# michel123456's relativity thread (from Time dilation dependence on direction)

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3 hours ago, michel123456 said:

My search is almost exactly 20 years old.

You seriously have been trying to understand special relativity for 20 years, and you still say things like this?

3 hours ago, michel123456 said:

the fact that relativistics effects are "real" and the huge difference between time - accumulating- and length - not accumulating-

Maybe you should give up.  Seriously, at some point you need to realize you're wasting your time.

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7 hours ago, michel123456 said:

So you are saying that as the returning twin stops, all of a sudden he comes back to his original dimensions (length contraction stops because motion stops) but the accumulated time gap is still there. Is that it?

Comes back to his original dimensions as measured by the other twin (any statement like this has to say in what frame the measurement is being made)

The frequency of his clock reverts, too. The accumulated time difference remains.

5 hours ago, michel123456 said:

Since then, not any one question has been answered.

On the contrary, your questions have been answered numerous times. Please don’t pretend otherwise. You have not absorbed the information, unfortunately.

Part of the problem is that when one jumps into the middle of a complex problem without understanding the basics, it usually doesn’t go very well. Another issue is avoiding the mathematical underpinnings.

5 hours ago, michel123456 said:

Worse, my single question (what is time?) has expanded logarithmically as for each answer more questions arise.

That’s philosophy (metaphysics), so you won’t get an answer in a physics thread, other than some variation of “time is what is measured by a clock”

5 hours ago, michel123456 said:

Generally I still remain surprised with the facility answers are given when it is obvious (to me) that something goes wrong. Like answers I get at this right moment (concerning lets' say ONE reality, the fact that relativistics effects are "real" and the huge difference between time - accumulating- and length - not accumulating- while at the same time it is argued that "you essentially rotate some portion of the space part into the time part", which says to me that Time & Space are essentially the same. But if you see no contradictions, no problem, no question, that must be me.

Yes. You have preconceived notions which are wrong, and refuse to abandon them. Instead, you try and fit things into your view, and that invariably fails, because nature doesn’t behave as you think it does, or want it to.

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2 hours ago, Bufofrog said:

You seriously have been trying to understand special relativity for 20 years, and you still say things like this?

Time. My quest is about time, not relativity.

2 hours ago, Bufofrog said:

Maybe you should give up.  Seriously, at some point you need to realize you're wasting your time.

Maybe.

Maybe you can help & explain me what about time?

37 minutes ago, swansont said:

Yes. You have preconceived notions which are wrong, and refuse to abandon them. Instead, you try and fit things into your view, and that invariably fails, because nature doesn’t behave as you think it does, or want it to.

Symmetry prevails.

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2 minutes ago, michel123456 said:

Time. My quest is about time, not relativity.

To understand time, you have to understand Relativity, because, at its heart, Relativity is all about the very nature of Time and Space.

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2 hours ago, michel123456 said:

.

Exactly.

6 hours ago, michel123456 said:

I have realized that the higher the level of the author (Nobel Prize like I. Prigogine) the more awaken about the unanswered questions, and lower the level ( not to mention anyone) less acceptance for questioning.

Let me mention some lower-level authors for you: Dirac, Pauli, Einstein, Fermi, Bohr, Schrödinger, Feynman, Schwinger, Yukawa, Yang, Parisi, Altarelli, Aspect, Zeilinger...

Every major physicist of the 20th/21st century except Prigogine, maybe.

There are no unanswered questions about special relativity. None.

When you start learning about relativity it's well worth spending some hours thinking about Alice and Bob experiments. All of the A/B thought-experiment paradoxes in SR have been solved. But if you know anything else about physics, you start seeing many paradoxes and peculiarities get resolved and you get motivated to learn it and think in relativistic terms simply because it makes huge progress everywhere in physics. Not least because it predicts antiparticles, getting energy from mass, it gives you the right properties of helicity and chirality of elementary particles, it explains the observed speed-dependent lifetimes for elementary particles.

It is also an inescapable consequence of Maxwell's equations. It gives you a consistent picture of elementary processes (congruences of events, instead of the same thing (eg. a collision) happening at two different coordinate times). If you saw a particle-antiparticle annihilation coming at you at different speeds, you would perceive charge not to be conserved, because one would disappear before the other did.

My suggestion: Try to learn some electromagnetism and you'll see how special relativity is necessary. The right expression for the energy of the electromagnetic field ~E2+B2 is a direct consequence of relativistic formulation, and it has the transformations properties of an energy under Lorentz transformations. The right expression for the Lagrangian of the EM field ~E²-B² is a Lorentz-relativistic invariant, not a Galilean invariant. Galilean invariance leaves magnetism unexplained.

It also explains why charge is a relativistic invariant and the conservation of charge being agreed upon by all inertial observers, as I said. It's not difficult to come up with tens more examples. It is watertight.

It contradicts your intuitions, it contradicts direct human intuitions, I know, most of us here know. But then, these intuitions are wrong.

Many people have tried to bring to your attention the Alice/Bob approach; others have tried a more formalism-related approach; others, experiments with muons, etc...

Your arguments remind me a lot of "arguments" against evolution on the grounds that we don't see anything intermediate between a whale and a dog, dismissing the wealth of evidence as if it were irrelevant.

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37 minutes ago, michel123456 said:

Time. My quest is about time, not relativity.

But you wrote this strange line.

3 hours ago, Bufofrog said:

the fact that relativistics effects are "real" and the huge difference between time - accumulating- and length - not accumulating-

What huge difference are you talking about??

Let's say a space ship flies to Alpha Centauri at 90% c.

The ship clock would say the trip took 2.1 years and a clock on Earth would say the trip took 4.9 years, so I guess you would say the time 'accumulated'.

However, the ship odometer would say the distance to Alpha Centauri was 1.9 ly and Earth would say the distance was 4.4 ly, so why would you say that distance doesn't 'accumulate?

It makes perfect sense that both time and distance dilate since they are both dimensions.

Edited for clarity.

Edited by Bufofrog
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On 4/26/2017 at 6:43 PM, Janus said:

It is important here not to confuse what someone visually sees happening to a clock approaching or receding with how fast that clock is actually ticking according to that person.

In the first case you have to account for the Doppler effect which is caused by the decreasing or increasing distance between you and the clock. This would happen even if the clock itself did not tick any faster or slower than your own.

Time dilation of the clock is a measure of how fast the clock is actually ticking compared to your own, and this is independent of its direction of movement.

When you are watching a clock approaching or receding, what you visually see is a combination of these two effects.

The math for what you will see is

fo=fs1vc1+vc

Where f0 is the observed tick rate.

fs is the source tick rate

v is the velocity of the clock relative to you(positive if receding and negative if approaching)

c is the speed of light.

So let's work out a couple of examples:

Assume you are situated 1 light hr from a stationary(to you) clock that is synchronized to your own clock (when your clock reads 12:00, it reads 12:00)

You looking at this clock when your clock reads 12:00 will visual see the clock as reading 11:00. (since it took the light carrying that image 1 hr to travel from the other clock, to get to you when your clock reads 12:00, it had to leave the other clock when it read 11:00)

A third clock travels from your position to the second clock at 0.8c

Using the formula above we find that you will see it tick 1/3 as fast as your own. It will arrive at the second clock in 1.25 hrs when the second clock reads 01:15. But because of the light travel time lag between you and the second clock, you will not see this until until your clock reads 02:15. This means the you will watch the third clock recede for 2.25 hrs by your clock while seeing it ticking 1/3 as fast and thus if it read 12:00 when it left you, you will see it arriving at the second clock reading 12:45. So in the 1:15 it took to make the trip it only ticked off 45 min. (remember, even though you didn't see the third clock arrive until your clock read 02:15, it had actually arrived an hour earlier.)

The direction had no effect on the time dilation or total accumulated time difference even though it did have an effect on what you visually saw happening to the clock.

In the example, the ticking rate for the outbound travel is 1/3.

The ticking rate for the return travel is 3.

So mathematically, everything is fine. I understand perfectly, I have no objections.

It means that as observed from point A (the departure point) the traveling clock ticks late when going away and ticks faster when coming back at the exactly reverse rate. Which means, if you replace the clock with a traveler, he will be observed from A as aging slowly when going away, and aging fast when going back, at the exact reverse rate so that everything ends perfectly fine. Symmetry is  maintained. That makes sense, I agree & applause. The maths are correct, Michel has no problem.

Why then do you believe that the returning traveler will have a different age than his twin brother at rest at point A? That's my problem.

17 minutes ago, Bufofrog said:

It makes perfect sense that both time and distance dilate since they are both dimensions.

Edited for clarity.

Time dilates & length contracts.

Edited by michel123456
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7 minutes ago, michel123456 said:

Time dilates & length contracts.

Yes, I should have said contracts.  That is the only comment, nothing about the fact that both time and length 'accumulate'?

You are not trying to understand, you are trying not to understand.  So I suppose you will spend the next 20 years not understanding.  Whatever floats your boat.

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1 hour ago, Bufofrog said:

Yes, I should have said contracts.  That is the only comment, nothing about the fact that both time and length 'accumulate'?

You are not trying to understand, you are trying not to understand.  So I suppose you will spend the next 20 years not understanding.  Whatever floats your boat.

When I read Swansont's post, I conclude that time accumulates while length contraction not. What do you read?

And I am trying to put people into thinking, not only parroting what they have been told.

Edited by michel123456
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1 hour ago, michel123456 said:

In the example, the ticking rate for the outbound travel is 1/3.

The ticking rate for the return travel is 3.

So mathematically, everything is fine. I understand perfectly, I have no objections.

It means that as observed from point A (the departure point) the traveling clock ticks late when going away and ticks faster when coming back at the exactly reverse rate. Which means, if you replace the clock with a traveler, he will be observed from A as aging slowly when going away, and aging fast when going back, at the exact reverse rate so that everything ends perfectly fine. Symmetry is  maintained. That makes sense, I agree & applause. The maths are correct, Michel has no problem.

Why then do you believe that the returning traveler will have a different age than his twin brother at rest at point A? That's my problem.

Because If A sees B's clock tick slow while he travels out to a distance of 1 light hour, he will see it ticking slow for 2 and 1/4 hrs and accumulate 45 min of time.

He will then see  B's clock tick 3 times as fast for 15 min.  End result, he sees B's clock accumulate 1 1/2 hrs in the time it takes for his own clock to tick off 2 1/2 hrs.

In order to get  a result where B's clock ticks off as much as A's, you would need to use the Newtonian equation for Doppler shift, or

f0 = fs (c/(1±v/c)       Where v is positive if the source is receding.

In this case, A would see B's clock tick  5/9 as fast on the outbound trip, accumulating 1 1/4 hrs in 2 1/4 hrs by A's clock, and then tick 5 times faster for 15 min, accumulating another 1 1/4 hrs, for a total of 2 1/2 hrs, the same that accumulates for A.

However, this is not what we measure in real life.  We measure a Doppler shift that matches the Relativistic version, which ends up giving an answer of less accumulated time for B.

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30 minutes ago, michel123456 said:

When I read Swansont's post, I conclude that time accumulates while length contraction not. What do you read?

And I am trying to put people into thinking, not only parroting what they have been told.

You were talking about the twin’s dimensions. Not the same thing.

Bufofrog explained how the length contraction “accumulates” - the distances you traverse are shortened as long as you travel. That distance will be shorter as measured by someone in a different frame, and the difference in those values increases as the duration of the trip increases.

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17 minutes ago, swansont said:

You were talking about the twin’s dimensions. Not the same thing.

Bufofrog explained how the length contraction “accumulates” - the distances you traverse are shortened as long as you travel. That distance will be shorter as measured by someone in a different frame, and the difference in those values increases as the duration of the trip increases.

I have no problem with Bufofrog's example. I have no problem with A observing time dilation. I have no problem with A observing length contraction.

I have a problem when the situation is not symmetric. In my view: what A observes must be the same as what B observes. If A observes B time dilated, then B must observe A as time dilated too, and of the exact same amount. Same for length contraction: length contraction as observed by A must be the same as length contraction observed by B. In the outbound travel and in the inbound. And when they meet together, A & B should have measured exactly the same thing concerning the other one. There is absolutely no reason why A should have a different age from B. No reason at all. The Frequency equation shows it.

The jump in another FOR and arguments about accelerometer is not valid. Both are inertials. If you want add a 3rd observer (called C) that moves parallel to the returning B traveler, and of the same age, traveling at the same velocity. This 3rd IS inertial (he didn't encounter any acceleration, he didn't made any U-turn). By definition, the age of traveler C will be the same of B at arrival time.

IOW there must be a flaw in Janus explanation, more specifically in the return travel, I think.

A logic flaw that I cannot spot, maybe an addition instead of a subtraction, something like that.

Edited by michel123456
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10 hours ago, Markus Hanke said:

I am not entirely sure what you mean. Basically, you’d calculate the interval in one frame first; you then apply Lorentz transformations to the time and space parts in order to “go” into the other frame, and check what this does to the overall interval. What you’ll find is that the interval remains the same - Lorentz transformations are just hyperbolic rotations in spacetime, so when you go from one frame to another, you essentially rotate some portion of the space part into the time part, or vice versa. This leaves the overall quantity unchanged.

I think I misunderstood. "The spacetime interval is invariant" was used to explain several things, I think including (paraphrasing) "why are lengths and times different in different frames?" and "how is that consistent?" If you consider a single space-like or time-like interval, it being invariant seems to demonstrate on its own that the measurements between frames are consistent, but doesn't give a reason why the measurements are different. I think what I missed is: it's that *all* intervals are invariant that explains why the measurements must be different (which can be demonstrated just by light-like intervals being invariant?).

3 hours ago, michel123456 said:

My quest is about time, not relativity.

You have a knack for showing no interest in explanations, but continuing to get people to put effort into giving them.

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1 hour ago, michel123456 said:

When I read Swansont's post, I conclude that time accumulates while length contraction not. What do you read?

And I am trying to put people into thinking, not only parroting what they have been told.

I think you are saying the universe really is the way it appears to your human senses. The "really is" part is philosophy, but since this is Physics section, you need to show some evidence that human senses alone are capable of observing the Universe as it "really is". All the evidence so far says our perception alone is inadequate.

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55 minutes ago, Janus said:

(...)

In order to get  a result where B's clock ticks off as much as A's, you would need to use the Newtonian equation for Doppler shift, or

f0 = fs (c/(1±v/c)       Where v is positive if the source is receding.

In this case, A would see B's clock tick  5/9 as fast on the outbound trip, accumulating 1 1/4 hrs in 2 1/4 hrs by A's clock, and then tick 5 times faster for 15 min, accumulating another 1 1/4 hrs, for a total of 2 1/2 hrs, the same that accumulates for A.

However, this is not what we measure in real life.  We measure a Doppler shift that matches the Relativistic version, which ends up giving an answer of less accumulated time for B.

Fine. Why the relativistic scenario does not end to be symmetric? While presenting a totally symmetric equation for frequencies? see my post above.

59 minutes ago, Janus said:

Because If A sees B's clock tick slow while he travels out to a distance of 1 light hour, he will see it ticking slow for 2 and 1/4 hrs and accumulate 45 min of time.

He will then see  B's clock tick 3 times as fast for 15 min.  End result, he sees B's clock accumulate 1 1/2 hrs in the time it takes for his own clock to tick off 2 1/2 hrs.

(...)

A sees B traveling 2 and 1/4 hrs while going away.

And you say that A sees B traveling back in 15 minutes? Or 2 and 1/4hrs  divided by 3 = 45 min? Why do you divide 45 minutes (as observed by the traveling clock)? instead of the time observed by A?

Edited by michel123456
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1 hour ago, michel123456 said:

Fine. Why the relativistic scenario does not end to be symmetric? While presenting a totally symmetric equation for frequencies? see my post above.

A sees B traveling 2 and 1/4 hrs while going away.

And you say that A sees B traveling back in 15 minutes? Or 2 and 1/4hrs  divided by 3 = 45 min? Why do you divide 45 minutes (as observed by the traveling clock)? instead of the time observed by A?

I think you found something nobody considers; there is symmetry in the twin paradox! How can the maths possibly work out? You might convince me?

Okay, let's say that a traveling twin travels outward for one hour while seeing Earth's clock appear to tick at half the rate. Then it returns with a symmetric trip and for one hour, it sees Earth's clock appear to tick at double the rate.

Half the rate for one hour, and double the rate for one hour. What is the total time that it sees ticking on Earth's clock, during its own 2-hour round trip? Could it be that you were right? If you answer this, it will prove that I'm a fool.

Edited by md65536
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.

(previous answer erased, obvious mistake)

56 minutes ago, md65536 said:

I think you found something nobody considers; there is symmetry in the twin paradox! How can the maths possibly work out? You might convince me?

Okay, let's say that a traveling twin travels outward for one hour while seeing Earth's clock appear to tick at half the rate. Then it returns with a symmetric trip and for one hour, it sees Earth's clock appear to tick at double the rate.

Half the rate for one hour, and double the rate for one hour. What is the total time that it sees ticking on Earth's clock, during its own 2-hour round trip? Could it be that you were right? If you answer this, it will prove that I'm a fool.

The half and double rate are observed from the Earth. From this FOR, the outbound & inbound travel will not be observed the same.

I suspect that the same goes for the traveling twin, he will not measure the same time for the outbound & for the inbound, because of the Doppler shift taking place only in 1 direction (toward the Earth).

So it is not "Half the rate for one hour, and double the rate for one hour" it is "Half the rate for one hour, and double the rate for less than an hour.

Edited by michel123456
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2 hours ago, michel123456 said:

I have no problem with Bufofrog's example. I have no problem with A observing time dilation. I have no problem with A observing length contraction.

I have a problem when the situation is not symmetric. In my view: what A observes must be the same as what B observes. If A observes B time dilated, then B must observe A as time dilated too, and of the exact same amount.

Which is what happens, as long as the situation is symmetric.

Quote

Same for length contraction: length contraction as observed by A must be the same as length contraction observed by B. In the outbound travel and in the inbound. And when they meet together, A & B should have measured exactly the same thing concerning the other one.

The distances are not the same, but that part of the scenario is not symmetric. One of them is moving relative to the turnaround location, but the other is not. But each does observe length contraction, of an equal factor, of any frame that is in  motion

Quote

There is absolutely no reason why A should have a different age from B. No reason at all. The Frequency equation shows it.

Of course there is. Acceleration is not inertial, and not symmetric.

Quote

The jump in another FOR and arguments about accelerometer is not valid. Both are inertials.

This is not true. Acceleration is not inertial.

Quote

If you want add a 3rd observer (called C) that moves parallel to the returning B traveler, and of the same age, traveling at the same velocity. This 3rd IS inertial (he didn't encounter any acceleration, he didn't made any U-turn). By definition, the age of traveler C will be the same of B at arrival time.

That’s true up until B turns around. Arrival at the turnaround. (But given that you don’t understand relativity, making the example more complicated is a losing proposition.)

B undergoes an acceleration, which is not inertial and destroys the symmetry of the problem.

Quote

IOW there must be a flaw in Janus explanation, more specifically in the return travel, I think.

A logic flaw that I cannot spot, maybe an addition instead of a subtraction, something like that.

Things aren’t wrong just because you can’t figure it out. That’s a fallacy - argument by personal incredulity.

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1 hour ago, michel123456 said:

The half and double rate are observed from the Earth. From this FOR, the outbound & inbound travel will not be observed the same.

I suspect that the same goes for the traveling twin, he will not measure the same time for the outbound & for the inbound, because of the Doppler shift taking place only in 1 direction (toward the Earth).

No symmetry then? Abandoned the thought experiment the very second it didn't show what you wanted? Thus proving me a fool for even trying despite this obvious outcome.

The trip that I described is realistic. If you travel outbound at one speed, and return at the same speed, it will take you the same time to make each leg of the trip. Galilean relativity even agrees with that.

2 hours ago, michel123456 said:

Why the relativistic scenario does not end to be symmetric? While presenting a totally symmetric equation for frequencies? see my post above.

8 hours ago, Bufofrog said:

You seriously have been trying to understand special relativity for 20 years, and you still say things like this?

Maybe you should give up.  Seriously, at some point you need to realize you're wasting your time.

Denial of science is a lifelong pursuit, you don't just give it up. The same reason that we would keep trying to explain relativity to someone with an amazing commitment to avoid understanding it, keeps someone else trying to show that relativity is wrong, even if he thinks the people he's trying to convince are "insane to believe such a thing."

Re. "at some point you need to realize you're wasting your time."... it's a battle of stubbornness, who will refuse for longer to give up? My bet's on Michel. You don't throw away 20 years invested in denying relativity, and risk it all by accepting something new now. You take it to the grave, regretting only that the insane people wouldn't listen.

Edited by md65536
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1 hour ago, michel123456 said:

.

(previous answer erased, obvious mistake)

The half and double rate are observed from the Earth. From this FOR, the outbound & inbound travel will not be observed the same.

I suspect that the same goes for the traveling twin, he will not measure the same time for the outbound & for the inbound, because of the Doppler shift taking place only in 1 direction (toward the Earth).

So it is not "Half the rate for one hour, and double the rate for one hour" it is "Half the rate for one hour, and double the rate for less than an hour.

Specifically

Half the rate for one hour, so the outbound traveler sees Earths clock clicking only 30 minutes.

Double the rate for returning: Earths click 60 minutes for a return travel of 30 minutes.

The traveler sees at his clock that he traveled 1h30 minutes, the same as the observer on Earth.

That is what I call "symmetry".

The trip lasted the same time for the traveler & for the guy at rest on Earth.

You have been fooled.

Edited by michel123456
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10 minutes ago, michel123456 said:

You have been fooled.

Absolutely! You are absolutely correct! It was all worth it for the laugh.

Sounds good. Your example has a traveler moving away for one hour, and then coming back in half an hour. What does Galilean relati ah forget it. Thanks for the laugh!

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49 minutes ago, md65536 said:

The trip that I described is realistic. If you travel outbound at one speed, and return at the same speed, it will take you the same time to make each leg of the trip. Galilean relativity even agrees with that.

Galilean relativity yes., but not Relativity (because of SOL).

But even with Galilean relativity, if the Earth is also moving in the direction of traveler B, but at a lower velocity (which is the equivalent of the delay caused by SOL), the outbound will not be the same with the inbound.

Because of the delay, the outbound will look longer than the inbound, it appears in all examples posted by Janus.on page 1 of this thread.

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59 minutes ago, michel123456 said:

Specifically

Half the rate for one hour, so the outbound traveler sees Earths clock clicking only 30 minutes.

Double the rate for returning: Earths click 60 minutes for a return travel of 30 minutes.

The traveler sees at his clock that he traveled 1h30 minutes, the same as the observer on Earth.

That is what I call "symmetry".

The trip lasted the same time for the traveler & for the guy at rest on Earth.

You have been fooled.

Why is the outbound trip one hour but the return is 30 minutes, according to the traveler’s clock? How is that possible?

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22 hours ago, michel123456 said:

which says to me that Time & Space are essentially the same

Exactly  Relativity treats time and space on equal footing - which not only underpins relativistic effects in the macroscopic realm, but also gives rise to the Dirac equation in QM, as well as the concept of spin for elementary particles.

22 hours ago, michel123456 said:

concerning lets' say ONE reality

Relativity is a purely classical model based on the notion of local realism, so of course there is only one reality. It couldn’t be any different. The one major difference compared to Newtonian mechanics is that measurements of time and space are now purely local concepts, so in order to describe a global reality, you need to consider a different set of quantities - namely those that are invariant.

22 hours ago, michel123456 said:

I have realized that the higher the level of the author (Nobel Prize like I. Prigogine) the more awaken about the unanswered questions, and lower the level ( not to mention anyone) less acceptance for questioning.

I think this is a gross overgeneralisation, because it depends on what the question is. The original question of the thread (What is time?) is a very good one, and many aspects and facets of it are indeed unanswered, or at the very least subject to ongoing debate and investigation. Only a fool would dispute this. I personally found the discussion we have had here on this subject to be interesting and constructive. Who would have though that a tea cup can spawn 14+ pages of constructive exchanges?
On the other hand though, the question as to whether the world is actually Lorentz invariant or not within the domain we can observe and probe (i.e. is SR a valid model?), and what that means in physical and mathematical terms, is not in any way subject to debate - this question has been definitively answered long ago, and the model itself is one of the most studied and well-understood in all of physics. It can even be formally proven that it is internally self-consistent, and that no physical paradoxes can be constructed using the axioms of SR. So you really are flogging a dead horse on this particular issue. This is honestly not meant to be personal but...the fact that relativity doesn’t make sense to you really is down to yourself. But you know you could fix that, if you wanted.

What is subject to debate however is how far exactly the domain of applicability of relativity extends, and what exactly happens once you go outside of it. This is an area of very intensive research right now.

9 hours ago, michel123456 said:

That is what I call "symmetry".

Symmetry means that a transformation that brings you from one frame to another is invertible - so if you apply the inverse of the same transformation, you arrive back in the original frame. In the case of the twin travellers, there will be at least one section of the journey when this does not hold.

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8 hours ago, swansont said:

Why is the outbound trip one hour but the return is 30 minutes, according to the traveler’s clock? How is that possible?

Because what the traveler sees is the same (the mirror) of what the observer on earth sees.

When the traveler goes out, as much the distance to the Earth increases, as much the delay increases too. And on the return trip, as much the distance reduces, so reduces the delay. The turning point will not be reached at the middle point (in time) of the travel. Although it will be the middle point in distance.

If the clock makes the U-turn at the middle point in time, at the end of the travel the clock will miss the Earth by a distance corresponding to the delay (if I am correct, this is more a guess than an accurate calculation).

Check with Janus example.

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