Wikipedia:Reference desk/Archives/Science/2021 January 20

= January 20 =

Alpha Centauri is getting closer
In The Week magazine's latest issue there is a note about a supposed discovery of a narrow radio beam originated in the spot in the sky where Alpha Centauri is supposed to be, and hitting the earth. Narrow in this case is related not to the space but to the frequency of the coming radiation. The authors speculate that since all natural sources of radiation are much wider in spectral sense, the source must be artificial.

Suppose further observations have proved that the source is really artificial, and naturally there will be a talk about sending a signal back. My question is: What will it take technically to accomplish, that is, to create a radio transmitter powerful enough that will send a continuous signal to another solar system like Alpha Centauri and surely reach the destination? Thanks, AboutFace 22 (talk) 03:34, 20 January 2021 (UTC)


 * Depends on how sensitive the receiver you are assuming at the other end is. --142.112.149.107 (talk) 06:18, 20 January 2021 (UTC)


 * If we knew they were expecting a signal from us, we probably already have the hardware we need.
 * Unfortunately, we just lost Arecebo, but there are a couple other transmitters that have been used for interstellar "active SETI" transmissions.
 * The NASA Deep Space Network is a candidate.
 * Looking over the List_of_interstellar_radio_messages, I don't see any directed at Alpha Centauri. Probably because it's not normally believed to be habitable. ApLundell (talk) 06:28, 20 January 2021 (UTC)


 * Arecibo would have been useless for this anyway. It could only point to declinations from -1 to +38 degrees, but α Centauri is at -61 degrees, too far south. The Canberra station of DSN could do it, if we assume a sufficiently sensitive receiver on the aliens' end, pointing to us at the time when our signal arrives. PiusImpavidus (talk) 09:33, 20 January 2021 (UTC)


 * Using the idea of a phased array, it should be possible to build a huge transmitter consisting of many small ones, which could be dispersed over a large area. See also the Karl G. Jansky Very Large Array and the Atacama Large Millimeter Array (which does not operate at large millimetres; rather at small ones), observatories based on the same idea except now serving as a huge receiver. Using a regular deployment pattern simplifies the computations but is not essential, but the scattered components must agree very precisely on the clock time. --Lambiam 08:14, 20 January 2021 (UTC)


 * As noted above, we could use the Deep Space Network to send a signal to Alpha Centauri (or other nearby stars); the question, therefore, is not whether we can, but whether we should (and there are many reasons why this is not a good idea). 2601:646:8A01:B180:6D2B:1920:798B:506B (talk) 10:42, 20 January 2021 (UTC)

Quote: "....and there are many reasons why this is not a good idea," Why? What are the reasons? AboutFace 22 (talk) 13:00, 20 January 2021 (UTC)
 * To put it over-simply, if they know we're here, they might come and eat us all! Less crudely, interstellar spaceflight on any significant scale might or might not be logistically possible, but given the degree of home-grown malicious trolling that we already suffer, should we risk even a slim possibility of subjection to perhaps much more advanced and capable informatic disruption from completely alien entities that might, for all we know, consider our very existence to be a terrible blasphemy. The genre of Science Fiction has been exploring such themes for decades: for an interesting recent example, see Liu Cixin's international award-winning The Three-Body Problem and its sequels. {The poster formerly known as 87.81.230.195} 90.200.40.9 (talk) 15:20, 20 January 2021 (UTC)

I think interstellar travel is technically impossible for any civilization, advanced or not. Communications ... probably. It would be interesting to get a signal from them and see what they can say. AboutFace 22 (talk) 20:35, 20 January 2021 (UTC)
 * Perhaps. Historically, though, pronouncements that something is technically impossible have a poor track record. --Trovatore (talk) 21:16, 20 January 2021 (UTC)


 * Stephen Hawking famously warned against expecting contact with aliens to have a happy ending. "If aliens visit us, the outcome would be much as when Columbus landed in America, which didn't turn out well for the Native Americans. We only have to look at ourselves to see how intelligent life might develop into something we wouldn't want to meet." --Lambiam 22:11, 20 January 2021 (UTC)

It is one thing to move a massless photon across 4.5 light years of space and another thing to move a massive spacecraft across a fraction thereof. The boundaries of physics are well known now and they tell us that space travel is impossible. The issue was closed more than 100 years ago. AboutFace 22 (talk) 02:35, 21 January 2021 (UTC)
 * That's complete bullshit. It's a very difficult engineering problem.  There is no strictly physical reason it can't be done, or at least it has not yet been elucidated. --Trovatore (talk) 03:55, 21 January 2021 (UTC)
 * Superluminal travel is currently (as it is understood) not just an engineering problem, the limit of the speed of light is a physical one of the universe, and well tested. -- Jayron 32 13:42, 21 January 2021 (UTC)
 * Interstellar travel does not necessarily require superluminal travel. --Trovatore (talk) 18:15, 21 January 2021 (UTC)
 * We can play these games all day. Have fun building your space ship to travel to Alpha Centauri.  Send us a post card when you get there.  -- Jayron 32 19:04, 21 January 2021 (UTC)
 * Advanced civilizations will be machine civilizations who travel by uploading the information in their electronic brains to machines at the destination. Civilizations may spread across the galaxy and beyond by sending signals to catch the attention of inferior biological creatures like us who then get all excited and try to look out for more signals. The signal may be followed by other signals containing simple messages. The messages after that will be about how to decode the more complex messages that are yet to come. This way the civilization will be able to communicate to us how to build their hardware that we need to use to run their software. The next message will then contain the software that we need to install. This is then one-way communication, they are simply going to repeat the entire sequence of messages over and over again without listening for replies. So, the civilization could be located in the Andromeda galaxy and we could download and run ET simply by picking up messages sent to us in a matter of days. But if we were to do that the machines we've build could end up taking over our planet. Count Iblis (talk) 03:58, 21 January 2021 (UTC)


 * 100 years ago? Gee. Did nobody tell NASA, The British Interplanetary Society, and the Breakthrough Starshot Initiative (Founded by Hawking and Zuckerberg)? ApLundell (talk) 04:06, 21 January 2021 (UTC)


 * The boundaries of physics are not at all well known. How would and could we know? The laws of physics (as they are understood now) tell us in fact that space travel is possible; Voyager 2 has now travelled a distance of 23 billion km through space. Perhaps you meant interstellar travel, which is more daunting, or FTL travel, which as far as we know now may be impossible because of theoretical limitations, but we do not know for sure if these are truly fundamental or reflect a limitation of our understanding. --Lambiam 04:30, 21 January 2021 (UTC)
 * Generation ships are certainly technically possible. And things like nuclear pulse propulsion can get you going at a nice clip. What's impossible based on current understanding is Star Trek-style casual zipping about the galaxy. --47.152.93.24 (talk) 04:36, 21 January 2021 (UTC)

The speed of Voyagers is 47 kilometers per second. That speed could not be reached through ordinary chemical acceleration. Planetary bypasses were employed. Even with a speed like this a distance to Alpha Centauri may be covered in 82,000 years. Does it tell you something? I suspect you all read too many SciFi novels or worse. How much would it cost to make a spacecraft like this? What could the second generation people on this craft say? They may well ask: Why are we here? Who made this strange decision for us? Let's turn back. This is just one of the maltitude of problems with spaceflight. AboutFace 22 (talk) 16:46, 21 January 2021 (UTC)
 * You made an assertion and the burden of proof is on you. You said that "the boundaries of physics" have made it a closed issue. But you haven't come remotely close to justifying that assertion.  You have only suggested ways that such travel might be attempted, and argued that those ways cannot practically work.  If you don't understand the difference, then you're missing something at the level of basic logic. --Trovatore (talk) 18:19, 21 January 2021 (UTC)


 * By showing a finite travel time by your own calculations, you yourself have already shown it's physically possible. I don't think you know the meaning of the words possible or impossible. Whether it's practical is a different matter. And that's just interstellar spaceflight. In fact, you went a step further and said "space travel is impossible". Might want to tell the crew of the ISS that. Fgf10 (talk) 19:57, 21 January 2021 (UTC)
 * Sure, there are colossal issues of sociology, psychology, engineering, economics, and so forth. But it doesn't break any physical laws to point yourself in a direction and go that way. We've got a big backyard to play in, so the logical focus will be on Solar System habitats first. --47.152.93.24 (talk) 23:02, 21 January 2021 (UTC)

Reflecting telescope Vs Refracting telescope
Which is the best telescope for astronomy purpose? Rizosome (talk) 14:15, 20 January 2021 (UTC)
 * That depends to some extent on what you are actually trying to do. The vast majority of modern large telescopes are reflectors, because mirrors or mirror assemblies can be built to much larger sizes than lenses. Lenses are restricted to about one meter in diameter (Yerkes Observatory), whereas we are now building reflecting telescopes up to 39 meter diameter (Extremely Large Telescope). --Wrongfilter (talk) 14:34, 20 January 2021 (UTC)
 * Yes, and don't forget that Aperture synthesis and Astronomical interferometer techniques only work for mirrors, so they will always beat refractors. Mike Turnbull (talk) 14:38, 20 January 2021 (UTC)
 * Also don't forget that optical interferometry is very hard and most optical telescopes are not interferometers. --Wrongfilter (talk) 14:52, 20 January 2021 (UTC)
 * This is akin to asking, "What type of automobile is best?" The answer depends on a lot of things; there is no universal answer. Try some Web searching for telescope ratings. Also, note that there's a third category, catadioptric scopes, which in turn has a bunch of subtypes. --47.152.93.24 (talk) 04:31, 21 January 2021 (UTC)