[image: ChronosTime.jpg]
*Chronos: *   God of Time
It is the mythological Universe's most troublesome heterograph:  Cronos,
the Titan who sired the gods, and Chronos, the god of time who, according
to Orphic tradition, preceded Chaos, the unbounded nothing that predated
the Universe.  Although they were entirely different in character,
appearance and time frame, these two old fellows have often been fused into
a single personification merely because their names sound exactly alike,
even though the spellings differ by a single letter.    Consequently, the
Titan Cronos is often depicted wielding a scythe, the implement most
closely associated with Death Reapers who employ this device to level all
life forms.    In fact, Cronos only possessed a sickle of adamant, with
which he castrated his father Ouranos in order to liberate his siblings
imprisoned in Gaia's womb.   The actual scythe holder was the venerable
Chronos who arose from Earth and water to sire Aether, Chaos and a
Primordial Egg.     The personification of the loftiest sky regions, Aether
became the rarefied air that only the gods could breathe. Such was the
potency of this air that it quickly overwhelmed any mortal who tried to
inhale it.   (In some cases, it induced such an intense ecstasy into
mortals that they soon fainted from the sensation before bursting into
flame.)    Chaos, as we have seen, was the nothing from which the cosmos
took form.  The Primordial Egg produced the hermaphroditic Phanes who
became the progenitor of all living beings.       So, Gaia and Ouranos
emerged from Phanes, as opposed to Chaos.
          Like most of the truly ancient deities, Chronos hardly ever
became involved in the lives of those he engendered.      Instead, Chronos
was an ageless deity who devoted himself to turning the vast "zodiac
wheel."     This wheel contained the twelve constellations through which
the Sun annually progressed.      This one task might seem to have been
light work for Chronos, considering that nothing can occur in the Universe
without time .   However, Chronos relinquished his control over most
aspects of his dominion to the "younger" generation.   Thanatos presided
over death; Eileithyia  was the goddess of childbirth and labor pains;
Demeter governed the harvest; Hebe was the goddess of youth; Geras was the
god of old age; the moon phases fell under Artemis' domain; Apollo guided
the Sun; and Astraeus  controlled the stars and planets.   Instead of
shouldering these burdens himself, Chronos delegated them to others and as
a consequence the mythological Universe became brilliantly well ordered.
Though often portrayed as an elderly man with a luxuriant mane and beard of
white hair, he is also seen as a combination of a man, a lion and a bull.
His daughter Ananke, a snake, was also said to have revolved around him and
the primordial egg before it hatched out Phanes and then ultmately every
living, breathing entity that populated Olympus and the world.

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THE DAILY ASTRONOMER
Thursday, October 8, 2020
Remote Planetarium 105:  General Relativity II - Gravitational Time Dilation

That gravity affects time is one of General Relativity's most startling
consequences.  The time flow in a region close to a massive body differs
from that farther away from it.   For instance, regard  two atomic clocks,
one located in Greenwich, England (sea level) and the other, NIST-F1, in
Boulder, CO (5,238 feet above sea level).    Based on atomic transitions,
these two clocks serve as highly precise time keepers that are so accurate
they have an expected error of about one second over 100 million years.*
  As these clocks occupy different altitudes, they differ slightly in their
time keeping.  The atomic clocks in Greenwich run 5 microseconds a year
more slowly than the Boulder unit, which is farther away from Earth.
This difference is a practical example of *gravitational time dilation,* the
time dilation resulting from the presence of massive objects.

[image: 1280px-Modern-Trade-Painter.jpg]
Want to live longer?  Climb a ladder.    Provided you remain at the
ladder's top, you will gain 90 billionths of a second over a 70 year
period.



[image: GxoYshjWvH7yXJ23Uymqnc-1200-80.jpg]
GPS (Global positioning system) satellites must be calibrated to take
gravitational time dilation into account.  As these satellites revolve
around Earth at an approximate altitude of 12,550 miles (medium Earth
orbit), their on-board clocks run about 35 microseconds a day faster than
clocks on Earth's surface.


*Time runs more slowly around a gravitational body than it does away from
it.*

To explain why gravity influences time, we must return to the Special
Relativity's fundamental postulate:     * the speed of light is constant in
all inertial reference frames.*

A light beam travels from one point to the other at a constant speed.
However, as we learned yesterday, a massive object distorts its local
space-time geometry.  The amount of material present (and its density)
determines the extent of this deformation.           Regard two sets of
points: a to b and c to d.

Points a and b, being far away from a massive body, are separated by a
perfectly straight line.     Points c and d are located within a field of
higher gravitational potential (near a massive body).    These points are
separated by a curved line resulting from the space-time deformation.   The
path separating a and b is shorter than that between c and d.   The light
follows a longer path between c and d in a stronger gravitational field
while maintaining the same rate of speed.  As a consequence, time dilates,
or runs more slowly, in the gravitational field.

[image: gravity time diagram.png]

The amount of material present determines the extent of this dilation.
To cite an extreme example, let's examine the region around a stellar mass
black hole, a region of space where the gravitational pull is so strong
that nothing can escape from it, not even light.  (Recall that black holes
form after the collapse of a highly massive star.)


[image: maxresdefault.jpg]

Imagine that you and a friend are stationed along a platform well outside
the black hole's Event Horizon, the surface boundary surrounding the black
hole that marks the region from which light cannot escape.      Your friend
decides to travel toward the hole while you remain safely on the platform
watching his progress toward it.      At first you don't notice anything
amiss.  He progresses smoothly and rapidly troward his doom.  However, as
he approaches the event horizon, his progress slows more and more as he
draws closer.      Then, astonishingly, he "freezes."    You watch him just
float there outside the black hole and never appears to enter it.**    Even
though time doesn't stop for him, from your perspective he is suspended
there forever.

From GPS satellites to atomic clocks to Black Holes, we find examples of
gravitational time dilation throughout the Universe.    Another remarkable
aspect of physical reality.

On Monday, we'll learn more about space-time curvature, including the
now-famous Eddington expedition in 1919 that provided the first direct
evidence supporting the theory that matter curves space.


*Quantum logic clocks and optical clocks have now been developed with
accuracies of one second for every one billion years.

**We are ignoring the powerful differential gravitational effects that
would literally rip your friend into his component subatomic particles long
before he reached the event horizon.

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