What is Spacetime?

  • #1
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What is Spacetime? I think this is an essential question in Cosmology. Some will accept, others don't.
Has spacetime a hidden property, is it a sub-material elastic object/background or ...? Is there one spacetime or are there spacetimes? Is spacetime the "Field" (das Feld) Einstein referred to?

The Stanford link http://plato.stanford.edu/entries/spacetime-holearg/ [/URL] shows some approaches.

Marcus pointed out in post [PLAIN] https://www.physicsforums.com/showpost.php?p=588977&postcount=7 [/URL] - referring to an article with Renate Loll - that: "According to Einstein, spacetime is flexible (malleable, ductile), but also at the same time continuous and smooth-----for example it does not have breaks (fractures, gaps, rips....)".

So:
1. Flexible (thus elastic),
2. Continuous and smooth,
3. No breaks (non-breakable).

In my speculative opinion spacetime is indeed a hidden object and it is non-breakable.
But how can we come from these properties to local, discrete packages (which seems independent)?
Well, by bending and penetrating parts of spacetime with each other you can create locally discrete zones (holons) where two or more parts of that object interact - and influence - each other. That creates locally friction (and kinetics effects) between the spacetime layers. Interaction in such system is a feedback approach: The local parts influence the larger hidden system and the hidden system influences the local packages.
The consequence of this approach is that we can not make a distinction between matter (fields) and energy (fields) because they all come from the same properties. Interconnectedness in incorporated in everything, and that's called "gravity". Our Universe is thus "restructured gravity'.

Attached image shows you the basic concept of the multi-layers spacetime. If the image is not loaded you can find that on this link [PLAIN] http://mu6.com/holons_2/genderless_to_duality.jpg [/URL].

But ... what is spacetime in your opinion?
 

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  • #3
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I've theorised a bit on the nature of space/time but no one can actually say what spacetime fabric is as of yet because there is no way of actually proving anything conclusively.
 
  • #5
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selfAdjoint said:
I think that thread about covers the cases, so why do we need this new one?
first I found the old one afterwards (via Google!), secondly since 2003 there are lot new members, thirdly I was touched by the way Renate Loll expressed her points in the article (in Dutch).
BTW she stays on the level of spacetime "ripples" ('plooien' in Dutch). If something in non-breakable then there can be also other geometric concepts like topogical caps like I explained. I wonder why she stays in the one dimensional thinking of ripples. Why not multi-layers created by only one layer?
 
  • #6
selfAdjoint
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Maybe because she doesn't find any need for multiple layers!
 
  • #7
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selfAdjoint said:
Maybe because she doesn't find any need for multiple layers!
:biggrin: Touché.
yes probably. Or she didn't imagined the concept. I e-mailed her last weekend and asked. No reply yet. But maybe she doesn't find any need to answer. :wink:
 
  • #8
marcus
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pelastration said:
first I found the old one afterwards (via Google!), secondly since 2003 there are lot new members, thirdly I was touched by the way Renate Loll expressed her points in the article (in Dutch).
BTW she stays on the level of spacetime "ripples" ('plooien' in Dutch). If something in non-breakable then there can be also other geometric concepts like topogical caps like I explained. I wonder why she stays in the one dimensional thinking of ripples. Why not multi-layers created by only one layer?

I was hoping someone who can read Dutch would read the original. I was guessing about some of the words and sometimes only made a very rough approximation in English. Even the title I did not translate exactly, I think! Any help you can give would be appreciated Pela.

I wonder if the dutch word 'plooien' is related to the english word DEPLOY (unfold) and EMPLOY (fold in) and maybe also the PLEATS in the fabric of a curtain or dress. havent checked, just guess.

Here is the Dutch text
www.phys.uu.nl/~loll/Web/press/knutselen.pdf

here is a thread with partial rough English translation
https://www.physicsforums.com/showthread.php?p=588977&postcount=6
 
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  • #9
marcus
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pelastration said:
Since nobody reacts, here is a link to a previous (2003) PF thread titled: What IS the spacetime fabric?
https://www.physicsforums.com/showthread.php?t=5732&page=1&pp=15


thanks for linking us back to that Good Old Thread of 2003! I had forgotten that thread. Somebody says that scientists have discovered that the color of spacetime is beige and that he thinks it is made of caramel pudding----one may refer to it not as the spacetime continuum but as "the spacetime FLAN'


In this thread a poster named Eh gave a germane quote from Einstein
https://www.physicsforums.com/showpost.php?p=69491&postcount=4


"Space-time does not claim existence in its own right, but only as a structural quality of the [gravitational] field"."

At one time I found a link to a source for that quote which gives context, but I posted the link and forgot where. Does anyone have a source?
 
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  • #10
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marcus said:
I was hoping someone who can read Dutch would read the original. I was guessing about some of the words and sometimes only made a very rough approximation in English. Even the title I did not translate exactly, I think! Any help you can give would be appreciated Pela.

I wonder if the dutch word 'plooien' is related to the english word DEPLOY (unfold) and EMPLOY (fold in) and maybe also the PLEATS in the fabric of a curtain or dress. havent checked, just guess.

Here is the Dutch text
www.phys.uu.nl/~loll/Web/press/knutselen.pdf

here is a thread with partial rough English translation
https://www.physicsforums.com/showthread.php?p=588977&postcount=6

OK Marcus I will translate this night some parts and later the others.
Indeed I believe "plooien' is related. We also use 'plooien' in the curtain or dress. Very similar toe 'plooien' is 'vouwen' (like folding something as an action, but also the ripples in the elbow or skin.)
 
  • #11
marcus
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pelastration said:
OK Marcus I will translate this night some parts and later the others.
Indeed I believe "plooien' is related. We also use 'plooien' in the curtain or dress. Very similar toe 'plooien' is 'vouwen' (like folding something as an action, but also the ripples in the elbow or skin.)

thanks, any amount of improvement, substantial, or even just a small correction, would help. but please do not work too hard, unless you enjoy translating! (it is not to lose sleep over :smile:)

the photograph of Loll that they put in that Handelsblad article looks very serious to me. I like this one better
http://perimeterinstitute.ca/images/marseille/marseille103.JPG
where she is out for a walk with friends. Lee Smolin took it.
 
  • #12
marcus
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marcus said:
"Space-time does not claim existence in its own right, but only as a structural quality of the [gravitational] field"."

At one time I found a link to a source for that quote which gives context, but I posted the link and forgot where. Does anyone have a source?

I found the old post with google,
https://www.physicsforums.com/showthread.php?t=5768
it has a link to a longer quote:


Something Einstein wrote in 1952 contains this quote

"Space-time does not claim existence on its own, but only as a structural quality of the field."

It is not especially easy to grasp the meaning, I suspect, but it might be worth thinking about. Eh quoted this in another PF thread and I was able to find an online reference in this Usenet post, which gives more context:

> However, I consider the ultimate words of Einstein on this matter
> to be the fifth appendix, added in 1952 (three years before his
> death), to the fifteenth edition of his book "Relativity: The
> Special and the General Theory." In that appendix, titled
> "Relativity and the Problem of Space," Einstein explicitly
> addresses the issue in question here. (Note that in the following
> "type (1)" space is Minkowski space.)>
> "If we imagine the gravitational field, i.e., the
> functions g_ik, to be removed, there does not remain a
> space of the type (I), but absolutely _nothing_, and
> also no 'topological space'. For the functions g_ik
> describe not only the field, but at the same time also
> the topological and and metrical structural properties
> of the manifold. A space of type (I), judged from the
> standpoint of the general theory of relativity, is not
> a space without field, but a special case of the g_ik
> field, for which -- for the coordinate system used,
> which in itself has no objective significance -- the
> functions g_ik have values that do not depend on the
> co-ordinates. There is no such thing as an empty space,
> i.e., a space without field. Space-time does not claim
> existence on its own, but only as a structural quality of the field"

The Usenet post by Paul Stewart is archived at
http://www.lns.cornell.edu/spr/2003-07/msg0052723.html
 
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  • #13
Hans de Vries
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marcus said:
I was hoping someone who can read Dutch would read the original. I was guessing about some of the words and sometimes only made a very rough approximation in English. Even the title I did not translate exactly, I think! Any help you can give would be appreciated Pela.


Marcus, The German version you have is an exact translation of the original
Dutch version. There's not that much physics in it since it's for a daily journal.

dutch: http://www.phys.uu.nl/~loll/Web/press/knutselen.pdf
german: http://www.phys.uu.nl/~loll/Web/press/NRCdeutsch.htm

The essential points (for english readers) can be found here:

http://focus.aps.org/story/v14/st13

Regards, Hans
 
  • #14
marcus
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Hans de Vries said:
Marcus, The German version you have is an exact translation of the original
Dutch version. There's not that much physics in it since it's for a daily journal.

dutch: http://www.phys.uu.nl/~loll/Web/press/knutselen.pdf
german: http://www.phys.uu.nl/~loll/Web/press/NRCdeutsch.htm

The essential points (for english readers) can be found here:

http://focus.aps.org/story/v14/st13

Regards, Hans

Hans, thanks for your interest! I've looked at the three sources you give here and must say the Focus article by Adrian Cho is good science journalism. Worth recommending as a quick introduction.

I'm interested in the popular press coverage too, for various reasons. I'd like to understand those articles better although there is not much science content. [for example: What is the feeling attached to the dutch word "knutselen" and the german translation of it "basteln"? My dictionary does not do such a good job with colloquialisms and nuances. It just says that "basteln" means to devote hard work to something.]
 
  • #15
Hans de Vries
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marcus said:
[for example: What is the feeling attached to the dutch word "knutselen" and the german translation of it "basteln"? My dictionary does not do such a good job with colloquialisms and nuances. It just says that "basteln" means to devote hard work to something.]

"knutselen" and "basteln" is a very good 1:1 translation. To English the title
would translate best I think to "Playing with quantum foam". However, the
word "playing" misses the aspect that you're actually trying to make/create
something while playing, like in "basteln" and "knutselen"

Regards, Hans
 
  • #16
selfAdjoint
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Hans de Vries said:
"knutselen" and "basteln" is a very good 1:1 translation. To English the title
would translate best I think to "Playing with quantum foam". However, the
word "playing" misses the aspect that you're actually trying to make/create
something while playing, like in "basteln" and "knutselen"

Regards, Hans


Mmm, just to get the sense rather than anything literal, how about "tossing"? You can toss a ball, or a salad, or a bowl on a potter's wheel.
 
  • #17
marcus
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Hans de Vries said:
"knutselen" and "basteln" is a very good 1:1 translation. To English the title
would translate best I think to "Playing with quantum foam". However, the
word "playing" misses the aspect that you're actually trying to make/create
something while playing, like in "basteln" and "knutselen"

Regards, Hans

Another possible English title or headline for the story could be

"Building with quantum foam"

I gather that basteln and knutselen have the sense of

"to do handcrafts, to make stuff or tinker with stuff for fun"

Arts-and-crafts handwork is certainly popular in the United States, there is currently a craze for knitting. Retired people take classes in ceramics and woodworking and weaving and there's all those Martha Stewart glue-gun decoration projects.

but I dont know any general verb for "to do recreational handcrafts".

So maybe the best is just "building with quantum foam"

BTW in the interview Loll says SOME PEOPLE talk about quantum foam, but that is not her choice of words. The journalist put the foam image in the title for his own journalistic reasons, not because that is a central analogy in Loll's work. Yes the title of the popular article is misleading, so what's new.

[MUCH LATER EDIT: this is an afterthought, how about

"Crafting the Quantum Foam"]
 
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  • #18
marcus
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thanks for linking us back to that Good Old Thread of 2003! I had forgotten that thread. Somebody says that scientists have discovered that the color of spacetime is beige and that he thinks it is made of caramel pudding----one may refer to it not as the spacetime continuum but as "the spacetime FLAN'


In this thread a poster named Eh gave a germane quote from Einstein
https://www.physicsforums.com/showthread.php?p=69491&postcount=4


"Space-time does not claim existence in its own right, but only as a structural quality of the [gravitational] field".

to take another look at the thread topic: what is spacetime?

if we go by the accepted wisdom (Einstein) spacetime does not exist, what exists is the GEOMETRY of it (the "field" is just a name for the geometry of spacetime)

so we have come again to the essential question: how can something have a shape if it doesnt exist.

or, as AE put it, if it cannot claim separate existence IN ITS OWN RIGHT. I am not sure what that means but anyway, if something does not exist in its own right as a real something, then how can it have a shape.

how can disembodied geometry exist of its own accord?


so then people go and put the responsibility for existing on MATTER and they say that spacetime geometry (the "field") is just the geometric relationships between bits of matter or between events involving bits of matter, like the collision between my foot and the floor when I take a step.

maybe it is too frustrating to think along these lines.

anyway this thread is supposed to be about what is spacetime, and I think
the people making the most progress now are the Triangulations (CDT) people and they say, if I understand right, that

Spacetime is NOT made of triangles. It just helps to use triangles when you quantize its geometry. And afterwards shrink the triangles down in size, if you can get some more computer time and it is practical to shrink them some more. But spacetime is not made of them, one just uses them because they work.

And they do not say it is "foam" or "fabric" or "fluid" or any kind of substance AFAIK. I didnt hear them use a material analogy. They just want to do the best they can to making a quantum dynamics of the GEOMETRY and not ask, for the time being AFAICS, what IT it is the geometry of.

Please let me know if you got a different impression about the CDT approach.
 
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  • #19
hellfire
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> If we imagine the gravitational field, i.e., the
> functions g_ik, to be removed, there does not remain a
> space of the type (I), but absolutely _nothing_,
I guess this depends on what one defines with “remove the gravitational field”. It seams that Einstein assumed that the removal of the gravitational field is equivalent to the removal of guv. Based on this assumption, the identification between gravitational field and spacetime seams to be a meaningful step, as guv = 0 has no physical meaning. I have no knowledge to question Einstein’s claims, but it seams not very obvious to me that the removal of the gravitational field is equivalent to the removal of the guv. As a first guess I would take “removal of gravitational field” equal to taking no matter sources Tuv = 0, or as a second guess taking zero Christoffel symbols, or may be a zero Riemann curvature tensor. None of them lead to guv = 0. I have to admit that it is unclear to me what “gravitational field” actually means in general relativity, which seams to be the key issue to arrive to such a conclusion:

> There is no such thing as an empty space,
> i.e., a space without field. Space-time does not claim
> existence on its own, but only as a structural quality of the field"
 
  • #20
nightcleaner
Hi Marcus

We have an intuitive idea of space and time from experiments we performed in the cradle with fingers and toes. You remember. There is no such intuitive understanding of gravitational field. I wonder why we should now accept the idea, put forward by Einstein in his years as a self-professed curiosity of socklessness, that the concept of gravitational field should be more fundamental than the intuitive concepts of space and time?

I can and do accept gratefully the notion, due to Einstein and Minkowski, that space and time are equivalent. This is not intuitively obveous. However I have found that space-time equivalence yields fairly easily to the following analysis: an increment of time is required to measure an increment of space. Hence there is no space without time. It remains to wonder if there is any time without space.

Curiously, this seems to reduce in mathematical terms to the meaninglessness of zero in the denominator of rational expressions for motion. I suggest that nothing exists which does not move (this from the idea due to Einstein that there is no preferred reference frame). So, any object has to possess a velocity, expressed as change in spatial position divided by change in temporal position. From this I deduce that taking the increment of time to zero results in the disappearance of the notion of "object." Or, equivalently, that zero time results in the notion of object being generalized to the notion of the universal set. If time is zero, nothing is everything. Everything is nothing.

Since we must posit that we exist, there is something. Hence time cannot go to zero. The notion of instantaneous velocity is a convenient and useful fiction. We do of course take change in distance to zero, meaning only that the measured object is moving in the same frame as the observer. Does this allow us to say that space-time equivalence means only that space and time are proportional, and not that they are identical?

I think not. (Please do not let that be my epitaph!) The notion of point-like particles is, I do think, like the notion of zero time, a useful fiction. I imagine that it was thoughts like these that led Planck to declare the action potential, and Heisenburg to declare the uncertainty principle.

That's my coffee.

On a personal note, if anyone wonders where I have been lately, the garden is planted and the cottage now has windows and a door to go with the floor, walls and roof. None of this is my doing, but it has been interesting to watch. Heat, telephone, and electricity may arrive eventually. Plumbing is that bucket in the corner.

Be well,

Richard
 
  • #21
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I suggest that nothing exists which does not move (this from the idea due to Einstein that there is no preferred reference frame).
What about at the center of the Universe? (The point where everything is moving away from).
 
  • #22
marcus
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nightcleaner said:
... the idea, put forward by Einstein in his years as a self-professed curiosity of socklessness,...

That's my coffee.

On a personal note, if anyone wonders where I have been lately, the garden is planted and the cottage now has windows and a door to go with the floor, walls and roof. None of this is my doing, but it has been interesting to watch. Heat, telephone, and electricity may arrive eventually. Plumbing is that bucket in the corner.

...

this is an interesting point. can one trust the judgement (about space, time, and other serious questions) of someone who wears no socks?

Hi Richard, congratulations on getting the garden in. sounds like the cottage is quite habitable now, apart from utilities.

Unless I'm confusing places, the cottage is out in the woods, by a slough, and (I thought) inaccessible in the winter. I think I remember that you left your espresso maker there last fall and had to go the whole winter without espresso---something that added savor to returning this spring.

Now it is sounding more year-round accessible and habitable. Am I confusing cabins and cottages? Will this cottage be made comfortable in all seasons? I must have missed part of the story.
 
  • #23
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People are often surprised to hear that space and time had a beginning. This is due to their everyday notions of these familiar concepts. They say, 'The Universe must be expanding into something'. However, physicists suggest that prior to the inception of our 4D spacetime, there was a spaceless, timeless state. This could perhaps be thought of as a completely blank state, but still something in a sense, where our spacetime could then expand into. The attributes attached to spacetime could not be applied to such a state, but it would nonetheless be something.

Anyways, the nature of space and time will surely be elucidated when a complete quantum gravity theory is formulated.
 
  • #24
nightcleaner
Hi Marcus, and all...

My cabin is built of natural aspen logs in a hundred year deep woods at the end of half a mile of twin ruts through swamps and drumlin fields. It is seven miles from pavement and another eight miles on secondary roads to town. It is accessible in winter by snowshoe but I don't do that much anymore since I broke my leg three years ago. It has been standing there for twenty-six years.

The cottage, on the other hand, was built this year from squared aspen timbers by my friend Mark, who twitched the logs out of the woods with his team of horses, milled them into timbers, planks, and boards on his property north of Grand Marais last summer, and brought them down the Northshore road on his two ton truck in the fall. Mark is a very resourceful fellow who loves to talk about theoretical physics. He homeschools his two daughters and is a master horseman. I will see them again at solstice in a couple weeks.

The cottage is twelve by sixteen with a sleeping loft and stands on concrete posts sunk four feet into gravel. It has a porch and a sheet steel roof with wide eaves. Makes me wish I played the banjo, or at least a harmonica. It is tucked under maple and birch trees and has a fine view of a cedar swamp. Fifty feet back from The Hill Road, a designated natural and scenic byway, only twelve miles to town in beautiful Clover Valley, a region of abandoned dairy farms now being resettled by a new wave of urban refugees.

The cottage belongs to my friend Peg, who built it so I would have a place to stay while careing for her gardens and animals. She loves her lifestyle, which she thinks of as "technicolor Amish," but wants the freedom to travel. I will maintain a work/rent relationship, a kind of tennant-farmer, no, sharecropper, no, indentured servant, no, serf, no, Kulak. I don't want to go any lower than that. Back to my peasent roots. My friend Micheal, who incidently also recently started in a position as caretaker (to a landholder in Chico, California), thinks of himself as Nomadic Eclectic Bohemian, which is perhaps descriptive of my own position, altho I would think Semi-nomadic in my case.

So the cabin belongs to me, is primitive and remote, and has the advantage of being totally isolated, nearest neighbor half a mile by trail, two miles by road. The cottage by contrast belongs to my friend Peg, is on a maintained road, and is a few hundred yards (over the ridge) from Peg's house. The two places are nine miles apart as the crow flys, but nearly twenty miles apart by road. In one, I am a freeholder, in the other I am a serf. That's life for you.

Marcus, I am a poor man, but free as long as I don't care for comfort, safety, or human relationship. For these things I must depend on the generosity of my betters. Thank you for being here, and for sharing the wealth of your library. Reminds me of a poem by Gary Snyder....something about "sorry I broke into your house last night, but I had to use the dictionary....."

Be well,

Richard
 
  • #25
nightcleaner
Curious6 said:
People are often surprised to hear that space and time had a beginning. This is due to their everyday notions of these familiar concepts. They say, 'The Universe must be expanding into something'. However, physicists suggest that prior to the inception of our 4D spacetime, there was a spaceless, timeless state. This could perhaps be thought of as a completely blank state, but still something in a sense, where our spacetime could then expand into. The attributes attached to spacetime could not be applied to such a state, but it would nonetheless be something.

Anyways, the nature of space and time will surely be elucidated when a complete quantum gravity theory is formulated.

Hi Curious6;

I too hope for an elucidation of space and time, but I am not patient enough to wait for someone else to report on it. I want to go there myself, put my stake in the ground, even if it happens that the survey lines shift and change, leaving me owner of a piece of swamp instead of the hilltop I thought I had. Oh well.

I would like to engage you in a conversation about the meaning of "beginning" in 4D spacetime. However I do not know where to start. But consider the big bang singularity, surely a unique spacetime event if there ever was one. Where did it happen? How is it that we must look to the ends of the universe in every direction to see it, by virtue of the cosmic microwave background energy, when surely all the Hubble shifts must point to a single place and time? Does the universe have definable edges and a determinable center? Can we go and sit there and look out equally in all directions to eternity? If you stand at the North Pole and proceed in a northerly direction, where will you go?

I am not trying to be a wise guy. If there is no answer, we must find better questions.

Be well,

Richard
 

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