28 March 2020 | New Scientist | 35

what happens when we make a measurement,

nor any explanation of what collapse is. Von

Neumann just imposed it as a way of plucking

a unique result out of the Schrödinger

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.

“It pulls a blanket over what a measurement

is and the process by which a measurement

changes the state of a quantum system.”

It isn’t surprising, then, that quantum

theorists have come up with various ideas

about what is going on beneath the blanket.

In the many worlds interpretation, for instance,

wave function collapse isn’t necessary. It says

that when a measurement is made, all possible

outcomes contained in the wave function are

realised in many separate worlds that branch

off from ours at the moment of measurement

so that there is a split rather than a collapse.

In another interpretation, often known as

Bohmian mechanics, the wave function is a

kind of spread-out force that guides a single

underlying reality in which particles always

have definite properties and positions that

are described by variables we can’t access.

Then there is an approach known as

“objective collapse” that says wave function

collapse is a real, physical process – albeit a

random one – and adds an extra bit to the

Schrödinger equation to account for that.

All such solutions have their own

Reality in

the making

Has quantum theory’s greatest mystery

been solved? Philip Ball investigates

I

N THE minuscule realm of atoms and
particles, it looks as though things exist
not so much as things at all, but as vague
clouds of possibilities. They seem to be here,
there and everywhere, or appear to be this and
that all at once – until you look at them. 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 physicists prefer to avoid the question
altogether. Others suggest that we need
to add something new to complete our
understanding of how our familiar physical
reality emerges from the quantum.
But what if the whole picture was there all
along, and we just weren’t looking carefully
enough? That’s the startling suggestion from
recent experiments that have, for the first
time, given us a glimpse inside collapse as it
happens. Physicists are still coming to terms
with what they have witnessed, and it is too
early to say for certain what it all means.
But already there are hints that the latest
results could finally point the way towards
the truth about how the world we know is
conjured from the quantum realm.
Quantum theory enjoys exalted status in
science because it describes the microscopic

world with peerless accuracy. It was 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 why their

energies are limited to particular values.

Physicist Erwin Schrödinger was one of those

who did the maths. He devised an equation

that describes such equivocal behaviour with

a mathematical entity known as the wave

function. 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.

A decade later, John von Neumann

introduced the idea that became known

as wave function collapse: that the selection

of a single outcome on measurement from all

the possibilities encoded in the wave function

happens randomly and instantaneously,

even though repeated measurements of

the same thing fit the odds predicted from

the Schrödinger equation. That picture

of a sudden, mysterious shift from many

possibilities to one is often identified with the

“Copenhagen” interpretation 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 make a

measurement.

There is no theoretical justification for wave

function collapse as the correct way to describe >