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Definition of Time

  1. Nov 27, 2013 #1
    Once I said "there is no such thing as time" in my high school physics class and got laughed at.

    I still don't understand how time can be anything "natural" though and I was wondering if someone could help me understand it. I get the concept of an arrow of time, and how our minds organise the sequence and structure of events and I get how it is asymmetrical.

    But I don't understand how time can be anything BUT a measurement of a change in entropy or sequence of events.

    When physicists talk about things like "spacetime" they talk about time as a dimension, in which events occur in a sequence. So, suddenly the time measuring the steps between events is part of space itself.

    When physicists talk about black holes, they say time "Freezes" once something crosses the event horizon. How can time freeze from one or any reference frame? And on that note how can it change at all?

    I get that there are schools of thought about what time is, but as a naive hobbyist it seems that time can be thrown into any old model, formula or theory and be bent to make it work. But to me I still don't see the effect of time being anything except something that we (as humans) need to have to understand things.

    So I guess the point I am getting at is that the definition of time seems to be retrofitted into different models and that the definition of time changes depending on the line of thinking it serves.

    My thought was, what if time is a concept invented by us/maths to help us understand things?

    In newtonian physics we have v = d/t - which is used just about everywhere. Can we define velocity without time? If we can't then that means things need time in order to move according to our models?

    Sorry for the long post, and I will sum it up in a tl;dr; question "Is there such thing as time, or is it just an invention of man?"

    Sorry if this is the wrong place to post this sort of thing.
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  3. Nov 27, 2013 #2


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    Let's talk about Newtonian physics first. Time is a concept invented by man. We define which clocks keep accurate time in a way that makes the laws of physics observationally true - ie. the laws of physics are able to predict what we observe. This is not a definition that is necessarily true, because of the requirement that the laws of physics match observation.

    As we go to special relativity, we see that the Newtonian concept of space and time is not quite right, because it doesn't match observations exactly. So we have new concepts of space and time in special relativity, a theory whose predictions match observation better. It is when speeds are slow that Newtonian physics is an good approximation to special relativity. But because Newtonian physics is only an approximation, we are pretty sure that Newtonian concepts are invented by man.

    Nowadays, we believe that all our theories so far are only approximations, and so all the concepts in these theories are inventions of man - useful inventions, because they help us predict the future.
    Last edited: Nov 28, 2013
  4. Nov 28, 2013 #3


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    When contemplating this question in the past, I've felt that the basis of what we call time just boils down to "change". It doesn't matter what kind of change, only that a difference can be discerned. It's the difference(s) that serve as the milestones we use to identify one time from another.

    If the Sun changed color three times every lunar cycle, or sixteen times a day, that would probably have influenced our concept of weeks and hours.

    If the universe consisted of nothing except an arrow that periodically 'reversed' it's direction' time might simply be defined as 'up' or 'down'...
  5. Nov 28, 2013 #4


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    Do you keep up with physics? You do know how we define a second right?

    To respond to the OP, physics isn't in the business of determining if time is natural or invented, we have a pretty good definition of what we call time and how frame of reference effects it.

    Think about it this way, we don't question whether "length" is man made, we understand it's a unit of measure. Do objects have lengths?

    No, velocity is completely dependent on time.
    Last edited: Nov 28, 2013
  6. Nov 28, 2013 #5


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    A second is currently defined as "the duration of 9 192 631 770 periods of the radiation corresponding to the transition between the two hyperfine levels of the ground state of the cesium 133 atom.". In this case, the transition is the type of change that represents this as 'time'.

    I don't see your point. The OP question was not, "what is the definition of a second?". Perhaps I read too much into it? One of us has misinterpreted the question.
  7. Nov 28, 2013 #6


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    No I think I just read the first line of your response and thought "wtf" to myself and replied. I've reread it and I understand the point you were trying to make.

    My bad.
  8. Nov 28, 2013 #7
    Ironically the difference in the two answers highlights (to some degree) the point I am trying to make.

    I can google the definition of a second though. And by definition what we perceive as time is just how long we wait between an event that takes place in cesium? A second is still an arbitrary thing, and if it has a physical face then it doesn't answer my OP - "How can time freeze on the bad side of an event horizon?"

    What happens if we through some our cesium clock into a black hole? Do we redefine our definition of a second?

    To be clearer, I guess I am asking. Is the concept of time man made, or is it something more tangible that would exist without our perception of it.

    Further more, can we create a model of the universe without 't' - can we express velocity without using time. Seems we just invented time because it is easier on our brains, because time is indeed intuitive. Yet it is still just how we measure how long we have to wait to observer one event after another.
    Last edited: Nov 28, 2013
  9. Nov 28, 2013 #8


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    It most certainly exists without our knowledge of it, as much as joules and watts and kilograms. They're all units of measure that describe some physical property. Physics isn't concerned with the why question, or the secret inner functioning of time per say, that's the realm of philosophy.

    The time freezes thing isn't quite true.

    The second is defined at sea level. :wink: look into gps and the corrections for their clocks, it's a very famous example.

    And no, no model can be created without t, at least not that I am aware of.
  10. Nov 28, 2013 #9
    Thanks very much for the answers. Perhaps Michio Kaku is just confusing me.

    What happens to time in a blackhole if it doesn't "freeze"?

    I worry that this guy's math isn't quite modelling the universe properly - like with what happens when we actually equate things to 0 or infinity - but I guess that is the age old argument of "Is math not sufficient for explaining the universe."

    Why doesn't physics explore the inner workings of time? I thought it did with special relativity and shooting a twin around the earth at near the speed of light.

    Has anyone tried to model the universe outside of Math?

    Sorry if I am abusing your helpful nature with all the questions.
  11. Nov 28, 2013 #10


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    @faddishworm: The direction you're heading is one that I have a keen interest in. If I'm following you, my responses would be considered in the realm of philosophy because there are no published papers or theories that drill deeper into time itself.

    At least I THINK that's the case. Quite often, as I learn more, I find that previous lack of understanding or problems resolving issues are based on misconceptions and erroneous presumptions on my part. Still, I do want to explore time further as I find time. :rolleyes:

    You indicated your important question is, "How can time freeze on the bad side of an event horizon?"

    I'm not certain that's an accepted 'standard' as a statement. Just as c represents a limit, I think time "freezing" is an ideal limit of the same math. Time would freeze at the singularity, which is where math breaks down, infinities come into play, and so on. I know someone will reply soon and point out any problems with what I've written. That's something I've come to enjoy! (I think it's what's referred to as an 'acquired taste'. :smile:
  12. Nov 28, 2013 #11
    Thanks for reply, yes I think I am teetering on the edge of philosophy here.

    Regarding Math breaking down, I think you are right, infinity has never made much sense to me either. Coming from a computer-science background I do know there is a limit to the numbers we can process and the whole "Big Number" thing, and I often wonder if what we model as infinity in mathematics is actually just an unfathomably huge number, and the same with 0 being unfathomably small. This makes sense if you think about a collapsed star's mass being (HUGE) but finite in the first place.

    To draw another parallel between the computer and physics disciplines, people on forums seem to be quite friendly here as well and its all about the sharing of knowledge. The more posts I can get to help me understand the better! Thanks again :)
  13. Nov 28, 2013 #12


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    As it happens, I learned BASIC in 10th grade, completed all graduate CCS at U-Mich as an undergrad, and made IT my career. Good to meet you, faddishworm!

    Regarding infinities: I strive to remember that infinity is indeed a concept at not a value. And there are different types of infinities. Aristotle was the first to distinguish between what he called potential and actual infinities. An example of a potential infinity is the set of positive integers, 1,2,3,... An actual infinity is more like analog information or a function, like a sine wave, where there are an infinite amount of points/values within a finite entity. So in computer science, arithmetic overflow might be related to a potential infinity, while divide by zero is more often associated with a function (A denominator that evaluates to zero) - an artifact of an actual infinity.
  14. Nov 28, 2013 #13


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    I believe time "freezes" at the event horizon, not the singularity. But no worries, an observer watching another observer falling into a black hole would never see them cross the event horizon.

    From here: http://imagine.gsfc.nasa.gov/docs/ask_astro/answers/011024a.html

  15. Nov 30, 2013 #14
    About time freeze, i found an interview of leonard susskind ( topic was black hole wars) on some podcast very useful. You should check that out.
  16. Nov 30, 2013 #15
    Would you agree that distance is not arbitrary? Would you agree that velocity is not arbitrary? What quantity relates these two?

    Time increments are man-made. Time itself is not.
  17. Dec 1, 2013 #16


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    Actually that's a very advanced view of time. It gives the arrow of time ie why its not like the other dimensions in it has only one direction, however it does not tell us what time is.

    Probably the most modern view is its simply a parameter we have in our equations to relate to things such as clocks we know do exist.

    But what it is - well that's another matter - probably best left up to the philosophers unless you can come up with something that actually makes testable predictions.

  18. Dec 1, 2013 #17
    Thanks Bill, I'm glad I'm not completely missing the point.

    To me it seems like the universe is like a big state machine.

    It would be very hard to work out a way to explain the universe without using time, we might even have to abandon Math as a language to convey the model all together.

    Thanks for your insight!
  19. Dec 1, 2013 #18


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