**Xenology:
An
Introduction to the Scientific Study of Extraterrestrial Life, Intelligence,
and Civilization**

**
First Edition**

**©
1975-1979, 2008 Robert A. Freitas Jr.
All Rights Reserved.**

Robert A. Freitas Jr., **Xenology: An Introduction to the Scientific
Study of Extraterrestrial Life, Intelligence, and Civilization**, First
Edition, Xenology Research Institute, Sacramento, CA, 1979; http://www.xenology.info/Xeno.htm

**17.5
Time Travel**

Interstellar journeying ultimately depends upon manipulating the formula D = V x T, where D is distance, V is velocity, and T is time. So far we have discussed only the possibilities of increasing V or of decreasing (or abolishing altogether) D. But there is a third option available. Alien or human technologists may somehow manage to reduce or otherwise circumvent T.*

One-way time travel into the future is no problem at all. Each of us is doing it right now, at the constant rate of 24 hours/day. The problem is how to alter that rate.

We’ve already taken a quick look at relativistic time
dilation. Time passes more slowly far moving astronauts than for those who are
standing still. For example, a person traveling at a constant 99%c through space
may also be said to be traveling through time at the reduced rate of 3.4 hours/day
(hours personal time/standard Earth observer day). In case anyone is interested,
the time dilation rate may be calculated according to the following relation:
T = t(1 - v^{2}/c^{2})^{1/2}, where T is shipboard time
and t is stationary observer time. And time dilation is not just some theoretician’s
pipe dream, either. In 1971 two American physicists, Joseph Hafele and Richard
Keating, decided to test Relativity and see if they could make themselves age
a tiny bit more slowly. They purchased tickets on a jumbo jet for an around-the-world
flight, taking with them an atomic clock, accurate to one billionth of a second,
to measure more precisely the passage of time. The aircraft flew at 600 mph,
or about 0.00009%c, circumnavigating the planet. When the trip was over the
moving clock -- and the two scientists themselves -- had lost more than 10^{-8}
seconds. That is, for most of the flight, personal time had been passing at
the decreased rate of 23.999 999 999 9996 hours/day.^{2891}

The time dilation caused by motion is a product of Special Relativity. But similar effects occur near massive objects. According to General Relativity, time moves more slowly close to matter than far from it. In other words, mass makes time slow down. For instance, on the surface of Earth we age about 24 hours/day. But in free space, far from any planet, time speeds up and we age slightly faster -- about 24.000 000 016 hours/day.**

Near a stellar-sized black hole we could expect time delays
ranging from miniscule to enormous (but watch out for the tides). Judicious
selection of proper hyperbolic orbits could get our aging rate down to microseconds/day
or less.^{2636} The tides can be avoided by seeking out a supermassive
galactic BH -- perhaps 10^{8} M_{sun} or so -- to perform the
experiment. Or, if we could collect or manufacture lots of extremely dense matter,
we will discover that time also runs slower inside a heavy mass,^{2014}
We might construct a hollow spherical shell of dense matter either on Earth
or in orbit, and we’d have a one-way time machine. There is no gravity
inside a hollow sphere due to mass in the shell, and no tides either. But time
would pass more slowly inside the passenger chamber. If he could see out, the
world might appear to flash by much as in the movie classic *The Time Machine*;
to outside observers looking in, the time traveler would seem to be moving in
slow motion.

If it is deemed useful for some reason, it may also be possible
to increase the rate at which time passes. A hollow sphere constructed of extremely
dense negative mass would greatly speed up time throughout its interior. This
could amount to years/day rates or more. A British playwright could enter the
fast-time chamber at 3 PM one afternoon, spend two weeks writing a play, and
emerge at 4 PM the same day in time for tea. This could have a number of interesting
technical and social consequences.^{2020}

What about travel into the past? Traditionally, this has been considered quite impossible because of violations of causality (the proper relationship between cause and effect) that could occur. Causality problems are commonly illustrated in terms of the Grandfather Paradox, which goes something like this: You build a time machine and use it to go back 80 years in time. Your grandfather, whom you dearly despise, is a babe-in-arms, so it is an easy matter to dispatch him. But now he doesn’t live to marry your grandmother, so your father is never born and neither are you. So you don’t exist; worse, the time machine that caused all the trouble doesn’t exist either since you weren’t around to build it. But then you couldn’t have gone back in time to kill your grandfather, so he lived, so you were born, so the time machine was built...etc. -- a real paradox.

Such difficulties do not deter the stout of heart and firm
of nerve. F.J. Tipler, a physicist associated with the University of Maryland,
recently published a paper in *Physical Review* (a highly respected journal)
in which he proposed that General Relativity can be used to design a two-way
time machine.^{2894} Tipler suggests the construction of a dense cylinder
of mass, spinning with a tangential velocity of at least 50%c at the circumference.
This, he claims, should give rise to a very unusual region of space-time --
called "multiply connected space-time" -- existing outside the physical
boundary of the mass itself.

According to Tipler, any past or present event in the known universe could be reached by passing through the "multiply connected" region near the middle, but outside, of the spinning cylinder. Starting at any point (x, y, z, t) and traveling at suboptic velocities around the special region in the proper way, a starship can return to point (x, y, z) at any time t ± Dt -- past, present, or future. Tipler makes his prediction explicit: "In short, general relativity suggests that if we construct a sufficiently large rotating cylinder, we create a time machine."

It remains to be seen whether Tipler’s proposal can withstand critical review, but it remains a tantalizing possibility nevertheless.

* The literature on time travel and the nature
of time is enormous. Interested readers are referred to Anderson,^{2795}
Asimov,^{2817} Gerrold,^{2819} Hoyle,^{2818} Snatsky,^{636}
Vonnegut,^{2480} and Wells^{2796} for fictional treatments of
time travel, and to Edwards,^{1166} Finkelstein,^{1502} Fraassen,^{905}
Gardner,^{644} Lucas,^{1859} Meerloo,^{903} Niven,^{2743}
Taylor,^{1190} and Whittrow^{1847} for conceptual and theoretical
treatments of time and time travel.

** For the mathematically-inclined reader,
the formula for the general relativistic time dilation rate (t) as a planetary
mass (M) is approached to a distance R is given by: t = T(1 - 2GM/Rc^{2})^{1/2},
where T is free-space time, G is the universal gravitation constant, and c is
the speed of light.

Last updated on 6 December 2008