This theory is basically that gravity is similar to other forces in that it's not solely attractive, but things of opposite "gravitational charge" (i.e. particles and antiparticles) repel.
If that holds, then an ambient gravitational field in an area would presumably be strengthened by virtual particle pairs aligning their gravitational dipole with that of the ambient field, quite neatly explaining the anomalous observations that brought about the dark matter theory in the first place.
There are more recent observations that this new theory doesn't yet explain, but it doesn't seem an insurmountable challenge.
Additionally, this theory makes one very testable hypothesis - that antimatter is oppositely-charged, gravitationally, to normal matter. If that's not the case, we should be able to determine that fairly soon (a few years, perhaps).
Why wouldn't we see this effect on earth, and have taken it into account when determining the strength of earth's own gravity?
We could be observing the impact of this without exactly realizing it. I won't speculate too much on this because I haven't read enough about it but it might make more sense in light of my next answer.
Would we see this effect with electric fields as well?
We absolutely see this effect with electric fields. In the last century we've grown to understand that in electromagnetism we don't simply deal with isolated electrons and positrons. These particles are actually surrounded with clouds of virtual electrons and photons which have an impact on the field (this is a simplified view but it's enough along the right lines to give you the right impression if you haven't studied physics). When we talk about the field around an electron we take this into account by assuming that all these contributions sum to give us the field that we actually observe. If you want to read more about this then http://en.wikipedia.org/wiki/Quantum_field_theory#Renormaliz... is a good starting point.
Could differently oriented electric and gravitational fields compete to orient a given dipole?
Yes, all relevant fields will have an impact on what's happening. I don't want to speculate too much beyond my knowledge though and when it comes to quantum gravity we're already there.
I know that this wasn't the deepest contribution but hopefully it will help shed some light to someone...
One thing I don't understand: wasn't there just discovered a ring of antimatter circling around the Earth? Wouldn't it just disperse into space if antimatter caused antigravity? Stupid question, I know, but just something that's beyond my comprehension.
No, those are charged particles interacting with the magnetic field of the earth. Gravity is such a ridiculously weak force that you can safely ignore it in situations like that.
Tiny amounts of antimatter were just discovered circling around Earth. The anti-grav effect on the antimatter particles is swamped by the effects of the Earth's gravity and magnetic field.
According to this theory, it would be the magnetic field alone--not the gravitational field--that is overwhelming the antigravity effect. After all, if these particles had an opposite "gravitational charge", then their experience of gravity wouldn't be attractive but would be equal in magnitude but opposite in orientation. Hence "anti"gravity.
The whole 'virtual photons' thing is a physical interpretation of the mathematics, but it is a mistake to conclude that this is how it 'actually' is. There is a mathematical model that works. I don't think we know how it 'actually' is. There is every reason to doubt that this sea of 'virtual photons' has any physical reality in an independently verifiable way. Occam's razor doesn't favor solutions that involve multitudes of invisible, unmeasurable, particles popping in and out of existence.
In the end, all we have are our mental models and experiments to back them up.... we'll never know what anything 'actually' is - all we can do is model things to whatever degree of accuracy we can.
A construction worker doesn't need to go around thinking about his hammer as being made up of mostly empty space, and of the electromagnetic forces that give it it's properties, it's a hammer! We all know what it "IS" right?
But it's not that. It's not even what we think it is. It's not even what the best physicists think it is.... and in the end, all we're left with are incomplete mathematical models, or models that are only accurate on a certain scale.
BTW - how do we then explain hawking radiation, if virtual particles are "just math" and not real?
Well, first and foremost: the existence of Hawking radiation hasn't been experimentally verified. There's no direct evidence (measuring radiation that could have no other source than the evaporation of the black hole) nor any indirect evidence (black holes losing mass at a rate that cannot be explained unless Hawking radiation is included). So although it seems to fit nicely into the picture, it may not exist at all.
A second type of answer is: I don't know and I admit that the physical picture of virtual particles hopping in and out of existence indeed nicely includes an explanation of Hawking radiation. However, I'm reminded of the texts that illustrate how many physical phenomena were considered to be satisfactorily 'explained' within, for instance, the aether model, even though it later turned out the model was fundamentally flawed and all of those physical interpretations were hogwash.
A third type of answer is: sometimes things simply aren't what they seem. For instance, we can describe phonons as if they're particles: they are described by the same statistics as 'real' particles are. However, they decidedly aren't particles. They aren't properties of individual particles either: they only exist in macroscopic, well-ordered, amounts of material. You could look at all individual atoms one by one and never arrive at phonons as partial explanations for their movements and properties. For that, you'd have to look at things at a different level.
That was E.E. "Doc" Smith's assumption in the Lensman series when "negaspheres" were developed (planetary scale antimatter objects), in Grey Lensman written circa 1939. They had an "opposite 'gravitational charge'", a tractor beam would repel them while a pressor beam would pull them in, etc. It would be amusing if this turned out to be true (well, ignoring the yet to be if ever invented beam technology).
I find it ironic that this article features a picture of data on the Bullet cluster, which is currently one of the strongest pieces of evidence favoring dark matter as a form of matter. Hajdukovic is quoted as saying that he's "currently expanding his theory to account for these observations." That translates as "I haven't found a compelling way of explaining them yet."
It's great that he's considering alternatives to the usual approach, and it would be fantastic to have a second viable model to test observations against: competing theories encourage open-mindedness. I'm really happy to have his work up on the arXiv. But I don't believe that a speculative idea like this merits attention in the mainstream press until it can at least give plausible explanations for the major successes of existing models.
This seems plausible to me. I'm not a physicist, but I think the case of dark matter can be compared to the case of the planet Vulcan.
In the early 20th century, in the heyday of discovering planets by observing existing planets and realizing that classical mechanics didn't quite predict their orbits correctly, it was observed that Mercury's orbit didn't fit the classical model well enough. Reasonably enough, it was presumed that there was another planet orbiting inside of Mercury's orbit. This planet was dubbed "Vulcan", and the fact that no one had ever observed Vulcan was explained away by the fact that it was clearly too close to the sun to be observed. This was all well and good until the early heyday of Einstein's theories, which made small but crucial changes to the mathematical models of classical mechanics. The orbit of Mercury was found to be perfectly consistent with Einstein's model.
The question of dark matter supposes a dichotomy: either our current understanding of physics is essentially accurate and there's a ton of fundamentally unobservable matter in the universe that explains any anomalous observations, or our model of physics isn't quite right yet. Intuitively, I think it's more likely our physics are off.
Either way our model of physics is off. Currently, the most reasonable explanation for dark matter is that it consists of some unknown, weakly-interacting particle. However, the standard model of physics predicts the existance of no such particle. Even this, the most mundane possibility, requires radical changes to our most fundamental physical theories.
That sounds even sillier. The most reasonable explanation is that there's some impossible matter around? At least the planet Vulcan was an ordinary planet that was just too close to the sun.
Again, I'm emphatically not a physicist. Stranger things have happened. It just smells funny to posit dark matter as a hypothesis.
The only thing that's not suspicious is the anomaly itself. Physics has always refined itself by looking at different scales. We were wrong about the quantum level the first time we looked at it, and we were wrong about even the planet Mercury, so it's to be expected that we're wrong about galaxy clusters.
"Impossible matter"? What's "impossible" about it? That we don't know what it is, and that it isn't predicted by our current models, doesn't make it "impossible." It means we're going to need to extend our models, which we've been doing on a regular basis for hundreds of years. Rob Knop sums this up nicely:
"In the face of evidence otherwise, many still insist that most of the Universe must be made up of baryonic stuff that interacts with other baryons and our familiar photons. Is this not just as much hubris as insisting that the Earth, where we live, must be the center about which all the other Solar System bodies orbit?" (http://scientopia.org/blogs/galacticinteractions/2011/08/14/...)
The dark matter hypothesis seems to have a great deal of explanatory power. The antigravity hypothesis, like other MOND theories, has a good fit for the specific phenomenon it was designed to tackle, but doesn't explain a lot of the other things that dark matter does. Ethan Siegel has a good post on "What Dark Matter's Alternatives Must Do" (http://scienceblogs.com/startswithabang/2011/08/what_dark_ma...) which points out some of the stuff that dark matter explains and rival theories don't (e.g. temperature fluctuations, hydrogen/helium ratio). He does presciently note that 'antimatter has negative mass' could explain more than traditional MOND theories but claims that this impacts other well-tested assumptions such as conservation of energy.
Of course, none of this is to say that the dark matter hypothesis won't eventually be disproved and go the way of Vulcan as you suggest. Ironically, though, it sounds to me like the MOND and antigravity theories are the Vulcans here: they solve only one specific problem, while dark matter fits with so much more.
That we don't know what it is, and that it isn't predicted by our current models, doesn't make it "impossible."
I was indulging in a bit of poetic license with that one.
I'll agree that we don't currently have a good theoretical alternative to the dark matter hypothesis. Much smarter and more qualified people than me have tried and failed to come up with one. But it still seems unsatisfying to me.
I remember being completely blown away in undergraduate physics classes by how much of the world can be exactly and precisely explained by various subject areas of physics (kinematics, gas laws, thermodynamics, etc.) Then at some point I ran in to a wall with quantum mechanics and particle physics where I had to throw up my hands. I was contemplating going to graduate school for a masters in physics but I decided against it. It's vexing that there are such disparities within the subject (not that there aren't in others as well, but Physics seems especially afflicted).
Within these 'disparities' lie new understanding, and awesome discoveries that will change the world. Just as quantum mechanics did at the beginning of the 20th Century...
The resolution of these disparities is part of the thrill of Science!
If that holds, then an ambient gravitational field in an area would presumably be strengthened by virtual particle pairs aligning their gravitational dipole with that of the ambient field, quite neatly explaining the anomalous observations that brought about the dark matter theory in the first place.
There are more recent observations that this new theory doesn't yet explain, but it doesn't seem an insurmountable challenge.
Additionally, this theory makes one very testable hypothesis - that antimatter is oppositely-charged, gravitationally, to normal matter. If that's not the case, we should be able to determine that fairly soon (a few years, perhaps).