Talk:Spacetime

Summaries of earlier Talk (see [1] (http://www.wikipedia.com/wiki.cgi?action=history&id=Space-time/Talk))

Contents

1 Space-time vs. Spacetime 2 The Spacetime four vector 1 New Talk 2 global positioning system? 3 more content 4 space-time expansion 5 spacetime dimensions

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)

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 (http://www.stardrive.org).

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)

---

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)

spacetime dimensions

Whilst editting symmetries in general relativity, 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)