
In the
summer of 1935, the physicists Albert Einstein and Erwin Schrödinger engaged in
a rich, multifaceted and sometimes fretful correspondence about the
implications of the new theory of quantum mechanics. The focus of their worry
was what Schrödinger later dubbed entanglement: the inability to describe two
quantum systems or particles independently, after they have interacted.
Until his
death, Einstein remained convinced that entanglement showed how quantum
mechanics was incomplete. Schrödinger thought that entanglement was the
defining feature of the new physics, but this didn't mean that he accepted it
lightly.
"I know
of course how the hocus pocus works mathematically," he wrote to Einstein
on 13 July 1935. "But I do not like such a theory."
Schrödinger's
famous cat, suspended between life and death, first appeared in these letters,
a byproduct of the struggle to articulate what bothered the pair. The problem
is that entanglement violates how the world ought to work. Information can't
travel faster than the speed of light, for one.

But in a
1935 paper, Einstein and his co-authors showed how entanglement leads to what's
now called quantum nonlocality, the eerie link that appears to exist between
entangled particles.
If two
quantum systems meet and then separate, even across a distance of thousands of
lightyears, it becomes impossible to measure the features of one system (such
as its original position, momentum and polarity) without instantly steering the
other into a corresponding state. Up to today, most experiments have tested
entanglement over spatial gaps.
The
assumption is that the 'nonlocal' part of quantum nonlocality refers to the
entanglement of properties across space. But what if entanglement also occurs
across time? Is there such a thing as temporal nonlocality?
The answer,
as it turns out, is yes.
Just when
you thought quantum mechanics could not get any weirder, a team of physicists
at the Hebrew University (HU) of Jerusalem reported in 2013 that they had
successfully entangled photons that never coexisted.

Previous
experiments involving a technique called 'entanglement swapping' had already
showed quantum correlations across time, by delaying the measurement of one of
the coexisting entangled particles; but Eli Megidish and his collaborators were
the first to show entanglement between photons whose lifespans did not overlap
at all.
Here's how
they did it.
First, they
created an entangled pair of photons, '1-2' (step I in the diagram below). Soon
after, they measured the polarization of photon 1 (a property describing the
direction of light's oscillation) – thus 'killing' it (step II). Photon 2 was
sent on a wild goose chase while a new entangled pair, '3-4', was created (step
III). Photon 3 was then measured along with the itinerant photon 2 in such a
way that the entanglement relation was 'swapped' from the old pairs ('1-2' and
'3-4') onto the new '2-3' combo (step IV).
Sometime
later (step V), the polarization of the lone survivor, photon 4, is measured,
and the results are compared with those of the long-dead photon 1 (back at step
II). The upshot? The data revealed the existence of quantum correlations
between 'temporally nonlocal' photons 1 and 4. That is, entanglement can occur
across two quantum systems that never coexisted.
What on
Earth can this mean? Prima facie, it seems as troubling as saying that the
polarity of starlight in the far-distant past – say, greater than twice Earth's
lifetime – nevertheless influenced the polarity of starlight falling through
your amateur telescope this winter. Even more bizarrely: maybe it implies that
the measurements carried out by your eye upon starlight falling through your
telescope this winter somehow dictated the polarity of photons more than 9
billion years old.
Lest this
scenario strike you as too outlandish, Megidish and his colleagues can't resist
speculating on possible and rather spooky interpretations of their results. Perhaps
the measurement of photon 1's polarization at step II somehow steers the future
polarization of 4, or the measurement of photon 4's polarization at step V
somehow rewrites the past polarization state of photon 1.
In both
forward and backward directions, quantum correlations span the causal void
between the death of one photon and the birth of the other. Just a spoonful of
relativity helps the spookiness go down, though. In developing his theory of
special relativity, Einstein deposed the concept of simultaneity from its
Newtonian pedestal.
As a
consequence, simultaneity went from being an absolute property to being a
relative one. There is no single timekeeper for the Universe; precisely when
something is occurring depends on your precise location relative to what you
are observing, known as your frame of reference. So the key to avoiding strange
causal behaviour (steering the future or rewriting the past) in instances of
temporal separation is to accept that calling events 'simultaneous' carries
little metaphysical weight.
It is only a
frame-specific property, a choice among many alternative but equally viable
ones – a matter of convention, or record-keeping. The lesson carries over directly to both
spatial and temporal quantum nonlocality. Mysteries regarding entangled pairs
of particles amount to disagreements about labelling, brought about by
relativity.
Einstein
showed that no sequence of events can be metaphysically privileged – can be
considered more real – than any other. Only by accepting this insight can one
make headway on such quantum puzzles. The
various frames of reference in the Hebrew University experiment (the lab's
frame, photon 1's frame, photon 4's frame, and so on) have their own
'historians', so to speak.
While these
historians will disagree about how things went down, not one of them can claim
a corner on truth. A different sequence of events unfolds within each one,
according to that spatiotemporal point of view. Clearly, then, any attempt at
assigning frame-specific properties generally, or tying general properties to
one particular frame, will cause disputes among the historians.
But here's
the thing: while there might be legitimate disagreement about which properties
should be assigned to which particles and when, there shouldn't be disagreement
about the very existence of these properties, particles, and events. These findings drive yet another wedge
between our beloved classical intuitions and the empirical realities of quantum
mechanics. As was true for Schrödinger and his contemporaries, scientific
progress is going to involve investigating the limitations of certain
metaphysical views.
Schrödinger's
cat, half-alive and half-dead, was created to illustrate how the entanglement
of systems leads to macroscopic phenomena that defy our usual understanding of
the relations between objects and their properties: an organism such as a cat
is either dead or alive. No middle ground there. Most contemporary philosophical accounts of
the relationship between objects and their properties embrace entanglement
solely from the perspective of spatial nonlocality.
But there's
still significant work to be done on incorporating temporal nonlocality – not
only in object-property discussions, but also in debates over material
composition (such as the relation between a lump of clay and the statue it
forms), and part-whole relations (such as how a hand relates to a limb, or a
limb to a person).
For example,
the 'puzzle' of how parts fit with an overall whole presumes clear-cut spatial
boundaries among underlying components, yet spatial nonlocality cautions
against this view. Temporal nonlocality further complicates this picture: how
does one describe an entity whose constituent parts are not even coexistent?
Discerning
the nature of entanglement might at times be an uncomfortable project. It's not
clear what substantive metaphysics might emerge from scrutiny of fascinating
new research by the likes of Megidish and other physicists.
In a letter
to Einstein, Schrödinger notes wryly (and deploying an odd metaphor): "One
has the feeling that it is precisely the most important statements of the new
theory that can really be squeezed into these Spanish boots – but only with
difficulty."
We cannot
afford to ignore spatial or temporal nonlocality in future metaphysics: whether
or not the boots fit, we'll have to wear 'em.
Elise Crull
is the assistant professor in history and philosophy of science at the City
College of New York. She's co-author of the upcoming book "The 'Einstein
Paradox': Debates on Nonlocality and Incompleteness in 1935".
This article
was originally published at Aeon and has been republished under Creative
Commons.
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