Wikipedia:Reference desk/Archives/Science/2017 October 18

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October 18

How (un)likely are these science fiction inventions to ever exist in this universe?

If the universe is real, non-supernatural and the things physicists think are ruled out really are.

1. Affordable cars like in the Fifth Element or Star Wars.

2. Earth-like gravity in space without accelerating, spinning or having a massive enough spaceship.

3. Rayguns that exceed the utility of current handguns in the average application (i.e. if everything's similar except hits damage twice the meat and magazines hold 1 shot they might not be better for most things (also ignoring price, sunk cost of already having a gun, how much or little they're taken seriously by potential victims, bans/raygun control or that coolness could make many switch a bit before it's rational since these are either mostly transient or not actually affecting how good they are))

4. Vehicles that look like rounded boxoids or simple geometric solids and have at least the speed, range, acceleration and maneuverability of cars combined with the altitude capabilities of spaceplanes. And can do any movement in any orientation as long as it doesn't subject it to uncomfortable g-forces or density-adjusted airspeeds or compressional heating above it's speed capabilities. Means of moving/hovering/turning/flying etc not visible to the naked eye from outside are acceptable. Like I don't know, microscopic particle accelerators coating the hull. Could it also plausibly do single-stage-to-orbit and back? Without losing >10% mass? Accelerate to 0.1c at 1g, turn around and stop at 1g? If it's limited to 1g and 0.1c and has to arrive stopped, what's the furthest it could plausibly go on 1% of it's mass lost as fuel? Sagittarian Milky Way (talk) 03:43, 18 October 2017 (UTC)

Whoa. InedibleHulk (talk) 03:46, 18 October 2017 (UTC)

1) Do you mean hovercraft ? They exist, but are quite impractical for most private civilian uses, as they would tend to blow down pedestrians as they pass by, and make a lot of noise.

Some can fly at least thousands of feet high unlike hovercraft. They're also not as loud and I think they were shown to dive in Star Wars. Korben Dallas pitched his cab into a dive and parked it vertically in the smog to hide. Sagittarian Milky Way (talk) 07:19, 18 October 2017 (UTC)

In that case it may be more of a ground effect vehicle. These can't normally fly far from the ground, but perhaps a variation on one with extendable wings could, at least for short periods, but less efficiently than when skimming the ground. StuRat (talk) 15:34, 19 October 2017 (UTC)

2) This would seem to violate the known laws of physics.

Could collimated graviton rays ever be shot out of the floor? Sagittarian Milky Way (talk) 06:53, 18 October 2017 (UTC)

3) Seems plausible. In particular, it could automatically target whoever you look at, or rapidly aim and fire at everyone in range. The lack of recoil would be quite a plus. For a power source, how about a nuclear reactor in a connected backpack ?

4) You might want to break this up into subparts. StuRat (talk) 06:02, 18 October 2017 (UTC)

As for number 4: Relativistic mass at 0.1c is 1.005 m₀, or, even at 100% efficiency, you need to spend about 0.5% of your original mass to accelerate to 0.1c. Braking has the same cost, so your minimum mass loss for going to 0.1 c and stopping is already 1% (plus a few digits I have rounded down). Since there will never be a 100% mass to kinetic energy conversion (part of the energy goes into your reaction mass), I'm pretty sure that number 4 will never happen, unless you use "cosmic energy" or invent a way to do regenerative braking relative to the cosmic microwave background (both of which, I'm sure, have been described in SF, but neither of which is possible within the scope of our scientific understanding). On the other hand, if you somehow manage to get up to 0.1c, there is basically no limit of how far you can go (well, how far your vehicle can go - you may run out of resources like air, water, food, or lifetime). --Stephan Schulz (talk) 07:53, 18 October 2017 (UTC)

And Stu, a nuclear backpack is not such a good idea. Nuclear reactors are still heat engines, and a portable version will be lucky to run at 30% efficiency. So for every Watt you fire at the enemy, you need to provide cooling to get rid of two Watts from your backpack. And nuclear reactors cannot be regulated up and down quickly (even if you use control rods to quickly interrupt the main chain reaction, this will create neutron poisons, and there still will be many unstable intermediate reaction products that keep producing decay heat), so you basically need to keep it running at full output all the time you may want to use the gun - in which case you need to get rid of even more waste energy. --Stephan Schulz (talk) 08:02, 18 October 2017 (UTC)

I agree that it would be dangerous, but then so were gasoline-filled "backpacks" use with flame-throwers. StuRat (talk) 14:16, 18 October 2017 (UTC)

Danger is fine. But if you want to feed your 1kW ray gun directly from a nuclear backpack (at, if you are lucky, 50% efficiency of conversion from electricity to "ray"), and have a 30% efficiency for conversion from nuclear pile heat to electricity, you need to get rid of 6kW of unwanted heat when you don't fire it, and 5kW of unwanted heat if you do. That's about 2-3 space heaters worth of heat generation. Not only will you look like a Christmas tree on steroids to any infrared seeker, it will be quite easy to burn yourself. And 1kW is a puny little death ray - neither Ming the Mercyless nor Dr. Doom would bee seen with something this powerless. --Stephan Schulz (talk) 17:30, 18 October 2017 (UTC)

True, but flame-throwers also generated lots of excess heat and made the wearer highly visible (IR detectors weren't much used in WW2, but then they weren't needed to spot somebody using one of those). StuRat (talk) 18:30, 18 October 2017 (UTC)

Which (as well as their tendency to engulf the user and anything nearby in flames when hit by weapons fire) is why they were never very useful, and are barely used anymore. Flamethrowers are a great example of Awesome but Impractical. --47.138.160.139 (talk) 21:09, 18 October 2017 (UTC)

Laser weapons (or maser, etc.) could have some advantages over projectile weapons. One would be not needing to adjust for distance drop, wind, etc., on sniper shots. Always hitting with the first shot would be quite an advantage. There are disadvantages, too, but I wouldn't go so far as to say that personal weapons of this type will never be practical for any application. I can also imagine a time when bullets can be tracked in flight and the source automatically targeted. This may make such weapons far less practical than they are now. StuRat (talk) 01:19, 19 October 2017 (UTC)

For (1) I would direct to flying car - "affordable" depends on who you are and how much worse (or better) things get. For (2) I can say that general relativity equates gravity and acceleration, so current thinking is that you cannot have gravity without acceleration. Simulating it with magnetic boots or some vastly more elaborate setup of nanotech fibers is obviously possible with varying degrees of plausibility. For (3) I would direct you to [1]. In general there is no fundamental-physics reason why you can't store and deliver as much energy as is in gunpowder and bullets using batteries and light, but of course practicality leaves much to be desired presently. Wnt (talk) 12:02, 20 October 2017 (UTC)

• I was going to ask a follow-up question: whether there was any point of having "We don't answer requests for opinions, predictions or debate" appear at the top of the page that this request for opinions, predictions or debate is on, but I think I already have the the answer.--Shirt58 (talk) 02:58, 21 October 2017 (UTC)

• What is possible isn't idle speculation, as it's based on the known laws of science. StuRat (talk) 04:55, 22 October 2017 (UTC)

Elliptical Orbit of planets

Why do the planets revolve around the sun in elliptical orbits and not in circular orbits? — Preceding unsigned comment added by Hemant1776 (talk • contribs) 11:30, 18 October 2017 (UTC)

A circle is merely a special case of an ellipse where the two foci are merged to a single point. Statistically, planets have an infinite number of ways to orbit in an ellipse, but only one circle, so it's really just a question of the math; it's basically almost impossible for it to work out to be a circle, as 1 out of infinity is basically never. --Jayron32 11:34, 18 October 2017 (UTC)

• As noted, a circle is also an ellipse.

This is due to gravity having an inverse square law behaviour. Johannes Kepler had already observed enough to work out this behaviour (but not to explain it), and that satellite's orbits "swept out equal areas in equal times". This inspired Isaac Newton to give his model of gravity an inverse square law behaviour, thus explaining Kepler's observations.Andy Dingley (talk) 12:07, 18 October 2017 (UTC)

Our article Kepler orbit, albeit a bit long and technical, has all the information - including a demonstration of why the orbits are elliptical (from Newton's second, which was discovered later as AD said, but it is conceptually simpler in that direction). Tigraan^{Click here to contact me} 12:12, 18 October 2017 (UTC)

I agree with both Jayron and Andy Dingley. An alternative perspective that appeals to me is to observe that the only force acting on a planet is its own weight (the gravitational force between the planet and the center of mass of the rest of the solar system, or even the universe.) If the only force acting on any object is its own weight, the mechanical energy of the object will be conserved. So the kinetic energy of the planet, and its potential energy, can vary up and down but its mechanical energy will remain constant, just like a pendulum swinging back and forth.

If a planet revolves around the sun in a circular orbit its potential energy will remain constant, and so will its kinetic energy, just like a pendulum hanging vertical and motionless. This is a special trivial case of all the different possibilities. There are infinite alternatives in which the planet has variable potential energy (but constant mechanical energy and therefore variable kinetic energy); and these infinite alternatives are associated with elliptical orbits. Dolphin (t) 12:19, 18 October 2017 (UTC)

• Feynman's Lost Lecture is worth a read too, for a graphical proof of Newton. This has been published as a small, cheap paperback. Andy Dingley (talk) 12:20, 18 October 2017 (UTC)
• Basically per above. But it should be mentioned that Newtonian mechanics only allows two orbital solutions to the two-body problem: the orbit of both is spherical around the barycenter; or elliptical around the barycenter. With massive bodies orbiting smaller bodies the barycenter will be typically within the massive body, so the motion of the planets around the Sun has an approximately stationery Sun. See Orbital eccentricity: spherical orbit are merely a special case with zero eccentricity, and not much likelihood as explained earlier.

Relativity also complicates things slightly, leading to apsidal precession; but this effect is usually very small. See Two-body problem in general relativity for a discussion.

There is no general solution to three or more bodies orbiting: see Three-body problem. But in practice the orbits of planets around the Sun are are approximately elliptical; perturbations (differences from the elliptical) are mostly due to the gravitational attractions of other bodies, especially the planets; see Perturbation theory. --Jules (Mrjulesd) 12:47, 18 October 2017 (UTC)

Nitpick: Newtonian mechanics only allows two solutions to the two-body problem: the orbit of both is spherical around the barycenter; or elliptical around the barycenter - only in the case of bounded trajectories. If the initial relative speeds are large enough, the kinetic energy is high enough to allow one body to escape the other's gravitational well (hyperbolic trajectories in the Kepler problem). Tigraan^{Click here to contact me} 13:10, 18 October 2017 (UTC)

True, I have amended it to "orbital solutions" --Jules (Mrjulesd) 13:13, 18 October 2017 (UTC)

It goes back to the formation of the solar system. Typically the planetary orbits have very low eccentricity (i.e. they are almost perfect circles). Their axes also have very low tilt and they revolve and rotate in the same direction. Their orbital planes are almost identical. This applies to the satellites (moons) as well. If some other factor intervenes eccentricities can be very large - for example the orbits of captured comets and the remnants of the disintegration of the fifth planet (the minor planets or asteroids). 82.14.24.95 (talk) 15:18, 18 October 2017 (UTC)

Although the "exploded fifth planet" idea was proposed when the asteroids were first being discovered, it has been discarded (see Asteroid belt#formation). The current concensus is that the ("true") planets were formed by the coalescence of asteroid-like 'planetesimals', but perturbations by Jupiter's gravity prevented this in the region of the current main asteroid belt. Some of the minor planets that formed in this region did later collide and fragment (as shown by orbital and spectroscopic analysis of the resulting Asteroid families), but no actual "fifth planet" ever formed there, though a Planet V that formed nearer to the Sun, and later scattered some of the asteroids before colliding with the Sun or Mars, has been proposed. {The poster formerly known as 87.81.230.195} 94.0.129.189 (talk) 09:28, 19 October 2017 (UTC)

Pitch-drop experiment and scientific method

I understand that in a scientific experiment you raise a hypothesis, contrast it with experimental data and draw a conclusion. How can you fit the pitch-drop experiment into this framework? Is it a scientific experiment at all? Indeed, how can you fit careful observation into the scientific method framework? Arent't they scientific? Are they just an accessory?--Hofhof (talk) 16:45, 18 October 2017 (UTC)

The term "pitch-drop experiment" might apply either to the ongoing long-term experiment that measures the flow of a piece of pitch over many years or to a recording of the descending frequency of a receding sound, the former seems likely. It is an example of a Long-term experiment where a slow phenomenon is observed to verify thoroughly a quantified replication of the predicted behaviour of a highly viscous liquid. Blooteuth (talk) 17:25, 18 October 2017 (UTC)

Most concepts are fuzzy. It's useful to be able to name and model parts of the world, but our models will always be vague around the edges (e.g. Sorites paradox). The pitch-drop experiment is and is not an experiment. By itself, it is just an observation or demonstration. If we assume that the hypothesis is "pitch is liquid", then it is an experiment. Don't get too hung up on these semantic issues. If you try to find the objectively correct answer, you're likely to miss the forest for the trees.

The scientific method is a simplified and fuzzy model which breaks the process of doing science into concrete steps. It's useful precisely because it is simplified--it's useful as a sort of checklist to make sure you're on the right track. These steps don't really exist, in the sense that scientists don't neatly follow the process step-by-step. At the "start" of the scientific process, when you don't really have any idea what's going on, you can't have a general theory, so you have to come up with a theory after you've made the observations. A good example is the discovery of the structure of DNA, perhaps the most famous scientific result. It was important science, but the "experiment" consisted of reading out the X-ray image of DNA. C0617470r (talk) 07:30, 19 October 2017 (UTC)

The scientific method as described above mostly refers to qualitative research, not so much to quantitative research. Sometimes we just want a number. So if the question is "What is the state of matter of pitch?", the hypothesis "Pitch is a liquid" is useful. If the question is "What is the viscosity of pitch?", there is no useful hypothesis, but we just take a measurement to get a number. It's still science. PiusImpavidus (talk) 08:12, 19 October 2017 (UTC)

The Pitch drop experiment has enabled a quantitative calculation that pitch has a viscosity approximately 230 billion (2.3×10¹¹) times that of water. Blooteuth (talk) 11:47, 19 October 2017 (UTC)

Right, I think it's worth emphasizing that the "scientific method" as described here is a bit of a lie to children. If you read the current table of contents of any modern high quality journal, you'll see many articles that don't fit in to this rubric.

And that's not even counting all the thousands of studies that go like this: 1)Hey this sounds neat, I think we can get it published 2) let's design an experiment to look at A and B 3) Huh... that didn't work out the way we thought it would, but we found some really cool stuff about C! Let's write up some ex post facto hypotheses! And then the (good, valid) paper get's written up using a useful narrative that happens to be a white lie. "C is very important in the field of X, and we hypothesize H1, H2 about it. Using our experiment, we show new evidence that H1 is true." This ends up looking like the caricature in the diagram above, but that's simply not what happened in many cases.

For further reading along these lines, see 'The Scientific Method' Is A Myth, Long Live The Scientific Method, and What’s Wrong With the Scientific Method?

Ultimately, if OP prefers to call the pitch drop experiment the "pitch drop demonstration", or "the pitch drop object of measurement and observation", that's fine too. But it's still science, and falls under the broad notion of experiment, if not this narrow, over-simplified schema. SemanticMantis (talk) 14:53, 19 October 2017 (UTC)

My favorite scientific theory is the explanation for why Damascus swords got strong when using the original hardening method. It goes as follows:

The sword is heated up to the color of the rising sun over the desert and then quickly inserted into a Nubian slave several times. The theory is that the strength of the slave is transferred to the sword by this process. Clearly supported by experiments any number of times. This must surely qualify as science. Jost0128 (talk) 11:47, 20 October 2017 (UTC)

I suppose you're joking, but no, it doesn't count as science. If anyone is not sure why not, please ask a new question. SemanticMantis (talk) 14:57, 20 October 2017 (UTC)

Already disproven by stabbing people with regular swords not making the sword stronger. Also unethical experiment and this isn't how it was done. A slave was pretty expensive so even if this was how it was done it'd add a lot of price to the sword. Sagittarian Milky Way (talk) 19:08, 20 October 2017 (UTC)

Is cosmic deflation possible, etc

Since cosmic inflation seems to exist or at least be possible, what about deflation? Also, can inflation inflate more in one direction than in another direction, for example more in one direction than in a direction perpendicular to it? Or maybe it could inflate a lot to the "north" but not as much to the "south" direction which is in 180 degrees opposite? Or can it inflate "north/south" but deflate "east/west?" Finally, can or does the time dimension inflate or deflate also, and how could that be observed? Thanks.144.35.45.45 (talk) 19:38, 18 October 2017 (UTC)

The inflation in principle can be anisotropic but it is considered unlikely. Ruslik_Zero 19:44, 18 October 2017 (UTC)

Many things are theoretically possible, but as Ruslik touched on, our observable universe is almost perfectly isotropic (a fancy science word that means "it looks the same in all directions"). Over the universe as a whole, you could say time "deflated" enormously immediately after the Big Bang, because (as described by general relativity) time moves more slowly in the presence of mass-energy. The universe was incredibly dense right after the Big Bang, and as it expanded in space, time would have started ticking more quickly. Of course, an observer inside the universe wouldn't have noticed any change, since you can only tell if your "clock" is ticking more or less quickly relative to another observer's. (That's why it's called "relativity".) Actually, the relativistic effects at such a scale are one of the current big problems in physics, because general relativity and quantum mechanics aren't compatible as currently understood. Until the end of the Planck epoch, we currently don't understand what was going on. It's believed we need a theory of quantum gravity for this. --47.138.160.139 (talk) 21:33, 18 October 2017 (UTC)

Hmm... our observable universe is almost perfectly isotropic - does it mean that as far as we can tell (within our instruments' precision etc.) the universe seems isotropic, or that it is slightly anisotropic (and we know it)? I thought it was the former, but that sentence reads more like the latter. Tigraan^{Click here to contact me} 11:36, 19 October 2017 (UTC)

The anisotriopies are incredibly tiny, and inflation theory believes them to be random quantum fluctuations immediately after the Big Bang that then got blown up in size as the universe inflated. See this video for more. --47.138.160.139 (talk) 06:00, 20 October 2017 (UTC)

Infrastructure civil engineering

If you want to learn about and develop a career in infrastructure civil engineering, which would give you the best experience out of client, contractor and consultancy? 94.10.251.123 (talk) 21:43, 18 October 2017 (UTC)

• We don't answer requests for opinions, predictions or debate. Dolphin (t) 11:29, 19 October 2017 (UTC)