Time Travel and the Laws of Physics
An Exercise in Speculation
by Joey Swails (c) 1992
Wouldn't it be wonderful if travel to another time was
possible. Time travel has been a mainstay of fantasy and
science fiction for almost a century, from H.G. Wells to
Star Trek. This short essay shall attempt to discuss the
concept in the framework of what is currently held to be
true in the realm of physical science, and how current
theories might apply.
I'm not a professional physicist, just a well read
amatuer with a couple of underclassman physics courses to my
credit. Nothing I have to say on the subject requires a
doctorate in Theoretical Physics to comprehend. I've always
been impressed by the fact that, unlike most of his
collegues, Einstein felt that the average intelligent person
could grasp the ideas of Relativity without the heavy
mathematics; to him it was an indication of the fundimental
truth of what he was working on. To that end, he wrote
several highly readable "layman's" books on the subject
himself. I gleaned EVERYTHING in this essay from other
writers, Einstein, Hawking and Sagan among them. And Arthur
C. Clarke, Larry Niven, and Isaac Asimov; these gentlemen,
while best known for their science FICTION writing, are also
excellent SCIENCE writers as well!
And now, from high atop the shoulders of giants...
Most people have an intuitive grasp of what is meant by
"time travel", at least in a personal sense. One activates a
machine or says the magic words and is "transported" bodily
to a different "point in time", be it the past or future.
Sounds simple. But in a universe that is observed to obey
certain "rules of behavior", in practice it may not be as
simple as it sounds.
Why should we assume that certain laws of physics should
apply to time travel? Aren't new theories of physics often
proposed that supersede older theories, showing them to be
incorrect? And therefore, couldn't even newer theories allow
for time travel to be possible?
The popular misconception is that when a new physical
theory is proposed, it renders preceding one's obsolete.
This may have been true in the case of Copernicus, where
what is being superceded is basically a myth. In modern
theoretical physics, the distiction is not so clear cut. The
value of a theory is in it's power to predict the behavior
of the universe by logical inference. Physical theories
apply logical rules to analyzing a specific facet of the
"universe of discourse", that reality which is accessable to
everyone, either directly through the senses, or "second
hand" through the use of insturments of measurement. To
invoke logic, the proposed theory need only be logically
self- consistent, even if what it descirbes applies only in
very limited cases.
When Einstein published his theories of Special and
General Relativity, the planets did not suddenly alter their
orbits. We still use Newtonian laws of motion to predict the
trajectories of spacecraft, and they appear to work quite
well WITHIN THEIR LIMITS. What happens is that the limits
are expanded, and new theories are proposed to describe
behavior outside the old law's limits. The old laws still
hold; new laws usually apply only to new areas of
observation. They build on, rather than eliminate, the old
So, let's speculate on some of the concepts of time
travel. For the purposes of speculation, we must make some
assumptions and see where they take us. For logic to work,
the only requirement is that the assumptions are internally
consistent, i.e. if something is true in case A(1) it is
also true in case A(2) - no changing the rules as you go
along. The overall assumption I'm making is that certain
laws of the universe will always hold, and time travel can
only be possible if NONE of them are violated in the
process. Specifically, I refer to the Conservation of Matter
and Energy; the Conservation of Motion; Entropy; and
Causality. Perhaps I'm nitpicking, but not really - we'll
leave Relativity and Quantum Mechanics aside for now and
concentrate on predictive theories that have meaning across
a broad spectrum of universal models, from Newton to
Also because Relativity tends to knock out time travel
in the first round, which wouldn't be much fun; and Quantum
Mechanics STILL can't find that damn tachyon particle
(predicted to travel "backwards in time"), and at the
quantum level, we can't even tell which way even NORMAL time
is "moving" anyway...
At least, not yet...
The Basic Paradox of Time Travel
When anyone speculates about time travel, one of the
first things encountered is known as the Grandfather
Paradox, the origination of which is credited to Einstein.
The basic idea is this:
I invent a time machine. Since I always hated my nasty
old grandfather, I travel back in time to when he was a
young boy and murder him. The perfect crime!
But I've gone and killed him before he met my
grandmother, so therefore I was never born, and of course
couldn't have invented a time machine.
So I couldn't have killed him. So he sires my father,
who sires me, who invents a time machine...
Me and my time machine must both exist and not exist.
There lies the paradox. Any process that interferes with
the past in a self-cancelling way is broadly called a
This is the big one: Causality. Our belief in ANY
physical laws at all presupposes cause-and-effect; if this,
then that. Maybe you don't WANT to kill your grandfather,
but the possibility would exist that you COULD, and that's
all that matters. The effect is made to come before the
cause, and causes the cause not to effect the effect.
Fundimentally illogical, and was reason enough for Einstein
to consider time travel impossible.
But it gets even stickier when we consider along with it
the next law we will speculate about - Conservation of Matter
Conservation Laws and Time Travel
The law basically states that matter and energy can be
converted into one another, but can NEVER be destroyed (as
in totally eliminated) from the universe. Conversely,
neither one can be created spontaneously out of nothing. A
time machine that travels from now to 1000 years ago, can be
said by any test available, to have appeared out of nothing,
and can be said to also have dissappeared INTO nothing (from
it's original starting point). This also goes for any energy
that made the trip with it. As long as it remains translated
out of it's original point in time, there's extra matter
running around. But it doesn't stop there.
Let's assume that I build a time machine in my workshop.
Then I travel back to the past, visit the workshop, and
remove or destroy some irreplacable part of the machine. How
then would I be able to build the machine? Paradox again!
Let's continue further along this line. I travel back in
time to 1pm yesterday and remain there for one hour, then
return. Upon returning, I again travel back to 1pm
yesterday. I meet myself! BIG paradox!
Some may want to believe in some "grand overall" theory
of Conservation. It goes something like this - since the
time machine disappeared in one time period, and reappeared
in another, there is still only one time machine in any one
place at a time, so some kind of "overall conservation" from
the beginning of time to infinity, is preserved. There's no
evidence that this should be the case, but no clear evidence
that it could NOT be, so let's be gracious. Now, a problem
arises when I time travel from various points in the future
to the SAME point in the past. If I assume I could go on
doing this indefinitely, a HUGE pile of doppelgangers begins
to accumulate at a single point in time, seemingly
replicating me and my time machine ad infinitum. Clearly a
violation of logic as well as conservation.
For MORE fun, let's say we all agree to return to the
same point in future space/time. Now, how do we all occupy
the same point in space/time?
Now I here someone in the last row say, "OK, perhaps the
process won't work if you try to go somewhere (or is it
someWHEN?) that the time machine already is." Then the
question arises, what constitutes "the time machine"? The
collection of atoms and energy packets that make up it's
physical structure? OK. But all of those components existed
in SOME form before I built a time machine out of them. Why
would matter behave differently once it was part of a time
machine? Does it mean I can't travel back to my workshop to
the time before I assembled the machine, when all I had were
component parts? Or I can't travel to the time at the
foundry where/when they cast the steel for the outer hull? A
particle of matter or energy has no "knowledge" of what it
is a "part of" at a given point of space/time (at least, an
isolated particle has never been observed to behave
differently due to where it "came from".) There's no clear
place to "draw the line" either it's drawn with no
exceptions (paradox) or it's not drawn at all
That leaves the Law of Conservation of Motion. Newton
described it first, but Einstein was the first to offer a
viable explanation for it. (I know, I promised "no
Relativity", but this will be over in a second.) A rigid
fourth dimensional continuum is required for his
explaination to work; and it DOES work - If E=mc^2 was
incorrect, thermonuclear bombs would not explode. I think we
can all agree that they DO. For anything to move around in
the time dimension, it must move faster than light,
acquiring infinite mass and energy along the way. Once you
EXCEED the speed of light, you're traveling backwards in
time - but GETTING THERE could be hazardous to your health.
Physical time travel clearly violates any law of motion,
as motion ALWAYS relates to time. This effects conservation
of motion, rules of kinetic energy, even the law of
gravity.(At least, any law of gravity I can think of!)
Entropy and Time Travel
Finally, there is entropy (sounds like a philosophical
statement, doesn't it?) - the tendency of matter and energy
to move from order to disorder, embodied by the Second Law
of Thermodynamics. This tendency toward randomization gives
us the most obvious impression of "moving forward in time".
It's not easy to observe the decaying orbit of a planet, or
the breakdown of a lambda particle, but try to get an
exploded bomb back into it's casing. That's entropy. It's
happening all the time, to everything in the universe, and
it's rate of decay for a given type of particle is steady in
the extreme. This makes things like atomic clocks and Carbon-
14 dating possible.
There's also the products of this breakdown to consider.
A human body going through it's biological processes is a
walking bundle of changing energy states, some changing to
higher states, but more changing to lower states - always a
net loss. Entropy again.
So say I travel to the past, and while there I'm
breathing the air of the past, breaking down the bonds of
the O2 molocules and binding them to carbon, deriving energy
for my metabolism from the process. So I'm exhaling
particles with lower energy states than they had when I
sucked them into my lungs (or less of them; a moot point,
because matter and energy are the same thing ultimately.)
When I return to my original time, how do I recover all of
the exhaled particles that now have an "incorrect" energy
state relative to all the other particles in their "new"
time-frame? I'd HAVE to, in order to perserve any idea of
universal entropy; if I leave them "behind" they stay there
until the end of infinity, and we never "balance the books"
of the total energy state of the Universe.
Quantum Mechanics and Time Travel
I suppose we're going to HAVE to bring up tachyons. I
know, it IS Quantum Mechanics, but the tachyon is the
darling of time-travel enthusiasts, so in anticipation of it
being brought up by SOMEBODY, let's discuss it a bit.
Tachyons, though not yet detected, are predicted to
exist by Quantum Theory. Let me dig out the physics text -
what physicists are looking for is the decay of a lambda
particle into a proton plus a pi meson. They predict they
will find a nonzero value for the beta component of the spin
of the proton (whew!), implying the release of a particle
with reversed spin and charge - meaning it's moving
"backward in time"!
The problem is they have not found such a reaction to
take place. Particle accelerators have been blasting away
at the particles in their cloud chambers for years now, and
the shy tachyon still refuses to "show itself". Most
scientists studying the subject now believe they will find
something wrong with the model that predicts the existence
But let's assume that they DO exist. Can a machine (or a
human body) "make like a tachyon" and boogie backward in
time? All you have to do is start moving backwards and wait
for the past to catch up with you.
The theory that predicts tachyons says that in order to
do that trick, the mass in question would have to instantly
reverse the spin and charge of every sub-atomic particle in
it. But since the mass is NOT made of tachyons in the first
place, to impose this on any other kind of particle will
have the distressing side-effect of converting the entire
mass into anti-matter. MAJOR fireworks...
This points out the error in the logic of "if tachyons
can move backwards in time, then why can't something ELSE do
it too?". If you subscribe to the theory that allows for the
existence of the backwards moving tachyon, you can't
selectivly ignore any of the corrolaries that go along with
the prediction. Tachyons are a LIMITED SPECIAL CASE, and QM
is full of them. Sauce for the tachyon is not necessarily
sauce for the proton, in a manner of speaking. It's this
plethora of limited special cases that makes coming up with
a Grand Unified Theory to cover ALL the cases so difficult
Relativity and Time Travel
Before anybody gets upset, let me point out that the
Relativity Laws provide the ONLY available means to travel
in time that we know about. It's called time dilation, and
it occurs during acceleration of mass. It's almost
undetectable at speeds we normally deal with, but
experiments with atomic clocks in space capsules have borne
out the theory - it happens. An accelerating mass
experiences the flow of time at a slower rate than a mass at
rest. Travel to Proxima Centauri, a trip of 4.3 light years,
and accelerate steadily to 9/10ths of lightspeed till you're
halfway there, then de-accelerate down to rest; now turn
around and go back. To you, the round trip took about 25
years. Returning to earth, you find that about 3000 years
have elapsed while you were gone! The closer you approach
lightspeed, the more drastic the effect. A one-way trip, and
only to the future, but it works!
Also, the mathematics of Relativity predict that some
strange things may occur to a mass that manages to travel at
a trajectory that passes the outer event horizon of a
rotating black hole (look up Penrose diagrams in a text on
Relativity for more about this effect.) Suffice it to say
that it MAY allow a mass to travel outside it's normal space-
time axis. The theory can't predict exactly HOW this effect,
if it exists, would manifest itself; it could mean
superluminal motion, it could mean time-travel, or it could
mean emerging into a different universe entirely. It might
also not follow the prediction at all (remember tachyons?
Theory says we should find them, but we haven't.)
Plus there are the practical problems. Converting enough
energy for a controlled fly-by of a black hole would require
a LOT of logs to throw on the fire, to say the least!
Something like the output of your average star should
suffice. And then there's the question of what to build the
hull out of - something that can handle the hellish
radiation spewing out of the event horizon, as well as the
acceleration, gravity pull and tidal forces involved. A few
gigatons of neutron star material should do quite nicely, I
would think. And also the problem of controlling the
trajectory that you take, which is what determines
where/when you will end up, calculated to many fiendish
decimal places. The tiniest of mistakes and you end up being
INTIMATELY aquainted with a black hole...
It may be fun to think about, but it's not likely that
this "loophole" in Relativity will lead to a practical time
The Letter of the Law
I've assumed throughout all my speculations here that
the physical laws involved cannot be broken. To be fair,
I've accepted that there can even be some slight bending in
the cases of Relativity and Quantum Mechanics, which really
means only that our understanding of them is incomplete.
I've been really stubborn, however, regarding Conservation
Laws. Is this justifiable? I believe it is.
One of Einstien's postulations was that the universe is
the same everywhere. The rules governing the behavior of
mass and energy are the same no matter where you are. And in
ALL known cases, the Laws of Conservation hold rigorously,
from the behavior of galaxies to the behavior of particles.
They are what Quantum scientists use to find new quantum
states. The existence of the neutrino was postulated using
them, and this very elusive particle has been successfully
detected, behaveing exactly as the Laws say it should.
If we allow exceptions, we have some serious problems
trying to find another way to explain all the phenomena
occuring in our universe. Conservation is a FACT, as cast in
immutable stone as anything like it can possibly be.
Any theory is an exercise in speculation, and must start
with assumptions. I chose the ones that allowed for the
reality of ghost particles and hydrogen bombs. I think I'm
in good company. But what truly matters is self -
consistancy. Make you own assumptions and see where they
take you. However, once you make them, stick to them, or you
can't expect truth to emerge.
A Few Direct Quotes
First, from the fictional character of Lazarus Long,
the oldest living human, (when last seen, over 2000 years
old!)created by Robert Heinlein:
"What are the facts? Again and again and again - what
are the FACTS? Shun wishful thinking, ignore divine
revelation, forget what "the stars foretell", avoid opinion
(especially your own), care not what the neighbors think,
never mind the "unguessable verdict of history" - what are
the facts, and to how many decimal places? You pilot always
into an unknowable future, and facts are your single clue.
GET THE FACTS!"
Lastly, from Isaac Asimov, a NON-fictional character
(but quite a character, nonetheless):
"It is not required of The Laws of the Universe that
they manifest themselves in a way that is convienient."
In other words, "anything is possible" might simply be
impossible. The universal dice just might be loaded in such
a way that things like time travel and faster-than-light
speeds are TRULY IMPOSSIBLE, now and forever, no matter how
much we may wish it were otherwise.
But take heart. There are enough wonderous, mind-
boggling things left in the Universe to keep us entertained
for a very long time to come...
...but please, no time machines.