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Thread: Space and time

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    Space and time

    Whenever I start thinking about length contraction and time dilation, at first I think I 'kinda' get it. Then, when I try to think about it in more detail I start noticing how I get it less than I did when I first started thinking about it. Also, I noticed that any deeper thinking about relativity leads me to fundamental questions: what are space and time? I mean, if I don't know precisely what those are, how can I even talk about relativity, or anything actually... So my question is, what is space and why does it make sense to talk about distances? What is the distance between two points in space? Is it simply what we read off of a meter stick, or is there more 'real' meaning? Likewise, what is time and why does it make sense to talk about time intervals? What is the time interval between two events? Is it simply what we read off of a clock, or is there more 'real' meaning? What is an event?
    Observing from the top of the Motion Mountain.

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    There is no space and time. It is spacetime.

    Remember the Pythagorean formula for right angle triangles? a^2 + b^2 = c^2

    Well, if you have a triangle in 3D, it's a^2 + b^2 + c^2 = d^2

    Well, if you have spacetime, you get, x^2 + y^2 + z^2 - (ct)^2 = ds^2

    Space and time work like a triangle in 4D dimensions. Time is an inseparable 4th dimension.

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    If time is an inseparable 4th dimension, why do clocks exist and why do physicists use clocks to measure time intervals?
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    Because, like Newtonian Mechanics for gravity, for most purposes they wotk just fine. At relativistic velocities or intense gravitational fields or precision GPS, not so good.

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    Quote Originally Posted by Matej Velko View Post
    If time is an inseparable 4th dimension, why do clocks exist and why do physicists use clocks to measure time intervals?
    I don't know what to tell you. I can't figure out where you're coming from.

    Do you want to know about the fringe limits of the theory of special relativity? Or do you want to know why we can safely ignore these limits in our daily lives? That second one is going to require some math.

    Here's what you do. You calculate the contracted length of something according to special relativity (call it a). Now you've got the uncontracted length (call it b). Do ((b-a)/b) * 100. That is the percentage difference from using special relativity versus galillean relativity. Try it for something like a car, a plane, a spaceship. What you'll find is that you've wasted your time and it's only above 10% the speed of light that you should bother with these things.

    How fast does the electron in hydrogen travel? Are you not surrounded by hydrogen(water)? Calculate the contracted circumference of the orbit. Would you use special relativity to calculate electronic hydrogen energy levels?

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    I don't know what fringe limits are and I understand the physics of length contraction and time dilation and their domain of application, my question is more philosophical than scientific, I'm just struggling with understanding fundamental notions of space and time.
    Observing from the top of the Motion Mountain.

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    Quote Originally Posted by Matej Velko View Post
    If time is an inseparable 4th dimension, why do clocks exist and why do physicists use clocks to measure time intervals?
    Yes, it is quite untrue to claim that just because space and time merge into a kind of manifold with various invariant properties, this means time and space are somehow "inseparable." Time and space are different, they have to be because otherwise there could be no invariant boundary between "timelike separated" and "spacelike separated" events. This is an irony in how relativity is often explained-- often it is claimed that having a universal speed limit (c) means that time and space are the same, because any space interval x can be turned into a time interval x/c (such as the concept of a "light year"). But actually, it's not that simple. If c were infinite (so we have Galilean relativity), then all events are timelike separated, and no events are spacelike separated-- any two events could be thought to have occurred at the same place by some possible observer. So Gailean relativity certainly enforces a huge difference between time and space-- everything that is invariant is timelike, and all observers agree on the time intervals involved. Einstein's relativity, on the other hand, creates a whole new category: spacelike separation. Now, for the first time, we have events that no observers could ever think happened at the same place, and no observers could ever think that one of those events could have been either a cause or an effect of the other. So relativity is not just about merging space and time, it is about rending spacetime apart, and creating a relationship between some events that we never even knew existed-- events that simply could never have any kind of direct bearing on each other. It is like opening up a category called "invariant spacelike separation", and pouring a bunch of events into that open hole. In that sense, it is a tearing open of time to put space in, but it is not saying time and space are the same thing-- causation only connects timelike separated events, and that's a difference.

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    Quote Originally Posted by Matej Velko View Post
    I don't know what fringe limits are and I understand the physics of length contraction and time dilation and their domain of application, my question is more philosophical than scientific, I'm just struggling with understanding fundamental notions of space and time.
    Everyone struggles with the philosophy of time. We don't even have a decent definition of time. Anyone who claims to understand time is lying.

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    Quote Originally Posted by ShinAce View Post
    Everyone struggles with the philosophy of time. We don't even have a decent definition of time. Anyone who claims to understand time is lying.
    At the time.

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    Quote Originally Posted by ShinAce View Post
    Everyone struggles with the philosophy of time. We don't even have a decent definition of time. Anyone who claims to understand time is lying.
    I don't struggle with the "philosophy" of time because I couldn't care less about it. To me, time is a self-evident concept by which we evaluate how fast or slowly something changes. The fact that I cannot come up with a literal definition of time without getting into circular reasoning doesn't stand in the way of my understanding it. That is, unless I am missing a critical point about what "understanding" means. Likewise with points, lines and planes.

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    Quote Originally Posted by Hornblower View Post
    I don't struggle with the "philosophy" of time because I couldn't care less about it. To me, time is a self-evident concept by which we evaluate how fast or slowly something changes. The fact that I cannot come up with a literal definition of time without getting into circular reasoning doesn't stand in the way of my understanding it. That is, unless I am missing a critical point about what "understanding" means. Likewise with points, lines and planes.
    I think time is not self-evident at all, if it were self-evident, it would be so to everyone. Time as a concept by which we evaluate how fast or slowly something changes makes sense to me and it is certainly useful to think that way about time. But why settle for ''understanding'' when we cannot even define it plausibly? The lack of plausible definition should make us question our understanding.
    Observing from the top of the Motion Mountain.

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    Quote Originally Posted by Matej Velko View Post
    deeper thinking about relativity leads me to fundamental questions: what are space and time? I mean, if I don't know precisely what those are, how can I even talk about relativity, or anything actually... So my question is, what is space and why does it make sense to talk about distances? What is the distance between two points in space? Is it simply what we read off of a meter stick, or is there more 'real' meaning? Likewise, what is time and why does it make sense to talk about time intervals? What is the time interval between two events? Is it simply what we read off of a clock, or is there more 'real' meaning? What is an event?
    What science can say about the nature of space and time is a different question from what you call the ‘real’ meaning of these terms. The primary discussion on this board is about what science can say based on empirical observation. Your term “real meaning” is a philosophical concept. For example, it can be argued that meaning is a human construct based on our personal or social perspective, not something inherent in matter.

    With time, the ‘real meaning’ given by the clock and calendar is rooted in our evolution as organisms within very long term stable cyclic patterns such as the day, the month, the year, and perhaps even the long slow orbital patterns that drive climate. The “real meaning” of time for us is defined by practical needs and effects, for which the objective findings of relativity are relevant but only distantly. Real meaning can even be psychological, with the speed of time seeming to vary according to our moods and situation.

    The famous existential philosopher Martin Heidegger argued in his book Being and Time (1926) that understanding the real meaning of space and time must occur within the perspective of human being in the world. He argued this means that care is the meaning of being, and our constructed views about importance and value, what we care about, actually determine our sense of meaning. The objective data measurement and laws of science provide essential input to human construction of world, but real meaning is as much about relations, perspectives and values as about facts.

    I mention this to illustrate that a call for a purely scientific meaning of space and time requires some agreed definition of meaning which is likely to depart from the usual sense.

    Ken G has discussed how previous philosophies such as Kant were wrong to argue that space and time provide the absolute a priori necessary conditions of experience based on Newton’s assumption that matter obeys the axioms of Euclidean geometry.

    Relativity has relativized our understanding. I fear that the philosophy of space and time still has some way to go to explain our seemingly earth-centred perspective on these fundamental realities against the scientific reality of the modern discovery of the nature of the universe. Our terrestrial units of temporal measurement solely apply on earth and are not relevant to any other cycles in the universe.

    Saint Augustine famously said that he understood time until he started to think about it. “What then is time? If no one asks me, I know what it is. If I wish to explain it to him who asks, I do not know.”

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    Quote Originally Posted by Matej Velko View Post
    I think time is not self-evident at all, if it were self-evident, it would be so to everyone. Time as a concept by which we evaluate how fast or slowly something changes makes sense to me and it is certainly useful to think that way about time. But why settle for ''understanding'' when we cannot even define it plausibly? The lack of plausible definition should make us question our understanding.
    There are three things we use often and yet I feel we might never understand. They are: time, energy, and information(in the physical sense).

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    Quote Originally Posted by Matej Velko View Post
    I think time is not self-evident at all, if it were self-evident, it would be so to everyone. Time as a concept by which we evaluate how fast or slowly something changes makes sense to me and it is certainly useful to think that way about time. But why settle for ''understanding'' when we cannot even define it plausibly? The lack of plausible definition should make us question our understanding.
    My educated hunch is that some of us are overthinking the concept of time itself, as opposed to phenomena we evaluate in terms of it. I still have no grasp about what it is about time, if anything, that you think we don't understand.

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    Quote Originally Posted by Hornblower View Post
    My educated hunch is that some of us are overthinking the concept of time itself, as opposed to phenomena we evaluate in terms of it. I still have no grasp about what it is about time, if anything, that you think we don't understand.
    You may be right... I think I may be overthinking it.. but then again if I leave time simply be a useful concept, I get a feeling like a might miss a great realization about time that would help me think more clearly about its properties. But I don't think that realization is likely, if possible, so I'll just leave it be
    Observing from the top of the Motion Mountain.

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    Quote Originally Posted by Hornblower View Post
    My educated hunch is that some of us are overthinking the concept of time itself, as opposed to phenomena we evaluate in terms of it. I still have no grasp about what it is about time, if anything, that you think we don't understand.
    Yet, it is already a brand of personal philosophy to conclude that all questions that are not understood are therefore of no importance.

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    Quote Originally Posted by Matej Velko View Post
    You may be right... I think I may be overthinking it.. but then again if I leave time simply be a useful concept, I get a feeling like a might miss a great realization about time that would help me think more clearly about its properties. But I don't think that realization is likely, if possible, so I'll just leave it be
    I would say that when a problem seems intractable, it needs to be broken down into smaller problems. If you cannot leap a stream, look for rocks in the stream that are only partially across. For example, on the issue of time, you might find it a more focused issue to ask, is it the passing of time that allows us to think, or is it our thinking that gives us a concept of the passing of time?

    If you want my take on that one, I'd point to the fact that even a young child easily arrives at a concept of time, and uses that concept to organize their world. Yet to go beyond that basic functional approach, even the greatest physicists and philosophers struggle. So either time is a really simple concept that we are "overthinking" to try to get any further on, or it is so fundamental to how we think that we would need to understand ourselves better to make headway on. Perhaps we can only get so far when we regard time as something external to ourselves, and understanding ourselves is always the hardest thing. A telescope can resolve craters on the Moon and rings around Saturn, without the slightest concept of itself or how it does it.
    Last edited by Ken G; 2016-May-10 at 02:10 PM.

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    Quote Originally Posted by Ken G View Post
    I would say that when a problem seems intractable, it needs to be broken down into smaller problems. If you cannot leap a stream, look for rocks in the stream that are only partially across. For example, on the issue of time, you might find it a more focused issue to ask, is it the passing of time that allows us to think, or is it our thinking that gives us a concept of the passing of time?

    If you want my take on that one, I'd point to the fact that even a young child easily arrives at a concept of time, and uses that concept to organize their world. Yet to go beyond that basic functional approach, even the greatest physicists and philosophers struggle. So either time is a really simple concept that we are "overthinking" to try to get any further on, or it is so fundamental to how we think that we would need to understand ourselves better to make headway on. Perhaps we can only get so far when we regard time as something external to ourselves, and understanding ourselves is always the hardest thing. A telescope can resolve craters on the Moon and rings around Saturn, without the slightest concept of itself or how it does it.
    Thank you Ken G. I think it is our thinking that gives us a concept of the passing of time as well because everything that really exists is the present moment (in the everyday life at least) and past and future exist only in our minds.
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    To me, space seems to be self replicating. It is expanding, getting bigger and bigger, so, it is either "stretching" or self replicating, making more of itself.
    Far away is close at hand in images of elsewhere...

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    Quote Originally Posted by Matej Velko View Post
    Thank you Ken G. I think it is our thinking that gives us a concept of the passing of time as well because everything that really exists is the present moment (in the everyday life at least) and past and future exist only in our minds.
    Our minds certainly play a key role in saying what has existed and what will exist, but it also plays a key role in saying what exists in the present as well! But I agree that the idea that "time passes" is one of the more obvious of our mental constructions. Einstein had something interesting to say along those lines:
    "...for us physicists believe the separation between past, present, and future is only an illusion, although a convincing one."
    One might wish to nitpick that when an illusion is sufficiently convincing, we don't call it an illusion any more, but I believe I understand what Einstein was getting at here-- though who knows the full measure of his point.
    Last edited by Ken G; 2016-May-10 at 09:21 PM.

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    If time stops for a photon, does time even exist for a photon?

    Does it take a photon any amount of time to be reflected or absorbed and re-emitted?

    Will the CMB eventually be zero?
    Last edited by DALeffler; 2016-May-10 at 11:28 PM. Reason: added CMB ?

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    Quote Originally Posted by Ken G View Post
    ...Einstein had something interesting to say along those lines:
    "...for us physicists believe the separation between past, present, and future is only an illusion, although a convincing one."
    This idea implies that the past, present and future are equally real, opening the problem of how space-time could be considered an unchanging eternal unity. The various meanings of eternity include outside time, lasting forever within time, and having permanent value. An indigenous American saying is that you are surrounded by eternity.

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    Quote Originally Posted by DALeffler View Post
    If time stops for a photon, does time even exist for a photon?
    It's a singular limit, so it depends on how you take that limit. I would be inclined to imagine the state of a photon by taking a limit of faster and faster particles with smaller and smaller rest masses. After all, we don't even know that a photon rest mass is exactly zero. If you do that, you find something interesting-- time progresses for every hypothetical particle in that limiting series in exactly the same way it does for us. It's just proper time, it works the same for everyone at any speed. So by this reasoning, we should not say time stops for a photon, we should say time is completely normal for a photon-- but if the photon is created and later destroyed in some finite time from our point of view, it's life will be very short indeed from it's own point of view. It's not that time stops, it's that things happen very quickly! But a photon that is never reabsorbed, so exists for infinite time in our frame, can be seen by that limiting process I just described to also exist for an infinite time in its own frame.
    Does it take a photon any amount of time to be reflected or absorbed and re-emitted?
    This is the key point. For photons, all the lengths that we regard as finite are length contracted into very short (or zero) distances for the photon. So it takes essentially no time in its own frame to cross these distances-- things happen fast for photons.
    Will the CMB eventually be zero?
    It will redshift more and more, becoming effectively zero when we cannot detect a single photon from it any more, but there will still always be a nonzero probability of detecting a CMB photon in any finite timeframe. That said, I imagine that alien astronomers 100 billion years from now will have a very hard time indeed knowing that there is any CMB at all.

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    Quote Originally Posted by Robert Tulip View Post
    This idea implies that the past, present and future are equally real, opening the problem of how space-time could be considered an unchanging eternal unity.
    Yes, relativity inspires the concept of a "world line" that is in some sense "made of" a series of events, all equally important or real. The proper time along the world line is simply a kind of labeling system for ordering the events, but the event being perceived in the "now" is conveyed no special importance, nor any non-arbitrary way to match it up as being "simultaneous" with the events on other people's world lines except at the places where they cross. All that is invariant is the set of events that can affect my "now", and the set of events I can affect by what I do "now." In effect, if I claim that I "only exist in the now", it is like saying that all the other moments of my life were a different person, as different from my personal existence in the "now" as that of Abraham Lincoln. Alternatively, if I wish to maintain that all those "nows" are in some sense "also me", then it makes no sense to claim I exist only in the present.
    An indigenous American saying is that you are surrounded by eternity.
    The ultimate total perspective vortex!
    Last edited by Ken G; 2016-May-11 at 02:37 PM.

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    And, since the Universe appears to be without bound and uniform in all directions, without a center, and each one of us has our own view of the Universe independent of others, each of us can consider ourselves to be the center of the universe for our own purposes.

    As Feynman said, "What do you care what other people think?"
    I'm not a hardnosed mainstreamer; I just like the observations, theories, predictions, and results to match.

    "Mainstream isnít a faith system. It is a verified body of work that must be taken into account if you wish to add to that body of work, or if you want to change the conclusions of that body of work." - korjik

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    Quote Originally Posted by Ken G View Post
    Yes, relativity inspires the concept of a "world line" that is in some sense "made of" a series of events, all equally important or real. The proper time along the world line is simply a kind of labeling system for ordering the events, but the event being perceived in the "now" is conveyed no special importance, nor any non-arbitrary way to match it up as being "simultaneous" with the events on other people's world lines except at the places where they cross. All that is invariant is the set of events that can affect my "now", and the set of events I can affect by what I do "now." In effect, if I claim that I "only exist in the now", it is like saying that all the other moments of my life were a different person, as different from my personal existence in the "now" as that of Abraham Lincoln. Alternatively, if I wish to maintain that all those "nows" are in some sense "also me", then it makes no sense to claim I exist only in the present. The ultimate total perspective vortex!
    All the other moments of my life are a different person. What do you have in common with ''yourself'' 10 years ago? Only memories, and those are just constructs of our minds.
    Observing from the top of the Motion Mountain.

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    Quote Originally Posted by Matej Velko View Post
    All the other moments of my life are a different person. What do you have in common with ''yourself'' 10 years ago? Only memories, and those are just constructs of our minds.
    And, legal obligations!

    Personhood is fairly well established construct. You might need to coin a new phrase to describe the distinction you're making

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    For those who enjoy science fiction scenarios, it is fun (and potentially interesting) to devise situations where the concept of personhood is much more strained than the one we normally find applicable. Concepts like cloning or teleporter malfunctions can raise interesting issues that suggest personhood is more of a state of being than a claim on a stream of events. But then, we don't have clones or transporters in this day and age, so the question is currently more moot.

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    Quote Originally Posted by grapes View Post
    And, legal obligations!

    Personhood is fairly well established construct. You might need to coin a new phrase to describe the distinction you're making
    I agree it is fairly well established and it is because the great majority of people have established it that way. That most certainly does not mean they're right. I'm not saying I know for certain I'm right, but it seems fairly conclusive to me that personhood is an illusion, not having a way to objectively test this. Personhood is established in a way that people identify themselves with their thoughts. However, thoughts are only objects of one's consciousness and do not make up a person. I have not yet realized that personhood is an illusion, but the reason is I still lack needed concentration to do so. I know it may seem a little unscientific, but I think it will not be for long once neuroscience has developed more.
    Observing from the top of the Motion Mountain.

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    Quote Originally Posted by Matej Velko
    I think time is not self-evident at all, if it were self-evident, it would be so to everyone.
    Perhaps it is self-evident to everyone who does not overthink it.
    Time as a concept by which we evaluate how fast or slowly something changes makes sense to me and it is certainly useful to think that way about time.
    That is a key point. It is useful for a wide range of purposes ranging from managing your everyday life to cutting-edge exercises in physics.
    But why settle for ''understanding'' when we cannot even define it plausibly?
    Because that lack of a definition that would satisfy your standard of plausibility does not make it less useful for the aforementioned purposes.
    The lack of plausible definition should make us question our understanding.
    All it makes me do is recognize some limitations of linguistic logic (alliteration intended). We develop an instinctive and useful grasp of time at a very young age, before learning the linguistic fine points of defining terms.
    Last edited by Hornblower; 2016-May-15 at 01:22 AM. Reason: Fix syntax lapses

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