What is "spacetime" really?

[Michael Mooney]

In answering, you may correctly assume that I have thorougly studied the background of the "spacetime" component of relativity theory. Yet the actual nature of the "medium" (or whatever it is *supposed* to be) has never been explained to my satisfaction.

 

I am familiar with all the usual graphic representations, for instance, including the balls in a blanket, making it sag and creating orbits in the smaller balls "rolling" around the big one. Like I said in my "intro"... cool graphics, but...

 

In the extreme example, black holes are said to "curve space" around them via the extreme gravity wells they create, from which not even light can escape.

I get that we don't know exactly how gravity works, but why posit an actual "fabric" and call it spacetime to "explain" it? Seems it only creates a theoretical concept for everyone who is intimidated by Einstein's genius to accept without question... a scientific version of the "fabric" of "The Emperor's New clothes."

 

I know also about the new satellite telemetry which is said to confirm "frame dragging" as solid evidence of distorted "spacetime" around Earth. Yet the irregular topography of Earth's gravitational field explains the anomalies of the tracked satellites' orbits without insisting on "spacetime distortion."

 

Same with the whirlpool effect around black holes. Material is clearly "going down the drain" into the black holes in a spin just like water, but why cite it as another confirmation of this mysterious *theoretical fabric*, "spacetime?

 

Enough for now. Anyone?

[Tormod]

You post a good question.

 

Nobody really knows what "spacetime" is (just as we have no good explanation of what exactly "time" is). We do however know a few things about how it behaves. On quantum levels, spacetime has different properties than on macroscopic levels. This much is known because of studies in fundamental particle physics (for example in the particle colliders).

 

Spacetime isn't an attempt to "create something where there is nothing". Rather it is an attempt at explaining what there *is* that comprises the space between "things" in the cosmos.

 

That objects bend the paths of light traveling through space was proven back in 1919. The only apparent explanation for this (since light is a massless particle) is that light travels "through" something. Since the aether had been disproved there was no need to add a cosmic "fluid".

 

There was however a need to unite space and time since it was apparent that light traveled through space, and that the speed of light was finite (another of Einstein's insights). Thus the way light shifts when it passes around an object can be used to measure the gravitational pull of that object. In relativity theory, gravitational pull is actually a distortion of spacetime. Spacetime is, however, four dimensional, and therefore the "sheet with holes" is used to simplify and make it possible to understand the concept.

 

This is an interesting Wiki entry:

Tests of general relativity - Wikipedia, the free encyclopedia

[Michael Mooney]

Thank you Tormod for you prompt reply.

But your answer does not seem to address the title question. Not only does

"Nobody really knows what 'spacetime' is"... I am questioning *whether or not 'it' actually exists other than a handy metaphore or concept... like "time"... the measured "duration" of any "event", though the ongong present is not naturally segmented into "units of time" in nature.

Rather you replied that "We do however know a few things about how *it* behaves." You posit "it" as already existing as an actual entity which exibits behavior. This assumes its existence, bypassing my question altogether.

Thank you for the Wikipedia link. I am familiar, in various degrees with the content it covers, but it does not verify spacetime as a malleable medium.

 

If space is actually the emptiness in which all observable phenomena take place, being nothing in and of itself, then the mystery of gravity (how it works) remains a mystery without the assertion that this "fabric" is indeed *something real* as in my reference to "The Emporerer's New Clothes."

 

You conclude by, again assuming the actual existence of "spacetime"... which, as I said begs the original question.

 

"In relativity theory, gravitational pull is actually a distortion of spacetime. Spacetime is, however, four dimensional, and therefore the "sheet with holes" is used to simplify and make it possible to understand the concept."

 

We all know that the vector of light is bent by gravity, as amply reviewed in the Wikipedia article. Yet you assert again that gravity distorts "spacetime" and, further, that "it" is "four dimensional." Yes, the graphics of the sagging sheet are fine as aids to understanding the vectors which gravity creates in light and massive objects. Yet the agent of gravity remains a mystery, and creating a theoretical "distorted fabric" adds no understanding to the mystery... especially if, in fact, space remains the emptiness in between "things" and "time" merely the measure of selected "event duration" in an "always-now-everywhere" universe.

Please answer without assuming a-priori the existential validity of spacetime as an actual medium... i.e., the point of the title question.

Thanks.

[modest]

You posit "it" as already existing as an actual entity which exibits behavior. This assumes its existence, bypassing my question altogether.

 

Special relativity works on flat Minkowski spacetime (as I'm sure you know). SR also works (we know, experimentally) in small areas of freefalling space along the gravitational potential of a body. This means the *geometry* of space is somehow fundamentally curved. This is explained in the Wiki for General Relativity from section 2 to section 4.

 

If you'd like to debate the ontological questions regarding spactime then the Q&A forum is probably not the best place. The Philosophy of Science forum would be better.

 

~modest

[Michael Mooney]

Special relativity works on flat Minkowski spacetime (as I'm sure you know). SR also works (we know, experimentally) in small areas of freefalling space along the gravitational potential of a body. This means the *geometry* of space is somehow fundamentally curved. This is explained in the Wiki for General Relativity from section 2 to section 4.

 

If you'd like to debate the ontological questions regarding spactime then the Q&A forum is probably not the best place. The Philosophy of Science forum would be better.

 

~modest

Thanks for another prompt reply. (I'll be gone for a few days after this reply.)

 

Yes, I am familiar will all the well known theories of spacetime.

The "freefalling space along the gravitational potential of a body" still posits space as a thing, and then you go on to reiterate the theory that *it* has a geometry which must be curved, because of the curved trajectory of objects which "fall" toward other attracting masses.

 

I am asking for answers to the properties of spacetime as an actual medium, *if* "it" is actually more than a juxtaposition of "space" as emptiness and "time" as measured event duration.

 

I do not consider the title question as an invitation to "debate the ontological questions regarding spactime" but simply to get a straight answer to "What is it?"

It seems to have begun as a theoretical mental graphic aid to understanding the effects of gravity (Einstein and Minkowski) and, in the course of time, became *reified* into an actual entity. The nature of this possible reification, or of the "entity itself" if it is an actual malleable medium is the subject of my original question.... which has not yet been addressed here, given that both replies so far are based on thea-priori assumption of the actual existence of the entity/medium (whatever) "spacetime."

 

Back in a few days. Hope I have clarified my inquiry sufficiently.

Michael

[freeztar]

Spacetime is not an "entity".

 

It seems that it is the "time" part which is bothering you. That is why Modest suggested an ontological discussion.

 

In fact, it might be best to start with the in-depth (and often belabored) discussion of time that already exists here.

 

http://hypography.com/forums/philosophy-of-science/3650-what-is-time.html

[modest]

I've moved the thread from the Q&A forum as these are most properly questions pertaining to the philosophy of science and are most likely to generate useful responses in this forum.

 

It seems to have begun as a theoretical mental graphic aid to understanding the effects of gravity (Einstein and Minkowski) and, in the course of time, became *reified* into an actual entity. The nature of this possible reification, or of the "entity itself" if it is an actual malleable medium is the subject of my original question....

 

You are correct, spacetime is not a "malleable medium". It is not an aether such as an aether is commonly thought. It is precluded from having any property which imparts motion on a material in a frame-dependent kind of way. This is explained by Einstein here:

 

Ether and the Theory of Relativity

 

Neither is spacetime a background that objects interact 'on'. Einstein believed and said that the only meaningful indication of space and time are through matter. If you were to remove all matter then you would be left with nothing—rather than being left with space and time. These two sentiments are summed up in these 2 quotes:

All our spacetime verifications invariably amount to a determination of spacetime coincidences. If, for example, events consisted merely in the motion of material points, then ultimately nothing would be observable but the meeting of two or more of these points.

People before me believed that if all the matter in the universe were removed, only space and time would exist. My theory proves that space and time would disappear along with matter.

These ideas are well-known arguments in the philosophy of science regarding what exactly space and time are. Look around this forum and you'll see many threads dedicated to this topic (freeztar points out a good one). There are very honest and valid questions regarding the ontology of spacetime. This is not an impediment to physicists referring to the concept as a "thing" as you suggest. But, they are good questions nonetheless.

 

~modest

[Doctordick]

Thank you Modest for posting a link with direct conformation of my major complaints with Einstein’s theory of relativity. I find it astounding that it is Einstein himself who brings these issues to the front. I was told for over fifty years (by many supposed physics authorities, professors et al) that Einstein’s theory of relativity was not an “ether” theory; whereas it always struck me as inherently an ether theory. Note the following quote taken from that presentation credited to Einstein seem to me to confirm my impressions.

What is fundamentally new in the ether of the general theory of relativity as opposed to the ether of Lorentz consists in this, that the state of the former is at every place determined by connections with the matter and the state of the ether in neighbouring places, which are amenable to law in the form of differential equations,; whereas the state of the Lorentzian ether in the absence of electromagnetic fields is conditioned by nothing outside itself, and is everywhere the same. The ether of the general theory of relativity is transmuted conceptually into the ether of Lorentz if we substitute constants for the functions of space which describe the former, disregarding the causes which condition its state. Thus we may also say, I think, that the ether of the general theory of relativity is the outcome of the Lorentzian ether, through relativation.

Or near the end of his presentation, he says:

Recapitulating, we may say that according to the general theory of relativity space is endowed with physical qualities; in this sense, therefore, there exists an ether.

Finally a major complaint I have always had with Einstein’s theory of relativity is that it is a it is a static theory in that the concept of dynamic motion is totally absent.

The idea of motion may not be applied to it.

Finally, he himself confirms that the very essence of my work is the goal everyone should be looking for.

Of course it would be a great advance if we could succeed in comprehending the gravitational field and the electromagnetic field together as one unified conformation. Then for the first time the epoch of theoretical physics founded by Faraday and Maxwell would reach a satisfactory conclusion.

If anyone is interested in understanding such a unification, I suggest they make an attempt to understand the posts I have been making to this forum.

 

Have a ball guys -- Dick

[modest]

Thank you Modest for posting a link

 

No problem

 

with direct conformation of my major complaints with Einstein’s theory of relativity.

 

While I’ve read the link in question and some of your complaints against Einstein, I’m unsure exactly what you’re referring to.

 

I find it astounding that it is Einstein himself who brings these issues to the front. I was told for over fifty years (by many supposed physics authorities, professors et al) that Einstein’s theory of relativity was not an “ether” theory; whereas it always struck me as inherently an ether theory.

 

Well this may just be a matter of semantics. GR is certainly not an “aether theory” in the traditionally used sense of the term. It’s not a 19th-century-like luminiferous aether as relativity, in fact, kills the idea of such a thing as is discussed in the link. But, GR does have action at a distance and it does have a field that is present in all of space. I think it was easer in the 1920’s to call these aether-like properties than it is now, but the implications (which Einstein discusses in the link) remain the same.

 

The semantics of the word "ether" in regards to SR vs. a Lorentzian ether is summed up here:

Einstein in later years proposed calling empty space equipped with gravitational and electromagnetic fields the "ether", whereby, however, this word is not to denote a substance with its traditional attributes. Thus, in the "ether" there are to be no determinable points, and it is meaningless to speak of motion relative to the "ether." Such a use of the word "ether" is of course admissible, and when once it has been sanctioned by usage in this way, probably quite convenient.

 

http://www.tc.umn.edu/~janss011/pdf%20files/ether.pdf

You would have to be very explicit in which properties of the theory you are criticizing, how exactly you would change them, and what the implications of those changes would be. Because, saying GR is an “aether theory” is far too inexact to be a criticism at all.

 

My intention in posting the link was addressing Michael’s question about spacetime being an “actual malleable medium”. This is not the case (as I believe nobody including Michale would disagre) and the link does a good job of explaining why no such "medium" can have the property of being “malleable”, or as Einstein puts it: any “mechanical quality”.

 

~modest

[Rade]

Here is my philosophic answer to the OP question:

 

Definition: 'time' = that which is intermediate between two moments (two nows)

Definition: 'space' = that which is intermediate between two existents (two objects)

 

therefore,

 

'spacetime' = that which is intermediate between moments of existence.

[Doctordick]

Definition: 'time' = that which is intermediate between two moments (two nows)

But now you will have to define a "moment".

Definition: 'space' = that which is intermediate between two existents (two objects)

And, in addition, you need to define "existence" and/or an object.

[

therefore,

One thus can not even get to the "therefore".

 

Have fun -- Dick

[modest]

But now you will have to define a "moment".

 

Very ture Doctordick. I'd prefer to call it an instant rather than a moment. If the "present instant" can be defined as the border between the past and future (as you sometimes say) then perhaps an "instant" is a present instant that is not necessarily present.

 

In other words, if "present instant" can be defined as the border between two things, then perhaps "instant" can be defined as the border between two things as well.

 

~modest

[Essay]

Originally Posted by Einstein

People before me believed that if all the matter in the universe were removed, only space and time would exist. My theory proves that space and time would disappear along with matter.

~modest

Hey, thanks for sharing!

That reminds me of my own take on this subject:

 

That spacetime is an artifact of matter/energy. Matter/energy generates spacetime. Spacetime is a dimension generated by the interaction of the other 8 (?) dimensions (which generate matter/energy).

 

It sure is hard to verbalize, but you should see my 'magination...

"...The Colors of Infinity."

NOVA | Hunting the Hidden Dimension | Links and Books | PBS

Colours of Infinity Links

 

~ :naughty:

[Michael Mooney]

Spacetime is not an "entity".

 

It seems that it is the "time" part which is bothering you. That is why Modest suggested an ontological discussion.

 

In fact, it might be best to start with the in-depth (and often belabored) discussion of time that already exists here.

 

 

Thank you for the link to the "Time" thread, freeztar, but I found the 58 or so pages a bit more than I could reasonably handle, give that so many posts were just capricious, witty, opinionated... not actually scientifically astute dialogue on what "time" is. Granted, any such contributions (or all of them) were lost in the pile of less-than-serious-or intelligent replies.

 

(A personal note: I have infrequent access to the internet, not unlimited time to peruse, so I will get right to it and try to reply to the other responses, in consecutive order, since iI last posted.)

 

First, I an left wondering whether anyone here understood my brief comments on time. Seems it is only an artifact of the human mind in that the *duration of a selected event* is arbitrarily dependent on the observer's selection of "an event" and how "long" the intended observation lasts... i.e., a nano-second, a millennium (based on number of Earth orbits) or a full ""Bang/Crunch" cosmological cycle, if one were to subscribe to that model.

An aside:

(What other model accounts for the origin of cosmos... if "something out of nothing" is taken to be as absurd as religious "creation-ism"... where it all came out of "God's magic hat... a myth without scientific credibility?)

 

Time is clearly the concept of "event duration" arbitrarily designated as above. "It" (same "it" as in "It is raining", i.e., no agent or entity involved) is always *now* everywhere... transcending local observation of light as limited by its speed limit. This is a "cosmic perspective"... a mental exercise much like Einstein's exercises but not limited to local perspective.

 

Anyway, since time is not actual but a nominal human artifact ( observed, conceptual seconds, minutes, days, years, millennia, etc) and space remains the emptiness between observable phenomena *in space*, my question stands unanswered.

 

What is "Spacetime" supposed to be? Does not gravity, its agent and active force still a mystery, remain the mystery of the movement of massive objects being pulled together as per the old "universal law of gravitation" (directly with massiveness and indirectly with distance... no distance limit...?) Does gravity need a "fabric, spacetime" to explain the observed effects... including "bent light" (its momentum acting exactly like mass) without the superfluous concept of "spacetime?"

Seems a violation of "Occam's Razor" to me to posit an unnecessary "fabric" which still doesn't contibute an explanation of gravity's active agent, its "action-at-a-distance."

 

This is all the time I have right now. Hope to be back soon to address the other replies. (Also no time to proof-edit... sorry, gotta go.

 

If the explanations lie in links nI have not yet read, I will study them and comment in my next post, which may be a few more days away.

Thank you.

 

Michael

Just a quick afterthought before I run:

Between "the furure" which is not yet real and *present* and *the past* which is not still real and *present* is the ongoing, timeless now... the eternal, omni-present present. "Units of time" are all mental/observational artifacts. A day is still one rev. and a year is still one orbit, etc., but there is nothing in all that to conrtibute to a "real fabric" which can be curved around masses with obvious gravity... nor into a "sphere of space" which captures light and all matter in all black holes.

 

See ya next time.

[Essay]

MM, don't think my "fractals links" have any overt connection to explaining this stuff. They only serve to help break linear, 3-D type preconceptions (though imho, fractals need to be worked into our equations of horizons, fields, etc.).

 

I should probably read the time thread too, but let me just add a bit:

 

Time shouldn't be thought of as a line of measurement (which is simply how we conceptualize it);

Time should be thought of only as change.

Without space to change (w/ the flux of matter/energy), what sense would time make?

Without time, what sense would space make, with no movement, perception, or interaction?

 

You're pursuing different semantic paths in describing space and time. It should be the same path.

Thinking of the emptyness between objects as "space," we should think of "now" as the emptyness between events (changes).

Interestingly as you localize space, the now increases in duration; or as you expand the space measured, then the now between events get very quick. ...or something like that....

 

~ ;)

 

p.s. If you want to pin down the what, of 'what' is spacetime, I would go with:

Spacetime is an illusion. :naughty:

[watcher]

imo, relativity is already here to tell us that nothing has independent existence.

space time matter mutually arise with one another.

you can't even define one without referring to the others.

 

for me, time is the gap between one state to another state.

the present state of the universe is the result of all the changes it underwent.

space and mass are energies of varying densities.

i see motion as a unified action that we breakdown into a relata of spacemass over time.

[Doctordick]

Hi modest, I didn't answer you earlier because I didn't think it worth the effort. To quote Michael, most responses to almost all threads are indeed “less-then-serious-or intelligent replies” and I suspect this thread is also headed down that same dark tunnel. That is why I made that suggestion via the “Creating a 'maturing' forum!” thread.

Well this may just be a matter of semantics. GR is certainly not an “aether theory” in the traditionally used sense of the term. It’s not a 19th-century-like luminiferous aether as relativity, in fact, kills the idea of such a thing as is discussed in the link.

Yes, I suppose it is a semantic issue. In my head, the essence of an “aether theory” is that it gives substance and/or existence to the background of reality (what is commonly referred to as empty space) and Einstein's theory in no way gets rid of such a concept. When you speak of the “19th-century-like luminiferous aether” you are doing little more than bringing in the supposed properties of that aether. I read articles all the time which give “properties” to Einstein's aether: some say it's “foamy” on a small scale or, rotation tends to “twist it about the rotating object”.

 

People invariably talk about space-time as if it exists as a real entity unto itself: that alone makes his theory an “aether theory”.

You would have to be very explicit in which properties of the theory you are criticizing, how exactly you would change them, and what the implications of those changes would be. Because, saying GR is an “aether theory” is far too inexact to be a criticism at all.

Not really. My fundamental complaint is that it is a thing which is hypothesized to exit. No reason (other than it the fact that the picture yields results you like) is given for that presumption at all. Suppose no such thing exists! If that is the case, the whole theory (as it is commonly understood) just goes down the tubes.

My intention in posting the link was addressing Michael’s question about spacetime being an “actual malleable medium”. This is not the case (as I believe nobody including Michale would disagre) and the link does a good job of explaining why no such "medium" can have the property of being “malleable”, or as Einstein puts it: any “mechanical quality”.

Clearly it is not “malleable”; in his picture it is a totally unchanging aether (motion is not an aspect of his “space-time” aether).

 

But let us get to the reason I am posting today.

Very [true] Doctordick. I'd prefer to call it an instant rather than a moment. If the "present instant" can be defined as the border between the past and future (as you sometimes say) then perhaps an "instant" is a present instant that is not necessarily present.

 

In other words, if "present instant" can be defined as the border between two things, then perhaps "instant" can be defined as the border between two things as well.

This post seems somewhat intelligent; however, I would see nothing wrong with simply referring to an “instant” being exactly the same as what I have defined to be a “present”: i.e., a change in the past (what is known). I seriously doubt that I used the word "between" but perhaps I did; I do get sloppy sometimes. The real problem here is that you have not defined what you (or Rade) mean by the word “between”.

 

How about you give me a definition for that term which does not depend upon your definition of space or time: i.e., a definition which is not dependent upon your presumption of what “space and time” are supposed to be. The issue here is, “what comes first”. If you are going to define “space-time”, you need to tell me exactly what it is without invoking usage of concepts such as space and/or time.

 

You show signs of being able to think; put a little time into the issue.

 

Have fun -- Dick