- 3047 - EINSTEIN - are his theories real life? One of the most mysterious components in the entire Universe is “dark energy“, which wasn’t supposed to exist. We had assumed that the Universe was a balancing act, with the expansion of the Universe and the gravitational effects of everything within it fighting against one another.
--------------- 3047 - EINSTEIN - are his theories real life?
- If gravity won over dark energy, the Universe would recollapse; if the expansion won, everything would fly away into oblivion. And yet when we made the critical observations in the 1990s and beyond, we found that not only is the expansion winning, but the distant galaxies we see speed away from us at faster and faster rates as time goes on. The expansion is accelerating?
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- Is Einstein’s cosmological constant the same as dark energy? Why has, over time, the term “dark energy” replaced the original term “cosmological constant?” Are the two terms identical or not?
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- When Einstein was working on a theory of gravity to replace and supersede Newton’s law of universal gravitation, we didn’t yet know very much about the Universe. Sure, the science of astronomy was thousands of years old, and the telescope itself had been around for the better part of three centuries.
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- We had measured stars, comets, asteroids, and nebulae; we had witnessed novae and supernovae; we had discovered variable stars and knew about atoms; and we had revealed intriguing structures in the sky, like spirals and ellipticals.
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- We didn’t know that these spirals and ellipticals were galaxies all unto themselves. In fact, that was only the second-most popular idea; the leading idea of the day was that they were entities, perhaps proto-stars in the process of forming, contained within the Milky Way galaxy, which itself comprised the entire Universe.
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- Einstein was looking for a theory of gravity that could be applied to anything and everything that existed, and that included the known Universe as a whole.
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- The gravitational behavior of the Earth around the Sun is not due to an invisible gravitational pull, but is better described by the Earth falling freely through curved space dominated by the Sun. The shortest distance between two points isn’t a straight line, but rather a geodesic: a curved line that’s defined by the gravitational deformation of spacetime.
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- The problem became apparent when Einstein succeeded in formulating his theoretical General Relativity instead of being based on masses exerting forces on one another infinitely fast across infinite distances, Einstein’s conception was vastly different.
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- First, because space and time were relative for each and every observer, not absolute, the theory needed to give identical predictions for all observers: what physicists call “relativistically invariant.”
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- That meant instead of separate notions of space and time, they needed to be woven together into a four-dimensional fabric: spacetime. And instead of propagating at infinite speeds, gravitational effects were limited by the speed of gravity, which in Einstein’s theory equals the speed of light.
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- The key advance that Einstein made was that, instead of masses pulling on each other, gravity worked by both matter and energy curving the fabric of spacetime. That curved spacetime, in turn, then dictated how matter and energy moved through it.
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- At each instant in time, the matter and energy in the Universe tells spacetime how to curve, the curved spacetime tells matter how to move, and then it does: the matter and energy moves a tiny bit and the spacetime curvature changes.
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- And then, when the next instant arrives, the same equations of General Relativity tell both the matter and energy and the spacetime curvature how to evolve into the future.
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- If Einstein had stopped there, he would have instigated a cosmic revolution. On the one hand and on one side of the equation, you had all the matter and energy in the Universe, while on the other hand and the other side of the equals sign in the equation, you had the curvature of spacetime.
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- Whatever the equations predict should tell you what happens next. When Einstein solved those equations a large distance away from a small mass, he got Newton’s law of universal gravitation back. When he got closer to the mass, he started to get corrections, which both explained the orbit of Mercury and predicted that starlight passing near the Sun during a total solar eclipse would be deflected.
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- If we assumed that the Universe was filled roughly evenly with matter, we could solve that scenario. What Einstein discovered was disconcerting: the Universe was unstable. If it began in a stationary spacetime, the Universe would collapse in on itself. So Einstein, to fix this, invented a “cosmological constant“ to get things back into balance.
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- In a Universe that isn’t expanding, you can fill it with stationary matter in any configuration you like, but it will always collapse down to a blackhole. Such a Universe is unstable in the context of Einstein’s gravity, and must be expanding to be stable, or we must accept its inevitable fate.
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- You have to understand where the idea of a cosmological constant comes from. There’s a very powerful mathematical tool that we use all the time in physics: a differential equation. Something as simple as Newton’s F = ma is a differential equation. All it means is that this equation tells you how something will behave in the next moment, and then, once that moment has elapsed, you can put those new figures back into the same equation, and it will go on to tell you what happens in the next moment.
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- A differential equation will tell you what happens to a ball rolling down a hill on the Earth. It tells you what path it will take, how it will accelerate, and how its position will change at every moment in time. Just by solving the differential equation describing the ball rolling down the hill, you can know precisely what trajectory it will take.
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- The differential equation tells you almost everything you’d want to know about the ball rolling down the hill, but there’s one thing it can’t tell you: how high the base level of the ground is. You have no way of knowing whether you’re on a hill atop a plateau, on a hill that ends at sea level, or on a hill that ends in a hollowed-out volcanic crater. An identical hill at all three elevations will be described by the exact same differential equation.
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- When we see something like a ball balanced atop a hill, this appears to be what we call a finely-tuned state, or a “state of unstable equilibrium“. A much more stable position is for the ball to be down somewhere at the bottom of the valley. But is the valley at zero, or some non-zero positive or negative value? The mathematics of a ball rolling down the hill is identical up to this additive constant.
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- That same problem shows up in calculus when you first learn how to do an indefinite integral; anyone who’s taken calculus will remember the infamous “plus C” that you have to add at the end. Einstein’s General Relativity isn’t just one differential equation, but a matrix of 16 differential equations, related in such a way that 10 of them are independent of one another.
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- But to each of those differential equations, you can add a constant, plus C, in a particular way, This became known as the cosmological constant. Perhaps surprisingly, it’s the only thing you can add to General Relativity, besides another form of matter or energy, that won’t fundamentally alter the nature of Einstein’s theory.
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- Instead of going with what the equations , Einstein threw the cosmological constant in there in order to “fix” what appeared to be an otherwise broken situation. If he had listened to the equations, he could have predicted the expanding Universe.
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- Einstein made a balanced universe theory not because it was allowed, but because, for him, it was preferred. Without adding a cosmological constant in, his equations predicted that the Universe should either be expanding or contracting, something that clearly wasn’t happening.
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- Instead, the work of others would have to overturn Einstein’s prejudicial choices, with Einstein himself only abandoning the cosmological constant in the 1930s, well after the expanding Universe had been observationally established.
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- While matter, both normal and dark, and radiation become less dense as the Universe expands owing to its increasing volume, dark energy, and also the field energy during inflation, is a form of energy inherent to space itself. As new space gets created in the expanding Universe, the dark energy density remains a constant acceleration.
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- The cosmological constant is unlike the types of energy we know of otherwise. When you have matter in the Universe, you have a fixed number of particles. As the Universe expands, the number of particles stays the same, so the density goes down over time.
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- With radiation, not only are the number of particles fixed, but as the radiation travels through the expanding Universe, its wavelength stretches relative to an observer that will someday receive it: its density goes down, and each individual quantum also loses energy with time.
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- The cosmological constant is a constant form of energy that is “intrinsic to space“. If we extrapolate back in time to when the Universe was younger, hotter, denser, and smaller the cosmological constant wouldn’t have been noticeable. It would have been swamped by the much larger effects of matter and radiation early on.
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- Only after the Universe has expanded and cooled so that the matter and radiation density drops to a low enough value can the cosmological constant finally appear.
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- Dark energy might turn out to be a cosmological constant. Certainly, when we take all of the observations we have so far, it appears that dark energy is consistent with being a cosmological constant, as the way the expansion rate changes over time agrees, within the uncertainties, with what a cosmological constant would be responsible for.
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- But there are uncertainties there, and dark energy could be:
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---------------------------- increasing or decreasing in strength over time,
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---------------------------- changing in energy density, unlike a cosmological constant,
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---------------------------- or evolving in a novel, complicated fashion.
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- Although we have constraints on how much dark energy could be evolving by over the past 6 billion years, we cannot definitively say it’s a constant. While the energy densities of matter, radiation, and dark energy are very well known, there is still plenty of room in the equation of state of dark energy. It could be a constant, but it could increase or decrease in strength over time as well.
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- By the end of the 2020s, we’ll have an enormous and comprehensive ground-based survey of the universe from the “Vera C. Rubin Observatory“. We’ll have an enormous suite of space-based data thanks to the “ESA’s Euclid Observatory” and NASA’s “Nancy Roman telescope“, which will see more than 50 times as much Universe as Hubble presently sees.
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- With all of this novel data, we should be able to determine whether dark energy is truly identical to what the very specific “cosmological constant” predicts, or whether it varies in any way at all.
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- The cosmological constant may be the same thing as dark energy, but it doesn’t need to be. Even if it is, we’d still like to understand why it behaves this particular way and not any other.
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- As 2020 comes to a close and 2021 dawns, it’s important to remember the most vital lesson of all: the answers to our deepest cosmic questions are written on the face of the Universe. If we want to know them, the only way is to put the question to our physical reality itself.
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- The past century witnessed spectacular upheavals to our understanding of space and time. The next decade holds the promise of taking these insights to an entirely new level.
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- Einstein established that if we are moving relative to one another, our watches will tick off time at different rates and our tape measures will have different lengths. At familiar speeds, the effects are tiny, explaining why no one had noticed.
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- But Einstein’s mathematical derivation of the conclusion was so simple, relying on nothing more advanced than high-school algebra, that the community of physicists quickly relinquished common intuition in favor of a startlingly new flexibility of space and time.
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- There is now no controversy that were you to take a round-trip journey at nearly light speed, upon landing back on Earth, you would find that you had traveled far into the future.
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- Einstein went further still, realizing that distortions to space and time could be yet more severe: Massive objects cause the fabric of spacetime to sag, somewhat like a bowling ball resting on a trampoline.
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- Such spacetime curvature then affects the motion of nearby objects, such as a planet moving near a star, leading to a new and demonstrably deeper understanding of gravitational force: Gravity is nothing but grooves in the spacetime fabric that guide an object’s motion.
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- These results constitute Einstein’s “special and general theories of relativity“, and have led to profound insights into the origin of the universe, the nature of blackholes and gravitational waves, ripples in the very fabric of spacetime.
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- Einstein’s Relativity is silent on what space and time actually are. When we speak of the spacetime fabric, is that a metaphor? Or, are space and time material things? And if so, can we identify the threads stitching the spacetime fabric? Current research suggests that the answers may not be far off.
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- Einstein’s work on the photoelectric effect, which was one of the earliest and most influential papers on what would become quantum mechanics, the curious laws that are most manifest in the microworld of atoms and particles.
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- As quantum mechanics developed, Einstein grew ever less comfortable with its unfamiliar portrayal of reality. He recoiled at a world governed by probabilities, musing that if this were truly how reality works, he’d rather be a croupier than a physicist. A coupier is a person in charge of a gambling table.
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- Einstein believed that he had pinpointed quantum mechanics’ Achilles heel. Relying on a feature of the theory known as “quantum entanglement“, Einstein argued that two distinct and widely separated particles could act as though they are single entity.
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- Were you to interact with one of the particles, quantum mechanics predicts that you would instantaneously influence measurable properties of the other, even though it is thousands of kilometers away. Einstein considered this implication of quantum theory to be ludicrous, famously deriding it with the characterization “spooky.”
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- Einstein unwittingly illuminated a quality of the microworld that may be essential to understanding the very fabric of spacetime. It is spooky.
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- Physicists have realized that quantum entanglement is relevant not only to particles within space but to space itself. Space is permeated by fields, electric, magnetic, nuclear, gravitational, and depending on their overall configuration, quantum mechanics establishes relationships between the properties of the fields at different locations.
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- When these fields have the least amount of energy possible, the emptiest that empty space can be, these relationships amount to quantum mechanics entangling different locations throughout empty space.
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- Quantum mechanics may help us understand the weave of the spacetime fabric, perhaps explaining why they exist at all. How wonderfully ironic that the very theory Einstein so thoroughly resisted may provide the deepest rationale for the foremost ingredients in his life’s work: space and time.
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- An observation of stars near the Sun during a solar eclipse found their apparent position shifted just as Einstein had predicted. Newton’s law of gravity, considered inviolable for over two centuries, had been repealed.
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- While Einstein’s gravity has passed every test so far, nobody knows for sure that it applies everywhere, under all conditions. In particular, there is no guarantee that general relativity reigns over the entire expanse of the universe.
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- Today general relativity has earned increasing popular notoriety as scientists have verified its more exotic predictions, including blackholes and the vibrations in space known as “gravitational waves“.
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- But general relativity’s string of successes may not be endless. It’s true that the theory describes the observable universe quite well. That description includes massive amounts of invisible mass, known as dark matter, along with a peculiar repulsive force, called dark energy, occupying all of space. But the dark matter and dark energy s’ existence is deduced from the assumption that general relativity is correct.
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- The main assumption is that general relativity is the underlying theory of gravity, If you don’t assume general relativity is in fact correct, then “evidence for the dark matter and dark energy may signal a breakdown of general relativity on cosmological scales.
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- In other words, it’s conceivable that there is no dark energy. If that’s the case, apparent evidence for its existence might actually be a sign that the true cosmic theory of gravity differs from Einstein’s. If so, the current picture of the cosmos would have to be drastically redrawn.
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- Physicists still have plenty of reason for confidence in general relativity’s reliability. It solved a knotty problem that had perplexed astronomers about the planet Mercury: a discrepancy in its orbit from that forecast by Newtonian gravity. Einstein announced his theory in 1915 as soon as he was able to show that it correctly predicted Mercury’s actual orbit.
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- Einstein’s key to solving the Mercury mystery was conceiving gravity as an effect of the geometry of space, or, space-time, since his earlier work had shown space and time to be inseparable.
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- Gravity is not a mutual tug of massive objects, Einstein said, but rather the result of a mass’s distortion of the space-time surrounding it. Objects orbit or fall into a massive body depending on how strongly the space-time around it is curved. Rather than responding to some attractive force, masses just follow the contours of space-time’s geometry.
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- Gravity as geometry led to the famous prediction verified in the 1919 eclipse. Einstein pointed out that the curvature of space-time near the Sun would cause light from distant stars to bend when passing nearby, changing the stars’ apparent positions as seen from Earth.
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- Einstein’s gravity has passed many additional tests, such as the spectacular detection of gravitational waves, reported in 2016. But it’s not possible to test the theory under all conceivable conditions. And experts have long suspected that general relativity can’t be right in realms of extremely high mass density.
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- At the center of a blackhole the theory’s equations no longer make sense, because they imply that matter density would become infinite.
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- Scientists on Earth can probe realms of fairly strong gravity, possibly offering clues. One project uses a network of telescopes to image the region near the outer edge of a blackhole, its “event horizon , the point of no return for anything falling in. Such images can provide details of how matter flows into the blackhole from its “accretion disk,” a ring of orbiting material outside the event horizon.
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- By analyzing the structure of the accretion flow, it should be possible to probe the structure of space-time and test whether it is consistent with general relativity.
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- Gravitational waves can also provide details of gravity under extreme conditions, as when two blackholes collide. Analyzing the space-time ripples emanating from such collisions could reveal possible flaws in general relativity’s predictions.
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- Unification of general relativity and quantum physics is widely considered as the most outstanding open problem in fundamental physics. Such a unifying theory, most experts believe, would entail some sort of modification to general relativity.
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- One way of modifying the theory would be incorporating a new energy field permeating space. The strength of such a field at different locations could alter general relativity’s predictions for matter’s behavior.
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- General Relativity requires that gravity travels at the speed of light. Gravitational waves provide a way to test that. In 2017, the merger of two neutron stars not only sent gravitational waves to Earth, traversing a distance of 130 million light-years, but also released bursts of electromagnetic radiation, including X-rays and gamma rays, which travel at precisely the same speed as light.
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- Arrival time for the electromagnetic rays and the gravitational waves showed their travel speeds to be identical, i.e. within one part in a quadrillion, ruling out many alternative gravity theories that predicted a difference.
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- An effect predicted by Albert Einstein has been identified in a double star system about 29,000 light years from Earth. This phenomenon, called a 'gravitational redshift,' has been well documented in our Solar System, but it's been more elusive farther away.
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- Scientists saw evidence for this effect in the X-rays from a system with a neutron star in close orbit with a companion star.
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- Gravitational redshifts are crucial for maintaining the accuracy of technologies like the global positioning system (GPS). What do Albert Einstein, the Global Positioning System (GPS), and a pair of stars 200,000 trillion miles from Earth have in common?
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- The effect from Einstein's General Theory of Relativity, the "gravitational redshift," has light shifting to redder colors because of gravity.
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- Using NASA's Chandra X-ray Observatory, astronomers have discovered the phenomenon in two stars orbiting each other in our galaxy about 29,000 light years (200,000 trillion miles) away from Earth.
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- While these stars are very distant, gravitational redshifts have tangible impacts on modern life, as scientists and engineers must take them into account to enable accurate positions for GPS.
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- The intriguing system known as “4U 1916-053” contains two stars in a remarkably close orbit. One is the core of a star that has had its outer layers stripped away, leaving a star that is much denser than the Sun.
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- The other is a neutron star, an even denser object created when a massive star collapses in a supernova explosion. These two compact stars are only about 215,000 miles apart, roughly the distance between the Earth and the Moon.
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- While the Moon orbits our planet once a month, the dense companion star in
“4U 1916-053” whips around the neutron star and completes a full orbit in only 50 minutes.
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- Analysis of X-ray spectra, the amounts of X-rays at different wavelengths found the characteristic signature of the absorption of X-ray light by iron and silicon in the spectra. In three separate observations with Chandra, the data show a sharp drop in the detected amount of X-rays close to the wavelengths where the iron or silicon atoms are expected to absorb the X-rays.
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- However, the wavelengths of these characteristic signatures of iron and silicon were shifted to longer, or redder wavelengths compared to the laboratory values found here on Earth . The researchers found that the shift of the absorption features was the same in each of the three Chandra observations, and that it was too large to be explained by motion away from us. Instead they concluded it was caused by gravitational redshift.
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- How does this connect with General Relativity and GPS? As predicted by Einstein's theory, clocks under the force of gravity run at a slower rate than clocks viewed from a distant region experiencing weaker gravity.
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- This means that clocks on Earth observed from orbiting satellites run at a slower rate. To have the high precision needed for GPS, this effect needs to be taken into account or there will be small differences in time that would add up quickly, calculating inaccurate positions.
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- All types of light, including X-rays, are also affected by gravity. An analogy is that of a person running up an escalator that is going down. As they do this, the person loses more energy than if the escalator was stationary or going up.
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- The force of gravity has a similar effect on light, where a loss in energy gives a lower frequency. Because light in a vacuum always travels at the same speed, the loss of energy and lower frequency means that the light, including the signatures of iron and silicon, shift to longer wavelengths.
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- This is the first strong evidence for absorption signatures being shifted to longer wavelengths by gravity in a pair of stars that has either a neutron star or blackhole. Strong evidence for gravitational redshifts in absorption has previously been observed from the surface of white dwarfs, with wavelength shifts typically only about 15% of that for
“4U 1916-053“.
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- Scientists say it is likely that a gaseous atmosphere blanketing the disk near the neutron star absorbed the X-rays, producing these results. The size of the shift in the spectra allowed the team to calculate how far this atmosphere is away from the neutron star, using General Relativity and assuming a standard mass for the neutron star.
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- They found that the atmosphere is located 1,500 miles from the neutron star, about half the distance from Los Angeles to New York and equivalent to only 0.7% of the distance from the neutron star to the companion. It likely extends over several hundred miles from the neutron star.
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- In two of the three spectra there is also evidence for absorption signatures that have been shifted to even redder wavelengths, corresponding to a distance of only 0.04% of the distance from the neutron star to the companion.
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- The effect from Einstein's General Theory of Relativity is called the "gravitational redshift," where light is shifted to redder colors because of gravity. Chandra’s X-ray Observatory, has discovered the phenomenon in two stars orbiting each other in our galaxy about 29,000 light years (200,000 trillion miles) away from Earth.
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- While these stars are very distant, gravitational redshifts have tangible impacts on modern life, as scientists and engineers must take them into account to enable accurate positions for GPS. As predicted by Einstein's theory, clocks under the force of gravity run at a slower rate than clocks viewed from a distant region experiencing weaker gravity.
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- This means that clocks on Earth observed from orbiting satellites run at a slower rate. To have the high precision needed for GPS, this effect needs to be taken into account or there will be small differences in time that would add up quickly, calculating inaccurate positions.
February 14, 2021 3047
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