A persistent cosmological puzzle has been
troubling physicists since 1917: what is the universe made of?
Complicating this already-mind-boggling question is the fact
that our best theories conflict with our observations of the
universe. Albert Einstein, according to scientific folklore,
felt a unique responsibility for introducing this entire
problem, reportedly referring to it as his "biggest blunder."
Essentially, Einstein's novel theory of general relativity
didn’t hold up when used to describe the universe as a whole.
General relativity described the "geometry" of spacetime as
being a trampoline-like surface; planets are heavy bowling balls
that distort the surface, creating curves. If a less heavy ball
(like a marble) was placed near the bowling ball, it would roll
along the surface just like the motion of planets in orbit.
Thus, orbits are explained not by a gravitational “force” but by
curvature in spacetime.
This proposal worked when considering small regions of
spacetime. But when Einstein applied it to the entire universe,
its predictions didn't fit. So, Einstein introduced the
"cosmological constant," a fixed value that represents a kind of
anti-gravity, anti-mass, and anti-energy, counteracting
gravity’s effects. But when scientists discovered that the
universe was expanding rather than static, as Einstein had
believed, the cosmological constant was set to zero and more or
less ignored. After we learned that the universe’s expansion is
accelerating, however, scientists could no longer conveniently
cancel out Einstein’s anti-gravity suggestion.
What was previously assumed to be empty space in the universe
now had to be filled with huge amounts of mysterious anti-energy
in order to explain observations of the universe’s
ever-quickening expansion. Even so, observations of the
universe’s expansion suggest that the energy is 60 to 120 orders
of magnitude lower than what recent quantum field theory
predicts.
What this means is that all of this extra energy is somehow
missing when we look at the universe as a whole; either it’s
effectively hidden or very different in nature to the energy we
do know about.
Today, theoretical physicists are trying to reconcile these
mysteries by examining the structure of so-called “spacetime” in
the universe at the smallest possible scale, with surprising
findings: spacetime might not be the trampoline-like plane
scientists once envisioned—it might be a foamy mess of bubbles
all containing mini-universes living and dying inside our own.
What is spacetime foam?
To try and solve the mystery of what fills the universe,
scientists have been exploring the possibility that it's
actually full of bubbles.
In 1955, influential physicist John Wheeler proposed that, at
the quantum level, spacetime is not constant but "foamy," made
up of ever-changing tiny bubbles. As for what these bubbles are
"made" of, recent work suggests that spacetime bubbles are
essentially mini-universes briefly forming inside our own.
The spacetime foam proposal fits nicely with the intrinsic
uncertainty and indeterminism of the quantum world. Spacetime
foam extends quantum uncertainty in particle position and
momentum to the very fabric of the universe, so that its
geometry is not stable, consistent, or fixed at a tiny scale.
Wheeler illustrated the idea of spacetime foam using an analogy
with the surface of the ocean, as retold by theoretical
physicist Y. Jack Ng at the University of North Carolina, Chapel
Hill, in an email:
Imagine yourself flying a plane over an ocean. At high altitudes
the ocean appears smooth. But as you descend, it begins to show
roughness. Close enough to the ocean surface, you see bubbles
and foam. Analogously, spacetime appears smooth on large scales;
but on sufficiently small scales, it will appear rough and
foamy.
Professor Steven Carlip at University of California, Davis,
published new research in September that builds on Wheeler's
quantum foam theory to show that spacetime bubbles could “hide”
the cosmological constant at a large scale.
“There are so many different proposals [to solve the
cosmological constant problem], and a good sign for my research
is that none of them is very widely accepted,” Carlip said in an
interview. “I thought it was worth looking for an approach that
was less ad hoc, that might come from things we knew or
suspected from elsewhere.”
The idea is that in spacetime foam, every point in spacetime has
the huge amount of vacuum energy—the lowest energy state
equivalent to "empty space"—predicted by quantum theory, but
behaves differently to other points. For any particular way in
which a point in spacetime is behaving, the exact opposite is
equally as likely to occur at another point in spacetime. This
is the feature of spacetime foam which “cancels out” the extra
energy and expansions at a tiny scale, resulting in the lower
energy that we observe at the scale of the whole universe.
For this to work, one has to assume that at the quantum level,
time has no intrinsic "direction." In other words, there is no
"arrow of time." According to Carlip, in the quantum world, this
isn't such a wild suggestion. “Most physicists would agree that
we don't know at a fundamental level why there's an arrow of
time at all,” he said. “The idea that it's somehow 'emergent' on
larger scales has been around for a long time.”
Carlip calls spacetime foam a “complex microscopic structure."
It can almost be thought of as an expanding universe formed by
tiny expanding and contracting universes at every point in
spacetime. Carlip believes it’s possible that over time, the
expanding areas of spacetime each replicate the complicated
structure, and are themselves filled with tiny universes at
every point.
Another paper published in August 2019 explores this scenario
more thoroughly. Authors Qingdi Wang and William G. Unruh at the
University of British Columbia suggest that every point in
spacetime cycles through expansion and contraction, like tiny
versions of our universe. Every point in spacetime, they say, is
a “microcyclic universe”, endlessly moving from singularity, to
a Big Bang, and finally collapse, on repeat.
The tiniest computers in the universe and a theory of everything
Quantum foam is having something of a moment, not just as a
solution to the Cosmological Constant Problem, but also to
address other enigmas in physics, like black holes, quantum
computers, and dark energy.
A forthcoming article by Ng suggests that spacetime foam holds
the key to finally unify and explain phenomenon at both a
quantum and cosmological scale, moving us towards the elusive
Theory of Everything. Such a theory would explain areas of
physics which are currently independent, and at times
conflicting, under one coherent framework.
Like Carlip, Ng also derives the large value for a positive
cosmological constant using a model of spacetime bubbles. But to
do so, he treats the "bubbles" in quantum foam as the universe’s
tiniest computers, encoding and processing information.
Remember: quantum foam contains bubbles of uncertainty in space
and time. To measure how "bubbly" spacetime is, Ng suggests a
thought experiment involving clocks clustered in a spherical
volume of spacetime which transmit and receive light signals and
measure the time it takes for the signals to be received.
“This process of mapping the geometry is a sort of computation,
in which distances are gauged by transmitting and processing
information," he wrote in his paper.
Using other known relationships between energy and quantum
computation, and the limit on mass inside the sphere to avoid
forming a black hole, Ng argued that the uncertainty built into
the quantum-scale universe that determines how accurately (or
inaccurately) we can measure the geometry of spacetime also
limits the maximum amount of information these bubble-computers
can store and their computing power.
Extending this result for the entire universe rather than an
isolated volume of spacetime, Ng shows that spacetime foam is
equivalent to dark energy and dark matter, since ordinary matter
would not be capable of storing and computing the maximum amount
of information he derives from the measurement task.
“The existence of spacetime foam, with the aid of thermodynamic
considerations, appears to imply the co-existence of a dark
sector (in addition to ordinary matter),” Ng told Motherboard.
“This line of research is not common within the physics
community, but it makes (physical) sense to me.”
The key takeaway from Ng's work is is: not only can spacetime
foam be measured and explored conceptually, but it can also
explain the acceleration of the universe by connecting quantum
physics, general relativity and dark energy. Ng believes a
Theory of Everything is within reach.
“Eventually what I’d like to explore and, more importantly, what
I would like to encourage others to explore, is to go beyond the
consideration of spacetime foam, and to see whether both quantum
mechanics and gravitation are emergent phenomena, and whether
thermodynamics (whose protagonist is entropy) holds the key to
understand the laws of nature," he said.
The future of foam research
Conceptually, spacetime foam reconciles and explains many of the
outstanding problems between quantum physics and cosmology.
Still, both Ng and Carlip are calling for more work to be done
to truly understand the nature of spacetime.
Carlip is working on a quantitative model of spacetime foam to
supplement the theoretical model currently on the table. He’s
calling the model “minisuperspace," and is hopeful that
physicists researching other approaches in the quantum-cosmology
intersection could find examples of the model in their own work,
if they know to look for it. To start with, Carlip says he’ll be
looking at some numerical simulations to support the foam model.
Going beyond a simple quantitative model will need an all hands
on deck approach. “I'd love to have people who are working on
various approaches to quantum gravity, string theory, loop
quantum gravity, asymptotic safety, etc., look for this kind of
phenomenon in their work to see if a connection can be made,”
Carlip said.
Ng echoed the desire for more dedicated research which spans
boundaries between different areas of theoretical physics. But
his hope is even grander: for a unified theory which ties
together quantum mechanics, gravity, and thermodynamics to
explain the universe's mysteries. |
