Don Lincoln has produce another fun video on the speed of gravity.
SPOILER: It has the same speed as the speed of light!
But what is more interesting in this video is a brief description of LIGO and gravitational interferometry and how gravitational waves are detected.
Enjoy!
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Neutron stars and stellar black holes are the final stages of evolution for large stars – with black holes being more massive than neutron stars. In theory, the maximum mass of a neutron star is about 2.1 solar masses. However, there is some indirect evidence that more massive neutron stars could exist. There is little evidence for the existence of black holes smaller than about 5 solar masses, leading to a mass gap in our observations of these compact objects.
What is intriguing about the August 2019 merger – dubbed GW190814 – is the mass of the smaller object, which appears to fall within this gap. “Whether any objects exist in the mass gap has been an ongoing mystery in astrophysics for decades,” says Charlie Hoy of the UK’s Cardiff University, who played a key role in analysing data from the detection and writing the paper that describes the observation, which has been published in The Astrophysical Journal Letters. “What we still don’t know is whether this object is the heaviest known neutron star or the lightest known black hole, but we do know that either way it breaks a record.”
The actual paper is available to be read for free here since it is an open access article.
Like I had said to the students in my astronomy classes, this is going to go down as the golden age of astronomy. Since the beginning of human history, we only had light as our only detector of the heavens. Now, we have not only neutrinos and high-energy cosmic rays, but also gravitational waves as our means to look at the heavens. We have three different and separate ways to look at our sky!
Zz.
[1] R. Abbott et al., The Astrophysical Journal Letters,896:L44(20pp), 2020.
"Above all, it is the unique configuration of that system, akin to the Earth-Moon-Sun system with the presence of a second companion (playing the role of the Sun) towards which the two other stars 'fall' (orbit) that has allowed to perform a stellar version of Galileo's famous experiment from Pisa's tower. Two bodies of different compositions fall with the same acceleration in the gravitational field of a third one."
"The pulsar emits a beam of radio waves which sweeps across space. At each turn this creates a flash of radio light which is recorded with high accuracy by Nançay's radio telescope. As the pulsar moves on its orbit, the light arrival time at Earth is shifted. It is the accurate measurement and mathematical modeling, down to a nanosecond accuracy, of these times of arrival that allows scientists to infer with exquisite precision the motion of the star," says Dr. Guillaume Voisin.
The team compared the position and velocity measurements from GRAVITY and SINFONI respectively, along with previous observations of S2 using other instruments, with the predictions of Newtonian gravity, general relativity and other theories of gravity. The new results are inconsistent with Newtonian predictions and in excellent agreement with the predictions of general relativity.
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The new measurements clearly reveal an effect called gravitational redshift. Light from the star is stretched to longer wavelengths by the very strong gravitational field of the black hole. And the change in the wavelength of light from S2 agrees precisely with that predicted by Einstein’s theory of general relativity. This is the first time that this deviation from the predictions of the simpler Newtonian theory of gravity has been observed in the motion of a star around a supermassive black hole.
Archibald and colleagues’ study breaks new ground because the gravitational energy inside a neutron star can account for as much as 20% of the body’s mass. The authors’ results therefore imply that the accelerations of gravitational energy and matter differ by no more than a few parts per 105 — a tenfold improvement over the bound from lunar laser ranging.
More importantly, the authors have provided what is known as a strong-field test of general relativity. Unlike the Solar System, for which Einstein’s theory predicts only small deviations from Newton’s theory of gravity, the motion of a neutron star in a gravitational field invokes full general relativity in all its complex glory. Einstein’s theory passes this strong-field test with flying colours.
This effect of gravitational lensing, which occurs in both strong and weak variants, represents the greatest hope we have of testing General Relativity on scales larger than the Solar System. For the first time, a team of scientists led by Tom Collett performed a precise extragalactic test of General Relativity, and Einstein's theory passed with flying colors.
For the first time, we've been able to perform a direct test of General Relativity outside of our Solar System and get solid, informative results. The ratio of the Newtonian potential to the curvature potential, which relativity demands be equal to one but where alternatives differ, confirms what General Relativity predicts. Large deviations from Einstein's gravity, therefore, cannot happen on scales smaller than a few thousand light years, or for masses the scale of an individual galaxy. If you want to explain the accelerated expansion of the Universe, you can't simply say you don't like dark energy and throw Einstein's gravity away. For the first time, if we want to modify Einstein's gravity on galactic-or-larger scales, we have an important constraint to reckon with.
The team fired pulses of neutrons at a chamber filled with either helium or xenon gas and monitored both the travel time of the neutrons through the gas and the neutrons’ scattering angles. From these measurements, they reconstructed the scattering process with the aid of simulations. They found that the scattering-angle distribution fit the predictions—based only on known laws of physics—for neutrons bouncing off gas nuclei. This result indicates that, within the sensitivity of the experiment, no unexplained force—be it modified gravity or another type of interaction—acts on length scales below 0.1 nm.
The Nobel Prize in Physics 2017 was divided, one half awarded to Rainer Weiss, the other half jointly to Barry C. Barish and Kip S. Thorne "for decisive contributions to the LIGO detector and the observation of gravitational waves".
The average of these fluctuations is a gravitational field that is consistent with Newton’s theory of gravity. In this model, gravity is born out of quantum mechanics, but is not in itself a quantum-mechanical force. It is what scientists call “semiclassical.” Until this theory is tested further, it will remain a semi-solution; while the idea does predict certain known phenomena, it doesn’t yet account for Einstein’s theory of general relativity.
Now, in a paper published in Physical Review Letters on February 11, the Laser Interferometer Gravitational-Wave Observatory (LIGO) and Virgo collaborations announce the detection of just such a black hole merger — knocking out two scientific firsts at once: the first direct detection of gravitational waves and the first observation of the merger of so-called binary black holes. The detection heralds a new era of astronomy — using gravitational waves to “listen in” on the universe.
In the early morning hours of September 14, 2015 — just a few days after the newly upgraded LIGO began taking data — a strong signal, consistent with merging black holes, appeared simultaneously in LIGO's two observatories, located in Hanford, Washington and Livingston, Louisiana.
Tentative new work from Julian Barbour of the University of Oxford, Tim Koslowski of the University of New Brunswick and Flavio Mercati of the Perimeter Institute for Theoretical Physics suggests that perhaps the arrow of time doesn’t really require a fine-tuned, low-entropy initial state at all but is instead the inevitable product of the fundamental laws of physics. Barbour and his colleagues argue that it is gravity, rather than thermodynamics, that draws the bowstring to let time’s arrow fly. Their findings were published in October in Physical Review Letters.
The team’s conclusions come from studying an exceedingly simple proxy for our universe, a computer simulation of 1,000 pointlike particles interacting under the influence of Newtonian gravity. They investigated the dynamic behavior of the system using a measure of its "complexity," which corresponds to the ratio of the distance between the system’s closest pair of particles and the distance between the most widely separated particle pair. The system’s complexity is at its lowest when all the particles come together in a densely packed cloud, a state of minimum size and maximum uniformity roughly analogous to the big bang. The team’s analysis showed that essentially every configuration of particles, regardless of their number and scale, would evolve into this low-complexity state. Thus, the sheer force of gravity sets the stage for the system’s expansion and the origin of time’s arrow, all without any delicate fine-tuning to first establish a low-entropy initial condition.
The gravitational force between two particles is given by Newton's law of gravity, which says that the force between two point masses is proportional to the product of the masses and inversely proportional to the square of the distance between them.
The electric and gravitational force laws are both inverse square laws, so if one computes the ratio of the forces between two bodies, the distances cancel. For the electron and proton, the gravitational force is 39 orders of magnitude weaker than the electrical force. This is the source of the myth that gravity is a much weaker force than electromagnetism.
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The absolute strength of the electromagnetic force is specified by a dimensionless parameter alpha called, for historical reasons, the fine structure constant. It is actually not a constant but varies with energy. However that variation is very gradual and for most practical purposes alpha can be taken to have a value of 1/137
Conventionally a dimensionless parameter alpha-G is defined to represent the gravitational force strength. It is proportional to the square of the proton mass and has a value 23 orders of magnitude less than alpha. So "officially," gravity is this much weaker than electromagnetism.
De Rosa, a physicist at Louisiana State University in Baton Rouge, knows he has a long night ahead of him. He and half a dozen other scientists and engineers are trying to achieve 'full lock' on a major upgrade to the detector — to gain complete control over the infrared laser beams that race up and down two 4-kilometre tunnels at the heart of the facility. By precisely controlling the path of the lasers and measuring their journey in exquisite detail, the LIGO team hopes to observe the distinctive oscillations produced by a passing gravitational wave: a subtle ripple in space-time predicted nearly a century ago by Albert Einstein, but never observed directly.
