Black Hole Analogue Discovered in South Atlantic Ocean
Vortices in the South Atlantic are mathematically equivalent to black holes, say physicists, an idea that could lead to new ways of understanding how currents transport oil and garbage across oceans
Black holes are regions of spacetime in which gravity is strong enough to prevent anything escaping, even light. These strange objects were first discovered in the early 20th century as mathematical solutions to the equations of general relativity. (It was not until much later that astronomers began to gather observational evidence of their existence.). One of the curious features of general relativity is that the same mathematics crops up in various other situations. In recent years, for example, physicists have worked out how to create invisibility cloaks by steering light around objects using metamaterials.
Today, George Haller at the Swiss Federal Institute of Technology in Zürich and Francisco Beron-Vera at the University of Miami in Florida have found another analogue of a black hole, this time in the world of turbulence.
The vortices that can form in turbulent water are a familiar sight. Edgar Allan Poe described just such a whirlpool in his short story “A Descent into a Maelstrom” which he published in 1841:
“The edge of the whirl was represented by a broad belt of gleaming spray; but no particle of this slipped into the mouth of the terrific funnel… “
In this passage, Poe describes one of the crucial feature of these rotating bodies of fluid: that they can be thought of as coherent islands in an incoherent flow. As such, they are essentially independent of their environment, surrounded by a seemingly impenetrable boundary and with little, if any, of the fluid inside them leaking out.
If you’re thinking that this description has a passing resemblance to a black hole, you’d be right. Haller and Beron-Vera put this similarity on a formal footing by describing the behaviour of vortices in turbulent fluids using the same mathematics that describe black holes.
In this picture, Poe’s “broad belt of gleaming spray” is exactly analogous to a photon sphere around a black hole. This is a surface of light which encircles a black hole without entering it.
Haller and Beron-Vera go on to show that each vortex boundary in a turbulent fluid contains a singularity, just like an astrophysical black hole.
That has important implications for the study of fluids and the identification of vortices, which are otherwise tricky to define and spot. In this case, it is simply question of looking for the singularity and the boundary that surrounds it.
And that’s exactly what Haller and Beron-Vera have done in the pattern of currents in the south west Indian Ocean and the South Atlantic. A well-known phenomenon in this part of the world is called the Agulhas leakage which comes from the Agulhas current in the Indian Ocean. “At the end of its southward flow, this boundary current turns back on itself, creating a loop that occasionally pinches off and releases eddies (Agulhas rings) into the South Atlantic,” they say.
These guys used satellite images of the South Atlantic Ocean from between November 2006 and February 2007 to look for vortices using a set of simple computational steps that spots black hole analogues.
In this three-month period they found eight candidates, two of which turned out to be black hole analogues containing photon spheres. “We have found exceptionally coherent material belts in the South Atlantic, filled with analogs of photon spheres around black holes,” they conclude.
That’s an interesting result that could have significant implications for our understanding of the way ocean currents transport material. Since anything that gets into these black holes cannot get out, this should trap any garbage, oil or indeed water itself, moving it coherently over vast distances. “Beyond the mathematical equivalence, there are also observational reasons for viewing coherent…eddies as black holes,” say Haller and Beron-Vera.
The work also raises the possibility that black hole analogues will occur in other situations, such as in hurricanes and not just on Earth. By this way of thinking, the Great Red Spot on Jupiter might well be the most famous black hole in the Solar System.
The powerful vortices of current have been described as ‘maelstroms’ and are ‘mathematical analogues’ for black holes – which is to say they do exactly the same with water that black holes do with light.
A whirlpool pictured by NASA.
The discovery could give new insights into how oceanic currents transport debris and may even have implications for climate change studies.
Astronomical black holes bend space and time into a perpetually collapsing vortex. Light itself bends around them, which enables astronomers to recognise their existence.
Similarly, these oceanic maelstroms funnel current into an almost permanent spiral, trapping debris, oil and potentially living creatures in a body of water. Hardly anything leaks out.
The scientists used Edgar Allen Poe’s 1841 story ‘A descent into a Maelstrom’ to describe their discovery:
“The edge of the whirl was represented by a broad belt of gleaming spray; but no particle of this slipped into the mouth of the terrific funnel…”
The ability to apply the same mathematical principles to water currents on earth as black holes in space is an unexpected side-effect of the theory of general relativity.
This phenomenon has been observed in the South Atlantic and South-western Indian Ocean, using satellite imagery designed to spot the aquatic equivalent of black hole currents. According to scientists, the maelstroms are prevalent in this area thanks to the southbound Agulhas current in the Indian Ocean.
In a three-month period, two perfect matches were found to mimic black hole behaviour, “We have found exceptionally coherent material belts in the South Atlantic, filled with analogues of photon spheres around black holes,” said George Haller, from the Swiss Federal Institute of Technology in Zürich and Francisco Beron-Vera at the University of Miami in Florida, who worked on the study.
Top panel: Evolution of black-hole eddies (extracted from 3 months of data) in the South Atlantic over a period …
The phrase ‘photon spheres’ refers to the light that is trapped around astronomical black holes, and in this instance is comparable to the rings of debris that are forming around these ocean maelstroms. As Haller and Beron-Vera’s findings note, ‘these vortices will capture and swallow nearby passively floating debris.’
In their research findings, Haller and Beron-Vora express their surprise at finding real-world examples that hold so closely to the theoretical equations. ‘Vortices in turbulence are often envisaged as rotating bodies of fluid, traveling as coherent islands in an otherwise incoherent ambient flow. This… …view is appealingly simple, yet challenging to apply in actual vortex detection.’
The maelstroms are detected by their rotating edges, which the scientists found were reliable indicators of the vortex within, based on pioneering research carried out by Stephen Hawking on black holes: ‘Intuitively, one expects that any…vortex in the fluid must contain such a singularity in its interior, just as all black holes are expected to contain Penrose-Hawking singularities. This expectation turns out to be correct’.
The singularities, as they have been termed, last for months at a time, moving across the ocean without interference from other currents.
Thus they can transport water of different temperatures and salinity to other areas of the ocean, potentially influencing the regional climate.
Haller and Beron-Vera found that the vortices transported water in a north-western direction 30% faster than had previously been reckoned – at a rate equating to 1.3 million cubic metres of water per second.
In addition, the maelstroms were found to occur four times deeper in the ocean than previously estimated; the study found examples as deep as 2000m below the surface.