The effort to unify quantum mechanics and general
relativity means reconciling totally different notions of time.

December 1,
2016

Theoretical physicists striving to unify quantum
mechanics and general relativity into an all-encompassing theory of quantum
gravity face what’s called the “problem of time.”

In quantum mechanics, time
is universal and absolute; its steady ticks dictate the evolving entanglements
between particles. But in general relativity (Albert Einstein’s theory of
gravity), time is relative and dynamical, a dimension that’s inextricably
interwoven with directions

*x*,*y*and*z*into a four-dimensional “space-time” fabric. The fabric warps under the weight of matter, causing nearby stuff to fall toward it (this is gravity), and slowing the passage of time relative to clocks far away. Or hop in a rocket and use fuel rather than gravity to accelerate through space, and time dilates; you age less than someone who stayed at home.
Unifying quantum mechanics and general relativity
requires reconciling their absolute and relative notions of time. Recently, a
promising burst of research on quantum gravity has provided an outline of what
the reconciliation might look like — as well as insights on the true nature of
time.

As I described in an
article this week on a new theoretical attempt to explain away
dark matter, many leading physicists now consider space-time
and gravity to be “emergent” phenomena: Bendy, curvy space-time and the matter within
it are a hologram that arises out of a network of entangled qubits (quantum bits of
information), much as the three-dimensional environment of a computer game is
encoded in the classical bits on a silicon chip. “I think we now understand
that space-time really is just a geometrical representation of the entanglement
structure of these underlying quantum systems,” said Mark Van Raamsdonk, a theoretical physicist
at the University of British Columbia.

In a
new paper, Erik Verlinde of the University of Amsterdam argues that dark matter
is an illusion caused by the holographic emergence of space-time from quantum
entanglement.

Researchers have worked out
the math showing how the hologram arises in toy universes that possess a
fisheye space-time geometry known as “anti-de Sitter” (AdS) space. In these warped worlds,
spatial increments get shorter and shorter as you move out from the center.
Eventually, the spatial dimension extending from the center shrinks to nothing,
hitting a boundary. The existence of this boundary — which has one fewer
spatial dimension than the interior space-time, or “bulk” — aids calculations by
providing a rigid stage on which to model the entangled qubits that project the
hologram within. “Inside the bulk, time starts bending and curving with the
space in dramatic ways,” said Brian Swingle of Harvard and Brandeis
universities. “We have an understanding of how to describe that in terms of the
‘sludge’ on the boundary,” he added, referring to the entangled qubits.

The states of the qubits evolve according to
universal time as if executing steps in a computer code, giving rise to warped,
relativistic time in the bulk of the AdS space. The only thing is, that’s not
quite how it works in our universe.

Here, the space-time fabric has a “de Sitter”
geometry, stretching as you look into the distance. The fabric stretches until
the universe hits a very different sort of boundary from the one in AdS space:
the end of time. At that point, in an event known as “heat death,” space-time
will have stretched so much that everything in it will become causally
disconnected from everything else, such that no signals can ever again travel
between them. The familiar notion of time breaks down. From then on, nothing
happens.

On the timeless boundary of our space-time bubble,
the entanglements linking together qubits (and encoding the universe’s
dynamical interior) would presumably remain intact, since these quantum
correlations do not require that signals be sent back and forth. But the state
of the qubits must be static and timeless. This line of reasoning suggests that
somehow, just as the qubits on the boundary of AdS space give rise to an
interior with one extra spatial dimension, qubits on the timeless boundary of
de Sitter space must give rise to a universe with time — dynamical time, in
particular. Researchers haven’t yet figured out how to do these calculations. “In
de Sitter space,” Swingle said, “we don’t have a good idea for how to
understand the emergence of time.”

One clue comes from theoretical
insights arrived at by Don Page and William Wootters in the 1980s. Page,
now at the University of Alberta, and Wootters, now at Williams, discovered
that an entangled system that is globally static can contain a subsystem that
appears to evolve from the point of view of an observer within it. Called a
“history state,” the system consists of a subsystem entangled with what you
might call a clock. The state of the subsystem differs depending on whether the
clock is in a state where its hour hand points to one, two, three and so on.
“But the whole state of system-plus-clock doesn’t change in time,” Swingle
explained. “There is no time. It’s just the state — it doesn’t ever change.” In
other words, time doesn’t exist globally, but an effective notion of time
emerges for the subsystem.

A team of Italian
researchers experimentally demonstrated this phenomenon in
2013. In summarizing their work, the group wrote: “We show how a static,
entangled state of two photons can be seen as evolving by an observer that uses
one of the two photons as a clock to gauge the time-evolution of the other
photon. However, an external observer can show that the global entangled state
does not evolve.”

Other theoretical work has
led to similar conclusions. Geometric patterns, such as the amplituhedron, that describe the
outcomes of particle interactions also suggest that reality emerges from
something timeless and purely mathematical. It’s still unclear, however, just
how the amplituhedron and holography relate to each other.

The bottom line, in Swingle’s words, is that
“somehow, you can emerge time from timeless degrees of freedom using
entanglement.”Time will tell.

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Natalie Wolchover is a senior writer at

*Quanta Magazine*covering the physical sciences. Previously, she wrote for*Popular Science*,*LiveScience*and other publications. She has a bachelor’s in physics from Tufts University, studied graduate-level physics at the University of California, Berkeley, and co-authored several academic papers in nonlinear optics. Her writing was featured in*The Best Writing on Mathematics 2015*. She is the winner of the 2016 Excellence in Statistical Reporting Award and the 2016 Evert Clark/Seth Payne Award for young science journalists.**SOURCE:**

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