标签: Relativity

As I am a bear of simple brain like Winnie the Pooh, many things confuse and confound me. One such topic is that of dark matter. When Sir Isaac Newton presented his Theory of Universal Gravitation in 1687, everyone quickly came to the conclusion that "this was it" – a theory that truly described the way in which the universe worked. After a while, however, they came to realise that the planet Mercury wasn't orbiting the Sun in quite the way it should. Since the folks of the time absolutely believed in the concept of Newtonian gravity, they looked for an explanation that would fit into this "world view." The solution they came up with was that there must be an undiscovered planet (which they called Vulcan) in orbit between the Sun and Mercury. Based on this proposal, many folks devoted huge amounts of effort and ingenuity trying to find this planet... that we now know does not exist. It was more than two hundred years later, in 1916, that Albert Einstein published his theory of General Relativity, whose description of space-time curvature sorted out the problem of Mercury and appeared to provide all of the answers (gravity-wise). For close to 100 years, General Relativity has been accepted by the majority of folks as fully describing gravity. But once again there's a problem. Astronomers have discovered that the stars at the edges of rotating galaxies are travelling much faster than they should be... so fast that they should fly off into space... but they don't. In order to address this, astronomers and physicists came up with the concept of Dark Matter. The idea in a nutshell is that Dark Matter is something we can't "see" or "taste" or anything like that... except through its gravitational interactions. Am I the only one who finds this conclusion to be a tad dubious? Thus it was that I was somewhat irked by a recent column in Discover Magazine, which was titled Largest Map of Dark Matter Across the Cosmos . The associated image (see below) came from the NASA Website ( Click here to see the original posting of this image and associated text).   A snippet from the article in Discover is as follows: Astronomers believe dark matter makes up a quarter of the universe, yet it does not absorb or emit light, and nobody has detected a particle of it. Fortunately, dark matter does reveal itself in a subtle way: As light approaches a clump of the mysterious stuff, it bends around it in a phenomenon known as gravitational lensing. The more massive the clump, the more the light bends. Later on, talking about a catalogue of gravitational lensing that has been compiled over the last five years, the article goes on to say: Their map, which covers 100 times as much sky as previous surveys, reveals giant heaps of dark matter enveloping galaxies. "Whenever there was a dark matter peak, there was a massive cluster of galaxies." (The quote was from astrophysicist Catherine Heymans who is one of the principles on the project.) Now call me "Mr. Fuddy Duddy" if you wish, but I don't find this argument ( "Wherever there was a dark matter peak, there was a massive cluster of galaxies" ) to be particularly convincing. The thing is that gravitational lensing is also associated with the real mass constituting the galaxies. Proponents of dark matter will counter that the dark matter is "wrapped around" galaxies. For myself, having read a lot of "stuff" from a lot of sources, I'm much more inclined to think that the answer lies in another direction, which is that the Einsteinian theory of gravity is incomplete, not the least that – thus far – we haven't managed to tie gravity into the other fundamental forces that manifest themselves at the quantum level. I don't know why, but I have a "gut feeling" that we are on the verge of making some really big discoveries that will dramatically change the way in which we view the universe. I cannot wait to see whatever developments come our way in the next 10 or 20 years. Watch this space...