How airplane wing really works.
Ask HN: What scientific phenomenon do you wish someone would explain better?
251–260 of 839 posts
Re: Ask HN: What scientific phenomenon do you wish someone would explain better?
#252Lot's of quantum related phenomenon here, but what keeps bothering me is that while I get it that light is both a wave and a particle, but I have no clue what that means. I mean, a wave of sound, is made from air particles, a wave of ripples in a pond is made of movement of water molecules. In the double slit experiment, it's explained that the single photon has to be "interfering with itself", so I don't get it if by being a "wave" it means that the single photon is basically a bunch of "magic" photon ghosts that behave like a wave, but once it is measured or any other reason to "collapse" these ghosts "disappear". I just don't get what the "wave" of light/radio wave is. Is it just an abstract concept of something that behaves like a wave but not the same as sound waves / ripples since we simply don't know? Or is it just a wave of these "not yet collapsed" probabilities of the photons locations that are interfering with each other right until we ask them to choose a location, then they just collapse magically into a single "real" photon. Another thing I don't get is in the double slit experiment, a LOT of the measure before, measure after, etc, are told to be thought experiments, but it's also claimed that someone managed to actually replicate them. Why isn't there a video showing it? I obviously believe they happened, and understand why more or less (e.g. in the one photon at a time experiment, it's spooky that over time you get the same pattern that indicates interference as if you shoot many) but the more spooky result is that thought experiment, that if you measure which slit the photon actually traveled through, you'll see 2 slits on the screen vs the famous pattern. e.g. you'll cause the wave to collapse back to particles. So any video of reproducing of that thought experiment or explanation why it's so hard to reproduce, will be super helpful.
Re: Ask HN: What scientific phenomenon do you wish someone would explain better?
#253Earlier quoted context omitted.
Well, if you're uncomfortable with LIGO, you're in good company. Once a whole decade went by in physics where people couldn't agree if it would work in principle or not. And it is true that a lot of common explanations are bad (e.g. "it's just a ruler" is not complete by itself because "why doesn't the light get stretched too?"). Nonetheless, today we have a variety of independent explanations. Maybe you'll find this…
Thank you for this link. I read it once and think I've gotten more of the picture and I'm going to re-read it to see if I can get more. What's baffling to me is everyone who has tried to explain the LIGO detector doesn't even realize this question exists. I've independently thought this question and when people start explaining LIGO to me, and I take the time to spell out the question, they realize they don't underst…
> If light is emitted at a constant wavelength independent of the stretching of the universe, doesn't that imply light is traveling through a higher spatial dimension, otherwise the emitter itself would be stretched with the universe and we'd never be able to observe differences in the speed of light? If I understand this paper, once light is emitted, it's "stuck" to space and will stretch along with it. But if the emitter wavelength stays constant doesn't that imply it's waving through a higher dimension?
I'm not totally sure what you mean by a higher dimension. The properties of the emitter (which is, e.g. a laser cavity) aren't affected by the gravitational wave because the emitter is a rigid body, which doesn't get stretched. (It's the same thing as described here: https://news.ycombinator.com/item?id=22990753 ) So it puts out light of a given frequency.
By contrast, LIGO is not a rigid body, because the mirrors at the ends of the arms hang freely, hence allowing gravitational waves to change the distance between them.
> What's baffling to me is everyone who has tried to explain the LIGO detector doesn't even realize this question exists. I've independently thought this question and when people start explaining LIGO to me, and I take the time to spell out the question, they realize they don't understand LIGO either.
Yup, it generally is the case in physics that over 95% of people who claim they can explain any given thing don't actually understand it! But the professionals are aware. I even know a LIGO guy who goes to popular talks armed with a pile of copies of the paper I linked.
Re: Ask HN: What scientific phenomenon do you wish someone would explain better?
#254What happens when you actually fall inside a black hole and what is the singularity. I never really understood what happened really when the guy fell inside it in Interstellar and how come he started seeing all those photos. I just accepted it as Hollywood bs. I know my question is based on a movie but would still like to know what will someone witness (assuming of course they somehow live)
> I never really understood what happened really when the guy fell inside it in Interstellar and how come he started seeing all those photos. I just accepted it as Hollywood bs. Coupled with people saying "but they had scientists on staff! they talked with scientists that makes it so cool and accurate, lets ignore that other part"
Well I think there are two things here. Yes, they had black hole experts work on the film and even generate the film's blackhole images (iirc) from Kip Thorne plugging in relativistic formulas into Mathematica. That is probably what your friends are referring to. Of course, any Hollywood science fiction movie also has some make believe in it. In this case the inside of the blackhole allowing him to communicate with his daughter and therefore save himself didn't make a ton of sense (I would assume paradox, but I think the laws of physics break down in the singularity so they used some extremely heavy hand waving). That part isn't what your friends are likely referring to.
I thought the music, acting, and triumph of humanity were pretty inspiring much like Star Trek can be despite the fact that most of the technology violates the laws of physics. You may have thought it was a terrible movie which is fine. I thought Star Wars Rogue One was one of the most boring films I've seen in the last decade, but a LOT of people loved that film.
Re: Ask HN: What scientific phenomenon do you wish someone would explain better?
#255Flight. Apparently "air flows faster on the top side of the wing, lowering the pressure" is an incomplete explanation; I even heard we don't completely understand why it works (?!?).
The now classic Bernoulli vs Newton debate.
"Air flows faster on top" is the Bernoulli explanation. The Bernoulli principle tells us that fast air means low pressure, and low pressure sucks the plane up.
Newton explanation is the idea that the wing pushes the air down, and by reaction, pushes the plane up. Based on Newton's third law.
In reality, both are correct. The Bernoulli explanation is more specific and the Newton one is more generic. But if you want the whole picture, you need the Navier Stokes equations. Unfortunately, these are very hard to solve, so even engineers have to use simplified models.
I personally prefer the Newton explanation. It explains less, but the Bernoulli one is confusing and results in many misunderstandings. For example, that air takes the same time to follow the top side and bottom side of the wing, which is completely wrong.
The common depiction also tends to hide the fact that the trailing edge of wings is at an downwards angle, even though it is the most important part. Nice profiles make wings more efficient, but the real thing that makes planes fly is that angle, called angle of attack.
Focusing on the profile rather than on the angle of attack leads to questions like "How can planes fly upside down?" (the answer is "by pointing the nose up", and that should be obvious). If you are just trying to understand how planes fly, forget about wing profile, it is just optimization.
Re: Ask HN: What scientific phenomenon do you wish someone would explain better?
#256Earlier quoted context omitted.
Disc Loading is just the force on the actuator disc (such as a rotor, in which case the force is the weight of the vehicle the rotor holds aloft) divided by the swept area (pi times r squared). It's used when discussing propulsive efficiency, as it's a proxy measurement for how much "work" each blade is doing. Because propeller/rotor blades are just high-aspect wings, if you have high disc loading your blades are at…
Thanks na85 Let's take a contrived example. I have a 4 engine high wing airplane with 2 bladed 55 inch props on each engine with 100kg force (1 kg-f is equal to 9.8 N) thrust per engine. Now, I need to make that a low wing airplane so need to change to 22 inch blades with the same thrust and I don't want to change engines. So I want to add more prop blades. How many more blades do I need to add?
Couple of points:
1) 100kg is a measure of mass, not force. Thrust is a force, not a mass. But let's say you have 100N of thrust per engine.
2) Why would changing from a high-wing to a low wing monoplane require you to add prop blades?
Re: Ask HN: What scientific phenomenon do you wish someone would explain better?
#257I wish I could get a clear explanation of why faster than light travel breaks causality. I have seen the math but intuitively I am having trouble grasping it especially when you have only two reference frames (most explanations will use three reference frames to show the violation)
Re: Ask HN: What scientific phenomenon do you wish someone would explain better?
#258Why are clouds flat on the bottom?
Because that's (roughly) the Lifted Condensation Level: https://en.wikipedia.org/wiki/Lifted_condensation_level
Water vapor around the LCL starts condensing and turning from a gas into liquid cloud droplets. This process happens considerably faster once it begins for a variety of reasons, so once you can have cloud droplets, you get a ton of cloud droplets - not a gradual transition from water vapor to cloud. It's almost like a light switch.
Most air masses are relatively homogenous anyways, so unless there are underlying processes causing things like undulatus asperatus, it will certainly appear very, very flat over a large area.
Re: Ask HN: What scientific phenomenon do you wish someone would explain better?
#259A non handwavy explanation of how does evolution result in such complicated and carefully orchestrated mechanisms, way beyond our engineering capability. The computer analogy of genetic algorithms certainly doesn't explain much, they are not very effective, and if they were, we should be able to generate marvels of engineering with GAs and current computational power.
A whole LOT of time, a whole lot of space.
So estimate 1 minor good random mutation per 10,000 population. Assume 1 major mutation per 1000 years, per 10,000 population.
Re: Ask HN: What scientific phenomenon do you wish someone would explain better?
#260Has someone that thought they were taking LSD ever turned into a permanent schizophrenic zombie or in a mental institution, or is it all urban legend. If someone that didn't know they were predisposed to mental illness, is it applicable to dismiss their experience in order to maintain how safe LSD is? If any of this is true, are there any sources aside from "my friend's friend's brother took too much and now he is...…
This entertaining article lists what happened to the early LSD researchers: https://slatestarcodex.com/2016/04/28/why-were-early-psyched...