Wikipedia:Reference desk/Archives/Science/2018 October 7

= October 7 =

What will be the most correct definition for light?
93.126.116.89 (talk) 09:45, 7 October 2018 (UTC) The light is a result of energy? "The light is a result of chemical reaction (fire) or physical energy (electricity)" is correct? 93.126.116.89 (talk) 23:41, 6 October 2018 (UTC)
 * That definition does not describe light specifically. Fire and electricity can also produce heat, which is different from light. Light is electromagnetic radiation with wavelenghts visible to the human eye (sometimes ultraviolet or infrared 'light' is also included). - Lindert (talk) 23:57, 6 October 2018 (UTC)
 * And light can be produced in many other ways than chemical reaction or electricity. "physical energy" is too vague to say what it includes. PrimeHunter (talk) 00:28, 7 October 2018 (UTC)
 * To expand on those replies, the statement is wholly wrong. Light is produced when something emits a photon. The photon is the gauge boson of the electromagnetic interaction. Photons are produced in an electromagnetic interaction. All baryons radiate photons all the time. That includes you and pretty much everything you interact with daily. Those photons are of too low a frequency for your eyes to detect, but if something gets hot enough, the photon frequency increases until they become visible. This is what happens in a fire, or an incandescent light bulb, or the Sun. Other processes also produce photons, one example being when electrons and holes recombine in a light-emitting diode.
 * Where do the photons come from? Well, they come from nowhere. They are created ex nihilo. However, for real (that is, non-virtual) photons, the energy of the photon does have to come from somewhere. If this bothers you, you are in good company. Niels Bohr reputedly said, "anyone who is not shocked by quantum theory has not understood it." But the universe doesn't care what we think about it. --47.146.63.87 (talk) 05:38, 7 October 2018 (UTC)
 * "Those photons are of too low a frequency for your eyes to detect, but if something gets hot enough, the photon frequency increases until they become visible." Does it mean that colors that we see in daily life contains high frequency photons? Id so, why does it happens, here is the color can be the coldest in the world and almost no energy in it?! And as I understand there should be some energy to make photons in high frequency. Isn't it? 93.126.116.89 (talk) 09:45, 7 October 2018 (UTC)
 * All photons have energy. Photon energy is directly proportional to the frequency. Visible light is in a small frequency interval around 430–750 terahertz. The frequency and energy of a photon can both be billions of times larger and smaller than this. See Electromagnetic spectrum. PrimeHunter (talk) 11:43, 7 October 2018 (UTC)
 * This [[Media:Plutonium_pellet.jpg]] is a lump of plutonium that is glowing orange black-body radiation because it surface has been heated to about 2500K, from natural decay. If you heated, say steel, in any manner to 2500K, it would glow with a similar colour, even after the source of the heat has been removed. This [[Media:Gaseous_tritium_light_source.jpg]] is tritium. The tritium emits electrons, which strike phosphor on the inside of the glass. This energises electrons in the atoms. They then loose that energy, emitting photons on specific frequencies. LongHairedFop (talk) 13:19, 7 October 2018 (UTC)

Erie-builts in Russia?
Were the Kharkiv Locomotive Factory 2D100 and 10D100 engines (as used on the TE3 and TE10 locomotives respectively) based on Fairbanks-Morse engine designs such as those used in their Erie-built series of locomotives? 2601:646:8A00:A0B3:AC2E:3471:38A3:4615 (talk) 02:01, 7 October 2018 (UTC)


 * I can't directly answer your question, but this article about tank motors suggests that the Soviet Union was a world leader in diesel engine design in the 1930s, so seem likely to have been able to design their own without having to copy anybody else. Alansplodge (talk) 08:12, 8 October 2018 (UTC)
 * Hmmm... Quora is sometimes an interesting read, but is not RS and Additionally, soviet B-2 was the first aluminium engine in the world, so it was kind of high-tech of that time and the British simply did not have the know-how to produce it. is especially dubious. The UK, the country which had invented the nickel-aluminium alloys twenty years earlier (see Y alloy and Hiduminium) and which fielded the most diesel tanks in the early part of the war (the Matilda ran on a pair of AEC diesel bus engines, because the UK had been operating large numbers diesel road vehicles since 1930. The Valentine also used this engine.); "did not have the know-how"?? Andy Dingley (talk) 09:08, 8 October 2018 (UTC)


 * I'm not an expert on Soviet loco engines, but AFAIK, they took a lot more inspiration (as did post WWII Britain) from the Junkers designs, than from Fairbanks-Morse. Opposed piston engines were not uncommon for large diesels in the 1930s, although they're seen (incorrectly) as esoteric today. Andy Dingley (talk) 09:09, 8 October 2018 (UTC)

Antarctic rescue
Were Kharkovchanka off-road vehicles ever used in rescue missions in Antarctica? 2601:646:8A00:A0B3:AC2E:3471:38A3:4615 (talk) 02:20, 7 October 2018 (UTC)


 * Read the article, follow the ref. Use Google Translate if you don't read Russian. --Guy Macon (talk) 06:03, 7 October 2018 (UTC)


 * not quite a rescue mission, but here one V. Sanin recounts how he and a couple of coworkers got lost in a snow-storm and found their way back thanks to Kharkovchanka's chain tracks.
 * Kharkivchanka is due to some over-eager Western editor who sympathizes with Ukrainians. The Ukrainian for Kharkovite FEM is Kharkiv'yanka, not Kharkivchanka, and this is what the Ukrainian article calls it. Someone please fix.  Aecho6Ee (talk) 15:55, 7 October 2018 (UTC)

Questions of Our Expanding Universe
Many currently published theoretical physicists reference a universe that is expanding at an increasing rate and which has been doing so since the 'Big Bang' that first created the universe 13.7bn years ago. They also reference the fact that when we 'look out into the universe around us' we see everything moving away from us. From these two ideas follow my two questions: 1. If everything is moving away from us, then that means, does it not, that we are at the 'centre' of the universe? If we are not at the centre then we must surely observe things moving towards us too. 2. If the universe's expansion since the Big Bang has been continuously accelerating then surely it must have been expanding very slowly indeed at the beginning if it is still expanding now. And because the universal constant, c, (incidentally, the speed at which light travels) is finite and known, then after such a long period of time surely terminal velocity of matter would have been reached. Jeremy — Preceding unsigned comment added by 82.34.208.156 (talk) 08:11, 7 October 2018 (UTC)


 * One of the easier ways of thinking about this is to consider evenly spaced dots on the surface of an inflating balloon - all of them spread away from each other and none of them get closer. It's not a perfect analogy, but it may help. Mikenorton (talk) 08:24, 7 October 2018 (UTC)
 * Edit conflict: The way in which the universe is expanding is not like an explosion where everything moves outward from a definite center. You can imagine it like this: take a balloon and draw two dots on it with a sharpie. The balloon is a 3D object but its surface is 2D. The relationship between the two points can be described in terms of their relative locations in that 2D space. Now blow the balloon up. As far as the two dots on the 2D surface of the balloon are concerned, they are moving away from each other, but there is no "center" of the balloon's 2D surface. We can mathematically describe objects in a space of more than 3 dimensions. For example, we can add a 4th axis to a regular Euclidean space. Then we can imagine an surface that is exactly 1 unit from the origin at every point. That object is a 3-sphere, the 4 dimensional analogue of sphere. The 3-sphere is 4D but its surface is 3D. If we "blow up" the 3-sphere in a way analogous to how we blew up the 3D balloon, we increase the volume of the 3-sphere's 3D surface in a way analogous to the 2D surface of the balloon increasing. And that's basically what's happening to our universe. The volume of our 3D space is increasing in all directions, all the time. It's not expanding out from any center that exists within out 3D space, and it's not expanding "into" anything. 139.194.67.236 (talk) 08:32, 7 October 2018 (UTC)
 * Actually, I've found the balloon model, while common, to be somewhat lacking given that it still depends on the balloon "expanding into" something else to visualize or see. There's an alternate view of the expanding universe that works by understanding the expansion of the universe like Hilbert's Infinite Hotel.  In the Hilbert Hotel, you have a hotel with an infinite number of rooms and guests in each one.  When a new guest arrives, the hotel manager just has every guest move down one room (the guest in room 1 moves to room 2, and room 2 to room 3, and so on) and the new guest now has an empty room (1) to move into.  You can understand the expanding universe on similar terms, except that there aren't new arriving guests.  In an infinite hotel, imagine that in the first hour, the guests move such that they leave an empty room between each guest, abandoning rooms as they expand out into the infinite hotel.  In the second hour, they repeat, so that over time, the rooms gradually become emptier and emptier.  They hotel is infinite, so there's nowhere outside the hotel for the guests to "move into", and yet over time the hotel gradually becomes less and less "dense" in people, as people spread out through the infinite hotel.  In the same way, an infinite and infinitely dense universe (as existed at the Big Bang) can gradually become less dense as it expands.  It isn't moving "into" anything, the "space" between the parts of it are created by the expansion itself.  -- Jayron 32 16:20, 9 October 2018 (UTC)
 * The OP's post contains only one question and its second section just states a deduction. In the expanding universe described by the above models, galaxies recede from us at velocities proportional to their distances from Earth. Astronomers observe galaxies' resulting Redshifts due to the Doppler effect. The limiting velocity c relative to Earth corresponds to the finite spherical size of our Observable universe. A different location in the Universe has its own observable spherical volume, different from ours. DroneB (talk) 14:05, 7 October 2018 (UTC)


 * Astronomy and thus cosmology are actually still in an very early stage of collecting data aka evidence! The Hubble Ultra-Deep Field, the first try to look as far as technology allows happened just 14 years ago. The Herschel Space Observatory is less than 10 years old. The Spitzer Space Telescope is looking for planets in other systems around us since 2003. So Astronomy is barley starting to explore with serious technology/instruments. The James Webb Space Telescope, which unfortunately still is in launch preparation, will be a huge step forward in that regard. In that sense lots of cosmology is more mainstream theories than verified science!! --Kharon (talk) 18:28, 8 October 2018 (UTC)


 * To put the answers very simply: yes, we are at the centre of the expanding universe, and so is every other point in the universe, and no, there is no terminal velocity — the metric of space can expand at any "speed".  Dbfirs  21:22, 8 October 2018 (UTC)
 * Have any objects been detected receding from us at greater than the speed of light? ←Baseball Bugs What's up, Doc? carrots→ 22:08, 8 October 2018 (UTC)
 * I believe so, yes, depending on what you mean by "receding". That is, if you look at the distance from our galaxy to one of these very distant galaxies at the same comoving time, and look at the rate that is increasing with respect to time, that rate is greater than c, and nevertheless we can see light from those galaxies.  You can argue about whether that counts as "receding faster than light". --Trovatore (talk) 22:26, 8 October 2018 (UTC)
 * I mean relative to us, as if we were the center of the universe, as per the OP's question. But are the distant objects simply moving fast, or are they accelerating? And if they are, what force would be behind the acceleration? ←Baseball Bugs What's up, Doc? carrots→ 22:55, 8 October 2018 (UTC)
 * Well, it's not so simple to figure out what "relative to us" means in this case, because we don't share an inertial reference frame with the distant galaxies. I was careful to word what I meant in a precise way.  Figuring out whether the distant galaxy is really moving away from us that fast, or whether it's just that more space is being added in between us, is trickier, arguably a question of interpretation. --Trovatore (talk) 03:01, 9 October 2018 (UTC)


 * Imagine you and I are each standing on a giant balloon, some distance apart. I send a steady stream of ants, walking towards you. Let's call it 100 ants per second. And lets say I have a clock, and as I send each ant, I write my time on the back of each ant. So I'm releasing a 100 ants per second, and after a while the first one reaches you. As long as neither of us is moving, and as long as the balloon isn't expanding... you see the ants arrive at 100 per second. You also see the time you see written on them (a delayed copy of my clock time) increasing at normal speed (you see it going up 1 second every second). Now we slowly start inflating the balloon. You and I are getting further apart. The ants have further to walk, and they take longer to reach you. I'm still releasing 100 ants per second, but you only receive 99 ants per second. (The extra ant is between us, still walking towards you.) As long as the balloon is inflated at the same slow rate, the distance between us increase at the same slow rate, and you keep receiving 99 ants per second. But now lets say the balloon starts getting inflated faster, the distance between us grows faster. You only receive 98 ants per second. Then 97 ants per second, and so on. After a while you're only getting 50 ants per second. (All the extra ants are strung out between us walking the extra distance.) Recall that I was writing my clock-time on each ant. You're only getting 50 per second, so the written-time is going up at half speed. When you look at the image of my clock, it seems to be running slower and slower. As the balloon inflates faster and faster, you only receive 10 ants per second, and later you only receive 1 per second. At a certain point I send an ant towards you, but it can't actually get any closer to you... the space between us is growing at the exact same speed that an ant walks. That is the exact moment when you and I moving apart at the speed of light. I keep sending ants towards you, but the balloon is expanding and carrying those ants away from you faster than the ant can walk. You never see any more of my ants. So.... as a galaxy moves away from you faster and faster, it looks like its clock going slower and slower. The image also starts stretching towards the red and getting fainter. The light stretches out to infra-red and then radio waves. Just as the galaxy is about to recede from you at the speed of light, it takes longer and longer for the image to reach you..... increasing towards an infinite amount of time for the image to reach you. It's clock appears to slow, infinitely close to a halt. You never loose sight of it, you forever see a frozen image of the stopped clock, and the light stretches towards infinitely dim infinitely long radio waves. It's forever visible to you, it just gets harder and harder to see it. So the answer is yes, we can see galaxies that are receding from us faster than the speed of light. But we're seeing old light, light that was sent when the galaxy was closer, light that was sent when galaxy was receding at less than the speed of light. The galaxy is receding faster than the speed of light, meaning we'll never see any of that light. We will forever see old images of the galaxy, and those images will slow towards a frozen clock-image, getting infinitely dimmer and fading towards infinitely long infinitely slow radio waves. A frozen image from the moment it was receding at exactly the speed of light. Alsee (talk) 11:24, 9 October 2018 (UTC)
 * That is a brilliant explanation. Thanks! --Stephan Schulz (talk) 17:44, 9 October 2018 (UTC)
 * I would like to second that. Both deeply illuminating and a pleasure to read. Jmchutchinson (talk) 20:36, 9 October 2018 (UTC)
 * Thirded! Double sharp (talk) 03:14, 10 October 2018 (UTC)
 * Infinited! Akld guy (talk) 23:51, 11 October 2018 (UTC)

Dark Energy
We read that Dark Energy constitutes approx. 68% of all energy in the universe (baryonic (real) matter, dark matter and dark energy). Well isn't that percentage quite obviously the ratio of the surface area of a sphere to its volume? In other words Dark Energy is not yet 'seen' because we're not looking in the right place. We 'expect' to find it on the surface of the universe and we need to look inside / under the 'surface'. [Analogously it would be like calculating the mass of our Earth as that land which we 'see' only on its surface]. — Preceding unsigned comment added by 82.34.208.156 (talk) 08:21, 7 October 2018 (UTC)
 * That's numerology and not even enough digits to indulge in good numerology. Plus the ration of a volume and an area is a length not a number. Dmcq (talk) 10:12, 7 October 2018 (UTC)


 * Dark matter is only assumed because without it the relation of the gravitation of all visible matter of our home galaxy and the rotation speed of our home galaxy (basically Keppler's law) does not add up. No prove of the existence of such matter or energy has been presented till today. --Kharon (talk) 18:02, 8 October 2018 (UTC)
 * I think of those postulated items as being the modern equivalent of the Aether. ←Baseball Bugs What's up, Doc? carrots→ 21:07, 8 October 2018 (UTC)


 * There is the difference that we know today "Aether" does not exist. In contrast we dont know if dark matter does or does not actually exist. But your right i would guess. Isnt it kinda funny and revealing to notice todays Scientists still sometimes boldly make something up like the Quacksalvers in the Medieval?! --Kharon (talk) 22:30, 8 October 2018 (UTC)
 * It is a human-brain tendency to try to "fill in the gaps" to make sense of something. I suppose it's not just human brains that do that, except most animals focus on survival and not on celestial mechanics. ←Baseball Bugs What's up, Doc? carrots→ 22:53, 8 October 2018 (UTC)
 * There is extremely strong evidence for "dark matter" as a real thing. The rotational anomaly could possibly be explained by something like MOND, but there is e.g. the Bullet cluster, which is very hard to explain without dark matter. Dark_matter has more information. The evidence for dark energy is more tentative. --Stephan Schulz (talk) 22:59, 8 October 2018 (UTC)
 * And the OP wasn't asking about dark matter anyway. Someguy1221 (talk) 00:42, 12 October 2018 (UTC)

The percentage of all energy that is explained by dark energy changes over time under the current standard model of cosmology. The fact that it is close to 68% right now should be considered coincidental. Someguy1221 (talk) 00:42, 12 October 2018 (UTC)