I am with you on this one Mike.

We can come up with all kinds of weird theories to account for observations, for instance we could come up with the theory that other human beings do not exist when we do not observe them, and the theory could still be made to match observations accurately, but is that desirable? Since we have another theory that says that other human beings exist even when we don’t observe them, does that mean that they both exist and do not exist when we don’t observe them? No, it has to be one way or the other, there is a way that reality is even if we don’t see the whole of it, otherwise everything both happens and not happens at the same time, everything both exists and does not exist at the same time, and everything stops making sense. There is existence rather than non-existence, and we can’t be mistaken about that. Whatever experiences we have, these experiences exist. Not all is relative.

So regarding special relativity, we can also account for observations in a weird way such that each twin ages more slowly than the other, but is that desirable? Is there really no underlying reality and everything is relative? No! We are not forced to see it that way. Many people believe or want us to believe that relativity proves everything is fundamentally relative, they are wrong. Special relativity can be formulated in a way such that there is one absolute reference frame, meaning that we have a theory that is empirically equivalent to special relativity (it makes all the same observable predictions), without all the stuff that doesn’t make sense. Where the theories differ is only in their description of what happens beyond our observations (for instance how much each twin is really aging at each moment of the trip).

That theory is the Lorentz aether theory. Contrary to a popular misconception, the concept of the aether has not been disproven. In fact, the cosmic microwave background radiation that permeates all space can be interpreted as an aether. And even Einstein had reintroduced an aether in his theory of general relativity.

According to the Lorentz aether theory, there is one absolute frame of reference. Then we carry out all calculations relative to this absolute frame of reference. If the absolute frame happens to be where the home twin is, then the traveling twin ages more slowly at every moment (because he is moving relative to the absolute frame). If the absolute frame happens to be the frame in which the traveling twin is on the first half of his trip, then the home twin ages more slowly on the first half (because he is moving relative to the absolute frame), but the traveling twin ages much more slowly on the second half (because he is moving twice faster relative to the absolute frame). No matter the state of motion of the home twin relative to the absolute frame, there is always one reality and the result is always that the traveling twin ages more slowly, with the same result as what special relativity predicts.

That theory explains all the experiments that are considered tests of special relativity, and it explains experiments such as the Sagnac effect or thought experiments such as the twin paradox in a way that is intuitive, whereas special relativity has to resort to convoluted explanations that leave a feeling of uneasiness and give rise to the sad mentality of “shut up and calculate”.

I believe that the Lorentz aether theory is not the end of the story, however for now it is an alternative to special relativity that works just as well experimentally and that makes sense. I believe special relativity is an approximation of a more correct theory, so in the same way I believe the Lorentz aether theory is an approximation of a more correct theory.

Special relativity postulates that light travels at the same velocity in all inertial frames and that physical laws are the same in all inertial frames, but if we accept these postulates then we have zero clue as to where to go further to find a more fundamental theory. Whereas the Lorentz aether theory explains these postulates by saying that light travels at the same velocity in all directions only in the absolute frame, that objects moving relative to that frame are length-contracted by a given amount that depends on their velocity in the aether, that clocks run slower by a given amount that depends on their velocity in the aether, and that gives us lines of inquiry to go further. We can ask why are objects physically contracted when they move in the aether, and why are processes such as those taking place in clocks slower when they are in motion in the aether? And move from there and search in that direction.

Why aren't we at the centre of the earth by now then? — ovdtogt

Mainly because of the reactive force/acceleration of the surface of the Earth upon one's feet and also partly because of the rotation of the Earth and the centrifugal/centripetal forces keeping us rotating with the Earth ( this is more prevalent at the equator than at the poles.)

Philosophy has entered into my thinking about the twin paradox in this way: when some physicists contend that simultaneity at a distance is meaningless, I have a philosophical problem with that. IF I were that traveler, I don't think I would be able to believe that she no longer EXISTS whenever I am not co-located with her. (And I doubt that many other physicists believe that either). BUT many physicists DO believe that she doesn't have a well-defined current AGE when he is separated from her (at least if he has accelerated recently). THAT'S the conclusion that I can't accept philosophically: it seems to me that if she currently EXISTS right now, she must be DOING something right now, and if she is DOING something right now, she must be some specific AGE right now. So I conclude that her current age, according to him, can't be a meaningless concept. That puts me at odds with many other physicists.

What say the philosophers on this forum? — Mike Fontenot

The way I think of it is that every object has a present moment, just as every object has a location.

People on Earth (as with most matter in the universe) age at about the same rate because we all move at similarly slow speeds relative to the speed of light. So for all practical purposes, we can suppose that Alice's present moment on Earth is the same as Bob's on Earth. That is, their planes of simultaneity are approximately the same wherever they are on Earth or whatever their velocity. And so Alice's age in her reference frame is the same as Alice's age in Bob's reference frame (within a tiny error range).

That explains our natural intuition (backed by everyday experience) that other people are a specific and unambiguous age right now.

However if Bob rockets away at close to the speed of light, the present moments for Alice and Bob diverge. Thus Alice's age at the turnaround point in Bob's reference frame will be different to Alice's age at the turnaround point in Alice's reference frame.

The philosophical point here, I think, is that we make a simplifying assumption regarding the present moment because of our everyday experience on Earth. But if that assumption is false (as SR would seem to indicate), then that has consequences for other concepts that depend on that assumption. Such as, for example, what it means for distant objects or events to exist right now. This idea is explored further with the Andromeda paradox.

The philosophical point here, I think, is that we make a simplifying assumption regarding the present moment because of our everyday experience on Earth. But if that assumption is false (as SR would seem to indicate), then that has consequences for other concepts that depend on that assumption. Such as, for example, what it means for distant objects or events to exist right now. This idea is explored further with the Andromeda paradox. — Andrew M

I think the way Penrose explains the situation makes it clear that it is only superficially paradoxical:

Two people pass each other on the street; and according to one of the two people, an Andromedean space fleet has already set off on its journey, while to the other, the decision as to whether or not the journey will actually take place has not yet been made. How can there still be some uncertainty as to the outcome of that decision? If to either person the decision has already been made, then surely there cannot be any uncertainty. The launching of the space fleet is an inevitability. In fact neither of the people can yet know of the launching of the space fleet. They can know only later, when telescopic observations from Earth reveal that the fleet is indeed on its way. Then they can hark back to that chance encounter, and come to the conclusion that at that time, according to one of them, the decision lay in the uncertain future, while to the other, it lay in the certain past. Was there then any uncertainty about that future? Or was the future of both people already "fixed"? —  Roger Penrose, The Emperor's New Mind

There is no "difference that makes a difference" here, and I think this is the important lesson, which also shows the silliness of those who bitch and moan about how counterintuitive and just wrong relativity is. The fact is that relativity does not contradict our everyday experience. Ask yourself, what would have been different from your point of view if simultaneity was absolute rather than relative?

Another thing to note is that this and other such thought experiments rather cavalierly assume that there is some specific surface of simultaneity associated with each observer. It may be argued that the assumption is natural, but there is no physical significance to it. The standard theory of relativity says that simultaneity is conventional; there is no fact of the matter about simultaneity of distant events.

The fact is that relativity does not contradict our everyday experience. Ask yourself, what would have been different from your point of view if simultaneity was absolute rather than relative? — SophistiCat

Indeed. The following Wittgenstein anecdote seems apt here.

[Wittgenstein] once greeted me with the question: 'Why do people say that it was natural to think that the sun went round the earth rather than that the earth turned on its axis?' I replied: 'I suppose, because it looked as if the sun went round the earth.' 'Well,' he asked, 'what would it have looked like if it had looked as if the earth turned on its axis?' — G. E. M. Anscombe, An Introduction to Wittgenstein's Tractatus

Another thing to note is that this and other such thought experiments rather cavalierly assume that there is some specific surface of simultaneity associated with each observer. It may be argued that the assumption is natural, but there is no physical significance to it. The standard theory of relativity says that simultaneity is conventional; there is no fact of the matter about simultaneity of distant events. — SophistiCat

OK, though SEP notes that "The debate about conventionality of simultaneity seems far from settled". It seems that what is important here, as with any thought experiment, is to be clear and upfront about the assumptions made.

There is no "difference that makes a difference" here, and I think this is the important lesson, which also shows the silliness of those who bitch and moan about how counterintuitive and just wrong relativity is. The fact is that relativity does not contradict our everyday experience. Ask yourself, what would have been different from your point of view if simultaneity was absolute rather than relative?

The standard theory of relativity says that simultaneity is conventional; there is no fact of the matter about simultaneity of distant events. — SophistiCat

For now. If we ever come to find out that faster-than-light communication is possible, then this relative simultaneity will be wrong. Sure people will still be able to invoke relative simultaneity in terms of this higher speed limit instead of the speed of light, but if we were all blind we could have also said that nothing travels faster than sound and we would disagree much more about say when a thunder occurred. We would have said that the time when the thunder originated is conventional, that we may choose it as having occurred a few seconds later or earlier, that there is no fact of the matter about it, and we would have been wrong. Not seeing things does not imply that they do not exist.

Similarly, one doesn’t have to assume that we won’t ever find anything that travels faster than light, so we don’t have to assume that what we see as conventional now will remain to be so in the future. And since absolute simultaneity is much more intuitive for most people, maybe those who complain about these people should be more humble, instead of trying to force the view that simultaneity is fundamentally conventional. Well, there is no maybe about it.

No, it has to be one way or the other, there is a way that reality is even if we don’t see the whole of it, otherwise everything both happens and not happens at the same time, everything both exists and does not exist at the same time, and everything stops making sense. — leo

[You are conflating the existence of an object with the knowledge of that object. Since our primary sensory input is vision, our awareness requires light transit time. For 'local' events, there is no significant delay. For distant objects their existence becomes less certain, based on your knowledge. A natural disaster, terrorist attack, etc., could occur. A star 100 ly distant may not be there, after becoming a nova, 90 yr ago. People make an assumption based on the condition that nothing new happens. Someone you know passes away. You assumed he was alive, when he wasn't, until you got a call making you aware.]

[In the 1905 paper by A. Einstein, he uses a simple example of electromagnetism, requiring only relative motion of the magnet and coil to produce the effects. It's not rocket science, just fundamental physics.
1. The 2nd postulate states, 'the speed of light is constant and independent of its source'.
The 'independent' is the most significant property. It is equivalent to, 'events don't move', thus light is emitted as if from a fixed position in space, the same environment as the Lorentz ether, and allowing the 'fixed stars' or the cmb to serve as a ref. frame. There are no known experiments that can reveal any differences in SR or LET.
2. The 1st postulate states 'the laws of physics are the same for all inertial reference frames.'
When A and B are in relative motion, A will conclude The B-clock rate is slower than the A-clock, and B will conclude The A-clock rate is slower than the B-clock.
That is not a contradiction if you understand postulate 1. No one would complain if both clocks showed the same time, but it's not about whether the times differ, but whether each observes the same physics! In reading Einstein's work, he includes a disclaimer:]
"That light requires the same time to traverse the same path A to M as for the path B to M is in reality neither a supposition nor a hypothesis about the physical nature of light, but a stipulation which I can make of my own freewill in order to arrive at a definition of simultaneity."
Relativity The Special and the General Theory
Albert Einstein 1961 Crown Publishers Inc. pg 23
[If an observer in an inertial ref. frame can assume a pseudo rest frame, then he would expect light transit times to be equal out and back. The assessment of the distant clock requires a poll using light to get a clock reading. Both clocks run at a constant rate, but the transit times are not equal, so the assigned times vary. The observer is coincident with the emission and detection, thus knows the event times accurately. The more distant the reflection event, the more uncertain its time.]
--------------------------------
There is no absolute ref. frame. Light speed is finite, thus universal time has been replaced with subjective time. Position is relative therefore motion must be relative. Newton was wrong about two states, motion or rest. Rest is a special state of motion when two ref. frames have the same velocity. They can be moving relative to other ref. frames, while simultaneously being at rest relative to each other.
Motion modifies measurement and perception. The world wasn't ready for that, or to replace absolute values with relative ones, and all are still not convinced.

In reading Einstein's work, he includes a disclaimer:
"That light requires the same time to traverse the same path A to M as for the path B to M is in reality neither a supposition nor a hypothesis about the physical nature of light, but a stipulation which I can make of my own freewill in order to arrive at a definition of simultaneity." — sandman

Indeed, but what he says is valid only as long as nothing travels faster than light. If we ever find something that travels faster than light, we could use it to measure the one-way speed of light, and then his postulate may not remain valid.

The famous twin paradox of special relativity involves a scenario where one twin (he) rockets away from the home twin (her), coasts to a far-away turnpoint, reverses course, coasts back, and comes to a halt when they are reunited. At the reunion, both twins agree (by inspection) that she is older than he is.

There is no dispute about the outcome at the reunion. But physicists DO differ about what HE concludes about HER current age DURING his trip. One school of thought is that he says that she is ageing more slowly than he is, on both the outbound leg and on the inbound leg, but that he concludes that she instantaneously ages by a large amount during the instantaneous turnaround. But that conclusion can be shown to imply that he will have to conclude that it is possible for her to instantaneously get YOUNGER when he changes speed in certain other ways. THAT result is abhorrent to many (maybe most) physicists. The most extreme reaction is to conclude that simultaneity at a distance is simply a meaningless concept. Other physicists react by embracing alternative simultaneity methods, that don't result in instantaneous ageing (either positive or negative).

So what does the above have to do with philosophy? For many physicists, there is no place for philosophy in special relativity. Philosophical arguments are usually banned on physics forums. But philosophy has always played a role in my thinking on the subject, even though I'm a physicist, not a philosopher (and I have only a VERY limited knowledge of philosophy). Philosophy has entered into my thinking about the twin paradox in this way: when some physicists contend that simultaneity at a distance is meaningless, I have a philosophical problem with that. IF I were that traveler, I don't think I would be able to believe that she no longer EXISTS whenever I am not co-located with her. (And I doubt that many other physicists believe that either). BUT many physicists DO believe that she doesn't have a well-defined current AGE when he is separated from her (at least if he has accelerated recently). THAT'S the conclusion that I can't accept philosophically: it seems to me that if she currently EXISTS right now, she must be DOING something right now, and if she is DOING something right now, she must be some specific AGE right now. So I conclude that her current age, according to him, can't be a meaningless concept. That puts me at odds with many other physicists.

What say the philosophers on this forum? — Mike Fontenot

Are we discussing simultaneity here?

Consider the following

Imagine a person B located equidistant from two light bulbs 1 and 2 and a third person located to the left of light bulb 1 like so A........................1.....B.....2

B has the switch that lights both bulbs which are connected with wires of equal length

Imagine now that B throws the switch. Electricity, at the speed of light travels through two wires, both of equal length, to the bulbs and turns them on. This must happen at the same time, i.e. the lights turn on in 1 and 2 simultaneously for observer B.

However, person A being closer to light bulb 1 than 2 will see 1 turn on first, followed by 2. In other words person A will see two simultaneous events (light bulbs 1 and 2 turning on) as non-simultaneous.

So simultaneity depends on the position of an observer relative to what is being observed even if all observers are in the same frame of reference. Note that all objects in the above thought experiment (persons A, B and the light bulbs 1, 2) are at rest relative to each other.

If we introduce motion, things could get complicated.

An important fact here is the constancy and the unsurpassable limit of the speed of light. Being the fastest mode for information transmission, it implies that circumstances will arise when simultaneous events will be perceived as not.

The following Wittgenstein anecdote seems apt here. — Andrew M

It's an eye-opener, isn't it? Keeping the phenomenological perspective in mind helps to not get oneself confused with language and clever abstractions and remember what really matters.

OK, though SEP notes that "The debate about conventionality of simultaneity seems far from settled". It seems that what is important here, as with any thought experiment, is to be clear and upfront about the assumptions made. — Andrew M

I take your point. It is arguable whether in SR one can find a synchrony that is in some sense objective (though GR throws a monkey wrench into that scheme and makes things much messier). But I don't think it helps much with the personal perspective, because any such objective synchronization procedure (such as the Einstein synchronization, which they say is considered to be the best candidate in SR) is still going to differ from your personal clock, which governs everything that happens to you, including your observations and your aging processes. Nor does it enable us to subvert the speed limit on communication (and thereby conventional causality), which is what usually restores intuitive sanity to abstract paradoxes of this sort.

Indeed, but what he says is valid only as long as nothing travels faster than light. If we ever find something that travels faster than light, we could use it to measure the one-way speed of light, and then his postulate may not remain valid. — leo

The clock synch convention sends light signals in opposite directions to equally distant clocks. A typical outside observer describes the light transit times as a long out and short return forward and a short out and long return backward. This is 'skew' symmetry, which says, the forward path rotated 180 deg equals the backward path. Assuming in one direction (forward out) light speed <c with a delay dt, then the backward return will also have a delay dt. I.e.any variation is not detectable with this symmetry, regardless of propagation speed

Mike, Hope you are still here. I just joined so am just able to reply.

First there it's easy to demonstrate there is a universal current present moment.

1. It's well known that everything in the universe continually travels through spacetime (combined space and time) at the speed of light.

2. As a consequence everything in the universe is continually traveling the same distance through spacetime as light does.

3. Thus the current distance everything has traveled through spacetime is the universal current present moment. This is the only moment that actually exists, and it's common to the entire universe.

4. Now the current proper time on any object's clock depends entirely on its own path through spacetime, not on how it is being observed from any other frame. Specifically all the distance it travels through spacetime is through time if it's path is inertial. However the amount of deviation from an inertial path reduces the distance it travels through time. Whatever the result an object's current proper time at the current common distance it has traveled through spacetime is the proper time it has in the universal current present moment.

5. There is a unique 1:1 invariant correlation between the current proper times of all clocks in the universe in this universal current present moment, which we all inhabit simultaneously.

6. It must be noted that this is independent of how observers in relative motion view each other's clocks. That is a matter of perspective, and in general relatively moving observers each view the time on each other's clocks ticking slower than their own. They DO NOT see the actual 1:1 current present moment proper time correlations except in specific cases such as being at rest in the same frame.

Hope you see this Mike!

Edgar L. Owen.

[...] in general relatively moving observers each view the time on each other's clocks ticking slower than their own. — Edgar L Owen

That's a special relativity result. The twin "paradox" doesn't require (or profit from) general relativity. And that result applies ONLY to perpetually inertial observers. An accelerating observer in the twin paradox scenario will find his home twin will be older than him at their reunion. So sometimes during the trip, he MUST conclude that she is ageing FASTER than he is. The co-moving inertial frames (CMIF) simultaneity method says that her age instantaneously increases by a large amount during his instantaneous velocity change at the turnaround. My new simultaneity method says that her age doesn't change at all during his turnaround, but that it does increase linearly for years after his turnaround, at a rate greater than his own rate of ageing. There are also two other simultaneity methods (Dolby and Gull, and Minguizzi) that give an increased ageing rate for her, without an instantaneous increase.

For details, see my webpage:

https://sites.google.com/site/cadoequation/cado-reference-frame

Hi Mike,

Sure, it's SPECIAL relativity. That sentence should read "In general, relatively (NOT GENERAL RELATIVITY) moving... Note I wrote relatively, not relativity!

It's easy to calculate the elapsed current proper time of a moving clock as it appears from the perspective from any frame. One just uses the equation dτ = √(dt^2 – dx^2) for any point (dt, dx) in the reference frame to find the proper time of the moving clock as it appears from the perspective of the reference frame.

That however does NOT give the actual proper time of the moving clock in its own frame at the current common spacetime distance everything in the universe currently occupies which is the true universal current moment.

My understanding from posters in other groups is that you also advocate a universal current present moment, and in that sense an absolute notion of a universal coordinate system based on that. Is that correct?

Edgar L. Owen

she instantaneously ages by a large amount during the instantaneous turnaround — Mike Fontenot

Wouldn't instantaneous change in age be violating a principle of relativity that is bandied around like juicy gossip, to wit that communication can't be faster than light? Are we entering the domain of quantum entanglement?

Madfool,

The 'instantaneous aging by a large amount isn't anything actual'. First, it only occurs when a physically impossible instantaneous deceleration - acceleration in the opposite direction is assumed to simplify the twin example. When a physically possible deceleration-acceleration is used the apparent aging sweeps continuously across the difference. And second this is only describing how the space twin SEES the earth twin's age. It does NOT describe the actual aging process of the earth twin which doesn't depend on how it's being observed in the least and is proceeding normally.

Edgar L. Owen

Time and the apple paradox: I put one apple in the fridge and the other on the sideboard. After a week the one on the sideboard had aged a lot whereas the one in the fridge had aged barely at all.

Conclusion: time passes more slowly in the fridge.

Only that's stupid, isn't it?

So why aren't physicists being that stupid when they reason that as an apple that moves at speed ages more slowly than one at rest therefore time passes more slowly at speed? Or is that not how they reason?

And second this is only describing how the space twin SEES the earth twin's age. — Edgar L Owen

This is a serious misrepresentation of what the theory says.
Upon the turnaround, the space twin sees pretty much the same exact time on the Earth twin clock as before. The clock doesn't suddenly appear to jump like your statement seems to imply.

Secondly, you throw around the term 'proper time' a lot, all in a way incompatible with the physics definition of the term. Proper time is a duration (a temporal length), not a reading at a particular moment.

noAxioms,

No, proper time is the current reading of a comoving clock, a clock moving with an observer. ELAPSED proper time is the proper term for a duration of proper time.

It was Mike, not me that said there was a sudden jump in how the traveling twin views the age of the earth twin. I just pointed out this is only under the a-physical simplification of instantaneous acceleration.

Edgar L. Owen

First there it's easy to demonstrate there is a universal current present moment. — Edgar L Owen

You're an absolutist and presentist I see, but claiming you can demonstrate it seems a bit too much. So you misrepresenting relativity theory is sort of a strawman tactic.

No, proper time is the current reading of a comoving clock, a clock moving with an observer. — Edgar L Owen

SR theory makes no reference to the concept of a 'current' anything. The definition also makes no reference to an observer being necessary. Just google 'proper time' and you get:

In relativity, proper time along a timelike world line is defined as the time as measured by a clock following that line. — wiki

OK, there are definitely some wrong definitions out there in non-physics sites like:

time measured by a clock that has the same motion as the observer. Any clock in motion relative to the observer, or in a different gravitational field, will not, according to the theory of relativity, measure proper time — dictionary.com

This seems to agree with your definition that only clocks accompanied by observers are proper, while in fact all clocks measure the proper time of their own worldlines, and in the example above, the clock simply is not measuring the proper time of said observer since it is a different worldline. Were the clock to be comoving with a different worldline, (not in the presence of the other object, but with the same motion and potential all the way), then it would measure the proper time of that object, but only in a frame in which its motion matched that other object. This wouldn't be true in all frames unless the object was completely inertial the whole time.

'Proper' implies frame independence. No frame need be specified to talk about the proper time along a specific worldline, or the proper length or proper mass of an object. These things don't change from one frame to the next.

It was Mike, not me that said there was a sudden jump in how the traveling twin views the age of the earth twin. I just pointed out this is only under the a-physical simplification of instantaneous acceleration.

Mike adds confusion by using absolute verbiage in a relative interpretation of events, but what he says is technically correct. So "he concludes that she instantaneously ages by a large amount during the instantaneous turnaround" is misleading but not wrong. He claims that one school of thought claims this, but I've never seen a physicist word it that way.
Let's say the trip takes 20 years for her and 10 years (5 each way) for him. Mike says the brother concludes that she ages 15 years during the turnaround. Worded more the way any relativist would, it should say: "Simultaneous with the event where the brother is furthest from Earth, the difference in her age in the outbound frame of the traveler and in the return frame of the traveler approaches 15 years as the time taken to turn around approaches zero".

That makes it much more clear that it is two different frames being referenced, and not anything actually happening to the girl back home caused by what the traveler is doing. Similarly, using the 'approaches' language gets rid of the instantaneous turnaround simplification that seems to offend you. In fact, the longer it takes the twin to turn around, the more the 15 year age difference of the sister is in the example I gave above. 15 years is a minimum difference given an arbitrarily powerful ship. Point is, there are two frames, and her age is very much at least 15 years different in those frames, simultaneous with the one twin's turnaround event. This is a simple example of relativity of simultaneity.

Notice no mention of 'current reading' of any clock in the twins scenario. There is no particular moment in the scenario that is the 'current' one. Special relativity denies a preferred frame, and with it a preferred moment. Absolute interpretations (Lorentz say) assert said preferred frame (not an inertial one), but even Lorentz didn't go so far as to assert a current (preferred) moment in time.

No Axioms,

I'm not an absolutist or presentist. Labeling thought generally misrepresents it. My theories are my own perhaps a new interpretation but completely compatible with relativity though not necessarily how it's interpreted (which varies anyway). I have around 12 books and 22 YouTube talks explaining my Complete Theory of Everything with free pdf's and links at http://edgarlowen.com/reality.shtml

[note that the sections on special relativity there are out of date and currently being updated]

Proper time is simply what any clock is currently reading. The presence of an observer is irrelevant though if there is a comoving observer he is aging at the same rate his clock is ticking.

You are correct that the elapsed proper time of a world line between two events is invariant and the same seen from any frame after a Lorentz transformation.

Anyway the twin example is pretty simple. A lot of people over complicate it...

1. The elapsed proper time of any clock depends entirely on its own motion through spacetime, not in the least to how it's being observed by any observer. If it follows an inertial path all its constant spacetime distance traveled is through time. Time passes at the same rate on all inertial clocks. However to the extent it deviates from an inertial path it travels a shorter distance through time due to the distance it deviates in space. This is what happens to the space-traveling twin.

2. The entirely separate issue is how relatively moving observers view each other's clocks. That is simply a matter of perspective and is easily calculated as dτ = √(dt^2 – dx^2) for any point (dt, dx) in the reference frame to find the proper time of the moving clock as it appears from the perspective of the reference frame.

That's about all there is to it.

Edgar L. Owen

One school of thought is that he says that she is ageing more slowly than he is, on both the outbound leg and on the inbound leg, but that he concludes that she instantaneously ages by a large amount during the instantaneous turnaround. — Mike Fontenot

I don't understand why he concludes she is instantaneously aging by a large amount at that turnaround. Suppose the turnaround were not instantaneous, but rather there's an instantaneous stop , a 10 minute delay to turnaround, followed by an instantaneous acceleration back to near light speed.

Shouldn't he conclude that during that 10 period, they are aging at (pretty close to) the same rate? As we shorten the turnaround interval towards zero, the same-rate-aging period gets shorter, and at a zero turnaround - the same-rate-aging period is zero.

What am I missing?

If he instantaneously changes his velocity from +v to zero, he will conclude that she instantaneously ages by half the amount she would have aged if he has gone from +v to -v. Then, during that 10 minutes at zero velocity, they would be ageing at the same rate. And during the change from zero velocity to -v, she would instantaneously age by the same amount of ageing that happened when going from +v to zero velocity. I.e., in the case where her she instantaneously ages by 60 years in the standard twin paradox, in your example, she would instantaneously age by 30 years at the beginning of the 10 minutes, then age at his rate for 10 minutes, and then age by another 30 years at the end of the 10 minutes. That is the result for the co-moving inertial frames simultaneity method.

Mike and Relativist,

Yes but of course this is only how the space twin VIEWS the aging of his earth twin. That doesn't affect the actual aging rate of the earth twin in the least which goes on completely as normal unaffected by how anyone else views it.

Edgar L. Owen

Yes but of course this is only how the space twin VIEWS the aging of his earth twin. That doesn't affect the actual aging rate of the earth twin in the least which goes on completely as normal unaffected by how anyone else views it. — Edgar L Owen

That's true, provided by "views" you mean what the traveling twin CONCLUDES about the home twin's current age. His conclusion about her current age is NOT what he sees on a TV screen showing her, because that image is "out of date" ... if they are separated by many light years, it took many years for that image to travel from her to him. I suspect you already know this. It's best to not refer to his "view" when you mean what he concludes, or what he computes her current age to be.

Mike,

Good point. It's what he computes or would see disregarding the time lag of light traveling between them. It's her apparent age in his reference frame as opposed to her actual age in her own reference frame.

Edgar L. Owen

The scenario is somewhat different, but this link provides a detailed, but fairly easy to understand explanation of the Twin Paradox.

In the Op, you said:

BUT many physicists DO believe that she doesn't have a well-defined current AGE when he is separated from her (at least if he has accelerated recently). THAT'S the conclusion that I can't accept philosophically: it seems to me that if she currently EXISTS right now, she must be DOING something right now, and if she is DOING something right now, she must be some specific AGE right now. So I conclude that her current age, according to him, can't be a meaningless concept. That puts me at odds with many other physicists. — Mike Fontenot

The animated charts in that link suggest to me that it's perfectly reasonable to consider there to be simultaneous points in time between the respective inertial frames (they can be mapped to one another), albeit that they proceed at a different pace.

If messages could be sent between the twins at infinite speed that would show simultaneity:
"What are you doing now?"
- "I'm reading Dante"
But for all practical purposes simultaneity cannot be determined.

It's what he computes or would see disregarding the time lag of light traveling between them. — Edgar L Owen

I would say that it's what he computes when he properly takes the image transit time into account.

It's her apparent age in his reference frame as opposed to her actual age in her own reference frame.

I wouldn't use the term "actual" for her perspective, and I wouldn't use the term "apparent" for his perspective. They each are equally correct in their perspectives. They disagree, but neither one is more correct than the other.

I'm not an absolutist or presentist. Labeling thought generally misrepresents it. — Edgar L Owen

I'm not representing it, just categorizing it. Presentism isn't one fixed belief system, so I'm not telling you your beliefs. I'm just reacting to your initial post asserting the ability to demonstrate a "universal current present moment", and then immediately following that assertion with a premise that assumes its conclusion (and doesn't seem to be true even the conclusion is).

Your website is full of presentist assertions, notably this:

All processor computations occur in the current universal present moment in a non-dimensional computational space in the same sense as computer programs define computational spaces.

It that (my bold) isn't presentism, I don't know what is. I'm not saying presentism is necessarily wrong, but you seem to be in denial about being in the category, like its something to be embarrassed about. The vast majority of people are presentists, even if most of them are unaware of the term or the alternatives.

My theories are my own perhaps a new interpretation but completely compatible with relativity though not necessarily how it's interpreted (which varies anyway). I have around 12 books and 22 YouTube talks explaining my Complete Theory of Everything

How nice that you publish your personal beliefs, but almost all of it seems to be falsified. Has any of this been reviewed by somebody competent in the respective fields? It seems not.

Proper time is simply what any clock is currently reading.

There is no 'currently' in the definition of proper time, and if proper time is described merely as what any clock reads, the description is too simple since our twins are reunited with the clocks reading different values, which is unexplained by this oversimplified statement. Yes, all clocks measure the proper time of that clock, or more correctly, of the worldline followed by that clock. Your statement needs to encompass that.

The presence of an observer is irrelevant

Good. Your wording sometimes left me wondering.

Anyway the twin example is pretty simple. A lot of people over complicate it...

1. The elapsed proper time of any clock depends entirely on its own motion through spacetime, not in the least to how it's being observed by any observer.

This wording presumes that there is a concept of motion through spacetime. Any non-presentist interpretation would not word it that way. "The proper time of any clock depends entirely on the worldline of the clock". Calling that 'motion' makes it sound like the rock is here in 2020 and hence 2019 has no rock or anything else, it having all moved on to the present. That contradicts relativity theory which would require, in any inertial frame, existing events to happen simultaneously with nonexistent (not current) events.
I say 'in any inertial frame' since no inertial frame foliates all of spacetime, hence any 'universal current present moment' cannot map to any inertial frame.

If it follows an inertial path all its constant spacetime distance traveled is through time.

Not in any coordinate system where it isn't stationary, so this is false. Yes, in a coordinate system where an inertial object is stationary, two events on that object's worldline are separated only by time, but in other coordinate systems (other reference frames), this is not so.

Time passes at the same rate on all inertial clocks.

Obviously false, as can be demonstrated by doing the twins experiment with a tag team of 2 clocks. All clocks are then inertial, but since the final comparison yields different values, some of the inertial clocks must be running at different speeds than another.

The entirely separate issue is how relatively moving observers view each other's clocks.

Well, you said this which is empirically incorrect.

relatively moving observers each view the time on each other's clocks ticking slower than their own.

Not true. If I look at an approaching clock, it will appear to run faster. Hence the blue shift of light from approaching objects like Andromeda. If your statement were true, everything in motion would appear to be red shifted, not just the receding stuff.