[geordief (OP)]

I have been told (forget the context) not to get fixated on light in relativity ,that It is more fundamental than that.

My question is whether we can show the union between space and time in other ways (physically ,perhaps rather than mathematically but that would be good too)

Do we have to go to the quantum realm to  find processes where em radiation  is not a consideration ?

Is spacetime an equally valid concept in quantum physics as it is in Classical physics?

Is it more or less assumed that the classical forces  are more or less branches of one same force?

[Halc]

The text of the post seemed pretty baffling to me, so I'll take a shot at the title question.

Spacetime is understood in relation to c, which is a constant speed.  It just happens that EM radiation propagates at that speed, as does several other things (like gravity waves), but the fundamental constant c is not particularly about light.

So when one does a Lorentz transformation to reorder a set of events to a different inertial frame, there is no EM radiation involved in this transformation.  It is simply a geometric exercise.

That is all a pretty classical description and I would not do a good job of trying to fit quantum physics into that description.

Three of the forces have been unified under one theory, but it is questionable if gravity is a force at all.  The inability to unite gravity with the others suggests that Einstein was on track in casting of it as curvature in spacetime rather than a force.

[geordief (OP)]

The text of the post seemed pretty baffling to me, so I'll take a shot at the title question.

Spacetime is understood in relation to c, which is a constant speed.  It just happens that EM radiation propagates at that speed, as does several other things (like gravity waves), but the fundamental constant c is not particularly about light.

So when one does a Lorentz transformation to reorder a set of events to a different inertial frame, there is no EM radiation involved in this transformation.  It is simply a geometric exercise.

That is all a pretty classical description and I would not do a good job of trying to fit quantum physics into that description.

Three of the forces have been unified under one theory, but it is questionable if gravity is a force at all.  The inability to unite gravity with the others suggests that Einstein was on track in casting of it as curvature in spacetime rather than a force.

Sorry for being baffling (it probably is a reflection of my confused mind)-and thanks for your answer.

But why is "spacetime understood in relation to c" then if it is not in connection with the speed of light?

Was it just plucked out of the air as a speed of propagation (of what?)   of undetermined actual value that came to be associated with the actual measured speed of light (and massless objects) in a vacuum?

It was Einstein ,wasn't it who was responsible for this step if I have described it correctly  (or approximately correctly).

Or was it actually Lorentz  who was responsible ,drawing formal mathematical conclusions from Einstein's  idea?

Or was it some kind of a hybrid offspring between the two men?

ps I wasn't considering gravity as one of the forces,just the electro-weak and strong  forces

[Halc]

But why is "spacetime understood in relation to c" then if it is not in connection with the speed of light?

c seems to be one of those fundamental constants in our universe, and it got its name 'light speed' because light happens to be one of those things confined by this fundamental process. 

Was it just plucked out of the air as a speed of propagation (of what?)

It was probably first measured by observation of light, and hence the name applied to the constant.  You find a reliable and readable clock at a large distance that changes and observe (via EM radiation) how the time on the clock appears to read sooner or later depending on how far away it is.  That's how it was done in Newton's time.

of undetermined actual value that came to be associated with the actual measured speed of light (and massless objects) in a vacuum?

Nobody suspected any fundamental constant back then.  It was first measured as light speed just like they measured sound speed, without suggesting that it was a fundamental limit.  Only with Lorentz/Minkowski/Einstein came the fact that c was a fundamental constant related to many non-EM aspects of the universe.

It was Einstein ,wasn't it who was responsible for this step if I have described it correctly  (or approximately correctly).

And others, yes. Einstein put all the pieces together first to form a complete theory.

Or was it actually Lorentz  who was responsible ,drawing formal mathematical conclusions from Einstein's  idea?

They drew on each other, but it was often more Einstein drawing on Lorentz's conculsions.  It would have been called the Einstein transformation if he had thought of it first.

Or was it some kind of a hybrid offspring between the two men?

A combined effort of several physicists, not all of which I can rattle off without looking up,.

ps I wasn't considering gravity as one of the forces,just the electro-weak and strong  forces

I don't think the unification of those theories falls from Einstein's work.  That is more in the quantum realm, and QFT has never been unified with GFT, hence the lack of unified field theory.  They're working on it.

I've seen the unification of EM and strong via early work on octonian theory rather than through quantum field theory.  If that can be done, further study in this area might be the path to a unified theory.
https://www.wired.com/story/the-peculiar-math-that-could-underlie-the-laws-of-nature/

[Janus]

The text of the post seemed pretty baffling to me, so I'll take a shot at the title question.

Spacetime is understood in relation to c, which is a constant speed.  It just happens that EM radiation propagates at that speed, as does several other things (like gravity waves), but the fundamental constant c is not particularly about light.

So when one does a Lorentz transformation to reorder a set of events to a different inertial frame, there is no EM radiation involved in this transformation.  It is simply a geometric exercise.

That is all a pretty classical description and I would not do a good job of trying to fit quantum physics into that description.

Three of the forces have been unified under one theory, but it is questionable if gravity is a force at all.  The inability to unite gravity with the others suggests that Einstein was on track in casting of it as curvature in spacetime rather than a force.

Sorry for being baffling (it probably is a reflection of my confused mind)-and thanks for your answer.

But why is "spacetime understood in relation to c" then if it is not in connection with the speed of light?

Was it just plucked out of the air as a speed of propagation (of what?)   of undetermined actual value that came to be associated with the actual measured speed of light (and massless objects) in a vacuum?

It was the behavior of light that led to the conclusion of the fundamental constant c. It was the clue that pointed us where we needed to go.  Even if this had not been the case, we would have eventually stumbled upon c as a limit as we learned to accelerate massive particles up to greater speeds. We would have found that they did not behave as expected by Newtonian Physics.  After enough experiments were performed, a pattern would have emerged that would have pointed us to c.
As to why we keep referring to c as "the speed of light", it just easier than constantly calling it " the fundamental invariant speed of the universe".      For much the same reason, people tend to use the term "Crescent wrench" when referring to any "open-ended adjustable wrench", even though "Crescent" is actually a just a brand name.

[Colin2B]

Just to add to what @Halc and @Janus said:

I have been told (forget the context) not to get fixated on light in relativity ,that It is more fundamental than that.

This may be a reference to the fact that Einstein’s first paper to mention SR was on moving electric/magnetic fields, what is known as electrodynamics. The ‘more fundamental’ issue is that these fields are 2 sides of the same coin and relativity makes this obvious. Light came into it because it is a moving em field and Maxwell had shown it to have a speed independent of the source or receiver - although he didn’t realise it at the time - and Einstein took this as an assumption.
What is really interesting is that much our electronics would not work if relativity was wrong.

Do we have to go to the quantum realm to find processes where em radiation  is not a consideration ?

no, em is just as important there, so is relativity. Quantum Field Theory is probably the most precise theory of natural phenomena ever developed and relativistic em fields are a very important part of it.

Is spacetime an equally valid concept in quantum physics as it is in Classical physics?

Initially Schrodinger’s view of the atom didn’t consider where the atom is or whether it is moving so relativity and spacetime didn’t come into it. When particle accelerators started throwing atoms and parts of atoms around it soon became apparent that relativity applied to these very small items and it was essential to take it into account. Indeed, it is yet another indication that relativity is correct.

[jeffreyH]

I have to disagree with the idea that gravity is not a force. Is there a force that draws opposite magnetic poles together or is it a curvature in the magnetic field? There is curl in the magnetic field so does that point to geometry over force?

Objects follow geodesics in a gravitational field. This can be thought of as a divergence from an inertial state. A change of direction is by default an acceleration. I can attach a rope to the side of a moving shopping trolley and pull on it to divert its path. That requires a force. Acceleration is always the result of a force. Geometry describes the effect of this force on objects. It is not the cause. Don't confuse the model with the observable phenomena.

[Halc]

I have to disagree with the idea that gravity is not a force. Is there a force that draws opposite magnetic poles together or is it a curvature in the magnetic field? There is curl in the magnetic field so does that point to geometry over force?

I don't think that can be modeled as curvature since different (otherwise interial) particles will take different paths depending on their mass/charge.  Not so with gravity where an inertial mass always moves in straight lines (a geodesic) regardless of mass or velocity.
This allows a geometric interpretation of that motion rather than a description in terms of force. It does not prove one interpretation over the other, but some of the discussion above reinforces the bent-space interpretation.

Objects follow geodesics in a gravitational field. This can be thought of as a divergence from an inertial state.

Well no, that's the whole point.  It isn't a divergence from an inertial trajectory.  Any non-geodesic is such a divergence and requires an actual force.

A change of direction is by default an acceleration.

Not in the case of a geodesic, which is not a change in direction in the space being considered.  A airplane has but two geodesic paths between any two points on the surface of earth, and only on those two paths can the airplane get from here to there without ever turning left or right.  It doesn't change direction on the surface of Earth even if it appears to on a paper map.

Acceleration is always the result of a force.

Well, that's a pretty good point. Why does a dropped rock accelerate in the geometric interpretation of gravity? Does it?? If it isn't a force, then it isn't turning, but it certainly is trading potential for kinetic energy, and a force seems necessary for that.  Not sure if the geometric model considers changes in kinetic energy due to changes in depth of a gravity field to be the same sort of acceleration that you'd get from a more classic force like EM.  I'm sitting here on Earth and I have a classic force pushing me up from the floor.  If that's the only force, why am I not accelerating?  Well, actually I am since I'm certainly being diverted from the geodesic that I'd otherwise be following.

[jeffreyH]

Einstein argued that if you were inside a box you would not be able to tell if you were in a freely falling frame in a gravitational field or in an inertial frame. In an accelerating frame an object would be emitting electromagnetic radiation. So does the free falling object in the box emit electromagnetic waves? Does the radiation increase with increasing speed? If it emits no radiation due to acceleration then it can be considered truly inertial. In which case Einstein is correct. Which is it?

[Halc]

Einstein argued that if you were inside a box you would not be able to tell if you were in a freely falling frame in a gravitational field or in an inertial frame. In an accelerating frame an object would be emitting electromagnetic radiation. So does the free falling object in the box emit electromagnetic waves? Does the radiation increase with increasing speed? If it emits no radiation due to acceleration then it can be considered truly inertial. In which case Einstein is correct. Which is it?

In an accelerating frame a charged object would emit EM radiation, which seems equivalent to an accelerating mass emitting gravity waves.  I'm not sure if either can be detected from inside a box accelerating with the object. Einstein pushed the whole bent-space interpretation (as opposed to force), and yet he predicted the gravity waves which emit from objects taking geodesic paths, which means yes, there is acceleration going on.  If that violated the equivalence principle, I think it would have been pointed out.  So I suspect such radiation cannot be detected from within the box accelerating with the charged mass.
I very much see your point though. You're looking for empirical differences between the two interpretations, which is far better than just asserting your opinion on the subject. I'm not sure if my response is correct.

[Janus]

Einstein argued that if you were inside a box you would not be able to tell if you were in a freely falling frame in a gravitational field or in an inertial frame. In an accelerating frame an object would be emitting electromagnetic radiation. So does the free falling object in the box emit electromagnetic waves? Does the radiation increase with increasing speed? If it emits no radiation due to acceleration then it can be considered truly inertial. In which case Einstein is correct. Which is it?

https://en.wikipedia.org/wiki/Paradox_of_radiation_of_charged_particles_in_a_gravitational_field

[jeffreyH]

The point about a charged particle radiating when falling freely is the most interesting. Since it would fall at a different rate to a neutral particle then a free falling frame must be equivalent to an inertial frame. Thanks for the link Janus. It clears up a lot of questions.

[Halc]

The point about a charged particle radiating when falling freely is the most interesting. Since it would fall at a different rate to a neutral particle then a free falling frame must be equivalent to an inertial frame. Thanks for the link Janus. It clears up a lot of questions.

But it doesn't.  The paradox says that it would seem to at first consideration, but the paradox was resolved with:

Fritz Rohrlich (1965),[6] who shows that a charged particle and a neutral particle fall equally fast in a gravitational field

Thus showing that motion under gravity is not acceleration.  Gravity is bent space, not a force.  A pseudo-force at best, like centrifugal force.

[jeffreyH]

The point about a charged particle radiating when falling freely is the most interesting. Since it would fall at a different rate to a neutral particle then a free falling frame must be equivalent to an inertial frame. Thanks for the link Janus. It clears up a lot of questions.

But it doesn't.  The paradox says that it would seem to at first consideration, but the paradox was resolved with:

Fritz Rohrlich (1965),[6] who shows that a charged particle and a neutral particle fall equally fast in a gravitational field

Thus showing that motion under gravity is not acceleration.  Gravity is bent space, not a force.  A pseudo-force at best, like centrifugal force.

You missed the point. It doesn't radiate therefore a freely falling frame is equivalent to an inertial frame. I see no paradox at all.

[Halc]

I am basing my comment on this one, which is confusing.

Since it would fall at a different rate to a neutral particle then a free falling frame must be equivalent to an inertial frame.

It says 'since [the charged particle] would fall at a different rate to a neutral particle' which would lead one to the opposite conclusion: that the principle of locality is wrong and gravity is a force.  But the two particles don't in fact fall at at a different rate.  The above comment is mistaken.  Maybe you forgot a 'not' somewhere in that sentence.  It seemed to be taken from the statement of the paradox as presented before the paradox was resolved in the 60's.

[jeffreyH]

It was my sloppy use of language. However, you still cannot say gravity isn't a force. I will explain my reasoning later.

[westernira]

Your opinion also resolve my issue and it really very helpful for me.

[jeffreyH]

What is inertia? It is objects moving uniformly through space. If two objects collide then we have a force. What is gravity? It is objects uniformly accelerating through space. If objects collide in free fall we get a force. However, it is the uniform motion  that makes the force of gravity undetectable. An evenly distributed force causes no detectable stress. This breaks down in the case of extreme tidal forces, where a force would become apparent.

An undetectable force is still a force. It is an all encompassing one. Which is why it ultimately traps light in the densest objects.

This might manifest itself geometrically but that does not make it a 'ficticious' force.

[yor_on]

I'd expect it to break down at Planck scale Geordie. That's a scale where light is presumed to take on Planck 'step' in one Planck 'time' as locally measured. It's such a ridiculously small unit that it doesn't even, ever, will be measurable for us.

Under that your guess is as good as mine.

[Halc]

What is inertia? It is objects moving uniformly through space.

That's inertial motion.  Inertia is something else, more or less equivalent to mass.
Two objects can have the same inertial motion but very different inertia, or they can have the same inertia but very different inertial motion.

What is gravity? It is objects uniformly accelerating through space.

Ouch...   No.  I can accelerate uniformly through space without involving gravity, and I can achieve non-uniform acceleration using only gravity.

I googled the question just to see what would come up, and found this interesting answer:
https://starchild.gsfc.nasa.gov/docs/StarChild/questions/question30.html

And some scientists think that it is made up of particles called gravitons which travel at the speed of light.

No scientist thinks that. The statement is blatantly wrong. Gravity waves, sure, but not gravity.
I'm used to finding crap on the internet, but on a nasa.gov site?  It sort of recovers from that statement to say at least that we do not know what gravity "is" in any fundamental way - we only know how it behaves.

If objects collide in free fall we get a force.

Or even if not in free fall.

However, it is the uniform motion  that makes the force of gravity undetectable. An evenly distributed force causes no detectable stress. This breaks down in the case of extreme tidal forces, where a force would become apparent.

Free fall is locally indistinguishable from inertial motion, yes.  A tidal test is not a local test since it requires a separation to detect a non-uniform gravitational field.  The difference need not be extreme at all.  A simple tidal test is pretty trivial on the space station, if they have a place where the air currents don't overpower where gravity is pushing a couple objects.

An undetectable force is still a force.
...
This might manifest itself geometrically but that does not make it a 'ficticious' force.

Agree with this. It might be a matter of interpretation, but the 'accelerating charged particle' thing seems to be a test distinguishing between the two interpretations. If so, they're not really interpretations anymore.