**Learning About Gravity**

**1.**

I was six
years old

when I had
my first lesson

I remember

I wanted to
fly and

I really
thought I could

I saw birds
doing it all the time and

it looked
easy to me

so I climbed
up on

the roof of
the garage and

stood at the
top

flapping my
arms

I wasn’t
afraid

when I
looked at the ground

before
leaping into the air

**2.**

a few years
later

the girl who
lived on the corner

moved away
and

it must have
been around

the sixth or
seventh grade

I thought

she would
always be there

I thought

we would
fall in love and

maybe

we’d get
married some day

but she left
and

I don’t
remember

if she even
told me goodbye

**3.**

it was the
middle of the night

when my
brother got killed and

two
policemen showed up

at our door
and

I remember

how I was
pulled from

my warm bed

sitting at
the bottom of the stairs

my parents
standing in the kitchen and

all the air

sucked out
of the room

as the men
in uniform

gave us the
news

Gravity -- Alessandro Rinaldi

When Aryabhata was 23 years old, in 499, he wrote a book which his disciple Bhaskara I called "Ashmakatantra" (or the treatise from the Aśmaka," a group of people who lived between the Narmada and Godavari rivers in central India) but in usually known as the "Āryabhaṭīya." It it sometimes referred to as "Arya-shatas-aShTa (Aryabhata's 108), because there are 108 terse verses in the text (in which each line is a mnemonic aid to a complex system) plus 13 introductory verses. Among his many scientific and mathematical innovations, he identified the force of gravity to explain why objects do not fall out when the Earth rotates. A century or so later Brahmagupta, who wrote in Sanscrit elliptic verse; he was the first to mention 0 as a number and he described "gruhtvaakarshan" as an attractive force. Modern work on gravitational theory began with Galileo Galilei in the late 16th and early 17th centuries, in which he showed that gravitational acceleration is the same for all objects. This insight led to Isaac Newton's 1687 deduction of the laws of gravity. His theory was used to predict the existence of Neptune based on motions of Uranus that could not be accounted for by the actions of the other planets. Calculations by John Couch Adams and Urbain Le Verrier predicted the general position of the planet, and Le Verrier's calculations are what led Johann Gottfried Galle to the discovery of Neptune. However, a discrepancy in Mercury's orbit pointed out its flaws, By the end of the 19th century it was known that its orbit showed slight perturbations that could not be accounted for entirely under Newton's theory. The issue was resolved in 1915 by Albert Einstein's new theory of general relativity, which accounted for the small discrepancy in Mercury's orbit. The equivalence principle, explored by a succession of researchers including Galileo, Loránd Eötvös, and Einstein, expresses the idea that all objects fall in the same way, and that the effects of gravity are indistinguishable from certain aspects of acceleration and deceleration. But Einstein ascribed the effects of gravitation to spacetime curvature instead of a force; matter changes the geometry of spacetime, and this is interpreted as gravity. In the following decades scientists realized that general relativity is incompatible with quantum mechanics; leading to hypotheses about gravitational waves as ripples in the curvature of spacetime that propagate likes waves at the speed of light.

ReplyDeleteOliver Heaviside discussed the possibility of gravitational waves in 1893 by using the analogy between the inverse-square law in gravitation and electricity. In 1905 Henri Poincaré first proposed gravitational waves (ondes gravifiques) that emanate from a body and propagate at the speed of light, as required by the Lorentz transformations. When Einstein published his theory of general relativity in 1915, he was skeptical of Poincaré's idea since they implied there were no "gravitational dipoles" but in 1916 envisioned gravitational waves transporting energy as gravitational radiation, a form of radiant energy similar to electromagnetic radiation. (Gravitational waves are incompatible with Newton's law of universal gravitation, since it is predicated on the assumption that physical interactions propagate at infinite speed.) However, the nature of Einstein's approximations led many (including Einstein himself) to doubt the result. In 1922, Arthur Eddington showed that two of Einstein's three types of waves were artifacts of the coordinate system he used and could be made to propagate at any speed by choosing appropriate coordinates, leading Eddington to jest that they "propagate at the speed of thought") and cast doubt on the physicality of the third type (which Eddington showed always propagates at the speed of light regardless of coordinate system). In 1936 Einstein and Nathan Rosen submitted a paper to "Physical Review" claiming gravitational waves could not exist; the "Review" sent their manuscript to Howard P. Robertson for review, and he reported that the singularities Einstein and Rosen postulated were merely the harmless coordinate singularities of the employed cylindrical coordinates. Einstein, unfamiliar with the concept of peer review, angrily withdrew the manuscript and never published in "Physical Review" again, but his assistant Leopold Infeld, who had been in contact with Robertson, convinced him that the criticism was correct and the paper was rewritten with the opposite conclusion (and published elsewhere). In 1956 Felix Pirani remedied the confusion caused by the use of various coordinate systems by rephrasing the gravitational waves in terms of observable Riemann curvature tensors.

ReplyDeleteIn 1974 Russell Alan Hulse and Joseph Hooton Taylor, Jr. discovered the first binary pulsar (a discovery that earned them the 1993 Nobel Prize in Physics). In 1979, results were published detailing measurement of the gradual decay of the orbital period of the Hulse-Taylor pulsar, which fitted precisely with the loss of energy and angular momentum in gravitational radiation predicted by general relativity. This led to gravitational-wave astronomy as an emerging branch of observational astronomy which aims to use gravitational waves to collect observational data about objects such as neutron stars and black holes, events such as supernovae, and processes including those of the early universe shortly after the Big Bang. In 1984 Kip Thorne, Ronald Drever, and Rainer Weiss formed a steering committee after the National Science Foundation asked the Massachusetts and California Institutes of Technology (MIT and Caltech) to join forces to lead a Laser Interferometer Gravitational-Wave Observatory (LIGO) project; however, it took a decade for the LIGOlLaboratory director Barry Barish to create the LIGO study, project plan, and budget, receive funding, and begin construction; LIGO, budgeted at US$395 million, became the NSF's most expensive project. The search for gravitational waves began in 2002. Three groups in 2005 independently solved the binary black hole problem and modeled the inspiral, merger, and ring-down of the phenomenon. In 2016, LIGO announced the detection in September 2015 of gravitational waves from a signal received from two black holes with masses of 29 and 36 solar masses merging about 1.3 billion light years away. During the final fraction of a second of the merger, it released more than 50 times the power of all the stars in the observable universe combined. The signal increased in frequency from 35 to 250 Hz over 10 cycles (5 orbits) as it rose in strength for a period of 0.2 second. The mass of the new merged black hole was 62 solar masses. Energy equivalent to three solar masses was emitted as gravitational waves. Later that year LIGO also announced a December detection of the merger of two smaller black holes.

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