What Is Space and Time?

A plain-language introduction for the curious reader. No physics or philosophy background is needed — only a willingness to look hard at two things you swim in every second of your life without ever noticing them.

The puzzle in 30 seconds

Space and time seem like the least mysterious things there are. Space is just the emptiness that things sit in; time is just the ticking that carries us from breakfast to bedtime. What could be simpler?

But start asking basic questions and the ground gives way. Is empty space a real thing — a giant invisible container that would still be "there" if you removed every atom in the universe? Or is "space" just a way of talking about how objects are arranged, so that with nothing in it there would be no space at all? Does time actually flow, like a river carrying the present moment forward — or is that feeling of flow a kind of illusion, and past, present, and future all equally real? And is the future already out there, fixed and waiting, or does it not yet exist?

These are not word games. Modern physics — Einstein's relativity above all — has genuinely surprising answers to some of them, and it has turned what looked like idle questions into some of the deepest problems in science. This page is a tour of the big ones.

Two things, three questions

Almost every puzzle about space and time is a version of one of three questions. (The technical section is organised around exactly these — see What Is the Philosophy of Space and Time?.)

• Are space and time real things, or just relationships? Is space a container, or only the pattern of distances between objects?
• Does time really pass? Is there a special moment called "now" that moves — or is that just how things look from inside?
• What decides the shape of space? Is it obvious and fixed (the geometry you learned in school), or could space be "curved," and how would we ever know?

Let's take them in turn.

Question 1: Is empty space a thing?

Here is a debate that has run for over 300 years, between two of history's greatest minds.

Isaac Newton said: yes, space is a real, self-standing thing — an infinite, invisible stage on which the whole show of the universe plays out. Even a totally empty universe would still have space.

Gottfried Leibniz said: no, that is a fantasy. "Space" is just a convenient word for how things are laid out relative to each other — this cup is 20 cm from that plate. Take away all the objects and you haven't got an empty container; you've got nothing at all. (See Substantivalism and Relationism.)

Who's right? Newton had a killer piece of evidence: spin a bucket of water and the surface climbs the sides. Something is clearly, physically different about spinning versus not spinning — you can feel it on a merry-go-round. But different relative to what, if the bucket were alone in an empty universe? Newton said: relative to space itself. That suggests space is real after all. Leibniz's followers have been trying to answer the spinning bucket ever since. (See Newton's Bucket.)

The strange thing is that this ancient argument is still alive — Einstein's theory of gravity reopened it in a dramatic new form, because in that theory space and time turn out to bend, and something that bends starts to look a lot like a real, physical thing.

Question 2: Does time actually flow?

This is the one that keeps philosophers up at night.

Everyone feels time flowing. The present moment seems to move: this instant is here, then it's gone, swept into the past, while the future rushes toward us. The past feels fixed and the future feels open.

But here is a troubling question: how fast does time flow? "One second per second" — but that's not a speed at all; it's like saying a stationary car moves "one metre per metre." Ask how fast the present moves and you can't give a real answer. That makes some philosophers suspect that the "flow" of time isn't a feature of the world at all — it's a feature of us, of what it's like to be a conscious being with a growing pile of memories. (See Temporal Passage.)

This leads to one of the wildest ideas in modern thought: the block universe. On this view, past, present, and future all equally exist. The universe is like a completed film reel or a loaf of bread, laid out whole in four dimensions (three of space, one of time). There is no special moving "now" — every moment thinks it is "now," the way every place is "here" to whoever is standing there. Dinosaurs and your great-grandchildren are just as real as you; they're simply located elsewhere in time, the way Australia is real but not here. (See The Block Universe.)

Why would anyone believe something so strange? Because of Einstein.

The surprise that changed everything: "now" is not universal

The single most mind-bending discovery about time came from Einstein's special relativity in 1905, and it is this: whether two distant events happen "at the same time" depends on how you're moving.

In everyday life we assume there's a single, universal "now" ticking away everywhere at once — that this instant on Earth is the same instant on a distant galaxy. Relativity says: there isn't. Two people walking past each other at different speeds will genuinely disagree about which faraway events are happening "right now," and — this is the shocker — neither of them is wrong. There is no fact of the matter about a single, universe-wide present moment. (See Relativity and the Reality of Simultaneity.)

This is devastating for the cozy idea that time flows and only the present is real — because whose present? If different observers can't even agree on what "now" includes, maybe there is no special "now" out there at all. That's the road to the block universe, where all moments are equally real. Many physicists and philosophers think relativity more or less forces this picture on us. Others resist. The argument is still going. (This is the deepest reason the "block universe" is taken seriously and not just science fiction.)

Two more surprises: slow clocks and short rulers

That single discovery — no universal "now" — has two famous side effects you may have heard of.

Moving clocks run slow. If someone speeds past you, everything about them — their watch, their heartbeat, their ageing — ticks slower than yours. And they say exactly the same about you. It sounds like a contradiction, but it isn't, because the two of you no longer agree on what "at the same time" means. This is not a trick of the eye; it is real and measured every day. The satellites behind GPS must correct their clocks for it, or your phone's map would wander off by kilometres within a day. Tiny particles called muons reach the ground from the upper atmosphere only because moving so fast stretches out their brief lives. And in the famous twin paradox, a twin who rockets away and comes back is genuinely younger than the one who stayed — not because either aged wrongly, but because they travelled different-length paths through time itself.

Moving rulers shrink. By the same token, a fast-moving object is measured as shorter, front to back, than when it sits still. Nothing is crushing it; it is the mirror image of the clock effect. Measuring a moving thing's length means catching where both its ends are at the same moment — and, once again, observers can't agree on what "the same moment" is.

We never notice any of this in ordinary life for one reason only: the effects are vanishingly small until you approach the speed of light. Near it they become enormous — and they are exactly the price of there being no universal "now." (See the relativity of simultaneity.)

Question 3: Can space be curved?

For 2,000 years, everyone assumed the geometry of space was the one Euclid wrote down: parallel lines never meet, the angles of a triangle add up to 180°, and so on. It seemed not just true but necessarily true — how could space be any other way?

Then two things happened. Mathematicians discovered that other, "curved" geometries are perfectly consistent — you can have a space where triangles add up to more or less than 180°. And Einstein discovered that our universe actually uses one. In his theory of gravity (general relativity), massive objects like the Sun bend the space and time around them, and what we call gravity — the Earth orbiting the Sun, an apple falling — is just things rolling along the curves. Space and time are not a fixed stage; they warp, stretch, and ripple. (See General Relativity.)

This has been measured: starlight bends as it passes the Sun, exactly as much as the curving of space predicts. So the shape of space isn't obvious, isn't fixed, and isn't decided by pure logic — it's a physical fact you have to go out and check. (See The Epistemology of Geometry.)

A few more rabbit holes

Once you pull on these threads, wonderful puzzles tumble out:

• How do we even measure time and space? There is no master clock or master ruler hanging in the universe. A second is officially defined as so many billion vibrations of a caesium atom, and since 1983 a metre is defined as the distance light travels in a tiny sliver of a second. So we now measure distance in terms of time — space and time are knitted together even in the definitions of our everyday units. (See Space and Time in Special Relativity.)
• Can you cross an infinite number of points? To walk across a room you must first cross half of it, then half of what's left, then half of that — infinitely many steps. So how do you ever arrive? This is one of Zeno's paradoxes, 2,500 years old, and answering it properly took the invention of modern mathematics. (See Zeno's Paradoxes.)
• Why does time have a direction? A cup shatters but never un-shatters; you remember the past but not the future. Yet the basic laws of physics work exactly the same forwards and backwards. So where does time's one-way arrow come from? The surprising answer traces all the way back to the extreme orderliness of the universe just after the Big Bang. (See The Arrow of Time.)
• Is time travel possible? Astonishingly, Einstein's equations permit loops in time, and the famous "grandfather paradox" (go back and prevent your own birth) turns out not to prove time travel is impossible — only that any trip to the past would have to be consistent with the past that already happened. (See Time Travel.)
• Did time have a beginning? If the Big Bang was the first moment, what was there "before"? The startling possibility — anticipated by Saint Augustine 1,600 years ago — is that the question is confused: there was no "before," because time itself began with the universe. (See Time, Cosmology, and the Beginning.)
• What is time, really? At the very frontier, when physicists try to combine Einstein's gravity with quantum theory, something bizarre happens: time disappears from the equations altogether. Some researchers now suspect that time is not fundamental at all, but something that emerges from a deeper, timeless reality. (See The Problem of Time.)

Why it matters

You might think none of this touches daily life — but the ideas are quietly enormous. If the block universe is right, then in some sense your whole life already exists, and death does not erase it. If "now" is not universal, then our deepest intuition about time is simply mistaken. And the fact that plain thinking about space and time could be overturned by experiment is one of the great lessons of science: even the things that seem most obvious, most built-in, most impossible-to-doubt, can turn out to work in ways no one imagined.

Space and time are the stage on which everything else happens. It turns out the stage has a plot of its own.

Ready for the details? The technical treatment starts at What Is the Philosophy of Space and Time?, and its companion sections on free will and whether God exists take the same plain-language approach to other deep questions.