Let's journey back to the summer of 1935, a time of intense intellectual discourse between eminent physicists Albert Einstein and Erwin Schrödinger. Their vibrant and often challenging exchanges revolved around the implications of the burgeoning theory of quantum mechanics.
At the heart of their debates was a concept Schrödinger later termed 'entanglement.' This idea encapsulates the impossibility of independently describing two quantum systems or particles post-interaction.
Einstein remained steadfast until his final days, asserting that entanglement underscored the incompleteness of quantum mechanics. Schrödinger, on the other hand, saw entanglement as the quintessence of the new physics, although he didn't necessarily embrace it wholeheartedly.
Schrödinger once wrote to Einstein in July 1935, "I comprehend how the mathematical trickery operates, but I can't endorse such a theory." His renowned thought experiment of Schrödinger's cat, caught in a limbo between life and death, was born from these letters as he tried to express his and Einstein's shared concerns.
Their main issue? Entanglement seemed to defy the established norms of the universe. For instance, it suggested that information could travel faster than light.
In a groundbreaking 1935 paper, Einstein and his co-authors illustrated how entanglement translated into quantum non-locality, an extraordinary connection apparent between entangled particles.
The paper proposed that if two quantum particles interact and become entangled, then separate across thousands of light-years, it becomes impossible to measure one's characteristics without the other instantly mirroring it.
Temporal Non-Locality
Traditionally, most experiments have examined entanglement across spatial distances. The "non-local" aspect of quantum non-locality has been associated with spatial entanglement. This raises an intriguing question: could particle entanglement occur over time as well? Could temporal non-locality be a reality?
As it turns out, the answer is affirmative.
Quantum mechanics, although already perplexing, becomes more so with the addition of quantum non-locality concept. This strangeness reaches its pinnacle when a team of physicists at the Hebrew University of Jerusalem revealed in 2013 that they had successfully entangled photons that never co-existed.
Earlier experiments using a method called "entanglement exchange" had revealed quantum correlations over time by delaying the measurement of one entangled particle. However, Eli Megidish and his team were the first to demonstrate entanglement between photons whose life-spans didn't overlap at all.
Here's how they did it:
First, they created a pair of entangled photons, "1-2" (Step I in the diagram below). Next, they measured the polarization of photon 1 (a property that describes the direction of light's oscillation)-then "killed it" (step II).
Photon 2 is forced onto a random path within the system while a new entangled pair, "3-4," is created (phase III). Photon 3 is then measured together with itinerant photon 2 so that the entanglement relation is "swapped" from the old pairs ("1-2" and "3-4") onto the new combo "2-3" ( step IV).
Some time later (step V), the polarization of the surviving lone photon, photon 4, is measured and the results are compared with those of the long-dead photon 1 (again in step II).
The result?
The data revealed the existence of quantum correlations between the "temporally nonlocal" photons 1 and 4, thus, entanglement can occur across two quantum systems that have never coexisted.
Okay, but what does this mean?
First aspect: Claiming that the polarity of starlight emitted in the distant past (say, more than twice the lifespan of Earth) affects the polarity of starlight visualized now through an amateur telescope complicates our lives badly.
Stranger still: This could imply that the measurements made by your eye on the starlight now passing through your telescope somehow show the polarity of photons older than 9 billion years.
Lest this scenario seem too bizarre, Megidish and his colleagues could not resist speculating on possible, spectral interpretations of their results.
Perhaps measuring the polarization of photon 1 in step II somehow directs the future polarization of photon 4, or measuring the polarization of photon 4 at step V somehow rewrites the past polarization state of photon 1.
In both directions forward and backward in time, quantum correlations span the causal gap between the death of one photon and the birth of the other.
Only by applying a little relativity does the disquiet diminish.
In developing his theory of special relativity, Einstein shifted the concept of simultaneity from its Newtonian pedestal; as a result, simultaneity went from being an absolute property to a relative property.
There is no single timekeeper for the Universe, Einstein argued; in fact, the concept of when something is happening depends on your precise position, in space and time, relative to what you are observing, and that is your frame of reference.
So the key to avoiding strange causal behavior (driving the future or rewriting the past) in cases of temporal separation is to accept that "simultaneous" call events have little metaphysical weight.
In short, temporal separation is just a frame-specific property, a choice among many alternatives but equally valid: a matter of convention or record keeping.
All this brings us directly to quantum non-locality, both spatial and temporal.
The mysteries concerning entangled pairs of particles are fundamentally discordant about labeling, because of relativity. Einstein showed that no sequence of events can be metaphysically privileged-that is, can be considered more real-than any other.
Only by accepting this insight can progress be made on these quantum conundrums.
The various frames of reference in the Hebrew University experiment (the laboratory frame, photon 1, photon 4, and so on) have their own "historians," so to speak, but while these historians disagree on how things turned out, none of them can claim a corner of truth: a different sequence of events unfolds within each, according to that spatiotemporal viewpoint.
Clearly, then, any attempt to assign frame-specific properties in general or to tie general properties to a particular frame will cause controversy among historians.
While there may be legitimate disagreement about which properties should be assigned to which particles and when, there should be no disagreement about the very existence of these properties, particles, and events.
These findings insert another wedge between our cherished intuitions of classical physics and the empirical realities of quantum mechanics.
As was true for Schrödinger and his contemporaries, scientific progress involves the study of the limits of certain metaphysical views. Schrödinger's cat, suspended between the two states of life and death, is designed to illustrate how the entanglement of systems leads to macroscopic phenomena that challenge our usual understanding of the relationships between objects and their properties: an organism like a cat is either dead or alive. There is no middle ground.
Most contemporary philosophical accounts of the relationship between objects and their properties embrace the entanglement solely from the perspective of spatial non-locality.
But there is still significant work to be done in incorporating temporal non-locality not only in discussions of object properties, but also in debates about material composition (such as the relationship between a piece of clay and the statue it forms) and partial relationships (how a hand relates to a limb, or a limb to a person).
For example, the "puzzle" of how parts fit into an overall whole assumes clear spatial boundaries between the underlying components, yet spatial non-locality warns against this view.
Temporal non-locality further complicates this picture: how do you describe an entity whose constituent parts are not even coexistent?
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