38 | New Scientist | 28 March 2020
AS
HLEY
MACK
EN
ZIEPhilip Ball is the author of
Beyond Weird: Why everything
you thought you knew about
quantum physics is... differentWhere does that leave our understanding
of how physical reality as we know it emerges
from the quantum realm? For Hans Mooij, a
specialist in quantum electronics at the Delft
University of Technology in the Netherlands,
experiments that reveal the fine details of
measurement “will inevitably lead to a
reinterpretation of quantum physics”,
even if it isn’t yet clear what will emerge.
For starters, says Mooij, the work requires “a
complete overhaul of the Copenhagen-related
interpretation with collapsing wave functions”- neither Bohr’s “don’t ask” mantra nor von
Neumann’s unexplained collapse seems
needed any more. The idea that measurement
involves some instantaneous, almost magical
transformation is now dead, says Cabello.
Just add ignorance?
Removing the phenomenon previously known
as collapse also seems to remove the key
motivation for the many worlds interpretation
of quantum theory, which seeks to avoid the
need for collapse at the cost of proliferating
worlds. “My personal view is that such an
interpretation is redundant,” says Carmichael,
but he admits “there is always wriggle room”.
In Carmichael’s view, what seems to be
emerging is something not far removed from
the objective-collapse picture, with the twist
being that the random kick that forces a
quantum system to decide on its state emerges
naturally from the environment, rather thanbeing added by hand as some putative, ad hoc
mathematical extension of standard quantum
theory. The equation of the most popular
objective-collapse model, he says, “is formally
equivalent to the version of QTT we used to
model the Yale experiment” – with the random
“collapse” term now coming from the
inclusion of quantum back-action.
Arguably, that would remove the common
objection to objective collapse – that
bolting-on a new term to the Schrödinger
equation is makeshift. But it isn’t that simple.
For some supporters of this model, the add-on
is essential and can’t be replaced simply with
the inscrutably random influence of the
environment. Without objective collapse,
“there is nothing really random, but only
ignorance about the exact state of the
environment”, says Daniel Sudarsky at the
Universidad Nacional Autónoma of Mexico.
“And adding ignorance into the picture can’t
possibly provide a better account of the state of
affairs than not adding ignorance.”
Ultimately, says Sudarsky, making the
environment the sole agent of collapse just
postpones its origin to ever expanding
definitions of the system: if you accept that
the entire universe can be considered one big
quantum system governed by the Schrödinger
equation, there’s nothing left to collapse it.
Given the long history of argument over the
right way to interpret quantum mechanics, it is
too much to expect the debate to be settled
easily. Theorist Roderich Tumulka at theUniversity of Tübingen in Germany points out
that QTT itself is agnostic about interpretations,
which suggests it may be possible to formulate
versions of Bohmian mechanics or many
worlds that don’t conflict with these new
observations. Indeed, some researchers insist
that the new results have no consequences for
the various ways to understand the meaning of
quantum mechanics. For Lev Vaidman at Tel
Aviv University in Israel, a long-term advocate
of the many worlds idea, the Yale experiments
“confirm that standard quantum mechanics
and the equipment work well but have zero
bearing on interpretations”.
Even if you aren’t partial to any of the exotic
alternatives to Copenhagen, one thing at least
is certain: Bohr’s insistence that the emergence
of physical reality from the quantum realm
is something that just happens, with no
questions asked, is no longer an option. We
can now watch measurement unfold, and we
don’t need collapse to describe it.
The challenge now is to see if these new
insights can be used to replace the magic
wand of collapse with a full theory of quantum
measurement. “I wish I was not retired and still
in full experimental flow,” says Mooij, “because
this will be extremely exciting.” ❚“ The idea
that quantum
measurement
involves an
almost magical
transformation
is now dead ”