2003-12-05 22:53:06 UTC
Scientific America and I am wondering (forgive my ignorance): have any
_serious_ essays been published solely on the subject of time travel
It's an obvious topic and I have read discussions and
_interpretations_ regarding this topic but did Kubrick talk about time
travel in interviews about 2001 in 1968. (I cannot find my 2001 book
November 24, 2003
Borrowed Time: Interview with Michio Kaku
A theoretical physicist contemplates the plausibility of time travel
By JR Minkel
A motion picture adaptation of Michael Crichton's time travel
adventure story Timeline opens November 26. Crichton cites theoretical
physicist Michio Kaku of the City University of New York as one
inspiration for the science behind the story. Kaku, a string theorist,
is the author of several physics books for a popular audience,
including Hyperspace and Visions: How Science Will Revolutionize the
21st Century, and host of a weekly science radio show. He recently
spoke with Scientific American.com about the possibility of time
travel and his thoughts on science and popular culture. An edited
transcript of that conversation follows.
Scientific American.com: How has speculating about time travel changed
over the years?
Michio Kaku: About 10 years ago, if you were a serious physicist
talking about time travel, you'd be laughed out of the scientific
establishment. People would snicker behind your back, your scientific
career would be ruined, and you wouldn't get tenure. In the last
decade or so, there's been a sea change with regards to the scientific
attitude toward time travel, and I think Michael Crichton picked that
up. And I tried to convey that in my book Hyperspace. Originally, the
burden of proof was on physicists to prove that time travel was
possible. Now the burden of proof is on physicists to prove there must
be a law forbidding time travel.
SA: When did scientists first start thinking about time travel in a
MK: In 1949 Einstein's colleague at Princeton was Kurt Gödel, one of
the greatest logicians of the last thousand years. Gödel found a
solution to Einstein's equations [of general relativity] in which the
universe rotated. And if the universe rotated, then in a rocket ship,
if you went around the universe, you would come back before you left.
Now Einstein was very troubled by this. The river of time, Newton
thought, was straight and uniform; it never deviated, it always flowed
at the same rate, and it carried everything in its way. Einstein comes
along and says, "Not so fast, the river of time meanders, speeds up
and slows down around stars and galaxies." The new wrinkle that Gödel
showed in 1949 was that the river of time could have whirlpools. These
are called "closed timelike curves." And in his memoirs, Einstein says
that yes, these are solutions to his equations, but we can dismiss
them on physical grounds: the universe expands; it doesn't rotate.
Then scientists looked back at earlier solutions to Einstein's
equations and found that there were other solutions which also allow
for time travel. In 1937 [W. J.] van Stockum took an infinitely long
cylinder that was spinning like a maypole and [it was later found
that] if you danced around the maypole you would come back before you
left. In 1963 Roy Kerr, a mathematician, found that a spinning black
hole collapses into a ring of compressed matter, not a dot. If you
fall through the ring, you could wind up backwards in time or perhaps
on another universe. The mathematicians call [such spaces] multiply
connected spaces. The physicists call them wormholes. In the late
1980s Kip Thorne at Caltech and his colleagues found yet another class
of Einstein's equations where these time machines were traversable.
Like an elevator connecting parallel universes, these solutions have
an up button and a down button. Under certain conditions, you can go
through them easily, just like in the movies. You can look through the
looking glass and then come back.
SA: Where would the wormhole come from in that case?
MK: We would get the wormhole by grabbing it from the vacuum, because
they're everywhere. We think that at very small distances, 10-33
centimeters, spacetime becomes foamy. The dominant structures at those
quantum distances are probably wormholes, little bubbles, universes
that pop into existence and then pop right back out of existence. Now
if you could manipulate [the so-called] quantum foam, then you could
go through one of these bubbles. And in Kip Thorne's original proposal
for a time machine, he said that maybe we would obtain a wormhole by
grabbing one of these bubbles and expanding it, stabilizing it with
SA: Negative energy?
MK: Negative energy is energy below the vacuum state, or the state of
motionless nothing. Let's say we have two parallel plates that are
uncharged. We say they are at a state of zero energy because nothing
moves. But when you actually calculate this [state using quantum field
theory], you have "virtual particles" that dance everywhere. These
virtual particles create a pressure that is greater outside the plates
than it is between the plates. Therefore the plates collapse. But the
plates were in a state of zero energy; therefore as they collapse,
they're going to a lower energy state. This is called the Casimir
effect. It is minuscule; it takes a laboratory of sophisticated
equipment to pick it up. But this exists. This is not science fiction.
We've seen negative energy in the laboratory, and this is what I think
Michael Crichton picked up on, that there is a kernel of truth there.
SA: The idea in Timeline is that you can "fax" particles into the
past. What is the kernel of truth there?
MK: In the last 10 years, there has been enormous progress in
something called quantum teleportation. This is not science fiction
anymore. Now, to be real, we're not talking about sending Captain Kirk
across space and time. But we are talking about sending individual
photons across space. In a few decades, maybe we will teleport the
first virus, if the virus consists of a few thousand molecules. But at
the present time, that's the limit of what we can do. And we can only
teleport things in space, not time. But the concept of faxing matter
is not totally out of the question. And that was also raised in my
book. So there is a little bit of truth there.
SA: In Timeline the characters travel back to France in A.D. 1357
because the wormhole happens to let out there. They have six hours to
return, but their six hours in the past are synchronized with the
present. How plausible is all that?
MK: It depends. There are many designs for time machines. Wormholes
from the vacuum would connect randomly with any point in space and
time, so the other end would connect God knows where. Probability-wise
the wormhole would be more likely to connect with the universe back in
time, rather than the present. And if the mouths of the wormhole are
stationary relative to each other, time will pass at the same rate at
SA: How practical would it be to build one of these time machines?
MK: In fact the energies we are talking about are the energies of
stars. It would take a civilization far more advanced than ours,
unbelievably advanced, to begin to manipulate negative energy to
create gateways to the past. But if you could obtain large quantities
of negative energy--and that's a big "if"--then you could create a
time machine that apparently obeys Einstein's equation and perhaps the
laws of quantum theory. You need string theory to ultimately control
all the divergences [i.e., to make sure a hail of gravitons doesn't
fry you when you open or close the time machine]. Some cynics say
quantum effects may still make the machine blow up. But at this point
the burden of proof has shifted: people who are skeptical of time
travel have to prove it's impossible. And so far they have failed.
SA: Wouldn't time travel lead to paradoxes?
MK: There are about four or five main classes of paradox. The most
famous is called the Grandfather Paradox, and that's when you go back
in time and kill your parents before you were born. If you kill your
parents before you were born, how could you be born and kill your
parents before you were born? There are two schools of thought on
this. First is the Russian school. Igor Novikov [of Copenhagen
University] is a well-known cosmologist. He proposes that free will is
somehow abridged by going backwards in time. Something happens to
prevent you from killing your parents before you are born. Or let's
say, for example, that you went backwards in time to when Queen
Elizabeth's forces defeated the Spanish Armada. And let's say you give
a submarine to the Spanish with machine guns; then of course you're
altering human history and we are all speaking Spanish now. Novikov
says that's not possible, because when you go backwards in time and
give the submarine to the Spanish, something prevents you. Well, my
attitude is, in the future, a vanced civilizations might simply mail
the submarine to the Spanish without any free will being abridged;
inanimate matter will go through the time machine and change the past.
That's why I tend to doubt the Novikov interpretation. It's simply too
much to assume that the laws of the universe conspire to prevent
SA: Then what resolves the paradox?
MK: I prefer the "many worlds" interpretation. [Editor's note: Quantum
physics describes a particle by a probabilistic wave function, such
that its position is indeterminate until the wave function "collapses"
and the particle assumes a definite, though randomly determined,
position.] The many worlds theory simply says that maybe the wave
[function] never collapses. Maybe the wave just keeps on bifurcating
every time it hits an obstacle. So the timeline is constantly
bifurcating because the wave is bifurcating all the time. We just
happen to be in one thread of this wave. And we have the illusion that
we are the only ones. In this other thread, they think they are the
only universe. The reality is, nobody's function has collapsed.
In time travel scenarios, you would simply go from one thread to the
next, one timeline to the next timeline. And the two look awfully
similar. If the many worlds theory is correct, it means that if you go
backward in time and kill your parents before you were born, they are
somebody else's parents. The timeline has diverged. Your parents gave
birth to you, in your universe, in your timeline. So if you have the
many worlds theory, there are no paradoxes, just different timelines.
SA: What's the value to physicists of thinking about time machines?
MK: In physics we have a theorem that if it's not forbidden, it's
mandatory. So when we postulate that we understand the laws of
everything, that means it must answer all "how" questions. It must
answer where did the universe come from, where did the big bang come
from, what is the singularity of a black hole? And here we have this
huge gap in the question of causality; attempts so far to create a
"chronology protection" hypothesis to forbid time travel have failed.
Therefore we don't really know the laws that well. When you look at
the calculation, it's amazing that every time you try to prove or
disprove time travel, you've pushed Einstein's theory to the very
limits where quantum effects must dominate. That's telling us that you
really need a theory of everything to resolve this question. And the
only candidate is string theory. So that's why we should study these
things, even if we can't build one of them for millennia.
SA: And does string theory give any insight so far into these
MK: No. String theory gives you trillions of solutions. Each solution
is a well-defined solution to Einstein's equations and the quantum
theory. So there is a multiverse [many possible universes, perhaps
coexisting] in string theory. However, string theory is also
compatible with the Copenhagen interpretation [an alternative to many
worlds]. So string theory does not rule out either interpretation.
Personally, I believe that whether or not the many worlds theory is
correct will be decided by string theory. And string theory seems to
lean toward the multiverse idea.
SA: So why do you think we haven't seen any time tourists?
MK: If you go down the road and see an anthill, do you go down to the
ants and say, "I bring you trinkets: I bring you nuclear energy, I
give you DNA technology?" The answer is no, and for the most part you
might even step on them. The distance between the ant and us,
scientifically speaking, is comparable to the distance between us and
a civilization that can manipulate the Planck energy [required to
probe very small distances and operate a time machine]. We are
arrogant and self-centered to believe that they would be interested in
us enough to want to visit us and give us technology. For the most
part, they may not care. However, I should point out that if one day
someone knocks on your door and claims to be a great great great great
granddaughter who has decided to visit you in the past, don't slam the
door. Because who knows? Maybe they have access to a time machine.
SA: How do you feel about the influence of popular culture and science
fiction on physics?
MK: Scientists, historically, are embarrassed by science fiction; they
want to distance themselves as much as possible. However, when you
read the biographies of great scientists, you realize that a lot of
them, as children, were fascinated by science fiction. I just finished
writing a biography of Einstein called Einstein's Cosmos (due in April
2004), and I had to look up the biographies of many great scientists.
I was shocked to find, for example, that Edwin Hubble, when he was a
young man, read Jules Verne. And he was fascinated by the concept of
going into outer space, the concept of going to the moon, stuff like
that. That childhood fascination was so great that he gave up a
promising law career to become an astronomer. So I think that even
though scientists are embarrassed to admit this, as children many were
influenced by Jules Verne and even Star Trek. I think there's nothing
to be ashamed of. That's one reason why we should take science fiction
But the other reason is to combat scientific ignorance in the general
public. Anything that promotes a kernel of science, even though it's
exaggerated and hyped by Hollywood, I think is a step forward. We in
the ivory tower ultimately have to realize that in some sense we have
to sing for our supper. The cancellation of the SSC was a wake-up call
for all high energy physicists. Unfortunately, I think that we
scientists have failed to engage the public. And I think that has
SA: How so?
MK: Take a look at George Gamow, who is now recognized as one of the
great cosmologists of the last hundred years. I speculate that he
probably didn't win the Nobel Prize because people could not take him
seriously. He wrote children's books. His colleagues have publicly
stated his writing children's books on science had an adverse effect
on his scientific reputation, and people could not take him seriously
when he and his colleagues proposed that there should be a cosmic
background radiation, which we now know to be one of the greatest
discoveries of 20th-century physics.
When Carl Sagan was engaging the public years ago, he was denied
admission to the National Academy of Sciences. In the debate, it came
out that many [scientists] could not take him seriously. They saw him
on television; how seriously can you take someone you see on
television? You see actors on television. So I think it had a negative
effect on his scientific career.
SA: It sounds like things have changed, though.
MK: The sea change came when Steven Hawking wrote the book A Brief
History of Time. He was a serious cosmologist who took the time to
write a book for the general public, and it was among the best-selling
books of all time. Even the publishing world had to take note of that
book. I think that has made it possible for more scientists to engage
the public, to the point where reputable scientists can write books
about science and not have to suffer like Gamow suffered decades ago.
SA: Does this have anything to do with your radio program and why you
MK: Yes, I like to engage the public because when I was in high school
I had all these questions about anti-matter, higher dimensions and
time travel. Every time I went to the library, every time I asked
people these questions, I would get some strange looks. Nobody could
answer any of these questions. So I said to myself as a child that
when I become a theoretical physicist, and I do research, I want to be
able to answer these questions for children who ask these questions
and get no answer.
SA: Do you have a favorite time travel movie?
MK: Oh, that's a hard one. There is a problem being a physicist, and
that is when you see these movies, you say, "Well, that's not right."
And it really ruins it. But I like the Back to the Future series. Here
was a movie where you actually saw the scientist building and doing
things; he was an essential character in the entire series. Doc Brown
was this crazy man, but at least they showed him. He was there. He was
making the series work.
SA: Even though in Hyperspace you say that the sort of time travel
found in Back to the Future wouldn't really work?
MK: Neither of the two theories [single or multiple timelines] is
compatible with slowly fading away as you slowly change the past. In
part two, Doc Brown does draw a timeline forking and he does say
explicitly that we went from one timeline to the next. I thought that
it was interesting that in the movie they take a position, and their
position is many worlds.
JR Minkel is a freelance writer based in New York City.
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