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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.
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.
The expected main sources of gravity waves are things like merging binary "star" systems where the stars are actually black
holes or neutron stars, and supernova explosions. However, these are relatively rare events.
So, for a gravity wave detector to see something, such an event must take place within the volume of space
where the detector has the sensitivity to detect something. That means for the original LIGO to detect something
we would have had to have been very lucky to have seen something.
With the upgraded version, the volume of space where LIGO will be sensitive is greatly expanded.
We have educated guesses for e.g. the occurrence rate of merging black holes. That can be used
to estimate how likely it is that a gravity wave detection would be made within a certain period of time
The current estimates give advanced LIGO a good chance of detecting something. (I'm too lazy
to check the actual numbers!)
So, if nothing is seen within a few years at the final sensitivity limit then people will have to reexamine
their estimates of event rates and/or general relativity.
Tricky as it is to create a gravitational detector, a gravitational radiator the emits significant power is a lot tougher. I seem to remember that thermonuclear bombs and asymmetric explosions generate a trivial amount of gravitational wave energy.
LIGO is looking for supernova scale sources - probably not a good idea to build one in our solar system.
That is the whole point of the soon to be launched LISA pathfinder project. It will have an arm-length on the order of a million kilometers. The long arm length and lack of seismic noise make it very sensitive to frequencies several orders of magnitude below that of LIGO. However, the long arm length also makes it insensitive to higher frequencies (when the wavelength is smaller than the arm length). So it works out such that LISA and LIGO frequency ranges don't overlap, and the complement each other quite well with sensitivity to different phenomena.