Massive orbiting disco ball to measure relativity with lasers

Italian astrophysicists, well-known as the party animals of their field, have decided that it would be fun to launch a disco ball into orbit and then shoot lasers at it. Besides giving the astronauts on the ISS a good excuse to get their boogie on (like they need one), the disco ball should also help measure one of the weirdest effects of general relativity to an accuracy of 1%.

The disco ball is called LARES (for Laser Relativity Satellite), and we don't keep calling it a disco ball just because it looks like one. LARES is a completely passive 14-inch tungsten sphere, and just like a disco ball, it doesn't actually do anything besides sit there (albeit in orbit) and reflect light. Each one of those holes that you can see in the picture above contains a corner reflector (or retroreflector), which will bounce any incoming beam of light from any direction off two mirrors and directly back to the source. The idea is that laser ranging stations down here on Earth will be able to fire at LARES, calculating its distance (and orbit) very, very precisely.

Precision is necessary because researchers are trying to use LARES to measure what's called frame-dragging (or the Lense-Thirring effect), which is what general relativity predicts will happen when something really big (like a planet) spins. In traditional Newtonian mechanics, objects such as satellites could theoretically orbit the Earth in the exact same way forever, whether or not the Earth was rotating, since all the satellites care about is gravity. But relativity says that massive objects can have measurable effects on the spacetime around them, and when a massive object (like the Earth) spins, it drags spacetime around with it a little bit in the same direction as it's spinning.

spacetime-frame-dragging.jpg

This local twist in spacetime can, theoretically, cause a lot of weird stuff to happen. For example, from the perspective of a distant observer, light that goes past the Earth in the same direction as the spin will move faster than light going past on the other side of the planet, counter to the spin. And it also means that an orbiting satellite will be very slowly dragged along with the twisting space, shifting its orbital plane in the same direction as the Earth is spinning.

Such orbital procession (as it's called) is very, very hard to detect: the frame-dragging effect only amounts to about one part in a few trillion. This is why LARES isn't designed to do anything besides keep a stable orbit and reflect lasers that, over time, will hopefully show that its orbit is shifting eastwards by a couple feet per year, and the exact result should help nail down the intensity of Earth's frame-dragging within 1%.

For this to work, the researchers will also need to be able to very accurately account for all of the gravity bumps in the Earth itself, like how passing over a big mountain can slightly skew a satellite's orbit. There's some skepticism that it's going to be possible for the Italian team to narrow down which orbital changes come from frame-dragging and which ones come from other sources, but the hope is that by 2016 or so, we'll have yet another experimental proof of general relativity and how utterly weird our universe can get.

LARES, via New Scientist

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