34 | New Scientist | 2 November 2019
physicists to accept than the existence of
the aether, she says. “All I have to do is change
how I think about space and time.”
Within a decade, Einstein had enshrined
the predictions of special relativity into his
general theory of relativity, which remains the
foundation of our modern theory of gravity
and how we understand the workings of the
universe on its largest scales. Yet there are a
number of signs that general relativity isn’t the
definitive picture of reality either. For one
thing, it is hopeless at describing the universe
at its smallest scales. Instead, physicists turn
to quantum theory to deal with particles and
their interactions. These two pictures of reality
work brilliantly in their own domains, but have
so far resisted being combined into a theory of
everything.One theory to rule them all
Many attempts, including string theory and
loop quantum gravity, have been made over
the past century to combine relativity with the
quantum world. None has so far succeeded.
One difficulty comes from the two theories
having such differing world views. In general
relativity, the fabric of space-time has to
be continuous. From the perspective of
quantum theory, however, many fundamental
quantities need to be discrete. Energy, for
example, has to come in small bundles rather
than in a constant flow. The same goes for
electric charge, momentum and a host of
other properties. Including, potentially, the
fabric of space-time itself.
For Ted Jacobson at the University of
Maryland, something has to give. “Ever since
I learned about quantum mechanics and
relativity, I thought that somehow space and
time should be quantised,” he says. The concept
of such atoms of space-time, tiny unsplittable
pixels of reality, seems unobjectionable enough.
Einstein’s relativity, however, posits that the
size of a measured object changes depending
on the observer. A shortest unit of space
sounds sensible in practice, says Jacobson,
“but which frame of reference is it shortest in?”
The trouble didn’t end there. In the 1990s,
strong hints emerged that string theory
and loop quantum gravity, when completed,
would feature a preferred frame of reference
too. These results led Jacobson and other
researchers to ask a forbidden question: what if
the aether – or something like it – weren’t dead,
but merely slumbering?In 2000, Jacobson and his colleague David
Mattingly published a variation on general
relativity, now known as Einstein-aether
theory, that allowed an aether-like frame
of reference to exist alongside general
relativity. It rapidly became a widely used
tool, says Diego Blas, a cosmologist at King’s
College London. Their flexible framework let
researchers predict the aether’s impact on the
ways in which gravity shows itself, such as the
rate at which the universe is expanding.
Checking the predictions against real data
didn’t immediately turn up signs of the aether,
but Jacobson was unsurprised. At least for tests
in the model’s early days, he says, “I always
expected that there would be no evidence”. But
absence of evidence is not evidence of absence.NASA/N. SMITH (UNIVERSITY OF CALIFORNIA, BERKELEY) AND NOAO/AURA/NSFIf experiments left reasonable room for an
aether to be hiding within the margins of error,
then it would be worth looking closer.
So far, no identifiable departures from
relativity have appeared in any actual data,
and new experiments are tightening up
those margins all the time. When the first
gravitational waves were detected in 2015,
these vibrations in the fabric of space-time
were found to travel at very close to the speed
of light. This matches Einstein’s 1915
predictions, making the broader predictions
made by Einstein-aether theory seem, once
again, like unnecessary additions. If such a
substance is there at all, according to the data,
it must be so well hidden that it might as well
not exist.The dark substances
that permeate the
cosmos could be the
aether in disguise