Is the difference between LIGO & Virgo and their "Advanced" versions really generational, or were these just planned incremental upgrades? I just learned the term &quot;third generation gravitational wave detector&quot; in answers to What would a kHz gravitational wave detector look like? (mountains on millisecond magnetars) How would it differ from LIGO/Virgo?
Wikipedia's Gravitational wave observatory; Interferometers lists LIGO and Virgo as 1st generation, and Advanced LIGO and Advanced Virgo as second generation.
Is the difference between the originals and the &quot;Advanced&quot; versions really generational or were these just planned incremental upgrades that were just waiting for funding once the basic systems had demonstrated they worked as planned?
I think this is largely a matter of what you decide is incremental as opposed to a generational change.
The basic location and overall size and detection principle of the interferometers did not change. An important change to the interferometer configuration was the addition of the signal recycling mirror that allows a certain amount of &quot;tuning&quot; of the frequency response. A homodyne readout scheme was employed instead of heterodyne - and the interferometer is run slightly off the dark fringe as a result. The test mass suspension system changed from being a single to a quadruple pendulum and the suspension wires are now silica rather than metal. The test masses themselves were increased from 11 kg to 40 kg which reduces the low frequency noise significantly.
I believe most of the other changes were essentially upgrades to the materials and technology being used. New mirrors, better lasers, improved vacuum, better seismic isolation etc.
I don't think it is anywhere near the generational change between aLIGO and the proposed Einstein Telescope, which will be in a different location, have a different size, different number of arms and geometry and will be operated cryogenically etc.
However, as pointed out by @mmeent, the improvement in sensitivity is about an order of magnitude in both cases, which is probably what justifies the &quot;next generation&quot; tag for going from LIGO to aLIGO and from aLIGO to the 3G detectors.