Spacetime doesn't really exist does it?

[andrewkirk]

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. For example, we say that spacetime is curved as if it is a thing in the universe that is curved. If hypothesising about multiverses we might talk about there being numerous distinct spacetimes as if they were all different things.

On reflection though, I have gravitated towards the following view:

1.- spacetime is just a mathematical model that's useful for predicting the interaction of mass-energy
2.- it has no independent existence of its own and without mass or energy there would be no meaningful concept of spacetime
3.- when we talk about spacetime being curved we are just talking about properties of the theoretical 4-dimensional manifold we use to determine what path light rays will follow, particles will fall etc.

Does this view make any sense?

 

[Dale]

1.- X is just a mathematical model that's useful for predicting Y2.- it has no independent existence of its own and without Y there would be no meaningful concept of X

Take almost any scientific concept X and you could make a similar statement.

 

[Mentz114]

Does this view make any sense?

It does to me. Surely all mathematics is abstraction ? What is amazing that nature can be so well described by it. Maybe mathematics contains all possible physical laws and it's just a matter of finding those that explain the data best.

Pretty soon I expect someone will point out that space exists ( we can measure it with rulers), and so does time because clocks measure it. But spacetime is a weird mixture of these with possible curvature or translational defects which look like abstractions to me.

 

[rogerl]

Spacetime can't not exist. Why. Just imagine the Big Bang. Spacetime grows from near Planck size to the entire universe with billions and billions of galaxies. How can something not real grow. Hence spacetime as manifold should be something that exist and can grow and morph. How can mere math models produce the Big Bang. Space actually expand. How can space expand if space (or spacetime) is not some kind of fabric like thing.

 

[bcrowell]

2.- it has no independent existence of its own and without mass or energy there would be no meaningful concept of spacetime

Gravitational waves have energy, and they consist of nothing but pure spacetime.

 

[marcus]

...
On reflection though, I have gravitated towards the following view:

1.- spacetime is just a mathematical model that's useful for predicting the interaction of mass-energy
2.- it has no independent existence of its own and without mass or energy there would be no meaningful concept of spacetime
3.- when we talk about spacetime being curved we are just talking about properties of the theoretical 4-dimensional manifold we use to determine what path light rays will follow, particles will fall etc.

Does this view make any sense?

I think it makes sense and it has a nontrivial content. Points in space or spacetime have no independent existence.
There is something called "the hole argument" that makes this clear. A geometry is an equivalence class of manifolds and metrics. It is not the unique correct metric that expresses the world's geometry. A geometry is more abstract, more like a web of relationships between events. There is no objectively real rubber sheet. I find it hard to say.

We talked about this here:
https://www.physicsforums.com/showthread.php?t=166997

Einstein said something about this when talking about the principle of General Covariance (diffeomorphism invariance):

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

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

for an online source see page 43 of
www.tc.umn.edu/~janss011/pdf%20files/Besso-memo.pdf[/URL]

In another context ( In the introduction of the paper on the perihelion motion presented on 18 November 1915), Einstein wrote
“[B]durch welche Zeit und Raum der letzten Spur objektiver Realität beraubt werden,[/B]”

“[i]... time and space are robbed of the last trace of objective reality[/i]”



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.
==================================

There is something meaningful here. It's not just empty words.

I find it hard to pinpoint exactly. Maybe he is saying that locations have no objective reality. the story is told in terms of relationships. Events are located in relation to other events.

Bohr said something like this: Physics is not about how nature is. It is about what we can say of it.

All we have are the measurments we can make, and the predictions and the observations.
We cannot make absolute statements about what nature is. [B]We can only say how it responds to measurements.[/B]

This also is a kind of relational point of view---coming from the other direction, from QM rather than from GR.

Ultimately a physical theory boils down to the finite information we have, and how these finite pieces of information are related. There is no spacetime continuum, and no wave function either. they are mathematical conveniences.

It's past my bedtime, Mr.Kirk. I've tried to respond to what you said, but may not have managed to be very coherent.

 

[rogerl]

I think it makes sense and it has a nontrivial content. Points in space or spacetime have no independent existence.
There is something called "the hole argument" that makes this clear. A geometry is an equivalence class of manifolds and metrics. It is not the unique correct metric that expresses the world's geometry. A geometry is more abstract, more like a web of relationships between events. There is no objectively real rubber sheet. I find it hard to say.

We talked about this here:
https://www.physicsforums.com/showthread.php?t=166997

Einstein said something about this when talking about the principle of General Covariance (diffeomorphism invariance):

At the end of the above thread you shared. There was this statement by a poster called Mejennifer and she said (no follow up to it):

"At any rate, I am not aware of any experiment that proofs or even suggests that reality is diffeomorphism invariant (whatever that might mean). And I think it is even doubtful if an experiment could ever determine that."

Is that true. No experiments can determine General Covariance (diffeomorphism invariance)? If not and experiments have proven it, is this proven categorically already?

If so, and spacetime points dont' have solidity and it is just the relationship of the geometry, then it's very highly likely that this universe is just a simulation in a computer or occurring in some distance horizon in the holographic principle and our universe some kind of projection. This makes sense in light of the fact that spacetime and the wavefunction don't really exist but just equations.

 

[Garth]

Gravitational waves have energy, and they consist of nothing but pure spacetime.

This statement will prove that spacetime does have an independent existence of its own ONCE gravitational waves have been detected.

As their detection has been sought so far for about forty years without success I would say it is still an open question...

Garth

 

[rogerl]

At the end of the above thread you shared. There was this statement by a poster called Mejennifer and she said (no follow up to it):

"At any rate, I am not aware of any experiment that proofs or even suggests that reality is diffeomorphism invariant (whatever that might mean). And I think it is even doubtful if an experiment could ever determine that."

Is that true. No experiments can determine General Covariance (diffeomorphism invariance)? If not and experiments have proven it, is this proven categorically already?

If so, and spacetime points dont' have solidity and it is just the relationship of the geometry, then it's very highly likely that this universe is just a simulation in a computer or occurring in some distance horizon in the holographic principle and our universe some kind of projection. This makes sense in light of the fact that spacetime and the wavefunction don't really exist but just equations.

Another thing. I was re listening to Brian Greene The Hidden Reality audio and he said something like this:

"If instead of zero, he entered a positive number on the third line, endowing the spatial fabric with a uniform positive energy, he found every region of space would push away from each other producing what most physicists thought impossible, repulsive gravity... "

Brian was describing the cosmological constant. If space was not an entity, where is the cosmological constant repulsive gravity contained, if not in the "fabric" of space? Einstein proposed this after 1920's so didn't it conflict his Hole Argument?

 

[A.T.]

Frequently it is referred to as if it is a thing that exists, like mass and energy.

If you frame it like this, then yes: It "exists", in the same sense in which "mass and energy" exist

1.- spacetime is just a mathematical model that's useful for predicting the interaction of mass-energy

The concepts called "mass" and "energy" are just parts of models as well.

 

[PhilDSP]

Due to the size of the material, it's difficult. I'd like to find a quote from Maxwell's treatise I remember where he points out that all mathematical models are ultimately suspect in representing all facets of reality. They sooner or later break down and misrepresent some aspect of reality (whatever that is, apart from what we can measure and sense)

 

[bcrowell]

This statement will prove that spacetime does have an independent existence of its own ONCE gravitational waves have been detected.

As their detection has been sought so far for about forty years without success I would say it is still an open question...

They haven't been detected directly, but we can be absolutely certain that they exist and carry energy, since we see energy disappearing from the Hulse-Taylor binary pulsar at exactly the rate predicted by GR.

 

[Garth]

They haven't been detected directly, but we can be absolutely certain that they exist and carry energy, since we see energy disappearing from the Hulse-Taylor binary pulsar at exactly the rate predicted by GR.

I am a relativist and 'believe' that spacetime together with gravitational waves 'really exist'; however I concede that, unless detected in controlled laboratory experiments such as LIGO, alternative theories, which explain gravitational wave energy loss (and spacetime cuvature effects) by other means, cannot be excluded.

We see energy disappearing from binary pulsars as predicted by GR, but that might be because GR emulates a deeper theory, which 'we' will call 'quantum gravity' for the sake of argument, that might explain gravitational forces, and the slowing down of the binary pulsar system, not as the effect of spacetime curvature but by the exchange of graviton virtual particles.

Garth

 

[nitsuj]

If you frame it like this, then yes: It "exists", in the same sense in which "mass and energy" exist


The concepts called "mass" and "energy" are just parts of models as well.

That's really well said.

Also, I think there are very well defined physical limitations for spacetime. That makes it seem really real to me.

Gravitational waves have energy, and they consist of nothing but pure spacetime.

That comment also makes it seem really real.

 

[bcrowell]

I am a relativist and 'believe' that spacetime together with gravitational waves 'really exist'; however I concede that, unless detected in controlled laboratory experiments such as LIGO, alternative theories, which explain gravitational wave energy loss (and spacetime cuvature effects) by other means, cannot be excluded.

We see energy disappearing from binary pulsars as predicted by GR, but that might be because GR emulates a deeper theory, which 'we' will call 'quantum gravity' for the sake of argument, that might explain gravitational forces, and the slowing down of the binary pulsar system, not as the effect of spacetime curvature but by the exchange of graviton virtual particles.

I would consider this a valid reason for skepticism if there was an alternative to GR that was (a) well-motivated, (b) consistent with current observations, and (c) described energy loss from the Hulse-Taylor system as being due to something other than gravitational radiation, and (d) made predictions that differed from GR's in some way that was testable.

Quantum gravity fails to satisfy c (and possibly d as well). Everybody doing current work on quantum gravity wants very much to make sure that whatever theory they manage to come up with reduces to GR in the classical limit. That means that if their theories were known not to have gravitational waves in them, they would already have been discarded as candidates for a theory of quantum gravity.

In any case, quantum effects are irrelevant here, because the Hulse-taylor system doesn't have any structure on the Planck scale, so it is purely classical, to an incredibly high degree of precision -- more classical than a basketball sailing through the air.

 

[harrylin]

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. For example, we say that spacetime is curved as if it is a thing in the universe that is curved. If hypothesising about multiverses we might talk about there being numerous distinct spacetimes as if they were all different things.

On reflection though, I have gravitated towards the following view:

1.- spacetime is just a mathematical model that's useful for predicting the interaction of mass-energy
2.- it has no independent existence of its own and without mass or energy there would be no meaningful concept of spacetime
3.- when we talk about spacetime being curved we are just talking about properties of the theoretical 4-dimensional manifold we use to determine what path light rays will follow, particles will fall etc.

Does this view make any sense?

1: Nearly so. Space-time has been a graphical representation of the mathematical description of positions and motions for centuries. Obviously that makes sense. And you might consider such concepts as "space" and "time" more abstract than "energy" and "mass" - but that is probably a matter of opinion.
2. Indeed, without our material world we would not have the means (or at least, not the same means) to define those concepts.
3. It is a fact that "curved spacetime" refers to the graphical (geometrical) description of the mathematical relationship - at least, that is the way people like Einstein used that kind of expressions. See for example http://www.bartleby.com/173/17.html : "space-time" describes the world of physical phenomena.

 

[rogerl]

Don't forget about the Inflaton field. It is said to inflate spacetime after the Big Bang. So if spacetime is not real but just pure aid for calculation, how does inflaton interact with something non-existent. Pls. address this Marcus because I still think it's possible spacetime is not real because if space points are real.. General Covariance is not possible like you said.

 

[andrewkirk]

Gravitational waves have energy, and they consist of nothing but pure spacetime.

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.

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?

 

[bcrowell]

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.

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]

instein said something about this when talking about the principle of General Covariance (diffeomorphism invariance):

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

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

for an online source see page 43 of
www.tc.umn.edu/~janss011/pdf%20files/Besso-memo.pdf[/URL]

In another context ( In the introduction of the paper on the perihelion motion presented on 18 November 1915), Einstein wrote
“[B]durch welche Zeit und Raum der letzten Spur objektiver Realität beraubt werden,[/B]”

“[i]... time and space are robbed of the last trace of objective reality[/i]”



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.
==================================

.[/QUOTE]

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]

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]

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]

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.

 

[Dale]

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]

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]

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..

https://www.physicsforums.com/showthread.php?t=386051&highlight=is+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]

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]

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]

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]

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 couldn't 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:

https://www.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?

 

[Dale]

Personlly I dislike mathematical description of anything

Then you should probably avoid physics.

 

[jtbell]

what it really is

And how do we decide whether a certain description of something is what it "really is?"

 

[Tanelorn]

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 don't 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]

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.

It's a grammatical error.

 

[PAllen]

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 don't 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.

 

[Dale]

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.

 

[Phrak]

"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.

Spacetime consists of the past, present and future. The future will exist. The present exists. The past existed. Would you provide experimental evidence that the past exists?

 

[Tanelorn]

PAllen, perhaps consider a mathemetical simulation which models the weather. Is it really providing us with a full or complete understanding of the fundermental Physics of the weather? Or is it just using a bunch of equations that give us most of the time numbers close enough to the real world? Reminds me a little of the Turing test; if the computer responds with the right answers did it truly understand anything, is it truly alive? I don't believe so, and I suspect it never will with the current approach.

I think the method of understanding inside your head is the most important thing. Even with mathemetics I convert to pictures inside my head to see dependencies etc.

 

[Tanelorn]

I doubt that the past still exists, or at least exists in our present. If time was like space then we could walk across the road to yesterday and enjoy that beer all over again.

 

[PAllen]

I doubt that the past still exists, or at least exists in our present. If time was like space then we could walk across the road to yesterday and enjoy that beer all over again.

Now you're getting into personal beliefs, not science. Suppose yesterday exists but no known process allows you to 'get there'. How do you distinguish this from 'not existing'? Certainly, the image from a billion years ago exists - that's what astronomers see. There is no plausible process to get into the center of neutron star and out, so I suppose you might argue that a neutron star has no center?

 

[PAllen]

PAllen, perhaps consider a mathemetical simulation which models the weather. Is it really providing us with a full or complete understanding of the fundermental Physics of the weather? Or is it just using a bunch of equations that give us most of the time numbers close enough to the real world? Reminds me a little of the Turing test; if the computer responds with the right answers did it truly understand anything, is it truly alive? I don't believe so, and I suspect it never will with the current approach.

I think the method of understanding inside your head is the most important thing. Even with mathemetics I convert to pictures inside my head to see dependencies etc.

Your head and mine can both do (I hope) mathematics. Our respective mathematics is more likely to coincide than natural language pictures.

As for the turing test, I have less than that to go on to believe that you are a real being with intelligence.

As for your weather example, all physics is approximation. Until we go well over (e.g.) 500 years without needing to modify any physics, we can assume that 'reality', whatever it is, is only aproximately modeled by our current physical theories, whether expressed with mathematics or parables.

 

[Dale]

Spacetime consists of the past, present and future. The future will exist. The present exists. The past existed. Would you provide experimental evidence that the past exists?

So you are concerned about the conjugation of the verb "to exist"? I have to say that this is one of the weakest arguments I have ever seen on the subject. Many natural languages don't even have verb tenses. In any case, even a statement like "the past exists" is not grammatically incorrect, and I assume that resorting to this argument is a tacit admission that you have no supporting observations.

Regarding evidence, all of the accumulated evidence for SR and GR can be interpreted as evidence that the past exists in the sense of being part of spacetime. There is plenty of evidence that spacetime existed over the last century during the collection of that data. Absent any new evidence to the contrary, we will make the usual scientific assumption that it continues to exist and that it will continue to exist.

 

[Dale]

PAllen, perhaps consider a mathemetical simulation which models the weather. Is it really providing us with a full or complete understanding of the fundermental Physics of the weather?

If you had a mathematical simulation which would accurately model every possible feature of the weather, then in what conceivable way could you say that our understanding of the weather is not "full or complete".

 

[jtbell]

And how do we decide whether a certain description of something is what it "really is?"

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 don't 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.

Indeed, I think this is off-topic for a physics forum. Maybe it is on-topic for a metaphysics or philosophy forum.