# Talk:Spacetime

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## What is spacetime?

Isn’t it important to distinguish between spacetime as a mathematical model and spacetime as an object in an external world? One is a thought inside an observer’s brain and the other is an object outside an observer's brain.Lestrade (talk) 22:41, 10 April 2013 (UTC)Lestrade

I can't think of any cases in which the lack of distinction would be confusing. It seems to me that the mathematical model is meant to represent the real world entity, and so any discussion thereof addresses the real world entity by describing it in terms of a mathematical model. Although, maybe I just haven't been able to think of any examples where the lack of distinction would cause genuine confusion; please mention any specific instances you can think of.
Crazyeirishman (talk) 15:05, 17 June 2013 (UTC)

Everyone tells me that spacetime is a real world entity, but I have yet to see or feel it in my daily comings and goings. I guess that the distinction between what is inside the brain (mathematical model) and what is outside the brain (real world entity) is some old, outdated notion that belongs in the dustbin of history.Lestrade (talk) 15:58, 17 June 2013 (UTC)Lestrade

Perhaps, but this is the place where we discuss the article, not the subject — see wp:talk page guidelines. If you have questions about the subject, you can go to our wp:reference desk/science. Good luck. - DVdm (talk) 06:19, 18 June 2013 (UTC)

I merely suggested that it is important for the article to distinguish between spacetime as a mathematical model and spacetime as a physical object. One is inside of someone's brain and the other is outside of someone's brain.Lestrade (talk) 15:04, 18 June 2013 (UTC)Lestrade

This would be trying to capture perception and subjectivity. This is more a topic of philosophy, and does not apply only to this example. Your contention is also debatable, and you'd need references anyway. The article should not cover meta-issues like this. — Quondum 15:56, 18 June 2013 (UTC)

In an article on “spacetime,” the definition of spacetime is a meta-issue? The difference between spacetime as a mathematical concept in someone’s mind might be different from spacetime as a physical entity that is curved, becomes warped when matter is near it, and is, itself, located in space. In this way, some naïve reader of the Wikipedia article may think that there are two kinds of spacetime: one a private idea in a mathematician's brain, and the other a curved, warped, public, physical object that is similar to a fabric or rubber sheet. Such a situation would lead to ambiguity and misunderstanding.Lestrade (talk) 00:36, 19 June 2013 (UTC)Lestrade

The article claims that spacetime is a mathematical model, not a physical entity. Is the concept of spacetime analogous to the concept of the electromagnetic field?Lestrade (talk) 15:15, 1 August 2013 (UTC)Lestrade
I don't see a problem. The first lede sentence does indeed define "spacetime" as a mathematical model. However, the very next sentences state that

From a Euclidean space perspective, the universe has three dimensions of space and one of time. By combining space and time into a single manifold, physicists have significantly simplified a large number of physical theories, as well as described in a more uniform way the workings of the universe at both the supergalactic and subatomic levels.

Which means that while "spacetime" may be a mathematical model, that model describes quite well the workings of the actual physical universe in which we reside. The only other possible way to state this that I can come up with is that "spacetime" is a physical attribute of the universe, but which is best explained using mathematical models. I don't see a substantive difference either way. — Loadmaster (talk) 00:05, 2 August 2013 (UTC)

I would think that an encyclopedia article on spacetime would tell a reader what spacetime is. Is it a mathematical model (inside your head) or a physical entity (outside your head)?Lestrade (talk) 02:55, 2 August 2013 (UTC)Lestrade

It is the former, a mathematical model, which is used to model the latter, the physical structure of our universe. As I said above, that is fairly clear from the first lede paragraph. — Loadmaster (talk) 22:25, 5 August 2013 (UTC)

## Block quote format

Some of the quotes under 'Spacetime in literature' seem too long not to be block quotes, especially the one from William Rowan Hamilton, since it includes single-quoted passages. The only reason I bring this up is because I was quickly scanning the article, saw a passage that didn't look very wikipedia-like, and had to look up and down several lines before I found any indication that it was from a quote. Has anybody else noticed this? Crazyeirishman (talk) 14:59, 17 June 2013 (UTC)

## Necessity of the "privileged character of 3+1 spacetime" section

Hi all, I'm currently working on this article in my sandbox in the very early steps of attempting to bring it to FA, and I believe that the section mentioned in this section header goes off on a tangent and distracts from the rest of this article; it doesn't really belong here. I'd like to hear some further opinions on this, and what should be done with this section if people don't think it should stay. Thanks! StringTheory11 (t • c) 02:58, 17 July 2013 (UTC)

The section isn't entirely bs, but I agree, it doesn't belong here as it stands. Try to shorten it down to a minimum and rinse out the more or less philosophical stuff. Statements like "The inverse square law doesn't produce stable orbits unless 3+1" and "EM theory looks different in other dimensions" are true and can be backed up rigorously.
Also, be warned! I don't think this article will make FA status ever. Try to get it to B+ or even A. That would be an accomplishment. YohanN7 (talk) 08:12, 17 July 2013 (UTC)
We can try :). StringTheory11 (t • c) 21:41, 20 July 2013 (UTC)
No ideas for now but looking forwards to StringTheory11 advancing the article up to FA, since he's an expert in the topic. Right now I'm editing articles related to spacetime on four vectors and in relativistic quantum mechanics, not to mention symmetries in quantum mechanics involving the Lorentz group and Poincaré group. M∧Ŝc2ħεИτlk 10:19, 17 July 2013 (UTC)
Eh, I'm not really an expert in the topic; I don't (yet) have a formal physics or astronomy education; everything I know is self-taught. But yeah, I'd be happy to work on one of those articles, if necessary. StringTheory11 (t • c) 05:51, 18 July 2013 (UTC)
Since nobody seems to have any objections, I'll get to work trimming it. StringTheory11 (t • c) 21:41, 20 July 2013 (UTC)
I agree with the other comments, but I would put it even more strongly: this article is not the place for what is essentially a summary of meandering supposition and, no doubt, circular reasoning. This entire section appears to be aimed at motivating/explaining/debating the necessity of this particular configuration of observed spacetime. I suggest removal of the section from the article, leaving at most a see-also link (if it is moved elsewhere, else simply delete it). If the content seems to have value, an article could be created for it, but IMO it'd take a lot of work to make it withstand an AfD. It makes for interesting speculative reading, but so does science fiction; it does not strike me as encyclopaedic material. It might, for example, belong better as a subsection of Anthropic principle#Applications of the principle if trimmed and properly sourced. — Quondum 01:16, 21 July 2013 (UTC)

## Article on time correction

Dear Editors,

I have an article on time correction that explains the dimension of time as a parameter for several special relativity calculations. I am hoping to add to basic concepts and add to some of my topics to space time if you can look over it. I am wondering if an editor can look at my article and suggest additions for space time please?

https://en.wikipedia.org/wiki/Wikipedia_talk:Articles_for_creation/Time_correction

Magravat (talk) 21:35, 19 February 2014 (UTC)MAgravatMagravat (talk) 21:35, 19 February 2014 (UTC)

Hi Magravat. I have added a header for your new section. I think your best bet is to go to Wikipedia talk:WikiProject Physics/Taskforces/Relativity with this. But do note that we cannot add our original research (see wp:NOR) to articles. For anything we like to add anywhere, we need solid reliable secondary sources (see wp:secondary sources). Good luck and cheers - DVdm (talk) 21:57, 19 February 2014 (UTC)

## An erroneous claim about spacetime geodesics

"Certain types of world lines (called geodesics of the spacetime) are the shortest paths between any two events, with distance being defined in terms of spacetime intervals." This claim is false. I've altered it twice, but both times the edit has been reverted. In Euclidean (and Riemannian) spaces, paths of (locally) stationary length are (locally) shortest paths. But the same is not true in Minkowski spacetime. Timelike geodesics are paths of longest temporal length. This is why, in the context of the "twin paradox" scenario, the twin whose motion is geodesic (unaccelerated) is older when they meet again than the twin whose motion is non-geodesic (accelerated). In Minkowski spacetime the length of spacelike geodesics is stationary, but they are neither longest nor shortest paths (e.g., they are shortest wrt variations confined to a spacelike hyperplane containing them, but longest wrt variations in a plane with timelike tangent vectors). This is all completely standard, but since it appears to be controversial, here are some references.

"A curve on $M$ that minimises its length $\int ds$... between two fixed points is actually a geodesic of for the metric g. \ldots When g is not positive definite, the argument is basically the same, but now geodesics do not minimize $\int ds$, the integral being what is called `stationary' for a geodesic" (Penrose, Road to Reality, pp 318-9)

"In the case of positive definite metrics, i.e. ones with signature of form (n, 0), we know, geodesics are *locally shortest* curves. The corresponding result for Lorentz metrics is that timelike geodesics are *locally longest* curves." (David Malament, [1], p. 8.)

The latest edit by [2] is still incorrect. Under no sensible construal of "shortest" are geodesics in Lorentzian spacetime shortest. In particular, spacelike geodesics are stationary but not extremal (and so not shortest -- in fact, for any two spacelike separated points in Minkowski spacetime, and for any positive number e, there is an everywhere spacelike curve connecting the points the length of which is less that e).— Preceding unsigned comment added by 86.9.64.18 (talkcontribs) 12:11, 6 March 2014‎ (UTC)

Please put new talk page messages at the bottom of talk pages and sign your messages with four tildes (~~~~). Thanks.
My edit does not say that geodesics in Lorentzian spacetime are shortest. Please feel free to improve the wording, but make sure it reflects the cited source. - DVdm (talk) 12:21, 6 March 2014 (UTC)
This is a tad confusing, not too clear in the article. I agree that it says "longest", but this is correct only for purely timelike worldlines. However, the reference uses the term "worldline" essentially to mean an arbitrary path in spacetime, not restricted to being timelike. And as the OP points out, the spacelike case seems to be considerably more complicated. We can either qualify the worldline as timelike and gloss over the rest, or we can sensitize the reader to the quirks of non-timelike geodesics. I favour something more sophisticated than a gloss. —Quondum 05:19, 7 March 2014 (UTC)