# What is time? Does it flow linearly? If so, how are we sure? [duplicate]

Distances in our world (classically at least) can be defined by considering our world to be a model for the Euclidean normed three dimensional space. But how can we set the notion of time on a firm footing? What are the first principles here?

Because almost everywhere in physics, we want to see how a system evolves with “time”. But when I think about it, I can’t find a clear understanding of what time is.

I’m not talking of “the arrow of time”, just time.

Before relativity also, physicists worked with time rates. But how can we be sure that a ticking device is measuring time linearly? As far as I can see, there must be a guaranteed (local, for relativity) existence at every place in the universe of some time keeping mechanism which is infinitely accurate and can be accessed by the observer to read off the time differences between events he observes.

What guarantees such existence?

Basically what I’m asking is against what the rate of time flow is measured. Or asking it is nonsense?

Is there some different way to firmly define this notion?

This is NOT the same question as the marked question, here.

Here, I ask how do we assure that a time keeping device ticks at “equal” intervals? How do we measure these “equal intervals” with? We surely can’t rely on our perception for it.

It is much like (to me) the problem of defining temperature in classical thermodynamics. We have to resort to statistical theory for a firm definition of it. We just can’t say that temperature is proportional to the height of mercury column in a thermometer. Because we’ll, it just isn’t...

• Possible duplicate of What is time, does it flow, and if so what defines its direction? Aug 3, 2019 at 19:03
• The rate or time flow is measured against changes in the world around us that we can observe. The official SI standard is based on a caesium atom's ground-state electron transition frequency, because it is considered objective, universal and accessible (fairly "easy" to verify anywhere). But you could measure time according to a ticking pendulum, a revolution of the Sun on our sky or a decaying apple tree. Theoretically, if time suddenly slowed down to, say, half the current rate, you wouldn't be able to know, because everything you measure time with would likewise be slowed down similarly. Aug 3, 2019 at 19:10
• @Steeven But how do we know that the Cs clock ticks at equal intervals? Or is it just that we define it to be at equal intervals?
– Atom
Aug 3, 2019 at 19:21
• @Atom That's the point. We don't. But we have to pick something as our reference to measure everything else against. If that reference itself is varying - if the Cs atom frequency suddenly changed on all Cs atoms on the planet - then we might notice because everything else, all clocks on earth, that have been set according to this reference still would agree with each other but suddenly not to the reference. But if other types of changes will or has happened, like if time changes universally, then we simply wouldn't be able to know because our reference would change along. Aug 3, 2019 at 19:30
• FWIW, here's a couple of quotes from the first chapter of "Gravitation": (1) "Good clocks make spacetime trajectories of free particles look straight" and (2)"Time is defined to make motion look simple!" Aug 3, 2019 at 22:26

Although the answer given above is good, I want to add something for both the OP and future readers of this post.

. . .But when I think about it, I can’t find a clear understanding of what time is.

Physics, as a mainstream science, doesn't seem to have much of an answer to questions about what time actually is. Physicists certainly wonder about the nature of time in their own spare time just as you seem to be wondering about it. But as far as normal experimental physics goes, being able to measure time with clocks is about as close to understanding time as we get. Scientists in general do not have much of any way to resolve issues about things above and beyond what they can measure.

• Note that the order of answers on stack exchange sites changes from time to time, so you should probably avoid phrases like "the answer given above." :) Aug 3, 2019 at 21:31

Time (specifically proper time) is the thing measured by clocks. We can be sure that equal time intervals measured by clocks are equal because we have defined time to be the thing that clocks measure. If a clock measured twice as much of something and if the thing that clocks measure is defined to be time then what it measured was by definition twice as much time.

Now, there is no such thing as an ideal clock, but there are some clocks that agree with each other better than other clocks. These clocks that agree with each other well over a long time are used as the standard clocks, and groups of them (at least 3) are used together to detect when one starts to fail electronically or mechanically. Other clocks that are less stable are compared to the standard clocks and adjusted as needed. Clocks that don’t agree with the standard clocks are fixed or removed.

• But suppose the group of clocks we chose were such that they ticked in unison but the length between successive ticks increased quadratically with each number of tick. It would mean that the lifespan of humans is continuously increasing. Then what would we do?
– Atom
Aug 3, 2019 at 20:13
• Not only that, but it would also imply violation of time translation symmetry in many cases. Like I gave you the above example.
– Atom
Aug 3, 2019 at 20:15
• Such clocks would never be chosen as the reference standards because their disagreement rate would grow linearly in time and we already have clocks whose disagreement rate is constant in time.
– Dale
Aug 3, 2019 at 20:44
• @Atom If you chose to define time using the quadratic clocks, wouldn't Newton's laws of motion (along with electrodynamics and so on) have to be modified? Newton's first law says that objects (in an idealized scenario) that are not affected by forces trace out a straight-line. Based on observation (like with frictionless air-tracks), we see that there is a unique time standard that parametrizes all such straight-line motions linearly with respect to time. Secondly, your proposal would conflict with the Cesium-133 time standard (and the transition rates would be non-linear in your idea). Aug 3, 2019 at 22:04
• @Atom Then the rate of human experience, aging, etc., would also increase quadratically and we wouldn't be able to observe the variation in ticks. If they didn't, we'd have picked that stuff as the clock in the first place. Aug 4, 2019 at 3:36

Much of the answer to your question is actually part of philosophy more than physics. "What is time?" is a famously difficult open ended question in philosophy. For example, philosophers will question whether time is discrete or continuous.

As for the practical scientist, what we observe is that the things we call clocks (i.e. hour glasses, watches, atomic clocks, etc.) behave in a way which is consistent with each other. If you look at the state of one clock, you can very reliably predict the state of another clock. And it seems the more "reliable" a clock is, the more predictable they are. Atomic clocks, for instance, are known to be remarkably consistent with respect to one another.

When we measure physical events, and reference this against the behaviors of these accurate clocks, we find that there is a very predictable pattern if we assume there is some variable, "time," which is ticked off linearly by these clocks. When we make this assumption, the laws of physics become "simple."

Thus we use it. Time can indeed be thought of as nothing more than a convenient analogy for physical behaviors. Or one may dabble into the philosophy to dig deeper.

Or, as an alternative, consider a reversed method of thinking. The thing scientists refer to as "time" is that which best fits the observations made about clocks. After all, science is an art of drawing conclusions about reality using data.

• Have you dived deeply into the philosophy behind this question? Do you think it is wrong if someone has not - hence why my answers are generally of unremarkable quality, because I'm doing wrong? Aug 4, 2019 at 3:03
• @The_Sympathizer I have dived into that philosophy, and you will note that it did not make it into the answer other than to link the words of others. Time is a particularly tricky universal, which one reason its a question that remains open. As for how deep one may dive, I will readily admit that I have trouble appreciating the nuanced differences between McTaggart's A-series and B-series time, which suggests my own dive has been shallow by some standards. Aug 4, 2019 at 19:59
• Myself, i see more value in understanding that there is a point where these questions turn into philosophy, and you are welcome to cross that line or not. That helps you appreciate the assumptions which go into science, when to assume them, and when to question them. Dive when the water looks cool and refreshing; don't dive if its murky and stangant. That way, no matter what, it was a pleasant journey. Aug 4, 2019 at 20:00

In a comment you wrote:

But suppose that the group of clocks we chose were such that they ticked in unison but the length between successive ticks increased quadratically with each number of tick.

You seem to have a mental model of our universe as being some sort of simulation that’s running on a computer in some meta-universe, which has a notion of “meta-time” that you are thinking of as the “true” time. And yes, in this mental model it’s possible to imagine the operators of the simulation programming their computer to speed up or slow down the speed at which the simulation is running according to a quadratic function, or indeed any other nonlinear function. For example, they could decide that on meta-Mondays and meta-Wednesdays they will pause the simulation entirely between meta-noon and meta-1 pm to go eat meta-lunch. How would we humans know that this has happened? Well, we wouldn’t, because we are living inside the simulation, so while the simulation was paused, we would be paused too. When it resumes, we would resume too, and to us it would seem that the moment after meta-1 pm came immediately after the moment before meta-noon. From our point of view we would experience a sense of time flowing “linearly” and continuously, despite the discontinuity when you look at things in the meta-time parameter.

The key to resolving this difficulty, and to answering your “how can we be sure” question, is to recognize that these sorts of questions are meaningless within the framework of scientific theories and the scientific method. Specifically, your speculative theory about time flowing quadratically with respect to a more “true” time variable lacks the property of falsifiability, which is one of the basic requirements a scientific theory must satisfy:

I shall require that [the] logical form [of the theory] shall be such that it can be singled out, by means of empirical tests, in a negative sense: it must be possible for an empirical scientific system to be refuted by experience.

— Karl Popper, Popper 1959. p 19

The point is that science deals only with scientific questions, and what makes a question scientific is if it can be tested via experiment. Your question belongs to a class of “what if” questions that involve assumptions about how the universe “really” works, or what is happening “behind the curtains” or “under the hood” or “outside the simulation”, which invariably cannot be tested by any experiment, and therefore are not scientific questions. They may be debated by philosophers, or sometimes by physicists if they are in a certain mood, but they are not really mainstream physics questions, and physics has nothing useful to say about them.

(With that being said, such questions seem to be very common on this site, and are often interesting and entertaining to think about, so I don’t mean to suggest that there’s anything wrong with your asking about this.)