B Does Spacetime Have Physical Existence?

[Uncle Thi]

Spacetime is the foundation of General Relativity, describing how mass and energy influence the curvature of space and the flow of time. However, an important question arises: Is spacetime a physical entity, or is it merely a mathematical framework?

Physics relies on measurable quantities and empirical evidence. Yet, when we discuss spacetime curvature, time dilation, and gravitational lensing, we often assume that space and time themselves are tangible, modifiable entities. But can these concepts be directly observed and measured, or are they just descriptions of how objects behave under certain conditions?

To explore this, I would like to raise three fundamental questions regarding space, time, and light in the context of spacetime theory.

1. Space: Does it have a physical structure to bend?

If spacetime is said to bend due to mass and energy, is space itself a physical entity that can actually bend?


2. Time: Can it stretch or contract if it has no physical existence?

If time is claimed to dilate, what exactly is the measurable unit of this dilation? Is there direct evidence beyond clock rate differences?


3. Light: Does it actually bend, or just change direction?

If light is bent by gravity, does that mean photons have mass? If not, how does gravity interact with them?

 

[PeroK]

Spacetime is the foundation of General Relativity, describing how mass and energy influence the curvature of space and the flow of time. However, an important question arises: Is spacetime a physical entity, or is it merely a mathematical framework?

This is more of a philosophical question than a question for physics.

Physics relies on measurable quantities and empirical evidence. Yet, when we discuss spacetime curvature, time dilation, and gravitational lensing, we often assume that space and time themselves are tangible, modifiable entities. But can these concepts be directly observed and measured, or are they just descriptions of how objects behave under certain conditions?

Space and time intervals can be measured. A second is defined in terms of transitions of a Caesium atomic clock; and a metre is defined as the distance light travels in vacuuum in some specified time.

To explore this, I would like to raise three fundamental questions regarding space, time, and light in the context of spacetime theory.

1. Space: Does it have a physical structure to bend?

If spacetime is said to bend due to mass and energy, is space itself a physical entity that can actually bend?

That question depends on how you define physical. If the definition of physical includes spacetime, then spacetime is physical; if not, then it isn't.

2. Time: Can it stretch or contract if it has no physical existence?

If time is claimed to dilate, what exactly is the measurable unit of this dilation? Is there direct evidence beyond clock rate differences?

Time dilation is a coordinate effect. Leaving that aside, the question is really philosophy rather than physics.

3. Light: Does it actually bend, or just change direction?

There is no such thing as an absolute direction - especially in curved spacetime. Light follows null geodesics in spacetime - which are the nearest things to straight lines in curved geometries. In fact, I would say that the concept of a straight line really only makes sense in Euclidean geometry. For example, is a line of longitude on the surface of the Earth a straight line?

If light is bent by gravity, does that mean photons have mass?

No.

If not, how does gravity interact with them?

Light follows null geodescics, which are the nearest things to straight lines. In any case, it's only in the Newtonian theory that gravity is an interaction between masses. Newtonian gravity is an approximation to a special case of General Relativity - not the other way round.

 

[Orodruin]

TLDR: It depends on what you put into the meaning of ”having physical existence”. Physics is about describing reality, not about philosophical debates of what is ”real”.

 

[Ibix]

I would say that spacetime is an object in the mathematical formulation of general relativity. Whether that means it's "real" or not is a matter of personal preference, not physics. Physicists will typically talk about it as real if they're using general relativity. If they're trying to replace general relativity they will usually talk about other entities and try to explain how spacetime emerges from their maths, just as you can derive Newton's force model of gravity from GR.

Generally the justification for believing in spacetime is post hoc: when we model geometry using a 4d manifold obeying Einstein's field equations, we make accurate predictions. When we finally figure out quantum gravity we'll probably replace it with something else. It may one day be replaced. Worrying about whether it's really real is kind of pointless unless you can go on to provide a consistent mathematical framework that matches all current observations of gravity to our available precision and makes different predictions for things we haven't measured yet. Then we can talk about whether those predictions are correct, and then (if they are) you can start worrying if the entities in that model are really real or not.

Is there direct evidence beyond clock rate differences?

Trying to treat "space" and "time" separately, as the OP's questions 1 and 2 do is never going to end well, but this is an interesting sub-question. What evidence of the behaviour of time could there be except the behaviour of clocks? Whatever time is, if you aren't using clocks you aren't measuring its behaviour (to paraphrase Einstein).

 

[Uncle Thi]

"That question depends on how you define physical. If the definition of physical includes spacetime, then spacetime is physical; if not, then it isn't.

In my view, the definition of 'physical' implies that an entity must objectively exist in the real world, and this physical entity must have measurable quantities.
---
Based on that definition, could you clearly define the following:

1. What is space?
2. What is time?
3. What is light?

 

[Ibix]

1. What is space?
2. What is time?
3. What is light?

By your definition, all three are non-physical abstractions in models of varying precision.

 

[PeroK]

In my view, the definition of 'physical' implies that an entity must objectively exist in the real world, and this physical entity must have measurable quantities.

That just shifts the definition of "physical" to the definition of "objectively exist". One problem arises if you can describe the same physical phenomena using different physical models. One example is reflection and transmission of light using Maxwell's equations and the classical electromagnetic field. The EM field must, therefore, exist (by your definition). Note that, contrary to popular belief, there are no photons in classical EM. So, they do not exist.

But, if we use the Quantum Mechanical model of lights, then we have a probabilistic wave function, the uncertainty principle and the EM field is quantized (i.e. can be described in terms of photons).

What is real depends on what theory you are using. You might argue that currently QM is more fundamental than classical EM, so you might argue that photons really exist and EM waves do not. But, what happens if a theory of gravity changes this - what happens to the things that used to really exist.

---
Based on that definition, could you clearly define the following:

1. What is space?
2. What is time?

Space and time are well defined within the theory of GR. In the single concept of spacetime.

3. What is light?

As above, light is EM radiation. Or, a certain type of state of the quantized EM field.

 

[Dale]

an entity must objectively exist in the real world

Do you have an “objectivelyexistometer” that can measure whether or not something objectively exists?

this physical entity must have measurable quantities

Spacetime clearly satisfies the measurable quantities requirement. You can measure spacetime with clocks, rulers, protractors, speedometers, radars, and the like.

1. Space: Does it have a physical structure to bend?

Spacetime does curve. Space isn’t separate under either the model or experiments.

Spacetime curvature is tidal gravity. Consider two objects released side by side from a hovering platform in space. Initially they are at rest with respect to each other, so their worldlines are parallel. Their accelerometers read 0, so the worldlines are straight. Using radar or rulers, the distance between them changes. This combination of facts implies curved spacetime. In flat spacetime parallel straight worldlines are always equidistant. Since parallel straight worldlines do not remain equidistant in tidal gravity, tidal gravity is curved spacetime

2. Time: Can it stretch or contract if it has no physical existence?

In common scientific definitions time (specifically proper time) is what a clock measures. So time is by definition something that can be experimentally measured by physical devices.

So a claim that time “has no physical existence” must define “physical existence” in a way that doesn’t include things that can be experimentally measured by physical devices. Such a definition of “physical existence” is so weak that it is hard to see why anyone would care about it outside of philosophers.

I would point out your own requirement “this physical entity must have measurable quantities”. The usual scientific definition of time clearly satisfies that.

3. Light: Does it actually bend, or just change direction?

What is the difference between “actually bending” and “just changing direction”? What kind of experiment could detect the difference?

 

[Dale]

The thread has been reopened. Participants are reminded that the purpose of PF is to educate regarding mainstream science as described by the professional scientific literature. Personal opinions that are in conflict with that are not permitted.

 

[Uncle Thi]

Do you have an “objectivelyexistometer” that can measure whether or not something objectively exists?

Spacetime clearly satisfies the measurable quantities requirement. You can measure spacetime with clocks, rulers, protractors, speedometers, radars, and the like.

Spacetime does curve. Space isn’t separate under either the model or experiments.

Spacetime curvature is tidal gravity. Consider two objects released side by side from a hovering platform in space. Initially they are at rest with respect to each other, so their worldlines are parallel. Their accelerometers read 0, so the worldlines are straight. Using radar or rulers, the distance between them changes. This combination of facts implies curved spacetime. In flat spacetime parallel straight worldlines are always equidistant. Since parallel straight worldlines do not remain equidistant in tidal gravity, tidal gravity is curved spacetime

In common scientific definitions time (specifically proper time) is what a clock measures. So time is by definition something that can be experimentally measured by physical devices.

So a claim that time “has no physical existence” must define “physical existence” in a way that doesn’t include things that can be experimentally measured by physical devices. Such a definition of “physical existence” is so weak that it is hard to see why anyone would care about it outside of philosophers.

I would point out your own requirement “this physical entity must have measurable quantities”. The usual scientific definition of time clearly satisfies that.

What is the difference between “actually bending” and “just changing direction”? What kind of experiment could detect the difference?

Einstein's field equations are often used to describe gravity in General Relativity. I would like to understand better: What is the unit of gravitational force in these equations? Does it have the same unit as Newtonian force (kg·m/s²)?

 

[PeroK]

Einstein's field equations are often used to describe gravity in General Relativity. I would like to understand better: What is the unit of gravitational force in these equations? Does it have the same unit as Newtonian force (kg·m/s²)?

There is no force in the Field Equations. They essentially replace Newton's first law, which describes the motion of an object subject to no forces.

 

[Ibix]

What is the unit of gravitational force in these equations?

There is no gravitational force in these equations. General relativity models gravity as the geometry of spacetime. Things moving in a gravitational field follow straight line paths through 4d spacetime (called "geodesics"), which usually produce curved lines when you pick a slicing of spacetime that you want to call "space" and project the straight lines onto them.

 

[phinds]

If spacetime is said to bend due to mass and energy, is space itself a physical entity that can actually bend?

The concept of spacetime "bending" is due to a misapplication of Euclidean Geometry in a situation where it does not apply. Spacetime is NOT described by Euclidean Geometry but by pseudo-Riemann Geometry. "Geodesics" in that geometry are the equivalent of Euclidean "straight lines" but if you apply a Euclidean straight line onto a Geodesic then only in special cases would you get a match, whereas in most cases the Geodesic would look bent relative to the Euclidean line. SO ... to call the line "bent" you have to be applying Euclidean Geometry, which, again, is not what describes space-time.

If you still want to call a space-time geodesic "bent" then sure, knock yourself out ... it IS, after all "bent" in Euclidean Geometry. If, on the other hand, you want to talk actual physics, use the math that applies to the situation.

 

[Uncle Thi]

There is no force in the Field Equations. They essentially replace Newton's first law, which describes the motion of an object subject to no forces.

According to your explanation: "There is no force in the Field Equations."
Can I understand this as: there is no gravitational force, and therefore, no unit for gravitational force?
Thank you for your explaination.

 

[phinds]

Can I understand this as: there is no gravitational force, and therefore, no unit for gravitational force?

That is correct. Gravity is not a force in GR, it is a name given to a property of space-time geometry, namely that objects that do not have any force acting on them follow geodesics.

If you have a rocket with propellant coming out the back end, that is a force on the rocket and the rocket does not follow a geodesic. If the rocket is in free-fall with no force on it, then it is following a geodesic. The International Space Station, for example, is in orbit around the Earth, following a Geodesic.

 

[Ibix]

Can I understand this as: there is no gravitational force, and therefore, no unit for gravitational force?

There is no gravitational force in general relativity, so asking for its unit is like asking what units I use to measure the weight of the colour purple. It doesn't really make sense.

In theories of gravity that do have a force of gravity, you use the same units as any other force.

 

[jbriggs444]

Einstein's field equations are often used to describe gravity in General Relativity. I would like to understand better: What is the unit of gravitational force in these equations? Does it have the same unit as Newtonian force (kg·m/s²)?

The field equations tell you how an unaccelerated object will move -- along a geodesic. If you want to get an object to follow a different trajectory you will need to apply a force to do so. The required force is the familiar one from Newton's second law: $F = ma$.

For instance, a book sitting on a table is not following a geodesic trajectory. It is being accelerated upward and away from that free fall trajectory by the contact force from the table beneath. The required force to maintain its actual trajectory is given by the product of the mass of the book $m$ and the required proper acceleration $g$.

To be technically correct, the "force of gravity" is not the upward supporting force of table on book. It is the fictitious downward force that we imagine holds the book in place despite that upward force.

 

[Dale]

Can I understand this as: there is no gravitational force, and therefore, no unit for gravitational force?

There is no gravitational force in GR, but any force has units of newtons in SI.

There are fictitious forces. Those, like any force, have units of newtons. The Newtonian gravitational force is a fictitious force in General Relativity

Note that there are formulations of Newtonian gravity where Newtonian gravity is also a fictitious force and Newtonian spacetime is curved. So this formulation characterizes gravity, not relativity.

 

[PeroK]

I'm losing it!

Take the astronauts on the ISS. They are "weightless" despite the force of gravity acting on them. They feel nothing. Whereas, someone standing on Earth feels the real force pushing them upwards.

Even in elementary classical mechanics, we can model an external acceleration as the equivalent of a gravitational force. For example, a pendulum inside an accelerating vehicle. It behaves exactly as it would if gravity acted at an angle. The "effective" gravity being the vector sum of the Newtonian gravity and the fictitious force associated with the external acceleration.

 

[PeroK]

PS even before you get to GR, the seeds of gravity as a fictitious force are sown.

 

[Uncle Thi]

The field equations tell you how an unaccelerated object will move -- along a geodesic. If you want to get an object to follow a different trajectory you will need to apply a force to do so. The required force is the familiar one from Newton's second law: $F = ma$.

For instance, a book sitting on a table is not following a geodesic trajectory. It is being accelerated upward and away from that free fall trajectory by the contact force from the table beneath. The required force to maintain its actual trajectory is given by the product of the mass of the book $m$ and the required proper acceleration $g$.

To be technically correct, the "force of gravity" is not the upward supporting force of table on book. It is the fictitious downward force that we imagine holds the book in place despite that upward force.

In this case, how is it related to the unit of gravitational force that I mentioned?

*"For example, a book resting on a table does not follow a geodesic trajectory. It is being accelerated upward, away from the free-fall trajectory, by the contact force from the table below. The force required to maintain its actual trajectory is determined by the product of the book's mass and the required proper acceleration.

Technically speaking, the "gravitational force" is not the upward supporting force of the table on the book. It is the fictitious downward force that we imagine to hold the book in place despite that upward force."*

 

[Uncle Thi]

Take the astronauts on the ISS. They are "weightless" despite the force of gravity acting on them. They feel nothing. Whereas, someone standing on Earth feels the real force pushing them upwards.

Even in elementary classical mechanics, we can model an external acceleration as the equivalent of a gravitational force. For example, a pendulum inside an accelerating vehicle. It behaves exactly as it would if gravity acted at an angle. The "effective" gravity being the vector sum of the Newtonian gravity and the fictitious force associated with the external acceleration.

Are there any direct experimental measurements of spacetime curvature that do not rely on its effects on mass and energy?

 

[jbriggs444]

In this case, how is it related to the unit of gravitational force that I mentioned?

It comes from Newton's second law. $F=ma$ (or $F=dp/dt$ if you prefer).

As @Dale said, in SI this is kilogram meters per second squared.

 

[PeroK]

Are there any direct experimental measurements of spacetime curvature that do not rely on its effects on mass and energy?

The detection of gravitation waves at LIGO might be the best direct experiment.

 

[julian]

Have people explored Einstein's hole argument and its implications for spacetime ontology? It’s a fascinating way to rethink how we understand coordinates and reality. I've discussed it before, but I'm quite busy at the moment. If you're interested, I highly recommend checking out Chapter 2 of Rovelli's book, available for free here: https://www.cpt.univ-mrs.fr/~rovelli/book.pdf.

 

[Uncle Thi]

It comes from Newton's second law. $F=ma$ (or $F=dp/dt$ if you prefer).

As @Dale said, in SI this is kilogram meters per second squared.

My question: Why is the unit of force kg·m/s² instead of another unit?

 

[Dale]

Are there any direct experimental measurements of spacetime curvature that do not rely on its effects on mass and energy?

All measuring devices are made of mass and energy, so the answer is trivially “no”. That “no” answer is not particularly meaningful.

My question: Why is the unit of force kg·m/s² instead of another unit?

That is really a question that belongs in the classical physics forum. It has nothing to do with relativity. The brief answer is that is the units required for dimensional consistency in Newton’s 2nd law.

 

[PeroK]

My question: Why is the unit of force kg·m/s² instead of another unit?

How much physics do you actually know?

 

[jbriggs444]

My question: Why is the unit of force kg·m/s² instead of another unit?

You can do $F=kma$ with slugs, furlongs and microfortnights if you choose. With the SI units, $k=1$ which makes life a bit easier.

 

[PeterDonis]

You can do general relativity with slugs, furlongs and microfortnights.

My high school AP chemistry teacher wanted us to use tons, furlongs, and fortnights. I even worked out values for physical constants in the TFF system:

http://peterdonis.net/misc/tffunits.html

 

[Uncle Thi]

You can do $F=kma$ with slugs, furlongs and microfortnights if you choose. With the SI units, $k=1$ which makes life a bit easier.

This answer doesn't make me fully understand the meaning. You should be careful with your response, as it might get a warning, lead to the post being locked, or end the discussion. I'm just trying to learn, but it seems like not having formal education is an issue here?
Thank you.

 

[PeterDonis]

You should be careful with your response, as it might get a warning, lead to the post being locked, or end the discussion.

I'm not sure where you're getting that from, but nothing @jbriggs444 has posted comes anywhere near meriting any of those things.

it seems like not having formal education is an issue here?

I don't know where you're getting that from either. You have been getting good responses in this thread.

 

[PeterDonis]

Technically speaking, the "gravitational force" is not the upward supporting force of the table on the book. It is the fictitious downward force that we imagine to hold the book in place despite that upward force."*

In GR, no force needs to "hold the book in place despite that upward force". The upward force pushes the book off of the geodesic (free-falling) trajectory it would otherwise follow, so that the book now has nonzero proper acceleration due to the upward force on it. That is the relativistic version of Newton's Second Law: any nonzero proper acceleration has to be caused by a force.

In GR, if there were truly a downward force on the book equal and opposite to the upward force on it, it would not have a nonzero proper acceleration and it would follow a different trajectory--the same trajectory it would follow in free fall, since the net force on it would be zero.

Note that in the above analysis I have been implicitly using a local inertial frame. It is true that in a non-inertial frame, such as the rest frame of the table, a "fictitious" force is usually said to be acting downward on the book to "hold it in place". But this fictitious force cannot be felt and does not correspond to any proper acceleration. And in GR, that means it isn't a force at all; in GR, only things that cause nonzero proper acceleration are considered to be forces. This is a much simpler and more physically reasonable viewpoint than the viewpoint of Newtonian physics, where the "gravitational force" is considered to be a real force--and then a separate ad hoc explanation needs to be given for why this force is not felt; objects in free fall in a gravitational field are weightless, feeling no force at all.

 

[pervect]

In my view, the definition of 'physical' implies that an entity must objectively exist in the real world, and this physical entity must have measurable quantities.
---
Based on that definition, could you clearly define the following:

1. What is space?
2. What is time?
3. What is light?

I have no idea how you think you can tell if something "objectively exists", especially via experiment (which is what science is primarily about), so I won't address this part of your question. You're on your own, enjoy.

I think things might go better if we discuss Euclidean geometry for a moment, and once we have a common understanding or an agreement to disagree about that, we can revisit relativity. At least I'll attempt to -as I wrote this, things started to branch out a bit.

One can measure distance with a ruler. So, I'd assume you would share the belief that it's a physical quantity. "Entity" seems to be a bit on the wrong track to me, I don't usually think of distance as an "entity". But that may be just the way I use the words.

I would say that geometry, which I think of as the study of distance, is more of a mathematical structure. So, we have "physical" distance, and the theoretical structure of Euclidean geometry that organizes it. Note that we have some closely related alternatives to Euclidean geometry, for instance spherical and hyperbolic geometries. And, somewhat importantly for later on, we have Riemannian geometry.

Now, let's move on to relativity.

The organization principle of special (and general) relativity basically replaces "distance" with the "Lorentz interval".

So, the obvious the next question would be - is the Lorentz interval real? An argument for, is that it's the same for all observers. Distance, in special relativity, is NOT the same for all observers. Proper time, the sort of time one measures with a clock, falls into the category of a Lorentz interval, and proper distance, the sort of distance one measures with a ruler, also falls in the category of the Lorentz interval. So I would put proper distance, proper time, and the Lorentz interval all into the "physical" category.

As an aside, I view most tensor quantities as "existing". This may not help if you are not familiar with tensor quantites. The key point of tensor quantities is that while they may have components that depend on the observer, the structure as a whole has rules that allow these components to be transformed between observers. Sorry if you're not familiar with tensors, this may be moot. If you are familiar and have some thoughts about whether tensors are "real", you could consider sharing these thoughts in the intersts of communication and discussion.

Going back to the Lorentz interval - I imagine one could come up with an argument that The Lorentz interval is a mathematical construct of some sort, but that's not my view. While one could argue about that, my personal reaction is that it is the sort of argument that doesn't go anywhere I find interesting, it's mostly about semantics.

This discussion may not help if you're not familiar with the Lorentz interval. Sorry about that if that's the case. Taylor & Wheeler's "space-time physics" talks about them a lot, I have always found "The Prable of the Surveyor" to be particular helpful, for whatever that's worth. Realistically, though, if you're not already familiar with the term and its implications, it would probably be a huge digression to talk about it. If you're interested in relativity, it'd be worth your time, it's just wouldn't be appropriate for this thread.

We can regard the organizational principles of the Lorentzian geometry, the geometry of the Lorentz interval, in the same category that we put Euclidean geometry, the study of distances. Special relativity is then a particular case of the geometry of the Lorentz interval, one that applies to "flat" spacetimes. General relativity opens things up to different geometries. General relativity basically applies Riemannian geometry to the Lorentz interval. (Some purists call it pseudo-Riemannian geometry for technical reasons).

The fact that one can apply the structure of a geometry (Euclidean or Riemannian) to multiple physical concepts is one argument for why I think of them as being organizational structures rather than physical entities.

So to recap the points I think are most important. "Distance" and "Lorentz intervals" are IMO physically measurable quantities which exist independent of the observer. The organization of either into a geometry is more mathematical. The same math can be used to organize different things, the math assumes some basic axioms, if the axioms fit the physics, we apply the structure of the math to the physical objects to draw conclusions.

 

[Uncle Thi]

How much physics do you actually know?

My physics knowledge is just at a high school level and what I've gathered from the internet."

I thought science was about critical thinking and questioning to find the truth, not just memorizing knowledge without reasoning. Is physics about open-minded exploration, or is it just about repeating what has been taught?"

By the way, what I’m presenting here is actually mainstream science in Vietnam!

According to my high school knowledge: A theoretical model without any physical quantities cannot be considered a physical model. Is that correct?

 

[Uncle Thi]

In GR, no force needs to "hold the book in place despite that upward force". The upward force pushes the book off of the geodesic (free-falling) trajectory it would otherwise follow, so that the book now has nonzero proper acceleration due to the upward force on it. That is the relativistic version of Newton's Second Law: any nonzero proper acceleration has to be caused by a force.

In GR, if there were truly a downward force on the book equal and opposite to the upward force on it, it would not have a nonzero proper acceleration and it would follow a different trajectory--the same trajectory it would follow in free fall, since the net force on it would be zero.

Note that in the above analysis I have been implicitly using a local inertial frame. It is true that in a non-inertial frame, such as the rest frame of the table, a "fictitious" force is usually said to be acting downward on the book to "hold it in place". But this fictitious force cannot be felt and does not correspond to any proper acceleration. And in GR, that means it isn't a force at all; in GR, only things that cause nonzero proper acceleration are considered to be forces. This is a much simpler and more physically reasonable viewpoint than the viewpoint of Newtonian physics, where the "gravitational force" is considered to be a real force--and then a separate ad hoc explanation needs to be given for why this force is not felt; objects in free fall in a gravitational field are weightless, feeling no force at all.

Does your example actually fit the meaning of spacetime?

 

[jbriggs444]

My physics knowledge is just at a high school level and what I've gathered from the internet.

The information delivered here is more reliable than pretty much any high school and definitely better than 99% of the popular science videos on the internet.

By the way, what I’m presenting here is actually mainstream science in Vietnam!

If true, that is unfortunate for Vietnam.

According to my high school knowledge: A theoretical model without any physical quantities cannot be considered a physical model. Is that correct?

A theoretical model that makes predictions that can be experimentally tested can be considered a physical model. If those predictions turn out to be uniformly correct then it may be considered a reliable model.

That said, not many people run around worrying about the defining characteristics of a "physical model". It is more useful to shut up and do physics.

Does your example actually fit the meaning of spacetime?

As a rule, what @PeterDonis writes is highly reliable. The content in the passage you quote is not at all controversial and definitely fits with spacetime as described by general relativity.

 

[phinds]

I thought science was about critical thinking and questioning to find the truth, not just memorizing knowledge without reasoning.

That is correct. If you think that what is going on here at PF is anything other than this, then you are badly misunderstanding what IS going on.

I am puzzled. Presumably you have joined PF so as to get answers to science questions from people who know what they are talking about and if that is so, then you have come to the right place. What I don't understand is why you seem to believe that we are NOT giving you the right answers to your questions.

If you have "learned" your science from popular science presentations, then that would explain why you have a poor understanding of existing science.

 

[Dale]

I thought science was about critical thinking and questioning to find the truth, not just memorizing knowledge without reasoning.

Science is about the scientific method. Critical thinking is great, but only when one actually has the experimental evidence in mind while doing so.

I recommend that you start here: https://math.ucr.edu/home/baez/physics/Relativity/SR/experiments.html

Scientists don’t accept relativity because we find it appealing. We accept it because of the strength of the experimental evidence supporting it.

 

[PeterDonis]

I thought science was about critical thinking and questioning to find the truth, not just memorizing knowledge without reasoning. Is physics about open-minded exploration, or is it just about repeating what has been taught?"

I think you are under a serious misapprehension about why you were asked how much physics you actually know. The question was not asking how much formal physics education you have. It was asking how much physics you know--as in, how much physics have you actually exercised critical thinking and questioning about, instead of just memorizing knowledge without reasoning.

By the way, in your definition of science, you left out one critical component: experiments. Science is about critical thinking and questioning to find the truth by doing experiments to test our models.

(This, by the way, should help to point you to good answers to the questions I pose to you at the end of this post. You are expected to use critical thinking to answer them.)

By the way, what I’m presenting here is actually mainstream science in Vietnam!

Do you have a reference?

According to my high school knowledge: A theoretical model without any physical quantities cannot be considered a physical model.

What are "physical quantities"? If I hand you a theoretical model, how do you tell whether it has "physical quantities" in it or not?

 

[PeterDonis]

Does your example actually fit the meaning of spacetime?

Um, yes?

What do you think "the meaning of spacetime" is, if you're not sure whether my example fits it?

 

[Uncle Thi]

According to my high school knowledge: A theoretical model without any physical quantities cannot be considered a physical model. Is that correct?

You still haven't answered my question directly, have you?
Thank you.

 

[Orodruin]

You can do $F=kma$ with slugs, furlongs and microfortnights if you choose. With the SI units, $k=1$ which makes life a bit easier.

That doesn’t make $k\neq 1$ as long as you use the corresponding derived unit of force (slug furlongs per microfortnight squared).

It is always F = ma, but you can get a numerical factor for the measured value from unit conversion. This is no stranger than 1 cm = 0.01 m.

 

[Dale]

You still haven't answered my question directly, have you?

Your question is not relevant to the current discussion. Often, as here, when a post contains multiple questions people will naturally focus on the more important ones, leaving less useful ones unanswered.

However, in the interest of directness:

A theoretical model without any physical quantities cannot be considered a physical model. Is that correct?

That is correct.

It is irrelevant since we are not discussing a “theoretical model without any physical quantities”.

Let me also ask you a direct question: do you believe that the statement “the table top is flat and the legs are perpendicular to the table top” is a physical statement which could be experimentally observed and measured?

 

[PeterDonis]

You still haven't answered my question directly, have you?

You haven't answered the questions I posed to you in response at the bottom of post #40.

 

[Uncle Thi]

In the case of black holes we understand that spacetime takes on a remarkable physical existence, spacetime in such cases even acts as a physical object with mass and rotation for an external observer. It is true that in your daily life you have a normal and smooth relationship with spacetime, but you will think differently if you move at speeds close to the speed of light, or fall into a black hole.

According to your explanation, should I understand that spacetime is a physical entity with mass? If so, in Einstein’s field equations (EFE), can you point out the physical quantity that represents the mass of spacetime? I have been searching for it for a long time but have not found it.

Einstein’s field equations (EFE) are written as:
Gμν + Λ gμν = (8πG / c⁴) Tμν

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Thank you.

 

[phinds]

According to your explanation, should I understand that spacetime is a physical entity with mass?

His "explanation" is wrong and should be ignored.

can you point out the physical quantity that represents the mass of spacetime?

There is no such thing.

I have been searching for it for a long time but have not found it.

Not surprising since there is no such thing.

 

[phinds]

@Uncle Thi you REALLY need to read some actual physics texts and stop with these pop-sci questions.

 

[PeterDonis]

According to your explanation

Please note that the post you were responding to there is not a good description of how spacetime is treated in GR.

 

[PeterDonis]

should I understand that spacetime is a physical entity with mass?

Not the way you mean.

in Einstein’s field equations (EFE), can you point out the physical quantity that represents the mass of spacetime?

There isn't one.

What is true is that a black hole is a solution of the EFE which is vacuum (i.e., no matter present), but which looks to an external observer like an object with mass--for example, you can put things in orbit around it and calculate the mass using Kepler's Third Law. But that is not the same as saying there is a quantity in the EFE that represents "the mass of spacetime". There isn't.