17.5 - Time Travel

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

First Edition

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

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 - v²/c²)^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 10⁸ 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 Wells²⁷⁹⁶ 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 Whittrow¹⁸⁴⁷ 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/Rc²)^1/2, where T is free-space time, G is the universal gravitation constant, and c is the speed of light.