Talk:Spacetime/Archive 2

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

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)

Clean up
When I was reading it I thought it was thediscussion page theres no continuity. I'll try and clean it up a bit but I dont thikn I could help with the content much, so I'm giving the warning I may not be able to fix the actual informative bits. wolfie 08:26, 20 May 2006 (UTC) It's proving to be impossibe without a understanding of what needs to be there and what doesn't.

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)

I think it came by trying to help people who dont understand space-time, or werent mathematic able and didnt uderstand what a dimension is. But i dont really know wolfie 08:36, 20 May 2006 (UTC)

I think that space/time does not exist as a continuum but no cunt will take me seriously, i have mathamatical forumalae which prove my theory and as of yet no professor has been able to disprove me. They shrug me off with a simple "it's impossible because Einstein or Newton or Galileo said so"
 * Woah, can we please tone down our language? --M e rovingian { T C @ } 12:14, 31 May 2006 (UTC)

Although I am new to contributing to Wikipedia, I have long been a great admirer of this wonderful resource as an English teacher for many years. Long may it continue serving humanity far into our future.

As is often the case, there can be complications to maintaining such a resource, especially since anyone in the entire world can contribute here. As we are all scholars, I make an appeal that we always respect each other as a matter of professionalism and that we treat each other with the utmost courtesy, especially when it comes to overwriting or deleting another contributor's work. If you'll allow me, I'd like to make the motion that any revision to another's work, must be accompanied by a reason posted in either the Section Edit Summaries or in the user's Talk sections. It is only fair. If such a wonderful resource as Wikipedia becomes no more than a battleground for our personal needs of aggression, then it will have completely lost its great value to the world. Thank you so much for your time and for allowing me to be a contributor. LachlanSosa 06:36, 15 March 2007 (UTC)

Definition of the spacetime interval
This article defines the space-time interval as s^2 = x^2 - (ct)^2, whereas wolfram defines it the other way around as s^2 = (ct)^2 - x^2. So are both correct? Is there any logic to using one over the other? I think we should mention both in the article, because it obviously effects how you classify spacelike, timelike, etc. Brendanfox 04:51, 2 November 2006 (UTC)

Famous explanations about space and time
In a nutshell, four different accounts of space and time could be described as follows:
 * Newton&mdash;space and time exist independently, apart from all things. They are God's sensations.
 * Leibniz&mdash;space and time are mental concepts of relations that are based on properties of things in themselves.
 * Kant&mdash;space and time are ways that the mind intuits the juxtaposition and succession of the appearances of objects.
 * 21st century science&mdash;space and time are, together, independently existing geometry that exists in yet another space and time. Space can be can be warped, twisted, flattened, curved, and bent within that other space. Time can be sped up or slowed down within that other time.

As believers in progressive development, we ignore the older explanations and wholeheartedly give our assent to the most recent.Lestrade 15:18, 26 October 2007 (UTC)Lestrade


 * What if the most recent is pseudo-scientific quackery from "theoretical astrophysicist"? Einstein's Spacetime was the motion of matter through space, not some foldable space fabric. Read Wikipedia today however, and you'd have the notion that Time is a magic trampoline covered in bowling balls ala the History Channels documentary on Einstein. --67.58.84.106 (talk) 17:21, 18 June 2009 (UTC)

Encyclopédie, of course...
Encyclopédie is the first printed book talking about time as a dimension. It was in 1754. The article "Dimension" was written by D'Alembert himself. Time as a fourth dimension in the Bulletin of the American Mathematical Society. Joseph Louis Lagrange (1736-1813 ; born French -because of his French father- at Torino in the Kingdom of Sardinia ; no Italy at that time...) was one of the first to work on this concept. "LAGRANGE" on the official website of the friends of the École Polytechnique. Excuse my poor English. 84.103.176.154 (talk) 02:31, 4 November 2008 (UTC)
 * The article by R.C. Archibald (1914) provides details on Lagrange and d'Alembert initiating the concept of spacetime. Primacy attributed to Edgar Allen Poe has been changed now to the proper writers. Thank you, contributor, for the information and links.Rgdboer (talk) 22:51, 16 June 2009 (UTC)

Talk page archival
I have taken the liberty to start archiving this talk page, keeping 5 threads visible. I hope I haven't made a mess of it. If I did something wrong, we can always revert to last known good. DVdm (talk) 12:34, 8 March 2010 (UTC)

Inappropriate wikilink to Continuum mechanics
User added a see-also-entry to Continuum mechanics. As this subject is not directly related the the subject of this article, I removed it, per Linking. Without providing a comment CES1596 undid my removal. As spacetime is nowhere mentioned in that article, I have reverted again. Comments? DVdm (talk) 18:43, 23 May 2010 (UTC)
 * Thank you for your comment. I thought it helpful to image spacetime continuum, but maybe Continuum (theory) is more appropriate. CES1596 (talk) 16:31, 24 May 2010 (UTC)
 * Hm, I don't think so. Perhaps, if that article would could contain something, but there is nothing there that isn't already here. DVdm (talk) 16:51, 24 May 2010 (UTC)

plural?!

 * Spacetimes are the arenas in which all physical events take place—an event is a point in spacetime specified by its time and place. For example, the motion of planets around the sun may be described in a particular type of spacetime, or the motion of light around a rotating star may be described in another type of spacetime. ...

Say what? It's a fundamental assumption of physics that the same rules apply throughout the observable universe; in other words, there's only one kind of spacetime. —Tamfang (talk) 08:20, 26 September 2010 (UTC)


 * If you read the article, you find that as a mathematical concept many spacetimes are possible. For example, we have the flat Minkowski spacetime and the Schwarzschild spacetime. You'll find some more spacetimes in the figure [[Image:Spacetime dimensionality.svg|thumb|300px|Properties of n+m-dimensional spacetimes]]. DVdm (talk) 09:32, 26 September 2010 (UTC)


 * How many of them coexist in the observed universe? —Tamfang (talk) 16:01, 28 September 2010 (UTC)


 * In the current context, as many as we can dream up, I guess. DVdm (talk) 16:40, 28 September 2010 (UTC)
 * Now I'm wondering what 'exist' means to you. —Tamfang (talk) 19:48, 28 September 2010 (UTC)
 * That's for me to know and for you to wonder, and thus off-topic on this article talk page :-) DVdm (talk) 20:39, 28 September 2010 (UTC)


 * The quoted passage might be improved by changing described to approximated. —Tamfang (talk) 16:08, 28 September 2010 (UTC)


 * The descriptions of the motions are of course approximations to/for the (essentially unknown) "real thing", but the motions are described in some spacetime, so, still in the current context of "arenas in which", I think the passage is properly formulated. DVdm (talk) 16:40, 28 September 2010 (UTC)


 * Of course it's only my assumption that the stars mentioned are all in this universe. If they're not, then obviously they can move in different types of spacetime, but that negation of a reasonable default assumption ought to be made explicit. —Tamfang (talk) 00:36, 30 September 2010 (UTC)

Planck units
In Planck units, the fundamental (minimal?) space-time interval is 2.275×10$−148$ m$3$s. Given that the present size and age of the Universe is about 1.482×10$98$ m$3$s, this works out to about 6.516×10$245$ Planck-scale space-time interval/volumes as the size/age of the Universe (assuming my calculations are correct). But I'm not sure where it's appropriate to mention this little bit of trivia. — Loadmaster (talk) 17:23, 29 September 2010 (UTC)


 * According to the holographic principle which most physicists think will apply in any theory of quantum gravity, if one wants to know how much information is contained in the sphere representing the observable universe, one should look at the 2D area of the surface of the sphere in multiples of the Planck area, not the 3D volume in multiples of the Planck volume. Also, it's not actually correct that the radius of the observable universe in light-years is equal to the age of the universe in years, while the age is about 13.75 billion years the radius is more like 46 billion light-years--see the discussion on the observable universe page. Hypnosifl (talk) 04:28, 3 October 2010 (UTC)


 * Oh, I just noticed that when you talked about the "present size and age" you weren't saying the age in years should equal the radius in light-years, but rather multiplying size by age...I'm not sure, but I'd guess it would be true according to the holographic principle that the information in the observable universe since the big bang would be about equal to sum of the planck area of the surface at each interval of Planck time since the Big Bang. Meanwhile, I see that the idea that the holographic principle applies to all volumes of spacetime, not just black holes, is a conjecture that might be false even if the black hole version is correct...towards the end of this article by Jacob Bekenstein, he writes In 1999 Raphael Bousso, then at Stanford, proposed a modified holographic bound, which has since been found to work even in situations where the bounds we discussed earlier cannot be applied. Bousso's formulation starts with any suitable 2-D surface; it may be closed like a sphere or open like a sheet of paper. One then imagines a brief burst of light issuing simultaneously and perpendicularly from all over one side of the surface. The only demand is that the imaginary light rays are converging to start with. Light emitted from the inner surface of a spherical shell, for instance, satisfies that requirement. One then considers the entropy of the matter and radiation that these imaginary rays traverse, up to the points where they start crossing. Bousso conjectured that this entropy cannot exceed the entropy represented by the initial surface--one quarter of its area, measured in Planck areas. This is a different way of tallying up the entropy than that used in the original holographic bound. Bousso's bound refers not to the entropy of a region at one time but rather to the sum of entropies of locales at a variety of times: those that are "illuminated" by the light burst from the surface. 


 * More on Buosso's bound, which is called the "covariant entropy bound", in this paper. But the last part of this paper may suggest that according to Buosso's conjecture the entropy of the observable universe can't just be calculated from the area of its surface: To interpret the CEH entropy itself as a maximum entropy is to invoke the holographic bound on the volume of the cosmic event horizon. Fig. 6 shows the holographic bound being violated between 10-40 and 10-20 seconds after the big bang, when the entropy of radiation is larger than the entropy of the cosmic event horizon. During this time interval the radiation within the CEH contained more entropy than the CEH itself. This happens because the holographic bound is not expected to hold on volumes much larger than the Hubble sphere. One can see in Fig. 4 that for very early times, the event horizon is much larger than the Hubble sphere. Any volume larger than the Hubble sphere is more dense than a black hole the same size. Violations of the standard holographic bound, such as the one seen here, motivated Bousso (1999) to propose a modified holographic bound, the Covariant Entropy Conjecture, which is not violated in the same way. On the other hand, this abstract suggests Buosso's covariant entropy bound would actually fail in a universe where negative energy densities (from dark energy, perhaps) can occur. More discussion on the covariant entropy bound and cosmology here, here, here, and here.So, seems to be a complicated and not well-resolved question, any discussion of the limits on the amount of information in the entire universe should probably go in the holographic principle article where the various issues could be discussed in more detail. Hypnosifl (talk) 17:54, 3 October 2010 (UTC)