We are all natural time travelers. From the moment we’re born to the moment we die,
we are trekking across time to witness events around us unique to our period of existence.
We watch, across our lives, cause and effect and the progression of time as we see it in
our frame of reference. Or you could put it a different way, we are stuck in time travel,
and as was said in Star Trek, time is the fire in which we burn, a paraphrase of a line
in a poem by Delmore Schwartz.
This has led to many notions throughout history. For the Romans time travel meant a return
to happier, more prosperous times. To the modern age, and H. G. Wells envisioned a time
machine, or even the exploration of whether time could stop and what it would be like.
This paralleled the science of the last 150 years, culminating in Einstein’s General
and Special relativity, which placed limits, but also opened up possibilities of time travel.
So dream as we might that time travel of one sort or another, the science of today does
allow for some possibilities and some realities regarding it. So here are ten strange ways
time travel could be possible.
Number 10. Future: Extreme Time Dilation
While we know that we are time travelers inching our way day to day into the future, one of
the great revelations of science has been that the rate at which you travel into the
future can change. This was once thought impossible, in the physics of Isaac Newton clocks ticked
at the same rate across the universe no matter where you were. A wrist watch would have ticked
at the same rate no matter if you were here or in the alpha centauri system or in a distant
galaxy.
Then came special relativity, which killed the clocks. The prediction of Einstein was
that clocks do not tick at the same rate, but depend on relativistic frames of reference.
In short, it depends on how fast you are moving, or if you’re in a gravity well, versus another
frame of reference where someone is not moving. So the net result is if you set two watches
digital or not, or atomic clocks, at exactly the same time and keep one here on earth,
and send the other at high speed through the universe and then do a u-turn and come back,
the clocks will not match when it gets back.
This is real. We actually have to correct the GPS system satellites for time dilation
or the clocks get out of sync. While this effect doesn’t negate the physics of Newton,
that’s all still in use, your vehicle whether a car, plane or a train must account for it,
it does point to Newtonian physics having been incomplete. But there is a huge implication
here, the faster you go, the more time dilation becomes evident. In short, given you are already
moving forward in time, the universe allows you to hack the rate of time you are traveling
into the future. And it gives you two different ways.
Yes, if you, as a human living right and you want to see what earth and Earth and Human
civilization will be like in 600 years, far beyond a current lifetime, the universe allows
you a way to do that. Just go very, very fast.
The challenges of getting up to relativistic speeds fast enough to experience this kind
of time dilation aside, it would require massive amounts of energy, and a whole lot of engineering,
you can in principle do it. But at the cost of the past, so far as we know, or at least
think we know, more on that in a bit, you couldn’t ever get back to your own time.
But if your past doesn’t matter, then you can see the future. Even the very far future
if you get as close to the speed of light as you can. Millions or more years.
Number 9. Future: Black Holes
Acceleration in relativistic physics is an interesting notion. You’d normally think
of it in everyday terms, such as stomping on the gas pedal of a car. But that’s not
the only acceleration possible in physics. If you think about it, gravity is really an
acceleration as well at least of sorts. When gravity pulls on you, it is in effect imparting
an acceleration of you towards the source of the gravity. Even if you’re on the surface
of that world and not moving, you are still in effect accelerating towards it, at least
as the force of gravity is concerned.
This means that time dilation also applies here. The more massive the object, the higher
its gravity is, and that means an increase in time dilation. This is especially evident
in the environment of a black hole, which was more or less shown in an exaggerated form
in the movie Interstellar, where time passed at a different rate when on the ocean wave
world than further out in orbit of the black hole. The reality is that for that to have
happened as shown, it would have been required to be hovering just above the event horizon
of the black hole, or at least the implied size of that particular black hole.
This makes black holes future machines. Spend time near one, and time will appear in your
frame of reference to tick normally, but in the universe at large huge amounts of time
may be passing in what to you is just a short amount of time. So when you leave the proximity
of the black hole, if you can, then you will be in the distant future compared to when
you went in.
Number 8. Future: Suspended Animation
The idea of waking a person up, such as they might be, after a period of being inanimate
is an old idea. Indeed, what is often credited as the first science fiction novel, and for
that matter horror novel at least in the modern sense on that genre. It’s Frankenstein by
Mary Shelley and the idea is simple. Grab a bunch of body parts from corpses including
a brain and shock it back to life and see what you get. Old parts living again, but
to not great results for better or worse.
This idea has partly left science fiction, or at least some hope with efforts to crogenically
freeze people at the point of death, or really shortly thereafter, in hopes of some day being
able to wake them up. That’s beyond a long shot, but there are ways to envision being
able to put humans into some form of suspended animation in order to revive them at a later
date. All of the problems with this aside, say it could be made to work. That person
becomes a time traveler in effect, having left the present the moment they went unconscious
to wake up in the future, whatever that date may be. While that may seem like cheating,
they’d be spending their non-animation here on earth with our normal time elapsing, it
still counts just as much as time dilation does. You’re just in time out, instead of
fast forward.
Number 7. Past and Future: Many Worlds
This isn’t really a method of time travel, rather a method for making backwards time
travel possible. The biggest issue with backwards time travel is causality violations and paradoxes,
such as the grandfather paradox. If you go back and convince your grandfather not to
have children, you should cease to exist, a theme explored in Back to the Future. But
the reality is that it actually depends on one interpretation of quantum mechanics, and
it may not be the case.
An alternative to it is the many worlds interpretation, where every possible reality splits off into
alternate realities. This myriad of alternate worlds can be only slightly different from
the one you experience, or they could be radically different. It may be that all paradoxes regarding
time travel are solved if the past or the future you visit represents an alternate reality
to the one you just left, and time travel really represents a leap between worlds.
This was however countered by Stephen Hawking, who suggested that even in the many worlds
interpretation, a single person should be self-consistent, and actually arrive in their
past. An even further suggestion is that if you did travel back in time in the many worlds
interpretation by using whatever method, you might end up as a spray of particles each
one ending up in a different reality. Not good.
Number 6. Past and Future: Alcubierre Drives
We often think of the Alcubierre warp drive as a method of faster than light travel. You
essentially isolate a piece of space-time around your starship, put a bunch of energy
and negative mass into it creating a warp field of space itself, which is not beholden
to the speed of light, that can move across the universe at whatever speed you want carrying
your ship inside it. The problem is, in relativity, anything that can travel faster than light
can in principle travel backwards in time. The alcubierre warp drive is not immune to
this.
This is a problem because of an idea originating with Hawking. The chronology protection conjecture,
and it states that anytime you try to build a time machine or anything like one, quantum
effects intervene to stop it. In short, the alcubierre warp drive may be a double edged
sword. If it’s possible to use it for faster than light travel, you may be able to do it.
But once you try to use it as a time machine, the accumulated quantum fluctuations and energy
could cause it to explode or collapse into a black hole. There has to be more than a
few hard horror sci fi stories in that one.
Number 5. Past: The Tipler Cylinder
So far we’ve covered future time travel, which in this universe is no big deal, and
hints that maybe past time travel may be possible but ambiguously so. But here we get into actual
past time travel possibilities that may exist within the rules of the universe. In this
case, we’re discussing the Tipler Cylinder or Tipler time machine. This one depends on
one of two things, infinity and or negative energy.
This was a weird one. The solution to the equations of general relativity that spawned
this idea were first formulated in 1924 and 1936, but the scientists Willem van Stockum
and Kornel Lanczos didn’t recognize the application to Einstein’s closed timelike
curves. These are interesting, CTCs represent space-time trajectories that allow you to
return to your starting point consistent with special relativity. This means that you could
journey into the future, but arrive back in the past, which might violate causality and
might not exist.
Frank Tipler recognized this in 1974 and proposed a cylinder of infinite length stretching across
the universe and rotating creating a frame dragging effect on space-time. The end result
is this, if you have a spacecraft accelerating in a certain direction along one of these
can travel along a CTC and end up in the past. But try building an infinitely long cylinder.
But if you add in negative energy, you might be able to shorten the length needed for the
effect. But we’ve never seen negative energy and have no idea how to produce it. But it
does at least provide a highly theoretically way for backwards time travel to be possible.
Number 4. Past: The Anti-Universe
One idea in recent cosmology is the concept of a two sided universe advanced by Neil Turok.
This idea essentially describes our universe extending out of one side the big bang, and
a possible second universe telescoping out from the other side of the big bang. This
two sided approach could involve an antiuniverse where the companion universe to our own is
running backwards in time.
It’s even possible that the backwards in time universe is made of antimatter, accounting
for the matter and antimatter asymmetry of our universe. Such a universe with a backwards
running clock might be otherwise identical to our own, with conceivable copies of us
running in a sort of anti-history running in reverse. If this anti universe could ever
be accessed, a tall order for sure, then it might provide a method for backwards time
travel to an identical, but backwards universe. The catch there, if it’s possible, is that
to go there costs you one thing: forward moving time. By going there, you would experience
backward moving time. Would you make that trade?
Number 3. Past: The One Electron Universe
In what must have been a truly amazing phone call in spring 1940, physicist John Wheeler
called Richard Feynman explaining that he had solved the mystery of why all electrons
have the same charge and mass. The answer, quite simply, was that they were all the same
electron.
This novel idea is based on worldlines that are traced across spacetime by every electron,
imagine it as a huge number of lines representing the existence of an electron across time,
or one line connecting from your birth to your death across your life. But we don’t
actually know that’s the case for electrons, so Wheeler thought why not instead of lines
a gigantic tangle, for which we see a slice of at any given time, something like if you
were to take a slice of a ball of string. Each of the strings you’d see in the slice
represent a single electron, or piece of string, passing around the ball. But they’re all
the same string in the end.
Wheeler pointed out that it could be that half of the lines go forward, and the other
half backwards and the backwards ones represent the positron, the anti-particle of the electron
moving backwards through time. Feynman noted that electrons in the universe seem to vastly
outnumber positrons, to which Wheeler hypothesized that they might be hidden within protons.
Nevertheless, Feynman did credit Wheeler with the thinking of positrons going backwards
in time and wrote further on it.
If positrons do represent backwards time travel, it’s difficult to see a use for it, other
than confirmation that some particles in nature can go backwards careening towards the big
bang. While we may never be able to use the effect, should it actually be the case, it
does provide proof of concept that the past is back there somewhere, and positrons may
be streaming towards it.
Number 2. Past and Future: Wormholes and Time
One of relativities most interesting possibilities is that of wormholes, which could consist
of gateways through space, perhaps through another dimension, that link two separate
points in space, but also possibly two separate points in time. Whether wormholes can be created
and the even bigger question of can they be controlled as to when or where the other side
leads are still unknowns. But there are ideas of creating traversable wormholes.
The major approach is to envision negative mass, if that’s possible to create. This
would allow for the creation of a stable wormhole, essentially held open through the use of the
negative mass. There are still many questions regarding wormholes, but they do offer one
of the more potentially viable ways to travel time in the far future.
Number 1 Past: Tachyonic Antitelephones
This is perhaps the strangest possibility in regards to past time travel options. One
of the strange attributes of relativity is that while it states that matter can’t ever
reach the speed of light due to an infinite energy requirement, and the pesky issue that
at the speed of light time stops, it doesn’t prohibit a mirror situation that could be
happening faster than light. This is to say that there could be particles traveling faster
than light that can’t ever slow down to meet it.
These hypothetical FTL particles would inherently be traveling backwards in time. It has been
suggested that such particles, should they actually exist, might be manipulatable and
could allow for essentially a telephone call to the past. You could send information backwards
in time, and if anyone could receive it, say you knew about the concept and built a receiver
and then just waited for someone from the future to call, then you could in principle
receive a message from the future. This of course creates paradoxes, and you’d never
have to debate about which stocks to invest in. But here is the odd possibility, if they
sent you a genetic plan for cloning a human from the future, you could in principle do
it. While not a very satisfying way to time travel back to the past as it would be a clone,
and not the original person, it’s still a very weird possibility.
Thanks for listening! I am futurist and science fiction author John Michael Godier and be
sure to check out my books at your favorite online book retailer and subscribe to my channels
regular, in depth explorations into the interesting, weird and unknown aspects of this amazing
universe in which we live.