36 | New Scientist | 24 August 2019
malleable space-time. By contrast, quantum
theory suggests that it must at some level
come in discrete chunks, or quanta, of space
or space-time.
We have at least half a dozen ways to get
part of the way across this divide, among
them string theory and loop quantum gravity.
Indeed, the latter idea gives precise predictions
for what the quanta of space-time must look
like. But we have no idea whether any of the
suggested routes are the right one because
none predicts an experimental test we can
perform with current technology.
Quantum theory and general relativity
clash in other ways, too, notably over the
nature of time. Relativity makes it impossible
to establish one objective “flow” of time of
the sort we perceive, with a past and a future
separated by a universally defined now.
Quantum theory, meanwhile, characterises
time as a metronomic “beat” set somewhere
outside the universe. So is our perception of
a flowing time real, or an illusion?Back to basics
There are other deep questions. The quantum
descriptions of the other three fundamental
forces – electromagnetism and the weak
and strong nuclear forces – can be bundled
together into the so-called standard model of
particle physics. But why do these three forces
have such very different strengths within the
standard model? Then there is the nature of
the dark matter and dark energy that dominate
the cosmos on a large scale, but which the
standard model doesn’t mention. These
questions and others concern how our
universe came to be, out of a vast number
of seemingly equally probable universes
allowed by the laws of physics.
To solve all these issues, we need to wipe
the slate clean, go back to the first principles
of quantum theory and general relativity,
decide which are necessary and which are
open to question, and see what new principles
we might need. Do that, and an alternative
description of physics becomes possible, one
that explains things not in terms of objects
situated in a pre-existing space, as we do now,
but in terms of events and the relationships
between them.
This endeavour starts with a few basic
hypotheses about the nature of space and
time. First, that the history of the universe
consists of events and the relationships
between them. Second, that time – in the
sense of causation, the process by which
future events are produced from presentevents – is fundamental. Third, that time is
irreversible: causation can’t go backwards,
and once an event has happened, it can’t
be made to unhappen. Fourth, that space
emerges from this description: events cause
other events, creating a network of causal
relationships. The geometry of space-time
arises as a coarse-grained and approximate
description of this network.
A fifth hypothesis is that energy and
momentum are fundamental features of
the universe, and are conserved in causal
processes. These five hypotheses define a class
of models called energetic causal set models
that my collaborator Marina Cortês of the
Royal Observatory in Edinburgh, UK, and
I introduced in 2013. I have since added a
sixth hypothesis, a version of the holographic
principle first stated by ’t Hooft. This says that
when two-dimensional surfaces are defined
in the emerging geometry of space-time,
their area gives the maximum rate by which
information can flow through them.
In this picture, every event is distinguished
by the information available to it about its
causal past. We call this the event’s sky because
it functions rather like the sky above us does.
The sky – or the horizon of our sight more“ To solve the
problems of
physics, we
need to decide
which of its
principles are
open to question”