Slashdot Mirror
US Restarts Hunt For Gravitational Waves With Advanced LIGO
schwit1 writes: The hunt for gravitational waves began again for the Laser Interferometer Gravitational-Wave Observatory (LIGO)-the largest instrument of its kind. The restart follows a five-year-long, US $200-million project to overhaul the experiment's detectors. Many physicists believe the revamped experiment, dubbed Advanced LIGO, will be the first to find direct evidence of gravitational waves: ripples in the fabric of space-time that can be created by, among other things, a pair of neutron stars or black holes orbiting each other.
Gravitational waves were first theorized in 1916 by Albert Einstein as a consequence of his general theory of relativity, which celebrates its centennial this year.
Schrödinger's cat can answer this: it's radiating energy and it's not radiating energy at the same time.
Schrödinger's cat can explain any physics question. Except when it can't.
At the same time.
Schroedinger's Brexit: The UK is both in and out of the EU at the same time!
If I remember correctly, the noise floor of the previous instrument was approximately the level of the signal they were looking for. A better detector may help.
Indeed. It's hard to overstate the sensitivity of these instruments, or the vulnerability of these instruments to noise. To take one example, here's an ArXiv preprint that calculates that the original LIGO detectors would need to be physically shielded from tumbleweeds, since the the impact of a wind-borne tumbleweed on the building exterior (100 feet from the detector) could produce a vibrational or gravitational transient sufficient to appear to be a spurious gravitational wave signal.
~Idarubicin
If I remember correctly, the noise floor of the previous instrument was approximately the level of the signal they were looking for.
A better detector may help.
As someone who used to work at LIGO Hanford (quite some time ago), I can confirm this. It was always planned to be two phases. The first stage was simpler, and was used to get through any teething issues. It actually used simpler mirror controls and detection systems than many other gravitational wave detectors around the world, and made up the sensitivity by being much longer (4km beam length compared to some that were just 300m). That let it get up to speed quickly with at least some chance of still being able to detect something. However, the only way they expected to get a good signal was either by being lucky, or if the estimates of the actual signal level were off and there were much stronger signals out there. But, as the site director I worked for liked to point out, every time we have opened up a new way to view the universe, we saw something unexpected. Optical telescopes, UV, IR, radio, etc. all saw something new. If there is something we don't expect out there, it might send a strong signal.
Since they didn't get lucky and get a clear signal, they moved on to the second phase and replaced the simple control systems with the more complex (and likely more fiddly) systems. Since a large part of the cost of the project was "baking" the 4km steel tubes in order to get a good enough vacuum, the upgrade of the mirrors and control systems was comparatively cheap. The advanced mirror controls are expected to match or exceed the detectors elsewhere, and combined with the much longer beam tubes it should show sensitivity far beyond anything else out there.
As an interesting side note, some parts of the Advanced LIGO upgrades have been installed at the Louisiana site for quite some time.. Specifically, the seismic isolation systems were upgraded soon after it came online. When surveys were initially done for the site, the seismic noise level was just fine, but soon after the interferometer came online the forest around them matured to the point where logging operations commenced. Even miles away, trees falling to the ground were enough to shake the instrument out of lock. When looking at plots of when the LA site was collecting data, you could clearly see dawn and dusk, because as soon as there was enough light to chop down a tree, the instrument became useless. This forced them to move up plans for the active isolation system originally scheduled for Advanced LIGO, and they installed it to replace the passive isolation system. It actually worked rather well, and gave them some experience with the system, which hopefully helped with installing it at the Hanford site.