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What is time?

  1. Apr 4, 2015 #1
    I watch a few videos on youtube saying that the time may be an illusion, what they mean by that?
    If time is an illusion, then why our brain knows when to sleep? and why the brain knows when to wake up (at the right time) And about the birds that agree to wake up at the same time every day? And why when we take melatonin, the brain regulates at the right time which we all should sleep? (between 9-10pm).
    I do not understand why they say "illusion", to me it seems VERY REAL.
  2. jcsd
  3. Apr 4, 2015 #2


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    Time is that property of the universe that prevents everything from happening all at once.

    Ok that was meant to be a joke. :) But it has a serious aspect, in that it seems to be impossible to define time in any way that isn't self referential.
  4. Apr 4, 2015 #3


    Staff: Mentor

    I am fond of Wilipedia's definition. "Time is a measure in which events can be ordered from the past through the present into the future."

    That is close to what uart said.

    The deep philosophical aspect of this is that without events, time cannot be defined. And quantum theory tells us that without energy, nothing can ever change. No change, no events. Therefore energy and time are closely intertwined.
  5. Apr 4, 2015 #4


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    Yeah I know what you're saying, but things like special relativity tell us that time certainly isn't how it intuitively appears to be. We know that time is somehow coupled with space, but I think that the ultimate nature of those most fundamental of all things, time and space, is still a mystery.

    I guess a good analogy would be that only a few hundred years ago people didn't know what temperature was. Yet certainly concepts of hot and cold, keeping ourselves warm and keeping certain food items cool so as not to spoil, must have been just as important to people back then. So people would have believed that they understood what hot and cold were, but fundamentally they didn't. (As in, they didn't know that the temperature of objects was due to random atomic vibrations).
    Last edited: Apr 4, 2015
  6. Apr 4, 2015 #5
    I was once mystified by time. Now I see it as nothing but counting something mechanical.

    Take a reference motion, such as the Earth rotation, and all of the "t"'s in physics equations could be replaced with "number of Earth rotations".

    Instead of "distance over time", speed could be defined as "distance between A and B" over a reference "distance between C and D".

    Time is currency for various movements (distances, rotations, and oscillations), just as money is currency for various commodities.

    Without movement, there is no time. Until I'm corrected of course.
  7. Apr 4, 2015 #6


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    This question comes up time after time after time here (pun intended), as can be seen by the list of "similar discussions" at the bottom of this page. Discussion almost always goes off in various directions, mostly philosophical, which goes against our general rule against woolly philosophical speculation. Therefore this thread is closed.
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