Have you always had a
burning desire to travel through time? Good news! You are technically already
doing it. You are moving through time at quite the speed, even if you’re
sitting comfortably at home on your sofa. There’s only one issue, you can only
move in just one direction.
The more pressing question
is can we travel towards the past? As far as we know, no. There is no known
mechanism that allows us to physically transmit matter or information from the
present into the past. Time travel requires going faster than the speed of
light and we haven’t found a hint that it’s possible. It is highly unlikely
that traveling to any arbitrary point in space-time can actually happen.
This could be an accurate if
somewhat disappointing “case closed” on the question, but there are two reasons
why it is important to chat about time travel. Number one, time travel would be
great and I totally want to do it. Number two, it is good to challenge one of
the biggest unknowns in physics: time.
What is time? We don’t know.
Why is time going in only one direction? We don’t know. Must it always move
forward? Maybe, but we don’t really know. Is time continuous or quantized? Is
it made by tiny but finite units? We don’t know. This is the state of our
ignorance regarding time.
Relativity Gives Us (Some)
Hope To Go Back To The Past
This ignorance has
consequences for our theories. Once theories are formulated, people like to
play with them and push them to the limit.
Black holes and the Big Bang
were once mere ideas out of general relativity. Could the same happen for time
travel? That's unlikely but in the interest of fairness, there are special
solutions that allow the creation of regions where it is possible to cheat
physics and go faster than the speed of light.
You can start simple and
have a solution for closed, time-like curves. You travel back in time simply
because time is a loop. Think Groundhog Day but with more math. Alternatively,
you can have a wormhole, a bridge between two distinct points in space and
time. The only issue is that both of these require conditions that we have not
witnessed in the known universe.
Another solution is a
little bit more forceful. You could technically move faster-than-light by
literally squeezing space-time in such a way that you cover huge distances in
brief intervals of time. This is the principle behind the Alcubierre Drive. If
this wasn’t crazy enough, another solution that allows for time travel requires
an infinitely long cylinder that spins. You could travel backwards or forwards in
time just by going around it. As it spins, the cylinder would twist space-time
around its axis and this would allow a hypothetical time machine to travel
backward or forward in time by following specific paths arouns the object.
The only conditions required
is the infinite length of the spinning pole and that's not really possible.
Once again, we don’t really know how to approach either solution in a realistic
way.
Quantum Mechanics Keeps Us
In The Present
But not all physics is so
happy-go-lucky with time travel. Relativity doesn’t play well with quantum
mechanics in many aspects, and this is one of them. Pretty much any attempt at
creating a quantum time machine always ends up violating some key principle of
the theory.
Some researchers have been
able to solve one of the time-traveling problems, the so-called grandfather
paradox. This is where you travel through time to go and kill your grandad
stopping you from existing and thus traveling through time. The solution sends
a quantum of information (qubit) back in time, but in a parallel universe,
creating a complex quantum system, where the qubit both exists and doesn’t
exist.
There has also been a more
damning finding from the quantum world that suggests that time-travel is indeed
impossible. Researchers have discovered pear-shaped nuclei that violate some
pretty solid laws of fundamental physics. These laws expect that particle
physics to be symmetric under certain transformations. For example, if you
change how your system looks like in a special mirror which invert all your
directions, particles physics should still behave in the same way, the
so-called parity symmetry.
Same if you look at it
backward or forward in time, time symmetry. Or if you switch matter for
antimatter, the charge symmetry.
But the pear-shaped nuclei,
won't look the same if you turn them upside down and then switch front and back
and left and right. It also looks different if your switch matter for
antimatter. Such violations are usually solved by combining the two together (Charge-Parity
symmetry) but this is one of the handful of examples where CP is violated. This
implies that the time symmetry is also violated. So only by combining CPT, this
can be explained. So this violation can only be fixed if time has a very
specific direction.
All In All, We Are None The
Wiser
All in all, time travel seem
like a pretty difficult and messy affair. We can’t prove beyond any doubt that
it’s impossible, but it’s not like we are swimming in evidence that suggests it
is possible. Obviously, by not understanding time itself, we lack a big chunk
of the puzzle.
This doesn't mean that
research on time travel is wasted, it can actually help us understand current
theories better. But be critical of people claiming they have found a way to travel
back in time.
Via IFLScience