Abstract or Physical?

[quaternion]

I saw the thread about the constancy of the speed of light, and it triggered a question, but I didn't want to try to wade into that conversation to ask my question, so I thought a separate thread might be apropos.

Is general relativity, spacetime, etc. founded on an abstraction (e.g. Einstein's field equations) or physical reality (e.g. measuring the speed of light)?

I know people will say it matches observations of physical reality, but you can't really "observe" things like spacetime. It's not like you can put a piece of spacetime in a test tube and play with it. You can't say, "Look here's how light behaves over here when it interacts with spacetime, and this is how it behaves over there without spacetime."

So, it seems to me the theory consists of, at least in part, some abstractions.

 

[sjastro]

I saw the thread about the constancy of the speed of light, and it triggered a question, but I didn't want to try to wade into that conversation to ask my question, so I thought a separate thread might be apropos.

Is general relativity, spacetime, etc. founded on an abstraction (e.g. Einstein's field equations) or physical reality (e.g. measuring the speed of light)?

I know people will say it matches observations of physical reality, but you can't really "observe" things like spacetime. It's not like you can put a piece of spacetime in a test tube and play with it. You can't say, "Look here's how light behaves over here when it interacts with spacetime, and this is how it behaves over there without spacetime."

So, it seems to me the theory consists of, at least in part, some abstractions.

The answer to your question is both.

General Relativity is based on an abstract mathematical idea of non Euclidean geometry applied to the real world.
Space-time has properties that can be tested such as its geometry.
Surveyors use triangulation to measure distances; the triangles add up to 180⁰ which is a property of flat space.
The Gravity Probe B was able to measure the degree of space-time curvature near Earth.
Gravitational waves have been detected using interferometers.
A passing gravitational wave causes space-time to stretch in one direction and contract in the 90⁰ direction which can be detected by the interferometer.

 

[quaternion]

The answer to your question is both.

General Relativity is based on an abstract mathematical idea of non Euclidean geometry applied to the real world.
Space-time has properties that can be tested such as its geometry.
Surveyors use triangulation to measure distances; the triangles add up to 180⁰ which is a property of flat space.
The Gravity Probe B was able to measure the degree of space-time curvature near Earth.
Gravitational waves have been detected using interferometers.
A passing gravitational wave causes space-time to stretch in one direction and contract in the 90⁰ direction which can be detected by the interferometer.

You're not actually measuring spacetime, but things in spacetime.

Regardless, since you're saying it's both physical and an abstraction, would it be possible to produce a theory that didn't contain an abstraction?

 

[Subduction Zone]

I saw the thread about the constancy of the speed of light, and it triggered a question, but I didn't want to try to wade into that conversation to ask my question, so I thought a separate thread might be apropos.

Is general relativity, spacetime, etc. founded on an abstraction (e.g. Einstein's field equations) or physical reality (e.g. measuring the speed of light)?

I know people will say it matches observations of physical reality, but you can't really "observe" things like spacetime. It's not like you can put a piece of spacetime in a test tube and play with it. You can't say, "Look here's how light behaves over here when it interacts with spacetime, and this is how it behaves over there without spacetime."

So, it seems to me the theory consists of, at least in part, some abstractions.

One can measure the speed of light in many different ways. For example the Michelson Morley experiment demonstrated that no matter which way one traveled the speed of light was constant to the observer.

That means if I fired off a beam of light and measured it here on Earth it would be going almost 300,000 km/sec relative to me, as if I was standing still. Meanwhile a person in a space craft moving at a speed of half of the speed of light away from Earth measured the speed of the same beam of light it would be moving at 300,000 feet per second relative to him.

And this has been tested and confirmed many times in many ways. In fact you have probably tested it yourself. Every time that you use GPS and it works it is confirming relativity.

 

[quaternion]

One can measure the speed of light in many different ways. For example the Michelson Morley experiment demonstrated that no matter which way one traveled the speed of light was constant to the observer.

That means if I fired off a beam of light and measured it here on Earth it would be going almost 300,000 km/sec relative to me, as if I was standing still. Meanwhile a person in a space craft moving at a speed of half of the speed of light away from Earth measured the speed of the same beam of light it would be moving at 300,000 feet per second relative to him.

And this has been tested and confirmed many times in many ways. In fact you have probably tested it yourself. Every time that you use GPS and it works it is confirming relativity.

General relativity is a geometric theory of gravity. i.e. that spacetime curves in the presence of matter. In confirming that the speed of flight is constant, are we confirming that spacetime is a thing, or are we confirming the physical interaction of matter, whereby spacetime is a convenient abstraction?

 

[Subduction Zone]

General relativity is a geometric theory of gravity. i.e. that spacetime curves in the presence of matter. In confirming that the speed of flight is constant, are we confirming that spacetime is a thing, or are we confirming the physical interaction of matter, whereby spacetime is a convenient abstraction?

Special relativity is more about the speed of light. General relativity is a theory that includes gravity.

Special relativity is confirmed by experiments on the order of the Michelson Morley experiment. General relativity can be confirmed with a ladder. Atomic clocks at different heights from the center of the Earth run at different speeds:

NIST Pair of Aluminum Atomic Clocks Reveal Einstein's Relativity at a Personal Scale

 

[sjastro]

You're not actually measuring spacetime, but things in spacetime.

That's not correct.
For example galaxies that have a redshift z > 1.5 have a recessional velocity that exceeds the speed of light c.
If we measured the velocity of galaxies in spacetime it can never exceed c.
Recessional velocity is an intrinsic property of spacetime, galaxies are carried by this Hubble flow without necessarily having to move within spacetime.

Regardless, since you're saying it's both physical and an abstraction, would it be possible to produce a theory that didn't contain an abstraction?

Yes it can happen.
Maxwell's equations for electromagnetism were developed on the work of experimenters such as Faraday and Ampere.

 

[quaternion]

That's not correct.
For example galaxies that have a redshift z > 1.5 have a recessional velocity that exceeds the speed of light c.
If we measured the velocity of galaxies in spacetime it can never exceed c.
Recessional velocity is an intrinsic property of spacetime, galaxies are carried by this Hubble flow without necessarily having to move within spacetime.

Again you're talking about measuring the things in spacetime (galaxies). Give me a way to measure empty spacetime - a measurement of only spacetime and nothing else - no reference to any other thing.

 

[sjastro]

Again you're talking about measuring the things in spacetime (galaxies). Give me a way to measure empty spacetime - a measurement of only spacetime and nothing else - no reference to any other thing.

In order to "measure" spacetime you need two reference points, the observer and the object as it is the spacetime between the two that is expanding.
Furthermore the rate of expansion of spacetime depends on this distance.
The larger the distance the greater the rate of expansion which a characteristic of metric or spacetime expansion where the Universe's scale factor is increasing with time.

 

[quaternion]

In order to "measure" spacetime you need two reference points, the observer and the object as it is the spacetime between the two that is expanding.
Furthermore the rate of expansion of spacetime depends on this distance.
The larger the distance the greater the rate of expansion which a characteristic of metric or spacetime expansion where the Universe's scale factor is increasing with time.

Yes, exactly. I'm not challenging the model. I understand it works.

My only point is that you're measuring the relationship between two material objects. You're not actually measuring spacetime. Given that is the case, I would consider spacetime an abstraction. You could just as easily say there is a space fairy who likes Riemannian Geometry flitting about causing these things to happen. The result would be the same.

But "spacetime" sounds more sciencey than "spacefairy".

 

[FrumiousBandersnatch]

I saw the thread about the constancy of the speed of light, and it triggered a question, but I didn't want to try to wade into that conversation to ask my question, so I thought a separate thread might be apropos.

Is general relativity, spacetime, etc. founded on an abstraction (e.g. Einstein's field equations) or physical reality (e.g. measuring the speed of light)?

I know people will say it matches observations of physical reality, but you can't really "observe" things like spacetime. It's not like you can put a piece of spacetime in a test tube and play with it. You can't say, "Look here's how light behaves over here when it interacts with spacetime, and this is how it behaves over there without spacetime."

So, it seems to me the theory consists of, at least in part, some abstractions.

Every theory is an abstraction - it's a model explanation for a set of observations.

Whether spacetime is real is a metaphysical question - our spacetime model fits what we can observe of whatever is out there very closely. Ultimately it may not be possible to say what is 'real' beyond how what we can observe behaves, any more than it makes sense to ask what the fundamental stuff of reality is 'made of'.

 

[Larniavc]

Yes, exactly. I'm not challenging the model. I understand it works.

My only point is that you're measuring the relationship between two material objects. You're not actually measuring spacetime. Given that is the case, I would consider spacetime an abstraction. You could just as easily say there is a space fairy who likes Riemannian Geometry flitting about causing these things to happen. The result would be the same.

But "spacetime" sounds more sciencey than "spacefairy".

Are the words “and therefore God” marching into view with undeserving confidence?

 

[quaternion]

Every theory is an abstraction - it's a model explanation for a set of observations.

Whether spacetime is real is a metaphysical question - our spacetime model fits what we can observe of whatever is out there very closely. Ultimately it may not be possible to say what is 'real' beyond how what we can observe behaves, any more than it makes sense to ask what the fundamental stuff of reality is 'made of'.

I agree.

Are the words “and therefore God” marching into view with undeserving confidence?

No.

 

[essentialsaltes]

I mean, velocity is an abstraction. It's not a thing like a rock. We don't need to leap toward Einstein to find people using concepts to describe the universe.

 

[quaternion]

I mean, velocity is an abstraction. It's not a thing like a rock. We don't need to leap toward Einstein to find people using concepts to describe the universe.

True.

 

[sjastro]

Yes, exactly. I'm not challenging the model. I understand it works.

My only point is that you're measuring the relationship between two material objects. You're not actually measuring spacetime. Given that is the case, I would consider spacetime an abstraction. You could just as easily say there is a space fairy who likes Riemannian Geometry flitting about causing these things to happen. The result would be the same.

But "spacetime" sounds more sciencey than "spacefairy".

It’s quite obvious you do not understand how it works.
In the case of the observer and galaxy the “relationship” between the two is that both are stationary in the same comoving frame of reference.
According to your logic there should be no observed redshift for this very reason.
The fact is a cosmological redshift is observed due to spacetime expansion and not the “relationship” between observer and galaxy.

Einstein emphasised the physical reality of spacetime by equating it with “aether”.

We may say that according to the general theory of relativity space is endowed with physical qualities; in this sense, therefore, there exists an aether. According to the general theory of relativity space without aether is unthinkable; for in such space there not only would be no propagation of light, but also no possibility of existence for standards of space and time (measuring-rods and clocks), nor therefore any space-time intervals in the physical sense. But this aether may not be thought of as endowed with the quality characteristic of ponderable media, as consisting of parts which may be tracked through time. The idea of motion may not be applied to it.

The physical reality of spacetime can be taken a step further by QFT (Quantum Field Theory) which is the application of Minkowski spacetime to Quantum Mechanics.
If every single atom, molecule, ion and photon is removed from a volume, the vacuum is nothing more than spacetime which turns out to be considerably more complicated than even what Einstein envisaged as it has properties such as being to exert pressure via the Casimir effect which has been verified in the lab.
Spacetime or a vacuum can never be totally empty.

 

[quaternion]

It’s quite obvious you do not understand how it works.

I don't have a degree in physics and never claimed any expertise. Your reply has a tinge of hostility that is unnecessary. If you can't suffer fools without physics degrees, don't talk to fools without physics degrees.

If you're going to continue the discussion, a simple clarification/explanation will suffice.

In the case of the observer and galaxy the “relationship” between the two is that both are stationary in the same comoving frame of reference.

OK.

According to your logic there should be no observed redshift for this very reason.

That's not what I said. I said I understand it works, but that it seemed to be explanation by abstraction rather than an explanation of existing physical things. I've had this discussion about comoving frames before, but the conversation dissolved without resolution. If you want to have a go, I'll listen.

Are you claiming a comoving frame is a physical thing? Is the proper frame a physical thing? Or are they simply transformations from one frame to another, one being more mathematically convenient than the other?

The physical reality of spacetime can be taken a step further by QFT (Quantum Field Theories) which is the application of Minkowski spacetime to Quantum Mechanics.

Hmm. As I've always understood, there are discontinuities between relativity and QM. Isn't something like relativity accurately describes the very large but not the very small and vice versa for QM. There might even have been something about one area of physics handling fast/slow and strong/weak forces better than another, but I don't recall it clearly. That, in and of itself, implies abstraction to me.

Regardless, it doesn't make me keen to jump from relativity to QM, but ... I guess.

If every single atom, molecule, ion and photon is removed from a volume ...

That's possible? That would be news to me. I got the impression from other posters who agreed that physics includes abstractions, that such ideal states weren't possible. But maybe I misunderstood.

... the vacuum is nothing more than spacetime which turns out to be considerably more complicated than even what Einstein envisaged as it has properties such as being to exert pressure via the Casimir effect which has been verified in lab.

Even if it has been done, isn't the quantum foam (isn't that what it's called?) still present? And isn't that something other than spacetime?

 

[sjastro]

I don't have a degree in physics and never claimed any expertise. Your reply has a tinge of hostility that is unnecessary. If you can't suffer fools without physics degrees, don't talk to fools without physics degrees.

If you're going to continue the discussion, a simple clarification/explanation will suffice.



OK.



That's not what I said. I said I understand it works, but that it seemed to be explanation by abstraction rather than an explanation of existing physical things. I've had this discussion about comoving frames before, but the conversation dissolved without resolution. If you want to have a go, I'll listen.

Are you claiming a comoving frame is a physical thing? Is the proper frame a physical thing? Or are they simply transformations from one frame to another, one being more mathematically convenient than the other?



Hmm. As I've always understood, there are discontinuities between relativity and QM. Isn't something like relativity accurately describes the very large but not the very small and vice versa for QM. There might even have been something about one area of physics handling fast/slow and strong/weak forces better than another, but I don't recall it clearly. That, in and of itself, implies abstraction to me.

Regardless, it doesn't make me keen to jump from relativity to QM, but ... I guess.



That's possible? That would be news to me. I got the impression from other posters who agreed that physics includes abstractions, that such ideal states weren't possible. But maybe I misunderstood.



Even if it has been done, isn't the quantum foam (isn't that what it's called?) still present? And isn't that something other than spacetime?

So far your responses have been repetitious and the reference to spacetime as spacefairy comes across as an exercise in ridiculing so I am going to leave it to other posters to answer your questions.

 

[FrumiousBandersnatch]

Are you claiming a comoving frame is a physical thing? Is the proper frame a physical thing?

They are frames of reference; i.e. abstract coordinate systems and the set of physical reference points that uniquely fix (locate and orient) them and standardize measurements within them.

Regardless, it doesn't make me keen to jump from relativity to QM, but ... I guess.

QM is generally thought to be more fundamental. IOW, GR is expected to supervene on or emerge from QFT (Quantum Field Theory).

Even if it has been done, isn't the quantum foam (isn't that what it's called?) still present? And isn't that something other than spacetime?

AIUI, there's an argument to be made that the quantum foam is spacetime, and there are various speculative ideas as to how that might be the case - usually involving gravity.

 

[quaternion]

They are frames of reference; i.e. abstract coordinate systems and the set of physical reference points that uniquely fix (locate and orient) them and standardize measurements within them.

That's what I understood them to be. Though simpler, I've worked with various types of coordinate transformations where some are more convenient than others. But, as long as all the coordinates within one system are orthogonal, everything works out just fine. The physics don't change from one system to another. It's just easier to do the calculations in one versus another.

Again, that's what I would expect here. The physics doesn't change between a comoving frame and a proper frame. It's just that the comoving frame is better suited to the issue. If that's not the case - if different physics are assumed in different frames, that would be an interesting discussion.

AIUI, there's an argument to be made that the quantum foam is spacetime, and there are various speculative ideas as to how that might be the case - usually involving gravity.

Then I'll wait for the issue to be settled. However, in the meantime, I'd wager that if spacetime is declared to be equivalent to quantum foam, a supporting abstraction will develop to explain why that is - there will be yet another turtle standing beneath and supporting the current one.

If you don't get the reference, I'm suggesting an infinite regress.