It’s quite an old question, and by now most of us have come across a number of videos or articles about what time is the space-time continuum, the theory of relativity, all of it. I’m writing this anyway, because I still do not know what time is. I know we keep track of our lives with it and that almost everything we do depends on it. Beyond that I’m not sure I know anything at all about time.
Having said that, I went looking for what physics actually tells. The surprise wasn’t how much we understand but how much we don’t, and how old some of those gaps are.
We measure it way better than we understand it!#
A second is no longer a slice of a day. Since 1967 it has been defined as 9,192,631,770 ticks radiation of an atom. Today’s best clocks are accurate to roughly one part in a billion billion.
They’re so precise that they can see time bending. In 2010, physicists at JILA raised one clock about 33 cm above another and watched the higher one tick faster, exactly as general relativity says it should. By 2022 the same group could measure that difference across a single millimetre of height.
So we can detect time running at different rates between the top and bottom of a pencil maybe but we can’t say what the thing we’re measuring is. A definition of the second is a method for counting, not an explanation.
There is no universal now#
This is the part Einstein settled for all of us a century ago, and it still hasn’t sunk in for most of us.
If you and I are moving relative to each other, we don’t agree on which events happen at the same time. Not because our clocks are bad, but because “at the same time” isn’t a property the universe has. Each observer slices spacetime into “now” at their own angle with respect to their motion, and all such slices are equally valid.
There is no cosmic present moment, no master clock ticking for everything at once. Einstein put it bluntly in a letter written shortly before his death - for physicists, the distinction between past, present and future is “only a stubbornly persistent illusion.”
Which leaves a strange gap. The universe has no now. I live in nothing else.
Why does it only go one way?#
Here’s the one thing that bothers me most.
Write down the fundamental laws of physics, and almost all of them work the same forward and backward. Newton’s laws, Maxwell’s equations, general relativity, the Schrodinger equation. You can watch a movie and play it back in reverse and nothing illegal happens.
Yet nothing in life is reversible. Cups shatter, coffee cools, people age. Where does that come from?
The standard answer we seem to be content with is the second law of thermodynamics, entropy. But that answer is a bit of a trick. Statistics alone would predict entropy rising in both directions from now. The only reason the past is different is that the universe started in a low entropy state and there is a bunch of explanations around the chance of life and probabilities, which I don’t fully understand. Just that the arrow of time isn’t explained by the laws. It’s inherited from a beginning we can’t yet explain.
Our two best theories can’t agree on what time does#
General relativity treats time as a dimension woven into spacetime, stretched/ bent by mass, with no specific direction and no universal rate.
Quantum mechanics treats time as something else entirely, an external parameter, a dial turned from outside the system, ticking along in the background while states evolve. This version of time is even more difficult for me to comprehend or imagine.
Try to combine the two and they seem to not converge. The Wheeler–DeWitt equation, written in 1967 as a first attempt at quantum gravity, describes the universe has no time variable in it at all. The equation says the state of the universe doesn’t change. It is, as far as the math is concerned - frozen.
This is the problem of time, and it has been open for several years.
Maybe time isn’t fundamental#
One way out is to stop treating time as a basic ingredient and start treating it as something that emerges, the way temperature emerges from molecules bouncing around.
Page and Wootters proposed in 1983 that a frozen universe can still contain time internally. Entangle a small clock system with the rest, and from inside, everything looks like it’s evolving normally. Nobody knows if any of this is right. But notice what it implies, time might not be a thing the universe has. It might be a thing that shows up when you’re inside.
We don’t even know whether time is continuous. Below the Planck time, about 10⁻⁴⁴ seconds, our theories stop returning sensible answers. Some approaches to quantum gravity suggest time comes in discrete grains. We have no way to check.
The part we actually live in#
Then there’s the gap I’m unsure if anyone is even trying to close.
Nothing in any equation describes the present moment. Nothing describes flow, or passage, or the feeling that the future is arriving. The equations give you a block of events with a coordinate labelled t, and that’s it. The thing we can maybe call “experiencing time” has no entry in the formalism.
There is also no convincing answer to why we remember the past and not the future because memories are records.
So, how much don’t we know?#
Lining it up, we don’t know whether time is fundamental or emergent, why it has a direction, what fixed the initial conditions resulted in one, how to reconcile the two ways our best theories handle it, whether it’s continuous or discrete, or why there’s a present moment at all.
What we do know is how to count it, very very well.
This is the thing I keep coming back to. Time is the one quantity we organise every single day around, build every plan on, and measure more precisely than anything else in existence But, it’s also among the least understood things in physics and life. We are all expert users of something nobody can define.
Does any of this change how you think about time, or is it one of those things that’s better left unexamined when the alarm goes off tomorrow?