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What Exactly is Time?

  1. Mar 10, 2009 #1
    I understand that time has effects and that time can be slowed by gravity and such but I dont understand what time realy is. Gravity is the bending of space and time but what is time? What causes time?
  2. jcsd
  3. Mar 10, 2009 #2
    Before trying to understand the GR definition of time, you must understand the classical description. Time is a coordinate. That is, it is simply a number that describes either when, or where something is. Immagine a ball bouncing around a room. You need exactly four numbers to describe the motion of the ball. You need three spacial coordinates, and the time coordinate for the time that the ball was at that point in space. The time coordinate is different from the spacial coordinates in that it only moves in the positive direction.
  4. Mar 10, 2009 #3
    I thought the General Reletivity says that an observer traveling near the speed of light would not agree with the time though. I also thought that time CAN travel backwoards if you have enough energy. The laws of Physics dont prevent time travel.
  5. Mar 10, 2009 #4
    If you want to really understand this, you need to learn classical first. You're want to run and you can't walk yet. Unfortunately, that's the way it is.
  6. Mar 10, 2009 #5
    Still, this is general physics and I think the question is still a good one. Otherwise, people wouldn't write stuff like:

    http://www.chronos.msu.ru/EREPORTS/rovelli_time.pdf [Broken]

    Maybe the OP thought that there is this a priori concept of time that lacks explanation? And
    that it might be the root of many problems? What do you guys think about the above paper?

    Last edited by a moderator: May 4, 2017
  7. Mar 10, 2009 #6
    My simple thought is that we human feel time as things happen sequentially and we experience that certain physical reaction takes a certain time. But how about photons point of view ? A photon travel at speed 'c', so its clock (if they carry one) looks frozen, meaning that how far it travels no time elapse from photon point of view. So photon does not have time and no aging -- which is quite different from us.
  8. Mar 10, 2009 #7
    This is a shameless plug for my favorite blog, pretty interesting little over my head but interesting
    http://blogs.discovermagazine.com/cosmicvariance/2009/03/08/the-envelope-please/ [Broken]
    Last edited by a moderator: May 4, 2017
  9. Mar 10, 2009 #8
    Time perception is an interesting subject. Not sure if it ties into physics though... Quantum physics maybe?
  10. Mar 10, 2009 #9
    quantum physics is physics
  11. Mar 10, 2009 #10


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    Good quote on Time

    Time is the quality of nature that keeps events from happening all at once.
  12. Mar 10, 2009 #11


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    I'm just going to quote myself, and say that the Rovelli paper is interesting, but I didn't really understand it (when I read it a few months ago). I'm going to have to have another look some time.

  13. Mar 10, 2009 #12
    no one will ever truly understand time.
  14. Mar 11, 2009 #13
    The direction of Time is shown in thermodynamics by the increase in entropy..
    that's my two cents
  15. Mar 11, 2009 #14
    I feel the thermodynamic definition of time is to narrow, yes entropy increases and we can't un-crack an egg but I think the fact that processes that are exactly the same will increase in entropy at different rates according to the same observer. Take an atomic clock on earth take one exactly the same and send it spinning around the earth as fast as you can. Few years later grab the one spinning around the earth and put it next to the one sitting in your lab and ...they are different. Same processes same "time" from the observers point of view different change in entropy.
  16. Mar 11, 2009 #15
    Time could simply be our perception of the 4th dimension. It is sort of like saying that the there are no spatial, time, etc... types of dimensions; there are simply dimensions and time might not be different from the 3 spatial dimensions...simply that we are unable to perceive 4 spatial dimensions, so any dimension after 3 is a different experience.
  17. Mar 11, 2009 #16
    We believe that the speed of light is c in every inertial refrence frame. If I had a cesium atomic clock at rest in my reference frame, and I counted x million-billion wavelengths of a certain atomic transition, wouldn't that qualify as measuring time, and indeed quantify what a second was in my reference frame?
  18. Mar 12, 2009 #17
    "What is time?" is not a proper question according to the use of the word "time." Examples of proper uses are:

    "What time is it?"
    "How much time do we have remaining?"
    "At what time will it arrive?"
    "How much time is this going to take?"
    "I don't have time."
    "This is a waste of time."
  19. Mar 12, 2009 #18
    Time as specified in SR is what governs all processes, while proper time is what we can actually use as a coordinate system, they are not the same.

    As an example the twin paradox is perfect to show the effects, by moving away and then back one twin becomes older than the other yet they they both have exactly the same coordinates. Therefore you must conclude that the time we measure and the proper time used as a coordinate system are not the same things.
  20. Mar 12, 2009 #19

    Andrew Mason

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    Here is my attempt: Time is a measure of the separation between events that occur at the same location (in a spatial frame of reference).
  21. Mar 12, 2009 #20
    What is time is prolly one of the most complex questions that could be asked ,
    i was wondering does time have a field. And if time is the fourth dimension
    the could we say using newtons laws for every action there is an equal but opposite
    reaction , if we can travel forward in time then we must be able to travel backwards in time.
  22. Mar 13, 2009 #21
    saying this though is going by the arbitrary value we give time here, namely the ticks of a clock! Think of how different the answers to these questions would be for me, a mere land dweller as opposed to someone in a weaker gravitational field/travelling super fast. This is a very profound question which isn't very easily answered, however i think there were a few good answers given hier, and i think the one about the reference frames was the best one given.
  23. Mar 13, 2009 #22
    ya it is a very complex question , Einstein thought he figured out what time was in
    1915 but then changed his mind in 1916
  24. May 4, 2009 #23
    Is time related to distance? I think what I'm asking is this. If I were to travel into space with a clock on board my ship and I were to look back at earth - at differing distances I would see earth moving in and out of daylight hours at differing frequencies. So the frequency of that transition in and out of daylight would depend on the distance I am away from earth. If the regularity of that flicker could be seen as say, every 10 minutes on my clock, but each flicker relates to an advance of 24 hours time on earth then my distance from earth could be calculated. The further the distance, then the faster the flicker into and out of daylight hours. It's as if everything has it's own time frame.

    It makes we wonder what it would be like to ride an electron around the orbit of an atom. Would that time frame have any correspondence to daylight hours?

    And on and on. When we look at galaxies spinning in the distance - it's a given that we are looking at the galaxies past. But if we can see it describe an entire orbit in, say, 24 hours, then we are not only looking back into time but at contracted time. Like a fast forward. Nothing whatsoever to do with real time on that object.
  25. May 5, 2009 #24
    space and time are one thing , if i move through space i am moving through time so i guess distance and time are related.
  26. May 5, 2009 #25


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    Welcome to PF!

    Hi rosie! Welcome to PF! :smile:
    Time is related to speed, but not to distance.

    The "regularity of the flicker" that you would see would be the same, no matter how far away you were.

    It changes when your speed (relative to the Earth) changes, because of Special Relativity, but so long as your spaceship stays at the same speed (doesn't even have to be in the same direction), the flicker rate will be the same. :smile:
    Every velocity has its own time frame, but every thing (in every position) doesn't.

    For example, an electron (assuming it has a "classical" circular orbit) has constant speed relative to the Earth, and so has a different "time frame" … not to be confused with a Special Relativity space-time frame of reference, which of course only applies to constant speed in a fixed direction. :wink:
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