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How does Time expand?

  1. Apr 22, 2008 #1
    Id like to know how time expands.
    Ive always heard that time and space are constantly expanding, and i could never understand how time expands, but i think that might have been because the ppl how tried to explain how time expands did it poorly or didn't understand themselves.

    Also im in 9th grade so try to keep out complicated mathematics and what not, Thx:)
     
    Last edited: Apr 22, 2008
  2. jcsd
  3. Apr 1, 2015 #2
    I do too have the same question. I understand that our universe expands spacially. If you take points A and B and measure their distances from C then in an interval of time measure the distances again, it shows that A and B grew away from point C at close to the same rate. So that's how we describe space expansion. But to say that time is expanding is very interesting. I can conceive time as an open plane that had been here before (and after) space expansion, and that energy came just to fill "time". Lol get that joke? Expansion of time could be conceptualized in my mind, but scientists agree that time is all relative to space. They think "there is no time where there isn't space." Which is a little closed-minded to me, believing that time started at the start of the spacial universe, or maybe I misunderstand. But this is where "spacetime is always expanding" comes from.
     
  4. Apr 1, 2015 #3
    As far as I know, only space expands. Space and spacetime are not the same thing.
     
  5. Apr 1, 2015 #4

    PeterDonis

    Staff: Mentor

    Correct.
     
  6. Apr 2, 2015 #5
    Isn't time just a measurement of objects moving through space? (example: it takes this long, for this object, to move this far, through space) If I am correct in this, then if there is no space, there is no time - because there is nothing to move through to be measured.
     
  7. Apr 2, 2015 #6

    PeterDonis

    Staff: Mentor

    No. An object that is not moving through space at all can still experience time. You experience time when standing still, don't you?

    One could argue that, without some sort of change happening, "time" as a concept has no meaning. But our universe does include things that are changing, so this is not an issue.

    One could also argue that "time" does not need to be thought of as something separate from the changing things: that "time" is already included, implicitly, if we have a complete description of how things change. Julian Barbour has written a number of papers developing this point of view. But "time" on this view is still a perfectly good concept; it's just defined in terms of other concepts.
     
  8. Apr 15, 2015 #7
    Thats what my statement referred to.
    When I am standing still, there is still movement within (blood flow, heartbeat, cellular) and aound me. (the world at large)
     
  9. Apr 15, 2015 #8

    PeterDonis

    Staff: Mentor

    Yes, but there's no reason why that must be true for any entity that can experience time. For example, you could be a robot with no internal moving parts; your robotic brain could run on electromagnetic impulses that do not involve any material object moving. Not all change involves motion of material objects.

    Yes, but again, there's no reason why that would have to be true for any entity that can experience time. You could be in an isolation chamber with no information coming in at all from the outside world. (You might need a brain better designed than a human's to cope with that kind of lack of external input, but again, there's no reason to think a human brain, with all its faults, is required to experience time.)
     
  10. Apr 17, 2015 #9

    How could any measurments be made to an object that doesn't "move" (have any motion)? That's not physics.

    A light clock can illustrate time "on its own" i.e. with out needing a concept that includes time (motion).

    Note that we cannot observe the photons motion.
     
  11. Apr 17, 2015 #10

    PeterDonis

    Staff: Mentor

    See my example of a robot brain running on electromagnetic impulses in post #8. The only things "moving" could be electromagnetic wave packets, which are not directly observable.
     
  12. Apr 17, 2015 #11
    Isn't electromagnetic wave packets another way of saying photons? That's the example I used for a light clock.
     
  13. Apr 17, 2015 #12

    PeterDonis

    Staff: Mentor

    Sort of. I don't know that we want to open the can of worms involved in trying to unpack that statement further.

    At any rate, since you said that we cannot observe the motion of photons, we're basically in agreement on that. My point was that, if that's true, then we can have objects that measure time (such as a light clock) without having any observable motion (the only things "moving" are EM wave packets/photons, whose "motion" can't be observed). Which gives an obvious answer to the question you posed that I was responding to:

     
  14. Apr 17, 2015 #13
    So what measurement could be made to something that doesn't move?

    I think the light clock is an ideal example, though is an idealized object. Where as the robot brain would have mass being displaced within its "brain" as the energy packets move around, a process I could tick my light clock against to measure motion (with my ruler too ofcourse).

    All this said, I think the statement of no motion means no time, is annologus to saying the "elsewhere" regions of a light cone don't exist. Of course observation of something by every observer isn't a qualifier for it to exist.

    From that I'd say for an idealized object that doesn't move it simply means the object doesn't move. It moving isn't a qualifier for time to exist. There seems to be an additional example in our scenarios, photons themselves.
     
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