What is spacetime made of?

[Confused2]

Consider a projectile motion, it can be described as..

1. The ball (object thrown) is following its worldlines in some co-ordinate system (x,y,z,t), this world line is parabolic (curved).

2. The ball is following a parabolic (curved path) due to deceleration force in vertical direction (gravity).

3. The ball is moving straight, in a path called geodesic, which is due to curving of spacetime.

4. Your take? If any.

The point is around a magnet the iron fillers will follow some curved path, in electromagnetic field a charged particle follows a curved path, all depending on the initial momentum too. It is the path followed under force. It is not some motion without force.

Can you solve a simple projectile motion in curved spacetime of GR without any reference to force ? Try that.

I should make clear GR isn't my thing. I've played with SR a bit and claim some 'feel' for spacetime - my attempts to introduce acceleration into SR haven't gone well. The accuracy of what comes next is best judged by the reaction of others.

The first experiment should be to go to the top of a high building while holding a cricket ball. Jump off and gently push the ball away. Observe the path of the ball ... (ignoring air resistance) it travels in a straight line. The mystery isn't so much what the ball is doing as why the street below is rushing up to meet it. The street is rushing up to meet the ball because the street is prevented from falling towards the centre of the Earthy by all the stuff underneath it.

If I were rash I'd say the parabola associated with projectile motion is the result of viewing an inertial object (the ball) from an accelerating frame. You could ask me why I think the presence of mass affects spacetime in any way and I would have to admit to having absolutely no clue whatsoever.

 

[exchemist]

You may be right but there are certain observations which could not be explained by GR. Then?


Pl read any text, since papers will be having complex maths so general articles, on gravitational lensing, one of the first and foremost observation which brought GR in limelight way back 1919 or so.

Now pl enlighten what is spacetime in those GR lensing images/sketches. Visual with some text will help.

It is not sufficient that only Rpenner knows here what is spacetime, many people talk of supporting spacetime without actually knowing what it is.

It depends what you mean by observations that cannot be explained by GR. GR, like any theory, can only be expected to account for phenomena that are within its remit. I am unaware of any prediction of GR within its remit that has failed so far. Do you have something in mind?

As for asking me what spacetime "is", I also take rpenner's view, that this seems to be a question outside science, bordering on metaphysical. What "is" an electron, after all? Or, what "is" EM radiation? All we can do is define these entities in terms of their predicted behaviour and their relationship to other similar entities. What we properly say in science is "nature seems to behave as if there are.........." Of course we don't actually say this every time - and we do come to trust many theories so implicitly that they become for us a sort of "reality". But if we ask, from a serious philosophical point of view, what they "really are", then all we can do is explain them in terms of other similar entities, which is a bit circular, (just like any dictionary definition of a word, in fact). Eventually we get to a point beyond which we cannot go. I think that is what we have with spacetime.

But that is also what we have with Newtonian gravitation. Nobody knows what "causes" the "force" or how it acts. It is just taken as a given, not to be challenged, right? You only think it is more physical than spacetime curving because you have been brought up with it, as we all have. It would be a ridiculous idea, except for the observational evidence that bodies appear to obey its rules - up to a point. So we accept it. It is in just the same category as spacetime being curved by mass, which again is experimentally observed via those classes of phenomena that Newton's gravitation fails to account for.

 

[The God]

Exchemist

Below talks of certain specific issues with prevalent cosmological model..


https://arxiv.org/abs/1604.05484

VII. CONCLUSIONS AND DISCUSSIONS
The standard ΛCDM model has a great success in explaining the observations of the CMB temperature anisotropies,
as well as the galaxies distribution and motion. The standard model of cosmology is based on the assumptions: the
validity of Einstein’s general relativity, and the cosmological principle. This model can explain most large-scale
observations with unprecedented accuracy. However, several directional anomalies have been reported in various
observations: the polarization distribution of the quasars, the velocity flow, the handedness of the spiral galaxies, the
anisotropy of the cosmic acceleration, the anisotropic evolution of fine-structure constant, including anomalies in the
CMB low multipoles, such as the CMB parity asymmetry. Although the confidence level for each individual anomaly
is not too high, the directional alignment of all these anomalies is quite significant, which strongly suggests a common
origin of these anomalies.
If these anomalies are due to cosmological effects , e.g. the alternative theory of gravity or geometry, the non-trivial
topology of the universe, the anisotropic dark energy or the particular large-scale fluctuation modes, they indicate
the violation of the cosmological principle. So, one should consider to build a new cosmological model to explain...

 

[The God]

It depends what you mean by observations that cannot be explained by GR. GR, like any theory, can only be expected to account for phenomena that are within its remit. I am unaware of any prediction of GR within its remit that has failed so far. Do you have something in mind?

As for asking me what spacetime "is", I also take rpenner's view, that this seems to be a question outside science, bordering on metaphysical. What "is" an electron, after all? Or, what "is" EM radiation? All we can do is define these entities in terms of their predicted behaviour and their relationship to other similar entities. What we properly say in science is "nature seems to behave as if there are.........." Of course we don't actually say this every time - and we do come to trust many theories so implicitly that they become for us a sort of "reality". But if we ask, from a serious philosophical point of view, what they "really are", then all we can do is explain them in terms of other similar entities, which is a bit circular, (just like any dictionary definition of a word, in fact). Eventually we get to a point beyond which we cannot go. I think that is what we have with spacetime.

But that is also what we have with Newtonian gravitation. Nobody knows what "causes" the "force" or how it acts. It is just taken as a given, not to be challenged, right? You only think it is more physical than spacetime curving because you have been brought up with it, as we all have. It would be a ridiculous idea, except for the observational evidence that bodies appear to obey its rules - up to a point. So we accept it. It is in just the same category as spacetime being curved by mass, which again is experimentally observed via those classes of phenomena that Newton's gravitation fails to account for.

Interesting...

Can you equate electron with spacetime in the context ?

Electron has mass, charge, inertia, spin etc...

What does spacetime have?

 

[The God]

It depends what you mean by observations that cannot be explained by GR. GR, like any theory, can only be expected to account for phenomena that are within its remit. I am unaware of any prediction of GR within its remit that has failed so far. Do you have something in mind?

As for asking me what spacetime "is", I also take rpenner's view, that this seems to be a question outside science, bordering on metaphysical. What "is" an electron, after all? Or, what "is" EM radiation? All we can do is define these entities in terms of their predicted behaviour and their relationship to other similar entities. What we properly say in science is "nature seems to behave as if there are.........." Of course we don't actually say this every time - and we do come to trust many theories so implicitly that they become for us a sort of "reality". But if we ask, from a serious philosophical point of view, what they "really are", then all we can do is explain them in terms of other similar entities, which is a bit circular, (just like any dictionary definition of a word, in fact). Eventually we get to a point beyond which we cannot go. I think that is what we have with spacetime.

But that is also what we have with Newtonian gravitation. Nobody knows what "causes" the "force" or how it acts. It is just taken as a given, not to be challenged, right? You only think it is more physical than spacetime curving because you have been brought up with it, as we all have. It would be a ridiculous idea, except for the observational evidence that bodies appear to obey its rules - up to a point. So we accept it. It is in just the same category as spacetime being curved by mass, which again is experimentally observed via those classes of phenomena that Newton's gravitation fails to account for.

I should make clear GR isn't my thing. I've played with SR a bit and claim some 'feel' for spacetime - my attempts to introduce acceleration into SR haven't gone well. The accuracy of what comes next is best judged by the reaction of others.

The first experiment should be to go to the top of a high building while holding a cricket ball. Jump off and gently push the ball away. Observe the path of the ball ... (ignoring air resistance) it travels in a straight line. The mystery isn't so much what the ball is doing as why the street below is rushing up to meet it. The street is rushing up to meet the ball because the street is prevented from falling towards the centre of the Earthy by all the stuff underneath it.

If I were rash I'd say the parabola associated with projectile motion is the result of viewing an inertial object (the ball) from an accelerating frame. You could ask me why I think the presence of mass affects spacetime in any way and I would have to admit to having absolutely no clue whatsoever.

Presence of mass affects the spacetime!!!

If so the spacetime got to be something.

Recently a small private jet was sent into spiral because few minutes before a jumbo jet (Big Mass) passed through the same part of sky. Conclusion : the big mass affected the air flow (background) and smaller private jet motion changed.

Now if a smaller spacecraft passes through a part of vacuum where a bigger spacecraft has just passed by, nothing will happen, because no background was affected, no background as such.

So when you say that mass affects the spacetime then it better have some property or it is mathematical entity. It is maths. Nothing metaphysical about this, Rpenner is playing with words, because he has no answer and he does not want to admit it. The question what is spacetime is a valid science question. One can ask what is electron, one can ask what is positive charge, one can ask what is mass, one can ask what force, momentum etc. In the same vein one can ask what is spacetime. Those who avoid this or call it metaphysics are constrained to do so.

 

[exchemist]

Interesting...

Can you equate electron with spacetime in the context ?

Electron has mass, charge, inertia, spin etc...

What does spacetime have?

Yes I suppose you can make an argument that more properties you can associate with an entity, the more "real" it may seem to you. But then is a vacuum not "real", too?

Spacetime just has the property that it responds to mass in a certain way.

Exchemist

Below talks of certain specific issues with prevalent cosmological model..


https://arxiv.org/abs/1604.05484

Yes but this calls into question the cosmological principle, not, so far as I can see, GR.

 

[Schmelzer]

As for asking me what spacetime "is", I also take rpenner's view, that this seems to be a question outside science, bordering on metaphysical.

Roughly, me too.

Except that I think that interpretations, which give some answers to such "what is" questions, are also a legitimate and important part of science, namely as starting points of future theory development. Theory development starts not from nothing, but from some interpretation of existing theories, with the aim to solve some problems. The problems are different in different interpretations. So, different interpretations give different directions of theory development.

And that to have different interpretations of the leading theories is important also for pedagogical reasons: The differences between the different interpretations tell us much better which part of the theory is supported by observation, and which parts are metaphysical. If two interpretations give different answers, above answers are essentially metaphysical. If there is only one interpretation, the metaphysical parts are much harder to distinguish from the physical parts.

 

[The God]

Spacetime just has the property that it responds to mass in a certain way.

Oops... So you are associating some property (which bends) to spacetime; what could that be? And you have brought it out of purely mathematical entity.

Yes but this calls into question the cosmological principle, not, so far as I can see, GR.

Pl read a bit more of non math part of article, it says our assumption regarding validity of GR or cosmogical principle need to be looked into...

 

[Confused2]

It strikes me that a rotation preserves all local properties but makes the same (unrotated) thing look different. I seem to recall rpenner posting about SR rotations in a now extinct forum. Probably my mistake but could there be some clues there?

 

[exchemist]

Roughly, me too.

Except that I think that interpretations, which give some answers to such "what is" questions, are also a legitimate and important part of science, namely as starting points of future theory development. Theory development starts not from nothing, but from some interpretation of existing theories, with the aim to solve some problems. The problems are different in different interpretations. So, different interpretations give different directions of theory development.

And that to have different interpretations of the leading theories is important also for pedagogical reasons: The differences between the different interpretations tell us much better which part of the theory is supported by observation, and which parts are metaphysical. If two interpretations give different answers, above answers are essentially metaphysical. If there is only one interpretation, the metaphysical parts are much harder to distinguish from the physical parts.

Sure. I don't mean to suggest there is no purpose to seeking a deeper layer of explanatory meaning to things, merely that however deep one goes, one has to stop somewhere, since observation will only provide evidence for a certain number of layers of theory. In that sense, science never provides ultimate answers. Or so it seems to me, at least.

 

[exchemist]

Oops... So you are associating some property (which bends) to spacetime; what could that be? And you have brought it out of purely mathematical entity.






Pl read a bit more of non math part of article, it says our assumption regarding validity of GR or cosmogical principle need to be looked into...

Sorry I've scanned it but could not immediately see anything calling GR into question. Can you refer me to a page number and a para on the page in question?

 

[The God]

Sorry I've scanned it but could not immediately see anything calling GR into question. Can you refer me to a page number and a para on the page in question?

Page #18, Possible interpretations.


A. Non-trivial topology of the universe

......
As mentioned before, the standard cosmological model is based on two assumptions: One is that Einstein’s general relativity correctly describes gravity, the other assumes the universe as homogeneous and isotropic on large scales. If we believe that the anomalies have a cosmological origin, at least one of these two assumptions will be broken.......

 

[Confused2]

Universe faulty/broken?

 

[exchemist]

Page #18, Possible interpretations.


A. Non-trivial topology of the universe

......
As mentioned before, the standard cosmological model is based on two assumptions: One is that Einstein’s general relativity correctly describes gravity, the other assumes the universe as homogeneous and isotropic on large scales. If we believe that the anomalies have a cosmological origin, at least one of these two assumptions will be broken.......

Ah yes I see, thanks.

But I think you have misunderstood the entire thrust of the paper. These possibilities are raised, presumably to show how much of physics would need to be rethought if the origins of the anisotropy were to be cosmological in origin, but then the authors come down in favour of non-cosmological explanations of the data they have studied. I quote the last part of the abstract, (my stress added in bolded part):

"In this paper, by considering the preferred axis in the CMB parity violation, we find that it coincides with the preferred axes in CMB quadrupole and CMB octopole, and they all align with the direction of the CMB kinematic dipole. In addition, the preferred directions in the velocity flows, quasar alignment, anisotropy of the cosmic acceleration, the handedness of spiral galaxies, and the angular distribution of the fine-structure constant are also claimed to be aligned with the CMB kinematic dipole. Since CMB dipole was confirmed to be caused by the motion of our local group of galaxies relative to the reference frame of the CMB, the coincidence of all these preferred directions hints that these anomalies have a common origin, which is not cosmological or due to a gravitational effect. The systematical or contaminative errors in observation or in data analysis, which can be directly related to the motion of our local group of galaxies, can play an important role in explaining the anomalies."

So basically they are saying they do not think they have evidence that GR needs revision.

Or at least, that would be my reading of it.

 

[The God]

Ah yes I see, thanks.

But I think you have misunderstood the entire thrust of the paper. These possibilities are raised, presumably to show how much of physics would need to be rethought if the origins of the anisotropy were to be cosmological in origin, but then the authors come down in favour of non-cosmological explanations of the data they have studied. I quote the last part of the abstract, (my stress added in bolded part):

"In this paper, by considering the preferred axis in the CMB parity violation, we find that it coincides with the preferred axes in CMB quadrupole and CMB octopole, and they all align with the direction of the CMB kinematic dipole. In addition, the preferred directions in the velocity flows, quasar alignment, anisotropy of the cosmic acceleration, the handedness of spiral galaxies, and the angular distribution of the fine-structure constant are also claimed to be aligned with the CMB kinematic dipole. Since CMB dipole was confirmed to be caused by the motion of our local group of galaxies relative to the reference frame of the CMB, the coincidence of all these preferred directions hints that these anomalies have a common origin, which is not cosmological or due to a gravitational effect. The systematical or contaminative errors in observation or in data analysis, which can be directly related to the motion of our local group of galaxies, can play an important role in explaining the anomalies."

So basically they are saying they do not think they have evidence that GR needs revision.

Or at least, that would be my reading of it.

The author says it's either A or B. I am inclined towards A and you are towards B. That's fair enough.

 

[exchemist]

The author says it's either A or B. I am inclined towards A and you are towards B. That's fair enough.

Well no, that is not what is being said. The research finding they are reporting to the world is that their analysis of the data is consistent with a non-cosmological explanation.

 

[The God]

Well no, that is not what is being said. The research finding they are reporting to the world is that their analysis of the data is consistent with a non-cosmological explanation.

Pl read the conclusion again. The puzzle is still open. It's either GR/cosmological problem or non cosmological. They hint or suggest non cosmological but do not conclude that way. It's open.

 

[exchemist]

Pl read the conclusion again. The puzzle is still open. It's either GR/cosmological problem or non cosmological. They hint or suggest non cosmological but do not conclude that way. It's open.

True, they cannot dismiss the possibility. But it is a bit perverse of you to cite this paper as evidence in support of GR being incomplete, when its purpose is to show the plausibility of a more mundane explanation.

 

[The God]

True, they cannot dismiss the possibility. But it is a bit perverse of you to cite this paper as evidence in support of GR being incomplete, when its purpose is to show the plausibility of a more mundane explanation.

Bad inference. You asked for something. I gave you the observations which question mainstream cosmology. They are precise observations with best of the equipments, indicating (not concluding) issues with prevalent models.

There is no perversity in this. In fact it would be perverse to set them aside lightly.

In #56 you dismissed, in #58 you put it correctly.

Pl get to the background of the data source, you won't find any mundane cause.

 

[exchemist]

Bad inference. You asked for something. I gave you the observations which question mainstream cosmology. They are precise observations with best of the equipments, indicating (not concluding) issues with prevalent models.

There is no perversity in this. In fact it would be perverse to set them aside lightly.

In #56 you dismissed, in #58 you put it correctly.

Pl get to the background of the data source, you won't find any mundane cause.

I rely on what the authors of the paper say in conclusion, as highlighted in bold in my post 54.