[HN Gopher] Monster gravitational waves spotted for first time
___________________________________________________________________
Monster gravitational waves spotted for first time
Author : throw0101c
Score : 264 points
Date : 2023-06-29 14:36 UTC (8 hours ago)
(HTM) web link (www.nature.com)
(TXT) w3m dump (www.nature.com)
| ivanhoe wrote:
| What's the value of amplitude of one such 'monster' g-wave?
| x3874 wrote:
| [flagged]
| beginning_end wrote:
| I'm curious what the amplitude of these waves are: what's the
| (order of magnitude) change in the distance to a 1000 light year
| distant pulsar, as the gravitational wave passes through?
|
| Edit: Being told by @Dr_CMingarelli on twitter that it's 10
| meters pr lightyear.
| kennywinker wrote:
| Per the wikipedia article:
| https://en.wikipedia.org/wiki/Gravitational_wave
|
| > Gravitational waves are not easily detectable. When they
| reach the Earth, they have a small amplitude with strain
| approximately 10^-21
|
| "strain" being the unit-less measurement they use for
| gravitational wave.
|
| The ones we measured just now, if I'm reading this article
| right
| (https://iopscience.iop.org/article/10.3847/2041-8213/acdac6),
| are being reported as 2.4x10^-15.
|
| I can say that's a lot larger, but I can't tell you much else
| about what that means.
| lucgommans wrote:
| You mean in like meters?
|
| > Each observatory has two light storage arms that are 4
| kilometers in length. [...] A passing gravitational wave will
| slightly stretch one arm as it shortens the other. [...] Even
| with such long arms, the strongest gravitational waves will
| only change the distance between the ends of the arms by at
| most roughly 10-18 m.
|
| With the nominal hair value being 75 um, `apt install qalc`
| tells me that's > 75 um / 10^-18 m (75
| * micrometer) / ((10^-18) * meter) = 7.5E13
|
| so a change in length 7'500'000'000'000 times smaller than the
| width of a hair
|
| https://en.wikipedia.org/wiki/Gravitational_wave ,
| https://en.wikipedia.org/wiki/Hair%27s_breadth
| beginning_end wrote:
| I was referring to the latest results using pulsars.
| kennywinker wrote:
| That's for the gravitational waves we can detect from LIGO.
| These new ones, as I understand it, are being detected by
| changes in pulsar frequency over 15 years of measurements -
| so they have a pretty different character than the ones
| wikipedia's talking about... I don't know how that translates
| to anything that makes any sense tho
| swayvil wrote:
| I hear that gravity is a contour in spacetime, or something like
| that.
|
| Does that mean that a gravity wave is a ripple in space or a
| travelling wave of pinched or stretched space?
|
| Also, assuming all that. I think we have 2 ways of squishing
| space this way : mass and acceleration. Are there any others?
| r721 wrote:
| Another discussion: https://news.ycombinator.com/item?id=36514521
| tannhaeuser wrote:
| Also, The second data release from the European Pulsar Timing
| Array I - The dataset and timing analysis [1] was published just
| today.
|
| [1]: https://arxiv.org/abs/2306.16224
| bilsbie wrote:
| Dumb question but why can a gravity wave escape a black hole but
| not a light wave?
| jessriedel wrote:
| Roughly speaking, it's because the waves are emitted from the
| region outside the black holes, being created by the in-
| spiraling of two black holds before/during their merger. Once
| the black holes merge and settle down, they stop emitting
| gravitational waves.
|
| But that's just a cartoon. Strictly speaking, the picture of a
| wave traveling with respect to a fixed background spacetime is
| only an accurate approximation when the wave is very weak. In
| the immediate neighborhood of a black hole merger, the
| approximation breaks down, and you just have to look how the
| whole spacetime itself is evolving (usually through
| simulation).
| Zoadian wrote:
| are g-waves causing the Galactic filaments?
| notaustinpowers wrote:
| No, they are not caused by the galactic filaments. We don't
| know exactly yet what all is causing gravitational wave
| background (GWB), but a theory is that it could be caused by
| supermassive black holes, or primordial black holes from the
| early universe, etc.
| Zoadian wrote:
| I asked the reverse. If the filaments are caused by
| gravitational waves.
| derefr wrote:
| What do these waves look like as they pass through us? Acoustic-
| like compression and expansion of particles as molecules
| temporarily reorient toward a "down" that is ever-so-slightly off
| from the Center of mass of the Earth?
|
| Also, I assume that these waves are very gentle sinusoids? Could
| the opposite -- a high-amplitude gravitational square wave -- be
| possible? What would it do to the things it passes through?
| jessriedel wrote:
| Yes, that's basically right.
|
| The gravitational way has a direction (say, z) in which its
| propagating. Within the plane perpendicular to that direction
| (x-y), a circular ring of particles will at at one moment
| experience squeezing in one direction (x) and stretching the
| perpendicular direction (y). As the wave passes through and you
| move from the peak of the wave to the trough, the directions
| reverse, so the first direction (x) stretches and the other
| direction (y) squeezes. By "stretching" and "squeezing" I mean
| instantaneous additional (positive and negative) acceleration
| on top of the (much, much larger) acceleration from the
| background gravitational field provided by the Earth.
|
| Here's a visualization:
|
| https://www.researchgate.net/publication/313828462/figure/fi...
|
| Just as a child can swing their legs at the resonant frequency
| of a swing to pump up their sinusoidal amplitude, a very weak
| gravitational wave can pump up a ring oscillator if it's
| oscillating at the ring's resonant frequency.
|
| Exactly square gravitational waves are of course not possible,
| just as for electromagnetic waves. (They would have infinite
| energy at the corners.) But in principle you could get a close
| approximation. However, spacetime is _incredibly_ stiff, and I
| think all the known real-world sources produce pretty smooth
| waves. I presume most violent events are mergers of existing
| black holes, and essentially always result from a smooth in-
| spiral rather, say, a sharp collision event. This is what the
| "chirp" signal looks like to the LIGO detector:
|
| https://www.youtube.com/watch?v=TWqhUANNFXw
|
| The effects of a square wave would be roughly as you would
| expect: instead of smoothly pumping up an oscillator, it would
| give it a sharp kick, just as with electromagnetism.
| nativeit wrote:
| Thank you for that! As an EE and radio enthusiast, I feel
| like I have a fairly good grasp of how gravitational waves
| behave from that description. The way in which the mechanics
| of these different energies all sort of share characteristics
| is rather beautiful.
| xattt wrote:
| > Just as a child can swing their legs at the resonant
| frequency of a swing to pump up their sinusoidal amplitude, a
| very weak gravitational wave can pump up a ring oscillator if
| it's oscillating at the ring's resonant frequency.
|
| So is it possible that a passing gravitational wave could
| initiate some natural process that otherwise might have not
| happened?
| mirekrusin wrote:
| It can at least trigger hn front page submission, this we
| know.
|
| You mean something like tectonic event trigger or something
| physics specific?
| xattt wrote:
| I was thinking more along the lines of a chemical or
| nuclear reaction (or some yet-to-be-conceptualized space-
| time reaction) at near an initiation point, but not quite
| there.
|
| I assume the gravity wave could push it in the direction
| for the reaction to initiate the reaction by shifting a
| subatomic element (like an electron orbital) into an
| otherwise impossible configuration on a scale of
| picometers for a split second.
| zeven7 wrote:
| These gravity waves aren't strong enough to pull us off
| Earth. Everything on Earth is already subjected to
| gravity, and it doesn't cause nuclear reactions. Gravity
| waves won't cause impossible configurations to become
| possible.
| taeric wrote:
| Could it impact orbits? Or tides?
| asdff wrote:
| Could you set up massive pendulums to convert this
| gravitational wave energy into mechanical energy? Would such
| a device diminish the strength of the wave downstream?
| thatcherc wrote:
| Yes! In principle at least, and only miniscule/undetectable
| amounts of energy. The first gravitational wave detectors
| build were these "Weber bars" [1] - big block of aluminum
| that were supposed to resonant mechanically (like a bell)
| when a gravitation wave of the right frequency passed
| through them.
|
| Looks like these things haven't detected a real
| gravitational wave, but if a strong enough one at the right
| frequency came through, they might start ringing like (very
| quiet) bells!
|
| [1] - https://en.wikipedia.org/wiki/Weber_bar
| derefr wrote:
| Acoustic waves propagate through what are essentially elastic
| deformations of the material they travel through. Can
| gravitational waves be thought of as propagating by
| elastically deforming spacetime?
|
| If this analogy holds, then can it be taken further? Acoustic
| waves dissipate their energy insofar as they trigger plastic
| deformation in a material. Could gravitational waves
| plastically deform... spacetime itself? Or would they just be
| deforming the material? Or is gravitational energy not
| dissipated into other forms of energy at all?
| jessriedel wrote:
| Acoustic waves dissipate their energy into non-acoustic
| (molecular) degrees of freedom. In the absence of matter, I
| think there are no other degrees of freedom for
| gravitational waves to dissipate into, unless they are
| strong enough to form a black hole (which they can do under
| extreme conditions, I think).
|
| (This is waaaay outside my expertise though, so take my
| answer with a grain of salt. Everything I've said in this
| thread is basically based off the rudimentary understanding
| from taking a GR course in grad school. I've never done
| research on this topic.)
| nyrikki wrote:
| Hard or impossible to visualize, but as the temporal
| dimension is so much larger, most of the deformity is in
| the time dimension.
|
| But yes, spacetime itself is jiggly like Jell-O.
|
| As we can't visualize 4d spacetime, most analogies will be
| wrong. But as photons don't experience time themselves,
| thinking about the geodesic path is probably less error
| prone than thinking of it as squishing physical objects.
|
| Objects being pulled towards slower flows is the intuition
| that matches the math best for me.
| sunshinerag wrote:
| what does "photos don't experience time themselves.."
| mean? why not?
| alfiopuglisi wrote:
| Photons travel at the speed of light, and at that speed,
| any "subjective" time is zero. In Einstein's theory of
| special relativity, the faster you go, the slower your
| proper time appears to an external observer. At the speed
| of light, this effect reaches infinity.
| criddell wrote:
| > the faster you go, the slower your proper time appears
| to an external observer
|
| From my perspective, it takes about 8 minutes for a
| photon from the sun to hit my eye. From the perspective
| of the photon, a little time has passed, no? Doesn't the
| atmosphere and passing through my glasses slow it down a
| wee bit? Can the photon "know" that its position has
| changed between emission and absorption? From the photons
| point of view, I must be very, very close to the sun,
| right?
| ben_w wrote:
| Light does slow in a medium, the statement presumes the
| light is in a vacuum.
|
| From the point of view of the photon, "forwards" is, like
| time, a null[0] dimension.
|
| [0] I may be using that word imprecisely, but I can't
| think of a better one.
| adgjlsfhk1 wrote:
| This isn't quite true. At a macro level, light slows
| down. At a micro level, photons travel at the speed of
| light, get absorbed and re-emitted, and change
| directions. These effects average out to looking like
| photons traveling slower.
| nyrikki wrote:
| No. From the photons perspective, there is no concept of
| time. Phase speed, group speed, shadows going faster than
| the speed of light, etc.. will all complicate using the
| concepts used to teach diffraction
|
| Massless particles being required to travel at the speed
| of light is perhaps a lens to think about it.
| derefr wrote:
| It takes time for a photon to move in your reference
| frame, but time within the photon's own reference frame
| is not advancing at all during that. Within the photon's
| reference frame, the photon exists instantaneously,
| simultaneously, at its emitter and absorber. Its whole
| existence "brings together" the spacetime it was emitted
| from and the spacetime it is absorbed into, at a single
| 4D pinch-point. It's like the whole universe is squished
| flat into two hyperplanes of "everything behind the
| photon at time of emission" and "everything ahead of the
| photon at time of absorption", and those two hyperplanes
| have no distance between them.
|
| > Doesn't the atmosphere and passing through my glasses
| slow it down a wee bit?
|
| When a photon is travelling through anything other than
| vacuum, it's not "slowed down." It's repeatedly being
| absorbed and re-emitted. (Or rather, it's being absorbed,
| and new photons that happen to be mostly equivalent are
| being emitted.) The refractive index of a material is
| effectively a measurement of the likelihood of
| absorption, times the average per-particle time-delay
| between absorption and re-emission.
| criddell wrote:
| So from the photon's point of view, the entire universe
| is a single point?
| ithkuil wrote:
| No because a single photon doesn't *experience" the whole
| universe but only the points where it's emitted and
| absorbed and you could say all the points in between
| along the geodesic between the emission and absorption
| events
| nyrikki wrote:
| Nothing that travels at light speed experiences time. For
| a photon, emission and absorption is a single event.
| cyberax wrote:
| > Or is gravitational energy not dissipated into other
| forms of energy at all?
|
| Extremely weakly. The mechanism is similar to acoustic
| waves, but the coupling constant is so small, that the
| amount of dissipated power would be insignificantly small.
|
| In theory, you can use gravitational waves to extract
| energy. For example, you can wait for the "compression"
| part of the wave, and push a cart uphill during it. Then
| let it slide downhill when the compression peak passes.
| You'll be able to extract some useful energy, because the
| distance that you pushed the car uphill will be shorter
| than the "normal" distance.
|
| You can make more elaborate systems on this principle. E.g.
| a tuned resonator: two orbiting masses with a period
| selected to match the frequency of the gravitational waves.
| treeman79 wrote:
| The earth going around the sun produces gravitational
| waves, and some of the energy is lost.
|
| If all of that energy could be harnessed; it would be
| sufficient to power a small toaster oven.
| staunton wrote:
| > If all of that energy could be harnessed; it would be
| sufficient to power a small toaster oven.
|
| Do you have a source? Sounds like an interesting
| calculation.
| ortusdux wrote:
| I recommend looking into LIGO and other similar experiments.
| They use laser interferometry to accurately measure the
| distance between two points, to an extreme degree. From LIGO's
| website:
|
| _Gravitational waves cause space itself to stretch in one
| direction and simultaneously compress in a perpendicular
| direction. In LIGO, this causes one arm of the interferometer
| to get longer while the other gets shorter, then vice versa,
| back and forth as long as the wave is passing. The technical
| term for this motion is "Differential Arm" motion, or
| differential displacement, since the arms are simultaneously
| changing lengths in opposing ways._
|
| _As described above, as the lengths of the arms change, so too
| does the distance traveled by each laser beam. A beam in a
| shorter arm will return to the beam splitter before a beam in a
| longer arm--as the wave passes, each arm oscillates between
| being the shorter arm and the longer arm. When they arrive back
| at the beamsplitter (where they re-merge), the light waves no
| longer meet up nicely; they are out of phase. Instead, they
| shift in and out of alignment for as long as the wave is
| passing._
|
| https://www.ligo.caltech.edu/page/what-is-interferometer
|
| https://en.wikipedia.org/wiki/LIGO
| samstave wrote:
| >*Gravitational waves cause space itself to stretch*
|
| Please ELI5 specifically and empiracally what "Space Itself"
| actually is.
|
| Would it be possible to build a 'galactic clock & Compass' -
| a "clock" to the regular pulses of a pulsar and the galactic
| direction the pulsar is in relation to the terrestrial
| compass (magnetic) on earth...?
|
| What is the pulsar with the most reliable timings?
| pantulis wrote:
| Space itself means the metric of spacetime: how you measure
| distances. Locally, if there was no gravity, you could
| think of space as a plain reticule where distances would be
| calculated through eucledian geometry: that means cartesian
| distances, parallel lines would be parallel and the angles
| of a triangle would sum 180, and the shortest path between
| two points is the straight line.
|
| General Relativity successfully establishes that the
| presence of mass distorts this, so it defines a
| mathematical object (the Einstein tensor) that reacts to
| the distribution of mass and energy and precisely describes
| the changes to the metric. For example it can model how the
| mass of the sun distorts the space so that light from
| distant stars appear to follow a curved path because very
| close to the sun a curved path is now the shortest path.
|
| The Einstein tensor defines how distances --and time-- are
| measured and it's the best mathematical model that we have
| about what "space itself" is. Future theoretical advances
| could take us forward and demonstrate that space itself
| emerges from other more fundamental elements, but this
| needs bridging quantum mechanics and gravity. We don't
| really know what space is made of, but scientists have
| precisely modelled how it reacts to mass (and energy) with
| utmost precision.
|
| NB: At cosmic scales the exercise becomes more difficult,
| as there is an expansion of the metric of spacetime that is
| not due to the presence of mass, in fact it is caused by
| the _absence_ of mass as it seems to be due the energy of
| empty space: the phenomenon called Dark Energy.
|
| Hope this helps!
| dangond wrote:
| Out of curiosity, from someone who has never worked with
| non-euclidean geometry, what does it mean for a path to
| be curved in non-Euclidean space? My outsider
| understanding of curvature is that the inside of a curve
| is shorter than the outside of the curve, whereas a line
| has the same length on either side (assuming we give
| these curves and lines some thickness). But, if the
| shortest path can be curved, what do we mean by curved?
| ajkjk wrote:
| Picture curves on the surface of the earth. They seem
| flat locally, but if you go a mile north, a mile east, a
| mile south, and a mile west, you don't end up _exactly_
| where you start. (In the northern hemisphere you end up a
| little east of where you start; in the southern, a little
| west.)
|
| Same thing in general relativity: the metric tensor
| measures the failure of closed loops on each axis to not
| close perfectly, the way they would in Euclidean space.
|
| Basically even as a small creature on earth you can
| 'figure out' about the curvature by carefully measuring
| small-ish loops. The same is true for spacetime, but the
| loops' deformities are even smaller.
| olddustytrail wrote:
| It's not too complicated. Get a round ball of some sort.
| When you draw on the surface, that's a "non-Euclidean
| space".
|
| Take a straight line down from the "north pole" of your
| ball to its equator. Draw another straight line around a
| quarter of the equator. Draw a third line back to the
| pole. You've just drawn a triangle with 3 straight lines
| and the angles add to 270 degrees.
|
| A non straight line is just not the shortest distance
| between two points on that surface.
| AdamH12113 wrote:
| I've found it's easier to think about this stuff in two
| dimensions. The surface of a sphere (or the Earth!) has
| non-Euclidean geometry.
|
| Imagine two people standing some distance apart from each
| other at the equator. They both begin walking in
| straight-line paths due south. At first, their paths are
| parallel. But as they move toward the south pole, they
| begin to drift closer to each other, as though their
| paths were curving towards each other. When they reach
| the south pole, they bump into each other. But they were
| both walking straight forward following the shortest path
| to the south pole the whole time. The curvature of the
| surface causes their initially-parallel paths to
| converge.[1]
|
| On a plane (which has Euclidean geometry), initially-
| parallel paths never converge.
|
| [1] Don't take this too literally; the real planet Earth
| is three-dimensional, and its gravity keeps us on the
| surface. But mathematically, it's possible to describe a
| curved two-dimensional space without referring to any
| higher dimensions. When I talk about "the surface of a
| sphere", that's what I mean -- the surface is the entire
| 2D space.
| urinotherapist wrote:
| If two people are in parallel, they will make two
| parallel circles. If two people aimed at a singe point,
| they are not in parallel.
|
| Space-time is 4d array: array of framebuffers. You can
| stretch your mathematical model all day long, but you
| knowledge must be _mapped_ to reality somehow. In model
| we have space-time, while in real world we have
| "physical vaccum" ("something nothing" or "phaccuum", for
| short). I prefer to name that thing "ether", because I
| like that word.
| AdamH12113 wrote:
| > If two people are in parallel, they will make two
| parallel circles. If two people aimed at a singe point,
| they are not in parallel.
|
| In spherical geometry, the equivalent of a straight line
| is a great circle. There are no parallel great circles.
| That's why I used the phrase "initially parallel" -- at
| the starting point, both people's paths are at a
| 90-degree angle to the great circle connecting their
| locations.
|
| I didn't want to get into "locally flat" vs. "globally
| curved" in something that started as an ELI5 thread.
| abecedarius wrote:
| Maybe it's worth adding that in this way of thinking
| (intrinsic geometry of the surface), great-circle paths
| have exactly the property the GP brought up about
| straight lines: neighboring paths aren't shorter on one
| side and longer on the other. (If you think of them as
| 3-d paths then there's a shorter path _below_ vs. longer
| _above_ , but that's not part of the intrinsic geometry.)
| pantulis wrote:
| I cannot be of help here but I'd say that your concept of
| curvature is too informal, but more formal maths can deal
| with it, take a look at the wikipedia page for
| "Geodesic", the maths are way above my head but the
| diagrams are cool :D
| bumby wrote:
| If this is ELI5, we hang around very different kinds of
| five year olds :-)
|
| Joking aside, thank you for the deeper explanation. The
| idea that spacetime isn't fundamental is a very non-
| intuitive concept given how we've evolved to interact
| with the world. Any suggested reading on this topic for
| laypeople?
| pantulis wrote:
| > Any suggested reading on this topic for laypeople?
|
| Carlo Rovelli is a bona fide theoretical physicist, very
| involved in the development of Quantum Loop Gravity (one
| of the attempted approaches to bridge GR and QM). Turns
| out he is also a good pop-sci writer, so I would begin
| there. His book "The Order of Time" deals with the nature
| of time, which is not about the nature of space, but then
| again reading it you see that the mental gimnastics are
| similar.
|
| I also found useful contributions from regular
| contributors at /r/cosmology (thank you /u/jazzwhizz) but
| it's less straightforward and alas, Reddit has its own
| issues.
| samstave wrote:
| I find /r/cosmotology a hairy subject hard to swallow...
|
| I think they push string theory too much, and try too
| hard to braid it into the fabric of our societies, with
| their little shops and what not... It gets everywhere,
| and tomorrow is their favorite day "Friday"!
| pantulis wrote:
| ChatGPT to the rescue!
|
| let's think about space like a giant, invisible
| playground. Normally, it's flat like your bedroom floor,
| where you can measure how far your toys are from each
| other with a ruler straight across. That's like when
| there's no gravity in space.
|
| But guess what happens when something really heavy, like
| a big bowling ball (that's like a star or planet) comes
| into your playground? It makes everything around it bend
| and curve. So, the distance between your toys is no
| longer a straight line. It's like when you throw a ball,
| it doesn't go straight, it goes in a curve.
|
| This bending is what a really smart guy named Einstein
| explained in a thing called General Relativity. He came
| up with a way to measure how space bends around heavy
| stuff.
|
| And you know what else? There's this really weird stuff
| called Dark Energy that's everywhere but we can't see it.
| It makes space grow bigger and bigger, not because of
| heavy stuff, but because there's a lot of empty room.
| Scientists are still trying to understand this, but it's
| like blowing up a balloon: even though there's no heavy
| stuff inside, the balloon still gets bigger!
| samstave wrote:
| Super helps! thanks - while I knew a bunch of pieces and
| tidbits, you cemented it for me - thank you.
|
| -
|
| TensorFlow
|
| >*General Relativity successfully establishes that the
| presence of mass distorts this, so it defines a
| mathematical object (the Einstein tensor) that reacts to
| the distribution of mass and energy and precisely
| describes the changes to the metric.*
|
| This leads me to think that TensorFlow was attempting to
| map the 'weight' among topics of intersecting interests,
| sciences, etc... and seeing who the "tensor warping" was
| most strong with and adding higher eval weights to things
| that "gravitated" to one another based on the
| informational difference in distance?
|
| (I dont know the nomenclature, but is that were using
| 'tensors' in AI/ML/whatever 'weights' come from?
| pantulis wrote:
| A tensor is a mathematical object that has a lot of uses
| beyond general relativity, of course. It's a little
| tricky to visualize (I think there are good YouTube
| videos) but tensors can be thought of as multidimensional
| arrays, and also the whole tensor algebra can be done in
| terms of multidimensional array operations.
|
| So reasoning about a neural network weights and
| operations in terms of tensors makes sense and I guess
| that's what the name Tensorflow comes from.
| samstave wrote:
| Yes, but your explaination helped me to better understand
| the other types of tensors in a way I can 'grok' it - as
| opposed to trying to be at the level of some google quant
| or whatever with ML understandings of weights.
|
| It helped my put my own internal visualizations to the
| understanding.
|
| and I had a weird peripheral memory on this from a
| thought I had whilst driving in 1999 where I was thinking
| of tensors in this way, but I didnt know what I was just
| daydreaming about... but apparently, it was einsteins
| tensors coupled with information theory - and while to me
| it was a pedestrians take on the premise - it turns out
| that that day dream was correct!
|
| And it all ties back to when I was ten years old and
| meditating on the Mind of God -- It all tied into one
| another - and you gave me some cord to pull these
| experiences together with understanding which I havent
| had in 40 years... so that was nice. Thanks.
|
| Anyone else recall daydreams from their past where their
| later understanding was confurmed, even though you were
| just "daydreaming at the time"?
| sidlls wrote:
| Take a sheet of elastic material, like a balloon, but
| square. Suspend the corners in clamps on posts so that it's
| "perfectly" flat. Draw a line that is the shortest distance
| between two points on the material: it will be a "straight"
| line like what you learnt in geometry class. Now place a
| marble in the center of the sheet and let it stretch the
| sheet. Now draw the shortest possible line connecting these
| two points: it will curve (and unless you chose by
| coincidence) it will likely not overlap with the first
| line: the "shortest distance" between the points has
| changed.
|
| There's another aspect to this: the expansion coefficient.
| One such model is that the coefficient depends only on
| time: as time passes, distances increase. To model this,
| draw the same line on the flat sheet of material, then
| expand it uniformly in all directions. The distance is
| still a straight line, but the line is longer after the
| expansion.
| readams wrote:
| It's not so much that you see a slightly different "down," but
| that space itself is changing such that the distance between
| e.g. your head and feet is (very) slightly altered.
| jessriedel wrote:
| The two descriptions are equivalent. By the equivalence
| principle, the wave looks locally like neighboring bodies see
| different directions of down (and also slightly different
| strengths of the force of gravity in that direction).
| truculent wrote:
| Dumb, but earnest question: if space itself changes, how can
| the distance change? What is the distance a measure of, if
| not space itself? What's the yardstick, speed of light?
| wlesieutre wrote:
| Yes, and light is actually how the LIGO and similar
| detectors can measure such small changes in distance. Light
| emitted from a laser is split into two beams traveling
| different paths, and then merged back together.
|
| When the light merges back together, if the two paths
| traveled took exactly the same distance (or an even
| multiple of the wavelength at least), then the beams add
| together constructively and you put back together the light
| from the laser.
|
| But if one path becomes longer or shorter the other, the
| light is out of phase with itself (peaks of the waves no
| longer line up with each other) and you can detect the
| interference between them.
|
| LIGO can detect a change in distance of less than one ten-
| thousandth the charge diameter of a proton.
|
| https://en.wikipedia.org/wiki/LIGO#/media/File:Gravitationa
| l...
| ISL wrote:
| The yardstick is indeed the light-travel time.
|
| Gravitational waves really do change the time it takes for
| light to travel between two points. We use light travel-
| times to measure distances, thus we say that the distance
| between the points has changed.
|
| If it feels counterintuitive for spacetime to be changing,
| that's good. It is outside our human experience and
| perception. The strongest gravitational waves ever observed
| by scientists passed through everyone who was alive in
| 2015. None of those people noticed before the instruments
| registered a detection.
| [deleted]
| jballanc wrote:
| >None of those people noticed before the instruments
| registered a detection.
|
| IIRC, the reason no one noticed is that even the
| strongest gravitational waves are only going to "stretch"
| space by something less than the diameter of a hydrogen
| atom.
| photonerd wrote:
| I think "look" is a misnomer here. It wouldn't really look like
| anything, as it's fundamentally all of space-time that is being
| stretched and rippled.
|
| There's nothing to really see as such.
| rkagerer wrote:
| Could one devise similar detectors for time dilation and
| expansion?
| ComputerGuru wrote:
| More reporting, for a more pedestrian overview:
|
| In a major discovery, scientists say space-time churns like a
| choppy sea
|
| https://www.washingtonpost.com/science/2023/06/28/gravitatio...
|
| (Archived: https://archive.is/AmRvg)
| sslayer wrote:
| Imagine if we could take advantage of the time-space
| potential/differences between gravitational areas to "skip"
| large parts of space. We would have to have a very precise
| gravity map, but could also get huge gravity potential boost!
| [deleted]
| TechBro8615 wrote:
| That WaPo article was also discussed [0] on HN yesterday
| (although most of the comments were about the garbage quality
| of the reporting).
|
| [0] https://news.ycombinator.com/item?id=36514521
| ComputerGuru wrote:
| Thanks for the link. I had issues with the WaPo article, but
| that tongue-in-cheek joke with dead-pan delivery wasn't one
| of them. Fortunately another commenter called it out as a
| joke as well.
| [deleted]
| anonym29 wrote:
| EDIT as a commenter below has kindly pointed out, our current
| models indicate that the answer to this question is "no", per
| https://www.youtube.com/watch?v=QMFLcmsjOBg
|
| ------
|
| This is all firmly outside my technical discipline, but aren't
| there some theories that faster-than-light travel might be
| achieved by bending spacetime around a spacecraft, as opposed to
| trying to propel the spacecraft through space?
|
| I really want to emphasize that this is entirely speculation, but
| is it possible that these gravitational waves could be the
| "ripples" produced in the wake of such faster-than-light travel,
| the same way a boat travelling through a body of water leaves
| ripples in the water behind it?
| zackmorris wrote:
| This video might help understand where mass comes from and how
| to potentially modulate it, because mass can be thought of as
| bound energy creating voids in the gluon field:
|
| https://m.youtube.com/watch?v=Ztc6QPNUqls
|
| The Higgs mechanism affects electrons, not quarks, and is only
| responsible for about 1% of matter's mass. Most mass comes from
| the binding energy between quarks, which creates flux tubes
| between quark-antiquark pairs. If we add more energy to pull
| quarks apart, eventually the total energy added exceeds the
| mass-energy equivalence of another quark-antiquark pair, so a
| new pair gets created from the vacuum. I believe this is
| related to the Casimir effect, but IANAP (physicist).
|
| Keep in mind that the mass-energy equivalence also applies to
| time. So like in the movie Interstellar, when they go down to
| the water planet, gravity is so high that time passes slower
| for them than the guy in the orbiting ship. In other words, the
| ship sees the landing craft move slower and slower as it
| approaches the surface. This difference in the speed of time
| near a gravity well is what slows the inner edge of a satellite
| slightly more than the outer, curving it along the path of the
| orbit, which from the satellite's perspective feels like a
| straight line at that velocity, because it's weightless and
| feels no other acceleration other than tidal force. So if
| someone could move large amounts of energy into a confined
| space with some kind of flux capacitor (is this a pun? I don't
| even know anymore), they could slow time there and create a
| virtual mass through mass-energy equivalence by E=mc^2. If they
| did it in front of the satellite, it would begin to increase in
| velocity towards that mass. So this is sort of a warp drive
| mechanism, although I don't know how you'd confine it, and the
| energies involved would be planet-scale to achieve 1 g of
| acceleration like near the Earth.
|
| Also if someone made a closed loop where electron-positron
| pairs were sent one way, then their energy was used to create
| quark-antiquark pairs sent the other way, there might be a 1%
| imbalance in mass due to the Higgs mechanism, which would add
| momentum to the loop opposite the direction of the heavier
| stream. Although due to conservation of momentum, I suspect
| that this wouldn't actually happen, because any momentum above
| light pressure should get lost to heat/entropy/etc. But it
| would be a fun experiment to try. The same experiment would
| also work just sending light energy photons one way and matter-
| antimatter pairs back the other way, but I've never seen a
| proof as to why this would or wouldn't beat light pressure.
| This would be a reactionless rocket, not a warp drive.
|
| If there's a gravity field like a gluon field, just with
| slightly different rules, then I don't see why it couldn't be
| modulated. In fact, I think that the dark matter strands
| connecting galaxies are densities where perhaps something like
| slowed neutrinos or axions collect and slow time. They could
| even be places where gravity "flows" along eddies left over
| from the Big Bang, although this seems strange to us because
| gravity normally only flows into gravity wells. There's also
| currently no explanation for the Hubble constant in the
| expansion of the universe, so perhaps something is creating
| space over time. So I don't see why space couldn't be created
| behind a craft to push it forward. We just don't know how.
|
| There are so many unexplored interactions like this, that I
| don't think any physicist can confidently say that warp drives,
| reactionless rockets and folding space are impossible. Which
| means that I give it 50/50 odds that some kind of sci-fi space
| engine will be invented within the next few decades, probably
| starting with a reactionless drive like the EmDrive, which (if
| it works) uses resonance to time the interaction of microwaves
| with the rebound of atoms in an asymmetric field, similar to
| the Biefeld-Brown effect explored by Thomas Townsend Brown in
| the 1920s, which was later found to just be an
| electrohydrodynamic (EHD) effect:
|
| https://en.wikipedia.org/wiki/Reactionless_drive
|
| https://en.wikipedia.org/wiki/EmDrive
|
| https://en.wikipedia.org/wiki/Biefeld-Brown_effect
|
| Unfortunately only physicists are privy to the mental
| associations which allow thought experiments like this.
| Textbooks leave us mainly theory and equations, not insights or
| abstractions. Physics formulas are like trying to understand
| the behavior of an app from its assembly language. So in a very
| real way, academic gatekeeping prevented almost everyone from
| contributing. For every divergent thinker like Einstein, there
| are 100 convergent thinkers who judge skeptically and crush
| ideas into oblivion.
|
| I'm just an armchair warrior full of derivative ideas who has
| never invented anything, who would love to run experiments like
| these. But just like you the reader, I'll spend the rest of my
| life making CRUD apps to make rent because billionaires have
| all the money, instead of getting to be like Dr. Gillian Taylor
| in Star Trek IV, suddenly able to explore every possibility
| under the freedom of UBI. That was a joke, but not really.
| AprilArcus wrote:
| >The Higgs mechanism affects electrons, not quarks
|
| It effects both. The LHC produces Higgs particles through the
| annihilation of top-antitop pairs, which works because the
| top quark couples strongly to the Higgs field.
| UncleSlacky wrote:
| You should join us at APEC: https://www.altpropulsion.com/
| colechristensen wrote:
| Are there theories? Yes!
|
| Do they require a long list of impossible things to work? Also
| yes!
|
| There are entirely valid solutions in general relativity which
| allow for an object in a pocket of spacetime with other
| spacetime warped around it in such a way that, more or less,
| space is moving but not the object.
|
| However _achieving_ the arrangement of spacetime to make this
| happens requires many things which are impossible, aren 't
| known to exist, or require something like all the energy in the
| Universe to achieve.
|
| Also there are no valid known solutions that transition from
| normal space to this special spacetime arrangement, so it could
| only exist if it always existed.
|
| So it comes down to: we're pretty sure such things are not
| actually possible but we know where to look and what problems
| to solve if it were. Occasionally we see a paper which removes
| some of the impossible things from the list.
|
| It's one of those "unlikely but maybe someday" kinds of things.
| tnowacki wrote:
| PBS Spacetime did an episode on this just recently!
|
| https://www.youtube.com/watch?v=QMFLcmsjOBg
|
| In the video, they mention that the models we have for FTL (by
| bending spacetime) wouldn't generate ripples in this way. We
| could however detect ripples from a really massive ship
| accelerating really really quickly.
| pedro_hab wrote:
| I just posted another PBS Spacetime video answering another
| question.
|
| I feel like a quasi-expert on these subjects because of it.
| lol
| anonym29 wrote:
| Thank you for sharing this :D
| psychphysic wrote:
| The energies involved are stupendous.
|
| It would be like being hit by a tsunami and wondering if a
| cruise ship caused it.
|
| But on a galactic scale.
| edgyquant wrote:
| Are not the energies required for a theoretical warp drive
| also stupendous?
| psychphysic wrote:
| Quite likely. I'd say it's at present beyond our
| comprehension.
|
| A big enough cruise ship, accelerating fast enough, could
| cause a tsunami but it'd be far beyond anything we could
| reason about sensibly.
|
| If they control that much energy, maybe it's waves from
| their equivalent of a microwave oven.
| QuadmasterXLII wrote:
| We're working with numbers with 30s and 40s in the
| exponent- plenty of room for stupendous things to be
| stupendously different
| akiselev wrote:
| The original Alcubierre paper came up with the energy
| equivalent of the entire mass of Jupiter, if I remember
| correctly.
| Retric wrote:
| Which as shocking as it sounds still small compared to
| large gravitational waves.
|
| The merger of black holes radiate something like 10% of
| that mass as gravitational waves. Start talking 2.5+
| million solar masses black holes and ~500,000 solar mass
| worth of energy in gravitational waves seems plausible
| though obviously rare.
| sidcool wrote:
| Theoretically it's impossible to travel through space at or
| more than speed of light. But space itself can move faster than
| light speed, and a warp drive would help something similar that
| you mentioned. That is possible theoretically. But the amount
| of energy or mass it needs is very high and no current
| technology (or in foreseeable future) can achieve it. So FTL
| remains a dream.
|
| My hunch based on nothing is that we will achieve FTL no
| earlier than 2250.
| automatic6131 wrote:
| It's fun to imagine the species in x-hundred years. My
| college physics professor once told us that in 500 years,
| physics professors will still teach Maxwell's equations in
| the format he was showing us. And honestly, I think he's
| right.
|
| Somethings we will do the same way for hundreds of years,
| like the wheelbarrow will still exist in 500 years as it has
| for likely the previous 5,000.
|
| Otoh, I doubt we will be going faster than light this
| millenium.
| nine_k wrote:
| We still teach the Newton laws that are comparably old. We
| also teach that they are approximate, but work flawlessly
| for household-scale speeds and masses.
| rkagerer wrote:
| Naw in another 500 they'll be antigravbarrows
| zzzeek wrote:
| there will be "physics professors" in 500 years, I see we
| are being optimistic about humanity
| SkyMarshal wrote:
| Yeah the only energy source that can produce enough energy to
| power an Alcubierre drive (warp drive) that humanity has ever
| even conceived is a matter-antimatter reactor. But we don't
| even fully understand matter, much less antimatter, and are
| pretty far from that. 2250 at the earliest is not an
| unreasonable estimate.
| ben_w wrote:
| Antimatter wouldn't be even close to powerful enough, and
| both matter and antimatter have got the wrong sign for
| every (or almost every, depending on which headline I
| trust) warp drive variation.
|
| As antimatter is as powerful as one can get, it not being
| powerful enough is a good reason to think it's not going to
| work.
| amelius wrote:
| How would someone _inside_ space make use of FTL movement of
| space itself?
| DexesTTP wrote:
| To simplify, the same way that a surfer on the sea can use
| the movement of the sea itself (waves) to surf!
|
| An Alcubierre drive (they're theoretical) would basically
| constantly compress the time curve of spacetime in front of
| the craft, allowing the craft to "ride" this compression as
| it moves forward, which means that the local speed of light
| of the craft is faster than the speed of light of an
| external observer. Note that the main issue we have is to
| find something that can compress space, and then to have it
| have enough energy for it not to be trivial (because 110%
| of the speed of light, while technically FTL speed, is
| still very slow for interstellar travel). And of course,
| while the existence of something that does this spacetime
| curve compression fits the math we have, we've yet to find
| a material or technique that actually does so.
| amelius wrote:
| Sorry, I don't get it.
|
| Imagine you are on a rubber ruler. You can move at most 1
| mark per second on the ruler. This is true regardless of
| how much the ruler is stretched or compressed.
|
| So to move from mark 1 to mark 100 will always take the
| same time at top speed, regardless of any
| stretching/compression.
| cglan wrote:
| Don't take my word at all, but I think in your analogy
| you can't imagine it as you're only allowed to go 1 ruler
| tick a second, but imagine you can only move 1mm per
| second. If you compress the rubber ruler you traverse
| more ruler per second than before while still going the
| Same speed
| mjburgess wrote:
| 'space' isnt a substance that can move at any speed.
|
| What's meant by the claim that 'space moves faster than
| light' is that extremely distant objects are moving away from
| each other, relative to each other, 'faster than light' --
| which is permitted, so long as that distance can _never_ be
| bridged by light.
|
| The claim amounts to, in other words, that the universe is so
| large that we can compare objects at distances greater than
| those light could travel between them, and if we do that,
| they travel faster than light.
|
| This is an "illusion in measurement" more than anything else.
| Nothing is travelling faster than light.
| TechBro8615 wrote:
| Here's an article that explains this distinction in detail:
| https://medium.com/the-infinite-universe/why-galaxies-
| recedi...
| gpderetta wrote:
| > space moves faster than light
|
| I think this simply refers to the metric expansion of the
| universe [1]. While nothing actually travels FTL, the
| distance between some objects really expands faster than
| FTL and is not an illusion.
|
| [1]
| https://en.m.wikipedia.org/wiki/Expansion_of_the_universe
| mjburgess wrote:
| The illusion is the impression that something moves
| faster than light.
|
| 'distances' arent literally 'expanding' -- this metaphor
| of expansion describes a shift in the matter distribution
| of the universe over time which _seems_ like an expansion
| of an underlying substance 'space'.
|
| This is the illusion. The metric _is just_ that matter
| distribution. And the _fact_ of its changing we call
| 'expansion'.
|
| This metaphorical, substantival language, creates a lay
| impression that some physical object moves faster than
| light.
| rkagerer wrote:
| So it can't fold on itself and form a wormhole that bridges
| light?
| SketchySeaBeast wrote:
| > My hunch based on nothing is that we will achieve FTL no
| earlier than 2250.
|
| I see you too have been forced to give PMs estimates when you
| don't understand the problem.
| actionfromafar wrote:
| And without a later bound, so a very reasonable estimation
| too.
| anonym29 wrote:
| "Did I say 2250? Sorry, I meant 22,500. Yeah, you know,
| with all the extra time needed to compile and
| everything..."
| devX3 wrote:
| "Not sure about this one but sounds to me like an 8"
| time0ut wrote:
| Is this before or after the estimate is rejected and
| arbitrarily cut in half?
| davrosthedalek wrote:
| Yeah, we really should have had FTL by 1125! You'll have
| to work overtime for the next sprint.
| sidcool wrote:
| Lol I feel you.
| wes-k wrote:
| Futurama had an episode about that!
|
| "I understand how the engines work now. It came to me in a
| dream. The engines don't move the ship at all. The ship stays
| where it is and the engines move the universe around it."
| --Cubert Farnsworth
|
| https://futurama.fandom.com/wiki/Dark_Matter_Engine
|
| https://www.youtube.com/watch?v=1RtMMupdOC4
| SketchySeaBeast wrote:
| > Whereas the original discovery spotted waves originating from
| the collision and merger of two star-sized black holes, the
| most likely source of the latest finding is the combined signal
| from many pairs of much larger black holes -- millions or even
| billions of times the mass of the Sun -- slowly orbiting each
| other in the hearts of distant galaxies. These waves are
| thousands of times stronger and longer than those found in
| 2015, with wavelengths of up to tens of light years. By
| contrast, the ripples detected since 2015 using a technique
| called interferometry are just tens or hundreds of kilometres
| long.
|
| Seems like they probably know where these are coming from. I
| imagine, like your boat analogy, that we can observe massive
| natural oceans swells and wouldn't notice the wake of a boat as
| it moves across the ocean.
| dylan604 wrote:
| >with wavelengths of up to tens of light years.
|
| from a layman's perspective, this sounds crazy cool that they
| were able to "see" this in the data. seems like one of those
| things that would be easy to miss from being scoped in and
| only discoverable after zooming back out. waaaaay out.
| RhodesianHunter wrote:
| We're using what we can measure/see of distant stars for
| these measurements, so it is indeed zoomed way out.
| chorsestudios wrote:
| These 'monster' gravitational waves have supposedly been
| 'spotted' by calculating disparities in pulsar timings. The
| article didn't have as much information as I was hoping - were
| any of these waves detected/confirmed by LIGO?
| gaoshan wrote:
| It's my understanding that LIGO is too small to be able to
| detect such waves, hence the use of pulsar timings.
| flqn wrote:
| [dead]
| bowsamic wrote:
| As others pointed out, LIGO is too small compared to the
| wavelength of the GW, but even if LIGO was extremely long, the
| technical noises (seismic, control system noises, gravity
| gradient noise, etc.) at such low frequencies are extremely
| high for ground-based detectors
| seventhson wrote:
| The wavelengths are too large for LIGO. The wave measurements
| were correlated with data collected from many pulsars.
|
| https://arstechnica.com/science/2023/06/nanograv-picks-up-si...
| chorsestudios wrote:
| Thank you for this article link, I found it much more
| informative than the parent article. The video at the bottom
| was well made.
| thallada wrote:
| How are we sure that pulsars are 100% consistent? How do we know
| the timing discrepancies are due to gravitational waves and not
| just tiny wobbles in the pulsar itself?
| pertymcpert wrote:
| FTA: they don't just use a single pulsar.
| TechBro8615 wrote:
| I understand the gist of this research: pulsars emit radio waves
| at regular frequencies, so by monitoring radio waves received
| from pulsars in the sphere surrounding us, we can measure
| correlated anomalies in their frequencies and infer that they
| were caused by large gravitational waves that effectively changed
| the shape of the transmission medium. That makes sense.
|
| But this is not measuring the gravitational wave itself. It's
| measuring the change in trajectory of the radio signals that are
| "riding" the wave. In the ocean analogy, it would be as if we
| were surrounded by a circle of floating turrets that each emitted
| floating darts at regular intervals in all directions. Then we
| would measure the time it took the darts to reach us, and from
| that we could infer the size of the waves the darts encountered
| along the way. But we never actually see the waves, only the
| darts.
|
| So my question is: how can we tell the difference between one
| really big wave, and many really small waves that would sum to
| the same effect? In other words, we know there is some
| waveform(s) that changed the velocity vector of the radio signal.
| But if there are multiple arrangements of waves that would
| produce the same change in signal, how do we pick the right
| arrangement?
| rkagerer wrote:
| _The timing of a single pulsar would not be reliable enough to
| detect gravitational waves. Instead, each collaboration
| monitors an array of dozens. As a result, they have found a
| signature called the Hellings-Downs curve, which predicts how,
| in the presence of gravitational waves coming from all possible
| directions, the correlation between pairs of pulsars varies as
| a function of their separation in the sky._
|
| Not sure it answers your question but my impression is they
| simulate results of all possible effects and then see which
| one(s) the data correlates with. So if there are multiple
| causes that could produce identical effects then I doubt they
| could distinguish between them.
| TechBro8615 wrote:
| That makes intuitive sense. So I guess the more pulsars we
| measure, the more accurately we can disambiguate between
| different possible waveforms.
|
| It reminds me of EEG (brain wave) measurement: a hairnet with
| 256 electrodes will have higher resolution than one with 128
| electrodes (ignoring all the issues with interference of the
| skull).
| [deleted]
| theonlybutlet wrote:
| A great non-technical explanation
|
| https://www.reddit.com/r/space/comments/14lpjnx/scientists_h...
| photochemsyn wrote:
| Astronomy Picture of the Day has a nice graphic of this:
|
| https://apod.nasa.gov/apod/ap230629.html
|
| Spacetime is incredibly resistant to deformation, hence the tiny
| displacements and the need for long-baseline laser interferometry
| to detect these waves.
|
| https://blogs.scientificamerican.com/life-unbounded/just-how...
| MeteorMarc wrote:
| See also Quanta Magazine: https://www.quantamagazine.org/an-
| enormous-gravity-hum-moves...
| 8lahaj wrote:
| I was hoping they were gravitational waves from a monster, darn
| title getting my hopes up.
| peterlk wrote:
| Hey dang! There are a ton of these threads showing up right now.
| Any chance we could get them merged or pinned in a comment?
| satellite2 wrote:
| It was yo mama getting a sandwich.
| m3kw9 wrote:
| This joke is probably the most this discovery will affect me in
| my lifetime
| FollowingTheDao wrote:
| If the distance between the earth and these stars can change,
| then we can change the distance between the earth and these
| stars.
| pedro_hab wrote:
| Mathematically yes, actually probably not.
|
| I'd suggest looking into Alcubierre Warp Drive, cool story on
| why the guy came up with it and shows how to wrap spacetime
| around a spaceship to make it go faster than light.
|
| The ship wouldn't go faster than light because it wouldn't move
| at all, the spacetime around it would.
|
| The channel below has quite a few other videos on the subject.
| I love it.
|
| https://www.youtube.com/watch?v=94ed4v_T6YM
___________________________________________________________________
(page generated 2023-06-29 23:00 UTC)