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| 5/25 |
| 2005/2/25 [Science/Physics] UID:36423 Activity:very high |
2/25 Is anti-matter and dark matter the same thing? Thx.
\_ Is our children learning?
\_ No they ain't. As to original question, not at all.
\_ No, you're wrong. Technically speaking dark-matter could
be the "same" as anti-matter if it turns out to be
composed of neutrinos. Since neutrinos are their own
anti-particle you could say that dark matter is composed
of anti-matter. Right now we don't know what dark matter
is. Perhaps it could be composed primarily of some sort
of anti-matter. Nobody knows.
\_ "More seldom than not, the movies gives us exquisite sex and
wholesome violence, that underscores our values. Every two
child did. I will."
\_ http://en.wikipedia.org/wiki/Antimatter
http://en.wikipedia.org/wiki/Dark_matter
\_ Dark matter is quite different from normal baryonic matter
and anti-matter. There are several views on what exactly
anti-matter is, but in general it can be though of as
matter that is composed of particles that are similar to
protons and electrons except that they have the opposite
charge (ex. positron is an electron with a + charge).
Dark matter is completely different. It interacts with
regular matter very weakly and is probably not composed
of any sort of known particles. Dark matter's presence
is mostly inferred from gravitational anomalies in the
rotation of galaxies. Some newer experiments are trying
to detect dark matter based on nuclear interactions, but
so far nothing has turned up.
\_ There was some /. story recently on something that looks like
an entire galaxy composed entirely of dark matter. Dark matter
has a bit of a 'magic blue smoke' quality to it, imo. -- ilyas
\_ I agree. The entire dark-matter/dark-energy discussion
reminds me a lot about the cosmic ether discussions prior
to SR. It seems like the universe is telling us something
fundamental and physicists want to shoe-horn it into the
standard model.
BTW, how did they detect a dark matter galaxy?
\_ I am not a specialist, so I don't know (the article didn't
really explain it well). Obviously using some indirect
way involving gravity, coupled with noticing there are no
stars there. -- ilyas
\_ If you are talking about the recent results re
VIRGOHI21, my understanding is that it was a
radio telescope search looking for H emission
lines. Also this isn't a dark matter galaxy
but a dark galaxy (basically a huge cloud of
H w/o many stars).
\_ I would be very surprised if you could get a cloud of
H to behave like a galaxy without any star formation.
-- ilyas
\_ How much dark matter is in a liter of the air next to you?
If none, how much dark matter is there in a liter of volume
of deep space?
If negligible, about how much volume of deep space would you
need to get one particle of dark matter?
\_ IIRC current estimates are that each second every square
meter of the earth passes through 1e9 dark matter particles.
\_ That means 1 liter has ~ one million dark matter particles!
I am swimming in dark matter!
\_ In comparison a liter of water has ~ 1e25 water
molecules.
\_ If we take 1 liter of air, I'm getting ~ 4 particles
of dark matter for every trillion molecules of
air. Is that right?
\_ Sort of. Dark matter is "there" but it doesn't
interact w/ regular matter. It just passes
through your body (and pretty much everything
else) as if it wasn't even there.
\_ Hmmm, so the motd is dark-matter. If we put ilyas and tom
together, would they explode?
\_ I think that you are confusing a few concepts. Dark
matter is weakly interacting and does not affect
normal matter except via its gravitation effect.
The following works a bit better: the motd is space-time,
ilyas is matter, tom is anti-matter. If they both
meet on the motd you will get an uncontrollable burst
of energy that will destroys everything in its path.
\_ If the only way to detect dark matter so far is from gravitational
anomalies, how do they know that dark matter is in the form of
discrete particles (or wave-particles like the way normal particles
are in quantum physics)?
\_ If you really want to know, look online in a source that does
not consist of computer science people. There is much
bullshit in the above answers, and I have become tired of
having flamewars with morons on the motd about physics.
Look on the websites of the various darkmatter searches out
there, and they should have good explanations of what they're
looking for and why.
\_ is dark matter like the Force?
\_ Not everyone agrees that dark matter is made of particles.
The leading theory is from the super-symmetry people who
think that one of the particles in their theory, the
neutralino, fits the dark matter bill.
FYI, SciAm had some decent articles on this topic a few
months back. |
| 5/25 |
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| en.wikipedia.org/wiki/Antimatter Paul Dirac's theory has b een experimentally verified and today a wide range of antiparticles have been detected. This is one of the few examples of a fundamental particl e being predicted in theory and later discovered by experiment. In principle, suf ficiently large quantities of antimatter could produce anti-nuclei of ot her elements, which would have exactly the same properties as their posi tive-matter counterparts. periodic table of anti-el ements" is thought to be, at best, highly unlikely, as the quantities of antimatter required would be, quite literally, astronomical. kinetic energy they will be able to escape the magnetic t rap, and it is therefore essential that the anti-atoms are produced with as little energy as possible. The symbol used to denote an antiparticle is the same symbol used to deno te its normal matter counterpart, but with an overstrike. For example, a proton is denoted with a "p", and an antiproton is denoted by a "p" wit h a line over its top (\bar{\mbox{p}}) . energy per unit mass is much great er than the chemical energy or even nuclear energy that can be converted today, using chemical reactions or nuclear fission/fusion. Generating a single antiproton is immensely difficu lt and requires particle accelerators and vast amounts of energymillion s of times more than is released after it is annihilated with ordinary m atter, due to inefficiencies in the process. Known methods of producing antimatter from energy also produce an equal amount of normal matter, so the theoretical limit is that half of the input energy is converted to antimatter. Counterbalancing this, when antimatter annihilates with ordi nary matter energy equal to twice the mass of the antimatter is liberate dso energy storage in the form of antimatter could (in theory) be 100% efficient. |
| en.wikipedia.org/wiki/Dark_matter red shift that light f rom distant heavenly bodies exhibits. The amount of ordinary matter seen in the universe is not enough for gravity to stop this expansion, and s o the expansion would continue forever in the absence of dark matter. order of magnit ude or so, and assuming that the visible material makes up only a small part of the cluster is the most straightforward way of accounting for th is. With gravitational theory and new computer analyses, astronomers have now been able to work out where the dark matter appears to be. The results are just what you would expect if dark matter and galaxies are clustered in exactly the same way. Galaxies themselves also show signs of being c omposed largely of dark matter for instance the rotation curves in and indeed the very existence of our galaxy's disc are most easily explaine d if the galaxy contains an extended dark matter halo. Coma cluster, based on the motions of the galaxies near the edge of the cluster. When he compared t his mass estimate to one based on the number of galaxies and total brigh tness of the cluster, he found that there was about 400 times more mass than expected. The gravity of the visible galaxies in the cluster would be far too small to keep such fast-moving galaxies bound, so something e xtra was required. Based on these conclusions, Zwicky inferred that there must be some other form o f matter existent in the cluster which we have not detected, which provi des enough of the mass and gravity to hold the cluster together. From there the search for this source of the sufficient gravity has comme nced. At present, the density of the universe (excluding dark matter) is estimated to be about one hydrogen atom per cubic meter of empty space. This is not enough density for the universe to collapse on itself. Howe ver, dark matter is said to form 9095% of all matter in the universe. Milky Way Galaxy we live in, with app roximately 1000 times as much dark matter as observed hydrogen. For comp arison, the Milky Way is believed to have roughly 10 times as much dark matter as ordinary matter. edit Alternative explanations An alternative to dark matter is to suppose that gravitational forces bec ome stronger than the Newtonian approximation at great distance. Anoth er approach, proposed by Finzi (1963) and again by Sanders (1984), is to replace the gravitational potential with the expression U=\frac{GM(1-Be^{-r/\rho})}{(1-B)r} where B and are adjustable parameters. However, all such approaches run into difficulties explaining the different behavior of different galaxi es and clusters, whereas one can easily describe such differences by ass uming different quantities of dark matter. strong nuclear force and so are incredibly dif ficult to detect. This is why they are such good candidates for hot dark matter. However, current bounds on the neutrino mass indicate that ordi nary neutrinos make only a small contribution to dark matter. To explain the small neutrino mass so-called sterile neutrinos can be added to the Standard Model. These sterile neutrinos are expected to be heavier than the ordinary neutrinos. Hot dark matter cannot explain how individual galaxies formed from the Bi g Bang. CO BE satellite is very smooth and fast moving particles cannot clump toget her on this small scale from such a smooth initial clumping. To explain small scale structure in the universe it is necessary to invoke cold dar k matter. Hot dark matter therefore is almost always discussed as part o f a mixed dark matter theory. |