[HN Gopher] New ways to catch gravitational waves
___________________________________________________________________
New ways to catch gravitational waves
Author : sohkamyung
Score : 223 points
Date : 2024-06-28 12:41 UTC (1 days ago)
(HTM) web link (www.nature.com)
(TXT) w3m dump (www.nature.com)
| demondemidi wrote:
| How does one aim a gravitational wave detector? Or is that not
| how they work?
| 4ad wrote:
| 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 wrote:
| 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
| BurningFrog wrote:
| You can also figure out the source direction by when the
| signal arrives at the different (currently three, on a good
| day) detectors.
| msarchet wrote:
| > 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/ligos-ifo
| sithadmin wrote:
| 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 wrote:
| 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 wrote:
| 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 wrote:
| Pretty spot on.
| simcop2387 wrote:
| > 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/ 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 wrote:
| > it's important to have multiple detectors around the world
|
| Or realllly tall detectors.
| butterNaN wrote:
| Similar discussion elsewhere on the internet:
| https://einsteinathome.org/content/how-can-they-aim-ligo
| jfengel wrote:
| 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 wrote:
| 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 wrote:
| Now that's the kind of clickbait title I can get behind!
| nudgeee wrote:
| 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_A...
| shellac wrote:
| 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 wrote:
| Ooof missed that, thanks!
| jjk166 wrote:
| LISA is mentioned in the 6th paragraph.
| cubefox wrote:
| > 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 wrote:
| 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 wrote:
| 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?
| serial_dev wrote:
| 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 wrote:
| 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 wrote:
| Will you distinguish it from noise? Surely those will be
| compressed and encrypted.
| gadilif wrote:
| 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).
| anyfoo wrote:
| I'm not sure "slow" matters that much. If (and that's a big
| if) there is advantage to announce yourself to other
| civilizations using gravity waves, instead of other means,
| then it probably does not matter too much if transmitting a
| reasonable amount of information takes a decade or so.
| You'd put it on repeat of course, and hope that interested
| civilizations don't have _too_ short of a lifespan to still
| make sense of it.
| BurningFrog wrote:
| I can't really imagine how such a transmitter works?
|
| Somehow moving planet sized objects around to create gravity
| waves?
|
| Of course, the cop-out "using their advanced tech we don't have
| a clue about" answer could actually be correct.
| vbezhenar wrote:
| Create tiny black holes and throw them into each other.
| qual wrote:
| I'm not sure I see what the advantages of communicating this
| way would be. The amount of energy required per bit transmitted
| would be astounding.
|
| I feel like, while theoretically possible, it's pretty much all
| downsides and no upsides. At least for communication purposes.
|
| However, your comment reminded me of an interesting PBS Space
| Time episode discussing the possibility of finding alien
| civilizations via the gravitational waves produced by their
| massive ships accelerating to near light speed.
|
| https://www.pbs.org/video/could-ligo-find-massive-alien-spac...
| gadilif wrote:
| Thanks for the PBS reference, interesting! As to the energy
| required, well, yes, I guess manipulating huge masses will be
| costly, but, if there is an efficient way to do this, then
| gravity waves are a parallel plane of communication. The
| analogy I heard once is about tribes communicating with smoke
| signals, while the air around them is filled with radio
| waves. Maybe we can't hear anyone out there because we're not
| listening to the right thing...
| colmmacc wrote:
| The medium is the message. If we detected a modulated signal
| in gravitational waves, it would be like an Iron Age tribe
| receiving a 100 foot tall perfectly polished stainless steel
| statue with ornate inscriptions and pictograms. It would be
| recognizable and within our conception, but it would also be
| a demonstration of development and access to resources beyond
| our imagination. That's the upside. In a galaxy of sparse and
| sparingly advanced civilizations, the message might be "fear
| us and stay away" in a way that EM would not convey.
| qual wrote:
| > _it would also be a demonstration of development and
| access to resources beyond our imagination. That 's the
| upside [...] In a galaxy of sparse and sparingly advanced
| civilizations, the message might be "fear us and stay away"
| in a way that EM would not convey._
|
| I think you've hit the crux the question. If there are only
| a few civilizations, I agree, that'd be an awe-inspiring
| deterrent.
|
| However, if you don't know how many civilizations there are
| that are similarly advanced to your own, sending out a big
| "we're here!" message may be quite risky.
|
| In terms of game theory, it's a sequential and incomplete
| information game. I think the smartest decision is to
| remain quiet.
| rvba wrote:
| I think the UN should force nations to remain quiet. Srop
| the signals that can be stopped, especially the ones
| designed to go far.
| 01acheru wrote:
| https://en.m.wikipedia.org/wiki/Dark_forest_hypothesis
| Larrikin wrote:
| Maybe that is the filter. If your society hasn't figured out
| the tech to do it efficiently, the rest of the galaxy doesn't
| care about what you have to say.
| kimbernator wrote:
| I've always been a bit confused about the idea that an
| advanced civilization would be uninterested in us because
| we've "only" reached the ability to communicate via radio
| waves. We're either a threat or an ally to these other
| civilizations, and if you were them and you detected a
| society that was clearly on course to eventually catch up,
| it would be in your best interest to treat them like one of
| those things ASAP so you can have a hand in their
| development.
|
| To me it feels most likely that our signals just have not
| had enough time to get to them.
| qual wrote:
| I think the "dark forest hypothesis" can apply here.
|
| The advanced alien civilization may indeed be interested
| in us, but still not consider it their best interest to
| act as soon as possible.
|
| If they decide to act, other civilizations (perhaps even
| more advanced) may decide to intervene in some way. A
| civilization that decides to reach out (in a friendly or
| hostile way) _also_ reveals their own location in the
| universe.
|
| The most risk adverse choice is probably to remain quiet,
| especially if they are millennia ahead of us
| technologically.
|
| https://en.wikipedia.org/wiki/Dark_forest_hypothesis
| pitaj wrote:
| You can detect gravitational waves based on the strain
| (amplitude) directly, rather than relying on intensity like
| electromagnetism.
|
| Because the amplitude is inversely proportional to the
| distance, but intensity is inversely proportional to the
| distance squared, this could allow for communication over
| longer distances.
| adrianN wrote:
| The upside is that space is quite transparent for
| gravitational waves.
| supportengineer wrote:
| What would a gravitational wave generator look like? A
| machine to "wiggle" an asteroid, or say a moon? What if you
| made a huge array of small machines that "wiggle", say, a
| bowling ball, in perfect sync.
| nyssos wrote:
| In principle, almost anything: any system of masses will
| emit gravitational waves with an intensity proportional to
| its mass quadrupole moment (which may be 0, as it is for
| rotationally symmetric systems). But the proportionality
| constant is _extremely_ small. Realistically you 're
| looking at stellar-mass objects, if not larger.
| supportengineer wrote:
| Hopefully we build a small transmitter and experiment
| with it, now that we have a receiver
| randomname93857 wrote:
| Nah, asteroid is not enough. you'd need to wiggle a couple
| or more large black holes in super close proximity. But who
| knows, may there be alternatives we are not aware of? Is it
| Higgs boson that gravitational field carrier particle ,
| similar to electrons for EM field? Maybe there is a way to
| mass-produce those and modulate gravity waves that way, eh
| ?
| floxy wrote:
| >I'm not sure I see what the advantages of communicating this
| way would be.
|
| Gravitational waves might be the best way to communicate
| between our world and the dark matter world/dark sector?
|
| https://en.wikipedia.org/wiki/Hidden_sector
|
| ...or maybe there is a lower noise floor for gravitational
| wave comms?
|
| >The amount of energy required per bit transmitted would be
| astounding.
|
| Has someone calculated this out? Or is it more of a "well we
| need an exceedingly sensitive instrument to detect some of
| the most energetic events in the universe from half-a galaxy
| away" gut-feel? Any reason something like a phased-array for
| directional comms / beam forming wouldn't work with
| gravitational waves?
| qual wrote:
| > _Has someone calculated this out? Or is it more of a
| "well we need an exceedingly sensitive instrument to detect
| some of the most energetic events in the universe from
| half-a galaxy away" gut-feel?_
|
| I was thinking about the energy required to transmit the
| gravitational waves, not receive them. Being able to move
| objects massive enough (stellar mass or more) to create
| detectable gravitational waves in a quick and precise
| enough manner to allow for communication would require
| mind-boggling amounts of energy.
| floxy wrote:
| Right, sorry I wasn't clear. If you were more interested
| in "local" communications, and less interested in
| broadcasting to the rest of the cosmos, do you still need
| gigantic amounts of energy at the transmitter? And how
| much power would you need, even if the energy is
| relatively high? For low power transmissions, what is the
| limiting factor on the receiving end? Or why can't you
| detect really low power transmissions? Can't get your
| receiver close enough to absolute zero, so thermal
| fluctuations kill receiver sensitivity? Background
| gravitation noise floor is too high across the band?
| Quantum fluctuations are a limiting factor? Can't make an
| X-ray/gamma-ray interferometer? "Antenna" size scales
| with length rather than area? Other?
|
| I suppose the ratio of Coulomb's constant to the
| gravitation constant (or something similar) govern the
| relative difficulty in using gravitational vs. EM? But
| that's not obvious to me that it would make gravitational
| wave communications inefficient in absolute terms.
| qual wrote:
| Oh, yeah sorry, I was thinking more along the lines of
| inter-galaxy communications!
|
| I definitely do not know enough about the topic to
| approach answering your questions, but I'd certainly be
| interested in knowing the answers. I really hadn't
| thought about it in that context.
| chatmasta wrote:
| Maybe you don't need to generate the waves, if you can
| collect them and redirect them into the shape you want.
| brianjlogan wrote:
| Since gravitational waves would still be restricted to the
| speed of light what would the benefit be?
|
| Would "obstacles" be circumvented? I would think interference
| would still be possible but instead of line of sight it would
| be large gravitational distortions (black hole, stars).
|
| Humility clause: I don't know what I'm talking about.
| fsmv wrote:
| This idea was featured in The Three Body Problem trilogy.
|
| The problem is modulating the signal. The only way is to move
| large masses quickly.
| imoverclocked wrote:
| Lots of naysayers but I can think of one very valid reason this
| might be the case. Our species, and many species on our planet,
| can see a very narrow band of the radiation spectrum. It's
| possible an advanced civilization never developed that but was
| instead far more intimate with gravitational energy instead. We
| are talking about the universe after all.
| jimbokun wrote:
| Did you read Death's End by Cixin Liu?
| elihu wrote:
| "Zorlax the mighty would like to connect on LinkedIn"
|
| https://xkcd.com/1642/
| ape4 wrote:
| This made (very lay-person) me wonder if you could detect quantum
| wave function events.
|
| https://physics.stackexchange.com/questions/275556/can-you-d...
| captainkrtek wrote:
| 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
| freeqaz wrote:
| The only dates available are during DEFCON. I would so love to
| go sometime though! Thanks for sharing -- this made my morning.
| captainkrtek wrote:
| I'd highly recommend it if you get the time, was an excellent
| tour to geek out on. Enjoy DEFCON!
| dudinax wrote:
| If you go to LIGO Hanford don't miss the B reactor tour near
| by. It's the first full scale nuclear reactor.
| mrlonglong wrote:
| It's entirely possible if it's extremely sensitive to detect the
| waves given off by alcubierrie warp drives crisscrossing the
| galaxies.
| cypherpunks01 wrote:
| 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_f...
| qual wrote:
| 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 wrote:
| 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.
| bowsamic wrote:
| 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
| openrisk wrote:
| 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
| jessriedel wrote:
| 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 wrote:
| 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 wrote:
| > Resonant bars were the first generation of gravitational
| wave detectors, period.
|
| Well, too bad, no one in the field refers to it that way
| (I'm a LIGO member). Jess is pointing out how the language
| is used, not saying that it's correct.
| defrost wrote:
| Last I checked David Blair's in the field, what with the
| sapphire clock development and his son being part of the
| 14 September 2015 first detection event.
|
| IIRC David talked about his niobium bar from 1976 as a
| first generation gravity wave detector - we certainly
| referred to it as a gravity wave detector in 1982 and it
| was talked about as such at the fifth Marcel Grossmann
| meeting.
|
| But hey, _you 're_ a LIGO member.
| bowsamic wrote:
| I've met David Blair and he's not someone I'd want to
| associate with at all
| jessriedel wrote:
| Openrisk was commenting on how the language is used by
| practitioners in the field, for which being a LIGO member
| is directly relevant. It's a mistake to try and mock him
| for doing that. It just drives actual experts even
| further away from HN.
| openrisk wrote:
| Combined with their second sentence that aims to
| discredit and erase an entire era / approach as crackpot,
| the implication is that this is indeed the "correct" way.
| There may be a lingering fear that the historical false
| detection claim could still tarnish the entire field but
| it feels overblown. This thread is about exploring "new
| ways" and it is very instructive to know and study the
| "old ways".
| jessriedel wrote:
| > The internal nomenclature of those working within a
| particular approach cannot restart the clock of an entire
| field?
|
| As bowsamic said, I'm telling you what the field's
| nomenclature _is_ , and then additionally explaining why it
| is used that way.
|
| > That certainly requires a reference. There were several
| teams around the world besides Weber pursuing the approach.
|
| They only pursued it after he published erroneous claims of
| discovery!
|
| https://physics.aps.org/story/v16/st19
|
| Look, there are groups around the world doing lots of
| things the majority think are foolish. (The DAMA experiment
| comes to mind.) It's good and healthy that this is allowed,
| and indeed there is a smooth continuum between "very
| unlikely but I guess _someone_ should be checking " and
| "lunacy". But we also shouldn't pretend all attempts are
| equally promising or reasonable.
|
| > 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.
|
| I'm not confusing anything. The people who have the
| expertise to estimate limits on the strongest plausible
| gravitational waves are called "astrophysicists" for a
| reason.
|
| > In this respect Weber was simply unlucky.
|
| An extreme example to illustrate why this sort of lazy "it
| might have worked" thinking is wrong: If I argue that there
| is a heretofore unknown law of physics that causes black
| holes to form at low energies as long as it's in Wilmington
| NC and it's on the 17th Wednesday of the year, and if I
| build a special detector in my basement in Wilmington that
| looks for the black hole signature on such a day, and if I
| claim to see a positive signal, most people will not say
| that I "built the first generation of table-top black hole
| detectors". So saying "it might have worked, who knows?"
| just isn't enough.
|
| It's not a coincidence that Weber both (1) thought it was
| worthwhile to look for gravitational waves at a strength
| that was widely thought to be impossible and (2) deluded
| himself into thinking he had made a positive
| identification. If he had only done #1, and if he had been
| clear that he was looking under the lamp post just in case
| there was surprising new physics, a defense of Weber would
| be more plausible. (Not necessarily convincing, but at
| least plausible.) But the fact that he also did #2 strongly
| undermines arguments that he was clear-eyed about what he
| was doing.
| awhitty wrote:
| Huh, I wonder if anyone's tried to validate this approach again
| now that we have LIGO (and presumably more precise equipment?).
| I know very little about the physics involved here, but the
| articles I found about Weber bars don't cite disagreement about
| the theory underpinning the experiment, so I'm curious if we
| expect a detectable effect with our current understanding?
|
| I also know very little about manufacturing Weber bars, but I
| could imagine it's cheaper to build 100s or 1000s of these and
| perform signal processing on them than building another LIGO.
| Or Weber bars in space?
|
| Just spitballing here
| sigmoid10 wrote:
| If you look at the lengths LIGO had to go to in order to
| eliminate background noise to even be able to theoretically
| detect gravitational waves, it seems very unlikely that a
| comparatively crude technology from the 80s could have
| achieved the same things. Like modern lasers or squeezed
| light states, which were mostly theoretical back then. If I
| remember correctly, Weber's device was claimed to show a huge
| amount of events per year, which would indicate tons of GW
| sources in our neighbourhood. LIGO and the rest of modern
| astronomy have since disproved that.
| adolph wrote:
| Sometimes noise turns out to be signal (see developments
| leading to Cosmic Microwave Background [0]). At this stage
| in gravitational wave detection is it well founded to
| consider noise as noise instead of information we cannot
| yet understand?
|
| 0. https://discovery.princeton.edu/2015/11/19/cosmic-
| background...
| mr_mitm wrote:
| Noise always has different sources, and when you
| eliminate noise you should always be conscious of the
| source. There should be a gravitational wave background
| analogous to the CMVB. Detecting it would be a sensation.
| umvi wrote:
| 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 wrote:
| Isn't space gravity carrier itself? Something like the water
| that has waves?
| simcop2387 wrote:
| 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 wrote:
| 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.
| atombender wrote:
| That would suggest space is "made of" something, so what
| would that be? If it's made of something, how is that
| "carrying" the wave, causing it to distort in the presence of
| mass?
| adolph wrote:
| Or that gravity is made of something traveling through the
| simcop2387 wrote:
| So I want to try to answer what I can, despite being a layman
| on this.
|
| Gravity exists, it manifests as the warping/geometry of space.
| This is in contrast to the other fundamental forces which get
| explained via Quantum Field Theory. That's the very high level
| difference of the two, our current understanding of gravity
| does not work the same way as the way everything else does, and
| so far we can't find a provable theory (yet) that makes the two
| work together at all scales.
|
| String theory purported as a way to create a quantum theory of
| gravity and explain everything else, but my understanding is
| that it's fallen out of favor because it mostly turned into a
| tunable mathematical framework that could just change to fit
| any observations that were made, so it doesn't have the same
| kind of predictive power that people want (i.e. too much
| freedom so it can be used to explain anything, not just
| everything). I believe this is where predictions about a
| possible gravitational force carrier generally come from, aka
| the graviton.
|
| Then there's theories like Loop Quantum Gravity, where the way
| it works is that space-time itself is quantized and that's how
| you get things to mesh because you can now use the same wave-
| function style of things that all other quantum theories use.
| Though I _think_ this doesn 't predict much about a quantum
| field for gravity on it's own.
|
| I believe one of the other things that runs into everything
| being difficult is that with relativity you end up with a lot
| of infinities in the equations and results and so there's a
| "new" kind of math for it that gets called "renormalization"
| that prevents them from coming out but it also has issues when
| translating between quantum theories and relativity.
| nyssos wrote:
| > it doesn't have the same kind of predictive power that
| people want (i.e. too much freedom so it can be used to
| explain anything, not just everything).
|
| That's the popsci version that's been disseminated, yes. It's
| not exactly wrong, but it's misleading.
|
| First a bit of background. Quantum field theories like the
| standard model are _effective_ theories, not fundamental
| ones. We know we don 't know the real high-energy physics, so
| we treat it as a black box and loosely speaking "average it
| out" as a new free parameter. This is analogous to how an
| engineer designing a bridge can ignore the fact that iron has
| a crystal structure and treat it as a continuum with bulk
| properties like tensile strength. In reality this having a
| particular tensile strength is a _state_ , not an intrinsic
| property, and you could end up with a different tensile
| strength if you melted the iron and let it resolidify (I'm
| not a metallurgist, substitute some other material if that's
| not true for iron), but we can build bridges without knowing
| that.
|
| In the same way, Standard Model is a particular form of
| "solidified" string theory. It's true that there are many,
| many, many others, but they're not _free_ parameters in the
| same way. You can write down perfectly reasonable looking
| quantum field theories that string theory can 't produce, and
| if our best effective theory was one of them then we would
| have good reason to reject string theory. But it's not.
|
| So the situation we're in is that we have some solid
| material, and we want to know what a single molecule of it
| looks like, but we can't see anything other than the bulk
| properties. What the "string theory is unfalsifiable" crowd
| is demanding is that whatever molecule we predict have only a
| few possible crystal structures. And maybe it does. That
| would be convenient. But sometimes nature inconveniences us:
| it might be some crazy carbon allotrope. It might be glass.
| superposeur wrote:
| These experiments confirm the _classical_ theory of gravity,
| which is Einstein's general relativity, just as Maxwell's
| equations are the classical theory of the electromagnetic
| field. Just as Maxwell's equations are perfectly adequate for
| waves of macroscopic intensity, GR is perfectly adequate for
| astrophysical gravity waves.
|
| A whole separate question is what is the quantum mechanical
| theory that has general relativity as its classical limit? For
| electromagnetism, quantum electrodynamics (understood in the
| 40's) is the quantized version of Maxwell's and predicts that
| electromagnetic energy measurement outcomes come in "chunks"
| (photons). But, although much is known about features of
| "quantum gravity" (like that gravitons will be massless, spin
| 2), there is famously no consensus yet about the precise
| theory.
|
| As to how to reconcile the force carrier picture with spacetime
| picture -- even classically one can consider an "overall"
| spacetime background geometry such as that created by the whole
| earth. Then consider little ripples perturbing this background.
| Gravitons are these little ripples turned on a quantized amount
| (heuristically). How the overall background itself gets formed
| as an "enormous pile of gravitons" will depend on the precise
| theory of quantum gravity. String theory does have a partial
| answer to this so can model such things as black holes
| quantumly.
| akira2501 wrote:
| We have two systems of physics. We can make predictions using
| either system. We cannot fully unify the two systems.
| cletus wrote:
| 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/Staffor...
| strangattractor wrote:
| 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 wrote:
| 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
|
| > Practically Free Primordial Gravitational Waves Detector
|
| https://www.youtube.com/watch?v=XfOxNJvSvf4
| westurner wrote:
| - "Kerr-enhanced optical spring for next-generation gravitational
| wave detectors" (2024)
| 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
| ImageXav wrote:
| 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/a....
| mnemotronic wrote:
| Here's something to get you ready ...
| https://youtu.be/DRRfhZFyDvg?feature=shared
| perlgeek wrote:
| 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?
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