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  1. NASA visualization shows a black hole's warped world Seen nearly edgewise, the turbulent disk of gas churning around a black hole takes on a crazy double-humped appearance. The black hole's extreme gravity alters the paths of light coming from different parts of the disk, producing the warped image. The black hole's extreme gravitational field redirects and distorts light coming from different parts of the disk, but exactly what we see depends on our viewing angle. The greatest distortion occurs when viewing the system nearly edgewise. Credit: NASA's Goddard Space Flight Center/Jeremy Schnittman This new visualization of a black hole illustrates how its gravity distorts our view, warping its surroundings as if seen in a carnival mirror. The visualization simulates the appearance of a black hole where infalling matter has collected into a thin, hot structure called an accretion disk. The black hole's extreme gravity skews light emitted by different regions of the disk, producing the misshapen appearance. Bright knots constantly form and dissipate in the disk as magnetic fields wind and twist through the churning gas. Nearest the black hole, the gas orbits at close to the speed of light, while the outer portions spin a bit more slowly. This difference stretches and shears the bright knots, producing light and dark lanes in the disk. Viewed from the side, the disk looks brighter on the left than it does on the right. Glowing gas on the left side of the disk moves toward us so fast that the effects of Einstein's relativity give it a boost in brightness; the opposite happens on the right side, where gas moving away us becomes slightly dimmer. This asymmetry disappears when we see the disk exactly face on because, from that perspective, none of the material is moving along our line of sight. Closest to the black hole, the gravitational light-bending becomes so excessive that we can see the underside of the disk as a bright ring of light seemingly outlining the black hole. This so-called "photon ring" is composed of multiple rings, which grow progressively fainter and thinner, from light that has circled the black hole two, three, or even more times before escaping to reach our eyes. Because the black hole modeled in this visualization is spherical, the photon ring looks nearly circular and identical from any viewing angle. Inside the photon ring is the black hole's shadow, an area roughly twice the size of the event horizon—its point of no return. This image highlights and explains various aspects of the black hole visualization. Credit: NASA's Goddard Space Flight Center/Jeremy Schnittman "Simulations and movies like these really help us visualize what Einstein meant when he said that gravity warps the fabric of space and time," explains Jeremy Schnittman, who generated these gorgeous images using custom software at NASA's Goddard Space Flight Center in Greenbelt, Maryland. "Until very recently, these visualizations were limited to our imagination and computer programs. I never thought that it would be possible to see a real black hole." Yet on April 10, the Event Horizon Telescope team released the first-ever image of a black hole's shadow using radio observations of the heart of the galaxy M87. Source: NASA visualization shows a black hole's warped world
  2. Satellites play chase to measure gravity, achieve picometer accuracy Satellites tracked to the picometer, test tech for gravitational wave detector. Enlarge NASA/JPL-Caltech The spinoffs from gravitational wave detectors are not just new scientific discoveries. The technology also has other uses. A good example of this is the gravity-measuring mission, GRACE Follow On, which was launched last year. The first reports on its laser rangefinder's performance have been released, and it makes for impressive reading. Gravitational wave detectors work by measuring tiny changes in the distance between two mirrors. Ripples in space-time cause a tiny oscillation in that distance, which is then detected by comparing the phase shift between light that has traveled between the two mirrors and light that has traveled along a path that was unaffected by the gravitational wave. To put it in perspective, a gravitational wave detector measures changes that are far smaller than the diameter of an atom and are more like the diameter of a single proton. The gravity of GRACE Similar technology found its way into space to increase the sensitivity of Earth-monitoring instruments. On Earth, we have stationary detectors that wait for gravitational waves to pass through them. In orbit, the detector is moving and can measure subtle changes in the Earth's gravitational field. That brings us to GRACE—a backronym for gravity recovery and climate experiment—a pair of satellites that orbited the Earth at a fixed distance from each other. Or they would be fixed if the Earth’s gravity didn’t change in time and space. GRACE used a radar system to measure the distance between the two satellites to track those changes, providing information about the gravitational field it was traveling through. Changes in that distance could be translated into local measurements of the acceleration due to gravity. This, in turn, is used to measure things like the volume of water in aquifers. GRACE turned out to be hugely useful but, like all satellite missions, eventually died. In this case, the battery on GRACE-2 failed, and the pair was cremated in the Earth’s atmosphere. The follow-up to GRACE, called GRACE-FO (follow on), was launched in 2018. GRACE-FO is mostly a copy of GRACE, but scientists took the opportunity to include a new distance-measuring tool that would outperform the old GRACE radar system. This, if successful, would mean that the measurement data would not put a limit on how we interpreted the data. Instead, models of the tides would need to be improved. It also just happens that the technology required to do this sort of station-keeping measurement is exactly the technology required for LISA (a proposed space-based gravitational wave observatory). The LISA pathfinder mission was hugely successful but, being a single satellite, could not test station-keeping technologies. So, in many ways, the GRACE-FO satellites are also a lightweight LISA pathfinder mission. Smooth running Since June 14, when contact was established with the satellites, scientists have been testing the laser range-finder system. And, to put it simply, it absolutely smashes the design specifications. Over a time period between 5 and 1,000 seconds, the system should be able to detect distance changes of 2 to 40 billionths of a meter between two satellites that are separated by 220km. However, the team reports sensitivity as low as 300 trillionths of a meter. To put this in perspective, the radar system on the original GRACE was sensitive to changes at about the 10 micrometer level. How do you achieve this sort of accuracy? In short, with lasers. More seriously, a laser beam on the master satellite is stabilized to have a very precise frequency. That laser is shot at the slave satellite. This in itself is an achievement because the laser has to be continuously aimed in the right direction (the radar system manages this with a fixed antenna). The slave satellite uses the incoming light (all 25nW of it) to do two things. First it uses a tiny amount to check that the laser is pointing in the right direction. The remainder of the light is used to set the phase of its own laser, which is sent back to the master satellite—the received laser light is too weak to just be reflected back. The light that is received at the master acquires a phase shift relative to the transmitted light that is proportional to the distance between the two satellites. Since the distance is continuously changing, this is measured as an additional frequency in the received laser spectrum. Hence, a frequency measurement of the incoming laser spectrum becomes a measurement of distance, which, in turn, becomes a measurement of acceleration due to gravity. In the short term, this means that GRACE-FO data will be even better than expected, and the modelers are going to have to get back to work. In the long term, it means that more of the technology for LISA will be validated with the additional benefit of a long-term robustness study. In the very short term, it gave me some good weekend reading. Source: Satellites play chase to measure gravity, achieve picometer accuracy (Ars Technica)
  3. Alternative theory of gravity makes a nearly testable prediction A massive simulation done with a "chameleon" theory of gravity. Enlarge / Galaxy clusters generated by the Universe simulator IllustrisTNG. TNG Collaboration From our current perspective, the Universe seems to be dominated by two things we find frustratingly difficult to understand. One of these is dark matter, which describes the fact that everything from galaxies on up behaves as if it has more mass than we can detect. While that has spawned extensive searches for particles that could account for the visual discrepancy, it's also triggered the development of alternative theories of gravity, ones that can replace relativity while accounting for the discrepancies in apparent mass. So far, these proposals have fallen well short of replacing general relativity. And they say nothing about the other big mystery, dark energy, which appears to be accelerating the expansion of the Universe. Instead, researchers have developed an entirely separate class of theories that could modify gravity in a way that eliminates the need for dark energy. Now, researchers have run simulations of galaxy and star formation using this alternative version of physics, and they found we might be on the cusp of testing some of them. Gravitational alternatives General relativity explains a broad range of phenomena, and it works well to describe the Universe as a whole, provided dark matter and dark energy exist as separate entities. Any alternatives to gravity have to account for everything that's explained by general relativity while also accounting for the additional effects of at least one of these two dark forces. A class of theories, collectively termed MOND (for Modified Newtonian Dynamics), is intended to do away with dark matter, but it struggles to account for things relativity handles with easy. And, when it comes to dark energy, MOND is silent, in part because it was originally developed before dark energy was known to be an issue. Instead, an entirely separate class of theories has been developed that handle gravity while eliminating the need for a separate dark energy. These are known as f(R) models and are commonly described as having a "chameleon" mechanism. That's because they posit an additional force that changes its behavior based on its surroundings. Where there's a lot of matter, the chameleon force is minimized, allowing it to blend in with its surroundings. As matter becomes sparse on larger scales, it starts to make its presence felt. That's why we can't detect any major deviations from relativity on Earth or near objects like neutron stars, but we do detect them when we start looking at the large scale structure of the Universe. The net result is an acceleration of the expansion of the Universe that's only apparent at large scales—just like dark energy. For any additions to physics to be successful, they have to make sense with what we know of relativity, plus handle details it can't. That makes it difficult to test, because the new models are already crafted to match existing data (and would be pretty pointless if they weren't). So, the trick is finding data we don't yet have, but could show a difference between relativity and these new models. To search for these sorts of discrepancies, a group of cosmologists at Durham University decided to plug some chameleon proposals into massive computational models that simulate the formation of structures ranging from stars to galaxy clusters. A model universe The researchers worked with the IllustrisTNG model, a mini-Universe that can simulate galaxy formation and evolution. Under standard conditions, the model controls this evolution in part by having everything obey general relativity. But for this test, the research team also ran a version where relativity was replaced with a chameleon f(R) version of gravity. (They also ran an exaggerated version of f(R) in order to accentuate the differences.) All models assumed the presence of large amounts of dark matter; remember it's MOND that hopes to get rid of that. Simulations were run under two conditions: with feedback from regular matter, and without. Unlike dark matter, regular matter ignites into stars and forms black holes, and those provide feedbacks that alter the behavior of nearby matter. The simulations indicated that the gas in the inner regions of galaxies doesn't feel the effect of modified gravity, behaving much as it would with general relativity. This includes gas flowing into the area near supermassive black holes that power active galaxies. In contrast, the outer regions of galaxies should show some differences because of the changes caused by the chameleon force. Here, additional stars are expected to form due to changes in the dynamics of gravity under the chameleon model. Too small Unfortunately, most of these effects are too small to create detectable differences between f(R) and general relativity. There is, however, one exception. The changes to the gas in the outer region of galaxies causes higher densities of gas to form there, which in turn increases the efficiency of cooling of that gas. It turns out that an instrument of the Square Kilometer Array radiotelescope will be sensitive to the altered properties of the gas. As a result, it may be able to pick up any deviations from general relativity. The other result that was significant here is the finding that, for chameleon models that are similar to general relativity, the effect of including feedbacks from regular matter is simply an additive effect; there are no further interactions with modified gravity. This would allow future computations to be considerably simplified. So, while we're not yet ready to start ruling out alternatives to general relativity, the new work highlights the sorts of things we need to do to be able to test potential replacements. Because relativity has been so successful and seemingly explains so much of what we see, there's not much space for alternatives to stake out a distinct identity. By putting in the effort to figure out where that rare space may reside, research like this sets up the possibility of ultimately putting some of our ideas to the test. And, well, if they fail the test, there's still dark energy Source: Alternative theory of gravity makes a nearly testable prediction (Ars Technica)
  4. This week, I settled down to watch the first episode of The 100. If you haven't seen the show, I'll just point out that it takes place in the near future (though it ran, on the CW, in the near past). For reasons that I won't get into, there is a spacecraft with a bunch of teenagers that is traveling from a space station down to the surface of the Earth. During the reentry process, one kid wants to show that he is the master of space travel and that he's awesome. So what does he do? He gets out of his seat and floats around as a demonstration of his mastery of weightlessness. Another teenager points out that he's being pretty dumb—and that he's going to get hurt very soon. OK, that is enough of the description of the scene so that we can talk about physics. The point is that there is one dude "floating" around in the spacecraft during reentry. Before I over-analyze this short scene, let me add a caveat about my philosophy on science and stories. I've talked about this before, so I'll just give a summary: The number one job for a writer of a show is to tell a story. If the writer distorts science in order to make the plot move along—so be it. However, if the science could be correct without destroying the plot, then obviously I'd prefer it. On to the over-analysis! What Causes Gravity? Obviously this scene has to do with gravity, so we should talk about gravity—right? In short, gravity is a fundamental interaction between objects with mass. Yes, any two objects that have mass will have a gravitational force pulling them together. The magnitude of this gravitational force depends on the distance between the objects. The further apart the objects get, the weaker the gravitational force. The magnitude of this force also depends on the masses of the two objects. Greater mass means a greater force. As an equation, this would be written as: In this equation, the masses are described by the variables m1 and m2 and the distance between the objects is the variable r. But the most important thing is the constant G—this is the universal gravitational constant and it has a value of 6.67 x 10-11 Nm2/kg22. That might seem like it's important, so let me give an example that everyone can relate to. Suppose you are standing somewhere and your friend is right there with you and you two are having a conversation. Since you both have mass, there is a gravitational force pulling the two of you together. Using rough approximations for distance and mass, I get an attractive force of 3 x 10-7 Newtons. Just to put that into perspective, this value is fairly close to the force you would feel if you put a grain of salt on your head (yes, I have an approximate value for the mass of one grain of salt). So, the gravitational force is super tiny. The only way we ever notice this force is if one of the interacting objects has a super huge mass—something like the mass of the Earth (5.97 x 1024 kg). If you replace your friend with the Earth and put the distance between you and your friend-Earth as the radius of the Earth, then you get a gravitational force of something like 680 Newtons—and that is a force you can feel (and you do). Is There Gravity in Space? Now for the real question. Why do astronauts float around in space unless there is no gravity? It sure seems like there is no gravity in space—it's even referred to as "zero gravity." OK, I've answered this before, but it's important enough to revisit the question. The short answer is "yes"—there is gravity in space. Look back at the gravitational equation above. What changes in that equation as you move from the surface of the Earth into space? The only difference is the distance between you and the center of the Earth (the r). So as the distance increases, the gravitational force decreases—but by how much does the gravitational force change? How about a quick estimation? Let's use an Earth radius of 6.371 x 106 meters. With this value, a person with a mass of 70 kg would have a gravitational force of 686.7 Newtons. Now moving up to the orbital height of the International Space Station, you would be an extra 400 km farther from the center. Recalculating with this greater distance, I get a weight of 608 Newtons. This is about 88 percent the value on the surface of the Earth (you can check all my calculations here). But you can see there is clearly gravity in space. Oh, here is some extra evidence. Why does the moon orbit the Earth? The answer: gravity. Why does the Earth orbit the Sun? Yup, it's gravity. In both of these cases, there is a significant distance between the two interacting objects—but gravity still "works," even in space. But why do astronauts float around in space? Well, they float around when in orbit—if there was a super tall tower reaching into space, they wouldn't float around. The "weightless" environment is caused by the orbital motion of the people inside a spacecraft or space station. Here is the real deal. If the only force acting on a human is the gravitational force, that human feels weightless. Standing on a tall tower would result in two forces (gravity pulling down and the tower pushing up). In orbit, there is only the gravitational force—leading to that feeling of weightlessness. Actually, you don't even need to be in orbit to feel weightless. You can be weightless by having the gravitational force as the only thing acting on you. Here is a situation for you to consider. Suppose you are standing in a stationary elevator at the top of a building. Since you are at rest, the total force must be zero—that means the downward gravitational force pulling down is balanced by the upward pushing force from the floor. Now remove the force from the floor. Yes, this is difficult but it can be accomplished. Just have the elevator accelerate down with the same acceleration as a free falling object. Now you will be falling inside an elevator. The only force is gravity and you will be weightless. Some people think this falling elevator is fun. That's why many amusement parks have a ride like The Tower of Terror. Basically, you get in a car that drops off a tower. During the fall, you feel weightless—but you don't crash at the bottom. Instead, the car is on a track that somehow slows down more gradually than if it smashed into the ground. They have one of these types of rides at the NASA center in Huntsville. went on this with my kids—it was actually scarier than I had imagined. How about another example? If you are in an airplane and the plane flies with a downward acceleration, everyone inside will be weightless. Even a dog. Check it out. In the end, there seems to be huge misunderstanding about gravity. I believe the reasoning follows like so: Astronauts are weightless in space. There is no air in space. Therefore, if there is no air, there is no gravity. This no-air/no-gravity idea pops up all the time in movies (incorrectly so). Here's how you'll see it: Some dude is floating around in space (that's OK) and then he enters the airlock of a spacecraft, still floating. The airlock door shuts and air is pumped into the chamber and boom—he falls to the ground because now there's gravity. Here is what it should look like—from the epic movie 2001: A Space Odyssey. SPOILER ALERT: Hal is crazy and won't open the pod-bay doors. Not even for Dave. Wow. That scene is pretty much perfect. They even have no sound until the air comes in. What Happens During Reentry? Now back to the events in The 100. The scene doesn't take place in orbit, it occurs during reentry. This is the part where the spacecraft enters back into the atmosphere and encounters an air resistance force (because there is air). Let me start with a simple force diagram showing the spacecraft at some point during this motion. Clearly, this not weightless. Yes, there is a gravitational force acting on everything—but there is also that air drag force that will make the spacecraft slow down as it moves down. If the human is going to stay inside the spacecraft, there must also be an extra force on that human (from the floor). So, not weightless—in fact, the human would feel more than normal gravity because of the acceleration. You already know this, though, because the exact same thing happens to you in an elevator. As the elevator is moving down and coming to a stop, it is also slowing down. During this time, you would feel a little bit heavier because of the force from the floor pushing on you. You aren't really heavier, you just feel that way because of the acceleration. Again, there is another movie example where someone gets this reentry physics right. It's from Apollo 13. Check it out. Notice the water falling from the ceiling. In this case, the capsule is moving downward at an angle. However, the air resistance force is pushing in the opposite direction of motion causing the spacecraft to slow down. But what slows down the water? The water does cling to the surface a little bit—but the acceleration is too much to keep it there and it "falls" towards the astronaut. Note that "falling" here doesn't mean straight towards the surface of the Earth but rather just in the opposite direction as the acceleration. Looking back at the scene from The 100, here's how they could fix the scene—and it's pretty simple. Have the bold floating guy move around before they get to reentry. Then the other guys fall as soon as the spacecraft starts to interact with the atmosphere. That wouldn't even change the plot—and it would be more scientifically accurate. source
  5. smallhagrid

    Too cute not to post here !

    And it really speaks for itself...!
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