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The Black hole Gargantua in ‘Interstellar’.
The Black hole Gargantua in ‘Interstellar’.

At the centre of it all, a black hole

A black hole causes some distortion in the character or behaviour of nearby stars

This year’s Nobel Prize in physics rewards research into the phenomenon of black holes. That’s greatly deserved and long overdue, in my opinion. But it’s hardly that the term is unknown. ‘Black hole’ is a familiar phrase in everyday conversation. For example, have you heard of a chain of patisseries known as “Theobroma"? Well, my wife has an irresistible tendency to be drawn inexorably into a Theobroma store anytime she passes within a 5km radius of one. You might say, and we do, that for her, Theobroma acts like a black hole.

There is one big difference, though. We know that objects that enter a real black hole can never leave. But after being drawn into Theobroma, my wife does eventually manage to emerge. With some difficulty, yes, because she’s usually laden with croissants and pain au chocolat, red velvet cake and several other bags filled with other assorted pastries.

Yummy edibles aside, the nature of black holes raises an interesting question: if we can’t see them, if nothing can escape them, if there is effectively nothing tangible about them in the sense that other physical objects are tangible — well, how do we even know they are there? Is there some way we can detect their presence?

Well, that’s more or less the problem that two of the three scientists who shared this year’s Physics Nobel were faced with and managed to solve. Andrea Ghez and Reinhard Genzel will share half of the Prize for discovering an enormous black hole at the centre of our Milky Way Galaxy. But how did they do that? How do you see something that can’t be seen?

One big clue: variation. If a star you’re looking at up in the sky varies in some way, that’s a good sign that it’s moving. If the variation is rhythmic, it’s likely rotating on its own axis, or orbiting around some other object. What could this other object be?

Use the analogy of a lamp post to understand this in one sense. Suppose you’re idly observing a distant lamp post late one night. You notice that every few seconds, like clockwork, it becomes slightly dimmer. Why does this happen? Of course, it could be a faulty bulb, but maybe you know that’s not the case. What other explanation is there?

Here’s one: there’s a smaller, darker object that’s flying steadily around the lamp post. Maybe a moth. When the moth is positioned between you and the lamp, it actually blocks off a part of the light from the lamp, thus making it dimmer. When it has rounded the lamp to the farther side, not visible to you, it no longer blocks light from the lamp. So as this moth circles the lamp, you who are observing from across the street will notice that the lamp’s brightness varies regularly. Maybe you won’t see the moth, but you’ll notice the dimming and brightening of the lamp. You’ll think about it and conclude: something is circling the lamp. (I explained this briefly in “The variation tells the tale", published 7 May 2015).

That’s one way we detect objects circling distant stars: because the stars’ brightness varies regularly. In fact, this is how we surmise that a star might have planets orbiting it. That is, we don’t actually see those orbiting planets. (It’s a star, it’s very far away, how can we hope to actually see much smaller and darker objects near it?) Instead, we detect and measure the effect they have on the stars they orbit.

This is analogous, in some ways, to how we can detect black holes.

Roger Penrose once spoke of a black hole — now known as Cygnus X-1 — that’s a companion to the blue supergiant star HDE 226868: the two orbit each other. So you might wonder, as the black hole passes in front of the star, do we detect a dimming of the star’s light? Does the brightness rise again as the black hole goes behind? Well, that might indeed happen. In fact, Cygnus X-1, being a black hole, might even capture all the light that streams towards us from the star that’s behind it, rendering the star totally invisible. The issue here is that this orbiting happens in such a way that as seen from the Earth, the two objects never actually eclipse each other.

So, it wasn’t by a regular dimming of HDE 226868’s brightness that we detected Cygnus X-1. (Well, it does dim slightly, but that’s almost by the way). Instead, it was by what Cygnus X-1 is doing to HDE 226868. Just as tides on Earth are caused by our orbiting Moon, Cygnus X-1 exerts a tidal pull on the material on the star’s surface. Of course, with the Earth’s tides, the Moon’s gravity is only strong enough to change sea levels by a few metres every few hours. But HDE 226868 finds its entire starry self being distorted into a teardrop shape, with material actually being drawn off its surface to flow towards Cygnus X-1.

The only plausible explanation for distortion on this scale is that Cygnus X-1 is a black hole.

The point here is that a black hole has an effect on the stars in its neighbourhood. This is hardly surprising, given its gigantic gravitational pull. So, even if we can’t see a black hole, we can infer the existence of one if we detect something peculiar, some distortion, in the character or behaviour of nearby stars.

This is just what Ghez and Genzel set out to find at the centre of the Milky Way. They trained the two largest optical telescopes in the world — the Keck Observatory in Hawaii and the Very Large Telescope in Chile — on a group of stars in a celestial region known as Sagittarius A*, near the centre of the galaxy and some 26,000 light years from us.

What their observations showed is that these stars all appear to be moving around an object we can’t see. In our solar system, the planets orbit the (very visible) Sun along nearly circular paths. If the stars in Sagittarius A* were on similar nearly circular paths, we might conclude that it was some relatively ordinary object they were orbiting and redouble our efforts to locate it. But the paths of these stars are not circles at all.

For example, after studying the motion of one of them, S2, for 27 years, Genzel’s team concluded that its orbit is “highly elliptical". That can be said of the other stars in that region as well. The invisible object is deforming these stars’ orbits.

“The S2 data," the team wrote, “are inconsistent with pure Newtonian dynamics." (“Detection of the gravitational redshift in the orbit of the star S2 near the Galactic centre massive black hole," R Genzel et al, Astronomy and Astrophysics, 26 July 2018).

Which is to say, Newton’s laws don’t apply there. That tells us the invisible object that is skewing these stellar orbits is almost certainly a black hole, which we need to analyse using Einstein’s General Theory of Relativity. Ghez and her team were able to calculate how heavy this black hole is: about 4 million times the mass of our Sun (“Measuring Distance and Properties of the Milky Way’s Central Supermassive Black Hole with Stellar Orbits", AM Ghez et al, The Astrophysical Journal, December 2008).

That’s one enormous black hole. Lucky it’s so far away. Or it might have had to compete with Theobroma for my wife’s attentions.

Once a computer scientist, Dilip D’Souza now lives in Mumbai and writes for his dinners. His Twitter handle is @DeathEndsFun

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