Aug 03 2010

Does the universe have its own “fifth column”… er, fifth force?

Category: Uncategorizedharmonicminer @ 9:37 am

Modern physics has for decades assumed that we know of the basic four forces in the universe, and that there are only those four, gravity, electromagnetism, weak nuclear and strong nuclear forces. The problem is that some observed aspects of the universe seem not to be explainable in terms of only those four forces, notably the continued expansion of the universe, and the way galaxies stay together without (apparently) enough mass to produce the gravitic effects that are observed.  So called “dark energy” and “dark matter” is the scientific community’s way of saying, “We don’t have a fuzzy clue what’s out there or why it behaves that way.”  Dark energy and dark matter are believed to be 95% of everything in the universe, with what we see and can directly measure being 5% or less. So some scientists are asking, Is a cosmic chameleon driving galaxies apart?

The basic idea for this fifth force was hatched in 2004 by Justin Khoury and Amanda Weltman, then members of a team led by well-known string theorist Brian Greene at Columbia University in New York City. String theory is the favoured route to unifying gravity, the odd one out among the four forces, with the other three under the umbrella of quantum mechanics. It is a great playground for devising new fields and forces. The theory is formulated in 11 dimensions, seven of which are assumed to be curled up so small that we cannot see them. Disturbances in those curled-up dimensions might make themselves felt as “extra” forces in the four dimensions of space and time we do see.

For this picture to make sense, the effects in the visible dimensions must match our observations of the universe. Khoury and Weltman proposed one way of doing this: an extra force could be transmitted by particles whose mass depends on the density of the matter around them. That way, its effects could remained veiled on Earth.

How would that work? Well, in quantum mechanics, the range of influence of a force depends largely on the mass of the particles produced by the associated force field: the lighter the particle, the longer the force’s range. Electromagnetic fields, for example, produce photons that have no mass whatsoever, so the range of the electromagnetic force is infinite. The particles that transmit the weak nuclear force, on the other hand, are extremely heavy and do not travel very far, confining the force to the tiny scales of the atomic nucleus. With the strong nuclear force, things are slightly more complex: the associated particles, called gluons, are massless but also have the ability to interact with themselves, preventing the force from operating over large distances.

Khoury and Weltman started from the observation that the average density of matter in Earth’s vicinity is very high in cosmic terms, at about 0.5 grams per cubic centimetre. Under these circumstances, they proposed, the particle that transmits the chameleon force would be about a billion times lighter than the electron. The force itself would then have a range of not more than a millimetre – small enough for its effects to have remained undetected in the lab so far.

In the wide open spaces of the cosmos, however, where a cubic centimetre contains just 10-29 grams of matter on average, the mass of the chameleon particle plummets by something like 22 orders of magnitude, producing a muscular force that could act over millions of light years. The lost mass is picked up as energy by the chameleon field.

You’ll pardon me… but I’m excited by the idea that there is still plenty of physics left undiscovered, and the universe is still a very mysterious place. And, by that same token, I suggest that all the scientists in the room be relatively circumspect, or maybe even downright humble, in their conjectures about the possibility that all this happened more or less “by accident”, and did not require a Designer.