Time travel can lead to some really weird situations. And one of the most studied is what is known as the causal loop or ‘paradox of predestination’. In reality, and despite the name, a causal loop does not contain any paradox, since this situation occurs when a hypothetical time traveler is caught in a series of events that inevitably lead him to repeat the loop over and over again.
Imagine, for example, that we travel to the past and give Einstein his famous equation E=mc² before he formulates it. Einstein publishes it, so that we, at present, find it in a textbook and bring it back to him, and so on and so on. We would have created a situation where the information, the formula, has no true origin.
Well, a new study has just shown that this type of causal loops are not only theoretical, but can actually occur in a greater number of possible universes than previously thought. And it is possible, although it is not yet entirely clear, that also in ours.
As many science fiction movies tell us, sending messages in time requires information to move faster than light, which as we know is the maximum speed possible in the universe. Universe. But in theoretical universes where causal loops are allowed, there is no need to resort to physics that breaks that law.
In their work, which appears in a study published on the arXiv preprint server, V. Vilasinifrom ETH Zurich, in Switzerland, and Roger Colbeckfrom the University of York, in the United Kingdom, mathematically modeled a set of theoretical universes in which, as in ours, there are people who can discern information and act accordingly, but in no case can they communicate faster than light.
In their simulations, the researchers did not require that the universes obey any particular physical law, not even gravity. And they found that causal loops could be mathematically possible in those universes, which are not particularly strange but rather closely resemble our own. It is true that such causal loops would perturb reality by removing the origin of some information, but they seem to be possible in these universes.
Two forms of causality
Vilasini explains that causality can be defined in two ways. The first describes how two agents are related to each other in space-time, the distance between them and whether each of them is in the future or in the past of the other. The second involves analyzing the flow of some information that passes between the two agents.
“Usually,” says Vilasini, “we say that correlation does not imply causation. Now we focus on the opposite, where causality does not imply correlation, or the ability of two agents to send signals to each other.” Which would be like being able to get Einstein to figure out his famous equation based on information received from the future but without directly communicating with him.
As the researcher explains, the causal loops examined do not necessarily lead to dramatic paradoxes, but show that past and future can be correlated in counterintuitive ways. Although whether causal loops can occur in our own Universe remains an open question.
Our Universe, with a well-structured space-time and in which nothing can move faster than light, has important similarities with the universes examined in the new study. But the three spatial dimensions in which we live can change the mathematics of causal loops enough to make them impossible altogether. Vilasini and Colbeck are still studying these dimensional effects, and it is very likely that they will soon publish a new study clarifying the issue.