We Thought Time Only Moved Forward. A Quantum Experiment Could Split It in Two.

This episode explores the quantum nature of time by merging Schrödinger's cat with the twin paradox, asking whether a single quantum particle could be both old and young at once. It explains that relativity and quantum mechanics disagree on what time fundamentally is, and that testing time dilation on truly quantum systems could reveal new clues. While experiments like the COW neutron interferometer confirmed gravity-induced phase shifts, they weren't decisive, and proposed "clock" experiments remain unperformed. The video highlights a new ion-trap proposal from Sorci and colleagues that puts a single ion in a superposition of slower and faster vibrational states, creating a superposition of different time-flow rates without spatial separation. This experiment is feasible with current NIST precision and, if successful, would suggest that time is not a universal background but a quantum property belonging to each branch of a wavefunction.

• ▶ 0:00 The episode introduces a combined thought experiment: merging Schrödinger's cat with the twin paradox to ask whether a "quantum twin" could be both old and young at the same time, exploring the quantum nature of time.
• ▶ 0:36 Announcements encourage viewers to like and comment, and note that Patreon supporters mainly join to sustain the Spacetime community rather than for perks.
• ▶ 1:15 A limited-edition "50 Years on Mars" collection is launched to celebrate the Viking 1 landing, featuring UV glow apparel and a mat showing all Mars lander locations.
• ▶ 1:47 Time is both intuitive and mysterious; our best theories—quantum mechanics and relativity—do not agree on what time actually is.
• ▶ 3:05 Relativity treats time and space symmetrically, but quantum mechanics treats time as special; we don't know if quantum objects can occupy two times or have two ages at once.
• ▶ 3:34 A key path forward is to study how relativity's strange distortions of time apply to truly quantum systems, possibly revealing new clues about time's nature.

• ▶ 3:39 Quantum twin paradox means sending a single particle in superposition along two routes with different time flows, not two separate twins or clocks.

• ▶ 5:02 Atomic clock experiments like Hafele–Keating confirm relativistic time dilation but are not quantum experiments about time itself.

• ▶ 6:58 Adding time dilation to an interferometer (e.g., paths at different altitudes) shifts interference, and which-path knowledge destroys or blurs the pattern.

• ▶ 7:46 A Mach–Zehnder interferometer with paths at different altitudes would let a particle “age” differently on each path, raising the possibility of observing a superposition of different time flows.

• ▶ 8:03 The COW experiment (1970s) already confirmed a gravity-induced phase shift for neutrons at different heights, but this is not decisive: Newtonian gravity alone can explain the same phase shift, analogous to the Gravitational Aharonov-Bohm effect.

• ▶ 9:28 Zych et al. proposed a more direct test using an internal quantum pendulum as a clock; if time dilation differs between paths, the two superposed components would get out of sync and effectively record which path they took, blurring the interference pattern. However, the experiment remains unperformed and interpretations still compete with pure phase-decoherence explanations.

• ▶ 12:10 A new proposal by Sorci and colleagues offers a way to observe a superposition of time in an actual quantum clock, using an ion trap instead of spatial interferometry.
• ▶ 14:26 The key idea: put the ion in a superposition of internal motional states (slower vs. faster vibrations), so motional time dilation creates a superposition of different rates of time flow — no large spatial separation needed.
• ▶ 16:18 The experiment is feasible with current NIST ion-clock precision; success would suggest time is not a universal background but a quantum property of each wavefunction branch.

• ▶ 0:00 Introduction and Announcements (0:00 - 1:47) - - Opening setup, channel announcements, and Mars 50th-anniversary merchandise.
• ▶ 1:47 Time in Physics and the Quantum Challenge (1:47 - 3:39) - - Contrasts Newtonian and relativistic time, leading to the difficulty of defining time in quantum mechanics.
• ▶ 3:39 Atomic Clocks and Quantum Interference (3:39 - 7:46) - - Covers atomic-clock experiments, double-slit interference, and Mach-Zehnder interferometer setups.
• ▶ 7:46 Gravitational Effects and Internal Clocks (7:46 - 12:10) - - Explores gravitational phase effects, internal-clock proposals, and the status of Zych et al.'s experiment.
• ▶ 12:10 New Ion Clock Proposal and Implications (12:10 - 17:18) - - Details Sorci et al.'s alternative ion-clock approach, its feasibility, and consequences for quantum time.