# Time as a physical dimension

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?

• "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
• This secondary source might aid in your understanding of Minkowski spacetime – Mr. Kennedy Mar 12 '17 at 7:57
• 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) – user19423 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. – user19423 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. – user19423 Mar 13 '17 at 9:14

19th Century proponents of Time as a fourth physical dimension included Gustav Fechner (under the pseudonym of Dr. Wises) and C.H. Hinton. The idea was popularised by H.G. Wells in his 1895 novel The Time Machine. Hermann Minkowski and Albert Einstein soon gave it its modern formulation. It is motivated by the idea of a "block Universe", of a mathematical description of spacetime spread out like a four-dimensional map on which we can trace our timelines, in much the same way we can trace a journey on an ordinary map.

Of course, the Minkowski metric of Time's imaginary multiplier and Einstein's shape-shifting relativistic coordinates provide a rather more sophisticated model of the block universe, but from the perspective of Time as a fourth dimension, that is all they do.

As for time, space and matter, the quartet of mass, energy, space and time are welded together by Relativity into a recursive and inseparable whole sometimes referred to as MEST. None makes any existential sense without all the others. For example, as Einstein explained, "Spacetime tells matter how to move, matter tells spacetime how to bend". So having one or more as an emergent property of the remainder is an attractive proposition. Various speculative ideas have been bounced around by theoretical physicists, including wider ideas such as Time as an emergent property of quantum thermodynamic systems (whatever that is) but none has yet to make any headway.

Some nuggets do appear. In string theory there are typically six extra dimensions, of which three, like time, have an imaginary metric. Their imaginary multiplier arises from a quite different principle and they are definitely spatial dimensions. This suggests that Time in the Minkowski metric is not a spatial dimension in the same way, though it offers no help as to what it might be instead.

Then there is the Twistor theory of Roger Penrose, in which time and space are replaced by something more like frequency. Technically twistor space is a Fourier-like transform of objects in ordinary spacetime, analogous to the way a sound frequency spectrum is a Fourier transform of the physical waveform in time. It suggests that space and time are indeed closely related.

So at the moment, we can say that space and time are closely related, but we are not sure how closely or in quite what ways they differ.

• Also, an important twist from Einstein's theory is the relativity of simultaneity, which perhaps makes presentism seem less appealing as a metaphysical idea (though it doesn't rule it out, one could suppose there is an objective truth about simultaneity from a metaphysical perspective even if no possible empirical experiment can distinguish which frame's definition of simultaneity is the true one). – Hypnosifl Jul 29 '20 at 20:48
• Although simultaneity is not absolute, the remarkable thing is that the flow of cause-and-effect is never affected; different observers merely disagree on how long they have to wait for the effect. There is a similar phenomenon in some geometries, such as affine, in which order is preserved but exact distances are not. Quantum physics offers far worse challenges to presentism, but that is going off-topic. – Guy Inchbald Jul 30 '20 at 7:00
• Order is preserved for causally-related events, but not for events outside of one another's light cones which cannot causally affect one another; for a pair of such events A and B, some frames will say A happened before B, some will say B happened before A, and there'll always be a frame that says they happened simultaneously. Presentism by definition requires a single objective truth about whether any pair of events A and B happened simultaneously or not. – Hypnosifl Jul 30 '20 at 7:29
• Exactly so. Time reversal can only occur with events which are not in a potentially causal relationship, i.e. are outside each other's time cones. – Guy Inchbald Jul 30 '20 at 9:36

Special relativity demonstrates that time and space are related in a fundamental way. Consider a muon created in the upper atmosphere. These particles will travel only about 456 meters (or 2.2 microseconds) before decaying yet survive to the Earth's surface and below. From the viewpoint of an observer on Earth the time dilation effect of special relativity results in time passing more slowly for the muon and it reaches the surface before decaying. For an observer riding along with the muon, the muon's clock is not slowed but the distance to the Earth's surface is foreshortened, and so the distance it must travel (in its reference frame) is less than the 456 meter decay distance. In both cases the muon survives to reach the Earth's surface, but the reason is due to time dialation (as viewed from the Earth reference frame) or distance foreshortening (muon reference frame). This means that time and space are two sides of the same coin, so to speak. It has an analogy in the wave-particle duality of light and matter in which an object (particle of matter or photon of light) exhibit both wave properties (e.g interference) and particle properties (e.g. photoelectric effect). It is not intuitive to regard something as having properties that seem mutually exclusive, but it is nonetheless the nature of our universe. It would not be correct to state that time is independent of physical dimension as theory and experiment demonstrate otherwise.