Dark Matter Is an Illusion, New Antigravity Theory Says

Previous discussion: http://news.ycombinator.com/item?id=2882670

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?

Would we see this effect with electric fields as well?

Could differently oriented electric and gravitational fields compete to orient a given dipole?

You've ask a couple of very deep questions there.

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...

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).