by Ethan Siegel
The idea of traveling back in time
has long fascinated humans,
such as in Back To The Future's Delorean DMC-12.
After decades of research,
we may have hit upon a solution that's physically possible.
It's one of the greatest tropes in movies, literature, and television shows:
the idea that we could travel back in time to alter the past.
From the time turner in 'Harry Potter' to 'Back to The Future' to 'Groundhog Day', traveling
back in time provides us with the possibility of righting wrongs in our own past.
To most people, it's an idea that's relegated to the realm of fiction, as every law of physics
indicates that motion forward through time is an absolute necessity.
Philosophically, there's also a famous paradox that seems to indicate the absurdity of such a
possibility:
if traveling backwards through time were possible, you'd be able to go back and
kill your grandfather before your parents were ever conceived, rendering your
own existence impossible.
For a long time, there seemed to be no way to go back.
But thanks to some very interesting properties of space and time in Einstein's General
Relativity, traveling back in time may be possible after all.
An illustration of the early Universe
as consisting of quantum foam,
where quantum fluctuations are large, varied,
and important on the smallest of scales.
Positive and negative energy fluctuations
can create minuscule, quantum wormholes.
NASA/CXC/M.Weiss
The place to start is with the physical idea of a wormhole.
In our known Universe, we have tiny, minuscule quantum fluctuations in the fabric of space-
time on the smallest of scales. These include energy fluctuations in both the positive and
negative directions, often very close by one another.
A very strong, dense, positive energy fluctuation would create curved space in one particular
fashion, while a strong, dense, negative energy fluctuation would curve space in exactly the
opposite fashion.
If you connected these two curvature regions together, you could - for a brief instant - arrive
at the notion of a quantum wormhole.
If the wormhole lasted for long enough, you could even potentially transport a particle through
it, allowing it to instantly disappear from one location in space-time and reappear in another.
Exact mathematical plot of a Lorentzian wormhole.
If one end of a wormhole is built out of positive mass/energy,
while the other is built of negative mass/energy,
the wormhole can become traversible.
Wikimedia Commons user Kes47
If we want to scale that up, however, to allow something like a human being through, that's
going to take some work.
While every known particle in our Universe has positive energy and either positive or zero
mass, it's eminently possible to have negative mass/energy particles in the framework of
General Relativity.
Sure, we haven't discovered any yet, but according to all the rules of theoretical physics,
there's nothing forbidding it.
If this negative mass/energy matter exists, then creating both a supermassive black hole and
the negative mass/energy counterpart to it, while then connecting them, should allow for a
traversable wormhole.
No matter how far apart you took these two connected objects from one another, if they had
enough mass/energy - of both the positive and negative kind - this instantaneous connection
would remain.
All of that is great for instantaneous travel through space.
But what about time?
Here's where the laws of special relativity come in.
If you travel close to the speed of light, you experience a phenomenon known as time
dilation.
Your motion through space and your motion through time are related by the speed of light:
the greater your motion through space, the less your motion through time.
Imagine you had a destination that was 40 light years away, and you were able to travel at
incredibly high speeds:
over 99.9% the speed of light.
If you got into a spaceship and traveled very close to the speed of light towards that star, then
stopped, turned around, and returned back to Earth, you'd find something odd.
Due to time dilation and length contraction, you might reach your destination in only a year,
and then come back in just another year. But back on Earth, 82 years would have passed.
Everyone you know would have aged tremendously.
This is the standard way time travel physically works:
it takes you into the future, with the amount of travel forward in time dependent
only on your motion through space.
Is time travel possible?
With a large enough wormhole,
such as one created by a supermassive black hole
connected to its negative mass/energy counterpart,
it just might be.
Wikimedia Commons user Kjordand
But if you construct a wormhole like we just described, the story changes.
Imaging one end of the wormhole remains close to motionless, such as remaining close to
Earth, while the other one goes off on a relativistic journey close to the speed of light. You
then enter the rapidly-moving end of the wormhole after it's been in motion for perhaps a
year.
What happens? Well, a year isn't the same for everyone, particularly if they're moving
through time and space differently!
If we talk about the same speeds as we did earlier, the "in motion" end of the wormhole
would have aged 40 years, but the "at rest" end would only have aged by 1 year.
Step into the relativistic end of the wormhole, and you arrive back on Earth only one year
after the wormhole was created, while you yourself may have had 40 years of time to pass.
