GW170817 and Multi-Messenger Astronomy Part 2

This is the second part of a two-part post. For the first part, see here.

The previous post described the gravitational wave event GW170817 (which took place on August 17, 2017) and how it was ultimately identified as a binary neutron star merger. In addition, it was associated with a gamma ray burst (designated GW170817) and imaged across the electromagnetic spectrum, an unprecedented and landmark event in the field of multi-messenger astronomy. Though it is intrinsically of interest to be able to both "see" (EM waves) and "hear" (gravitational waves) an astrophysical event, what are some other conclusions to be drawn from the merger?

One simple conclusion requires nothing more than a quick calculation, but verifies a foundational principle of physics that while almost universally assumed, had never been directly proven. This principle states that both electromagnetic waves and gravitational waves travel at the speed of light in a vacuum, about 3*10⁸ m/s. Recall that the merger is estimated to have taken place about 130 million light-years away. This means that both the gravitational wave signal and the gamma ray burst both took about 130 million years to travel from the source to detectors on Earth. Despite their long journey, they arrived within a few seconds of each other. Now, we cannot be certain exactly when the gamma ray burst was emitted, due to our incomplete understanding of how a binary neutron star merger would work. However, it is likely that the neutron stars must first collide (marking the end of the gravitational wave signal) before emitting a burst of gamma radiation. Moreover, this initial high-energy burst was estimated by most models to occur no more than a few minutes after the merger. Therefore, dividing the amount by which the signals could have drifted apart over their travel time, we obtain bounds on the "speed of gravity" relative to the speed of light. Even with conservative assumptions, these observations prove that the two speeds very likely differ by no more than one part in a trillion (0.0000000001%) and probably several orders of magnitude less than this. Theoretically, they are equal, but never before has this been measured with such incredible precision.

In a similar vein, the merger allowed other tests of various aspects of general relativity and field theory, such as the influence of gravitational waves on the propagation of electromagnetic field. The data all confirmed the current understanding of general relativity and set very tight bounds on possible deviations, better than those ever achieved in the past.

The detection of the merger also taught us about the very structure of neutron stars. Unlike black holes, which (to our knowledge) are effectively points of mass, neutron stars are on the order of a few miles across. Considering their mass (usually 1-2 solar masses), they are exceedingly dense, but nevertheless their physical size affects how the gravitational wave event unfolds. When the two objects get very close to one another, their mutual gravitational attraction is expected to cause tidal deformations, i.e. warping of their shape and mass distribution. In theory, information concerning the deformation is encoded in the measured waveforms.

The figure above (click to enlarge), while rather technical, gives some idea as to how exactly the gravitational wave data constrain the structure of the neutron star. The statement |χ| < 0.05 in the diagram indicates that the entire figure is made presupposing that the neutron stars were not spinning too fast (which our knowledge of neutron star systems suggests is a very reasonable assumption). The two axes measure the magnitude of two parameters Λ₁ and Λ₂ that measure how much the larger and the smaller neutron stars, respectively respond to tidal deformation. In other words, smaller values of the parameters (toward the lower left) mean denser and more compact neutron stars, as indicated. More on what these parameters actually mean can be found in the original paper here.

Next, the darker shades of blue represent values considered more likely given the shape of the gravitational wave signal. This is a probability distribution, and lighter shaded areas were not ruled out with certainty, but simply deemed less likely. The uncertainty in the original masses contributes to the uncertainty in this diagram. Finally, the gray shaded "stripes" indicate the predictions of several different theoretical models of neutron stars. These are distinguished by their different equations of state, which specify how mass, pressure, density, and other properties of neutron stars relate to one another. The varying predictions of these models show just how little was definitively known about neutron stars. Analysis of the merger event suggested that the "SLy" and "APR4" models were more accurate than the rest (at 50% confidence) and that the "MS1" and "MS1b" models are unlikely to be correct (with more than 90% confidence). No model was ruled out for sure, but the data above suggest that neutron stars are more compact than most models predicted.

The gamma ray burst that followed the merger also contained some information concerning how these mergers actually occur and the physics of when and why high-energy radiation is released. Notably, the gamma ray burst was a single short pulse (lasting under a second) with no discernible substructure. It was difficult to draw conclusions from this limited sample, but explaining the nature of the pulse and the delay may require a dense layer of ejecta from each of the neutron stars to momentarily impede electromagnetic radiation from the merger. It would take some time for gamma rays to penetrate this cloud of debris until they finally burst through.

Moreover, among the known population of gamma ray bursts, GW1701817A was relatively dim. This may have been due to the main "jets" of energy not being along our line of sight; most of the burst is hypothesized to have been released along the original axis of rotation of the two bodies. The discrepancy may in part have been due to observational bias, since brighter events are more likely to be observed. In such cases, the Earth likely was directly facing the angle of peak gamma ray emission. Detecting an event "off-center" elucidates somewhat the structure and extent of the these jets.

The image above (click to enlarge) was originally from this paper. It demonstrates schematically some different theories explaining the relative dimness of the gamma ray burst event and the structure of the jets along the axis of rotation. Earlier theories postulated a relatively uniform jet, as shown in the first scenario. If this is the case, our line of sight may have been outside the jet, but relativistic effects allowed us to see a smaller amount of the radiation. Other explanations postulate that the jet has some internal structure and "fades" with increasing angle from the axis (ii) or that the interaction of the jet with surrounding matter produces a secondary cocoon of radiation (iii). A final scenario is simply that this event was a few orders of magnitude dimmer than other known gamma ray bursts for some intrinsic reason, although the authors deem this unlikely.

Without the background information provided by the gravitational wave signal (the component masses of the merger, the timing of the merger, etc.), little of the above could be gleaned from the gamma ray signal. Nor would it be possible with only one of the two to conduct the precision tests of fundamental physics described earlier. These are examples of the power of multi-messenger astronomy. Having both an eye and an ear to the cosmos will continue to yield fundamental insights into the nature of our universe.

Sources: https://journals.aps.org/prl/pdf/10.1103/PhysRevLett.119.161101, https://arxiv.org/pdf/1710.05834.pdf, http://iopscience.iop.org/article/10.3847/2041-8213/aa91c9/pdf

GW170817 and Multi-messenger Astronomy

The first astronomers had only their own eyes as tools, and visible light was their only source of information. Recent instruments have broadened our sight to include all types of electromagnetic radiation, from radio waves to X-rays and gamma rays. Each part of the spectrum is suited to different types of observations and gave us incredible new insight into the cosmos. However, the second decade of the 21st century saw the advent of a fundamentally new kind of astronomy: the detection of gravitational waves.

Gravitational waves, as discussed in a previous post, are the "ripples" in spacetime that propagate in response to the acceleration of massive objects (stars, black holes, and the like). All objects with mass produce these waves, but the vast majority are far too small to detect. It was only with the advent of extremely sensitive instruments that the first detection of gravitational waves was made by LIGO (the Laser Interferometer Gravitational-Wave Observatory) in 2015. This detection, and its immediate successors, were of binary black hole merger events, in which two black holes orbiting one another spiraled inwards and finally combined into a single, larger black hole. The last moments before merging brought exceptionally colossal objects (weighing perhaps dozens of solar masses) to great accelerations, the perfect recipe for producing strong gravitational waves detectible across the cosmos. However, these cataclysmic events were quite dark: little electromagnetic radiation was emitted, and no "visual" evidence for these events accompanied the wave signal. Something quite different occurred in 2017.

On August 17, 2017 at 12:41 UTC, the LIGO detectors at Hanford, Washington and Livingston, Louisiana and the Virgo gravitational wave detector in Italy simultaneously measured an event as shown above (click to enlarge). The two LIGO frequency-time diagrams clearly show a curve that increases in frequency before disappearing at time 0. This corresponds to two inspiraling objects orbiting one another faster and faster before merging finally occurs and the signal stops. In the Virgo diagram, the same line is not very visible, but further analysis of the data nevertheless expose the same signal from the noise. The gravitational wave event, designated GW170817, was genuine.

Having three detectors at different points on the Earth measure the event allowed a better triangulation of the location of the source than had occurred previously (when LIGO and Virgo were not simultaneously active).

The above figure shows a visualization of the celestial sphere (representing all possible directions in the sky from which the signal could have come) and locations from which the signal data suggest the signal originated. The green zone is the highest probability region taking all three instruments into account. This area is still 31 square degrees, quite large by astronomical standards. Fortunately, corroboration of the event came immediately from an entirely separate source.

The above figure (click to enlarge) shows at the bottom the same gravitational wave signal from before. The rest of the data come from the Fermi Gamma-ray Space Telescope and the International Gamma Ray Astrophysics Laboratory, both satellites in Earth orbit. As their names suggest, they search the cosmos for astrophysical sources of high-energy gamma rays. In particular, they monitor the cosmos for gamma-ray bursts (GRBs), especially intense flashes of radiation that typically accompany only the most explosive events, such as supernovae. As the figure shows, less than two seconds after the gravitational wave signal stopped (indicated the merger of two orbiting objects), there was an elevated count of gamma rays in each detector across the different photon energy levels. The source of this burst is indicated by a reticle in the celestial sphere figure above, lying right within the estimated location of the merger! It appeared that this merger had an electromagnetic counterpart! Further, analysis of the gravitational waves indicated that the masses of the two objects were around 1.36-2.26 and 0.86-1.36 solar masses (these were the uncertainty ranges), respectively, not heavy enough for black holes. What was going on?

The conclusion drawn from these events was that the merger was not of black holes, but of neutron stars, compact remnants of large stars that were yet not massive enough to collapse into black holes. An artist's conception of a binary neutron star black hole merger is shown above. Following the initial identification of the event, countless telescopes around the world trained on the event the very same day after a notice was released around 13:00 UTC, hoping to observe more following the merger.

And they were not disappointed. Less than a day after the initial gamma ray burst had faded, the source began to appear at other frequencies, and remained bright for several weeks before fading. The above figure shows the Hubble image of the merger's host galaxy, NGC 4993. This galaxy is at a distance of roughly 130 million light-years, and even at this distance, the collision of the neutron stars was clearly visible against the billions of other stars. Finally, the chart below demonstrates just how well documented the event was:

Many different instruments took images in X-rays as well as ultraviolet, visible, infrared, and radio waves. The horizontal axis indicates the rough timeline of events (on a logarithmic scale) in each part of the electromagnetic spectrum, stretching from less than a day to several weeks after the merger. Several representative images of NGC 4993 and the source within are shown at bottom.

Without extensive collaboration within the astronomical community, collecting this wealth of data on this binary neutron star merger would not have been possible. This marked the first time in history that a single event was measured in both gravitational waves and electromagnetic waves, not to mention how thoroughly the merger was photographed across the spectrum. This coordinated observation is known as multi-messenger astronomy, and may have profound implications on our future understanding of the universe. Some of what we learned from the binary neutron star merger is discussed in the next post.

Note: Most of the figures above are taken from the open access papers detailing the discovery and analysis of the binary neutron star merger. For further reading on the event, links to these papers may be found in the sources below.

Sources: https://journals.aps.org/prl/pdf/10.1103/PhysRevLett.119.161101, https://arxiv.org/pdf/1710.05834.pdf, http://iopscience.iop.org/article/10.3847/2041-8213/aa91c9/pdf

Neutrinos and Their Detection 2

This is the second part of a two part post. For the first part, see here.

The discovery of neutrinos led to a rather startling realization concerning the omnipresence of these particles. Scientists have known since the early 20th century that stars such as the Sun generate energy through nuclear fusion, especially of hydrogen into helium. In addition to producing radiation that eventually leads to what we see as sunlight, every one of these reactions releases neutrinos. As a result, the Earth is continually bathed in a stream of neutrinos: every second, billions of neutrinos pass through every square centimeter of the Earth's surface. A vast, vast majority of these pass through the planet unimpeded and resume their course through space, just as discussed in the previous post. As we will see, studying the properties of these solar neutrinos later led to an revolutionary discovery.

In 1967, an experiment began that had much in common with many of the neutrino experiments to come. Known as the Homestake experiment after its location, the Homestake Gold Mine in South Dakota, the main apparatus of the experiment was an 100,000 gallon tank of perchloroethylene (a common cleaning fluid) located deep underground, nearly a mile below the Earth's surface. The purpose of holding the experiment underground was to minimize the influence of cosmic rays, which would react with the perchloroethylene and produce experimental noise. Cosmic rays do not penetrate deep underground, however, while neutrinos do. The immense volume of liquid was necessary to obtain statistically significant data from the small rate of neutrino interactions. The number of argon atoms that were produced through the reaction was measured to determine how many reactions were occurring.

Simultaneously, physicists made theoretical calculations using knowledge of the Sun's composition, the process of nucleosynthesis, the Earth's distance from the Sun, and the size of the detector to estimate what the rate of interactions should have been. However, the results were not consistent with the data collected from the experiment. Generally, theoretical estimates were around three times as large as the actual results. Two-thirds of the expected reactions were missing! This disagreement became known as the "solar neutrino problem."

The models of the Sun were not at fault. In fact, the cause of the problem was an incorrect feature of the otherwise quite powerful Standard Model of Particle Physics, namely that neutrinos have mass. As far back as 1957, Italian physicist Bruno Pontecorvo considered the implications of neutrinos having mass.

He and others realized that neutrinos with mass would undergo what is known as neutrino oscillation when traveling through space. For example, an electron neutrino emitted from nuclear fusion would become a "mix" of all three flavors of neutrinos: electron, muon, and tau. When a solar neutrino reaches Earth and interacts with matter, it only has roughly a 1 in 3 chance of "deciding" to be an electron neutrino. This would explain the observed missing neutrinos, since the Homestake detector only accounts for electron neutrinos.

For the remainder of the 20th century, several more experiments were performed to investigate whether neutrino oscillation was in fact the solution to the solar neutrino problem. One experiment that was crucial in conclusively settling the matter was Super-Kamiokande, a neutrino observatory located in Japan. Like the Homestake experiment, it was located deep underground in a mine and consisted of a large volume of liquid (in this case, water).

When neutrinos interact with the water molecules in the detector, charged particles are produced that propagate through the chamber. These release radiation which is amplified and recorded by the photomultipliers that surround the water tank on every side. The number of photomultipliers allows a more detailed analysis of this radiation, yielding the energy and direction of origin for each neutrino interaction. It was this added precision that helped to resolve the solar neutrino problem: neutrinos indeed have mass and undergo oscillation. This discovery led to Japanese physicist Takaaki Kajita (who worked on the Super-Kamiokande detector as well as its predecessor the Kamiokande detector) sharing the 2015 Nobel Prize in Physics.

The exact masses of the different flavors of neutrinos are not yet known, nor do we completely understand why they have mass. However, despite the mysteries of particle physics that remain, further applications of neutrino detection continue in a different field: astronomy. The use of neutrinos to observe extraterrestrial objects is known as neutrino astronomy. In theory, if one could accurately measure the direction from which every neutrino arrives at Earth, the result would be an "image" of the sky highlighting neutrino sources. In reality, the scattering that occurs in detectors such as Super-Kamiokande when incoming particles hit and change direction limits angular resolution and so few interactions occur that there are insufficient samples to construct such an image. Only two extraterrestrial objects have ever been detected through neutrino emissions, in fact: the Sun, and a nearby supernova event, known as SN1987a after the year in which it took place. Theoretical calculations indicate that sufficiently bright supernovae may be located with reasonable accuracy using neutrino detectors in the future.

There is one major advantage to using neutrinos as opposed to light in making observations: neutrinos pass through nearly all matter unimpeded. The above discussion indicated that the Sun is a neutrino source. This is true, but not fully precise; the solar core is the source of the neutrinos, as it is where fusion occurs, and its radius is only about a quarter of the Sun's. There is no way to see the light emanating from the core because it interacts with other solar particles. However, we can see the core directly through neutrino imaging. In fact, the data from the Super Kamiokande experiment should be enough to approximate the radius in which certain fusion reactions take place. Future detectors could tell us even more about the Sun's interior.

Neutrino astronomy is still a nascent field and we do not yet know its full potential. Further understanding and detection of neutrinos will tell us more about the fundamental building blocks of matter, allow us to peer inside our own Sun, and measure distant supernovae.

Sources: http://www.sns.ias.edu/~jnb/SNviewgraphs/snviewgraphs.html, https://arxiv.org/pdf/hep-ph/0410090v1.pdf, http://slideplayer.com/slide/776551/, https://www.bnl.gov/bnlweb/raydavis/research.htm, https://arxiv.org/pdf/hep-ph/0202058v3.pdf, https://j-parc.jp/Neutrino/en/intro-t2kexp.html, https://arxiv.org/pdf/1010.0118v3.pdf, https://www.scientificamerican.com/article/through-neutrino-eyes/, https://arxiv.org/pdf/astro-ph/9811350v1.pdf, https://arxiv.org/pdf/1606.02558.pdf

Neutrinos and Their Detection

Neutrinos are a type of subatomic particle known both for their ubiquity and their disinclination to interact with other forms of matter. They have zero electric charge and very little mass even compared to other fundamental particles (though not none, more on this later) so they are not affected by electromagnetic forces and only slightly by gravity.

Since neutrinos are so elusive, it is not surprising that their existence was first surmised indirectly. In 1930, while studying a type of radioactive decay known as beta decay, physicist Wolfgang Pauli noticed a discrepancy. Through beta decay (shown above), a neutron is converted into a proton. This is a common process by which unstable atomic nuclei transmute into more stable ones. It was known that an electron was also released in this process. However, Pauli found that this left some momentum unaccounted for. As a result, he postulated the existence of a small, neutral particle (this properties eventually led to the name "neutrino"). The type emitted in this sort of decay is now known as an electron antineutrino (all the types will be enumerated below).

However, they were speculative for some decades before a direct detection occurred in 1956 in the Cowan-Reines Neutrino Experiment, named after physicists Clyde Cowan and Frederick Reines.

The experiment relied upon the fact that nuclear reactors were expected to release a large flux of electron antineutrinos during their operation, providing a concentrated source with which to experiment. The main apparatus of the experiment was a volume of water that electron antineutrinos emerging from the reactor would pass through. Occasionally, one would interact with a proton in the tank, producing a neutron and a positron (or anti-electron, denoted e⁺) through the reaction shown on the bottom left. This positron would quickly encounter an ordinary electron and the two would mutually annihilate to form gamma rays (γ). These gamma rays would then be picked up by scintillators around the water tanks. To increase the certainty that these gamma ray signatures in fact came from neutrinos, the experimenters added a second layer of detection by dissolving the chemical cadmium chloride (CdCl) in the water. The addition of a neutron (the other product of the initial reaction) to the common isotope Cd-108 creates an unstable state of Cd-109 which releases a gamma ray after a period of a handful of microseconds. Thus, the detection of two gamma rays simultaneously and then a third after a small delay would definitively indicate a neutrino interaction. The experiment was very successful and the rate of interactions, about three per hour, matched the theoretical prediction well. The neutrino had been discovered.

The Standard Model of particle physics predicted the existence of three "generations" of neutrinos corresponding to three types of particles called leptons.

The above diagram shows the three types of leptons and their corresponding neutrinos. In addition to this, every particle type has a corresponding antiparticle which in a way has the "opposite" properties (though some properties, such as mass, remain the same). The electron antineutrino discussed above is simply the antiparticle corresponding to the electron neutrino, for example. The discoveries of the others occurred at particle accelerators, where concentrated beams could be produced: the muon neutrino in 1962, and the tau neutrino in 2000. These results completed the expected roster of neutrino types under the Standard Model. In its original form, though, the Standard Model predicted that all neutrinos would have exactly zero mass. Note that this hypothesis (though later proved incorrect) is not disproven by the fact that neutrinos account for the "missing momentum" Pauli originally identified; massless particles, such as photons (particles of light), can still carry momentum and energy.

All of the neutrino physics described so far concerns artificially produced particles. However, these discoveries were only the beginning. Countless neutrinos also originate in the cosmos, motivating the area of neutrino astronomy. For more on this field and its value to both astronomy and particle physics, see the next post (coming January 22).

Sources: http://www.astro.wisc.edu/~larson/Webpage/neutrinos.html, http://hyperphysics.phy-astr.gsu.edu/hbase/particles/cowan.html, https://perimeterinstitute.ca/files/page/attachments/Elementary_Particles_Periodic_Table_large.jpghttp://www.scienceinschool.org/sites/default/files/articleContentImages/19/neutrinos/issue19neutrinos10_xl.jpg, http://www.fnal.gov/pub/presspass/press_releases/donut.html

The Detection of Gravitational Waves

For an introduction to gravitational waves, see here.

Before 2016, a nobel prize had already been rewarded for an observation that was consistent with, and seemed to confirm, the existence of gravitational waves. In 1974, Russell Hulse and Joesph Taylor discovered a very compact binary system of objects at a distance of 21,000 light years, consisting of two neutron stars orbiting one another. One of the bodies was also a pulsar, meaning that the radiation beams emitted from its poles periodically point toward Earth as it rotates. Since the rotation rate of a neutron star changes only very slowly over time, pulsars are fairly precise clocks. However, Hulse and Taylor detected that the pulses did not reach Earth precisely on time, but varied slightly from the expected arrival time. They were sometimes sooner, sometimes later in a regular pattern, indicating that the pulsar in question was in fact part of a binary system.

The above diagram depicts the binary system consisting of pulsar B1913+16 and its companion, another neutron star. No radiation from the companion has been observed on Earth, indicating that its poles oriented away from us. However, its presence can be inferred from the fact that the pulsar moves farther and closer to Earth in a short, regular period, indicating an orbit. The difference in arrival times is about 3 seconds, indicating that the orbit is about 3 light-seconds across. Further, the orbital period is 7.75 hours.

This discovery provided an excellent opportunity to confirm the predictions of general relativity: such a compact system with rapidly orbiting masses would radiate fairly large quantities of gravitational radiation. However, direct detection was well beyond 1970's technology. Instead, Taylor observed the pulsar system over a number of decades, and found the following:

Since the discovery of the pulsar, its orbital period had been decreasing very slowly, though steadily and measurably, by about 35 seconds over a timespan of 30 years. This is very little relative to the total period of 7.75 hours, but the data matched the predictions of general relativity almost precisely: as energy was lost to gravitational waves, the neutron stars gradually spiral inward toward one other as their orbits becoming shorter and shorter. This remarkable confirmation of a prediction of relativity won Hulse and Taylor the Noble Prize in physics in 1993.

And there the matter sat. Though detectors grew more and more advanced, no direct detections of gravitational waves were made for over 20 years. This all changed in 2015.

On September 14, 2015, at 09:50:45 UTC, shortly after LIGO (the Laser Interferometer Gravitational-Wave Observatory) resumed activity following an upgrade, the two detectors in Washington State and Louisiana picked up a transient gravitational wave signal, the first ever observed by humankind. The announcement of the discovery was made several months later, on February 11, 2016.

The above image shows the signals recorded at Hanford, Washington (left) and Livingston, Louisiana (right). The signals are also superimposed on the right to demonstrate their similarity. The horizontal axis is time, measured relative to 09:50:45 UTC on that day. The reader may notice that the event was distinguishable from the surrounding noise in the detector for only about 0.05 seconds (the third row charts the residual noise after the theoretical waveform in the second row is subtracted out). The final row shows the rapid increase in gravitational wave amplitude during the event and the subsequent silence. The vertical dimension in the first several rows is the relative strain on the detectors, or the amount by which the different arms of LIGO were stretched or compressed by the ripples in spacetime. The scale for these axes measures strain by parts in 10^-21. This corresponds to extraordinarily minute changes in length: the 4 kilometer arms of the LIGO detector changed by only about 10^-18 meters, only about one thousandth the diameter of a proton!

The theoretical wave form above was a simulation of the event that generated the gravitational waves: the final in-spiraling and ultimate merging of two black holes. The increasing frequency and amplitude of the signals corresponds to the final moments of the collapsing system as the two black holes orbit faster and faster and tighter and tighter around one another before finally combining. Further, the signals at the two detectors were separated by 6.9 ms, smaller than the light travel time between the sites of 10 ms. The delay between the arrival times allows the direction of the source to be identified.

This image shows the region in the sky from which the signals likely originated. The colors indicate the confidence that the source lay within the indicated region: purple is the 90% confidence region and yellow the 50% confidence region. The uncertainty arises from the fact that there were two detectors, and not the three required for a full triangulation.

In addition to the location of the source, the analysis of the waveform yields more. The distance of the system was roughly 1.2 billion light-years, meaning that the merger that we are just now observing occurred over a billion years ago. The two black holes had respective masses of about 36 and 29 solar masses, while the final black hole after the merger weighed in at 62 solar masses. This corresponds to a loss of about 3 solar masses, which was all converted into energy released as gravitational waves as the holes merged. The magnitude of this cataclysm can scarcely be overstated: at its peak, the rate of energy release was an estimated 3.6x10⁴⁹ W, greater than the radiation emitted from all stars in the observable universe combined!

In addition to being a resounding confirmation of general relativity, the observation was the first truly direct detection of black holes: the fact that such massive objects came within hundreds of kilometers of one another indicates that they had extremely high densities, densities only possible in black holes. But while significant, cosmologists were already nearly certain that both gravitational waves and black holes existed. However, this discovery marks the opening of a brand new field of astronomy. Gravitational waves, which pass unimpeded through nearly anything over nearly any distance, allow us to "hear" cosmic events that we could not have detected before. In theory, these waves could allow us to observe the earliest stages of the universe, before it became transparent to electromagnetic radiation. In 2016, 100 years after Einstein predicted gravitational waves, we took the first step towards seeing the universe in a new way.

Sources: http://articles.adsabs.harvard.edu/cgi-bin/nph-iarticle_query?1989ApJ...345..434T&data_type=PDF_HIGH&whole_paper=YES&type=PRINTER&filetype=.pdf, https://journals.aps.org/prl/pdf/10.1103/PhysRevLett.116.061102, http://resources.edb.gov.hk/physics/articlePic/InterestingTopics/BinaryStars_pic04E.gif

Cosmic Rays 3

This is the final post in a series on cosmic rays. For the first, see here.
We began by showing another graph of the cosmic ray flux at various energies.

It so happens that this graph very nearly follows an inverse cube relationship (which appears straight on log-log graphs). That is, cosmic rays with double the energy are about eight times rarer in the cosmos. The above diagram also shows the small deviations from the inverse cube relationship that occur at high densities, known as the "knee" and the "ankle".

The first major deviation from the inverse cube law, the so-called "knee", occurs around 10¹⁵ eV. Theoretically, this is near the maximum energy that a charged particle could have and still be contained in the Milky Way by the galaxy's galactic halo. The Earth receives a slightly larger flux of these energies than expected because particles slowed to this energy remain in the Milky Way while higher energy particles do not. The transition to extragalactic particle origins forms the "ankle" of the graph.

Extragalactic Cosmic Rays

Cosmic rays with energies more than about 10¹⁵ electron volts seldom, if ever, originate in our galaxy. Such cosmic rays could originate from particularly powerful supernovae, or, accelerated by the same mechanism of magnetic shock waves, from two colliding galaxies. However, it is likely that most cosmic rays originate in active galactic nuclei, such as quasars.

The above is an artist's rendering of a quasar. As the name suggests, quasars are very similar to microquasars, except much larger (similar features include the accretion disk and relativisitic jets, both illustrated above). The compact object involved, rather than being a neutron star or stellar black hole, is a supermassive black hole, the kind found at the center of most galaxies (including the Milky Way). Such black holes often exceed one million solar masses.

The characteristic property of active galactic nuclei is that they consume matter at an extraordinarily rapid rate. As a result, the energy and radiation released far outshine the rest of the galaxy. In fact, many quasars are hundreds or thousands of times more luminous than our entire Milky way, corresponding to a luminosity sometimes exceeding a trillion suns. However, over time, the supermassive black hole consumes all nearby matter, and the galaxy becomes dormant, becoming in many respects like our own. For this reason, quasars tend to be young galaxies, and thus the ones we see tend to be quite distant (that is, we see them as they were a long time ago). Even the nearest quasars are still billions of light-years away.

Cosmic rays from these distant quasars may have energies up to about 5x10¹⁹ eV (50 million trillion eV). However, this value is the theoretical upper-limit for the energy of cosmic rays from distant sources, also known as the Greisen-Zatsepin-Kuzmin Limit (GZK limit). Computed independently in the 1960's by Kenneth Greisen, Georgiy Zatsepin, and Vadim Kuzmin, the GZK limit comes about from the interactions between cosmic ray particles and the cosmic background radiation (see also here), the leftover radiation from shortly after the Big Bang which permeates the Universe. In theory, if cosmic rays with higher energy travel a sufficient distance, interactions with the cosmic background radiation will reduce them to the GZK limit.

However, this is not the end of the story. It has been confirmed that cosmic rays with even higher energies have hit the Earth, though such events are rare: cosmic rays with energy exceeding the GZK limit hit a given square kilometer of the atmosphere only about once per decade. The most commonly accepted explanation for these particles is that they have not yet traveled far enough for the cosmic background radiation to slow them down, and therefore come from sources within about 200 million light-years.

Recently, scientists have proposed that quasar remnants (galactic nuclei that were formerly active) could eject such particles, though being relatively quiet in electromagnetic emissions. Unlike quasars, so-called "retired quasars" are very common in our region of the cosmos, some within 100 million light-years. Very massive, rotating supermassive black holes are the most probable candidates for the origin of these cosmic rays.

The elliptical galaxy M60 (above) is one plausible source for cosmic rays with energies exceeding the GZK limit. The galaxy is only about 55 million light-years away, and harbors a supermassive black hole of 4.5 billion solar masses, among the largest known.

Likely the current record-holder for the most energetic cosmic ray ever observed was aptly named "Oh-My-God" particle. This particle (most likely a proton) was observed in Utah on October 15, 1991. It had an energy of approixmately 3x10²⁰ eV, roughly six times the GZK limit. Since this energy is about 50 J, it is comparable to the kinetic energy of a pitched baseball, all in a single particle! Upon Earth impact, this particle was traveling at roughly 99.99999999999999999999951% of the speed of light. However, the Oh-My-God particle, and other similar cosmic rays, are no threat to Earth or its indigineous life, due to their infrequency and their interactions with the atmosphere before reaching Earth's surface.

Nevertheless, studying the composition and energy of cosmic rays is very important to astrophysics. By observing cosmic rays in addition to collecting images using electromagneitc radiation, we obtain a more complete picture of our Universe.

Sources: http://www.cosmic-ray.org/reading/uhecr.html, http://www.boseinst.ernet.in/capss/doc/cosmic_primer.pdf, http://www.spaceflightnow.com/news/n0204/23quasars/, http://imagine.gsfc.nasa.gov/docs/features/news/22apr02.html, http://en.wikipedia.org/wiki/Oh-My-God_particlehttp://astronomy.swin.edu.au/cms/astro/cosmos/g/Greisen-Zatsepin-Kuzmin+Limit, http://arxiv.org/abs/0910.4168, http://science.gsfc.nasa.gov/662/boldt/BoldtSymp_Loewenstein.pdf

Cosmic Rays 2

This is the second part of a two-part post about cosmic rays. For the first part, see here.

The first post dealt primarily with the composition of cosmic rays and the abundance of different types. The other major characteristic of cosmic rays is their energy.

The figure above illustrates the abundance of cosmic rays of different energies on a logarithmic scale. The x-axis shows the energy of cosmic rays in electron volts. An electron volt (eV) is a unit of energy, defined to be the energy required to move an electron through an electric potential of 1 volt. Protons, for example, the most common type of cosmic ray, have a rest energy of 9.38x10⁸ eV, at the bottom end of the chart above. Even at rest, protons are considered to have energy given by the mass-energy relationship E = mc². Protons (and other cosmic rays such as atomic nuclei) of larger velocity have even greater energies.

The y-axis of the chart indicates cosmic ray flux. Flux, generally speaking, refers to the quantity of something passing through a given surface. In this case, the flux refers to the number of cosmic rays of a certain energy passing through a given area of the Earth's atmosphere. For example, (as labeled on the graph) "1 m^-2 s^-1" indicates that one cosmic ray passes through each square meter of the Earth's atmosphere (viewed as a spherical surface) every second.

The blue line indicates the relationship between the energy of cosmic rays and the frequency with which they impact Earth (measured by flux). The fact that the curve is decreasing represents that higher energy cosmic rays are rarer. Uncertainty in the values causes the curve to widen for higher energies.

Finally, the graph is separated into three sections, indicating the typical origins of cosmic rays of different energies. The leftmost zone, the yellow zone, refers to solar cosmic rays, i.e. those originating in the Solar System.

Solar Cosmic Rays

Solar cosmic rays, also known as Solar Energetic Particles (SEP's), have energies up to a few gigaelectron volts (~10¹⁰ eV). The Sun emits these particles during coronal mass ejections (CME), or explosions of the Sun's atmosphere.

The above image shows a solar flare, which is similar but distinct from a CME in that most radiation released in electromagnetic (the explosion is bright) rather than cosmic rays. Solar flares do release cosmic rays, but to a lesser extent. Mass ejections, on the other hand, release huge quantities of SEP's, some of which are acclerated to over half the speed of light (these have the greatest energies). These events can produce particles that can interfere with the performance of satellites by penetrating their outer skin. The volume of high-energy radiation produced by the strongest events, such as the CME of August 1972, would be fatal to a human outside of the Earth's magnetic field.

Very rarely do solar (and other) cosmic rays reach the surface of the Earth, and, when they do, there are too few to cause radiation damage. However, by the chart above, there is an average of about 1000 such particles impacting every square meter of the surface of the outer atmosphere (outside the magnetic field) every second. Thus, there is potential for satellite damage, especially when this value increases during CME's.

Galactic Cosmic Rays

Galactic cosmic rays (GCR's) are cosmic rays originating from outside the Solar System, but within the Milky Way galaxy. Though they may have energies similar to those of solar cosmic rays, they also may be more energetic, falling into the range 10¹⁰ eV to 10¹⁵ eV, as illustrated by the blue zone of the figure.

The primary sources for GCR's are supernova remnants; the magnetic fields of these supernovae may accelerate ejected particles to over 99.9% of the speed of light. Through a process known as diffusive shock acceleration, the shock waves of the magnetic field of an exploding supernova emits impart massive amounts of energy to accelerate charged particles. However, other objects, such as so-called microquasars, also produce very high energy cosmic rays.

Microquasars are compact objects, such as white dwarfs, neutron stars, or black holes (see here) which have an ordinary companion star in a binary system. The gravitational pull of these objects sucks plasma off of their companion stars, adding them to an accretion disk as shown. Though a process not entirely understood (but known to involve the magnetic fields of the compact objects) matter from the accretion disk is shot out of the polar regions into relativistic jets, so named for the extremely high speeds of particles therein. Cosmic rays in these jets may be extremely energetic, on the order of 1-1000 TeV (10¹²-10¹⁵ eV). An example of a microquasar in the Milky Way is Cygnus X-3, named for being the third brightest X-ray source in the constellation Cygnus.

An X-ray image of Cygnus X-3

Though Cygnus X-3 is the third brightest in X-rays from Earth, it is more distant than Cygnus X-1 and Cygnus X-2, at a distance of 37,000 light-years (though still in the Milky Way). Further, it emits some of the highest-energy cosmic rays known to originate in the Milky Way galaxy, up to 1000 TeV. Note that the most energetic artificially accelerated particles produced on Earth by the Large Hadron Collider (LHC) have energies on the order of 10 TeV. Since the Earth receives about one particle with energy 1000 TeV per square meter per year, our planet is constantly bombarded by particles moving faster than anything manmade particle colliders have produced.

The final category of cosmic rays (corresponding to the purple area of the figure above) is extragalactic cosmic rays. They are discussed in the final post of this series, coming March 22.

Sources: http://www.euronuclear.org/info/encyclopedia/r/rest-energy.htm, http://solarphysics.livingreviews.org/open?pubNo=lrsp-2006-2&page=articlesu15.html, http://helios.gsfc.nasa.gov/sep.html, http://preppercentral.com/wp-content/uploads/2013/11/sun-big-solar-flare.jpg, http://hesperia.gsfc.nasa.gov/sftheory/spaceweather.htm, http://hyperphysics.phy-astr.gsu.edu/hbase/starlog/cygx3.html#c1, http://helios.gsfc.nasa.gov/gcr.html, http://hyperphysics.phy-astr.gsu.edu/hbase/relativ/releng.html, http://www.andrewcollins.com/pics/cygx3_415k.jpg,

Cosmic Rays 1

The Earth constantly is bombarded with a large amount of radiation from the cosmos. Much of this radiation is electromagnetic, coming in the form of radio waves, microwaves, infrared rays, visible light, ultraviolet rays, X-rays, and gamma rays. All of these types, however, are composed of photons, massless particles which travel at the speed of light. Other subatomic particles also compose incoming radiation, falling into the umbrella of "cosmic rays".

We cannot take detailed images of the cosmos with cosmic rays like we can with electromagnetic radiation. This is because the particles composing this radiation are charged, and thus change direction and speed when influenced by the magnetic fields of the Sun and Earth. As a result, we cannot directly ascertain the direction from which these particles come. Nevertheless, cosmic rays help us understand the composition of objects in the cosmos. In addition, since the particles making up cosmic rays have mass, they often require huge amounts of acceleration to reach us. The fact that the particles reached Earth at all reveals clues about the nature of the objects from which they came.

Cosmic rays have two main characteristics: composition and energy. We explore each in turn.

Cosmic rays are generally composed of the nuclei of atoms, though other particles appear as well, such as electrons. The extreme energy and speed of these former atoms stripped them of their electrons, leaving positively charged nuclei and negatively charged electrons, each of which are influenced by magnetic fields, as described above. Of these types of particles, hydrogen and helium nuclei (protons and alpha particles), the lightest two elements, are by far the most abundant in cosmic rays. Further, the proportions of hydrogen and helium nuclei in cosmic rays, ~90% and ~9%, respectively, are very consistent with the proportion of atoms in the Universe which are hydrogen and helium.

The above diagram illustrates the relative abundance of the chemical elements (in increasing order) in the Universe, as estimated through indirect means such as spectroscopy, an observational method which uses facts about how different elements absorb and reflect light to discern the composition of celestial objects. Note the relative rarity of lithium, beryllium, and boron, despite the fact that these are very light elements (atomic numbers 3, 4, and 5). The reason for this lack is that these elements are not heavily produced during stellar fusion, occurring only as intermediates in nuclear reactions that either produce helium or heavier elements like carbon. However, the nuclei of these three elements compose 0.25% of cosmic rays, more than a million times greater than what we would expect from the Universe's composition!

It is believed that these elements are disproportionately represented in cosmic rays due to collisions between "original" cosmic rays containing protons and atoms in the interstellar medium, such as carbon or oxygen. Such interactions may produce energetic lithium, beryllium, or boron nuclei as a byproduct, and these particles subsequently reach Earth. The fact that the products of these collisions are energetic and thus more likely to reach Earth explains why these elements are disproportionally represented in cosmic rays. The nuclei of elements of higher atomic numbers also appear more frequently in cosmic rays than we would "expect" from the abundance graph above. This shows that cosmic rays tend to come from sources rich in heavy elements, such as supernovae.

Antiparticles also make infrequent appearances in cosmic rays. An estimated 0.01% of cosmic rays are composed of antimatter. In fact, cosmic rays led to the original discovery of antimatter. In 1928, Paul Dirac, using the mathematics of quantum theory, including Schrödinger's Wave Equation, predicted the existence of antimatter. Antimatter was at the time an undiscovered class of particles each of which would be an "opposite" counterpart to an ordinary particle (opposite in some properties, such as charge, but not in others, such as mass, which must always be positive or zero). One example is the antielectron, also known as the positron, which would have the same mass as the electron, but a positive, rather than negative, charge. This charge, through opposite, is of the same magnitude as the electron's. In 1932, Carl D. Anderson observed a cosmic ray composed of a positron, using a device known as a cloud chamber.

Antimatter, however, does not exist in abundance in the Universe. Matter and antimatter particles annihilate on contact, producing energy, so due to a slight matter-antimatter imbalance following the Big Bang (the origins of which are still unknown), the known Universe is dominated by matter. However, just as matter and antimatter particles can annihilate and form energy, energy may also spontaneously (during certain quantum interactions) be converted into a particle-antiparticle pair through a phenomenon known as pair production.

This diagram illustrates the formation of an electron (e^-) and its antiparticle the positron (e⁺) from a gamma ray photon (γ). This instance of pair production often happens when a high-energy photon impacts the nucleus of an atom. The number of cosmic rays composed of antimatter which impact the Earth over a given period helps us to estimate the frequency of these reactions in space. Cosmic rays also often cause chain reactions in the Earth's atmosphere. In fact, this is how these rays are often detected.

The above image illustrates the cascade of particles that can occur when a cosmic ray (especially a powerful one) hits Earth's atmopshere. The interaction proceeds from top to bottom and begins when the cosmic ray impacts an atom (all atoms involved are denoted by circles). This produces several short-lived particles known as pions (with the symbols π⁺, π^-, and π⁰ depending on the charge). The left branch indicates the decay of the neutral pion into gamma rays, each of which forms an electron-positron pair through pair production, (presumably through more interaction with atmospheric matter) releasing other gamma rays which do the same. The center shows the main decay mode of the pion into a muon (denoted μ) and a muon antineutrino (not shown) or the corresponding antiparticles (if the initial pion charge is reversed). Muons too decay after around 2.2 microseconds, leaving their characterisitc signiture of particles. Finally, the right branch indicates how incoming cosmic rays might impact and destablize atomuc nuclei. These nuclei then emit protons, neutrons, or tiny particles called neutrinos. Sometimes the emitted neutrons and protons hit other atoms, and the chain reaction continues.

The length of such a chain reaction depends on the energy of the original cosmic ray. Sufficiently energetic cosmic rays can cause huge chain reactions which reach the Earth's surface, allowing us to observe them. To see how energy affects the behavior of cosmic rays and can reveal more about their origins, see the next post.

Sources: http://helios.gsfc.nasa.gov/cosmic.html, http://imagine.gsfc.nasa.gov/docs/science/know_l1/cosmic_rays.html, http://hyperphysics.phy-astr.gsu.edu/hbase/astro/cosmic.html#c2, http://imagine.gsfc.nasa.gov/docs/science/know_l2/cosmic_rays.html, http://abyss.uoregon.edu/~js/glossary/cosmic_rays.html, http://www.algebralab.org/practice/practice.aspx?file=Reading_RelativeAbundanceElementsUniverse.xml, http://jtgnew.sjrdesign.net/stars_fusion.html, http://www.whoi.edu/cms/files/ksims/2006/10/Cosmo_lect_2006_4SPP_14851.pdf, http://cosmos.phy.tufts.edu/~zirbel/ast21/sciam/AntiMatter.pdf, http://neutronm.bartol.udel.edu/catch/cr2b.gif,

Gravitational Waves 2

This is the second part of a two-part post on gravitational waves. For the first part, see here.

The previous post introduces gravitational waves, and discusses early attempts at their detection, such as LIGO. Despite LIGO's failure to detect these waves, new instruments promise to increase precision, allowing us to find weaker gravitational waves, and other indirect methods have yielded results. Overall, these techniques will give us new methods of observing our Universe.

LIGO, as well as other early laser interferometer observatories such as VIRGO (a similar detector in Italy), are sometimes known as the "first generation" of gravitational wave detectors. During LIGO's operation, upgrades led to modest increases in sensitivity. However, after the temporary cessation of operations in 2010, more major upgrades were made to LIGO including heavier mirrors and more powerful lasers, which will increase sensitivity and reduce background noise caused by thermal energy. The new Advanced LIGO began operation in 2016, and should have a range of hundreds of millions of light years, ten times that of the original design (see diagram below). These upgraded observatories were the "second generation" of detectors.

The original LIGO could detect gravitational wave sources only within our Local supercluster and its neighbors (small gray sphere), but Advanced LIGO was able to scour an volume of space 1000 times as large for gravitational wave signals (the entire scope of the figure above).

The above diagram shows the actual and estimated sensitivities for different gravitational wave detectors, including LIGO, VIRGO, and their respective upgrades. The x-axis of the graph is the frequency of the gravitational wave (gravitational waves have different frequencies in the same way that electromagnetic waves do) and the y-axis indicates the intensity of the waves. The detectors exhibit different sensitivities to different frequencies; curves that dip lower indicate better detectors. The Einstein GW Telescope is a proposed "third-generation" laser interferometer concept, still in design phase. This design would have the facility be underground to reduce seismic noise and cryogenically cooled to prevent thermal vibrations from altering the distance between mirrors.

Another "third-generation" design concept which would theoretically yield numerous detections is the Laser Interferometer Space Antenna (LISA), a spaced-based model.

LISA would consist of three separate spacecraft, which would create a equilateral triangle of side length 5 million kilometers (3.1 million miles). The above diagram is an artist's conception. This triangle would trail the Earth in heliocentric orbit, and would be very sensitive to different frequencies of gravitational waves than ground-based detectors like LIGO.

The above figure shows that LISA would detect much longer wavelengths than Advanced LIGO (due to LISA's enormous arms). Advanced LIGO could only discover very high frequency oscillations, such as neutron stars rotating very close to each other just before colliding. Such systems are rare and short-lived, since neutron star systems contract and ultimately collide. However, LISA could detect more slowly orbiting binary systems, long before their final collision. These are very common, and many are already known through other means of observation, guaranteeing that LISA would find many sources if it functions correctly.

There are unfortunately no definite plans for launching LISA, but a small test mission, known as LISA Pathfinder, launched in 2015. This small probe contained a tiny interferometer meant to test the LISA concept in space and evaluate the proposal's feasibility.

Ultimately, the most important goal of gravitational-wave observatories is to peer farther into the early universe than could be possible with telescopes measuring electromagnetic radiation. Using ordinary visual telescopes (of sufficient power), we can view objects billions of light years away (seeing them as they were billions of years ago, since it takes light a year to travel each light-year). However, there is a fundamental limit to how far these telescopes can see. Before 380,000 years after the Big Bang (or about 13.8 billion years ago), the temperature of the Universe was too high for electrons to combine with atomic nuclei into atoms, and, since electrons scatter electromagnetic radiation, the Universe was opaque. Thus the "oldest" light in the Universe is from 380,000 years after the Big Bang; it is called the Cosmic Microwave Background (CMB), and traditional telescopes cannot see farther. However, gravitational wave astronomy has the potential to receive signals from earlier periods and study them directly, leading to a greater understanding of the Big Bang.

For an update on recent developments in the detection of gravitational waves, see here!

Sources: https://www.advancedligo.mit.edu/summary.html, http://www.ligo.caltech.edu/docs/G/G080303-00.pdf, http://www.et-gw.eu/, http://www.physik.hu-berlin.de/qom/research/freqref/lisa, http://lisa.nasa.gov/, http://cosmology.berkeley.edu/~yuki/CMBpol/CMBpol.htm, http://www.theguardian.com/science/2014/mar/17/primordial-gravitational-wave-discovery-physics-bicep, http://www.nytimes.com/2014/03/25/science/space/ripples-from-the-big-bang.html?_r=0, "An Ear to the Big Bang" from The Scientific American October 2013 issue

Gravitational Waves 1

Gravitational waves, in brief, are the propagations of gravitational fields through space. Before dealing with gravitational waves directly, we attempt to provide historical context and a way to visualize how the waves work.

In 1865, James Clerk Maxwell (1831-1879) published a paper outlining his theory of electromagnetism, compiling and uniting earlier work into a single theory explaining the properties of both electricity and magnetism. For example, it deals with the properties objects possessing positive and negative charges, and the forces they exert on their environments (electric and magnetic fields). This theory also describes electromagnetic waves, or propagating changes in the electromagnetic field. Such waves are characterized by their wavelength and amplitude.

The above simplified diagram of a wave shows its wavelength and amplitude. We also define the frequency of a wave as the number of oscillations per second. In the diagram above, the wave has a frequency of 2 Hz. For electromagnetic waves, amplitude corresponds to intensity of the wave, and wavelength to type (or in the case of visual light, color). The continuous interval of electromagnetic wavelengths is known as the electromagnetic spectrum and includes many familiar types of radiation, including radio waves, microwaves, infrared rays, visible light, ultraviolet rays, X-rays, and gamma rays (all of these types are discussed in the link above).

Such waves are produced when charged objects move through space, causing a change in electric field. When a charge has moved, it will not exert the same forces on its surroundings as it had previously. The change in field is "carried" by waves, which move at the speed of light, a finite (though very fast) speed. The diagram below shows an example of electromagnetic wave production by a dipole, or a pair of equal and opposite charges.

As the charges oscillate up and down, an electromagnetic wave is produced, and propagates away from the dipole (the blue and red parts of the oscillation are the electric and magnetic field components, respectively). Since the oscillation is periodic, the wave signal it produces is also periodic. The magnitude of the charges forming the dipole determines the amplitude of the generated wave. Also, the frequency of the oscillation determines the frequency of the electromagnetic waves.

At the beginning 20th century, though Maxwell's theory had supplanted earlier understandings of electromagnetism, Newton's was still the dominant paradigm for gravitation. The theories did have similarities, among them the fact that both electromagnetic and gravitational forces shrank with distance in inverse proportion with the square of this distance (F ~ 1/r²). However, while there were both attractive and repulsive electromagnetic forces, gravity was always an attractive force. Another crucial difference was that, as shown above, electromagnetic fields move at the speed of light. Newton's theory, though, simply assumed that all bodies pulled instantaneously on one another. Albert Einstein (1879-1955) developed the theory of general relativity in 1916 and resolved this difference. His theory predicted that differences in gravitational fields would move analogously to electromagnetic fields: using gravitational waves. Further, these postulated gravitational waves would travel at the speed of light.

Gravitational waves, in Einstein's theory, would also be produced in an analogous manner to electromagnetic waves. Instead of oscillating charges, oscillating masses would produce the waves. For example, two massive bodies (such as black holes) orbiting one another at close range would produce gravitational radiation, as in the diagram below.

The conception above illustrates how gravitational waves move away from the orbiting system in all directions. Unlike their electromagnetic counterparts, gravitational waves travel undisturbed through matter, and as a consequence are much more difficult to detect. Nevertheless, they do have a subtle effect on the matter which they pass through. The medium through which gravitational waves travel is the fabric of space itself. The diagram above illustrates distortions of a two-dimensional space fabric; in reality, gravitational waves would cause small "ripples" in our three-dimensional space.

Beginning in the 1960's scientists on Earth have constructed increasingly sophisticated gravitational wave detectors. The first variety were known as Weber bars, or large bars of metal which, if sufficiently isolated from the surrounding environment, could oscllate as gravitational waves passed through them. However, the waves had to be very strong to be detected, and the original models were not up to the task. More modern Weber bars have been supercooled to temperatures very near absolute zero to reduce outside vibrations and increase their sensitivity.

Another method for identifying incoming gravitational waves is known as laser interferometry.

Laser interferometry works by using light beams to measure distances. In the usual design (diagram above), a laser creates a beam of light which is split by a beam-splitting mirror into two beams which travel down the two perpendicular arms of the interferometer. On the return trip, if the arms are exactly the same length, the beams interfere with one another in such a way that all the light travels back to the laser. If, however, the arms have slightly different lengths, some light will be reflected by the beam-splitter into a detector.

The Laser Interferometer Gravitational-Wave Observatory (LIGO) wass one project making use of a laser interferometer to detect gravitational waves. In each of LIGO's facilities (there is one in Louisiana and one in Washington) there was a laser inteferometer with arms four kilometers (2.5 miles) long. Theory held that when a gravitational wave passes through the detector, it distorts space and actually alters the lengths of the arms slightly. Since the arms are perpendicular, the distortions are different, and sufficiently strong waves should then cause the laser beams to be enough out of sync to send light to the detector. There were two LIGO stations to weed out false data and to determine which way gravitational waves moved through the Earth. Despite the precision of LIGO, it did not make any unambiguous detections during its operation (2002-2010).

Despite these setbacks, concepts for more precise instruments and new detectors have since been developed, and the road to gravitational wave detection has also proceeded through more indirect means (see the next post).

Sources: http://www.britannica.com/EBchecked/topic/242499/gravity-wave, http://rsta.royalsocietypublishing.org/content/366/1871/1849.full, http://www.tapir.caltech.edu/~teviet/Waves/differences.html, http://www.geo.mtu.edu/~scarn/teaching/GE4250/EM_wave_lecture.pdf, http://ned.ipac.caltech.edu/level5/ESSAYS/Boughn/figure1.gif, http://upload.wikimedia.org/wikipedia/commons/3/35/Onde_electromagnetique.svg, http://www.vias.org/wirelessnetw/img/wndw-print_img_3.png, spaceplace.nasa.gov, http://en.wikipedia.org/wiki/Gravitational-wave_detector, http://www.learner.org/courses/physics/visual/visual.html?shortname=ligo_interfermometer, http://www.ligo-la.caltech.edu/LLO/overviewsci.htm

Degenerate Matter: Black Holes

This post concerns black holes in the context of degenerate matter. For an introduction to degenerate matter, followed by a description the various stages of a stellar remnant's collapse preceding the possible formation of a black hole, see here.

White dwarfs, neutron stars, and hypothetical exotic stars are examples of objects of various masses within which some force counteracts gravity and stops collapsed stellar cores from shrinking further. However, above about 3.5-4 solar masses, no known force can counteract gravity. The collapsed star shrinks even further, until its escape velocity, the velocity necessary to leave the object from its surface, reaches the speed of light. At this stage, nothing can escape the body, not even the fastest particles in the Universe: photons. The collapsed star has become a black hole.

As far as is known, no further phenomenon can halt the contraction of the stellar remnant. Under its own weight, the black hole would collapse to become infinitely small and infinitely dense: such an object is called a singularity. Such singularities are consistent with the theory of relativity, but it is not known whether a singularity would be compatible with quantum mechanics; an infinitely dense object seems contrary to any known particle behavior.

Despite the complexities of black hole formation, the structure of a black hole is very simple. In some ways, as we shall see, it is even too simple. From long distances away, a black hole exerts gravity just the same as any other object. For example, light near a black hole undergoes gravitational lensing just as light around neutron stars does (see below).



The above is an impression (not a real image) of gravitational lensing. The gravity of a black hole is so great that it bends light, and therefore causes the view of objects behind the black hole to be distorted.

Near to the black hole, the only noticeable features are the accretion disk and the event horizon. The accretion disk is simply infalling material, sucked in by the black hole's gravity. Such matter is often accelerated to enormous speeds, and releases high energy radiation (X-rays and gamma rays) before it falls into the black hole. This is why black holes can be detected, as no radiation (with a possible exception, see below) is emitted from the hole itself.

The event horizon, which is the real "edge" of the black hole, is the boundary beyond which the velocity needed to escape the black hole exceeds the speed of light. At the center of the area bounded by the event horizon is the actual stellar remnant, the composition of which, as remarked above, is unknown.

The formation of a black hole has been discussed, but do these objects ever die? In fact, they are predicted to "evaporate" by emitting Hawking radiation, named after the physicist Stephen Hawking, who first proposed the process in 1974. Though no particles can escape a black hole, certain fluctuations of space can spontaneously create particle-antiparticle pairs near the event horizon. When this happens, one of the particles can escape by a process known as quantum tunneling. The precise nature of this radiation is unknown, but the rate of emission is so slow that the background radiation of the Universe gives more energy to most black holes than they lose through Hawking radiation. However, the amount of radiation emitted is inversely proportional to a black hole's size, so a small black hole (below stellar mass) could evaporate in our current Universe, if one existed. Black holes that are stellar remnants, however, will remain in existence for trillions of years, as the cosmic background radiation is still energetic enough to insure that they take in more mass then they give off. If the Universe's expansion continues, black holes are likely to be the last large objects in the Universe in the very far future (perhaps about 10⁴⁰ years from now).

The characteristics of a black hole are its mass, spin (or angular momentum), and electric charge. The latter two of these yield interesting phenomena. When a black hole is spinning, it provides angular momentum to the matter circling it, giving rise to a special region called the ergosphere (see below).



The ergosphere is a region outside the event horizon. Any matter in this region is subjected to not only the gravity of the black hole but also the drag of spacetime itself resulting from the angular momentum of the black hole. Therefore, even though the escape velocity in the region is lower than the speed of light, the sum of this and the additional momentum causes any matter in the region to be moved in the direction of rotation of the black hole. This occurs in such a way that a particle in the ergosphere would have to move at superliminal (over the speed of light) speeds to stay stationary (with respect to an outside frame of reference).

With regard to the electric charge of a black hole, some theories of the universe, notably string theory, acknowledge the possibility of the existence of what is called a magnetic monopole, essentially analogous to "a bar magnet with only one end". Even particles such as electrons, though possessing a net electric charge, have, due to their spin, a typical magnetic dipole called a magnetic moment, which obeys the laws of magnetism. A monopole would have to be composed of some unknown particle, as all elementary particles known to date have magnetic dipole moments. Black holes may be composed of these monopoles, as they are likely to be made of some other elementary particle. Note that these ideas are quantum mechanical. If black holes are true singularities, it may preclude this possibility.

The above are a few exotic phenomena that can arise in black holes. However, returning to the characteristics of the black hole, the three listed above are the only ones known that can be calculated by an outside observer, i.e. by the black hole's effect on its environment. From this viewpoint, one could precisely determine the nature of a black hole with only three parameters. Yet, if an object, say an apple, were to fall into a black hole, there would be no way of knowing afterward that this had occurred! One could record the mass contribution of the apple to the hole, but there is no way of recovering its shape, its color, etc., from observation of the black hole. The information that the apple carried is lost.

Or is it? Many principles of physics are contrary to this assumption. Classical physics and relativity both imply reversibility, that is, they imply that a "simulation" of the universe could be run just as well forward in time or backward. The amount of information in the universe must remain constant, or, in running time backward, one could have no idea of whether an apple, an orange, or any other object of equivalent mass had fallen into the black hole. Furthermore, an important quantum mechanical equation, the wave equation, totally determines of the probabilities of quantum states at any past or future time given the wave equation of a system at the present time. Therefore there can be no "collapse" of many states into one; the scenario with the apple and that with the orange cannot have the same outcome.

Many resolutions to this paradox have been posited. Some state that the information would be conserved in some (initially) non-observable fashion: the information would "leak out" slowly over time as black holes Hawking radiation, or that black holes, at the end of their life, would release all their stored information in a single burst, or even that the information is transported, by means of the singularity through a hole in spacetime, to another universe. However, all three of these violate some aspect of our current understanding of information conservation.

If information leaks out, there is still a time delay in which it is not known. It seems unlikely that all of the information is emitted at the end of a black hole's lifetime, as many theories put a limit on the amount of information that can be stored in a finite volume of space. Finally, if the information is transported to another Universe, the information is not conserved in any local, or obtainable, sense. There is a final alternative, however. It is possible that the fluctuations of the event horizon itself would store the impressions of the incoming (or outgoing) particles. Note that this requires the projection of the information in a four-dimensional space (three spatial dimensions plus time) onto a three-dimensional space (the surface of the event horizon is a two-dimensional surface, which again changes over time), but this poses no problem, and has sound mathematical justification; for a sufficiently "well-behaved function" on a space, the behavior of the function within a region is completely determined by the values of the function on its boundary. This result is known as Green's theorem, and its application to the projection of information onto the event horizon is known as the holographic principle.

Of course, this does not mean we could practically access this information, but simply states that it is, in theory, possible. Other obstacles prevent one from ever reaching the event horizon. One notable phenomenon is relativistic time dilation.



Any massive object creates a depression in space, and a black hole creates an especially steep depression; in fact, if singularities exist as predicted by relativity, the depression would actually be a hole in space, as illustrated above. Due to the symmetry between time and space, again predicted by relativity, time is accordingly distorted near a black hole. If one were to watch another object, or person, approach the event horizon, they would appear to slow down as they neared the edge of the black hole. In fact, from the perspective of an outside observer, they would take infinitely long to cross the horizon itself. From the viewpoint of the object, however, time does not slow, and the crossing of the horizon takes place in finite time.

Of course, no object could actually survive this crossing, but would rather be torn apart by gravitational pull. In addition, no one could actually watch the entire descent, as the radiation that renders one visible, when travelling from the object to the observer, uses energy to move "uphill" in the black hole's gravity field. Lower energy light, for example, is red, so the radiation is said to have red-shifted. By the time an object is close to the horizon, it is only visible in radio waves, and eventually, not at all.

The key concept of degenerate matter is the interplay between gravitation and quantum mechanical forces, in particular how they oppose one another. Black holes, the culmination of the process of stellar collapse, represent the crux of the differences between "large-scale" physical theories, i.e. relativity, and "small-scale" theories, i.e. quantum mechanics. Singularities are present in relativity as "pathological" points in spacetime, where density becomes infinite. However, in light of quantum mechanics, singularities are seem to be contradiction, resulting in exotic phenomena including, but not limited to, those listed above.

Solving the mysteries of degenerate matter and black holes in particular is one of the main outstanding problem in modern physics, and will continue to shape our understanding of the universe for years to come.

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Sources: http://astrofacts.files.wordpress.com/2009/07/rouge-black-hole.jpg, No-hair theorem on Wikipedia