## Where does time cease being continuous and become *granular*?

One of the greatest problems in physics today is understanding what happens when space-time transitions from a continuous description to a discrete one. Is there an abrupt change or is there a gradual transformation? And where does this change occur?

Our experience of space-time is continuous, as described by classical physics, without gaps or discontinuities. But for some quantum gravity models, space-time is more “granular” at tiny scales (Planck scale), acting as a variable mesh of solids and voids.

The separation between these two descriptions creates problems for physicists: How can they describe gravity that is explained so well by classical physics, according to quantum mechanics?

*SEE ALSO: Earth’s Core Is Younger Than Its Surface Due to the Curving of Space-Time*

Quantum gravity is an area of study which currently has no unified theories. There are “scenarios,” which offer possible interpretations of quantum gravity, but they still need to be confirmed experimentally.

One of these problems states that if space-time is granular beyond a certain scale, then there must be a basic scale — a fundamental unit of time that cannot be broken down into anything smaller. However, this is a hypothesis that clashes with Einstein’s theory of special relativity.

Even suggesting the existence of this basic scale violates Lorentz invariance — a quantity that remains unchanged after rotations and changes of velocity — the fundamental principle of special relativity.

So how can these differences be reconciled?

Physicists either have to hypothesize violations of Lorentz invariance, or they must find a way to avoid the violation and find a scenario that is compatible with both granularity and special relativity. So far, the former has been the preferred approach.

A team of researchers, led by Stefano Liberati, a SISSA professor, decided to develop a scenario that preserves special relativity but also introduces a new possibility: that physics, at a certain point in space-time, can not only be affected by what happens close to that point, but also what happens at regions very far from it.

What is unique about this scenario is that is it actually experimentally testable. "To develop our reasoning we worked side by side with the experimental physicists of the Florence LENS. We are in fact already working on developing the experiments," explained Liberati in a press release.

With these measurements, Liberati and his colleagues may be able to identify the boundary or transition zone where space-time transitions from a continuous to a granular state.

The results, published in *Physical Review Letters*, could pave the way for a whole new form of physics.

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