[Paleopsych] NYT: Remembrance of Things Future: The Mystery of Time
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Tue Jun 28 18:41:18 UTC 2005
Remembrance of Things Future: The Mystery of Time
["A Trip Forward in Time. Your Travel Agent: Einstein." appended.]
By DENNIS OVERBYE
There was a conference for time travelers at M.I.T. earlier this
I'm still hoping to attend, and although the odds are slim, they are
apparently not zero despite the efforts and hopes of deterministically
minded physicists who would like to eliminate the possibility of your
creating a paradox by going back in time and killing your grandfather.
"No law of physics that we know of prohibits time travel," said Dr. J.
Richard Gott, a Princeton astrophysicist.
Dr. Gott, author of the 2001 book "Time Travel in Einstein's Universe:
The Physical Possibilities of Travel Through Time," is one of a small
breed of physicists who spend part of their time (and their research
grants) thinking about wormholes in space, warp drives and other
cosmic constructions, that "absurdly advanced civilizations" might use
to travel through time.
It's not that physicists expect to be able to go back and attend
Woodstock, drop by the Bern patent office to take Einstein to lunch,
see the dinosaurs or investigate John F. Kennedy's assassination.
In fact, they're pretty sure those are absurd dreams and are all
bemused by the fact that they can't say why. They hope such extreme
theorizing could reveal new features, gaps or perhaps paradoxes or
contradictions in the foundations of Physics As We Know It and point
the way to new ideas.
"Traversable wormholes are primarily useful as a 'gedanken experiment'
to explore the limitations of general relativity," said Dr. Francisco
Lobo of the University of Lisbon.
If general relativity, Einstein's theory of gravity and space-time,
allows for the ability to go back in time and kill your grandfather,
asks Dr. David Z. Albert, a physicist and philosopher at Columbia
University, "how can it be a logically consistent theory?"
In his recent book "The Universe in a Nutshell," Dr. Stephen W.
Hawking wrote, "Even if it turns out that time travel is impossible,
it is important that we understand why it is impossible."
When it comes to the nature of time, physicists are pretty much at as
much of a loss as the rest of us who seem hopelessly swept along in
its current. The mystery of time is connected with some of the
thorniest questions in physics, as well as in philosophy, like why we
remember the past but not the future, how causality works, why you
can't stir cream out of your coffee or put perfume back in a bottle.
But some theorists think that has to change.
Just as Einstein needed to come up with a new concept of time in order
to invent relativity 100 years ago this year, so physicists say that a
new insight into time - or beyond it - may be required to crack
profound problems like how the universe began, what happens at the
center of black hole or how to marry relativity and quantum theory
into a unified theory of nature.
Space and time, some quantum gravity theorists say, are most likely a
sort of illusion - or less sensationally, an "approximation" - doomed
to be replaced by some more fundamental idea. If only they could think
of what that idea is.
"By convention there is space, by convention time," Dr. David J.
Gross, director of the Kavli Institute for Theoretical Physics and a
winner of last year's Nobel Prize, said recently, paraphrasing the
Greek philosopher Democritus, "in reality there is. ... ?" his voice
The issues raised by time travel are connected to these questions, Dr.
Lawrence Krauss, a physicist at Case Western Reserve University in
Cleveland and author of the book "The Physics of Star Trek," said.
"The minute you have time travel you have paradoxes," Dr. Krauss said,
explaining that if you can go backward in time you confront
fundamental issues like cause and effect or the meaning of your own
identity if there can be two of you at once. A refined theory of time
would have to explain "how a sensible world could result from
something so nonsensical."
"That's why time travel is philosophically important and has
captivated the public, who care about these paradoxes," he said.
At stake, said Dr. Albert, the philosopher and author of his own time
book, "Time and Chance," is "what kind of view science presents us of
"Physics gets time wrong, and time is the most familiar thing there
is," Dr. Albert said.
We all feel time passing in our bones, but ever since Galileo and
Newton in the 17th century began using time as a coordinate to help
chart the motion of cannonballs, time - for physicists - has simply
been an "addendum in the address of an event," Dr. Albert said.
"There is a feeling in philosophy," he said, "that this picture leaves
no room for locutions about flow and the passage of time we
Then there is what physicists call "the arrow of time" problem. The
fundamental laws of physics don't care what direction time goes, he
pointed out. Run a movie of billiard balls colliding or planets
swirling around in their orbits in reverse and nothing will look
weird, but if you run a movie of a baseball game in reverse people
Einstein once termed the distinction between past, present and future
"a stubborn illusion," but as Dr. Albert said, "It's hard to imagine
something more basic than the distinction between the future and the
The Birth of an Illusion
Space and time, the philosopher Augustine famously argued 1,700 years
ago, are creatures of existence and the universe, born with it, not
separately standing features of eternity. That is the same answer that
Einstein came up with in 1915 when he finished his general theory of
That theory explains how matter and energy warp the geometry of space
and time to produce the effect we call gravity. It also predicted,
somewhat to Einstein's dismay, the expansion of the universe, which
forms the basis of modern cosmology.
But Einstein's theory is incompatible, mathematically and
philosophically, with the quirky rules known as quantum mechanics that
describe the microscopic randomness that fills this elegantly curved
expanding space-time. According to relativity, nature is continuous,
smooth and orderly, in quantum theory the world is jumpy and
discontinuous. The sacred laws of physics are correct only on average.
Until the pair are married in a theory of so-called quantum gravity,
physics has no way to investigate what happens in the Big Bang, when
the entire universe is so small that quantum rules apply.
Looked at closely enough, with an imaginary microscope that could see
lengths down to 10^-33 centimeters, quantum gravity theorists say,
even ordinary space and time dissolve into a boiling mess that Dr.
John Wheeler, the Princeton physicist and phrasemaker, called
"space-time foam." At that level of reality, which exists underneath
all our fingernails, clocks and rulers as we know them cease to exist.
"Everything we know about stops at the Big Bang, the Big Crunch," said
Dr. Raphael Bousso, a physicist at the University of California,
What happens to time at this level of reality is anybody's guess. Dr.
Lee Smolin, of the Perimeter Institute for Theoretical Physics in
Waterloo, Ontario, said, "There are several different, very different,
ideas about time in quantum gravity."
One view, he explained, is that space and time "emerge" from this
foamy substrate when it is viewed at larger scales. Another is that
space emerges but that time or some deeper relations of cause and
effect are fundamental.
Dr. Fotini Markopoulou Kalamara of the Perimeter Institute described
time as, if not an illusion, an approximation, "a bit like the way you
can see the river flow in a smooth way even though the individual
water molecules follow much more complicated patterns."
She added in an e-mail message: "I have always thought that there has
to be some basic fundamental notion of causality, even if it doesn't
look at all like the one of the space-time we live in. I can't see how
to get causality from something that has none; neither have I ever
seen anyone succeed in doing so."
Physicists say they have a sense of how space can emerge, because of
recent advances in string theory, the putative theory of everything,
which posits that nature is composed of wriggling little strings.
Calculations by Dr. Juan Maldacena of the Institute for Advanced Study
in Princeton and by others have shown how an extra dimension of space
can pop mathematically into being almost like magic, the way the
illusion of three dimensions can appear in the holograms on bank
cards. But string theorists admit they don't know how to do the same
thing for time yet.
"Time is really difficult," said Dr. Cumrun Vafa, a Harvard string
theorist. "We have not made much progress on the emergence of time.
Once we make progress we will make progress on the early universe, on
high energy physics and black holes.
"We are out on a limb trying to understand what's going on here."
Dr. Bousso, an expert on holographic theories of space-time, said that
in general relativity time gets no special treatment.
He said he expected both time and space to break down, adding, "We
really just don't know what's going to go."
"There is a lot of mysticism about time," Dr. Bousso said. "Time is
what a clock measures. What a clock measures is more interesting than
A Brief History of Time Travel
"If we could go faster than light, we could telegraph into the past,"
Einstein once said. According to the theory of special relativity -
which he proposed in 1905 and which ushered E=mc² into the world and
set the speed of light as the cosmic speed limit - such telegraphy is
not possible, and there is no way of getting back to the past.
But, somewhat to Einstein's surprise, in general relativity it is
possible to beat a light beam across space. That theory, which
Einstein finished in 1916, said that gravity resulted from the warping
of space-time geometry by matter and energy, the way a bowling ball
sags a trampoline. And all this warping and sagging can create
shortcuts through space-time.
In 1949, Kurt Gödel, the Austrian logician and mathematician then at
the Institute for Advanced Study, showed that in a rotating universe,
according to general relativity, there were paths, technically called
"closed timelike curves," you could follow to get back to the past.
But it has turned out that the universe does not rotate very much, if
Most scientists, including Einstein, resisted the idea of time travel
until 1988 when Dr. Kip Thorne, a gravitational theorist at the
California Institute of Technology, and two of his graduate students,
Dr. Mike Morris and Dr. Ulvi Yurtsever, published a pair of papers
concluding that the laws of physics may allow you to use wormholes,
which are like tunnels through space connecting distant points, to
travel in time.
These holes, technically called Einstein-Rosen bridges, have long been
predicted as a solution of Einstein's equations. But physicists
dismissed them because calculations predicted that gravity would slam
Dr. Thorne was inspired by his friend, the late Cornell scientist and
author Carl Sagan, who was writing the science fiction novel
"Contact," later made into a Jodie Foster movie, and was looking for a
way to send his heroine, Eleanor Arroway, across the galaxy. Dr.
Thorne and his colleagues imagined that such holes could be kept from
collapsing and thus maintained to be used as a galactic subway, at
least in principle, by threading them with something called Casimir
energy, (after the Dutch physicist Hendrik Casimir) which is a sort of
quantum suction produced when two parallel metal plates are placed
very close together. According to Einstein's equations, this suction,
or negative pressure, would have an antigravitational effect, keeping
the walls of the wormhole apart.
If one mouth of a wormhole was then grabbed by a spaceship and taken
on a high-speed trip, according to relativity, its clock would run
slow compared with the other end of the wormhole. So the wormhole
would become a portal between two different times as well as places.
Dr. Thorne later said he had been afraid that the words "time travel"
in the second paper's title would create a sensation and tarnish his
students' careers, and he had forbidden Caltech to publicize it.
In fact, their paper made time travel safe for serious scientists, and
other theorists, including Dr. Frank Tipler of Tulane University and
Dr. Hawking, jumped in. In 1991, for example, Dr. Gott of Princeton
showed how another shortcut through space-time could be manufactured
using pairs cosmic strings - dense tubes of primordial energy not to
be confused with the strings of string theory, left over by the Big
Bang in some theories of cosmic evolution - rushing past each other
and warping space around them.
Harnessing the Dark Side
These speculations have been bolstered (not that time machine
architects lack imagination) with the unsettling discovery that the
universe may be full of exactly the kind of antigravity stuff needed
to grow and prop open a wormhole. Some mysterious "dark energy,"
astronomers say, is pushing space apart and accelerating the expansion
of the universe. The race is on to measure this energy precisely and
find out what it is.
Among the weirder and more disturbing explanations for this cosmic
riddle is something called phantom energy, which is so virulently
antigravitational that it would eventually rip planets, people and
even atoms apart, ending everything. As it happens this bizarre stuff
would also be perfect for propping open a wormhole, Dr. Lobo of Lisbon
recently pointed out. "This certainly is an interesting prospect for
an absurdly advanced civilization, as phantom energy probably
comprises of 70 percent of the universe," Dr. Lobo wrote in an e-mail
message. Dr. Sergey Sushkov of Kazan State Pedagogical University in
Russia has made the same suggestion.
In a paper posted on the physics Web site
arxiv.org/abs/gr-qc/0502099, Dr. Lobo suggested that as the
universe was stretched and stretched under phantom energy, microscopic
holes in the quantum "space-time foam" might grow to macroscopic
usable size. "One could also imagine an advanced civilization mining
the cosmic fluid for phantom energy necessary to construct and sustain
a traversable wormhole," he wrote.
Such a wormhole he even speculated, could be used to escape the "big
rip" in which a phantom energy universe will eventually end.
But nobody knows if phantom, or exotic, energy is really allowed in
nature and most physicists would be happy if it is not. Its existence
would lead to paradoxes, like negative kinetic energy, where something
could lose energy by speeding up, violating what is left of common
sense in modern physics.
Dr. Krauss said, "From the point of view of realistic theories,
phantom energy just doesn't exist."
But such exotic stuff is not required for all time machines, Dr.
Gott's cosmic strings for example. In another recent paper, Dr. Amos
Ori of the Technion-Israel Institute of Technology in Haifa describes
a time machine that he claims can be built by moving around colossal
masses to warp the space inside a doughnut of regular empty space into
a particular configuration, something an advanced civilization may be
able to do in 100 or 200 years.
The space inside the doughnut, he said, will then naturally evolve
according to Einstein's laws into a time machine.
Dr. Ori admits that he doesn't know if his machine would be stable.
Time machines could blow up as soon as you turned them on, say some
physicists, including Dr. Hawking, who has proposed what he calls the
"chronology protection" conjecture to keep the past safe for
historians. Random microscopic fluctuations in matter and energy and
space itself, they argue, would be amplified by going around and
around boundaries of the machine or the wormhole, and finally blow it
Dr. Gott and his colleague Dr. Li-Xin Li have shown that there are at
least some cases where the time machine does not blow up. But until
gravity marries quantum theory, they admit, nobody knows how to
predict exactly what the fluctuations would be.
"That's why we really need to know about quantum gravity," Dr. Gott
said. "That's one reason people are interested in time travel."
But what about killing your grandfather? In a well-ordered universe,
that would be a paradox and shouldn't be able to happen, everybody
That was the challenge that Dr. Joe Polchinski, now at the Kavli
Institute for Theoretical Physics in Santa Barbara, Calif., issued to
Dr. Thorne and his colleagues after their paper was published.
Being a good physicist, Dr. Polchinski phrased the problem in terms of
billiard balls. A billiard ball, he suggested, could roll into one end
of a time machine, come back out the other end a little earlier and
collide with its earlier self, thereby preventing itself from entering
the time machine to begin with.
Dr. Thorne and two students, Fernando Echeverria and Gunnar
Klinkhammer, concluded after months of mathematical struggle that
there was a logically consistent solution to the billiard matricide
that Dr. Polchinski had set up. The ball would come back out of the
time machine and deliver only a glancing blow to itself, altering its
path just enough so that it would still hit the time machine. When it
came back out, it would be aimed just so as to deflect itself rather
than hitting full on. And so it would go like a movie with a circular
In other words, it's not a paradox if you go back in time and save
your grandfather. And, added Dr. Polchinski, "It's not a paradox if
you try to shoot your grandfather and miss."
"The conclusion is somewhat satisfying," Dr. Thorne wrote in his book
"Black Holes and Time Warps: Einstein's Outrageous Legacy." "It
suggests that the laws of physics might accommodate themselves to time
machines fairly nicely."
Dr. Polchinski agreed. "I was making the point that the grandfather
paradox had nothing to do with free will, and they found a nifty
resolution," he said in an e-mail message, adding, nevertheless, that
his intuition still tells him time machines would lead to paradoxes.
Dr. Bousso said, "Most of us would consider it quite satisfactory if
the laws of quantum gravity forbid time travel."
A Trip Forward in Time. Your Travel Agent: Einstein.
By DENNIS OVERBYE
When H. G. Wells published his epochal novel "The Time Machine" in
1895, time travel was outlawed by the laws of physics. But that was
Newtonian physics, and everything changed 10 years later with
Einstein's theory of relativity.
That theory - which ushered in the age of E=mc² and set the speed of
light, 186,000 miles per second, as the cosmic speed limit - allows
for time travel to the future, physicists say. Here's how:
One consequence of Einstein's theory is that a clock in motion will
always appear to run slowly compared with one at rest (and since all
motion is relative the clock at rest will appear to go slowly from the
vantage of the one moving).
This leads to the famous "twin paradox" in which one twin is rocketed
at high speed across the galaxy and back home. Even at a velocity
close to the speed of light, the journey would take tens of thousands
of years from the vantage point of Earth, but because of his high
relative motion the astronaut would age more slowly than he or she
would than on Earth, and would return home only a few years older. His
twin would be long dead.
In effect the astronaut would have traveled into the future, said Dr.
J. Richard Gott, a Princeton astrophysicist.
The slowing clock prediction has been confirmed by flying atomic
clocks around Earth on jets.
"If you take a plane east around the world you will come back 59
nanoseconds younger than if you had stayed home," Dr. Gott said.
The record holder for this type of travel, he said, is the Russian
astronaut Sergei Krikalev, who came back from 748 days orbiting in the
Mir space station a full one-fiftieth of a second younger than he
would have if he had stayed on the ground.
In his 1905 paper Einstein predicted that because of the rotation
speed of Earth, clocks would also run slower at the Equator than the
poles, but that turned out to be wrong.
In a recent article in Physics Today, Dr. Alex Harvey of Queens
College in New York and Dr. Engelbert Schucking of New York University
pointed out that Einstein had not taken account of an effect of
general relativity, then 10 years in the future, which says that
clocks run slower the more deeply in a gravitational field they sit.
The rotation of Earth causes it to bulge at the Equator, lifting
clocks there and making them run slightly faster relative to those at
the poles by just enough to compensate for the extra speed.
So the two effects just cancel out, and clocks at the Equator and
poles run at the same speed. "It's a deep coincidence," Dr. Gott said.
The two effects could be combined for an even deeper trip into the
future by going to Mercury, which is both deep in the gravitational
field of the Sun and also zooming around it at high speed. A 30-year
stay there, Dr. Gott said, would save 22 seconds of an astronaut's
A few seconds might not sound like much of a trip in time, but Dr.
Gott points out that astronauts haven't been that far into space,
either. The Moon, humanity's most distant destination so far, is only
about 1.3 light-seconds away, about like hopping over the Atlantic, he
"The astronauts are the Lindberghs of time travel," Dr. Gott said.
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