The quantum double slit experiment remarkably illustrates the counterintuitive nature of reality at the smallest scales. Originally conducted with electrons, modern variations employ photons, atoms, and even larger molecules like buckyballs. By transmitting quantum particles through a barrier with two parallel slits, and then observing the resulting interference pattern, seminal insights emerge on quantum superposition, wave-particle duality, and quantum measurement.

## Introducing the Quantum Double Slit Experiment

In the classic double slit setup, quantum entities like photons or electrons pass through two narrowly spaced slits in an otherwise impenetrable barrier, with detectors recording where they land on a registration surface like a photographic plate or CCD camera. After many particles traverse the double slits, an interference pattern consisting of light and dark fringes emerges reflecting constructive and destructive interference between the two possible quantum paths.

Intriguingly, this wave-like interference persists even when emitting just one quantum particle at a time, as demonstrated experimentally. Yet curiously detecting which slit each particle travels through collapses the interference. Simply observing destroys the superposition between trajectory states—a perplexing consequence in quantum mechanics.

## The Quantum Measurement Problem

This disappearance of interference triggered by identifying particles’ paths highlights perhaps the most vexing question underpinning quantum theory – the measurement problem. Math describing smooth quantum evolution governed by Schrödinger’s equation differs radically from the sudden unpredictable state changes upon measurement. Why does measurement itself seemingly inject randomness and “collapse” coherent superpositions into definite observable outcomes?

Various complex interpretations attempt to reconcile this split between quantum calculation and reality. Most physicists adopt simple Copenhagen pragmatism – states exist in probabilistic limbo until measured. Yet doubts linger about whether measurement merely reflects human ignorance versus actively influencing reality. This centres on the crucial question – does measurement create reality or merely uncover pre-existing properties?

## Retrocausality Hints at Quantum Time Symmetry

Intriguingly, several modern variations of the iconic double slit experiment add a “quantum eraser” which lets experimenters recover lost interference fringes even after detecting which slit particles passed through. This suggests measurement doesn’t irrevocably collapse superpositions into concrete outcomes manifesting only at detection. Perhaps quantum time symmetry allows future detector settings to alter prior states.

Some interpretations of the quantum double slit experiment incorporate retrocausality – events in the future influencing past events. Philosophically this upends conventional notions of causal flow while aligning with relativistic spacetime where no universal “now” separating past from future exists. Quantum retrocausality permits particle self-interference after detection by erasing “which-path” information. Thus quantum time symmetry restores coherence enabling results to mirror no measurement occurred, though each particle earlier had determined trajectories.

## What is Time in the Quantum World?

Quantum temporal oddities observed in double slit experiments compound quantum theory’s already counterintuitive nature. Conceptually it remains challenging to accept future measurement selections can alter past states to already recorded data. Classically this invites time travel causal paradoxes. Yet quantum systems play by different rules where absolute befores and afters don’t constrain events.

Findings from quantum double slit setups support time-symmetric formulations of quantum mechanics in contrast to standard calculations always evolving forward temporally. At quantum scales, time likely exhibits much stranger properties than the uniform directional flow we perceive classically. These experiments hint time within the quantum realm behaves less like an arrow and perhaps more like a multiplying, splintering tree where branches interconnect.

Ongoing double slit experiment variations continue revealing the peculiar qualities of time within quantum theory. Work by Dean Radin and others investigates whether human consciousness itself exhibits quantum temporal effects. As knowledge expands experimentally, hope remains to resolve quantum mysteries like measurement and temporal symmetry to fully unlock the deep quantum nature of reality.