CRONOS
The End of Time
The Next Revolution in Physics
JULIAN BARBOUR
A revolutionary new theory that attacks one of the foundation
stones of science--the existence of time
Richard Feynman once quipped that "Time is what happens when
nothing else does." But Julian Barbour disagrees: if nothing
happened, if nothing changed, then time would stop. For time is
nothing but change. It is change that we perceive occurring all
around us, not time. Put simply, time does not exist.
In this highly provocative volume, Barbour presents the basic
evidence for a timeless universe, and shows why we still experience
the world as intensely temporal. It is a book that strikes at the heart
of modern physics. It casts doubt on Einstein's greatest contribution,
the spacetime continuum, but also points to the solution of one of the
great paradoxes of modern science, the chasm between classical
and quantum physics. Indeed, Barbour argues that the holy grail of
physicists--the unification of Einstein's general relativity with
quantum mechanics--may well spell the end of time.
Barbour writes with remarkable clarity as he ranges from the
ancient philosophers Heraclitus and Parmenides, through the giants
of science Galileo, Newton, and Einstein, to the work of the
contemporary physicists John Wheeler, Roger Penrose, and Steven
Hawking. Along the way he treats us to enticing glimpses of some
of the mysteries of the universe, and presents intriguing ideas about
multiple worlds, time travel, immortality, and, above all, the illusion
of
motion.
The End of Time is a vibrantly written and revolutionary book. It
turns our understanding of reality inside-out.
"This book is gold.... Barbour leaves his mark on every topic
he considers, including the arrow of time and the origins of the
Big Bang...his book is a masterpiece."--The New York Times
Book Review
"The orderly flow of events may really be as much an illusion
as the flickering frames of a movie. And according to
independent physicist Barbour's new book, even the apparent
sequence of the flickers is illusory.."--Time
New and recent titles of related interest:
General Science
Julian Barbour is a theoretical physicist who has worked on
foundational issues in physics and astronomy for 35 years. His first
book, the widely praised The Discovery of Dynamics, has recently
been republished in paperback. In 2000 the Association of American
Publishers awarded The End of Time its prestigious award for
excellence in the Physics & Astronomy section. Julian Barbour, a
theoretical physicist, has worked on foundational issues in physics
for 35 years. He _ is the author of the widely praised Absolute or
Relative Motion?: Volume I, and is working on the second volume.
Book review by Anthony Campbell. Copyright © Anthony Campbell
(2001).
There have been many books about the paradoxes of time, but this one
is far and away more paradoxical than most, for its thesis is
that time doesn't exist at all. This idea is explored in great detail,
mostly in relation to physics but the philosophical and even the
theological implications are touched on in an Epilogue. In fact, Barbour
himself is something of a paradox, for he is a respected
theoretical physicist who has remained independent, supporting himself
and his family by translation while working on his ideas
about time whenever he could. However he continued to keep in touch
with mainstream physics and he acknowledges help from a
number of well-known scientists, including Lee Smolin among others.
The book, he tells us, is intended to interest that fabled beast, the
"general reader", but he also expects his colleagues to look over
his shoulder. He mentions Roger Penrose and Richard Dawkins as other
scientists who have written similar types of books, and
his own will certainly bear comparison with theirs . Like those authors,
Barbour is an excellent writer, and his interests include
philosophy, art, and literature as well as science so that his view
of his subject is multidimensional; no nonsense about "two
cultures" here.
In the first part of the book, Barbour sets forth a number of basic
ideas which are then progressively elaborated in later parts. The
central mystery that he confronts in his first three chapters is best
described in his own words.
"The main aim is to introduce a definite way
of thinking about instants of time without having to suppose that they
belong to something that flows relentlessly
forward. I regard instants of time as real things, identifying them with
possible instantaneous arrangements of all
the things in the universe. They are configurations of the universe. In
themselves, these configurations are perfectly
static and timeless. But how and why can something static and
timeless be experienced as intensely dynamic
and temporal?"
These instants of time Barbour calls "Nows" or "time capsules". Examples
include long-term memory stored within the brain,
fossils and geological records, and the human body, which contains
within itself more time capsules nested one within another
(cells, genes). The universe is composed of things like this, and it
exists within what Barbour calls Platonia, which is an
unimaginably vast configurational space. To help us understand what
this means, Barbour uses the analogy of Triangle Land: the
different ways that triangles can be arranged in a configurational
space. Much poring over of diagrams is needed here and
elswhere in the book if his argument is to be grasped, and my attempt
to do so wasn't helped initially when a Necker cube illusion
made me think that a diagram was meant to represent a solid cube whereas
it's really supposed to be hollow. A note in the legend
might have made this clearer.
In fact, this book does demand close attention from the reader, in spite
of the beautiful clarity of the text. The difficulties are of at
least two kinds. One comes from the complexity of the physics. The
theory that Barbour advances has profound implications for
both quantum mechanics and general relativity, and these implications
are examined in considerable depth. Part of Barbour's thesis
is that his approach will reconcile these two fundamental scientific
theories, something that ultimately baffled Einstein and has still
not been achieved. Certain more technical or more peripheral sections
are enclosed in boxes, so the reader can skip them if
necessary, and other material is placed in the notes at the end. Even
so, Barbour pays his reader the compliment of thinking that he
or she is willing to come to terms with some pretty deep ideas.
The other. and even more profound, kind of difficulty arises from the
nature of the theory itself. We feel as if we have our being in
time. If there is no time, what meaning can we attach to the notions
of past or future, and - even more difficult to accommodate -
our peception of motion? Barbour suggests that what we see as motion,
in a leaping cat or a diving kingfisher, is really a series of
still photographs, which are somehow brought together by the brain
to produce an illusion of movement. Of possible relevance here
is the very interesting fact that in certain kinds of brain damage
the ability to perceive objects in motion is lost. Barbour mentions
this, but not the equally interesting observation, recorded by Oliver
Sacks, that some patients suffering from post-encephalitic
Parkinsonism found themselves frozen in time for years, until released
from this state, though only temporarily, by the drug
levodopa.
Trying to picture oneself in a timeless state is probably something
like a fish would feel if it tried to picture itself out of the water.
Our language, of course, has no vocabulary to describe this, and Barbour
finds himself repeatedly forced to use temporal language
to describe his theory, even though he acknowledges that this is just
shorthand. Indeed, even if he is right, will it ever be possible to
feel that he is? The analogy that comes to mind here is with the shift
from a geocentric to a heliocentric universe that took place in
the sixteenth century; no doubt many people, and not only churchmen,
found that hard to come to terms with, but the imaginative
shift from a time-based to a timeless universe would be incomparably
bigger.
But is Barbour right? This is a mainly technical question for physicists
and cosmologists, but he does suggest some ways in which
the theory can be tested; in other words, it is intended to be physics,
not metaphysics. Neverthless, profound metaphysical
questions are inevitably raised. Does free will exist? Is there room
for a Creator? Where is Heaven (and Hell)? Is time travel
possible? Doesn't the denial of motion and change take all the joy
out of life? Some of these are old chestnuts, of course, but they
would need to be radically re-evaluated if Barbour's theory were to
become widely accepted. And, of course, the field is open for
anyone who wants to have a go; Barbour merely opens the way for us
to think about the implications. For himself, he finds that his
ideas, and those of Lee Smolin (who wrote "The Life of the Cosmos"),
tend towards pantheism. "The whole universe ... is the
closest we can get to a God."
DISCOVER Vol. 21 No. 12 (December 2000)
Table of Contents
From Here to Eternity
Imagine a universe with no past or future, where time
is an illusion and everyone is immortal. Welcome to
that world, says physicist Julian Barbour
By Tim Folger
Time seems to stand still in south newington, a secluded village
ringed by rolling
green hills about 20 miles north of Oxford, England. The 1,000-year-old
baptismal font in the town's church, the thatch-roofed houses,
and the tidy
gardens along narrow lanes all appear unchanged by the passage
of centuries.
Standing on the roof of the church's bell tower on a warm, late-summer
day,
Julian Barbour, a theoretical physicist with some extraordinary
notions about the
nature of time, points to his home, known as College Farm, which
borders the
ancient church.
"It looks almost exactly as it did when it was built 340 years
ago," says Barbour.
"The barn is also from the 17th century. Virtually all the houses
you see around
are from about 1640 to 1720. The long, low house is the one
I grew up in.
That's my parents' house. It dates from about 1710 to 1720."
The entire scene is
so placid one can't help but imagine that Barbour's childhood
home, as well as
the village and the surrounding landscape, will remain unchanged
for the next
340 years.
Such utter quiescence suits Barbour, who is convinced the static
harmony of
South Newington extends past the horizon to the universe at
large. In his view,
this moment and all it holds— Barbour himself, his American
visitor, Earth, and
everything beyond to the most distant galaxies— will never change.
There is no
past and no future. Indeed, time and motion are nothing more
than illusions.
In Barbour's universe, every moment of every individual's life—
birth, death,
and everything in between— exists forever. "Each instant we
live," Barbour
says, "is, in essence, eternal." That means each and every one
of us is immortal.
Like the perpetually unmoving lovers in Keats's "Ode on a Grecian
Urn," we
are "for ever panting, and for ever young." We are also for
ever aged and
decrepit, on our deathbeds, in the dentist's chair, at Thanksgivings
with our
in-laws, and reading these words.
Barbour fully realizes how outrageous the notion of a world without
time
sounds. "I still have trouble accepting it," he says. But then,
common sense has
never been a reliable guide to understanding the universe— physicists
have
been confounding our perceptions since Copernicus first suggested
that the sun
does not revolve around Earth. After all, we don't feel the
slightest movement
as the spinning Earth hurtles through the void at some 67,000
miles per hour.
Our sense of the passage of time, Barbour argues, is just as
wrongheaded as
the credo of the Flat Earth Society.
Barbour has been preoccupied with studying the basic properties
of time for
four decades. It's an issue he believes most theoretical physicists
have
ignored."Given what a fascinating thing time is, it's surprising
how few
physicists have made a serious attempt to study time and say
exactly what it is,"
he says. "It's an unusual gap." At the outset Barbour didn't
think he would have
any fresh insights he could bring to the topic. "I don't regard
myself as being at
all talented. I struggle to do equations," he says, laughing.
"But I just got very
interested in the subject and found that very few people have
really thought
seriously about it."
Perhaps Barbour himself wouldn't have been able to devote nearly
40 years of
his, well, time to the problem if it hadn't been for his unique
background. Unlike
most of his colleagues, he doesn't work at a university or a
government lab— he
is one of the world's few freelance theoretical physicists.
Nevertheless, his
credentials are solid, and prominent physicists take him— and
his
unconventional ideas— quite seriously.
"He has some wild ideas, but he definitely knows what he's talking
about when
it comes to these fundamental issues," says Carlo Rovelli, who
works at the
Center for Theoretical Physics in Luminy, France. Lee Smolin,
a theoretical
physicist at Pennsylvania State University, agrees: "Barbour
is one of the few
people I know who went out on their own and succeeded in doing
several things
that were important and would not have been easy to do in a
conventional
career."
After receiving his doctorate in physics from the University
of Cologne in 1968,
Barbour, who is now 63, decided he didn't want to follow a traditional
academic
career, with the inevitable pressure to publish or perish. So
he supported his
wife and four children by translating Russian scientific articles
and worked on
physics on the side, publishing scholarly papers every few years.
Outside
academia, he was free to explore his interest in time without
worrying about
tenure or funding for what might seem an arcane pursuit.
Until recently, Barbour's provocative work was little known beyond
a rarefied
circle of physicists. That changed earlier this year with the
publication of his
latest book, The End of Time, in which he presents his case
for a universe
where time, despite all appearances to the contrary, plays no
role.
Barbour's central argument is that a mistaken belief in the reality
of time
prevents physicists from achieving their ultimate goal: the
unification of the
submicroscopic atomic world of quantum mechanics with the vast
cosmic one
of general relativity. The problem arises because each theory
provides a
radically different conception of time, and physicists simply
don't know how to
reconcile the two views. Until they do, they will never have
one seamless
theory of the universe comprising the very smallest objects
to the very largest.
And certain middling-sized objects— human beings— will never
understand the
true nature of time and existence.
What makes the two versions of time so different? Time in the
quantum realm
has no remarkable properties at all. In theories of quantum
mechanics, time is
essentially taken for granted; it simply regularly ticks away
in the background,
just as it does in our own lives. Like a clock at a sporting
event, it provides an
invisible framework in which events unfold. That's not the case
in Einstein's
general theory of relativity.
To describe the universe on the largest scale, Einstein had to
weave time and
space together into the very fabric of the universe. As a result,
in general
relativity, there is no invisible framework, no clock ticking
outside the universe
against which to measure events. How could there be? Time and
space joined
together have weird consequences: Space and time curve around
stars and
other massive bodies and make light bend away from straight-line
paths. Near
black holes, time seems to slow down or even come to a full
stop.
Barbour is not alone in recognizing that the pictures of time
in general relativity
and quantum mechanics are fundamentally incompatible. Theoretical
physicists
around the world, spurred by Nobel dreams, sweat over the problem.
But
Barbour has taken perhaps the most unorthodox approach by proposing
that the
way to solve the conundrum is to leave time out of the equations
that describe
the universe entirely. He has been obsessed with this solution
for more than 10
years, since he learned of a vexing mathematical tour de force
by a young
American physicist named Bryce DeWitt.
DeWitt, with the help of the eminent American physicist John
Wheeler,
developed an equation in 1967 that apparently melded quantum
mechanics with
general relativity. He did this by taking the principles from
quantum mechanics
that describe the interactions of atoms and molecules and applying
them to the
entire universe, a mind-bending feat not unlike trying to make
a jockey's suit fit
Michael Jordan.
Specifically, DeWitt hijacked the Schrödinger equation,
named for the great
Austrian physicist who created it. In its original form, the
equation reveals how
the arrangement of electrons determines the geometrical shapes
of atoms and
molecules. As modified by DeWitt, the equation describes different
possible
shapes for the entire universe and the position of everything
in it. The key
difference between Schrödinger's quantum and DeWitt's cosmic
version of the
equation— besides the scale of the things involved— is that
atoms, over time,
can interact with other atoms and change their energies. But
the universe has
nothing to interact with except itself and has only a fixed
total energy. Because
the energy of the universe doesn't change with time, the easiest
of the many
ways to solve what has become known as the Wheeler-DeWitt equation
is to
eliminate time.
Most physicists balk at that solution, believing it couldn't
possibly describe the
real universe. But a number of respected theorists, Barbour
and Stephen
Hawking among them, take DeWitt's work seriously. Barbour sees
it as the
best path to a real theory of everything, even with its staggering
implication that
we live in a universe without time, motion, or change of any
kind.
Strolling in the meadows of oxford's Christ Church College with
Julian Barbour,
time and motion seem undeniable. Towering cumulus clouds float
overhead,
ferried by a gentle breeze. Children run and shout in the same
field where Alice
Liddell, the girl who inspired Lewis Carroll's Alice's Adventures
in
Wonderland, often played. How can there be no time, no movement?
Barbour
settles his tall, lean frame into the grass, readying himself
for a long explanation
to yet another skeptic. He begins with what seems a most straightforward
proposition: Time is nothing but a measure of the changing positions
of objects.
A pendulum swings, the hands on a clock advance. Objects— and
their
positions— he argues, are therefore more fundamental than time.
The universe
at any given instant simply consists of many different objects
in many different
positions.
That sounds reasonable, as it should, coming from a thoughtful
gentleman like
Barbour. But the next part of his argument— the crux of his
view— is much
harder to swallow: Every possible configuration of the universe,
past, present,
and future, exists separately and eternally. We don't live in
a single universe that
passes through time. Instead, we— or many slightly different
versions of
ourselves— simultaneously inhabit a multitude of static, everlasting
tableaux that
include everything in the universe at any given moment. Barbour
calls each of
these possible still-life configurations a "Now." Every Now
is a complete,
self-contained, timeless, unchanging universe. We mistakenly
perceive the
Nows as fleeting, when in fact each one persists forever. Because
the word
universe seems too small to encompass all possible Nows, Barbour
coined a
new word for it: Platonia. The name honors the ancient Greek
philosopher who
argued that reality is composed of eternal and changeless forms,
even though
the physical world we perceive through our senses appears to
be in constant
flux.
Before allowing himself to be interrupted by the stream of questions
he knows
will come, Barbour continues to press his point. He likens his
view of reality to a
strip of movie film. Each frame captures one possible Now, which
may include
blades of grass, clouds in a blue sky, Julian Barbour, a baffled
Discover writer,
and distant galaxies. But nothing moves or changes in any one
frame. And the
frames— the past and future— don't disappear after they pass
in front of the
lens.
"This corresponds to the way you remember highlights of your
life," Barbour
says. "You remember very vividly certain scenes as snapshots.
I remember
once, very tragically, I had to go to a man who had shot himself.
And I still have
no difficulty in recalling the scene of opening the door just
to where he was at
the foot of the stairs and seeing him there with the gun and
the blood. It's still
imprinted as a photograph on my mind. Many other memories I
have take that
form. People have strong visual memories. If it's not just a
snapshot, it might be
a few stills of a movie you recall. Think of perhaps your most
vivid memories.
You don't think of them as just lasting a second. You see them
as snapshots in
your mind's eye, don't you? They don't fade— they don't seem
to have any
duration. They're just there, like the pages of a book. You
wouldn't ask how
many seconds a page lasts. It doesn't last a millisecond, or
a second; it just is."
Barbour calmly awaits the inevitable sputtering objections.
Don't we then somehow shift from one "frame" to another?
No. There is no movement from one static arrangement of the universe
to the
next. Some configurations of the universe simply contain little
patches of
consciousness— people— with memories of what they call a past
that are built
into the Now. The illusion of motion occurs because many slightly
different
versions of us— none of which move at all— simultaneously inhabit
universes
with slightly different arrangements of matter. Each version
of us sees a
different frame— a unique, motionless, eternal Now. "My position
is that we
are never the same in any two instants," Barbour says. "Obviously,
as
macroscopic human beings, we don't change much from second to
second. And
there's no question that we're the same people. I mean only
an extreme
madman would deny that," he says reassuringly. "To that extent,
it's true that
we do move from one Now to another. But in what sense can you
say we're
moving? The way I see it, not exactly the same information content,
but nearly
the same information content, is present in many different Nows."
Nothing
really moves, he says.
"The information content or the consciousness that makes us aware
of being
ourselves, of having a certain identity, is just present in
many different Nows.
There are two things that distinguish my position from what
people might just
intuitively think. First of all, the Nows are not on one timeline.
They're just
there. And second, there is nothing corresponding to motion.
I'm taking a very
radical position on that. I'm saying the Nows are really like
snapshots. The
impression of motion only arises because the snapshots have
got an
extraordinarily special structure." We are part of that special
structure.
For all the apparent complexity of his scheme, Barbour believes
that it provides
the simplest way to merge quantum mechanics and relativity into
a single theory
of the universe. Like all physicists, he strongly believes that
mathematically
elegant explanations tend to be true, even if they conflict
with common sense. "I
think the approach I'm proposing does deserve to be taken seriously,"
he says.
"It would be extremely rash and stupid to say it's definitely
right, but there's an
inner logic to these ideas. They're very natural. If we want
to put quantum
mechanics and general relativity together, what is the simplest
way that could
be done? I believe it is the way I've proposed. And I believe
it is essentially the
way that Bryce DeWitt discovered in 1967 when he found his infamous
equation."
Barbour stands and brushes some grass from his pants. He has
to meet his
wife, Verena, for dinner and looks at his watch, grinning as
he does so. "This is
what comes of saying there is no time— I have to pull my own
leg sometimes,"
he says.
Walking to a fashionable new restaurant on Oxford's old High
Street, Barbour
talks about how his ideas have changed his perceptions of the
world. "I think it's
completely wrong to say that the world was created in the Big
Bang and that it
was the unique creation event." Barbour hastens to add that
there exists an
eternal Now that contains the Big Bang, but he sees it as just
one of an infinite
array of Nows existing alongside this instant on High Street.
"Immortality is all
around us," he says. "Our task is to recognize it."
How does the physics community react to such ideas? Physicists
who know
Barbour's work agree that it shouldn't be dismissed out of hand.
At a physics
conference in Spain, Barbour conducted an informal poll. He
asked how many
of the physicists believed that time would not be a part of
a final, complete
description of the universe. A majority were inclined to agree.
Don Page, a cosmologist at the University of Alberta in Edmonton
who
frequently collaborates with Stephen Hawking, raised his hand
that day. "I think
Julian's work clears up a lot of misconceptions," says Page.
"Physicists might
not need time as much as we might have thought before. He is
really
questioning the basic nature of time, its nonexistence. You
can't make technical
advances if you're stuck in a conceptual muddle." Strangely
enough, Page feels
that Barbour might actually be too conservative. When physicists
finally iron
out a new theory of the universe, Page suspects that time won't
be the only
casualty. "I think space will go too," he says cryptically.
Like Page, Carlo Rovelli applauds Barbour for forcing physicists
to think about
things they may have taken for granted. "It's time to go back
to the big
questions," he says. "We need a new way to think about the world.
There are
major philosophical challenges, and Julian is a part of that."
Barbour, meanwhile,
is still developing his theory. With Niall Ó Murchadha,
an Irish physicist, he is
attempting to formulate a modification of general relativity
in which not only
time but also distance plays no role. In particular, his theory
would predict that
the universe, being static, is not expanding. The main evidence
that physicists
have for the expansion— the pervasive stretching of the spectra
of light from
distant galaxies known as the cosmic redshift— would instead
be explained by
the gravitational effects of neutron stars and black holes.
"If you want the wildly optimistic scenario," he says, "in which
the Irishman and
I develop this theory, make this prediction, and it turns out
to agree with
observations, then we would really be in the big time."
The parish church next to Barbour's home contains some of the
rarest murals in
England. One painting, completed in about 1340, shows the murder
of Thomas à
Becket, the 12th-century archbishop whose beliefs clashed with
those of King
Henry II. The mural captures the instant when a knight's sword
cleaves
Becket's skull. Blood spurts from the gash. If Barbour's theory
is correct, then
the moment of Becket's martyrdom still exists as an eternal
Now in some
configuration of the universe, as do our own deaths. But in
Barbour's cosmos,
the hour of our death is not an end; it is but one of the numberless
components
of an inconceivably vast, frozen structure. All the experiences
we've ever had
and ever will have lie forever fixed, set like crystalline facets
in some infinite,
immortal jewel. Our friends, our parents, our children, are
always there. In
many ways it's a beautiful and comforting vision. But the question
still nags:
Could it possibly be true? Only time will tell.
Is There Life After Death?
Julian Barbour is convinced we are all immortal. Unfortunately,
in a timeless
universe immortality does not come with the same kind of perks
that it does on
Mount Olympus. In Barbour's vision, we are not like Greek gods
who remain
forever young. We still have to buy life insurance, and we will
certainly seem to
age and die. And instead of life after death, there is life
alongside death. "We're
always locked within one Now," Barbour says. We do not pass
through time.
Instead, each new instant is an entirely different universe.
In all of these
universes, nothing ever moves or ages, since time is not present
in any of them.
One universe might contain you as a baby staring at your mother's
face. In that
universe you will never move from that one, still scene. In
yet another universe,
you'll be forever just one breath away from death. All of those
universes, and
infinitely many more, exist permanently, side by side, in a
cosmos of
unimaginable size and variety. So there is not one immortal
you, but many: the
toddler, the cool dude, the codger. The tragedy— or perhaps
it's a blessing— is
that no one version recognizes its own immortality. Would you
really want to be
14 for eternity, waiting for your civics class to end?
As odd as this vision of a timeless world might seem, Barbour
believes there is
something stranger still to ponder: the very fact of our existence.
"Creation and
the fact that anything is— this for me is the complete mystery,"
he says. "The
fact that we are here is totally mysterious."
— T.F.
1st Law and Newtonian space and time.
One of the most important consequences of the First Law is that it defines what we mean by an inertial frame of reference.
An inertial reference frame is a reference frame where isolated bodies
are seen to move
in straight lines at constant velocity.
An observer at rest with respect to an inertial frame of reference is
called an inertial observer. The laws of physics devised by
Newton take a particularly simple form when expressed in terms of quantities
measured by an inertial observer (such as positions,
velocities, etc.). For example, an inertial observer will find that
a body on which no forces act moves in a straight line at constant
speed or is at rest.
All motion occurs in space and is measured by time. In Newton's model
both space and time are unaffected by the presence or
absence of objects. That is space and time are absolute, an arena where
the play of Nature unfolds. In Newton's words,
Absolute space in its own nature, without relation to anything external, remains always similar and immovable.
...absolute and mathematical time, of itself, and from its own nature,
flows equally without relation to anything external,
and by another name is called duration.
Space and time were taken to be featureless objects which served as
a universal and preferred reference frame (see Fig. 4.9 for
an illustration). A consequence of this is that a given distance will
be agreed upon by any two observers at rest with respect to
each other or in uniform relative motion, for, after all, they are
just measuring the separation between two immovable points in
eternal space. In the same way a time interval will be agreed upon
by any two observers for they are just marking two notches on
eternal time.
Figure 4.9: Illustration of Newton's concept of space. The grids represent
space
which are unaffected by the presence and properties of the objects in it.
Newton's assumptions about space and time are the foundation of his
theory of Nature and were accepted due to the enormous
successes of the predictions. Eventually, however, experimental results
appeared which disagreed with the predictions derived
from Newton's theory. These problems were traced to the fact that these
basic assumptions are not accurate descriptions of space
and time (though they do represent a very good approximation): space
and time are not absolute (Chaps. 6, 7) . The
realization that Newton's theory required revisions came to a head
at the beginning of the XXth century. In the two decades from
1905 to 1925 a completely new framework was constructed and has now
replaced Newton's ideas. These theories comprise the
special and general theories of relativity and quantum mechanics.
Do we know that the current theories of space and time are the truth?
The answer is no: we do know that the current theories
explain all the data (including the one explained by Newton and more),
but we cannot determine whether they represent the
ultimate theories of Nature. In fact, we expect them not to be the
last word as there are many unexplained questions; for example,
why should the proton be precisely 1836.153 times heavier than the
electron? Why should space have 3 and not 25 dimensions?
etc.
But in the 17th century there was no inkling of these problems and very
few scientist questioned Newton's hypothesis. In particular
Newton constructed his mechanics to comply with Galilean relativity:
an observer in uniform motion with respect to another
cannot, without looking outside his laboratory, determine whether he
is at rest or not. And even if he looks outside, he cannot
decide whether he is in motion or the other observer is. In fact for
two inertial observers moving relative to each other the
question, ``which of us is moving?'' is un-answerable and meaningless.
The only thing to be said is that they have a certain relative
velocity.
Traveling Through Time NOVA ONLINE
by Clifford Pickover
What is time? Is time travel possible? For centuries, these
questions have intrigued mystics, philosophers, and
scientists. Much of ancient Greek philosophy was
concerned with understanding the concept of eternity, and
the subject of time is central to all the world's religions and
cultures. Can the flow of time be stopped? Certainly some
mystics thought so. Angelus Silesius, a sixth-century
philosopher and poet, thought the flow of time could be
suspended by mental powers:
Time is of your own making;
its clock ticks in your head.
The moment you stop thought
time too stops dead.
The line between science and mysticism sometimes grows thin. Today physicists
would agree that
time is one of the strangest properties of our universe. In fact, there
is a story circulating among
scientists of an immigrant to America who has lost his watch. He walks
up to a man on a New York
street and asks, "Please, Sir, what is time?" The scientist replies, "I'm
sorry, you'll have to ask a
philosopher. I'm just a physicist."
Most cultures have a grammar with past and future tenses, and also demarcations
like seconds and
minutes, and yesterday and tomorrow. Yet we cannot say exactly what time
is. Although the study of
time became scientific during the time of Galileo and Newton, a comprehensive
explanation was
given only in this century by Einstein, who declared, in effect, time is
simply what a clock reads. The
clock can be the rotation of a planet, sand falling in an hourglass, a
heartbeat, or vibrations of a
cesium atom. A typical grandfather clock follows the simple Newtonian law
that states that the
velocity of a body not subject to external forces remains constant. This
means that clock hands
travel equal distances in equal times. While this kind of clock is useful
for everyday life, modern
science finds that time can be warped in various ways, like clay in the
hands of a cosmic sculptor.
Science-fiction authors have had various uses for time
machines, including dinosaur hunting, tourism, visits to
one's ancestors, and animal collecting. Ever since the time
of H.G. Wells' famous novel The Time Machine (1895),
people have grown increasingly intrigued by the idea of
traveling through time. (I was lucky enough to have chats
with H.G. Wells' grandson, who told me that his
grandfather's book has never been out of print, which is rare
for a book a century old.) In the book, the protagonist uses a
"black and polished brass" time machine to gain mechanical
control over time as well as return to the present to bring
back his story and assess the consequences of the present
on the future. Wells was a graduate of the Imperial College
of Science and Technology, and scientific language permeates his discussions.
Many believe Wells'
book to be the first story about a time machine, but seven years before
22-year-old Wells wrote the
first version of The Time Machine, Edward Page Mitchell, an editor of the
New York Sun, published
"The Clock That Went Backward."
One of the earliest methods for fictional time travel didn't involve a
machine; the main character in
Washington Irving's "Rip van Winkle" (1819) simply fell asleep for decades.
King Arthur's daughter
Gweneth slept for 500 years under Merlin's spell. Ancient legends of time
distortion are, in fact, quite
common. One of the most poetic descriptions of time travel occurs in a
popular medieval legend
describing a monk entranced for a minute by the song of a magical bird.
When the bird stops
singing, the monk discovers that several hundred years have passed. Another
example is the
Moslem legend of Muhammad carried by a mare into heaven. After a long visit,
the prophet returns to
Earth just in time to catch a jar of water the horse had kicked over before
starting its ascent.
Time travel is possible
Today, we know that time travel need not be confined to myths, science
fiction, Hollywood movies, or
even speculation by theoretical physicists. Time travel is possible. For
example, an object traveling
at high speeds ages more slowly than a stationary object. This means that
if you were to travel into
outer space and return, moving close to light speed, you could travel thousands
of years into the
Earth's future.
Newton's most important contribution to science was his mathematical definition
of how motion
changes with time. He showed that the force causing apples to fall is the
same force that drives
planetary motions and produces tides. However, Newton was puzzled by the
fact that gravity
seemed to operate instantaneously at a distance. He admitted he could only
describe it without
understanding how it worked. Not until Einstein's general theory of relativity
was gravity changed
from a "force" to the movement of matter along the shortest space in a
curved spacetime. The Sun
bends spacetime, and spacetime tells planets how to move. For Newton, both
space and time were
absolute. Space was a fixed, infinite, unmoving metric against which absolute
motions could be
measured. Newton also believed the universe was pervaded by a single absolute
time that could be
symbolized by an imaginary clock off somewhere in space. Einstein changed
all this with his
relativity theories, and once wrote, "Newton, forgive me."
Einstein's first major contribution to the study of time occurred when
he revolutionized physics with his "special theory of relativity" by
showing how time changes with motion. Today, scientists do not see
problems of time or motion as "absolute" with a single correct answer.
Because time is relative to the speed one is traveling at, there can
never be a clock at the center of the universe to which everyone can
set their watches. Your entire life is the blink of an eye to an alien
traveling close to the speed of light. Today, Newtonian mechanics
have become a special case within Einstein's theory of relativity.
Einstein's relativity will eventually become a subset of a new science
more comprehensive in its description of the fabric of our universe.
(The word "relativity" derives from the fact that the appearance of the
world depends on our state of motion; it is "relative.")
We are a moment in astronomic time, a transient guest of the Earth.
Our wet, wrinkled brains do not allow us to comprehend many
mysteries of time and space. Our brains evolved to make us run from
saber-toothed cats on the American savanna, to hunt deer, and to
efficiently scavenge from the kills of large carnivores. Despite our
mental limitations, we have come remarkably far. We have managed
to pull back the cosmic curtains a crack to let in the light. Questions
raised by physicists, from
Newton to Kurt Gödel to Einstein to Stephen Hawking, are among the
most profound we can ask.
Is time real? Does it flow in one direction only? Does it have a beginning
or an end? What is eternity?
None of these questions can be answered to scientists' satisfaction. Yet
the mere asking of these
questions stretches our minds, and the continual search for answers provides
useful insights along
the way.
Exactly what is it that we are
measuring?
<P>In his book "Companion to the Cosmos" John Gribbin
states that
everybody knows what time is, but nobody can explain what it is, and
that
in physics, the important thing about time is that it provides a reference
system (a set of coordinates) in which events can be ordered. One event
comes before or after another in this system. He goes on to say that
it
is important that, although this defines an<B> </B>arrow of time,
there
is no suggestion anywhere in the laws of physics that time actually
flows
from the past through the present and into the future. An interesting
point
that Gribbin goes on to make is that all times have equal status and
that
this shows up most forcefully in Einstein's Special Theory of Relativity
where time is regarded as a fourth dimension, on an equal footing with
the
familiar three dimensions of space. He says that you can imagine all
of
space and time represented as a four dimensional spacetime map, on
which
all of history, the present and the future of the universe can be represented.</P>
<P>If Einstein's Special Theory of relativity is right, then according
to
Gribbin this raises interesting questions about the nature of destiny
and
freewill - is the future 'already there' in some sense, just waiting
for
our consciousness to move over it? But Gribbin goes on to say that
the uncertainty
inherent in <A HREF="Quantum%20mechanics">Quantum Mechanics</A><B>
</B>suggests
that a better theory of space and time, merging relativity and quantum
theory,
may restore the vagueness of the future to the description of spacetime.</P>
<P>So did that help at all? Do you now have a feeling for what time
actually
is? Test your knowledge and answer me this. When does the present become
the past and the future become the present? If I mention the word 'future'
you think of some distant event, maybe next year, next week, tomorrow,
an
hour from now. How about a minute from now? that is still obviously
the
future, and a second from now? well yes, it is still in the 'future'.
A
thousandth of a second, a millionth of a second? A what precise time
interval
does the future merge into the present and for how long before it merges
into the past? Does time unfold in a smooth continuous flow or in small
discreet steps? With the absence of time is the concept of 'always'
possible?
How long does the present last and precisely what does 'now' mean?</P>
<P>It would appear to be impossible to define 'now' as a precise
moment
in time that could be used as a universal standard. It seems to be
a personal
experience as perceived by the individual, our own unique frame of
reference.
In this sense time would appear to be a unique event to every observer,
and this is indeed in agreement with relativity. Even the speed at
which
time passes is individual to the observer depending on their speed
and on
the strength of any gravitational field they may happen to be in. The
future
to is also unique to the observer depending on when the individual
enters
the future light cone of an event. (See Stephen Hawking's 'A Brief
History
of Time) So time appears to us to be something that we all experience
differently
and not as some sort of universal standard. In this sense then, time
is
not something real, but only something that we experience, in the same
way
that we can experience pain although pain itself only exists in our
minds
and not in the 'real' world. In other words, time is dependent on our
consciousness.
Would then, for example, the universe continue to expand and evolve
if it
did not contain sentient beings that could observe it? (I think that
this
question has the same merit as asking - if a tree fell down in the
forest
and there was nobody there to hear it, would it make a noise?) The
answer
would be it doesn't matter, it would be meaningless to ask! If there
were
no sentient beings in the universe, the question could not be raised!</P>
<P>If we look at time from the opposite view point we can consider
the implications
of time not existing. The only condition that I can imagine for time
not
existing is in an infinite eternal nothing as described in the previous
page. In this context the concept of 'always' is a fundamental requirement
of the very existence of nothing. If the universe does indeed exist
in nothing
it must always exist in nothing, without a past, present or future
because
these terms could not exist. See <A HREF="Origin%20of%20the%20universe%20">Where
did the universe come from?</A> This would suggest that 'always'
requires
the absence of time.</P>
<P>An interesting observation regarding time as being relative is
that individually
we have no way of determining the rate at which time is passing. An
astronaut
travelling at near light speed would not be able to detect that time
had
slowed down, it would only be apparent on his return to base when he
compares
his clock with the base clock and observes a difference. While traveling
at near light speed our astronaut has no means of detecting that time
has
slowed down, everything would appear to be perfectly normal aboard
his space
craft. (A look out of the window would reveal that something was amiss,
but our astronaut could not prove if it is him that is moving or the
objects
passing by!) A recent television program demonstrated the relationship
of
movement and time to good effect by setting two atomic clocks to show
the
same time to within a very accurate degree. One of the clocks was sent
by
'plane from London to Tokyo and back while the other remained in the
studio.
The two clocks were compared on return and the clock that had remained
in
the studio was shown to be running slightly ahead. What does this simple
experiment tell us? It tells us that the effect is real, time really
does
run slower for a moving object, it isn't just in our heads, clocks
are not
sentient!</P>
<P>So is the effect of time running at different speeds a 'real '
effect
irrespective of whether or not it is being measured? No. We do not
know
what time is doing when we are not measuring it. We would be making
foolish
assumptions if we assumed that time always behaved the same way regardless
of whether or not it was being observed. We do not know! It is the
same
as the behaviour of electrons in Quantum Mechanics, we do not know
what
they are doing when we are not observing them. We can only know what
is
taken place while we are observing.</P>
<P>So is time 'real'?</P>
<P>I dislike the idea that the future may be 'already there' just
waiting
for us to pass over it, for as John Gribbin states, where does that
leave
destiny and freewill? See <A HREF="Free%20Will">Do we have free
will? </A>If
on the other hand the future is not already there, then where does
that
leave the special theory of relativity ? If time is 'real' in the same
way
that the other three dimensions are real, this would indicate that
it does
exists and is there waiting for our consciousness 'to move over it'.
How
much flexibility that allows for the future I do not know, but can
only
hope that the uncertainty in quantum theory allows us a free choice
of actions.</P>
<P>I think that time is the biggest mystery of them all, and that
it is
somehow tied into and dependent upon our consciousness.</P>
<P>Not much of an answer is it? But how do you explain something
that is
so apparent to us and so completely necessary to our every experience
in
life, yet has no form? It is as though it <B>exists</B> only
in our minds
even though its <B>effects</B> are <B>perceived</B> to
be everywhere throughout
the universe.</P>
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