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midtskogen
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16 Oct 2017 01:29

Watsisname wrote:
Source of the post What predictive power did epicycles have which help justify the proposition?

Besides predicting the positions in the sky of each of the planets and explaining the lack of violent winds if the Earth were moving, epicycles predicted changes to the distances/brightnesses of the planets, they predicted Earth as the centre of the universe, they predicted the heavenly spheres.  All is falsifiable, though in the case of the latter two predictions not directly with the technology of antiquity.  It's not hard to explain why the model was not challenged much for about a millennium and a half.

I think this discussion boils down to whether the successful predictions of dark energy are really independent of the accelleration, or they're all merely all consequences of the same property of the universe.  it's easy not to like dark energy because it appears not so tangible.  It's hard to describe what exactly it is besides what it does, and it's hard to prove its existence by in a laboratory experiment.  Maybe a few centuries from now people will laugh at dark energy.  Or maybe they celebrate the remarkable insight that was made with our limited knowledge of our time.
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16 Oct 2017 12:20

 
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midtskogen
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16 Oct 2017 13:46

So, Watsisname guessed right.  No, not the sock.

These days it was surely the most anticipated astronomical discovery.  And so no surprise either that the press release was so carefully orchestrated, perhaps well prepared even before the discovery.  It better lead to a whole new series of similar discoveries!   ESA's head's on the block.
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16 Oct 2017 20:31

It is an incredibly big moment for gravitational wave astronomy.  We have detected several black hole mergers already, but now for the first time we have a gravitational wave event observed coincident with electromagnetic emission.  Black hole mergers are violent events, but the violence is almost purely in space-time and invisible to our eyes.  But a neutron star merger both shakes the space-time and also makes a bright flash.  This is one of the best demonstrations one could ask for in terms of verifying that the gravitational wave detections are real events tied to astrophysical sources. :)

Also exciting is that this combination of gravitational + electromagnetc observations allows for new, potentially high precision tests of general relativity and cosmological parameters.  For example, we can use this as another way to measure the Hubble constant.  This one event yields a best estimate of H0 = 70km/s/Mpc (plus 12 or minus 8), which is well consistent with Planck data.

GW170817: Observation of Gravitational Waves from a Binary Neutron Star Inspiral

Multi-messenger Observations of a Binary Neutron Star Merger

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midtskogen
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16 Oct 2017 21:56

I'm counting 4218 authors for that paper.  Is that some kind of new record?
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Watsisname
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16 Oct 2017 22:45

Almost.  I think this one still holds the record at 5154.  It's so funny to see it authored as

G. Aad et al.*     

Followed by "*Full author list given at the end of the article.", which goes for 24 pages out of the 33. 
 
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16 Oct 2017 23:30

I think this practice dilutes the author term somewhat.  Certainly, this discovery has involved a lot of people, all with some contribution (hopefully no "honorary" authors, or "we'll include you in the author list if you include us in your list" stuff), and nobody wants a fight for the pen, or have multiple papers basically publishing the same discovery.  I've been listed as an author of peer reviewed papers which I didn't actually write anything in, but parts were based on work that I had done.  I wouldn't have objected to having my name in an acknowledgement section instead (if possible, also stating my actual contribution, but that might take up precious space).  But, my professional career doesn't depend much on getting published, so I don't need to care much, and I don't keep a list.  I think that I have at least carefully read through all papers that I've "authored" before they were submitted, which may be more than what those 4218 or 5154 authors did, but obviously not been able to verify the results that weren't mine.
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26 Oct 2017 06:11

A comet in a hyperbolic orbit has been discovered, having an eccentricity of ~1.2. If this is confirmed by further observations, it would appear we've found our first comet from interstellar space, having formed in (and ejected from) an extrasolar planetary system.

First Known Interstellar Comet
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26 Oct 2017 06:58

So apparently there is the possibility of an actual picture of Sagittarius's event horizon soon? This is very perplexing to me. If we can photograph objects at that distance, why aren't there any photos of nearby exoplanets yet?
 
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26 Oct 2017 08:45

Interesting. You can see by the technique used as described in the main webpage that's an incredibly time-consuming process, and this case it's a first of a kind. It's also aimed at detecting radio emissions of the surrounding matter, not visible light, so I don't think it could be used for direct observation of exoplanets unless there's a considerable amount of energy involved. Infrared light seems to be a better option, so yay, James Webb telescope!

We do have detected exoplanets by direct imaging (https://en.wikipedia.org/wiki/List_of_directly_imaged_exoplanets), even if we are still missing some details :)
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26 Oct 2017 10:04

Oh, radio emissions. Article I read didn't mention that.  :roll:
 
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26 Oct 2017 15:47

Yeah, the Sagittarius A* event horizon must be observed in radio, because visible light from the galactic center is almost completely blocked by the intervening dust, whereas the radio wavelengths can pass through.

Resolving such a small structure on the sky also requires interferometry, where a signal is recorded at the same time from widely separated locations (a wide "base-line").  The light waves arrive slightly differently at these different locations, and those differences can be used to reconstruct the image at a resolution comparable to that of a single telescope whose diameter is equal to that baseline distance.

Interferometry-based observations are not new (for example ALMA took this image of a protoplanetary disk using the same principle), but the Event Horizon Telescope is the first time the technique has been applied on this kind of scale.

To image details on an exoplanet, one must achieve sub-milliarcsecond resolution (Jupiter at 4LY subtends an angle of 0.76 milliarcseconds).  In optical wavelengths, this would require a baseline distance of over 150 meters, and a limiting magnitude of at least +20.  The best that any current optical interferometer can do is about 7th magnitude, or up to 11th in infrared.  This is why we don't yet have interferometry-based images of exoplanet surfaces -- the angular resolution is reachable, but the sources are too faint.

The shadow of the Sagitarrius A* black hole has an angular diameter of about 50 microarcseconds, and the optimal wavelength to observe it in is 1.3mm.  The required baseline distance for this observation is about 6600 kilometers -- about the radius of Earth!  So it is just barely doable with a global network of radio telescopes.  And we really luck out here because the inner part of the black hole's accretion disk is very bright at this wavelength, so the network is able to observe it easily.
 
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26 Oct 2017 23:40

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Watsisname
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27 Oct 2017 04:16

I look forward to all the conspiracy theories this is sure to spark...  :?
 
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27 Oct 2017 04:32

Watsisname, it's on a collision course with the Earth but don't worry the shadow government sent a special team with a hyperdrive generator and it will take a path through hyperspace when it intersects the Earth.
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