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What is time ?

  1. Sep 20, 2007 #1
    What is "time"?

    Hey All,
    I have 2 questions about time.
    First, when someone says that time is the fourth dimension, are they talking about plotting on 4 planes at once, instead of 3 or less? I'm just starting precalculus in college, just so you know. I understand how a point can be plotted with 3 planes at the same time, like x y and z coordinates. So, is time just a 4th plane? Or are they talking about a physical dimension that I can't see?

    2nd question. I was thinking about time travel- how I could go back in time and get rich,etc. Just as a daydream.
    But then it dawned on me, what the expletive is time? Is it just a concept, or a real physical thing? I'm not even sure what questions to ask. Lol.

    Whatever anyone has to say about time, so that I can undertand it a little bit more, all is greatly appreciated.
  2. jcsd
  3. Sep 20, 2007 #2


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    I find this a very slippery subject, so I'll try to explain a bit; but I might say something stupid or unintentionally start a discussion... my apologies in advance.

    It's very hard to imagine a fourth dimension, as we have always thought in three dimensions, grew up in them, live in them and see them around us all day. Think about an ant living on a piece of paper, or the surface of a ball. Though this is obviously a thee-dimensional situation, the ant is constrained to a two-dimensional surface. We can go anywhere on that surface, but he will never be able (assuming he can't jump) to go into the third dimension. Probably, he will not even be able to imagine it.

    Your post makes me think you don't really know what a dimension is, so let me give it a try...

    First consider a line. That's a one-dimensional object. If I want to uniquely define a point on my line, I only need one coordinate (that is, after agreeing with you on an origin and orientation of the x-axis, I can call out a single number and we will agree on which point I meant).
    Now take a plane. It's two dimensional. We need two coordinates to fix a point uniquely (for example, an x and y coordinate). We can use more, but then there will be some redundant information (I'm assuming the x and y axis are orthogonal to keep things simple).
    Note that I can fill out a plane, by drawing a line (one dimensional object) in it, and at each point of that line draw a line (one dimensional object) perpendicular to it.

    Now the three dimensions in which we live. Three dimensional means, as you will have guessed by now, we need three coordinates to fix a point, e.g. (x, y, z). We can fill out a 3-d space by drawing a line (1d-object, for example, the z-axis) and at each point of that line make a plane (2d object).

    Now on to four dimensions, and for simplicity, take time to be the fourth dimension. We will need four coordinates, the time t and three space coordinates. If you will, fill out the four-dimensional space by drawing a line through it (a "time" axis), and at each point on that line stick a three dimensional space.

    In modern physical theories, space and time are united in a sort of four-dimensional thing called spacetime, but still time is a bit odd. Most notably, because while in space we can move anywhere we like, we seem to be stuck in a constant speed in the "forward" direction in the time. I think it is generally agreed upon, since the theory of special relativity has been widely accepted, that time travel is not possible. By way of thought experiment you could try to think about what would happen if you could, but you would get all kinds of paradoxes. For example, causality would be violated (most famous example: if you kill your mother before you were born, you would never have been born, therefore; you would never have been able to travel back in time when you were older to kill your mother, so she would have lived... and you would have been born to go back in time to kill her, etc.).

    Actually, it's hard to tell what time is. It's a fundamental quantity that can be viewed it as part of spacetime (just as a plane or a line would be a part of 3D space). Actually time just exists and all we can do is measure it (by making clocks) and experience it (even if I stay at exactly the spot where I am now, the world will change and I will age and I will never be typing this exact text again, because I have already done it in the past :smile:)
  4. Sep 20, 2007 #3
    I was under the impression GTR allows for time-travel, hence the emergence of objects like the Einstein-Rosenberg bridge.
  5. Sep 20, 2007 #4
    Thanks for the reply. You're right about my confusion about understanding dimensions. I'm going to ask one of my teachers to explain dimensions to me on a blackboard. Is this a problem that I should ask a physics teacher to explain, or a math teacher? Is there a point in math that I'd learn this, or in physics? At what point would that be?
  6. Sep 20, 2007 #5


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    Whether you should ask your question to your math or your physics teacher depends on whether you want a mathematical or a physical dimension. I'd suggest your math teacher, because you first have to get the mathematical concept right, before you try to apply it to physics.

    In mathematics, a "dimension" (usually with a real-number basis) means grossly "one of the numbers you need to describe a point in a set". For a point on a line, you need 1 number, for a point in a plane, you need 2 numbers, and for a point in space, you need 3 numbers. That's why we say that space is 3-dimensional, because with 3 numbers, you can pinpoint any specific point in space (if you've defined a reference system that allows you to "execute" these numbers and "walk to the point", such as an x,y and z axis together with means to measure and to displace in parallel ways).

    Now, "an appointment" needs "a point in space" (3 numbers) and a "point in time" (1 number). As such, one can consider the "set of all appointments" to be a 4-dimensional set. Example: 4th floor, 2nd street, building 15, at 15hr tomorrow.

    Einstein called "the set of all appointments", spacetime. And he called "appointments" actually "events". As such, spacetime is a 4-dimensional set, because its basic "points" which are events or appointments, are to be described with 4 real numbers.
  7. Sep 21, 2007 #6
    The simplest depiction of spacetime is the Minkowski cone, which traces the equation x2+y2+z2=(ct)2, where c is the speed of light. Relative to the origin, objects inside the cones (timelike) are massive and move at less than the speed of light, objects on the cones (lightlike) are massless and move at c, objects outside the cones (spacelike) would move faster than c.
  8. Sep 21, 2007 #7
    For me, to make 'things' easier when they don't need to be 'complex', remember that time can be replaced as a 'parameter' that can be graphed as 'y'.

    There's a whole bunch of definitions of 'time' (in physics, and the vernacular)--I think maybe to try to use the 'definition' of 'time' that is specific for each area that you go into-----just like the word "work".
  9. Sep 21, 2007 #8
    It's important to understand that physics never says "what" something is, but only what happens in interactions with it. If you're a philosophy student, you might call it an extreme form of structuralism. Therefore, defining time physically can only be reduced to a list of its properties. However, whether the properties usually listed are actually describing the same "thing" is left open. In particular, a lot confusion in special relativity (and even worse in general relativity) is down to the fact that what people thought of as one thing gets fragmented into multiple things. For example, we might characterise time as something that always increases, we can measure through a clock, and something against which we can do dynamics. However, there's nothing that says these three properties are describing the same thing. In special relativity, the latter 2 are forced apart. In a microscopic statistical state, the first 1 isn't necessarily true (microscopic clocks tend to run backwards sometimes). Further fragmentation is possible. Always remember that physics deals in *models*. Concepts are grounded in particular models, and are not valid outside of them. However, truly clever physics occur when we do try and push ideas outside of their established boundaries: that's how we usually come up with new models, with more refined concepts.
  10. Sep 21, 2007 #9
    OK--so a physicist now "never says "what" something is", and they don't ask (or not supposed to) "why something happens"?----I can't wait for the next statement of what "physicists never (fill in the blank)"

    aren't these types of "never/don't" statements more for applied physicists?----(it seems like when talking 'theory' (physics, etc.) you should be able to structure a sentence in just about any form--to see where it may lead).
  11. Sep 21, 2007 #10
    Any answer which says "this is what it is" can't be tested. It's only the "what does it do" questions which can be tested. Yes, it is a bit extreme as far as positivism goes, but quite often people who aren't physicists have this view that we can actually answer everything in a sort of oracle manner. Confusion spreads quickly when, like I said, we extend the concepts from one model to another.

    However, we all shorthand and say "blah blah *is* blah blah". Usually, we assume that we're talking to an audience who will understand the context.

    Theorists, even more than applied physicists, have to be careful about the range of validity of concepts. A quick look at the Ehrenfest paradox in SR shows that even the best physicists of the day will make a bucket load of hidden assumptions that invalidate the reasoning.
  12. Sep 21, 2007 #11
    hmm--I don't know how to respond--maybe to say that if you read any discussion from just about anyone (on any level) on the concepts and 'what's going on in/around/near/?' a black hole, if you used your parameters, there may be only hand waving involved and very few words spoken.
  13. Sep 21, 2007 #12
    how does quantum mechanics define/describe time?

    EDIT: Found the answer I was looking for: Time quanta is a hypothetical concept. In the modern quantum theory (the Standard Model of particle physics) and in general relativity time is not quantized.
    Last edited: Sep 21, 2007
  14. Sep 22, 2007 #13


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    In quantum mechanics time is not really tied to space as it is in relativity.
    So while position and momentum are operators (with eigenkets and their own set of commutation relations and the whole formal mathematical bunch), time is merely just a label with no specific meaning.
    (Of course, in any theory unifying QM and Rel. this would have to be different I suppose.)
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