[HN Gopher] The Nanohertz Gravitational-Wave Detection Explained
___________________________________________________________________
The Nanohertz Gravitational-Wave Detection Explained
Author : raattgift
Score : 147 points
Date : 2023-08-12 07:44 UTC (1 days ago)
(HTM) web link (physics.aps.org)
(TXT) w3m dump (physics.aps.org)
| chrisweekly wrote:
| Fantastic explainer. The illustrations are both charming and
| helpful.
|
| It's reminiscent of other excellent multi-modal "explainers",
| e.g. Julia Evans (https://jvns.ca), https://betterexplained.com,
| Randall Munroe (https://xkcd.com), http://inception-explained.com
| ...
|
| And on this topic of mixing text with imagery, I highly recommend
| Nick Sousanis's amazing "Unflattening"
| (https://archive.org/details/unflattening0000sous)
| mariushop wrote:
| outstanding delivery
| dilaver wrote:
| "First, [gravitational waves] have really long wavelengths."
|
| Why?
| [deleted]
| speff wrote:
| l = v / f
|
| The velocity (v) is the speed of light for gravitational waves,
| which is already a really big number. If the frequency (f) is
| based on the period of the black holes which circle around each
| other, then I assume one rotation happens over a long period of
| time. Long period -> low frequency -> small denominator which
| makes the wavelength (l) even longer.
| geon wrote:
| Some rotations are fast. LIGO detects gravitational waves at
| 200-10000 Hz.
|
| That's kind of fast. Just imagine a couple of black holes
| orbiting each other that fast.
| walnutclosefarm wrote:
| But the higher frequency waves detected by LIGO are not
| caused by two bodies orbiting their common center of mass
| at a distance, but rather by two much smaller masses - a
| few to a few tens times the mass of our sun - than the ones
| described in the cartoon. These masses orbits have decayed
| and they are spiraling into each other merging. The Short
| waves we detect only occur in the final bit of the spiral
| and merge - we see only the final milliseconds of the
| merger, and just a few wave crests.
| ooterness wrote:
| The idea of something with ten solar masses moving so
| fast still terrifies me. It's just mind-boggling to think
| about something at that scale completing an orbit in the
| blink of an eye.
|
| I am reminded of Randall Monroe taking about the sheer
| energy of a supernova:
|
| > Which of the following would be brighter, in terms of
| the amount of energy delivered to your retina:
|
| > A supernova, seen from as far away as the Sun is from
| the Earth, or
|
| > The detonation of a hydrogen bomb pressed against your
| eyeball?
|
| > Applying the physicist rule of thumb suggests that the
| supernova is brighter. And indeed, it is ... by nine
| orders of magnitude.
|
| https://what-if.xkcd.com/73/
| defrost wrote:
| Not a bad question at all really. In
| principle, gravitational waves could exist at any frequency.
| The speed, wavelength, and frequency of a gravitational wave
| are related by the equation c = lf, just like the equation for
| a light wave. For example, the animations shown
| here oscillate roughly once every two seconds. This would
| correspond to a frequency of 0.5 Hz, and a wavelength of about
| 600 000 km, or 47 times the diameter of the Earth.
|
| https://en.wikipedia.org/wiki/Gravitational_wave
|
| To get a short wavelength requires a high frequency, to get an
| observable gravity wave requires a very large mass.
|
| We haven't yet seen a Super Massive Black Hole orbiting with
| (say) an Mercury orbit radius at a thousand times a second.
|
| There is another quote from that wikipedia article:
| Stephen Hawking and Werner Israel list different frequency
| bands for gravitational waves that could plausibly be detected,
| ranging from 10^-7 Hz (very slow) up to 10^11 Hz (very very
| fast).
|
| The faster the wave the shorter the wavelength and the greater
| the difficulty in detection (as the amplitude likely lessens
| and with distance falls below our current direct means).
|
| You'd have to chase the Hawking-Israel paper for their thoughts
| on short wavelength high frequency gravitational wave sources
| and how they believe they might plausibly be detected .. I
| anticipate some devil in the detail.
|
| http://library.lol/main/F92F35CD83F13A6021FC2385BBA171B0
|
| ( _perhaps wikipedia misquoted that high frequency_ )
| tzs wrote:
| >> The speed, wavelength, and frequency of a gravitational
| wave are related by the equation c = lf, just like the
| equation for a light wave [...]
|
| > To get a short wavelength requires a high frequency
|
| For light that is assuming a vacuum. The more general
| equation is c' = lf where c' is the speed of light in the
| medium the light is traveling through. c' <= c. Hence, for
| light, you can get a shorter wavelength without having to
| raise the frequency if you work with the light in a medium
| with a lower c'.
|
| Is there anything similar with gravitational waves?
| dotnet00 wrote:
| Probably not because the medium is the same spacetime
| everywhere and there isn't anything we know of that slows
| down or blocks gravity.
| 9dev wrote:
| Now wouldn't that make for a neat science fiction plot
| setup.
| bloopernova wrote:
| What an utterly delightful and eminently understandable
| presentation of a complex subject!
|
| I know that it would take decades or centuries to gather enough
| data to make discoveries. But I wonder if one day a scientist
| will be looking at these results and see something that makes
| them think _" that's odd..."_ And suddenly our understanding of
| the universe is turned upside down yet again!
| jack-bodine wrote:
| If gravitational waves can be detected by looking at pulsars,
| then what is the purpose of LIGO and ground-based gravitational
| wave observatories? Is there any difference in the waves detected
| by LIGO and those from observing pulars?
| venusenvy47 wrote:
| Those two methods observe waves of much different
| wavelength/frequency, and the mechanisms that create such
| different wavelengths are assumed to be different. So
| cosmologists are studying different things by looking at
| different wavelengths.
| Cerium wrote:
| For one we didn't have a way to measure the speed of
| gravitational waves before [1].
|
| [1] https://physics.stackexchange.com/questions/622729/did-
| ligo-...
| magicalhippo wrote:
| As mentioned in the sibling comment, different frequencies
| means probing different things.
|
| I found this talk[1] rather nice to explain the NANOGrav
| experiment, and at 6:45 there's a very nice plot that shows
| where NANOGrav fits in compared to the other gravitational wave
| experiments and which type of sources the various experiments
| can probe.
|
| [1]: https://pirsa.org/20100068
| _Microft wrote:
| LIGO detects gravitational waves in the frequency range of
| hundreds of Hertz (10^2 Hz), which are produced in the last
| moments of the in-spiraling of merging neutron stars or
| blackholes.
|
| Nanohertz are 10^-9 Hz which relate to a rotational period of
| decades of years.
|
| https://en.wikipedia.org/wiki/Gravitational-wave_astronomy
| zackees wrote:
| [dead]
| aaron695 wrote:
| Very well done
|
| So pulsars are not regular at the tick level as they can spin
| faster and slower but have 100 nanoseconds accuracy (Way worse
| than an atomic clock) when doing an average profile -
|
| https://astronomy.swin.edu.au/cosmos/p/Pulsar+Timing
|
| This is a cool line -
|
| "In effect this allows changes in the relative distance between
| the pulsar and Earth to be computed to an accuracy of 30 metres
| (~100 feet)"
| beders wrote:
| I still don't understand what the carrier of gravitational waves
| is.
|
| How are the waves propagated? Space is not an elastic fabric or
| water where water molecules transfer potential energy.
|
| Aren't gravitational waves a hint that spacetime is not
| fundamental, but an emergent property of something else?
| pfdietz wrote:
| Gravitational waves follow because space time and gravity are
| described by equations that admit waves as solutions. Thinking
| about what space time is "made of" is contrary to what
| mathematical physics is about. This was a realization with
| electromagnetism: early attempts tried to interpret the EM
| field as something mechanical, but it was eventually realized
| that math making predictions was what one should focus on, and
| not try to create some sort of more comfortable interpretation
| in terms of materials or gears or something.
| beders wrote:
| Not looking for a comfortable interpretation.
|
| The confusing thing here is that spacetime warps in the
| presence of mass.
|
| But no mass is transmitted in a gravity wave.
|
| So how do the waves travel?
| raattgift wrote:
| The "carrier" is the metric tensor. Roughly, at every
| infinitesimal point in a spacetime like ours there is a
| tensor value which encodes the distance from that point to
| its neighbours along all four orthogonal dimensions. "The
| metric" of a spacetime is usually taken to mean an
| integration of the metric tensor values at each point along
| some set of points.
|
| Let's draw a supermassive black hole binary. In the rough
| <https://en.wikipedia.org/wiki/Minkowski_diagram>
| schematics below, each equal-mass black hole is an O and we
| show an observer ("y" for "you") who sees the binary's
| circular orbit edge-on. The y axis is time, oriented with
| the future towards the start of this comment. The x axis is
| _one_ spatial dimension. The diagram-angle from the binary
| to the observer is meant to be 45 degrees, representing a
| null aka lightlike separation.
|
| Let's consider two snapshots of the binary's mutual orbit:
|
| 1. y OO
|
| 2. y 8
|
| The "OO" vs "8" is an abuse of notation; read the OO as the
| orientation where "y" sees one eclipsing the other, and the
| 8 as the orientation where y sees the two distinctly.
|
| "y" is at a spatial remove. But "y" is drawn a couple of
| lines up because of the finite propagation speed c. The
| orientation OO...y or 8...y is not felt by y at the time,
| but later. A _null curve_ connects the binary "at the
| time" with the "but it's felt later at" observer.
|
| We then integrate all the infinitesimal points along the
| null curve between y and the centre of mass ("COM") of the
| binary and find that the null curve in diagram 1 is
| shorter* than the null curve in diagram 2, because there is
| more mass between y and the COM in the first diagram.
|
| Above, "y" is pointlike and y would free-fall towards the
| centre of mass. In y's proper time the free-fall would be
| faster during orientation 1 than during orientation 2.
| Since the binary is in a mutual orbit, there is a speeding-
| up and slowing-down of the free-fall measured by y in y's
| proper time.
|
| Now we consider some extended-body stuff. Things above and
| below the SMBHB orbital plane will tend to fall towards the
| plane. If "y" has some height and the binary isn't treated
| as practically pointlike, the various parts of now line-
| like y want to free-fall to the black hole closest to them.
| In orientation OO the top and bottom of the y will squash
| inwards. In orientation 8 the top and bottom of the y will
| stretch up and down. Interpolate through the orientations
| between OO and 8. The "height" here is along one axis in
| the plane perpendicular to the binary's orbital plane; a
| "y" with "width" would be extended in that plane in the
| orthogonal axis. In orientation 8 there is a stretch along
| that axis compared to OO.
|
| The magic is all in the orientation of the
| observer/gravitational-wave-detector to the two mutually-
| orbiting bodies, and the integration of the metric tensor
| between every part of the observer and every part of the
| binary. This is ... generally analyitically intractable, so
| one does numerical methods ("NR", numerical relativity) or
| one works in the approximation known as linearized gravity.
| Most non-specialists, and even many members of the
| LIGO/Virgo/KAGRA et al collaborations who have encountered
| the mathematics of gravitational waves did so in the
| context of linearized gravity, and only sometimes encounter
| NR and (assuming they aren't writing the NR codes) even
| then don't think too hard about how the "block universe"
| spacetime described by the field equations are split into
| 1+3 dimensions.
|
| So, in summary, the crucial thing is the length of the null
| curves connecting every part of the y with every part of
| the binary. The periodic rotation of the binary causes the
| lengths and angles of those curves to oscillate, shortening
| and spreading or lengthening and narrowing. Looking closely
| at a large number of such null curves at once, one can
| successfully model them in bulk as obeying the massless
| wave equation, even though nothing actually propagates (the
| whole spacetime-filling metric tensor field is solved all
| at once; it only "evolves" or "propagates" when we think in
| terms of the initial values formalism or a 1+3 formalism).
|
| Finally, here is an pulsar-timing-array astrophysicist
| (<https://www.aoc.nrao.edu/~tcohen/>) doing an intepretive
| dance (after a bit of explanation):
|
| https://www.youtube.com/watch?v=uH91gSI4ELs&t=0s
|
| (there are further links in the video description).
|
| - --
|
| * someone is bound to pounce on "shorter" given that the
| interval of a null curve is always 0 (thus the name and
| from that flows crazy wrong ideas about "photons experience
| no time"), but we can use the very much timelike worldlines
| of the (components of the) binary and observer and retarded
| time to construct a notion of the length of the null curves
| connecting them. Eric Poisson has gory technical details
| for anyone actually interested, in the context of the self-
| force formalism which is more suitable than linearized
| gravity for high-mass-ratio binaries, especially as the
| binary hardens.
|
| PS. "Fabric of spacetime" is a cliche that causes me to
| grit my teeth. I grit harder when someone who has worked
| with the Einstein Field Equations writes or says it. It's
| not only overused, it's misleading to non-experts (and
| sometimes even to experts). Spacetime is not a substance.
| It does not "distort", a solution to the EFEs is what it
| is; "distortions" imply comparing the solution to some
| alternative that one likes better (e.g. the spacetime
| without any matter, despite any non-zero stress-energy in
| the solution). It is at best a mathematical container for
| coincidences ("events" where two objects at the same point
| in spacetime can have e.g. their velocities usefully
| compared) and interactions like collisions, scatterings or
| the formation of molecular bonds.
|
| Unfortunately, colloquially or informally it's very hard to
| talk about gravitational waves without talking about the
| stretching and squashing of some region of _space_ (not
| spacetime), because people (even experts) internalize
| "fabric" and similar metaphors, because formalisms and
| approximations to general relativity used in studying
| inspiralling-binary sources of gravitational waves split
| spacetime into space + time, and don't invite the
| consideration of solutions of the geodesic equation (for
| tractability reasons).
| ajkjk wrote:
| A simple point that might clear up the confusion is: note
| that the electromagnetic field warps in the presence of
| charge, yet no charge is transmitted in an electromagnetic
| wave either.
|
| Explanation: the wave is the warping itself. If a charge or
| mass exists, the field is (permanently) warped to contain
| it, in a way that doesn't propagate (although it can be
| mathematically described as an exchange of particles). But
| if the charge or mass _accelerates_ , then the warping
| changes, and the information about that acceleration
| propagates away. Basically the wave is other charges/masses
| 'finding out' about a distant change in the velocity of a
| charge.
| snarkconjecture wrote:
| There are many ways for spacetime to warp, which can be put
| into two categories. The simpler kind, Ricci curvature, is
| the only kind of curvature in <4 dimensions and is produced
| by mass-energy, momentum, pressure, and shear stress,
| according to general relativity. The other kind, Weyl
| curvature, only exists in 4 or more spacetime dimensions
| and can exist in a vacuum.
|
| Gravitational waves are Weyl-curvature distortions of
| spacetime that propagate in a vacuum according to general
| relativity.
|
| (Also, gravitational waves _do_ carry a little bit of
| energy, so they cause a small amount of Ricci curvature,
| but this is a secondary effect.)
| raattgift wrote:
| I'm not sure why you want to draw attention to non-
| Lorentzian spacetimes in this context.
|
| > ... that propagate ...
|
| "propagate".
|
| That requires a decomposition of spacetime into
| space+time, and of course the decomposition of the
| Riemann curvature tensor, the setting of a background
| value for the Weyl tensor, and the use of perturbation
| theory.
|
| But if you're going down that path, why not use the
| metric tensor? g_munu = eta_munu + h_munu + h.o.t. is
| standard in post-Newtonian expansion approximations, and
| in particular
| https://en.wikipedia.org/wiki/Linearized_gravity (which
| doesn't track the higher-order terms).
|
| The Weyl curvature tensor C_abcd is useful in
| understanding that in a spherical region of space (not
| spacetime, so really we're in the land of extracting
| 3-Cotton-York C_ab) where a GW is incident suffers not
| from a volume deficit but from an ellipsoidal stretch-
| squash. But conceptual understanding of and calculation
| are... well, not really on speaking terms. Even theorists
| who take the full covariant theory seriously will
| decompose further, into e.g. an electrical and magnetic
| part, and add further structure to match the worldlines
| to the Raychaudhuri equation in shear and vorticity.
|
| > they cause a small amount of Ricci curvature
|
| ???
|
| If nothing else, I think you'd need to choose between
| explaining this or explaining why "Ricci curvature is
| produced by [matter but] Weyl curvature ... can exist in
| a vacuum" (or choose neither).
|
| The sticky bead apparatus is a breaking of the T_munu = 0
| vacuum condition.
| turndown wrote:
| >Space is not an elastic fabric
|
| Space is an elastic fabric, if Einstein's explanation of a
| spacetime dimension is correct. This would mean that gravity is
| the warping and stretching of this dimension.
|
| If it is not then the other solution is in quantum field
| theory, which would state that gravity is a field with a
| messenger particle, the graviton, in which case we are just
| detecting a group of gravitons as they pass by.
| beders wrote:
| The confusing thing here is that space time warps in the
| presence of mass.
|
| But no mass is transmitted in a gravity wave.
|
| So what is the fabric made of?
| semi-extrinsic wrote:
| No mass is transmitted in a gravity wave, just the same way
| that no mass is transmitted when the Sun's gravity pulls on
| the Earth.
|
| And just the same way that no cell phone towers are
| transmitted when you use your phone.
|
| It's entirely correct to say that the electromagnetic field
| warps in the presence of moving charges, sending out EM
| waves like in wireless communication. And the EM waves can
| travel without any medium, even in hard vacuum. What is the
| electromagnetic field made of?
| ianburrell wrote:
| Space is the fabric.
|
| Moving electric charges produce electromagnetic waves
| (light). No charge is transmitted with light, but it can
| cause charges to move. Space is also the fabric for
| electromagnetic waves.
| vl wrote:
| We use words to describe abstractions which try to model
| reality as we observe it.
|
| In case of _" space time warps in the presence of mass"_
| it's important to understand that this is just a model.
| Another way to explain it that there is no space-time per
| se, only masses (with special case of photon et al) and
| masses interact. What we mean by _space-time_ is that if
| there was a tiny mass in this given point, it would
| experience given force. Or, again, modeled differently,
| travel along given path.
|
| But if there is nothing to interact with, there is no
| "space-time" per se in this point, after all, it is an
| abstraction to describe interactions.
|
| So, to sum up, masses interact, space-time is an
| abstraction to conveniently describe how they interact.
| Gravitational wave is two massive masses rotating and
| shaking third small mass as a result of distance changes.
| ajuc wrote:
| When you move mass around - the spacetime curvature
| responds with light-speed delay. I thought gravitational
| waves are just that - delayed change of curvature of space-
| time in response to the mass distribution changing in one
| place?
| consumer451 wrote:
| Along similar lines, I wonder if gravitational waves can be
| focused.
| raattgift wrote:
| Yes, foreground massive objects can magnify gravitational
| waves emitted by a background source (the foreground objects
| can also induce a beating pattern on the background waves, or
| split the background waves into multiple wavefronts). The
| "foreground" emitter in a massive cluster of thousands of
| galaxies can be deep inside the cluster, and those are
| probably the gravitationally-lensed gravitational-wave
| sources we will identify first [more below at [1]].
|
| A good starting point for a non-specialist is probably
| <https://astrobites.org/2021/05/25/gw-lensing/>.
|
| (You could compare a specialist paper <https://www.aanda.org/
| articles/aa/full_html/2020/11/aa38730-...> which describes
| the lensingGW software package
| https://gpagano.gitlab.io/lensinggw/ ).
|
| - -- [1] When a candidate gravitational wave (GW) detection
| is found at LIGO et al., an electromagnetic (EM) detection
| may follow. Neutron stars collide explosively, generating a
| huge burst of very high-frequency (gamma ray) EM. Black holes
| don't explode but their accretion structures can interact
| very strongly producing high-frequency (X-ray) EM. These
| bright high-frequency EM flashes decay into a glow that can
| last for many days after a merger. Consequently they are
| looked for, as part of the overall project of multi-messenger
| astronomy (where EM and GW are two "messengers"; neutrinos
| and so forth make it "multi-").
|
| Close to the source, peak EM tends to lag a bit behind the
| merger, mostly because of how the EM is produced at some
| distance outside merged black holes. Additionally, EM
| scatters/refracts/etc with dust and gas that one finds deep
| inside galaxies, which are more transparent to GW than to EM.
| Consequently in general GWs will be detected before EMs from
| the same source. Candidate GW detections provoke searches
| across the electromagnetic spectrum, since EM may arrive
| hours or even days later.
|
| GW from an inspiral and merger in general have much much
| longer wavelengths than the X-rays from the gas around the
| merging objects. Black holes merge roughly when their
| horizons touch, and horizon size is proportional to mass. GW
| wavelength is proportional to the orbital diameter, which
| must be larger than the sum of horizon radiuses, so
| supermassive black hole binary (SMBHB) mergers have much
| longer-wavelength GW than stellar-mass black hole binary
| mergers.
|
| The relatively short wavelengths of electromagnetic radiation
| will follow the laws of geometrical optics and in particular
| will experience Shapiro delay. GW wavelengths, being much
| longer, will not. This explains a further delay on EM from
| SMBHB mergers that are deep within a massive galaxy or
| cluster of galaxies.
|
| Foreground lenses (e.g. massive galaxies that the source does
| not live within) will further increase the delay suffered by
| EM produced by the merger that sourced the GW. Moreover,
| since foreground lenses can split the wavefronts of the GW
| and EM, there may be ~four detections, at different times, of
| the same merger in GW and EM. Alternatively, the foreground
| lens can magnify the source merger, but with the delays
| described above and geometric optics vs not, the focal point
| for GW and for EM will differ.
|
| All of this is active research, close to cutting edge, so
| sadly is mostly to be found via a literature search on
| Einstein lensing of gravitational waves. AFAICT most "pop
| sci" attempts to summarize some of the academic literature is
| simply _awful_. Hopefully I 'm merely just _not great_ above.
| :- /
| pfdietz wrote:
| They should be focusable by objects with mass, just as light
| is. A difference is that gravitational waves could be focused
| by the Sun's core, and so would come to a focus closer to the
| Sun than light would be, which would have to be focused by
| the entire Sun (as any light passing through the Sun itself
| is blocked.)
| nwallin wrote:
| So forget about waves for a second. Forget about spacetime.
| Forget about general relativity.
|
| The force of gravity is correlated to the mass of the object
| divided by the square of the distance to the object. The higher
| the mass, the lower the curvature. The larger the distance, the
| lower the curvature.
|
| The force of gravity doesn't change instantaneously. The speed
| is the speed of light. If you move the Sun around using
| Sufficiently Advanced Technology, (SAT) the orbit of the Earth
| won't change until 8 minutes later, when the gravity has had
| time to propagate from the Sun to the Earth.
|
| So what does it feel like if you're on the Earth and somebody
| is using SAT to wiggle the Sun around? The force felt from the
| Sun will go up and down. The math that you would use to
| quantify this changing force is the same math you'd use to
| characterize sound waves, or water waves, or electromagnetic
| waves. So we call this changing tidal acceleration
| 'gravitational waves'.
|
| That's how gravitational waves were described in the 19th and
| early 20th century, before general relativity. Not long after
| Einstein published his paper on General Relativity, he adapted
| the same process via which gravitational waves propagate in
| Newtonian F = m1 m2 G / r^2 gravity to General
| Relativity/curvature of spacetime gravity. The math is nasty,
| and it wasn't really ironed out until decades later, but the
| theoretical predictions of what gravitational waves will look
| like in any given detector was worked out well before we
| actually went and detected them.
|
| > Space is not an elastic fabric
|
| It's a 4D/(3,1) Lorentzian manifold. If you want a better
| analogy than the stretched elastic fabric analogy,
| unfortunately the next step is to grab a textbook and slog
| through the math.
| [deleted]
| dotnet00 wrote:
| Space is like a fabric, it's a literal compression and
| stretching of space as the wave propagates through it. The
| distances physically get shorter and longer as the wave passes
| by.
|
| It doesn't necessarily mean that spacetime cannot be
| fundamental, as there's no rule that says that space itself
| must emerge from some sort of underlying medium (eg the way
| fabric is made of threads).
| beders wrote:
| What is stretching/compressing it?
|
| Spacetime is warped by mass but no mass is transmitted. So
| what exactly is being carried along?
| pseudosudoer wrote:
| You're essentially asking the same question after it's been
| answered. Space-time is the carrier that acts like a
| fabric, and some large cosmological mass is distorting that
| fabric.
|
| The distortion of the fabric of space-time ripples
| throughout the entire universe at the speed of light, and
| spreads it's effect proportional to the inverse square
| related to distance.
| Sharlin wrote:
| The surface of water is warped by a rock thrown into a
| pond, but no rock is transmitted by the waves. The
| electromagnetic field is disturbed by accelerating charges
| but no charges are transmitted by the EM waves.
|
| In any case, mass is not the only thing that warps
| spacetime. The components of the stress-energy tensor [1]
| include energy (which includes mass) density, energy flux,
| momentum density, and momentum flux, the latter of which is
| composed of pressure and shear stress components.
|
| [1]
| https://en.wikipedia.org/wiki/Stress%E2%80%93energy_tensor
| dotnet00 wrote:
| Are you perhaps mis-applying how we say that photons carry
| the electromagnetic force?
|
| If so, for now we believe that gravity is simply the result
| of the warping of spacetime, with no known carrying
| particle. That's kind of the hurdle between uniting gravity
| with the quantum world.
|
| If say, it turns out that there is such a thing as a
| graviton - which carries gravity, then gravitational waves
| would be a bunch of gravitons.
| eis wrote:
| I don't think they are a hint that spacetime is not
| fundamental. But I do think spacetime has to be some kind of
| real physical reality.
|
| The modifications of spacetime that we see as effects of
| gravity are relative changes to our immediate surroundings or
| reference frame.
|
| Similarly how you can't tell who is actually stationary and who
| is moving when two objects are in freefall and all you can note
| is the relative speed between the two, it would be equally
| valid to say the objects inside spacetime are getting distorted
| relative to spacetime.
| jrmg wrote:
| I think you're looking for a 'ether'? Many people have asked
| similar questions about all sorts of physical phenomena over
| the years.
|
| https://en.wikipedia.org/wiki/Aether_theories
| whoknowsidont wrote:
| I've always wanted to ask this really dumb question, so I'll just
| ask it here (laugh or answer, your choice):
|
| Would there be gravitational waves so huge that we'd have trouble
| detecting them? As in, if there was a huge, massive gravitational
| wave and we were in the process of "riding" it how would we ever
| know or detect such a thing?
|
| Would this possibly (not plausibly) be an answer to some of the
| weirdness and contradictions we have observed in astronomy over
| the past few decades?
| krastanov wrote:
| You can see "sensitivity" plots for the various methods we have
| for observing gravitational waves. The horizontal axis is
| usually in terms of the frequency of the wave and the vertical
| axis is how sensitive we are to them. This article is about
| "nanohertz" waves, which means their wavelength is roughly
| 109sec x speed of light, i.e. 30ish light years. So, it seems
| we can detect really "big" gravitational waves. If you look
| through the aforementioned plots you will see that sensitivity
| drops off significantly for "more astronomical" wavelengths, so
| there are definitely scales at which we can not detect, as you
| suggested.
|
| There might be some interesting ways to introduce very long
| gravitational waves as solution of current discrepancies in our
| understanding of cosmology, but they are also probably
| introducing more discrepancies than they solve. I would guess
| people have considered these ideas, because "cosmological scale
| general relativity" is pretty heavily researched and
| mainstream.
| raattgift wrote:
| > so huge that we'd have trouble detecting them
|
| Gravitational waves can be arbitrarily long because patterns in
| the cosmic microwave background suggest the early universe had
| gravitational radiation with wavelengths of arbitrary size,
| including quite long waves. The metric expansion of space has
| stretched those "primordial gravitational waves" (PGW), so even
| short-wavelength-in-early-universe PGW can have periods of many
| years at about the present size of the universe.
|
| Additionally, most models of cosmic inflation stretch PGW, with
| some of the stretched wavelengths being made longer than the
| distance to the cosmic horizon (Initially, before inflation,
| PGWs can have practically unlimited length or can be almost
| arbitrarily short, with the shortest winding up not so short
| once inflation has ended), so at modern times the super-
| horizon-length primordial waves would have periods of many
| billions of years.
|
| Any masses in a ~binary orbit (stars to galaxies, down to
| grains of dust and up to collections of galaxies like in the
| local group, <https://en.wikipedia.org/wiki/Local_Group>, the
| "dumbbell" shape is relevant) generates gravitational waves
| with a frequency comparable to the orbital period. In our sky
| there are galaxies that appear to be in mutual orbits with
| orbital periods of many millions of years.
|
| The pulsar timing arrays in the cartoons linked at the top of
| the page look for gravitational waves with periods of months to
| years. It takes several orbital periods to be statistically
| comfortable that a gravitational wave with that period
| (nanohertz frequency) has been detected. Those are probably
| generated by supermassive black holes in hard mutual orbits. If
| we soften such an orbit, increasing the semimajor axis (or
| radius for a circular orbit), the orbital period grows, so we
| have to watch the array of pulsars longer (and be more wary of
| so-called red noise and other long-term signal contamination).
|
| > some of the weirdness and contradictions we have observed in
| astronomy
|
| Unlikely. Do you have any particular weirdness and/or
| contradiction in mind?
| whoknowsidont wrote:
| Thank you for this incredibly informative and detailed
| answer! It's going to take a few passes (and following some
| links) to put everything together.
|
| >Do you have any particular weirdness and/or contradiction in
| mind?
|
| Not in particular or specifically. I was just imagining that
| if there were such a thing possible, it surely would create
| some weird anomalies in our measurements.
|
| The idea that something could warp our spacetime so
| dramatically that we would perceive it as being the normal
| state of things, even though the "warp" is ultimately
| temporary, is for some reason incredibly interesting to me.
| zadwang wrote:
| Excellent illustration. After reading it I thought this looks
| like the PHD style. And I checked the author, who IS Jorge Cham.
| About 22 years ago I was reading his Piled higher and deeper,
| PHD, series and bought several books of his. It is a great
| feeling to see that he is still doing comics. Thanks Jorge!
| eande wrote:
| Is there a similar illustration to the Quantum Entanglement
| phenomena? Something I yet to have to fully understand how that
| is possible?
| turndown wrote:
| Say your car and another car got in a crash, but you both
| panicked and drove away(that is, you did not inspect the
| damage.)
|
| You get home and finally summon the courage to look at your
| car. It's busted. With that information you can infer that the
| other car is busted too, and you don't need to ever come in
| contact again with the other car to know this fact _only that
| at some point in the past your car and theirs interacted._
| swid wrote:
| This explanation infers a hidden variable, which is not how
| quantum entanglement works.
|
| IE - it sounds like the damage occurs at the time of impact,
| and you just decided to look at a later point in time. But
| that isn't what happens with quantum entangled particles. The
| particles will have opposite spin when the entangled state
| collapses, but measurement will affect the angle of the spin
| in a way that proves spin is not preselected when the
| particles were initially entangled and local to one another.
|
| In your story, looking at your own car would have an
| observable effect on the entangled car, even though it is far
| away. But you also cannot even tell it was entangled with the
| other car without comparing the two over more traditional
| channels!
|
| Edit: Since a graphic was requested, I found this image
| hosted by JPL, which shows a related phenomenon (quantum
| teleportation) to what I described, and hopefully dispels the
| car crash explanation which doesn't imply "spooky action at a
| distance".
|
| https://www.jpl.nasa.gov/news/researchers-advance-quantum-
| te...
| brookst wrote:
| Really great explainer!
|
| Naive question: how do we know it is gravity specifically
| distorting space? Could there be other forces or phenomena not
| related to mass?
| teraflop wrote:
| The theory of general relativity says that "gravity" and
| "distorted space(-time)" are one and the same phenomenon. So
| it's a question of whether you believe that theory is correct.
|
| We've known for a long time that GR predicts that when black
| holes and/or neutron stars collide, they should emit
| gravitational waves with a very specific shape. The LIGO and
| Virgo observatories detected faint signals that seem to match
| this shape, which gives us confidence that the theory is a
| correct explanation of the data.
|
| This adds to the mountains of other evidence for GR, and
| complements it by showing that GR still holds in environments
| where space-time curvature is extremely strong.
|
| Technically, we don't know for sure that it's impossible for
| GWs to be produced by other sources than moving masses, but we
| have neither theory nor evidence to suggest such a thing is
| possible.
| florbo wrote:
| > So it's a question of whether you believe that theory is
| correct.
|
| You shouldn't _believe_ in a theory, they simply explain what
| we think we know, which always changes!
| uwagar wrote:
| you think it could be wrong?
| brookst wrote:
| Thank you for the wonderful explanation!
| leetrout wrote:
| I had never heard of "cosmic strings".
|
| https://en.wikipedia.org/wiki/Cosmic_string
| layer8 wrote:
| It would be nice if there was a plain-text version for
| accessibility.
| NickC25 wrote:
| I don't have a deep understanding of physics and would have no
| chance at understanding the details of this if it was in a
| traditional academic paper, and not presented so well. This is
| absolutely fantastic!
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