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| Illustration, Credit: istock |
If wormholes in space exist, they look a lot like black holes from a
particular angle, physicists claim, raising the possibility we’ve seen
examples of this long-sought phenomenon without knowing it.
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.
Reference:
Valentin Deliyski et al, Polarized image of equatorial emission in horizonless
spacetimes: Traversable wormholes, Physical Review D (2022).
DOI: 10.1103/PhysRevD.106.104024