- 3680 - GRAVITATIONAL LENSING - can bend a light beam? When Albert Einstein developed the equation for E = m *c^2 in 1905 he showed energy and mass to be equivalent. He realized at the time that light was energy and like mass it would be pulled by the force of gravity. Gravity would bend a light beam that passed by a large enough mass.
---------- 3680 - GRAVITATIONAL LENSING - can bend a light beam?
- Einstein’s paper with these predictions was published in 1936.
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- In 1919 Eddington proved Einstein to be correct by photographing a star behind the edge of the Sun during a solar eclipse. As the star moved near the Sun’s edge it shifted position for a time and then returned to its original spot after it passed the edge. The shift in position was not very much, only 1.75 arc seconds, but this is exactly what Einstein had predicted from his equations.
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- The Full Moon is ½ degree diameter in the sky. A degree has 60 minutes of arc and an arc minute has 60 seconds of arc. A degree therefore has 3600 arc seconds. So the moon is about 1,800 arc seconds in diameter. A dime viewed face-on at a distance of one mile would have an angular diameter of 2 arc seconds. So, 1.75 arc seconds that Eddington measured was a very small amount.
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- Other measurements have also confirmed gravity’s effect on light. Mercury orbits near the Sun and the orbit we measure has a precision, or offset, at the perihelion, the orbit closest to the Sun, that is 43.03 arc seconds just as the calculations predict. And, a radar beam reflected off Mercury will be slowed on its return trip to Earth due to the pull of the Sun’s gravity by .00016 seconds, just as the equations predict.
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- These are small changes but, what happens when light passes by a very, very large mass. The bending should be much greater according to these same equations. If we get a mass as big as a galaxy or a cluster of galaxies the gravity is strong enough to bend light much like a glass lens.
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- The equations are different because we are dealing with the speed of light which uses equations of relativity. In the case of a glass lens the light is slowed down as it passes through another medium other than air. The outer part of the convex lens is thinner than the middle part so light is slowed more at the center than at the edge.
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- The light wave is like a marching band that turns a corner, the outside rows have to march faster than the inside row in order to make the bend. So the math is much different for a gravitational lens than for a glass lens although the effect is the same. Light is focused to create an image different that what is really there. A magnified image for example.
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- Even with clusters of galaxies all that gravity does not bend light very much, still 10’s of arc seconds. That is why gravity lens do not work with near by objects. The bent light beam has to travel billions of miles before it can converge at a focal length point matching the observer’s eye. Most of the bent light does not reach our eye. But, if conditions are just right the magnification could be enormous. It is possible to see planets the size of Earth orbiting far off stars.
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- The gravitational lensing effect was first discovered in 1979 when astronomers observed two images of the same quasar located behind the lens. The image distortion depends on the alignment of the lens between us and the object. With perfect alignment centered on the lens the image is distorted into a perfect ring, called the Einstein Ring.
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- This ring was first observed in 1988. If the alignment is partial, not prefect, the image becomes an elongated arc. In 1987 an arc was measured to be 300,000 light years long and 2,000 lightyears wide, making it one of the largest objects known in the Universe. Of course, it is only an image. The real galaxy lies far beyond and behind the lens and is normal galaxy size.
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- In 1984 astronomers measured a quasar that has a known and predictable variation in brightness. Then realized that they had seen the same quasar event in 1982, 1.5 years earlier. It was the same quasar. The second image had taken a longer light path due to gravitational lensing.
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- This was a fortunate happening because astronomers could use the time delay to calculate the distances the light beam traveled. This in turn allowed them to calculate Hubble’s constant to be 77 kilometers per second per mega parsec.
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- A mega parsec is 3.26 lightyears. 77 km/sec is how fast the distant galaxy or quasar is accelerating for ever 3.26 lightyears distant it lies. The further away the faster the universe is expanding.
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- A variety of other measurements have sense been made and the Hubble Constant has been calculated at 86 km/sec/mpc. The equation is:
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------------------- Recessional velocity = Hubble Constant * distance.
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- This is simply the equation for a straight line with the slope of the line denoted by the Hubble Constant.
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- So, if we can use the redshift of a light source to measure recessional velocity we can use this simple equation to determine the distance to that light source.
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- In addition to calculating distances with gravitational lensing astronomers are also able to calculate masses. By measuring the angle of the bend in the light beam astronomers can calculate the gravity of the mass that caused it. These calculations are how astronomers determined the Universe was 90% Dark Matter. The mass they calculated was 9 times bigger than could be accounted for in all the visible matter in the gravitational lens.
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- These same results are calculated when the rotation of galaxies are studied. In our solar system planets closer to the Sun, the center, orbit faster and planets farther from the Sun orbit slower. This does not happen in a galaxy. Stars far out from the massive galaxy center are moving at the same speed regardless how far out they are. This rotation can only be accounted for if there is much more mass in the galaxy than is unaccounted for n visible matter, it must be Dark Matter.
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- Current calculations are that the Universe is 23% Dark Matter, 73% Dark Energy, and only 4% ordinary matter that we see and understand. We are dealing with a Universe 96% of which we do not know what it is or how it works. Nothing like new challenges for young minds.
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- We are learning that energy tells spacetime how to curve and spacetime tells energy how to move in orbital motion and in gravitational lensing.
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- Gravitational lensing has revealed nearby clusters of galaxies that are ordinarily too dim to see. Measurement of these galaxy clusters finds 75% Dark Matter, 20% hot gas, and 5% visible matter (stars).
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- The conclusion is that Dark Matter is everywhere we just do not know what it is. Calculations are that the Dark Matter in the Earth is about the size and weight of a bowling ball, but spread throughout the earth somehow, we just can’t see it. It is still dark to us.
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September 14, 2022 GRAVITATIONAL LENSING - bends light? 581 3680
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