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The laws of physics allow for the creation of 'closed
time–like curves', structures in spacetime like wormholes that loop backwards
in time. On the quantum scale, scientists say these can be created using
entangled particles |
Time machines may seem better suited to science fiction than the physics lab, but experts say this futuristic technology could become a reality.
Researchers have revealed how time travel could really work
by using the laws of quantum physics.
While their method won't let you hop back to the time of the
dinosaurs, scientists say it could be possible to send messages into the past.
The researchers even say this mind–bending technique would
work just like in Christopher Nolan's sci–fi epic, Interstellar.
In the movie, an astronaut played by Matthew McConaughey
sends a message to his daughter in the past by moving the hands on her watch.
Although the reality wouldn't be so cinematic, the
researchers argue that this 'causal loop' resembles the way real time travel
would work.
Co–author Dr Kaiyuan Ji, a researcher at Cornell University,
told New Scientist: 'The father remembers how the daughter decodes his future
message.
'So he can instruct himself on what is the best way to
encode the message.'
It might seem surprising, but there is actually nothing in
the laws of physics as we understand them that makes time travel impossible.
According to the laws of general relativity, which are the
best description of the universe we have, everything moves through the fabric
of space and time on a set path.
One possible path that something can follow is known as a
closed time–like curve (CTC).
Something travelling on a CTC moves into the future before
looping back on itself via the past to end up exactly where it started.
The laws of physics allow for these loops to form, but
actually making one on a large scale requires twisting spacetime with a
literally infinite amount of energy.
However, on the very, very small scale, structures like CTCs
might form naturally through the laws of quantum physics.
On the quantum level, two particles can become 'entangled',
which means that what happens to one particle affects the other even if they
are light–years apart.
One possible way to explain this effect, which Einstein
called 'spooky action at a distance', is to say that one particle is actually
sending messages backwards in time to the other.
Rather than assuming that they are part of one massive
system or that they are sending information faster than light speed, the
particles' 'sensitivity' is explained by their receiving messages in the past
that tell them how to react later.
That might sound absolutely mad, but in 2010, scientists
actually came up with a way of mimicking closed time–like curves using
entangled particles.
Professor Seth Lloyd, a quantum physicist from the
Massachusetts Institute of Technology, says: 'It was the equivalent of sending
a photon a few nanoseconds backwards in time, and having it try to kill its
former self.'
What this creates is a bit like a telephone with a direct
connection to another device a few moments earlier.
In theory, you could use something like this to pass
messages back to yourself in the past.
Just like a real phone line, the connection on a CTC isn't
always going to be perfect, and noise or disruption will make it hard to pass
information with 100 per cent accuracy.
Professor Lloyd says: 'Nobody's built an actual physical,
closed time–like curve, and there are reasons to think it's very hard to make
one. But all channels are noisy.'
This is where an insight from Matthew McConaughey in
Interstellar comes in handy.
In their new paper, accepted for publication in Physical
Review Letters, Professor Lloyd and his co–authors write: 'The father, who is
in the future, may retrieve his memory of past events he has witnessed, even
including the daughter's decoding of the message which he is about to send!
'It would thus not be surprising that he will consult his
memory of the daughter's decoding when encoding his message, so as to maximize
the efficiency of the communication.'
Essentially, if you've already watched someone struggle to
piece together your garbled message, you should know how to send it so that
it's easier for them to decode.
Even if the connection is very noisy, a backwards
time–travelling message would still be legible.
The slightly weird conclusion of this is that sending
messages backwards in time is likely to be clearer than sending a message in
normal time.
Although no one has built a real closed time–like curve,
Professor Lloyd says it should be fairly easy to turn this new idea into an
experiment on the quantum level.
That could let scientists investigate how information is
transmitted through 'noisy channels' and even improve real–life communication
methods.
WHAT IS QUANTUM ENTANGLEMENT?
In quantum physics, entangled particles remain connected so
that actions performed by one affects the behaviour of the other, even if they
are separated by huge distances.
This means if you measure, 'up' for the spin of one photon
from an entangled pair, the spin of the other, measured an instant later, will
be 'down' - even if the two are on opposite sides of the world.
Entanglement takes place when a part of particles interact
physically.
For instance, a laser beam fired through a certain type of
crystal can cause individual light particles to be split into pairs of
entangled photons.
The theory that so riled Einstein is also referred to as
'spooky action at a distance'.
Einstein wasn't happy with theory, because it suggested that
information could travel faster than light.