No one has yet managed to travel through time – at
least to our knowledge – but the question of whether or not such a feat would
be theoretically possible continues to fascinate scientists.

As movies such as The Terminator, Donnie Darko, Back
to the Future and many others show, moving around in time creates a lot of
problems for the fundamental rules of the Universe: if you go back in time and
stop your parents from meeting, for instance, how can you possibly exist in
order to go back in time in the first place?

It's a monumental head-scratcher known as the
'grandfather paradox', but a few years ago physics student Germain Tobar, from
the University of Queensland in Australia, worked out how to "square the
numbers" to make time travel viable without the paradoxes.

"Classical dynamics says if you know the state
of a system at a particular time, this can tell us the entire history of the
system," said Tobar.

"However, Einstein's theory of general
relativity predicts the existence of time loops or time travel – where an event
can be both in the past and future of itself – theoretically turning the study
of dynamics on its head."

What the calculations show is that space-time can
potentially adapt itself to avoid paradoxes.

To use a topical example, imagine a time traveler
journeying into the past to stop a disease from spreading – if the mission was
successful, the time traveler would have no disease to go back in time to
defeat.

Tobar's work suggested that the disease would still
escape some other way, through a different route or by a different method,
removing the paradox. Whatever the time traveler did, the disease wouldn't be
stopped.

Tobar's work isn't easy for non-mathematicians to
dig into, but it looks at the influence of deterministic processes (without any
randomness) on an arbitrary number of regions in the space-time continuum, and
demonstrates how both closed time-like curves (as predicted by Einstein) can
fit in with the rules of free will and classical physics.

"The maths checks out – and the results are the
stuff of science fiction," said physicist Fabio Costa from the University
of Queensland, who supervised the research.

The research smoothed out the problem with another
hypothesis, that time travel is possible but that time travelers would be
restricted in what they did, to stop them creating a paradox. In this model,
time travelers have the freedom to do whatever they want, but paradoxes are not
possible.

While the numbers might work out, actually bending
space and time to get into the past remains elusive – the time machines that
scientists have devised so far are so high-concept that for they currently only
exist as calculations on a page.

We might get there one day – Stephen Hawking
certainly thought it was possible – and if we do then this new research suggests
we would be free to do whatever we wanted to the world in the past: it would
readjust itself accordingly.

"Try as you might to create a paradox, the
events will always adjust themselves, to avoid any inconsistency," said
Costa.

"The range of mathematical processes we
discovered show that time travel with free will is logically possible in our
Universe without any paradox."

Reference: Classical and Quantum Gravity.

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