This question was closed as being opinion-based on the physics site. So I thought, time being a popular subject in philosophy, to ask it here and find out about some philosophical opinions.

Time can be associated with irreversibility. A broken egg can't reassemble. Most, if not all, processes are irreversible and this is associated with time going in one direction. Ŕemarkably, living organisms seem to behave contrary to this. They even give birth to the egg which can't reassemble when broken. Which of course doesn't mean that time goes backward. In the larger context the irreversible process is one including the Sun (directly or indirectly). So, while entropy decreases locally, globally the process is an irreversible one and evolving towards higher entropy.

Such processes can be compared with (quasi) periodic processes, called clocks (of which in the ideal case it can't be decided if it goes forward or backward). It's this clock time that's used on the spacetime manifold of relativity. One can imagine such a clock to be placed at all points in space, thus adding the time-dimension connected to space by the invariant lightspeed c.

So there are irreversible, unidirectional, thermodynamic processes on one side and the (quasi) periodic process quantifying it by observing how manny periods have passed between two moments in the process.

The question arises though, are the irreversible processes constituting time or are they evolving in time, which seems the case in general relativity, time being the clock. It's confusing since the very concepts of "a process" or "evolving" already seem to contain time. Which maybe can be resolved by assuming processes constituting time. Or is the clock time, as part of the spacetime continuum more fundamental and existing even without processes taking place in it? Is maybe the field of virtual particles involved? I mean it's the superposition of all energies and momenta and bidirectional in time.

What can be said about this?

  • Comments are not for extended discussion; this conversation has been moved to chat.
    – Geoffrey Thomas
    Commented Aug 5, 2022 at 7:14
  • @nielsnielsen "Because philosophy is unconstrained by any requirement to mirror the real world" I disagree. That's were physics usually is, how to put it, eeeeh, quiet? Physics as practiced by most, doesn't judge about the true nature of reality. It uses math to explain experimental results and empirical data. Using models which are not necessarily the true state of affairs. I think it's philosophy that addresses this question. Of course you gotta have knowledge of physics. So it goes even deeper than physics. Your turn.
    – Gerald
    Commented Aug 5, 2022 at 7:17

2 Answers 2


There are two distinct topics here, about time and the arrow of time.

The universe is a 4D space in which matter resides, and because of the geometry of space and the nature of matter, the matter apparently takes the forms of wiggly lines/curves that all tend to extend in roughly the same direction, between 'past' and 'future' ends of the universe. This direction is called 'time'.

So to answer the question "What is time?" it is helpful to first consider the question "What is space?" It's essentially the same question, but probably easier to answer because it's not confused by some of the odd-man-out features of time. For some reason, it seems to evoke a lot less interest.

Space has a bunch of geometrical concepts like 'length' and 'angle' that we can then extend to the time direction to build concepts like 'duration' and 'speed'. This is really just geometry. "What is space and why does it have these geometrical properties?" is deep and mysterious, but having accepted that it exists, figuring out the consequences of its rules is just routine maths. 'Duration' is no more mysterious than 'length', 'clocks' no more mysterious than 'rulers'.

The question "What is the arrow of time?" is really a question about the behaviour of matter in the universe - the shapes of those wiggly lines. The arrow of time is what allow us to have memories, and drives thermodynamic progress. It is also more complicated and mysterious.

If we consider all the possible configurations matter might take, almost all of them look like nothing is happening. It starts random, ends random, and is random at every point in between. It's a box full of particles bouncing around, endlessly. There is continual change at the microscopic level, but it always changes into something that looks very similar. And if you play the tape backwards, it looks just the same.

Our universe is extremely peculiar in this regard in that that patterns matter makes at one end (the past) are extremely well-ordered and non-random. Nobody knows why. The odds of it happening by chance are mind-bogglingly low. It no doubt has something to do with the physics of how the universe started. But so far as I know, nobody has come up with any plausible explanation as to why.

Given an ordered past, the most likely way for matter to behave is to spread out randomly into the future, like holding a bundle of fine threads in one hand at one end, and letting the other ends blow in the wind. This gives us our irreversible processes, and we can harness the progression from order to disorder to drive useful machinery (like clocks and organisms). The progression to disorder impresses the past behaviour of a thread on its future, but not vice versa, so we only have memories of the past and not the future. This in turn gives us the illusions of 'moving through time' and 'living in one moment'. Most of our philosophical questions about time arise from the constrained perspective this gives us.

Unfortunately, the ultimate answer to our questions is still lost in the distant past. In the part of space and time we can see, we can trace back all the peculiar properties of our perception of time to this business of the matter being far more ordered than we have any right to expect at the past boundary. But as we look further and further back, the question just gets pushed back too. We know of no way that order of such magnitude can arise spontaneously. We cannot create it. (Devices like refrigerators only move it around.) All we can do is exploit the order we have inherited from our unknown past.

In summary - I think you are really asking about the arrow of time, and that question is not really about time itself but a property of the history of matter, evolving in time. However, it's probably true to say that it's the source of a lot of our questions about our experience of 'time', which is what we often mean by the word. So a case could be made for either answer. It depends what aspects of 'time' you're talking about.

  • Thank you for this informative answer. All in the words! The greatest mystery to me is why time goes forward.
    – Gerald
    Commented Aug 4, 2022 at 17:03
  • 1
    It's an interesting answer, but I'll note that you give no argument for why anyone should think time really is as you describe it rather than merely being conveniently formalized in that way. In science, we routinely use macroscopic properties in our theories, even though it is understood that the world isn't really that way, the macroscopic properties are just averages of microscopic properties. Why couldn't something similar be true of time? Commented Aug 4, 2022 at 19:20
  • Try Conformal Cyclic Cosmology, for a candidate theory explaining early-universe low entropy. The separation of geometric time & thermodynamic time only seems to be that between General Relativity & Quantum Field Theory - so, reconciliation between them is widely expected.
    – CriglCragl
    Commented Aug 8, 2022 at 21:11

The question arises though, are the irreversible processes constituting time or are they evolving in time? ... What can be said about this?

Two views apparently corresponding to “relationism with respect to time” and “absolutism with respect to time” respectively, (see SEP below). Relationalism is clock time based on the resonant frequency of caesium (or other regular motion); absolutism is the god's-eye view within which caesium clock rates differ depending on local gravity and inertia. Two types of time, complementary and different. However, the absolutist view is a hot topic and widely viewed as bereft of theoretical validity, e.g. "Special relativity eliminates absolute time" - Wikipedia

SEP, Time - 2. Reductionism and Platonism with Respect to Time

Aristotle and Leibniz, among others, have argued that time is not independent of the events that occur in time. This view is typically called either “reductionism with respect to time” or “relationism with respect to time”, since according to this view, all talk that appears to be about time can somehow be reduced to talk about temporal relations among things and events. The opposing view, normally referred to either as “Platonism with respect to time” or “substantivalism with respect to time” or “absolutism with respect to time”, has been defended by Plato, Newton, and others. On this view, time is like an empty container into which things and events may be placed; but it is a container that is independent of what (if anything) is placed in it.

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