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Illustration of massive objects warping space-time
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A mathematical test for the nature of space-time – the fabric of physical reality – may be the first step towards novel computer-like devices that process information using gravity.
Is space-time an unchanging expanse, or can it be warped in ways that affect a signal travelling through it? According to Albert Einstein’s theory of special relativity, it is static – but his theory of general relativity reveals something completely different. In this framework, massive objects make space-time dimple and curve, like when a ball is dropped onto a taut sheet, which could change the path of a signal moving nearby.
Eleftherios-Ermis Tselentis at the Brussels Polytechnic School in Belgium and Ämin Baumeler at the University of Lugano in Switzerland have now developed a mathematical test for whether space-time in any given region is unchanging or not.
They analysed a scenario where three or more people exchange information by messaging each other. They asked whether it is possible to tell if one of the people – nicknamed Alice, Bob and Charlie – could change the way that information travels by warping space-time. Could Alice receive a message meant for Bob because the region of space-time that the signal travelled through got distorted? Could she reverse causality for Charlie and Bob – so that Bob might receive a response from Charlie before even messaging him – by messing with space-time near her?
Tselentis and Baumeler derived an equation that could help Alice, Bob and Charlie know when these situations are possible. After several rounds of sending messages to each other, they could tally up who got what message when, then plug that data into the equation.
The result would reveal whether they had been communicating in a setting where manipulating space-time was an option. This mathematical framework was general enough that the threesome wouldn’t have to know anything about where in space they are or use any non-standard messaging devices.
Baumeler says that general relativity was understood as a successful description of our physical reality for decades, but we still lacked a rigorous mathematical connection between changing space-time and the flow of information. Understanding information flow, and related concepts, forms the foundations of computer science.
In this sense, he says, his team’s work may be a very early step towards using gravitational effects – moving masses around and distorting space-time – for computation.
“If we are going to use the mysteries of physics to do computing, why not try general relativity?” says Pablo Arrighi at Paris-Saclay University in France. He says that other researchers have considered extreme ideas like throwing a computer into a black hole so the warping of space-time near the edge of the black hole causes time to slow down and enable otherwise impossibly long calculations to conclude.
But the new theory stands out because it doesn’t focus on any special devices or theories of space, which means it could be applied to a broad range of situations, says Arrighi. Building a “gravity information” device, however, currently seems impractical, he says.
Tselentis and Baumeler also say that much more work is needed before they can design a practical device. The calculations in their recent work rely on fantastical situations – picture Alice moving a whole planet to sit between Charlie and Bob, for example. For their ideas to find practical use, they need to better understand the effects of gravity at much smaller scales.
Gravity produces notoriously weak signals for objects that aren’t extremely massive, which is why you never feel the effect of space-time warping around, say, a pencil on your desk. Still, some devices such as clocks made from extremely cold atoms can detect these effects. Future development of such devices – combined with advances in theories connecting gravity and information – could lead to more practical applications of the mathematical work by Tselentis and Baumeler.
Their research could illuminate the connections between the ways that different paradigms – information theory and special relativity – deal with causality, says V. Vilasini at the University of Grenoble Alpes in France. Because the new work engages with ideas like inverting the order of events, it raises questions about seemingly basic notions like what an event – for instance, Alice pushing a button to send a message – actually is, she says.
In her view, the next step is to more fully integrate the researchers’ approach with the theory of general relativity, which would allow for further investigations of space-time’s nature.
“Could astrophysical phenomena like black hole mergers, which produce gravitational waves that reach Earth, hold physically meaningful signatures of the type of correlations studied in this work, and could this allow us to study how they warp space-time?” she asks.
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