Wikipedia:Reference desk/Archives/Science/2019 February 10

= February 10 =

Will it ever be possible to invent a time machine?
Persononthinternet (talk) 01:00, 10 February 2019 (UTC)


 * The encyclopedia has this article, but a good answer to your question without inline citations might be, "yes." Tamanoeconomico (talk) 02:17, 10 February 2019 (UTC)


 * Or maybe "no." Only time will tell. And keep in mind that forward time travel is a given. But read some of Hawking's work for a reasonable explanation of why backward time travel is illogical. ←Baseball Bugs What's up, Doc? carrots→ 04:32, 10 February 2019 (UTC)


 * You are traveling forward in time as I write this. --Guy Macon (talk)


 * Assumption #1: For the sake of argument, assume that inventing a time machine is possible.


 * Assumption #2: For the sake of argument, assume that actions performed in the past by a time traveler change the future (our present).


 * Is the above assumptions are true, if someone invents a time machine and it becomes widely used, time travelers will keep going back and changing the future (our present) until they make a change that prevents that particular time machine from being invented.


 * So a few days later somebody else invents a time machine, and the process repeats until the new time machine doesn't get invented.


 * Given the above assumptions, time travel is possible but never gets invented.


 * A similar argument hold for the question of whether we are living in a computer simulation:


 * For the sake of argument, assume that we are living in a computer simulation and whoever is running it doesn't want us to find out.


 * Someone figures it out.


 * The simulation owner loads a backup copy from before someone figures it out and kills the person who figured it out before he has a chance to do so.


 * Given the above assumptions, figuring out that we are living in a computer simulation is possible, but nobody ever figures it out. --Guy Macon (talk) 16:32, 10 February 2019 (UTC)


 * In the block time view of the nature of time, the past, present and future are all equally real. Each moment in time can be interpreted as parallel timeless universes. The notion that we're all traveling forward in time is then an illusion as time doesn't really exist. This view is strongly suggested for by relativity. Also quantum mechanics can be formulated in such a way when using the two-state formalism, see here. Count Iblis (talk) 19:27, 10 February 2019 (UTC)

Everyone stop! I've come back to warn you--answering this request for speculation will have dire consequences! For one, it really annoys people and they organize to shut down the Ref Desks. Crap, it's already happening! You finally, really did it! You maniacs, you blew it up!

Ok, all joking aside, OP, you must understand that as this is WP:NOTAFORUM, we can't engage in discussion about what might happen at some far future date. Our relevant article on the topic, as mentioned by Tamanoeconomico, is Time travel. That article gives a concise summary of various physical models which have been proposed for how time travel could occur. Do note, however, that often when physicists talk about "time travel" in this sense, they are typically not speculating about a device that could send an intact person, or even necessarily coherent information, back to an earlier state in time. For more information, you may want to consider searching our archive (which, respectfully, you are meant to do before posting an inquiry here) since this has come up time and time again over the years and you're unlikely to find substantially new or more compelling references or explanations this time around. Nevertheless, I did take a moment to pull up some off-project references that you may wish to take a look at:, , , , , , and  Snow let's rap 07:36, 11 February 2019 (UTC)

There are relevant articles like Tipler cylinder and in a sense the Alcubierre drive (since AFAIK it doesn't respect any absolute frame of reference when exceeding light speed but not going back in time). Those who have tampered in precognition may have some familiarity with the usual time-travel paradoxes. (I see time travel does mention the first two) Wnt (talk) 01:19, 13 February 2019 (UTC)

Why absorbance is measured with Arbitrary Units
I read the pages Arbitrary unit and Absorbance (with google translate) but I don't understand a thing:

For the Lambert-Beer law Absorbance is without unit of measurement; And more: the definition of absorbance (the negative of 10 logartitm of the trasmittance; the trasmittance is the division about two irradiances, so is without unit of measurement) want that she is without unit of measurement.

But there is a problem: if I undertand correct, the absorbance have got a problem: is not possible to predict the real value that will result. The absorbance (not like other undimensional (without unity of measure) quantity like pH, molar fraction etc... that hasen't got the problem of prediction of real value) is like a not undimensional quantity (is like as she has got a measurement unit, but in real she hasn't got it), so we all report her in a graphhycs with the unity of measurement arbitrary unit, what is not a real unity of measurement, but a artificial imposition.

Sorry for this long premise where I write like a mentally retarded (I'm Italian, I don't know how to express these concepts with few correct words), but it was important for my question: Why (because?) absorbance have got this problem of unpredictability value? What were is inside absorbance that create this uncertainty, with the result we need to use the arbitrary unit?

There are other undimensional quantity with this problem?

Thank you, sorry for the umiliant English, good morning--87.5.142.3 (talk) 09:51, 10 February 2019 (UTC)
 * That article is difficult to read! The absorbance unit is dimensionless. To see this you calculate it by logarithm of ((incident power) ÷ (transmitted power)). The unit of power will be Watts. But when you divide Watts by Watts you get no dimension or dimensionless. For example assume there are 10 Watts of radiation hitting a piece of paper and 1 Watt comes through the other side. The calculation is log10(10÷1) or 1. If the material is completely transparent the you will get a value of 0. Contrary to what you say, it will often be possible to calculate absorbance of different materials, and optical engineers will be interested in absorbance of glass or other camera filters, climatologists are interested in absorbance of the Earth's atmosphere, and astrophysicists will study absorbance of the plasma in stars. Graeme Bartlett (talk) 10:51, 10 February 2019 (UTC)
 * The Absorbance article says that it is a mistake to call its units "arbitrary units". The unit for absorbance is really a dimensionless quantity. Graeme Bartlett (talk) 10:58, 10 February 2019 (UTC)
 * Yes, absorbance in dimensionless because derives from a ratio, but is important using the dimension "arbitrary units" because its numerical value is not "univocal": the pure definition of absorbance does not guarantee that you and I, for the same substance and for the same wavelength of light, will measure the same transmittance / absorbance value . For other dimensionless quantity is different, for example molar fraction . The operative definition of absorbance leaves margins of arbitrariness against which it is not possible to predict which number will come out (same reference). I don't understand why the absorbance have got this peculiarity, and not other dimensionless quantity.--87.5.142.3 (talk) 13:03, 10 February 2019 (UTC)


 * The talk comment on Italian Wikipedia is misleading you. Absorbance is not a property of a substance, but depends on thickness and direction etc. If you do not know these facts, you will not be able to reproduce the measurement. But that does make the units "arbitrary units". But say I have a ND2 photographic filter, and we say that visible light is going perpendicular to the surface through it. Then anyone could reproduce the absorbance as log104. Graeme Bartlett (talk) 21:45, 10 February 2019 (UTC)

My feeling is that the use of arbitrary units could reflect (or emphasize) practical issues with the set-up. This is reflected in web hits like where someone argues that "Parameters such as detector efficiency, sample thickness, geometric corrections, solution concentration can involve large uncertainties so the horizontal scale may be unknown by as much as an order of magnitude." Initially I felt a bit skeptical -- the usual control, a cuvette of water, should do reasonably well to constrain these, and results are reasonably though by no means perfectly reproducible. I mean alright, maybe you don't know your path length exactly, but how much can you be off by really? BUT -- the figure you get for the attenuation of a blank cuvette is not 100% in absolute terms! We know obviously that much of the light is lost. So the attenuation is not 100% in real dimensionless ratio of emitted to absorbed, but in "arbitary units" of emitted/absorbed set to be 100% for a blank cuvette. And still ... they're not arbitary in terms of cuvette of water/cuvette of water. Maybe it's a matter of philosophy. In any case, bear in mind that extinction coefficient, indeed molar extinction coefficient is something else again. The goal of all the math is to indeed get at a fixed characteristic property of a substance, e.g. DNA or RNA for the classic OD 260-280 ratio. Wnt (talk) 22:02, 10 February 2019 (UTC)

2009 satellite collision
Per 2009 satellite collision, the satellites collided at a speed of 42,000 km/h. How it came that after over 12 years both satellites retained such speed and momentum, with Kosmos 2251 having no propulsion system according to our article? Was it some sort of gravity assist? 212.180.235.46 (talk) 11:52, 10 February 2019 (UTC)
 * It's fundamental to orbital dynamics that satellites in approximately-circular orbits of the same orbital radius will have approximately the same speeds. (This is less simple for elliptical orbits, as they don't have a constant altitude.) Their velocities can be different, as they could be travelling in opposite directions.
 * So, whether their speed is changing or constant, we know that satellites in similar orbits will have to have similar speeds.
 * It also indicates that for 'air traffic control', satellites all need to orbit in the same direction, or else to be separated by altitude. Otherwise the risk of such accidents increases.
 * As to how they maintained their speed, then that's just Newton's 1st Law, and a lack of strong forces to decelerate them. The altitude is such that air resistance is negligible and tidal effects likewise. Their orbits wouldn't decay appreciably for a much longer time that this. Andy Dingley (talk) 12:39, 10 February 2019 (UTC)


 * Both orbits where at 789 kilometers (490 mi) according to the collision article. The Iridium satellite, which is part of a group of satellites orbits at approximately 17,000 mph (27,000 km/h) on that height. Actually all satellites at the same height need a similar orbital speed to stay in that orbit. So the other one ran the same speed.
 * Because of the high difference of the orbital inclination (tilt) between both orbits, i guess more than 60°, the collision happened at roughly 1.6 times the speed.(27,000 km/h x 1.6 = 43,000 km/h.) --Kharon (talk) 13:11, 10 February 2019 (UTC)

Elimination of the air travel
That unbelievable statement shows the quality of thinkers at the new House of Representative, but what if? Suppose it is implemented in the United States only. How would it affect the Global Warming?

If instead the ground transportation of any kind, including driven by gasoline and diesel engines is eliminated? What is next: horses? I want to know if some realistic numbers could be attached to both "ideas."

There was a period in English history at the end of the 19th century right before the invention of the automobile when horses dominated British commerce and people movement. The streets in London were piled up with manure. I wonder how it affected the Global Warming in progress already at that time?

Thanks, - AboutFace 22 (talk) 22:37, 10 February 2019 (UTC)


 * Maybe we could ask Donald to uninvent the wheel? Martinevans123 (talk) 22:39, 10 February 2019 (UTC)


 * For those unacquainted with the minutiae of US domestic politics, an overview of the Democrats' Green New Deal is here. The actual proposal is to "build out high-speed rail at a level where air travel stops becoming necessary". Alansplodge (talk) 23:10, 10 February 2019 (UTC)


 * Yes, it makes HS2 look like a train set. Martinevans123 (talk) 08:49, 11 February 2019 (UTC)


 * There's nothing wrong with ideas. As to "realistic" numbers, the Republican majority in the Senate is high on that list. ←Baseball Bugs What's up, Doc? carrots→ 23:48, 10 February 2019 (UTC)


 * The idea of replacing air travel with evacuated tunnels (essentially space travel with direct support to remove the inefficiency of rockets) is really very old. We actually have an article vactrain that details some (not all) of these proposals.  The difference now is that people are thinking literally about building hyperloop trains.  Theoretically at least it should be possible to recover every last bit of energy used to move the trains by regenerative braking into room temperature superconductors (getting closer every day! ) so that it would be, to an approximation, a way to travel without using energy.  I don't think it will happen because the infrastructure would cost billions and a handful of petty terrorists with a high powered rifle apiece could probably make it all worthless.  But it's a noble aspiration. :) Wnt (talk) 03:36, 11 February 2019 (UTC)


 * Passenger plane fuel efficiency has developed quite well. Read Fuel economy in aircraft for some examples.


 * Politics would be much wiser to cap automobile power or make it much more costly. Many modern cars have engines with 300-700 hp - often more than 18 wheeler heavy duty trucks - and accordingly very bad fuel economy. Just imagine in the 1950s Mercedes-Benz W120 where luxury sedan! They weighted 1.2 tons and had only 38–43 hp and needed only 1.5 gallons on 100km! Of course you needed 90 seconds to reach a top speed of 120km/h but in most countries that is already close to or beyond the speed limit. WTF you need 700 hp in a sedan for? Beat Porsches at acceleration ? --Kharon (talk) 05:43, 11 February 2019 (UTC)
 * Aircraft today are significantly more efficient than those of yesteryear, but the problem innate to aircraft is you have to fight gravity every step of the way, which inevitably takes more energy (and thus produces more ) than remaining on the ground. Some numbers:   Additionally, aircraft presently rely on chemical fuels, while trains can be electrified and therefore powered by low-emissions energy sources where available. --47.146.63.87 (talk) 08:14, 11 February 2019 (UTC)
 * The streets in London were piled up with manure. I wonder how it affected the Global Warming in progress already at that time? Not at all, as the horses ate food that pulled carbon out of the air. When broken down, the carbon simply returned back to the atmosphere; it was carbon-neutral. This was before the large-scale use of chemical fertilizer produced with fossil energy, so that wasn't a factor. Anthropogenic global warming is caused by us digging up and burning stuff that contains carbon from hundreds of millions of years ago, thus releasing that carbon into today's atmosphere.
 * Also, minor quibble: animal power was the dominant source of land transport until the 1920s at least. Early automobiles were expensive and unreliable and didn't cope well with the poor roads of the time. --47.146.63.87 (talk) 08:14, 11 February 2019 (UTC)
 * But another quibble, very little cargo was moved any distance by road, everything from eggs to coal was moved by rail in the UK before the Beeching cuts of the 1960s, road transport was just for getting stuff to the railway station. "Rail was the most popular means of transport for goods and people throughout the Victorian era and well into the 20th century. In a sense, rail set the tone for 19th century 'progress' and made possible the entrepreneurial successes and excesses of the Industrial Revolution".  Alansplodge (talk) 11:17, 11 February 2019 (UTC)
 * And before those smelly noisy trains there were canals. Quiet, efficient, and very low carbon footprint. Matt Deres (talk) 14:47, 11 February 2019 (UTC)
 * They weren't that efficient. How many donkey equine-animal-pulled canal barges, (or human powered!) are necessary to move the goods that a small crew can move via freight train?  The cost of feeding and maintaining a donkey equine animal, paying every one of those donkey equine animal drivers, etc.  is a stupidly inefficient system.  The only current downside (and it is a big one) of trains right now is the fossil-fuels angle, but as the rate of renewable and non-combustion based fuel sources are improving, eventually that won't be a problem.  -- Jayron 32 19:04, 11 February 2019 (UTC)
 * Canals are now great carbon offsetters, of course, as they are fully of wrecked cars, shopping trolleys and dead bodies? Martinevans123 (talk) 19:11, 11 February 2019 (UTC)
 * Historic canal reborn as low-carbon cargo route (BTW, we Britons used horses not donkeys, one is still operating). Alansplodge (talk) 20:32, 11 February 2019 (UTC)
 * See also Electric aircraft. Count Iblis (talk) 19:06, 11 February 2019 (UTC)
 * The Erie Canal, at least, used horses and mules, not donkeys. ←Baseball Bugs What's up, Doc? carrots→ 00:38, 12 February 2019 (UTC)
 * So fixed. -- Jayron 32 16:19, 13 February 2019 (UTC)
 * "When broken down, the carbon simply returned back to the atmosphere; it was carbon-neutral." While it may have been carbon neutral in the sense that moles of carbon in equaled moles of carbon out, it was taken out of the atmosphere as carbon dioxide and returned to the atmosphere as methane. Methane is a much more potent greenhouse gas than carbon dioxide, so it increased AGW. 202.155.85.18 (talk) 03:58, 14 February 2019 (UTC)