I second that - and for even more unrelated info, I believe while the sun has 99% of the mass of the solar system, it only has 1% of the angular momentum.
99.2%, according to Nine Planets, probably the most well-respected general astronomy reference on the web.
Just another thought on the nature of dark matter, I recently went to a public lecture by Professor Michio Kaku in Birmingham (UK). He suggested that one possibility for dark matter comes from string theory, particularly the 11 dimensional brane theory (M theory?).
To keep it relatively simple he suggests that branes (dimensional membranes or parallel universes in effect) can be close together in space, but be completely undetectable except by gravity. One part of string theory suggests that gravitons (if they exist) should not be bound to their brane of origin and should be the only particle capable of trvelling between branes, thus the dark matter we detect could be the gravitation of a “nearby” brane.
I hope that I’ve a) explained this ok and b) Not misquoted Prof. Kaku.
:rolleyes:
M-Theory can explain anything…and prove nothing.
I’m not clear about this “particle capable of traveling between branes” thing, though; that seems, well, a definitional contradiction. Do you have the date or title of the colloquium? I looked online but coudn’t find anything that matched up.
Stranger
What is this ‘big particle’ hypothesis of dark matter that the Wiki article mentions?
The Sun definitely has the bulk of the mass in the Solar System, but angular momentum is a bit trickier to pin down, since it depends on frame of reference (or at least, some of it does). The Sun does rotate, and therefore has some angular momentum to which it unambiguously lays claim, but that rotational angular momentum is, indeed, small compared to the whole Solar System. Most of the angular momentum in the Solar System is in the Sun-Jupiter system, and it’s not entirely straightforward to divide that up into the Sun’s share and Jupiter’s share.
The idea that “dark matter” is due to the effect of matter on other branes is not new, and cannot be completely discredited, but there’s not yet any real way to test it, and there’s no evidence for it, either.
Dark matter does seem to clump on at least some scales, and appears to be more prevalent in galaxies. In fact, one key in modern models of galaxy formation is that the dark matter, being relatively unhindered by radiation pressure, clumped first, and provided the gravitational seeds for the normal matter to clump around. This is a large part of the reason why dark matter is suspected to be cold, since hot dark matter doesn’t clump as well.
A substance which interacted only gravitationally would, indeed, be dark matter, if it existed, but inflationary cosmology predicts that any such substance should be exceedingly sparse (as in, perhaps a single particle in the entire observable Universe) in our present, post-inflation times. So most models for the dark matter assume that it interacts via the strong and/or weak forces, as well.
And finally, the class of objects of which Pluto is the best-known member is the Kuiper belt objects, but they’re mostly icy, not rocky.
There’s a good book–lessee, here it is, Voyage To The Great Attractor–that describes the efforts and eventual success of the “Seven Samuari” in searching for large scale astronomical structures. It’s as interesting for the process of collaboration between scientists as it is for the science, but it does a good job of explaining the various theories for unseen (dark) matter, univeral inflation, and irregularities in the commic microwave background that help astrophysicists and cosmologists piece out what lead to developing objects like stars and galaxies. It’s an easy read, too; a good combination of non-technical science and anecdotes.
Stranger
Thanks, learned my something new for the day.
And thanks all who answered my tangental question. It helped me put the dark matter question into a little perspective.
Yeah, the picture increased my understanding of dark matter by uh, oh, some or no percent. Definitely one of those. It’s really beautiful, but then I’m a minimalist.
That is more or less the gist of what Kaku said in that lecture, apart from, I think, the partcles travelling between branes bit.
Stranger on a Train, the lecture was one of a series of public lectures that Kaku’s doing to publicise his new book. Kaku’s new book – Parallel Worlds
Thanks for clearing that up, I could not remember the date of the lecture. Sorry about the misleading particle bit, it was not intentional.
Can anyone clear up my confusion about how gravity can supposedly travel between branes?