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Right, you obviously can't just do away with the relativistic effects, but what I mean is, the thing that makes the difference between being symmetric and asymmetric with different directions is those things but with the added Doppler effect, right? Because otherwise if you don't factor in the delay with light and the seeming delay in the photons you use to measure events, its the same as the scenario I was talking about before where they both see each other dilate exactly the same.

Not really. You don't need photons at all, just the velocities. If the twins separate and exchange no information until they reunite, their clocks will show the same difference in aging that you'd get if they were constantly observing each other.

Their paths through spacetime really are asymmetrical, whether watched or not.

Edit: Or, to interpret what you said a different way: If the delay of light wasn't what it is, relativity wouldn't work the way it does. So I don't know if you could meaningfully describe the scenario without a delay of light.

I guess that looking for the resolution only in the long durations of relative velocity is like considering only time dilation, and looking only at the turnaround is like looking only at relativity of simultaneity. The resolution of the paradox is in all of these things considered together. What occurs, how it's seen, who feels it, what the clocks say, these are all related and tell the whole story in a consistent way, but no one of these aspect makes sense isolated from the others.

Edited by md65536

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Not really. You don't need photons at all, just the velocities. If the twins separate and exchange no information until they reunite, their clocks will show the same difference in aging that you'd get if they were constantly observing each other.

Their paths through spacetime really are asymmetrical, whether watched or not.

Edit: Or, to interpret what you said a different way: If the delay of light wasn't what it is, relativity wouldn't work the way it does. So I don't know if you could meaningfully describe the scenario without a delay of light.

I guess that looking for the resolution only in the long durations of relative velocity is like considering only time dilation, and looking only at the turnaround is like looking only at relativity of simultaneity. The resolution of the paradox is in all of these things considered together. What occurs, how it's seen, who feels it, what the clocks say, these are all related and tell the whole story in a consistent way, but no one of these aspect makes sense isolated from the others.

Ok so just to be clear, it's because there is delay in how long the information takes to reach Earth that is the reason we see the asymmetry? Otherwise if the information reached Earth instantaneously, it would show symmetry?

Is it possible to even more closely relate the phenomena to the Doppler effect by going on to say that the information relayed to Earth on the frequency of events (like the periodic ticking of a clock) that takes place "piles up" in a sort of temporal blue shift which would account for what you called "ticking faster," similar only in concept to conventional blue shifting of a photon's actual frequency?

Edited by SamBridge
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Ok so just to be clear, it's because there is delay in how long the information takes to reach Earth that is the reason we see the asymmetry? Otherwise if the information reached Earth instantaneously, it would show symmetry?

Is it possible to even more closely relate the phenomena to the Doppler effect by going on to say that the information relayed to Earth on the frequency of events (like the periodic ticking of a clock) that takes place "piles up" in a sort of temporal blue shift which would account for what you called "ticking faster," similar only in concept to conventional blue shifting of a photon's actual frequency?

Well now we're well beyond anything I can usefully comment on.

The reason we see the asymmetry is that the situation is inherently asymmetrical. I can't say what would happen if there was no delay of light. You'd probably have to make up a whole new set of rules, and it wouldn't match anything real.

I don't see how anything piles up. In the standard interpretation you can have many photons en route to an observer, interspersed across the distance to the source, and the photons at different locations carrying info from the source as it looked at different times. Rather than "piled up" I'd say that since information doesn't travel instantly, any changes take time to propagate. That is certainly related to the asymmetry here. A change in velocity can have an immediate effect on the accelerating observer, but not on the remote observer.

How the relativistic Doppler effect relates is simply that it describes the Lorentz transformation while accounting for delay of light.

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Well now we're well beyond anything I can usefully comment on.
The reason we see the asymmetry is that the situation is inherently asymmetrical.

So if the Doppler effect doesn't matter and "its just the way it is", what of the other things you have not mentioned so far "causes" the situation to be asymmetric?

I can't say what would happen if there was no delay of light.

But that's like saying you can't figure out what would happen with gravity in a perfect sphere just because perfect spheres don't exist in reality. With your knowledge of mathematics, when you set the speed of light equal to infinity or if you take a limit with it approaching infinity in this scenario, what happens? How do the other variables change?

I don't see how anything piles up. In the standard interpretation you can have many photons en route to an observer, interspersed across the distance to the source, and the photons at different locations carrying info from the source as it looked at different times. Rather than "piled up" I'd say that since information doesn't travel instantly, any changes take time to propagate.

So then why does it matter if the information takes time to propagate if there's no added effects from the delay? Like I was saying originally, and you said, the effects of time dilation and length contraction are the same regardless of direction, so there's no reason for the dilation alone to cause asymmetry as both Earth and the rocket would see each other traveling at the same velocity from their own respective frames of reference, which means there's some more important factor about the delay in light and direction that you haven't brought up yet.

I could see how things might maybe "pile up" in this scenario since I don't have yet enough information to completely rule it out. If you just look at how photons normally blue shift, their frequency increases as a source heads towards you, and since we use photons to measure the frequency in periodic events like the ticking of a clock, the photon's blue sift could match up with the rate at which one measured the clock ticking from a source once the photons got there to show an illusion of a traveling clock ticking faster because the photons should be in closer intervals like you would see in sound waves or the front of a sonic boom. The only problem is, I can only model that type of thing for objects going slower than light. Since the speed of light has to remain constant, and if we say a source heading towards us emits a photon every one of its own measured seconds, the photons must be at least 1 light-second apart in distance no matter what, because any less distance traveled in a second by the photons would imply the photons traveled slower than one light-second per second, which, is less than the speed of light. But, with anything less than the speed of light which doesn't have to be measured as traveling at c even .9999999 the speed of c, it would "pile up." in some amount. I guess what I'm saying is like an optical blue shift, I can only say without further information that there is temporal one that makes the clock appear as though it was ticking faster based on the detail you added where you flat out said the clock is "ticking faster on other half of the trip". It could make sense to explain how the clock could appear to tick faster on the way back towards Earth while keeping the effects of time dilation and length contraction the same in both directions with some tweaking, unless you explain the details I was asking about in which case I would have no need for this type of model.

Here's what makes sense: I'm on Earth and I observe someone traveling away from me near the speed of light to have a slower clock.

Here's what should make perfect sense and I don't know if or ow it doesn't make sense: The someone traveling away from me observes me traveling away from them and thus observes me to have the same dilation and contraction that I observe them to have, therefore the clocks should ultimately match at the end of the trip, I mean you can't have both be slower than each other at the same time.

And here's what doesn't make sense anymore: The clocks don't match up, and it's because the rocket ship had to turn around and face the other direction. It has something to do with a relativistic Doppler effect which I'm told isn't temporal and "it's just the way it is" with no easily understandable, causal explanation.

where it says " but he sees it running fast during the Inbound Leg because each flash has a shorter distance to travel" which gives me hope that I'm not completely lost.

Why, even after calling the Doppler effect an illusion, is the situation is still asymmetrical by nature? I observe the traveling twin traveling near the speed of light, she observes me traveling near the speed of light, and the Doppler shift is only an illusion, so it should in reality be symmetric, but according to you and maybe the article, the illusion apparently isn't too far from the truth, and the situation actually is in reality for some reason asymmetric regardless of the illusion, or in other words, it doesn't just "appear" asymmetric, it actually is and the twin actually is younger just as they would be if they stood near a black hole and came back, and that's what doesn't make sense to me, especially since they can explain it just fine without using a pseudo gravitational field that would come from the equivalence principal.

Edited by SamBridge
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The only asymmetry that matters is the difference in speed profiles, as drawn on a Minkowski diagram. The profile (path) that varies from the the constant speed (straight line) one, loses more time. It's permanent and results in a differnce in aging.

In the common simple case, only one twin changes speed, thus the speed change has to be assigned to that profile. It's a coincidence, but not a cause. If both twins changed speed, the aging would be decided by the inertial portions of the profile.

Doppler shift is nothing more than watching clocks.If both count ticks, the answer is only apparent IF they reunite.

There is a paper in post 23 that explains this, if anyone is interested.

Of course, if it's explained, there would be no debate.

Edited by phyti
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The only asymmetry that matters is the difference in speed profiles, as drawn on a Minkowski diagram. The profile (path) that varies from the the constant speed (straight line) one, loses more time. It's permanent and results in a differnce in aging.

In the common simple case, only one twin changes speed, thus the speed change has to be assigned to that profile.

But that argument alone doesn't work because of relativity itself. If I observe someone traveling away from me, they will observe me traveling away from them.The traveling twin should still measure Earth's clock slowing down because the traveling twin see's Earth accelerating away from them to near speed of light even if everyone on Earth isn't being pushed into their seats, and if the clock didn't slow down and length didn't contract, there wouldn't be a reason for Earth not to accelerate right past the speed of light. It has to be more complicated than that.

The Doppler shift drawn in Minkowski space only seems to account for the illusion that the Earth twin sees of the traveling twin and not an actual real, physical component of what's happening.

Now if I "assumed" special relativity didn't work in the other direction, it might make sense to say everyone can agree, even the traveling twin, that only the traveling twin is traveling. But I know the spacial and temporal metric contractions work both ways based on what different sources have been saying and the traveling twin doesn't see themselves traveling, only their environment and outside objects.

Someone said something about "only one is accelerating like when someone gets pushed into the seat of your car." The supposed force that "pushes you into the seat of your car" is a fictitious force caused by an accelerated frame of reference, that's why I asked what the "fictitious force" of this scenario is, what is the thing that could only happen if someone was accelerating for sure that would account for the age difference as an additional effect that Earth couldn't be afflicted by? We know lorentz's transformations work the same in both directions, so both have to observe each other being affected from relativistic effects in some way, but Earth confirms the twin is younger from these effects, similar to being in a higher gravitational field (though it is not necessary to explain it as such) because of the addition effect of _______________ (fill in the blank) caused by the traveling twin's acceleration.

Edited by SamBridge
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But that argument alone doesn't work because of relativity itself. If I observe someone traveling away from me, they will observe me traveling away from them.The traveling twin should still measure Earth's clock slowing down because the traveling twin see's Earth accelerating away from them to near speed of light even if everyone on Earth isn't being pushed into their seats, and if the clock didn't slow down and length didn't contract, there wouldn't be a reason for Earth not to accelerate right past the speed of light. It has to be more complicated than that.

The Doppler shift drawn in Minkowski space only seems to account for the illusion that the Earth twin sees of the traveling twin and not an actual real, physical component of what's happening.

Now if I "assumed" special relativity didn't work in the other direction, it might make sense to say everyone can agree, even the traveling twin, that only the traveling twin is traveling. But I know the spacial and temporal metric contractions work both ways based on what different sources have been saying and the traveling twin doesn't see themselves traveling, only their environment and outside objects.

Someone said something about "only one is accelerating like when someone gets pushed into the seat of your car." The supposed force that "pushes you into the seat of your car" is a fictitious force caused by an accelerated frame of reference, that's why I asked what the "fictitious force" of this scenario is, what is the thing that could only happen if someone was accelerating for sure that would account for the age difference as an additional effect that Earth couldn't be afflicted by? We know lorentz's transformations work the same in both directions, so both have to observe each other being affected from relativistic effects in some way, but Earth confirms the twin is younger from these effects, similar to being in a higher gravitational field (though it is not necessary to explain it as such) because of the addition effect of _______________ (fill in the blank) caused by the traveling twin's acceleration.

You keep ignoring the "third leg" of the SR stool: The Relativity of Simultaneity.

Assume the traveling twin flies by the Earth at 0.6 c. He is heading towards a planet 10 ly from Earth (as measured from Earth). when he is next to the planet, he reverses and heads back towards Earth, both he and the Earth set their clock to zero at the moment they pass each other. The planet and the Earth has previously synced their own clocks.

We will distinguish between the time each observer visually sees,and the the "real-time" on the clock. So for example if two clocks are 1 light hr apart and motionless with respect to each other, and one visually reads a time of 1 hr on the other clock, he knows that the real-time on the other clock is actually 2 hrs, because it took 1hr for the reading on the clock to reach him from the other clock.

According to the Earth, the traveler takes 16 2/3 yrs to reach the planet (Both the clocks on Earth and on the Planet read 16 2/3 yrs real-time upon his arrival) and 16 2/3 years to return for a total trip time of 33 1/3 yrs. During which time, the traveler ages at a rate of 0.8(time dilation) and has aged 26 2/3 yrs on his return.

That's the easy part.

According to the traveler, the Distance from Earth to planet is 8 light yrs. (length contraction), also due to the Relativity of Simultaneity, the clock on the Planet does not read 0 (real-time) as he passes the Earth, but already reads a time of 6 yrs(real-time).

I'll explain that last bit. As the traveler passes the Earth, he visually sees the same time reading for the other planet (-10 yrs). Now for someone on the Earth, all they have to do is add 10 yrs to this to get what the "real-time" it is at that moment on the planet. They can do this because the planet and the Earth do not move with respect to each other.

However, in the case of the Traveler, things are different. He knows that the planet is 0.8 light yrs away as he passes the Earth. (or, at the very least, will know when he reaches it in 13 1/3 years at 0.6c) But that means it had to be further away when the light he sees left it. By backtracking both the light he sees(which travels at c with respect to him) and the planet's motion of 0.6 c with respect to him, he can determine how far away the planet was from him when the light left the planet and how long ago by his clock. He also knows that due to time dilation, the Planet's clock ticks slower than his own by a factor of 0.8 Putting this all together (the -10 yrs reading he sees, how long ago by his clock the light carrying that information left the planet and the time dilation factor for the planet's clock), he can determine what real-time it is at the planet the moment he passes the Earth. This will not be the same answer as someone on the Earth.

Okay, so now it will take 13 1/3 years by clock to cross the light years between Earth and planet (or if you prefer for the Planet to cross the distance to come to him.) The planet's clock ticks off 0.8 x 13 1/3 year = 10 2/3 years during this time. Since it started at 6 yrs when the traveler left Earth, it now reads 16 2/3 yrs. The Earth clocks has also ticked off 10 2/3 years and reads 10 2/3 yrs ar this moment. Both traveler and planet will visually see the Earth clock as reading 6 2/3 years

The traveler changes velocity so that he and the Earth are now moving towards each other at 0.6c We will assume that he does it in so short a time that he never leaves the vicinity of the planet. The planet clock still reads 16 2/3 yrs, the traveler clock, 13 1/3 yrs and both still read 6 2/3 years on the Earth clock.

However, as before, when leaving Earth, while the real-time readings on the Earth and planet clock are the same according to someone on either Earth and Planet, they are not so according to the traveler; there will be a 6 year difference in the real-time readings of the two clocks. And since he is now heading towards the Earth, it will the Earth clock that reads 6 yrs ahead of the planet one, again due to the Relativity of Simultaneity. So, according to real-time, it is now 12 2/3 years on Earth.

It again takes 13 1/3 years by the traveler's clock for he and Earth to meet up again,during which time, the Earth clock undergoes time dilation and ticks off 10 2/3 years, and reads 33 1/3 years for his 26 2/3 years when they meet back up.

The point is that due to the Relativity of Simultaneity, if you have two clocks separated by distance, inertial frames with different velocities that run parallel to the line separating the clocks will disagree as to the real-time difference in the reading of those clocks. And you have to include the Relativity of Simultaneity into any discussion of the Twin paradox.

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You keep ignoring the "third leg" of the SR stool: The Relativity of Simultaneity.

Assume the traveling twin flies by the Earth at 0.6 c. He is heading towards a planet 10 ly from Earth (as measured from Earth). when he is next to the planet, he reverses and heads back towards Earth, both he and the Earth set their clock to zero at the moment they pass each other. The planet and the Earth has previously synced their own clocks.

We will distinguish between the time each observer visually sees,and the the "real-time" on the clock. So for example if two clocks are 1 light hr apart and motionless with respect to each other, and one visually reads a time of 1 hr on the other clock, he knows that the real-time on the other clock is actually 2 hrs, because it took 1hr for the reading on the clock to reach him from the other clock.

According to the Earth, the traveler takes 16 2/3 yrs to reach the planet (Both the clocks on Earth and on the Planet read 16 2/3 yrs real-time upon his arrival) and 16 2/3 years to return for a total trip time of 33 1/3 yrs. During which time, the traveler ages at a rate of 0.8(time dilation) and has aged 26 2/3 yrs on his return.

That's the easy part.

According to the traveler, the Distance from Earth to planet is 8 light yrs. (length contraction), also due to the Relativity of Simultaneity, the clock on the Planet does not read 0 (real-time) as he passes the Earth, but already reads a time of 6 yrs(real-time).

I'll explain that last bit. As the traveler passes the Earth, he visually sees the same time reading for the other planet (-10 yrs). Now for someone on the Earth, all they have to do is add 10 yrs to this to get what the "real-time" it is at that moment on the planet. They can do this because the planet and the Earth do not move with respect to each other.

However, in the case of the Traveler, things are different. He knows that the planet is 0.8 light yrs away as he passes the Earth. (or, at the very least, will know when he reaches it in 13 1/3 years at 0.6c) But that means it had to be further away when the light he sees left it. By backtracking both the light he sees(which travels at c with respect to him) and the planet's motion of 0.6 c with respect to him, he can determine how far away the planet was from him when the light left the planet and how long ago by his clock. He also knows that due to time dilation, the Planet's clock ticks slower than his own by a factor of 0.8 Putting this all together (the -10 yrs reading he sees, how long ago by his clock the light carrying that information left the planet and the time dilation factor for the planet's clock), he can determine what real-time it is at the planet the moment he passes the Earth. This will not be the same answer as someone on the Earth.

Okay, so now it will take 13 1/3 years by clock to cross the light years between Earth and planet (or if you prefer for the Planet to cross the distance to come to him.) The planet's clock ticks off 0.8 x 13 1/3 year = 10 2/3 years during this time. Since it started at 6 yrs when the traveler left Earth, it now reads 16 2/3 yrs. The Earth clocks has also ticked off 10 2/3 years and reads 10 2/3 yrs ar this moment. Both traveler and planet will visually see the Earth clock as reading 6 2/3 years

The traveler changes velocity so that he and the Earth are now moving towards each other at 0.6c We will assume that he does it in so short a time that he never leaves the vicinity of the planet. The planet clock still reads 16 2/3 yrs, the traveler clock, 13 1/3 yrs and both still read 6 2/3 years on the Earth clock.

However, as before, when leaving Earth, while the real-time readings on the Earth and planet clock are the same according to someone on either Earth and Planet, they are not so according to the traveler; there will be a 6 year difference in the real-time readings of the two clocks. And since he is now heading towards the Earth, it will the Earth clock that reads 6 yrs ahead of the planet one, again due to the Relativity of Simultaneity. So, according to real-time, it is now 12 2/3 years on Earth.

It again takes 13 1/3 years by the traveler's clock for he and Earth to meet up again,during which time, the Earth clock undergoes time dilation and ticks off 10 2/3 years, and reads 33 1/3 years for his 26 2/3 years when they meet back up.

The point is that due to the Relativity of Simultaneity, if you have two clocks separated by distance, inertial frames with different velocities that run parallel to the line separating the clocks will disagree as to the real-time difference in the reading of those clocks. And you have to include the Relativity of Simultaneity into any discussion of the Twin paradox.

It will take some to read over to interpret everything you said correctly, but initially what it sounds like is one of the important differences is that the traveling twin says it's a shorter distance due to the relativistic effects which is maybe how we could say the traveling twin is younger though the asymmetry. The thing is, I just can't help but think about the fact that the traveling twin still sees Earth traveling away at the same speed, and then observes Earth to be returning and then heading towards themselves and why that situation alone is asymmetrical, like if the distance didn't matter, just in any scenario, why the symmetry is broken in reality and not via illusion because of a photon delay in those two instances alone. Earth twin see's traveling twin heading away, traveling twin see's Earth twin heading away at the same speed, and they both see each move in a semi circle around the star when the direction is changing, then both see each other heading towards one another, and in that scenario events, how do we know only the traveling twin has experienced more time dilation or traveled dramatically less measured distance to make them younger?

Edited by SamBridge
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But that argument alone doesn't work because of relativity itself. If I observe someone traveling away from me, they will observe me traveling away from them.The traveling twin should still measure Earth's clock slowing down because the traveling twin see's Earth accelerating away from them to near speed of light even if everyone on Earth isn't being pushed into their seats, and if the clock didn't slow down and length didn't contract, there wouldn't be a reason for Earth not to accelerate right past the speed of light. It has to be more complicated than that.

The Doppler shift drawn in Minkowski space only seems to account for the illusion that the Earth twin sees of the traveling twin and not an actual real, physical component of what's happening.

Now if I "assumed" special relativity didn't work in the other direction, it might make sense to say everyone can agree, even the traveling twin, that only the traveling twin is traveling. But I know the spacial and temporal metric contractions work both ways based on what different sources have been saying and the traveling twin doesn't see themselves traveling, only their environment and outside objects.

Someone said something about "only one is accelerating like when someone gets pushed into the seat of your car." The supposed force that "pushes you into the seat of your car" is a fictitious force caused by an accelerated frame of reference, that's why I asked what the "fictitious force" of this scenario is, what is the thing that could only happen if someone was accelerating for sure that would account for the age difference as an additional effect that Earth couldn't be afflicted by? We know lorentz's transformations work the same in both directions, so both have to observe each other being affected from relativistic effects in some way, but Earth confirms the twin is younger from these effects, similar to being in a higher gravitational field (though it is not necessary to explain it as such) because of the addition effect of _______________ (fill in the blank) caused by the traveling twin's acceleration.

The mutual observations of clocks are symmetrical, but demonstrate doppler shifts, not aging or accumulated time. Aging can only be determined when the clocks are reunited.

In the simple case, The cast is A(naut) and E(arth).

E records: A leaves, moves inertially for t1, reverses direction (due to rocket) for t2, and returns in t3.

A records: E leaves, moves inertially for t1', reverses direction (due to g-field) for t2', and returns in t3'.

Note, each has an explanation for the non inertial part of the trip, and A senses it, using the equivalence principle, and E does not.

The A times will be less, since his speed profile varies from the constant speed of E.

This is the point where you have to avoid the road map interpretation of the Minkowski drawing. It's not necessarily a longer path. It's a different speed profile or path, and the relation is t = x/v, i.e., faster speed, less time. (At least one segment will be faster than that of E.)

Consider acceleration as establishing the rate of time dilation.

Acceleration is not fictitious, since it can be measured (every time you weigh yourself).

What if A and B are both anauts in ships, and at max separation, each accelerates to rejoin, which one counts?

This is an example showing acceleration is not a factor.

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The mutual observations of clocks are symmetrical, but demonstrate doppler shifts, not aging or accumulated time. Aging can only be determined when the clocks are reunited.

In the simple case, The cast is A(naut) and E(arth).

E records: A leaves, moves inertially for t1, reverses direction (due to rocket) for t2, and returns in t3.

A records: E leaves, moves inertially for t1', reverses direction (due to g-field) for t2', and returns in t3'.

Note, each has an explanation for the non inertial part of the trip, and A senses it, using the equivalence principle, and E does not.

The A times will be less, since his speed profile varies from the constant speed of E.

This is the point where you have to avoid the road map interpretation of the Minkowski drawing. It's not necessarily a longer path. It's a different speed profile or path, and the relation is t = x/v, i.e., faster speed, less time. (At least one segment will be faster than that of E.)

Consider acceleration as establishing the rate of time dilation.

Acceleration is not fictitious, since it can be measured (every time you weigh yourself).

What if A and B are both anauts in ships, and at max separation, each accelerates to rejoin, which one counts?

This is an example showing acceleration is not a factor.

The equivalence principal isn't necessary to describe it and that link I posted didn't use it, so an explanation with out it should be simpler, ultimately. But, otherwise the only other way I could maybe see it making sense with it is if you create an additional distortion in space-time around the traveling twin like a gravity field itself to show the traveling twin's acceleration put them in a higher degree of curvature away from Earth's frame of reference. But the same previous problem arises where if say, I'm 1/3 of a radian away in curvature from someone, then that someone will observe me being 1/3 of a radian away from them (not that it works that simply) and so there's still no clear place where the asymmetry comes from. Someone mentioned previously that it was symmetrical for the first half of the trip which if true would mean there's a more real component to the Doppler effect, and if not, back to square one.

Edited by SamBridge
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The equivalence principal isn't necessary to describe it and that link I posted didn't use it, so an explanation with out it should be simpler, ultimately. But, otherwise the only other way I could maybe see it making sense with it is if you create an additional distortion in space-time around the traveling twin like a gravity field itself to show the traveling twin's acceleration put them in a higher degree of curvature away from Earth's frame of reference. But the same previous problem arises where if say, I'm 1/3 of a radian away in curvature from someone, then that someone will observe me being 1/3 of a radian away from them (not that it works that simply) and so there's still no clear place where the asymmetry comes from. Someone mentioned previously that it was symmetrical for the first half of the trip which if true would mean there's a more real component to the Doppler effect, and if not, back to square one.

I don't understand the continuing confusion. It's been explained several times in this thread that the whole situation in SR involves all three of time dilation, length contraction, and relativity of simultaneity. The two twins measure the local time until a turnaround event as different. The two measure the length that the other has traveled away as different. Only one of the twins measures a change in relative simultaneity. None of these things is symmetric. Neither is the overall Doppler effect, velocity profiles, and proper acceleration, consistent with the 3 main things.

It makes no sense to look at just one of the three and say "There must be more". ALL THREE together form a consistent picture.

What makes you think the situation is symmetrical at all in the first place? It is because some one detail of SR is symmetrical (their relative closing velocity, or the time dilation factor, or the color of their hair or any other of many single details that you might focus exclusively on). So SR says that one detail is symmetric, but SR also says the situation is not symmetric, if you look at the whole thing and consider all three of time dilation, length contraction, and relativity of simultaneity together. It makes no sense to have one without the others. You're only thinking the situation is symmetrical because of what SR tells you, yet you refuse to consider all of what SR is saying. It's mind boggling. It's as if the words "relativity of simultaneity" mean nothing, and so they can be safely ignored, and instead just repeat "There must be something more to this!"

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I don't understand the continuing confusion.

Symmetry.

It's been explained several times

Not very clearly.

IThe two twins measure the local time until a turnaround event as different.

And why? Saying "time slows down near the speed of light" doesn't explain why it isn't symmetric, both observers observe each other moving away from each other near the speed of light, that's the whole problem. Beyond some illusion of photon red-blue shifts, what accounts for the physical asymmetry of the aging, when alternatively, they should both see the same level of time dilation and length contraction in each other?

What makes you think the situation is symmetrical at all in the first place?

Normal relativity. The concept that everyone is stationary from their own frame of reference and only see other things moving.

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And why? Saying "time slows down near the speed of light" doesn't explain why it isn't symmetric, both observers observe each other moving away from each other near the speed of light, that's the whole problem. Beyond some illusion of photon red-blue shifts, what accounts for the physical asymmetry of the aging, when alternatively, they should both see the same level of time dilation and length contraction in each other?

"Time slows down near the speed of light" doesn't explain it just like "it'll affect your insurance premiums" doesn't explain what happens if you set fire to a fireworks factory---it is just one part of it, it is not ALL of it.

The situation is physically asymmetric. You've set it up that way. You've set it up so that one twin turns around. THAT is the cause of the asymmetry. Yes, there are some symmetries (such as the relative time dilation factor as they are receding, or any other aspect up to the point of the turnaround). Just because the results of SR correlate with the asymmetry in the situation, doesn't mean that is CAUSES the asymmetry.

Consider this: Two twins leave Earth in different directions at the same speed. Each travels a proper time of one year and turns around and they reunite back at Earth. Suppose gamma relative to Earth is 2, each way. Then the twins each aged 2 years (while Earth aged 4). Now these twins are in a symmetrical situation, and their relative aging is the same. This is a symmetrical situation! Now take this situation, and change it so that SR predicts a difference in aging in the twins. THERE IS THE CAUSE OF YOUR ASYMMETRY. Whatever you did to make the situation asymmetrical, that is the cause. In the original situation, you have one twin turn around while the other doesn't. THAT is the cause of the asymmetry. The relativistic effects that occur due to the asymmetry are NOT the cause of it!

Can you use the words "relativity of simultaneity" in your attempts at explaining what's happening, so that at least we know that you're trying to fit this essential concept into your understanding of the situation? If you don't yet see that it's important, you'll get a lot further ahead a lot quicker by looking up what it is and what it means, instead of trying to figure it out without it. I feel like you're trying to understand SR while insistently avoiding SR.

Edited by md65536
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Beyond some illusion of photon red-blue shifts, what accounts for the physical asymmetry of the aging

Acceleration. In the basic scenario, only one of them accelerates. Acceleration is not relative.

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Yes, there are some symmetries (such as the relative time dilation factor as they are receding, or any other aspect up to the point of the turnaround).

But that contradicts what you're saying and I have no idea where you're coming from. You say it's the same before receding, then you say its different, then the same, then different. What am I suppose to make of that?

"you have one twin turn around while the other doesn't. THAT is the cause of the asymmetry.

But both observe each other from turning around, there isn't only one turning around. When the traveling twin circles the star from Earth's view to turn around, the traveling twin actually sees Earth turning around instead, they don't see themselves turning around, and it's that principal that is why I can't account for the asymmetry. Still don't see how the physical, non-illusionary happening of of the traveling twin experiencing less time isn't symmetric the whole way or only in the first half or not in only the first half or whatever you're saying now. No matter what you say, it has to be true in some way that both Earth and the traveling twin observe similarities in how they are traveling from each other's frame of reference. If Earth observes the twin heading towards them, then the traveling twin observes Earth heading towards them instead which means they should both observe the same relativistic effects of each other. If Earth observes the traveling twin accelerating away, the traveling twin must instead observe Earth accelerating away and it can't be any other way, and that's why I don't see where the asymmetry is.

Can you use the words "relativity of simultaneity" in your attempts at explaining what's happening, so that at least we know that you're trying to fit this essential concept into your understanding of the situation? If you don't yet see that it's important, you'll get a lot further ahead a lot quicker by looking up what it is and what it means, instead of trying to figure it out without it. I feel like you're trying to understand SR while insistently avoiding SR.

People can't agree on when they see something happen which I believe we've discussed as it arises from relativistic effects. As far as I know, you said and the article said it fits into this as some illusion created by photon blue shifts to account for the asymmetry aside from fitting into the length contraction and time dilation.

Acceleration. In the basic scenario, only one of them accelerates. Acceleration is not relative.

Yes many others have made that point, but still not a clear explanation. What about acceleration does this to create the asymmetric situation? Why is it that everyone agrees a certain object is accelerating, and what about this agreement makes the situation asymmetrical when the traveling twin observes the environment and Earth accelerating instead?

Edited by SamBridge
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Yes many others have made that point, but still not a clear explanation. What about acceleration does this to create the asymmetric situation? Why is it that everyone agrees a certain object is accelerating, and what about this agreement makes the situation asymmetrical when the traveling twin observes the environment and Earth accelerating instead?

The earth twin is not accelerated. You can tell when you are in an accelerated frame.

If you had a Foucault pendulum, you would see it slowly change direction over the course of several hours. That's one way we know the earth is rotation (and is not an inertial frame, but we're ignoring this for now). But if the earth were to change direction, the pendulum would show this. That's just one of many ways that one can tell if one has undergone an acceleration. Accelerations are not relative. The symmetry is broken when one object accelerates and the other does not.

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If Earth observes the twin heading towards them, then the traveling twin observes Earth heading towards them instead which means they should both observe the same relativistic effects of each other.

And when does each first observe this happening in your example, according to SR (according to the math)? Use any simplifications you want.

Are the moments just before the rocket's relative velocity changes according to an Earth observer, and just before the Earth's relative velocity changes according to a rocket observer, simultaneous in any frame? If so, which? Is that symmetrical?

Edited by md65536
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Are the moments just before the rocket's relative velocity changes according to an Earth observer, and just before the Earth's relative velocity changes according to a rocket observer, simultaneous in any frame? If so, which? Is that symmetrical?

Probably not, but I still need an explanation to get around the fact that the traveling twin observes Earth traveling in semi circle instead and so should still observe the same Doppler shift of Earth. Ultimately, the traveling twin is going to have more time dilation from Earth's frame of reference making them younger, but I don't see the physical, non-illusion mechanism for how, only in the moment of acceleration. I could see potentially how with the equivalence principal if you treat the object as though its in a higher gravitational field, but the physical, non-illusion part of the relativistic Doppler shift and simpler changing to moving towards to increase time dilation of the traveling twin from supposedly any frame and thus make the traveling twin younger is what I don't see.

And when does each first observe this happening in your example, according to SR (according to the math)? Use any simplifications you want.

At whatever respective time they say it takes light to travel the distance between them.

The earth twin is not accelerated. You can tell when you are in an accelerated frame.

See you keep repeating that, but that statement itself isn't the issue, I already know its true and you also keep repeating it. It's how that concept fits in to simply taking a hypothetically instantaneous moment around a star to explain how the physical reduction in aging of only the traveling twin, why that statement is physically true to physically account for what's going on.

Edited by SamBridge
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At whatever respective time they say it takes light to travel the distance between them.

No. For one thing, they calculate the timing differently regardless of the travel time of light (due to relativity of simultaneity). For another thing, (only) the twin who turns around sees the change in relative velocity immediately.

The situation is asymmetric, as explained in many ways. It is measured differently, seen differently, felt differently, timed differently. The symmetrical slowing of clocks and the resulting "paradox" is only a partial application of SR. You know the situation is asymmetrical, but you think that applying a part of SR keeps it symmetrical. However, a full application of SR (considering time dilation, length contraction, and relativity of simultaneity) resolves the paradox and shows you where the differences are. You are fighting every explanation that involves SR, yet you keep demanding an explanation of SR. I don't think you'll get much further without understanding the theory a bit more. Keep in mind that you're so sure that the other twin's clock ticks slower, but why? Just because it is predicted by SR? Do you see the physical mechanism for it? If so, what is it? And if not, then why do you require a physical mechanism for the other predictions of SR while rejecting the other theoretical predictions and explanations? Eg. the traveling twin does not remain in an inertial frame, so the "slowing of other clocks" doesn't apply without relativity of simultaneity.

If you work through some examples with numbers, it might give you a concrete understanding of the important concepts and make it impossible to brush them aside.

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See you keep repeating that, but that statement itself isn't the issue, I already know its true and you also keep repeating it.

I keep repeating it because you keep asking the question about symmetry and acceleration. If you want a different answer, you need to ask a different question.

It's how that concept fits in to simply taking a hypothetically instantaneous moment around a star to explain how the physical reduction in aging of only the traveling twin, why that statement is physically true to physically account for what's going on.

The acceleration allows you to declare who was actually moving and who was stationary. In essence, it lifts the "relative" part of relativity. We know the rocket twin was the one moving, we know his/her clock (i.e. time) ran slow. Absent an acceleration, both can claim those things about the other.

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. Keep in mind that you're so sure that the other twin's clock ticks slower, but why? Just because it is predicted by SR? Do you see the physical mechanism for it? If so, what is it?

Those are the exact things you haven't answered in detail, all you keep repeating some doppler analogy while changing what you say about how it works in and then saying "that's just how SR works."

The acceleration allows you to declare who was actually moving and who was stationary. In essence, it lifts the "relative" part of relativity. We know the rocket twin was the one moving, we know his/her clock (i.e. time) ran slow. Absent an acceleration, both can claim those things about the other.

Right, that's what I got, but like what physical effect is there? If someone is in an accelerated frame of reference, they might experience, say, a fictitious force that causes a different measurement. What is that force in this scenario? What about turning around and that instantaneously small moment of acceleration that makes the entire situation asymmetric? Or how about before they turned around? They had to accelerate to get to near the speed of light but still stayed at a constant velocity once they got to that speed?

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Right, that's what I got, but like what physical effect is there? If someone is in an accelerated frame of reference, they might experience, say, a fictitious force that causes a different measurement. What is that force in this scenario?

There is no force. Time dilation and length contraction are artifacts of c being invariant.

What about turning around and that instantaneously small moment of acceleration that makes the entire situation asymmetric?

We just went through this. The asymmetry is the acceleration, which is not relative. One twin accelerates. The other does not.

Or how about before they turned around? They had to accelerate to get to near the speed of light but still stayed at a constant velocity once they got to that speed?

In this scenario the setting of the clocks occurs after the spaceship twin is moving at the speed given in the problem. There will be dilation prior to that, but the amount can be made trivially small by making the acceleration last for a vanishingly small amount of time.

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Right, that's what I got, but like what physical effect is there? If someone is in an accelerated frame of reference, they might experience, say, a fictitious force that causes a different measurement.

Theoretically a change of relative length contraction and relative simultaneity causes the effect.

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We just went through this. The asymmetry is the acceleration, which is not relative. One twin accelerates. The other does not.

So say we do know which one accelerates, but what about that small acceleration changes everything? Or is it actually asymmetrical prior to that as well as md6 was flip-flopping with?

Edited by SamBridge
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So say we do know which one accelerates, but what about that small acceleration changes everything? Or is it actually asymmetrical prior to that as well as md6 was flip-flopping with?

It breaks the symmetry. It allows you to know whose clock was running slow; while the symmetry exists there is no answer to that.

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