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The nature of time

  1. May 30, 2008 #1
    I know I am probably missing the point, but something is not clicking for me in understanding time as a dimension.

    Isn't time just an arbitrary measurement of distance?

    It's always just us 'counting hippopotamuses' while watching something moving from point A to point B (we don't have to use seconds or minutes, because we just made those up for our convenience, didn't we?)

    I understand that if we want to accurately describe an event, we need to give an x,y,z and a "time dimension" measurement. But how does measuring or describing something make it a dimension?

    I guess I don't really understand the concept of "past" either. All the matter in the universe exists in the state we see around us right now. The matter no longer exists in the state it was in yesterday. You could never go back in time because all of the matter has changed. The time we experience is just the accumulation of our current, "now" experience of the matter around us.

    I'm on the verge of a long ramble, maybe someone can set me straight before I confuse myself too much more?
  2. jcsd
  3. May 30, 2008 #2


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    i'm still of the opinion that, despite all they tell us about it in SR and GR, that there is a qualitative difference between time as a physical quantity and length as another physical quantity. not to say that there ain't a relationship between the two, but they ain't exactly the same "stuff". outside of the event horizon of a black hole, i don't think there is an "arrow of distance" in the same manner that there is an "arrow of time". i guess i still feel that there are four fundamental or base dimensions of physical quantity being length, time, mass, and electric charge. all other physical quantity can be derived from those.

    i dunno, maybe properties like "charm" or whatever it is that quarks take on is another dimension of physical "stuff". beats me.
  4. May 30, 2008 #3
    thanks for your reply, r b-j,

    When you say "four fundamental or base dimensions of physical quantity being length, time, mass, and electric charge," it still does not clear up for me how time is considered a physical entity, quality or quantity.

    Let me try to 'splain: think of a single point containing all matter. Without physical distance, there is no point A or point B, no "between", so there can be nothing to measure, and no "time" can "exist" because time is only conceptual construct to help us understand changes in the current state of the universe. With no changes in state (no change to the length, mass or charge) would there be any way to experience time?

    I'm not sure I've got this bit right either, but wouldn't a change in mass or charge be caused by a change in the "length" between electrons and their atoms?
    Last edited: May 30, 2008
  5. May 30, 2008 #4
    Maybe this is a better question: Is time a dimension or a concept?

    I am looking out my window watching a new leaf appear on a nearby tree. Yesterday there was no leaf. So the "time" measurement for leaf growth is one per day.

    But what does that mean? It means that I am seeing one leaf for every 40070 km the earth moves (that's the diameter of the planet.) So you can see that I am measuring the "time" it takes for the leaf to grow against the distance the earth moves. So the rate for leaf growth can also be expressed as "one leaf per 40070km." (Isn't that what we imply if we say "per day?")

    If we break it down a bit more, leaf growth rate is really a just distance measurement too. It's a measure of the movement of molecules and chemicals required to create cells and to construct a leaf.

    In this example, hauling all those chemicals up the tree and stitching them together to create a leaf happens at the same rate as the planet moving 40070 km.
    So we've taken time completely out of the equation. ("Rate" just means an amount of one thing considered in relation to a unit of another thing, in this case 1 leaf vs. 1 planet rotation.)

    So if time is really just the comparison of two distances, how can it be a dimension?

    utterly confusededed.
  6. May 30, 2008 #5
    Confused? Take a moment to laugh and relax, because you are not alone.
    The subject of dimensions is incredibly difficult, and part of this problem is that we would like the concept to be nice and tidy with clear progressions when in fact it is not.

    While the 1st-3rd dimensons can be described and characterized by aspects of distance vectors, the higher ones do not. For whatever reason, there seems to be a radical shift of characterization after the 3rd-dimension, and in such a way that seems to be hard to grasp; as if reality itself alters.
  7. May 30, 2008 #6


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    and, by dimensions, i didn't mean the x, y, z (or t) axis thing. hell, the string theorists tell us there are 11 of those dimensions anyway. i meant more of what dimensional analysis is about. as far as i can tell, we should have 4 base dimensions of physical stuff and any other physical quantity can be expressed in terms of those four. of those four, time is different than length. not just another length axis. it has qualitative differences. at least one qualitative difference (the "arrow of time" for which there is no counterpart regarding length).
  8. May 30, 2008 #7


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    well, a change in charge or mass of the fundamental particles would force a change in the Bohr radius. that's true.

    why "or"? it's both. hell, everything is a concept. maybe take a look at the wikipedia article on it:


    it deals with it pretty well.

    it means that along this single dimensioned axis we call "time", something at this fixed spatial point (if you neglect the movement and rotation of the Earth) is different at one "place" along that time axis than at another "place". likewise, during a single moment of time, there is something different along some spatial axis "x" than there is at another place.
  9. May 30, 2008 #8
    Yeah, it's a big can-o-worms.



  10. May 31, 2008 #9
    One more link from Discover Magazine called "Newsflash: Time May Not Exist."


    I've pondered about the nature of time as well, and I was just going to ask a similar question. If it is a dimension, and traversable like any other dimension, what keeps it going "forward"? Is it going forward? How could I tell that it's going forward for everyone? If it's not a dimension, then what is it, exactly, and how do things change? Are things changing?

    Furthermore, the measurement of time itself bothers me. How can you measure something that is a priori to our making sense of the universe?

    According to Wikipedia, the definition of a second is:
    "the duration of 9 192 631 770 periods of the radiation corresponding to the transition between the two hyperfine levels of the ground state of the caesium 133 atom."

    But if a period is the amount of time it takes for a wavelength to pass, then the definition of a second is recursively defined by time. How do you know the frequency of "the radiation corresponding to the transition between two hyperfine levels of the ground state of Caesium 133" if you can't define the x-axis as time to begin with? Unless somehow you can count oscillations without time.

    Now I'm confused as well, zazaphys. I'm in waaay over my head.
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