<p>I think this is largely a matter of what you decide is incremental as opposed to a generational change.</p>
<p>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.</p>
<p>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.</p>
<p>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.</p>
<p>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.</p>

CC BY-SA 4.0
2021-04-24T17:42:59.033Z
The improvement in strain sensitivity from initial LIGO to advanced LIGO is similar as the projected improvement from aLIGO to 3g detectors.
TimRias
CC BY-SA 4.0
2021-04-24T09:24:30.450Z
2021-04-27T14:30:05.450Z
2021-04-27T14:30:05.450Z
ProfRob
<p>I don't feel that ProfRob's answer does the step to advanced LIGO justice.</p>
<p>First, it is important to recognize that LIGO, from its inception,  was envisioned as a two-stage project. From <a href="https://www.ligo.caltech.edu/system/media_files/binaries/386/original/LIGOHistory.pdf" rel="nofollow noreferrer">Caltech`s LIGO lab</a>:</p>
<blockquote>
<p>... and in 1989, Vogt, Drever, Fred Raab, Thorne and Weiss submitted a joint Caltech/MIT proposal for LIGO construction to the NSF.The proposal envisioned building LIGO facilities at two sites, and then operating in them a pair of initial interferometers based on proven technology, with a sensitivity where gravitational waves might be detected, followed by advanced interferometers based on more advanced technology, with a high probability of detecting waves. This two-stage approach has been essential to LIGO’s 2016 success. The technological leap from prototypes to advanced interferometers was too great to be carried out in a single step.</p>
</blockquote>
<p>Building an Michelson interferometer isn't hard, it is literally done by first year physics students around the world. The hard part is sufficiently isolating the interferometer against environmental noise that the minute signal produced by a gravitational wave can be isolated.</p>
<p>By 2010, initial LIGO had reached the absolute limit of what was possible with the technology upon which it was built, reaching a strain noise of <span class="math-container">$1.5\times 10^{-22} Hz^{-1/2}$</span>. To do better would require an almost entirely new instrument. Advanced LIGO was designed to give an order of magnitude improvement in strain sensitivity over intial LIGO. Achieving this required changing almost every part of the instrument, including heavier test masses, more powerful lasers, and a completely redesigned suspension system. Essentially the only parts that carried over from initial LIGO were the facilities in which the instruments were installed, and parts of the vacuum system (which were designed with the ultimate goal of building advanced LIGO in the first place. Construction of advanced LIGO started in 2008 and took over 6 years to complete.</p>
<p>In this sense the step between initial and advanced LIGO can be best compared to the relationship between the <a href="https://en.wikipedia.org/wiki/Large_Hadron_Collider" rel="nofollow noreferrer">LHC</a> and its predecessor <a href="https://en.wikipedia.org/wiki/Large_Electron%E2%80%93Positron_Collider" rel="nofollow noreferrer">LEP</a>, which had occupied the same tunnel at CERN before it.</p>
<p>As work on advanced LIGO started, people also start thinking about would be necessary to make another order of magnitude step in strain sensitivity. As the goal was to make a similar step from advanced LIGO (and advanced Virgo in Europe) as had been made from initial LIGO and Virgo to their advanced versions, these detectors where dubbed &quot;third generation&quot; (3G) detectors.</p>
<p>There is currently two 3G detectors proposed. The Einstein Telescope in Europe and Cosmic Vision in the US. Einstein Telescope is currently the furthest in planning and is envision as a triangular interferometer with 10km arms build underground. Cosmic Vision is closer to a scaled up version of LIGO with an L-shaped layout with 40km arms.</p>
<p>The realization of these 3G detectors is still very far off. In the meantime more incremental improvements to the current ground based detectors are planned. First of all, advanced LIGO is not yet at its full design sensitivity which it is expected to reach in its 4th observation run. Beyond that further upgrades to LIGO labelled &quot;A+&quot; have been approved.</p>
<p>People have also considered what would be the maximum sensitivity that can be squeezed out of the existing LIGO facilities, if one were to build an almost entirely new instrument (again) using all the technology that is foreseen for 3G detectors, including moving to cryogenics to reduce thermal noise (as is currently being applied in the Japanese <a href="https://en.wikipedia.org/wiki/KAGRA" rel="nofollow noreferrer">KAGRA</a> detector). This hypothetical design is called &quot;LIGO Voyager&quot;. It is sometimes also referred to as a third generation detector, although since it wouldn't be quite at the level of Cosmic Explorer and the Einstein Telescope it is now more commonly referred to as a &quot;2.5G&quot; instrument.</p>

CC BY-SA 4.0
2021-04-27T12:55:54.680Z
Isn't aLIGO expected to achieve design sensitivity sometime this year or next?
Daddy Kropotkin
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2021-04-27T13:03:37.420Z
@DaddyKropotkin It should indeed be O4 (O5 will be the A+ upgrades). Unfortunately, O4 seems to be pushed back to the second half of 2022 (at least).
TimRias
CC BY-SA 4.0
2021-04-27T13:10:43.703Z
Neat! Also, is LISA not a third gen detector? And what about the many decihertz detectors being planned such as TianGO?
Daddy Kropotkin
CC BY-SA 4.0
2021-04-27T13:12:59.760Z
No, normally the term "3G" is reserved for ground based interferometers. See also: https://gwic.ligo.org/3Gsubcomm/documents/gwic_3g_science_case_Sep_2020.pdf
TimRias
CC BY-SA 4.0
2021-04-27T14:14:49.203Z
I listed all the improvements that had been made between aLIGO from LIGO. I don't see that I missed anything significant out other than the length of time it took. I initially voted to close this question as a matter of opinion. So it has proved.
ProfRob
CC BY-SA 4.0
2021-04-27T14:19:51.360Z
@ProfRob Your question seemed to over value cosmetic superficial outward changes like geometry and location, over technological miracle working on things like the suspension.
TimRias
CC BY-SA 4.0
2021-04-27T14:25:35.540Z
As I said. "I think this is largely a matter of what you decide is incremental as opposed to a generational change." The change in geometry and arm length is neither cosmetic or superficial. The change in location requires the building of an entire new instrument. In addition the Einstein Telescope will change ALL of the things that were changed between LIGO and aLIGO. There was no intention to do down the miracle that is aLIGO.
ProfRob
CC BY-SA 4.0
2021-04-27T14:29:15.033Z
@ProfRob Moreover, as explained in my answer, what is considered "generational" in this context is *defined* by the step from LIGO to advanced LIGO. The term third generation  as used for ET was chosen specifically to refer to this step.
TimRias
CC BY-SA 4.0
2021-04-27T14:36:41.063Z
The geometry, and being under ground are "superficial"  in the sense that they are not defining features of what is considered a third generation observatory, as evidenced by both ET (and underground triangular configuration) and CE (an L-shaped above ground facility) being considered 3G observatories).
TimRias
CC BY-SA 4.0
2021-04-28T18:41:07.563Z
^ Exactly. The 3G detectors will not be hindered by the seismic noise wall, for example, as is the signature of ground-based detectors.
Daddy Kropotkin
CC BY-SA 4.0
2021-06-22T03:02:48.533Z
fyi I’ve just added a bounty to [How can there only be “11 phonons” in the mirrors of LIGO interferometers?](https://astronomy.stackexchange.com/q/44409/7982)
uhoh
CC BY-SA 4.0
2021-04-27T09:23:40.763Z
2021-04-27T13:02:14.533Z
2021-04-27T13:02:14.533Z
TimRias
<p>I just learned the term &quot;third generation gravitational wave detector&quot; in answers to <a href="https://astronomy.stackexchange.com/q/43518/7982">What would a kHz gravitational wave detector look like? (mountains on millisecond magnetars) How would it differ from LIGO/Virgo?</a></p>
<p>Wikipedia's <a href="https://en.wikipedia.org/wiki/Gravitational-wave_observatory#Interferometers" rel="nofollow noreferrer">Gravitational wave observatory; Interferometers</a> lists LIGO and Virgo as 1st generation, and <a href="https://en.wikipedia.org/wiki/LIGO#Advanced_LIGO" rel="nofollow noreferrer">Advanced LIGO</a> and <a href="https://en.wikipedia.org/wiki/Virgo_interferometer#History" rel="nofollow noreferrer">Advanced Virgo</a> as second generation.</p>
<p>Is the difference between the originals and the &quot;Advanced&quot; versions really <em>generational</em> or were these just <em>planned incremental upgrades</em> that were just waiting for funding once the basic systems had demonstrated they worked as planned?</p>

CC BY-SA 4.0
2021-04-26T12:57:03.810Z
What does "really generational" mean? And "the plans" for how LIGO works were constantly being updated. It is expected to reach "design sensitivity" sometime soon https://journals.aps.org/prd/pdf/10.1103/PhysRevD.102.062003
Daddy Kropotkin
CC BY-SA 4.0
2021-04-27T09:26:04.330Z
@DaddyKropotkin I didn't invent the term "generation" in terms of GW detection technology nor assign 1st, 2nd or 3rd to different designs, these classifications are somewhat established. My question is about the appropriateness of saying that the difference between these detectors original configurations and their advanced configurations are really a generational change or not. The two answers posted so far seem to accept "how generational" as a viable question, though interestingly have different positions. Anyway, I've just added the `terminology` tag to the question to highlight this.
uhoh
CC BY-SA 4.0
2021-04-27T12:54:44.977Z
Okay, so then the answer is by definition yes, since "generations" is defined as successive upgrades from the original construction to the design construction. That's why i asked for you to clarify, since you're question seemed, as is, trivial.
Daddy Kropotkin
CC BY-SA 4.0
2021-04-27T14:56:29.200Z
@DaddyKropotkin in Stack Exchange the OP decides what post *best answers* their question. If you feel you have the best answer, then consider posting it as a Stack Exchange answer and let folks vote on it. I think you'll need to support your assertion that successional generation changes in a gravitational wave detectors are anything that can be considered upgrades.
uhoh
CC BY-SA 4.0
2021-04-27T23:58:37.770Z
I'm just trying to arrive at a better posed question because I don't understand the OP qusetion. A "generation" refers to a suite of upgrades/improvements, I think. https://dcc.ligo.org/public/0006/P0900255/005/P0900255-v5.pdf
Daddy Kropotkin
CC BY-SA 4.0
2021-04-28T01:17:52.557Z
@DaddyKropotkin That's extremely helpful! It sounds to me more like they threw everything away and built a whole new system rather than upgraded the existing system. "Advanced LIGO is the name given to the next generation of detectors that will use the existing vacuum envelopes at the two LIGO sites in Louisiana and Washington state (See Figure 1). These new detectors will include improved seismic isolation, suspensions, optics, lasers, and all other hardware subsystems."
uhoh
CC BY-SA 4.0
2021-04-28T18:38:08.820Z
Cool! Yes, indeed. They basically kept the same chassis of the old tunnels but improved essentially all the technical aspects.
Daddy Kropotkin
<observational-astronomy><gravitational-waves><terminology><instruments>
Is the difference between LIGO & Virgo and their "Advanced" versions really generational, or were these just planned incremental upgrades?
uhoh
# Is the difference between LIGO & Virgo and their "Advanced" versions really generational, or were these just planned incremental upgrades?

(Asked by: username_0 on 2021-04-24)

I just learned the term "third generation gravitational wave detector" in answers to [What would a kHz gravitational wave detector look like? (mountains on millisecond magnetars) How would it differ from LIGO/Virgo?](https://astronomy.stackexchange.com/q/43518/7982)

Wikipedia's [Gravitational wave observatory; Interferometers](https://en.wikipedia.org/wiki/Gravitational-wave_observatory#Interferometers) lists LIGO and Virgo as 1st generation, and [Advanced LIGO](https://en.wikipedia.org/wiki/LIGO#Advanced_LIGO) and [Advanced Virgo](https://en.wikipedia.org/wiki/Virgo_interferometer#History) as second generation.

Is the difference between the originals and the "Advanced" 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?

**Tags:** observational-astronomy, gravitational-waves, terminology, instruments
> **Comments:**
> **username_1**: What does "really generational" mean? And "the plans" for how LIGO works were constantly being updated. It is expected to reach "design sensitivity" sometime soon https://journals.aps.org/prd/pdf/10.1103/PhysRevD.102.062003  
> **username_0**: @DaddyKropotkin I didn't invent the term "generation" in terms of GW detection technology nor assign 1st, 2nd or 3rd to different designs, these classifications are somewhat established. My question is about the appropriateness of saying that the difference between these detectors original configurations and their advanced configurations are really a generational change or not. The two answers posted so far seem to accept "how generational" as a viable question, though interestingly have different positions. Anyway, I've just added the `terminology` tag to the question to highlight this.  
> **username_1**: Okay, so then the answer is by definition yes, since "generations" is defined as successive upgrades from the original construction to the design construction. That's why i asked for you to clarify, since you're question seemed, as is, trivial.  
> **username_0**: @DaddyKropotkin in Stack Exchange the OP decides what post *best answers* their question. If you feel you have the best answer, then consider posting it as a Stack Exchange answer and let folks vote on it. I think you'll need to support your assertion that successional generation changes in a gravitational wave detectors are anything that can be considered upgrades.  
> **username_1**: I'm just trying to arrive at a better posed question because I don't understand the OP qusetion. A "generation" refers to a suite of upgrades/improvements, I think. https://dcc.ligo.org/public/0006/P0900255/005/P0900255-v5.pdf  
> **username_0**: @DaddyKropotkin That's extremely helpful! It sounds to me more like they threw everything away and built a whole new system rather than upgraded the existing system. "Advanced LIGO is the name given to the next generation of detectors that will use the existing vacuum envelopes at the two LIGO sites in Louisiana and Washington state (See Figure 1). These new detectors will include improved seismic isolation, suspensions, optics, lasers, and all other hardware subsystems."  
> **username_1**: Cool! Yes, indeed. They basically kept the same chassis of the old tunnels but improved essentially all the technical aspects.  

---

## 2 answers

### Answer by username_2 on 2021-04-24. Score: 4

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 "tuning" 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 "next generation" tag for going from LIGO to aLIGO and from aLIGO to the 3G detectors.

> **Comments:**
> **username_3**: The improvement in strain sensitivity from initial LIGO to advanced LIGO is similar as the projected improvement from aLIGO to 3g detectors.  

---

### Answer by username_3 on 2021-04-27. Score: 3

I don't feel that ProfRob's answer does the step to advanced LIGO justice.

First, it is important to recognize that LIGO, from its inception, was envisioned as a two-stage project. From [Caltech`s LIGO lab](https://www.ligo.caltech.edu/system/media_files/binaries/386/original/LIGOHistory.pdf):

> ... and in 1989, Vogt, Drever, Fred Raab, Thorne and Weiss submitted a joint Caltech/MIT proposal for LIGO construction to the NSF.The proposal envisioned building LIGO facilities at two sites, and then operating in them a pair of initial interferometers based on proven technology, with a sensitivity where gravitational waves might be detected, followed by advanced interferometers based on more advanced technology, with a high probability of detecting waves. This two-stage approach has been essential to LIGO’s 2016 success. The technological leap from prototypes to advanced interferometers was too great to be carried out in a single step.

Building an Michelson interferometer isn't hard, it is literally done by first year physics students around the world. The hard part is sufficiently isolating the interferometer against environmental noise that the minute signal produced by a gravitational wave can be isolated.

By 2010, initial LIGO had reached the absolute limit of what was possible with the technology upon which it was built, reaching a strain noise of $1.5\times 10^{-22} Hz^{-1/2}$. To do better would require an almost entirely new instrument. Advanced LIGO was designed to give an order of magnitude improvement in strain sensitivity over intial LIGO. Achieving this required changing almost every part of the instrument, including heavier test masses, more powerful lasers, and a completely redesigned suspension system. Essentially the only parts that carried over from initial LIGO were the facilities in which the instruments were installed, and parts of the vacuum system (which were designed with the ultimate goal of building advanced LIGO in the first place. Construction of advanced LIGO started in 2008 and took over 6 years to complete.

In this sense the step between initial and advanced LIGO can be best compared to the relationship between the [LHC](https://en.wikipedia.org/wiki/Large_Hadron_Collider) and its predecessor [LEP](https://en.wikipedia.org/wiki/Large_Electron%E2%80%93Positron_Collider), which had occupied the same tunnel at CERN before it.

As work on advanced LIGO started, people also start thinking about would be necessary to make another order of magnitude step in strain sensitivity. As the goal was to make a similar step from advanced LIGO (and advanced Virgo in Europe) as had been made from initial LIGO and Virgo to their advanced versions, these detectors where dubbed "third generation" (3G) detectors.

There is currently two 3G detectors proposed. The Einstein Telescope in Europe and Cosmic Vision in the US. Einstein Telescope is currently the furthest in planning and is envision as a triangular interferometer with 10km arms build underground. Cosmic Vision is closer to a scaled up version of LIGO with an L-shaped layout with 40km arms.

The realization of these 3G detectors is still very far off. In the meantime more incremental improvements to the current ground based detectors are planned. First of all, advanced LIGO is not yet at its full design sensitivity which it is expected to reach in its 4th observation run. Beyond that further upgrades to LIGO labelled "A+" have been approved.

People have also considered what would be the maximum sensitivity that can be squeezed out of the existing LIGO facilities, if one were to build an almost entirely new instrument (again) using all the technology that is foreseen for 3G detectors, including moving to cryogenics to reduce thermal noise (as is currently being applied in the Japanese [KAGRA](https://en.wikipedia.org/wiki/KAGRA) detector). This hypothetical design is called "LIGO Voyager". It is sometimes also referred to as a third generation detector, although since it wouldn't be quite at the level of Cosmic Explorer and the Einstein Telescope it is now more commonly referred to as a "2.5G" instrument.

> **Comments:**
> **username_1**: Isn't aLIGO expected to achieve design sensitivity sometime this year or next?  
> **username_3**: @DaddyKropotkin It should indeed be O4 (O5 will be the A+ upgrades). Unfortunately, O4 seems to be pushed back to the second half of 2022 (at least).  
> **username_1**: Neat! Also, is LISA not a third gen detector? And what about the many decihertz detectors being planned such as TianGO?  
> **username_3**: No, normally the term "3G" is reserved for ground based interferometers. See also: https://gwic.ligo.org/3Gsubcomm/documents/gwic_3g_science_case_Sep_2020.pdf  
> **username_2**: I listed all the improvements that had been made between aLIGO from LIGO. I don't see that I missed anything significant out other than the length of time it took. I initially voted to close this question as a matter of opinion. So it has proved.  
> **username_3**: @ProfRob Your question seemed to over value cosmetic superficial outward changes like geometry and location, over technological miracle working on things like the suspension.  
> **username_2**: As I said. "I think this is largely a matter of what you decide is incremental as opposed to a generational change." The change in geometry and arm length is neither cosmetic or superficial. The change in location requires the building of an entire new instrument. In addition the Einstein Telescope will change ALL of the things that were changed between LIGO and aLIGO. There was no intention to do down the miracle that is aLIGO.  
> **username_3**: @ProfRob Moreover, as explained in my answer, what is considered "generational" in this context is *defined* by the step from LIGO to advanced LIGO. The term third generation  as used for ET was chosen specifically to refer to this step.  
> **username_3**: The geometry, and being under ground are "superficial"  in the sense that they are not defining features of what is considered a third generation observatory, as evidenced by both ET (and underground triangular configuration) and CE (an L-shaped above ground facility) being considered 3G observatories).  
> **username_1**: ^ Exactly. The 3G detectors will not be hindered by the seismic noise wall, for example, as is the signature of ground-based detectors.  
> **username_0**: fyi I’ve just added a bounty to [How can there only be “11 phonons” in the mirrors of LIGO interferometers?](https://astronomy.stackexchange.com/q/44409/7982)  

---

