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.]
Hi VR. I believe the need for dark matter comes from the anomalous falloff in the rate of rotation as one goes from the center of a galaxy out to the edges, so the mass of the black hole in the center doesn't come into play. The effect of dark matter comes from a mass that is much more spread out than that of a black hole.
What are you urging here, surrender to ignorance? Science stumbles and bumps its way, converging upon an ever-better understanding of the universe.
Somehow, this bothers some of you unduly. I need to explain that. You're not upset that science has to revise the picture, but that it's clearly getting somewhere. On FR, there's lots of cheering for the gaps, rooting for ignorance. In fact, our ignorance is routinely exaggerated to avoid dealing with facts that have been established for well over a century. A strange position!
That's correct. Black holes have been inferred at the centers of several galaxies from star motions near the centers. The hitch is that stars' galactic orbital velocities don't generally seem to drop off with distance from the center as fast as one might expect.
There's a great website on the Milky Way's black hole at Black Hole Slide Show. The slide show includs an actual movie of stars, not an animation, of stars in motion around the black hole. (You can download the movie from Black Hole Movie.)
That's correct. Black holes have been inferred at the centers of several galaxies from star motions near the centers. The hitch is that stars' galactic orbital velocities don't generally seem to drop off with distance from the center as fast as one might expect.
There's a great website on the Milky Way's black hole at Black Hole Slide Show. The slide show includes an actual movie of stars, not an animation, of stars in motion around the black hole. (You can download the movie from Black Hole Movie.)
It seems to me that the humble ones would be the ones still looking for answers -- the scientists.
I have no problem whatsoever with scientific progress, and you won't find a post of mine anywhere where I take a position which even remotely suggests otherwise.
The most beautiful thing we can experience is the mysterious. It is the source of all true art and science.
Albert Einstein, What I Believe
As long as the brain is a mystery,
The universe will also be a mystery
Santiago Ramón Y Cajal
Subjective existence, baby. Why is there something rather than nothing? How did we get here? Those are the only "scientific problems" that count. We are no closer to solving them than was Plato.
Are you a hardcore materialist, moonman? In the past, that has not been my impression of you. Indeed, scientific inquiry is an ennobling and, usually, humbling enterprise. It deserves high praise. But another strain of thought has recently come to the fore, one which assumes knowledge of final truths. Perhaps it is best expressed by Nobel prize winning physicist Steven Weinberg, who wrote that he is all in favor of "a dialogue between science and religion, but not a constructive dialogue. One of the great achievements of science has been, if not to make it impossible for intelligent people to be religious, then at least to make it possible for them not to be religious. We should not retreat from this accomplishment." I interpret Weinberg's use of the term "religion" as synonomous with "respect for mystery."
Hardcore materialists, as I define them, believe that "intelligent" people have solved the problem of existence, and that there are no unknown unknowns. To me, such thinking is foolishness beyond reckoning.
It is constant in a vacuum. I'm not aware of that having changed.
I guess this is related to the mysterious gravitation effects on our deep space probes. I also wonder if this is related to the flavor changing solar neutrinos. It is troubling to me that a particle so inert to interaction with matter that it can pass through light years of lead and yet it "coyly" metamorphoses in ~93 million miles of vacuum.
Actually, that is the result of Fundamentalist Christians punishing scientists who tried to believe in God who were also advancing science because their discoveries and theories were 'detrimental' to the fundamentalist doctrine.
Although I am no scientist, I am considered less of a Christian by the fundamentalists who engage in the crevo debates and by those whom I know IRL because I "believe in" evolution (not being ignorant, etc.)
You are correct, of course. Maintenence of truth requires constant vigilence.
You might want to read this article.
Determining the Rotation Curve of the Milky Way Galaxy
The rotation curve of a system is a plot of the orbital speed versus the distance from the center of the system. For different objects, or systems of objects, different rotation curves result. For example, the rotation curve of a disk is a line with a positive slope, because as you move outward on the disk, the angular velocity remains constant. A planetary system has a different kind of rotation curve. The angular velocity is not constant in this case, and the velocity decreases as the inverse of the square root of distance. The third type of rotation curve, and the one exhibited by spiral galaxies, is a flat rotation curve.
A flat rotation curve implies that the mass increases as distance from the center increases. The rotation curve, when plotted, did not agree with previous estimates, which were made based on the luminous matter in the galaxy. It was therefore concluded that there is much more matter in the galaxy than is luminous.
Since the advent of radio astronomy, the 21-cm emission line of HI has been a vital part of the study of the kinematics of the Milky Way. It was predicted in the early 1940’s and was first observed in 1951. This line is produced by a “flip” in the spin of an electron. Most of the Galaxy is optically thin to the 21-cm line, making it an excellent resource for determining the rotation curve of the Galaxy.
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