Is there really such a thing as dark matter? Also if there is, surely its gravity properties are quantum and therefore their collective mass may not produce the gravity needed for them to exist? So how does gravity work in the quantum world? Why do all of the many attempts of find dark matter fail? What else could explain the missing mass of the universe?
I think cosmologists, astronomers and theoretical physicists made an enormous error when they coined the term 'dark matter'. So many people think dark matter must be mysterious and alien and magical. If they would have called it 'space dandruff', maybe people would actually read something about it and not get so hysterical.
A more precise term is non-baryonic matter, but it doesn't have the cachet of dark matter. The confusion is compounded by the fact that most (around 90%) of ordinary (baryonic) matter happens to be dark also.
It would be safe to say that around 90 percent (at least) of the known mass in the universe is in the form of stars in galaxies. Bright shining stars.
Hi alex, Dark matter is already known to exist! We call them neutrinos. They don't have an electric charge, so they don't couple directly to the electromagnetic field (technical note: they can interact indirectly). The crazies who rave about dark matter being unphysical inevitably fail to inform you that we already know of particles that are "dark". Neutrinos are so ethereal that their existence was initially inferred from missing energy (sound familiar?) in beta decays. Only very recently have we even been able to determine that neutrinos do indeed have mass. I should also mention that there are a host of more mundane reasons why we sometimes can't see ordinary matter. Neutrinos are almost certainly not the more famous "dark matter" everyone talks about, but the point is that the idea isn't crazy. Sometimes seeing nothing at all can be very telling. Today dark matter is a successful and consistent part of our picture of the universe. Moreover, we have plenty of candidates for exotic dark matter and high hopes that the Large Hadron Collider will shed some light on the issue. P.S. No one knows how gravity works when quantum effects are important.
"It would be safe to say that around 90 percent (at least) of the known mass in the universe is in the form of stars in galaxies. Bright shining stars. " Completely wrong. About 5% of the universe is baryonic matter and shining stars make up about 10% of the baryonic matter (i.e. 0.5% of the universe). "Dark matter is already known to exist! We call them neutrinos." Partially true. However, according to current theory, neutrinos cannot account for most of the non-baryonic matter. (I don't remeber the upper limit, but it is somewhere in the 10% to 20% range of the non-baryonic matter.)
Hi mathman, What I said was completely true. Neutrinos are a kind of dark matter. More to the point, I never said neutrinos account for most of the dark matter. I specifically said the opposite.
Physics Monkey is correct. Neutrinos are a form of "dark matter". I quote below from a 1970 High School physics text: "The description of Beta decay in terms of the transformation of a neutron in the nucleus is part of one of the most fascinating stories in modern physics: the prediction and eventual discovery of the particles called the neutrino and the antineutrino. Quantitative studies of the energy relations in Beta decay during the 1920s and 1930s raised a difficult and serious question. Methods were devised for determining the energy change in a nucleu during Beta decay. According to the principle of conservation of energy, the energy lost by the nucleus should be equal to the energy carried off by the Beta particle. But, the kinetic energy of the Beta particles had a whole range of measured values, all smaller than the amount of energy lost by the nucleus: some of the energy lost by the nucleus seemed to have disappeared. Measurements made on a large number of Beta-emitters indicated that on the average about two-thirds of the energy lost by the Beta-decaying nuclei seemed to disappear. Attempts to find the missing energy failed. For example, some physicists thought that the missing energy might be carried off by gamma-rays; but no such gamma-rays could be detected experimentally. The principle of conservation of energy seemed to be violated in Beta decay. Similar discrepancies were found in measurements of the momentum of the emitted electron and the recoiling nucleus. As in the case of the experiments that led to the discovery of the neutron, physicists tried very hard to find an alternative to accepting the failure of the principles of conservation of energy and momentum. The Austrian physicist Wolfgang Pauli suggested in 1933 that another, hitherto unnoticed particle, is emitted in Beta decay along with the electron, and that this particle carries off the missing energy and momentum. This hypothetical particle could have no electric charge, because the positive charge of the proton and the negative charge of the Beta particle together are equal to the zero charge of the original neutron. The mass-energy balance in the decay of the neutron indicated that the rest mass of the hypothetical particle should be very small - much smaller that the mass of an electron, and possibly even zero. The combination of zero electric charge, and zero or nearly zero mass, would make the particle extremely hard to detect. The Italian physicist Enrico Fermi called the suggested particle the neutrino ("little neutral one" in Italian). ... ... Finally, in 1956, neutrinos were detected in an experiment using the extremely large flow of neutrinos that comes out of a nuclear reactor. ... " I've read elsewhere that 'normal' matter (stars, interstellar H2 gas, etc.) accounts for 5% of the mass of the universe, 'dark' matter accounts for 25%, and 70% is in the form of the "false vacuum" which underlies our vacuum of space. I don't know whether that is correct or not, but some advanced theorists have posited as such. Of course, nowadays, we know of many different types of neutrinos, associated with electrons, positrons, various muons, etc. Conceivably, the "dark matter" is from other forms of particles (WIMPs, etc.), or something not yet conceived. Good question.
I've read that in one theory that gravity (and only gravity) "leaks" out of our universe into others, thus accounting for the weakness of gravity compared to the other forces. Is it possible that the gravity effects of dark matter could be gravity "leaking" into our universe from another?
YES! NARRATOR: Randall tried to calculate how gravity could leak from our membrane Universe into empty space, but she couldn't make it work. Then she heard the theory that there might be another membrane in the eleventh dimension. Now she had a really strange thought. What if gravity wasn't leaking from our Universe but to it? What if it came from that other universe? On that membrane, or brane, gravity would be as strong as the other forces, but by the time it reached us it would only be a faint signal. Now when she reworked her calculations everything fitted exactly. End.
Reminds me of a sci fi story I read in the 1970s. I forget the author, Asimov?, Clarke? It posited another universe, with slightly different properties of physics, constants, etc., in which creatures had evolved which were of 3 sexes (call them A, B, C), not our two (male, female). Accordingly, they could have 3 different sexual unions (AB, AC, BC) of two partners, which was called flitting. Finally, all 3 could join in sex, but they became permanently bound, forming a single entity (a 'hard one') little noticed by the others. They were worried because some of our universe was leaking into theirs, and so the 'hard ones' were contacting our scientists. Pure nonsense, but it was an interesting story by one of our noted sci-fi writers. Anyone else read that story?
If we are going to understand how the universe really works, "ALL" the Sci-FI stuff has got to be taken out!!! What happens to all the Baryonic Matter that the SMBH's are 'feeding' on?????????????????????????? Leave out the Sci-Fi, just consider the processes that happen in the workings of the universe! Humans CANNOT be in worm holes!!!
In the 1940s, I believe it was Asimov who wrote an interesting sci-fi story, in which one of the concepts he put forth was the idea that future civilizations would have these things called artificial satellites, and that if they were launched properly, and at a sufficient distance, they could be placed so that they were constantly directly overhead of any spot on Earth, as the Earth rotated. I'm not certain if he called it a geo-synchronous oribt or not, but he predated the science by several decades in his sci-fi novel. Personally, I too dislike sci-fi that strays from actual physics, but we should not be too hasty to judge.
Asimov. The Gods Themselves. One of his best works. The aliens deliberately initiated the exchange between universes, because their star was dying, they had no interstellar capability, and they needed energy. The Earth hero was worried because he calculated that the exchange would result in the Sun going supernova due to the local change in fundamental constants (no, his name wasn't Paul Dixon Please Register or Log in to view the hidden image!). That was Clarke, in 1945, in a letter to Wireless World magazine: ...Both of these developments demand nothing new in the way of technical resources; the first and probably the second should come within the next five or ten years. However, I would like to close by mentioning a possibility of the more remote future--perhaps half a century ahead. An ``artificial satellite'' at the correct distance from the earth would make one revolution every 24 hours; i.e., it would remain stationary above the same spot and would be within optical range of nearly half the earth's surface. Three repeater stations, 120 degrees apart in the correct orbit, could give television and microwave coverage to the entire planet. I'm afraid this isn't going to be of the slightest use to our post-war planners, but I think it is the ultimate solution to the problem.
I have used a conservative definition of the term "known matter". If we, as some of you have not feared to do so, blur the line between what is reasonably known and what lies in your dreams, we can imagine that anything at all is not only possible but has already come true. One of your colleagues, Prof. Irwin Correy, has published a theory in which 105% of the mass in the universe is invisible. You believe it, don't you?
Thanks Pete! It's been quite a few years since I read the story when it first came out (1972) and my memory of the entire plot has grown a little fuzzy. Likewise, with Clarke I only read about his letter, not the letter itself, though he is generally credited as being the first person to have described geosynchronous satellites and the ability to have continous coverage of events around the globe. It would be interesting to see how many achievements in science were predated by descriptions of them in sci-fi novels. 20,000 Leagues Beneath the Sea would be one novel with quite a few, and there must be quite a few others. Why not start a thread along those lines? Walter
Since this thread is called Dark Matter and my book is called Dark Visitor and you stated a preference for scientific "science fiction," you may want to look at the web page under my name, and its subpage that lists all the physics I have woven into a cosmic horror story. - Rapid return of new permanent ice age, to be caused by a "Dark Visitor" (probably a small balck hole) passing by our solar system with no warning of its approach except the slight perturbations now noticed by the book's astronomer to the orbit of Pluto. (His analysis predicts Ice Age will start in Northern Hemisphere's winter of 2008.) You can read book for free - web site tells how and why I wrote the book.
