Posted on 01/15/2002 7:02:17 AM PST by PatrickHenry
Once upon a time -- a bit more than 100 years ago -- many scientists believed that seemingly empty space wasn't empty at all, but was filled with a substance called luminous ether. This mysterious stuff, never seen in any laboratory on Earth, was thought to explain how gravity from one celestial body could affect another.
By the end of the 19th century, though, luminous ether had gone the way of countless other scientific misconceptions. Today, another mysterious substance beguiles astronomers, and this one isn't going away. In fact, it's been at the forefront of cosmological theories for decades. It's called dark matter, and it is now widely accepted by astronomers as the stuff most of the universe is made of.
"We've known that it exists for more than 25 years," says astronomer Virginia Trimble of the University of California Irvine. "But we don't know what the hell it is."
How can astronomers be so certain of something they have never seen? The answer comes from observations of how stars and galaxies move, studies that have been going on for more than 50 years. Within spiral galaxies, individual stars and clouds of gas are orbiting faster than they should if they were only being affected by the gravity of the galaxy's visible matter. The same is true for clusters of galaxies: The motions of individual galaxies can't be explained by the gravity of what astronomers can see.
To explain these observations, astronomers have deduced that galaxies are surrounded by vast halos of a different, unseen kind of matter.
This so-called dark matter is invisible to us because it does not radiate energy. But it does have mass, and that means it can supply the extra gravity necessary to hold galaxies, and clusters of galaxies, together. Even in the bizarre world of cosmology, it's a strange proposition.
But is dark matter the only explanation?
Perhaps scientists don't entirely understand the way gravity works; perhaps Isaac Newton's famous law of gravitation needs some revising. But that idea, says the University of Arizona's Chris Impey, is not very popular.
"Definitely most astronomers are extremely unwilling to give up Newton's law," he says. "So it's essentially a choice of two evils: You either hypothesize that Newton's law is wrong, and that our knowledge of the gravity theory is incomplete. Or, you hypothesize a fundamental microscopic particle that has never been detected in any physics lab, whose properties are only constrained by these astronomical observations. Which is a pretty uncomfortable position for physicists to be in."
Still, as Trimble explains, dark matter is the lesser of the two evils, simply because it requires fewer departures from accepted physics.
To explain the observations by revising the theory of gravity, astronomers would have to identify a few different effects, each of which would operate at a different distance scale. But with dark matter as the explanation, Trimble says, "You only need one Tooth Fairy."
[The rest is omitted, but you can visit the source and read it all.]
Yet another question revealing my ignorance: Kepler described the orbits of the planets around our sun. Do Kepler's laws hold for stars orbiting the galaxy? Wouldn't you need a solid rotating disk of a galaxy to perform that way? Well, maybe not.
That's not in a vacuum.
It holds for a central potential. If most of the mass is concentrated at the center of the galaxy--which is where the brightness is concentrated--then it will hold.
The speed of a star around the galaxy is 2 pi times the radius (assume circular orbits), divided by the period. The period, per Kepler, is proportional to the radius to the 3/2 power. So the speed should be proportional to the -1/2 power. Instead, the speed is roughly independent of radius.
Wouldn't you need a solid rotating disk of a galaxy to perform that way?
In a solid rotating disk, the speed is proportional to the distance from the axis.
The differential seems to be turning fine. Maybe it's your transmission?
True enough, but brightness isn't necessarily a good indication of where the mass is. Spiral galaxies, in particular, have enormous amounts of dust in and between their visible arms. The dust in the arms both adds to the mass there and makes them dimmer. The centers are mostly dust-free. This was known even before accurate measurements of velocity distributions, and is accounted for when astronomers complain about "missing mass."
As a lawnmower mechanic, I can attest to the truth of this statement, just examine a Snapper rear-engine riding mower "transmission."
Don't a lot of scientist critisize (sp?) religious types when they use this type of logic to prove the existance of God?
"There's no tangible proof, but we can see God's effects so there must be a God."
"There's no tangible proof, but we can see dark matter's effects, so there must be dark matter."
If this type of logic is unacceptable to imply the existance of God, why should it be acceptable to imply the existance of anything else?
There is no finding there that the speed of light in a vacuum is not constant.
They are not the same kind of argument. Science says: "Look, we have countless observations to tell us how gravity works; and using those observations, we can predict how things will behave in a gravitational field." Okay so far? Now science goes on to say: "We see some behavior that, in accordance with all our past evidence, leads us to conclude that there is some extra masss which is causing the behavior."
What that is, in big words, is inductive reasoning from specific observations to obtain a general rule; then using that general rule to conduct deductive reasoning. Further, if any conclusion from this process turns out to be false, science will modify or reject the general rule to make it consistent with the evidence.
The religious argument, on the other hand, leaves out the inductive reasoning part. There are just proclaimations of doctrines, and then deductive conclusions derived from them, which conclusions are not subject to revision; and there is no evidentiary foundation to support the whole structure. A very different mode of thinking from that which is employed in science. So you can't say that science is using the identical methods of theology. They're not the same at all.
So supposedly space is distorted and the big guy just kinda wallows around and rolls around an indentation, or warp caused by the heavier sun. But the sun is moving too, so how does the warp move so durn fast?
The great thing about FR is that, no matter what the subject or the level of detail, there's always someone who can relevantly expand upon it. I dumbed it down, probably unnecessarily, for the sake of clarity. It is true that the stars in the galaxy were never expected to move in exactly a central potential.
Because the models of dark matter that fit the data are few and testable, while the models of God that fit the data are limited only by the imagination and credulity of man--in other words, infinite--and are not testable.
The heavier it is, the stronger gravity acts on it. There is just enough gravity to balance the centripetal force. Ko-inky dink?
Either way, Whether it acts instantaneously like a radio-control car or is a permanent warp in the continuum, it in effect is always there and Jupiter just rides in the groove like a Victrola needle. However you want to look at it.
Disclaimer: Opinions posted on Free Republic are those of the individual posters and do not necessarily represent the opinion of Free Republic or its management. All materials posted herein are protected by copyright law and the exemption for fair use of copyrighted works.