⧉ Two-time physics
A second time dimension breaks things we rely on. A handful of physicists think the breakage can be tamed — and that the tamed version might explain puzzles in ours.
What goes wrong
With two times, a complete snapshot of the present doesn't fix the future for general starting data (Craig and Weinstein, 2009); energy becomes a direction in a plane, so particles could decay into heavier ones (Dorling, 1970); and quantum versions tend to produce 'ghosts' — states with negative probability. Established as mathematics.
Bars: a hidden symmetry
Itzhak Bars (from about 1998) proposes a world with four space and two time dimensions plus a gauge symmetry (called Sp(2,ℝ)) that removes the ghosts and leaves only one effective time. Different familiar one-time theories then appear as different 'shadows' of one two-time theory. Speculative
Newer proposals
Pettini and colleagues (2025) use a (3, 2) spacetime to model quantum entanglement without faster-than-light influences, and propose a test (2026). Kletetschka (2025) proposes three time dimensions and claims to reproduce particle masses — heavily covered in the press; whether the masses are predictions or fits is disputed. Speculative
How to judge them
The same way as anything else: which of the five hurdles do they clear, what new thing do they predict, and what result would overturn them? Each has an Atlas entry with its scores.
A challenge to think with
See it in the labs
Sources: Craig & Weinstein, Proc. R. Soc. A 465 (2009); Dorling, Am. J. Phys. 38 (1970); Bars, Class. Quantum Grav. 18 (2001); Pettini et al., Phys. Rev. Research 7 (2025), arXiv:2606.12457; Kletetschka (2025).