A causal loop is a classic time travel conundrum. If you send information to the past – say, you give Albert Einstein the formula E=mc ² before he theorises it himself, then he publishes it and you go on to find it in a textbook – you would create a situation in which the information has no true origin.
A new analysis shows that this type of causal loop is possible in more theoretical universes than had previously been expected.
In most science-fiction scenarios, sending messages back in time requires information to move faster than the speed of light. But in theoretical universes where causal loops are allowed, such law-breaking physics isn’t required, though it is unclear if these loops would be possible in our universe.
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at ETH Zurich in Switzerland and Roger Colbeck at the University of York, UK, mathematically modelled a set of theoretical universes where all that is known is that there are people who can discern information and act on it, but who can’t communicate faster than light.
The researchers didn’t require the universes to obey any specific physical laws, such as how gravity works. They found that causal loops could be mathematically possible in universes that they didn’t theorise as being particularly odd or exotic from the start.
Such causal loops would disturb reality by removing the origin of some information, but they seem to be possible in universes with one spatial dimension.
Vilasini explains that causality can be defined in two ways. The first incorporates how two agents are related to each other in space-time, the distance between them and whether they are in each other’s future or past. The second involves analysing the flow of some information passing between the two agents.
“Typically, we say that correlation does not imply causation. We now focused on the converse, where causation does not imply correlation, or the ability of two agents to send signals to each other,” says Vilasini.
Somewhat mind-bogglingly, this case is like being able to cause Einstein to discover his famous equation based on information from his future without ever directly communicating with him.
Vilasini says that the causal loops examined don’t necessarily lead to dramatic paradoxes, but they do show that past and future can be correlated in counter-intuitive ways. Whether causal loops can happen in our universe is still an open question, she says.
Our universe, with space-time structured the way it is and where nothing can move faster than light, is similar to the universes examined in the new analysis. But the three spatial dimensions in our universe may change the mathematics of causal loops just enough to make them impossible. Vilasini and Colbeck are still studying these dimensional effects.
at the University of Hong Kong says that while the analysis provides a general framework for examining causal loops, the exact details of the physical mechanisms that would bear them out are the biggest question still to answer.
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