# The Traveler's Perspective in the Twin "Paradox"

Discussion in 'Physics & Math' started by Mike_Fontenot, Jan 4, 2014.

1. ### Mike_FontenotRegistered Member

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It is remarkable that, more than 100 years after the birth of the special theory of relativity, that there remains considerable controversy and disagreement about the traveler's perspective in the traditional twin "paradox" scenario. Specifically, at each instant of his life, what does the traveler conclude about the current age of (and current distance to) any given distant object or person (in the (assumed flat) universe)?

One view is that whenever any observer (whether inertial of not) is not co-located with some other person, that there simply IS no meaning to the concept of that other person's current age. Another view is that the current age of a distant person DOES have a meaning for a (perpetually-inertial) observer, but that it has NO meaning for an observer who (sometimes) accelerates.

Among those who believe that the traveler IS entitled to his own perspective, some nevertheless believe that the current age of a distant person, for an observer who sometimes accelerates, has no DEFINITE value ... it is "discretionary". In other words, the observer is free to CHOOSE the current age of a distant person from among several different possibilities (or perhaps, from an unlimited number of possibilities).

Others believe that the the current age of a distant person, according to the traveler, is some definite value that is "non-negotiable" and non-discretionary. Among this group, some believe that that definite value can only be properly determined by using the general theory of relativity, via the equivalence principle. Conversely, others believe that, in the (assumed) absence of any significant masses within the spatial region of interest, that the special theory of relativity is all that is needed to provide that definite current age of the distant person. But, even among this latter group, there is disagreement as to WHAT that definite value of the current age IS.

Who is right?

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3. ### Mike_FontenotRegistered Member

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I am personally in the above quoted latter group (traveler's perspective is definite and non-negotiable, and determinable purely via special relativity).

There are two reasons why I don't believe that the traveler's perspective is arbitrary, "negotiable", and/or "meaningless". The first reason is philosophical: IF the home twin (she) exists AT ALL at some given instant of the traveler's (his) life when he is distant from her, then it would seem that she must be doing SOMETHING definite at that instant of his life. And that SOMETHING defines a unique instant in HER life.

The second reason is that the simultaneity result given by the Lorentz equations isn't just some meaningless age: it is the result that the traveler can determine from his own elementary calculations, when he receives a sequence of radio messages from her, giving her current age at the time she transmits the message. If he properly allows for her ageing during the transit of the message, and adds that to her announced age in her message (when she TRANSMITTED that message), in order to get her age when he RECEIVES that message, then he will get the same result given by the Lorentz equations. I show the process that the traveler must use, to properly determine her ageing during the transit of her message, in my paper

"Accelerated Observers in Special Relativity", PHYSICS ESSAYS, December 1999, p629.

The above comments apply to the case where each twin is perpetually inertial (i.e., neither of them ever accelerate), so that there is no doubt that each twin is entitled to use the Lorentz equations (and the time-dilation result). But I also show, in the above paper, that regardless of how the traveler accelerates, that his conclusions (at each instant of his life) about the current age of the home twin, is ALWAYS exactly the same as the conclusion of a perpetually-inertial person who happens to be momentarily co-located and mutually-stationary with the traveler at that instant. That means that the traveler himself is entitled to use the Lorentz equations to determine simultaneity at each instant of his life, whether he is accelerating then or not. I call that momentarily-stationary inertial reference frame the "MSIRF". (It is usually referred to by others as "the co-moving inertial frame", but I prefer my term). This same method of obtaining the traveler's perspective, of the current age of the home twin, has been used by others. For example, Taylor and Wheeler used the same approach in their example (Example 49) in their "Spacetime Physics" book, pp. 94-95. And Brian Greene in his NOVA series (and in his book) on the "Fabric of the Cosmos" also used the same approach.

The simultaneity result given by the Lorentz equations can also be determined by an equation I derived long ago that I call "the CADO equation". I originally coined the acronym "CADO" just to save time having to repeatedly write the phrase " the Current Age of a Distant Object". The CADO equation gives exactly the same answer as is given by the Lorentz equations (as it must, since it was derived from the Lorentz equations) ... it's just easier and faster than using the Lorentz equations, and it is less likely to be miss-used in applications. The CADO equation is especially simple and quick to use when all the velocity changes are the idealized instantaneous changes, but it is also fairly easy to use for piecewise-constant finite accelerations (perhaps alternating with "coasting" segments with no acceleration). It is even valid for any acceleration profile whatsoever, although in continuously-varying-acceleration scenarios, numerical integration will generally be required. The CADO equation is fully explained and illustrated on the webpage

[I'm not yet allowed to post a link ... interested readers might be able to find the webpage by googling "CADO equation"]

and its derivation (as well as a good bit of additional information) are given in my previously referenced paper.

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5. ### dumbest man on earthReal Eyes Realize Real LiesValued Senior Member

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Hello, Mike_Fontenot, and welcome to SciForums.

Time is a Human Construct. And the Special Theory of Relativity, will always have "controversy and disagreement" among its adherents.

If your Posts are not pegged as proselytizing, you may get some lively discussions.

Be prepared, though! There are some very intelligent Posters on this Forum, but it is the Internet, so...well, some Posters do not even understand the term "relative" when applied to "speed", let alone "time" or "chronological aging"!

Again, Mike_Fontenot, welcome to SciForums.

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7. ### paddoboyValued Senior Member

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Not an expert here, they'l be along shortly, but one of the main postulates with SR is that each FoR is as real and as valid and as relevant as any other.
So I see your above statement as wrong.
The equivalence principal tells us that each twin [the traveller and the stay at home] sees the other as aging more slowly, when in the inertial state.
The fact that the traveler needs to accelerate/decelerate and to turn around and come back home then differentiates between them.
The actual reasons why that happens I'm not sure, perhaps to do with the energy in decelerating and/or accelerating.
Central Postulates of Relativity:
The laws of physics are the same for all uniformly moving observers.
The speed of light is the same for all observers.
Consequences:
Different observers measure different times, lengths, and masses.
Only space/time is observer independent.
Newton's Universe

In Newton's view:
The universe keeps absolute time.
Objects move through absolute space.
Universe looks the same to all observers, regardless of how they move through it.

If there is any disagreement with the mainstream view, then one of our experts will discuss shortly.
All in all, from my basic position, I don't see too much of a problem within the mainstream view.
As usual though, there are always nutjobs about that will try and make a name for themselves and purport to know more then all the experts since 1905.
We have them here as well.

8. ### dumbest man on earthReal Eyes Realize Real LiesValued Senior Member

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"Not an expert here...each FoR is as real and as valid and as relevant as any other...So I see your above statement as wrong...The actual reasons why that happens I'm not sure, perhaps...there are always nutjobs about that will try and make a name for themselves and purport to know more then all the experts since 1905...We have them here as well".

Again, like I said before Mike_Fontenot, welcome to SciForums!

9. ### paddoboyValued Senior Member

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21,803

These are recognised by there "äbsolute" claim that they are right and all the experts are wrong, and the persistent asking of self evident and nonsense questions.
As is well known, although everyone has the right to say what they think, each must also be responsible for what they say.
But that is digressing from your original question, and you will recognise the nut job situation if and when it arises.

10. ### paddoboyValued Senior Member

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21,803
Time is real enough.......It is the measuring of time that is a human construct.
The observations of time dilation is evident to that fact.

11. ### UndefinedBannedBanned

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Hi Mike_Fontenot. Welcome.

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Good luck.

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Just reading through before logging out again, but have to comment briefly on this from you...
Sorry, mate, but you've got that the wrong way round. What is real enough is MOTION. What we measure is MOTION and then COMPARE different motions measured.

The 'time' we abstractly construct is based on that comparative motion parameters/measurements, and we use ONE of said motions for a REFERENCE STANDARD RATE OF CHANGE/MOTION to arrive at the various 'times' in any one study/observation. The physical rate of change of state attending MOTION in/through SPACE is what affects the rate of change within a body/process inherent to the entity in motion as a whole. Be careful about 'glib' assumptions/explanations that have been 'doing the rounds' for so long that they are inadvertently taken on face value without the underlying understandings/dynamics that give rise to such 'glib abstractions/explanations'. Maxila will explain it to you in detail if need further supporting arguments (I haven't time now for prolonged discussions/explanations).

Gotta go! Cheers and happy new year paddoboy, Mike_Fontenot, everyone!

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12. ### paddoboyValued Senior Member

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I don't believe I have. How about time in relation to a photon?
It doesn't exist, does it? Yet the photon is moving at the maximum rate.
Time separates events, to put it simply, and its reality can be seen in time dilation.

13. ### UndefinedBannedBanned

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1,695
Just came in to fix some typos elsewhere and saw your response. Thanks.

I don't have time to go into it more than to say: I long ago pointe out that even in the photon PROPAGATION process there is a RATE of change involved. The photon is a perturbation feature in an underlying E-M field pervading all space. The E-M field 'oscillations' which manifest as 'photons' are changes which have a certain rate of transition from space location to space location at those scales involved. Hence he TIME involved for photons is NOT 'zero', but the characteristic rate of E-M field oscillation cycles as the photon feature passes along the field from location to the next until absorbed and the perturbation becomes part of the energy-space content/dynamics parameters of that other feature which absorbed its energy-space oscillation rate properties. The 'time' is always there IF motion of any kind is there (it is especially observable when the 'cycles' involved in the PROCESSES involved can be measured. In the case of the photon, its E-M field 'oscillatory state is what the PROPAGATION is, else the field that does not oscillate does not transmit a photonic perturbation and there IS NO photon or motion of same to observe (only then can you say there is no motion and hence no 'time' to be abstracted about its motion since there IS no motion or photon). Gotta go again! Won't come in again until tomorrow or next day. Cheers and stay safe in the new year, paddo!

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14. ### paddoboyValued Senior Member

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I still don't quite see it that way undefined.....
Time is flowing non-stop, and we follow it with clocks and calendars......All we can say in reference to your definition re time and motion, is that the flow of time is represented through change, such as the circular motion of the moon around Earth etc etc.
The other notion we should be familiar with, is time is most associated to the concept of space.

But I also see this more as a philosophical argument, as there appears to be a few valid definitions of time about.
Being a basic layman, I just prefer the simple one.

Henceforth space by itself, and time by itself,
are doomed to fade away into mere shadows,
and only a kind of union of the two will preserve an independent reality.
Hermann Minkowski:

Last edited: Jan 5, 2014
15. ### dumbest man on earthReal Eyes Realize Real LiesValued Senior Member

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In my near 60 years of doing "time" on Planet Earth - "time" has been most associated with measurement, scheduling and aging.

Pink Floyd – Time

Ticking away the moments
That make up a dull day
Fritter and waste the hours
In an off-hand way

Kicking around on a piece of ground
In your home town
Waiting for someone or something
To show you the way

Tired of lying in the sunshine
Staying home to watch the rain
You are young and life is long
And there is time to kill today

And then the one day you find
Ten years have got behind you
No one told you when to run
You missed the starting gun

And you run and you run
To catch up with the sun
But it's sinking

Racing around
To come up behind you again

The sun is the same
In a relative way
But you're older

Shorter of breath
And one day closer to death

Every year is getting shorter
Never seem to find the time

Plans that either come to naught
Or half a page of scribbled lines

Hanging on in quiet desperation
Is the English way

The time is gone
The song is over
Thought I'd something more to say

Home
Home again
I like to be here
When I can

When I come home
Cold and tired
It's good to warm my bones
Beside the fire

Far away
Across the field
Tolling on the iron bell
Calls the faithful to their knees
To hear the softly spoken magic spell...

by: DAVID GILMOUR, NICHOLAS MASON, ROGER WATERS, RICK WRIGHT

16. ### paddoboyValued Senior Member

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21,803
Isn't it? From what I have read [and I always try and sort out the wheat from the chaff as we have a heap of rubbish on the net] a photon experiences no time in its own FoR.
I still see time existing even in the absence of motion...It still certainly exists in a static Universe according to Sean Carroll.

_____________________________________________________________________________________________
" There’s different moments in the history of the universe and time tells you which moment you’re talking about. And then there’s the arrow of time, which give us the feeling of progress, the feeling of flowing or moving through time. So that static universe in the middle has time as a coordinate but there’s no arrow of time. There’s no future versus past, everything is equal to each other."

Even in empty space, time and space still exist. Physicists have no problem answering the question of “If a tree falls in the woods and no one’s there to hear it, does it make a sound?” They say, “Yes! Of course it makes a sound!” Likewise, if time flows without entropy and there’s no one there to experience it, is there still time? Yes. There’s still time. It’s still part of the fundamental laws of nature even in that part of the universe. It’s just that events that happen in that empty universe don’t have causality, don’t have memory, don’t have progress and don’t have aging or metabolism or anything like that. It’s just random fluctuations."
Sean Carroll:
______________________________________________________________________________________________

Which reminds me of the way Hawking thought of time and its three arrows as illustrated in his book "A Brief History of Time"
[A long time since I have read it though.....

17. ### brucepValued Senior Member

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4,098
You'll always see the speed of light expressed as a coordinate speed. The reason is the proper speed of light is infinite while the proper time is 0. It comes in useful when you can solve the metric for 0 and get an important theoretical prediction. I agree with Sean Carroll. It's real natural phenomena to me. Wanting to find reasons why it isn't seems to be counterproductive. But that's how it is.

18. ### paddoboyValued Senior Member

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Makes perfect sense to this little old layman!!

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Thanks.

19. ### dumbest man on earthReal Eyes Realize Real LiesValued Senior Member

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So...Mike_Fontenot, this is what you get on SciForums after about a day or so :

Is this what you were expecting?

Like I said, welcome to SciForums, Mike_Fontenot!

20. ### brucepValued Senior Member

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4,098
All you said just means you don't understand relativity. SR is the special circumstance where the metric we use on the manifold is Minkowski. That's a good thing and it isn't controversial unless you don't know what it means. It means that most the local physics can be evaluated using SR. Tangent to every point on the differentiable [dynamic] manifold the spacetime approximates to flat. It doesn't have anything to do with how much mass is in the local region. Other than to determine how large a space we can approximate as flat before we have to account for the effects of gravity on experimental measurements. There's no mystery [controversy] for those who know how the theory works and what predictions it makes. The LHC experiment is being conducted in the tangent space using the Minkowski metric. The GPS has to take into account miniscule gravitational effects because of the temporal accuracy required to get the result we want.

21. ### brucepValued Senior Member

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4,098

It's what I've come to expect from you. Hopefully the site will catch wind of your 'stalking and trolling' members just because you can't admit you're wrong. Thread to thread. Stalking troll. There was no wisdom for you to accrue over the years? You 'bearassedme' when you said you're a member of my generation.

22. ### Mike_FontenotRegistered Member

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81
I WOULD have liked the various responders to put themselves in one of the groups I described in my first posting, and then provide one or more reasons why they believe that group is the correct one (like I did in my second posting). Or perhaps to put themselves in an additional group that they feel I failed to describe. Or to suggest that one or more of the groups I described be further subdivided into subgroups, and to put themselves in one of those subgroups. In some cases, I can GUESS which group the responder belongs in, but I'd prefer to hear where each responder thinks he belongs, and why he feels that that is the correct group.

23. ### paddoboyValued Senior Member

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Not too much to worry about brucep......His posts are all the same, no scientific substance, no answers to any questions, just sniping, trolling, and stalking.
Did you see the following post of his?

That statement along with his many other self gratutitious remarks, certainly label him for what he is. *shrug*