Frequently Asked Questions

Gravitational waves

What are gravitational waves?
Gravitational waves are ripples in spacetime that travel through the Universe.

To better understand what we are talking about, let us consider a water pond. If you perturb a medium with an object, like the surface of a pond with a stone, you produce "ripples" that propagate as waves through that medium. Now, substitute that object with two massive and compact stars, like two black holes, and the medium with the spacetime itself. Instead of waves on the water, you will have gravitational waves.

Why are gravitational waves important?
If you remember the analogy of the water in a pond, you can get some information on it just by looking at the propagating waves that you have generated. For example, their intensity is going to be related to both the weight of the stone and also the consistency of the water; waves dissipate more quickly when the water is very muddy. Also, the shapes of these waves can tell you about how far away that stone has hit the water and about the presence of other rocks in between. In a similar way, gravitational waves give us information on what has produced them and on the very nature of the fabric of space-time through which they are propagating. In some cases, this is the only way that we have to learn about phenomena that happened in very distant regions of the Universe or that involved objects that are not observable with any other means, like black holes.

What are the sources of gravitational waves?
Unlike the fabrics we are commonly used to, space-time is very, very stiff. Only very violent phenomena in the Universe can produce gravitational waves with intensities that are actually detectable by us. In turn, the development of instruments sensitive enough to detect them is a formidable task. So far, the only sources of gravitational waves that we have been able to detect are the coalescence of binary compact star systems, like those involving black holes or neutron stars. These are what remains of stars with large masses. Unlike our Sun, they don't emit an appreciable amount of light (with the notable exception of pulsars, which behave like "lighthouses" and appear as blinking stars) and have masses multiple times larger than that of the sun, concentrated in a few tens of kilometers. Their collision releases a huge amount of energy, a significant part of which is in the form of gravitational waves. Other possible sources include the aforementioned pulsars and the Big Bang itself.

What can gravitational waves tell us about the Universe?
Gravitational waves constitute a formidable tool with which to study the Universe. They can probe the very nature of its fabric and provide information about some of the most violent phenomena happening in it, such as the collision of black holes or the Big Bang. Moreover, some of these phenomena are too distant to be observed with any other means, like the electromagnetic radiation that could be absorbed or confused with the foreground in its path from the source toward us.

The Advanced Virgo detector

What is Advanced Virgo?
Advanced Virgo is the upgraded version of the Virgo gravitational wave detector, a 3-km Michelson interferometer located near the city of Pisa, Italy. It started taking scientific data in 2017 for the second joint observing run (O2), together with the similar detectors of LIGO in the United States.

How sensitive is Virgo?
Interferometers are extremely sensitive instruments with which to measure relative differences in distance, and, in particular, differences in the relative sizes of their arms. They are exceptionally well suited to the detection of gravitational waves, the effect of which on them produces a differential strain on their arms. To detect gravitational waves, they need to be sensitive enough to be able to detect variations in the kilometre-long arms of the order of 1E-18 metres. This corresponds to one thousandth of the size of the nucleus of an atom. Or, if you prefer, to find, from Earth, a needle on the star nearest to the Solar system. Quite impressive, right?

What is the difference between Virgo and LIGO?
The main difference between the Advanced Virgo and Advanced LIGO detectors is the way the reduction of seismic noise has been implemented. In Virgo, this is achieved by multiple stages of inverted pendula, a mostly passive device called a superattenuator, isolating each of the main optical components of Virgo from ground motion and environmental noise. Instead, in LIGO, the reduction of seismic noise is provided by a hybrid active-passive system that tracks the vibrations of the ground and corrects them.

What are glitches?
Glitches are transient excesses of noise, which, in turn, is everything at the detector output that is not produced by gravitational waves. These are typically very rapid, lasting about a second or less. It is particularly convenient to display them by means of spectrograms, which allow their classification on the basis of their morphology and to find coincidences and correlations with other channels.

Where do glitches come from?
The short answer is that we do not know exactly, and this is why this project is so important. To be more precise, for some classes of glitches we have many possible known sources of noise, such as vibrations of parts of the apparatus induced by microseismic noise, which needs to be precisely identified in order to find mitigation strategies.

What are auxiliary channels?
The detectors are constantly monitored by a very large number of controls and sensors, which are recorded by these auxiliary channels. Comparing the noise excess in the main strain channel, aimed at measuring gravitational wave signals, with the excess in the auxiliary channels, allows us to identify what has astrophysical origins, and is not present in the auxiliary channels, and what is coming from instrumental malfunctions or disturbances from the physical environment of the observatory.

The GWitchHunters Project

What is the goal of GWitchHunters?
The goal of this project is to engage citizens, shoulder by shoulder with the researchers, in the improvement of gravitational wave detectors through the characterization of their transient noise sources. In turn, this should lead to more sensitive detectors for the study and the understanding of the Universe surrounding us. Even simple tasks, like identifying excesses of noise or similarities in various noise features, are incredibly valuable in the unveiling of the causes of it and in helping to guide mitigation strategies by the scientists working directly on the detectors.

What are the Mobile Challenges?
Even small gestures can provide a huge contribution in the task of improving current gravitational wave detectors. Mobile challenges are special tasks that you can complete by using just the tip of your finger, wherever you are, and are specifically meant for mobile devices, such as smartphones or tablets. In designing them, we have tried to maintain all of the engagement and fun of the desktop version.

How can I help Virgo and LIGO scientists with GWitchHunters?
The answers that you are going to submit to the proposed tasks are incredibly valuable to us. They can help us develop new strategies to identify noise transients, such as glitches, classify them and find correlations between various detector channels. This is basically what researchers working at the various detector sites do constantly. But now, through GWitchHunters, we have formed a global team of citizens and scientists for just this purpose. Are you ready to join us?

Why do we need the people to do this task?
The most direct and honest answer is that we, as humans, are way better than machines at accomplishing this task. Recognizing the features and similarities of the noise in the images that this project proposes is really not that different from reading the handwriting of various people, like doctors or six-year-old children. Or recognizing the species of some animals or plants just by looking at random pictures or photographs, with many elements of confusion in the foreground. It is very hard to explain what elements matter the most for our decision, and similarly to code an algorithm able to reproduce our reasoning. We are able to do this because we have been trained for all of our lives to accomplish this task and make similar decisions in a myriad of situations. This is why humans are more suited than machines for this specific role, and in general for the improvement of our instruments and knowledge.

What is the importance of computers for our Science?
From the previous answer, one might wonder: Are computers at all relevant for our Science? Of course they are. After the unique contribution that people can provide, we can indeed attempt to train computers to reproduce our own decisions. We do this not by programming them with an algorithm that encodes why we arrived at our conclusions, but by asking them to mimic our choices. This is feasible once we have obtained a sufficiently large and general set of examples to learn from. Then, once this training has been completed, our computers can be very helpful to our Science in automatizing and speeding up all the repetitive stuff. This is a very fascinating aspect of modern Artificial Intelligence development and its uses in solving practical problems from everyday life and more.