RedOrGreen

Yes, in a way.

Think of a single dish radio telescope as a one-pixel camera, where measuring the emission intensity at each point in the sky lets you build up a map. Typically, this is done with high resolution on the frequency axis, which is used to map Doppler shifts for spectral lines of Hydrogen, for example [1].

With a rooftop antenna, it's not likely to be a very sensitive map, though. You'll see the Sun, and its easy to see the Milky Way transit overhead, but other than that ...

[1] https://sites.google.com/site/galfahi/

Very nice, even with no planes overhead. But it looks like the map orientation is reversed (off by 180 degrees)? Real bug, or just me?

Oof, yes, I didn't even mention corporate compliance.

Paraphrasing: "If you don't want to, don't sideload apps, no one is forcing you to" - do people really not see the problem? It's not that technically unsophisticated users will want to sideload apps. They don't know / don't care / have different things to worry about.

But they want their Facebook. Or SnapChat, or Insta, or TikTok, or whatever.

Once other app stores are allowed, there's nothing stopping Meta (for example) from revoking their existing apps, and requiring the use of the Facebook App Installer for access to Facebook. They've paid people in the past to use their Onavo VPN app to bypass Apple's privacy controls, so this would be unsurprising.

Of course, that's just an example; replace with the next SnapChat, TikTok, whatever. If that's the thing that teens want, and the way to get it is to click a bunch of "Yes I agree" dialogs, they'll happily do it. And now suddenly some developer has access to all your family financials through your teen.

If your solution is "well, people shouldn't do that then", you might not understand teens. (Or grandparents. Or regular people.)

I came to this thread a bit too late, but if you're interested in the actual signal that Stella found, take a look at the spectrum vs time on the NASA release [1] - third plot on the page, with a convenient slider that lets you flip back and forth to see the previously known plasma oscillation events and the newly discovered continuous signal. Notice the huge change in scale!

[1] https://www.nasa.gov/feature/goddard/2021/as-nasa-s-voyager-...

Allow me to gently contradict the previous responders. The community of radio astronomers is feeling the loss of Arecibo very keenly.

I've mentioned this in a previous comment [1] so I won't rehash it again, but here [2] is a statement from the NANOGrav collaboration - we have been using the Arecibo telescope on a weekly basis (along with the Green Bank telescope) to observe a large number of radio pulsars for over 15 years now, and when we announce the first detection of very low frequency gravitational waves, Arecibo data will play a critical role.

This loss really hurts. The US community is scrambling, and we are going to have to rely on the generosity of our international partners going forward - there's just not going to be enough telescope time to do all the science that we'd judged as very important through competitive review processes.

(And that's before we get into the unique radar capability, etc.)

[1] https://news.ycombinator.com/item?id=25154898 [2] http://nanograv.org/announcement,/press/2020/12/02/Arecibo.h...

I'm too sad today to engage with the discussion here, but I'll just mention, for those of you who think that Arecibo has either "outlived its usefulness" or been supplanted by FAST in China:

Here's Arecibo on the cover of Nature in 2018: [1]. Yes, we are doing similar work at FAST as well, but one is not a replacement for the other.

And here's a link to NANOGrav: [2]. I promise that you'll hear more about NANOGrav in a year or two, depending on how publication timelines work out. And it wouldn't be possible without Arecibo - now that it's gone, we have to seriously contemplate how to move on beyond our 15-year data set (already in the can).

[1] https://twitter.com/nature/status/951423495107481602 [2] http://nanograv.org/

I'm too sad today to engage with this, but you're simply wrong about Arecibo "having done all the useful things it could do ages ago, and not finding anything new".

Here, for example, is Arecibo on the cover of Nature, in 2018: [1]. And you might want to look into NANOGrav [2]: you'll be hearing a lot more about it (I promise) in one or two years, depending on publication timelines.

[1] https://twitter.com/nature/status/951423495107481602 [2] http://nanograv.org/

If the full hundred hours was such a huge risk, why not take, like, 5-10 photos first to see if you get much of anything? Seems like they would have been able to know pretty early on whether it was going to be worth it.

This has been answered already but just to pull some threads together - the suggestion to take a few exposures first and see what comes out is precisely backwards. They already knew, going into it, that what they were looking for was at the limits of sensitivity, so nothing would show up until they accumulated enough observing time.

In the absence of systematic effects, our signal to noise ratio improves as the square root of integration time. Think of a bucket accumulating photons - the signal accumulates in linear proportion to the integration time, while the RMS fluctuations increase as the square root of the integration time, so the signal to noise ratio improves as T/sqrt(T) = sqrt(T).

So the idea was to accumulate enough exposure that the faint galaxies would become visible at the telescope performance limits - can't get to something scientifically useful by imaging only the first few exposures. It might have made a pretty enough picture, sure, but not an informative one, compared to what we could already do from huge ground-based facilities.

HST is in a 96-minute orbit, up to half of which is blocked by the Earth, depending on where in the sky you point it. Getting time on it is an extremely competitive process - the recent cycle had over a thousand proposals, requesting over 24,000 orbits worth of observing time, and they had only 2700 orbits to allocate [1].

This sort of proposal pressure (over 9:1 oversubscription) leads to very conservative proposals, where we request absolute minimums and emphasize that even a non-detection will lead to publishable science in such and such a way.

Getting over 60 orbits to point at a blank field would have been frankly impossible through the regular time allocation process, not because the science wasn't excellent, not because it wouldn't be recognized as excellent, but because it would crowd out other science that could be done with far fewer orbits per project.

The telescope administrators explicitly recognized this problem when they set up the Director's Discretionary Time process (and this is common to all telescopes now) - sometimes you need someone with the authority to just roll the dice on a long shot.

[1] https://www.stsci.edu/contents/newsletters/2019-volume-36-is...

This is a comparison of images, not just catalogs of sources in images. If you look at the image in the linked article, you can easily spot one of the stars that has "gone missing".

Looking at the brightness of the source that went missing, it is hard to suggest instrumental error - much fainter stars remain detectable in both epochs.

And fraud, really? Who would bother to fake stars in catalogs and plates in the 1950s, and why?

Sure, that's an excellent point. In case you read late replies: we recognize this, and parameterize the burst energy [1] by a beaming scale factor, \Delta\Omega/4\pi. (It still drops off as the inverse square, though, as long as you're not in the near field.)

The problem is, while you can reduce the energy requirement by making the beams narrow, that increases the total number of sources by the same factor. When we say that there are between 5000-10,000 FRBs all over the sky every single day, we are referring to FRBs beamed towards us. If you apply a 10% beam, your energy requirement drops by 10x, sure, but the source count goes up by 10x.

Right now, we don't have a large enough plausible progenitor population, even at 1x, for these FRBs. It's a really fun problem.

[1] See, e.g, the Methods section in https://www.nature.com/articles/nature20797 (or https://arxiv.org/abs/1701.01098)

We measure the energy of signals received at our radio telescopes - our units are Janskys, which correspond to 10^-26 watts per square metre per hertz of bandwidth, and we can calibrate our telescopes using noise diodes or known sources in the sky.

So for FRBs, we get the received brightness, and if we can measure the distance, we can use the basic inverse square scaling to estimate how much energy it had at the source. (Why inverse square? Because the area of a sphere is 4 pi R^2 - if you go twice as far away, the signal is spread out over 4 times the area, and so on.)

The key problem is a distance estimate for the FRBs - we have a pretty precise measurement for FRB 121102 because we identified its host galaxy and measured the redshift to that galaxy, so we know how far away it is. That's now been done for 2 other sources. For the rest, we can use the pulse dispersion, which measures the integrated column density of electrons along the line of sight, along with models for our galaxy and the intergalactic medium, to get some idea. It's not as good, but better than nothing.

Yes, we're not going to do optical VLBI soon, but the reason is deeper... I just wrote out a long comment [1].

[1] https://news.ycombinator.com/item?id=19674563

Calling it a "really fast camera" elides much of the actual difficulty. We're trying to tag individual wavefronts of light at different telescopes, record them, and then play them back at a central "correlator" with the appropriate delays so that the waves come to a focus.

For a wavelength of 1.3 mm, we'd want the time tagging to be better than a quarter of the wavelength at least - say 0.3 mm. The speed of light is 300 mm/ns (a foot per nanosecond is the shorthand beloved of circuit and chip designers). So, for 0.3 mm, we're going to have to get down to a wavefront tagging accuracy of 0.001 ns.

No clock is going to get there, but if we can get ~close enough, we can use a procedure called fringe fitting to determine the clock corrections by looking at the wavefronts. (Does it line up this way? How about this way? How about now? Yes, it's as laborious as it sounds, but computers, eh.)

This is all in the calibration of data, before we do the Fourier inversion to create images - the magic of radio interferometry is that we can record the signal to disk while preserving phase. Optical photons can not be recorded and played back with phase preserved - optical interferometry has to split up the photon streams and send different parts to be correlated against streams from other telescopes, so you run out of signal quickly. Meanwhile, we can record radio waves at the 27 VLA dishes, say, and play them back for correlation on all 27*26/2 = 354 baselines, no problem. That's why radio VLBI is a thing, but not optical VLBI.

Even as a professional radio astronomer, the underlying physics is deep and almost magical.

(I haven't watched the video, but I do have professional expertise on this topic.)

With interferometry, you're getting an incomplete sampling of the Fourier transform of the sky image, and if you just invert the samples, you get what we call a "dirty" image.

But you know your sampling of the Fourier plane exactly, since that's just a function of the projected baselines between every pair of telescopes during the observation, so you can create a "dirty beam" - now all you have to do is remove the effects of the dirty beam from the dirty image. Of course, that's a deconvolution problem, and given that you don't have all the information - you sampled it - it can never be exact.

But it can be very good! There are very sophisticated radio synthesis image deconvolution algorithms, including CLEAN and Maximum Entropy. For Maximum Entropy methods, you can apply a Bayesian prior on your images - most of the time, the prior we apply is a blank sky (seriously!) but if we have other constraints that we can use (e.g., the approximate size of the region with extended emission), Bayes tells us that we would be remiss not to use it.

If you look at this image [1] from Paper IV [2], we show the image results from different techniques on different observing days. Those are the inputs to what is the "consensus image" - you can check how close they all are to each other.

Does that make sense...?

[1] https://iopscience-event-horizon.s3.amazonaws.com/2041-8205/... [2] https://iopscience.iop.org/article/10.3847/2041-8213/ab0e85

For pulses or bursts of emission, we like compact objects, just from light-travel-time arguments. I would say that the leading models involve very young, highly magnetic neutron stars ("magnetars"), either embedded in their birth supernova remnants / wind nebulae, or in the environments of massive black holes (as in the center of our Galaxy).

There are good reasons for these specific models to be favored. But honestly, we don't know for sure yet. Could it be cosmic strings? Evaporating microscopic black holes? Blobs of gas being flash-annihilated by drifting into the beams of active galactic nuclei? Theorists have models that seem to be OK on paper, and we're working hard to kill their models with new observations.

(One thing I'll say - and I say this as a firm optimist about the prospect for aliens - this is NOT aliens.)

It's a full interferometer, although for the pulsar and FRB modes, they are tiling the field of view with many, many phased array beams. (Yes, the computation requirements are horrendous.)

Would you care to elaborate on what you find objectionable about Face ID? Just like the fingerprint reader, all data is stored on the device secure enclave, and does not leave the phone. And it's been pretty much fast enough for me since the X. (I don't notice much change in speed for the XS but it's probably a bit faster.)

Yes, the authors discuss the implications for scalar-tensor theories, and it doesn't look great for many of them. The Nature News and Views article by Clifford Will [1] offers a decent overview.

[1] https://www.nature.com/articles/d41586-018-05549-4

We can do even better some of the time - for example, we can get to ~few nanosecond timing precision for pulsars like J1909-3744 [1], which is equivalent to measuring (changes in) the path length to that pulsar to a few meters or better.

[1] https://arxiv.org/abs/1406.4716

I'm too close to this topic for a chatty comment, but I'll just point to this IAU meeting: "Pulsar Astrophysics: The Next 50 years"[1]. It was held this September at Jodrell Bank, which has always had deep involvement with pulsar astronomy. Jocelyn Bell delivered the opening talk, and she got a sustained standing ovation. It was just a wonderful moment.

(And yeah, she wasn't a mere grad student - after her initial discovery and after it was dismissed by everyone around her, to borrow a phrase, she persisted.)

[1] http://pulsarastronomy.net/iaus337/

I hold out hope the NS-NS rumor is true, that paper is just taking a bit longer to prepare.

Mark October 16th on your calendar.

I guess this is cool, but not really that novel a result (and yeah, it's an ATel, not a paper yet, although that's just a matter of time). We've seen similar burst storms from this FRB source before, which is what allowed us to localize it in the first place. And I'm disappointed that they used the Hubble Deep Field - we have very nice deep images [1] of this field already.

But yes, the Breakthrough digital hardware is really pretty neat!

[1] https://www.nytimes.com/2017/01/04/science/fast-radio-burst-...

And they haven't even tracked down the location of the source as well as it sounds. They detected a burst, and found a fading radio source in that field of view, but there's a decent chance (<6% or <0.1%, depending on which of their estimates you prefer) that it's just coincidence.

Alas, if it's just coincidence, the rest of the (beautiful!) paper falls apart.