Is it time or duration?

Question

Taking this post: "Is there a proof of existence of time?", as a starting point. Therein was mentioned that there is confusion between:

"time" and "flow of time".

There was a comment (of mine) that the confusion is not between time and flow of time (which are equivalent), but between time and duration of which one is a dimension (i.e duration).

Given the importance of the problem of time in General Relativity and Quantum Gravity.

Having made this disctinction is an important step, since duration can easily be considered as a dimension (with the proper $c$ factor) along with other space dimensions, than actual time (or flow of time).

Can we say that time parameter/dimension in SR/GR actually represents not event time but duration (i.e time-interval)?

By the way this would elucidate the wick-rotation method, as transforming from "duration" to "frequency" representation.

(Not to mention that one can have as many duration dimensions in a manifold as one wants with no conceptual or definition problems like when one attempts that with extra time dimensions, per some theoretical proposals)

This is also getting kinda language-y. Wiktionary says duration is "an amount of time or a particular time interval". By this definition your question would not make a lot of sense. Smolin says time is real when the past and the future is nonexistent, while the present is existent. But for that kind of discussion we should rather migrate to Philosophy SE. — Void, Aug 20 '14 at 18:56
This question appears to be off-topic because it is overly pedantic and about English language word meanings. — Brandon Enright, Aug 20 '14 at 19:56
@BrandonEnright, i certainly not consider that way but i can understand where you are getting. In my defense i will simply say it is not just a matter of terminology. It is a conceptual matter (at least). Time (as event time) is different from duration (or time-interval) and this elucidates the other question refered to, and similar ones about time paradoxes. Remember that in SR specificaly time dilation is actually time-interval dilation, i.e duration dilation. it all makes sense this way. — Nikos M., Aug 20 '14 at 21:07
@BrandonEnright: How can a scientific reasoning be "overly pedantic"? You mean that it leaves too little room for unconstrained interpretation, or what? What is wrong with achieving maximum precision in science? — bright magus, Aug 21 '14 at 05:57
@NikosM.: You are absolutely right and in order to see it you only need to consider what is an event. Can anything happen in no time? — bright magus, Aug 21 '14 at 06:00
@brightmagus I don't think this question is about scientific reasoning. I think it's reading way too much into English instead of any actual physical concept. I meant "pedantic" ind a pejorative sense. — Brandon Enright, Aug 21 '14 at 06:01
@BrandonEnright: Judging other people's intentions is a very tricky business and you better be 100% sure before you do that. — bright magus, Aug 21 '14 at 06:05
@brightmagus I agree we should try to stick to the physics wherever possible. I don't see any physics in this question. It's natural that others don't agree and this is a good example of why each member only gets one close vote on a question. — Brandon Enright, Aug 21 '14 at 06:13
@BrandonEnright: You see, to me this is just the essence of physics: To understand a given phenomenon as much as possible in order to be able to draw correct conclusions and not be led astray. Sadly, in contemporary physics such an approach is often called philosophical or even metaphysical. If I had to pick the single most destructive idea in science that would be Feynman's (?) slogan: "Shut up and calculate!" — bright magus, Aug 21 '14 at 06:31
I think the question is largely meaningless, but I don't see why it should be closed and I've voted to reopen. — John Rennie, Aug 21 '14 at 16:01
@JohnRennie, well thanx! btw i would not expect from you (given the referenced post and your interest in SR/GR) to consider this as meaningless question, but anyway.. — Nikos M., Aug 21 '14 at 16:04
@NikosM. If you take a vector pointing in the time direction then duration is just a scalar that gives the length of the vector. I don't understand why you consider it a significant quantity. — John Rennie, Aug 21 '14 at 16:05
@JohnRennie, interesting, if this re-opens it could be an answer as well — Nikos M., Aug 21 '14 at 16:07

Mathias · Accepted Answer · 2014-08-20T22:33:12.597

Duration is certainly a more physical concept than time.

Duration is something you may measure between timelike separated events while time is always something you compute by adding up duration measurements + an arbitrary constant to fix the origin.

Duration is experimental and relational while time (e.g. GPS time) is an abstract a posteriori construction.

For these reasons I think your proposal is correct.

It could happen that advanced theories get rid of most of Time, but at some point these advanced theories will need to have some room for durations (even if only in a limit).

Beyond this quantitative component of Time, or rather more fundamentally, there is also the more qualitative notion of ordering of timelike separated events, linked to causality. The ordering does not require a continuous flow, e.g. discontinuous "pre-time" can work for this purpose. In particular if durations are discrete in a way or another. This would be quite a different concept for time.

@BrandonEnRight : in such a domain as modern gravitation theory, it is normal to have discussions on fundamental concepts because they need to be questioned and understood to see better what is useful in the postulates of the theory under construction. I understand you want to contain the pseudo-scientific spontaneous trends of discussion, but this not the case here. And about reverting to philosophy SE, I would say yes if we were into metaphysics (discussion on non-experimentable concepts). But here we are still in physics as all the assertions lend themselves to experimental tests, at least in principle

thanks for liking the question :), what do you mean in the last 2 lines "discontinuous "pre-time" can work for this purpose. In particular if durations are discrete in a way ice another. This would be quite a different concept for time" ?? — Nikos M., Aug 20 '14 at 22:14
@Nikos M. You could imagine a partial ordering relation between events without the concept of duration. This partial ordering is what I mentioned as pre-time. — Mathias, Aug 20 '14 at 22:29
see a paper of Smolin on relational approaches to physics and relativiy in particular — Nikos M., Aug 20 '14 at 23:07
i really like the parts: "Duration is experimental and relational" and "It could happen that advanced theories get rid of most of Time, but at some point these advanced theories will need to have some room for durations", although i would leave the time flow for a formulation of events (taking into acount thermodynamiocs et al) having quite different concepts, but this is not for now — Nikos M., Aug 20 '14 at 23:38

score 1 · Answer 2 · answered Aug 20 '14 at 22:28

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Okay, I am going to try and give this a shot, but this is most probably not going to be a decisive answer.

Let us operate with the term event time and duration and consider only special relativity (SR). The conclusions of general relativity should be the same for reasonable space-times. (e.g. without closed time-like curves etc.)

We expect event time to identify the exact relation to all other events, i.e. at a given event time, we choose for events with a smaller time to be in the past and the ones with a larger time to be in the future. Notice that this notion of time does not really need any quantitative measure, it is more a question of a certain topological ordering of events.

However, the famous Rietdijk-Putnam argument shows, using relativity of simultaneity, that there is no natural global classification of such an order of events (at least without invoking a privileged class of observers). In special relativity, there is thus no global notion of event time.

It would seem that event time would make sense in the lightcone but it is not so. We can always order causally connected events, but for non-zero time-like intervals between them, there will always be adjacent events in their lightcones that are causally disconnected. I.e. there is never a nonzero volume of space with unique event time. The only case of event time in SR as I define it is the proper time on a single time-like curve and it's homeomorphisms.

As for duration, we want it to measure an amount of time passed. But how do we do it? We must compare the amount of time to a certain physical process. Galileo used his heartbeat, but we would use the cycles of radiation in the caesium atoms. In this sense, the physical process must always be happening in a certain frame of reference and special relativity tells us (and this is verified by experiment e.g. through mean decays of particles) that the duration of any physical process is happening with a stable rate with respect to the proper time in a given frame of reference.

Different observers will thus through time dilation have different notions of duration of processes observed in their surroundings. It is pretty easy to show that once again not even the ordering of magnitude of duration of physical processes is universal. As in the case of event time, there is no natural global definition of duration and a natural fixed comparison of durations is only possible on the world-line of the observer but nowhere else.

To conclude, there is thus actually no unambiguous time dimension/coordinate in relativity, be it event time or duration. You need four numbers to specify your event, but none of these are uniquely identifiable with either space or time (without a mixed-in part of the other one) unless you specify the observer who is asking that question.

answered Aug 20 '14 at 22:28

Void

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i like it, a couple of comments: 1) it is not necessary (esp for a duration dimension) to be globally defined (actually i consider this as elucidating many paradoxes) 2) in SR all dimensions can be taken to be intervals in fact (not absolute coordinates), 3) past and future is exactly at the crux of the problem (and it gets worse in GR) (travel back in time and all, 2nd law coming into play), cont.. – Nikos M. Aug 20 '14 at 22:43
..cont i dont think we have this problem with a duration approach (although there are others, like what a negative duration means and definition of event-time out of durations, simultaneity might play a part here). – Nikos M. Aug 20 '14 at 22:44
..cont not to mention all those time-"sth" uncertainty relations which are always about durations – Nikos M. Aug 20 '14 at 22:46
do you think there will be a problem (in theoretical results or experiment) if considering time dimension in SR as duration? this is unclear in your answer – Nikos M. Aug 20 '14 at 22:56
Well, once again, language. We have dimensions of space. In that sense, duration is no natural universal dimension, at least in SR. It would be meaningful only in a world of objects all at rest with respect to a particular reference frame. On the other hand, we have dimensions/measures of events over physical intervals. But for these, duration is tautologically valid, because we define it as such. SR "only" tells us such a definition is always consistent and measurable in all reference frames. ... – Void Aug 21 '14 at 09:12
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... So yes, in a purely relational (but not in the coordinate-organization) sense, duration is the only consistent dimension. – Void Aug 21 '14 at 09:13
@Void: "On the other hand, we have dimensions/measures of events over physical intervals." Why "on the other hand"? You don't agree with the common claim in contemporary physics that time is actually no different a dimension to physical ones? If it's fully "equivalent" then it should be treated the same here, shouldn't it? – bright magus Aug 21 '14 at 11:38
@brightmagus "On the other" hand because I identify the "dimension of space" with a coordinate labeling of all events. Whereas the dimension of an "object $\to$ event" is the measure of it's extent. The "spatial" dimension talks about possible descriptions whereas the measure of event extent talks about the meaning of quantities obtained by measurement actions. We need four coordinate numbers for specifying an event, and we thus talk about four dimensions. But none of these can be even identified as either space or time. They are only space-time coordinates, there is no separation. – Void Aug 21 '14 at 12:53
@Void: That's the kind of anwer I expected from current mathematical physics, ie. yes and no. – bright magus Aug 21 '14 at 13:22
@brightmagus Well, in the end physics only tells you about physical actions and measurements. The language it provides for describing it is only a language, a code, or labeling useful for organization of observational knowledge. A question about the language itself is non-physical and will not have a distinct answer based on physics unless there is a hidden question about observable phenomena or physical actions underneath. – Void Aug 21 '14 at 15:09
@Void: completely not true. Mislabeling leads to misusing and misunderstanding mathematics. And this is especially true for physics (not much so for mathematics, which is now pretending to be physics). – bright magus Aug 21 '14 at 16:45
Let us continue this discussion in chat. – Void Aug 21 '14 at 16:53

score 0 · Answer 3 · answered Aug 21 '14 at 01:34

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The answer is more simple than you think. Time is that, which is measured by (technologically suitable) clocks. Physical theories will simply tell you how clocks behave under certain conditions. This is purely descriptive. There is not a single physical theory out there, that gives a microscopic description of time, although the similarity of time with irreversible thermodynamic processes SUGGESTS, that it can be derived from a state counting argument in a microscopic theory of spacetime using methods from statistical mechanics. Such a suggestion is far from being a useful theoretical framework, of course.

answered Aug 21 '14 at 01:34

CuriousOne

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i'll have to disagree with that. Plus (the question is now on hold), but did not answer whether one can indeed take the time dimension in SR/GR to be duration (without any further complications) – Nikos M. Aug 21 '14 at 13:58
@CuriousOne - Sure time is what is measured by clocks. But even though no physical theory give a microscopic description of time yet, I have to point that there is neither a single theory that allows a consistent, predictive approach for quantum gravitation. So discussing foundational aspects is necessary, and time cannot escape it. One has to assess alternatives to each postulate, one by one to derive results to be tested by experiments. So Time could require a microspcopic description to accomodate quantum and gravity. You should avoid closing discussions without constricutive argument. – Mathias Aug 21 '14 at 19:35
@NikosM.: You can disagree with that all you want. When you go into an actual physics laboratory, you will inevitably find, that time is measured by clocks. Some of those clocks are radioactive and muon decays and similar physical effects, but they are all localized physical phenomena that agree (within the limitations of their accuracy) on what "time" is. There is no other definition for it in physics. – CuriousOne Aug 21 '14 at 19:48
@Mathias: You wanted to know what time is and what physics can say about it. I told you just that. There may, in some future, be a consistent theory of quantum gravity (I and plenty of people who are actually knowledgable about the subject doubt even that for very good technical reasons), that has absolutely nothing to do with the definition of time as we use it today. That time is measured by clocks is not a postulate. It's the physical reality of time measurement. If you want to talk about time in physics, you have to talk about clocks and how they depend on the rest of the universe. – CuriousOne Aug 21 '14 at 19:51
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@CuriousOne - I think you don't get my point. I agree with the role of clocks. What I suggest is that there might be something more profound than time as a coordinate yet to be discovered. This might be (or not but one has to try to know) an ingredient of quantum gravity. And I think that duration rather than time could lead to it. – Mathias Aug 21 '14 at 20:30
@Mathias: Sorry for the misunderstanding. I do not understand time as a geometric variable, at all. General relativity does that, and as a result it can't explain any more than Classical Mechanics "what time is". My point is, that time, as of now, is defined operationally by how we measure it, and that certain aspects of how our clocks behave can be described without a microscopic explanation of it. I did, by no means, attempt to deny that there is a proper microscopic dynamic that causes "good" clocks to show a consistent "flow of time". At this point we simply haven't found it, yet. – CuriousOne Aug 21 '14 at 20:54
@CuriousOne, the "operational definition of time" is indeed time duration. As such i can take the comment as implying that when time parameter is (taken as) duration, there is no complication either in theory or experiment. The whole point (per Mathias answer) is that duration is indeed more physical (or experimental if you like) and de-confuses the whole flow thing (e.g "and time flows wrt to what?") – Nikos M. Aug 27 '14 at 20:54
@NikosM.: There are no other times but the operational ones that we measure by clocks. Everything else is a human fantasy that we develop about those times in theories and models. – CuriousOne Aug 28 '14 at 03:42
@CuriousOne, well i would not like to turn this into a chat, but i am interested on where you stand (i cant really make from the answer), on (time) evolution. is it an illusion (aka GR wins) or a (tangible) reality (aka 2nd Law wins), or sth else? This is simplistic of course, but just as rough outline. – Nikos M. Aug 28 '14 at 23:55
@NikosM.: Good question... my gut feeling is, that (on macroscopic scales) time, spacetime and gravity are thermodynamic remnants of a dynamics about which we know very little. I think that solves several difficulties at once, e.g. the irreversibility problem (which is a natural part of thermodynamics), the dimension problem, the hierarchy problem for gravity. A thermodynamic approach may also explain the big bang and inflation as a phase transition and it gives a much more rosy tinge to the future of the universe. Since I am not a theoretician, I might just be talking nonsense, though. – CuriousOne Aug 29 '14 at 19:53

Is it time or duration?

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