Reality in
the Dll1king
Has quantum theory's greatest mystery
been solved? Philip Ball investigatesI
N THE minusc:ule:realmofatoms and
particles, it looks as though things exist
not so much as things at all. but as vague
clouds ofpossibOitles. They seem to be here,
there and everywhere, or appear to be this and
thatallatonce-untll'JOUlookatthem. Then
the quantum haze is suddenly distilled into
something definite and describable, a thing
we recognise as "real".
That much we know. The trouble is that
quantum mechanics, the theory that describes
this uncertain world. has been mostly silent
about how the so-called "collapse" from fuzzy
probabilities to solid certainties happens.
Some phyaldats prefer to avoid the question
altogether. Others suggest that we need
to add sometlllng new to complete our
understanding of how ourfamiliarphysiad
:r:ealityemezgesfromthequantum.
Butwha.tifthewholepicturewutbereall
along, and we just weren't looking cuefully
enough? That's the startling suggestion from
:recent experiments that have. for the first
time, given us a glimpse inside collapse as tt
happens. Physidats are still coming to terms
with what they have witnessed. and it is too
early to say for certain what it allmean1.
But alreadythe:r:e are hints that the latest
:results could finally point the waytowards
thetrutbabouthowthewoddweknowis
conjUn!dfromthequantumrealm.
Q.uantum theory enjoys exalted status in
science because it describes the microscopic
world with peerless accuncy. Itwu developed
in the 1920s to explain why subatomic
particles, such as electrons, seem to sometimes
behave like waves, while light waves can show
particle-like behaviour-and. whytheir
energies are lfmtted to particularvalue1.
Physicist Erwin ScbrOdJngerwas one of those
whodidthemaths.Hedeviaedanequation
that describes such equivocal behaviour with
a mathematical entity knownutbewave
func:ticm. This allows you to calculate reliably
the odds on which of the various possible
properties, such as location, will be observed
if a quantum object is measured.
Adecadelater,JohnvonNeumann
introduced theideatbatbecameknown
as wave function collapse: that the selection
ofasingleoutcomeonmeuurementfromail
the possibilities encoded in the wave function
happens randomly and instantaneously,
even though repeated measurements of
thesamethingfittheoddspmtictedfrom
the Schrodinger equation. That picture
of a sudden, mysterious shift from many
possibilities to one is often identified with the
"Copenhagen" interpn!tatlon of quantum
mechanics. That is despite the fact that Niels
Bohr, one of the main architects of that
interpretation, preferred to avoid entirely the
question of what happens when we mate a
measurement.
'lhere is no theoretical justificationforwaw
functionmllapseastheconectwaytodescribewhat happens whenwemakeameasurement.
noranyexplanationofwhatmllapseis. Von
Neumannjustlmposedttas a way of plucking
a uniqueresuhout oftheSdu&Unger
equation, and in doing so papered over a
huge hole at the heart of quantum theory.
Collapse is "an inherently mysterious
notion", says Zlatko Minev at Yale University.
'1t pulls a blanket over what a measu:r:ement
isandtheproc:essbywhicbameasurement
changes the state of a quantum system."
It isn't surprising, then, that quantum
theorists have come up with various ideas
aboutwha.tis going on beneath the blanket
ID themanyworJda intelpretation, for instaruie,
wave function collapse isn'tneceasary. It says
thatwhenameuurementismade,allpossibl.e
outcomes contained in thewavefunctionme
:realised in many separate worlds that bianch
offfromoursatthemomentofmeasurement
so that there is a split rather than a collapse.
In another inteqm!mtion, often known as
Bohmianmechanics, thewavefundionisa
kind of spMad-outfon:e that gWd.es a single
undedyjngrealityin which particles always
have definite properties and positions that
are described by variables we can't access.
Then thereisanapproachknownas
•objective collapse" that says waw function
collapse is a:reaI. physical process-albeit a
nmdomone-andaddsanextra bit to the
SchrDdingerequation to account for that.
All such solutions have their own >