Dark matter and gravity. Doug sent a note about it and I interrupted your reply and said something can be done with that.
I was sitting here wondering what I was going to begin with and I couldn’t remember what Doug had said. I was surprised you interrupted. Very odd. It has only happened a couple of times and it makes me suspicious that I have two personalities; myself and my writer self. Fortunately you don’t exhibit yourself, Amorella, except in writing or rarely in subtle muscular movement surrounding the eyeball.
I don’t know how you can do anything with dark matter and gravity. No one knows what either is exactly. Here is the selection from Doug’s note:
Dick,.. . . Dark matter has really made me question if we have any true understanding of gravity. Supposedly without dark matter there would be no galaxies and thus no life. Yet there is no clue as to what dark matter is. All we seem to know is that it clumps together and that is where the galaxies form and exist.
Doug
When you (Amorella) interrupted I was thinking . . . this stuff is too deep, I wouldn’t know how to use this in a discussion or in the books . . . this is Doug all the way . . . throwing things out for me to think about . . . part of his sense of humor as well as his great interest in the mysteries of science.
Here is my yesterday’s reply to Doug:
Thanks! You are always stirring me up . . . dark matter and gravity . . . and the thinking . . .
Hello, Doug. Dark matter and gravity. I may be able to do something with that. - Amorella
I don't know, Doug. She just popped in. I'll let you know what happens. Dick
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It is interesting that I “popped in” not “popped up”.
That’s true, but, you popped into the conversation. You could have still popped up from my head.
You are always interested in prepositions in these matters.
I am. Figures of speech are set by unconscious standards; this is my feeling. I am probably generalizing but I think there is something to it. Popping into my head and popping up from my head are two different things – like Jesus walking on the water or Jesus walking around the water. Sometimes it makes a huge difference in translation and/or comprehension.
Take a break, post, and respond to Kay H.’s morning email. – Amorella.
She has good questions, and I am not sure I can answer them properly.
I will help you. – Amorella.
Carol is off to lunch with her friends leaving you with the cat, who is presently having one of several mad dashes of energy for the day – probably before her afternoon nap.
A nap sounds like a good idea.
No nap, you worked on correcting the phone directory numbers and have yet to run it off. Carol returned and you took a walk in Rose Hill Cemetery, now you are over at the Streets of West Chester waiting for Carol in front of Chico’s. Looking for one more piece of clothing for your trip West.
You took some time to play a game of computer chess. You won on fast speed. First time a full game on MBA.
I need to practice. I haven’t played much since the late nineties. It’s at the computer’s weakest level, which is fine for me because I am making enough mistakes already. Nothing beats chess for a computer game.
You have moved to inside Barnes and Noble so let’s go to the web. Dark matter first...From NASA you have this:
“When the Universe was young, it was nearly smooth and featureless. As it grew older and developed, it became organized. We know that our solar system is organized into planets (including the Earth!) orbiting around the Sun. On a scale much larger than the solar system (about 100 million times larger!), stars collect themselves into galaxies. Our Sun is an average star in an average galaxy called the Milky Way. The Milky Way contains about 100 billion stars. Yes, that's 100,000,000,000 stars! On still larger scales, individual galaxies are concentrated into groups, or what astronomers call clusters of galaxies.
The cluster includes the galaxies and any material, which is in the space between the galaxies. The force, or glue, that holds the cluster together is gravity -- the mutual attraction of everything in the Universe for everything else. The space between galaxies in clusters is filled with a hot gas. In fact, the gas is so hot (tens of millions of degrees!) that it shines in X-rays instead of visible light. In the image above, the hot X-ray gas (shown in pink) lying between the galaxies is superimposed on an optical picture of the cluster of galaxies. By studying the distribution and temperature of the hot gas we can measure how much it is being squeezed by the force of gravity from all the material in the cluster. This allows scientists to how much total material (matter) there is in that part of space.
Remarkably, it turns out there is five times more material in clusters of galaxies than we would expect from the galaxies and hot gas we can see. Most of the stuff in clusters of galaxies is invisible and, since these are the largest structures in the Universe held together by gravity, scientists then conclude that most of the matter in the entire Universe is invisible. This invisible stuff is called 'dark matter', a term initially coined by Fritz Zwicky who discovered evidence for missing mass in galaxies in the 1930s. There is currently much ongoing research by scientists attempting to discover exactly what this dark matter is, how much there is, and what effect it may have on the future of the Universe as a whole.”
And, then,
“. . . . By fitting a theoretical model of the composition of the Universe to the combined set of cosmological observations, scientists have come up with the composition that we described above, ~70% dark energy, ~25% dark matter, ~5% normal matter. What is dark matter?
We are much more certain what dark matter is not than we are what it is. First, it is dark, meaning that it is not in the form of stars and planets that we see. Observations show that there is far too little visible matter in the Universe to make up the 25% required by the observations. Second, it is not in the form of dark clouds of normal matter, matter made up of particles called baryons. We know this because we would be able to detect baryonic clouds by their absorption of radiation passing through them. Third, dark matter is not antimatter, because we do not see the unique gamma rays that are produced when antimatter annihilates with matter. Finally, we can rule out large galaxy-sized black holes on the basis of how many gravitational lenses we see. High concentrations of matter bend light passing near them from objects further away, but we do not see enough lensing events to suggest that such objects to make up the required 25% dark matter contribution.”
Both from: science.nasa.gov/
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The above are the definitions we will use here. Below is a definition of gravity. – Amorella.
History of gravitational theory
Equivalence principle
The equilvalence principle, explored by a succession of researchers including Galileo, Lorand Eotvos, and Einstein, expresses the idea that all objects fall in the same way. The simplest way to test the weak equivalence principle is to drop two objects of different masses or compositions in a vacuum, and see if they hit the ground at the same time. These experiments demonstrate that all objects fall at the same rate when friction (including air resistance) is negligible. More sophisticated tests use a torsion balance of a type invented by Eötvös. Satellite experiments are planned for more accurate experiments in space.
Formulations of the equivalence principle include:
• The weak equivalence principle: The trajectory of a point mass in a gravitational field depends only on its initial position and velocity, and is independent of its composition.
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• The Einsteinian equivalence principle: The outcome of any local non-gravitational experiment in a freely falling laboratory is independent of the velocity of the laboratory and its location in spacetime.
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• The strong equivalence principle requiring both of the above.
The equivalence principle can be used to make physical deductions about the gravitational constant, the geometrical nature of gravity, the possibility of a fifth force, and the validity of concepts such as general relativity and Brans-Dicke theory.
General relativity
In general relativity, the effects of gravitation are ascribed to spacetime curvature instead of a force. The starting point for general relativity is the equivalence principle, which equates free fall with inertial motion, and describes free-falling inertial objects as being accelerated relative to non-inertial observers on the ground. In Newtonian physics, however, no such acceleration can occur unless at least one of the objects is being operated on by a force.
Einstein proposed that spacetime is curved by matter, and that free-falling objects are moving along locally straight paths in curved spacetime. These straight paths are called geodesics. Like Newton's first law of motion, Einstein's theory states that if a force is applied on an object, it would deviate from a geodesic. For instance, we are no longer following geodesics while standing because the mechanical resistance of the Earth exerts an upward force on us, and we are non-inertial on the ground as a result. This explains why moving along the geodesics in spacetime is considered inertial.
Einstein discovered the field equations of general relativity, which relate the presence of matter and the curvature of spacetime and are named after him. The Einstein field equations The are a set of 10 simultaneous, non-linear, differential equations. The solutions of the field equations are the components of the metric tensor of spacetime. A metric tensor describes a geometry of spacetime. The geodesic paths for a spacetime are calculated from the metric tensor.
Notable solutions of the Einstein field equations include:
▪ The Schwarzschild solution, which describes spacetime surrounding a spherically symmetric non-rotating uncharged massive object. For compact enough objects, this solution generated a black hole with a central singularity. For radial distances from the center, which are much greater than the Schwarzschild radius, the accelerations predicted by the Schwarzschild solution are practically identical to those predicted by Newton's theory of gravity.
▪ The Reissner-Nordstrom solution, in which the central object has an electrical charge. For charges with a geometrized length, which are less than the geometrized length of the mass of the object, this solution produces black holes with two event horizons.
▪ The Kerr solution for rotating massive objects. This solution also produces black holes with multiple event horizons.
▪ The Kerr-Newman solution for charged, rotating massive objects. This solution also produces black holes with multiple event horizons.
▪ The cosmological Friedmann-Lemaitre-Robertson-Walker solution, which predicts the expansion of the universe.
The tests of general relativity included the following:
▪ General relativity accounts for the anomalous perihelion precession of Mercury.
▪ The prediction that time runs slower at lower potentials has been confirmed by the Pound-Rebka experiment, the Hafele-Keating experiment, and the GPS.
▪ The prediction of the deflection of light was first confirmed by Arthur Stanley Eddington from his observations during the Solar eclipse of May 29, 1919. Eddington measured starlight deflections twice those predicted by Newtonian corpuscular theory, in accordance with the predictions of general relativity. However his interpretation of the results was later disputed. More recent tests using radio interferometric measurements of quasars passing behind the Sun have more accurately and consistently confirmed the deflection of light to the degree predicted by general relativity.
▪ The time delay of light passing close to a massive object was first identified by Irwin I. Shapiro in 1964 in interplanetary spacecraft signals.
▪ Gravitational radiation has been indirectly confirmed through studies of binary pulsars.
▪ Alexander Friedmann in 1922 found that Einstein equations have non-stationary solutions (even in the presence of the cosmological constant). In 1927 Georges Lemaitre showed that static solutions of the Einstein equations, which are possible in the presence of the cosmological constant, are unstable, and therefore the static universe envisioned by Einstein could not exist. Later, in 1931, Einstein himself agreed with the results of Friedmann and Lemaître. Thus general relativity predicted that the Universe had to be non-static—it had to either expand or contract. The expansion of the universe discovered by Edwin Hubble in 1929 confirmed this prediction.
Gravity and quantum mechanics
In the decades after the discovery of general relativity it was realized that general relativity is incompatible with quantum mechanics. It is possible to describe gravity in the framework of quantum field theory like the other fundamental forces, such that the attractive force of gravity arises due to exchange of virtual gravitons, in the same way as the electromagnetic force arises from exchange of virtual photons. This reproduces general relativity in the classical limit. However, this approach fails at short distances of the order of the Planck length where a more complete theory of quantum gravity (or a new approach to quantum mechanics) is required. Many believe the complete theory to be string theory or more currently M-theory, and, on the other hand, it may be a background independent theory such as loop quantum gravity or causal dynamical triangulation.
From: Wikipedia
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Now what, Amorella? I will have to continue to go over this several times just to make sure I get the basics for the books. I don’t see how any of this can be used in The Rebellion.
Sleep on it. Post for now. – Amorella.
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