7 ms·

New ways to catch gravitational waves

233 points by sohkamyung 2y ago

demondemidi 2y ago
How does one aim a gravitational wave detector? Or is that not how they work?
- 4ad 2y ago
  Gravitational waves detectors, just like any kind of antennas have anisotropic sensitivity. By having multiple detectors with different orientations you can determine the source of the signal. But you can't aim them, no, at least not any current or planned ones. They are simply too big (many kilometers) and require extreme stiffness. Everybody knows of LIGO, but it's actually three detectors that work together, LIGO, Virgo, and KAGRA.
  - _Microft 2y ago
    There is also a project called GEO600 in Germany that is collaborating with LIGO (and therefore also Virgo/KAGRA). This detector is much smaller though. https://en.m.wikipedia.org/wiki/GEO600 https://en.m.wikipedia.org/wiki/GEO600
  - BurningFrog 2y ago
    You can also figure out the source direction by when the signal arrives at the different (currently three, on a good day) detectors.
- msarchet 2y ago
  > LIGO can't point to specific locations in space Since LIGO doesn’t need to collect light from stars or other objects or regions in space, it doesn't need to be round or dish-shaped like optical telescope mirrors or radio telescope dishes. Nor does it have to be steerable, i.e., able to move around to point in a specific direction. Instead, each LIGO detector consists of two 4 km (2.5 mi.) long, 1.2 m-wide steel vacuum tubes arranged in an "L" shape (LIGO's laser travels through these arms), and enclosed within a 10-foot wide, 12-foot tall concrete structure that protects the tubes from the environment. > A mirror at the vertex of the arms splits a single light beam into two, directing each beam down an arm of the instrument Mirrors at the ends of the arms reflect the beams back to their origin point where they are recombined to create an interference pattern called 'fringe https://www.ligo.caltech.edu/page/what-is-ligo https://www.ligo.caltech.edu/page/what-is-ligo https://www.ligo.caltech.edu/page/ligos-ifo https://www.ligo.caltech.edu/page/ligos-ifo
- sithadmin 2y ago
  Intuitively, it would seem that one wouldn't 'aim' a gravitational wave detector. The inputs are omnidirectional, and direction-finding would be accomplished by triangulation based on an array of detectors.
- serial_dev 2y ago
  You can't aim our current grav wave detectors. In a very simplified form, it's two very long tubes with laser in it, you aren't pointing these buildings like you could do with even very large telescopes. The detectors have different sensitivity depending on the direction of the waves, so if a wave comes from a "bad" direction (perpendicular to both arms, e.g. directly from above), a particular detector might not detect anything, even if the wave is strong. This is why (amongst many other reasons) it's important to have multiple detectors around the world (e.g LIGO has two locations in the US, you also have Virgo in Italy, and they do collaborate), this way you can assure in theory that you can "see" every wave, no matter which direction it's coming from. (AFAIK).
  - throwawaymaths 2y ago
    Having only two detectors was always a sketchy affair given the tremendous amount of filtering done by the teams. Now with three detectors, if the correlataed event count between pairs of detectors doesn't diminish from one detector events with the same ratio as from 2-3 detectors (high school statistics), then we will know that event detection is real.
  - pwatsonwailes 2y ago
    Pretty spot on.
  - simcop2387 2y ago
    > The detectors have different sensitivity depending on the direction of the waves, so if a wave comes from a "bad" direction (perpendicular to both arms, e.g. directly from above), a particular detector might not detect anything, even if the wave is strong. Unless there's such a thing as polarized gravitational waves, https://www.ligo.org/science/Publication-O1StochNonGR/ https://www.ligo.org/science/Publication-O1StochNonGR/ which might exist but are hard to discern with current detectors from what I understand. It'd be really cool to learn that there's such a thing as vector and scalar polarized gravitional waves.
  - tonynator 2y ago
    > it's important to have multiple detectors around the world Or realllly tall detectors.
- butterNaN 2y ago
  Similar discussion elsewhere on the internet: https://einsteinathome.org/content/how-can-they-aim-ligo https://einsteinathome.org/content/how-can-they-aim-ligo
- jfengel 2y ago
  A gravitational wave moves at the speed of light. So if you compare arrival times from several widely-spaced detectors, you can get a pretty good idea of what direction the wave came from. Even a single detector has two arms at 90 degrees to each other, which can give you a rough idea of where in the sky it came from. But now that we have multiple detectors online we get better and better about spotting the origin.
- akira2501 2y ago
  It's a triangulation problem. As long as the two detectors are perpendicular with each other you can triangulate the source of the signal received by both of them.
andsens 2y ago
Now that’s the kind of clickbait title I can get behind!
nudgeee 2y ago
I’m amazed there was no mention of LISA [0] — a space-based gravitational wave detector using 3 satellites flying in formation 2.5 million Km apart! Seriously cool engineering, planning to launch in 2035. [0] https://en.m.wikipedia.org/wiki/Laser_Interferometer_Space_Antenna https://en.m.wikipedia.org/wiki/Laser_Interferometer_Space_A...
- shellac 2y ago
  It is mentioned: > ...Researchers are now working on several next-generation LIGO-type observatories, both on Earth and, in space, the Laser Interferometer Space Antenna;...
  - nudgeee 2y ago
    Ooof missed that, thanks!
- jjk166 2y ago
  LISA is mentioned in the 6th paragraph.
- cubefox 2y ago
  > LISA was first proposed as a mission to ESA in the early 1990s. I remember reading about LISA when I was a little kid. Back then it was projected to launch in the far future of 2015. Now I would be surprised if it actually launches in 2035.
  - ajford 2y ago
    The university I studied at had a Gravitational Wave center under the Physics dept, and all the professors would joke about how LISA had been "less than a decade away" for 20 years.
gadilif 2y ago
So, going from a very narrow frequency band (up to 1000Hz) to a much wider range, which can theoretically encode information (e.g. frequency modulation)... Hmm, I'm wondering if comms over gravity is something a sufficiently advanced civilization might consider using, and should we be looking for that 'Hello, world' in some 'natural frequency' like we're doing for EM radiation?
- jimbokun 2y ago
  Did you read Death's End by Cixin Liu?
- serial_dev 2y ago
  If we are going all in on imaginary civilizations, even a very narrow (well, not that narrow) frequency band could be used to encode information, couldn't they?
  - gadilif 2y ago
    Yes, but not efficiently (it will be very slow) and will be hard to detect because of noise (the article mentions the 'cacophony' of gravity noise). This is similar to EM radiation, so, you do frequency modulation (or amplitude, but that seems harder with gravity - you'll need to modify mass...), so you have a base high frequency on top of which you add lower frequency. The substraction later gives you a clean signal (or, Gravity Radio).
    - vbezhenar 2y ago
      Will you distinguish it from noise? Surely those will be compressed and encrypted.
      - gadilif 2y ago
        Some of it, definitely. But if the purpose is to seek others and announce yourself (like we did with Voyager 1 and 2), then you'll want it to be plain. Also - Public radio transmissions are not encrypted (as your purpose is to have as many listeners as possible... You monetize on ads).
deleted 2y ago
[deleted]
ape4 2y ago
This made (very lay-person) me wonder if you could detect quantum wave function events. https://physics.stackexchange.com/questions/275556/can-you-detect-when-wave-function-collapse-occurs https://physics.stackexchange.com/questions/275556/can-you-d...
captainkrtek 2y ago
Just a plug that you can tour the LIGO facilities for free! I visited a couple years ago and got a tour of the Hanford facility, included a lecture beforehand as well. Really awesome people and got to tour the entire facility, even going to the control room. https://www.ligo.caltech.edu/WA/page/lho-public-tours https://www.ligo.caltech.edu/WA/page/lho-public-tours
- freeqaz 2y ago
  The only dates available are during DEFCON. I would so love to go sometime though! Thanks for sharing -- this made my morning.
  - captainkrtek 2y ago
    I’d highly recommend it if you get the time, was an excellent tour to geek out on. Enjoy DEFCON!
- dudinax 2y ago
  If you go to LIGO Hanford don't miss the B reactor tour near by. It's the first full scale nuclear reactor.
mrlonglong 2y ago
It's entirely possible if it's extremely sensitive to detect the waves given off by alcubierrie warp drives crisscrossing the galaxies.
cypherpunks01 2y ago
I was surprised that the article doesn't mention LISA at all. I had thought that's the next stage gravitational wave detector on the horizon? https://www.esa.int/Science_Exploration/Space_Science/LISA_factsheet https://www.esa.int/Science_Exploration/Space_Science/LISA_f...
- qual 2y ago
  It's mentioned both in the article, and in the comments here where someone else thought it isn't mentioned. They didn't use the acronym ("LISA"), but instead spelled out the entire thing. >Researchers are now working on several next-generation LIGO-type observatories, both on Earth and, in space, the Laser Interferometer Space Antenna; [...]
tea-coffee 2y ago
Any physicists in here that could layout a path of going from rudimentary first year level physics knowledge to being able to understand on a deeper level topics such as gravitational waves? These articles are interesting but are very abstract when you do not have knowledge from first principles.
- deleted 2y ago
  [deleted]
- bowsamic 2y ago
  I’m a scientist who works on LIGO and next generation detectors and I don’t have a deep knowledge of the physics behind gravitational waves either! But realistically you should pick up some lecture notes on general relativity: https://www.damtp.cam.ac.uk/user/tong/gr.html https://www.damtp.cam.ac.uk/user/tong/gr.html
openrisk 2y ago
Something already lost in the twisted passages of history is that the first generation of gravitational wave detectors was of an entirely different design than the current interferometers [1]. It never worked and Weber's claim to have detected gravitational waves from SN1987A in 1987, was widely discredited... [1] https://en.wikipedia.org/wiki/Weber_bar https://en.wikipedia.org/wiki/Weber_bar
- jessriedel 2y ago
  FYI, within the field, “first generation” now refers specifically to the first version of LIGO (not “advanced LIGO”). It’s not just that Weber bars didn’t work, it’s that they couldn’t work according to most physicists (but Weber obviously disagreed).
  - openrisk 2y ago
    Umm, not sure what this clumsy attempt to rewrite science history is about? The internal nomenclature of those working within a particular approach cannot restart the clock of an entire field? Resonant bars were the first generation of gravitational wave detectors, period. > they couldn’t work according to most physicists That certainly requires a reference. There were several teams around the world besides Weber pursuing the approach. You make it sound as if the physics behind these bars was crackpot. Fyi, you may be confusing physicists with astronomers. The prevailing notion in astro circles was indeed that the known pathways to gravitational waves and with calculations made using GR would be too weak to be detected. (Incidentally astronomers were notoriously hostile to any GR research anyway. Why spend money in a high risk esoteric field if you can keep doing the same stuff over and over?) But a new observational window can always throw a surprise. A previously unknown class of sources may hit you in the face despite your primitive instrument. Historically this happened repeatedly with x-rays, radio etc. Or the theory you use to make predictions may be deficient and not telling you the whole story... In this respect Weber was simply unlucky. The universe is in a sense slightly more "conventional" and less exciting than it could have been.
    - bowsamic 2y ago
umvi 2y ago
Probably a dumb question, but... is it basically proven then that gravity doesn't exist (it's effects are just a result of spacetime's geometry?). Because it seems like these gravitational waves experiments show that spacetime exists and has measurable geometry. Yet every time quantum mechanics comes up everyone talks about how we haven't found the gravity force carrier yet which doesn't make sense to me if gravity doesn't exist and is a consequence of the geometry of spacetime.
- mati365 2y ago
  Isn't space gravity carrier itself? Something like the water that has waves?
  - simcop2387 2y ago
    I believe they're referring to the idea of a gravitational force carrying particle, like photons for electromagnetism, W/Z bosons for the weak force, and gluons for the strong force. Typically this gets called a graviton but they've never been observed and the theories predicting them as far as I know don't predict that we can detect them in any meaningful/practical way right now which is also one of the problems for issues with a quantum theory of gravity. I've always wondered (but not done the research/reading) on how that would mesh with black holes since you'd need gravitons to escape to mediate the curvature of space-time but that'd seemingly (to me) require them to be able to either ignore the curvature of space-time or travel faster than the speed of light in order to do so. And I believe that those two options there are actually mathematically equivalent as far as the consequences of things go.
    - superposeur 2y ago
      This question isn’t actually anything to do with quantum mechanics. One can already ask, for GR, can a gravity wave generated by a wiggling object that has already passed the event horizon propagate back out through the event horizon? The answer from gr is unambiguously no. Effectively, all complicated goings-on inside the black hole are washed out into just a couple parameters for an outside observer: total mass, total charge, total angular momentum. All the matter contained by the black hole does still make its gravitational effect felt on outside objects through these three parameters. This observation in fact is what inspired wheeler to ask his grad student bekenstein what then happens to the entropy of a cup of tea thrown into the black hole — how to reconcile with 2nd law of thermo. Which in turn was the start of the very long story of black hole thermodynamics.
cletus 2y ago
I just want to add something about how sensitive LIGO is. The gravity waves it is detecting are equivalent to measuring the distance from Earth to Alpha Centauri with a variation the width of a human hair. Anyway, these techniques are aimed at detecting different types of gravitational waves, not necessarily about simply increasing sensitivity. I don't know what dictates the frequency of a gravitational wave. Truth be told, I still don't get what expanding space or gravitational waves really are but then again I'm just an idiot who doesn't understand tractor calculus [1]. [1]: https://www.math.auckland.ac.nz/mathwiki/images/c/cf/StaffordMSc.pdf https://www.math.auckland.ac.nz/mathwiki/images/c/cf/Staffor...
- strangattractor 2y ago
  If one takes the width of the plow or seeder one is tractoring and divide the length of the field by that width one will get the number of times the tractor goes back and forth across the field to completely seed or plow it. If the tractor travels at constant velocity V_tractor m/s and multiplies by the width of the plow or seeder and the time in seconds the tractor took to plow or seed the field one gets the total area of the field expressed in meters^2. This can be extended to a tractor traveling at V(t) if infinitesimal units dt of area are summed. This is known as the Fundamental Theorem of Tractor Calculus and is the basis of Tractor Field Theory;)
consumer451 2y ago
I just learned of a new proposal to use an already planned probe, as a gravitational wave detector. Maybe I am missed it, but this does not appear to be covered in TFA. > Bridging the micro-Hz gravitational wave gap via Doppler tracking with the Uranus Orbiter and Probe Mission: Massive black hole binaries, early universe signals and ultra-light dark matter https://arxiv.org/abs/2406.02306 https://arxiv.org/abs/2406.02306 > Practically Free Primordial Gravitational Waves Detector https://www.youtube.com/watch?v=XfOxNJvSvf4 https://www.youtube.com/watch?v=XfOxNJvSvf4
westurner 2y ago
- "Kerr-enhanced optical spring for next-generation gravitational wave detectors" (2024) https://news.ycombinator.com/item?id=39957123 https://news.ycombinator.com/item?id=39957123 - "Physicists Have Figured Out a Way to Measure Gravity on a Quantum Scale" with a superconducting magnetic trap made out of Tantalum (2024) https://news.ycombinator.com/item?id=39495482 https://news.ycombinator.com/item?id=39495482
ImageXav 2y ago
A more niche but nonetheless interesting method that I was hoping to see discussed was magnetism. Gravitational waves are expected to decay into photons in intense magnetic fields. Or so I was told by one of my physics professors back in the day. I did understand the math somewhat back then, but it is beyond me now. It does however seem as though some people are still exploring this avenue [0]. [0] https://indico.cern.ch/event/1074510/contributions/4519384/attachments/2327235/3964731/Aldo_Ejlli_13_10_2021.pdf https://indico.cern.ch/event/1074510/contributions/4519384/a....
mnemotronic 2y ago
Here's something to get you ready ... https://youtu.be/DRRfhZFyDvg?feature=shared https://youtu.be/DRRfhZFyDvg?feature=shared
perlgeek 2y ago
Why is there no hope of raising the upper frequency limit of the current detection methods? 1kHz seems pretty low for an interferometer. Is it limited by the wavelength associated with the 1kHz, which becomes smaller than the length of the light path through the interferometer?
zeristor 2y ago
So if you can set black holes to collide if a large gravitational wave passes by, that would be stimulated emission and amplification... A bit tricky to find something that could reflect a gravitational wave, to make an equivalent of a gravitational wave laser.

233 points by sohkamyung 2y ago

demondemidi 2y ago

How does one aim a gravitational wave detector? Or is that not how they work?

4ad 2y ago
Gravitational waves detectors, just like any kind of antennas have anisotropic sensitivity. By having multiple detectors with different orientations you can determine the source of the signal. But you can't aim them, no, at least not any current or planned ones. They are simply too big (many kilometers) and require extreme stiffness. Everybody knows of LIGO, but it's actually three detectors that work together, LIGO, Virgo, and KAGRA.
- _Microft 2y ago
  There is also a project called GEO600 in Germany that is collaborating with LIGO (and therefore also Virgo/KAGRA). This detector is much smaller though. https://en.m.wikipedia.org/wiki/GEO600 https://en.m.wikipedia.org/wiki/GEO600
- BurningFrog 2y ago
  You can also figure out the source direction by when the signal arrives at the different (currently three, on a good day) detectors.
msarchet 2y ago
> LIGO can't point to specific locations in space Since LIGO doesn’t need to collect light from stars or other objects or regions in space, it doesn't need to be round or dish-shaped like optical telescope mirrors or radio telescope dishes. Nor does it have to be steerable, i.e., able to move around to point in a specific direction. Instead, each LIGO detector consists of two 4 km (2.5 mi.) long, 1.2 m-wide steel vacuum tubes arranged in an "L" shape (LIGO's laser travels through these arms), and enclosed within a 10-foot wide, 12-foot tall concrete structure that protects the tubes from the environment. > A mirror at the vertex of the arms splits a single light beam into two, directing each beam down an arm of the instrument Mirrors at the ends of the arms reflect the beams back to their origin point where they are recombined to create an interference pattern called 'fringe https://www.ligo.caltech.edu/page/what-is-ligo https://www.ligo.caltech.edu/page/what-is-ligo https://www.ligo.caltech.edu/page/ligos-ifo https://www.ligo.caltech.edu/page/ligos-ifo
sithadmin 2y ago
Intuitively, it would seem that one wouldn't 'aim' a gravitational wave detector. The inputs are omnidirectional, and direction-finding would be accomplished by triangulation based on an array of detectors.
serial_dev 2y ago
You can't aim our current grav wave detectors. In a very simplified form, it's two very long tubes with laser in it, you aren't pointing these buildings like you could do with even very large telescopes. The detectors have different sensitivity depending on the direction of the waves, so if a wave comes from a "bad" direction (perpendicular to both arms, e.g. directly from above), a particular detector might not detect anything, even if the wave is strong. This is why (amongst many other reasons) it's important to have multiple detectors around the world (e.g LIGO has two locations in the US, you also have Virgo in Italy, and they do collaborate), this way you can assure in theory that you can "see" every wave, no matter which direction it's coming from. (AFAIK).
- throwawaymaths 2y ago
  Having only two detectors was always a sketchy affair given the tremendous amount of filtering done by the teams. Now with three detectors, if the correlataed event count between pairs of detectors doesn't diminish from one detector events with the same ratio as from 2-3 detectors (high school statistics), then we will know that event detection is real.
- pwatsonwailes 2y ago
  Pretty spot on.
- simcop2387 2y ago
  > The detectors have different sensitivity depending on the direction of the waves, so if a wave comes from a "bad" direction (perpendicular to both arms, e.g. directly from above), a particular detector might not detect anything, even if the wave is strong. Unless there's such a thing as polarized gravitational waves, https://www.ligo.org/science/Publication-O1StochNonGR/ https://www.ligo.org/science/Publication-O1StochNonGR/ which might exist but are hard to discern with current detectors from what I understand. It'd be really cool to learn that there's such a thing as vector and scalar polarized gravitional waves.
- tonynator 2y ago
  > it's important to have multiple detectors around the world Or realllly tall detectors.
butterNaN 2y ago
Similar discussion elsewhere on the internet: https://einsteinathome.org/content/how-can-they-aim-ligo https://einsteinathome.org/content/how-can-they-aim-ligo
jfengel 2y ago
A gravitational wave moves at the speed of light. So if you compare arrival times from several widely-spaced detectors, you can get a pretty good idea of what direction the wave came from. Even a single detector has two arms at 90 degrees to each other, which can give you a rough idea of where in the sky it came from. But now that we have multiple detectors online we get better and better about spotting the origin.
akira2501 2y ago
It's a triangulation problem. As long as the two detectors are perpendicular with each other you can triangulate the source of the signal received by both of them.

andsens 2y ago

Now that’s the kind of clickbait title I can get behind!

nudgeee 2y ago

I’m amazed there was no mention of LISA [0] — a space-based gravitational wave detector using 3 satellites flying in formation 2.5 million Km apart! Seriously cool engineering, planning to launch in 2035. [0] https://en.m.wikipedia.org/wiki/Laser_Interferometer_Space_Antenna https://en.m.wikipedia.org/wiki/Laser_Interferometer_Space_A...

shellac 2y ago
It is mentioned: > ...Researchers are now working on several next-generation LIGO-type observatories, both on Earth and, in space, the Laser Interferometer Space Antenna;...
- nudgeee 2y ago
  Ooof missed that, thanks!
jjk166 2y ago
LISA is mentioned in the 6th paragraph.
cubefox 2y ago
> LISA was first proposed as a mission to ESA in the early 1990s. I remember reading about LISA when I was a little kid. Back then it was projected to launch in the far future of 2015. Now I would be surprised if it actually launches in 2035.
- ajford 2y ago
  The university I studied at had a Gravitational Wave center under the Physics dept, and all the professors would joke about how LISA had been "less than a decade away" for 20 years.

gadilif 2y ago

So, going from a very narrow frequency band (up to 1000Hz) to a much wider range, which can theoretically encode information (e.g. frequency modulation)... Hmm, I'm wondering if comms over gravity is something a sufficiently advanced civilization might consider using, and should we be looking for that 'Hello, world' in some 'natural frequency' like we're doing for EM radiation?

jimbokun 2y ago
Did you read Death's End by Cixin Liu?
serial_dev 2y ago
If we are going all in on imaginary civilizations, even a very narrow (well, not that narrow) frequency band could be used to encode information, couldn't they?
- gadilif 2y ago
  Yes, but not efficiently (it will be very slow) and will be hard to detect because of noise (the article mentions the 'cacophony' of gravity noise). This is similar to EM radiation, so, you do frequency modulation (or amplitude, but that seems harder with gravity - you'll need to modify mass...), so you have a base high frequency on top of which you add lower frequency. The substraction later gives you a clean signal (or, Gravity Radio).
  - vbezhenar 2y ago
    Will you distinguish it from noise? Surely those will be compressed and encrypted.
    - gadilif 2y ago
      Some of it, definitely. But if the purpose is to seek others and announce yourself (like we did with Voyager 1 and 2), then you'll want it to be plain. Also - Public radio transmissions are not encrypted (as your purpose is to have as many listeners as possible... You monetize on ads).

deleted 2y ago

[deleted]

ape4 2y ago

This made (very lay-person) me wonder if you could detect quantum wave function events. https://physics.stackexchange.com/questions/275556/can-you-detect-when-wave-function-collapse-occurs https://physics.stackexchange.com/questions/275556/can-you-d...

captainkrtek 2y ago

Just a plug that you can tour the LIGO facilities for free! I visited a couple years ago and got a tour of the Hanford facility, included a lecture beforehand as well. Really awesome people and got to tour the entire facility, even going to the control room. https://www.ligo.caltech.edu/WA/page/lho-public-tours https://www.ligo.caltech.edu/WA/page/lho-public-tours

freeqaz 2y ago
The only dates available are during DEFCON. I would so love to go sometime though! Thanks for sharing -- this made my morning.
- captainkrtek 2y ago
  I’d highly recommend it if you get the time, was an excellent tour to geek out on. Enjoy DEFCON!
dudinax 2y ago
If you go to LIGO Hanford don't miss the B reactor tour near by. It's the first full scale nuclear reactor.

mrlonglong 2y ago

It's entirely possible if it's extremely sensitive to detect the waves given off by alcubierrie warp drives crisscrossing the galaxies.

cypherpunks01 2y ago

I was surprised that the article doesn't mention LISA at all. I had thought that's the next stage gravitational wave detector on the horizon? https://www.esa.int/Science_Exploration/Space_Science/LISA_factsheet https://www.esa.int/Science_Exploration/Space_Science/LISA_f...

qual 2y ago
It's mentioned both in the article, and in the comments here where someone else thought it isn't mentioned. They didn't use the acronym ("LISA"), but instead spelled out the entire thing. >Researchers are now working on several next-generation LIGO-type observatories, both on Earth and, in space, the Laser Interferometer Space Antenna; [...]

tea-coffee 2y ago

Any physicists in here that could layout a path of going from rudimentary first year level physics knowledge to being able to understand on a deeper level topics such as gravitational waves? These articles are interesting but are very abstract when you do not have knowledge from first principles.

deleted 2y ago
[deleted]
bowsamic 2y ago
I’m a scientist who works on LIGO and next generation detectors and I don’t have a deep knowledge of the physics behind gravitational waves either! But realistically you should pick up some lecture notes on general relativity: https://www.damtp.cam.ac.uk/user/tong/gr.html https://www.damtp.cam.ac.uk/user/tong/gr.html

openrisk 2y ago

Something already lost in the twisted passages of history is that the first generation of gravitational wave detectors was of an entirely different design than the current interferometers [1]. It never worked and Weber's claim to have detected gravitational waves from SN1987A in 1987, was widely discredited... [1] https://en.wikipedia.org/wiki/Weber_bar https://en.wikipedia.org/wiki/Weber_bar

jessriedel 2y ago
FYI, within the field, “first generation” now refers specifically to the first version of LIGO (not “advanced LIGO”). It’s not just that Weber bars didn’t work, it’s that they couldn’t work according to most physicists (but Weber obviously disagreed).
- openrisk 2y ago
  Umm, not sure what this clumsy attempt to rewrite science history is about? The internal nomenclature of those working within a particular approach cannot restart the clock of an entire field? Resonant bars were the first generation of gravitational wave detectors, period. > they couldn’t work according to most physicists That certainly requires a reference. There were several teams around the world besides Weber pursuing the approach. You make it sound as if the physics behind these bars was crackpot. Fyi, you may be confusing physicists with astronomers. The prevailing notion in astro circles was indeed that the known pathways to gravitational waves and with calculations made using GR would be too weak to be detected. (Incidentally astronomers were notoriously hostile to any GR research anyway. Why spend money in a high risk esoteric field if you can keep doing the same stuff over and over?) But a new observational window can always throw a surprise. A previously unknown class of sources may hit you in the face despite your primitive instrument. Historically this happened repeatedly with x-rays, radio etc. Or the theory you use to make predictions may be deficient and not telling you the whole story... In this respect Weber was simply unlucky. The universe is in a sense slightly more "conventional" and less exciting than it could have been.
  - bowsamic 2y ago

umvi 2y ago

Probably a dumb question, but... is it basically proven then that gravity doesn't exist (it's effects are just a result of spacetime's geometry?). Because it seems like these gravitational waves experiments show that spacetime exists and has measurable geometry. Yet every time quantum mechanics comes up everyone talks about how we haven't found the gravity force carrier yet which doesn't make sense to me if gravity doesn't exist and is a consequence of the geometry of spacetime.

mati365 2y ago
Isn't space gravity carrier itself? Something like the water that has waves?
- simcop2387 2y ago
  I believe they're referring to the idea of a gravitational force carrying particle, like photons for electromagnetism, W/Z bosons for the weak force, and gluons for the strong force. Typically this gets called a graviton but they've never been observed and the theories predicting them as far as I know don't predict that we can detect them in any meaningful/practical way right now which is also one of the problems for issues with a quantum theory of gravity. I've always wondered (but not done the research/reading) on how that would mesh with black holes since you'd need gravitons to escape to mediate the curvature of space-time but that'd seemingly (to me) require them to be able to either ignore the curvature of space-time or travel faster than the speed of light in order to do so. And I believe that those two options there are actually mathematically equivalent as far as the consequences of things go.
  - superposeur 2y ago
    This question isn’t actually anything to do with quantum mechanics. One can already ask, for GR, can a gravity wave generated by a wiggling object that has already passed the event horizon propagate back out through the event horizon? The answer from gr is unambiguously no. Effectively, all complicated goings-on inside the black hole are washed out into just a couple parameters for an outside observer: total mass, total charge, total angular momentum. All the matter contained by the black hole does still make its gravitational effect felt on outside objects through these three parameters. This observation in fact is what inspired wheeler to ask his grad student bekenstein what then happens to the entropy of a cup of tea thrown into the black hole — how to reconcile with 2nd law of thermo. Which in turn was the start of the very long story of black hole thermodynamics.

cletus 2y ago

I just want to add something about how sensitive LIGO is. The gravity waves it is detecting are equivalent to measuring the distance from Earth to Alpha Centauri with a variation the width of a human hair. Anyway, these techniques are aimed at detecting different types of gravitational waves, not necessarily about simply increasing sensitivity. I don't know what dictates the frequency of a gravitational wave. Truth be told, I still don't get what expanding space or gravitational waves really are but then again I'm just an idiot who doesn't understand tractor calculus [1]. [1]: https://www.math.auckland.ac.nz/mathwiki/images/c/cf/StaffordMSc.pdf https://www.math.auckland.ac.nz/mathwiki/images/c/cf/Staffor...

strangattractor 2y ago
If one takes the width of the plow or seeder one is tractoring and divide the length of the field by that width one will get the number of times the tractor goes back and forth across the field to completely seed or plow it. If the tractor travels at constant velocity V_tractor m/s and multiplies by the width of the plow or seeder and the time in seconds the tractor took to plow or seed the field one gets the total area of the field expressed in meters^2. This can be extended to a tractor traveling at V(t) if infinitesimal units dt of area are summed. This is known as the Fundamental Theorem of Tractor Calculus and is the basis of Tractor Field Theory;)

consumer451 2y ago

I just learned of a new proposal to use an already planned probe, as a gravitational wave detector. Maybe I am missed it, but this does not appear to be covered in TFA. > Bridging the micro-Hz gravitational wave gap via Doppler tracking with the Uranus Orbiter and Probe Mission: Massive black hole binaries, early universe signals and ultra-light dark matter https://arxiv.org/abs/2406.02306 https://arxiv.org/abs/2406.02306 > Practically Free Primordial Gravitational Waves Detector https://www.youtube.com/watch?v=XfOxNJvSvf4 https://www.youtube.com/watch?v=XfOxNJvSvf4

westurner 2y ago

- "Kerr-enhanced optical spring for next-generation gravitational wave detectors" (2024) https://news.ycombinator.com/item?id=39957123 https://news.ycombinator.com/item?id=39957123 - "Physicists Have Figured Out a Way to Measure Gravity on a Quantum Scale" with a superconducting magnetic trap made out of Tantalum (2024) https://news.ycombinator.com/item?id=39495482 https://news.ycombinator.com/item?id=39495482

ImageXav 2y ago

A more niche but nonetheless interesting method that I was hoping to see discussed was magnetism. Gravitational waves are expected to decay into photons in intense magnetic fields. Or so I was told by one of my physics professors back in the day. I did understand the math somewhat back then, but it is beyond me now. It does however seem as though some people are still exploring this avenue [0]. [0] https://indico.cern.ch/event/1074510/contributions/4519384/attachments/2327235/3964731/Aldo_Ejlli_13_10_2021.pdf https://indico.cern.ch/event/1074510/contributions/4519384/a....

mnemotronic 2y ago

Here's something to get you ready ... https://youtu.be/DRRfhZFyDvg?feature=shared https://youtu.be/DRRfhZFyDvg?feature=shared

perlgeek 2y ago

Why is there no hope of raising the upper frequency limit of the current detection methods? 1kHz seems pretty low for an interferometer. Is it limited by the wavelength associated with the 1kHz, which becomes smaller than the length of the light path through the interferometer?

zeristor 2y ago

So if you can set black holes to collide if a large gravitational wave passes by, that would be stimulated emission and amplification... A bit tricky to find something that could reflect a gravitational wave, to make an equivalent of a gravitational wave laser.