Space-time, the so-called fabric of physical reality, may be made of tiny, discrete pieces stitched together. A preliminary analysis of the way matter orbits quasars suggests we could find evidence of this cosmic quilt in the universe鈥檚 most extreme neighbourhoods.
Much of the understanding we have of the make-up of our universe comes from probing matter at smaller and smaller scales. Think of fluids, says at Princeton University. Many equations that correctly capture how fluids behave were developed without accounting for what every one of their atoms does individually. Historically, researchers first assumed that fluids were continuous, but later found that they are collections of discrete atoms. Gorard suggests we may be at such an inflection point for space-time itself.
If space-time is discrete, akin to being pixelated, we couldn鈥檛 notice without zooming in to the smallest possible scale. Quantum theory suggests that is the Planck length 鈥� about 100 billion billion times smaller than a proton 鈥� which has been impossible to directly observe. But Gorard says we may be able to see signatures of space-time鈥檚 discreteness anyway.
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He calculated how a discrete space-time would affect particles around incredibly bright supermassive black holes called quasars. Quasars release a lot of radiation because they are surrounded by an accretion disc of gas, which heats up as it is falling towards the black hole. 鈥淚 was trying to work out what would be the effect on things like black hole accretion if space-time were discrete and would that lead to observable signatures. The answer is, tentatively, yes,鈥� says Gorard.
He found that the places where discrete pieces of space-time touch would be like a pebble in the road for a particle, making some of them change trajectory and move away from the black hole鈥檚 edge instead of falling in. This would add more matter to the disc, and make some quasars brighter than conventional physics theories predict.
However, he says he is still working out the details of how, for instance, a quasar鈥檚 rotation may affect the process, and how to precisely account for electromagnetic fields.
at the Radboud University in the Netherlands says the idea of discrete space-time is 鈥渨ishful thinking鈥� for many physicists because it would resolve some instances in quantum field theory where calculations produce impossible infinite values. Often, space-time discreteness is an assumption of a theory, as it is in Gorard鈥檚 work, rather than something that emerges from known physics laws, she says. Additionally, Loll says this quasar analysis may not correctly include quantum effects that could change particles鈥� behaviour near the edge of a black hole.
She is not the only sceptic. 鈥淚 don鈥檛 see how this idea can make sense. Any viable discreteness would be at a very small scale compared to the scale relevant for the physics of accretion,鈥� says at the University of Maryland.
Gorard, however, is confident in his work. He says that some black holes that are unusually large and have strangely big accretion discs have been discovered already, which may be the beginning of a trend that could eventually support his theory. With advances in multi-messenger astronomy 鈥� a technique that combines observations in many wavelengths and on different scales 鈥� Gorard believes that his predictions may become testable soon.
Proving that space-time is discrete would be no simple task, but if it is true, we could be opening the door to new, or more evidence-based, theories that aim to combine quantum theory and Albert Einstein鈥檚 theory of general relativity. Doing so would bridge the gap between these crucial pillars of physics and produce a long-sought theory of quantum gravity.
