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Cell["Why Study Gravity?", "Title"],
Cell[CellGroupData[{
Cell["George E. Hrabovsky", "Author"],
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Cell["\<\
So, my wife\[LongDash]Dianna\[LongDash]said to me, \[OpenCurlyDoubleQuote]The \
way you do all of this stuff is boring!\[CloseCurlyDoubleQuote] Meaning \
gravitational research and teaching. I was offended, I was hurt, I was\
\[LongDash]stopped by the realization that she might be right. It is easy to \
imagine people grounded in the real world finding things in gravitational \
research a little esoteric. We use phrases like Killing vector fields, event \
horizons, cosmological horizons, and such; even students can find it daunting \
to listen to two researchers talk\[LongDash]its like a new language is being \
used. The conversation continued with a flurry of comments back and forth \
that I will not relate here, not least wise because they do not paint me in a \
positive light. This conversation led\[LongDash]ultimately\[LongDash]to the \
question that Di (as my wife likes to be called) asked, and started this \
writing, \[OpenCurlyDoubleQuote]Why do you find gravity so interesting?\
\[CloseCurlyDoubleQuote] What I responded with was good, and well thought out\
\[LongDash]you will find it below. Here is what I should have said.
\tGravitational research is exciting! Why? Because gravity holds our planet \
together, keeps us from flying off the planet into space, and keeps our \
planet from hurling itself off into deep space where it would freeze up and \
kill us all. Without it the universe would not exist in any way we can \
imagine.\
\>", "Text"]
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Cell["What is Gravity?", "Section"],
Cell["\<\
The plain truth of the matter is that we don\[CloseCurlyQuote]t know what \
gravity is. We can descibe it in many ways. But when it comes to the ultimate \
explanation, we have nothing; it is a deep mystery. This is one very good \
reason to study it!\
\>", "Text"],
Cell["\<\
\tHow can we descibe gravity? We can say that it is an interaction that \
occurs at a distance, this causes objects far apart to pull towards each \
other\[LongDash]here gravity is thought of as a force. The first theory of \
gravity, posed by Newton, was stated in this way.\
\>", "Text"],
Cell["\<\
\tWe can say that gravity exists everywhere in space. In this way it is a \
field theory. By understanding the field concept, we can attempt to \
understand gravity.\
\>", "Text"],
Cell["\<\
\tThere were successes and failures with the gravitational field model. \
Albert Einstein, in his attempt to adapt special relativity to accelerations \
discovered another model of gravitation. His idea of flat spacetime in \
special relativity turned out to be ineffective in coping with gravity. This \
led to the notion that spacetime curves around a density of matter and energy \
in a volume of spacetime. What does this mean? It means that distances near a \
gravitating body get shorter, though only from the point of view of an \
outside observer. In a similar manner time seems to run slower the closer you \
get, from the point of view of an outside observer. In this way the speed of \
light is preserved. Note that the underlying coordinate system distorts in a \
gravitational field. This is the only force that has been proposed that \
behaves this way. The coordinate system, and the underlying space that it \
represents is a dynamical player in the processs.\
\>", "Text"],
Cell["\<\
\tThe mathematics describing all of this is very intense and we will be \
attacking it slowly\[LongDash]systematically increasing its difficulty level \
as we go.\
\>", "Text"],
Cell["\<\
\tSo the big question is why bother with something so complicated. What is it \
that keeps those of us who study this stuff marching through the labyrinth?\
\>", "Text"]
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In one sense it is inspiring to study how the theory evolved from very \
primitive ideas\[LongDash]from our modern perspective\[LongDash]to the \
current state of general relativity and quantum gravity. It is interesting \
that general relativity is not conceptually far removed from the theory of \
Newton, even though the mathematical framework is totally different. So one \
reason to study gravity is to experience the change in views and methods as \
our understanding became more sophisticated. This process continues today.\
\>", "Text"],
Cell["\<\
\tThere can also be a pressure to find out what is really going on after \
seeing so many examples of descriptions of the theory like what you would see \
on Nova, or read about in Scientific America. After seeing so many \
descriptions there is a desire to really understand what is going on.\
\>", "Text"],
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"\tOne process that is fascinating is what happens to stars as they get old. \
As they burn their hydrogen fuel into heavier elements they produce less \
radiation pressure to balance the gravity that holds the star together. This \
causes gravity to collapse the star. If the mass of the star is beyond a \
certain point, the star will collapse to a stage where the electrons in the \
atoms cannot really tell which atom they are part of any more. This is called \
",
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" and is very similar to a crystal. The core of such a star behaves like a \
giant crystal, such a star is called a ",
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". If the mass of the star is larger than that the collpase will continue to \
the point where individual neutrons within the atomic nuclei cannot tell \
which nucleus they are part of, we call this ",
StyleBox["neutron degenerate matter",
FontSlant->"Italic"],
". This is very weird stuff with bizarre properties. Such a star is called a \
",
StyleBox["neutron star",
FontSlant->"Italic"],
". There are many speculative stages beyond this, if the star is massive \
enough collapse will continue beyond all of the stages to the ultimate \
collapse, where the star no longer exists in any way that we can understand. \
All of its mass is collapsed into a point and the gravity is so strong that \
not even light can escape it, what we call a ",
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FontSlant->"Italic"],
"."
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