Spacetime, physical or not really?

In summary: However, the theory of general relativity is all about inertial movement in geodesics in curved spacetime. There is no relation to a field in its terminology in the particle theory sense because a field is a means (form of) of energy that mediates energy transfer or interaction. Do you consider spacetime to be a field in this sense?Spacetime might be considered as a field in this sense because it can mediate energy transfer or interaction between objects.
  • #1
ZirkMan
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I try to get the gist of the Special and General theories of relativity for more than two years now. And I still don't understand if the geometrical explanation really explains or just describes what is physically going on when talking about all the relativistic effects (like gravitation, time dilation and space contraction).

This is what I mean by the difference between "explains" and "describe":

Are the theories of relativity physical theories, in a sense that the mechanism of the relativistic effects can be explained by energy transfer or interaction (in whatever form), like I believe any physical theory should and does?

A related question is if spacetime itself is a form of energy and for example gravitational attraction can be exlained as an energy interaction between objects and curved spacetime or the curved spacetime is just an abstraction that let us model the outcome of some other physical (in the defined sense) processes but is not the physical process itself that is responsible for the relativistics effects?
 
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  • #2
Special relativity is not really about time dilation or length contraction. It is about a symmetry in the laws of physics. Newtonian physics had a symmetry called Galilean symmetry. Special relativity replaces the Galilean symmetry of Newtonian physics with Lorentz symmetry.

General relativity is a theory of gravity. The gravitational field interacts with matter, and matter interacts with the gravitational field. Matter carries localizable energy, but the gravitational field does not. It is possible in some cases to say that the grvitational field carries non-local energy.
 
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  • #3
atyy said:
Special relativity is not really about time dilation or length contraction. It is about a symmetry in the laws of physics. Newtonian physics had a symmetry called Galilean symmetry. Special relativity replaces the Galilean symmetry of Newtonian physics with Lorentz symmetry.
Well, take for example the twin paradox. One of the twins comes back to Earth and is evidently younger than the other twin that stayed on the Earth. Can you tell me what physical law (that involves exchange or transformation of energy) and by what mechanism caused the difference in age of the twins? Something physical happened there and a physical explanation is needed to explain how. I'm afraid that the answer that laws of physics follow the Lorentz symmetry doesn't quite explain how the change in age happened. It only states that the result in the age is to be expected.
It's a subtle but important difference that makes a distinction between a theory that explains by describing a mechanism of how known objects transform from one form to another and a theory that describes consequences of some other processes that it really doesn't take into account (or care about).

Edit:
To me the difference between the theory of Special or General relativity and another theory (that I hoped to learn instead) is like a difference between a theory that predicts that when you make a fire you will feel warm and a theory that says that when you make a fire the chemically interacting gasses that make it radiate also infrared electromagnetic radiation that will interact with atoms and molecules in your body in such a way that it will make them jiggle more and this is what you will perceive as warmth.
Unfortunately I see that the both of the relativity theories are more of the first kind than the second. Or am I wrong? (I'd rather be wrong :)

atyy said:
General relativity is a theory of gravity. The gravitational field interacts with matter, and matter interacts with the gravitational field.

You use a well defined term of "field" from particle physics and yet the General relativity is all about inertial movement in geodesics in curved spacetime. There is no relation to a field in its terminology in the particle theory sense because a field is a means (form of) of energy that mediates energy transfer or interaction. Do you consider spacetime to be a field in this sense?
atyy said:
Matter carries localizable energy, but the gravitational field does not. It is possible in some cases to say that the grvitational field carries non-local energy.
Can you elaborate on this? What do you mean by that "the gravitational field" carries non-local energy?
 
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  • #4
ZirkMan said:
Are the theories of relativity physical theories, in a sense that the mechanism of the relativistic effects can be explained by energy transfer or interaction (in whatever form), like I believe any physical theory should and does?
I think that according to this unusual definition of "physical theory" SR does not qualify. I would see that as a clear failing of the definition, not the theory.
 
  • #5
Your question about the "physical mechanism" of ageing in the twin paradox is interesting. It's too hard for me to answer off the top of my head.

ZirkMan said:
You use a well defined term of "field" from particle physics and yet the General relativity is all about inertial movement in geodesics in curved spacetime. There is no relation to a field in its terminology in the particle theory sense because a field is a means (form of) of energy that mediates energy transfer or interaction. Do you consider spacetime to be a field in this sense?

I'm using field in the sense of classical field theory, with spacetime being a field. General relativity is not formulated fundamentally as geodesics in curved spacetime. Rather there are a bunch of fields like the electromagnetic field and spacetime, and equations describing their motion. The equations that describe the motion of the electromagnetic field are Maxwell's equations, and the analogous equation for spacetime is the Einstein Field Equation. This is not a field in the particle physics sense, where gravity is a spin-2 field on flat spacetime.

ZirkMan said:
Can you elaborate on this? What do you mean by that "the gravitational field" carries non-local energy?

Yau gives a short introduction here.
 
  • #6
ZirkMan said:
One of the twins comes back to Earth and is evidently younger than the other twin that stayed on the Earth. Can you tell me what physical law (that involves exchange or transformation of energy) and by what mechanism caused the difference in age of the twins?

The traveling twin has to fire rockets at some point in his trajectory in order to come back to Earth; the Earth twin can stay in free fall the whole time. (We are assuming that the Earth twin is really floating in space near the Earth, perhaps in a far orbit, so the question of the Earth's own gravity doesn't enter into it and the "Earth" twin can be in free fall the whole time.) Does the firing of rockets by the traveling twin qualify as "exchange or transformation of energy"?
 
  • #7
atyy said:
ZirkMan said:
You use a well defined term of "field" from particle physics and yet the General relativity is all about inertial movement in geodesics in curved spacetime. There is no relation to a field in its terminology in the particle theory sense because a field is a means (form of) of energy that mediates energy transfer or interaction. Do you consider spacetime to be a field in this sense?
I'm using field in the sense of classical field theory, with spacetime being a field. General relativity is not formulated fundamentally as geodesics in curved spacetime. Rather there are a bunch of fields like the electromagnetic field and spacetime, and equations describing their motion. The equations that describe the motion of the electromagnetic field are Maxwell's equations, and the analogous equation for spacetime is the Einstein Field Equation. This is not a field in the particle physics sense, where gravity is a spin-2 field on flat spacetime.
To emphasize atyy's point, tensors are sometimes called tensor fields to emphasize the point that they are defined over some extended region of spacetime. In GR, the spacetime is represented by the manifold and "gravity" is a tensor field (the metric) on the manifold. You can also have matter fields represented by the stress-energy tensor. The Einstein field equations give the relationship between the matter fields and the gravity fields.

They are all classical fields, not quantum fields.
 
  • #8
PeterDonis said:
The traveling twin has to fire rockets at some point in his trajectory in order to come back to Earth; the Earth twin can stay in free fall the whole time. (We are assuming that the Earth twin is really floating in space near the Earth, perhaps in a far orbit, so the question of the Earth's own gravity doesn't enter into it and the "Earth" twin can be in free fall the whole time.) Does the firing of rockets by the traveling twin qualify as "exchange or transformation of energy"?
Sure it does. Nothing else really happens or?
So the theory of Special relativity will say that (I'm quoting PAllen #9 from t=558360) "the path with greater deviation from an inertial path will accumulate less time".
Great, case closed. But does this answer really satisfy you? "Make a fire, you will feel warmth." A really practical theory it is. You can build a civilization based on this prediction of the theory.
You can make a satellite GPS navigation possible when you combine the Special and General theory of relativity. A remarkable practical achievement.
But does the theory explain how the fire will make you feel warmth? Does it explain how the increased movement of the rocket in space provided by the energy from the rocket engines will lead to real slower movement of every particle that is moving with it? Of course all that only relative to the "stationary twin". Or it will just say that it will happen? You tell me.
 
  • #9
ZirkMan said:
Well, take for example the twin paradox. One of the twins comes back to Earth and is evidently younger than the other twin that stayed on the Earth. Can you tell me what physical law (that involves exchange or transformation of energy) and by what mechanism caused the difference in age of the twins?

Answering your question directly may not help you, because you're asking the wrong questions, probably based on false assumptions about what relativity is about. But here goes anyway. The law that makes one twin age less does not involve any exchange or transformation of energy. It's more of a mathematical law, than a physical law.

In Euclidean geometry, we have the triangle inequality. If you have a triangle with sides AB, AC, and BC, you can say that the length of AB, |AB| is greater than the sums of the lengths of the sides AC+BC, i.e |AB| <= |AC|+|BC|, where I use the || notation to indicate that one is measuring length.

The space-time geometry of special relativity is not Euclidean, but Miinkowskian. This sort of geometry has curves in both space and time, the former curves are called "spacelike" curves, and the later "timelike" curves. The length of a space-like curve on a diagram represents a physical length that you'd measure with a ruler. The length of a time-like curve on a space-time diagram represents a physical time , also known as a proper time, that you'd measure with a clock.

It's a feature of the geometry that the triangle law holds unmodified for spacelike curves , i.e. for spacelie curves |AB| <= |AC|+|CB|, while the triangle inequality is reversed for timelike curves, i.e. for timelike curves |AB| >= |AC|+|CB|. Going into the details of how the geometry works would take a very long post to do well (and you'd do better to study a textbook than to try to learn it from posts), so this is just a very brief sketch of what's actually needed to answer your question.

The point is that the triangle inequality, and the triangle inequality alone, is sufficient to say that if you have two twins that separate and reunite, the twin that travels in a straight line , going directly from A to B, will age more than the twin that makes a stop at some point C which is not on the line AB. (If point C is on AB, then the equality part of the inequality is used, and the two twins age the same).

You'll need more than this one property (the triangle inequality) of special relativity to get anywhere. I'd suggest "Space-time physics' by Taylor & Wheeler. You can find the first few chapters of an earlier edition on the web. Length contraction and time dilation are NOT the whole story of special relativity. The piece that usually goes missing is the relativity of simultaneity. I'm not sure why the piece goes missing, but it happens a lot. I guess people don't understand what "relativity of simultaneity" means, so they ignore the concept. "Just a string of words". Unfortunately, it's an important concept, and you get into all kinds of confusion if you ignore it.
 
  • #10
ZirkMan said:
[..] To me the difference between the theory of Special or General relativity and another theory (that I hoped to learn instead) is like a difference between a theory that predicts that when you make a fire you will feel warm and a theory that says that when you make a fire the chemically interacting gasses that make it radiate also infrared electromagnetic radiation that will interact with atoms and molecules in your body in such a way that it will make them jiggle more and this is what you will perceive as warmth.
Unfortunately I see that the both of the relativity theories are more of the first kind than the second. Or am I wrong? (I'd rather be wrong :) [..]

Quite right - Newton would have called those theories, as well as quantum mechanics, "mathematical" theories of physics. And his theory was in part also just mathematical: he gave equations but no physical explanation for gravitation.

However, even a mathematical theory can hint at what may be "hidden" underneath it. In the case of GR, roughly two physical interpretations have been forwarded (that is to say that I don't know of a third one): either a physical "space" (or "ether") which determines the local speed of physical processes ("time"), or a physical "spacetime" (or "block universe") that governs everything. The first relates to a Lorentzian interpretation, and the second relates to a Minkowskian interpretation (although he was already dead when GR saw the light) - or as you say, a geometrical explanation. Depending on who you ask or which papers you read, Einstein favoured at certain times one or the other (or maybe both?); and that was possible because GR is a principle theory, and not based on a hidden physical model.

Addendum, I had missed:
I'm afraid that the answer that laws of physics follow the Lorentz symmetry doesn't quite explain how the change in age happened. It only states that the result in the age is to be expected.
It's a subtle but important difference that makes a distinction between a theory that explains by describing a mechanism of how known objects transform from one form to another and a theory that describes consequences of some other processes that it really doesn't take into account (or care about).

Pervect already provided the geometrical explanation. The Lorentzian explanation (if he really understood this) was given by Langevin in the sections of p.47 and 50-53 of http://en.wikisource.org/wiki/The_Evolution_of_Space_and_Time
 
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  • #11
ZirkMan said:
Can you tell me what physical law (that involves exchange or transformation of energy) and by what mechanism caused the difference in age of the twins?
I have a rectangular desk, the path from one corner to the opposite corner is longer if it goes straight along the edges than if it goes diagonally across. Can you tell me what physical law that involves exchange or transformation of energy causes the difference in path length?
 
  • #12
pervect said:
The law that makes one twin age less does not involve any exchange or transformation of energy. It's more of a mathematical law, than a physical law.
Ok, this is what I needed to know. My problem with this is more conceptual than physical, I guess. And I'm bumping to this problem from various sides again and again (as maybe DaleSpam will remember).

The problem is that I cannot see how a mathematical law operating within a seemingly man-made framework (spacetime) can have an influence on physical reality. If it really has an influence and can interact with it, that must mean that it or a part of it must be physically real too. If this is true and spacetime through its geometry really influences matter and energy then it means that the spacetime is real in the sense too. But when I ask this question I usually get the answer that atty mentioned here and DaleSpam confirmed. That the spacetime "is not a field in the particle physics sense". Then I am back again scratching my head trying to understand how can something non-physical have a physical influence.
pervect said:
You'll need more than this one property (the triangle inequality) of special relativity to get anywhere. I'd suggest "Space-time physics' by Taylor & Wheeler. You can find the first few chapters of an earlier edition on the web. Length contraction and time dilation are NOT the whole story of special relativity. The piece that usually goes missing is the relativity of simultaneity. I'm not sure why the piece goes missing, but it happens a lot. I guess people don't understand what "relativity of simultaneity" means, so they ignore the concept. "Just a string of words". Unfortunately, it's an important concept, and you get into all kinds of confusion if you ignore it.
Maybe you are right and deeply understanding the concept of "relativity of simultaneity" will help me to understand how the experimentally observed and theoretically predicted geometry of spacetime is a necessary and the only one way of how to maintain principle of causality in the Universe (I guess you wanted to hint to this). From this imperative I can imagine that the spacetime geometry and its effect on matter/energy will come out as necessary and logical. Thank you for your explanation and recommendation.
 
  • #13
ZirkMan, if you are interested in Special Relativity what pervect is a perfect way to formally introduce yourself to SR with little - no math.

I found the right-angle triangle explains a lot and is a great way to introduce spacetime diagrams (which are a great way to introduce "relativity of simultaneity").

The classic "light-clock" of two mirrors with a photon bouncing back and forth is a great visual of the right-angle triangle "in action".

With all that being said, I found that actually playing with the simple math of the Pathagorean Theory and playing with spacetime diagrams the concepts of SR are fairly easily "discovered".

As far as the question does SR explain or simply desrcibe relativistic effects, I'd say the two postulates of SR "explains" the "mechanism" that determines relativistic effects.

Of course this is not the same as explaining the mechanics of the postulate itself. But you have to stop asking why at some point and just accept something as being fundamental.
 
  • #14
DaleSpam said:
I have a rectangular desk, the path from one corner to the opposite corner is longer if it goes straight along the edges than if it goes diagonally across. Can you tell me what physical law that involves exchange or transformation of energy causes the difference in path length?
This is a very good question and analogy. I will need to think about this.
 
  • #15
ZirkMan said:
The problem is that I cannot see how a mathematical law operating within a seemingly man-made framework (spacetime) can have an influence on physical reality. If it really has an influence and can interact with it, that must mean that it or a part of it must be physically real too. If this is true and spacetime through its geometry really influences matter and energy then it means that the spacetime is real in the sense too. But when I ask this question I usually get the answer that atty mentioned here and DaleSpam confirmed. That the spacetime "is not a field in the particle physics sense". Then I am back again scratching my head trying to understand how can something non-physical have a physical influence.
Do you somehow think that geometry is not physical?

Consider a table. You can make all sorts of material statements about it, it has a certain mass, a given temperature, etc. You can also make all sorts of geometric statements, it has a certain length, surface area, etc.

Do you believe that the material statements are "physical" while the geometric statements are non-"physical"? If so, I would suggest that your definition of "physical" needs some adjustment.

PS sorry about excessive furniture analogies :smile:
 
  • #16
DaleSpam said:
Do you somehow think that geometry is not physical?

Consider a table. You can make all sorts of material statements about it, it has a certain mass, a given temperature, etc. You can also make all sorts of geometric statements, it has a certain length, surface area, etc.

Do you believe that the material statements are "physical" while the geometric statements are non-"physical"? If so, I would suggest that your definition of "physical" needs some adjustment.

PS sorry about excessive furniture analogies :smile:

Geometry of a physical object like any piece of furniture is of course physical.

But geometry of an invisible object that is not made by anything physical (virtual particles aside, we are talking about space as a dimension now) and one of its dimensions is temporal (which is even a higher order of abstraction) is another thing.
Since spacetime is not physical (like a table or an electromagnetic field is) I do not believe its geometry is physical either.
 
  • #17
ZirkMan, why can I drink coffee on an aeroplane without spilling it, just as if I were stationary on the ground? What is the physical mechanism that makes going very fast just the same as not moving?
 
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  • #18
ZirkMan said:
Geometry of a physical object like any piece of furniture is of course physical.
A table also has other geometrical features, such as duration (lifetime), etc. Those features of the table are just as physical as the other geometric features of the table. Do you agree with that?

ZirkMan said:
But geometry of an invisible object that is not made by anything physical (virtual particles aside, we are talking about space as a dimension now) and one of its dimensions is temporal (which is even a higher order of abstraction) is another thing.
Since spacetime is not physical (like a table or an electromagnetic field is) I do not believe its geometry is physical either.
At this point I am just talking about the geometry of classical macroscopic material objects, not spacetime, fields, or quantum weirdness.
 
  • #19
ZirkMan said:
The problem is that I cannot see how a mathematical law operating within a seemingly man-made framework (spacetime) can have an influence on physical reality.
Humans develop mathematical models of the observable reality. The mathematical models don't influence reality. This applies to all of physics. not just GR.
 
  • #20
atyy said:
Special relativity is not really about time dilation or length contraction. It is about a symmetry in the laws of physics. Newtonian physics had a symmetry called Galilean symmetry. Special relativity replaces the Galilean symmetry of Newtonian physics with Lorentz symmetry.

General relativity is a theory of gravity. The gravitational field interacts with matter, and matter interacts with the gravitational field. Matter carries localizable energy, but the gravitational field does not. It is possible in some cases to say that the grvitational field carries non-local energy.

First is right, second is only half right. GR is an extension of the principal of relativity to all frame, not just inertial. This is accomplished by replacing all equations with tensor equations. It was via thought experiment that once can conclude gravity = warped space-time if you assume the equivalence of inertial mass and gravitational mass. So, GR firstly extends SR into all coordinate systems (frames of reference), and secondly describes gravity via a correlation between accelerated reference frames and frames under gravity's pull.
 
  • #21
atyy said:
Matter carries localizable energy, but the gravitational field does not.
I agree with the second part but not with the first.

Could you explain what you mean by the first part with the understanding that where there is matter there is also a gravitational field.
 
  • #22
Passionflower said:
I agree with the second part but not with the first.

Could you explain what you mean by the first part with the understanding that where there is matter there is also a gravitational field.

Yes, there is that ambiguity that the stress energy of matter always requires the metric to be defined. But what I meant can be clarified by the "vacuum" of GR, where there is curvature but no matter. In those places the stress-energy tensor is zero, even though the curvature isn't.
 
  • #23
atyy said:
ZirkMan, why can I drink coffee on an aeroplane without spilling it, just as if I were stationary on the ground? What is the physical mechanism that makes going very fast just the same as not moving?
It's called inertia. And it's the reason why physical laws appear to be the same in all inertial frames. Details of how this is possible I would like to know too.
 
  • #24
DaleSpam said:
A table also has other geometrical features, such as duration (lifetime), etc. Those features of the table are just as physical as the other geometric features of the table. Do you agree with that?
At this point I am just talking about the geometry of classical macroscopic material objects, not spacetime, fields, or quantum weirdness.
Well you can construct "a geometry" out of any set of variables that change as a function of basically anything that you are able to define. Be it a table or a concept of a Pink fairy Unicorn from the Netherworld. But the fact that an object has an geometry doesn't mean that the geometry has an impact of how physical objects behave.
The geometry of a table might have an impact of how many tables you are able to get into a car or how it rolls down stairs. The geometry of the concept of a Pink fairy Unicorn will most probably have zero influence on anything physical.
So it's really important when talking about an object's geometry to also say which kind of object we are talking about. Otherwise we are not able to assess if its geometry is physical (has an physical impact and consequences) or not.

Now let's come back to spacetime. I will use another analogy. Because spacetime is neither a table nor a concept of a Pink fairy Unicorn. It's more like a shadow because its very existence depends on existence of physical objects. So in a sense it is connected to the physical reality of matter and energy and one cannot live without the other. But does the spacetime have a physical impact? Does a shadow have an impact of whether a person will come through a door or how it rolls down stairs? I will leave you to decide as this is the question I am asking.
 
  • #25
ZirkMan said:
[..] Then I am back again scratching my head trying to understand how can something non-physical have a physical influence. [..]

Obviously something non-physical can not have a physical influence; that's a wrong path to go!

Meanwhile in this thread you got presented two explanations: physical 4D space (the "geometrical" explanation) and physical 3D space (the "ether" explanation). I had forgotten that there also exist what appear to be intermediate explanations, such as by Harvey R. Brown. And as you can see from the following link, one quickly enters philosophy with that kind of discussions:
http://ndpr.nd.edu/news/25025-physical-relativity-space-time-structure-from-a-dynamical-perspective/
 
  • #26
ZirkMan said:
I try to get the gist of the Special and General theories of relativity for more than two years now. And I still don't understand if the geometrical explanation really explains or just describes what is physically going on when talking about all the relativistic effects (like gravitation, time dilation and space contraction).

This is what I mean by the difference between "explains" and "describe":

Are the theories of relativity physical theories, in a sense that the mechanism of the relativistic effects can be explained by energy transfer or interaction (in whatever form), like I believe any physical theory should and does?

A related question is if spacetime itself is a form of energy and for example gravitational attraction can be exlained as an energy interaction between objects and curved spacetime or the curved spacetime is just an abstraction that let us model the outcome of some other physical (in the defined sense) processes but is not the physical process itself that is responsible for the relativistics effects?

I wonder the same things. I've learned to do the math and come up with the right answer, but that doesn't mean I understand it. Here is my best shot as of today, for better or worse.

The reason relativity seems so strange is the environment in which our form of life evolved. There are plenty of natural clocks, and the idea of "distance" is also perfectly natural. The concepts of time and distance are very basic and useful to us. Electronic clocks at sea level all run at the same speed. So no wonder we got the idea that time was the same everywhere. But this is not the norm, just a peculiarity of the surface of a planet.

Surfaces of planets are very exceptional environments. The norm is some place in space surrounded by unblinking stars. Nothing changes. Everything is moving, the constant and repeatable distances of Earth don't exist. If distance has little meaning and utility, then velocity has little meaning as well. If you are using hydrogen wavelengths as a clock, then the observation that all clocks run differently would be fundamental. That's what they do.

In such an environment what DOES matter is energy, rest mass, and wavelength. This you would have. You will notice that if you define quantities in these terms then things work naturally. Expend twice as much energy to move yourself someplace, and you get there twice as fast. This simple relationship always holds. Use a hydrogen wavelength from a local star as a clock: that will give you some idea of relative velocity and clue you in on the effects of gravity So...what's the problem?

The problem is that distance, time, and velocity are derivative concepts of the fundamentals. Using them is a lot like having no concept that the Earth is a sphere and navigating using a Mercator projection. Why do I have to make all these screwy corrections? (Clearly no force is at work making the shortest path between two points a curve on the map.)

One can learn to think of the special relativity corrections as four-dimensional rotations around an imaginary axis. Sort of. That is, it is better than the Mercator style, but I can't claim that I visualize this. I've learned what to do in the common cases, that's all. It seems to me that there is the same problem: we are forcing the distance and universal time concepts on there, and it just isn't a comfortable fit.

What really is fundamental are certain second order differential equations. That's what is maintained, and our time and distance concepts are solutions to those equations. That means in order to get numbers you have to do two integrals, and those are not easy. But that is what time and distance really are: solutions to non-linear second order differential equations. When you look at it that way, then perhaps you no longer expect to get simple results. I am told that in general relativity that even relative velocity is not well-defined: it's path dependent. If we expect it to behave in a simple way, we will be disappointed.

So if time and distance are derivative concepts falsely assumed to be simple, why do we use them? Easy: we need them to design lab equipment, and to measure and express the results. It's part of our language. Time and distance may not be simple, but they certainly are useful here on Earth. Not only that, all of our pre-1900 physics was built around these concepts and they work very well down here. We aren't going to throw that away for something that is much harder to use and doesn't necessarily come up with useful numbers.
 
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  • #27
harrylin said:
Obviously something non-physical can not have a physical influence; that's a wrong path to go!
Well obviously many people here don't think as clearly as you :smile:
And I need to make sure that they see that this is not the way to go or that they have a point that I have missed.

harrylin said:
Meanwhile in this thread you got presented two explanations: physical 4D space (the "geometrical" explanation) and physical 3D space (the "ether" explanation). I had forgotten that there also exist what appear to be intermediate explanations, such as by Harvey R. Brown. And as you can see from the following link, one quickly enters philosophy with that kind of discussions:
http://ndpr.nd.edu/news/25025-physical-relativity-space-time-structure-from-a-dynamical-perspective/
Thank you. I have sent the article to my Kindle and I will surely reflect on the topic in another thread. To go deeper into that would be off-topic now.
 
  • #28
Note modern physics is not about explaining reality but just describing the symmetry in the equations. And to get used to it.. merge with the equations and be one with it. This is the reason why full fledged physicists like Max Tegmark begins to believe that the universe is some kind of mathematical structure... see:

http://discovermagazine.com/2008/jul/16-is-the-universe-actually-made-of-math

When you believe something so hard your mind becomes one with it.. even if it weren't true.. for example.. Children believing in Santa Claus or tooth fairy or the thousands living in mental institutions.
 
  • #29
This thread has meander into the nonsensical and personal tastes. This has very little physics content.
 
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1. Is spacetime a physical entity or just a mathematical concept?

Spacetime is a physical entity that describes the combination of three dimensions of space and one dimension of time. It is not just a mathematical concept, as it can be observed and measured through various physical phenomena such as gravity and the bending of light.

2. How does spacetime differ from the concept of space and time separately?

Spacetime differs from the concept of space and time separately in that it is a unified entity that describes the interwoven fabric of the universe. Space and time are no longer considered separate and absolute, but rather interconnected and relative to the observer's frame of reference.

3. Is spacetime affected by gravity and other physical forces?

Yes, spacetime is affected by gravity and other physical forces. According to Einstein's theory of general relativity, gravity is not a force between masses, but rather a curvature of spacetime caused by the presence of mass and energy. This curvature affects the motion of objects in spacetime.

4. Can spacetime be warped or distorted?

Yes, spacetime can be warped or distorted. As mentioned before, the presence of mass and energy can cause curvature in spacetime, leading to the warping or distortion of the fabric of the universe. This is observed in the bending of light around massive objects and the effects of gravity on the motion of celestial bodies.

5. How does the concept of spacetime impact our understanding of the universe?

The concept of spacetime has greatly impacted our understanding of the universe. It has revolutionized our understanding of gravity and led to the development of theories such as general relativity and the Big Bang theory. It also helps explain the behavior of objects in the universe and has led to discoveries such as black holes and gravitational waves.

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