24 August 2019 | New Scientist | 37the probabilities, uncertainties and spooky
interactions of quantum physics. They only
ever occur between simple systems such as
single particles on a microscopic scale because
only these can have similar views. Large,
complex systems with many degrees of
freedom – you, me, Schrödinger’s cat – will have
a unique causal past. For us, the closer we are
in space or space-time, the more similar our
view will be. Proximity matters at the classical
scale in a way it doesn’t at a quantum scale.
In a series of recent papers, my collaborators
and I have also shown how to describe an
interaction among the members of each
ensemble that results in the ensemble’s
quantum state evolving in time according
to the laws specified in quantum mechanics.
That gives a simple and elegant solution to
the measurement problem.
There remains the question of what
happens with systems of an intermediate size,
whose causal pasts aren’t unique, but which
might have an intermediate degree of causal
relationship with things far away in space.
These, I predict, should be described by
a tweaked version of quantum physics in
which the superposition principle fails to hold
exactly. It is possible that experimentalists
can construct such systems, and test this
prediction, using the tools of quantum
information. If we can create sufficiently large
and complex entangled states, which would
have no or only a few natural copies within
the universe, our picture predicts that their
evolution in time will deviate from that
predicted by quantum mechanics.
More details need to be filled in. This is just
a sketch of how we might go beyond today’s
quantum picture and construct a unified
physics that sidesteps the fundamental
problems we currently see ourselves facing,
while preserving the best of what we have.
No doubt it isn’t correct in every detail, and
others may come along with other, entirely
different ideas. But the current impasse in
physics suggests that it is only through bold
ideas that we will move forward. ❚events, the more likely they are to interact.
The overall effect of choosing the pair with
the most similar views as parents pushes
both out of the present and into the past.
Removing two very similar views and creating
a new view that is a synthesis of both – and
hence different from both – has the effect of
increasing the total diversity of the views of
all events in the universe. A measure of the
total diversity of an ensemble of views is a
quantity we invented in the late 1980s with
Julian Barbour at the University of Oxford.
We called it the variety of the system.
All this has intriguing consequences.
The views are chosen and evolve precisely so
that the total variety evolves to its maximum –
and it turns out that this exactly reproduces
the dynamics of quantum theory.
You can begin to see how this works.
Similarity of views only implies nearness
in emergent space-time for large, complex
events. If an event has a very simple recent
causal past, there may be other simple events
with similar pasts that aren’t necessarily
nearby in the emergent space-time. Yet by
the principle of similarity, they have a high
probability of interacting with each other.
Einstein and others since have proposed
that quantum wave functions describe
collections, or “ensembles”, of systems
defined by properties they share, but it has
never been clear whether these ensembles
truly exist. In this “real ensemble” picture,
they do. The continual, brazenly non-local
interactions between simple, causally related
objects widely distributed in space explain allLee Smolin is a theoretical physicist
at the Perimeter Institute in Waterloo,
Canada. He is author of Einstein’s
Unfinished Revolution: The search for
what lies beyond the quantumgenerally – is a snapshot of what we see at
any one instant, a two-dimensional surface
formed by photons of different colours,
informing us of our relationships with the
things around us. Because nothing travels
faster than the speed of light, only things
within an event’s sky can influence it,
so the sky is also a view of its causal past.
Sky’s the limit
This picture allows us to describe how
information and energy flow through
events as the universe evolves. Ted Jacobson
at the University of Maryland in the US and
Thanu Padmanabhan at the Inter-University
Centre for Astronomy and Astrophysics in
Pune, India, have independently shown that
the sixth hypothesis, together with the first
law of thermodynamics, which governs
the amount of useful energy available to a
process, can be used to derive the equations
of general relativity, and hence gravity.
Their work assumes that space-time is
always smooth. By marrying their reasoning
with the picture of a prototypical discrete,
quantum space-time in our models, we can
derive both general relativity and smooth
space-time as emerging from a dynamically
evolving causal network.
As well as providing the seed of a quantum
picture of gravity, this immediately solves
the problem of the flow of time in Einstein’s
cosmos. In a causally defined universe, the
most basic interaction is the creation of an
event when two “parent” events come together
to make something new happen. At each stage
in the construction of a space-time history,
the future doesn’t exist. But we can postulate
a limit to the number of events any parent
event can give birth to. Events that have had
their full allotment of progeny cannot have
any further direct influence on the future,
and are relegated to the past: time flows.
The most exciting prospect, which Cortês
and I have been exploring over the past few
years, is that quantum theory might also
emerge from this picture. That comes from
building energetic causal set models to answer
the key question of which events interact.
Events differ from one another in that each
has a different sky, a different view of its causal
past. We can define a measure of how similar
two events’ views are, and pick the pair with
the most similar views to be the parents of the
next event. The idea is that the similarity of
views can play the role that distance in space
does in conventional classical and relativistic
physics. The more similar the views of two
A MANIFESTO FOR
A NEW REALITYSix hypotheses are needed to begin to
rewrite physics with causation at its core –
and perhaps solve the problems of quantum
theory and relativity (see main story).- The history of the universe consists of events
- Time causation is fundamental
- Causation doesn’t go backwards: events
don’t “unhappen” - Space is constructed from the web of
causation between events - Energy and momentum are conserved when
events cause other events - The amount of information that can flow
between events through emerging space
is determined by that space’s area