Wikipedia:Reference desk/Archives/Science/2020 May 26

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May 26

Why Large Synoptic Survey telescope launching for mapping milky way galaxy?

Not a serious question: disruptive user copying questions from other sites.
The following discussion has been closed. Please do not modify it.
Most famous Hubble space telescope failed to map milky way galaxy? So why Large Synoptic Survey telescope launching for mapping milky way galaxy? Ram nareshji (talk) 09:11, 26 May 2020 (UTC) The Hubble Space Telescope was not designed to map large portions of the sky – it has a very small field of view and it would take ages for it to map the Milky Way. You need different types of telescope for performing different tasks. --Wrongfilter (talk) 09:38, 26 May 2020 (UTC) If it is not designed to map large portions of the sky, then what it is designed for? Is it just a test telescope before launching Large Synoptic Survey telescope? Ram nareshji (talk) 11:35, 26 May 2020 (UTC) It was designed to observe individual objects at high angular resolution. --Wrongfilter (talk) 11:39, 26 May 2020 (UTC) Courtesy links: the Hubble Space Telescope, launched into orbit in 1990, and the Vera C. Rubin Observatory (previously called the Large Synoptic Survey Telescope), a ground-based instrument due to commence observations this year. Cutting-edge telescope design advances considerably over several decades, and as the previous responders have said, the two instruments operate in totally different circumstances and perform very different kinds of observations. {The poster formerly known as 87.81.230.195} 90.202.168.206 (talk) 13:19, 26 May 2020 (UTC) Mybe this comparison can help: you need a wide angle lens for your camera if you want to take a picture of the whole Symphonic Orchestra (this is LSST), but you need a telephoto lens if you want a good portrait of the third violinist from left, and this is Hubble. Two different instruments for two different tasks. 2003:F5:6F08:8200:3179:F503:CF2E:D769 (talk) 18:40, 28 May 2020 (UTC) Marco PB

How far can a spider descend hanging from its silk on a windless day?

Likely variety-dependent of course. Sagittarian Milky Way (talk) 16:06, 26 May 2020 (UTC)

No farther than the ground. Googling the subject "length of spider silk", here's an item about an African species whose thread can extend 25 yards or more.[1] ←Baseball Bugs ^{What's up, Doc?} carrots→ 19:07, 26 May 2020 (UTC)

Don't forget holes! InedibleHulk (talk) 19:35, 26 May 2020 (UTC)

@Sagittarian Milky Way: The article linked above by Baseball Bugs refers further to: Agnarsson I, Kuntner M, Blackledge TA (2010) Bioprospecting Finds the Toughest Biological Material: Extraordinary Silk from a Giant Riverine Orb Spider. PLoS ONE 5(9): e11234. https://doi.org/10.1371/journal.pone.0011234 --CiaPan (talk) 09:35, 27 May 2020 (UTC)

You may be asking about the strength to weight ratio of spider silk, i.e. how many feet of silk does the silk have sufficient tensile strength to hold up (plus the spider's weight). If you look in that article, there is a table of specific strengths for different materials, that includes spider silk. You should be able to do the math from there. 2601:648:8202:96B0:9BC:E0D4:15B8:4B0A (talk) 05:56, 29 May 2020 (UTC)

What would a camera show at this specific situation related to speed of light?

Imagine there is a train moving in a circle motion near the speed of light with a camera inside it. Inside that small circle, there is a small building with a computer and a screen that will receive the data and show at the screen what the camera is recording. The circle has a small radius, so the distance between the camera the computer is always small despise the train move at near light speed.

Anyway at this very specific situation, what the computer monitor would show?2804:7F2:686:36B7:2910:A06E:9111:2C9D (talk) 23:07, 26 May 2020 (UTC)

How many frames per second is your camera recording? ←Baseball Bugs ^{What's up, Doc?} carrots→ 01:58, 27 May 2020 (UTC)

It is easier to imagine a recording camera, where you watch the recording afterwards. We need to suspend disbelief; a real camera would get crushed by the centripetal force needed to keep it on track. (See also Wikipedia:Reference desk/Miscellaneous#Calculating "g" forces.) Here is a video by Carl Sagan showing a simulation of what a voyager would see near lightspeed. The circularity of the track should not make an essential difference.  --Lambiam 13:39, 27 May 2020 (UTC)

Yes, it'll show what the camera sees. The challenge for our quasi-magical recording and transmittal setup is it has to compensate for the blueshift/redshift of the transmission from the relativistic Doppler effect. But if we've already built a train system that can withstand relativistic speeds this is just another simple matter of engineering. To the questioner: is there some effect you were expecting? If you can help us understand your thinking, we can probably point you at where to look to learn more. --47.146.63.87 (talk) 02:11, 30 May 2020 (UTC)

Is there any atmosphere around? A train moving at near light speed in the air will very soon turn into a hot fireball, and if your circle is "small" it will also burn up building, computer and monitor. This is what happens to meteors, and they are much slower than the speed of light. If there is no air, consider the effect of time dilation. If you are using mobile phone or WiFi to communicate, then they fail to work if there is too much Doppler shift. Graeme Bartlett (talk) 13:04, 31 May 2020 (UTC)

The train will move in what is effectively a vacuum donut, because air molecules will have no time to rush in before the train comes around again. There should be no Doppler effect, as the distance between the moving transmitter and a centrally located receiver remains constant. But time dilation does play a role, so assuming a fixed transmitter frequency in the reference frame of the train, the receiver will have to tune to a lower frequency. And the video shown on a stationary screen will also run slower than the on-board recording, so the frame rate needs adjustment too.  --Lambiam 11:58, 1 June 2020 (UTC)

@Lambiam: We are talking about radio communication here, not acoustic, and travelling at almost light speed, so the Relativistic Doppler effect may be more relevant. See the transversal effect section, and arbitrary movement as well. --CiaPan (talk) 12:28, 1 June 2020 (UTC)

Relativistic Doppler effect § One object in circular motion around the other states that this can be analyzed using simple time dilation arguments.  --Lambiam 12:44, 1 June 2020 (UTC)

I think you're vehemently agreeing with each other. If I'm understanding it correctly the relativistic Doppler effect occurs when two observers have a difference in their relative velocities that is a significant fraction of c ("relativistic speeds" in other words). Then the effects of time dilation and length contraction kick in, causing the observers to perceive transmissions from each other differently then if they were both at rest relative to each other (or moving at non-relativistic speeds, i.e. good old Newtonian mechanics). --47.146.63.87 (talk) 22:03, 2 June 2020 (UTC)