Time Reborn
By
James Gleick The New York Review of Books, June 6, 2013
Edited by Andy Ross
Lee Smolin aims to convince us that time is real: "Not only is time real,
but nothing we know or experience gets closer to the heart of nature than
the reality of time."
Thus he contradicts Einstein. The past is gone
and the future is open. Things change, and time is our name for the
reference frame in which we organize our sense that one thing comes before
another.
Clocks measure time. In fact you can define time as what
clocks measure. For Newton, the cosmic clock ticks invisibly and inexorably,
everywhere the same. He needed absolute time and space to define his terms
and express his laws. With them he built an entire cosmology.
Newton
reified time. When a scientist records the position of the Moon, the result
is a table of numbers representing both space and time. Representing the
orbit of the Moon in Cartesian coordinates makes it a curve in space and
time, a mathematical object in a timeless configuration space.
Smolin: "The method of freezing time has worked so well that most physicists
are unaware that a trick has been played on their understanding of nature."
We have inherited the idea of timeless truths from Plato. A leaf fades
from green to brown, but greenness and brownness are immutable. Here in the
sublunary world everything is subject to change and nothing is perfect. But
in the mathematical world, truth exists outside of time.
Smolin:
"Whatever we most admire and look up to — God, the truths of mathematics,
the laws of nature — is endowed with an existence that transcends time."
We reenter time when we accept uncertainty. The prototype for thinking
in time is Darwinian evolution. Natural processes lead to genuinely new
organisms, new structures, new complexity, and new laws of nature. All is
subject to change. Laws are not timeless.
The faith in timeless laws
of nature is part of the appeal of the scientific enterprise. It is a vision
of transcendence akin to the belief in eternity that draws people to
religion. The explanations for our world lie in another, more perfect world.
But perhaps timeless laws of nature are no more real than perfect circles.
The cosmic clock of Newton (or God) ticks no more. Einstein broke it. He
did this by pointing out that every observer has a reference frame, and each
reference frame includes its own clock. Simultaneity is not meaningful. Now
is relative. No observer has access to the now of any other observer.
Everything that reaches our senses comes from the past.
Thus space
and time are wedded. One cannot be measured independently of the other.
Spacetime becomes indispensable. Time is frozen into the 4D block. Only to
the gaze of my consciousness, crawling upward along the world line of my
body, does a section of the world come to life as a fleeting spatial image
that continuously changes in time.
Smolin: "Everything we experience,
every thought, impression, action, intention, is part of a moment. The world
is presented to us as a series of moments. We have no choice about this. No
choice about which moment we inhabit now, no choice about whether to go
forward or back in time. No choice to jump ahead. No choice about the rate
of flow of the moments. In this way, time is completely unlike space."
For space, the deeper reality is a network of relationships. Things are
related to other things. They are connected, and the relationships define
space. Smolin believes that time is fundamental but space an illusion. The
real relationships that form the world are a dynamical network. The network
can and must evolve over time.
Time runs one way. The universe grows
ever more structured and complex, in apparent contradiction to the second
law of thermodynamics. Smolin says the second law of thermodynamics applies
to any isolated system within the universe but not to the universe as a
whole. In a universe where time is real and fundamental, complexity evolves
and systems become more organized.
By declaring space to be
secondary, Smolin avoids contradicting general relativity. If size and
location are relative, then time doesn't need to be. A preferred global time
extends throughout the universe and defines a boundary between past and
future. Now need not be the same to different observers, but it retains its
meaning for the cosmos.
Smolin: "The world remains, always, a bundle
of processes evolving in time."
AR This view of time is strongly reminiscent of
that I developed in my 2006 paper "About time" (chapter 13 of my book
Mindworlds). In fact I
sent a copy of the paper to Smolin, but he didn't reply. Perhaps this new
position is the result.
Physics: The Limits 1
By David J. Gross Wired, June 2013
Edited by Andy Ross
When I was a graduate student in California, experimentalists were
constantly discovering new atomic particles and quantum field theory was
failing to explain them. For quantum mechanical theories to be consistent
with the constraints of special relativity, we picture the interactions
between charged particles as flowing through a quantum mechanical field, a
spatial field. Ripples in the field can be treated as electromagnetic waves
or radiation or light. And these ripples can also be described as particles
that transmit the forces of nature through space.
When I was at
Berkeley, the framework of quantum field theory could calculate the dynamics
of electromagnetism. It could roughly describe the motion of the weak
nuclear force, radiation. But it hit a brick wall with the strong
interaction, the binding force. The experimenters were banging protons,
hoping to find direct evidence of quarks. Protons are bags of quarks, but
there is no such thing as an individual quark. We glimpse them only
indirectly, by measuring the energies and momentums emerging from proton
collisions.
Using quantum field theory, my colleagues and I predicted
certain patterns in the proton collision detritus. To our surprise, the
calculations showed that the invisible quarks are not purely mathematical
abstractions, but particles that can move about freely inside the proton
when they are close together. And we learned that as the distance between
the quarks increases, the force binding them together also increases. It was
the only explanation of the strong force that could be calculated. The
Standard Model is a very precise, reductionist theory.
Physics
explains the world around us with incredible precision and breadth. But
further explanation is highly constrained by what we already know. String
theory is a model, a framework, part of quantum field theory. And there are
frustrating theoretical problems in quantum field theory that demand
solutions. Our model of spacetime might be a derived concept. It seems to
emerge from a more fundamental physical process that informs the
mathematical pictures drawn by string theory and quantum field theory.
Physics: The Limits 2
By Margaret Wertheim Aeon, June 2013
Edited by Andy Ross
Things at the subatomic level are simultaneously particles and waves. They
appear to us as two different categories of being. Physics itself is riven
by the competing frameworks of quantum theory and general relativity, whose
differing descriptions of our world mirror the waveparticle tension. Where
quantum theory describes the subatomic realm as a domain of individual
quanta, all jitters and jumps, general relativity depicts happenings on the
cosmological scale as a stately flow of smooth spacetime.
Relativity
and quantum theory each pose philosophical problems. Are space and time
fundamental qualities of the universe, as general relativity suggests, or
are they byproducts of something even more basic, something that might arise
from a quantum process? Looking at quantum mechanics, huge debates swirl
around the simplest situations. The dilemma posed by waveparticle duality
is the tip of an epistemological iceberg on which many ships have been
broken and wrecked.
The manyworlds interpretation of quantum theory
proposes that every time a subatomic action takes place the universe splits
into multiple copies, with each new world representing one of the possible
outcomes. The equations are taken to be the fundamental reality. The fact
that the mathematics allows for gazillions of variations is seen to be
evidence for gazillions of worlds.
This kind of reification of
equations strikes some humanities scholars as childishly naive. At least it
raises questions about the relationship between our mathematical models and
reality. Many important discoveries have emerged from revelations within
equations, but it is hard not to feel skeptical about the idea that the only
way forward now is to accept an infinite cosmic landscape of universes that
embrace every conceivable version of world history.
The late British
anthropologist Mary Douglas studied taboo rituals that deal with the
unclean. All languages parse the world into categories, and all category
systems contain liminal confusions, and she proposed that such ambiguity is
the essence of what is seen to be impure or unclean.
Cultures can be
categorized in terms of how well they deal with linguistic ambiguity. Some
cultures accept the limits of their language by understanding that there
will always be things that cannot be cleanly parsed. Others become obsessed
with ever finer levels of categorization. Perhaps what we are encountering
here is not so much the edge of reality, but the limits of the physicists’
category system.
According to Galileo Galilei and others, nature was
a book written by God, who had used the language of mathematics because it
was transcendent and timeless. But to articulate a more nuanced conception
of what physics is, we need to abandon the loaded metaphor of the cosmic
book and focus on the creation of physics as a science.
Much of
physics involves finding ways to measure physical phenomena. Physics is an
ever more sophisticated process of quantification that multiplies and
diversifies the ways we extract numbers from the world, thus giving us the
raw material for our quest for patterns or laws.
To a large degree,
progress in physics has been made by slowly extending the range of phenomena
we can measure. The discovery of electromagnetic waves was a triumph of
quantification. James Clerk Maxwell showed that magnetic and electric fields
were linked by a precise set of equations that enabled him to predict the
existence of radio waves. The quantification of these fields has led to the
whole world of modern telecommunications.
Light acts like a wave, yet
experiments show that under many conditions it behaves like a stream of
particles. And particles of matter can sometimes behave like waves.
Electrons are clearly particles, yet in orbiting around atoms they behave
like waves. Waveparticle duality is a core feature of our mathematical
descriptions of our world, but the universe remains whole.
Returning
to quantum theory and relativity, subatomic particles can be entangled. Once
particles are entangled, what we do to one immediately affects the other,
contradicting a basic premise of special relativity. Entanglement suggests
that either quantum theory or special relativity, or both, will have to be
rethought. We are in a mire of contradiction and need some new physics.
Subjective experience might not be amenable to mathematical law. Many
paradoxes relating to relativity and quantum theory focus on the issue of
time, and our mathematical descriptions of time conflict with our lived
experience of time. Lee Smolin says we must change them.
SpaceTime Divorce?
By Anil Ananthaswamy New Scientist, June 2013
Edited by Andy Ross
Relativity and quantum mechanics differ radically in form and content. Sean
Carroll: "One of the tensions comes from the fact that the relation between
space and time is very, very different in general relativity than it is in
quantum mechanics."
In 1905, Einstein wove space and time into the 4D
fabric of spacetime. Here and now mean different things to people moving at
different speeds. In 1916, he said massive objects curve spacetime, and
measurements of lengths and times depend on the strength of the prevailing
gravitational field.
In quantum mechanics, an object's state is
described by a wave function in an abstract Hilbert space that encompasses
all the possible states of the object. The Schrödinger equation tells us how
the wave function evolves in time, moving from one state in its Hilbert
space to another. Time is not part of the Hilbert space. We measure the
evolution of a quantum state to the beat of an external clock.
The
status of space depends on what you measure. The wave function of an
electron orbiting the atomic nucleus has the spatial property of distance
from the nucleus. But the wave function describing the quantum spin of an
isolated electron has no mention of space. Abhay Ashtekar: "This is one
sense in which there are attributes of physical systems which don't refer to
space, but which change in time. One could say that for those attributes,
time is more fundamental than space."
Relativity says space and time
together form the fabric of reality. Quantum mechanics treats time and space
differently, with time occasionally seeming more fundamental.
String
theory needs at least 10 spacetime dimensions to be mathematically
consistent. But according to Juan Maldacena's "antide Sitter/conformal
field theory correspondence" (AdS/CFT), you can sometimes swap the 10D
representations of string theory that include gravity for a more tractable
4D representation that dispenses with gravity.
The time dimension
seems unchanged, but space is transformed: a point in the 4D world
translates to multiple points within the 10D world. Carroll: "In this
example it seems perfectly clear that space is not fundamental. It is very,
very different depending on what description of the world you are using."
Joe Polchinski has doubts. The AdS/CFT correspondence is only valid for
a negatively curved spacetime with hyperbolic geometry. No one has yet
worked out an AdS/CFTlike correspondence for our spacetime. Also, if you
want to let information escape from a black hole, quantum theory says a
"firewall" of highenergy radiation appears just inside the event horizon.
General relativity says anything going past a black hole's event horizon
should encounter nothing but gently curved spacetime. If you want to keep
quantum mechanics intact and avoid a firewall too, something else must give,
such as the limiting speed of light. Steve Giddings: "This does point to the
fact that we may be missing something in our conceptual description."
Polchinski's team turned to Maldacena's conjecture. They put a black
hole into a volume of negatively curved spacetime. There the 4D physics of
an observer on the surface of the volume should be able to account for the
physics of an observer deep inside the 10D bulk of a black hole. Instead,
what the two observers see is described by two different quantum theories.
Polchinski: "I want to shake people's faith in AdS/CFT."
But string
theory is just one approach. Loop quantum gravity arose when Ashtekar
rewrote Einstein's equations of general relativity using a quantum
mechanical framework. Working with Lee Smolin and Carlo Rovelli, he arrived
at a picture in which spacetime is smooth down to the Planck scale, where
you see loops of gravitational field lines.
Loop quantum gravity
provides a different perspective on space and time. Chunks of space, one
Planck length to a side, appear first in the theory, while time pops up only
later as an expression of the relationships between other observable
physical properties. Ashtekar: "Somehow space might emerge first, and time
is born by observing relations between various physical subsystems."
Giddings has been trying to describe a black hole using a network of
interconnected Hilbert spaces that do not presuppose the existence of space
or time. He showed last year how time can emerge relationally. A concept of
space also emerges from his calculations.
Polchinski: "The direction
that light rays travel in is neither space nor time. We call it null. It's
on the edge between space and time. A lot of people have this intuition that
in some sense the existence of these null directions might be more
fundamental than space or time."


