Talk:Spacetime

Summaries of earlier Talk (see [1])

History of the Concept of Spacetime

There's something off about this section, and I can't quite put my finger on what it is. I think it's the tone of it...but I'm not sure. For instance, the following statement: "Since time is used to measure change and events, and if there are no events or changes, is the measurement of time necessary?" This question isn't exactly relevant to this discussion because we do live in a universe full of events and changes. The same thing holds true with the quote from aristotle. I'm going to move this stuff to its own section.

While the tone of the above quote is kind of funky, I actually disagree with the statement, "Space is the distance between two objects." I can pick two points in space that are not composed of anything, with no matter between them, and there is still space between those two points. I'm going to delete this part. I'd love to hear some other opinions on this though, I'd like to know where this all came from.:)Pkeck 15:38, 30 January 2006 (UTC)

Experimental evidence for quantised spacetime

Double slit in time experiment. http://physicsweb.org/articles/news/9/3/1/1?rss=2.03Cbr20/E This surely is experimental evidence that that matter/energy is not just quantised over spatial dimensions (as shown by the classical double slit experiement) but over the time dimension as well. ie: spacetime does have a quantum nature.

http://xenz.stumbleupon.com/

Space-time vs. Spacetime

Examples of use of spacetime:

• Weisstein's encyclopedia http://www.treasure-troves.com/physics/
• D. J. Griffiths' Introduction to Electrodynamics (Upper Saddle River, N. J.: Prentice-Hall, 1989)
• numerous books with spacetime in title
  • E. F. Taylor and J. A. Wheeler, Spacetime Physics (San Francisco: W. H. Freeman, 1966)
• Caltech class: "Spacetime 101"
• .edu matches online are almost exclusively for spacetime

Examples of use of space-time:

• Brehm & Mullin, Introduction to the Structure of Matter (ISBN: 047160531X)
• Merriam-Webster http://www.m-w.com
• space-time about four times as many hits as spacetime on AltaVista

Since Wikipedia is not a dictionary, it should stick to one spelling. Thus I removed the comment "(alternatively, space-time)". Space-time already redirects to Spacetime, so all is well. —Herbee 20:31, 11 Nov 2004 (UTC)

It is Wikipedia's task to report information as it is. While it is acceptable to prefer one spelling throughout the entire article, if some references give "spacetime" and others give "space-time," then Wikipedia should at least mention at the beginning of the article that both are acceptable instead of hiding one spelling. This has nothing to do with Wikipedia not being a dictionary. —Lowellian (reply) 06:43, 19 September 2005 (UTC)

The Spacetime four vector

The space-time four vector is:

[x y z ct]

the use of the vector as:

[x y z ict]

was common about 40 years ago or so, but it is now considered archaic and the former notation is prefered.--BlackGriffen

However, there are certainly physicists who prefer the i notation, including Dr. Jack Sarfatti.

The Wikipedia should present the mainstream state of a field as much as possible, presenting relevant dissenting views as such when they arise. The use of the i is just a lazy way to make finding the "length" squared of a four vector feel like finding the length of any other vector (dot product the vector on to itself). It is, however, just as easy to define a new "length" operator for four vectors that doesn't require complex numbers.

And if bandying about names is the game, a quick look at the Feynmann Lectures on Physics explanation of four vectors shows nary an i, even though he had a discussion of using c=1.--BlackGriffen

I concur: both 'space-time' and the 'i' notation appear to be common early usage, and physicists everywhere seem now to have standardised on both 'spacetime' and the 'i'-less notation -- The Anome

1: Is there a free 3d prog that does nice, uniform renderings of some of this math?

2: Might it be a good idea to use a sidebar (can even use a msg tag for multiple article consistency) which describes some of the core foundational concepts, required for laymen learners like myself to have related study materiale at hand. Finding links in the text lacks structure that a weeded list would have. -SV(talk)

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New Talk

We can read in this article:

"Strictly speaking one can also consider events in Newtonian physics as a single spacetime. This is Galilean-Newtonian relativity, and the coordinate systems are related by Galilean transformations. However, since these preserve spatial and temporal distances independently, such a spacetime can be decomposed unarbitrarily, which is not possible in the general case." - is that right? Is unarbitrarily or arbitrarily? I've some difficulties with english but I'm trying to translate this to portuguese and I can't understand this... Is it an error?... Say something, please. -- Manuel Anastácio 20:29, 15 Jun 2004 (UTC)

Hopefully, sorted. --Eddie 19:51, 31 Dec 2004 (UTC)

global positioning system?

GPS?? WTF?? - Omegatron 03:38, Dec 16, 2004 (UTC)

more content

should there be something on spacetime compactification ? I don't know much about this, but perhaps some1 out there could contribute to this. It would link in nicely with the spacetime topology section .

Mpatel

space-time expansion

I've heard quite a few article discussions mention the expansion of spacetime. IT'S JUST AS PREPOSTEROUS AS TIME DILATION! How can something that is NOT a physical entity be expanded? Scorpionman 02:14, 20 May 2005 (UTC)

I hope I've been able to help address your concerns at Talk:Time dilation, but please leave additional comments should you require clarification. I admit that time dilation is a very unusual concept, but I don't understand why you think that spacetime expansion is preposterous. It doesn't seem very strange to me, and it fits astronomical observations quite nicely. You can imagine spacetime to be like a rubber sheet on which the matter and energy of our universe is distributed. As the sheet expands in all directions, from the point of view of someone in a galaxy, all the galaxies are receding from each other. — Knowledge Seeker দ 21:22, 23 May 2005 (UTC)

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Knowledge Seeker is comfortable with analogies of two-dimensional rubber sheets as four-dimensional spacetime. Isn't it relaxing to have such closure?

Are you asking me or others? Yes, I suppose it is relaxing, in a sense. Of course, the rubber-sheet analogy is flawed in many ways. But if one can imagine a rubber sheet being stretched in both dimensions, and one can grasp/accept the idea of a four-dimensional structure, I don't see how one would not be able to imagine the structure expanding in all four dimensions. Or what are you getting at exactly? This seems to have little do with Scorpionman's question. — Knowledge Seeker দ 02:15, September 5, 2005 (UTC)

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Take Knowledge Seeker's rubber sheet. Stretch it high. Stretch it wide. Now you have a four-dimensional rubber sheet. "It doesn't seem very strange to me." Closure.

spacetime dimensions

Whilst editting spacetime symmetries, I remembered that a spacetime can (in principle) have dimensions other than 4. Most people use 4D spacetimes, but many researchers use 10D spacetimes etc; it even makes sense to talk of 3 dimensional spacetimes (2 space dimensions + 1 time). I think we need to redefine spacetime to accomodate these common uses - I don't know why I never spotted this before. Roughly, isn't an n-dimensional spacetime a manifold etc... with n-1 space dimensions + 1 time dimension ?

Mpatel 10:50, 10 Jun 2005 (UTC)

There are also models containing more than one dimension of time! I recall seeing a table in a Scientific American article that went so far as to identify which systems of x space dimensions and y time dimensions would support currently known physics, or something of that nature. ‣ᓛᖁ♀ᑐ 06:55, 21 September 2005 (UTC)

It's probably better to stick to the 4D definition, and then mention that other uses of the word can accomodate things like 3D spacetime diagrams and higher spacetime dimensions in other theories. ---Mpatel (talk) 11:57, 19 October 2005 (UTC)

Ambiguity

The first sentence of the article asserts that a space-time continuum is "time and three-dimensional space treated together as a simple four-dimensional object." If it is an object, one wonders if this object has a location. Where is this object?

Relative to what? As I understand it, spacetime is a four-dimensional mathematical object which is used to describe the universe. Since the universe is (typically defined to be) everything there is, you can't locate this "object" anywhere within the universe, so asking "where" it's located doesn't make sense. But I'm no expert... - dcljr (talk) 03:46, 14 November 2005 (UTC)

Technicality

This article is disappointing. Spacetime is not a subject that requires a great deal of knowledge to understand, but the text treats it as if it is. I would expect the average reader to feel vaguely lost by the end of the second paragraph, and then completely lost upon scrolling past the table of contents to be confronted by dense and mostly unexplained mathematical jargon. ‣ᓛᖁ♀ᑐ 09:31, 21 September 2005 (UTC)

Then stop complaining and do something about it... . You're right about the concept of spacetime not being a difficult one to grasp. However, the article is about spacetime, not about spacetime for the layman. Anything and everything (we believe to be factual) about spacetime must be included; this is an encyclopedia article, not a brief excursion into spacetime. ---Mpatel (talk) 15:20, 21 September 2005 (UTC)

I disagree with your assertion that this is "not about spacetime for the layman". Every article in Wikipedia should be written for a general audience (AKA, "the layman"). Technical info only of use to experts can also be included, but not at the price of excluding a basic intro that everyone can follow. StuRat 00:07, 22 September 2005 (UTC)

I agree with Eequor. This article is too utterly technical that no one other than the guy who wrote it understands. Please make it more easy to understand. Spawn Man 23:49, 21 September 2005 (UTC)

I stuck a "toe in the water", by adding the intro line:

Spacetime is the concept that space and time are not separate entities, but are both interrelated.

StuRat 00:20, 22 September 2005 (UTC)

It is true that every article in WP should be for the layperson. But, WP also allows proofs of mathematical theorems to be written - not every layperson is gonna read those pages and understand them. In any case, when you stuck your toe in the water, you gave an incorrect definition (it's imprecise, at the very least). I repeat, the article is about 'spacetime', in all it's glorious meanings (for the layperson, for the general scientist, for the physicist etc...). As such, I agree that the introduction should not be technical, but the article will inevitably contain technical information, as otherwise the totality of the meanings of 'spacetime' will not be covered - if we want an encyclopedic article, then that's what it should be. If the article gets too long, then maybe new articles on 'spacetime in relativity' etc. should be created. However, let's not forget that the concept of 'spacetime' arose from relativity theory in the first place, and hence this important historical connection should very definitely be mentioned at the start. ---Mpatel (talk) 10:34, 23 September 2005 (UTC)

I disagree with your edit summary comment "not true that all pages should be written for laypeople", but that isn't what you said in the body of the text. Perhaps you meant "not true that all pages should be written ONLY for laypeople", to which I would agree. The body of your text agrees with what I said. That is, that a basic intro "for laypeople" should be included, with info for experts included later. In the case of proofs, the layperson may only understand what is being proven, not the entirety of the proof. BTW, what exactly was wrong with my def ? StuRat 12:08, 23 September 2005 (UTC)

As the notice says, making articles accessible is not about removing the technical information. Proofs and formulas do have their place. ‣ᓛᖁ♀ᑐ 12:06, 23 September 2005 (UTC)

My comment about the definition being incorrect was that it was incomplete (I was thinking of the technical details too - i.e. spacetime being a manifold). I was hoping for something like, 'Spacetime is a concept that views space and time as interrelated rather than as separate entities. The concept was formulated by Hermann Minkowksi soon after Albert Einstein developed the theory of special relativity.' As far as the basic idea of spacetime is concerned, that's about all we need for an introduction (I think). Later in the article, we need to mention the maths of spacetime, the physics of spacetime etc. ---Mpatel (talk) 13:10, 23 September 2005 (UTC)

I think the manifold concept is beyond the layman, so should not be placed in the intro, but rather later, in the body. StuRat 13:38, 23 September 2005 (UTC)

I know; I didn't say (or suggest) that it should. ---Mpatel (talk) 13:53, 23 September 2005 (UTC)

So if you're not saying my def was incorrect because it didn't include a manifold reference, do you mean it's incorrect because it doesn't include the history of the term ? StuRat 14:10, 23 September 2005 (UTC)

What I'm saying is this: your definition of spacetime is ok for a layperson (and this is what should be at the start), but not for a specialist (who will be satisfied by the manifold stuff later on in the article). I mentioned the historical bit only to indicate what I think should also be at the start. ---Mpatel (talk) 15:42, 23 September 2005 (UTC)

Ok, glad to know you think it's incorrect but should be at the start, LOL. StuRat 16:02, 23 September 2005 (UTC)

A short introduction to spacetime

From any physical theory that contains a set of elements called space and a set of elements called time, one can create the Cartesian product of space and time and this product is called spacetime. An element of spacetime is called an event. Hence, an event is an ordered pair (where,when) where where represents an element in space (a point in space) and when represents an element in time. Two events in spacetime are called simultaneous if they have the same element of time as the second element in the ordered pair. Two events in spacetime occur at the same place if they have the same element as the first element of the ordered pair.

In Newtonian physics, space has the additional structure of a three-dimensional Euclidean space, and time has the structure of a one-dimensional Euclidean space. Newtonian spacetime is the Cartesian product of the three-dimensional Euclidean space and the one-dimensional Euclidean space. However, one does not make the product into a four-dimensional Euclidean space, one keeps the three-dimensional Euclidean structure on space, and the one-dimensional Euclidean structure on time. The three-dimensional Euclidean space structure on space allows one to identify points in space with ordered triples $(x,y,z)$ of real numbers by choosing some (not unique) coordinate system. Given a unit length, this coordinate system may be chosen so that the square of the distance $D$ between any two points $(x_{1},y_{1},z_{1})$ and $(x_{2},y_{2},z_{2})$ is

$D^{2}=(x_{2}-x_{1})^{2}+(y_{2}-y_{1})^{2}+(z_{2}-z_{1})_{.}^{2}$

Similarly, the one-dimensional Euclidean space structure on time allows one to identify elements in time with the set of real numbers by choosing some (not unique) coordinate system. Given a unit time duration, the identification of time with the real numbers may be chosen so that the square of the time difference $T$ between events occuring at times $t_{1}$ and $t_{2}$ is given by

$T^{2}=(t_{2}-t_{1})_{.}^{2}$

In Newtonian spacetime, the distance D and the time duration between the events $(x_{1},y_{1},z_{1},t_{1})$ and $(x_{2},y_{2},z_{2},t_{2})$ is not combined to produce the product metric (which would give the square of the spacetime distance as $D^{2}+T^{2}$). $D$ and $T$ are considered separately.

In special relativity, Minkowski spacetime (the spacetime of special relativity) is determined by the property that the set of events (the elements of the spacetime) can be specified by an ordered quadruple $(x,y,z,t)$ and the existence of a spacetime interval $I$ between any pair of events $e_{1}=(x_{1},y_{1},z_{1},t_{1})$ and $e_{2}=(x_{2},y_{2},z_{2},t_{2})$whose square is given by

$I^{2}=(x_{2}-x_{1})^{2}+(y_{2}-y_{1})^{2}+(z_{2}-z_{1})^{2}-(t_{2}-t_{1})_{.}^{2}$

In both Newtonian spacetime and Minkowski spacetime events can be specified by four real numbers. However, in Newtonian spacetime, the spacetime consists of a product of space and time (and all observers agree with the splitting of the spacetime into its space component and it time component). In Minkowski spacetime, each observer can choose to split the spacetime into a space component (a three-dimensional Euclidean space) and a time component (a one-dimensional Euclidean space). Different observers split spacetime into different space and time components, but they all agree that spacetime has four dimensions and that the square of the spacetime interval between a pair of events has a fixed value (once a unit interval has been selected).

In general relativity, the spacetime is curved. Minkowski spacetime is flat and the spacetime of general relativity is curved. In a manner that is analogous to the way that a sphere in three-dimensional Euclidean space can be approximated by a tangent plane near the point of contact between the plane and sphere, the curved spacetime of general relativity has the same type of relation to flat Minkowski spacetime as does the sphere to a tangent plane. The difference between the curved spacetime of general relativity and the flat spacetime of Minkowski spacetime is reflected in the (Riemann) curvature tensor.

In general, spacetime represents the part of a physical theory that is supposed to correspond to the common sense notion of space and time. After the ideas of relativity, space and time do not appear to be independent of each other and independent of other physical entities. What space and time are (and what spacetime is) is not known. In classical physics, spacetime is Newtonian spacetime. In special relativity, spacetime is Minkowski spacetime. In general relativity, spacetime is a curved four-dimensional space satisfying Einstein's field equations. Each physical theory has its own version of spacetime.

Ref: Newtonian Mechanics, Special Relativity, General Relativity, etc.

Reorganised article

I've reorganised the article to bring out the relativity ideas more clearly. The maths section still needs a lot of work. Perhaps some more on the concept of spacetime in non-relativistic theories is needed. ---Mpatel (talk) 08:52, 22 October 2005 (UTC)

• I'm a bit dubious about calling spacetime an object, instead of a model. It's not a physical object, nor is it a computer science object. Why this change? RaulMiller 00:03, 23 October 2005 (UTC)

Hi RaulMiller.

Actually, I didn't change 'model' to 'object'. I changed 'coordinate system' to 'object', because spacetime is definitely not a coordinate system - spacetimes are independent of any observers and hence coordinate systems: nature doesn't care whether we measure the properties of a spacetime (e.g. time intervals) with clocks and rulers; all that matters is that spacetime exists independently of anyone who chooses to measure it's properties or not. The word 'model' is still there in the first sentence. Perhaps 'object' could be changed to 'entity' or maybe something better. ---Mpatel (talk) 08:37, 29 October 2005 (UTC)

Quantized vs. Quantificated

Someone, I don't know why, decided that "Quantized" (refering to quantum effects as in quantum theory) should be replaced with "Quantificated" (which looks like an informal variant on quantification). Since the context is the question "Is spacetime quantized", and it's pretty clear already that quantities are involved in locating events, I don't think the question is meaningful when using the psuedo-word "quantificated". If I'm totally off base here, could someone spell out why, in more detail, we should wonder if space time is quantificated? Also, why we should not concern ourselves with whether or not it's quantized? RaulMiller 03:38, 3 November 2005 (UTC)