Science
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Day 11: What If We Put Clocks in Space?
In 1977, three years before GPS launched, the engineers building the satellites had to make a decision.
The clocks they were about to put in orbit were going to run faster than the clocks on the ground. By about 38 microseconds per day.
That sounds like nothing, but over 24 hours of GPS operation, an uncorrected clock would put you 11 kilometers off your actual position.
They had two options:
- Adjust the time signal on the ground, applying the correction as the data came back down.
- Pre-tune the clocks on the satellites to run slow by exactly the right amount, so that by the time relativity sped them back up, they’d tick at the right rate.
GPS chose option two.
They built the clocks to run at 10.22999999543 MHz instead of the nominal 10.23 MHz, so that orbital relativity speeds them up to ~10.23 MHz by the time the signal hits your phone.
The correction is baked in.
That’s what putting clocks in space looks like. One decision, and now everyone on Earth gets both navigation and time from the same signal.
This post is about the impact of that decision.
Why Clocks in Orbit Run Faster
Two relativistic effects act on a GPS satellite clock, and they push in opposite directions.
Special relativity slows the satellite clock down because it’s moving fast. General relativity speeds it up because it sits in weaker gravity than the ground. Gravity wins. Net result: the satellite clock gains about 38 microseconds per day.
Sounds like nothing. But uncorrected, that 38 microseconds drifts your GPS position by 11 km in 24 hours. Within a day of launch, GPS would be useless for anything more precise than “are you in the right country.”
This was known before launch. It was tested. It works.
What GPS Time Actually Is
GPS time is its own scale, started at midnight on January 6, 1980, and ticking continuously since. No leap seconds. No time zones.
The relationship to the other scales is fixed and simple:
GPS = TAI − 19 seconds (constant since launch) GPS = UTC + 18 seconds (today)GPS−TAI never changes. GPS−UTC grows every time UTC gets a leap second, and freezes after the 2035 leap-second abolition.
It is, in every meaningful sense, the most accurate clock in your daily life. And you’ve never seen it.
What It’s Used For
GPS time runs almost everything that needs precise timing in modern civilization, but it’s invisible because nobody consumes it directly.
- Finance. US and EU regulators (MiFID II, SEC) require trading firms to timestamp orders to microsecond precision. GPS-disciplined oscillators are how.
- Telecom. Cellular base stations need their carrier frequencies aligned across the network. GPS clocks them. Without GPS, your phone would struggle to hand off between towers.
- Power grid. Phasor Measurement Units monitor the AC waveform across the entire grid, synchronized to GPS. This is how grid operators detect instabilities before they cascade into blackouts.
- Datacenters. Stratum-1 NTP servers are typically GPS-disciplined. Every clock you’ve ever checked on a computer ultimately traces back, through several layers of network sync, to a GPS receiver in someone’s rack.
- Aviation, surveying, autonomous vehicles, drones, scientific instruments, particle physics. Anything built since 1995 that needs accurate timing or positioning, which is essentially everything.
The civilian world runs on GPS time. It just doesn’t admit it.
What It Didn’t Solve
Putting clocks in space solved navigation.
It did not solve timekeeping.
Your watch is still on local time. Your calendar uses civic dates with leap seconds buried in the UTC. You’re reading a clock face anchored to a Roman calendar, a Babylonian 24-hour day, and an Earth rotation that nobody can predict.
GPS time is great if you are a satellite, a financial trader, a power-grid engineer, a fighter jet, or a cell tower.
It is not great if you are trying to know what time to pick up your kid from school.
For that, you still need wall time, which still needs UTC, which still needs leap seconds, which still needs Earth’s wobbling rotation.
We built absurdly precise atomic clocks. We launched them into orbit. We baked relativity corrections into the silicon. We covered the planet in time signals accurate to nanoseconds.
And your meeting is still at 3 PM on Tuesday.
GPS quietly handles the part it needs to handle. But all of this assumes you’re on Earth.
Where This Goes
Earth orbit needs relativity corrections. The Moon needs more. Mars needs different ones still.
The further you get from Earth, the more “GPS-style time” stops being a solution.
Tomorrow: if an hour is an Earth measurement, so how do you tell time on a planet that doesn’t have them?
Sources
- Error analysis for the Global Positioning System — Wikipedia
- GPS time — Wikipedia
- Schriever Space Force Base — Wikipedia
- Phasor measurement unit — Wikipedia
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/ Science / Infrastructure / 30daysoftime / Timekeeping / Gps / Relativity
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Day 9: A Clock You Can Use to Measure a Mountain
Yesterday I said cesium was about to be deposed. Today’s the deposition.
The successor is the optical lattice clock. It is so precise that, it can detect the altitude difference between two desks in the same office.
That last sentence sounds crazy but Its what optical clocks do.
Here’s how.
More ticks per second
The trick of the optical lattice clock is simple in concept: use a finer ruler.
A cesium atom oscillates at about 9 × 10⁹ Hz (~9 GHz, microwave). That’s how many “ticks” per second the clock has to count to keep time.
A strontium-87 atom has a clock transition at about 4 × 10¹⁴ Hz (~430 THz, visible light). That’s 50,000 times more ticks per second.
Same second. More tick marks on the ruler.
Faster clock makes better time.
Faster clocks are better than slower ones.
Faster clocks make for better timekeeping.
See the difference and why we need to upgrade the second?
Today’s best optical clocks drift about one second over the age of the universe. 13.8 billion years. Our best cesium clocks are nowhere near that, drifting the same second in roughly 300 million years instead. About a hundred times worse.
The magic wavelength trick
So if optical is obviously better, why didn’t we do this first?
Because it’s hard. Two reasons, really.
First, you can’t just point an optical clock at a free-floating atom. Atoms in flight have all sorts of velocity, Doppler-shifting the frequency you measure. You have to hold them still. But the only way to hold an atom still is to grip it with something, and gripping it with a laser changes the energy levels you’re trying to measure. Catch-22.
Second, even if you could pin the atom, you couldn’t count that fast. Microwave at 9 GHz fits inside what electronics can divide down and tick off. Visible light at 400+ THz does not. We had no way to count optical-frequency oscillations until the optical frequency comb showed up in 1999, work that won Hänsch and Hall the 2005 Nobel.
In 1967 when the CGPM picked cesium, none of this existed. The laser was seven years old. Laser cooling wouldn’t be demonstrated until the late 1970s. Frequency combs were 30+ years away. Cesium at 9 GHz wasn’t the best clock you could imagine. It was the best clock you could build.
The fix came in 2003 from a Japanese physicist named Hidetoshi Katori. He proposed trapping the atoms in a standing-wave laser pattern, a 3D optical lattice, like an egg crate made of light, and tuning the lattice laser to a specific frequency called the magic wavelength.
At the magic wavelength, the lattice light affects both energy levels of the clock transition by exactly the same amount. The trap is invisible to the clock. The atoms are held still, but the energy levels they emit at are unperturbed. You get to measure the atomic transition cleanly while the atom sits frozen in midair.
Optical lattice clocks are a beautiful piece of physics. Do not ask me about the math behind them.
Which atom?
Two main candidates have been thinking about for the title of “next SI second”:
- Strontium-87. Run in optical lattices by labs around the world: NIST/JILA in Boulder, NPL in the UK, RIKEN in Japan, SYRTE in Paris. About 1 part in 10¹⁸ uncertainty. Currently the favorite.
- Ytterbium-171. Comparable accuracy, different sensitivity profile (different ways the clock can go wrong, which is good for cross-checking against strontium).
There are also trapped-ion optical clocks, which hold a single ion in an electric field instead of a lattice of neutral atoms. The aluminum-ion logic clock at NIST has hit about 1 part in 10¹⁹. That’s one second of drift in 300 billion years. The amount of drift is older than the universe itself. That sounds pretty good to me.
What this gets you
Optical clocks are sensitive enough to see general relativity at human scale.
What does that mean? It means you can put two clocks on different shelves and watch one tick slower than the other.
Einstein said clocks deeper in a gravitational field run slower than clocks higher up. Near Earth, the effect is about 1 part in 10¹⁶ per meter of altitude. A clock on the floor runs slower than a clock on the table. For most of human history this was a theoretical curiosity. Now it’s a measurement.
In 2022, a JILA strontium clock measured a 1 millimeter height difference as a frequency shift. One millimeter. The thickness of a credit card. The clock could tell which side of the card it was sitting on, from gravitational time dilation alone.
This opened a new field: relativistic geodesy. Put an optical clock at two locations, compare frequencies, and you’ve directly measured the gravitational potential difference between them. Detect underground oil, magma movement, ice sheet melt, anything that shifts mass around the planet.
Cesium can’t do this.
The next “second”
The BIPM is planning to redefine the SI second around an optical transition by 2030. Same dance as 1955: measure the new number against the current cesium standard, freeze it at that precision, declare it the new second.
Four layers of backwards-compatibility instead of three. The fingerprints accumulate.
Where this goes
We’ve now spent three days on how a second is built. The next question is the one the calendar dodges: which second?
You need a reference point. A zero. An origin.
Tomorrow we’ll talk about where you start counting from, and why astronomers picked noon on January 1, 2000.
Sources
- Optical clock — Wikipedia
- Hidetoshi Katori — Wikipedia
- Quantum logic clock — Wikipedia
- Relativistic geodesy — Wikipedia
- Bothwell et al., Nature 2022 — millimeter-scale gravitational redshift
- Gravitational time dilation — Wikipedia
- Jun Ye — Wikipedia
- Epoch (astronomy) — Wikipedia
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/ Science / Time / Physics / 30daysoftime / Metrology
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Day 8: 9,192,631,770
Today’s number is 9,192,631,770.
That’s how many times a cesium-133 atom oscillates between two specific energy states in one second. Or, more precisely, it’s how many times we decided a cesium atom does that in one second. The number is doing some heavy lifting that the universe didn’t ask for.
This post is about the number, how it got picked, and how we use it to measure time.
The chain
To know what a second is, you need a way to measure one. For most of human history, the answer was: “look at Earth and count” or earth++ for the programmers reading this.
One second = 1/86,400 of a mean solar day.
That worked for sundials and pendulum clocks. It stopped working in the 1930s, when good quartz clocks revealed that Earth’s rotation isn’t uniform. Yesterday, we talked about how. Earth’s rotation is not a good measurement of time. I mean good in the sense of precise, because there are so many variables that go into the rotation and impact it. We don’t have an accurate way to measure and predict how those variables change.
In 1960, astronomers tried again. Because why not? Wheel, reinvented.
They picked Earth’s orbit instead of its rotation, which is far more stable:
One second = 1/31,556,925.9747 of the tropical year for 1900.
That weird denominator was chosen specifically so the new “ephemeris second” came out the same length as the old mean solar second. Don’t break clocks … like ever.
Great in principle, terrible in practice. To know what time it was, you had to consult a 60-year-old astronomical table. It doesn’t work with the clock in the lab. The ephemeris second won the philosophy and lost the engineering.
But wait, maybe there’s a third option.
How I wish it was Cesium-123 and not Cesium-133, but alas. The 133 is the mass number: 55 protons + 78 neutrons. Cesium-133 also happens to be the only stable isotope of cesium, so it kind of picked itself.
The cesium handoff
Cesium-133 has two slightly different ground-state energies. Drop an atom from one to the other and it spits out a photon at a specific frequency, call it f. If you can build a clock that locks itself to that frequency, you have a timekeeper that doesn’t depend on the Earth, the Sun, or any astronomer’s table. Every cesium-133 atom in the universe agrees on f to absurd precision, because quantum mechanics doesn’t have local variants.
The number for f is approximately 9.192631770 GHz. So if you count 9,192,631,770 oscillations, exactly one second has elapsed.
In 1967, the General Conference on Weights and Measures (the CGPM, the body that gets to define units) voted to make that the official definition. The SI second became:
The duration of 9,192,631,770 periods of the radiation corresponding to the transition between the two hyperfine levels of the ground state of the cesium-133 atom.
No Earth, no Sun, no tables. Just an atom.
Why that exact number
The number 9,192,631,770 is not a fundamental constant of the universe. It was measured. By two specific guys. In 1955.
Their names were Louis Essen and Jack Parry, and they worked at the National Physical Laboratory in the UK. They had just built the first cesium clock that actually worked. The question was: how many cesium ticks fit in one ephemeris second? They measured it carefully. Their answer, published in 1958 with William Markowitz and Robert Hall:
1 ephemeris second = 9,192,631,770 ± 20 cesium periods.
Nine years later, the CGPM took that number, dropped the uncertainty, and made it the definition. From then on, the second was defined as exactly 9,192,631,770 ticks. The plus-or-minus twenty disappeared.
This means three things:
- The cesium second was chosen so it would equal the ephemeris second.
- The ephemeris second was chosen so it would equal the mean solar second.
- The mean solar second was 1/86,400 of an Earth day.
The three layers of don’t break the previous standard. If Essen and Parry had been off by one in their measurement, the entire SI second would be slightly different today, and every clock and computer and GPS satellite would be calibrated to that other version.
The number is a historical fingerprint, not a physical constant.
It records, with eleven decimal-digit precision, exactly how well two physicists in 1955 could compare a cesium clock to an Earth-orbit calculation.
Every GPS satellite, every NTP server, every timestamp on every photo you’ve ever taken, they’re all calibrated to the limits of what Essen and Parry pulled off with 1955 equipment. The universe doesn’t care about your iPhone. Your iPhone cares about two guys at the NPL.
How precise are we now
Modern cesium fountains, the latest generation, where atoms are laser-cooled and tossed gently upward through a microwave cavity, falling back down under gravity like a slow ballistic juggling act, hit accuracies of about one part in 10¹⁶. That’s roughly one second of drift per 300 million years.
NIST-F2 in Boulder, the U.S. primary clock, is one of these. About a dozen others sit in metrology labs across France, Germany, the UK, Japan, and China. They all report their measurements to the BIPM in Paris, which combines them into a weighted average called TAI, International Atomic Time. TAI is what your phone’s clock is ultimately disciplined to, through a long chain of NTP servers and GPS signals.
Everything you do that involves time:
- sending a message,
- taking a photo with a timestamp,
- syncing with a calendar,
- getting a stock trade priced
It all traces back to a few hundred atomic clocks averaging each other in real time.
What comes next
Cesium won’t be the way we measure the second forever. It’s about to be deposed.
Optical lattice clocks, using strontium or ytterbium atoms instead of cesium, operating at visible-light frequencies instead of microwave, are now about a hundred times more precise. They lose one second per about 30 billion years, which is roughly twice the age of the universe.
The BIPM is planning to redefine the SI second around an optical transition, possibly by 2030.
When that happens:
- Mean solar second = 1/86,400 of an Earth day
- Ephemeris second chosen to match the mean solar second
- Cesium second chosen to match the ephemeris second
- Optical second chosen to match the cesium second ← the new one
Three layers of backwards-compatibility will become four.
Tomorrow we’ll look at those optical clocks, what they are, how they work, and how they let us measure what cesium can’t.
Sources
- Louis Essen — Wikipedia
- William Markowitz — Wikipedia
- Second — Wikipedia
- Ephemeris time — Wikipedia
- Atomic clock — Wikipedia
- International Atomic Time — Wikipedia
- NIST-F2 — Wikipedia
- Optical clock — Wikipedia
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Day 7: A Day Is Not 24 Hours
Last week we talked about what time is, what it might not be, and what your brain does to make you feel it. This week we measure.
The obvious place to start is to look up at the sky. Humans did this for several thousand years before they realized how badly the sky was lying to them.
The Sundial Problem
The oldest clock is a literal stick in the figurative ground.
Watch the shadow move. When the shadow is shortest, the sun is overhead. That’s noon.
For a long time, this was good enough. If you wanted to coordinate a meeting in ancient Egypt, you and your buddy could both look at the sun and agree on roughly when to show up. Our Civilizations were built on this.
Here’s the problem. If you mark where noon falls on a sundial every day for a year, and compare it to a clock that ticks steady seconds, the sundial drifts. Sometimes the sun is “late.” Sometimes it’s “early.” Over the course of a year, the gap swings by up to about sixteen and a half minutes one way (early November) and just over fourteen minutes the other (mid-February).
This is called the equation of time, and it has two main causes.
First, Earth’s orbit isn’t a circle. It’s an ellipse. We move faster when we’re closer to the sun (around January 3rd) and slower when we’re farther away (around July 4th). When we’re moving faster, the sun appears to drift across the sky faster, and noon comes sooner than the clock predicts.
Second, Earth’s axis is tilted. The sun doesn’t ride along the equator, it rides along the ecliptic at a 23.5 degree angle. That tilt distorts the projection of the sun’s motion onto our daily rotation, which means the sun runs ahead of the average for parts of the year and behind it for others.
If you graph the equation of time across a year, you get a wobbly figure-eight called the analemma. You’ve probably seen it on a globe somewhere and ignored it. It’s the actual shape of “noon” over a calendar year.
So if you want a 24-hour clock that doesn’t drift around with the seasons, you can’t use a sundial directly. You have to average. The result is called mean solar time, the time you’d see if the sun behaved itself.
Two Kinds of Day
Hopefully following along so far, because now we need to talk about what a “day” is. There are two ways to define it and they disagree.
The solar day is what you’d guess. Sun is straight overhead; rotate Earth until the sun is straight overhead again. That’s one day. About 24 hours.
The sidereal day is what astronomers use. Pick any distant star; rotate Earth until that star is back in the same position in the sky. That’s one sidereal day.
A sidereal day is 23 hours, 56 minutes, and 4.09 seconds. Almost exactly four minutes shorter than a solar day.
Why? Because Earth is doing two things at once.
While you spin on your axis, you’re also moving around the sun. By the time you finish one full rotation relative to the stars, you’ve also moved a tiny bit along your orbit. The sun has effectively shifted in the sky from your perspective. You have to rotate a tiny bit further to point at the sun again.
That tiny bit further takes about four extra minutes. Add it up over 365 days and it equals exactly one full rotation. That’s why a year has one more sidereal day than solar days. The arithmetic comes out clean. The universe is just doing this weird double-counting thing where one of your rotations gets eaten by your orbit.
If you’re an astronomer trying to point a telescope at a star, sidereal time is what you want. The star is in a fixed place in inertial space; your dome needs to compensate for Earth’s actual rotation, not for “where the sun appears to be.”
If you’re a person trying to know when to eat lunch, solar time is what you want. The sun is the thing your body cares about.
These two definitions don’t reconcile. They are answering different questions.
Earth Doesn’t Tick Steadily
Even after you average the equation of time and pick which kind of day you want, Earth still doesn’t make a great clock.
Earth’s rotation is slowing down. Tidal friction with the Moon transfers angular momentum outward, the Moon drifts farther away (about 3.8 centimeters per year, measured by bouncing lasers off Apollo-era retroreflectors), and our days get longer by roughly 1.7 to 2.3 milliseconds per century. Slow, but cumulative. A really, really long time ago, a day was about 22 hours.
So we know, Earth’s rotation is jittery in the short term. The atmosphere (air mass) sloshes around with weather. Ocean currents shift mass around because hot water weighs less than cold water. There is some coupling between the outer core and the mantle that yanks the rotation rate around. It’s very hard to predict all these factors in advance. All of these factors cause the the length of a day to fluctuate from one week to the next.
For a long time none of this mattered. If a day was off by a few milliseconds, who cares? Sundials don’t have that resolution.
The moment it started mattering was when we got better clocks than the those based on the Earths rotation.
How We Measure Earth’s Rotation Today
I hope you are ready to learn some astronomy.
The most precise measurement of Earth’s rotation right now comes from watching distant quasars, supermassive black holes billions of light years away whose positions in the sky are effectively fixed. A technique called Very Long Baseline Interferometry, or VLBI, uses arrays of radio telescopes spread across continents to triangulate Earth’s exact orientation against these quasars.
That’s worth reading again. The way we figure out what time it is on Earth is by triangulating against the cores of ancient galaxies billions of light years away.
VLBI pins down Earth’s orientation to the level of microseconds and millimeters. It’s how we know, day by day, by exactly how many milliseconds the planet ran fast or slow. It’s how we know, to staggering precision, exactly how badly the planet underneath us is failing to be a steady clock.
That measurement, and what we did about it, is going to matter in a bit, in future articles. This week on Time is all about how we measure time.
Tomorrow: the second we use today isn’t measured by Earth at all. It’s measured by an atom that doesn’t care which planet you’re on.
Sources
- Equation of time — Wikipedia
- Sidereal time — Wikipedia
- Earth’s rotation — Wikipedia
- Seeing the Light: lunar laser ranging — Eos
- VLBI — NASA Earthdata
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/ Science / Time / 30daysoftime / Astronomy
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Day 5: Time Is Not the Same for a Fly
Try to swat a fly. Even with a flyswatter. You will fail more often than you succeed.
You’re not slow. You’re not bad at this. The fly is living in a slower-motion version of the world than you are.
The Refresh Rate of Perception
Biologists have a clean way to measure how fast an animal experiences time. It’s called the critical flicker fusion threshold, or CFF.
Here’s the experiment. You flash a light at increasing speeds. At some point, the flashes blur together and look like a steady beam. The frequency where discrete flashes become continuous light is the CFF. It’s basically the refresh rate of an animal’s visual system.
A Hertz (Hz) is the number of times
repeats per second. We will go into how a second is defined in the post on Day 8. For humans, CFF is around 60 Hz. That’s why old TVs ran at 60 frames per second. Beyond that, you can’t tell the difference between flickering and steady. Fun fact, this is also why dogs, with a CFF around 75 Hz, were unimpressed with old CRT TVs. The 60 Hz refresh looked like a strobe to them.
For a housefly, CFF is around 250 to 300 Hz.
That means a fly’s brain is processing roughly five times as many visual frames per second as yours. When you swing a flyswatter at 5 meters per second, you see one smooth motion. The fly sees an event playing out across hundreds of crisp, individual snapshots. By the time your hand is halfway there, the fly has watched a slow-motion replay of your intent and made a decision about which direction to dodge.
You are not fast. The fly is.
Why It Varies
In 2013, a research group led by Kevin Healy at Trinity College Dublin published a paper that nailed down the pattern. CFF, they showed, scales inversely with body size and directly with metabolic rate. Smaller and faster-burning means a higher CFF, which means slower-feeling time.
The reasons are physical.
Smaller animals have shorter nerves. A fly’s visual signal travels micrometers from eye to brain. A blue whale’s motor command has to run thirty meters down an axon. Big bodies just can’t run fast neural loops, because the signal arrives too late.
And the energy cost is brutal. Maintaining a high refresh rate means firing photoreceptors over and over, pumping ions back across cell membranes, burning ATP. Small fast-metabolizing animals can afford that. Slow large animals can’t, and don’t need to.
So evolution settles each species at the CFF that fits its niche.
A Field Guide to Other Nows
A rough sense of how the rest of the animal kingdom experiences time:
- Housefly: ~250 to 300 Hz. Your hand moves in slow motion.
- Songbird: ~100 to 140 Hz. Necessary for darting between branches in a forest.
- Dog: ~75 to 80 Hz. Saw old TVs as strobes.
- Human: ~60 Hz. The baseline. Why film at 24 fps looks smooth to us.
- Cat: ~55 Hz. Slightly slower than us.
- Sea turtle: ~15 Hz. The world looks fine because it doesn’t need to move fast in it.
- Deep-sea fish: 10 to 15 Hz. Cold, dark, energy-scarce. Slow-mo for them.
Values compiled from Healy et al. 2013 and the Lafitte et al. 2022 systematic review of CFFs across 156 species.
Every species in this list is processing the same physical reality. They are just sampling it at radically different rates.
Predator vs. Prey
Who has a higher CFF, the predator or the prey?
The question matters because it shapes survival. A songbird being chased by a hawk that sees more frames per second than the hawk does has more reaction time. It picks up the swoop earlier and gets out of the way. Evolution rewards that across generations, and prey CFFs climb in response. Predators have to catch up. The chase pushes the numbers higher on both sides.
There’s a ceiling. CFF is expensive. Every additional frame a nervous system resolves costs energy, and each species can only afford to perceive as fast as its metabolism can fuel. The energy budget caps the arms race.
CFF also factors into communication, though we don’t fully understand how most animals communicate. But imagine a small, fast-perceiving animal signaling at frequencies higher than its slower predators can resolve. To the predator, the message is a blurred smear. To the recipients, it’s a clear sequence of flashes.
Encrypted messages, too fast for your enemies to read.
Subjective Lifespan
Here’s a thought experiment.
A mayfly’s adult life is a single day. A bowhead whale lives over 200 years. If you measure lifespan by sensory frames processed rather than clock time, you can imagine the gap closing. The mayfly burning hot and short, the whale burning slow and long, each living some comparable subjective stretch.
Has actually measured CFFs for mayflies or whales? I couldn’t find any definitive studies on the topic. The “all animals live equally long subjective lives” line you’ll find online, but not in a science backed study. Still, the question is fun to chew on.
I think the hypothetical whale still beats the hypothetical mayfly in terms of number of sesnory frames but, either way, it’s clear we need more studies on sensory frames and lifespan!
So What Is Your Now?
Your subjective present is roughly 1/60th of a second wide. That’s the slice of reality your visual system can resolve as a single moment. The concept of “now” depends on what animal we are talking about. The flow of time you feel is built out of these slices of sensory frames.
Tomorrow we’ll talk about how your brain stitches the frames together to create the present, and what happens when that machinery breaks.
Sources
- Flicker fusion threshold - Wikipedia
- Time perception - Wikipedia (covers species differences)
- Healy, K., McNally, L., Ruxton, G. D., Cooper, N., & Jackson, A. L. “Metabolic rate and body size are linked with perception of temporal information,” Animal Behaviour 86 (2013): 685–696
- Lafitte, A., Sordello, R., Legrand, M., Nicolas, V., Obein, G., & Reyjol, Y. “A flashing light may not be that flashy: A systematic review on critical fusion frequencies,” PLOS ONE (2022)
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/ Science / Time / 30daysoftime / Perception / Biology
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The World's Strongest Cable Is One Atom Thick
I fell down a rabbit hole this morning. It started with a simple question: how far are we really from a space elevator? This really is a feasibility question, and I"m convinced that the answer is probably no, but for a really fascinating reason.
The Basic Problem
A space elevator needs a cable stretching from Earth’s surface to geostationary orbit, so about 36,000 kilometers up. That cable has to hold its own weight, plus whatever cargo you’re lifting, plus deal with a counterweight extending beyond geostationary orbit that’s constantly trying to pull away to balance everything out.
The heavier the cable, the more stress it puts on itself. So you need something impossibly light and impossibly strong. Carbon nanotubes were the go-to candidate for years, but even their theoretical limits fall short of what’s needed. The forces involved are just too extreme.
Enter Graphene
Graphene is similar to carbon nanotubes, but instead of a tube, it’s a flat hexagonal lattice of carbon atoms… so essentially a really long sheet, or aka a ribbon.
The world’s strongest cable isn’t round. It’s flat, and it’s one atom thick.
That’s kind crazy to think about, that the strongest material we can come up with is literally as thin as matter gets.
Why It Still Won’t Work (Probably)
Even if graphene has the right properties on paper, we have a number of impossibly hard engineering problems.
Manufacturing: We can’t produce a continuous ribbon of graphene anywhere close to the lengths needed. We’re talking thousands of kilometers of perfectly formed, single-atom-thick material. So not happening.
You don’t want one ribbon anyway: Even if you could manufacture a single ribbon that long, you wouldn’t want to. You need redundancy. If one section gets damaged — and at 36,000 km of exposure to space debris, micrometeorites, and atmospheric forces, damage is a when, not an if. You need backup ribbons to bear the load. So you’d need separate sections, separate lengths.
The bonding problem: And this is where I think the whole concept falls apart. There’s no reliable way to bond or clamp separate graphene ribbons together. Think about it… how do you reliably verify that something one atom thick is actually bonded properly? Every single joint, every repair, becomes a critical failure point. You can’t just slap a section in place and hope for the best.
To do that in a reproducible, verifiable way, at scale, in space, repeatedly. I think that’s the thing that makes the space elevator functionally impossible. Not conceptually impossible, but practically impossible.
The Climber Problems
I’m deliberately ignoring all the problems the climber introduces (the mechanism that actually lifts cargo up the cable). There’s a whole separate set of headaches there. But the cable itself is the fundamental challenge, and it comes down to this:
We might have identified the right material. Graphene’s properties are genuinely remarkable. But knowing what to build something out of and knowing how to build it are two very different things.
/ Science / Space / Engineering
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The universe is a giant board game. Space is the board, matter is the pieces, and logic is the rules. Language is our rulebook of how we describe how the board works. You can’t separate the rules from the board; if you had a different board, the rules would be different too.
When you get to the “what ifs,” it becomes a thought experiment, not grounded in science.
/ Philosophy / Science / Metaphysics