When Shruti Paranjape,
a physicist then at the University of California, Davis,
saw a slide at a conference last December that listed which quantum theories shared “hidden” zeros,
she felt a flash of recognition.
They were all theories in which it was possible to combine two copies of amplitudes of one theory to make an amplitude of another theory
— a somewhat mysterious operation known as the "double copy".
She and her collaborators showed that if one could double copy a theory,
then that theory would have the zeros Figueiredo had found.
It’s another hint that more theories might be brought into the geometric fold.
The original surfaceology group is following up on indications that their curves know about much more than just amplitudes for colored particles.
The typical procedure is to draw only curves that don’t cross themselves.
But if you include the self-intersecting curves, the researchers noticed, you get a strange-looking amplitude,
which turns out not to describe collisions
between particles
but rather tangled interactions between longer objects known as strings.
Thus, surfaceology appears to be another route to string theory,
a candidate theory of quantum gravity that posits that quantum particles are made of vibrating strings of energy.
“This formalism, as far as we can tell,
contains string theory but allows you to do more things,” Arkani-Hamed said.
Surfaceology might also account for gravitons,
the particles thought to impart the gravitational force.
While working out how much each curve would contribute to a trace phi cubed amplitude,
the group came across curves that were unavoidable but that didn’t change the final answer.
If the surface had holes, these curves circled around the holes forever,
never taking an exit.
From the space-time perspective, these curves capture events beyond the purview of trace phi cubed theory:
colorless particles that the researchers believe could eventually describe gravitons.
That would be a crucial step toward Arkani-Hamed’s ultimate aspiration of developing a novel theoretical framework for quantum gravity.
“We don’t yet have a complete working picture of something gravitational,” Arkani-Hamed said.
“But there are more and more hints that gravity is going to come along.”
There’s more to quantum gravity than gravitons,
which would represent just the mildest ripples in space-time.
A full theory would need to go beyond ripples to describe what happens when stars collapse and form black holes,
warping the space-time fabric to oblivion.
It should also account for how space-time came into existence during the Big Bang.
Feynman diagrams capture only the minimal ripples of a quantum field and nothing more.
So the full picture
— what physicists call a “nonperturbative” theory
— might be beyond the reach of the geometric paleophysics that these researchers are exploring.
“I would be surprised if somehow this told us how to build space-time,” said Daniel Harlo,
a theoretical physicist at the Massachusetts Institute of Technology.
“My bias is that all the good stuff in quantum gravity is nonperturbative.”
Harlow is pursuing another popular research program,
known as #holography, that seeks to capture space-time in its entirety,
including the interiors of black holes,
by treating it as a hologram of quantum particles moving around in one lower dimension.
