Although it's a common theme in science fiction and fantasy, there are logical reasons for time travel to seem mind-boggling. For example, if somebody could skip back and forth in time, the "grandfather paradox" could arise.
If somebody could go back in time, he or she could kill his or her ancestors and thus, prevent his or her own birth. But if an individual did not exist, due to having killed off ancestors, he or she could not then go back in time. Hence, he or she would in fact, exist. Rinse and repeat would set up an infinite loop of paradoxes. Another simple "argument" against time travel is that we have never, so far as is known, been visited by travellers from the future.
Discounting such conundrums, many of the laws of physics should work, regardless of the direction in which time is running. The Theory of Relativity says that if an object is travelling at speeds approaching "c", as the speed of light is known, time slows down for that object. In fact, modern satellites circle the Earth fast enough for time-dilation effects to be allowed for in synchronising clocks between satellite and ground control.
Relativity suggests that the speed of light (equal to about 300,000 km per second) cannot be exceeded in a normal space-time continuum ("Galilean Space"). If c is exceeded, time starts running backwards for an object travelling at above light-speed.
There is a possible way to exceed "c": a large object could travel through a "wormhole" or a closed time-like curve (CTC). This wormhole is a loop, which takes the object back to the same spot in space at an earlier point of time. Such an object may then meet an earlier version of itself. Speculatively, such wormholes could exist in the vicinity of black holes.
Where very small objects are concerned, quantum theory suggests that there may be ways for particles to travel back in time. The Heisenberg Uncertainty Principle states that it is impossible to determine the exact momentum (momentum is a function of mass and velocity) and exact position of a particle at the same instant. Either momentum or position can be determined precisely. But not both.
The fuzzy nature of quantum systems (where a particle could also be a wave) allows room for developing time-travel hypotheses. Quantum systems work as a set of probabilities as exemplified by Schrodinger's Cat, which is both alive and dead at the same time. This probabilistic effect helps resolve paradoxes. To stretch the feline analogy, in a quantum system, the time-traveller could both kill and not kill ancestors.
What would happen if time-travel is indeed possible? Some rather strange effects could be expected. An Australian team from the University of Queensland has gone some distance in investigating these possibilities.
In a clever set of simulations that are misleadingly being referred to as "proof" of time-travel, this team has used single photons - light particles - to test possible time-travel effects at quantum level. The team was led by Physics professor Tim Ralph and it included his PhD student, Martin Ringbauer, and several others.
The team looked at two possibilities. In one, a photon is put into a wormhole and goes back into the past and interacts with an earlier version of itself. It is not possible to actually experimentally create a wormhole, though it is possible that CTCs exist, according to current theory.
The Australian team "pretended", for want of a better word, that two photons could be used to represent a single photon. This is akin to two different actors playing the same character at different moments in time, like Marlon Brando and Robert De Niro in the Godfather movie series.
In the other case the Australians set up, a photon A moves through normal time-space. A meets and interacts with another photon, B, which is supposed to be permanently trapped inside a wormhole (B is not really trapped inside a wormhole). A and B are supposed to be the same particle at different moments in time.
The scientists are guessing that the mathematical aspects of these situations would be very similar. In terms of some quantum mechanics effects, these two cases should be identical. So by studying this case, it would be possible to develop clues as to possible time-travel effects. By shining a laser through crystals, the lab generated photon pairs, which were passed through an optic-entangling gate. These photons were measured and counted.
So what do we have here? A team of scientists assumes, for argument's sake, that time travel is possible. It cannot set up a lab experiment because one pre-requisite - the wormhole or CTC - is impossible to create. So it simulates a case that is as close as it can get to this time-travel situation.
This doesn't prove time-travel is possible. But it does allow for experimental observations and mathematical calculations that simulate time-travel, if it is possible. One of the odd findings is that the Heisenberg Principle could break down in such a situation. This is amazing, if true. Another finding, which should interest quantum computer developers, is that time-travel would also make it possible to decrypt quantum cryptographic systems.
This doesn't answer the basic question - is time-travel on the cards? Nobody has yet discovered a CTC, or worked out a method for creating one. But it does suggest that some very unusual conditions of space-time would be required to get there.
Disclaimer: These are personal views of the writer. They do not necessarily reflect the opinion of www.business-standard.com or the Business Standard newspaper
