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What is time?

  1. Oct 14, 2012 #1
    Hi All! I'm Armenia,learning in the faculty of Physics in our State University. Recently while one of lectures we talked about time,what is it. The lecturer said that it's something that is moving on without stopping but I disagreed and said that no time exists, time is just a way of arranging events and nothing else.
    This is my opinion and I'd like to know yours. Thanks beforehand!
     
  2. jcsd
  3. Oct 14, 2012 #2

    Drakkith

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    Staff: Mentor

    Warning: Previous threads on what time is have always degenerated into speculation and ramblings that have no place on PF. Per the rules of the forum, our belief on what time is does not matter, it is how science defines and uses it. Please, do not post things like "time is flowing" or "time doesn't exist", as you are 100% wrong according to science, which is what this forum is for. ALSO, this is about the PHYSICS of time, not the biological PERCEPTION of time. It is a well known fact that organisms can perceive the passage of time differently, and such discussion is better off in the Biology subforum than here.

    Currently time is viewed as the 4th dimension, with the other 3 being space. What this means is that it takes 3 different numbers to define the location of an event in a coordinate system, and a further number to define its location in time. One of the things that happens in General and Special relativity is that we can assign a "four velocity" to an object, which replaces it's velocity in 3 dimensions with a 4d view, in such a way as to make the speed of light a constant in every inertial frame of reference. Commonly we use the phrase "we always travel at c through spacetime" meaning that our four velocity is always c and that increases in spatial velocity relative to another object come at the cost of our velocity through time, hence time slows down for moving objects.

    http://en.wikipedia.org/wiki/Four-velocity

    This is the only view on time that I know is actually useful and relevant according to science. If anyone else knows another feel free to post about it. I'm not sure if there are some quantum theories out there that view it differently or something.
     
  4. Oct 14, 2012 #3
    Albert einstein wrote that "time is what a clock measures." In physics you aren't going to do much better than that.
     
  5. Oct 14, 2012 #4
    Time is what we measure with a clock; space is what we measure with a ruler.

    we have a lot more to learn about each.

    'Time is nature's way of keeping everything from happening at once.'

    Nobody knows, for example, if each is continuous or discrete.

    "no time exists, time is just a way of arranging events and nothing else. "

    This is a superficial view....Time, for example, mixes with
    space at different velocities according to special relativity. In additional, general relativity tells us gravitational potential also causes changes, warps or curvature, in space and time.
    [This relates to four velocity mentioned abopve by Drakkith.]

    Try these on for size:

    Concept of time

    On the reality of time and the evolution of laws
    Speaker(s): Lee Smolin
    Abstract: There are a number of arguments in the philosophical, physical and cosmological literatures for the thesis that time is not fundamental to the description of nature. According to this view, time should be only an approximate notion which emerges from a more fundamental, timeless description only in certain limiting approximations. My first task is to review these arguments and explain why they fail. I will then examine the opposite view, which is that time and change are fundamental and, indeed, are perhaps the only aspects of reality that are not emergent from a more fundamental, microscopic description. The argument involves several aspects of contemporary physics and cosmology including 1) the problem of the landscape of string theory, 2) cosmological inflation and the problem of initial conditions, 3) the interpretation of the “wavefunction of the universe,” and the problem of what is an observable in classical and quantum general relativity. It also involves issues in the foundations of mathematics and the issue of the proper understanding of the role of mathematics in physics. The view that time is real and not emergent is, I will argue, supported by considerations arising from all these issues It leads finally to a need for a notion of law in cosmology which replaces the freedom to choose initial conditions with a notion of laws evolving in time. The arguments presented here have been developed in collaboration with Roberto Mangabeira Unger .
    -- http://pirsa.org/08100049/


    http://arxiv.org/abs/0903.3832
    "Forget time"
    Authors: Carlo Rovelli
    (Submitted on 23 Mar 2009 (v1), last revised 27 Mar 2009 (this version, v3))
    Abstract: Following a line of research that I have developed for several years, I argue that the best strategy for understanding quantum gravity is to build a picture of the physical world where the notion of time plays no role. I summarize here this point of view, explaining why I think that in a fundamental description of nature we must "forget time", and how this can be done in the classical and in the quantum theory. The idea is to develop a formalism that treats dependent and independent variables on the same footing. In short, I propose to interpret mechanics as a theory of relations between variables, rather than the theory of the evolution of variables in time.


    http://arxiv.org/abs/0711.0757
    Universal quantum mechanics
    Steven B. Giddings
    "There is no intrinsic notion of time or history in this description. Such notions may emerge for certain UQM theories in certain states."
     
    Last edited: Oct 14, 2012
  6. Oct 14, 2012 #5
    Perhaps better to say that science has nothing to say on whether 'time is flowing' or 'time exists', mainly because these two assertions are not at all well defined.

    According to the theory of relativity, time is a dimension like the 3 space dimensions, but crucially different in that the square of a time period is the negative of the square of a spatial distance (times a constant factor).
     
  7. Oct 14, 2012 #6
    Just to expand on what TGlad said: I've heard time described as 'the dimension that moves backwards'.

    What this means is that in special relativity the only mathematical difference between time and the three spatial dimensions is that the sign of time (positive or negative) is the opposite of the three spatial dimensions (whether you choose time to be negative and the three spatial dimensions to be positive or vice-versa is up to you- it ends up making no difference).

    Edit (the next morning): Drakkith, I can post what I can later on but I'm busy right now- what I'm talking about is the signature of the Minkowski tensor, I'll post what I can later.
     
    Last edited: Oct 15, 2012
  8. Oct 14, 2012 #7

    Drakkith

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    Staff: Mentor

    Can I get a little math to show me what you guys mean Vorde?
     
  9. Oct 15, 2012 #8
    A quote from Kurt Godel
    "There is no now in science."


    thanks for the links natty 1
     
  10. Oct 15, 2012 #9
    The Minkowski tensor which is used in special relativity is a diagonally symmetric 4x4 matrix:
    ##
    \begin{pmatrix}
    -1&0&0&0\\
    0&1&0&0\\
    0&0&1&0\\
    0&0&0&1\\
    \end{pmatrix}
    ##
    This matrix is labeled [itex]\eta[/itex] and the inner product between two four-vectors v and u is [itex]\eta[/itex][itex]\alpha[/itex][itex]\beta[/itex]u[itex]\alpha[/itex]v[itex]\beta[/itex] = d[itex]\tau[/itex]2 where [itex]\tau[/itex] is the proper time.
    You can compare this with the normal euclidean metric diag(1,1,1) to say that in four dimensions, time is the dimension whose sign is negative.

    However, the matrix:
    ##
    \begin{pmatrix}
    1&0&0&0\\
    0&-1&0&0\\
    0&0&-1&0\\
    0&0&0&-1\\
    \end{pmatrix}
    ##
    returns the exact same results. And so to take time as the negative dimension and the other three as positive dimensions or vice-verse is a matter of taste. My textbook uses the first definition and said that the first is more common amongst relativistic physicists and the latter is more common with particle physicists.
     
  11. Oct 15, 2012 #10
    Note

    Quite so. Also vanhees called it recently an arrangement of events and I roughly agree
    - "physicsforums.com/showthread.php?p=410753" [Broken]5

    However, that is a rather abstract (mathematical) way of putting it. Probably time measurement started with the counting of days and the division of days in periods of hours (which is a rather primitive way of putting it).

    In physics we do time measurements by comparing the progression of physical processes with a reference process (a "clock", which usually is repetitive). So, from an experimental physicists point of view, time is a measure of the progression of physical processes; that enables arranging sequences of events compared to reference events as well as comparing time periods.

    Note the stark difference between what one might call the "physical" view such as expressed by Naty1 (and Einstein), with what one might call the "mathematical" view such as expressed by Drakkith (and Minkowsi).
     
    Last edited by a moderator: May 6, 2017
  12. Oct 15, 2012 #11
  13. Oct 15, 2012 #12

    Drakkith

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    Staff: Mentor

    Re: Note

    I don't really see a big difference. The mathematical view is simply using math to describe what we observe, just like all science does.
     
  14. Oct 15, 2012 #13
    Re: Note

    I'd say there is a difference, though I lean towards the mathematical answer.

    The mathematical view tends to say that an interpretation that is mathematically consistent and agrees with experiment is enough, whereas the physical view tries to find reason behind it. Or at least a philosophical explanation.
     
  15. Oct 15, 2012 #14

    Drakkith

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    Staff: Mentor

    Re: Note

    Wouldn't you use both to do so? I don't think there have been very many major discoveries in science in the last 200 years without a mathematical framework to describe them. I don't think you can say that the mathematical view is separate from the physical view IF you want to talk about how science views time. Both are integral in my opinion.
     
  16. Oct 15, 2012 #15
    I'd use the first part of what you said to justify why I think the mathematical interpretation is more important. I would say that some of the things Naty1 said are examples of trying to give justification for physical phenomena above and beyond the reach of math and physics.
     
  17. Oct 16, 2012 #16
    Re: Note

    I fully agree with that; for example Maxwell used both to do so, while Einstein relied more on mathematics but nevertheless stressed that his math related to clocks and rulers. However, there is sometimes a confusion between mathematical things (such as mathematical dimensions) and physical things (such as spatial dimensions, based on rulers). Saying that time is a 4th dimension suggests that time is merely a mathematical object; however it has physical meaning. Thus, I would say that time is a physical concept as elaborated in post #10, and which can be mathematically described as elaborated in post #2.
     
    Last edited: Oct 16, 2012
  18. Oct 21, 2012 #17
    Re: Note

    What you've just mentioned is actually a good example of the following: Einstein relied heavily on mathematical formalism later in life, while he was trying (in vain) to formulate a unified field theory. He was actually most successful when he regarded the *physical* considerations first, as he did with SR and GR. Relativity is a very axiomatic (in the logical sense) theoretical framework because of this. At least that's the account one reads in Walter Isaacson's biography of him.
     
  19. Oct 22, 2012 #18
    Fascinating. I am glad you can all agree that time is a physical entity that can exist without the need for consciousness or perception. I do find those debates rather drab.

    I have also heard it said that the direction of time is defined by the direction of increasing entropy. I must say I find it difficult to believe this is true. Because the 2nd law is the only one that seems to have a prefered direction (dictated by statistics) does this mean it is the cause of that direction? Would love to know what your thoughts on that are.
     
  20. Oct 22, 2012 #19
    I have heard the same. But I think that the direction of time is inherent in the physical definition of time; there is no need to call on entropy.
     
  21. Oct 22, 2012 #20
    "I have also heard it said that the direction of time is defined by the direction of increasing entropy."

    That's not literally correct. Statistically they usually move in the same direction...but entropy MAY decrease...it's, however, usually improbable.

    http://en.wikipedia.org/wiki/Entropy_(arrow_of_time [Broken])
     
    Last edited by a moderator: May 6, 2017
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