Thursday, September 01, 2011

"Dark Matter is an Illusion" summary in National Geographic News gets something a little wrong

There was an interesting article from National Geographic News yesterday:
"Dark Matter Is an Illusion, New Antigravity Theory Says"
by Ker Than
I thought I'd post a link to the primary source here. I also wanted to point out that the explanation Ker Than gave got something really important wrong and consequently diminished the elegance of the proposed theory.

Here's the primary source:
"Is dark matter an illusion created by the gravitational polarization of the quantum vacuum?"
by Dragan Slavkov Hajdukovic
Astrophysics and Space Science 334(2):215--218
DOI: 10.1007/s10509-011-0744-4
Ker Than, the National Geographic News reporter, got it a little mixed up in this part of the NatGeo article:
All of these electric dipoles are randomly oriented—like countless compass needles pointing every which way. But if the dipoles form in the presence of an existing electric field, they immediately align along the same direction as the field.

According to quantum field theory, this sudden snapping to order of electric dipoles, called polarization, generates a secondary electric field that combines with and strengthens the first field.
This electric analogy states that electric dipoles align and strengthen electric fields, but that's incorrect. Electric dipoles weaken surrounding electric fields. In particular, the positive end of the dipole goes toward the "negative end" of the field and the negative end of the dipole goes toward the "positive end" of the field. So the two fields subtract from each other, not reinforce. This is summarized in the primary source (that I'll quote below).

[ note that magnetic dipoles align and reinforce surrounding magnetic fields because there are no magnetic monopoles. That is, magnetic field lines are continuous; they don't terminate. Consequently, magnetic dipoles are torqued to align their fields. Electric dipoles are driven by the motion of their monopolar ends ]

What Ker Than missed was that in this model of "gravitational charge", it is the case that opposites repel and likes attract. That's why you (matter) are attracted to earth (also matter). However, anti-matter and matter would repel each other. Moreover, if you had a matter–antimatter virtual pair (as quantum field theory says you do in a vacuum of space), that dipole would align because its "positive" end would be pulled toward the positive end of the gravitational field (and vice versa for its negative end). This alignment would strengthen the resulting field.

Here's the relevant snippet from the bottom of the first column of page 2 of the article:
In order to grasp the key difference between the polarization by an electric field and the eventual polarization by a gravitational field, let's remember that, as a consequence of polarization, the strength of an electric field is reduced in a dielectric. For instance, when a slab of dielectric is inserted into a parallel plate capacitor, the electric field between plates is reduced. The reduction is due to the fact that the electric charges of opposite sign attract each other. If, instead of attraction, there was repulsion between charges of opposite sign, the electric field inside a dielectric would be augmented. But, according to our hypothesis, there is such repulsion between gravitational charges of different sign.

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