Physics
Science
41 rabbit holes · 17 people exploring
Common starting questions
- Was the Pioneer anomaly ever explained?
- Is 432 Hz actually more natural than 440?
- How small a height difference can an atomic clock detect?
- What is ground effect in F1?
- Why does bunny hopping make you go faster?
Rabbit holes
- you can measure the brain's magnetic field, which is absurdly faint
Brain fields are in the femtotesla range, somewhere around a billion times weaker than Earth's. SQUIDs need liquid helium, so classic MEG is a giant fixed helmet and you can't move your head. Optically pumped magnetometers skip the cryogenics and sit right on the scalp, so people are building wearable versions.
Quinn Harlow · 6 sources
- a second is 9,192,631,770 wiggles of a cesium atom
Dev Malhotra · 5 sources
- water drops skate on a hot pan because they're floating on steam
Cooking question that turned into a physics evening. Past a certain temperature the bottom of the drop flash-boils and the drop rides on its own layer of vapor, so it lasts way longer than it would on a merely hot pan.
Ilya Smirnov · 5 sources
- the Pioneer anomaly turned out to be heat
Dev Malhotra · 6 sources
- leap seconds are being retired
Ilya Smirnov · 4 sources
- quantum gravity sensors can find a tunnel under a road
A Birmingham team took an atom-interferometer gravity gradiometer outdoors and detected a buried tunnel from the surface. Ordinary gravity sensors struggle outside because ground vibration swamps the signal; comparing two atom clouds cancels most of it.
Quinn Harlow · 6 sources
- the 432 Hz thing, and why A is 440 anyway
Ama Mensah · 6 sources
- clocks can now see time run differently across a millimeter
Quinn Harlow · 6 sources
- the Vasa sank 1,300 m into its maiden voyage
Top-heavy warship, the king kept wanting more guns, a stability test (30 sailors running side to side) almost capsized it and they launched anyway. Raised in 1961 almost intact because the Baltic has no shipworms.
Nikolai Volkov · 4 sources
- ground effect: banned in 1983, back in 2022, still bouncing
Shape the underside of the car like an upside-down wing and the low pressure under it pulls the car into the track. Lotus cracked it in the late 70s, with sliding skirts sealing the sides, and F1 mandated flat floors from 1983.
Dev Malhotra · 7 sources
- bunny hopping is a physics bug that became a skill
Ludovica Ferri · 5 sources
- Fresnel lenses made lighthouses visible for miles
Nikolai Volkov · 4 sources
- some old camera lenses are radioactive
Some mid-century lenses used thorium oxide in the glass for a high refractive index without much dispersion. Over decades the radiation turns the glass yellow-brown. Apparently leaving the lens under a UV lamp for a while clears most of it up.
Fern Achterberg · 5 sources
- the first numerical weather forecast was done by hand and was spectacularly wrong
During WWI, while driving ambulances in France, Richardson worked out a six-hour forecast for a patch of central Europe by hand. It took weeks and predicted an absurd pressure change. But the method was basically right; the starting data needed smoothing. He also imagined a 'forecast factory' of 64,000 human computers in one giant hall.
Tomás Rinaldi · 6 sources
- color photos of the Russian Empire from the 1910s that look like yesterday
He took three black-and-white exposures in quick succession through red, green and blue filters. Combine them and you get color. The Library of Congress has the glass plates and recombined them digitally.
Fern Achterberg · 6 sources
- stellarators look like someone crumpled a donut on purpose
A tokamak gets the twist in its magnetic field partly by driving a huge current through the plasma, which can go unstable. A stellarator gets it entirely from the shape of the coils, which is why they look insane. Spitzer came up with it at Princeton in 1951 and it lost to the tokamak for decades, partly because nobody could build coils that precise.
Dev Malhotra · 6 sources
- why is the night sky dark?
Dev Malhotra · 5 sources
- racing tyres only work inside a temperature window
Jonas Becker · 5 sources
- Prince Rupert's drops survive a hammer and explode if you snip the tail
Drip molten glass into cold water and you get a tadpole you can hit with a hammer on the head. Bend the thin tail and the whole thing turns to powder. The outer skin is locked in compression and the core in tension, and snapping the tail lets the tension loose all at once.
moth · 5 sources
- the Brabham fan car raced once, won, and disappeared
Officially the fan was for engine cooling. Unofficially it sucked the air out from under the car. Lauda won the 1978 Swedish GP with it, and Brabham withdrew it after one race even though the FIA had said it could keep running it that season.
Dev Malhotra · 6 sources
- Pikes Peak: racing up a mountain where the air runs out
Twelve and a half miles, over 156 turns, finishing above 4,300 m. Fully paved since 2011, which sent times tumbling. An electric VW took the overall record in 2018, partly because electric motors don't lose power in thin air the way combustion engines do.
Dev Malhotra · 5 sources
- how do we know quantum randomness isn't just hidden information?
Dev Malhotra · 6 sources
- the wingnut that kept flipping on a space station
Spin a rigid object around its axis with the middle moment of inertia and it periodically flips over; the other two axes are stable. Toss your phone spinning end over end and you'll see it.
Dev Malhotra · 6 sources
- Does hot water freeze faster than cold?
Haruto Sato · 4 sources
- ball lightning was basically folklore until someone accidentally filmed one
Centuries of eyewitness reports, glowing spheres floating through rooms, and almost no instrument data. The one good measurement is a spectrum recorded by chance in 2012 by researchers studying ordinary lightning in western China. It showed silicon, iron and calcium, which fits the idea that it's vaporized soil burning. Still no consensus.
Dev Malhotra · 5 sources
- the sun's core makes less heat per cubic meter than a compost heap
The core produces apparently a few hundred watts per cubic meter, roughly like a compost pile or a lizard's metabolism. The sun is bright because it is unimaginably big, not because each bit is intense.
Arash Mohammadi · 6 sources
- magenta isn't in the rainbow
Quinn Harlow · 6 sources
- EUV lithography vaporizes tin droplets with lasers 50,000 times a second
Molten tin droplets are hit twice by a laser (once to flatten, once to vaporize) to make plasma that emits 13.5 nm light. Everything after that has to be mirrors in a vacuum because EUV is absorbed by nearly everything, including air.
Annika Brandt · 4 sources
- the blue glow in reactor pools is basically a sonic boom made of light
Charged particles moving faster than light travels in water throw off a cone of light, the way a jet drags a shock wave behind it. Nothing is breaking relativity, light is just slow in water.
Quinn Harlow · 6 sources
- the great molasses flood
1919: a tank of 8.7 million liters of molasses burst in Boston and a wave of it moved through the streets. 21 people died. Locals claimed the neighborhood smelled sweet on hot days for decades.
Yuki Tanabe · 4 sources
- what does a sprinkler do if you run it backwards
Samuel Okafor · 6 sources
- LIGO is limited by quantum noise, so they squeeze the light
LIGO measures changes in arm length around a ten-thousandth of a proton's width. At that point the random arrival of individual photons is a real noise source. Squeezing trades more uncertainty in one property of the light for less in the one you care about. LIGO has been squeezing since 2019, and since 2023 in a frequency-dependent way.
Dev Malhotra · 6 sources
- the summer of LK-99
I refreshed for replication videos for about a week straight in August 2023. Half-levitating flakes, theory papers appearing in days, everyone suddenly an expert in apatite crystal structures.
Dev Malhotra · 7 sources
- a lab experiment that has produced nine drops since 1927
Pitch looks solid and shatters if you hit it with a hammer, but it's a liquid hundreds of billions of times more viscous than water. A funnel of it at the University of Queensland has dripped nine times since it was set up in 1927.
Dev Malhotra · 5 sources
- graphene was isolated with sticky tape
Geim's lab had 'Friday night experiments', side projects with no grant behind them. One was peeling graphite with tape until a flake was one atom thick. That was 2004; the Nobel came in 2010.
Quinn Harlow · 6 sources
- What the Saturn V actually burned per second
Five F-1 engines burning through more than 12 tonnes of propellant a second. And they solved combustion instability partly by detonating explosives inside running engines.
Ethan Cole · 6 sources
- in 1991 one particle hit the atmosphere with the energy of a thrown baseball
It was seen by the Fly's Eye detector in Utah, roughly 3×10^20 eV. That's tens of joules carried by one subatomic particle. Nobody knows what accelerates them that hard, and the sources should be relatively nearby, which makes it weirder that nobody can point to them.
Arash Mohammadi · 6 sources
- how old is the light from the stars I can actually see
Ethan Cole · 5 sources
- what 'fusion ignition' at NIF actually meant
Dev Malhotra · 6 sources
- the 'Woodstock of Physics' was a 1987 superconductor session that ran past 3am
The American Physical Society added a last-minute evening session on the new ceramic superconductors and people packed the hall and overflowed into the corridors. Talks were limited to about five minutes each.
Arash Mohammadi · 6 sources
- how the James Webb telescope stays cold enough to work
Theo Martin · 6 sources