When we hear the phrase ‘time travel’, the first thing that we usually think of is travelling backwards into the past, perhaps to see our past selves. This version of time travel is the one that is popularized by science fiction movies.
What we seek to discuss here is the feasibility of time travel, both in the forward and backward directions.
Forward Time Travel
Forward is the default direction for time travel, and all of us are doing that. We are going forward into the future, moving in time. What about moving far into the future, ahead of others?
This is indeed possible according to Einstein’s Theories of Relativity.
According to special relativity, if an observer moves close to the speed of light, time passes slowly for them. As a result, the moving observer is effectively moving ahead in time.
According to general relativity, a similar result can be seen when an observer is in a strong gravitational field. So, time moves slowly for a person in a strong gravitational field. This happens as gravity distorts spacetime and leads to something known as gravitational time dilation.
Backward Time Travel
What about travelling backwards? This question is harder to answer. There is no law in physics that explicitly forbids it. But physicists haven’t seen see any solid evidence for it.
One of the biggest issues to address with regards to backward time travel is causality: the cause always comes before the effect, in terms of time.
If backwards time travel were possible, a person could hypothetically go back and kill their own grandparent, before their parents were even conceived, thus preventing the person himself from being born. This creates what is called the “grandfather paradox” – basically meaning that the person changes their past, which violates causality.
The person here is the effect and the grandfather is the cause – the person going back and killing the grandfather would violate the “cause preceding the effect”.
Special relativity seems to forbid faster than light travel on similar grounds. A signal sent faster than light could potentially go backwards in time in some frames, which would make it possible for the “effect” to come before the “cause”.
General relativity also has some constraints on backwards time travel based on causality violation.
There are many paradoxes that can be envisioned when it comes to causality violations, such as the tachyonic antitelephone – a hypothetical machine that lets one send signals to the past.
Getting Past Problems of Causality
Some philosophers and physicists have tried to solve the problem of causality when it comes to time travel.
One suggestion is the parallel universe explanation – when a person travels back in time, they enter an alternate universe and cannot influence the past of their own universe.
Another suggestion is Novikov’s Self-Consistency principle, which means that any event of the past is unchangeable, and any changes that are made by time travel are technically already part of the history. This brings to mind the time travel scenes from the movie Harry Potter and the Prisoner of Azkaban.
Possibilities for Backward Time Travel
As we have seen, the biggest objection here is the violation of causality. If we are willing to suspend disbelief for a moment and assume that causality violation is possible, then there are a few situations where backward time travel could be possible.
Wormholes are tunnels that connect regions of space-time in general relativity. They are found in some solutions of the equations of general relativity. However, the existence of wormholes is debated.
A special category of wormholes are traversable wormholes, that is, wormholes that can be travelled both ways. Such wormholes can potentially be used to travel backwards into past only if they are stable
However, they need a form of matter that is called exotic matter (that has negative energy density) to stabilize them. Such matter is so far only hypothetical, though negative energy states could exist on a quantum scale.
Closed Timelike Curves (CTCs)
CTCs are solutions to the equations of general relativity that involve causal loops. Causal loops are formed the worldline of an object intersects with itself.
In simple terms, this means that a cause and effect can occur at the same time, with nothing to distinguish which happens first. While this is theoretically possible within general relativity, it seems that is highly unfeasible physically.
Cosmic strings are massive hypothetical structures that are found in the universe. They are supposed to be topological defects that have formed early on in the expansion of the universe. They are similar to defects formed during crystal formation.
Cosmic strings are extremely dense and narrow objects. It is possible to envision scenarios that could lead to time travel by travelling around cosmic strings. They could also potentially be used to stabilize wormholes, similar to exotic matter.
However, there is no conclusive experimental evidence for the existence of cosmic strings yet.
While backward time travel is not physically ruled out, it seems right now that it is at least unfeasible.
It remains to be seen whether the solutions of general relativity that permit backward travel are merely mathematical oddities, or whether they correspond to actual physical situations.
It is hoped that the development of a theory of quantum gravity could help arrive at a solution for the matter.
Though it is possible to find solutions in general relativity that allow for backward time travel, it seems right now that it is at least unfeasible.
Forward time travel is not feasible currently as we can neither achieve light speed nor get into very strong gravitational field. The possibility of backward time travel itself is a question as it doesn’t even have proper proofs.