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What is time exactly

  1. Nov 25, 2007 #1
    first off, sorry if this is in the wrong place. iam not sure exactly were this topic should be put so i posted it in general physics

    anyway, i was interested to know what time is. i mean its used in formulas i know, so obviously its something, but does it really exist.

    i was originally thinking that was just some measurement that we pulled out of our hats because the formula's are wrong without it. but then i think about how its possible to go back in time, which makes me think that there has to be something more to it than just some made up measurement.

    i would appreciate all your thoughts on time in general :)
  2. jcsd
  3. Nov 25, 2007 #2
    I liked your train of thought better before you assumed time-travel was possible.
  4. Nov 25, 2007 #3


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    We can't really know that traveling backwards in time is impossible until we know what time is.
  5. Nov 26, 2007 #4
    well more assuming its probable, but not for living beings. more or less for particles instead.

    i dont mind opinions but what do we actually know about time?
  6. Nov 26, 2007 #5

    It relates to space, velocity and mass in a meaningfull way.

    Time is not like a switch, that you can turn on or off, or a variable you can zero out. Think of it this way, if an object has mass and any velocity relative to any other arbitrary object (including an external observer) then time is a function of the observation.

    Just like a Scottie dog you measure the length of while he/she/it is taking it's leisure at a local fire hydrant gives you a number the same puppy doing the same thing at the same "time" if measured from orbit the puppy is just about a doggy hair shorter not due to a rounding error, but by nature of the fact that in orbit the measurer is moving at several thousand miles per hour and there is a tiny, but predictable, difference.

    Read "Einstien's Dreams" and see if you can pick which dream reflects the real world.
  7. Nov 26, 2007 #6
  8. Dec 21, 2007 #7
    Is it safe to say that time is the speed of light which varies depending on the strength of the gravitational field in GR?
  9. Dec 21, 2007 #8


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  10. Dec 21, 2007 #9
    Thats what confuses me. Time is said to run slower near strong gravtational fields. Light redshifts to adjust distance vrs. time changes to keep c constant and as a result clocks in different places run at different rates. Are you saying that the speed of light does not slow when it is redshifted, but the atomic clock does slow.
  11. Dec 22, 2007 #10
    Im pretty sure thats a yeah. C is constant.
  12. Dec 22, 2007 #11


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    The speed of light is always equal to 'c' if you use local clocks and local rulers.

    While the modern approach to GR always uses local clocks and rulers to measure speeds, sometimes you'll see people doing things differently, especially if they aren't doing GR, but something more down-to-earth.

    There are at least two different basic sorts of time, but people don't always carefully distinguish between the two sorts. One sort of time is "coordinate time". The other sort of time is "proper time", which is the sort of time a clock measures. So when one asks "what is time", one could mean several things, because the term is used ambiguously. In addition there can be several different coordinate systems used, there are several sorts of coordinate time possible. (Example: TCB vs TCG - this gets technical quicly, but if you are interested in the details, see for example http://aa.usno.navy.mil/publications/docs/Circular_179.pdf).

    "Gravitational time dilation", which depends on gravitational potential rather than the gravitational field, really describes the relationship between a certain type of coordinate time, and proper time.
    Last edited: Dec 22, 2007
  13. Dec 22, 2007 #12
    The concept of time comes from our awareness of change. It is most obvious that objects around us are in flux, and our inherent sense of this is necessary for interacting with the world. We have refined our notion of time by finding (or fabricating) objects that undergo cyclical change, and we measure the passage of time by counting cycles. We measure time by measuring change.

    From this it is clear that traveling "forward" in time is no problem, because our concept of change (cause --> effect) is a forward progression. But traveling backwards is problematic: What does changing backwards mean if all change is forward by definition?

    Next we invent Thermodynamics.
  14. Dec 29, 2007 #13
    I'd like to learn more about the culture of time in the manner this post was establishing.
  15. Dec 31, 2007 #14
    I would say time could be defined as a series of irreversible processes, all of which as more similar to the others as possible.
  16. Dec 31, 2007 #15
    To grasp a little bit more on what we know, so far, about time you must first establish which time you are talking about. There are two main views that seem to try to explain time; the philosophical that says that time is nothing more than an illusion of consciousness, and the scientific that says that time is measurable. All other descriptions tend to branch from these two ideas.

    This forum isn't designed for the philosophical so I won't speak much about it but to sum it up they say that time is an illusion of motion, progression, cause and effect, etc. Everbody has differing reasons for why it's an illusion, but agree that it's an illusion of some sort. Basically time from this perspective boils down to nothing more than being the conscious observation of the relationship between two or more objects in repititious motion: ie the earth around the sun = a year.

    The scientific approach is more detailed and complicated but still does not give a concrete theory of time. (I would suggest further reading "About Time" by Paul Davies. It simplifies a lot of the complexity of the scientific time.) One of the big questions about time that must be figured out is whether time can be measured in static blocks, because we can't currently measure units time to a high enough accuracy this question is left to theorists and tends to border science and philosophy. Some believe that time is the sequence or transition of these static events.

    The idea of time being used for calculations was embedded by Newton. But he believed in a constant time, one that he derived from his theological beliefs. This was an assumption that lasted until Einstein who threw relativity at the concept of time into the picture. This where the possibility of time travel comes into play. This is another area where science meets philosophy b/c it raises the paradoxical question of if you go back in time can you kill your grandfather and therefore not exist? I personally don't think so but there is yet to be scientific or mathematical evidence for why you can't.
  17. Dec 31, 2007 #16


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    Time is about ten past nine where I am :D
  18. Jan 8, 2008 #17
    As far as i know, "time" is the extra dimension that we need to add in our usual 3-D geometry to explain gravity in relativity. As i think, the backward travel in time is prohibited by the sets of lorentz transformation (its improper transformation-that makes v>1, weinberg approach (c=1), since proper homogeneous lorentz transformations always have v< 1). The main problem i find is this-How do we relate the time we use in daily life with that in General Relativity theory?
  19. Jan 8, 2008 #18
    the man who sees the world in his 50s the same when he was in 20s has wasted his 30 years.
  20. Jan 11, 2008 #19

    Shooting Star

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    Why not? We may hit upon it by accident. Do we have to understand what goes on in our bodies to live?

    Why so?

    I've heard many people say that time dilation is valid for atomic time, not for biological time, whatever that means. They simply cannot accept the fact that humans may age at different rates.
  21. Jan 11, 2008 #20
    The British Science writer Michael Hanlon has made som very interesting comments on this question in his book "10 questions science can't answer (yet)"
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