Yesterday, we loitered around the ancient mythological realm. Today, we sail as far afield as we can go without knocking against the plexiglass bubble at the Universe's outer edge. At a subscriber's behest, we'll expound on 'gravitational lensing,' that strange phenomenon one can only observe at the highest levels of physical reality.* Simply, gravitational lensing is the distortion or even multiplication of a distant object's image caused by space time contortions that massive foreground objects induce. Even though Albert Einstein predicted this effect with his General Theory of Relativity (1916), it was only observed in the late 1970s.
Gravitational lensing is only explicable once one understands General Relativity's description of gravity. Contrary to Newton's assertion that gravity was a force mediated between massive objects, Einstein determined gravity to be a property of spacetime,** not of particles. A massive object, such as the Sun, Earth or even a galaxy, distorts its local space-time geometry, producing indentations or "wells" that attract other massive bodies. Those bodies, of course, are surrounded by their own self-generated indentations, and through these are similarly attractive. The magnitude of this distortion depends on the object's mass and size, which makes sense. A dense cannon ball bends a taut rubber sheet to a greater extent than a ball bearing. Similarly, a galaxy distorts spacetime far more than a single star or even star cluster.
These distortions do not merely affect matter, but light as well. As electromagnetic radiation propagates through spacetime, any curvature within it will alter a photon's travel path. This effect might be slight, such as the Sun's bending influence on the starlight passing near it,*** or it could be profound, as when a quasar's light is bent or multiplied as it passes an entire galaxy. A quasar, or "quasi-stellar" object, is believed to be the highly energetic centers of early galaxies, properly known as 'active galactic nuclei.' These are amongst the most distant objects ever observed, meaning, of course, that we're seeing them as they were in the remote past. (The closest known active quasar, 3C 273, is 2.4 billion light years away.)
"Einstein's Cross."
Image captured by the Hubble Space Telescope
Astronomers discovered the first example of this long scale gravitational lensing in 1979 by observing the "Twin Quasars" QSO 0957+561 A (QSO 0957+561 B These two quasars were found to have the same exact properties, including their red shifts (the measure of how fast they recede from us.) Finding two quasars that are exactly alike and at the same distance (8.7 billion light years) is exceedingly improbable. Scientists finally realized that they were observing two images of the same quasar! A giant elliptical galaxy and other members within its cluster created two quasar apparitions separated by six arc seconds. Although by the late 1970s', General Relativity was widely accepted, this discovery offered yet more proof to substantiate it.
Perhaps the most famous example of gravitational lensing is "Einstein's Cross." This 'cross' consists of four different images of the same quasar! These images are arranged as though defining the four cardinal points, thereby producing a cross shape. The actual quasar, Q2237+030, is 8 billion light years away. The lensing occurs when the quasar light moves around a galaxy 'merely' 400 million years from us. The gravitational lensing is so powerful that the one quasar appears to us as a quintuplet!
Through the cosmos we see examples of how malleable spacetime can be in the right circumstances. Einstein's Cross, which one can find with a sufficiently powerful telescope (18" diameter or greater), is a dramatic illustration of how an object as large as a galaxy can produce spectacular mirages. When we see the four points of Einstein's Cross, we see the split light of something as it appeared nearly three billion years before our own solar system formed. That it is hidden in the sky within an unremarkable region between Pegasus and Aquarius shows that astronomy is as delightful as damnit.
*Heavens above, when we refer to 'highest levels of physical reality,' we are not kidding. At this extreme cosmological realm, even entire galaxies are as particles in a teeming swarm.
**Spacetime," We bandy this term about every so often, so every so often we should explain it. Before Einstein set things right, physics regarded space and time as separate and absolute. They were unchangeable and disconnected. Puzzle out one of those ball off a cliff physics problems and you find the ball accelerates through immutable space throughout constant time intervals. However, this neat appearance of absolutism is a situational accident, caused by the ball descending at low speed within a weak gravity field. (Yes! Compared to the big league entities, Earth is a gravitational weakling.) However, take that same ball and let it descend above a neutron star: suddenly, life isn't so tidy and one realizes that time and space both distort dramatically at rapid velocities and in high gravity regions. Space and time are therefore shown to be manifestations of one continuum - spacetime.
***In 1919, Sir Arthur Eddington led an expedition to South America to observe the star field behind an eclipsed Sun. As Einstein predicted, the star's positions were slightly altered as they passed through the Sun's gravity well. This experiment provided conclusive proof that gravity distorts a light's path.
Astronomers discovered the first example of this long scale gravitational lensing in 1979 by observing the "Twin Quasars" QSO 0957+561 A (QSO 0957+561 B These two quasars were found to have the same exact properties, including their red shifts (the measure of how fast they recede from us.) Finding two quasars that are exactly alike and at the same distance (8.7 billion light years) is exceedingly improbable. Scientists finally realized that they were observing two images of the same quasar! A giant elliptical galaxy and other members within its cluster created two quasar apparitions separated by six arc seconds. Although by the late 1970s', General Relativity was widely accepted, this discovery offered yet more proof to substantiate it.
Perhaps the most famous example of gravitational lensing is "Einstein's Cross." This 'cross' consists of four different images of the same quasar! These images are arranged as though defining the four cardinal points, thereby producing a cross shape. The actual quasar, Q2237+030, is 8 billion light years away. The lensing occurs when the quasar light moves around a galaxy 'merely' 400 million years from us. The gravitational lensing is so powerful that the one quasar appears to us as a quintuplet!
Through the cosmos we see examples of how malleable spacetime can be in the right circumstances. Einstein's Cross, which one can find with a sufficiently powerful telescope (18" diameter or greater), is a dramatic illustration of how an object as large as a galaxy can produce spectacular mirages. When we see the four points of Einstein's Cross, we see the split light of something as it appeared nearly three billion years before our own solar system formed. That it is hidden in the sky within an unremarkable region between Pegasus and Aquarius shows that astronomy is as delightful as damnit.
*Heavens above, when we refer to 'highest levels of physical reality,' we are not kidding. At this extreme cosmological realm, even entire galaxies are as particles in a teeming swarm.
**Spacetime," We bandy this term about every so often, so every so often we should explain it. Before Einstein set things right, physics regarded space and time as separate and absolute. They were unchangeable and disconnected. Puzzle out one of those ball off a cliff physics problems and you find the ball accelerates through immutable space throughout constant time intervals. However, this neat appearance of absolutism is a situational accident, caused by the ball descending at low speed within a weak gravity field. (Yes! Compared to the big league entities, Earth is a gravitational weakling.) However, take that same ball and let it descend above a neutron star: suddenly, life isn't so tidy and one realizes that time and space both distort dramatically at rapid velocities and in high gravity regions. Space and time are therefore shown to be manifestations of one continuum - spacetime.
***In 1919, Sir Arthur Eddington led an expedition to South America to observe the star field behind an eclipsed Sun. As Einstein predicted, the star's positions were slightly altered as they passed through the Sun's gravity well. This experiment provided conclusive proof that gravity distorts a light's path.