Let’s start with a basic assumption: Time is made up of an infinite number of moments.
You can break a day into hours, hours into minutes, minutes into seconds, and so on. So if you go keep going, you’ll eventually get singular moments—tiny little slivers of time. And time as we know it is just all those little slivers arranged one after another. It’s like Legos, except it’s made out of time instead of physical blocks.
That’s a common way of thinking about time. But way back around 490 BC, a philosopher named Zeno said it led to a paradox—a way of thinking about time that contradicts itself.
Zeno’s paradox
Imagine an arrow flying through the air from an archer’s bow to a target. Zeno says that if time is made up of an infinite series of moments, the arrow is never really in motion. Aristotle, another Greek philosopher, explained Zeno’s argument:
The flying arrow is at rest, which result follows from the assumption that time is composed of moments …. he says that if everything when it occupies an equal space is at rest, and if that which is in locomotion is always in a now, the flying arrow is therefore motionless.
In other words, if time is really just a bunch of individual moments, then in each one the arrow is still: at each single point in time, it’s in a single place. If it’s not moving during any single moment, and time is just a bunch of those moments, then the arrow is never really moving. Motion is an illusion, Zeno says.
Come on, now
Now, you may be thinking to yourself: that’s just silly. Of course the arrow moves! I can watch it go from one place to the other!
And that’s a good point! But what Zeno’s paradox shows us isn’t that motion is an illusion. It’s that the assumptions he makes about time are wrong. If imagining that time is a bunch of individual moments added together leads us to conclude that motion isn’t real, but we know motion is real, then we must be wrong about time being a bunch of individual moments.
So how should we think about time?
Enter Einstein
When Zeno was writing, he was mostly doing thought experiments. But the way we study things like time has changed: now we have modern science. And one of our greatest scientists, Albert Einstein, changed the way we think about time with his theory of special relativity.
Relativity is a much more complicated topic than we can cover here, but space.com sums up one of the main findings, saying that Einstein “found that space and time were interwoven into a single continuum known as space-time. Events that occur at the same time for one observer could occur at different times for another.”
That theory has led to a lot of debates among philosophers of time. Before Einstein, we thought of time as objective thing. If you think about an event, like the time you stubbed your toe, you could give specific, objective measurements to it. It happened two days ago, or seven minutes ago, or you’re going to do it next week (if you like to plan minor injuries).
But Einstein’s theory finds that time isn’t the same for everyone. If that’s true, some philosophers argue, then events can’t be absolutely simultaneous. And Ned Markosian, writing in the Stanford Encyclopedia of Philosophy, explains what those philosophers say:
According to that theory (the argument goes), there is no such thing as absolute simultaneity. But if there is no such thing as absolute simultaneity, then there cannot be objective facts of the form “t is present” or “t is 12 seconds past”. Thus, according to this line of argument, there cannot be objective facts about [time], and so the passage of time cannot be an objective feature of the world.
Zeno might have been wrong about motion being an illusion, but according to these thinkers that’s only because he thought time was objective, and it’s not.
If they’re right, then time is not actually a thing that exists out there in the world. It has to do with you—with your own perception of the world.
The debate isn’t settled, of course. Philosophers never stop debating things like time, and scientists are adding new wrinkles everyday as they work to understand subjects like quantum mechanics. And as those findings change our understanding of the world, it changes everything else with it.