In a major discovery, scientists say space-time churns like a choppy sea(washingtonpost.com) |
In a major discovery, scientists say space-time churns like a choppy sea(washingtonpost.com) |
https://arstechnica.com/science/2023/06/nanograv-picks-up-si...
Astrobites articles are usually written by post-grad students working in the relevant area, with the writing level targeted at undergrad astrophysics students. This one was written by NANOGrav collab members, all (or at least most) of whom directly worked on the results that will be all over the press today and tomorrow, and maybe hits a level for an advanced undergrad student or so.
The journal these papers are published in is open access: no paywall.
The Ars article focused on the NANOGrav raft of papers; NANOGrav is just one of the several pulsar timing array collaborations that are in this synchronized un-embargoing of papers. There is substantial overlap between, say, the NANOGrav papers in the Ars Technica article and the joint InPTA/EPTA set which includes https://arxiv.org/abs/2306.16225 https://arxiv.org/abs/2306.16214 and https://arxiv.org/abs/2306.16226 but also enough non-overlap to encourage reading all of them (and to refresh one's foundational knowledge by consulting e.g. Burke-Spolaor et al 2019 "The astrophysics of nanohertz gravitational waves", <https://link.springer.com/article/10.1007/s00159-019-0115-7>).
Unfortunately I find myself griping about a focus on -- to be rude -- only the USA-based-institutions-dominated collaboration (<https://nanograv.org/collaboration/partner-institutions>, although of course there are non-USA and non-North American institutions and researchers in that collab too). Relatively USA-institution-light collabs will chase press attention in different markets and in different ways, of course, but there is scant sign of them in the Ars article (which at least names and links to IPTA) or in the WaPo one (which only lists "Europe, India, Australia and China", not naming the actual collabs).
The inter-collab two year plan on synchronization of paper releases (the "3P+ agreement") itself is something of a story; I wonder if it's going to get much attention in anyone's press.
That said, I don't mind either Ouellette's or Achenbach & Jaggard's reporting. I think they serve amateur enthusiasts well enough that their summaries of the results and science don't need to be nitpicked. Other than that both articles use the unhelpful cliché "fabric of space[-]time" (the latter also has "fabric of the cosmos", "fabric of space and time", because variety is important right?), grrrrrrrr.
They can and do replace submitted URLs under numerous conditions, including not only clickbait but superior articles.
I'll do so here, though being ~15 h after initial submission, it's largely a moot point.
Ranting over poor link quality in the thread itself is not useful in this regard.
Why arent we using midjourney for obligatory space article pictures?
World’s most replaceable job award goes to Astronomy editorial artists
You? I have absolutely no idea why you did not do this and I wish you would! It'd be even better if you could make some kind of "Astronomy.com's RSS feed -> yieldcrv's midjourney prompt" but one step at a time.
To me it’s especially clear that using AI to make art is immoral when you use it for a purpose that you would have previously hired a human artist for. Harvesting work from artists without permission and then using that to replace them is pretty scummy.
"The gravitational wave background, as described by the astrophysicists, does not put any torque on everyday human existence. There is not a weight-loss discovery in here somewhere. A burble of gravitational waves cannot explain why some days you feel out of sorts. But it does offer potential insight into the physical reality we all inhabit."
This is just embarrassing for the Washington Post; the scientists who worked so long for this result deserve better science journalism.
The odds of this being a good article were pretty low.
WashPo is many things... but good journalism and scientific journalism is not two.
I might be huge comparatively but still too small to measure it's effects maybe?
I get that on Earth we don't seem to have any crazy gravitational effects from the "churning" of spacetime. However, the universe is vast. There might be some parts of spacetime that churn more than it does on Earth.
Can someone with more of a physics based background explain:
Does this mean with the appropriate detection tools, we might actually start finding hyperspace lanes?
i.e. Rather than creating spacetime bubbles with warp engines[0], we could instead "sail" / "surf" certain parts of spacetime for FTL?
That is far short of the usual 5-sigma (1-in-3,500,000) threshold. So I guess interesting results but not really a major discovery?
Though in that event, I suspect you might be having other priorities.
sounds very much like the Warp.
Of course the movement is so small and subtle that these movements are more like vibrations. Also known as the sound of the universe aka OM in Hindu scripture.
Gravitational radiation can form caustics, so in one way, yes. This was an area of theory that Bondi and Pirani were interested in during the 60s to 80s. Caustics are meant in the sense of the bright spots in https://en.wikipedia.org/wiki/Caustic_(optics) but rather than some "rogue wave" you might get a black hole.
More recently mathematical-relativity studies of the stability of Kerr black holes included a 900+ page paper https://arxiv.org/abs/2205.14808 which among other things considers something like a rogue wave passing through a spinning black hole, and asking: does the black hole swallow up some of the wave? or does part of the wave get entrained into a sort of accretion disc? in that disc do caustics appear, and if so do they lead to black holes? if so, do those black holes fall in, fly away, or coalesce into long-term orbiting bodies around the original black hole? These questions were asked because "stability" means "if perturbed (e.g. by a powerful gravitational wave), does an initially Kerr-like black hole relax back into a Kerr-like black hole, or does it become something very much not like Kerr?".
Caustics may also form by a powerful gravitational wave gravitationally-lensing around some massive object like a heavy galaxy cluster. And if that massive object has enough angular momentum, it's possible the caustic will evolve into a significant gravitational wave enhanced somewhat analogously to a slingshot manoeuvre by a spacecraft.
Conversely, we can start with a rogue wave existing, and see what it does.
Bondi and Pirani were also interested in spacetimes with enormous gravitational wave impulses. One family of those is the "sandwich wave", https://royalsocietypublishing.org/doi/10.1098/rspa.1959.012... (1959) (put "https://sci-hub.se/ in front of that URL if you need to).
Loosely, it's called "sandwich" because the spacetime is pretty bland (like white bread?) or even flat on either side of the wave, which divides the spacetime into three parts (including the wave itself).
Thirty years later they showed that the sandwiches are pretty unhealthy as they tend to destroy everything in the universe they pass through (any set of finitely-but-widely-separated galaxy clusters in such a spacetime will end up colliding with each other in finite time). https://royalsocietypublishing.org/doi/10.1098/rspa.1989.001... (also on sci-hub, and with nice late-1980s diagrams).
Fortunately, I don't think anyone has any idea how to generate a sandwich wave - it's just a feature of the spacetime put in by hand.
Unfortunately, we could be in a sandwich spacetime and not know it yet. A sci-fi writer might speculate about a preferred orbital plane for a large number of (very-early-universe-born) supermassive black hole binaries building up an approximate sandwich wave over the course of billions of years. Or perhaps speculate that the apparent stochastic background that's the topic of the fine link at the top where I guess you get "churning" from is not caused principally by supermassive black hole binaries but by some tensor-mode reheating during cosmic inflation (or a false-vacuum-to-real-vacuum first-order (not as in recent Star Wars, or is it?) phase change), and that this effect happened to generate a sandwich wave which has already destroyed galaxies far far away. A hard-working hard science fiction writer might even be able to force a cosmologist to concede that such a picture might be consistent with observation, including the pulsar timing array results in today's news.
A sandwich wave would zip through our galaxy at the speed of light, but the sandwich-wave-induced collapse of the galaxy into a caustic might take enough time that you could really what's-the-opposite-of-enjoy the experience. Pleasant dreams.
I'm struggling to understand how a sandwich wave causes "everything in an infinite spatial volume becomes known in a finite time". Armchair physicist here.
Crazy that this stuff has already been studied 40 to 60 years ago.
Watching some 68 stable millisecond pulsars for pulse-delays over a long time let the NANOGrav collaboration spot quite a lot of gravitational radiation from heavy bodies (heavy as in billions of solar masses each) in mutual orbits lasting months to years.
LIGO, Virgo, and Kagra are sensitive to much quicker mutual orbits, where a single orbit is a fraction of a second. This also implies a much lower maximum mass for the bodies, somewhere between about 2 and 30 solar masses. A pair of bodies of lower mass can have a quicker orbit; heavier bodies will collide and merge instead.
> sail or surf
The sources of the type of gravitational radiation NANOGrav (and other pulsar timing array collaborations like CPTA, EPTA and PPTA) is sensitive to are scattered randomly across the sky at random distances and with orbits in random orientations. There is unlikely to be a directional bias in our part of the universe.
The results are close to what everyone expected, although there may be support for a greater than expected count of supermassive black hole (SMBH) binaries. Binarization channels -- the ways in which a pair of initially-separated SMBHs go into ~months-long orbits with one another will get more study. This is of particular use in understanding the formation and merger of galaxies.
Ultimately this is a mostly confirmation of 3-spacelike+1-timelike dimension General Relativity as a good theory for our universe at the largest scales. "Hyperspace lanes" have more dimensions right in the name, so I guess the NANOGrav 15-year study results seem to leave them in the land of fantasy.
At best, one can imagine a variety of non-prosaic explanations for the unexpectedly high count of gravitational waves could mean sources other than SMBHs (some early universe phase transitions, that our spacetime has topological defects, unexpected action during cosmic inflation). None of that really invites realistic thinking about "sailing" or "surfing" any more than a random scattering of SMBH binaries. The prosaic explanation for the unexpectedly large number of SMBH-like signals is simply that black holes tended grow large in dustier regions than expected.
Finally, for the most part (i.e., in not-very-dusty regions of space) light waves and gravitational waves move at the same speed, so at first glance it doesn't seem likely that gravitational waves would help one with FTL travel.
That is, "General Relativity's right about gravitation yet again". Interesting, because it's yet another way of confirming the theory (at the linear level and a flavour of the equivalence principle), but not at all startling.
What's intriguing is that pulsar timing arrays ("PTA") like NANOGrav, CPTA, EPTA, InPTA, PPTA etc may be seeing more gravitational waves than expected. Assuming the sources are all SMBH binaries in ~month-long mutual orbits, then maybe there are more SMBHs than expected, or they binarize more commonly than expected or earlier in the universe than expected, or through some unexpectedly popular channel other than galaxy mergers. Pulsar timing arrays appear likely to produce increasingly useful data about galaxy formation, mergers, and evolution.
I think that having both LIGO (Hz-kHz, so ~stellar mass) and PTAs (~nHz, and much larger mass) giving views into different parts of the spectrum in gravtiational wave astronomy is pretty major. That LIGO appeared to work at all was awfully cool; and now we have these large international PTA collabs appearing to work too. I am tempted to compare high-end early 20th century optical telescopes at the dawn of radio telescopes: same electromagnetic spectrum, but sensitivity to different wavelengths, with different wavelengths associated with different phenomena and different objects.
PS: I like InPTA's set of cartoons at https://twitter.com/InPTA_GW/status/1674208503395934208 (thread).
I wonder how many science experiments were done on earth yesterday. It was probably more than a thousand. This is why if you are doing science, you should always do the sciences that will give amazing headlines if you are right. Maybe you will be one of that day's lucky one in a thousand!
There has been 1 pulsar-timing experiment looking for gravitational waves and it produced this positive, 1-in-1000 result. Seems extremely promising.
With sound waves you could use a really super thin piece of metal that vibrates to sound itself, in which you could harvest sound energy. But gravitational waves? I don't know how easy that would be.
Maybe a super long and rigid cable, positioned thousands of kilometers apart that bends ever so slightly to changes in their anchors.... though I'm sure any energy harvested from that would have gravitational waves as their least contributing factor.
> Armchair physicist here
I've started and aborted a couple of ELI-n where n < ~ 24, and I'm not sure this one is especially satisfying either, but I think my previous attempts were just confusing and went beyond your implied question.
Causality in a sandwich wave spacetime is weird. All the null geodesics from a point can eventually refocus on some future point in the afterzone. We can take that simple fact (from Penrose initially) and contrive a sandwich wave spacetime that shows off the Bondi&Pirani words that confused you.
If you do an isotropic pulse of light in the beforezone, as expected (beforezone being just the flat spacetime of special relativity) it flies outwards in an expanding spherical shell. In a flat spacetime without a sandwich wave, you'd never see any of it again in the absence of a reflection. However, in the sandwich spacetime the gravitational wave can roll over you (and this expanding shell of light) such that in the afterzone all the light that you flashed out isotropically, eventually lands all at once on your eyeball, no mirrors required.
Moreover, the null geodesics out of multiple different points can all be refocused on just one future point. So you can fire off dozens, millions of isotropic flashes in the beforezone, and fry your eyeball (in the future, in the afterzone) to a crissssp.
Going further, one can set up a sandwich spacetime so that the light from every event in the beforezone lands on one point in the afterzone, or on a succession of time-ordered points. In this way, an observer in the afterzone can in finite time receive light from every light-emission in the beforezone, even if the beforezone is infinite.
Finally, I am using "light" loosely; to keep the sandwich spacetime flat except for the wavezone, instead of light we need massless test particles that feel gravitation only passively. They don't exert an active influence on the gravitational wave or on the focal point in the afterzone, even if there is an awful lot of them. More technically, we need the stress-energy tensor to be 0 everywhere in the whole (vacuum) sandwich spacetime, otherwise we will have a lot of work[1] exploring the contrived spacetime's insensitivity to perturbations from matter (which generates curvature after all, and a heavy enough amount can distort the wavezone). Matter here includes light.
- --
[1] there is a literature in that area, with Mustafa Halilsoy being fairly prominent in studies of (matter) waves including standard electromagnetism interacting with the sandwich (gravitational) wave, which in turn invited thoughts about (non-sandwich) gravitational waves interacting with the sandwich wave, which is also in the literature. I'm not likely to ELI any of that here though.
Same with Getty’s
is that the approved use then?
As for AI art specifically, the problem isn’t really that artists are being replaced. The problem that their own work is being used to replace them, and yet they are not getting credited or compensated by midjourney (etc).
> It's not immoral, it's just [how] economies work.
Do you believe that every aspect of "economies" (or, to simplify the question, let's say just current day capitalism to exclude things like communism and historic economies from the equation) must automatically be moral, or perhaps amoral since you only said "not immoral"?
Seems to me like either a badly thought out claim, or a bad faith argument to justify your main claim about AI art, rather than an actual justification for believing AI art is not immoral.
(Side note - from the POV of my comment anyway, though I suppose technically my comment is the side-note and this is back on the main topic: if you genuinely haven't noticed any debate over the morality or not or AI art, you've not been following AI coverage in mainstream & tech news publications, nor reading the huge number of HN threads where people disagree over whether or not AI image generators training on human art without compensating the human creators should be considered immoral IP theft or should be considered the same as a human studying great artists while working on improving their own art).
Same with Getty’s
is that the approved use then? or does the goal post move
It makes sense you can't clone exact types of art, but mixing and mashing two or more to make something totally new and derivative is fine.
Now back to the morality of the Getty and Adobe models: did those artists know their work was going to be used for training AI art models? From what I understand there are many artists who licensed their art to Adobe Stock who are upset that their work is being used for this purpose.
Keep in mind this is a usage of the art that literally didn’t exist when the artists licensed their art to Adobe. I would argue it’s a fundamentally different use case.
and once that’s done all arguments disintegrate as people stop going to artists and photographers anymore either way?
But thanks for assuming that I'm arguing in bad faith despite not making an argument for either side!