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Does time exist?

  1. May 31, 2012 #1
    Are there 3 dimenionsional objects that exist for a moment and cease to exist everytime they change or are they 4 dimenionsional objects that persists because time exists? To better illustrate imagine a timeline, I today don`t know the answer and I later kow the answer. Opposed to the idea that I am 2 people because of these 2 events. I can here because I want science answer. What does science have to say?
  2. jcsd
  3. May 31, 2012 #2

    Ken G

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    I don't think science has much to say about what exists, that is the job of philosophy (and that's a bit of a rub, because science is designed to be objective and demonstrable, but philosophy has a hard time aspiring to either of those!). What science tells us is under what conditions it behooves us to model time as existing (like a world line in relativity), and under what conditions it behooves us to model time as a parameter or index used to reference an experimental outcome (like in quantum mechanics). Do the successes or failures of either approach tell us if time exists or not? Not in any scientific sense, but it certainly is a good launching point for the philosophical inquiry. In a very real way, science was originally invented by philosophy to accomplish exactly that purpose, but it has since taken on a life of its own with other goals like achieving some mastery over our environment (which it does without ever needing to know, or even caring, whether or not time exists).
  4. May 31, 2012 #3
    I just think of time as a coordinate, and real world objects are three dimensional which move in four dimensions. Think of a shadow moving across a curved surface, the shadow is completely two dimensional, but it can still move in three dimensions.

    Although it's a bad analogy because time as a dimension is different from the spacial dimensions, and honestly I don't think anyone can truly visualize or understand in any satisfactory way what time actually represents physically.
  5. May 31, 2012 #4
    What does behoove mean? I looked it up and can't seem to understand how it fits into the sentences.

    What is a worldline, are you refering to string theory?

    What does "experimental outcome (like in quantum mechanics)" mean?

    Is time just movement so only the present exists and an object is not connected to its past in other words it is 2 different objects? Sorry if I am repeating myself.
    Last edited: May 31, 2012
  6. Jun 1, 2012 #5

    Ken G

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    "Behooves us" means "benefits or befits us to do." I guess it means it gives us hooves or something! You were probably looking at the "it is our duty to do" element, I was stressing more the beneficial element, like it is our duty to do physics in such a way as to benefit humanity.
    A worldline is the track of a particle through spacetime, so it carries the aspect of "existent time" that you were talking about-- the worldline exists at all times and at all the places it visits, it is a set of events that does not need time to "sweep out" in successive order. It is a geometric construct.
    In quantum mechanics, time is not an observable, nor does it sweep out from one moment to the next, it is simply a way to index an observation-- we say the probability of outcome A at time t is p(A,t). So to predict an observation, we reference what the clock reads-- but we don't care if there were any times before or after that, it's just a way of labeling which observation we are trying to predict. Note that is not at all how time is treated in relativity, where it has a geometric meaning that requires all times to exist to define the meaning of the spacetime manifold whose geometric properties are under consideration. Being geometric, time is treated on a similar footing with space, but in quantum mechanics, posiition is an observable and time is a way of indexing what position measurement is being referred to.
    That question is probably deeper than either relativity or quantum mechanics, but those theories do give a different flavor to how they inspire answers to that philosophical question. I would say that QM inspires an answer more along the lines that the object has no continuous existence in time-- we say a particle can go through two slits without having to go through one or the other or even both, it is simply indeterminate which slit it went through. But relativity inspires an answer more along the lines that the object does have a continuous existence at all times along its world line, indeed the whole concept of a "world line" suggests that a particle is a one-dimensional entity, rather than a zero-dimensional entity. String theory takes the dimensionality even farther, but I'm not any kind of expert on that.

    My point is simply that physics is not a monolithic structure that provides answers to questions like that-- it is a body of useful theories, and we choose what theory we want in what situation and for what purpose. Different theories inspire different answers to the various philosophical questions one might hope physics would answer-- and that's exactly why physics doesn't really answer philosophical questions, rather it shows that questions like that (that combine philosophy and science) probably don't have unique answers in the first place. But they are still good questions to ask-- not because they have a single answer, but because their multiple answers are an informative way to understand the various theories of physics.
  7. Jun 1, 2012 #6
    Lets say you have a ball it is eventually going to fall down doesn't that mean that time matters? I don't have time today to read carefully through your post, I got there are 2 main theories, so if this Contradicts than ignore it. So does it make a differnece in the object you use is say a human or a ball or light?
  8. Jun 1, 2012 #7

    Ken G

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    It certainly makes a difference if the object is a human, because then we are doing more than just predicting behavior-- we have perceptions to account for. This complicates the meaning of time a great deal-- because a human has a concept of the experience of time, and that's where the idea that time "marches along" one moment to the next comes in. In quantum mechanics, say the two-slit experiment, we are usually only interested in a particular time-- the time the particle hits the screen. The experiment gives us no information about what happened at earlier times, so it leaves those happenings entirely indeterminate, and there is no need to "tell a story" about what happened to the particle before it hits the screen. But a human would have perceptions all along that path, so any human being could not be used in a two-slit experiment, they are constantly observing their status and drawing inferences and forming memories. So time means something quite a bit different for a human than for a particle. There may also be a difference in the meaning of time for a human, and for a macro object like a boulder, but this is less clear and will get you into tricky philosophical waters.
  9. Jun 1, 2012 #8
    Good answers already, I'll just add...
    >>Are there 3 dimenionsional objects that exist for a moment and cease to exist everytime they change or are they 4 dimenionsional objects that persists because time exists

    Is there a difference in your two views? Is there any conceivable experiment which would show a different answer based on which of these is right?
    If not then it is just a difference of interpretation.
  10. Jun 1, 2012 #9
    If you record the double slit experiment and watch it at different time does it make a difference ? Also are you saying that humans do exist in 4d? What is it about the human mind that causes the double slit experiment? Does something have to be consious? What about an monkey or a dog or a bird or a plant?
  11. Jun 1, 2012 #10
    "What is it about the human mind that causes the double slit experiment?"
    Nothing. That is an over-interpretation of the Copenhagen version of quantum theory
    "Does something have to be conscious?"
    "What about an monkey or a dog or a bird or a plant?"
    The observer just refers to a macroscopic interaction, i.e. with anything big.
  12. Jun 1, 2012 #11
    What about gravitational time dilation, that affects GPS satellites?

    Isn't proof enough that time exists? If it didn't exist, how could it be affected by gravity?
  13. Jun 1, 2012 #12

    Ken G

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    Sure-- if you measure the location of the particle before it encounters the slits, it will change the interference pattern it creates on the other side of the slits.
    Humans have a perception that we need to account for. That perception is of 4d. How we account for that perception is up to the philosophers!
    Experimental outcomes are interpreted in our minds. We don't really know how dogs or birds or plants would interpret such an experiment.
  14. Jun 1, 2012 #13


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    I think this depends highly on what you consider time to be and how you define it. Does time exist? Since a clock measures time just like a ruler measures distance I would say yes. But Ken makes some good points in that depending on how you look at something one definition of time may not apply.
  15. Jun 1, 2012 #14

    Ken G

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    Yes, I would say the first order of business is establishing what criterion you will accept for establishing what "exists." That is a time-honored philosophical question, and has spawned quite a few schools of thought over the years! You could begin your survey by looking up the term "ontology."
  16. Jun 1, 2012 #15


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    Time is already treated at a level on par with space in relativity. In quantum mechanics, there have been some experiments with temporal double slit diffraction, akin to the Young's (spatial) double slit experiment. If you regard space as real, then you should regard time as real also.
  17. Jun 1, 2012 #16
    If gravity can affect how two different clocks measure time, then I think time is a physical entity.

    If time wasn't a physical entity, how could it be affected by gravity?
  18. Jun 1, 2012 #17
    Who has said it isn't?
  19. Jun 2, 2012 #18

    Ken G

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    There's no point trying to decide if time is or is not a physical entity until one offers their definition of what a "physical entity" needs to be. For example, I can easily give a reason that we should think of time as a physical entity (two observers moving the same way between two events will measure the same result for the elapsed time), and I can easily give a reason why we should not think of it that way (two observers moving differently between two events will measure different results for the elapsed time). We could argue that the "physically existent" entity is actually (ct)2-x2, because that is the invariant quantity that always comes out the same, yet that is not something that we ever actually measure by itself! So the issue is far from clear. What exists-- what we measure, yet depends on us, or what does not depend on us, but we do not measure?
  20. Jun 6, 2012 #19
    Physics talks about time slowing down when reaching the speed of light and time slowing down when approaching a black hole and time stopping upon entering a black hole and the theoretical possiblility of going back or forward in time and also about the Arrow of Time.

    I think time is more of a relative measurement of events. The number of Sunrises, the point miday when the sun is near its apex. The number of heartbeats in a year, the vibrations of an atomic clock. As such, if you are properly shielded and nearing a black hole or nearing the speed of light would your heart slow to a point that it nearly stops, I don't think so. If, again with proper shielding, when you entered a black hole would time stop. If it does then you couldn't actually enter the black hole, you would forever be in some never land on no time. What would your heart do in that situation.

    It is said that time did not exist before the big bang. We don't know that. It certainly began for what we call our Universe, but what if something did start the big bang, then their concept of time would be the time before the big bang and the time after the big bang.

    If time is a measurements of events, then you cannot go backwards in time because those events have already happened and you cannot go forward because those events have not happened yet, therefore the Arrow of Time.

    What are your thoughts?
  21. Jul 2, 2012 #20
    Does the present exist for a human and it is different person or does the past, present and future all exists for a human and it is the same person overtime?

    I think according to relativity and einstein they all exist, correct?

    According to quantum mechanics only the present exist, correct?

    Since humans are composed of electrons does that mean that weird quantum effects can take place like particle entaglement and that a human could have effects of the double slit experiment inside the body? If that is true would this affect persistence of an individual?
    Last edited: Jul 2, 2012
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