Greetings from Toronto, where I’m visiting UofT to talk about dark energy, the arrow of time, and other obsessions of mine. Which has prevented me from as yet writing the long-awaited second installment of “Unsolicited Advice,” the one that will tell you how to choose a graduate school. It is that time of year, after all.
In the meantime, check out this nice post at Anthonares on Lunar laser ranging. The Apollo astronauts, during missions 11, 14, and 15, were sufficiently foresighted to bring along reflecting corner mirrors and leave them behind on the Moon’s surface. Why would they do that? So that, from down here on Earth, we can shoot lasers at the lunar surface and time how long it takes for them to come back. Using this data we can map the Moon’s orbit to ridiculous precision; right now we know where the Moon is to better than a centimeter. This experiment, called Lunar laser ranging, teaches us a lot about the Moon, but it also teaches us about gravity. The fact that we can pinpoint the location of the Earth’s biggest satellite and keep track of it over the course of years provides us with a uniquely precise test of Einstein’s general relativity.
You might think that general relativity is already pretty well tested, and it is, but clever folks are constantly inventing alternatives that haven’t yet been ruled out. One example is DGP gravity, invented by Gia Dvali, Gregory Gabadadze, and Massimo Porrati. This is a model in which the observable particles of the Standard Model are confined to a brane embedded in an infinitely large extra dimension of space. Unlike usual models with compact extra dimensions, the extra dimension of the DGP model is hidden because gravity is much stronger in the bulk; hence, the gravitational lines of force from an object on the brane like to stay on the brane for a while before eventually leaking out into the bulk.
The good news about the DGP model is that it makes the universe accelerate, even without dark energy! This is one of the things that I talked about at my colloquium yesterday, and I hope to post about in more detail some day. The better news is that it is potentially testable using Lunar laser ranging! The claim is that the DGP model predicts a tiny perturbation of the Moon’s orbit, too small to have yet been detected, but large enough to be within our reach if we improve the precision of existing laser ranging experiments. People are hot on the trail of doing just that, so we may hear results before too long.
Not to get too giddy, the bad news about DGP is that it may be a non-starter on purely theoretical grounds. There are claims that the model has ghosts (negative-energy particles), and also that it can’t be derived from any sensible high-energy theory (see Jacques’s post). I haven’t examined either of these issues very closely, although I hope to dig into them soon. Maybe if I could quite traveling and sit down and read some papers.