What is the intuition behind time being a physical dimension? I read the phrase "in cosmology terms, far away means long ago" somewhere and I got to thinking what if time is an emergent property of the interaction between matter and gravity. How can we think of time outside of space and matter? Or does it not need to be? Or am I just confusing matter and physical dimensions?

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    "Spacetime" per Minkowski. Check out "Raum und Zeit" ("Space and Time") – Mr. Kennedy Mar 11 '17 at 20:04
  • @Mr.Kennedy just finished reading the space and time chapter from the link. What part of the Mathematics of it all, prohibits the existence of t < 0? And if it doesn't, isn't the mathematical approach ill suited for our intuitive thinking of time (where we can't wrap our heads around t < 0)? First let me know if my question makes logical sense, and then go ahead with any thoughts on it. Because I'm not sure I totally understood minkowski after the past light cone and the future light Cone part. No prior background in advanced geometry – Ravi Shankar Mar 12 '17 at 7:50
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    This secondary source might aid in your understanding of Minkowski spacetime – Mr. Kennedy Mar 12 '17 at 7:57
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    Wow. Thanks. That clarifies a lot. Atleast as a foundation for understanding relativity – Ravi Shankar Mar 12 '17 at 8:20

The intuition behind time being a physical dimension is in the experience of being able to "travel" through it. However, in daily life we only seem to be able to move in one direction and at one speed. In theory, we enter the realm of traveling through time forward, backward, and at different speeds. Thus it has traits of physical dimensions. The phrase, "in cosmology terms, far away means long ago", can at least refer to what we see. Because light takes a specific amount of time to reach us, we are actually seeing an image of something which existed in the past. The farther away it appears to be, the older the image which we see. One way we can think of time outside of space and matter is by thinking of space and matter being inside time. Time does not necessarily need space or matter to exist. If space and matter exist, they do so for a length of time. Matter exists in the physical dimensions. The physical dimensions describe matter. Thus, there are two uses of the term "physical dimensions". The first is the name of a place, the second is a measurement.

  • Welcome to philosophySE! Good answer :) Do we know that time does not necessarily need space or matter to exist? Are space and time indistinguishable from spacetime (and this no matter how many dimensional aspects we may describe them with)? Per Max Tegmark, time without space is at the very least completely unpredictable. – Mr. Kennedy Mar 11 '17 at 21:32
  • Thank you! Conceptually, time could exist by itself. Time would only need itself to exist because existence occurs for an amount of time. Already a new concept, existence, is introduced. If there were matter in existence, which then ceased to exist for a certain amount of time, then new matter came into existence, time would exist by itself during this period. It may very well be that time is simply a unit of measure of the occurrence and existence of things, rather than an entity unto itself. In trying to stay within philosophy, I can't say much more without going into physics. – takintoolong Mar 11 '17 at 22:07
  • Certainly epistemic considerations of time are inadequate as Minkowski demonstrates with spacetime, and ontologically it is conceivable that time could exist independently of time, however, how would we know that we knew that time existed so? As for physics - use whichever tools are adequate to the occasion of resolving your query! :) If you haven't, do read Minkowski's "Raum und Zeit" - he's great! – Mr. Kennedy Mar 11 '17 at 22:20
  • @takintoolong Can time not be thought of as the rate of change of existence? Because we can calculate the age of the universe only due to the "changes" (expansion of space) that we observed. If the universe was static (nothing ever changed) , so as to speak, would time be an entity then? – Ravi Shankar Mar 12 '17 at 4:37
  • Mr. Kennedy and Ravi Shankar, you are both assuming our own existence. For us to "know", "calculate", or "observe", we must exist for some period of time. I think time is a measurement of change. I think the universe always has been changing, always is changing, and always will change. – takintoolong Mar 12 '17 at 8:18

There are various theories whereby spacetime (the whole ball of wax) is emergent, e.g., https://arxiv.org/abs/1504.00464 Reading between the lines of your question, I'd guess the best way for you to think about it intuitively is as follows. What's more fundamental: "objects" or "events"? To best interpret time as just another dimension, instantaneous "events" would be the better answer. Then, say, "eating breakfast" and "eating lunch" are two events, separated not only by the distance between the two restaurants you ate at, but by the several hours difference when you ate the meals. In this kind of view, "objects" are just highly-correlated (and kind-of-continuous) sequences of events.

  • From my understanding of your answer, events are "objects" +"that moment in time". But objects can be percieved independently of all "monents of time". Can "a moment in time" be perceived independent of all objects existing "then"? Or is that just a limitation of my thought? – Ravi Shankar Mar 12 '17 at 7:56
  • @RaviShankar Your remark, "objects can be percieved independently of all 'monents of time'" would be wrong under this interpretation. See, e.g., "Events as Fundamental Entities in Physics", link.springer.com/article/10.1007/BF02724247 (though, sorry, I'm not seeing a free pdf offhand). And, no, the observation of an event >>defines<< a "moment in time", and objects pretty much correspond to sequences of events (though the word "sequence" itself pretty much implies a pre-existing/pre-defined time notion, so I'm playing too fast-and-loose with ideas that require a paper-length discussion) – John Forkosh Mar 12 '17 at 11:26
  • @RaviShankar re-reading your comment, your remark "events are 'objects' + 'that moment in time'" seems to suggest you're thinking of objects as fundamental. In this interpretation, that's backwards. Events are fundamental, objects are "just in your head", essentially emergent, a convenient way of organizing events so your mind/brain can make sense of them as simply as possible. Of course, this is only possible because nature, in the classical regime, exhibits events that can be organized as "highly correlated 'sequences'" in the first place. – John Forkosh Mar 12 '17 at 11:41
  • So, what would the length of a moment in time be? – takintoolong Mar 13 '17 at 0:58
  • @takintoolong Seriously? No such thing. And that's regardless of whether or not we're considering (space)time as "emergent". Typically, given two events, say e1 and e2, there'd be a time t(e1,e2) between them which, in most interpretations, would be observer-dependent (an invariant observer-independent interval between events involves both space and time). But "length of a moment" is meaningless, at least to me. – John Forkosh Mar 13 '17 at 9:14

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