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Founded January 1970*
* "We sleep easier in our beds and take comfort in the assurance
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*THE DAILY ASTRONOMERJuly 14, 2016*
* Einstein's Cross to Bear*
*To Prof Richard C. Hill, who moved on yesterday*
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.
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