30daysoftime
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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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Day 6: Your Brain Is Lying to You About Time
Yesterday I told you a fly’s now is much shorter than yours and a whale’s is much longer. Today I want to talk about how wide your now actually is, and why none of it is what your brain is showing you.
The short answer is that your brain is editing.
The Saddleback
In 1882, a philosopher writing under the pseudonym E. R. Clay noticed something obvious that nobody had said cleanly before. The “present” of physics, a knife-edge with zero duration separating past from future, is not the present you actually experience.
When you hear a melody, you are not perceiving one note at a time and then assembling them from memory. You are hearing the melody. The first three notes are still vividly present even as the fourth arrives. They haven’t dropped into recall yet. They’re still in the room.
Clay called this experienced now the specious present. Specious because it isn’t really the present, it’s a stretched window that the brain treats as a single perceptual moment.
William James picked the idea up in 1890 and gave it the metaphor people still use:
“The practically cognized present is no knife-edge, but a saddle-back, with a certain breadth of its own on which we sit astride, and from which we look in two directions into time.”
You don’t sit on a point. You sit on a saddle. From the saddle, you can see a little way into the just-past and a little way into the just-coming. All of it feels like now.
How wide is the saddle? Modern cognitive science puts the boundary at roughly 2 to 3 seconds. Beyond that, things start to feel like memory rather than experience.
The 3-Second Rule Shows Up Everywhere
A German chronobiologist named Ernst Pöppel spent decades chasing this number across human behavior, and what he found is uncanny.
Spoken phrases across all studied languages cluster around 3 seconds. Lines in poetry, going back to ancient Greek hexameter, sit in that range. When you stare at an ambiguous image like the Necker cube, your brain flips its interpretation every 2 to 3 seconds, automatically. Even people clapping along to music will, without prompting, fall into clusters of beats roughly 3 seconds wide.
This isn’t because the universe has a 3-second resonance. It’s because that’s the size of cognitive chunk your brain processes as a coherent now. Everything you build out of moments, language, music, attention, conscious experience, is laid down in saddles roughly that wide.
3 syllable words are more likely to be used, and thus carry more meaning than 13 syllable words.
The Lie About Order
It gets worse. Inside the saddle, your brain is actively rewriting the order of events.
Light and sound travel at different speeds. The signals from your eyes and ears take different times to reach your cortex. If your brain just delivered each signal to consciousness whenever it arrived, the world would feel much different, with everything out of sync.
So instead, your brain buffers. Within about an 80-millisecond window, it pulls events back into alignment. What you experience as “now” is the output of that stitching, not the raw signal.
Another way to think about this is that if a flash and a bang happen within 80 ms of each other, the brain rewrites the timing and presents them as simultaneous, regardless of which one your neurons actually processed the “event” first.
You don’t experience the truth. You experience the post-production cut.
The clean stream of reality, where dialogue lines up with mouth movements and you can catch a ball without thinking about it, is a fiction the brain is generating in real time.
When the Editor Breaks
The cleanest evidence that the present is a construction is what happens when the constructor breaks.
There’s a rare condition called akinetopsia, motion blindness. People with akinetopsia, usually after damage to a brain region called V5, lose the ability to perceive continuous motion. They see the world as a series of static snapshots. A car is here, and then it’s there, and they never see it travel between. Water pouring from a pitcher looks frozen, like ice. A dog mid-leap is a still photograph.
These people are not blind. Their eyes work. What’s broken is the part of the brain that stitches the saddleback together. Strip that out, and you don’t see “time” anymore. You see disconnected frames.
The flow of time is something your brain is doing. Not something it’s perceiving.
Why a Year Feels Shorter Every Year
Now for the part that’s going to depress you.
In 1877 a French philosopher named Paul Janet pointed out something nobody likes admitting. The reason a year felt long when you were five and feels short when you’re forty is mathematical. At five, a year is one-fifth of your entire existence. At forty, it’s one-fortieth. The same calendar interval is a much smaller fraction of who you are.
This is intuitive but it isn’t the whole picture. The deeper explanation, from the psychologist Robert Ornstein in 1969, is about memory density. The brain reconstructs how long a past period felt by counting how many distinct memories it can pull from it. Childhood is packed with first times: first day of school, first bike ride, first betrayal. Memory traces are dense. In hindsight, the period feels enormous.
Adulthood is the opposite. You drive the same route to the same job and eat the same lunch. The brain, energy-thrifty as ever, throws most of that away. When you look back at the last year, there isn’t much to find. The conclusion your brain delivers: that year barely happened.
If you’ve ever come back from a two-week vacation in a new country and felt like you’d been away for a month, you’ve seen the other side of this. Novelty stretches the look-back. Routine erases it.
The way to slow down the rest of your life is to keep making first memories.
The Slow-Motion Crash Is a Lie Too
People who survive car crashes routinely report that time slowed down. The world stretched. Their hands moved through molasses. The popular explanation is that adrenaline ramps up neural processing, your brain “speeds up” in danger, and so the world appears to slow down.
It’s a great story, and it’s wrong.
The neuroscientist David Eagleman tested this directly in the early 2000s by getting volunteers to free-fall from a 31-meter tower into a net. While they fell, they wore a wristwatch that flashed numbers faster than the human eye can normally read. Eagleman’s bet: if perception really speeds up in fear, the falling subjects should be able to read the watch.
They couldn’t. Their perception didn’t speed up at all.
But afterwards, asked to estimate how long the fall lasted, they all overestimated dramatically. The fall felt long in retrospect.
The slow-motion is a memory effect. In a crisis, your brain switches into high-density recording mode. It lays down vastly more memory traces per second than usual. When you reconstruct the experience afterward, all those dense memories make it feel like the event took forever. You weren’t seeing slowly. You were remembering richly.
The present moment was the same as always. The look-back is the lie.
Where We Go From Here
We’ve spent a week on what time is, what it might not be, and how your brain assembles the experience of it.
Tomorrow we leave all of that behind. For the next two weeks, time stops being a thing you feel and becomes a thing you measure. The hard sciences are coming. We’ll start with how human beings figured out how long a day actually is, and why the answer changed depending on the century.
Sources
- Specious present - Wikipedia (E. R. Clay and William James)
- Temporal Consciousness - Stanford Encyclopedia of Philosophy
- Ernst Pöppel - Wikipedia (3-second window research)
- Akinetopsia - Wikipedia (motion blindness and V5)
- David Eagleman - Wikipedia
- Stetson, C., Fiesta, M. P., & Eagleman, D. M. “Does Time Really Slow Down during a Frightening Event?” PLOS ONE 2(12): e1295 (2007)
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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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Day 4: Does the Future Already Exist?
Yesterday I left you with a question, today we are getting stuck in. If the block universe is right, and my death is already sitting in the loaf at coordinates I haven’t reached yet, then in what sense am I choosing anything?
It turns out this isn’t a new problem. People were freaking out about it 2,400 years ago.
Aristotle’s Sea Battle
Around 350 BCE, Aristotle wrote a short text called On Interpretation. Most of it is dry logic. But in Chapter 9 he stops to consider something that has been bothering philosophers ever since.
Imagine someone says: “There will be a sea battle tomorrow.”
Is that statement true right now?
It seems like it has to be either true or false. That’s basic logic. A statement and its negation can’t both be true. One of them has to be the case.
But if it’s true right now that there will be a sea battle tomorrow, then the sea battle is already locked in. The admirals can deliberate, the sailors can train, the ships can be prepared or not prepared, but the battle is happening, because the statement was true before they did any of that. And if the statement was false right now, then no matter what anyone does, no battle is possible. Deliberation is pointless either way.
This is the original fatalism problem. Aristotle didn’t have a block universe yet. He didn’t need one. He just noticed that if statements about the future already have truth values, the future is already decided.
His escape was to say: future-tensed statements don’t yet have truth values. They become true or false as time passes. The statement “there will be a sea battle tomorrow” isn’t true or false today, it just isn’t yet. Today’s truth is silent on tomorrow’s events.
That’s a tidy answer, and most philosophers since haven’t bought it. Bivalence, the idea that every proposition is either true or false, is hard to give up.
The Block Universe Version
Fast forward 2,300 years. Einstein hands us the block universe. Now we don’t need to worry about statements, we can talk about the actual thing. Tomorrow’s sea battle is sitting at its spacetime coordinates whether anyone says anything about it or not. Tomorrow’s you is sitting at its spacetime coordinates whether you’ve made up your mind or not.
So when you sit down at lunch and choose between the salad and the pizza, is anything actually being decided? Or is the salad-eating version of you already there in the loaf, and your “deliberation” is just the part of the loaf where the neurons fire?
This is the worry that makes the block universe feel like a horror movie.
Compatibilism, or: It’s Not as Bad as It Sounds
The standard response is associated with philosophers J.J.C. Smart and David Lewis, and it goes like this.
Yes, the future is fixed. But “fixed” isn’t the same as “forced.”
When we say tomorrow’s you eats a salad, what does that mean? It means tomorrow’s you deliberated, weighed the options, and chose the salad. The block universe doesn’t bypass your decision. Your decision is what’s written into the block. The reason the future slice shows you eating a salad is because the present slice, the one doing the deliberating right now, picks the salad.
You are not a passenger on a fixed track. You are part of the track-laying. Your choosing is a real causal node in the structure, not a piece of theater performed over a predetermined script.
Compare it to the past. The past is fixed too. Yesterday’s choices are locked in. We don’t usually feel like that’s a problem, because we remember making them. The block universe says the future has the same status as the past, with one difference: you don’t remember it yet.
This doesn’t fully comfort everyone, and I get it. The libertarian objection, made forcefully by Peter van Inwagen, is that genuine free will requires the ability to do otherwise. If the block already shows you eating a salad, there’s no sense in which you could have eaten pizza. Counterfactually, sure, in a possible world where your desires were different, you’d pick pizza. But in this world, the salad is already there. The pizza branch was never on the menu.
I find the compatibilist response more convincing than the libertarian one. But I also notice that I would say that, because I want to keep eating salad and feel like I picked it.
Quantum Mechanics Isn’t Going to Save You
A lot of people, when they first encounter this problem, reach for quantum mechanics. Surely the universe is fundamentally indeterministic at the smallest scales. Surely that means the future isn’t fixed, that quantum events ripple up into our brains and give us real openness.
This rescue doesn’t work, and the reason is sharp.
If your decision to eat the salad was caused by a random quantum event in your brain, then your decision was random. Randomness isn’t agency. If a quantum fluctuation makes your arm jerk and you punch someone, you didn’t choose to punch them. You twitched.
Indeterminism gives you unpredictability. It doesn’t give you authorship. Free will, if it means anything, has to mean you did it, not that a die was rolled inside your skull.
Some philosophers like Robert Kane have tried to thread the needle here, proposing that quantum indeterminism happens at moments of intense deliberation, and that the agent’s “effort of will” resolves the indeterminacy. I find this hard to believe, because it just relocates the mystery. How does the effort of will resolve the indeterminacy? If we knew, we’d be done. We don’t.
Tomorrow is not today’s set of problems, because today’s problems are in the process of being solved.
Sources
- Future Contingents - Stanford Encyclopedia of Philosophy (Aristotle’s sea battle)
- Fatalism - Stanford Encyclopedia of Philosophy
- Compatibilism - Stanford Encyclopedia of Philosophy
- Peter van Inwagen - Wikipedia (author of An Essay on Free Will, 1983)
- Robert Kane - Wikipedia (author of The Significance of Free Will, 1996)
- J. J. C. Smart - Wikipedia
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Day 3: Einstein Made Us A Loaf
When Albert Einstein’s lifelong friend Michele Besso died in March 1955, Einstein wrote a letter to Besso’s family. In it, he wrote one of the most quoted sentences in the philosophy of time:
“People like us, who believe in physics, know that the distinction between past, present, and future is only a stubbornly persistent illusion.”
Einstein died a few weeks later.
What he was pointing at has a name now. The block universe.
The 4D Loaf
The block universe says reality is a four-dimensional structure. Three dimensions of space, one of time, stitched together into a single thing. Every event that ever happened, that’s happening, that ever will happen, all of it exists. Equally real. The Big Bang is over there in one corner of the loaf. The heat death of the universe is at the other end. Your tenth birthday is somewhere in the middle. Your death is somewhere too.
Nothing flows. Time doesn’t pass. The loaf just is.
This sounds insane. It is also where the math points.
Why Physicists Buy It
The argument is special relativity, and once you see it, it’s hard to unsee.
Einstein showed that simultaneity, the idea that two events happen at the same time, depends on how you’re moving. If you and I are at rest with respect to each other, we’ll agree on what’s happening “right now.” But if I get on a train and you stay on the platform, my “now” and your “now” start to disagree. Events you consider simultaneous, I won’t. And not because either of us is wrong. We’re both reading our clocks correctly. The universe just doesn’t have a single shared “now.”
Take that one step further. If my present and your present can disagree, whose present is the real one? Mine? Yours? Some third observer’s? The only answer that doesn’t pick favorites is: all of them. And if all of them are equally real, then the past and future they each describe must also be real, because one observer’s future is another observer’s now.
Philosophers Hilary Putnam and C.W. Rietdijk worked this out in the 1960s. Their argument, roughly: if my “now” overlaps with your “now,” and your “now” overlaps with someone else’s, and that someone else’s “now” overlaps with an event in my future, then by transitivity that future event exists right now. Not metaphorically. Actually exists.
Therefore, if you take special relativity seriously, the future is already here.
“Now” Is Just “Here”
The block universe has a clean way to talk about what feels like flow. “Now” works the same way “here” does.
“Here” doesn’t pick out some metaphysically special location in space. It just means the place I am. Other places are equally real, even though they’re not here. New York exists when I’m in San Francisco. I don’t need to be there for it to be there.
“Now” is the same. The moment I am. Other moments are equally real, even though they’re not now. 1955 exists when I’m in 2026. Einstein doesn’t need to be alive for 1955 to be a real place in the loaf.
This view is called eternalism: past, present, and future, all equally real. It’s the natural ontology of the block universe, and most working physicists, when asked, will admit they think something like this.
The Objection That Doesn’t Go Away
There’s an objection, and it’s the one your gut has been making since the first paragraph.
It doesn’t feel like a block. It feels like time flows. It feels like the present is special, the past is gone, and the future is open. We make choices. We anticipate. We regret. None of that lines up with a frozen 4D loaf where everything is already written.
Philosophers who take this objection seriously are called presentists. They say only the present is real. The past was, the future will be, but right now only this moment exists. This is closer to common sense, but it has a hard time with relativity. If only the present exists, whose present? The presentist owes us an answer, and most of the answers involve denying relativity in ways physicists find suspicious.
For today, the block universe gets to make its argument unopposed.
Rovelli’s Wrinkle
Carlo Rovelli I think mostly buys the block? He doesn’t think there’s a fundamental flow. He’s also not satisfied with leaving it there.
If there’s no flow at the bottom, why does it feel so vividly like there is? His answer is emergence. Flow is real the way temperature is real. There’s no such thing as the temperature of a single atom. Temperature emerges when you have a lot of atoms, statistics, and a viewer who’s coarse-grained enough to perceive averages instead of individual particles. Time’s flow, in Rovelli’s view, is similar. It emerges from entropy, from our memory pointing one way, from our being the particular kind of system we are.
That isn’t an answer that satisfies everyone. The philosopher Tim Maudlin has spent decades arguing that fundamental temporal passage is real, that the block universe view throws away something that ought to stay. I’m sympathetic. But Maudlin is not, today, winning.
So What Does This Mean For Me?
Here is the question that should have been tickling that noggin.
If the block universe is right, if my death is already sitting in the loaf at coordinates I haven’t reached yet, then in what sense am I choosing anything? If my actions tomorrow are already there, written into the geometry, am I just walking down a track that’s been laid?
But that’s tomorrow’s past, sorry, post.
Sources
- Being and Becoming in Modern Physics - Stanford Encyclopedia of Philosophy
- Time - Stanford Encyclopedia of Philosophy
- Rietdijk–Putnam argument - Wikipedia
- Michele Besso - Wikipedia (source for the Einstein letter quote)
- The Order of Time - Wikipedia (Carlo Rovelli, 2018)
- Tim Maudlin - Wikipedia (author of The Metaphysics Within Physics)
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Day 2: A Philosopher Argued Time Doesn't Exist
In 1908, a British philosopher named J.M.E. McTaggart published a paper called “The Unreality of Time.” The argument is exactly what it sounds like. He thought he had proven, with logic, that time is an illusion. Not “feels different than we think.” Not “isn’t fundamental.” Unreal.
Almost 120 years later, people are still arguing about whether he was right. People have written entire dissertations on McTaggart, and yet here I am trying to reduce his paper to a blog post. I’ll do my best to make it make sense.
Most modern philosophers think the argument fails, but the way it fails that matters. McTaggart carved up the conceptual landscape that every philosopher of time has been working in since. If you want to argue about time at all, you’re using his vocabulary, even if you’re trying to refute him.
Two ways to talk about time
McTaggart starts by noticing something obvious once you see it: there are two different ways we order events in time.
The first way is fixed relationships. The Battle of Hastings was earlier than the moon landing. The moon landing was later than the Battle of Hastings. These facts will be true forever. The relationship doesn’t change. He calls this the B-series.
The second way is changing properties. The moon landing used to be in the future. Then it was in the present. Now it’s in the past. The event itself didn’t change, but its tense did. He calls this the A-series.
You use both every day without noticing. “I have a meeting at 3pm” is B-series, it’s at 3pm whether you say it on Monday or next year. “I have a meeting in two hours” is A-series, that statement was true at 1pm and false at 4pm.
So far, no philosophy emergency. Two ways of describing time. Cool.
The argument
McTaggart’s argument has two steps, and the trick is how they trap each other.
Step one: For time to be real, you need change. Stuff has to actually become other stuff. Without change, you don’t have time, you have a frozen catalog of events. Fair enough.
Step two: The B-series can’t give you change. The relationship between Hastings and the moon landing never changes. Nothing in the B-series ever becomes anything else. It’s a static ordering.
So if you want change, you need the A-series. Events have to actually move from future to present to past. That’s where the change lives.
And this is where it gets weird. The A-series is contradictory.
Every event in the A-series has to be past, present, and future at some point. The moon landing was future before 1969, present on July 20, 1969, and is now past. So it has all three properties. But past, present, and future are incompatible. An event can’t be all three.
The obvious response: well, it has those properties at different times. Future first, then present, then past. Not all at once.
McTaggart was ready for that. If you say “at different times,” you’re using time to explain time. You’ve assumed the thing you’re trying to define. The A-series was supposed to be what makes time real, and now you’re using time to fix the A-series. Circular.
So: change requires the A-series. The A-series is contradictory. Therefore no change, therefore no time.
Does it work?
Mostly people think it doesn’t. But the responses split into camps that are still arguing.
Some philosophers say the A-series is real and McTaggart’s contradiction objection is bad. They’re called A-theorists. They argue that having different temporal properties at different times isn’t circular, it’s just what time means.
Others say the B-series is enough, and you don’t need real change in the way McTaggart thought. The “change” we observe is just a feature of how we experience the sequence. They’re called B-theorists. To them, the moon landing being past from where we sit and future from where Buzz Aldrin sat in 1968 are both just facts about a four-dimensional structure that doesn’t itself move.
I’ll get into this in more detail in the next post, while trying to tie it back to Rovelli’s work on time.
Actually, hang on a minute, this is my post.
McTaggart Was Right All Along
He noticed that we use two different vocabularies for time, and we never worked out how they fit together. We just slide between them depending on what we want to say.
That sliding is everywhere once you notice it. “It’s been five minutes.” B-series. “I’ll do it tomorrow.” A-series, scoped to “now.” “I have a 3pm.” B-series. Sometimes in the same sentence, two different ways we think about time.
Rovelli argues that the features we associate with time are emergent. They don’t exist at the subatomic level. Time runs differently in different gravitational fields, which is strange if time is supposed to be a fundamental property of the universe rather than something that arises from how we observe it.
Therefore, time is a human construct, and a British philosopher figured it out in 1908.
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