MESSAGE
| DATE | 2005-06-30 |
| FROM | Ruben Safir
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| SUBJECT | Subject: [NYLXS - HANGOUT] Remembrance of Things Future: The Mystery of Time
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Next Time you can't make the deadline, try this!
By DENNIS OVERBYE
There was a conference for time travelers at M.I.T. earlier this spring.
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 trailing
off.
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
the world."
"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 experience."
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 will laugh.
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
past."
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
relativity.
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,
Berkeley.
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 you
thought."
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 at
all.
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
them shut.
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 up.
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."
Saving Grandpa
But what about killing your grandfather? In a well-ordered universe,
that would be a paradox and shouldn't be able to happen, everybody
agrees.
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 plot.
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."
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