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aries1988 : physics   22

夏天这么热,穿衣服干什么?|小南都知道

平时,我们待的环境里面绝大多数都没有超过我们的体温,因此人体一直不断地在向外辐射热量。这个点其实挺反三观的。在绝大多数时候,通过汗液蒸发等方式散热的都只是很小一部分,真正的占了最主要的部分,反而是我们平时都忽略掉的辐射散热。传导散热次之,而人体和空气通过热传导散热则几乎可以忽略不计。而辐射这么重要,就是因为其散热的功率,和温度的四次方成正比。

黑色吸收了绝大部分的辐射热量,这些热量集中在羽毛的表层,只要环境风速较大,就可以迅速地把热量全部带走。

前面提到浅色的衣服反射电磁波的能力也比较强,所以它反射人体自己的辐射的能力也比较强,更多的热量反而返回给了人体,在外界风较大(>3m/s)的情况下,相比深色就不能发挥更好的作用了。
explained  thermo  Physics  summer  clothing  howto  radiation 
12 weeks ago by aries1988
Turbulence, the oldest unsolved problem in physics

because our understanding of turbulence over time has stayed largely ad-hoc and limited, the development of technology that interacts significantly with fluid flows has long been forced to be conservative and incremental. If only we became masters of this ubiquitous phenomenon of nature, these technologies might be free to evolve in more imaginative directions.

Motions of fluids are usually hidden to the senses except at the interface between fluids that have different optical properties.

For example, you can see the swirls and eddies on the surface of a flowing creek but not the patterns of motion beneath the surface.

The Navier-Stokes equation is difficult to solve because it is nonlinear. This word is thrown around quite a bit, but here it means something specific. You can build up a complicated solution to a linear equation by adding up many simple solutions. An example you may be aware of is sound: the equation for sound waves is linear, so you can build up a complex sound by adding together many simple sounds of different frequencies (harmonics). Elementary quantum mechanics is also linear; the Schrödinger equation allows you to add together solutions to find a new solution.

The idea is that the interesting dynamics occur at larger scales, and grid points are placed to cover these. But the subgrid motions that happen between the gridpoints mainly just dissipate energy, or turn motion into heat, so don’t need to be tracked in detail. This approach is also called large-eddy simulation (LES), the term eddy standing in for a flow feature at a particular length scale.

The idea is that, while the low-speed solution is valid at any speed, near a critical speed another solution also becomes valid, and nature prefers that second, more complex solution. In other words, the simple solution has become unstable and is replaced by a second one. As the speed is ramped up further, each solution gives way to a more complicated one, until we arrive at the chaotic flow we call turbulence.
turbulence  Physics  science  history  explained  example  LES 
december 2018 by aries1988
How Do You Take a Picture of a Black Hole? With a Telescope as Big as the Earth - The New York Times
near the core, that fog forms a great glowing Frisbee that rotates around a vast dark sphere. This is the supermassive black hole at the core of the Milky Way, the still point of our slowly rotating galaxy. We call it Sagittarius A*, that last bit pronounced “A-star.” The black hole itself is invisible, but it leaves a violent imprint on its environment, pulling surrounding objects into unlikely orbits and annihilating stars and clouds of gas that stray too close.

the inaugural run of the Event Horizon Telescope (E.H.T.), a virtual Earth-size observatory designed to take the first picture of a black hole. The E.H.T. uses a technique known as very long baseline interferometry (V.L.B.I.), in which astronomers at observatories on different continents simultaneously observe the same object, then combine the collected data on a supercomputer. The E.H.T.’s director, Shep Doeleman, a radio astronomer with the Harvard-Smithsonian Center for Astrophysics, likes to call the E.H.T. “the biggest telescope in the history of humanity.” It has the highest resolution of any astronomical instrument ever assembled. It’s sharp enough to read the date on a nickel in Los Angeles from New York, to spot a doughnut on the moon and, more to the point, to take a picture of the black hole at the center of our galaxy — or, at least, its shadow.

The effort to get that picture speaks well of our species: a bunch of people around the world defying international discord and general ascendant stupidity in unified pursuit of a gloriously esoteric goal. And in these dark days, it’s only fitting that the object of this pursuit is the darkest thing imaginable.

The physicist Werner Israel put it better when he described a black hole as “an elemental, self-sustaining gravitational field which has severed all causal connection with the material source that created it, and settled, like a soap bubble, into the simplest configuration consistent with the external constraints.”

what a black hole would look like if illuminated by the glow from the superheated matter swirling around it. He did his calculations by feeding punch cards into a primitive computer. He drew the results by hand. His black-and-white images looked like twisted depictions of a black Saturn, with a ringlike accretion disk warped like taffy.

That this shadow might be visible from Earth depended on an astonishing set of circumstances. Earth’s atmosphere happens to be transparent to the electromagnetic radiation — in this case, certain microwaves — shining from the edge of the black hole, even though it blocks radiation of slightly longer and shorter wavelengths. The interstellar gunk lying between Earth and the galactic center also becomes transparent at those frequencies, as do the clouds of superheated matter just outside the black hole, blocking a view of the event horizon. Later in life, Fulvio Melia compared this alignment to the cosmic accidents that give us total solar eclipses. The moon is just the right size, in just the right orbit, at just the right distance from Earth that now and then it blocks the sun entirely. Fulvio wasn’t religious, but these coincidences were so unlikely that he couldn’t help but feel that the black-hole shadow was meant to be seen. The universe had arranged for humans to see to the nearest exit.

it’s hard to reconcile two conflicting theories if you can’t find something wrong with either one, and quantum theory, like general relativity, has passed every test. As a result, scientists have been looking for ever-more-extreme situations in which to test these theories. That led them to black holes.

To avoid poisoning one another’s minds — so that no one could accidentally nudge another group into seeing a black-hole shadow that wasn’t really there — these groups worked in isolation, making images using different algorithms and techniques, trying hard to discredit anything that looked too sharp, too clean, too likely to be the product of wishful thinking.
telescope  astronomy  scientist  world  project  Physics  law  research  relativity  quantum 
october 2018 by aries1988
How many dimensions are there, and what do they do to reality? – Margaret Wertheim | Aeon Essays

Yet the notion that we inhabit a space with any mathematical structure is a radical innovation of Western culture, necessitating an overthrow of long-held beliefs about the nature of reality. Although the birth of modern science is often discussed as a transition to a mechanistic account of nature, arguably more important – and certainly more enduring – is the transformation it entrained in our conception of space as a geometrical construct.

What is so extraordinary here is that, while philosophers and proto-scientists were cautiously challenging Aristotelian precepts about space, artists cut a radical swathe through this intellectual territory by appealing to the senses. In a very literal fashion, perspectival representation was a form of virtual reality that, like today’s VR games, aimed to give viewers the illusion that they had been transported into geometrically coherent and psychologically convincing other worlds.

In Newton’s world picture, matter moves through space in time under the influence of natural forces, particularly gravity. Space, time, matter and force are distinct categories of reality. With special relativity, Einstein demonstrated that space and time were unified, thus reducing the fundamental physical categories from four to three: spacetime, matter and force. General relativity takes a further step by enfolding the force of gravity into the structure of spacetime itself. Seen from a 4D perspective, gravity is just an artifact of the shape of space.

General relativity says that this warping is what a heavy object, such as the Sun, does to spacetime, and the aberration from Cartesian perfection of the space itself gives rise to the phenomenon we experience as gravity.

Whereas in Newton’s physics, gravity comes out of nowhere, in Einstein’s it arises naturally from the inherent geometry of a four-dimensional manifold; in places where the manifold stretches most, or deviates most from Cartesian regularity, gravity feels stronger. This is sometimes referred to as ‘rubber-sheet physics’. Here, the vast cosmic force holding planets in orbit around stars, and stars in orbit around galaxies, is nothing more than a side-effect of warped space. Gravity is literally geometry in action.

Aristotle was right – there are indeed logical problems with the notion of extended space. For all the extraordinary successes of relativity, we know that its description of space cannot be the final one because at the quantum level it breaks down. For the past half-century, physicists have been trying without success to unite their understanding of space at the cosmological scale with what they observe at the quantum scale, and increasingly it seems that such a synthesis could require radical new physics.

Like Newton’s world picture, Einstein’s makes space the primary grounding of being, the arena in which all things happen. Yet at very tiny scales, where quantum properties dominate, the laws of physics reveal that space, as we are used to thinking about it, might not exist.
physics  dimension  maths  future  science 
january 2018 by aries1988
How quantum superposition could unravel the ‘grandfather paradox’ | Aeon Videos
could the concept of quantum superposition remove what seems so paradoxical from this tale of time travel and murder once and for all?
time  video  quantum  Physics  fun  thinking  experiment 
june 2017 by aries1988
BBC - Earth - Organisms might be quantum machines
This is the same kind of resonance you might experience when you sing in the shower – certain notes sound a lot louder and fuller than others. Hitting the right radio wave frequency will make the electron vibrate more vigorously in the same way.

Vanillin smells of vanilla, but eugenol, which is very similar in shape, smells of cloves. Some molecules that are a mirror image of each other – just like your right and left hand – also have different smells. But equally, some very differently shaped molecules can smell almost exactly the same.“

Olfaction requires a mechanism that somehow involves the actual chemical composition of the molecule,” he says. “It was that factor that found a very natural explanation in quantum tunnelling.”

Making use of quantum effects in biologically inspired photovoltaic cells, for instance, could give solar panels a huge boost in efficiency. “At this very moment there is quite a lot of activity in organic photovoltaics, to see whether with natural or artificial structures one can have an enhanced efficiency that exploit quantum effects.”
photosynthesis  photovoltaic  energy  biology  machine  quantum  physics  nature  future 
august 2016 by aries1988
The 100 best nonfiction books: No 21 – The Structure of Scientific Revolutions by Thomas S Kuhn (1962)
The American physicist and philosopher of science coined the phrase ‘paradigm shift’
science  history  Physics 
july 2016 by aries1988
From relativity to quantum theory – our physical world explored through coffee | Aeon Videos
From relativity to quantum theory – our physical world explored through coffee — via @aeonmag
Physics  coffee  talk  life  discovery  thinking 
june 2016 by aries1988
The Space Doctor’s Big Idea - The New Yorker
There once was a doctor with cool white hair. He was well known because he came up with some important ideas. He didn’t grow the cool hair until after he was done figuring that stuff out, but by the time everyone realized how good his ideas were, he had grown the hair, so that’s how everyone pictures him. He was so good at coming up with ideas that we use his name to mean “someone who’s good at thinking.”
Physics  scientist  story  explained 
november 2015 by aries1988
Michio Kaku: Is God a Mathematician?
Don't miss new Big Think videos! Subscribe by clicking here: http://goo.gl/CPTsV5 Michio Kaku says that God could be a mathematician: "The mind of God we…
physics  maths 
may 2015 by aries1988
Some Racing Raindrops Break Their 'Speed Limit'
Some raindrops travel faster than they should, say scientists who suggest the superterminal drops may mess with rainfall estimates.
phd  Physics  mecaflu 
february 2015 by aries1988
Joel Lebowitz, la physique de l’engagement
Que se passe-t-il à l’intérieur d’un échantillon de cet objet, si une de ses extrémités est chauffée et l’autre mise dans la glace d’un seau à champagne ? La réponse n’est toujours pas connue, mais elle illustre l’une des motivations de cet éternel chercheur, toujours très actif. Comment expliquer les effets macroscopiques de la matière à partir de ces composants microscopiques ? Comment décrire les systèmes qui ne sont pas à l’équilibre thermodynamique ? Ou, dans le cas présent, peut-on déduire l’équation de propagation de la chaleur, connue depuis Fourier au XIXe siècle, d’après le comportement des particules élémentaires à l’intérieur de l’échantillon ?

Il raconte aussi qu’un collègue physicien, pour la 100e conférence Rutgers, a proposé de baptiser une nouvelle unité du nom de « Joel », comme il y a des ampères ou des volts. Ce serait l’unité de travail d’un chercheur.
scientist  Physics  materiaux  american  france  research  work 
november 2014 by aries1988
If entropy always increases, how is life possible? - Quora
Life is not the only example of spontaneous order generation. Crystals are highly ordered, and form all by themselves. They go from a high-entropy state to a low-entropy one, giving off heat in the process. (And conversely, when they melt, they absorb heat.)
life  explained  god  physics 
november 2013 by aries1988
探索量子世界的里程碑——解读2012年诺贝尔物理学奖-三联生活周刊
60年之后,两位物理学家就是通过高超的实验手段,以不同的方式探测和控制光子与负离子的量子态,揭示了量子力学最奇特的一面,并以此获得了物理学的最高荣誉。
现代计算机传递数据的基本单位是一个比特,其数值只能是0或者1,而科学家们努力构建的量子计算机,正是利用了量子的叠加态,可以同时具有0和1两种状态,这称为“量子比特”。因此,一个量子比特可以同时具有00、01、10和11四个数值。量子计算机将可以同时进行多个运算,因此其速度与普通计算机相比将会大幅度提高。
通过世界上最精确的原子钟,一个人即使是高度变化30米,或是以10米/秒的速度进行运动,时间对于他流逝的速度变化都可以测量出来——这将是验证广义相对论对于时空特性的描述的绝佳工具。
from:kindle  physics 
april 2013 by aries1988
Loom Junior • Your morning dose of cosmic perspective: a...
A more realistic simulation of the cosmos (with moving mesh algorithms) via
video  astro  physics 
august 2012 by aries1988
物理学步入禅境:缘起性空
朱清时:在二十一世纪开始的时候,以弦论为代表的物理学真正步入缘起性空的禅境了。回头再看一下本文起头的那则消息,不难明白为何人们难以听懂霍金的那么生动的报告,原因就是物质是实体的观念在人们的心中太执着了!
science  physics  talk  religion 
august 2012 by aries1988
什么是“松弛时间”? : 万物皆流
以上这段话不仅从正面简短介绍了“松弛时间”的概念,还适时解答了一般人很容易产生的困惑,包括本文开头提出的那几个疑问。在一个教材里面,加上这么一小段话,并不影响教材的篇幅或者印刷,但是却对学生今后的发展作用重大。由于这一介绍松弛时间一般性的定义,因此它不会像“粘度除以模量”那样无法延伸到其他场合。所有场合下出现的“松弛时间”,都可以拿以上那段话的内容去理解。这才是学习应该达到的效果。
relaxation_time  physics 
june 2012 by aries1988
复活节闲扯:一场激动人心的数学公开挑战赛
最速降线问题
“想象一个小球,仅受重力,从点 A 出发沿着一条没有摩擦的斜坡滚至点 B。怎样设计这条斜坡,才能让小球在最短的时间内到达点 B?”
maths  physics 
april 2011 by aries1988

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