බටහිර භෞතික විද්‍යාවේ අර්බුදය
මේ ලිපිය පසුගිය මාර්තු 4
වැනි දා ක්වොන්ටා සඟරාවේ (Quanta Magazine) පළවූවකි.
අප රටේ ඉන්නා දසදහසකට අධික විද්‍යාඥයන්ගේ දැන ගැනීම පිණිස එය පල කරනවා. මෙහි ගණිත
සමීකරණ කිසිවක් නැහැ. ඕනෑම පඬි නැට්ටකුට කියව ගන්න පුළුවන්. ඒත් ලිපිය තරමක් දිගයි. බටහිර භෞතික විද්‍යාවේ
ඉහළ ම තැන්වල ඇති අර්බුදය එහි සාකච්ඡා කෙරෙනවා. බටහිර භෞතික විද්‍යාව පසුපස නොන්ඩි
ගසමින් යන ඊනියා සමාජයීය විද්‍යාවන් ඔහේ ගියා දෙන්.
මා
මෙහි ඇති සියලු කරුණු ගැන එකඟ වන්නේ නැහැ. මේ ඊනියා යථාර්ථය සත්‍යය සොයා යෑමේ ප්‍රතිඵලයක්.
කුන්ගේ සුසමාදර්ශ කතාවලින් වැඩක්
නැහැ.
A Deepening
Crisis Forces Physicists to Rethink Structure of Nature’s Laws
By NATALIE WOLCHOVER
For three decades, researchers hunted in vain for new elementary
particles that would have explained why nature looks the way it does. As physicists
confront that failure, they’re reexamining a longstanding assumption: that big
stuff consists of smaller stuff.
ucture of Scientific Revolutions, the philosopher
of science Thomas Kuhn observed that scientists spend long periods taking small
steps. They pose and solve puzzles while collectively interpreting all data
within a fixed worldview or theoretical framework, which Kuhn called a
paradigm. Sooner or later, though, facts crop up that clash with the reigning
paradigm. Crisis ensues. The scientists wring their hands, reexamine their
assumptions and eventually make a revolutionary shift to a new paradigm, a
radically different and truer understanding of nature. Then incremental
progress resumes.
For several years, the particle physicists
who study nature’s fundamental building blocks have been in a textbook Kuhnian
crisis.
The crisis became undeniable in 2016,
when, despite a major upgrade, the Large Hadron Collider in Geneva still hadn’t
conjured up any of the new elementary particles that theorists had been
expecting for decades. The swarm of additional particles would have solved a
major puzzle about an already known one, the famed Higgs boson. The hierarchy
problem, as the puzzle is called, asks why the Higgs boson is so lightweight —
a hundred million billion times less massive than the highest energy scales
that exist in nature. The Higgs mass seems unnaturally dialed down relative to
these higher energies, as if huge numbers in the underlying equation that
determines its value all miraculously cancel out.
The extra particles would have explained
the tiny Higgs mass, restoring what physicists call “naturalness” to their
equations. But after the LHC became the third and biggest collider to search in
vain for them, it seemed that the very logic about what’s natural in nature
might be wrong. “We are confronted with the need to reconsider the guiding
principles that have been used for decades to address the most fundamental
questions about the physical world,” Gian
Giudice,
head of the theory division at CERN, the lab that houses the LHC, wrote in 2017.
At first, the community despaired. “You could feel
the pessimism,” said Isabel
Garcia Garcia,
a particle theorist at the Kavli Institute for Theoretical Physics at the
University of California, Santa Barbara, who was a graduate student at the
time. Not only had the $10 billion proton smasher failed to answer a
40-year-old question, but the very beliefs and strategies that had long guided
particle physics could no longer be trusted. People wondered more loudly than
before whether the universe is simply unnatural, the product of fine-tuned
mathematical cancellations. Perhaps there’s a multiverse of universes, all with
randomly dialed Higgs masses and other parameters, and we find ourselves here
only because our universe’s peculiar properties foster the formation of atoms,
stars and planets and therefore life. This “anthropic argument,” though
possibly right, is frustratingly untestable.
Many particle physicists migrated to other
research areas, “where the puzzle hasn’t gotten as hard as the hierarchy
problem,” said Nathaniel Craig, a theoretical physicist at UCSB.
Nathaniel Craig and Isabel Garcia Garcia
have probed how gravity might help reconcile nature’s vastly different energy
scales.
Jeff Liang
Some of those who remained set to work
scrutinizing decades-old assumptions. They started thinking anew about the
striking features of nature that seem unnaturally fine-tuned — both the Higgs
boson’s small mass, and a seemingly unrelated case, one that concerns the
unnaturally low energy of space itself. “The really fundamental problems are
problems of naturalness,” Garcia Garcia said.
Their introspection is bearing fruit.
Researchers are increasingly zeroing in on what they see as a weakness in the
conventional reasoning about naturalness. It rests on a seemingly benign
assumption, one that has been baked into scientific outlooks since ancient
Greece: Big stuff consists of smaller, more fundamental stuff — an idea known
as reductionism. “The reductionist paradigm … is hard-wired into the
naturalness problems,” said Nima Arkani-Hamed, a theorist at the Institute for Advanced
Study in Princeton, New Jersey.
Now a growing number of particle
physicists think naturalness problems and the null results at the Large Hadron
Collider might be tied to reductionism’s breakdown. “Could it be that this
changes the rules of the game?” Arkani-Hamed said. In a slew of recent papers,
researchers have thrown reductionism to the wind. They’re exploring novel ways
in which big and small distance scales might conspire, producing values of
parameters that look unnaturally fine-tuned from a reductionist perspective.
“Some people call it a crisis. That has a
pessimistic vibe associated to it and I don’t feel that way about it,” said
Garcia Garcia. “It’s a time where I feel like we are on to something profound.”
What Naturalness Is
The Large Hadron Collider did make one
critical discovery: In 2012, it finally struck upon the Higgs boson, the
keystone of the 50-year-old set of equations known as the Standard Model of
particle physics, which describes the 17 known elementary particles.
The discovery of the Higgs confirmed a
riveting story that’s written in the Standard Model equations. Moments after
the Big Bang, an entity that permeates space called the Higgs field suddenly
became infused with energy. This Higgs field crackles with Higgs bosons,
particles that possess mass because of the field’s energy. As electrons, quarks
and other particles move through space, they interact with Higgs bosons, and in
this way they acquire mass as well.
After the Standard Model was completed in
1975, its architects almost immediately noticed a problem.
When the Higgs gives other particles mass,
they give it right back; the particle masses shake out together. Physicists can
write an equation for the Higgs boson’s mass that includes terms from each
particle it interacts with. All the massive Standard Model particles contribute
terms to the equation, but these aren’t the only contributions. The Higgs
should also mathematically mingle with heavier particles, up to and including
phenomena at the Planck scale, an energy level associated with the quantum
nature of gravity, black holes and the Big Bang. Planck-scale phenomena should
contribute terms to the Higgs mass that are huge — roughly a hundred million
billion times larger than the actual Higgs mass. Naively, you would expect the
Higgs boson to be as heavy as they are, thereby beefing up other elementary
particles as well. Particles would be too heavy to form atoms, and the universe
would be empty.
For the Higgs to depend on enormous
energies yet end up so light, you have to assume that some of the Planckian
contributions to its mass are negative while others are positive, and that
they’re all dialed to just the right amounts to exactly cancel out. Unless
there’s some reason for this cancellation, it seems ludicrous — about as
unlikely as air currents and table vibrations counteracting each other to keep
a pencil balanced on its tip. This kind of fine-tuned cancellation physicists
deem “unnatural.”
Within a few years, physicists found a
tidy solution: supersymmetry, a hypothesized doubling of nature’s elementary
particles. Supersymmetry says that every boson (one of two types of particle)
has a partner fermion (the other type), and vice versa. Bosons and fermions
contribute positive and negative terms to the Higgs mass, respectively. So if
these terms always come in pairs, they’ll always cancel.
The search for supersymmetric partner
particles began at the Large Electron-Positron Collider in the 1990s.
Researchers assumed the particles were just a tad heavier than their Standard
Model partners, requiring more raw energy to materialize, so they accelerated
particles to nearly light speed, smashed them together, and looked for heavy
apparitions among the debris.
Meanwhile, another naturalness problem
surfaced.
Merrill Sherman for Quanta Magazine
The fabric of space, even when devoid of
matter, seems as if it should sizzle with energy — the net activity of all the
quantum fields coursing through it. When particle physicists add up all the
presumptive contributions to the energy of space, they find that, as with the
Higgs mass, injections of energy coming from Planck-scale phenomena should blow
it up. Albert Einstein showed that the energy of space, which he dubbed the
cosmological constant, has a gravitationally repulsive effect; it causes space
to expand faster and faster. If space were infused with a Planckian density of
energy, the universe would have ripped itself apart moments after the Big Bang.
But this hasn’t happened.
Instead, cosmologists observe that space’s
expansion is accelerating only slowly, indicating that the cosmological
constant is small. Measurements in 1998 pegged its value as a million million
million million million times lower than the Planck energy. Again, it seems all
those enormous energy injections and extractions in the equation for the
cosmological constant perfectly cancel out, leaving space eerily placid.
Gravity … mixes physics at all length
scales — short distances, long distances. Because it does that, it gives you
this way out.
Nathaniel Craig
Both of these big naturalness problems
were evident by the late 1970s, but for decades, physicists treated them as
unrelated. “This was in the phase where people were schizophrenic about this,”
said Arkani-Hamed. The cosmological constant problem seemed potentially related
to mysterious, quantum aspects of gravity, since the energy of space is
detected solely through its gravitational effect. The hierarchy problem looked
more like a “dirty-little-details problem,” Arkani-Hamed said — the kind of
issue that, like two or three other problems of the past, would ultimately
reveal a few missing puzzle pieces. “The sickness of the Higgs,” as Giudice
called its unnatural lightness, was nothing a few supersymmetry particles at
the LHC couldn’t cure.
In hindsight, the two naturalness problems
seem more like symptoms of a deeper issue.
“It’s useful to think about how these
problems come about,” said Garcia Garcia in a Zoom call from Santa Barbara this
winter. “The hierarchy problem and the cosmological constant problem are
problems that arise in part because of the tools we’re using to try to answer
questions — the way we’re trying to understand certain features of our
universe.”
Reductionism Made Precise
Physicists come by their funny way of
tallying contributions to the Higgs mass and cosmological constant honestly.
The calculation method reflects the strange nesting-doll structure of the
natural world.
Zoom in on something, and you’ll discover
that it’s actually a lot of smaller things. What looks from afar like a galaxy
is really a collection of stars; each star is many atoms; an atom further
dissolves into hierarchical layers of subatomic parts. Moreover, as you zoom in
to shorter distance scales, you see heavier and more energetic elementary
particles and phenomena — a profound link between high energies and short
distances that explains why a high-energy particle collider acts like a
microscope on the universe. The connection between high energies and short
distances has many avatars throughout physics. For instance, quantum mechanics
says every particle is also a wave; the more massive the particle, the shorter
its associated wavelength. Another way to think about it is that energy has to
cram together more densely to form smaller objects. Physicists refer to
low-energy, long-distance physics as “the IR,” and high-energy, short-distance
physics as “the UV,” drawing an analogy with infrared and ultraviolet
wavelengths of light.
In the 1960s and ’70s, the particle
physics titans Kenneth
Wilson and Steven
Weinberg put
their finger on what’s so remarkable about nature’s hierarchical structure: It
allows us to describe goings-on at some big, IR scale of interest without
knowing what’s “really” happening at more microscopic, UV scales. You can, for
instance, model water with a hydrodynamic equation that treats it as a smooth
fluid, glossing over the complicated dynamics of its H2O molecules.
The hydrodynamic equation includes a term representing water’s viscosity — a
single number, which can be measured at IR scales, that summarizes all those
molecular interactions happening in the UV. Physicists say IR and UV scales
“decouple,” which lets them effectively describe aspects of the world without
knowing what’s going on deep down at the Planck scale — the ultimate UV scale,
corresponding to a billionth of a trillionth of a trillionth of a centimeter,
or 10 billion billion gigaelectron-volts (GeV) of energy, where the very fabric
of space-time probably dissolves into something else.
Kenneth Wilson, an American particle and
condensed matter physicist who was active from the 1960s until the 2000s,
developed a formal mathematical method for describing how properties of a
system change depending on the scale at which they are measured.
Cornell University Faculty Biographical
Files, #47-10-3394. Division of Rare and Manuscript Collections, Cornell
University Library.
“We can do physics because we can remain
ignorant about what happens at short distances,” said Riccardo
Rattazzi,
a theoretical physicist at the Swiss Federal Institute of Technology Lausanne.
Wilson and Weinberg separately developed
pieces of the framework that particle physicists use to model different levels
of our nesting-doll world: effective field theory. It’s in the context of EFT
that naturalness problems arise.
An EFT models a system — a bundle of
protons and neutrons, say — over a certain range of scales. Zoom in on protons
and neutrons for a while and they will keep looking like protons and neutrons;
you can describe their dynamics over that range with “chiral effective field
theory.” But then an EFT will reach its “UV cutoff,” a short-distance,
high-energy scale at which the EFT stops being an effective description of the
system. At a cutoff of 1 GeV, for example, chiral effective field theory stops
working, because protons and neutrons stop behaving like single particles and
instead act like trios of quarks. A different theory kicks in.
How Steven Weinberg
Transformed Physics and Physicists
AUGUST 11, 2021
READ LATER
Importantly, an EFT breaks down at its UV
cutoff for a reason. The cutoff is where new, higher-energy particles or phenomena
that aren’t included in that theory must be found.
In its range of operation, an EFT accounts
for UV physics below the cutoff by adding “corrections” representing these
unknown effects. It’s just like how a fluid equation has a viscosity term to capture
the net effect of short-distance molecular collisions. Physicists don’t need to
know what actual physics lies at the cutoff to write these corrections; they
just use the cutoff scale as a ballpark estimate of the size of the effects.
Typically when you’re calculating
something at an IR scale of interest, the UV corrections are small,
proportional to the (relatively smaller) length scale associated with the
cutoff. The situation changes, though, when you’re using EFT to calculate a
parameter like the Higgs mass or the cosmological constant — something that has
units of mass or energy. Then the UV corrections to the parameter are big,
because (to have the right units) the corrections are proportional to the
energy — rather than the length — associated with the cutoff. And while the
length is small, the energy is high. Such parameters are said to be
“UV-sensitive.”
The concept of naturalness emerged in the
1970s along with effective field theory itself, as a strategy for identifying
where an EFT must cut off, and where, therefore, new physics must lie. The
logic goes like this: If a mass or energy parameter has a high cutoff, its
value should naturally be large, pushed higher by all the UV corrections.
Therefore, if the parameter is small, the cutoff energy must be low.
Some commentators have dismissed
naturalness as a mere aesthetic preference. But others point to when the
strategy revealed precise, hidden truths about nature. “The logic works,” said
Craig, a leader of recent efforts to rethink that logic. Naturalness problems
“have always been a signpost of where the picture changes and new things should
appear.”
What Naturalness Can Do
In 1974, a few years before the term
“naturalness” was even coined, Mary K. Gaillard and Ben Lee made spectacular
use of the strategy to predict the mass of a then-hypothetical particle
called the charm quark. “The success of her prediction and its relevance to the
hierarchy problem are wildly underappreciated in our field,” Craig said.
That summer of ’74, Gaillard and Lee were
puzzling over the difference between the masses of two kaon particles —
composites of quarks. The measured difference was small. But when they tried to
calculate this mass difference with an EFT equation, they saw that its value
was at risk of blowing up. Because the kaon mass difference has units of mass,
it’s UV-sensitive, receiving high-energy corrections coming from the unknown
physics at the cutoff. The theory’s cutoff wasn’t known, but physicists at the
time reasoned that it couldn’t be very high, or else the resulting kaon mass
difference would seem curiously small relative to the corrections — unnatural,
as physicists now say. Gaillard and Lee inferred their EFT’s low cutoff scale,
the place where new physics should reveal itself. They argued that a recently
proposed quark called the charm quark must be found with a mass of no more than
1.5 GeV.
The charm quark showed up three months
later, weighing 1.2 GeV. The discovery ushered in a renaissance of
understanding known as the November revolution that quickly led to the
completion of the Standard Model. In a recent video call, Gaillard, now 82,
recalled that she was in Europe visiting CERN when the news broke. Lee sent her
a telegram: CHARM HAS BEEN FOUND.
In 1974, Mary K. Gaillard (pictured here
in the 1990s) and Ben Lee used a naturalness argument to predict the mass of a
hypothetical elementary particle called the charm quark. The charm was
discovered months later.
AIP Emilio Segrè Visual Archives
Such triumphs led many physicists to feel
certain that the hierarchy problem, too, should herald new particles not much
heavier than those of the Standard Model. If the Standard Model’s cutoff were
up near the Planck scale (where researchers know for sure that the Standard
Model fails, since it doesn’t account for quantum gravity), then the UV
corrections to the Higgs mass would be huge — making its lightness unnatural. A
cutoff not far above the mass of the Higgs boson itself would make the Higgs
about as heavy as the corrections coming from the cutoff, and everything would
look natural. “That option has been the starting point of the work that has
been done in trying to address the hierarchy problem in the last 40 years,”
said Garcia Garcia. “People came up with great ideas, like supersymmetry,
compositeness [of the Higgs], that we haven’t seen realized in nature.”
Garcia Garcia was a few years into her
particle physics doctorate at the University of Oxford in 2016 when it became
clear to her that a reckoning was in order. “That’s when I became more
interested in this missing component that we don’t normally incorporate when we
discuss these problems, which is gravity — this realization that there’s more
to quantum gravity than we can tell from effective field theory.”
Gravity Mixes Everything Up
Theorists learned in the 1980s that
gravity doesn’t play by the usual reductionist rules. If you bash two particles
together hard enough, their energies become so concentrated at the collision
point that they’ll form a black hole — a region of such extreme gravity that
nothing can escape. Bash particles together even harder, and they’ll form a
bigger black hole. More energy no longer lets you see shorter distances — quite
the opposite. The harder you bash, the bigger the resulting invisible region
is. Black holes and the quantum gravity theory that describes their interiors
completely reverse the usual relationship between high energies and short
distances. “Gravity is anti-reductionist,” said Sergei
Dubovsky,
a physicist at New York University.
Quantum gravity seems to toy with nature’s
architecture, making a mockery of the neat system of nested scales that
EFT-wielding physicists have grown accustomed to. Craig, like Garcia Garcia,
began to think about the implications of gravity soon after the LHC’s search
came up empty. In trying to brainstorm new solutions to the hierarchy problem,
Craig reread a 2008 essay about naturalness by Giudice, the CERN
theorist. He started wondering what Giudice meant when he wrote that the
solution to the cosmological constant problem might involve “some complicated
interplay between infrared and ultraviolet effects.” If the IR and the UV have
complicated interplay, that would defy the usual decoupling that allows
effective field theory to work. “I just Googled things like ‘UV-IR mixing,’”
Craig said, which led him to some intriguing papers from 1999, “and off I
went.”
It’s a time where I feel like we are on to
something profound.
Isabel Garcia Garcia
UV-IR mixing potentially resolves
naturalness problems by breaking EFT’s reductionist scheme. In EFT, naturalness
problems arise when quantities like the Higgs mass and the cosmological
constant are UV-sensitive, yet somehow don’t blow up, as if there’s a
conspiracy between all the UV physics that nullifies their effect on the IR.
“In the logic of effective field theory, we discard that possibility,” Craig
explained. Reductionism tells us that IR physics emerges from UV physics — that
water’s viscosity comes from its molecular dynamics, protons get their
properties from their inner quarks, and explanations reveal themselves as you
zoom in — never the reverse. The UV isn’t influenced or explained by the IR,
“so [UV effects] can’t have a conspiracy to make things work out for the Higgs
at a very different scale.”
The question Craig now asks is: “Could
that logic of effective field theory break down?” Perhaps explanations really
can flow both ways between the UV and the IR. “That’s not totally pie in the
sky, because we know that gravity does that,” he said. “Gravity violates the
normal EFT reasoning because it mixes physics at all length scales — short
distances, long distances. Because it does that, it gives you this way out.”
How UV-IR Mixing Might Save
Naturalness
Several new studies of UV-IR mixing and
how it might solve naturalness problems refer back to two papers that appeared
in 1999. “There is a growth of interest in these more exotic, non-EFT-like
solutions to these problems,” said Patrick Draper, a professor at the University of Illinois,
Urbana-Champaign whose recent work picks up where one of the 1999 papers left
off.
Draper and his colleagues study the CKN bound, named for the authors of the ’99 paper, Andrew
Cohen, David B. Kaplan and Ann
Nelson.
The authors thought about how, if you put particles in a box and heat it up,
you can only increase the energy of the particles so much before the box
collapses into a black hole. They calculated that the number of high-energy
particle states you can fit in the box before it collapses is proportional to
the box’s surface area raised to the three-fourths power, not the box’s volume
as you might think. They realized that this represented a strange UV-IR
relationship. The size of the box, which sets the IR scale, severely limits the
number of high-energy particle states within the box — the UV scale.
They then realized that if their same
bound applies to our entire universe, it resolves the cosmological constant
problem. In this scenario, the observable universe is like a very large box.
And the number of high-energy particle states it can contain is proportional to
the observable universe’s surface area to the three-fourths power, not the
universe’s (much larger) volume.
That means the usual EFT calculation of
the cosmological constant is too naive. That calculation tells the story that
high-energy phenomena should appear when you zoom in on the fabric of space,
and this should blow up the energy of space. But the CKN bound implies that
there may be far, far less high-energy activity than the EFT calculation
assumes — meaning precious few high-energy states available for particles to occupy.
Cohen, Kaplan and Nelson did a simple calculation showing that, for a box the
size of our universe, their bound predicts more or less exactly the tiny value
for the cosmological constant that’s observed.
Their calculation implies that big and
small scales might correlate with each other in a way that becomes apparent
when you look at an IR property of the whole universe, such as the cosmological
constant.
Draper and Nikita Blinov confirmed in another crude calculation last year
that the CKN bound predicts the observed cosmological constant; they also showed that it does
so without ruining the many successes of EFT in smaller-scale experiments.
The CKN bound doesn’t tell you why the UV
and IR are correlated — why, that is, the size of the box (the IR) severely
limits the number of high-energy states within the box (the UV). For that, you
probably need to know quantum gravity.
Other researchers have looked for answers
in a specific theory of quantum gravity: string theory. Last summer, the string
theorists Steven Abel and Keith
Dienes showed how UV-IR mixing in
string theory might
address both the hierarchy and cosmological constant problems.
A candidate for the fundamental theory of
gravity and everything else, string theory holds that all particles are, close
up, little vibrating strings. Standard Model particles like photons and
electrons are low-energy vibration modes of the fundamental string. But the
string can wiggle more energetically as well, giving rise to an infinite
spectrum of string states with ever-higher energies. The hierarchy problem, in
this context, asks why corrections from these string states don’t inflate the
Higgs, if there’s nothing like supersymmetry to protect it.
Video: The Standard
Model of particle physics is the most successful scientific theory of all time.
In this explainer, Cambridge University physicist David Tong recreates the
model, piece by piece, to provide some intuition for how the fundamental
building blocks of our universe fit together.
Emily Buder/Quanta Magazine;
Kristina Armitage and Rui Braz for Quanta Magazine
Dienes and Abel calculated that, because
of a different symmetry of string theory called modular invariance, corrections
from string states at all energies in the infinite spectrum from IR to UV will
be correlated in just the right way to cancel out, keeping both the Higgs mass
and the cosmological constant small. The researchers noted that this conspiracy
between low- and high-energy string states doesn’t explain why the Higgs mass
and the Planck energy are so widely separated to begin with, only that such a
separation is stable. Still, in Craig’s opinion, “it’s a really good idea.”
The new models represent a growing grab
bag of UV-IR mixing ideas. Craig’s angle of attack traces back to the
other 1999 paper,
by the prominent theorist Nathan Seiberg of the Institute for Advanced Study and two
co-authors. They studied situations where there’s a background magnetic field
filling space. To get the gist of how UV-IR mixing arises here, imagine a pair
of oppositely charged particles attached by a spring and flying through space,
perpendicular to the magnetic field. As you crank up the field’s energy, the
charged particles accelerate apart, stretching the spring. In this toy
scenario, higher energies correspond to longer distances.
Gravity is anti-reductionist.
Sergei Dubovsky
Seiberg and company found that the UV
corrections in this situation have peculiar features that illustrate how the reductionist
arrow can be spun round, so that the IR affects what happens in the UV. The
model isn’t realistic, because the real universe doesn’t have a magnetic field
imposing a background directionality. Still, Craig has been exploring whether anything like it
could work as
a solution to the hierarchy problem.
Craig, Garcia Garcia and Seth Koren have
also jointly
studied how
an argument about quantum gravity called the weak gravity conjecture, if true,
might impose consistency conditions that naturally require a huge separation
between the Higgs mass and the Planck scale.
Dubovsky, at NYU, has mulled over these
issues since at least 2013, when it was already clear that supersymmetry
particles were very tardy to the LHC party. That year, he and two collaborators
discovered a new
kind of quantum gravity model that solves the hierarchy problem;
in the model, the reductionist arrow points to both the UV and the IR from an
intermediate scale. Intriguing as this was, the model only worked in
two-dimensional space, and Dubovsky had no clue how to generalize it. He turned
to other problems. Then last year, he encountered UV-IR mixing again: He found
that a naturalness problem that arises in studies of colliding black holes
is resolved by a “hidden” symmetry that links low- and
high-frequency deformations of the shape of the black holes.
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Like other researchers, Dubovsky doesn’t
seem to think any of the specific models discovered so far have the obvious
makings of a Kuhnian revolution. Some think the whole UV-IR mixing concept
lacks promise. “There is currently no sign of a breakdown of EFT,” said David
E. Kaplan,
a theoretical physicist at Johns Hopkins University (no relation to the author
of the CKN paper). “I think there is no there there.” To convince everyone, the
idea will need experimental evidence, but so far, the existing UV-IR mixing
models are woefully short on testable predictions; they typically aim to
explain why we haven’t seen new particles beyond the Standard Model, rather
than predicting that we should. But there’s always hope of future predictions
and discoveries in cosmology, if not from colliders.
Taken together, the new UV-IR mixing
models illustrate the myopia of the old paradigm — one based solely on
reductionism and effective field theory — and that may be a start.
“Just the fact that you lose reductionism
when you go to the Planck scale, so that gravity is anti-reductionist,”
Dubovsky said, “I think it would be, in some sense, unfortunate if this fact
doesn’t have deep implications for things which we observe.”
