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!

Passage of time

  1. Jun 18, 2004 #1
    I've been thinking.. If time is what a clock measures, at what rate does time pass? By definition, the same rate as the clock's hand moves, but that's just circular. Then, there's also "everything is stationary wrt itself", so how does any change occur at all? It just dawned on me that it is related to memory. That is, passage of time or any change can be perceived, because we can compare the current status to the previous one in our memory. If we didn't remember a moment ago, we wouldn't have a concept of time, or life or anything for that matter. So it seems that memory (anything that can store information) must be very important, I didn't realize this before. Does anyone have some thoughts on this, or know of a web site where this line of thought is explored?
  2. jcsd
  3. Jun 18, 2004 #2


    User Avatar
    Staff Emeritus
    Gold Member

    Everything is spatially stationary wrt itself. The same is not true of temporal movement. Unless frozen to absolute zero, there is always change of some sort and increasing entropy in closed systems. Time is the dimension through which this change (or movement, if you want to call it that) occurs.
  4. Jun 18, 2004 #3


    User Avatar
    Science Advisor
    Gold Member

    How do you define any unit is the problem here? The simple answer is you define it by defining it; if two clocks 'tick' at the same rate (ignoring SR for a minute) they are essenmtially measuring the same thing, just as if two metre rules are the same length they are measuring the same thing.
  5. Jun 18, 2004 #4
    I'm becoming confused. Everything is spatially stationary because "here is here", and temporally stationary because "now is now". Isn't experienced proper time different than movement in time dimension?
  6. Jun 18, 2004 #5
    Even though the two clocks produce the same result, they are not measuring the same thing.
    Time is not an entity it is a perception directly related to an objects environment. Each clock is measuring its own time.

    If you stretch your arms out horizontally, your finger tips would experience time faster than your head because your head is closer to the main mass of your body, and subsequently a larger gravitational force.
    The difference would be infinitely small, but there would be a difference.
  7. Jun 18, 2004 #6


    User Avatar
    Science Advisor
    Gold Member

    I said to ignore relativty, but in that case we simply refine it so that the clocks are local and at rest to each other.
  8. Jun 20, 2004 #7
    Okay, so the clocks consist of exactly the same quantity of mass, are both at rest to each other and both have exactly the same gravitational force exerted on them - you take relativity out of the equation.

    The clocks are still not measuring the same thing.
    The movement of the mechanics/electronics of each clock is syncronised, but that is all.

    The measurement of time is directly related to the components of the clock. Not the cogs and springs, not even the atoms, but the energy that comprises all the subatomic particles.

    Even if one of the clocks is running slow, is measurement would be inacturate, but the clock would still preceive time the same as the other, it just wouldn't display it the same.
  9. Jun 20, 2004 #8
    time = action

    there's no other way to measure time except in the form of "something happening"

    glad someone else besides me "gets" this, wespe :D
  10. Jun 20, 2004 #9


    User Avatar
    Science Advisor
    Gold Member

    In that case they are essirtially measuring time on different scales, the only useful defintion of time is that which a 'clock measures'
  11. Jun 20, 2004 #10
    That would imply that time is defined by the movement within a mechanical device.
    Clocks provide you with a device that has a mechanical movement with a constant velocity. The amount of angular rotation of the cogs and hands at that velocity is how time is measured, but it is not a definition of time.

    For a true definition of time you shouldn't ignore relativity, but rather use it.

    A clock here on earth measures time at a different rate to one aboard an orbiting satelite. Why ?
    The easiest answer is Relativity, but that doesn't tell you very much.

    A true definition of time must be constant both here and in orbit.

    If we take the simplest particle, the photon, and assume for a moment that rather than travelling through empty space that is being transmitted through a granular space, then the transmission of photon energy from one grain to the next would give you the basic definition of time.
    The more compressed the grains (gravity well) the longer it will take for the photon to travel a set distance, but each grain is still transmitting the energy at the same rate, there's just more grains to be traversed.
    The more dispersed the grains get (a satelite on orbit) the shorter it will take the photon to travel - less grains - same transmission rate.

    Since the photon is unsigned energy, whatever rules it has to adhere to, so too all the subatomic particles.

    This would give you the most fundemental definition of time, its value would be the same anywhere in the Universe.
  12. Jun 20, 2004 #11


    User Avatar
    Science Advisor
    Gold Member

    A clock only measures it's proper time.

    We know that there is no global defitnion of time as it would not be Lorentz invariant.
  13. Jun 20, 2004 #12
    This is what I meant in post #4. A clock doesn't measure movement in time dimension.

    Jcsd, do you think the following statement is okay?
    Everything is spatially stationary wrt itself because "here is here",
    and temporally stationary wrt itself because "now is now"

    Actually my emphasis in this thread was: if we didn't remember a momement ago, we couldn't compare it to now, and we wouldn't perceive a passage of time. More like philosophy than physics.
    Last edited: Jun 20, 2004
  14. Jun 22, 2004 #13
    There are three basic states of time.
    1. The Past.
    2. The Future.
    3. The Here and Now.
  15. Jun 22, 2004 #14
    Am I supposed to extract anything useful out of that? What do you mean by "state of time"? If these are the basic ones, what are the more complex "states of time"? Why do you include "Here" with now?
  16. Jun 22, 2004 #15
    There is only one state of time and that is NOW

    Everything else is a measurement of elapsed time.
    • Past -> positive elapsed time
    • Future -> Negative elapsed time
  17. Jun 24, 2004 #16
    I see a clock as being nothing more than a device which relates numbers to time in such a way as to make it easy for the general population to understand a difficult to explain phenomenon.

    Clocks, and calenders too for that matter, are nothing more than number based systems which makes the average Joe's life easier and more productive. For example; if you had no idea what year you were born you could not possibly know how old you are. Thanks to the calender, average Joe knows he's 35 years old, or whatever the case may be.

    Similarly we can use an egg timer to cook an egg for 7 minutes, but wtf is 7 minutes besides 7 full rotations of the second hand on the clock?
  18. Jun 24, 2004 #17
    What do you mean by state of time. Perhaps that is where you lose me. What does this statement mean?

    You seem hung up on the word measurement. How Newtonian of you. Time is more than its measurement by mankind.

    I agree with geistkiesel. The most fundament awareness of time is the present. This is followed by the past and then the future. There are more complex states of time, but you seem not ready for them yet, as you seem hung up on measuring time.

    The universe is not all at the same time. Time does not flow at constant speed, and no two entities move at the same rate through time. I have no idea what you mean by there is only one state of time, now. What do you mean?

    You use the word elapsed. Elapsed is a verb expressed in the past tense. The past tense refers to time that has occurred in the past. Yet, you would speak of past time in the future, and use the word negative to do that. I challenge this. What do you mean?
  19. Jun 24, 2004 #18
    This is a Newtonain concept. In Einsteinian physics, time is much more than what a clock measures. Are you not aware of this? Are you not aware of space-time? I recommend that you investigate this.
  20. Jun 24, 2004 #19
    No physical theory can answer this question. Time flows uniformly even in the theory of relativity but there is a component of spacetime that can be curved as warping of time but this warpage of time is linked to the warpage of space. The two warpages cannot be separated into rate of time and rate of space.

    This is the time's arrow. And theoretically, there are four arrows. And nobody can answer these mysteries of one directional arrows.

    1. Thermodynamics arrow - heat flows from hot to cool. The 2nd law. The increase of entropy.

    2. Electromagnetic arrow - EM radiation always emanates outward from the source never inward.

    3. Cosmology arrow - The expansion of the universe. The domination of redshifts over blueshifts.

    4. Psychology arrow - We remember the past but not the future.
  21. Jun 24, 2004 #20
    well if my clock measures different values than your clock (they are identical, but due SR effects), then my time is different than your time, which means time is relative. that's what I understand from time is what clock measures.

    I am aware of spacetime but in spacetime diagrams there's a formula like sqrt(dx^2-dt^2) to find the elapsed proper time, so you don't just use dt as elapsed time, so I don't know why the 4th dimension is labelled time.

    Actually I don't know what I'm talking about so feel free to embarras me.

    I was trying to relate passage of time with memory but no one was interested so let the thread die..
  22. Jun 24, 2004 #21
    Clocks measure clock time.

    Your time is different from everyone else's time, always.

    If you and I are unable to recognize that our time is different, then we can use an irrelevant third object, a clock, to define for us a shared, objective time. We can use the irrelevant clock time for its simplicity of undersanding and measuring.

    This objective time is extremely useful. However, neither your time nor my time is dependant on the clock time. Each person's space passes through its time at its own rate. That is the nature of space-time.

    Clocks are useful to give a context to relative times. However, as useful as clocks are, their use ignores the fact that objective time is approximate, and the lost information is the most important information.
  23. Jun 24, 2004 #22
    I disagree completely. Calendars track the solar and lunar cycles. Without calendars, how could society, and the people within it, maintain awareness of the cycles of the month and the year?

    Without awareness of the cycles of the day, month, and year, our species would not have the level of consciousness, the level of science, or the level of religion that it has now.
  24. Jun 24, 2004 #23
    if "time is what a clock measures" then "clocks measure time" not "clocks measure clock time"

    even if we are in the same inertial frame?

    What is this lost information? Are we talking about Einstein's relativity or are you promoting a different theory?
  25. Jun 24, 2004 #24
    Do you subscribe to this? This is a pretty shallow understanding of time.


    If a clock is measuring time, then it must be involved in motion through space, as there is no other way for a clock to "measure" time. If a clock is in its own motion through space, doing whatever enables it to measure time, then it is involved in motion that other objects that are observing the clock are not. Since motion through space-time is dependent upon both motion through space and motion through time, and since there is a discrepancy in motion through space due to whatever motion through space it is that is enabling the clock to measure time, there must be a differential in motion through time between a clock and any object that observes it.
    Last edited: Jun 24, 2004
  26. Jun 24, 2004 #25
    What is your exact definition of time?

    ok let's think of a light clock: light continually bouncing back and forth between two parallel mirrors. If I am stationary wrt the mirrors, what would be the error in the measurement of this clock?
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook