Sagan on Time Travel
Carl Sagan, the astronomer, Pulitzer Prize-winning author, and legendary
popularizer of science, gave this interview during the making of "Time Travel."
True to form, he discusses arcane aspects of the field—from how you define
time to what it might look like inside a wormhole—with flair and a
refreshing dash of humor. Sagan was David Duncan Professor of Astronomy and
Space Sciences and director of the Laboratory for Planetary Studies at Cornell
University when he died in 1996.
NOVA: Let's start with the crux of the matter. What for you is time?
Sagan: Ever since St. Augustine, people have wrestled with this, and there are
all sorts of things it isn't. It isn't a flow of something, because what does
it flow past? We use time to measure flow. How could we use time to measure
time? We are stuck in it, each of us time travels into the future, one year,
every year. None of us to any significant precision does otherwise. If we could
travel close to the speed of light, then we could travel further into the
future in a given amount of time. It is one of those concepts that is
profoundly resistant to a simple definition.
NOVA: Do you think that backwards time travel will ever be possible?
Sagan: Such questions are purely a matter of evidence, and if the evidence is
inconsistent or insufficient, then we withhold judgment until there is better
evidence. Right now we're in one of those classic, wonderfully evocative
moments in science when we don't know, when there are those on both sides of
the debate, and when what is at stake is very mystifying and very profound.
If we could travel into the past, it's mind-boggling what would be possible.
For one thing, history would become an experimental science, which it certainly
isn't today. The possible insights into our own past and nature and origins
would be dazzling. For another, we would be facing the deep paradoxes of
interfering with the scheme of causality that has led to our own time and
ourselves. I have no idea whether it's possible, but it's certainly worth
NOVA: Would you like it to be possible?
Sagan: I have mixed feelings. The explorer and experimentalist in me would very
much like it to be possible. But the idea that going into the past could wipe
me out so that I would have never lived is somewhat disquieting.
NOVA: On that note, can you describe the "grandfather paradox?"
Sagan: The grandfather paradox is a very simple, science-fiction-based apparent
inconsistency at the very heart of the idea of time travel into the past. It's
very simply that you travel into the past and murder your own grandfather
before he sires your mother or your father, and where does that then leave you?
Do you instantly pop out of existence because you were never made? Or are you
in a new causality scheme in which, since you are there you are there, and the
events in the future leading to your adult life are now very different? The
heart of the paradox is the apparent existence of you, the murderer of your own
grandfather, when the very act of you murdering your own grandfather eliminates
the possibility of you ever coming into existence.
Among the claimed solutions are that you can't murder your grandfather. You
shoot him, but at the critical moment he bends over to tie his shoelace, or the
gun jams, or somehow nature contrives to prevent the act that interrupts the
causality scheme leading to your own existence.
NOVA: Do you find it easy to believe the world might work that way—that is,
self-consistently—or do you think it's more likely that that there are
Sagan: It's still somewhat of a heretical ideal to suggest that every
interference with an event in the past leads to a fork, a branch in causality.
You have two equally valid universes: one, the one that we all know and love,
and the other, which is brought about by the act of time travel. I know the
idea of the universe having to work out a self-consistent causality is
appealing to a great many physicists, but I don't find the argument for it so
compelling. I think inconsistencies might very well be consistent with the
NOVA: As a physicist, what do you make of Stephen Hawking's chronological
protection conjecture [which holds that the laws of physics disallow time
Sagan: There have been some toy experiments in which, at just the moment that
the time machine is actuated, the universe conspires to blow it up, which has
led Hawking and others to conclude that nature will contrive it so that time
travel never in fact occurs. But no one actually knows that this is the case,
and it cannot be known until we have a full theory of quantum gravity, which we
do not seem to be on the verge of yet.
One of Hawking's arguments in the conjecture is that we are not awash in
thousands of time travelers from the future, and therefore time travel is
impossible. This argument I find very dubious, and it reminds me very much of
the argument that there cannot be intelligences elsewhere in space, because
otherwise the Earth would be awash in aliens. I can think half a dozen ways in
which we could not be awash in time travelers, and still time travel is
NOVA: Such as?
Sagan: First of all, it might be that you can build a time machine to go into
the future, but not into the past, and we don't know about it because we
haven't yet invented that time machine. Secondly, it might be that time travel
into the past is possible, but they haven't gotten to our time yet, they're
very far in the future and the further back in time you go, the more expensive
it is. Thirdly, maybe backward time travel is possible, but only up to the
moment that time travel is invented. We haven't invented it yet, so they can't
come to us. They can come to as far back as whatever it would be, say A.D.
2300, but not further back in time.
Then there's the possibility that they're here alright, but we don't see them.
They have perfect invisibility cloaks or something. If they have such highly
developed technology, then why not? Then there's the possibility that they're
here and we do see them, but we call them something else—UFOs or ghosts or
hobgoblins or fairies or something like that. Finally, there's the possibility
that time travel is perfectly possible, but it requires a great advance in our
technology, and human civilization will destroy itself before time travelers
I'm sure there are other possibilities as well, but if you just think of that
range of possibilities, I don't think the fact that we're not obviously being
visited by time travelers shows that time travel is impossible.
NOVA: How is the speed of light connected to time travel?
Sagan: A profound consequence of Einstein's special theory of relativity is
that no material object can travel as fast as light. It is forbidden. There is
a commandment: Thou shalt not travel at the speed of light, and there's nothing
we can do to travel that fast.
The reason this is connected with time travel is because another consequence of
special relativity is that time, as measured by the speeding space traveler,
slows down compared to time as measured by a friend left home on Earth. This is
sometimes described as the "twin paradox": two identical twins, one of whom
goes off on a voyage close to the speed of light, and the other one stays home.
When the space-traveling twin returns home, he or she has aged only a little,
while the twin who has remained at home has aged at the regular pace. So we
have two identical twins who may be decades apart in age. Or maybe the
traveling twin returns in the far future, if you go close enough to the speed
of light, and everybody he knows, everybody he ever heard of has died, and it's
a very different civilization.
It's an intriguing idea, and it underscores the fact that time travel into the
indefinite future is consistent with the laws of nature. It's only travel
backwards in time that is the source of the debate and the tingling sensations
that physicists and science-fiction readers delight in.
NOVA: In your novel Contact, your main character Eleanor Arroway travels
through a wormhole. Can you describe a wormhole?
Sagan: Let's imagine that we live in a two-dimensional space. We wish to go
from spot A to spot B. But A and B are so far apart that at the speed of light
it would take much longer than a generational time or two to get there as
measured back on world A. Instead, you have a kind of tunnel that goes through
an otherwise inaccessible third dimension and connects A and B. You can go much
faster through the tunnel, and so you get from A to B without covering the
intervening space, which is somewhat mind-boggling but consistent with the laws
of nature. And [the theoretical physicist] Kip Thorne found that if we imagine
an indefinitely advanced technical civilization, such a wormhole is consistent
with the laws of physics.
It's very different from saying that we ourselves could construct such a
wormhole. One of the basic ideas of how to do it is that there are
fantastically minute wormholes that are forming and decaying all the time at
the quantum level, and the idea is to grab one of those and keep it permanently
open. Our high-energy particle accelerators don't have enough energy to even
detect the phenomenon at that scale, much less do anything like holding a
wormhole open. But it did seem in principle possible, so I reconfigured the
book so that Eleanor Arroway successfully makes it through the center of the
galaxy via a wormhole.
NOVA: What do you think it would be like to travel through a wormhole?
Sagan: Nobody really knows, but what Thorne has taught me is that say, for
example, you were going through a wormhole from point A to point B. Suppose
point B was in orbit around some bright star. The moment you were in the
wormhole, near your point of origin A, you would see that star. And it would be
very bright; it wouldn't be a tiny point in the distance. On the other hand, if
you look sideways, you would not see out of the wormhole, you would be in that
fourth physical dimension. What the walls of the wormhole would be is deeply
mysterious. And the possibility was also raised that if you looked backwards in
the wormhole you would see the very place on world A that you had left. And
that would be true even as you emerged out of the wormhole near the star B. You
would see in space a kind of black sphere, in which would be an image of the
place you had left on Earth, just floating in the blackness of space. Very
Alice in Wonderland.
NOVA: Your inquiries about space travel for Contact sparked a whole new
direction in research on time travel. How does that make you feel?
Sagan: I find it marvellous, I mean literally marvellous, full of marvel, that
this innocent inquiry in the context of writing a science-fiction novel has
sparked a whole field of physics and dozens of scientific papers by some of the
best physicists in the world. I'm so pleased to have played this catalytic role
not just in fast spaceflight but in the idea of time travel.
NOVA: How do you feel being responsible for bringing time travel perhaps a step
Sagan: I don't know that I've brought time travel a step closer. If anyone has
it's Kip Thorne. But maybe the joint effort of all those involved in this
debate has at least increased the respectability of serious consideration of
the possibility of time travel. As a youngster who was fascinated by the
possibility of time travel in the science-fiction novels of H.G. Wells, Robert
Heinlein, and others, to be in any way involved in the possible actualization
of time travel—well, it just brings goose bumps. Of course we're not really
at that stage; we don't know that time travel is even possible, and if it is,
we certainly haven't developed the time machine. But it's a stunning fact that
we have now reached a stage in our understanding of nature where this is even a
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