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Spacetime doesn't really exist does it?

by andrewkirk
Tags: cosmology, epistemology, spacetime
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bcrowell
#19
Apr6-11, 07:06 PM
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Quote Quote by andrewkirk View Post
That's an interesting perspective on it. Do you think they are different from electromagnetic waves in this regard, given that an EM wave consists of an E and a B field, each of which varies over time and space in accordance with the solution of a wave equation, whereas a grav wave is a gravitational field that varies over time and space in accordance with the solution of a different wave equation.
Well, I wouldn't really describe it as an oscillating gravitational field, since it's a phenomenon described by GR, and GR doesn't really describe gravity in terms of a field. GR deals with tensors, and the gravitational field is not a tensor. GR describes a gravitational wave as an oscillating curvature of spacetime.

Quote Quote by andrewkirk View Post
When you say the waves have energy I interpret this to mean that:
1. The system that emits the waves loses energy by emitting them; and
2. The wave can perform work on another body/system

Is there more to it than that?
Sure, I think that's fine. GR doesn't have a general principle of conservation of energy that applies to all spacetimes, but it does have one in special cases such as asymptotically flat spacetimes. That means that it covers the radiation from the Hulse-Taylor system, for example.
yogi
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Apr7-11, 12:49 AM
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Quote Quote by marcus View Post
instein said something about this when talking about the principle of General Covariance (diffeomorphism invariance):

“Dadurch verlieren Zeit & Raum den letzter Rest von physikalischer Realitšt. ..."

Thereby time and space lose the last vestige of physical reality”.

for an online source see page 43 of
http://www.tc.umn.edu/~janss011/pdf%...Besso-memo.pdf

In another context ( In the introduction of the paper on the perihelion motion presented on 18 November 1915), Einstein wrote
durch welche Zeit und Raum der letzten Spur objektiver Realitšt beraubt werden,

... time and space are robbed of the last trace of objective reality



To summarize, both quotes are from Nov-Dec 1915, one is from a paper on perihelion motion and the other is from a letter to Moritz Schlick a few weeks later.
==================================

.
Einstein seems to have drifted to a different perspective as he aged - some of the quotes from his later years conform with the notion of a substantive spatial reality
andrewkirk
#21
Apr7-11, 01:16 AM
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Marcus thanks for your interesting post and link to the other PF thread "what expands if there's no rubber". In fact that question - assuming you are referring to cosmological theories of expansion/inflation (I'm still unclear whether they are supposed to be different from one another) - is one of the main motivations for my question.

The popularised explanations of expansion use the ant on a balloon metaphor. In those cases it is possible to make a clear distinction between motion of the ant along the balloon and expansion of the balloon - eg just imagine a grid drawn on the balloon. However, when we are talking about spacetime, given two galaxies A and B, the distance between which is increasing at speed v m/s, can we decompose that into components (not necessarily simply additive) u from normal relative motion and w from expansion of the spacetime between them? I don't even know what I mean by "normal relative motion" in the preceding sentence, but the popularised descriptions of cosmological expansion appear to imply that such a thing can be sensibly defined.
harrylin
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Apr7-11, 04:29 AM
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Quote Quote by yogi View Post
Einstein seems to have drifted to a different perspective as he aged - some of the quotes from his later years conform with the notion of a substantive spatial reality
That isn't necessarily in contradiction with your quote, I think. With SR and GR, space and time got a reduced objective reality because our perspective influences our descriptions. However that lack of objectiveness doesn't rob space from subjective reality: if our description of something depends on our perspective, that doesn't make it "unreal" or non-existent!

Cheers,
Harald

Note: the Besso-memo link didn't work for me and I did not find it on http://www.tc.umn.edu/~janss011/ ...
Phrak
#23
Apr7-11, 06:28 AM
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Quote Quote by andrewkirk View Post
Having studied GR as far as Einstein's tensor equation and a bit beyond, I'm just doing a bit of a double-take wondering what we mean when we talk about spacetime.

Frequently it is referred to as if it is a thing that exists, like mass and energy.
Of course spacetime doesn't exist. I see this over and over again, ad nauseum. However, no explanations of this logical fallacy seems to penetrate the well educated mind.
DaleSpam
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Apr7-11, 06:41 AM
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Quote Quote by Phrak View Post
Of course spacetime doesn't exist. I see this over and over again, ad nauseum. However, no explanations of this logical fallacy seems to penetrate the well educated mind.
Please cite one experimental or cosmological observation that supports this statement.
Naty1
#25
Apr7-11, 07:45 AM
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Great discussion.....

One specific reply first:
how does inflaton interact with something non-existent.
It is hypothesized that during the inflationary period space and time, spacetime, did not yet exist. Otherwise how could inflation proceed at "speeds" greater than "c".

But I think this a reasonable point regarding the cosmological constant.

I still remember several years ago when I posted a similar question to the one here from the OP and listed maybe a dozen reasons why I though spacetime does "exist"....and was lambasted, ridiculed and, well.....I felt like a conservative at Columbia University must feel.

I don't recall anyone agreeing that it was possible spacetime is something , so again I say "Thanks, Ben Crowell"!!!

Regardless, it's good to see an actual discussion, mostly representing points of view, rather than hurling accusations.

I don't know what "exist" means, but it always seemed pretty obvious that spacetime is as much of an entity as energy or mass or gravity.,,or entropy!!!!

For example how can dark energy or vacuum energy exist in "empty" space(time) if spacetime is just a theoretical artifact, if it is "nothing" "real". How does nothing curve and bend, time dilate and length contract?? And if they doesn't "exist", why do we think "nothing" is expanding...in the universe.

Also, it seems horizons, such as black hole, Unruh effect and cosmological, suggest there is more to spacetime than we might understand completely.

In any case I'm not sure we have touched yet on what may be the origin of all "existence":
related to Verlinde's central notion that information is needed to derive gravity and that, separatelyand more generally, entropy/thermodynamics are simply a subset of information theory. And from information likely flows all the apparently separate entities we observe in this "existence".

Maybe the more general question is "Does information really "exist""??
Naty1
#26
Apr7-11, 08:33 AM
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I just searched "Is spacetime real"....there are hundreds of threads....!!!

I also came across a thread I started with "What is a particle.".....about a year ago..

http://www.physicsforums.com/showthr...spacetime+real


For who think "particles are real" but spacetime isn't, check out the discussion...but beware there are mathematical discussions galore....including some posts about local versus global "reality"....

I am reminded "reality" or "exist" are really tough, perhaps impossible, to pin down especially in the classical sense......Maybe that's my new perspective...
PhilDSP
#27
Apr7-11, 09:58 AM
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Quote Quote by Naty1 View Post
Maybe the more general question is "Does information really "exist""??
Just a thought: something being observed (either from within or without) that is undifferentiated has no information (to the observer at least). It seems to be a strange kind of magical act that can tease out something that is differentiated, qualitatively or quantitatively, from something that is generally undifferentiated.

Space seems to be generally undifferentiated (isotropic, etc.,), but it seems that it can be disturbed in finite regions and in finite segments of time in a way that produces differentiated qualities. It could be argued that those disturbances generate information and that the qualitative and quantitative differentiations are information.
dm4b
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Apr7-11, 10:48 AM
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Quote Quote by Garth View Post
This statement will prove that spacetime does have an independent existence of its own ONCE gravitational waves have been detected.

Garth
I think it will suggest that spacetime has an existence, but not necessarily an independent existence.

Gravitational waves are made when massive objects interact in space, then the waves propagate away. No mass, no gravitational waves.

Could a Universe exist that was completely empty of mass, or could spacetime exist without mass, or objects? According to Penrose's new idea, a Universe cannot even continue to exist with just radiation in it.

I personally suspect spacetime and mass are somehow dependent upon each other - you cannot have one without the other. Although, it's all still an open matter, I suppose.
bcrowell
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Apr7-11, 11:34 AM
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Quote Quote by Naty1 View Post
It is hypothesized that during the inflationary period space and time, spacetime, did not yet exist. Otherwise how could inflation proceed at "speeds" greater than "c".
This argument doesn't work. Even in plain old FRW cosmologies with no inflation, the proper distances between galaxies increases at speeds greater than c. In fact, we can even observe galaxies whose proper distance from us is, and always has been, increasing at a rate greater than c: Davis and Lineweaver, Publications of the Astronomical Society of Australia, 21 (2004) 97, msowww.anu.edu.au/~charley/papers/DavisLineweaver04.pdf Standard models of inflation take place within a spacetime.
Tanelorn
#30
Apr7-11, 12:16 PM
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Excellent thread this, and quite surprising to find that there are such differing views on the subject. The question of "What is Space" has bothered me for years and I couldnt understand why no one had cracked this puzzle yet, is anyone even working on it? Coincidentally I started a thread about it just 2 days ago without seeing this thread:

http://physicsforums.com/showthread.php?t=487911


Personally, I dislike mathematical descriptions of things, I prefer to understand what something really is. eg. I would not expect to see a mathematical equation for some obscure aspect of my car, instead I want to reach an understanding at the deepest level what my car really is. I was pretty good at maths at one time, but now I am afraid I think in terms of ideas and pictures. And also I would not accept that my car does not exist, even though I have been down that path myself! In fact I think my post was deleted when I suggested that space time didnt actually exist, or at least it seems to have got lost :)

Perhaps space / space time is a medium in which various fields and energies can exist and propagate and which was created during inflation, and which continues to inflate today? Expanding into.... what?

Perhaps we have to write down every known property of space, eg. The constants of nature. What it is composed of? Does it have any energy of its own? How is it affected by and how it affects other things? How it has changed over time, How it was made, and in what if anything it is contained?

Is the creation and expansion of space time analogous in some way to the burning of a blank CD, creating information where there was none before, even if it is all zeros?
DaleSpam
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Apr7-11, 12:21 PM
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Quote Quote by Tanelorn View Post
Personlly I dislike mathematical description of anything
Then you should probably avoid physics.
jtbell
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Apr7-11, 12:22 PM
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Quote Quote by Tanelorn View Post
what it really is
And how do we decide whether a certain description of something is what it "really is?"
Tanelorn
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Apr7-11, 12:46 PM
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Well in reply, I would suggest that many of the concepts that we discuss in these posts are communicated mainly verbally and with charts and pictures and as a result we communicate a deeper understanding of all aspects of an issue, especially the fundermental concepts and where the emphasis or heart of the matter lies. Granted the Mathematics and equations are a tool for high precision. For those who are fully conversant with the specific mathematics it is also a form of language, but I dont believe that these ideas can only be communicated mathematically. In fact I frequently witness people having to resort to using words to explain the true meaning and intent of each mathematical statement. When was the last time that the conclusion of a paper was written in mathematics? Anyway I think this is off topic.
Phrak
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Apr7-11, 02:35 PM
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Quote Quote by Phrak View Post
Of course spacetime doesn't exist. I see this over and over again, ad nauseum. However, no explanations of this logical fallacy seems to penetrate the well educated mind.
Quote Quote by DaleSpam View Post
Please cite one experimental or cosmological observation that supports this statement.
It's a grammatical error.
PAllen
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Apr7-11, 02:53 PM
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Quote Quote by Tanelorn View Post
Well in reply, I would suggest that many of the concepts that we discuss in these posts are communicated mainly verbally and with charts and pictures and as a result we communicate a deeper understanding of all aspects of an issue, especially the fundermental concepts and where the emphasis or heart of the matter lies. Granted the Mathematics and equations are a tool for high precision. For those who are fully conversant with the specific mathematics it is also a form of language, but I dont believe that these ideas can only be communicated mathematically. In fact I frequently witness people having to resort to using words to explain the true meaning and intent of each mathematical statement. When was the last time that the conclusion of a paper was written in mathematics? Anyway I think this is off topic.
I disagree. Though I mostly use words, handwaving etc. natural language are parables, math the 'reality' when it comes to physics.

So, another 'reality debate'. In physics, which is 'real' the words or the math? I guess my view is physics about constructing mathematical models that, combined with correspondence rules between mathematical objects and measurements, successfully predict observations.
DaleSpam
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Apr7-11, 03:10 PM
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Quote Quote by Phrak View Post
It's a grammatical error.
"Spacetime" is a noun, "exist" is a verb, so "Spacetime exists." is not a grammatical error. You will have to do better than that. Again, please substantiate your claim with any observational evidence which supports it.


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