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What is time?

  1. Jun 29, 2012 #1
    Good afternoon fiends.
    I am wondering what exactly is time. Some say, quite inaccurately which flows irrespective of everything.
    I am pretty confuse in it.
    Thank you in advance.
  2. jcsd
  3. Jun 29, 2012 #2


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    Previous threads on this very same topic have had ugly endings, for the threads and for some of the participants involved. I strongly suggest that all participants (and the OP) heed the PF Rules that everyone had agreed to before proceeding. Do not say that you have not been warned.

  4. Jun 29, 2012 #3
  5. Jun 29, 2012 #4
    Time is one of those things in science that is difficult to understand. But so is distance and mass, for example, if you stop and think about them. Each has its unique characteristics.

    One good way to think about time: "Time is natures way of keeping everything from happening at once".

    Time SEEMS to pass at some fixed constant rate; distance also seems to be fixed, say, between two objects on a table.

    Yet Einstein showed us neither is 'absolutely' correct: time and distance varying according to different observers. It is the speed of light that is constant!! In fact, what one rapidly moving observer sees as 'time' another rapidly moving observer interprets as distance. All this is incorporated into special relativity. That can also be confusing!!
  6. Jun 29, 2012 #5


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    There are a number of related, and yet some what different concepts of time in physics.

    Time is a coordinate in general relativity. It is special in that it has an opposite sign in the metric to the spacial coordinates.

    Proper time is a bit closer to what most people think of as time. It's what you would measure with a clock. It's also the time that shows up as parameter in non-relativistic quantum mechanics, and you can think of it as the "age" of something.

    And then there is time in sense of sequence of events. This is the tricky one. With the above two, forward and backwards directions in time are equivalent. And there is no "flow". Time as a coordinate just is. However, if you have a sequence of events that are casually related, you can always say that event 1 happened first, then event 2, then event 3. This is what we understand as the flow of time. You from five minutes ago definitely comes before you now, because you can think back to how things were five minutes ago.

    This one isn't understood, and the reason for all of the confusion about the concept of time. The only hint we have is the entropy. Entropy always increases, so maybe entropy increase defines the time flow. It'd also make sense with information flow, as storing information requires an entropy increase. But this hardly takes care of all the questions, and it's practically untestable at this stage. So it's all very speculative.

    If you want to know more about time as coordinate in space-time or about proper time, and why the clocks in different coordinate systems don't always match, this is a good place to ask these questions. If you want to understand why the time "flows", you won't get anything beyond guesses and speculation, so this is not a good place to discuss them. You'll just have to accept that nobody really understands it yet.
  7. Jun 29, 2012 #6
    Time is natures way of keeping everything from happening at once. ;->
  8. Jun 29, 2012 #7


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    yeah thats my favourite quote :)

  9. Jun 29, 2012 #8


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    Einstein held the throroughly pragmatic view that time is what clocks measure. But, of course he convincingly argued there was no such thing as an 'absolute' clock. For an engaging discussion, see http://www.iep.utm.edu/requires/.
  10. Jun 29, 2012 #9
    baby don't hurt me

    oh wait, wrong song
    There's already been about 100+ threads on this if you use the search function though
  11. Jun 29, 2012 #10

    nature has autonomy??!?!?
  12. Jun 30, 2012 #11
    Does this means there's place where all happening once as they say time is frozen in black hole. Or does it happened like BB where time started from no time.
  13. Jun 30, 2012 #12
    Time is a figment of the imagination, IE Zeno Paradox and or Arrow Paradox.
  14. Jun 30, 2012 #13


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    so Z, can you be more specific about what to avoid?

    it's interesting, but i went to the Wikipedia entry and they had recently had a good definition that just got yanked.

    Time is the indefinite continued progress of existence and events that occur in apparently irreversible succession from the past through the present to the future.1 Time is a component quantity of many measurements used to sequence events, to compare the durations of events and the intervals between them, and to quantify rates of change of quantities in material reality or in the conscious experience.

    in raw wiki language they, until recently, had:

    '''Time''' is the indefinite continued [[sequence|progress]] of [[existence]] and [[event (philosophy)|events]] that occur in apparently [[irreversible process|irreversible]] succession from the [[past]] through the [[present]] to the [[future]].<ref>http://www.thefreedictionary.com/in+time</ref> [Broken] Time is a component quantity of many [[measurement]]s used to [[sequence]] events, to compare the durations of events and the intervals between them, and to [[quantification|quantify]] rates of change of quantities in [[scientific realism|material reality]] or in the [[consciousness|conscious]] [[qualia|experience]].<ref name=DefRefs01>

    i thought that was a pretty good initial definition, before getting into what "time" means to various disciplines (like "time" as a 4th dimension in spacetime in physics or the conceptual reversibility of time regarding elementary anti-particles). i dunno why they recently took that out.
    Last edited by a moderator: May 6, 2017
  15. Jun 30, 2012 #14


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    I don't agree. Whatever time may be it is as real as distance is. Otherwise clocks wouldn't move, particles wouldn't decay, etc.
  16. Jun 30, 2012 #15
    time is not a problem for the physical universe to have to grapple with...

    time only becomes a problem when ''we'' invoke it with our conscious needs to reconcile a function or system that can't be understood....which then becomes a metaphysical issue

    it appears that entropy and decay is not winning the war in the observable universe and ''time'' will ''travel'' in a forward motion for us, or millenium beyond to ''measure''...that is possibly a big reason why time is vexing ...
  17. Jun 30, 2012 #16


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    And again, what?
  18. Jul 1, 2012 #17
    hello Drakkith and all .. sorry about the abstract nature of my posts...

    ..we experience the passing of time because it is important to our functioning and beyond that, to our ability to measure experimental data....but does the ''unconscious'' universe in the course of its function ''need'' that mandate ?

    time is just a constant ... it cannot be stretched, dilated or stopped...it's only when we ask too much of its reason for being, that our understanding fails...and who has never pondered the question .. ''is there no beginning, nor end, to time ?''......
    Last edited: Jul 1, 2012
  19. Jul 1, 2012 #18


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    The "need" for time is as valid as the "need" for distance in my opinion.

    Perhaps we look too deep into it.
  20. Jul 1, 2012 #19
    yes i agree Drakkith .. the problem i find both with time and distance is the infinites (or infinities) the measurements thereof bring with them ..

    problem being that we are repulsed mathematically and intellectually with the concept of something being infinite (we will never believe in a perpetual motion machine operating on our planet)...

    .we want concrete answers to our endeavours, and the ''open ended'' nature of time and distance tend to make a mockery of our need for the constraints that we would innately prefer to exist ....
    Last edited: Jul 1, 2012
  21. Jul 1, 2012 #20
    Taking the output t of a clock and putting it in equations like any other spatial parameters hide the differences between t and the other spatial parameters. There is no freedom to move in the reverse time direction. It is why the physical approaches which express changes not in terms of a spatial-like dimension t but using a relational approach as first proposed by Leibniz might be more appropriate.
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