- 4389 - ATOMS AND PROTONS - Physicists have begun to explore the proton as if it were a subatomic planet. Cutaway maps display newfound details of the particle’s interior. The proton’s core features pressures more intense than in any other known form of matter. Halfway to the surface, clashing vortices of force push against each other. And the “planet” as a whole is smaller than previous experiments had suggested.
----------------------------------- 4389 - ATOMS AND PROTONS
- The quest to understand the particle that
anchors every atom and makes up the bulk of our world. Over decades, researchers have meticulously
mapped out the electromagnetic influence of this positively charged particle.
But in the new research, physicists are instead mapping the proton’s
gravitational influence, the distribution of energies, pressures and shear
stresses throughout, which bend the space-time fabric in and around the
particle.
-
- The researchers do so by exploiting a
peculiar way in which pairs of photons, particles of light, can imitate a
“graviton”, the hypothesized particle that conveys the force of gravity. By
pinging the proton with photons, they indirectly infer how gravity would
interact with it, realizing a decades-old dream of interrogating the proton in
this alternative way.
-
- Physicists have learned a tremendous amount
about the proton over the last 70 years by repeatedly hitting it with
electrons. They know that its electric charge extends roughly 0.8 femtometers,
or quadrillionths of a meter, from its center. They know that incoming
electrons tend to glance off one of three “quarks” that buzz about inside it.
-
- They have also observed the deeply strange
consequence of quantum theory where, in more forceful collisions, electrons
appear to encounter a frothy sea made up of far more quarks as well as
“gluons”, the carriers of the so-called strong force, which glues the quarks
together.
-
- All this information comes from a single
setup: You fire an electron at a proton, and the particles exchange a single
“photon”, the carrier of the electromagnetic force, and push each other away.
This electromagnetic interaction tells physicists how quarks, as charged
objects, tend to arrange themselves. But there is a lot more to the proton than
its electric charge.
-
- The gravitational side of the proton is a
matrix of properties of the proton called the “energy-momentum tensor”. The energy-momentum tensor knows everything
there is to be known about the particle.
-
- In Albert Einstein’s theory of general
relativity, which casts gravitational attraction as objects following curves in
space-time, the energy-momentum tensor tells space-time how to bend. It
describes the arrangement of energy (or, equivalently, mass) the source of the
lion’s share of space-time twisting. It also tracks information about how
momentum is distributed, as well as where there will be compression or
expansion, which can also lightly curve space-time.
-
- If we could learn the shape of space-time
surrounding a proton we could infer all the properties indexed in its
energy-momentum tensor. Those include the proton’s mass and spin, which are
already known, along with the arrangement of the proton’s pressures and forces,
a collective property physicists refer to as the “Druck term,” after the word
for pressure in German. This term is as important as mass and spin, and nobody
knows what it is.
-
- The usual scattering experiment fires a
massive particle at a proton and let the two exchange a “graviton”, the
hypothetical particle that makes up gravitational waves, rather than a photon.
But due to the extreme weakness of gravity, physicists expect graviton
scattering to occur 39 orders of magnitude more rarely than photon scattering.
Experiments can’t possibly detect such a weak effect.
-
- The new scheme to measure gravity is to
fire an electron lightly at a proton, it usually delivers a photon to one of
the quarks and glances off. But in fewer than one in a billion events,
something special happens. The incoming electron sends in a photon. A quark
absorbs it and then emits another photon a heartbeat later.
-
- The key difference is that this rare event
involves two photons instead of one, both incoming and outgoing photons. If we could collect the resulting electron,
proton and photon, we could infer from the energies and momentums of these
particles what happened with the two photons. And that two-photon experiment
would be essentially as informative as the impossible graviton-scattering
experiment.
-
- Photons are ripples in the electromagnetic
field, which can be described by a single arrow, or vector, at each location in
space indicating the field’s value and direction. Gravitons would be ripples in
the geometry of space-time, a more complicated field represented by a
combination of two vectors at every point. Capturing a graviton would give
physicists two vectors of information. Short of that, two photons can stand in
for a graviton, since they also collectively carry two vectors of information.
-
- During the moment that elapses between when
a quark absorbs the first photon and when it emits the second, the quark
follows a path through space. By probing this path, we can learn about
properties like the pressures and forces that surround the path. We should obtain indirect access to how a
proton should interact with a graviton.
-
- From their index of space-time-bending
properties, the team extracted the elusive Druck term. They found that in the heart of the proton,
the strong force generates pressures of unimaginable intensity. 100 billion trillion trillion pascals, or
about 10 times the pressure at the heart of a neutron star. Farther out from
the center, the pressure falls and eventually turns inward, as it must for the
proton not to blow itself apart.
- This finding has no bearing on whether
protons decay, however, which involves a different type of instability
predicted by some speculative theories.
-
- Physicists found that close to its core,
the proton experiences a twisting force that gets neutralized by a twisting in
the other direction nearer the surface. These measurements also underscore the
particle’s stability. The twists had been expected based on theoretical work.
-
- Now they’re using these tools to calculate
the proton’s size in a new way. In traditional scattering experiments,
physicists had observed that the particle’s electric charge extends about 0.8
femtometers from its center (that is, its constituent quarks buzz about in that
region).
-
- But that “charge radius” has some quirks.
In the case of the neutron, the proton’s
neutral counterpart, in which two negatively charged quarks tend to hang out
deep inside the particle while one positively charged quark spends more time
near the surface, the charge radius comes out as a negative number.
-
- The new approach measures the region of
space-time that’s significantly curved by the proton. They calculated that this
radius may be about 25% smaller than the charge radius, just 0.6 femtometers.
-
- Conceptually, this kind of analysis smooths
out the blurry dance of quarks into a solid, planetlike object, with pressures
and forces acting on each speck of volume.
Precisely measuring the energy-momentum tensor would require much higher
collision energies than reseachers can produce.
-
- The proton is more than its quarks; it also
contains gluons, which slosh around with their own pressures and forces. The
two-photon trick cannot detect gluons’ effects. Sharper gravitational maps of both the
proton’s quarks and its gluons may come in the 2030s when the Electron-Ion
Collider, an experiment currently under construction at Brookhaven, will begin
operations.
-
- In the meantime, physicists are pushing
ahead with digital experiments. A team is computing the behavior of quarks and
gluons starting from the equations of the strong force. In 2019, they estimated
the pressures and shear forces, and in October, they estimated the radius,
among other properties. So far, their digital findings have broadly aligned
with Jefferson Lab’s physical ones.
-
- Even the blurry glimpses of the proton
attained so far have gently reshaped researchers’ understanding of the
particle. At CERN, the European
organization that runs the Large Hadron Collider, the world’s largest proton
smasher, physicists had previously assumed that in certain rare collisions,
quarks could be anywhere within the colliding protons. But the gravitationally
inspired maps suggest that quarks tend to hang out near the center in such
cases.
-
- The new maps may also offer guidance toward
resolving one of the deepest mysteries of the proton: why quarks bind
themselves into protons at all. There’s an intuitive argument that because the
strong force between each pair of quarks intensifies as they get further apart,
like an elastic band, quarks can never escape from their comrades.
-
- But protons are made from the lightest
members of the quark family. And lightweight quarks can also be thought of as
lengthy waves extending beyond the proton’s surface. This picture suggests that
the binding of the proton may come about not through the internal pulling of
elastic bands but through some external interaction between these wavy,
drawn-out quarks.
-
- It’s not a definite answer, but it points
toward the fact that these simple images with elastic bands are not relevant
for light quarks.
-
March 15, 2024 ATOMS AND PROTONS 4389
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