**HOW**do you reconcile the two pillars of modern physics: quantum theory and gravity? One or both will have to give way. A new approach says gravity could emerge from random fluctuations at the quantum level, making quantum mechanics the more fundamental of the two theories.

Of our two
main explanations of reality, quantum theory governs the interactions between
the smallest bits of matter. And general relativity deals with gravity and the
largest structures in the universe. Ever since Einstein, physicists have been
trying to bridge the gap between the two, with little success.

Part of the
problem is knowing which strands of each theory are fundamental to our
understanding of reality.

One
approach towards reconciling gravity with quantum mechanics has been to show
that gravity at its most fundamental comes in indivisible parcels called
quanta, much like the electromagnetic force comes in quanta called photons. But
this road to a theory of quantum gravity has so far proved impassable.

Now Antoine
Tilloy at the Max Planck Institute of Quantum Optics in Garching, Germany, has
attempted to get at gravity by tweaking standard quantum mechanics.

In quantum
theory, the state of a particle is described by its wave function. The wave
function lets you calculate, for example, the probability of finding the
particle in one place or another on measurement. Before the measurement, it is
unclear whether the particle exists and if so, where. Reality, it seems, is
created by the act of measurement, which “collapses” the wave function.

But quantum
mechanics doesn’t really define what a measurement is. For instance, does it
need a conscious human? The measurement problem leads to paradoxes like
Schrödinger’s cat, in which a cat can be simultaneously dead and alive inside a
box, until someone opens the box to look.

One
solution to such paradoxes is a so-called GRW model that was developed in the
late 1980s. It incorporates “flashes”, which are spontaneous random collapses
of the wave function of quantum systems. The outcome is exactly as if there
were measurements being made, but without explicit observers.

Tilloy has
modified this model to show how it can lead to a theory of gravity. In his
model, when a flash collapses a wave function and causes a particle to be in
one place, it creates a gravitational field at that instant in space-time. A
massive quantum system with a large number of particles is subject to numerous
flashes, and the result is a fluctuating gravitational field.

“A
spontaneous collapse in a quantum system creates a gravitational field at that
instant in space-time”

It turns
out that the average of these fluctuations is a gravitational field that one
expects from Newton’s theory of gravity (arxiv.org/abs/1709.03809). This
approach to unifying gravity with quantum mechanics is called semi-classical:
gravity arises from quantum processes but remains a classical force. “There is
no real reason to ignore this semi-classical approach, to having gravity being
classical at the fundamental level,” says Tilloy.

“I like
this idea in principle,” says Klaus Hornberger at the University of
Duisburg-Essen in Germany. But he points out that other problems need to be
tackled before this approach can be a serious contender for unifying all the
fundamental forces underpinning the laws of physics on scales large and small.
For example, Tilloy’s model can be used to get gravity as described by Newton’s
theory, but the maths still has to be worked out to see if it is effective in
describing gravity as governed by Einstein’s general relativity.

Tilloy agrees.
“This is very hard to generalize to relativistic settings,” he says. He also
cautions that no one knows which of the many tweaks to quantum mechanics is the
correct one.

Nonetheless,
his model makes predictions that can be tested. For example, it predicts that
gravity will behave differently at the scale of atoms from how it does on
larger scales. Should those tests find that Tilloy’s model reflects reality and
gravity does indeed originate from collapsing quantum fluctuations, it would be
a big clue that the path to a theory of everything would involve semi-classical
gravity.

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