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.]
To many religious people, "respect for mystery" means not asking about it at all. The scientist's natural reaction to any mystery is to want to investigate it and figure it out, so that it will be a mystery no longer. It's no virtue to "respect" mysteries to the point where they aren't questioned. In fact, it strikes me more as a form of disrespect. To learn about something is to cherish it.
Hardcore materialists, as I define them, believe that "intelligent" people have solved the problem of existence, and that there are no unknown unknowns.
Obviously, anyone who thinks there is nothing more to be discovered would not be interested in scientific research. What would be the point? We hardcore materialists believe that there are no unknowable unknowns worth considering.
Ah, there's the rub.
I wish they had described the physical implications of a "flat" rotation curve. Evidently I had quite the wrong picture of how galaxies rotate, one corresponding to the non-flat rotation curve of a disk.
I assume the positive slope of that line in that example comes from the increasing linear velocity--the angular being constant. If the line height at a given radius is linear velocity, that would mean that the outer parts of a galaxy (with its flat rotation curve) are indeed rotating much slower (angular velocity) than the inner parts after all, which doesn't sound like the problem befuddling astronomers at all.
That's right. The speed of the stars is constant throughout much of the galaxy, unlike a record album, in which the outer edge moves at a faster speed than the innermost groove.
which doesn't sound like the problem befuddling astronomers at all.
It's a huge problem, as according to Kepler's laws, the speed should be inversely proportional to the square root of the radius. It's not.
So name an unknowable unknown that's worth my time to ponder.
OTOH, I know you would not pursue any unknowable unknown worth considering. I presume you wouldn't even know that you didn't know.
The change is actually quite instructive, because it says something about my understanding of man as a limited, contingent agent, and your apparent understanding of him as capable of rendering everything "worth considering" knowable.
I concur. Thanks, Herr Physiker, it is wise to get out of the way of the pros so I can add nothing. I also found another paper on the calculation of galactic angular velocity. The Disk Rotation of the Milky Way Galaxy
Aye, but the religious are in that boat, too. The key difference lies in our treatment of the knowable unknown: how did the physical universe develop, how does the machinery of life work, how does the brain perform the act of consciousness, how do particles come to have the masses they do...
The rotation of the disk of the Milky Way Galaxy is analyzed. It rotates neither as a solid disk, nor in accordance with individual keplerian orbits. Rather the disk executes a form of differential rotation which suggests that a considerable fraction of its mass resides in the outer portions of the Galaxy.
Turntable needs oil. The Intelligent Maintainance Man is asleep on the job.
...no miracle there; that happens naturally in any refractive medium...
and even stop it cold, and then re-start its' motion on command.
It "stops" light in exactly the same sense that a tape recorder "stops" sound. It's a technological breakthrough, don't get me wrong, but it's no challenge to our understanding of nature.
... And to what purpose.
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