# What is the best paradox in physics

## Time travel possible without paradox

**Prohibited interventions: Time travel leads to causal paradoxes according to common assumption - to physically impossible changes in the timeline. But there is a solution, as two mathematicians have now discovered. According to this, a kind of recalibration of linked events automatically ensures that all actions of a time traveler still lead to the same end result. At least mathematically, the “grandfather paradox” can therefore be avoided.**

Time travel is technically impossible - and possibly always will be. But purely physically, the steady flow of time can be tricked. As early as 1949, the mathematician Kurt Gödel discovered that Albert Einstein's field equations allow so-called closed timelike curves (CTC) under certain conditions. In theory, they allow an object to return to its own past - in other words, a journey through time.

This would be possible, for example, through a wormhole, a black hole, whose enormous gravitational effect warps space-time so strongly that a closed loop is created.

### The hurdle of the grandfather's paradox

The problem, however, is that if time travel is possible, then causal paradoxes could arise. A classic example is the grandfather paradox: if a time traveler kills his grandfather in the past before he has fathered his father, he himself should never be born. The time traveler therefore did not exist in his time and could not travel back either. Classical physics forbids such causal paradoxes.

Another example: "Imagine going back in time to save patient Zero from being infected with the SARS-CoV-2 coronavirus," says Germain Tobar of the University of Queensland. "If this succeeded, the pandemic would not take place and there would be no motivation to undertake this journey through time at all."

In the world of quantum physics, there are loopholes that resolve the paradox because particles there are governed by probabilities. If the grandfather were a photon, he would still be alive in a small part of his probability distribution despite being killed. But in classical physics and thus in the macroscopic world, the causal paradox was not considered solvable.

### There is a solution - at least in math

Now, however, Tobar and his colleague Fabio Costa have found a way to solve the causal paradox - at least mathematically. The key to this is provided by a consideration that takes into account both local events - the killing of the grandfather - and related events. "When multiple local regions communicate with each other in the presence of a time loop, there are a number of scenarios that give the actor complete freedom of action without the occurrence of logical inconsistencies such as the grandfather paradox," the researchers explain.

In other words: the linked events rearrange themselves and thus avoid a paradox. "The range of mathematical processes that we have discovered shows that time travel is possible with free will in our universe - without a paradox," said Tobar. “The mathematics for this is correct.” The time traveler can therefore kill his grandfather and still be born in the future.

### The events recalibrate

"In the example with patient zero of the corona pandemic, this means that you can try to save the patient from the infection!" Explains Tobar. “But by doing this you become infected yourself and now patient zero in turn, or your action causes someone else to become infected. That means: the pandemic would start no matter what you do. "

Whatever the time traveler does: As a result of his action, the events simply recalibrate themselves in such a way that the end result is the same. The universe almost automatically ensures that the timeline is maintained - by shifting the balance of the local processes. On a mathematical level, this behavior of spacetime could solve the grandfather paradox.

However, whether this also works in the real universe has yet to be investigated, the researchers admit. (Classical and Quantum Gravity, 2020; doi: 10.1088 / 1361-6382 / aba4bc)

Source: University of Queensland

October 2, 2020- Nadja Podbregar

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