The known universe is full of exciting things like black holes, hypernovae, and neutron star mergers. All of these, however, seem tame compared to elements that physicists think may exist but have yet to find. Perhaps chief among them are wormholes, which theoretically join parts of space and time, allowing those who enter them a shortcut to distant places.
The possibility of wormholes came as a huge relief to science fiction writers, otherwise cut off from the star systems they wished to explore by physical laws preventing faster-than-light travel. Many physicists are skeptical of their existence, or at least that three-dimensional objects could pass through them unscathed, but sheer luck was enough for the writers to drive a spacecraft, or at least a novel, through them.
However, as telescopes advance, one question becomes more troubling: If wormholes are real, why haven’t we found any? Four Bulgarian physicists offered an answer in Physical Review D: maybe we have them and just haven’t recognized them.
The vast majority of black holes we have identified are known either by their gravitational effects on the stars around them or by the jets of matter erupting from their accretion disks. If any of them were in fact wormholes, it’s unlikely we know. However, the Event Horizon Telescope Collaboration’s observation of the polarization around M87* and its tracking on Sagittarius A* is another matter. In these cases, we saw the shadow of the object itself against its event horizon, and might hope to notice something if we were really looking at a wormhole.
The possibility of wormholes is exciting enough for physicists that 12 papers have appeared on ArXiv.org exploring the concept since the beginning of November. However, as Sofia University’s Petya Nedkova and her co-authors note, we don’t know what they would look like.
The article seeks to address this issue and concludes that, viewed from a high angle, wormholes would look like nothing we have seen. For small tilt angles, however, the authors believe a wormhole would show “a very similar polarization pattern” to a black hole. Therefore, M87*, viewed at an estimated angle of 17°, could be a wormhole and we wouldn’t know.
That’s not to say we’re doomed to not be able to tell wormholes from black. “More significant distinctions are observed for strongly lensed indirect images, where the polarization intensity in wormhole spacetimes can reach an order of magnitude compared to the Schwarzschild black hole,” the authors write. The lens in question here does not come from a massive object between us and the hole creating a gravitational lens. Instead, it’s from the the trajectories of the photons being distorted by the immense gravitational field of the hole, causing them to make a partial loop around the hole before heading towards us.
The situation becomes more complicated if we assume, as the authors do, that matter or light can pass back and forth through a wormhole. If so, they expect “signals from the region beyond the throat may reach our universe.” These will alter the polarized image of the disc we see around the hole, with light emerging from elsewhere having distinct polarization properties. This could provide what the authors call “a characteristic signature for detecting wormhole geometry.”
Besides the interest of finding wormholes to confirm their existence, and the fact that they could make interstellar travel possible, it is a good idea to be able to distinguish them from black holes before getting too close. . “If you were nearby, you would know too late,” Nedkova told New Scientist. “You will get to know the difference when you die or cross over.”
The authors acknowledge that their conclusions are drawn from a “simplified model of a magnetized fluid ring” orbiting the black hole. More advanced models may reveal differences that could be used to distinguish the wormhole from the black hole in other ways.
The article is published in Physical Review D.
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