# Control the behavior of a Black Hole

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Is it possible to manipulate the size of a black hole or how a black hole behaves if scientists were able to synthesize hawking radiation on demand. That is, if I could find a way to imitate hawking radiation in a missile would this missile decrease the size of the black hole I fire the missile at? I am thinking provided I create enough hawking radiation it should have an effect on the black hole size once the missile detonates. That is if it is true that hawking radiation can make the black hole disappear as it does one the mini black holes who die because of hawking radiation.

Hawking radiation isn't some special kind of radiation. It's perfectly ordinary "thermal" radiation, mainly photons, with a few neutrinos and electrons and positrons if the temperature is high enough. Black holes shrink when they emit it, not when the absorb it. So aiming simulated Hawking radiation (or any other kind of radiation) into a black hole just makes it bigger.

## Black Holes Simulated in a Tank of Water Reveals “Backreaction” for the First Time

It’s hard to make a black hole in the lab. You have to gather up a bunch of mass, squeeze it until it gravitationally collapses on itself, work, work, work. It’s so hard to do that we’ve never done it. We can, however, make a simulated black hole using a tank of water, and it can tell us interesting things about how black holes work.

Water simulations of black holes are possible because the mathematics that describes the behavior of water is similar to the mathematics that describes the behavior of things like gravitational waves. Gravitational interactions occur in fluid-like ways, so you can use a fluid to study them. There are limitations to these water models, however, so you need to be careful when studying water simulations.

One problem with water models of black holes is that you need to drive the simulation to keep it going. Suppose you want to study how matter might be captured by a black hole. You can simulate the black hole by a vortex of water, similar to the tornado-like swirl you sometimes see when draining a bathtub. To keep the vortex going, you have to power your system so that the pattern stays stable long enough for you to get good data.

Because of this, it was generally thought that water models couldn’t exhibit an effect that should occur with real black holes, known as backreaction. Backreaction occurs when there is an interaction where an object reacts back with its environment. For example, as a black hole captures matter its mass increases. This increase in mass changes how the black hole warps space around it, thus changing the surrounding space slightly. Backreaction is an important phenomenon, but it is subtle and difficult to study.

A water vortex simulating a black hole. Credit: University of Nottingham

Recently, however, a team has found that backreaction can be seen in water simulation models. The research studied how a background of gravitational waves could interact with a rotating black hole. In their water model, they created a water vortex simulating a black hole and then generated a ripple of waves toward the vortex. The reaction between the vortex and ripples caused the vortex to grow more quickly than it ordinarily would. In this way, gravitational waves could accelerate the growth of a black hole through a backreaction effect.

In the water simulation, the backreaction was strong enough the team would visibly see the water level of their tank drop when it occurred, proving that the reaction can occur on short time scales.

While this study is interesting on its own, the work also shows that backreaction must be taken into account with many water simulations. Usually, it has been assumed that water vortex simulations can assume a stationary background, meaning any backreaction can be ignored in the model. This work shows how that assumption might not work when studying other black hole effects such as Hawking radiation.

It will be a while before real black holes can be made in the lab. Fortunately, water simulations such as this one still have plenty to teach us.

Reference: Goodhew, Harry, et al. “Backreaction in an analogue black hole experiment.” Physical Review Letters 126.4 (2021): 041105

## Genome-wide association studies

Four billion years of natural selection crafted the refined machinery we all share — encoded in most of our DNA — as well as carefully selected room for variation — encoded in a minority of DNA differences. If the 3.2 billion nucleotides in our DNA would fit into a 300-page book, the differences between two random people would barely add up to two pages. Many decades of research in twins and family members suggest that considerable portions of differences in human behavior are associated with some of the tiny differences within those two pages.

It is hard to uncover the evolutionary stories behind these differences, but it would probably help to first find out how these genetic differences exactly give rise to the diversity in our behavioral repertoire. Recent advances in genetics research allow us to link specific DNA nucleotides on those two pages to complex behavioral outcomes. Studies that link genetic variation on a molecular level with complex traits are called genome-wide association studies (GWAS). In a GWAS, millions of single DNA nucleotides are tested one by one in order to quantify their relationship with the most complex of human traits, including behavior.

Professor Karin Verweij and I recently published an article in Nature Human Behavior, in which we review what we have learned so far from GWAS on human behavior and what steps we need to take to learn more. Here, I will summarize some highlights from our article and reflect on their societal relevance.

## Black Hole’s Tug on Space Pulls Fast-Moving Jets in Rapid Wobble

Jets of fast-moving material shot from the area surrounding a black hole are wobbling so fast that their change in direction can be seen in periods as short as minutes, and astronomers say it’s happening because the rotating black hole’s powerful gravitational pull is dragging nearby space itself along with it.

“We’ve never seen this effect happening on such short timescales,” said James Miller-Jones, of the Curtin University node of the International Centre for Radio Astronomy Research (ICRAR), who led a team using the National Science Foundation’s Very Long Baseline Array (VLBA).

The team studied V404 Cygni, a black hole nine times more massive than the Sun, nearly 8,000 light-years from Earth. The black hole is drawing in material from a companion star with a mass about 70 percent that of the Sun. As the material streams toward the black hole, it forms a rotating disk, called an accretion disk, surrounding the black hole.

In such systems, the disk becomes denser and hotter with decreasing distance from the black hole. Either the innermost portion of the disk or the black hole itself launches jets of material outward away from the disk. The astronomers said V404 Cygni’s jet material moves as fast as 60 percent of the speed of light.

Such a rapid wobble, called precession, as that in V404 Cygni has not been seen before in other such systems. To explain that phenomenon, the scientists said, requires using an effect of Einstein’s general theory of relativity. That theory says that massive objects like black holes distort space and time. Further, when such a massive object is spinning, its gravitational influence pulls space and time around with it, an effect called frame-dragging.

In V404 Cygni, the black hole’s spin axis is misaligned from the plane of its orbit with the companion star. That causes the frame-dragging effect to warp the inner part of the disk, then pull the warped portion around with it. Since the jets originate from either the inner disk or the black hole, this changes the jet orientation, producing the wobbling observed with the VLBA.

“This is the only mechanism we can think of that can explain the rapid precession we see in V404 Cygni,” Miller-Jones said. “You can think of it like the wobble of a spinning top as it slows down, only in this case, the wobble is caused by Einstein’s general theory of relativity,” he added.

While V404 Cygni’s accretion disk is about 10 million kilometers wide, Miller-Jones pointed out that only the inner few thousand kilometers is warped. That inner part also is puffed up by strong radiation pressure into a doughnut shape that precesses as a rigid body.

The jets’ rapid direction changes meant that the astronomers had to change their observation strategy. Normally, astronomers will produce a single image using data collected over as much as several hours, like a long time exposure.

However, “These jets were changing so fast that in a four-hour image we saw just a blur,” said Alex Tetarenko, a recent Ph.D graduate from the University of Alberta and currently an East Asian Observatory Fellow working in Hawaii.

To capture the rapid motion, the researchers made 103 individual images, each about 70 seconds long, then combined them to make a movie.

The result, according to Greg Sivakoff, of the University of Alberta, indicates that similar behavior could be found in other objects.

“We were gobsmacked by what we saw in this system — it was completely unexpected,” said Sivakoff. “Finding this astronomical first has deepened our understanding of how black holes and galaxy formation can work. It tells us a little more about that big question: ‘How did we get here?'”

V404 Cygni first came to astronomers’ attention in 1938, when it experienced an outburst, and got its designation as a “variable star.” Another outburst was observed in 1989, and follow-up studies revealed a previously-unnoticed outburst in 1956.

NASA’s Swift satellite detected a new outburst on June 15, 2015, triggering a worldwide observing effort. The VLBA observations began on June 17, 2015, and continued through July 11 of that year.

Miller-Jones, Tetarenko, and Sivakoff, along with colleagues from around the world, are reporting their results in the scientific journal Nature.

The National Radio Astronomy Observatory is a facility of the National Science Foundation, operated under cooperative agreement by Associated Universities, Inc.

## People

Dr. Satyapal is a professor in the Department of Physics and Astronomy at George Mason University. Professor Satyapal received her B.S degree in Physics from Bryn Mawr College and her Ph.D in Physics and Astronomy from the University of Rochester. She was a postdoctoral researcher at NASA Goddard Space Flight Center. Prior to joining Mason, she was an instrument scientist for the James Webb Space Telescope at Goddard and Space Telescope Science Institute. Her research centers on understanding the connection between the growth and evolution of supermassive black holes and the host galaxies in which they reside. She utilizes space- and ground-based multi-wavelength data from Chandra, XMM-Newton, WISE, Spitzer, the Very Large Array (VLA), Gemini, the Large Binocular Telescope (LBT), and Keck. Professor Satyapal is the recipient of numerous awards including the Presidential Early Career Award. Click here for more information about her research.

Dr. Satyapal is currently working on the following projects: intermediate mass black holes, dual supermassive black holes, and supermassive black holes in bulgeless and low mass galaxies. In recent years, she has been enjoying teaching quantum mechanics, statistical mechanics, and waves and radiation.

### Melissa Bierschenk

Melissa is a graduate student at George Mason University and holds a B.S. in physics from the University of South Alabama and an M.S. in physics from the University of Minnesota Duluth. While at UMD, Melissa focused on generating Post-Newtonian waveforms of coalescing binary black holes and producing animations of these orbits with its resulting gravitational waves. Her current research involves looking at the MIR spectroscopical properties of 200 AGN from the BASS Survey using the Spitzer telescope. The goal is to understand how the characteristics of the circumnuclear dust change with the accretion properties of the supermassive black hole. Melissa was awarded the Outstanding Graduate Teaching Assistant Award from the UMD Physics & Astronomy Department for the 2019-2020 academic year.

### Jenna Cann

Jenna is a graduate student at George Mason University, and holds a B.S. in Astronomy, also from GMU. Her work focuses on determining new diagnostics, such as emission lines coming from highly ionized elements called coronal lines, to search for lower mass SMBHs in dwarf galaxies using the photoionization modeling code Cloudy and instruments such as NIRSPEC and NIRES at Keck Observatory. She is the recipient of several prestigious awards, such as the National Science Foundation Graduate Research Fellowship, Cosmos Club Foundation Cosmos Scholar, and a Sigma Xi Grant-in-Aid of Research. She is also very active in outreach and conducts tours at the GMU Observatory.

### Luis Fernandez

Luis is a Cuban-American born in Miami, Florida who is a non-traditional student. He received his A.S. in Emergency Medical Services at Miami Dade College where he obtained his paramedic’s licence and worked at Palmetto General Hospital for 5 years before going back to school to receive his B.S. in Physics at Florida International University (FIU). Luis is currently a graduate student at George Mason University. Using Radio and X-ray data from observatories such as the Very Large Baseline Array (VLBA) and Swift/Bat, his work examens the feedback of Active Galactic Nuclei (AGN). Currently he is working on a 6 month survey of NGC2992. Luis was awarded the FIU Physics Department’s S-STEM scholarship 2 years consecutively during his undergraduate studies.”

### Lara Kamal

Lara is a student at George Mason University and holds a B.S. in Computer Science with an emphasis in Machine Learning. She is currently working on a second degree in Physics also at GMU. Her work focuses on using Machine Learning Algorithms, such as regression, classification, feature reduction, and neural network on Cloudy data, a photoionization modeling code, to determine the mass of the blackhole given a set of emission lines. Lara is a recipient of the Computer Science Outstanding Student Achievement and Academic Achievement in Mathematics Awards.

### William Matzko

William is currently a graduate student at George Mason University. As an undergraduate, he double majored in Physics and Astronomy and subsequently earned his B.S. in Physics and Astronomy from George Mason in 2019. His past research experience focused on exoplanets, including simulating time-series data for the EarthFinder mission concept study and validating transiting exoplanets. Currently, his research is focused on understanding the behavior of outflows in galaxies as a function of their merger stage. William is the recipient of several awards, including a travel grant from the Undergraduate Student Travel Fund (USTF) and an outstanding research award from George Mason’s Department of Physics and Astronomy. Beginning in the fall of 2019, he will be one of George Mason’s observatory coordinators.

### Jeffrey McKaig

Jeffrey holds a B.S in applied physics and applied mathematics from Christopher Newport University. There his research focused on general relativity and black holes with a concentration on the creation of extragalactic jets. He also researched Compton scattering in the high field regime at Thomas Jefferson National Accelerator Facility in partnership with Old Dominion University. Jeffrey joined GMU to study Active Galactic Nuclei (AGN) from the X-ray perspective and is currently studying the effect of polar dust on X-ray spectra from AGNs using high resolution simulations. This polar dust was discovered after high resolution interferometry of AGNs in the mid-infrared found a large component extending in the polar direction, which is not expected by the classical unified model of AGNs . Studying this polar component of dust in the X-ray regime could act as a unique probe of the kinematics of the polar gas. These simulations will provide a fundamental benchmark for future high spectral resolution X-ray instruments, such as those onboard XRISM and Athena.

### Ryan Pfeifle

Ryan is a graduate student at George Mason University and holds a B.S. in Physics – with an emphasis in astrophysics – also from GMU. Utilizing X-ray observatories such as Chandra, XMM-Newton, and NuSTAR, his work focuses on the search for dual AGN in mid-IR preselected late-stage galaxy mergers, characterizing the levels of AGN obscuration in late-stage mergers, as well as understanding AGN triggering and fueling in mergers versus isolated galaxies. Ryan is the recipient of a several awards, including the Sigma Xi Grant-in-Aid of Research (GRIAR) and two competitive International Travel Grants (ITGs) offered by the GMU Associate Provost for Graduate Education. Outreach is also a very important activity for Ryan, and he regularly conducts class and public tours at the GMU observatory.

### Emma Schwartzman

Emma is a graduate student at George Mason University, and holds two bachelor’s degrees – one in Astronomy and one in Physics – from the University of Maryland. She also works at the U.S. Naval Research Laboratory, where her work focuses on the study of galaxy clusters, cluster mergers, and other radio emitters, using ground-based radio interferometry and satellite-based high energy X-ray detectors. She spends the majority of her time performing data analysis for characteristics like source location, size, and spectral characteristics. She primarily works with radio data calibration packages such as CASA and WSCLEAN, as well as X-ray packages such as CIAO. Teaching is also a passion for Emma, and she enjoys her work as a teaching assistant for undergraduate physics and astronomy classes.

### Jim Williams

Jim earned his B.S. in Computer Science from the University of Michigan and his M.S. in Computer Science from American University. He served as a naval intelligence officer and then, as a civilian, held several computer science and programming jobs in both private industry and government. Among other projects, he wrote a compiler for a new computer language, modeled the effects of nuclear weapons, evaluated options for arms control treaties, and designed a Unix computer network. He retired from the Department of Defense in 2013. As a GMU student, he is interested in using X-rays to study active galactic nuclei (AGNs). His current research is on modeling the soft excess in AGNs.

### Jacqueline Fischer

Jackie Fischer is an astrophysicist, formerly in the Radio/Infrared/Optical Sensors Branch of the Remote Sensing Division and led the Infrared – Submillimeter Astrophysics & Techniques Section. She received a B.Sc. degree in physics from the Hebrew University of Jerusalem and a Ph.D. in astronomy from the State University of New York in Stony Brook. She began at NRL as a National Research Council postdoctoral associate and joined the Laboratory in 1988. She has worked on a number of infrared instrumentation projects: she was the technical manager of the HYDICE Hyperspectral Digital Collection Experiment, led the optical specification team for the ASTROCAM astrometric infrared imager for U.S. Naval Observatory, and was a member of the Infrared Space Observatory ( ISO ) Long Wavelength Spectrometer team. She was appointed as the Herschel Optical System Scientist and as a member of the Herschel Science Team in 2001. Her research interests are in the area of the evolution of galaxies and in particular, on the role that galaxy mergers play in the morphological transformation of galaxies, most recently concerning the structure of the nuclear regions of gas-rich galaxy mergers and in particular on the discovery of massive molecular outflows in gas-rich galaxy mergers and the implications for understanding galaxy feedback in these systems.

### Mario Gliozzi

Dr. Gliozzi’s research activity is focused on the investigation of the physical conditions of matter around black holes based on the analysis and interpretation of X-ray data from different classes of active galactic nuclei (AGN especially Radio Galaxies, Blazars, Narrow Line Seyfert 1 galaxies, and true type 2 AGN). He is interested in the unification of black hole systems at three different levels:

1) The unification among the different classes of AGN , with particular focus on the radio-loud/radio quiet dichotomy

2) The unification between active and normal galaxies , investigated by studying the X-ray nuclear properties of bulgeless galaxies and Low-power AGN , which may represent the link between powerful AGN and normal galaxies.

3) The unification between AGN and Galactic Black Hole systems , with particular emphasis for the variability properties .

Dr. Gliozzi is currently associated with the following research projects: supermassive black holes in bulgeless and low mass galaxies.

### Claudio Ricci

Dr. Ricci is an Assistant Professor at the Nucleo de Astronomia of the Universidad Diego Portales, in Santiago, Chile and a long-term visiting professor at the Kavli Institute for Astronomy and Astrophysics at Peking University, in China, and an affiliated faculty at George Mason University, Fairfax, USA, where his the graduate supervisor of several students.

His research is focussed on the structure and evolution of the material around supermassive black holes, combining X-ray spectroscopy with multi-wavelentgth observations. He is part of the core team of the Swift/BAT AGN Spectroscopic Survey (BASS), and of the science team of the hard X-ray NASA mission NuSTAR.

## X-Ray Properties of Black-Hole Binaries

AbstractWe review the properties and behavior of 20 X-ray binaries that contain a dynamically-confirmed black hole, 17 of which are transient systems. During the past decade, many of these transient sources were observed daily throughout the course of their typically year-long outburst cycles using the large-area timing detector aboard the Rossi X-Ray Timing Explorer. The evolution of these transient sources is complex. Nevertheless, there are behavior patterns common to all of them as we show in a comprehensive comparison of six selected systems. Central to this comparison are three X-ray states of accretion, which are reviewed and defined quantitatively. We discuss phenomena that arise in strong gravitational fields, including relativistically-broadened Fe lines, high-frequency quasi-periodic oscillations (100–450 Hz), and relativistic radio and X-ray jets. Such phenomena show us how a black hole interacts with its environment, thereby complementing the picture of black holes that gravitational wave detectors will provide. We sketch a scenario for the potential impact of timing/spectral studies of accreting black holes on physics and discuss a current frontier topic, namely, the measurement of black hole spin.

## Department of Physics and Astronomy

As a top-tier research university, there's no shortage of opportunities to get hands-on experience. Our physics faculty and students conduct both theoretical and experimental physics both on-campus and at major laboratories around the world.

### Research Groups with Current Faculty

With so many recent, fundamental discoveries (e.g., dark energy, dark matter, black holes, extrasolar planets), our understanding of the size and future of the universe is changing, making this the golden era of astronomy/ astrophysics research.

Astrophysical environments offer unique opportunities to study the behavior of matter under extreme conditions that are often impossible to attain in Earth-based laboratories. To understand the objects and events in the cosmos, astrophysicists combine knowledge from diverse areas of physics, mathematics, statistics, and image processing. Research students in Astrophysics thus obtain a broad and well-rounded education.

The astrophysics group at USC is engaged in research in extragalactic astrophysics and observational cosmology. Our work focuses on quasars, distant galaxies, intergalactic matter, and the evolution of these objects with cosmic time.
Some of the key scientific questions we are trying to address are: how did the cosmic abundances of the chemical elements build up with time? How did the processes of star formation and gas consumption progress in galaxies? How did the structure and shapes of galaxies get established over billions of years?

Our research uses primarily optical, infrared, and ultraviolet facilities, and is funded by the NSF and NASA. We use a wide range of observing facilities in Chile, Hawaii, Arizona, and New Mexico, especially the Magellan Clay telescope, the Very Large Telescope (VLT), the Gemini telescopes, the Keck telescopes, and the Apache Point Observatory (APO), In addition, we use the Hubble Space Telescope and the Spitzer Space Telescope to access parts of the electromagnetic spectrum that are attenuated by the Earth's atmosphere. Our group has attracted several USC students, and includes close collaborators at a number of institutions worldwide.

The condensed matter group has varied activities in the interdisciplinary areas of condensed matter physics, material science, and nanotechnology. We have a group of eight condensed matter people in the department engaged in a wide variety of research projects, some of which is described below.

Yaroslaw Bazaliy is engaged in the theoretical study of the behavior of nanomagnets in the framework of the new research area called spintronics. Spintronics uses spin currents and spin density similar to the way in which the electric current and charge are used in ordinary electronics.

Mas Crawford’s group studies magnetism and magnetic materials, researching new approaches to measure the fundamental properties of magnetic materials, specifically at nanometer length scales and picosecond time scales.

Rick Creswick's research covers both the foundations of statistical physics and condensed matter physics. Currently he is investigating analogues of the famous "spin echo" in systems of charged particles. These systems exhibit a symmetry that allows their time evolution to be reversed, and therefore offers an interesting laboratory in which to study the thermodynamic arrow of time. In collaboration with the particle astrophysics group, he is studying the feasibility of using various materials for low temperature bolometers and the possibility that channeling by ions recoiling from collisions with WIMPs (weakly interacting massive particles) may help reveal the presence of dark matter.

Scott Crittenden’s group works on bacteria that generate electricity as a natural byproduct of metabolism with the purely biological nanowires they produce and on the development of new techniques for scanning probe microscopy to explore material properties at the atomic scale.

Timir Datta’s group projects involve high temperature superconductivity, mesoscopic quantum transport, deterministic chaos, and the effects of disorder in linear and non-linear systems, and experimental measurements of gravity.

Milind Kunchur’s group is involved in two main areas: (1) Phenomena in superconducting nanowires and nanostructured thin films at ultra-short time scales and under extreme conditions. (2) Psychophysics, auditory neurophysiology, and high-fidelity audio.

Yuriy Pershin works in the field of computational/theoretical physics. His current research concerns investigation of charge and spin transport in molecules, semiconductor structures and other submicron electronic devices. He is also interested in different aspects of transport in biological systems.

Yanwen Wu's group investigates the optical properties of nanomaterials. There are three main research directions that 1) characterize and control the coupling dynamics of a hybrid plasmonic/quantum dot system for applications in photonics and optical information processing, 2) study the interaction between the ferroelectric and ferrimagnetic components in multiferroic heterostructure nanowires using second harmonic generation, and 3) functionalize ferroelectric polymers as a dynamic platform to control and manipulate nanomaterials such as quantum dots and 2D materials.

We have built up a significant set of shared use equipment that is officially part of the Nanocenter although most of it is actually in physics faculty laboratories. For major equipment, we have multiple atomic force microscopes, a bacterial fermenter, a confocal microscope, two SEMs, one with 1 nm resolution, a femtosecond pulsed laser, multiple thermal, plasma, and e-gun evaporators, a reactive ion etcher, and multiple low temperature dilution refrigerators.

Interdisciplinary collaboration is common we work with people in Chemistry, Mechanical Engineering, Electrical Engineering, the Medical School, and the History department.

• Yaroslaw Bazaliy
• Thomas M. Crawford
• Richard J. Creswick
• Scott R. Crittenden
• Timir Datta
• James M. Knight
• Milind N. Kunchur
• Yuriy Pershin
• Yanwen Wu

Our group has been at the forefront in addressing some the most fundamental questions in quantum mechanics. Topics for which the group has gained worldwide recognition include non-locality aspects such as the Aharonov-Bohm effect, geometric and topological aspects such as the Aharonov-Anandan phase, and new approaches to quantum measurement such as protective and weak measurements. These efforts have lead to directly testable new predictions and applications in diverse fields such as chemistry, condensed matter physics, elementary particle physics, astrophysics, and cosmology.

The very fundamental particles found in nature are leptons (electrons and their charged cousins, and neutrinos) and quarks. The Standard Model (SM) describes how these interact via the strong and electroweak forces. Our group studies neutrinos which have puzzling behavior unexplained by the Standard Model we also study the nature of the SM forces, with emphasis on searches for new phenomena that may lie beyond.

In particular, we focus on studies of particles that contain the bottom and charm quarks such decays may provide evidence of fundamentally new particles or sources of CP symmetry violation. This and other fundamental research is conducted using data collected by the Belle and Belle II experiments at KEK in Japan.
We also focus on further studies of neutrino physics.

• Sanjib Mishra
• Roberto Petti
• Milind Purohit
• Carl Rosenfeld
• Jeff Wilson

The physics of hadrons and nuclei is based on the strong interaction. There are two experimentally verified perturbative quantum field theories that describe nuclear phenomena: perturbative Quantum Chromodynamics (pQCD) at small distances which is governed by gluon fields and Chiral Perturbation Theory (ChPT) at larger distances which is governed by pion fields. However, the non-abelian nature of the strong interaction gives rise to a non-perturbative "confinement regime" at intermediate distances where more than 98% of the mass of normal matter is generated. A major goal of present day nuclear physics is to understand the connection of the two perturbative regimes and the transition from one to the other. The medium energy nuclear physics group at the University of South Carolina (USC) is devoted to find and carry out the most pressing experiments using electromagnetic probes that broaden our understanding of the nuclear force in the confinement regime. The group's activities are therefore concentrated on in-medium modifications of hadronic properties and baryon spectroscopy. The research program uses multi-GeV photon and electron beams at the Continuous Electron Beam Accelerator Facility (CEBAF) located at the Thomas Jefferson National Accelerator Laboratory (JLab).

One of the fundamental forces in nature is the strong interaction, which is responsible for (among other things) the existence of atomic nuclei. The particles that take part in the strong interaction are called hadrons. The main research theme of the Nuclear Theory Group is the study of hadrons and their aggregates such as nuclei and neutron stars. This line of study is of great interest in its own right, but its importance is further augmented by the following aspects:

It sheds light on the relation between the hadronic phenomena and the underlying fundamental interaction among quarks, which are the basic building blocks of hadrons.
The understanding of many astrophysical phenomena depends on our knowledge of relevant nuclear reactions.
The experimental studies of the fundamental processes (e.g., neutrino oscillation experiments) require input from hadron and nuclear physics for their accurate interpretations.

Currently, a main thrust of our research is directed to the application of effective field theory (known as chiral perturbation theory) to nuclear systems with the view to giving accurate predictions to the cross sections for various hadronic, electromagnetic and weak-interaction processes, in particular for those which are relevant to astrophysics and/or neutrino oscillation experiments.

Particle Astrophysics focuses on phenomena in astrophysics and cosmology associated with the properties of elementary particles ranging from neutrinos to Weakly Interacting Massive Particles (WIMPS), hypothesized as the Cold Dark Matter (CDM). The USC group was early in the field and made the first terrestrial CDM search. CDM is needed to explain the dynamics of galaxies and important features of cosmological models used to explain the evolution of the universe. The gravitational effects of CDM on the velocity distribution of stars in spiral galaxies, is well established. It was motivated by the discovery in 1933 by Fritz Zwicky that far more mass is needed to explain the dynamics of Globular Clusters than appears in stars and dust. In 1985, the USC group, inspired by the astrophysics group at Max Planck Institute in Munich, led the first terrestrial search for the CDM in the Homestake goldmine in Lead, South Dakota. The USC has also led several searches for elementary particles called axions emitted by the sun. Axions result in the theory by Roberto Peccei and Helen Quinn that explains why the strong interactions of quantum chromodynamics, do not violate charge-parity (C-P) symmetry. The USC group now concentrates on the MAJORANA, and CUORE Experiments which are searches for the exotic zero-neutrino nuclear double-beta decay (0νββ − decay) which is only possible if neutrinos have mass and are their own antiparticles (Majorana particles). (0νββ − decay also violates the law of lepton-number conservation. Neutrino oscillation experiments imply that neutrinos may well have enough mass to allow this decay to be measurable, but they can only measure mass differences. The measurement of the decay rate would determine the absolute masses of all three neutrino mass eigenstates.

• Frank T. Avignone III
• Richard J. Creswick
• Vincente E. Guiseppe
• Carl Rosenfeld
• David J. Tedeschi

Theoretical physics group (Altschul, Gudkov, Mazur, and Schindler) is involved with research in several areas of theoretical physics ranging from quantum aspects of gravity and cosmology, neutron physics and CP, P and T non-conservation in nuclear reactions to exotic physics from beyond the standard model such as breaking of Lorentz invariance.

The professor Altschul's research focuses on the possibility of exotic physics beyond the standard model of particle physics such as Lorentz symmetry breaking. He is working on astrophysical tests of relativity. This work concerns obtaining limits on Lorentz invariance violation (LIV) from synchrotron and inverse Compton sources, the limits on neutron LIV from pulsar timing.

Professor Gudkov is working on theoretical problems related to the experimental program in fundamental neutron physics at the SNS such as neutron beta-decay, parity and time reversal violation effects. This work is connected with the search for possible extensions of the standard model. His research subjects range from the applications of a neutron interferometric methods to subjects such as constraining non-Newtonian models of gravity at the nanometer scale, that has emerged as a result of phenomenological applications of string models, to scattering of ultra cold neutrons on nano-size bubbles, neutron beta decay in effective field theory and CP-violation effects in nuclear reactions.

The professor Mazur's research focuses on quantum aspects of gravity and cosmology. His work on quantum mechanics of black holes and black hole thermodynamics has led to the theory of gravastars. Gravastars are ultra-cold and superdense thermally stable (positive heat capacity) macroscopic quantum objects that are the final state of gravitational collapse of matter. One may think of them as quantum superfluid droplets. It is the superfluid nature of gravastars that offers the signature distinguishing them from classical black holes. The present observational methods are reaching the limits that will allow to test the gravastar scenario for the final state of gravitational collapse. Mazur and his collaborators have also discovered the connection between quantum field theories on de Sitter space and conformal field theories (CFT) on the boundary, that is the so-called dS/CFT correspondence. Some applications of the dS/CFT results are the extension of the Harrison, Zeldovich and Peebles-Yu scaling in two-point correlations of the primordial density fluctuations, and in the microwave background radiation, to the general case of three-point and higher correlations.

Professor Schindler is working on problems in hadronic physics related to the strong and weak interactions. He has worked on theoretical methods to describe the properties of single protons and neutrons as well as the interaction between two and more nucleons. His current research focuses on fundamental symmetries in two- and few-nucleon systems. This work is related to ongoing experimental efforts at neutron facilities such as the Spallation Neutron Source at Oak Ridge National Laboratory.

## The Black Hole Files with Camille Carlisle

Avid black hole enthusiast and S&T Science Editor Camille M. Carlisle explores new research on these enigmatic potholes in spacetime. She’ll dive into the behavior of the leviathans that lurk in galaxies’ hearts and sometimes blaze as quasars, analyze the smaller ones that smash together and make LIGO quiver, and even muse on questions like what lies beyond the event horizon and whether there are marauding black holes in the Milky Way. Because who doesn’t love black holes?

## Thanksgiving

Black holes are regions of spacetime where, according to the rules of Einstein’s theory of general relativity, the curvature of spacetime is so dramatic that light itself cannot escape. Physical objects (those that move at or more slowly than the speed of light) can pass through the “event horizon” that defines the boundary of the black hole, but they never escape back to the outside world. Black holes are therefore black — even light cannot escape — thus the name. At least that would be the story according to classical physics, of which general relativity is a part. Adding quantum ideas to the game changes things in important ways. But we have to be a bit vague — “adding quantum ideas to the game” rather than “considering the true quantum description of the system” — because physicists don’t yet have a fully satisfactory theory that includes both quantum mechanics and gravity.

The story goes that in the early 1970’s, James Bardeen, Brandon Carter, and Stephen Hawking pointed out an analogy between the behavior of black holes and the laws of good old thermodynamics. For example, the Second Law of Thermodynamics (“Entropy never decreases in closed systems”) was analogous to Hawking’s “area theorem”: in a collection of black holes, the total area of their event horizons never decreases over time. Jacob Bekenstein, who at the time was a graduate student working under John Wheeler at Princeton, proposed to take this analogy more seriously than the original authors had in mind. He suggested that the area of a black hole’s event horizon really is its entropy, or at least proportional to it.

This annoyed Hawking, who set out to prove Bekenstein wrong. After all, if black holes have entropy then they should also have a temperature, and objects with nonzero temperatures give off blackbody radiation, but we all know that black holes are black. But he ended up actually proving Bekenstein right black holes do have entropy, and temperature, and they even give off radiation. We now refer to the entropy of a black hole as the “Bekenstein-Hawking entropy.” (It is just a useful coincidence that the two gentlemen’s initials, “BH,” can also stand for “black hole.”)

Consider a black hole whose area of its event horizon is . Then its Bekenstein-Hawking entropy is

where is the speed of light, is Newton’s constant of gravitation, and is Planck’s constant of quantum mechanics. A simple formula, but already intriguing, as it seems to combine relativity (), gravity (), and quantum mechanics () into a single expression. That’s a clue that whatever is going on here, it something to do with quantum gravity. And indeed, understanding black hole entropy and its implications has been a major focus among theoretical physicists for over four decades now, including the holographic principle, black-hole complementarity, the AdS/CFT correspondence, and the many investigations of the information-loss puzzle.

But there exists a prior puzzle: what is the black hole entropy, anyway? What physical quantity does it describe?

Entropy itself was invented as part of the development of thermodynamics is the mid-19th century, as a way to quantify the transformation of energy from a potentially useful form (like fuel, or a coiled spring) into useless heat, dissipated into the environment. It was what we might call a “phenomenological” notion, defined in terms of macroscopically observable quantities like heat and temperature, without any more fundamental basis in a microscopic theory. But more fundamental definitions came soon thereafter, once people like Maxwell and Boltzmann and Gibbs started to develop statistical mechanics, and showed that the laws of thermodynamics could be derived from more basic ideas of atoms and molecules.

Hawking’s derivation of black hole entropy was in the phenomenological vein. He showed that black holes give off radiation at a certain temperature, and then used the standard thermodynamic relations between entropy, energy, and temperature to derive his entropy formula. But this leaves us without any definite idea of what the entropy actually represents.

One of the reasons why entropy is thought of as a confusing concept is because there is more than one notion that goes under the same name. To dramatically over-simplify the situation, let’s consider three different ways of relating entropy to microscopic physics, named after three famous physicists:

• Boltzmann entropy says that we take a system with many small parts, and divide all the possible states of that system into “macrostates,” so that two “microstates” are in the same macrostate if they are macroscopically indistinguishable to us. Then the entropy is just (the logarithm of) the number of microstates in whatever macrostate the system is in.
• Gibbs entropy is a measure of our lack of knowledge. We imagine that we describe the system in terms of a probability distribution of what microscopic states it might be in. High entropy is when that distribution is very spread-out, and low entropy is when it is highly peaked around some particular state.
• von Neumann entropy is a purely quantum-mechanical notion. Given some quantum system, the von Neumann entropy measures how much entanglement there is between that system and the rest of the world.

These seem like very different things, but there are formulas that relate them to each other in the appropriate circumstances. The common feature is that we imagine a system has a lot of microscopic “degrees of freedom” (jargon for “things that can happen”), which can be in one of a large number of states, but we are describing it in some kind of macroscopic coarse-grained way, rather than knowing what its exact state actually is. The Boltzmann and Gibbs entropies worry people because they seem to be subjective, requiring either some seemingly arbitrary carving of state space into macrostates, or an explicit reference to our personal state of knowledge. The von Neumann entropy is at least an objective fact about the system. You can relate it to the others by analogizing the wave function of a system to a classical microstate. Because of entanglement, a quantum subsystem generally cannot be described by a single wave function the von Neumann entropy measures (roughly) how many different quantum must be involved to account for its entanglement with the outside world.

So which, if any, of these is the black hole entropy? To be honest, we’re not sure. Most of us think the black hole entropy is a kind of von Neumann entropy, but the details aren’t settled.

One clue we have is that the black hole entropy is proportional to the area of the event horizon. For a while this was thought of as a big, surprising thing, since for something like a box of gas, the entropy is proportional to its total volume, not the area of its boundary. But people gradually caught on that there was never any reason to think of black holes like boxes of gas. In quantum field theory, regions of space have a nonzero von Neumann entropy even in empty space, because modes of quantum fields inside the region are entangled with those outside. The good news is that this entropy is (often, approximately) proportional to the area of the region, for the simple reason that field modes near one side of the boundary are highly entangled with modes just on the other side, and not very entangled with modes far away. So maybe the black hole entropy is just like the entanglement entropy of a region of empty space?

Would that it were so easy. Two things stand in the way. First, Bekenstein noticed another important feature of black holes: not only do they have entropy, but they have the most entropy that you can fit into a region of a fixed size (the Bekenstein bound). That’s very different from the entanglement entropy of a region of empty space in quantum field theory, where it is easy to imagine increasing the entropy by creating extra entanglement between degrees of freedom deep in the interior and those far away. So we’re back to being puzzled about why the black hole entropy is proportional to the area of the event horizon, if it’s the most entropy a region can have. That’s the kind of reasoning that leads to the holographic principle, which imagines that we can think of all the degrees of freedom inside the black hole as “really” living on the boundary, rather than being uniformly distributed inside. (There is a classical manifestation of this philosophy in the membrane paradigm for black hole astrophysics.)

The second obstacle to simply interpreting black hole entropy as entanglement entropy of quantum fields is the simple fact that it’s a finite number. While the quantum-field-theory entanglement entropy is proportional to the area of the boundary of a region, the constant of proportionality is infinity, because there are an infinite number of quantum field modes. So why isn’t the entropy of a black hole equal to infinity? Maybe we should think of the black hole entropy as measuring the amount of entanglement over and above that of the vacuum (called the Casini entropy). Maybe, but then if we remember Bekenstein’s argument that black holes have the most entropy we can attribute to a region, all that infinite amount of entropy that we are ignoring is literally inaccessible to us. It might as well not be there at all. It’s that kind of reasoning that leads some of us to bite the bullet and suggest that the number of quantum degrees of freedom in spacetime is actually a finite number, rather than the infinite number that would naively be implied by conventional non-gravitational quantum field theory.

So — mysteries remain! But it’s not as if we haven’t learned anything. The very fact that black holes have entropy of some kind implies that we can think of them as collections of microscopic degrees of freedom of some sort. (In string theory, in certain special circumstances, you can even identify what those degrees of freedom are.) That’s an enormous change from the way we would think about them in classical (non-quantum) general relativity. Black holes are supposed to be completely featureless (they “have no hair,” another idea of Bekenstein’s), with nothing going on inside them once they’ve formed and settled down. Quantum mechanics is telling us otherwise. We haven’t fully absorbed the implications, but this is surely a clue about the ultimate quantum nature of spacetime itself. Such clues are hard to come by, so for that we should be thankful.

## Contents

Loeb was born in Beit Hanan, [13] Israel in 1962. He took part in the national Talpiot program before receiving a PhD in plasma physics, at age 24, from the Hebrew University in Jerusalem, in 1986. From 1983 to 1988, he led the first international project supported by the U.S. Strategic Defense Initiative. Between 1988 and 1993, Loeb was a long-term member at the Institute for Advanced Study at Princeton, where he started to work in theoretical astrophysics. In 1993, he moved to Harvard University as an assistant professor in the department of astronomy, where he was tenured three years later. [4] [6] [2]

Loeb has written eight books and authored or co-authored about 800 papers on a broad range of research areas in astrophysics and cosmology, [2] [5] including the first stars, the epoch of reionization, the formation and evolution of massive black holes, the search for extraterrestrial life, gravitational lensing by planets, gamma-ray bursts at high redshifts, 21-cm cosmology, the use of the Lyman-alpha forest to measure the acceleration/deceleration of the universe in real time (the so-called "Sandage–Loeb test" [14] ), the future collision between the Milky Way and Andromeda galaxies, [15] the future state of extragalactic astronomy, [16] astrophysical implications of black hole recoil in galaxy mergers, [17] tidal disruption of stars, [18] and imaging black hole silhouettes. [19] [3] In 2010, he wrote a textbook entitled How Did the First Stars and Galaxies Form? [20] [21]

In 1992, Loeb suggested, with Andy Gould, that exoplanets could be detected through gravitational microlensing. In 1993, he proposed the use of the C+ fine-structure line to discover galaxies at high redshifts. In 2005, he predicted, in a series of papers with his postdoc at the time, Avery Broderick, how a hot spot in orbit around a black hole would appear their predictions were confirmed in 2018 by the GRAVITY instrument on the Very Large Telescope which observed a circular motion of the centroid of light of the black hole at the center of the Milky Way, Sagittarius A*. In 2009, Broderick and Loeb predicted the shadow of the black hole in the giant elliptical galaxy Messier 87, which was imaged in 2019 by the Event Horizon Telescope. In 2013, a report was published on the discovery of the "Einstein Planet" Kepler-76b, [22] the first Jupiter size exoplanet identified through the detection of relativistic beaming of its parent star, based on a technique proposed by Loeb and Gaudi in 2003. [23] In addition, a pulsar was discovered around the supermassive black hole, Sagittarius A*, [24] following a prediction by Pfahl and Loeb in 2004. [25] Also, a hypervelocity star candidate from the Andromeda galaxy was discovered, [26] as predicted by Sherwin, Loeb, and O'Leary in 2008. [27] Together with his postdoc, James Guillochon, Loeb predicted the existence of a new population of stars moving near the speed of light throughout the universe. [28] Together with his postdoc John Forbes and Howard Chen of Northwestern University, Loeb made another prediction that sub-Neptune sized exoplanets have been transformed into rocky super-Earths by the activity of Milky Way's central supermassive black hole Sagittarius A*. [29]

Together with Paolo Pani, Loeb showed in 2013 that primordial black holes in the range between the masses of the Moon and the Sun cannot make up the dark matter. [30]

Loeb led a team that reported tentative evidence for the birth of a black hole in the young nearby supernova SN 1979C. [31]

In collaboration with Dan Maoz, Loeb demonstrated in 2013 that biomarkers, such as molecular oxygen ( O
2 ), can be detected by the James Webb Space Telescope (JWST) in the atmosphere of Earth-mass planets in the habitable zone of white dwarfs. [32]

### Early universe Edit

In a series of papers with his students and postdocs, Loeb addressed how and when the first stars and black holes formed and what effects they had on the young universe.

Together with his former student Steve Furlanetto, Loeb published a textbook, The First Galaxies in the Universe. [33]

In 2013, Loeb introduced the new concept of "The Habitable Epoch of the Early Universe", [34] [35] and mentored Harvard undergraduate, Henry Lin, in the study of industrial pollution on exoplanets as a new method to search for extraterrestrial civilizations. [36] In April 2021, Loeb presented an updated summary of his ideas of life in the early universe. [37]

### Panspermia Edit

In 2020, Loeb published a research paper about the possibility that life can propagate from one planet to another, [38] followed by the opinion piece "Noah’s Spaceship" about directed panspermia. [39]

### ʻOumuamua Edit

In December 2017, Loeb cited ʻOumuamua's unusually elongated shape as one of the reasons why the Green Bank Telescope in West Virginia should listen for radio emissions from it to see if there were any unexpected signs that it might be of artificial origin, [40] although earlier limited observations by other radio telescopes such as the SETI Institute's Allen Telescope Array had produced no such results. [41] On December 13, 2017, the Green Bank Telescope observed the asteroid for six hours. No radio signals from ʻOumuamua have been detected. [42] [43]

On October 26, 2018, Loeb and his postdoctoral student Shmuel Bialy submitted a paper exploring the possibility of the interstellar object ʻOumuamua being an artificial thin solar sail accelerated by solar radiation pressure in an effort to help explain the object's non-gravitational acceleration. [44] [45] [46] Other scientists have stated that the available evidence is insufficient to consider such a premise, [47] [48] [49] and that a tumbling solar sail would not be able to accelerate. [50] [51] In response, Loeb wrote an article detailing six anomalous properties of ʻOumuamua that make it unusual, unlike any comets or asteroids seen before. [52] [53]

On November 27, 2018, Loeb and Amir Siraj, an undergraduate student at Harvard College, proposed a search for ʻOumuamua-like objects which might be trapped in the Solar System as a result of losing orbital energy through a close encounter with Jupiter. [54] They identified four candidates (2011 SP25, 2017 RR2, 2017 SV13, and 2018 TL6) for trapped interstellar objects which could be visited by dedicated missions. The authors pointed out that future sky surveys, such as with Large Synoptic Survey Telescope, could find many more. [55]

In public interviews and private communications with reporters and academic colleagues, Loeb has become more and more vocal regarding the prospects of proving the existence of alien life. [56]

On April 16, 2019, Loeb and Siraj reported the discovery of the first meteor of interstellar origin. [12]

In 2006, Loeb was featured in a Time magazine cover story on the first stars, and in a Scientific American article on the Dark Ages of the universe. In 2008, he was featured in a Smithsonian magazine cover story on black holes, and in two Astronomy magazine cover stories, one on the collision between the Milky Way and the Andromeda Galaxy and the second on the future state of our universe. In 2009, Loeb reviewed in a Scientific American article a new technique for imaging black hole silhouettes. Loeb received considerable media attention [61] after proposing in 2011 (with E.L. Turner) a new technique for detecting artificially-illuminated objects in the Solar System and beyond, [62] and showing in 2012 (with I. Ginsburg) that planets may transit hypervelocity stars or get kicked to a fraction of the speed of light near the black hole at the center of the Milky Way. [63]

Science magazine published an article about Loeb's career in April 2013, [64] and Discover reviewed his research on the first stars in April 2014. [65] The New York Times published a science profile of Loeb in December 2014. [66] In May 2015, Astronomy posted a podcast of an hour-long interview with Loeb in its series entitled "Superstars of Astronomy". [67] In April 2016, Stephen Hawking visited Loeb's home and attended the inaugurations of the Starshot and Black Hole Initiatives that Loeb leads. [68]

Loeb's eBook on Kindle details his career path from childhood on a farm with interests in philosophy to chairing the Harvard astronomy department and directing the ITC, and includes opinion essays on the importance of taking risks in research and promoting diversity. Loeb writes opinion essays on science and policy. [69] [70]

On January 14, 2021 Loeb appeared on the Lex Fridman Podcast (#154), [71] on January 16, 2021, on the Joe Rogan Experience podcast (#1596). [72]