An ordinary door swings both ways, but the physicists rigged a one-way door for atoms — enter at will, but leaving is off-limits. The atoms were cooled nearly to absolute zero — in such deep cold, they merge into a single cloud. This cloud was placed in a neat lattice of laser beams, adding a tiny irregularity, like a step at the threshold. Due to mutual jostling, the atoms start spontaneously flowing in only one direction.
The whole trick is that moving forward has far more ways than going backward, and nature picks the most likely path. Thus appears an invisible boundary, analogous to a black hole horizon. But instead of gravity, what's at play is the growth of disorder: the system itself becomes one-way, as if space curved to favor a single direction. Most striking — all this happens with no energy input, just from a bias in probabilities.
The atoms in the experiment were only a few billionths of a degree above absolute zero. In such cold, they become a single collective cloud governed by quantum laws.
In Interstellar, the hero enters a black hole but can’t escape or send a signal. This real atomic experiment builds a tiny model of that fateful boundary — not out of space, but out of probabilities.