Time
-
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)
I’d appreciate a follow. You can subscribe with your email below. The emails go out once a week, or you can find me on Mastodon at @[email protected].
-
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)
I’d appreciate a follow. You can subscribe with your email below. The emails go out once a week, or you can find me on Mastodon at @[email protected].
-
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.
I’d appreciate a follow. You can subscribe with your email below. The emails go out once a week, or you can find me on Mastodon at @[email protected].
-
30 Days of Time
I’m a big audiobook guy. Ear reading instead of sight reading, as I like to call it. Lately I’ve been ear reading The Order of Time by Carlo Rovelli, and it has wrecked me in the best possible way.
Rovelli’s argument, roughly, is that a uniform, universal flow of time does not exist at the fundamental level. The features we associate with time, a direction, a presentness, a duration, are tied to our perspective and to thermodynamics. We don’t fully understand the microstate of the universe and his argument is that the flow of time isn’t even there to be known. That’s a hell of a thing to think about when sitting in traffic between two city buses on your way to picking up fried chicken for dinner.
So I got curious. Not just about the physics, but about all the neighboring topics. Calendars. Daylight Saving Time. Leap seconds. The fact that GPS satellites have to correct for relativity or your maps app stops working. The fact that Mars uses its own day. The fact that we voted in 2022 to abolish the leap second by 2035, and most people did not notice.
I’ve decided to spend the next 30 days writing about it. One post a day. Welcome along.
The shape of the series
I don’t want to spoil the destination, so I’ll just sketch the journey. The series breaks into four rough movements, one per week.
Week 1 — What time is. The philosophy is older than the physics. Does time flow, or is it a frozen 4D block we move through? Why does your brain experience “now” the way it does? Do other animals even live in the same time you do? This is the squishy, weird, fun stuff. No equations, lots of questions.
Week 2 — How we measure it. Sundials to cesium to optical lattice clocks. The second is a defined unit now, and that decision had consequences. We’ll talk about why your phone’s clock is more accurate than any clock that existed when your grandparents were born, and what that accuracy is for.
Week 3 — How computers handle it. Unix time, NTP, ISO 8601, time zones, DST. The infrastructure quietly holding the digital world together, and the bugs that fall out of it twice a year. If you’ve ever shipped code that broke at 2 a.m. on the second Sunday of March, this week is for you.
Week 4 — Where it breaks. Leap seconds, calendar reform, DST politics, the slow drift between atomic and astronomical time. The current system is more held-together-with-tape than most people realize. By the end of the week I want us both asking the same question.
I’m not going to tell you what that question is yet. Part of the fun is getting there together.
What this is, and what it isn’t
This isn’t a textbook. It isn’t a research project or a reading log either. It’s a series of posts on a topic I’m interested in.
I’m not a physicist, so I’ll do my best to make sure the posts aren’t wrong. If you spot a correction, replying on Mastodon is the best way to let me know.
Why 30 days? Why not? I may do more later if a particular topic wants more room. But 30 days on a single subject is probably enough for most people who show up here for programming posts. Instead, you’re getting a 30-day divergence into physics, astrophysics, and a little philosophy. Week 3 covers computer time, so there’ll be some programming in there. Just not a lot of it for a month.
My research notes live in an Obsidian vault. I’m keeping them private for now, but I may share more by the time the series wraps.
How to follow along
New post every day for 30 days, starting now.
RSS is the easiest if you want every post as it lands. Email is a weekly digest, good if you’d rather catch up on Sundays than see every post the moment it drops. Mastodon is where each post lands on social, and any replies there show up back on the blog as comments: @[email protected].
Pick whichever annoys you least.