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Aerial view of the Virgo gravitational wave detector, located in Cascina, near Pisa (Italy). Virgo is a giant Michelson laser interferometer with 3 km long arms, which completes the two 4 km LIGO detectors.Nicola Baldocchi / Virgo Collaboration
For three years, humanity has been experiencing a new type of astronomy in relation to tradition. We are no longer just detecting light with a telescope, or neutrinos with huge particle detectors, to give us an eye on the Universe. In addition to these, we also see, for the first time, the undulations inherent in space itself: gravitational waves. The LIGO detectors, now completed by Virgo and soon joined by KAGRA and LIGO India, have extra-long arms that lengthen and contract when the gravitational waves pass through them, giving rise to a detectable signal. But how does it work? Amrish Pandya wants to know, asking:
If the wavelength of light stretches and shrinks with space-time, how can LIGO detect gravitational waves? [Those waves] stretch and contract the two arms of the LIGO detector and thus the light waves in both arms [must] stretch and contract too. The number of wavelengths of light in each arm would not remain the same? [gravitational waves] undetectable?
It's one of the most common paradoxes that people think about when they consider gravitational waves. Let's dive to find the resolution!
Basically, a system like LIGO or LISA is just a laser, fed by a beam splitter, sent by two identical perpendicular paths, then recombined to create an interference pattern. As arm lengths change, the pattern changes as well.LIGO collaboration
The way a gravitational wave detector works, like the LIGO, is:
- two long arms of exactly equal lengths and exact multiples of a particular light wavelength are created,
- these weapons are evacuated from all material so that there is a perfect vacuum inside,
- the coherent light (of the same wavelength) is divided by a beam splitter into two perpendicular components,
- one is sent one arm and the other is sent the other,
- the light is reflected (several thousands) of times between the two ends of each arm,
- and then the light is recombined, where it creates a pattern of interference.
If the arm lengths are the same and the speed along both arms is the same, then everything moving in both perpendicular directions will arrive at the same time. But if there is an effective headwind / downwind in a sense, there will be a lag in the arrival times. Image Credit: via https://www.ligo.caltech.edu/page/ligos-ifo.LIGO Scientific Collaboration
If the interference pattern remains absolutely constant in the absence of a gravitational wave signal, you know that you have correctly configured your detector. You know that you have taken noise into account; you know that you have set up your experience correctly. This is the struggle that LIGO has been leading for about 40 years: the attempt to correctly calibrate its detector and to bring the level of sensitivity back to a point where it will be able to detect a real gravitational signal.
The magnitude of these signals is incredibly low, and that is why it has been so difficult to reach the necessary clarifications and clarifications.
LIGO's sensitivity over time, compared to the design sensibility and design of Advanced LIGO. The "spikes" come from different sources of noise.Amber Stuver of Living LIGO
Once you are there, you are ready to search for your real signal. Gravitational waves are unique among the different types of radiation produced in the universe. Instead of detectable signatures that can interact with particles, gravitational waves echo into the fabric of space.
Instead of monopolar radiation (such as charge transport) or a dipole (with oscillating fields, such as electromagnetic), gravitational waves are a form of quadrupole radiation.
And instead of having electric and magnetic fields in phase perpendicular to the direction of wave propagation, the gravitational waves stretch and compress alternately the space that they cross in mutually directions. perpendicular.
Gravitational waves propagate in one direction, alternately developing and compressing the space in mutually perpendicular directions, defined by the polarization of the gravitational wave.Mr Pössel / Einstein Online
That's why we built our detectors the way we built them. When a gravitational wave passes through a detector such as LIGO, one of the arms will compress while the other expands, and vice versa, according to a mutually oscillatory pattern. LIGO detectors are deliberately placed at different angles and at different places on the surface of the Earth. Thus, whatever the orientation of the wave, at most one detector will be immune to the gravitational wave signal.
In other words, regardless of the orientation of the gravitational wave, there will always be a detector that will undergo a shortening of the arm while the other will extend, predictably and oscillatory, as the wave passes through the detector.
So what does it mean for light? The light always moves at the same speed, at constant speed: cor 299,792,458 m / s. It is the speed of light in the vacuum and LIGO has vacuum chambers inside the two arms. Indeed, when a gravitational wave passes through each arm, lengthening or shortening the arm, it also lengthens or shortens the wavelength of the light inside the arm.
This seems to be a problem on the surface: if the light lengthens or shortens as the arms lengthen or shorten, the total interference pattern must remain unchanged at the passage of time. l & # 39; wave. At least that is what you would have intuition.
The five black hole-black hole fusions discovered by LIGO (and Virgo), with a sixth signal, insufficiently significant. The largest black hole observed by LIGO, to date, was 36 solar masses, prior to melting. However, galaxies contain supermassive black holes millions, even billions of times the mass of the Sun, and although LIGO is not sensitive, LISA will be. As long as the frequency of the waveform corresponds to the amount of time that the beam passes through the detector, we can hope to extract it.LIGO / State Caltech / Sonoma (Aurore Simonnet)
But that's do not how it works. The wavelength of light, which strongly depends on how your space changes as a gravitational wave passes, is not important for the interference pattern. What is important is the amount of time that light passes through the arms!
When a gravitational wave passes through one of the arms, the effective length of the arms changes, which changes the distance that each laser beam must travel. An arm will lengthen, which will prolong the movement time of the light, while the other will shorten, thus reducing the time of travel of the light. As the relative arrival times change, we see an oscillatory pattern in the way the reconstructed interference pattern moves.
This figure shows the reconstructions of the four confident gravitational wave signals and one candidate (LVT151012) detected by LIGO and Virgo as of October 17, 2017. The most recent black hole detection, GW170814, was observed in all three detectors. Note that the duration of the merge is ridiculous: from hundreds of milliseconds to about 2 seconds maximum.LIGO / Virgo / B. Farr (University of Oregon)
When the beams come together, there is a difference in the amount of time they have traveled, and thus a detectable shift in the resulting interference pattern. The LIGO collaboration itself has published an interesting analogy for this:
[…] Now imagine that you and a friend want to compare the time it takes you to drive to the arms and back of the interferometer. You agree to travel at 1 mile per hour. Just like LIGO's laser light waves, you leave the corner station exactly the same time and travel exactly at the same speed. You should see each other exactly at the same time, shake your hand and go ahead. But let's say that you leave and halfway through, a gravitational wave passes. One of you now has a longer distance to travel, while the other has a shorter distance to go. This means that one of you will come back before the other. As you reach out to shake your friend's hand, they are not there! Your hand has been disturbed! Since you know how fast you travel each, you could measure how much time your friend needed to arrive and then determine how much time he had left to travel late.
When you do this with light, as opposed to a friend, the measurement you use is not a delay in the arrival (because the difference is 10-19 meters), but a change in the observed interference pattern.
When both arms have exactly the same length and no gravitational wave passes, the signal is zero and the pattern of interference is constant. As the arm lengths change, the signal is real and oscillating, and the interference pattern changes over time in a predictable manner.NASA Space Place
This is true: light redshifts and blue-shifts occur when a gravitational wave crosses the space that it occupies. When the space compresses, the wavelength of the light is compressed, making it more blue; as it becomes scarce, the wavelength stretches, making it redder. But these changes are transient and relatively unimportant, at least in relation to the difference in path length that light must travel.
This is the essential point in all of this: the long wavelength red light and the short wavelength blue light take the same time to travel the same distance, even if it takes more peaks and more. of hollow. light to do it. The speed of light in the void is not affected by the wavelength of light. The only factor that influences the pattern of interference is the distance that the light must travel.
The longer the wavelength of a photon, the lower it is in energy. But all photons, regardless of the length of the wave / energy, move at the same speed: the speed of light. The number of wavelengths needed to cover a specified distance may change, but the light travel time is the same for both.NASA / Sonoma State University / Aurore Simonnet
This is the changing distance in the path lengths when a gravitational wave passes through a detector that determines the shift in interference patterns that we see. As the wave crosses, one direction of the arms will lengthen, while the other will shorten simultaneously, which will require a relative displacement of the trajectory lengths and light travel times of the two arms.
As light passes through them at the speed of light, changes in wavelength are unimportant; when they meet again, they are at the same place in the space-time and their wavelengths will therefore be identical. What matters is that a beam of light spends longer in the detector, and when they end up, they are now out of phase. That's where the LIGO signal comes from and how we detect gravitational waves!
Send your Ask Ethan questions to startswithabang on gmail dot com!
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Aerial view of the Virgo gravitational wave detector, located in Cascina, near Pisa (Italy). Virgo is a giant Michelson laser interferometer with 3 km long arms, which completes the two 4 km LIGO detectors.Nicola Baldocchi / Virgo Collaboration
For three years, humanity has been experiencing a new type of astronomy in relation to tradition. We are no longer just detecting light with a telescope, or neutrinos with huge particle detectors, to give us an eye on the Universe. In addition to these, we also see, for the first time, the undulations inherent in space itself: gravitational waves. The LIGO detectors, now completed by Virgo and soon joined by KAGRA and LIGO India, have extra-long arms that lengthen and contract when the gravitational waves pass through them, giving rise to a detectable signal. But how does it work? Amrish Pandya wants to know, asking:
If the wavelength of light stretches and shrinks with space-time, how can LIGO detect gravitational waves? [Those waves] stretch and contract the two arms of the LIGO detector and thus the light waves in both arms [must] stretch and contract too. The number of wavelengths of light in each arm would not remain the same? [gravitational waves] undetectable?
It's one of the most common paradoxes that people think about when they consider gravitational waves. Let's dive to find the resolution!
Basically, a system like LIGO or LISA is just a laser, fed by a beam splitter, sent by two identical perpendicular paths, then recombined to create an interference pattern. As arm lengths change, the pattern changes as well.LIGO collaboration
The way a gravitational wave detector works, like the LIGO, is:
- two long arms of exactly equal lengths and exact multiples of a particular light wavelength are created,
- these weapons are evacuated from all material so that there is a perfect vacuum inside,
- the coherent light (of the same wavelength) is divided by a beam splitter into two perpendicular components,
- one is sent one arm and the other is sent the other,
- the light is reflected (several thousands) of times between the two ends of each arm,
- and then the light is recombined, where it creates a pattern of interference.
If the arm lengths are the same and the speed along both arms is the same, then everything moving in both perpendicular directions will arrive at the same time. But if there is an effective headwind / downwind in a sense, there will be a lag in the arrival times. Image Credit: via https://www.ligo.caltech.edu/page/ligos-ifo.LIGO Scientific Collaboration
If the interference pattern remains absolutely constant in the absence of a gravitational wave signal, you know that you have correctly configured your detector. You know that you have taken noise into account; you know that you have set up your experience correctly. This is the struggle that LIGO has been leading for about 40 years: the attempt to correctly calibrate its detector and to bring the level of sensitivity back to a point where it will be able to detect a real gravitational signal.
The magnitude of these signals is incredibly low, and that is why it has been so difficult to reach the necessary clarifications and clarifications.
LIGO's sensitivity over time, compared to the design sensibility and design of Advanced LIGO. The "spikes" come from different sources of noise.Amber Stuver of Living LIGO
Once you are there, you are ready to search for your real signal. Gravitational waves are unique among the different types of radiation produced in the universe. Instead of detectable signatures that can interact with particles, gravitational waves echo into the fabric of space.
Instead of monopolar radiation (such as charge transport) or a dipole (with oscillating fields, such as electromagnetic), gravitational waves are a form of quadrupole radiation.
And instead of having electric and magnetic fields in phase perpendicular to the direction of wave propagation, the gravitational waves stretch and compress alternately the space that they cross in mutually directions. perpendicular.
Gravitational waves propagate in one direction, alternately developing and compressing the space in mutually perpendicular directions, defined by the polarization of the gravitational wave.Mr Pössel / Einstein Online
That's why we built our detectors the way we built them. When a gravitational wave passes through a detector such as LIGO, one of the arms will compress while the other expands, and vice versa, according to a mutually oscillatory pattern. LIGO detectors are deliberately placed at different angles and at different places on the surface of the Earth. Thus, whatever the orientation of the wave, at most one detector will be immune to the gravitational wave signal.
In other words, regardless of the orientation of the gravitational wave, there will always be a detector that will undergo a shortening of the arm while the other will extend, predictably and oscillatory, as the wave passes through the detector.
So what does it mean for light? The light always moves at the same speed, at constant speed: cor 299,792,458 m / s. It is the speed of light in the vacuum and LIGO has vacuum chambers inside the two arms. Indeed, when a gravitational wave passes through each arm, lengthening or shortening the arm, it also lengthens or shortens the wavelength of the light inside the arm.
This seems to be a problem on the surface: if the light lengthens or shortens as the arms lengthen or shorten, the total interference pattern must remain unchanged at the passage of time. l & # 39; wave. At least that is what you would have intuition.
The five black hole-black hole fusions discovered by LIGO (and Virgo), with a sixth signal, insufficiently significant. The largest black hole observed by LIGO, to date, was 36 solar masses, prior to melting. However, galaxies contain supermassive black holes millions, even billions of times the mass of the Sun, and although LIGO is not sensitive, LISA will be. As long as the frequency of the waveform corresponds to the amount of time that the beam passes through the detector, we can hope to extract it.LIGO / State Caltech / Sonoma (Aurore Simonnet)
But that's do not how it works. The wavelength of light, which strongly depends on how your space changes as a gravitational wave passes, is not important for the interference pattern. What is important is the amount of time that light passes through the arms!
When a gravitational wave passes through one of the arms, the effective length of the arms changes, which changes the distance that each laser beam must travel. An arm will lengthen, which will prolong the movement time of the light, while the other will shorten, thus reducing the time of travel of the light. As the relative arrival times change, we see an oscillatory pattern in the way the reconstructed interference pattern moves.
This figure shows the reconstructions of the four confident gravitational wave signals and one candidate (LVT151012) detected by LIGO and Virgo as of October 17, 2017. The most recent black hole detection, GW170814, was observed in all three detectors. Note that the duration of the merge is ridiculous: from hundreds of milliseconds to about 2 seconds maximum.LIGO / Virgo / B. Farr (University of Oregon)
When the beams come together, there is a difference in the amount of time they have traveled, and thus a detectable shift in the resulting interference pattern. The LIGO collaboration itself has published an interesting analogy for this:
[…] Now imagine that you and a friend want to compare the time it takes you to drive to the arms and back of the interferometer. You agree to travel at 1 mile per hour. Just like LIGO's laser light waves, you leave the corner station exactly the same time and travel exactly at the same speed. You should see each other exactly at the same time, shake your hand and go ahead. But let's say that you leave and halfway through, a gravitational wave passes. One of you now has a longer distance to travel, while the other has a shorter distance to go. This means that one of you will come back before the other. As you reach out to shake your friend's hand, they are not there! Your hand has been disturbed! Since you know how fast you travel each, you could measure how much time your friend needed to arrive and then determine how much time he had left to travel late.
When you do this with light, as opposed to a friend, the measurement you use is not a delay in the arrival (because the difference is 10-19 meters), but a change in the observed interference pattern.
When both arms have exactly the same length and no gravitational wave passes, the signal is zero and the pattern of interference is constant. As the arm lengths change, the signal is real and oscillating, and the interference pattern changes over time in a predictable manner.NASA Space Place
This is true: light redshifts and blue-shifts occur when a gravitational wave crosses the space that it occupies. When the space compresses, the wavelength of the light is compressed, making it more blue; as it becomes scarce, the wavelength stretches, making it redder. But these changes are transient and relatively unimportant, at least in relation to the difference in path length that light must travel.
This is the essential point in all of this: the long wavelength red light and the short wavelength blue light take the same time to travel the same distance, even if it takes more peaks and more. of hollow. light to do it. The speed of light in the void is not affected by the wavelength of light. The only factor that influences the pattern of interference is the distance that the light must travel.
The longer the wavelength of a photon, the lower it is in energy. But all photons, regardless of the length of the wave / energy, move at the same speed: the speed of light. The number of wavelengths needed to cover a specified distance may change, but the light travel time is the same for both.NASA / Sonoma State University / Aurore Simonnet
This is the changing distance in the path lengths when a gravitational wave passes through a detector that determines the shift in interference patterns that we see. As the wave crosses, one direction of the arms will lengthen, while the other will shorten simultaneously, which will require a relative displacement of the trajectory lengths and light travel times of the two arms.
As light passes through them at the speed of light, changes in wavelength are unimportant; when they meet again, they are at the same place in the space-time and their wavelengths will therefore be identical. What matters is that a beam of light spends longer in the detector, and when they end up, they are now out of phase. That's where the LIGO signal comes from and how we detect gravitational waves!
Send your Ask Ethan questions to startswithabang on gmail dot com!
