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| Can gravity create light? (Image credit: dani3315 via Getty Images) |
Gravity can turn itself into light, but only if space-time behaves in just
the right way, a research team has found.
Under normal circumstances, you cannot get something from nothing.
Specifically, the Standard Model of particle physics, the reigning theory
that explains the subatomic zoo of particles, usually forbids the
transformation of massless particles into massive ones. While particles in
the Standard Model constantly change into each other through various
reactions and processes, the photon — the massless carrier of light — cannot
normally change into other particles. But if the conditions are just right,
it is possible — for example, when a photon interacts with a heavy atom, it
can spontaneously split off to become an electron and a positron, both of
which are massive particles.
With this well-known example in hand, a team of theoretical physicists,
writing in a paper posted March 28 to the preprint
database arXiv, asked if gravity itself could transform into other particles. We normally
think of gravity through the lens of general relativity, where bends and
warps in space-time influence the motion of particles. In that picture, it
would be very difficult to imagine how gravity could create particles. But
we can also view gravity through a quantum lens, picturing the gravitational
force as carried by countless invisible particles called gravitons. While
our picture of quantum gravity is far from complete, we do know that these
gravitons would behave like any other fundamental particle, including
potentially transforming.
To test this idea, the researchers studied the conditions of the extremely
early universe. When our cosmos was very young, it was also small, hot and
dense. In that youthful cosmos, all forms of matter and energy were ramped
up to unimaginable scales, far greater than even our most powerful particle
colliders are capable of achieving.
The researchers found that in this setup, gravitational waves — ripples in
the fabric of space-time generated by collisions between the most massive
cosmic objects — play an important role. Normally, gravitational waves are
exceedingly weak, capable of nudging an atom through a distance less than
the width of its own nucleus. But in the early universe, the waves could
have been much stronger, and that could have seriously influenced
everything else.
Those early waves would have sloshed back and forth, amplifying themselves.
Anything else in the universe would have gotten caught up in the push and
pull of the waves, leading to a resonance effect. Like a kid pumping their
legs at just the right time to send a swing higher and higher, the
gravitational waves would have acted as a pump, driving matter into tight
clumps over and over again.
The gravitational waves could also affect the electromagnetic field. Because
the waves are ripples in space-time itself, they don't limit themselves to
interactions with massive objects. As the waves continue to pump, they can
drive radiation in the universe to extremely high energies, causing the
spontaneous appearance of photons: gravity generating light itself.
The researchers found that in general, this process is rather inefficient.
The early universe was also expanding, so the standard patterns of
gravitational waves would not have lasted long. However, the team found that
if the early universe contained enough matter that the speed of light was
reduced (the same way light travels more slowly through a medium such as air
or water), the waves could have stuck around long enough to really get
things going, generating floods of extra photons.
Physicists do not yet fully understand the complicated, tangled physics of
the early universe, which was capable of achieving feats never observed
since. This new research adds one more strand to the rich tapestry: the
capability for gravity to create light. That radiation would presumably then
go on to influence the formation of matter and the evolution of the
universe, so working out the full implications of this surprising process
could lead to new revolutions in our understanding of the earliest moments
of the cosmos.