Clocks, rulers... and an issue for relativity

[Tim88]

 

 

 

You forgot to tell us what the traveler's simultaneity lines read between the 2010.2 earth year and the 2013.8 earth.

(You refer to Langevin p51 but that's only the story of the light beams reaching the observers. That's not the issue. We are interested in what the traveler simultaneity lines read during turnaround.)

 

During outbound journey the traveler's simultaneity lines read a real physical earth clock. That clock has less time than the traveler's clock. Same story for the inbound journey. No discussion about that. (In case you doubt: put a set of cameras along a railway track, synchronise the shutter of all the camera's. The camera takes a picture of the passing moving clock an inch in front of the camera lens: a real physical clock with hands showing less time.)

 

Does -per traveler simultaneity lines- the earth clock time turns fictitious at 2010.3 and return being real at 2013.8? You must be joking.

Or do you consider ALL the earth clock readings per simultaneity lines fictitious? If a car drives fast enough I feel -during a split second-the shorther moving car touching between two fingers. Time dilation and length contraction are real. Fly at high speed to the moon and you will be there after 1 minute of your life because the distance to cover contracted. Nothing fictitious. And in earth simultaneity (3D space) the traveler aged only one minut of his life. Nothing fictitious.

 

PS.

It's possible the traveler's simultaneity lines during turnaround are not straight lines (if we don't use momentarily co-moving inertial frames during deceleration/acceleration), but that doesn't change the issue; they still read real earth clock times. and have to cover times between 2010.3 and 2013.8

 

I did not forget to tell anything and my answer on your questions is no. [edit: and note that the traveler can freely choose his reference system; but that's again another topic!] Regretfully, I see that you totally missed my point... in my earlier illustration the pilots were also seeing real coastlines. I'll try a last illustration.

 

Two men are standing on the ground, and one is asking the other how classical mechanics works, as he has some doubts if the theory is self consistent. The other man explains it as follows: look, he says, if I now turn around then in my reference system the Earth is circling around me, and also the planets are swirling about wildly. I see it and I can plot this in a space-time diagram, so the wild acceleration of the planets is real.

 

Do you think that such is a good physical description of what happens according to classical mechanics, and that it serves to convince the other that classical mechanics is self consistent?

Edited September 13, 2016 by Tim88

[Celeritas]

Tim88,

Very sorry, circumstances have me moving very slowly as late ...

Wrt Langevin … I was talking about what exists IN only the B spacetime system, not an analysis based upon received light after the delay in its transit (eg Langevin's analysis), although that is no problem.

 

Wrt fictitious force … You’ve suggested that the superluminal relocation of twin A in the B system (at turnabout) is caused by a fictitious force. The only force "there is", is B’s proper acceleration(s). Twin A is always inertial. If the wild shifts of twin A (wrt location and time-readout) within only the B spacetime system are properly related to B’s transitioning of inertial frames, then A’s wild shifts are explained. If it can be shown that that force accounts for the wild shifts of A, then there is no fictitious force causing A’s superluminal shift (or rapid advancement of time-readout). B’s own thruster burn produces a change in relative motion, and hence a change in relativistic effects, wrt A. The force is real, and it is B’s measurable inertia.

 

Wrt valid physical description … While other analyses exist, I used the analysis of momentarily co-located and co-moving inertial reference frames (MCCIRF) wrt twin B. You pointed out that the sequential concatonation of an infinite collection-of-infitesimals of MCCIRFs (wrt B) is not any single real POV. I would say this … My scenario used a single B clock always at the B origin, and assumed twin B proper accelerations were instant. As such, my position is that the collective of MCCIRF infitesimals constitute how twin A exists in the twin B spacetime system. I mean, at any momentary co-location, twin B and the corresponding MCCIRF POV are at relative rest, and also receiving essentially the same EM from twin A. As such, the relativistic effects required within the B system during turnabout (that do not occur in the twin A system), must be the direct result of twin B’s own frame-transitioning, the related force being B’s own inertia at turnabout. I consider the analysis a valid representation of the B spacetime system (far as the A-clock within it), and therefore a valid representation of reality in that respect.

The stars seem to fly superluminally across the night sky, if one does not account for earthly rotation. A consideration that appropriately considers forces, reveals the earth rotating and the stars essentially fixed. Certainly, twin B rotates in spacetime when properly accelerating. Twin B may use the twin A system as the reference for all motion, if he so chooses, or he may use his own. However, using his own not-always-inertial system without consideration of force, is to consider kinematics alone, and so spacetime predictions could not be correct, nor consistent with the predictions of all inertial POVs (who do all agree). The MCCIRF method builds a twin B POV from inertial POVs, and thus must be consistent with all inertial observers. As such, it explains how B's frame-transitioning at turnabout relates to the always-inertial POVs. That is, the mapping of twin A in the B spacetime system is a function of B's proper acceleration during turnabout. It's built-in, using the MCCIRF method. This is not to say that the MCCIRF is the only method.

Wrt violations of physics … Twin A is always inertial, and so there is no violation of c there. Twin B is always at v<c wrt A, and so no violation of c there. The only real question, is … does the superluminal shift (or wildly fast time readout advancement) of twin A in only twin B’s spacetime system violate the cosmic speed limit? I say no …

 

The superluminal shifting of A within the B system, is caused only by a rapid shift in B’s own POV, and the theory requires it. Otherwise, B could not successfully predict that A ages more than he (for say a pre-planned scenario), upon return. To suggest such wild shifts cannot be real, is to say that relativistic effects as per SR cannot be real, and no worthy relativist believes that. All that’s required to understand how twin A can move superluminally in the B system without being a violation of physics, is to understand that all moments in time co-exist as inches on a ruler do. Ie it’s a continuum of BOTH space and time, not just space. Twin A essentially fast-forwards along his own dynamically rotating worldline in ONLY the B spacetime system during B’s own transitioning-of-frames at turnabout, and his own inertia (a measurable force, hence real) is the source of that. Yet, in A's own spacetime system, the rate of proper-time is always steady as usual. The wild shifts of A in B's system are due to a change in the way B measures space and time, ie a twin B POV change, as the direct result of a change in B's own state of motion.

VandD's twin's related spacetime figure (in the prior post) presents this rather elegantly.

 

Best regards,

Celeritas

Edited September 14, 2016 by Celeritas

[Tim88]

 

Tim88,

Very sorry, circumstances have me moving very slowly as late ...

Wrt Langevin … I was talking about what exists IN only the B spacetime system, not an analysis based upon received light after the delay in its transit (eg Langevin's analysis), although that is no problem.

 

Wrt fictitious force … You’ve suggested that the superluminal relocation of twin A in the B system (at turnabout) is caused by a fictitious force. The only force there is, is B’s proper acceleration(s). Twin A is always inertial. If the wild shifts of twin A (wrt location and time-readout) within only the B spacetime system are properly related to B’s transitioning of inertial frames, then A’s wild shifts are explained. If it can be shown that that force accounts for the wild shifts of A, then there is no fictitious force causing A’s superluminal shift (or rapid advancement of time-readout). B’s own thruster burn produces a change in relative motion and hence in relativistic effects wrt A. The force is real, and it’s B’s measurable inertia.

 

Wrt valid physical description … While other analyses exist, I used the analysis of momentarily co-located and co-moving inertial reference frames (MCCIRF) wrt twin B. You pointed out that the sequential concatonation of an infinite collection-of-infitesimals of MCCIRFs (wrt B) is not any single real POV. I would say this … My scenario used a single B clock always at the B origin, and assumed twin B proper accelerations were instant. As such, my position is that the collective of MCCIRF infitesimals constitute how twin A exists in the twin B spacetime system. I mean, at any momentary co-location, twin B and the corresponding MCCIRF POV are at relative rest, and also receiving essentially the same EM from twin A. As such, the relativistic effects required within the B system during turnabout (that do not occur in the twin A system), must be the direct result of twin B’s own frame-transitioning, the related force being B’s own inertia at turnabout. I consider the analysis a valid representation of the B spacetime system (far as the A-clock within it), and therefore a valid representation of reality in that respect.

The stars seem to fly superluminally across the night sky, if one does not account for earthly rotation. A consideration that appropriately considers forces, reveals the earth rotating and the stars essentially fixed. Certainly, twin B rotates in spacetime when properly accelerating. Twin B may use the twin A system as the reference for all motion, if he so chooses, or he may use his own. However, using his own not-always-inertial system without consideration of force, is to consider kinematics alone, and so spacetime predictions could not be correct, nor consistent with the predictions of all inertial POVs (who do all agree). The MCCIRF method builds a twin B POV from inertial POVs, and thus must be consistent with all inertial observers. As such, it explains how B's frame-transitioning at turnabout relates to the always-inertial POVs. That is, the mapping of twin A in the B spacetime system is a function of B's proper acceleration during turnabout. It's built-in, using the MCCIRF method. This is not to say that the MCCIRF is the only method.

Wrt violations of physics … Twin A is always inertial, and so there is no violation of c there. Twin B is always at v<c wrt A, and so no violation of c there. The only real question, is … does the superluminal shift (or wildly fast time readout advancement) of twin A in only twin B’s spacetime system violate the cosmic speed limit? I say no …

 

The superluminal shifting of A within the B system, is caused only by a rapid shift in B’s own POV, and the theory requires it. Otherwise, B could not successfully predict that A ages more than he (for say a pre-planned scenario), upon return. To suggest such wild shifts cannot be real, is to say that relativistic effects as per SR cannot be real, and no worthy relativist believes that. All that’s required is to understand how twin A can move superluminally in the B system without being a violation of physics, is to understand that all moments in time co-exist as inches on a ruler do. Ie it’s a continuum of BOTH space and time, not just space. Twin A essentially fast-forwards along his own dynamically rotating worldline in ONLY the B spacetime system during B’s own transitioning-of-frames at turnabout, and his own inertia (a measurable force, hence real) accounts for that. In A's own spacetime system, the rate of proper-time is always steady as usual. The wild shifts of A in B's system are due to a change in the way B measures space and time, ie a twin B POV change, as the direct result of a change in B's own state of motion.

VandD's twin's related spacetime figure (in the prior post) presents this rather elegantly.

 

Best regards,

Celeritas

 

 

Hi Celeritas, this has nothing to do with methods or figures and we clearly agree about the physical explanation. It's mainly about the meaning of words, and how to phrase things when we explain physics.

The issue came up in the spun off thread, when Michel remarked that he understood from some posts the contrary to what I explained. I thus started looking for phrasings that I consider improper, inconsistent physical descriptions which can be misunderstood as implying the contrary of what you and I clearly agree on.

 

However the discussion belongs to two different threads: this thread concerning consistent physical descriptions, and the spun off thread concerning suggested multiple realities. Also, you understood my illustrations wrongly: for example, I did not suggest that anything is caused by a fictitious force, but argued that your way of explaining corresponds to such descriptions which are not physically sound (fictitious things are things that do not exist in physical reality).

 

I will ponder about replying in detail in this thread or in the other thread; it depends in part on robinpike, if he comments on this issue or not.

And I will wait for your comments on my two other illustrations.

 

Best Regards,

Tim88

Edited September 14, 2016 by Tim88

[michel123456]

 

(...)

The issue came up in the spun off thread, when Michel remarked that he understood from some posts the contrary to what I explained. I thus started looking for phrasings that I consider improper, inconsistent physical descriptions which can be misunderstood as implying the contrary of what you and I clearly agree on.

(...)

Tim88

It continues.

I am confused by Langevin, I will open a new thread that will not disturb this excellent one.

---------------------

done. http://www.scienceforums.net/topic/98501-lost-in-langevins-language/

Edited September 14, 2016 by michel123456

[VandD]

 

To visualize the importance of relativity of simultaneity for the symmetry of length contraction, I added a long spaceship, length at rest = length of atmosphere at rest.

The muon sits on the nose (or flies alongside rocket's nose). Relative speed of rocket = relative speed muon

Mr Green is in rocket, Mr Red at rest in atmosphere.

 

First diagram: In Mr Green"s 3D space of simultaneous events "the collection of simultaneous rocket events" has rest length. The collection of simultaneous atmosphere events in Mr Greens 3D space has shorter length than the collection of simultaneous events of the "at rest" atmosphere in Mr Red's 3D space.

 

Second diagram: In Mr Red's 3D space of simultaneous events the "collection of simultaneous atmosphere events" has rest length. The collection of simultaneous rocket events in Mr Red's 3D space has shorter length than the collection of simultaneous events of the "at rest" rocket in Mr Green's 3D space.

 

The "at rest rocket" for Mr Green and the "at rest atmosphere" for Mr Red have different slope in 4D spacetime. I.o.w. they are not part of the same 3D space of simultaneous events.

 

Event C (for example a baby born on earth surface/lower atmosphere) happens after event B (a bird sits a split second on the roof).

For Mr Green the nose of the rocket (= muon) hits the atmosphere (=event A) when the baby is born (event C). No bird on the roof (the bird was on the roof some time ago).

For Mr Red the nose of the rocket (= muon) hits the atmosphere (=event A) when the bird sits on the roof (event B). No baby is born (yet).

Michel, have you made any effort in understanding what's shown in a Loedel or Minkowski diagram?

 

Have you tried Lorentz Transformations?

Edited September 14, 2016 by VandD

[michel123456]

1.Michel, have you made any effort in understanding what's shown in a Loedel or Minkowski diagram?

 

2.Have you tried Lorentz Transformations?

1 yes I hope so, see below

2.yes

 

I am still struggling with simultaneity, that is to say: the difference between simultaneity lines and lines representing ray of lights. It seems to me that in some explanations simultaneity is understood as events that are observed as happening at the same time and that is not my understanding. Maybe it is worth a separate thread.

[Janus]

1 yes I hope so, see below

2.yes

 

I am still struggling with simultaneity, that is to say: the difference between simultaneity lines and lines representing ray of lights. It seems to me that in some explanations simultaneity is understood as events that are observed as happening at the same time and that is not my understanding. Maybe it is worth a separate thread.

Simple example: You have two observers(Stan and Morton) with a relative speed between them. At the moment they pass each other they both simultaneously see flashes that originated from points that were equidistant from Stan at the moment of origin. We will call the origin of one flash Event A, the origin the other flash event B and the beams arriving at Stan and Morton as event C.

 

Then the two space time diagrams below show events as they occur according to Stan and Morton. Stan's view is on the left and Morton's in on the right.

 

 

According to Stan, the flashes, having originated at points an equal distance from him, originated at the same time since he sees them at the same time, both having occurred an equal time before event C..

For Morton, who also sees the flashes at the same time, the flashes could not have originated simultaneously. In order for the flashes to arrive at his eyes at the same time, he had to be much further from event B when it occurred than he was to event A when it occurred. The light from event B took longer to get to him than the light from event A, so event B must have taken place before event A.

 

Even though both Stan and Morton see the flashes at the same time, they come to different conclusions as to whether the origin of the flashes were simultaneous.

Edited September 14, 2016 by Janus

[VandD]

Here I put all the information in one diagram.

Check how the green simultaneity lines frame (Stan's) and the blue simultaneity lines (Morton's) move through spacetime.

 

 

I am still struggling with simultaneity, that is to say: the difference between simultaneity lines and lines representing ray of lights.

They are totally different.

The lines representing ray of light represents the path of a photon going from one location to another.

A simultaneity line represents all the events that occur simultaneously (and thus exist) for an observer at one specific moment in time out of observer's life

It seems to me that in some explanations simultaneity is understood as events that are observed as happening at the same time

This is only if the events are located at the same distance from you. If lights from those events reach you at the same instant of time, then the events occured simultaneously for you (your ref system).

 

when you look at the stars, all that light hit your retina at the same time, but because the distances are different fom your eye, the events when the light left the star didn't occur simultaneously for you.

 

Below I added the paths of light leaving the event 'nose of rocket / muon' hits upper atmosphere of previous discussion.

Following the yellow lines you can read where and when the photons are is both frames.

You slowly get the picture why I love spacetime diagrams?

Edited September 14, 2016 by VandD

[Celeritas]

 

Hi Celeritas, this has nothing to do with methods or figures and we clearly agree about the physical explanation. It's mainly about the meaning of words, and how to phrase things when we explain physics.

 

I see. I went thru thread thread more closely from the OP. Semantics.

 

 

The issue came up in the spun off thread, when Michel remarked that he understood from some posts the contrary to what I explained. I thus started looking for phrasings that I consider improper, inconsistent physical descriptions which can be misunderstood as implying the contrary of what you and I clearly agree on.

 

Indeed, that's always a problem when folks are trying to learn the theory. I was there

 

 

However the discussion belongs to two different threads: this thread concerning consistent physical descriptions, and the spun off thread concerning suggested multiple realities.

 

Got it, thanx.

 

 

Also, you understood my illustrations wrongly: for example, I did not suggest that anything is caused by a fictitious force, but argued that your way of explaining corresponds to such descriptions which are not physically sound (fictitious things are things that do not exist in physical reality).

 

I'm assuming that you are referring to the wild shifts in the A-clock location and time-readout in only the twin B spacetime system during B's proper-turnabout? Said effects are logically deduced, but not directly measured. I do understand that on the grounds of "not measurable", one might argue it is not real. I disagree on that particular matter, however I do understand the complaint

 

 

I will ponder about replying in detail in this thread or in the other thread; it depends in part on robinpike, if he comments on this issue or not.

And I will wait for your comments on my two other illustrations.

 

I've looked thru the 2 threads, and did not notice your 2 illustrations you mention here. Could you please give me a hyperlink to those, and I'll take a look. Thanx

 

Best regards,

Celeritas

[Tim88]

 

I see. I went thru thread thread more closely from the OP. Semantics.

 

Indeed, that's always a problem when folks are trying to learn the theory. I was there

 

A decade ago I was there too, even though I was supposed to know the theory. Probably most of us where there.

I'm assuming that you are referring to the wild shifts in the A-clock location and time-readout in only the twin B spacetime system during B's proper-turnabout? Said effects are logically deduced, but not directly measured. I do understand that on the grounds of "not measurable", one might argue it is not real. I disagree on that particular matter, however I do understand the complaint

 

 

I've looked thru the 2 threads, and did not notice your 2 illustrations you mention here. Could you please give me a hyperlink to those, and I'll take a look. Thanx

 

Best regards,

Celeritas

 

My objection, as I tried to illustrate, is really about the phrasing. I gave three illustrations, one of which you apparently interpreted inversely of how I meant it (is the rotating frame according to classical mechanics suited for not just geometrical, but also for physical descriptions?); the other two are in that same post and in a following one.

 

- post 247, the "PS" http://www.scienceforums.net/topic/97466-clocks-rulers-and-an-issue-for-relativity/page-13#entry942987

- post 251 http://www.scienceforums.net/topic/97466-clocks-rulers-and-an-issue-for-relativity/page-13#entry943345

[robinpike]

I will try to be as clear as possible so that my use of the terms “apparent” and “real” are not misunderstood. By way of example, the following context shows how I intend to use these terms…

 

If I measure the height of my brother, who say is standing on the far side of an athletics stadium, by sighting him against a ruler held at arm’s length, his height perhaps comes out as 1 inch. Although that is a real measurement, it is only an apparent height. To determine his proper height, I can do some calculations based on distance and perspective and from those infer that his height is 6 feet, but this would still be an apparent height (this apparent height could be the same as the real height, but it doesn’t have to be).

 

To be sure of getting his real height, I need to walk up to my brother and measure his height while standing next to him, which say comes out as 6 feet. This measurement of 6 feet then is his real height (although you might note that this value is only real with respect to the feet and inches marked on my ruler).

 

If my brother then does a lap around the track and on returning to me, I measure his height again when he is standing next to me and find that his height is now 5 feet 11 inches (his shoes fell off while running) – that measurement is now the new real value. We know that it is a new real value because the ruler has not changed - so therefore it is his height that has changed.

 

But his running coach disagrees with my measurement – using his ruler the coach stands next to my brother and measures my brother’s height as 5 feet 10.5 inches. So which measurement is to be taken as my brother’s real height?

 

To side-step this dilemma, the coach and I use our respective measurements before and after my brother ran around the track, to determine only whether my brother’s height got shorter, or got taller, or did not change. So now the coach and I can agree: my brother’s height got shorter and that this change in his height was a real change. This avoids any arguments as to the amount that his height changed, and who has the real measure of feet and inches on their ruler.

 

===============================================================

 

I thank people for the diagrams posted, but if I may, I would like to continue with a verbal description of the issue (bearing in mind the above example of how I want to use the terms apparent and real).

 

To briefly summarise the travelling clock scenario…

 

The initial conditions are that the travelling clock and the stay at home clock are stationary next to each other, both ticking at the same rate, and both showing the same time. The final position is that the travelling clock has completed its round trip and is again stationary next to the stay at home clock, both ticking at the same rate, but now the travelling clock shows less time than the stay at home clock.

 

When the travelling clock goes away and comes back to the stay at home clock, the one thing that can be stated with certainty as being real, is that the travelling clock lost time with respect to the stay at home clock.

 

When the travelling clock starts its journey and accelerates away from the stay at home clock, somewhere along its journey the travelling clock must lose time compared to the stay at home clock (rather than the stay at home clock gaining time), because the loss in time is real and nothing changes for the stay at home clock.

 

From the point of view of both clocks, during the first part of its journey, the travelling clock is moving away from the stay at home clock. But once its acceleration stops, who can say which clock is moving and which clock is stationary? In fact, while the clocks are moving away from each other, both clocks see the other clock as running slow.

 

After the travelling clock has turned around to return back to the stay at home clock, both clocks now see the other clock as running fast, although there is a period at the beginning of the return trip when the travelling clock sees the stay at home clock running fast while the stay at home clock still sees the travelling clock running slow.

 

===============================================================

 

Note that there is no issue with the calculations of relativity, as these agree with the amount of time lost by the travelling clock, with respect to the stay at home clock. The issue is a logical contradiction that appears to be caused by relativity, revealed when a method is chosen as to how the travelling clock loses the time. Working through the chosen method reveals a contradiction in logic – which at first simply suggests that the chosen method is invalid. So another method is chosen, and again the same contradiction in logic is found, and so on. This leads to the general conclusion that any chosen method will result in the logical contradiction. But the loss in time is real so there must be a method by which the travelling clock loses the time. This suggests that the fault lies somewhere else – and the obvious candidate for that is relativity itself.

 

To demonstrate the logical contradiction, I will start with the (most) obvious method of how time is lost: the travelling clock’s time runs at a slower rate (overall) during its round trip. Let’s see how this assumed method causes the issue by using one stay at home clock and two travelling clocks.

 

By the way, if you dislike the examples that I have chosen, by all means post an alternative, describing how the travelling clock loses time and where in its journey that loss in time occurs, and I will show how the logical contradiction applies to your example.

 

The two travelling clocks are initially stationary and synchronized against the stay at home clock. They start the round trip by accelerating side-by-side away from the stay at home clock (i.e. they increase their relative speed with respect to the stay at home clock). Since the assumption (in this example) is that the real loss in time is due to the travelling clock’s time running slow, during the trip to the half-way point, their clocks will now be running slow (with respect to the stay at home clock).

 

At the half-way point, one of the travelling clocks de-accelerates (i.e. reduces its speed with respect to the stay at home clock) to the point where it becomes stationary with respect to the stay at home clock. This means that the travelling clock’s time now runs at the same rate as the stay at home clock’s rate of time.

 

But when the travelling clock de-accelerated, it effectively accelerated away from the second travelling clock (i.e. it increased its relative speed with respect to the second travelling clock). But such an acceleration is just a repeat of the scenario when the travelling clocks first accelerated away from the stay at home clock (although now with respect to different reference frames). This means that the de-accelerated clock’s time has to run slow with respect to the second travelling clock’s rate of time, which already is running slow with respect to the stay at home clock’s rate of time – AND yet the de-accelerated clock’s rate of time has to be at the same rate as the stay at home clock’s rate of time (because they are stationary with respect to each other).

 

If these two conditions were apparent effects – such a scenario would be possible. But the loss in time is real and so these two conditions cannot occur together – hence a logical contradiction has occurred.

 

You can try to avoid this logical contradiction by explain the travelling clock’s real loss in time by other methods, but I think they will all fail for the same reason.

 

For example, try the loss in time is because the travelling clock’s progress through space-time is shorter than the stay at home clock’s progress through space-time. This fails at the turn around point because the de-accelerated clock’s progress through space-time has to be shorter with respect to the second travelling clock’s progress through space-time, which already is shorter with respect to the stay at home clock’s progress through space-time – AND yet the de-accelerated clock’s progress through space-time has to be at the same rate as the stay at home clock’s progress through space-time (because they are stationary with respect to each other).

.

[Endy0816]

Not going to work out as being a paradox if you go by the math/reality.

Edited September 15, 2016 by Endy0816

[robinpike]

Not going to work out as being a paradox if you go by the math/reality.

 

Can you give details of the reality - that will enable me to comment - thanks.

[Celeritas]

I will try to be as clear as possible so that my use of the terms “apparent” and “real” are not misunderstood. By way of example, the following context shows how I intend to use these terms…

 

If I measure the height of my brother, who say is standing on the far side of an athletics stadium, by sighting him against a ruler held at arm’s length, his height perhaps comes out as 1 inch. Although that is a real measurement, it is only an apparent height. To determine his proper height, I can do some calculations based on distance and perspective and from those infer that his height is 6 feet, but this would still be an apparent height (this apparent height could be the same as the real height, but it doesn’t have to be).

 

To be sure of getting his real height, I need to walk up to my brother and measure his height while standing next to him, which say comes out as 6 feet. This measurement of 6 feet then is his real height (although you might note that this value is only real with respect to the feet and inches marked on my ruler).

 

If my brother then does a lap around the track and on returning to me, I measure his height again when he is standing next to me and find that his height is now 5 feet 11 inches (his shoes fell off while running) – that measurement is now the new real value. We know that it is a new real value because the ruler has not changed - so therefore it is his height that has changed.

 

But his running coach disagrees with my measurement – using his ruler the coach stands next to my brother and measures my brother’s height as 5 feet 10.5 inches. So which measurement is to be taken as my brother’s real height?

 

To side-step this dilemma, the coach and I use our respective measurements before and after my brother ran around the track, to determine only whether my brother’s height got shorter, or got taller, or did not change. So now the coach and I can agree: my brother’s height got shorter and that this change in his height was a real change. This avoids any arguments as to the amount that his height changed, and who has the real measure of feet and inches on their ruler.

 

 

You must also compare your, your brother's, and his coach's rulers for a relative length comparison, before and after each jog.

 

Best regards,

Celeritas

[robinpike]

 

You must also compare your, your brother's, and his coach's rulers for a relative length comparison, before and after each jog.

 

Best regards,

Celeritas

 

By choosing to measure only whether my brother's height has changed after his jog - and not by how much - removes the necessity to compare rulers. As long as nothing happens to the rulers (that is they do not move in the context of relativity), then each ruler can be used to detect whether my brother's height has changed or not changed.

Edited September 16, 2016 by robinpike

[Celeritas]

 

Note that there is no issue with the calculations of relativity, as these agree with the amount of time lost by the travelling clock, with respect to the stay at home clock. The issue is a logical contradiction that appears to be caused by relativity, revealed when a method is chosen as to how the travelling clock loses the time. Working through the chosen method reveals a contradiction in logic – which at first simply suggests that the chosen method is invalid. So another method is chosen, and again the same contradiction in logic is found, and so on. This leads to the general conclusion that any chosen method will result in the logical contradiction. But the loss in time is real so there must be a method by which the travelling clock loses the time. This suggests that the fault lies somewhere else – and the obvious candidate for that is relativity itself.

...

 

By the way, if you dislike the examples that I have chosen, by all means post an alternative, describing how the travelling clock loses time and where in its journey that loss in time occurs, and I will show how the logical contradiction applies to your example.

 

Per SR ...

 

in A's spacetime system ... moving clocks must tick slower per any inertial POV, and twin A (on Earth) is always inertial. As such, it matters not that twin B changes in his own state of motion, ie he moves relatively during both thruster burns and coasting. Only at one instant at turnabout (say at planet X) does twin B momentarily arrive back into the twin-A frame, but even then B's clock ticks at the same rate as A's at that instant, not faster. So the B clock always ticks slower, except for an instant. Over the roundtrip, B clock MUST age less.

 

in B's spacetime system ... B properly accelerates to depart from A, and so the relative velocity builds. Twin B then records the separation between Earth and planet X in motion, and any moving-length contracts from its proper length. The greater the v, the closer planet X gets. We can assume virtually instant B accelerations, for simplicity. The shorter the distance to planet X (ie x'), the shorter the time to get there at v (since t' = x'/v). The same applies for the return leg of B's trip. Over the roundtrip, B must age less than A.

 

As such, both twins agree that B ages less than A.

 

Best regards,

Celeritas

Edited September 16, 2016 by Celeritas

[bvr]

Not going to work out as being a paradox if you go by the math/reality.

 

 

But it's not a mathematical problem, it's a logical issue. You can't say "do the math, forget the logic".

I am impressed, I don't see an error in robinpike's reasoning.

 

 

 

As such, bot twins agree on the outcome, per SR.

 

 

 

Nobody doubts about that. The problem is here:

 

At the half-way point, one of the travelling clocks de-accelerates (i.e. reduces its speed with respect to the stay at home clock) to the point where it becomes stationary with respect to the stay at home clock. This means that the travelling clock’s time now runs at the same rate as the stay at home clock’s rate of time.

 

But when the travelling clock de-accelerated, it effectively accelerated away from the second travelling clock (i.e. it increased its relative speed with respect to the second travelling clock). But such an acceleration is just a repeat of the scenario when the travelling clocks first accelerated away from the stay at home clock (although now with respect to different reference frames). This means that the de-accelerated clock’s time has to run slow with respect to the second travelling clock’s rate of time, which already is running slow with respect to the stay at home clock’s rate of time – AND yet the de-accelerated clock’s rate of time has to be at the same rate as the stay at home clock’s rate of time (because they are stationary with respect to each other).

 

 

 

After the de-acceleration, B runs at the same rate as A, but also slow wrt to C, wich travelled with him and still runs slow wrt to A.

[Tim88]

Hi Robin, as your post is long and consists of two, or even three parts, I'll answer in two posts. I quickly read your whole post and now will slightly improve some of your phrasings in bold. Possibly that already matters for what follows.

 

[bold face: tim88]

 

I will try to be as clear as possible so that my use of the terms “apparent” and “real” are not misunderstood. By way of example, the following context shows how I intend to use these terms…

 

If I measure the height of my brother, who say is standing on the far side of an athletics stadium, by sighting him against a ruler held at arm’s length, his height perhaps comes out as 1 inch. Although that is a real measurement, it is only an apparent height. To determine his proper height, I can do some calculations based on distance and perspective and from those infer that his height is 6 feet, but this would still be an apparent height (this apparent height could be the same as the real height, but it doesn’t have to be).

 

To be sure of getting his real height, I need to walk up to my brother and measure his height while standing next to him, which say comes out as 6 feet. This measurement of 6 feet then is his real height (although you might note that this value is only real with respect to the feet and inches marked on my ruler). Note that here the assumption was made that the ruler is not affected by its displacement (temperature, gravitational potential, etc); however there is no need to displace it.

 

If my brother then does a lap around the track and on returning to me, I measure his height again when he is standing next to me and find that his height is now 5 feet 11 inches (his shoes fell off while running) – that measurement is now the new real value. We know that it is a new real value because the ruler has not changed - so therefore it is his height that has changed.

 

But his running coach disagrees with my measurement – using his ruler -with which also nothing happened- the coach stands next to my brother and measures my brother’s height as 5 feet 10.5 inches. So which measurement is to be taken as my brother’s real height?

 

To side-step this dilemma, the coach and I use our respective measurements before and after my brother ran around the track, to determine only whether my brother’s height got shorter, or got taller, or did not change. So now the coach and I can agree: my brother’s height got shorter and that this change in his height was a real change. This avoids any arguments as to the amount that his height changed, and who has the real measure of feet and inches on their ruler.

 

===============================================================

 

I thank people for the diagrams posted, but if I may, I would like to continue with a verbal description of the issue (bearing in mind the above example of how I want to use the terms apparent and real).

 

To briefly summarise the travelling clock scenario…

 

The initial conditions are that the travelling clock and the stay at home clock are stationary next to each other, both ticking at the same rate, and both showing the same time. The final position is that the travelling clock has completed its round trip and is again stationary next to the stay at home clock, both ticking at the same rate, but now the travelling clock shows less time than the stay at home clock.

 

When the travelling clock goes away and comes back to the stay at home clock, the one thing that can be stated with certainty as being real, is that the travelling clock is lagging behind with respect to the stay at home clock.

 

When the travelling clock starts its journey and accelerates away from the stay at home clock, somewhere along its journey the travelling clock must lag behind on the stay at home clock (rather than the stay at home clock gaining time), because the loss in time is real and nothing changes for the stay at home clock.

 

Now we assume that after the acceleration has ended, the traveller sets up an independent reference system S1 with his rulers and clocks; the stay at home observer maintains his Earth based reference system S0.

From the point of view of both reference systems, during the first part of its journey, the distance between the clocks is increasing. But now, who can say which clock is moving and which clock is stationary? In fact, while the clocks are moving away from each other, both reference systems assume that the other clock is running slow.

 

After the travelling clock has turned around to return back to the stay at home clock, the traveller may choose to again set up a new, independent reference system S3 with his rulers and clocks in their new state of motion.

Both reference systems (S0 and S3) now see the other clock as running fast; a direct comparison of received frequencies shows that the frequency received from the far away clock is higher than that of the local clock. However, accounting for the Doppler effect, each infers that the other clock is running slow. There is a period at the beginning of the return trip when the travelling system sees the stay at home clock running fast while the stay at home system still sees the travelling clock running slow.

[..]

 

If you agree with the modifications, I will next consider your contradiction arguments.

[robinpike]

Tim88 - sure your clarifications are most helpful. The key thing to keep in mind when discussing the steps in the travelling clock's round trip, is that the loss in time for the travelling clock is a real change. Of course, there are apparent changes going on as well - and clarity is required to not inadvertently merge these two things together.

 

 

But it's not a mathematical problem, it's a logical issue. You can't say "do the math, forget the logic".

 

 

 

By the way bvr, thank you for taking time to understand the points in my post - much appreciated.

[Tim88]

Retake for clarity:

robinpike, on 15 Sept 2016 - 4:39 PM, said:

[bold face: tim88]

 

I will try to be as clear as possible so that my use of the terms “apparent” and “real” are not misunderstood. By way of example, the following context shows how I intend to use these terms…

 

If I measure the height of my brother, who say is standing on the far side of an athletics stadium, by sighting him against a ruler held at arm’s length, his height perhaps comes out as 1 inch. Although that is a real measurement, it is only an apparent height. To determine his proper height, I can do some calculations based on distance and perspective and from those infer that his height is 6 feet, but this would still be an apparent height (this apparent height could be the same as the real height, but it doesn’t have to be).

 

To be sure of getting his real height, I need to walk up to my brother and measure his height while standing next to him, which say comes out as 6 feet. This measurement of 6 feet then is his real height (although you might note that this value is only real with respect to the feet and inches marked on my ruler). Note that here the assumption was made that the ruler is not affected by its displacement (temperature, gravitational potential, etc); however there is no need to displace it.

 

If my brother then does a lap around the track and on returning to me, I measure his height again when he is standing next to me and find that his height is now 5 feet 11 inches (his shoes fell off while running) – that measurement is now the new real value. We know that it is a new real value because the ruler has not changed - so therefore it is his height that has changed.

 

But his running coach disagrees with my measurement – using his ruler -with which also nothing happened- the coach stands next to my brother and measures my brother’s height as 5 feet 10.5 inches. So which measurement is to be taken as my brother’s real height?

 

To side-step this dilemma, the coach and I use our respective measurements before and after my brother ran around the track, to determine only whether my brother’s height got shorter, or got taller, or did not change. So now the coach and I can agree: my brother’s height got shorter and that this change in his height was a real change. This avoids any arguments as to the amount that his height changed, and who has the real measure of feet and inches on their ruler.

 

===============================================================

 

I thank people for the diagrams posted, but if I may, I would like to continue with a verbal description of the issue (bearing in mind the above example of how I want to use the terms apparent and real).

 

To briefly summarise the travelling clock scenario…

 

The initial conditions are that the travelling clock and the stay at home clock are stationary next to each other, both ticking at the same rate, and both showing the same time. The final position is that the travelling clock has completed its round trip and is again stationary next to the stay at home clock, both ticking at the same rate, but now the travelling clock shows less time than the stay at home clock.

 

When the travelling clock goes away and comes back to the stay at home clock, the one thing that can be stated with certainty as being real, is that the travelling clock is lagging behind with respect to the stay at home clock.

 

When the travelling clock starts its journey and accelerates away from the stay at home clock, somewhere along its journey the travelling clock must lag behind on the stay at home clock (rather than the stay at home clock gaining time), because the loss in time is real and nothing changes for the stay at home clock.

 

Now we assume that after the acceleration has ended, the traveller sets up an independent reference system S1 with his rulers and clocks; the stay at home observer maintains his Earth based reference system S0.

From the point of view of both reference systems, during the first part of its journey, the distance between the clocks is increasing. But now, who can say which clock is moving and which clock is stationary? In fact, while the clocks are moving away from each other, both reference systems assume that the other clock is running slow.

 

After the travelling clock has turned around to return back to the stay at home clock, the traveller may choose to again set up a new, independent reference system S3 with his rulers and clocks in their new state of motion.

Both reference systems (S0 and S3) now see the other clock as running fast; a direct comparison of received frequencies shows that the frequency received from the far away clock is higher than that of the local clock. However, accounting for the Doppler effect, each infers that the other clock is running slow. There is a period at the beginning of the return trip when the travelling system sees the stay at home clock running fast while the stay at home system still sees the travelling clock running slow.

[..]

 

 

Tim88 - sure your clarifications are most helpful. The key thing to keep in mind when discussing the steps in the travelling clock's round trip, is that the loss in time for the travelling clock is a real change. Of course, there are apparent changes going on as well - and clarity is required to not inadvertently merge these two things together.

 

OK, your next description has some similarities with the last part of my post #116 :

http://www.scienceforums.net/topic/97466-clocks-rulers-and-an-issue-for-relativity/page-6#entry940714

But I'll continue with your description here, doing just as with the preceding part:

 

robinpike, bold face text by tim88

===============================================================

 

Note that there is no issue with the calculations of relativity, as these agree with the amount of retardation of the travelling clock, with respect to the stay at home clock. The issue is a logical contradiction that appears to be caused by relativity, revealed when a method is chosen as to how the travelling clock retards. Working through the chosen method reveals a contradiction in logic – which at first simply suggests that the chosen method is invalid. So another method is chosen, and again the same contradiction in logic is found, and so on. This leads to the general conclusion that any chosen method will result in the logical contradiction. But the retardation is real so there must be a method by which the travelling clock loses the time. This suggests that the fault lies somewhere else – and the obvious candidate for that is relativity itself.

 

I think that with "method" you effectively mean interpretation, or explanation.

 

 

robinpike, bold face text by tim88

 

To demonstrate the logical contradiction, I will start with the (most) obvious method of how the clock is retarded: the travelling clock’s time runs at a slower rate (overall) during its round trip. Let’s see how this assumed method causes the issue by using one stay at home clock and two travelling clocks.

 

By the way, if you dislike the examples that I have chosen, by all means post an alternative, describing how the travelling clock loses time and where in its journey that retardation occurs, and I will show how the logical contradiction applies to your example.

[..]

 

Here I perceive a possible inconsistency. The keyword is "overall": the average clock rate over the whole voyage is obviously and factually reduced. The "absolute", the fact that everyone agrees on, is the retardation of the traveling clock, for reasons that are judged differently by choosing different reference systems. You should not expect agreement between different systems about where in its journey that retardation occurs, just as you did not expect the coach and you to agree on who has a good ruler.

 

I temporarily stop here, for in what follows it looks as if you assume that according to everyone, "the real loss in time is due to the travelling clock’s time running slow, during the trip to the half-way point". Not so; that is not an "absolute" on which everyone agrees.

 

Anyway, I like your analysis, it's this kind of analysis that enhances physical understanding.

[J.C.MacSwell]

 

 

Note that there is no issue with the calculations of relativity, as these agree with the amount of time lost by the travelling clock, with respect to the stay at home clock. The issue is a logical contradiction that appears to be caused by relativity, revealed when a method is chosen as to how the travelling clock loses the time. Working through the chosen method reveals a contradiction in logic – which at first simply suggests that the chosen method is invalid. So another method is chosen, and again the same contradiction in logic is found, and so on. This leads to the general conclusion that any chosen method will result in the logical contradiction. But the loss in time is real so there must be a method by which the travelling clock loses the time. This suggests that the fault lies somewhere else – and the obvious candidate for that is relativity itself.

 

To demonstrate the logical contradiction, I will start with the (most) obvious method of how time is lost: the travelling clock’s time runs at a slower rate (overall) during its round trip. Let’s see how this assumed method causes the issue by using one stay at home clock and two travelling clocks.

 

By the way, if you dislike the examples that I have chosen, by all means post an alternative, describing how the travelling clock loses time and where in its journey that loss in time occurs, and I will show how the logical contradiction applies to your example.

 

The two travelling clocks are initially stationary and synchronized against the stay at home clock. They start the round trip by accelerating side-by-side away from the stay at home clock (i.e. they increase their relative speed with respect to the stay at home clock). Since the assumption (in this example) is that the real loss in time is due to the travelling clock’s time running slow, during the trip to the half-way point, their clocks will now be running slow (with respect to the stay at home clock).

 

At the half-way point, one of the travelling clocks de-accelerates (i.e. reduces its speed with respect to the stay at home clock) to the point where it becomes stationary with respect to the stay at home clock. This means that the travelling clock’s time now runs at the same rate as the stay at home clock’s rate of time.

 

But when the travelling clock de-accelerated, it effectively accelerated away from the second travelling clock (i.e. it increased its relative speed with respect to the second travelling clock). But such an acceleration is just a repeat of the scenario when the travelling clocks first accelerated away from the stay at home clock (although now with respect to different reference frames). This means that the de-accelerated clock’s time has to run slow with respect to the second travelling clock’s rate of time, which already is running slow with respect to the stay at home clock’s rate of time – AND yet the de-accelerated clock’s rate of time has to be at the same rate as the stay at home clock’s rate of time (because they are stationary with respect to each other).

 

If these two conditions were apparent effects – such a scenario would be possible. But the loss in time is real and so these two conditions cannot occur together – hence a logical contradiction has occurred.

 

You can try to avoid this logical contradiction by explain the travelling clock’s real loss in time by other methods, but I think they will all fail for the same reason.

 

For example, try the loss in time is because the travelling clock’s progress through space-time is shorter than the stay at home clock’s progress through space-time. This fails at the turn around point because the de-accelerated clock’s progress through space-time has to be shorter with respect to the second travelling clock’s progress through space-time, which already is shorter with respect to the stay at home clock’s progress through space-time – AND yet the de-accelerated clock’s progress through space-time has to be at the same rate as the stay at home clock’s progress through space-time (because they are stationary with respect to each other).

.

It is easy to set up a contradiction in the logic if you start with a false assumption. In this case you are assuming all frames have the same absolute time. Read the bold without that assumption and there is no contradiction.

[Endy0816]

Yeah, really helpful to draw triangle(s) to represent what is seen by who.

 

Logic works, just have to avoid mistakes.

Edited September 16, 2016 by Endy0816

[robinpike]

 

The two travelling clocks are initially stationary and synchronized against the stay at home clock. They start the round trip by accelerating side-by-side away from the stay at home clock (i.e. they increase their relative speed with respect to the stay at home clock). Since the assumption (in this example) is that the real loss in time is due to the travelling clock’s time running slow, during the trip to the half-way point, their clocks will now be running slow (with respect to the stay at home clock).

 

At the half-way point, one of the travelling clocks de-accelerates (i.e. reduces its speed with respect to the stay at home clock) to the point where it becomes stationary with respect to the stay at home clock. This means that the travelling clock’s time now runs at the same rate as the stay at home clock’s rate of time.

 

But when the travelling clock de-accelerated, it effectively accelerated away from the second travelling clock (i.e. it increased its relative speed with respect to the second travelling clock). But such an acceleration is just a repeat of the scenario when the travelling clocks first accelerated away from the stay at home clock (although now with respect to different reference frames). This means that the de-accelerated clock’s time has to run slow with respect to the second travelling clock’s rate of time, which already is running slow with respect to the stay at home clock’s rate of time – AND yet the de-accelerated clock’s rate of time has to be at the same rate as the stay at home clock’s rate of time (because they are stationary with respect to each other).

 

If these two conditions were apparent effects – such a scenario would be possible. But the loss in time is real and so these two conditions cannot occur together – hence a logical contradiction has occurred.

 

 

 

It is easy to set up a contradiction in the logic if you start with a false assumption. In this case you are assuming all frames have the same absolute time. Read the bold without that assumption and there is no contradiction.

 

In what way do you mean I am assuming all frames have the same 'absolute time'? I note that the two travelling clocks lose time with respect to the stay at home clock's time. When the first travelling clock de-accelerates, it loses time with respect to the second travelling clock, and by the same action, returns to not losing any further time with respect to the stay at home clock. Since the second travelling clock is losing time with respect to the stay at home clock, is there anyway that both can be achieved? I am suggesting that it is not possible (within the remit of relativity).

 

 

 

Retake for clarity:

 

 

 

 

OK, your next description has some similarities with the last part of my post #116 :

http://www.scienceforums.net/topic/97466-clocks-rulers-and-an-issue-for-relativity/page-6#entry940714

But I'll continue with your description here, doing just as with the preceding part:

 

 

I think that with "method" you effectively mean interpretation, or explanation.

 

[..]

 

Here I perceive a possible inconsistency. The keyword is "overall": the average clock rate over the whole voyage is obviously and factually reduced. The "absolute", the fact that everyone agrees on, is the retardation of the traveling clock, for reasons that are judged differently by choosing different reference systems. You should not expect agreement between different systems about where in its journey that retardation occurs, just as you did not expect the coach and you to agree on who has a good ruler.

 

I temporarily stop here, for in what follows it looks as if you assume that according to everyone, "the real loss in time is due to the travelling clock’s time running slow, during the trip to the half-way point". Not so; that is not an "absolute" on which everyone agrees.

 

Anyway, I like your analysis, it's this kind of analysis that enhances physical understanding.

 

 

What everyone agrees on is that the travelling clock loses time on returning to the stay at home clock.

 

With reference to the travelling clock and the stay at home clock, if the travelling clock does not lose time during the outward portion of the trip (and on the return portion of the trip) - then when does it lose time? Is your concern that different observers can see that loss in time in different parts of the travelling clock's journey?

 

Note that the argument only needs to consider the real loss in time of the travelling clock - not the apparent gains / loss in time as seen by different observers.

 

In my discussion I chose the point of view of the first travelling clock when it de-accelerated because it is this clock and at this point in its journey that the logical contradiction can be seen.

 

Of course, please think up ways to test and challenge this logical argument and I will try to rebuff them.

[J.C.MacSwell]

 

 

 

In what way do you mean I am assuming all frames have the same 'absolute time'? I note that the two travelling clocks lose time with respect to the stay at home clock's time. When the first travelling clock de-accelerates, it loses time with respect to the second travelling clock, and by the same action, returns to not losing any further time with respect to the stay at home clock. Since the second travelling clock is losing time with respect to the stay at home clock, is there anyway that both can be achieved? I am suggesting that it is not possible (within the remit of relativity).

 

 

 

 

Loses time why? For this essentially instantaneous acceleration/deceleration the two travelling clocks in this scenario are not displaced enough to have a significant change in time...just the rate going forward as measured by each. Compare with respect to the stay at home clock. The greater displacement effects a significant shift to the future through the deceleration.

Edited September 16, 2016 by J.C.MacSwell

[Tim88]

[..]

 

What everyone agrees on is that the travelling clock loses time on returning to the stay at home clock.

 

With reference to the travelling clock and the stay at home clock, if the travelling clock does not lose time during the outward portion of the trip (and on the return portion of the trip) - then when does it lose time? Is your concern that different observers can see that loss in time in different parts of the travelling clock's journey? [..]

 

Yes, exactly. In my earlier post that I also linked to, I presented a reference frame according to which the traveling clock ticks faster than the stay at home clock on that part of the trip.

 

 

Note that the argument only needs to consider the real loss in time of the travelling clock - not the apparent gains / loss in time as seen by different observers.

 

In my discussion I chose the point of view of the first travelling clock when it de-accelerated because it is this clock and at this point in its journey that the logical contradiction can be seen.

 

Of course, please think up ways to test and challenge this logical argument and I will try to rebuff them.

 

OK, I will do a retake (maybe not now but tomorrow), with a little rephrasing to clarify that it's just a perspective that should not matter for your argument.