Time Without a Clock, and the Tiny Universe That Learned Before and After

How do we know something came before?

If you were locked inside an isolated world, with no calendar, no clock, and no way to look beyond its boundaries, how would you know that time was passing at all?

The question sounds like a philosophical riddle, but for physicists it has a very concrete meaning. Professor Giovanni Barontini of the University of Birmingham conducted an experiment that made it possible to test this problem in the laboratory. To do so, he created a special system of about 24,000 rubidium atoms cooled almost to absolute zero. Under such extreme conditions, the atoms form a Bose-Einstein condensate. This is an unusual state of matter. A vast number of particles begin to behave as if they were a single shared quantum object.

The researcher placed the atoms in an optical trap, a system in which light helps hold them in a controlled location. He then divided the whole arrangement into two regions. 1 remained visible to the researchers; the other was hidden. Even so, the atoms could still move between the two parts.

A small universe made of rubidium atoms

During the tests, the number of atoms in the observed part of the system periodically fell and rose. Mathematically, this resembled cosmological models in which the universe passes through phases of contraction and expansion. The physicists compared these oscillations to processes reminiscent of the Big Bang and the Big Crunch.

The experiment, described in Physical Review Research, does not yet answer the question of what time really is. It does, however, show something highly significant: even in complete isolation from an external clock, we can still determine the order of events — “before” and “after.” It is enough to observe the changes taking place inside the system itself.

Time without a clock: the puzzle remains open

This discovery in physics matters because the problem of time is among the greatest obstacles to uniting the 2 major theories of modern science: general relativity and quantum mechanics. Barontini’s experiment shows that ideas previously known mainly from mathematical models can begin to be tested in the real world of the laboratory.

As a result, scientists do not have to rely only on equations. They can examine similar concepts in practice, under controlled conditions. In the future, such experiments may help us better understand the origins of the universe, quantum gravity, or the physics of black holes. For now, they show something more modest, but deeply intriguing: a small cloud of ultracold atoms can become a tool for studying 1 of physics’ greatest puzzles — time without a clock.

Read this article in Polish: Czym jest czas, gdy nie ma zegara? Fizycy wpadli na intrygujący trop