8legs: Physical review letters

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#1 out of 2

1d ago

A University of Toronto team reports photons exiting a rubidium cloud before they enter it, indicating negative time in transit.
The researchers fired a weak laser beam through the cloud and tracked tiny phase shifts to read atomic excitations during transit.
The finding emerged after averaging about 1 million runs across multiple parameter sets, totaling around 70 hours of data collection.
Physicists described the negative time as an effect explainable by standard physics, with no faster-than-light signaling involved.
The study relates to earlier work showing photons can appear to exit before entering due to group delay, now supported by direct atomic readouts.
The team emphasizes the result does not enable practical time manipulation or faster-than-light communication.
The experiment used a cloud of cold rubidium atoms to study photon transit and atomic excitation time.
The publication in Physical Review Letters confirms peer review after long circulation as a preprint.
The researchers aim to extend measurements to scattered photons to test balance in excitation time.
The authors acknowledge the result is primarily a fundamental quantum property with no immediate technology applications.

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#2 out of 2

14h ago

Scientists derive exact backward-channel capacity for noisy retrocausal communication, giving concrete limits for information sent back in time.
Amplified probabilistic teleportation is identified as the optimal one-shot strategy for retrocausal communication.
The scenario uses a daughter in the past storing quantum information and a father in the future encoding after retrieving that memory.
The model enforces self-consistent histories to avoid paradoxes in backward-time signaling.
Findings link retrocausal limits to black-hole information questions, including final-state evaporation models.
The work provides exact one-shot capacities and asymptotic limits for repeated uses of the channel.
Noisier real-world channels are handled, moving beyond idealized, perfectly noiseless backward time models.
The study references black-hole evaporation links, notably Horowitz–Maldacena final-state proposals.
Practical takeaway: researchers can calculate retrocausal information ceilings for realistic noisy channels.
The article ties the theory to experiments using postselected teleportation and entangled photons.

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