The known universe is full of exciting things like black holes, hypernovas, and merging neutron stars. All of those, however, look tame compared to items physicists think might exist but have yet to find. Perhaps chief among these are wormholes, which theoretically join parts of space and time, allowing those who enter them a shortcut to distant locations.
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 they exist, or at least that three-dimensional objects could pass through them unscathed, but the mere chance was enough for writers to drive a spaceship, or at least a novel, through.
As telescopes advance, however, a question becomes more troubling: if wormholes are real, why haven’t we found any? Four Bulgarian physicists have proposed an answer in Physical Review D: maybe we have and just haven’t recognized them.
The vast majority of black holes we have identified are known either from their gravitational effects on stars around them, or from the jets of material shooting out of their accretion disks. If any of these were actually wormholes, it’s unlikely we would know. However, the Event Horizon Telescope Collaboration’s observation of the polarization around M87* and its follow-up on Sagittarius A* are a different 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 sufficiently exciting to physicists that 12 papers have been posted to ArXiv.org exploring the concept just since the start of November. However, as Petya Nedkova of the University of Sofia and co-authors note, we don’t know what they would look like.
The paper seeks to address that and concludes that, seen at high angles, wormholes would look like nothing we have seen. For small inclination angles, however, the authors think a wormhole would show “a very similar polarization pattern” to a black hole. Consequently, M87*, seen at an estimated angle of 17°, could be a wormhole and we wouldn’t know.
That’s not to say we are doomed to not be able to tell wormholes from black. “More significant distinctions are observed for the strongly lensed indirect images, where the polarization intensity in the wormhole spacetimes can grow up to an order of magnitude compared to the Schwarzschild black hole,” the authors write. The lensing referred to here is not from a massive object between us and the hole creating a gravitational lens. Instead, it is from the paths of photons being distorted by the immense gravitational field of the hole, causing them to complete a partial loop around the hole before heading toward us.
The situation is complicated further if we assume, as the authors do, that material or light could pass in either direction through a wormhole. If this is the case, they expect that “signals from the region beyond the throat are able to reach our universe.” These will change the polarized image of the disk we see around the hole, with the light emerging from elsewhere having distinct polarization properties. This could provide what the authors call “a characteristic signature for the detection of wormhole geometry”.
Besides the interest in finding wormholes to confirm they exist, and the fact they might make interstellar travel possible, it's a good idea to be able to distinguish them from black holes before getting too close. “If you were nearby, you would find out too late,” Nedkova told New Scientist. “You’ll get to know the difference when you either die or you pass through.”
The authors acknowledge 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 wormhole from black hole in other ways.
The paper is published in Physical Review D.