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What is a thorough definition of time in terms of how it causes the universe to progress and only moves in one direction? Is something as abstract as time comprehendable to us beyond a measurement? What would have to be done to cause time to progress at a different rate from what has been experienced by us?

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    The view that time is a mental construct is widely accepted in philosophy: time is based on memory and expectation. In such sense, some writers say that time is the perception of change, which is quite interesting; think on this: if you perceive nothing, you would completely lose the sense of time. Regarding the direction of time, it seems also a mental fact, with a spatial equivalence: parts are always smaller than the whole. Seems obvious, but it shouldn't be.
    – RodolfoAP
    Commented Feb 4, 2022 at 17:29
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    I don't think you can make the notion of time having a "rate" coherent. Rate is how much of something happens over a given amount of time. A "rate" of time would be how much time happens over a given amount of time. To the extent that a rate makes sense, it has to be 1. Commented Feb 4, 2022 at 18:49
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    I think you want a theory of time, not a "definition". Time does not "cause" anything even for those that take a realistic view of it, and its directionality is actively studied in physics, see arrow of time. The "rate of time" can be altered by accelerating to near light speeds, see time dilation. For a review of philosophical theories of time see SEP, Time. This question needs more focus on something not easily found in encyclopedias.
    – Conifold
    Commented Feb 4, 2022 at 23:02
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    @RodolfoAP your comment also made me think about the perception of no change, that would also screw our perception of time. Then i realized that as thoughts are themselves perception of a change in our mental state, the perception of no change does not even make sense. Interesting.
    – armand
    Commented Feb 5, 2022 at 3:03
  • Aristotle " the number of movement ( change) with respect to before and after ( i.e. anteriority / posterioriry)". On can quantify movement under various respects. Time is one respect under which movement can be quantfed. Commented Feb 6, 2022 at 11:39

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From Plato to Einstein, time has been thought of by many. Everyone knows what time is. That's why I wonder what the big mystery is.

Time can be quantified by a clock. That's a periodic process, like a pendulum, with a device attached on which you can see how many periods have past. Like an odometer for spatial distance.

Einstein made the clock an objectively existing feature of reality. He associated a time axis with the clock. On different points on the axis the clock of an observer in the associated frame shows different values at different points on the axis. Dependendent on his state of motion, the clock of an observer tic-tacs at different rates. Not for the observer but according to someone who sees him moving. For the moving observer, the clock around his hip-hop neck tic-tacs always at the maximum rate. He hip-hops through time only and for an observer moving with the speed of light relative to him, he hip-hops through space only, without hipping through time. Put differently, an observer always moves through spacetime at the speed of light. Which is kind of misleading as he doesn't move through time at all. Einstein though objectified it as something through which you can move, which is obvious nonsense. It's the clock around your neck ticking and not you moving through it. The famous gamma factor is introduced in the Lorenz transformations which allow you to calculate the rate of the clock and distances in a moving frame, seen from a rest frame. The Galilean transformations are retrieved if the speed of light would be infinite (which, by the way, would result in all things happening at once and the non-existence of mass).

The different tic-tac rates are a consequence of the speed of light being the same for all observers. In gravity fields, if you stay at rest at a "point", the clocks tic-tac at different points at different rates. A clock on the surface of the Earth tic-tacs slower than in free space, and on the event horizon of a black hole (quantum entangled with the infalling stuff) the clock has stopped, as seen by a faraway observer (not at the clock itself). If you fall into a hole, you are almost at the same time radiated into space by Hawking radiation.

Time can't go backwards. If all motion were reversed quantum wave functions will decollapse and there would be no begin conditions of matter, only end conditions to which was aimed for with unreal precision. Entropic time, next to which the clock is placed to quantify it, is unidirectional, the clock can tick in both directions.

That being said, the perfect clock is an illusion. It's a fact of nature that there is no perfectly periodic process. Even the atomic clock has no constant period. Only the clock present at the pre-inflationary Planck era was perfectly periodic, comparable with Aristotle's perfect circular motion, and this clock had no direction in time yet, as entropic time didn't take off yet. Because particles are not pointlike, space and time can't reduce to a point.

So, the pre-inflationary Planck cell can be compared with Aristotle's objective unmoved mover and the perfect circular motion. Our friend was ahead of his "time"!

We can conclude that there are two different types of time. The entropic time, ET, and the clock time, CT. ET starts from the reversible CT around the Planck-sized 4D singularity. Time oscillates between + and - 10exp-43 (Planck time). Only back then a truly periodic process existed. This process had no direction in time yet, like one cannot say if a perfect pendulum swings forward or backward in time. When the circumstances around the 3D Planck volume (a tiny 3D curled up torus in 6D space, rendering it a 3D appearance) are right ET will take off and the CT is no more physically existent (though it can be approximated by, say, a caesium clock). The circumstance of the (apparent) 3D Planck cell is determined by two 3D predecessing universes accelerating away on the (apparent) 4D space, influencing the singularity at the center, a Planck-sized 4D wormhole connecting two infinite 4D spaces on which two 3D universes can pop up from the singularity.

So we finally arrive at the solution of the problem of time. The CT-ET hypothesis. We observe two mutual exclusive but dependent times. They are absent in each other's presence, but can't exist without each other.

You write:

"What is a thorough definition of time in terms of how it causes the universe to progress and only moves in one direction?"

Time doesn't cause anything. Forces cause distances between particles to change. These changes can be compared with a processes that are periodic, like a pendulum or oscillations in atoms, like in a caesium clock. If you add an extra device to the periodic motion you can read how many oscillation have occurred. We then say the (irreversible) process took x cycles (seconds) in the (reversible) process placed on the side of it. It's impossible to measure time exactly because no process exists in the universe that is truly periodic. The period time always varies in... eeeh... time! You can compare a clock with an odometer for distances. It not only shows the progress of time but also how much it has progressed. Like the odometer shows what a ruler shows and also the distance traveled.
A truly periodic process has no direction in time. You can't tell by looking at a perfect pendulum if time goes forward or backwards, which is also the case when you look at elementary processes in nature. The only truly periodic process exists around the singularity that existed before cosmic inflation. There was no temporal direction yet. The Planck-sized virtual bubble existed without real particles present yet. This state constituted a perfect clock. But there was nothing real yet to measure the time of. Only when real irreversible processes took off, these could be temporarily measured. But that perfect clock was gone then. Or isn't it? Virtual quantum bubbles are still present all around us. In a Feynman diagram they are represented by a closed line with an arrow. As circles on which a particle runs around, back and forth in time, without fixed direction.

So time doesn't cause anything, nor do we move in it. It's a usefull image though: Time flowing like a river or we flowing in a boat on top of it. We can indeed (mentally) put an ideal clock aside of all real processes. But that's all. Einstein objectified time by introducing an extra dimension (inducing a quasi Euclidean metric in the absence of gravity by multiplication of time with i). But a particle doesn't really move through it like it moves through space. A particle at rest in an inertial frame is said to move with the speed of light through time. In one second (on the ict axis) it is said to have traveled about 300 000 km. on that axis. What truly happened though is that the hand on the clock has moved one second and this has an equivalent of light having traveled 300 000 km. The speed of light is central to relativity. Speed. You can't use it to define time. Light is the instantaneous connection between matter parts. In Newtonian mechanics its speed is infinite. But Newton didn't realize it has to be finite to prevent all things from happening at once, which in a sense they do, as time and space seem to disappear for photons, for which no rest frame can be found. Very strange indeed. But logical at the same time.

"What would have to be done to cause time to progress at a different rate from what has been experienced by us?"

In a black hole, time seems to stand still wrt us. If you fall in though you get entangled with the horizon (or just above it). You get spaghettified in the bkink of an eye and all the information of your constituent particles (their momenta and positions) is radiated away into Hawking radiation. The photons lead directly back to the particles that got spaghettified and disappeared during evaporation. So only the information about your particles continues to exist, while the particles themselves are gone after evaporation. This take a blink for you while I (far away on the outside) see it spread out over a long time.

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  • "Only the clock present at the pre-inflationary Planck era was perfectly periodic, comparable with Aristotle's perfect circular motion, and this clock had no direction in time yet, as entropic time didn't take off yet" Citation please. The universe is full of exact clocks, ticking in precise ways, constrained by the speed of light. We can gain exact insights about distant blackholes colliding exactly because phenomena can all be linked to the same restframe, within the errors of our measurements. Similarly with standard-candles in cosmology.
    – CriglCragl
    Commented Feb 5, 2022 at 15:56
  • @CriglCragl There are no exact periodically ticking clocks.
    – Pathfinder
    Commented Feb 5, 2022 at 16:02
  • @CriglCragl "The short answer is, we don't know" But I do.
    – Pathfinder
    Commented Feb 5, 2022 at 16:04
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    What on Earth is a virtual propagator loop? What do they have that nothing else does? What do you mean by exact or inexact, error bars? Or some kind of absolute frame of reference? You said a bunch of things, gave no reference or evidence for them, & I am asking you to provide these. How is that too complicated for you?
    – CriglCragl
    Commented Feb 5, 2022 at 16:59
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    So virtual particles. How can they even be said to have regular periodic behaviour if by definition they can only be indirectly observed? What are you basing your idea of them on? Feynman interpreted antiparticles as going backwards in time. How would going forwards and backwards in a loop be a clock? More basic than a photon, in what sense, by what criteria?
    – CriglCragl
    Commented Feb 5, 2022 at 17:08
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Suarez ( Metaphysical disputations, 50) proposes theses distnctions :

  • Internal time : time is the mode of duration (continuaton in being) of " successive beings" , that is of those beings such that movements, processes, actions of production that are not " tota simul" ( wholly present a a single moment); this is internal time ( proper to each process, movement).

Note : in short, internal time is " duratio / duration" [genus] "successiva/ successive" [specific difference ]

Note : according to this definition, substances ( permanent beings, wholly present at each moment) are not temporal by themselves; they are in time only inasmuch as they undergo changes

  • External time

    • the mesure of time in the first sense , that is, the movement of haevenly bodies ( in particular, the Sun , or the Moon), and the number that yields the application of this measure; according to Suarez, external time so understood corresponds to Aristotle's definition of time " the number of movement with respect to before and after"

    • imaginary time, seen as the eternal and necessary flow of the present , of the " now" , movement that can be thought of as the displacement of a point on an infinite straight line; briefly Newton's "absolute time"

Note : according to Suarez, no form of external time is a genuine real entity ( that is a mind independent one).

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Time is an incredibly broad subject in philosophy, physics, and psychology. Within each subject there are multiple definitions too. Plus there’s anti-realist views about the sciences, adding further definitions.

For physics at least statistical mechanics, general relativity, and quantum mechanics provide definitions.

Statistical mechanics says entropy is overwhelmingly likely to increase. This provides directionality to time. But statistical mechanics isn’t every interaction. Small systems of quantum particles don’t have any directionality, and there is still time. There things change, just not according to stat mech, thus no entropic arrow in those systems.

This is one way to see how to decompose time into “increasing entropy” and just “change”. We can have time without entropy changing.

Still, we do observe time’s arrow. So why is there one (observed) direction, when statistical mechanics only says “overwhelmingly likely-not guaranteed”, and quantum systems need not have an arrow? The extra bit of science needed is, to hypothesize a very low entropy prior/initial condition must have occurred(according to the most popular account). There are many more ways for a system to change toward higher entropy than lower. And there were a lot of particles, too many for a small system of quantum particles to dominate. So it is the laws of physics, which don’t give a directionality alone, plus macroscopic low entropy prior conditions (Big Bang) that provides this notion of time and time’s arrow.

But then we can still talk about GR. In GR the only things that “exist” are events and the light-cone structure. There is no “start” or initial condition. Just adjacent events in spacetime. The Big Bang is just a prior condition then, not an initial one. Prior in that it’s spacetime coordinates are distant to ours. Not prior in some external dominion of just time.

Then finally psychology. Where we have at least 4 areas of the brain that tell different aspects of time. David Eagleman is a popular neuroscientist who talks about these areas. We can say from psychology and philosophy we experience a flow of time. But how do we cast a flow of time to something possibly static like general relativity, which according to itself, does not obviously provide a way for time to flow? There are just coordinates of infinitesimal points (events) in a lightcone background.

So we have at least 3 distinct aspects of time. Time as simply change, time’s directionality, and the flow of time.

We don’t have a theory of everything to weave together these different theories of time. There is also the endless problem of scientifically explaining phenomenological experience (experience of time and of flow of time).

This is my attempt at a neutral, cursory lay of the land. There are many competing views on time. Many philosophers do take as fundamental time evolving from the present, and our scientific theories are most certainly not final.

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Einstein and the philosopher Bergson famously had a debate about time. And famously, Bergson came off worst. This is not surprising as Einstein had only just revolutionised physics with his notion of special relativity with upended the Newyonian absolute time and space. However, Einstein, famously yet again had strong disagreements with Quantum Mechanics which also revolutionised our notions of space, time and matter on the microscopic scale.

On reflection, Bergsons notion of duration is closer to how time is thought in quantum mechanics and is closer to our own intuition compared to the block time of Einstein's universe. It's something that the physicist Smolin has only just recently understood, stating that time is open.

Bergson said all physical events contain a memory of the past:

"Duration is essentially a continuation of what no longer exists into what does exist. This is real time, perceived and lived. Duration therefore implies consciousness and we place consciousness at the heart of things for the very reason that we credit them with a time that endures."

Sheldrake, a philosopher of science (he trained in evolutionary biology) commented:

Whitehead was probably the first philosopher to recognise the radical implications of quantum physics. He realised that the wave theory of matter destroyed the old idea of material bodies as essentially spatial, existing at points in time but without any time existing within them. According to quantum physics, every primodorial element of matter is "an organised system of vibratory streaming of energy." A wave does not exist in an instant, it takes time; it's waves connect the past and future. He thought of the physical world as made up not of material objects but actual entities or events. An event is a happening or a becoming. It has time within it. It is a process, not a thing. As Whitehead put it, 'An event realising itself displays a pattern'. The pattern 'requires a duration involving a definite lapse of time and not merely an instantaneous moment.'

As Whitehead made clear, physics itself was pointing to the conclusion that Bergson had already reached. There is no such thing as timeless matter. All physical objects are processes that have time within them, an inner duration. Quantum physics shows that there is a minimum time period for events, because everything is vibratory and no vibration can be instantaneous. The fundamental units of nature, including photons and electrons, are temporal as well as spatial. There is no 'nature at an instant.'

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Time is a measure of change then accordingly if there is no change there is no time. Space and time are related as fabric of spacetime. Mass tells spacetime how to bend and this bending(warping) of spacetime cause gravity(most of the gravity we here experience on earth is due to time curvature) So time curvature affects gravity likewise it can be said that gravity can also affect passage of time(that is the reason for slowing down of time near to gravity) As well as speed of an object in space also affects the passage of time for it(for objects moving with high speeds time slows down and stops if it reaches speed of light) So rate of time changes with speed and gravity.

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This can be a "understandable definition of time", following my initial comment, suggesting that time is the perception of change (google for "time as perception of change"), which is a quite new philosophical idea (last 50 years, possibly). Let's start with a mental experiment.

If you are locked in jail, absolutely isolated from the external world, in complete silence and obscurity, you soon lose the notion of time. In such circumstance, you cannot tell if time passes fast or slow. You cannot know the hour, the date, etc. You just see that nothing changes, and that is probably hard to bear with. In the long run, as a consequence, you will probably stop moving, lose the sense of space, then, geometry (the rational intuition of forms and measures), then, arithmetic (the rational intuition of objects and relations). Perhaps, then, logic (the capability of reasoning). Research such consequences, although they are also product of social isolation: spatial orientation degrades, memory and concentration are affected, and even brain shrinking might occur[1].

The experiment leads to the following conclusions:

  • Time is a subjective experience. It is the measure of change. The more things change, the more the impression that time passes fast, which is evident: there is more information to process, and memory of massive events near in time could appear to have occurred long time ago. Evidently, if nothing changes (e.g. when you wait for the elevator), time seems to advance slowly.
  • Day to day experience exhibits certain patterns of change, which produce the internal sense of time. This means that time is determined by the patterns of change we perceive.
  • The same issue from a different perspective: nature exhibits certain patterns of behavior, which we perceive and organize as if they would occur in sequence (it is evidently not so, but we need to create a sequence, that's how we get home everyday), an order (we cannot tell if the bullet was shot first, or the trigger was pulled first, but we need an order which makes knowledge useful).
  • If you have a fast mind, then you live more. That's just a fact, not a theory. That's why being productive, agile, fast is better: because it provides more living time. On contrary, if you have a slow brain, then the clocks ticks faster for you.
  • Causality is just the habit of two events in time. If I hear a bell, and then, lunch is ready, I will tend to think that the bell causes lunch to be served (I mean, Pavlov's dog behavior is that of causality, which applies also to us). But lightning fire does not cause the kettle to boil. Both are unrelated facts (considering that every atom in the universe is constantly interacting with any other atom). But our minds put them in a sequential line, in an order (first, fire), and creates a relationship between them.
  • Time is not the only intuition that depends on change. If you cannot move (including the eyes), you can't experience space. If you have an acid pH in your mouth, you will have more difficulty to detect acid taste. If your eyes don't move (try focusing your eyes in a point in the room for a minute), you will soon see all as if it would have the same color. Odor, audition, touch, smell follow all the same principle: we perceive change, not state.

[1] https://www.amacad.org/publication/law-neuroscience-case-solitary-confinement

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"What then is time? If no one asks me, I know what it is. If I wish to explain it to him who asks, I do not know." - Saint Augustine, in The Confessions

The short answer is, we don't know. Relativity has proved intractable to the methods that have unified the other three fundamental forces into Quantum Field theory, that is canonical quantisation, ie keeping the symmetries but applying discontinuous energy states.

The Wheeler-DeWitt equation goes the furthest in confirmed verified science to combining the quantum and classical pictures, and it is not time-dependent, implying time is not fundamental, but emergent, and creating the problem of time. Approaches such as Loop Quantum Gravity and Constructor theory are investigating how time and space could be emerging from a more fundamental strata.

Einstein's perspective was that time is clocks, specifically light-clocks, that it relates to the fundamental speed-limit of the universe, the speed of light. Einstein did not originally propose thinking of time as a dimension, Minkowski did, and won him to that view. Our deepest understanding of dimensions relates to Noether's theorem, that sets of symmetries and conservation laws are formally equivalent, and collectively constitute what we call dimensions. Space and time are solely the domain of Relativity, and seem to be deeply linked to gravity. In the quantum world they are taken as an assumed background, and so far only special relativity has been integrated, meaning we don't know how gravity works at quantum scales, what strength it has there, or even whether antimatter has gravity of the opposite sign.

In the quantum world, almost all interactions are time reversible, except some weak-force interactions where a higher combined charge-parity-time symmetry is maintained - this is linked to the excess of matter over antimatter. The Arrow Of Time emerges from asymmetric average changes over classical scales and time of assemblies of quantum objects, giving us the important idea of entropy and it's increase as describing the characteristic trends we experience as time moving forwards. Entropy is often described as increasing disorder, but we can also describe it as information spreading out about events that happen, and that helping to understand why time travel an reversal don't happen - information not spreading out is the same as not experiencing effects of time.

The quantum coherence limit is only weakly defined by scale, but exactly described by when a system ceases being isolated and information about it leaks out, for instance in the case of a visible scale quantum system, or the recent admittedly contested case of bringing a tardigrade into a quantum state. From entanglement to delayed-choice quantum erasers, we can understand quantum behaviour by where information has arrived.

Personally, I incline towards presentism, that past and future only exist in now, I see that as the only way to understand the special qualities of the present moment as distinct from past and future. We can think of space and time as emergent symmetries in a fundamentally timeless system, resolving wuestions of the beginning of time and the origin point of Creation.

See related discussions:

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The concept of time is an abstraction. Physics does not say what time is, physics defines how to measure time: Time is the physical quantity which an observer measures by his clock.

  • According to Special Relativity the clocks of different observers may measure different time segments between the same pair of events. Therefore the „speed of time“ measured by different observers depends on the relative velocity of the observers.

    According to General Relativity the „speed of time“ also depends on the strenght of the gravitation at the location of each observer. As a consequence, one can change the speed of time by changing the velocity of the clock and/or by placing the clock in a strong gravitational field.

    See „time dilatation“ at

    https://en.wikipedia.org/wiki/Special_relativity#Time_dilation

  • For an example of a non-classical view onto time see the challenging conception of time according to loop quantum gravity https://arxiv.org/abs/1802.02382

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