When I consider my own existence with respect to time I can imagine three possibilities:

(1) Time extends infinitely into the past. In this case, how can the present, with me in it, exist, since there would be an infinite period of time that existed before the present. That seems illogical.

(2) Time had a starting point from which everything in our universe evolved to the present (t=1). That possibility would require something (God) to start the clock ticking (t=0). But in this case we're back to square 1 with the problem of an infinite past. So the only possibility that makes sense to me is a third possibility

(3) Where time is some sort of abstraction in human consciousness that we need in order to relate cause and effect.

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    "(1) Time extends infinitely into the past. In this case I don't understand how the present, with me in it, could exist, since there would be an infinite period of time that existed before the present." -- I simply do not comprehend this argument, though it's very common. Imagine the real number line extending indefinitely to the left and indefinitely to the right. You are at some point on that line. There you are. Why is this a problem? Your human mind imagines a qualitative difference between the infinite past and the infinite future; but look at the line. It's the same in both directions.
    – user4894
    Mar 24, 2014 at 23:24
  • 1
    >There you are. Why is this a problem? I have difficulty comprehending an infinite time line. Imagine a series of events going back in time in which C (present state) follows B (previous state), etc. But if the series of events is infinite how could C (present event) ever be reached? I just can't get my head around the idea of an infinite number of states existing prior to the present one. At least not in any meaningful way. Does your explanation simply show that it's mathematically possible to represent such a system? Mar 25, 2014 at 0:23
  • related
    – infatuated
    Aug 6, 2014 at 16:15
  • @user4894: space is not the same as time;even in GR where time is geometrised together with space it still has a distinctive character. Sep 10, 2014 at 13:09
  • 1
    Can you comprehend an infinite time line extending into the future? That's easier, intuitively, although it's truly no less problematic than an infinite history. Sep 10, 2014 at 20:08

14 Answers 14


A popular saying is: "Time doesn't exist, only clocks exist."

Time, like all units of measurement, are abstractions.

In physics, the purest way of comparison is the direct one: How long is this thing compared to that thing. It yields a fraction of some sort, which most importantly is invariant.

This fraction has no units, is directly bound to the original question and never varies, no matter our frame of reference or unit of measurement.

The thing is, if we can't measure one of the things before tomorrow, and we can't measure the other thing after today. That is when we introduce units of measurements, because units of measurement co-vary.

When we change our frame of reference, say by pretending everything is twice as big (effectively cutting all our reference units in half), all off our measurements go up by a factor of two.

Therefore, comparing the measurements to one another still yields an invariant fraction!

Time is measured in seconds, which is defined as some high number of periods in microwave radiation emitted by Cesium atoms cooled to some low temperature under some special conditions, in an atomic clock.

But atomic clocks are subject to time dilation courtesy special and general relativity.

If you put an atomic clock on a spacecraft (which has been done, by the way) and send it into orbit, the lower gravity will make the space clock disagree with an identically calibrated twin left on earth.

So when we say "it takes N seconds" we really mean "if I put an atomic clock next to this thing, the dial on the atomic clock would show the number N when this process has reached the end state."

Humans feel time passing because of some physical processes in our brains behave like (very inaccurate, compared to atomic ones) clocks.

But what does time then abstract? Entropy.

The universe is continually descending into informational chaos, usually in the form of heat. Entropy starts in the laws of quantum mechanics where a certain ubiquitous interaction causes loss of information; this can be proven to be an instance of Liouville's Theorem.

Some cosmologists and epistemological mathematicians currently believe that the final formulation of quantum mechanics will not include "time" as a parameter anywhere, and that time will be a derivable quantity.

  • "the lower gravity will make the space clock disagree with an identically calibrated twin left on earth." I was under the impression that the changes in relative time occurred due to speed, not gravity. Are there multiple effects here, and is the atomic clock in space actually experiencing a different time, or just "malfunctioning" due to gravity changes?
    – Cain
    Aug 5, 2014 at 16:40
  • @Cain Gravity and acceleration are indistinguishable per general relativity. The clock is not "malfunctioning."
    – Malcolm
    Aug 5, 2014 at 18:50
  • there's more to time than "Entropy". there is also Causality. the arrow of time. Jun 5, 2016 at 2:59

Makes sense. (2-3) are popular ways to go. I'd like to address your worry in (1).

Time extends infinitely into the past. In this case I don't understand how the present, with me in it, could exist, since there would be an infinite period of time that existed before the present.

Consider the domain of nonpositive integers (Z = {0, -1, -2, ...}). Interpret 0 as 'now', and precedence as '<' on Z. If we use Z to model time, we can satisfy your hypothesis that:

(1) Time extends infinitely into the past: ¬∃i ∈ Z ( ∀j∈ Z ( i ≤ j ) ).

Now, while this means that an infinite period of time precedes the present moment, we still have it that:

(2) The present exists: ∃i ∈ Z: i = 0.

Usually rational or real numbers are used to model time, but this model is sufficient for our purposes.

  • Unfortunately I'm no good at math. Are you saying that it's possible to mathematically represent an infinite period of time preceding the present moment? See my comment above. Mar 25, 2014 at 0:32
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    Yes. My point is exactly what User4894 has said above. While I share your worry, I think the unintuitiveness of completed infinities shouldn't stop us. Large part of mathematics is able to proceed only because of the acceptance of infinities of higher and higher orders. For example, according to a famous theorem of Cantor's, if you take an infinite set like nonnegative integers and you isolate the set of all of its subsets, you will end up with a larger set. This means that there are infinities that have elements that come after all the element of other infinities! Mar 25, 2014 at 1:27
  • This went against the intuitions of lots of great minds in the past, but mathematicians have learned to embrace the paradise Cantor created for us. There are numbers w, w+1, etc. that come after the series 0, 1, 2, ... ends. There are more interesting numbers, but you need to start exploring some logic and set theory first in order to be able to appreciate them. Mar 25, 2014 at 1:30

(3) Definitely.

I'd liken it to a crystallization process. Or maybe even condensation.

Time does not exist. Only change exists. We can observe change by comparing the now to the what we remember.

Relativistic physics talks about "time" speeding up or slowing down depending on how fast you travel. All that changes is the speed of change of the object that travels faster or slower.

If you take an analog clock for example. If you don't watch it for "some time" it will have made several 24h rotations. If you don't know if it's day or night outside you might not even know what "time" it is or if the time is correct. It's just some made up instrument to give us something to hold onto in the ocean of change.


I will attempt to present the stance of physicist Julian Barbour, who presented a compelling answer to this question in a lecture I saw him give in Oxford (this lecture).

He is firmly not in the camp of the latter viewpoint in your title, but I wouldn't necessarily put him in the former camp either. He puts his view of time in terms of the primary/secondary quality distinction and refers to Galileo, who in 1623 classified attributes such as color and sound as secondary characteristics of matter that are perceiver-dependent. Rather than photons entering eyes or vibrations entering ears, in the case of time we have our perception of the successiveness of events streaming through the system we know as consciousness. These progressions correspond with an increase in system complexity, with all consciousnesses experiencing a flow of time along a line of increasing complexity.

All in all a fascinating viewpoint that I've hardly done justice here. After the lecture, he recommended that people watch this video for a more in-depth look (assuming they didn't want to read The End of Time or any of his papers) so I will pass that along as well.

Lastly, I'll add something because it made me laugh and because it is bound to come up if you consider the implications of this position: When someone asked him what all this means about free will, he said “I'm not sure it matters much. It feels free anyway, and I'm excited to find out what I'll do next.”


I think there may be a 4th option which is closely related to your option (2). That is, time started at the big bang and has existed independently of our minds since then forming our Einstein-Minkowski 'time-cone'.

Of course, what came before the big bang, or if this is even a question that makes sense, is up for debate but there certainly would be no need to posit a God figure to make this work and so you could avoid your first cause infinite regress.


Time as you perceive it is naturally a mental construct, but it seems to correspond to an element of reality.

"In a complete theory there is an element corresponding to each element of reality. A sufficient condition for the reality of a physical quantity is the possibility of predicting it with certainty, without disturbing the system" - EPR

However, we know reality is absurd

"The theory of quantum electrodynamics describes Nature as absurd from the point of view of common sense. And it fully agrees with experiment. So I hope you can accept Nature as She is - absurd." - Feynman

So there you have it, a fourth possibility that time corresponds to an element of reality and absurdly continues infinitely into the past.

Other people here proposed to view time as the domain of integers or as an axis of real numbers, but how is causality represented in such an analogy?

  • To answer your question: causality can be represented by imposing further axioms on the usual field axioms. Unfortunately I don't know enough about causality to discuss the plausibility of this approach, but I don't see an immediate reason why this cannot be done. Aug 3, 2014 at 17:56

Is time a mental abstraction or does it exist outside consciousness?

The latter. Time exists independently of us.

In all likelihood, time exists independently of one's consciousness. That is to say, while it may be that the experience which we refer to as the passage of time is in part created by our brains, there is still a physical process which it corresponds to, and this is something which cannot be suspended or initiated by our brains.

While it's possible that time only exists when there is something there to 'observe' it, time certainly still exists when people aren't around, as can be verified by simply using a video/tape recorder to record events in an area which is devoid of people or even all intelligent life (within a certain radius). Alternatively, there is any number of subtle signs in everything which suggest that physical processes proceed without direct human/animal observation.

Infinite timelines

Does time stretch infinitely backward or have a starting point?

You say that either of these possibilities are less preferable to the option of time merely being an abstraction of the brain, and I understand this if you associate the idea of a non infinite timeline specifically with the idea of a deity. Also your option 1) (infinitely backwards timeline) involves the idea of infinity, which indeed may be internally contradictory in some way.

However, the possibility of time having a definite start point does not necessitate that there is some "entity" out there which initiated the timeline.

That being said, I don't know how to differentiate between option 1) and 2). All that we know is that the answer to 3) is that time proceeds without us, in some shape or form.


At the level of common sense one can say that as time is not physical like matter, it s certainly an abstraction. One cannot touch feel or see time except as an idea. But for all practical purposes time exists outside of our consciousness and it can be measured by a clock. But philosophically answering the question is very difficult. Words like mental construct, abstraction, existence, consciousness, independence will have to be defined or at least properly described and the inter-relations between them will have to be established. Good information on time and the philosophy of time can be found at wikipedia - http://en.wikipedia.org/wiki/Time, Internet encyclopaedia of Philosophy - http://www.iep.utm.edu/time/ and Stanford Encyclopaedia of Philosophy.


You've hit upon one of the lines of thinking by Kant.

In his thinking both options (1) & (2) are true; to Kant this suggests this is an actual contradiction, an antinomy in his language, and which is irresolvable; by us; one can say we are at the limit of thought, of the human capacity to reason - hence a critique of pure reason; establishing thus limits.

Contradictions & antinomies in the main current of European philosophy are severely frowned upon; there is an assumption, perhaps since the Enlightenment that reason can solve all, that the world in its entirety is an ordered and intelligible whole. The mysterium tremendum has only be looked at in the proper fashion, in right spirit of sustained enquiry for it to reveal itself as less tremendous and less mysterious.

We can suppose that the ontological tissue of the world, the fabric of reality is intelligible but we can also suppose by being finite creatures that we must then be epistemlogically finite; and between these two thoughts, which are separate thoughts and also the same thought is an antinomy; an antinomy that is nonetheless no less an antinomy for being dynamic.

Kant declared himself a new Copernicus; who recentred Philosophy in a new orbit; he swapped the role of subject and object; time and space lose there object-ive character and become subject-ive; they are the forms of intuitions that the mind has such that it can experience at all; they are the conditions for experience; but by being the substrate of subjective experience they also retain their objective character; this appears as an antinomy, but it is a resolvable one.

The phenomenal realm is both the objective and the subjective realm as normatively understood. Kant by outlawing noumena, by rendering it unspeakable and unsayable is outlawing the numinous. From Spinozas emanationist ontology he retains the mode of extension and of thought; the other modes, the infinity of them - the attributes of God and God himself are placed permanantly behind a veil.

In Ghazalis Occasionalism; not only is God required to create the world; he is required to create it from moment to moment; or rather to sustain it; to propel it along through the dynamic of natural law - his law (we may also call it the law of Vishnu); thus also the law of cause and effect.

Kant, in a Promethean move takes this out of the hands of God and into the mind of Man; but that part of the mind that is inaccessible to us; not Freuds unconcious; nor something lower; but something that is immanent to mind. Can one put time out of mind? One can certainly write the words...

What was once a triad: Man - God - Object; is now a dyad: Man - Object; Kant banished God as once God banished man; then after the exile - execution: the Nietzschian gesture.


The closest thing that time abstracts is motion. The passage of time is always tracked by some form of motion, be it clock ticks, days, seasons, ageing etc. If the motion of everything in the universe where to cease, time is frozen. Since time directly correspond to periodic (repetitive motion), one can argue that it is physical and not a pure mental construct. But if you ask if time is as 'real' as space, well it's not.

Ref: motionmountain.net

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    It seems that Time, as tracked/recorded in some sort of motion, would be a meaningless abstraction in an empty universe. Imagine a universe devoid of all energy/matter save for a single photon. Where is that photon going, and how fast is it moving? Going and moving compared to what? Mar 30, 2014 at 9:28
  • That's why we can talk about the beginning of time, coinciding with bigbang. Before big bang time couldn't be tracked or existed
    – AIB
    Mar 30, 2014 at 9:41

You've presented a false dichotomy: just because something is a "mental construct" doesn't mean that it can't also be a real property of the physical universe.

Take length as an example. "Length," as such, doesn't exist physically. It's not something you can see or touch. Nevertheless, it is a real quality of physical objects--it exists "out there" and not just in our heads. In this respect, length is like any other measure--it's an abstraction in the sense that it does not exist in and of itself, but it nevertheless describes real qualities of physical objects.

Time is no different. The more you understand of physics, the less sense it makes to think of time "extending" in any direction. Time isn't a medium, it's a measure, and what it measures is change. As for what change itself is, well...that's still an unanswered question!


(1) is a little too loaded or existential of a question. If we shoot a classical arrow through the air, we can slice up each moment of its flight time into infinitely many infantecimal moments. In a sense, time extends infinitely into the present moment. What is "continuous" motion but the mathematical model of the rational number? In one second, there are an uncountable number of fractional sub-seconds; to say it another way, you can split 1.000...0 into an infinite number of fractions.

The concept of "infinite" is an abstract mental construct. By very definition, "infiniteness" cannot be measured. By that admission, it might be tempting to conclude it is a fabrication. It might be an idea that only exists inside the human mind. Another one of our 'constructs' to provide order and explanation to what we describe as "uncountable" processes and rules of logic.

That doesn't mean it doesn't exist or isn't correct, there's more than enough scholarly work to suggest it is a natural and 'real' thing either.

  • Uncountable is a technical term in mathematics which does not apply here (the rationals are countable). Also, I wonder what you mean by measure. Infinite sets can be compared in size, classified, etc. in ways that some might consider a form of measurement.
    – Era
    Jun 2, 2016 at 23:44

Summations in math are completely true irregardless of "time" so the question itself has no meaning. Yes there is "time" but like gravity "it exists but doesn't mean all that much" until you start talking vast distances (say between Planets.) Yes, "time mattered" when your average speed of travel was at best 4mph across rough terrain going from say Philadelphia to Boston.

But once you fly an SR-71 from LA to New York in 45 minutes time really doesn't matter all that much.

It mattered a lot in the Railroad business as you didn't want two trains travelling on the same tracks in opposite directions "at the same time." But "speed of travel" (time) has been solved by aircraft. Our "modern age" is very much defined by "being put in a hurry" though which perhaps can be explained by the difficult distances "travel" encountered in North America when going by land to anywhere. ("Time is money.")

This was never really true in Great Britain or Germany though as the terrain, distances and History favored those "with all the time in the World."

Americans were in a hurry...there was gold in California and silver in Nevada. "To get it you had to get there...and fast" apparently...


3 is correct. Time is a abstraction, a filing system used to arrange events and memories into a logical system of cause and effect.

Per relativity, space and time are not discrete entities, but a single construct called spacetime. Saying something is X km long is equal to saying it is Y seconds long. All units of time are based on and can be reduced to measures of distance.

Time is a mental construct used to make sense of movement. Movement produces the sensation and experience of time, not the other way around.

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    "Saying something is X km long is equal to saying it is Y seconds long." is an inaccurate statement.
    – Dave
    Jun 2, 2016 at 23:24
  • @Dave is correct: Mathematically, what we call spacetime is a 4D manifold called Minkowski Space. What you're describing is as if spacetime were simply a 4D manifold, but Minkowski Space has additional structure which relates the spacial and temporal dimensions to one another (giving rise to relativistic concepts like the light cone).
    – Era
    Jun 2, 2016 at 23:49
  • I have yet to hear a definition for a unit of time that does not reduce to distance nor circular/infinite regress in nature. Jun 10, 2016 at 16:30

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