stars, that are close enough that they or­bit around a shared mass.

“The first, and prob­a­bly still [the] strong­est, ev­i­dence for black holes is that we see bi­nary star sys­tems that go through phases where they are ex­tremely lu­mi­nous in X-rays,” says Prof Fender.

“They are far more lu­mi­nous than a nor­mal star, and the most ef­fi­cient way of re­leas­ing such amounts of en­ergy that we can con­ceive of in the uni­verse is that mat­ter gets ex­tremely hot as it falls down to­wards a black hole.”

But then there are pe­ri­ods when this bright X-ray emis­sion fades away, and this re­veals a fur­ther clue – one first pro­posed by Michell back in the 18th cen­tury.

“What you can then see is the star be­ing swung around in an or­bit in space and yet you can’t Rob Fender pro­fes­sor of astro­physics at Brasenose Col­lege, Ox­ford see the com­pan­ion star which is mak­ing it do this. The only sat­is­fac­tory ex­pla­na­tion is that there is es­sen­tially a dark star there,” says Prof Fender.

Un­til rel­a­tively re­cently, the tell-tale light show was not vis­i­ble from Earth, be­cause our at­mos­phere ab­sorbs X-rays. It was only when NASA launched the first X-ray-de­tect­ing satel­lite in 1970 that the sig­na­ture of the first con­firmed black hole was spot­ted.

The po­si­tion of black hole Cygnus X-1 – in our galaxy, a mere 6,000 light years from the Sun – is also be­trayed by the pres­ence of its part­ner star, HDE 226868, which is about 25 times the mass of our Sun and cir­cles it in a five­and-a-half-day or­bit.

Work over the past decade or so has also changed the be­lief that any mat­ter that crossed the “event hori­zon” of a black hole – its point of no re­turn – was lost for good.

In fact, says Prof Fender, “what we find is that when mat­ter falls down, as well as get­ting hot and pro­duc­ing th­ese X-rays, for rea­sons that we frankly still don’t un­der­stand it pro­duces very pow­er­ful jets of mat­ter and en­ergy that fire away from the black hole in op­po­site di­rec­tions”.

Are we in any dan­ger of be­ing swal­lowed by a black hole? Al­most cer­tainly not, says Prof Fender. There’s a huge one at the cen­tre of our galaxy, about three or four mil­lion times the mass of our Sun, and its size and po­si­tion be­trayed by the flock of stars in 10-year or­bits around it, but that’s 25,000 light years away from Earth.

“In all prob­a­bil­ity there is a black hole within about 20 or 30 light years of us that we just haven’t found yet,” says Prof Fender. “But there’s still no threat to us.”

Luck­ily, our own Sun is sim­ply too small to be­come a black hole. In­stead, it will slowly run out of fuel and shrivel into a so-called “white dwarf” – though not, sci­en­tists be­lieve, be­fore ex­pand­ing dra­mat­i­cally in its death throes and scorch­ing away all life on Earth.

Luck­ily, we have a while to make plans: about three bil­lion years.

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