Tag Archives: #force

How To Discover New Forces? (Physics / Astronomy)

Jeff Dror and colleagues in their recent paper, considered the possibility that black holes in binaries are charged under a new long-range force and found that these forces can be detected with the help of pulsar-timing arrays. In particular, they showed that, in the presence of a new force, the spectral index of the stochastic gravitational wave background (SGWB) spectrum is modified, thus making the measurement of the spectral shape a powerful test of fundamental physics. Their study recently appeared in Arxiv.

Supermassive black hole binary mergers produced a stochastic gravitational wave background (SGWB) which can be detectable by pulsar timing arrays. PTAs use the extremely stable timing of successive light pulses from pulsars to detect gravitational waves (GWs) in the form of correlated timing distortions. In the presence of GWs, the observed time between pulses deviates from the stable rhythm in the frame of the source. The correlation of these deviations between pulsars exhibits a characteristic dependence on their angular separation, known as the Hellings and Downs curve, and this is considered the hallmark of a GW detection.

Recently, Zaven Arzoumanian and colleagues search for an isotropic stochastic gravitational-wave background (GWB) in the 12.5-year pulsar timing data set collected by the North American Nanohertz Observatory for Gravitational Waves. This maybe the first signal of the SGWB from SMBH mergers. While the signal does not yet conclusively exhibit the Hellings and Downs angular dependence, upcoming datasets from NANOGrav and other collaborations will be able to confirm their discovery.

Now, Jeff Dror and colleagues point out that, the study of the SGWB from SMBH mergers will open an entirely new observable for particle physics: the spectral shape of the SGWB. They showed that, in the presence of a new force, the standard power-law prediction for the SGWB spectrum is modified, presenting the spectral index as a robust prediction.

Supermassive black holes and their environments can acquire charge due to high-energy particle production or dark sector interactions, making the measurement of the spectral shape a powerful test of fundamental physics.

— told Jeff Dror, postdoc at UC Santa Cruz and first author of the study

They also mentioned that pulsar timing arrays are rapidly improving in sensitivity. For identical pulsars, the signal-to-background ratio of a pulsar timing array analysis scales,

where, T is the observation time and Np is the number of pulsars.

The large scaling with observation time suggested that NANOGrav will be able to significantly improve the estimate of the spectral index and amplitude as it continues observing the current pulsar set.

In addition, they suggested that, combining the 12.5-year NANOGrav data with the European Pulsar Timing Array (EPTA) and Parkes Pulsar Timing Array (PPTA) datasets may be enough to detect the Hellings and Downs correlation function between pulsars, which, if observed, would confirm the first detection of a stochastic GW background. Once a discovery is made, the measurement of the spectral index will be critical to measure the charges of the SMBHs and search for additional forces.

FIG. 1. A comparison of the spectral index as measured in the NANOGrav 12.5-year data set to the value predicted by merging supermassive charged black hole binaries. The shaded and bounded regions correspond to the 1σ and 2σ posteriors derived by the NANOGrav Collaboration. The black lines correspond to charged binaries under a new long-range vector force with different values of the dipole-strength parameter γ, assuming the potential-strength parameter is negligible (α = 0). Since this spectrum is not strictly a power law, they evaluated the spectrum at roughly the peak sensitivity of NANOGrav, f = (5 yr)-¹. © Jeff Dror et al.

Moreover it has been suggested that, pulsar timing arrays are particularly well suited to measure stochastic GW spectra at frequencies of order nHz–100 µHz. If we shall be able to confirm consistent spectral index and amplitude across this wide range of frequencies, it would be a remarkable confirmation of gravity-only mergers.

On the other hand, if a new force is present with a mediator mass above the pulsar timing range and below that of higher frequency detectors, it would show up as an observable break in the spectrum. This displays the critical complementarity between the different GW searches.

Other GW experiments may also detect stochastic binary merger backgrounds. In particular, LISA is expected to see a stochastic background of white dwarf, neutron star, and lighter black hole binary mergers.”

— told Jeff Dror, postdoc at UC Santa Cruz and first author of the study

Finally, they note that since the GW spectrum from SMBH binaries is yet to be discovered, it is possible that SMBHs have charges so large that new force is strong relative to gravity. In this case, we may uncover additional signals in the SGWB.

Firstly, for sufficiently large dark charges, a repulsive force will stall the merger on cosmological timescales. This could reduce the SGWB amplitude below lower bounds estimated for gravity-only mergers.

Secondly, while gravitational radiation tends to rapidly circularize binaries, dipole radiation can have the opposite effect as the binary passes through the mediator mass threshold and can have a dramatic effect on the spectrum.

All these effects they will gonna study in the future work.


Reference: Jeff A. Dror, Benjamin V. Lehmann, Hiren H. Patel, Stefano Profumo, “Discovering new forces with gravitational waves from supermassive black holes”, Arxiv, pp. 1-10, 2021. https://arxiv.org/abs/2105.04559


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Weak Force Has Strong Impact on Nanosheets (Physics)

Rice lab finds van der Waals force can deform nanoscale silver for optics, catalytic use.

You have to look closely, but the hills are alive with the force of van der Waals.

Rice University scientists found that nature’s ubiquitous “weak” force is sufficient to indent rigid nanosheets, extending their potential for use in nanoscale optics or catalytic systems.

Changing the shape of nanoscale particles changes their electromagnetic properties, said Matt Jones, the Norman and Gene Hackerman Assistant Professor of Chemistry and an assistant professor of materials science and nanoengineering. That makes the phenomenon worth further study.

A transmission electron microscope image by Rice University scientists shows a silver nanoplate deformed by a particle, forming flower-shaped stress contours in the material that indicate a bump. Changing the shape of the material changes its electromagnetic properties, making it suitable for catalysis or optical applications. Courtesy of The Jones Lab

“People care about particle shape, because the shape changes its optical properties,” Jones said. “This is a totally novel way of changing the shape of a particle.”

Jones and graduate student Sarah Rehn led the study in the American Chemical Society’s Nano Letters.

Van der Waals is a weak force that allows neutral molecules to attract one another through randomly fluctuating dipoles, depending on distance. Though small, its effects can be seen in the macro world, like when geckos walk up walls.

“Van der Waals forces are everywhere and, essentially, at the nanoscale everything is sticky,” Jones said. “When you put a large, flat particle on a large, flat surface, there’s a lot of contact, and it’s enough to permanently deform a particle that’s really thin and flexible.”

A transmission electron microscope image at left and a color map version at right highlights deformations in silver nanosheets laid over iron oxide nanospheres. Rice University scientists determined that van der Waals forces between the spheres and sheets are sufficient to distort the silver, opening defects in their crystalline lattices that could be used in optics or catalysis. (Credit: The Jones Lab/Rice University)

In the new study, the Rice team decided to see if the force could be used to manipulate 8-nanometer-thick sheets of ductile silver. After a mathematical model showed them it was possible, they placed 15-nanometer-wide iron oxide nanospheres on a surface and sprinkled prism-shaped nanosheets over them.

Without applying any other force, they saw through a transmission electron microscope that the nanosheets acquired permanent bumps where none existed before, right on top of the spheres. As measured, the distortions were about 10 times larger than the width of the spheres.

The hills weren’t very high, but simulations confirmed that van der Waals attraction between the sheet and the substrate surrounding the spheres was sufficient to influence the plasticity of the silver’s crystalline atomic lattice. They also showed that the same effect would occur in silicon dioxide and cadmium selenide nanosheets, and perhaps other compounds.

“We were trying to make really thin, large silver nanoplates and when we started taking images, we saw these strange, six-fold strain patterns, like flowers,” said Jones, who earned a multiyear Packard Fellowship in 2018 to develop advanced microscopy techniques.

“It didn’t make any sense, but we eventually figured out that it was a little ball of gunk that the plate was draped over, creating the strain,” he said. “We didn’t think anyone had investigated that, so we decided to have a look.

“What it comes down to is that when you make a particle really thin, it becomes really flexible, even if it’s a rigid metal,” Jones said.

In further experiments, the researchers saw nanospheres could be used to control the shape of the deformation, from single ridges when two spheres are close, to saddle shapes or isolated bumps when the spheres are farther apart.

They determined that sheets less than about 10 nanometers thick and with aspect ratios of about 100 are most amenable to deformation.

Rice University scientists found the ubiquitous, “weak” van der Waals force was sufficient to indent a rigid silver nanosheet. The phenomenon suggests possible applications in nanoscale optics or catalytic systems. Courtesy of The Jones Lab

The researchers noted their technique creates “a new class of curvilinear structures based on substrate topography” that “would be difficult to generate lithographically.” That opens new possibilities for electromagnetic devices that are especially relevant to nanophotonic research.

Straining the silver lattice also turns the inert metal into a possible catalyst by creating defects where chemical reactions can happen.

“This gets exciting because now, most people make these kinds of metamaterials through lithography,” Jones said. “That’s a really powerful tool, but once you’ve used that to pattern your metal, you can never change it.

“Now we have the option, perhaps someday, to build a material that has one set of properties and then change it by deforming it,” he said. “Because the forces required to do so are so small, we hope to find a way to toggle between the two.”

Co-authors of the paper are graduate student Theodor Gerrard-Anderson, postdoctoral researchers Liang Qiao and Qing Zhu, and Geoff Wehmeyer, an assistant professor of mechanical engineering.

The Robert A. Welch Foundation, the David and Lucile Packard Foundation and the National Science Foundation supported the research.

References: Sarah M. Rehn, Theodor M. Gerrard-Anderson, Liang Qiao, Qing Zhu, Geoff Wehmeyer, and Matthew R. Jones, “Mechanical Reshaping of Inorganic Nanostructures with Weak Nanoscale Forces”, Nano letters, 2020. https://pubs.acs.org/doi/10.1021/acs.nanolett.0c03383 https://doi.org/10.1021/acs.nanolett.0c03383

Provided by Rice University

New Glue Sticks Easily, Holds Strongly, And is a Gas to Pull Apart (Material Science)

Temporary glues may not steal headlines, but they can make everyday life easier.

Sticky office notes, bandage strips and painter’s tape are all examples of products that adhere to surfaces but can be removed with relative ease.

A temporary adhesive based on molecular solids is strong enough to hold a chemistry PhD candidate, but can be released without force through the use of heat in a vacuum. Photo courtesy of Nicholas Blelloch.

There’s only one drawback. To remove any of those adhesives, the glued surfaces need to be pulled apart from each other.

Dartmouth research has discovered a class of molecular materials that can be used to make temporary adhesives that don’t require force for removal. These non-permanent glues won’t be available as home or office supplies, but they can lead to new manufacturing techniques and pharmaceutical design.

“This temporary adhesive works in an entirely different way than other adhesives,” said Katherine Mirica, an assistant professor of chemistry at Dartmouth. “This innovation will unlock new manufacturing strategies where on-demand release from adhesion is required.”

The Dartmouth research focuses on molecular solids, a special class of adhesive materials that exist as crystals. The molecules in the structures are sublimable, meaning that they shift directly from a solid to a gas without passing through a liquid phase.

The ability to bypass the liquid phase is the key to the new type of temporary adhesives. The adhesive sticks as a solid but then turns to a vapor and releases once it is heated in a vacuum environment.

“The use of sublimation–the direct transition from solid to vapor–is valuable because it offers gentle release from adhesion without the use of solvent or mechanical force,” said Mirica.

Previous Dartmouth research was the first to identify how molecular solids can act as temporary adhesives. According to new research, published in the academic journal Chemistry of Materials, the class of molecules that can be used to make these new-generation materials is wider than previously thought.

“We’ve expanded the list of molecules that can be used as temporary adhesives,” said Nicholas Blelloch, a PhD candidate at Dartmouth and first author of the paper. “Identifying more materials to work with is important because it offers expanded design strategies for bonding surfaces together.”

The research team says the new temporary adhesives can be useful in technical applications such as semiconductor manufacturing and drug development.

When making computer chips, silicon components need to be temporarily bonded. The use of a strong adhesive that releases through sublimation can allow for the development of smaller, more powerful chips since tapes requiring forceful pulling would no longer be required.

In pharmaceuticals, the design principles highlighted through this work can help the development of smaller, faster-acting pills. The adhesives can also be helpful in the design of nano- and micromechanical devices where the use of tape is not possible.

The finding also gives researchers more flexibility in developing temporary adhesives.

“Identifying more molecules with adhesives properties refines our fundamental understating of the multi-scale and multi-faceted factors that contribute to the adhesive properties of the system,” said Blelloch

Most common temporary adhesives that are used in the home or office are polymers, long chemical chains that create strong bonds, but can be difficult to be pulled from surfaces.

If polymers can be described as long chemical strands that easily tangle, molecular solids are more like individual chemical beads that sit atop each other. Both can be made to adhere, but there are tradeoffs.

Polymers used to make super glues tangle so well that they form exceedingly strong bonds that are difficult to pull apart. Sticky office notes and painter’s tape are also polymers, but with much less holding strength. They also require a peeling or ripping action to remove the bond.

Molecular solids being studied by the Dartmouth team can be as strong as temporary, polymer-based adhesives. The advantage of the new glues is that they not only adhere easily, they can be released without force, and without disturbing the bonded surfaces.

References: Nicholas D. Blelloch, Haydn T. Mitchell, Carly C. Tymm, Douglas W. Van Citters, and Katherine A. Mirica, “Crystal Engineering of Molecular Solids as Temporary Adhesives”, ACS, 2020. https://pubs.acs.org/doi/10.1021/acs.chemmater.0c01401 https://doi.org/10.1021/acs.chemmater.0c01401

Provided by Dartmouth College

About Dartmouth

Founded in 1769, Dartmouth is a member of the Ivy League and offers the world’s premier liberal arts education, combining its deep commitment to outstanding undergraduate and graduate teaching with distinguished research and scholarship in the arts and sciences and its leading professional schools: the Geisel School of Medicine, the Guarini School of Graduate and Advanced Studies, Thayer School of Engineering and Tuck School of Business.

Weather On Jupiter And Saturn May be Driven By Different Forces than on Earth (Planetary Science)

A trio of researchers, two with Harvard University, the other the University of Alberta, has found evidence that weather on Saturn and Jupiter may be driven by dramatically different forces than weather on Earth. In their paper published in the journal Science Advances, Rakesh Kumar Yadav, Moritz Heimpel and Jeremy Bloxham describe computer simulations showing that major weather systems on Jupiter and Saturn might be driven by internal rather than external forces, resulting in outcomes such as the formation of large anticyclones like Jupiter’s famous red spot.

Credit: CC0 Public Domain

Weather on Earth is primarily driven by processes that take place in a thin layer of the atmosphere near the planet’s surface. For many years, it has been thought that similar processes drive weather on other planets, such as Jupiter and Saturn. In this new effort, the researchers demonstrate that such theories may be wrong.

The work involved creating two simulations to mimic conditions on Jupiter and Saturn. Rather than assuming weather patterns are driven by turbulence just above the surface, the researchers programmed their simulations to take into account turbulent convection occurring in spherical shells as they rotate. In one such simulation, which they called the “thin shell” approach, the simulation was used to reproduce what happens with convection layers on gas giants such as Saturn and Jupiter—events they note have very little interaction with the planet’s magnetic field. They found that the simulation showed cyclones, zonal jets and anticyclones forming spontaneously on both Jupiter and Saturn. The second simulation, which they called the “thick shell” approach, was programmed to mimic the interactions by the planet’s inner dynamo and the outer hydrodynamic layer. It showed plumes being ejected from the magnetic layer, which gave rise to what they describe as pancake-shaped weather patterns close to the surface.

The researchers suggest that some of the weather patterns on both planets are likely driven by jet streams and processes below the surface. They also suggest their simulations show that the famous red spot may have formed when the planet’s dynamo region set off processes that resulted in the production of large anticyclones in the atmosphere.

References: Rakesh Kumar Yadav, Moritz Heimpel, Jeremy Bloxham, “Deep convection–driven vortex formation on Jupiter and Saturn”, Science Advances, vol. 6 no. 46, 2020. Link: https://advances.sciencemag.org/content/6/46/eabb9298 https://doi.org/10.1126/sciadv.abb9298

This article is originally written by Bob Yirka on Phys.org and is republished here under common creative licenses

Why there is no speed limit in the superfluid universe? (Physics / Quantum)

Physicists from Lancaster University have established why objects moving through superfluid helium-3 lack a speed limit in a continuation of earlier Lancaster research.

Researchers found the reason for the absence of the speed limit: exotic particles that stick to all surfaces in the superfluid. Credit: Lancaster University

Helium-3 is a rare isotope of helium, in which one neutron is missing. It becomes superfluid at extremely low temperatures, enabling unusual properties such as a lack of friction for moving objects.

It was thought that the speed of objects moving through superfluid helium-3 was fundamentally limited to the critical Landau velocity, and that exceeding this speed limit would destroy the superfluid. Prior experiments in Lancaster have found that it is not a strict rule and objects can move at much greater speeds without destroying the fragile superfluid state.

Now scientists from Lancaster University have found the reason for the absence of the speed limit: exotic particles that stick to all surfaces in the superfluid.

The discovery may guide applications in quantum technology, even quantum computing, where multiple research groups already aim to make use of these unusual particles.

To shake the bound particles into sight, the researchers cooled superfluid helium-3 to within one ten thousandth of a degree from absolute zero (0.0001K or -273.15°C). They then moved a wire through the superfluid at a high speed, and measured how much force was needed to move the wire. Apart from an extremely small force related to moving the bound particles around when the wire starts to move, the measured force was zero.

Lead author Dr Samuli Autti said: “Superfluid helium-3 feels like vacuum to a rod moving through it, although it is a relatively dense liquid. There is no resistance, none at all. I find this very intriguing.”

PhD student Ash Jennings added: “By making the rod change its direction of motion we were able to conclude that the rod will be hidden from the superfluid by the bound particles covering it, even when its speed is very high.” “The bound particles initially need to move around to achieve this, and that exerts a tiny force on the rod, but once this is done, the force just completely disappears,” said Dr Dmitry Zmeev, who supervised the project.

This article is republished from science daily

References: S. Autti, S. L. Ahlstrom, R. P. Haley, A. Jennings, G. R. Pickett, M. Poole, R. Schanen, A. A. Soldatov, V. Tsepelin, J. Vonka, T. Wilcox, A. J. Woods, D. E. Zmeev. Fundamental dissipation due to bound fermions in the zero-temperature limit. Nature Communications, 2020; 11 (1) DOI: 10.1038/s41467-020-18499-1 link: https://www.nature.com/articles/s41467-020-18499-1

You Don’t Weigh The Same Everywhere (Physics)

Though your mass stays consistent no matter where you are, your weight can fluctuate. You’d weigh less standing at the equator than you would at a pole. Thank centrifugal and centripetal forces!

WHAT’S GOING ON HERE?

Imagine you’re carrying a plastic bag filled with oranges. If you swing that bag over your head at the right speed, the oranges will stay in the bag, and even try to round out the circle again. Swing it too fast, and the oranges might bust out through the bottom of the bag and get flung halfway across the room. Congrats, you just learned about centrifugal and centripetal forces!

Centrifugal force is what would cause the oranges to bust out of the bottom of the plastic bag (“the apparent force, equal and opposite to the centripetal force, drawing a rotating body away from the center of rotation, caused by the inertia of the body,” according to the American Heritage Dictionary). Centripetal force (“the component of force acting on a body in curvilinear motion that is directed toward the center of curvature or axis of rotation”) is what makes the oranges want to keep looping around in a circle.

This is why you weigh less standing at the equator than at a pole. At the equator, centripetal forces are acting on you as you spin around the center of the Earth. This spinning keeps you from flying off into space. At a pole, that force isn’t acting on you because you’re not rotating at such an intense speed. Also, at a pole, you’re closer to the center of Earth (it’s not a perfect sphere!), so gravity is pulling you down with just a tad more strength. But the effect it has on your weight isn’t too extreme — you’d weigh about 0.5 percent more at a pole. So if you weighed 200 pounds at a pole, you’d be 199 pounds at the equator.

Where you are on Earth isn’t the only element that can affect your weight. Altitude also has an effect. The gravitational force exerted on you is inverse to the square of your distance from the planet’s center — in other words, 1/R². As you move further from the Earth’s center, say, by climbing a mountain, you’d become ever so slightly lighter. If you moved further toward its center, perhaps by venturing down into Death Valley, you’d be a fraction of a percent heavier.

But the effect is much less than the difference between being on the equator and a pole. The Earth’s radius at the equator is 6,378 kilometers. If you were to climb a 5-kilometer mountain — something like Mount Kilimanjaro — it would put you 6,383 kilometers from the planet’s center, and your weight would have decreased by a factor of (6,378 / 6,383)2, or 0.9984 — basically, an 0.2 percent difference. To reach the same change you’d find by moving from a pole to the equator, that mountain would have to be a whopping 32 kilometers (20 miles) tall. That’s near the top of the ozone layer.


The Strong Force Is What’s Holding The Entire Universe (Chemistry / Universe)

Particle physicists might seem like a dry bunch, but they have their fun. Why else would there be such a thing as a “strange quark”? When it comes to the fundamental nuclear forces, though, they don’t mess around: the strongest force in nature is known simply as the “strong force,” and it’s the force that literally holds existence together.

To find out what the strong force is, you need to have a basic understanding of what physicists call the elementary particles. Let’s start with an atom—helium, for example. A helium atom has two electrons zipping around a nucleus made up of two neutrons and two protons. For most high-school chemistry classes, that’s where the tiny particles end.

But you can zoom even further into the atom: those protons and neutrons are a class of particle called hadrons (à la the Large Hadron Collider!), which are made up of even smaller particles called quarks. Quarks are what’s known as an elementary particle, since they can’t be split up any further. They’re as small as things get. There are two types of elementary particles; the other is the lepton. Quarks and leptons each have six “flavors”, and each of those have an antimatter version. (The electrons in our helium atom are a flavor of lepton, so we’re as zoomed in on them as is possible.) Heady stuff! Check out the diagram below if you’re getting lost.

Following so far? There are four more parts to this puzzle we call the Standard Model, which is the theory of all theories when it comes to particle physics. Those parts are the fundamental forces. Two are probably familiar: gravity is the force between two particles that have mass, and electromagnetism is the force between two particles that have a charge. The two others are known as nuclear forces, and they’re less familiar because they only happen on the atomic scale. Those ones are known as the weak force and the strong force. The weak force operates between electrons and neutrinos (another kind of lepton), but of course, it’s the strong force we’re here to talk about.

The strong force is what binds quarks together to form hadrons like protons and neutrons. Physicists first conceived of this force’s existence to explain why an atom’s nucleus can have more than one positively charged proton and still stay together — if you’ve ever played with magnets, you know that a positive charge will always repel another positive charge. Eventually, they figured out that the strong force not only holds protons together in the nucleus, but it also holds quarks together in the protons themselves. The force actually comes from a type of force-carrier particle called a boson. (Surely you remember the 2012 discovery of the Higgs boson?) The particular boson that exerts this powerful force is called a “gluon”, since it “glues” the nucleus together (we told you that physicists were a fun bunch).

Here’s what makes the strong force so fascinating: unlike an electromagnetic force, which decreases as you pull the two charged particles apart (think of magnets again!), the strong force actually gets stronger the further apart the particles go. It gets so strong that it limits how far two quarks can separate. Once they hit that limit, that’s when the magic happens: the huge amount of energy it took for them to separate is converted to mass, following Einstein’s famous equation E = mc2. That’s right—the strongest force in the universe is strong enough to turn energy into matter, the thing that makes up existence as you know it. We learned some particle physics, everyone. Who needs a snack?


References: (1) https://home.cern/science/accelerators/large-hadron-collider (2) http://abyss.uoregon.edu/~js/ast123/lectures/lec07.html (3) https://home.cern/science/physics/standard-model (4) https://home.cern/science/physics/higgs-boson (5) https://www.pbs.org/wgbh/nova/einstein/lrk-hand-emc2expl.html