Discover and read the best of Twitter Threads about #FridayPhysicsFun
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#FridayPhysicsFun – “Launch him/it into the sun!” My husband was griping over breakfast about the expression: surely this is not a good way of disposing of anything?
tvtropes.org/pmwiki/pmwiki.…
tvtropes.org/pmwiki/pmwiki.…
Is it actually possible to make something that will hit the “surface” of the sun?
The first problem of throwing something into the sun is that you need to hit it. Earth is moving 30 km/s, so you need to give the object this velocity in the opposite direction to make it fall straight in. That is a lot more than Earth escape velocity.
#FridayPhysicsFun - @KarimJebari and me have a new paper that is putting the longterm into longtermism. It is about doing good for the environment a billion years hence. link.springer.com/article/10.100…
The main point is that if we survive or leave automation in place, can extend the lifespan of the biosphere by perhaps a billion years or more. Ecocentric ethics places value on the Earth’s biosphere, so it should approve of this. But this thread is about the physics involved.
Why is the biosphere in trouble? The main reason is that the sun is getting brighter. As hydrogen is fused into helium, it ends up as a dense helium core with higher pressure and temperature. That increases the rate of hydrogen fusion. 
#FridayPhysicsFun - I wrote an answer on Physics Stack Exchange about moving the Earth outwards to compensate for solar brightening. physics.stackexchange.com/questions/7227…
The first consideration of how to move the planet was made by Archimedes boast: "Give me a place to stand and with a lever I will move the whole world." physics.stackexchange.com/questions/4831…
Christoph Grienberger in 1603 proposed gearing powered by a treadmill, allowing it to be raised veeeerrryyy slowly. He got the rough number of gears right by modern reckoning. bbc.com/future/article…
#FridayPhysicsFun – Coolest fact I learned this week: under some conditions light-emitting diodes can be more than 100% efficient, and act as refrigerators.
Normally energy conversion introduces losses: there is a production of entropy turning high-quality energy (e.g. mechanical motion, electricity) into disordered low-quality (e.g. heat), and turning low-quality into high-quality is less efficient.
This is why normally any device promising more than 100% efficiency is fake. Thermodynamics does not allow it.
#FridayPhysicsFun – I got a question today: are there pyroclastic flows from volcanoes elsewhere in the solar system?
Pyroclastic flows are the result of explosive volcanic eruptions, a mix of hot gas and volcanic matter hurtling downslope at potentially more than 100 km/h, burning and suffocating everything. Basically a landslide mixed with hot, dense gas and dust.
en.wikipedia.org/wiki/Pyroclast…
en.wikipedia.org/wiki/Pyroclast…
Why do landslides move in the first place? The Varnes classification suggests there are falls, topples, slides, spreads and flows.
geology.cz/projekt681900/…
onlinepubs.trb.org/Onlinepubs/sr/…
geology.cz/projekt681900/…
onlinepubs.trb.org/Onlinepubs/sr/…
#FridayPhysicsFun - @maartengm brought up cutting as a topic, and it is an interesting one. How do knives work? How sharp can you go? Is it the path to Royalty?
A knife is nearly a wedge. Wedges, one of the six simple machines, work (1) as a double incline, spreading out the splitting force over a long distance, and (2) by focusing the pushing force onto a thin edge. hyperphysics.phy-astr.gsu.edu/hbase/Mechanic…
The amplification of force, the mechanical advantage, is 1/tan(α) where α is the tip angle. Make α approach zero and very little force is needed to separate the target.
en.wikipedia.org/wiki/Mechanica…
en.wikipedia.org/wiki/Mechanica…
#FridayPhysicsFun – This year’s Nobel Prize in Physics went to Manabe, Hasselmann (climate) and Parisi. Media will focus on the climate stuff, because it is easy to explain. But what did Parisi do? And why does it matter for machine learning? nobelprize.org/prizes/physics…
A lot of this is way beyond me: I am not good enough at statistical mechanics to explain or use this directly. But I can see the shadows cast on the landscape by these results, and they are awesome. nobelprize.org/uploads/2021/1…
The start is spin glasses: solids where some atoms have spins that affect other spins, but in a disordered way (rather than the neat all parallel spins in ferromagnetic materials). They try to minimize their energy, but there will be frustration.
#FridayPhysicsFun – Stretching the definition of physics a fair bit, sigmoid growth curves are useful… except for predicting the future.
Sigmoids get the name from being S-shaped curves, taking the name from Greek letter sigma. They are also called logistic curves, ogives, s-curves, Gompertz curves, Bass curves, Verhulst growth...
en.wikipedia.org/wiki/Logistic_…
en.wikipedia.org/wiki/Logistic_…
First, an initially accelerating growth period, leading up to a turning point. Then the growth slows and the curve tends towards an asymptote (or maximum/ saturation level). There are many formulas that give such curves.
#FridayPhysicsFun – I am back home in the apartment where I grew up on the 11th floor. That is about 30 m down to the street, and as a kid I often considered the fate of toys dropped from the balcony. How does falling really work? en.wikipedia.org/wiki/Hagalund
The schoolbook answer is that the gravitational force F=mg accelerates the object as per Newton’s second law of motion F=ma and the falling object has an acceleration a=g because the mass factor cancels from both equations.
The velocity becomes v(t)=gt at time t, and the distance travelled d(t)=(1/2)gt^2. I remember kid-me inverting the later formula to t=sqrt(2h/g) and checking by dropping marbles that they took about 2.47 s to hit the ground. Fortunately nobody got hurt.
#FridayPhysicsFun – Normal crystals consist of atoms or molecules arranged in a regular lattice. Recently there has been experimental demonstrations of 2D Wigner crystals – crystals made of just electrons.
quantamagazine.org/physicists-cre…
quantamagazine.org/physicists-cre…
The idea is pretty old: Eugene Wigner proposed in 1934 that electrons would repel each other and if the density was low enough form a lattice. The repulsion dominates over the kinetic energy and makes it “solid”.
en.wikipedia.org/wiki/Wigner_cr…
en.wikipedia.org/wiki/Wigner_cr…
Too high density and they “quantum melt” as the kinetic energy dominates and the lattice dissolves. Too high temperature and they melt normally because of thermal vibration. 3D Wigner crystals need a lower density than 2D crystals to solidify.
#FridayPhysicsFun – This has been a month full of space news, with billionaires in space, nail biters when Nauka and Hubble glitched, test firing of the Super Heavy booster for Starship. But where are the limits to going *fast*? flic.kr/p/56Zfnf
Obviously Einstein blocks us at lightspeed, and Tsiolkovsky makes it expensive to use fast rockets. But I am more interested in locomotion in general.
There is a classic finding that animal speeds increase with body mass by the about 0.2th power – up to a point of about 119 kg. Elephants are not faster than cheetahs. …lpublications.onlinelibrary.wiley.com/doi/abs/10.111… doi.org/10.1002/ecy.20…
#FridayPhysicsFun – coming home after 6 months of absence implied a lot of dusting. Still almost nothing compared to the dust in Messier 64, the "Black Eye Galaxy". What and why is dust? en.wikipedia.org/wiki/Black_Eye…
“Dust” refers to fine particles of solid matter. It can be almost anything. But the smallness makes it behave different from larger pieces.
Household dust is dead skin cells, various fibres, pollen, mites, soil particles etc. The composition depends a lot on where you are and who you are: the bacterial content is about half determined by humans.
ehp.niehs.nih.gov/doi/10.1289/eh…
linkinghub.elsevier.com/retrieve/pii/S…
ehp.niehs.nih.gov/doi/10.1289/eh…
linkinghub.elsevier.com/retrieve/pii/S…
#FridayPhysicsFun - the EmDrive has failed some fairly rigorous tests. So no neat reactionless space propulsion. But why were most people so confident that it really did not work even before this?
popularmechanics.com/science/a35991…
popularmechanics.com/science/a35991…
The EmDrive purports to produce thrust by reflecting microwaves inside a conical cavity, producing an uneven force on the device that would make it move - in violation of momentum conservation.
en.wikipedia.org/wiki/EmDrive
en.wikipedia.org/wiki/EmDrive
Momentum in Newtonian mechanics is just defined as the product of mass and velocity. The change of momentum is equal to the net force on it. In relativistic mechanics one often takes the force law as the definition.
#FridayPhysicsFun - Is water stable or potentially explosive? The answer may depend on whether one can construct a computer out of fluid.
Fluid motion is described by the Navier-Stokes equations. They are non-linear and really tricky to solve.
quantamagazine.org/what-makes-the…
quantamagazine.org/what-makes-the…
One problem mathematicians would like to solve is whether an initial state of finite fluid velocities will remain finite, or whether it could evolve into something with a singularity.
#FridayPhysicsFun - Last week I gave a talk about Karl Popper's critique of historicism and how this strikes at macrohistory and future studies. But what does physics say we cannot predict?
Classical mechanics has "Laplace's demon" (born in 1814): it knows all the positions and momenta of every particle in the universe, the full set of mechanical laws, and should then in principle be able to predict the future state at any point in time.
en.wikipedia.org/wiki/Laplace%2…
en.wikipedia.org/wiki/Laplace%2…
Like most thought experiment demons (Descarte's, Maxwell's, Darwin's, and so on) it mostly exists to be exorcised. newyorker.com/books/under-re… springer.com/gp/book/978331… degruyter.com/princetonup/vi…
#FridayPhysicsFun - One of the weirdest physical effects I know about is the Casimir effect (in my mental ranking it is just a step down from the Aharanov-Bohm effect). physicsworld.com/a/the-casimir-…
If you place two uncharged conductive surfaces close together in vacuum, there is an attractive force between them. Why? Because the vacuum between them has less energy than the vacuum outside them. en.wikipedia.org/wiki/Casimir_e… scholarpedia.org/article/Casimi…
Empty space, according to quantum field theory, is full of possible electromagnetic waves and they all have a finite zero-point energy. However, normally the only thing that matters is differences between this energy and fields with actual waves.
#FridayPhysicsFun - Today I made a loaf of bread. I also learned that bread spontaneously forms heat pipes that move heat and moisture more efficiently. And that the internal structure kind of imitates the large scale structure of the universe.
The heat pipe info is from Nathan Myhrvold and Francisco J. Migoya in their book Modernist Bread, based on earlier research by food scientists.
physicsworld.com/a/the-physics-…
physicsworld.com/a/the-physics-…
When you heat dough in the oven, at first the surface heats up and starts to dry out. Water diffuses outward, and there is likely some capillary action causing wicking too.
#FridayPhysicsFun: what *is* that bright thing?
Everybody has seen light caustics since they are everywhere: reflections and refractions in glasses and cups, the net pattern cast by sunlit waves on walls and boats, rainbows & halos.
Caustics happen when a lot of light rays get bundled together. The term caustic comes from the Greek word kaustos for "burnt" - the concentrated light at the focal point of a magnifying glass is hot. en.wikipedia.org/wiki/Caustic_(…
