It was
an unlikely place to be
at 4:30 a.m., since I'm
not much on celebrations
and take minimal notice
of most every holiday.
Yet, a few years back,
on a rainy Dec. 31
morning, I stood in
Times Square, together
with a handful of other
early revelers, awaiting
images on a giant screen
of festivities on
Kiribati, the first
inhabited place on earth
to welcome the new year.
I was, as I recognized
through the fog of
exhaustion and the hazy
steam billowing from
manhole covers,
re-enacting a struggle
I'd been engaged in for
decades.
Time
dominates experience. We
live by watch and
calendar. We eagerly
trade megahertz for
gigahertz. We spend
billions of dollars to
conceal time's bodily
influences. We
uproariously celebrate
particular moments in
time even as we quietly
despair of its passage.
But
what is time? To
paraphrase Justice
Potter Stewart, we know
it when we see it —
but certainly, a few
years into the 21st
century, our
understanding of time
must be deeper than
that. By now, you'd
think, science must have
figured out why time
seems to flow, why it
always goes in one
direction and why we are
uniformly drawn from one
second to the next. The
fact is, though, the
explanations for these
basic features of time
remain controversial.
And the more physicists
have searched for
definitive answers, the
more our everyday
conception of time
appears illusory.
According
to Isaac Newton, writing
in the late 17th
century, "time
flows equably without
reference to anything
external," meaning
that the universe is
equipped with a kind of
built-in clock that
ticks off seconds
identically, regardless
of location or epoch.
This is the intuitive
perspective on time, so
it's no wonder that
Newton's words held sway
for more than 200 years.
In the
early part of the 20th
century, however, Albert
Einstein saw through
nature's Newtonian
facade and revealed that
the passage of time
depends on circumstance
and environment. He
showed that the
wristwatches worn by two
individuals moving
relative to one another,
or experiencing
different gravitational
fields, tick off time at
different rates. The
passage of time,
according to Einstein,
is in the eye of the
beholder.
Numerous
terrestrial experiments
and astronomical
observations leave no
doubt that Einstein was
right. Nevertheless,
because the flexibility
of time's passage
becomes readily apparent
only at high speeds
(near the maximum
possible speed, that of
light) or in strong
gravitational fields
(near a black hole),
nature lulls us into
believing Newton's rigid
conception. And so it's
not surprising that
nearly 100 years after
Einstein's
breakthroughs, it
remains a great
challenge, even for
physicists, to
internalize his
discoveries fully.
But
the cost of adhering to
Newton's description of
time is high. Like
believing the earth flat
or that man was created
on the sixth day, our
willingness to place
unjustified faith in
immediate perception or
received wisdom leads us
to an inaccurate and
starkly limited vision
of reality.
For
one thing, relativity
lays out a blueprint for
time-travel to the
future. Were you to
board a spaceship, head
out from earth at
99.999999 percent of
light speed, travel for
six months and then head
back home at the same
speed, your motion would
slow your clock,
relative to those that
remain stationary on
earth, so that you'd be
one year older upon your
return — while
everyone on earth would
have aged about 7,000
years. Or, were you to
venture into space again
and spend a year
hovering a dozen feet
above the edge of a
black hole, whose mass
was 1,000 times that of
the sun, the strong
gravitational field
would slow your clock so
much that on your return
to earth, you'd find
that more than a million
years had elapsed.
To be
sure, executing this
strategy for catapulting
yourself forward in time
is beyond what we can
now achieve, but
scientists routinely use
high-energy accelerators
to propel particles,
like electrons and
protons, to nearly the
speed of light, slowing
their internal clocks
and thereby sending them
to the future. Though
unfamiliar, forward
time-travel is an
unavoidable feature of
relativistic reality.
Relativity
also upends the way we
traditionally organize
reality. Most of us
imagine that reality
consists of everything
that exists right now
— everything that
would be found, say, on
a hypothetical
freeze-frame image of
the universe at this
moment. The history of
reality could thus be
depicted by stacking one
such freeze-frame image
on top of the one that
came before it, creating
a cosmic version of an
old-time flip-book. But
this intuitive
conception assumes a
universal now, another
stubborn remnant of
Newton's absolutist
thinking.
Let me
explain. Clocks that are
in relative motion or
that are subject to
different gravitational
fields tick off time at
different rates; the
more these factors come
into play, the further
out of synchronization
the clocks will fall.
Individuals carrying
such clocks will
therefore not agree on
what happens when, and
so they will not agree
on what belongs on a
given page of the cosmic
flip-book — even
though each flip-book
provides an equally
valid compendium of
history.
Under
these rules, what
constitutes a moment in
time is completely
subjective. This is
unfamiliar, and hence
hard to accept, because
we all experience the
same gravitational field
(the earth's), we all
travel extremely slowly
compared to light's
speed (even the space
shuttle never comes
close to exceeding a
ten-thousandth of light
speed) and we all
compare our conception
of reality to beings
who, by cosmic
standards, are nearby.
But by using our
understanding to relax
these measures, if only
hypothetically, we learn
that our experiences
belie the truth.
For
example, if you and I
were sitting next to
each other, our
freeze-frame images of
the present would be
identical. But were you
to start walking, the
mathematics of
relativity shows that
the subsequent pages of
your flip-book would
rotate so that each one
of your new pages would
angle across many of
mine; what you'd
consider one moment in
time — your new notion
of the present — would
include events I'd claim
to have happened at
different times, some
earlier and some later.
As we
pass each other in the
street, this rotation is
imperceptibly tiny;
that's why common
experience fails to
reveal the discrepancy
between our respective
senses of past, present
and future. But just as
a tiny angular shift
will cause a rocket to
miss a distant target by
a large margin, the tiny
angular shift between
our notions of now
results in a significant
time discrepancy if our
separation in space is
substantial. If instead
of being next to me, you
were 10 light years away
(and moving at about 9.5
miles an hour), what you
consider to have
happened just now on
earth would include
events that I'd
experienced about four
seconds later or earlier
(depending on whether
your motion was toward
or away from earth). If
you were 10 billion
light years away, the
time discrepancy would
jump to about 141 years.
In
this latter case, your
subsequent flip-book
pages, your notion of
the present — a notion
that agreed with mine
until you started
walking — would
include Abraham Lincoln
on the day the
Emancipation
Proclamation took effect
(if you walked away from
me), or the victor of
the hotly contested
presidential election of
2144 preparing for his
inaugural (if you walked
toward me). That's not
to say that you could
save Lincoln's life or
analyze mid-22nd century
American presidential
politics; at such
enormous distances it
takes signals, even
traveling at light
speed, a long time to
make the trip. But the
point is that even
ordinary motion, when
considered over vast
distances, results in a
marked change in our
conception of reality,
revealing how thoroughly
subjective the temporal
categories of past,
present and future
actually are.
In a
very specific way, then,
this realization
shatters our comfortable
sense that the past is
gone, the future is yet
to be and the present is
what truly exists.
Einstein was not
hardened to the
difficulty of absorbing
such a profound change
in perspective. Rudolf
Carnap, the philosopher,
recounts Einstein's
telling him that
"the experience of
the now means something
special for man,
something essentially
different from the past
and the future, but this
important difference
does not and cannot
occur within
physics." And
later, in a condolence
letter to the widow of
Michele Besso, his
longtime friend and
fellow physicist,
Einstein wrote: "In
quitting this strange
world he has once again
preceded me by just a
little. That doesn't
mean anything. For we
convinced physicists the
distinction between
past, present, and
future is only an
illusion, however
persistent."
Some
physicists and
historians see these as
declarations laced with
poignant hyperbole.
Perhaps they are. It's
hard to know whether
Einstein was
"convinced" to
such a deep level that
he had remolded his
emotional sense of time
to reflect his
understanding of
relativistic reality.
But regardless of
whether Einstein had
succeeded, his remarks
articulated the
challenge — to allow
carefully reasoned and
experimentally verified
investigations of the
universe, however
discomfiting their
conclusions, to inform
our lives with the same
force as experience.
When
quantum mechanics, the
tremendously successful
theory of atoms and
subatomic particles, is
taken into account, the
challenge becomes
greater still. Quantum
mechanics has, at its
core, the uncertainty
principle, which
establishes a limit on
how precisely particular
features of the
microworld can be
simultaneously measured.
The more precise the
measurement of one
feature (a particle's
position for example),
the more wildly
uncertain a
complementary feature
(its velocity) becomes.
Quantum uncertainty thus
ensures that the finer
the examination of the
microworld, the more
frantically its physical
features fluctuate, and
the more turbulent it
appears to be.
For
subatomic particles,
these fluctuations are
well understood
mathematically and have
been precisely
documented
experimentally. But when
it comes to time and
space, the fluctuations
speak to the very limits
of these familiar
concepts. On extremely
short time intervals
(about a tenth of a
millionth of a
trillionth of a
trillionth of a
trillionth of a second)
and distance scales
(about a billionth of a
trillionth of a
trillionth of a
centimeter), quantum
fluctuations so mangle
space and time that the
conventional ideas of
left/right,
backward/forward,
up/down, and
before/after become
meaningless.
Scientists
are still struggling to
understand these
implications, but many
agree that just as the
percentages in political
polls are average,
approximate measures
that become meaningful
only when a large
respondent pool is
canvassed, so
conventional notions of
time and space are also
average, approximate
concepts that become
meaningful only when
considered over
sufficiently large
scales. Whereas
relativity established
the subjectivity of
time's passage, quantum
mechanics challenges the
conceptual primacy of
time itself.
Today's
scientists seeking to
combine quantum
mechanics with
Einstein's theory of
gravity (the general
theory of relativity)
are convinced that we
are on the verge of
another major upheaval,
one that will pinpoint
the more elemental
concepts from which time
and space emerge. Many
believe this will
involve a radically new
formulation of natural
law in which scientists
will be compelled to
trade the space-time
matrix within which they
have worked for
centuries for a more
basic "realm"
that is itself devoid of
time and space.
This
is such a perplexing
idea that grasping it
poses a substantial
challenge, even for
leading researchers.
Broadly speaking,
scientists envision that
there will be no mention
of time and space in the
basic equations of the
sought-for framework.
And yet — just as
clear, liquid water
emerges from particular
combinations of an
enormous number of H20
molecules — time and
space as we know them
would emerge from
particular combinations
of some more basic,
though still
unidentified, entities.
Time and space
themselves, though,
would be rendered
secondary, derivative
features, that emerge
only in suitable
conditions (in the
aftermath of the Big
Bang, for example). As
outrageous as it sounds,
to many researchers,
including me, such a
departure of time and
space from the ultimate
laws of the universe
seems inevitable.
A
hundred years ago today,
the discovery of special
relativity was still 18
months away, and science
still embraced the
Newtonian description of
time. Now, however,
modern physics' notion
of time is clearly at
odds with the one most
of us have internalized.
Einstein greeted the
failure of science to
confirm the familiar
experience of time with
"painful but
inevitable
resignation." The
developments since his
era have only widened
the disparity between
common experience and
scientific knowledge.
Most physicists cope
with this disparity by
compartmentalizing:
there's time as
understood
scientifically, and then
there's time as
experienced intuitively.
For decades, I've
struggled to bring my
experience closer to my
understanding. In my
everyday routines, I
delight in what I know
is the individual's
power, however
imperceptible, to affect
time's passage. In my
mind's eye, I often
conjure a kaleidoscopic
image of time in which,
with every step, I
further fracture
Newton's pristine and
uniform conception. And
in moments of loss I've
taken comfort from the
knowledge that all
events exist eternally
in the expanse of space
and time, with the
partition into past,
present and future being
a useful but subjective
organization.
Yet my
presence in Times Square
that rainy morning —
losing sleep to mark an
arbitrary moment in the
passage of what I truly
believe to be a
derivative concept —
attests to the power of
convention and
experience. Regardless
of our scientific
insights, we will still
mourn the evanescence of
life and be able to
thrill to the arrival of
each newly delivered
moment. The choice,
however, of whether to
be fully seduced by the
face nature reveals
directly to our senses,
or to also recognize the
reality that exists
beyond perception, is
ours.
Brian
Greene, a professor of
mathematics and physics
at Columbia, is author
of "The Elegant
Universe" and the
forthcoming "The
Fabric of the Cosmos:
Space, Time and the
Texture of
Reality."