1. Limited time only! Sign up for a free 30min personal tutor trial with Chegg Tutors
    Dismiss Notice
Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

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


    User Avatar
    Staff Emeritus
    Science Advisor
    Education Advisor

    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


    User Avatar
    Science Advisor

    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


    User Avatar
    Science Advisor
    Gold Member
    2017 Award

    yeah thats my favourite quote :)

  9. Jun 29, 2012 #8


    User Avatar
    Science Advisor
    Gold Member

    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


    User Avatar

    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


    User Avatar

    Staff: Mentor

    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


    User Avatar

    Staff: Mentor


    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


    User Avatar

    Staff: Mentor

    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.
  22. Sep 6, 2012 #21
    I haven't read the posts in this thread, but the topic is right for yet another question about what time is. I watched the following movie:


    Between 13:45 (especially from 15:30) and 16:25 they describe how time passes slower as you move faster through space. But note that "time passes slower" means "the mechanical movement of things (like the arms of a clock, or perhaps the motion of you body parts, like your feet while walking) slows down" (or the wheels of a car, or the compressor blades in an airplane's jet engine).

    But since you need your feet moving at a certain speed to keep you velocity constant, or the wheels of the car spinning at a certain rate, they can't possibly be moving slower as we accelerate. That would look quite funky, as if you were sliding across the pavement of the road while your feet were standing still (and thus breaking the laws of physics)... So what do we REALLY mean when we say that "time passes slower"?

    By answering that question, I think we take a step closer to the answer of what time is.

    My attempt to an answer is the following... Everything (look at it at an microscopic level) is constantly moving through space at the speed of light. However, "standing still" simply means oscillating (vibrating) in random directions around a certain point in space. This oscillation is what we use to measure time in atomic clocks.

    Motion in a certain direction is achieved by altering the probability of the oscillation to occur in that direction. The more we move in this "well defined" direction, the less "speed" or "energy" is left for motion in any other direction, and thus "time moves slower" (but it really isn't, it's in some sense constant).

    This is my own model of reality, the way I understand things, and I think it works beautifully together with both relativity and quantum mechanics. The wave function in QM defines the "oscillation".

    Any thoughts about this?
  23. Sep 6, 2012 #22
    Interesting to note that entropy increase defines a "forward" direction in time, and time is affected by gravity.

    Is there a link between thermodynamics and gravity? Indeed, could it be possible that gravity is an emergent property?
  24. Sep 6, 2012 #23
    If you agree on my model of time (see my previous post), there are two different definitions of time. The first is the one you can measure - the vibration of particles. This is the "time" which is affected by gravity (actually, it's just the wave function that is affected by gravity). But time in this sense is not a real property of anything, it does not exist.

    I know that the following is a very non-scientific statement, since science is about things you can measure... But real time can't be measured, and is "moving forward" at a constant pace. The word "forward" in this case only means that particles change states randomly within the space defined by the wave function, at a constant pace. Entropy and the direction of time are emergent properties of this process.
  25. Sep 6, 2012 #24
    Time does not exist. There is only a now- the present. Time is the measure of change. Think about it - one second is the change of the second hand from one point to another. We all perceive time differently based on our consciousness. The more conscious we are of something and change occurring, then the more we experience the present. Because those who are more conscious experience more of the present they essentially experience time in a greater quantity. For example, a person who is barely conscious doesn't experience the present continuously but in small amounts, leading to that person having a sensation that more time has passed when in actuality they just have not been perceiving the present to its full potential.
    Have you ever tried solving a problem with your full attention/focus? Sit down and time yourself doing something that requires a lot of focus and attention and you will feel that a lot of time has passed when actuality it has not. Or you could say you "experience" more of the present relative to how much a second hand has changed.
    We all know people who are great problem solvers, who think "fast", and exceptionally smart. Turns out, from all the people I know like this, they are usually self-unaware of how they behave showing signs of arrogance. However, they behave in that manner because their full consciousness is given to and concentrated on their surroundings and whatever problem they are solving or what they are reading or who they are talking to. This leaves little room to be self-conscious of what they're doing.
    Once you realize there exists only a present will you then truly have control of your brain power. People like I just described also show good memorization skills, largely because the more conscious you are of something the better chances you have of remembering it. Living in the present brings you a strong conscious and powerful mind.
    Last edited: Sep 6, 2012
  26. Sep 6, 2012 #25


    User Avatar

    Staff: Mentor

    It also means that nuclear decay half lives are longer, which is not mechanical. This is one way to verify the validity of Special Relativity. Take a radioactive substance with a known half life and accelerate it to a very very high velocity, then measure the decay rate. It will be reduced by a rate equal to the amount of time dilation.

    No, this is not what would happen. If you were able to run at 0.99c, your feet would be moving VERY quickly, fast enough to propel you at 0.99c relative to the ground. They HAVE to, otherwise you wouldn't be moving. Keep in mind that your feet are stationary with respect to the ground whenever they touch it and are propelling you forward.

    Your model makes absolutely no sense in regards to real physics, nor are personal theories and models allowed on PF. The wave function is a mathematical formula in quantum mechanics describing the quantum state of a particle and how it behaves. This does NOT include any such vibrations at c.

    The possibility may exist, but current models don't say anything about that.

    What use is it to bring something to a science forum that isn't capable of being supported by observation and evidence?
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook