# Twins paradox explained without forces

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6 hours ago, swansont said:

If I move an arbitrary distance, the clocks don't read the same time.

The criticism of a setup like this is that it doesn't work, in general, and was contrived to get a certain result.

Another thing the OP's scenario lacks (as suggested by between3and26characterslon) is that there is no situation where all of the clocks can be compared side-by-side to confirm that they run at the same rate. You haven't actually demonstrated that it's relativity that is causing the different rate, as opposed to a clock that just runs slower.

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5 minutes ago, swansont said:

Another thing the OP's scenario lacks (as suggested by between3and26characterslon) is that there is no situation where all of the clocks can be compared side-by-side to confirm that they run at the same rate. You haven't actually demonstrated that it's relativity that is causing the different rate, as opposed to a clock that just runs slower.

But the geometric lengths of the world lines OP describes between AB (where A and B pass) and BC is 4 years, between BC and AC is 4 years, and between AB and AC is 10 years, calculated using special relativity. Are you getting a different answer, or not able to get that answer, or are you proposing a different cause other than relativity that is giving you that answer?

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33 minutes ago, md65536 said:

But the geometric lengths of the world lines OP describes between AB (where A and B pass) and BC is 4 years, between BC and AC is 4 years, and between AB and AC is 10 years, calculated using special relativity. Are you getting a different answer, or not able to get that answer, or are you proposing a different cause other than relativity that is giving you that answer?

My point is if you never compare the clocks in a single frame, you can't show that the different clock rates are due to relativity.

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2 hours ago, swansont said:

My point is if you never compare the clocks in a single frame, you can't show that the different clock rates are due to relativity.

The different clock rates are a direct prediction of relativity. The proper times along the given world lines are invariant. How does comparing clocks in a single frame have any bearing on that? Can you give an example of how the different clock rates are due to something other than relativity, yet is still consistent with relativity?

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It's a validation that the clocks give consistent time with other. In order to eliminate the possibility one clock runs slower than the other for reasons other than that due to relativity.

Edited by Mordred

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3 hours ago, md65536 said:

The different clock rates are a direct prediction of relativity. The proper times along the given world lines are invariant. How does comparing clocks in a single frame have any bearing on that? Can you give an example of how the different clock rates are due to something other than relativity,

Sure. It’s straightforward. You make a clock with a different frequency.

3 hours ago, md65536 said:

yet is still consistent with relativity?

As Mordred said, it’s experimental validation.

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1 hour ago, swansont said:

Sure. It’s straightforward. You make a clock with a different frequency.

As Mordred said, it’s experimental validation.

Does that mean that for a world line whose proper time between two events is 4 years (for example clock B's world line as per OP), you wouldn't be able to tell if a clock that measured 4 years between the two events on that world line was running properly in accordance with relativity, or was running too slow (or fast) for some other reason, unless you can compare it side-by-side with another clock?

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I really don't understand the issue of ensuring experimental validation as a step. Particularly when you wish to ensure accuracy. You want to quarantee that the 4 years you mentioned is as accurate as possible. The worldline could be 3.5 years but the clock error could give a result of 4 years.

Edited by Mordred

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8 hours ago, md65536 said:

Does that mean that for a world line whose proper time between two events is 4 years (for example clock B's world line as per OP), you wouldn't be able to tell if a clock that measured 4 years between the two events on that world line was running properly in accordance with relativity, or was running too slow (or fast) for some other reason, unless you can compare it side-by-side with another clock?

Yes. If your issue with the paradox is that relativity doesn't explain it, you need the experiment to eliminate the possibility that the clocks are simply running at different inherent rates. Experiments need to be able to confirm what you're looking for while also eliminating alternate explanations.

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My understanding of SR (limited though it may be) is that it relies on strict application of rules. I believe SR only makes sense to an inertial observer, one who can be considered at rest and about which measurements are made/calculated.

In the experiment as stated by OP are we to assume that these are naturally occurring clocks and have been traveling in straight lines at constant speeds since the beginning of time and come together for the experiment. The unlikeliness of such an event is irrelevant, what matters is that none of the three clocks in the experiment can be considered inertial (or all three of them are in their own right)

or

Are they manufactured clocks which started off from inertial frames and have accelerated towards another inertial frame

or

Did all three clocks start of at rest relative to the same inertial frame in which case it is just the twins paradox

I think SR only makes sense if there is a single inertial frame of reference, is this not the case?

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On 1/21/2020 at 12:58 PM, scuddyx said:

Is this explanation ok that avoids using forces?  Thanks

Rather than twin flying it is best to consider synchronised clocks.  This avoids talking about acceleration at certain times is the flight.  Special relativity doesn’t require an understanding of acceleration – this is dealt with in general relativity.

It is not necessary for an actual twin to follow the out and back path or to experience an acceleration at the turning point.  Outgoing and incoming spaceships could simply exchange clock readings or videos when they pass each other.  Clock A stays on earth and is synchronised to clock B on a spaceship flying past (3/5c).  After 5 earth years the spaceship will be at 3 light years and synchronises the clock C on a spaceship flying in the opposite direction. This clock will arrive back at earth after another 5 earth years.  The moving clock will only show that a total of 8 years has elapsed.

As the twins move apart both will see the other age slower by the same amount (4/5).  This is after the time has been corrected for the video to travel between them.

When the clocks are synchronised at the turning point there is a change in the frame of reference.  For the spaceships to continue watching each other (videos and heart pings) the clocks need to be resynchronised.

As they pass by the outbound rocket twin will relay the total number of pings counted so far to the inbound rocket and then the inbound rocket will continue counting all the way to Earth.  Outbound rocket twin will be getting low frequency pings and the inbound rocket will be getting high frequency pings.  The pings represent the heartbeats so the number of heartbeats by Earth twin will be greater and therefore Earth twin will be older.

On the inward journey, there is a new meaning of simultaneity.  There is a new clock synchronisation.  Remember there is no background absolute time.  The synchronisation of clocks must include the time for the clock reading to travel between them at the speed of light.

The outgoing spaceship (B) receives pings from the clock left on Earth (A) and from the clock on returning spaceship (C).  Clock C will be slow due to time dilation so that when it arrives at Earth it will show a total of 8 years has elapsed whereas the clock left on Earth will show 10 years has elapsed.  This is consistent with the twin paradox.  Note that clocks A and B will continue to run slow when compared to each other.  This is consistent with special relativity as there is no absolute time reference.

The Twin Paradox doesn’t explain what Time IS – but tells us a few things about it:

1. There is no absolute time reference – each inertial frame has its own independent Time

2. There is no contradiction that two inertial frames observe the time in the other moving more slowly

3. Einstein was wrong. The Twin Paradox can be explained without invoking accelerations

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30 minutes ago, scuddyx said:

each inertial frame has its own independent Time

Independent of what?

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15 hours ago, between3and26characterslon said:

I believe SR only makes sense to an inertial observer, one who can be considered at rest and about which measurements are made/calculated.

SR is well capable of handling accelerated frames as well; it’s just that the relationships between such frames are more complicated than simple Lorentz transforms. Where SR breaks down is when gravity is involved, i.e. when the region of spacetime in question is not flat.

15 hours ago, between3and26characterslon said:

I think SR only makes sense if there is a single inertial frame of reference, is this not the case?

All relativistic effects are relationships between frames; you would never observe any such effects, if you looked only at a single inertial frame.

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11 hours ago, studiot said:

Independent of what?

The clocks in every inertial frame of reference run independently of the clocks in every other inertial frame of reference.

In addition, there is no absolute time reference that each inertial frame can be compared to.

4 hours ago, Markus Hanke said:

SR is well capable of handling accelerated frames as well; it’s just that the relationships between such frames are more complicated than simple Lorentz transforms. Where SR breaks down is when gravity is involved, i.e. when the region of spacetime in question is not flat.

All relativistic effects are relationships between frames; you would never observe any such effects, if you looked only at a single inertial frame.

Yes SR is capable of handling accelerating frames.

The point I am making is that the twin paradox can be explained without resorting to acceleration.

Many twin paradox explanations just end by saying the spaceship carrying one twin experiences forces as it turns around - leaving the novice thinking the forces somehow corrupt the mechanical workings of the clocks.

The twin paradox gives profound insights into the nature of time.

20 hours ago, between3and26characterslon said:

My understanding of SR (limited though it may be) is that it relies on strict application of rules. I believe SR only makes sense to an inertial observer, one who can be considered at rest and about which measurements are made/calculated.

In the experiment as stated by OP are we to assume that these are naturally occurring clocks and have been traveling in straight lines at constant speeds since the beginning of time and come together for the experiment. The unlikeliness of such an event is irrelevant, what matters is that none of the three clocks in the experiment can be considered inertial (or all three of them are in their own right)

or

Are they manufactured clocks which started off from inertial frames and have accelerated towards another inertial frame

or

Did all three clocks start of at rest relative to the same inertial frame in which case it is just the twins paradox

I think SR only makes sense if there is a single inertial frame of reference, is this not the case?

Special relativity asserts that the speed of light is the same in all inertial frames.

The Lorentz transformations are a set of equations that relate the space and time coordinates between inertial frames.

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13 hours ago, scuddyx said:

3. Einstein was wrong. The Twin Paradox can be explained without invoking accelerations

Objections raised here notwithstanding...

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9 hours ago, Markus Hanke said:

SR is well capable of handling accelerated frames as well; it’s just that the relationships between such frames are more complicated than simple Lorentz transforms. Where SR breaks down is when gravity is involved, i.e. when the region of spacetime in question is not flat.

All relativistic effects are relationships between frames; you would never observe any such effects, if you looked only at a single inertial frame.

In the link above it seems to me that they are asserting the twin paradox (previously I stated SR) is the observable results of events within an inertial frame. For twins to be twins they must have been at rest relative to each other, or more formally both at rest relative to the same inertial frame of reference (IFOR), at some point i.e. the moment before they were born. Then within this IFOR one twin will accelerate, travel at speed, turn around and come back whilst the other one remains stationary.

So if the OP is suggesting that all clocks are synchronised whilst at rest relative to the same IFOR, are then moved to locations where they have the relative velocity and distance as set out in the OP and obey the timings and measurements as set out then is the OP, in part, right? However this is just a more complex twins paradox.

I am struggling with this though: if clocks which originated from outside the IFOR matched their velocity and position with the synchronised clocks they should give the same result, but this is not consistent with any explanation of SR I can find, to wit, the link above.

Edited by between3and26characterslon
Changed a statement into a question

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16 minutes ago, between3and26characterslon said:

In the link above it seems to me that they are asserting the twin paradox (previously I stated SR) is the observable results of events within an inertial frame. For twins to be twins they must have been at rest relative to each other, or more formally both at rest relative to the same inertial frame of reference (IFOR), at some point i.e. the moment before they were born. Then within this IFOR one twin will accelerate, travel at speed, turn around and come back whilst the other one remains stationary.

So if the OP is suggesting that all clocks are synchronised whilst at rest relative to the same IFOR, are then moved to locations where they have the relative velocity and distance as set out in the OP and obey the timings and measurements as set out then the OP is, in part, right. However this is just a more complex twins paradox.

I am struggling with this though: if clocks which originated from outside the IFOR matched their velocity and position with the synchronised clocks they should give the same result, but this is not consistent with any explanation of SR I can find, to wit, the link above.

Thanks for the link in your post.  Like most twin paradox explanations it says the spaceship carrying one twin experiences forces as it turns around and finishes with the question "what role does the acceleration play in this?"

The twin paradox can be explained without resorting to acceleration by merely by showing there is simply a change in inertial frame. No forces (GR) are required. SR is sufficient.

The twin paradox is extremely useful as it shows that there is no absolute time reference, each inertial frame has its own independent time and that there is no contradiction that two inertial frames observe the time in the other moving more slowly.

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If you have a change in inertial frame then you likely underwent an acceleration. For example the mathematics I posted previously shows that during acceleration the travelling twin is in a non inertial reference frame. When the twin stops accelerating then he is in a new inertial frame than that previous to accelerate.

Forces are involved with acceleration in accordance to the laws of inertia ie f=ma

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9 minutes ago, Mordred said:

If you have a change in inertial frame then you likely underwent an acceleration. For example the mathematics I posted previously shows that during acceleration the travelling twin is in a non inertial reference frame. When the twin stops accelerating then he is in a new inertial frame than that previous to accelerate.

Forces are involved with acceleration in accordance to the laws of inertia ie f=ma

Did you not read OP's description of the experiment? There is no acceleration. Are you able to understand that case? 3 inertial clocks, passing each other at 3 separate events.

On 1/23/2020 at 7:00 AM, between3and26characterslon said:

My understanding of SR (limited though it may be) is that it relies on strict application of rules. I believe SR only makes sense to an inertial observer, one who can be considered at rest and about which measurements are made/calculated.

SR involves many measures that are "invariant", and can be made sense of by all observers, including accelerated ones. Other measurements can be calculated for different observers. An accelerating observer can usually be treated as having a "momentarily comoving inertial frame" at any instant.

On 1/23/2020 at 3:52 AM, swansont said:

Yes. If your issue with the paradox is that relativity doesn't explain it, you need the experiment to eliminate the possibility that the clocks are simply running at different inherent rates. Experiments need to be able to confirm what you're looking for while also eliminating alternate explanations.

I disagree. The proper time on a world line is invariant. You don't need to compare two clocks to measure it.

Also I think you misunderstand. Of course relativity resolves the twin paradox, neither I nor OP is arguing against that. The measurements described in OP's experiment are 1) completely consistent with special relativity, and 2) the only possible values that are consistent with each other (if you change one of the values like speed, distance, or time, you'd have to change another to keep it consistent). There's no need for alternative explanations, unless you disagree with SR.

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1 hour ago, md65536 said:

I disagree. The proper time on a world line is invariant. You don't need to compare two clocks to measure it.

This isn't about relativity, as such, but about doing a proper experiment. If you don't do that comparison you haven't demonstrated that the differences aren't in the hardware. The clocks are uncalibrated. Such an experiment wouldn't pass peer-review.

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4 hours ago, md65536 said:

Did you not read OP's description of the experiment? There is no acceleration. Are you able to understand that case? 3 inertial clocks, passing each other at 3 separate events.

SR involves many measures that are "invariant", and can be made sense of by all observers, including accelerated ones. Other measurements can be calculated for different observers. An accelerating observer can usually be treated as having a "momentarily comoving inertial frame"

Did you not read his comment concerning changing inertial reference frames ?

That isn't describing a commoving inertial frame.

Let's stick to how inertial frames are applied compared to non inertial frames for a turnaround twin.

4 hours ago, scuddyx said:

Thanks for the link in your post.  Like most twin paradox explanations it says the spaceship carrying one twin experiences forces as it turns around and finishes with the question "what role does the acceleration play in this?"

The twin paradox can be explained without resorting to acceleration by merely by showing there is simply a change in inertial frame. No forces (GR) are required. SR is sufficient.

In particular in regards to this post which I was replying to.

Now you tell me how a commoving inertial frame handles the turnaround to preserve an inertial frame.

Particularly since commoving inertial frames is a technique to specifically handle acceleration.

However let's deal with a further issue.

There  are  two  quantities  called  “acceleration”:  Three-acceleration  and  four-acceleration.  Three acceleration  is  defined as the  derivative  of the  coordinate  velocity  with respect  to  coordinate  time.  It is  a relative  acceleration  which  can  be  transformed away.

Four-acceleration  is  defined  as  the  derivative  of  the  four-velocity  with  respect  to  proper  time.  It  is  an absolute  acceleration  which  cannot  be  transformed  away.  Four-acceleration  is  the  acceleration  of  a particle  as  measured  in  an  instantaneous  inertial  rest  frame  of  the  particle.    Particles  falling  freely  have vanishing  four-acceleration.  A non-vanishing  four-acceleration  is due  to  non-gravitational forces.

Centrifugal acceleration has non vanishing four acceleration.

Edited by Mordred

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scuddyx,

So you point out that the Time-Handoff scenario using 3 inertial clocks (A, B, and C) produces the same aging outcome of an instantaneous turnabout (impossible) in the classic twins scenario, assuming (of course) that only the legs of transit common to both scenarios are considered. You therefore imply that acceleration may play no role in the relative aging.

It's true that the all inertial Time-Handoff scenario produces the very same relative aging as the Twins scenario of instantaneous turnabout.  But what the Time-Handoff scenario does not present, is what the Twin B records "for cosmic clocks in his own spacetime system during his own turnabout". It's there, that acceleration's role in the Twins scenario is explained. A proper acceleration produces a dynamic change in his own sense-of-simultaneity wrt all other cosmic bodies. For any complete description of relativity, the "entire experience" of all observers must be presented.  And the more difficult experience to explain is that of he who properly accelerates, because other factors come into play (dynamic change in relative simultaneity) that do not exist in all inertial scenarios.

There is always a debate as to whether acceleration plays any role in relative aging, since SR relates the relative measure of space and time on relative speed alone. It may have been pointed out already, however there can be no change in relative speed, without a change in at least one observer's own state of motion (acceleration).  For the case of a non-instantaneous turnabout, how can you show that the predictions made by both twins A & B will concur wrt the relative aging outcome?  You will find that the extent of aging of any clock is simply the accrual of proper time between the events (in the Twin's case, leave earth event, and return to earth event).  Since each observer declares himself the reference for all motion, and passing only thru time, his time accrual is nothing more or less than the length of his own worldline between those 2 events.  The events, just come to him as he waits. Because both twins (A & B) exist at both events, a valid relative aging comparison may be made between them.  The acrrued duration between the events is dependent on nothing but the length of one's own worldine in spacetime.

Best Regards,

Celeritas

Edited by Celeritas

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2 hours ago, Mordred said:

Did you not read his comment concerning changing inertial reference frames ?

I ask again, what is accelerating? Nobody has described a physical thing accelerating, not OP nor anyone else.

OP's experiment describes 3 inertial frames. I disagree with the description of "changing inertial frames" (unless someone can explain what changes inertial frames). For what OP is describing, it suffices to say "we are considering two different inertial frames" to describe clocks B and then C. If OP's explanation relies on a "change" then I disagree with that, because as Markus Hanke has suggested, one can compare the geometric lengths of the observers’ world lines in spacetime, and I say you get the same answer that OP gave.

I agree with the critique of OP's wording ("turnaround", "synchronized", "change" etc) because this is a topic that even some people who have expert understanding of relativity will refuse to accept, and anything ambiguous or interpretable in an unfavorable manner will be nitpicked to death. If OP's "explanation" relies on interpretation then I don't care about it.

However the results of the description of OP's experiment are predicted as described, by special relativity. That is, unless you purposefully ignore a reasonable interpretation of what OP describes and invent something else (like clocks slowed for some other reason than SR's time dilation, which by the way I think is a ridiculous justification for doubting the predictions of SR). That is why a topic like this is better presented with precise language and only claims that are incontrovertible. On the other hand, I doubt even then, that incontrovertible predictions of SR would be accepted here if they didn't fit with preconceived ideas.

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I don't think you realize I replied to his later post which I quoted in my last post.

If no acceleration is involved then one can simply apply the equivalence principle. However where would the paradox arise if all observers consider themself as being in an inertial frame as opposed to at rest as per SR.

In GR all frames are inertial to begin with.

Edited by Mordred

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3 hours ago, Mordred said:

If no acceleration is involved then one can simply apply the equivalence principle. However where would the paradox arise if all observers consider themself as being in an inertial frame as opposed to at rest as per SR.

It's described in the first post in this thread. There is no paradox in any twin experiment, only a surprising or confusing result of SR. As described by OP, paraphrased, the length of the inertial path from AB (where A and B pass) to BC plus the length of the inertial path from BC to AC, totals 8 years of proper time (whether or not a single clock follows the entire world line), while the length of the inertial path from AB to AC is 10 years long, for the given speeds and distance. Do you disagree with that? If so, what is your calculation? (Remember that nothing here accelerates.) That difference in path lengths may be similarly surprising? One might argue that this thread proves that it's confusing.

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