7 ms·
Uhhhh this statement in the article about halfway down is incredible! “You need to be able to tell the difference between one wave front and the next, and if t
by wizardforhire 8y ago
Uhhhh this statement in the article about halfway down is incredible!
“You need to be able to tell the difference between one wave front and the next, and if the next wave front is 1.3 mm behind and traveling at the speed of light, then you need to reliably distinguishing between events 4 picoseconds apart (4 trillionths of a second). So, every telescope needs a shiny new atomic clock and a really fast camera. You begin to get a sense of why the data consolidation looks more like a cargo shipment than an email attachment; trillions of snapshots every second of not just the waves you’re looking for, but also the waves that will cancel out once all the data is processed. Add to that the challenge of figuring out (generally after the fact) where every detector is to within a fraction of a millimeter even as their orientation changes and the Earth rotates, and you’ve got a problem.”
Just stupefyingly complex and amazing!
- 317070 8y agoAlso, you are looking at a piece of the sky the size of 5 atoms at armslength.
- benj111 8y agoOr the thickness of a human hair at 500km (from the article)
- 317070 8y agoYes, but I did not use that one as I don't like that visualization. My reptilian brain made that sound way bigger than it actually is. As the horizon is only 5-10km away, I cannot really estimate a distance of 500km. I can see the problem of measuring 5 atoms height at arms length though, even when I cannot imagine the size of an atom. Black holes are small things. They're just very heavy.
- Filligree 8y agoBlack holes are normally small things. The furthest one out of these is larger than our solar system, however.
- ben_w 8y agoMessier 87* has a Schwarzschild radius of ~17.784 light hours. The scale of the universe is enough that the mother of my ex had a panic attack when watching a YouTube video about things much smaller than that — hypergiant stars.
- irrational 8y agoWhat was the cause of the panic attack?
- dclowd9901 8y agoProbably the innate fear of infinity
- ben_w 8y agoFrom what I understood, the realisation that the Earth, that everything and everyone she’d ever known, was an invisible speck next to an invisible speck compared to the largest star.
- Izkata 8y agoAhh yes, the Total Perspective Vortex.
- ikeyany 8y agoSome people don't react well to having their ego obliterated.
- perfmode 8y agoFunny enough: two of my lay friends had panic attacks when i described to them the distance to the nearest black hole.
- NikkiA 8y agoThe ISS is 'about' 500km away, that makes it fairly easy to visualise for me.
- dragontamer 8y agoThat's a good point. Perhaps we need to just use things that are well understood. The moon is about 350,000 to 400,000 km away. By my estimate (of other people's estimate), we're looking at a spot in the sky roughly the size of a dime (US 10-cent coin) on the moon. I think my reptilian brain understands the distance of the moon and the size of a dime.
- ben_w 8y agoHmm. My inner eye can’t really get a handle on the distance to the moon. I know it’s 33 Earth-diameters away, that it’s 400 times further than my longest cycle ride, but my mind somehow keeps shortening every distance longer than I can go in a day under my own power to similar levels of “quite far”.
- dragontamer 8y agoYou can get a sense of scale from mountain landscapes or city skylines. As you get closer to the city, you get a better sense of scale and size based off of how much the city has grown or shrunk. The moon is very far away, but it DOES grow and shrink as it comes closer and further away from us. Using the size differential, you can get an innate feel to the size of the moon. Technically speaking, just driving towards (or away from) the Moon will have you traveling closer / further away from it, and give you a sense of scale. https://en.wikipedia.org/wiki/Lunar_distance_(astronomy)#/media/File:Lunar_perigee_apogee.png https://en.wikipedia.org/wiki/Lunar_distance_(astronomy)#/me... The next time you drive to a major city or large, recognizable landscape, keep an eye on how big and small mountains (or buildings) are and how quickly they move parallax to the foreground. It really does give an instinctive sense of scale. Train this instinct well enough, and you can use it on the moon.
- ben_w 8y agoThe trouble with driving (or trains) is that my unthinking processing treats the speed as constant and shrinks the distances. Even cycling 1080 km along the Rhine, from the North Sea to the Swiss not-quite-Alps had that foreshortening, though to a lower degree.
- Cthulhu_ 8y agoOr an orange on the moon (from a youtube video IIRC?)
- mehrdadn 8y agoThanks for this analogy, it's far more enlightening than the human hair version.
- okcando 8y agoInteresting. I have precisely zero intuition for the scale of five atoms!
- rangibaby 8y agoHow many football fields is that?
- cjsawyer 8y agoLet’s just round to 0.
- anticensor 8y agoSmaller than kickoff dot.
- tim333 8y agoThe size of a football 27,000,000 football fields away.
- jkmcf 8y agoAmerican or European?
- bacon_waffle 8y agoThe specific mention of the box of hard drives in the press release, was because there are no flights in/out of South Pole (where the 10m South Pole Telescope is) for ~9 months in winter, and the Internet access there is very expensive and not very high bandwidth.
- leowoo91 8y agoAny similarities with an electron microscope?
- jcims 8y agoThis is why you aren't likely to be doing VLBI in the visible light spectrum any time soon. Your wavelength is three orders of magnitude smaller, so the equipment you use to capture phase information has to be three orders of magnitude faster and the correlation is likely to require at at least three orders of magnitude more capacity.
- RedOrGreen 8y agoYes, 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 https://news.ycombinator.com/item?id=19674563
- afturner 8y agoIs there any information on the tech behind this? As a software developer this is horrifying and fascinating
- RedOrGreen 8y agoCalling 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.
- acqq 8y agoThanks! > 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 Why is that for the optical photons, when it’s “deeper” than just a higher frequency as you answered elsewhere?
- ummonk 8y agoI can’t speak to any physical limitations that this poster seems to be speaking to (I only studied physics theory so I’m not too sharp on the details of lab / experimental devices / apparatuses) but I would guess intuitively that a fundamental limitation is that radio tends to be more coherent, so you aren’t relying on individual photons but rather aggregating a bunch of photons to measure a wave. In contrast, optical light tends to be incoherent (unless from a laser), so you have to measure individual photons and they aren’t really correlated with each other (so interferometry doesn’t work).
- hinkley 8y ago... and the tectonic plates move, and that fucking delivery truck for the people down the road keeps going past...