Time travel is a common staple in science fiction films and on TV. However, is it really possible? And how can you bypass the so-called "grandfather paradox," which basically states that if you go back in time and change things, it also changes the future?
A group of physicists from the University of Queensland simulated time travel in a lab by using the concept from Einstein's theory of general relativity that states that a powerful gravitational field, such as that found in a black hole, could create a situation where spacetime curves around itself. This "closed timelike curve" (CTC) allows for time travel.
However, if this is true, there is also the theory of the "grandfather paradox." This is a hypothetical scenario where someone uses a CTC to travel into the past and kill their own grandfather. The result is that this person no longer exists because they've prevented their own birth.
Most physicists think that CTCs are impossible because of their ability for creating paradoxes. However, in 1991, theoretical physicist David Deutsch suggested that these scenarios avoid paradoxes because, at least on a quantum scale, particles don't follow the strict rules of probability.
"It's intriguing that you've got general relativity predicting these paradoxes, but then you consider them in quantum mechanical terms and the paradoxes go away," says University of Queensland physicist Tim Ralph. "It makes you wonder whether this is important in terms of formulating a theory that unifies general relativity with quantum mechanics."
In a lab, physicists studied sending such a particle back in time.The particle created by the flip of a switch on a machine, could do one of two things: go back in time to flip the switch on the particle-generating machine that created it or it could not flip the switch. You would think that if the particle did not flip the switch the particle would stop existing, right? But because a particle must come out of a CTC with the same properties it had when it entered, even if it does not flip the switch, it still comes out of that time loop unchanged. It's a weird concept, but it goes in line with what we know of quantum mechanics.
For consistency, physicists used clones of pairs of polarized photons. They sent one pair through the CTC simulation. When it came out of the time loop, they compared it to its clone: both pairs were identical, signifying that the journey did not change the time traveling pair. The physicists performed this simulation multiple times, and each time, the results were the same.
"The state we got at our output, the second photon at the simulated exit of the CTC, was the same as that of our input, the first encoded photon at the CTC entrance," says Ralph. "Of course, we're not really sending anything back in time but [the simulation] allows us to study weird evolutions normally not allowed in quantum mechanics."
Of course, this still doesn't mean that humans can travel through time, affect the past and then emerge unscathed, but it does have implications in the area of quantum cryptography.
