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DoctorOfSpace
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05 Feb 2018 13:28

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Watsisname
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05 Feb 2018 18:05

 ^Good video.  First for awareness of mislabeled products, second because I love that method of calculating laser wavelength. :)  It is very precise, typically limited most by uncertainty in the spacing of the diffraction grating.  Or slit width if you happen to use the single/double slit method.

One other thing to be aware of about different wavelength and power lasers is that the relationship between apparent brightness of the beam/dot and its true intensity is very deceptive.  Eye sensitivity to light varies by wavelength: peaking at ~550nm (green) and dropping quickly in the red and violet.  My 120mW violet laser looks about as bright as a typical <5mW green laser, but will quickly sting bare skin.  Definitely not eye safe.
 
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05 Feb 2018 23:22

Have been tempted to buy a powerful laser, but I have no use for one outside of curiosity.  I have guns laying around and I feel fine with that, but a high power laser just seems like more of an immediate hazard.
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06 Feb 2018 00:49

There are interesting experiments in optics that can be done with lasers (my avatar image being an example), but besides that and pointing at things they're mostly curiosity.

Power levels above 100mW get pretty sketchy.  Too many people treat them like toys or shine them at things they shouldn't.  Treating a high power laser the same way as a firearm is a mindset I encourage in anyone who wishes to use mine.  In particular, a firearm that fires continuously and reflects.  At those power levels a momentary lack of awareness can lead to injury.  

High power green is nice for visibility.  I also like the 405nm violet wavelength as it is short enough to induce a variety of fluorescence. :)
 
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06 Feb 2018 21:29

Over here there is a ban on lasers above 1 mW.  You must have a permit to own one, and to get the permit you need a reason (beyond using it as a toy).  The reason for the ban was that too many people did use it as a toy (pointing at airplanes, etc).
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06 Feb 2018 21:34

midtskogen wrote:
Source of the post (pointing at airplanes, etc)

That is a felony in the states, as it should be.
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15 Feb 2018 19:07

 
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17 Feb 2018 19:14



Fantastic talk by Rainer Weiss -- one of the guys who was instrumental in developing the techniques used in gravitational wave astronomy.  Long video but worth it if you have some time to kill.  He presents a lot of interesting information and insights that I had not encountered before. :)
 
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18 Feb 2018 14:26

Interesting talk.  There was one thing that I didn't understand (well, besides what went completely over my head).  So the frequency gives up the masses of binary merges, and from the masses one can calculate the absolute amplitude, and the observed amplitude will then reveal the distance.  But doesn't also the distance influence the observed frequency for really distant sources due to the expansion of the universe, making the distance calculation impossible without some assumptions?
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18 Feb 2018 15:58

midtskogen wrote:
Source of the post So the frequency gives up the masses of binary merges, and from the masses one can calculate the absolute amplitude, and the observed amplitude will then reveal the distance.  But doesn't also the distance influence the observed frequency for really distant sources due to the expansion of the universe, making the distance calculation impossible without some assumptions?

Good question!  The expansion of the universe does cause a frequency shift of the signal, so from frequency alone it would be impossible to distinguish a difference in the mass of the gravitational wave source from a difference in distance.

The combination of frequency and amplitude is what breaks out of this entanglement.  The amplitude of a gravitational wave is proportional to the source mass, and also inversely proportional to the distance (the intensity of the wave obeys inverse square law, but intensity is the square of the amplitude, and amplitude is what the interferometer measures).  So by using frequency and amplitude together we can directly measure the mass and distance1 to the source, but we do not directly know how much the signal was redshifted.  Determining the redshift is what requires some prior knowledge of the relationship between distance and redshift by cosmic expansion.

An exception to this is if the gravitational wave event is also observed electromagnetically, as was the case with the neutron star merger.  With that event we were able to isolate the host galaxy, so we could directly measure both the distance of the source (from gravitational waveform) and its redshift (from electromagnetic spectrum), which allowed an independent measure of the Hubble Constant. :)


1: To be precise, this technique gives us the luminosity distance.  There are several definitions of distance in cosmology, in order to account for cosmic expansion in different ways.  Luminosity distance is defined so that the inverse square law is correct.  Comoving distance what you would measure if you froze the expansion and stretched out a ruler between here and there.  There are also angular diameter distance (defined so that angular size obeys Euclidean rules), and light-travel distance (defined by the time it took for light to make the trip).  In general, these all have different values in a non-static universe.  Cosmic expansion sucks.
 
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19 Feb 2018 11:46

Watsisname wrote:
Source of the post  With that event we were able to isolate the host galaxy

I might be remembering incorrectly, but didn't they say that the galaxy was identified because it has the right distance?
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19 Feb 2018 15:39

Not so much by having the right distance, because the distance determination from the gravitational waveform has fairly big error bars.  Rather, the possible source region on the sky was smaller, and the distance was closer, which meant there were not too many galaxies in that region to check to see if there was an electromagnetic event coincident and consistent with the gravitational wave signal.


To explain: the neutron star merger was detected by 3 gravitational wave observatories, which provided smaller uncertainty for the location of the source on the sky: down to 28 square degrees.  This is still a pretty big swath of sky, but much better than earlier GW events observed with just two detectors.  (This is one of the major benefits of having more detectors -- isolation of the source location on the sky improves significantly.)

Image

The other benefit, and what you are probably remembering, is that this event also had a fairly close distance as determined by the waveform: a luminosity distance of 40Mpc, with an uncertainty of +8 and -14.  (So it could be anywhere between 26 and 48Mpc).  A big uncertainty, but not very far away.  There aren't so many galaxies that lie in that volume of space.  So it was not difficult to determine a match with the electromagnetic counterpart (kilonova).

For comparison, the first gravitational wave event had a luminosity distance of 410Mpc with uncertainty of +160 and -180 (between 230 and 570 Mpc!) and region on the sky of 600 square degrees.  No way could we unambiguously connect that to a source galaxy.  That volume contains a vastly greater number of galaxies.
 
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21 Feb 2018 14:57

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21 Feb 2018 22:18

I look forward to the episode of trebuchets. :P
 
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26 Feb 2018 03:04

Applied Science makes a high temperature super-conductor.  Very nice look at the process, which is not actually that complicated, but very tedious and difficult to get right.

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