1 Twins and space station

The SR effects - GR say the traveling west bound twin's clock was running faster not slower.

The earth's equator is rotating at 1000 MPH east and the twin 500 MPH west so it would make sense that the twins clock will run faster as in the big picture he is moving 500 MPH slower.

It would make sense if there was a preferred FoR. I am okay with a preferred FoR. Do not mean to speak your mind but I believe you are not okay with a preferred FoR. Is there a way to have a moving clock run faster without a preferred FoR ?

2 Twins and space station

Actually neither clock runs faster, and neither clock runs slower -- ALL clocks run at their usual rate, regardless of how they might move or where they might be located. But when measured in an inertial frame relative to which a clock is moving, it is OBSERVED to run slower than identical clocks at rest in the inertial frame.

IOW: "time dilation" does NOT affect the clock itself, it is a geometrical projection of the interval between a moving clock's ticks onto the inertial frame used for the measurement.

>	It would make sense if there was a preferred FoR.

There is no "preferred frame" in the usual sense of it being somehow "special" in the dynamics. But HUMANS who make calculations definitely prefer to use an inertial frame, as calculations are simpler in such coordinates than in non-inertial coordinates. Such human preference, of course, is irrelevant to nature.

Tom Roberts

3 Twins and space station

4 Twins and space station

>>	Actually neither clock runs faster, and neither clock runs slower -- ALL clocks run at their usual rate, regardless of how they might move or where they might be located.

>	If you try to measure time with two identical clocks and the two don't measure the same than IMO you should disgard the slowest.

Why? Identical clocks should have equal weight in the measurement. But note that for this to apply they must be measuring THE SAME time -- in the "twin paradox" they don't.

>	That is why the GPS clocks are continuously synchronised?

They aren't. Small updates (~ few nanoseconds) are uploaded once a day; these are dwarfed by the GR correction to their internal dividers (~ 38 microseconds/day).

>	If you want to measure the revolution time of two objects and you place on each a clock to measure this, don't you want to be sure that the definition of 1 year for both is the same?

If they are different objects rotating differently, then the best you can do is use identical clocks. Then the "definition of 1 year" will be the same for both CLOCKS. Whether that corresponds to either object's rotation depends on the objects.

"1 year" is a measure of time, not rotation.

>>	IOW: "time dilation" does NOT affect the clock itself, it is a geometrical projection of the interval between a moving clock's ticks onto the inertial frame used for the measurement.

>	I doubt this.

Then you should learn about Special Relativity. It is SOLIDLY established experimentally.

>	Length contraction (if it exists?) is as far as I see, a geometrical projection and not something physical.

"Length contraction", like "time dilation" is purely a geometrical projection, in both SR and GR. So the object being observed is not physically affected. But still, such projections can have physical consequences (e.g. a ladder fits through a narrow doorway only if the geometrical projection of its length onto the doorway is smaller than the latter's width).

Tom Roberts

5 Twins and space station

When you measure the duration between two events (i.e. the start from position A and the arrival time at position B) using two clocks, IMO the duration should be the same. In a "twin paradox" type experiment both A and B are the same. One clock stays on earth at A. The other clock travels in a straight line away from A to a point C and back to A. What real experiments show is that the measured durations are not the same. The moving clock shows the shortest duration (# of ticks)

> >	That is why the GPS clocks are continuously synchronised?

>	They aren't. Small updates (~ few nanoseconds) are uploaded once a day; these are dwarfed by the GR correction to their internal dividers (~ 38 microseconds/day).

For GPS clocks you can do a simular experiment. One GPS clock you keep on earth and one other you bring in orbit for 1 year and you bring it back. Also here the question is: is the duration the same? I doubt this.

In fact you should do one experiment to test both.
1) One clock stays for one year on earth,
2) One clock travels for half a year away and for half a year back (fast)
3) One clock which travels for one year around the earth (stays in orbit) when they meet which clock shows longest duration (highest # of ticks) and which the shortest?

> >>	IOW: "time dilation" does NOT affect the clock itself, it is a geometrical projection of the interval between a moving clock's ticks onto the inertial frame used for the measurement.

> >	I doubt this.

>	Then you should learn about Special Relativity. It is SOLIDLY established experimentally.

Specific what type of experiment do you have in mind to demonstrate "time dilation"? Is it one above?

> >	Length contraction (if it exists?) is as far as I see, a geometrical projection and not something physical.

>

"Length contraction", like "time dilation" is purely a geometrical projection, in both SR and GR. So the object being observed is not physically affected. But still, such projections can have physical consequences (e.g. a ladder fits through a narrow doorway only if the geometrical projection of its length onto the doorway is smaller than the latter's width).

But has that anything to do with the speed of the ladder?

I proposed a different experiment.
The length of the train is 1 km. The track is straight There are two lights almost one 1 km apart. An observer is at a distance perpendicular to half the distance between these two lights. (at 500 m)
The train starts from the left at high speed towards the right. Will the observer see both lights on when the train is in between those two lights? If yes then there is length contraction.

Nicolaas Vroom

6 Twins and space station

That would be a nice experiment, but it cannot be done. GPS clocks fail if subjected to more than two g's acceleration. They are shut off during launch because of that and would experience more than 2 g's on reentry.

Another factor is that their vehicle doesn't have reentry capability, but I suppose a shuttle could pick one up. It would be useless, though, because the clock wouldn't operated during reentry.

A proper test would have to be done with both clocks in space. They would be synchronized when both are together and then one would be accelerated (gently) to a different trajectory and then brought back later and compared.

>	Also here the question is: is the duration the same? I doubt this.

So does every scientist who understands relativity.

>

In fact you should do one experiment to test both. 1) One clock stays for one year on earth, 2) One clock travels for half a year away and for half a year back (fast) 3) One clock which travels for one year around the earth (stays in orbit) when they meet which clock shows longest duration (highest # of ticks) and which the shortest?

It most likely won't be done because (1) every knowledgeable person knows what will happen, (2) satisfying the whims of doubters is a waste of time and money, and (3) spacecraft are better used discovering what we DON'T know.

>	Specific what type of experiment do you have in mind to demonstrate "time dilation"? Is it one above?

(1) The postulates of special relativity are well-established experimentally, (2) the derivation of the LT is logically correct and predicts TD and the twin paradox, and (3) there is abundant experimental evidence to support various aspects of both SR and GR and none to refute them.

So why would you continue to doubt?

7 Twins and space station

That means that all scientist agree that moving clocks (once synchronised) run slower? At the same time this also means that these moving clocks continuously require some form of synchronisation to be 'usefull' ? The issue is that you can explain that just by studying the inner workings of a clock and based on the assumption that the speed of light in one coordinate system is the same in all directions. That means if you have clocks with move all in the x direction with different speeds, these speeds are important for the ticking rate and not the speed of light.

> >	In fact you should do one experiment to test both.

> >	1) One clock stays for one year on earth, 2) One clock travels for half a year away and for half a year back (fast) 3) One clock which travels for one year around the earth (stays in orbit) when they meet which clock shows longest duration (highest # of ticks) and which the shortest?

>	It most likely won't be done because (1) every knowledgeable person knows what will happen, (2) satisfying the whims of doubters is a waste of time and money, and (3) spacecraft are better used discovering what we DON'T know.

What I try to compare is the behaviour of a twin type experiment with an as much as possible identical GPS type of experiment. That means one clock stays at home and the two other two clocks move under different circumstances. For any of these clocks, if they run behind, then IMO this is a physical issue and require continuous synchronisation. What I'm supposed to understand that all these clocks run normal.

> >	Specific what type of experiment do you have in mind to demonstrate "time dilation"? Is it one above?

>	(1) The postulates of special relativity are well-established experimentally,

IMO we should start from the experiments and try to explain them (as simple as possible)

>	(2) the derivation of the LT is logically correct and predicts TD and the twin paradox, and (3) there is abundant experimental evidence to support various aspects of both SR and GR and none to refute them.

I'm not claiming that SR and GR are wrong. I have my doubts if you need all of them to explain the trajectories of the stars in our Galaxy or the planets in our solar system.

>	So why would you continue to doubt?

Part of the problem is the more I try to read, the more I try to understand and discuss the issues involved are becoming less clear. Sometimes the responds indicate that Einstein had it wrong. Also many books have it wrong.

As such I have problems with this sentence:

> >	"Length contraction", like "time dilation" is purely a geometrical projection, in both SR and GR. So the object being observed is not physically affected. But still, such projections can have physical consequences (e.g. a ladder fits through a narrow doorway) etc.

What is "Length contraction"? Is it physical? yes or no? What is "Time dilation"? Is it physical? What are twin type experiments? Are they physical?

Part of the problem is what and how do you measure something. If you use a rod (example) but the length changes during the measuring process then you have to be careful. To measure the rate of a clock with an other clock you also have to be careful.

In some sense Newton's Law is very straight forward. The book Newton's Prinicpia emphasizes this. Newton's Law starts in some sense that at each instant for the whole of the Universe there is a now. At that same instant you can calulate the sum of all the forces active for each object considered. Is the sum is zero the particle will continue to move in the same direction. If it's not the direction will change. Using that concept you can calculate the trajectories of the planets around the sun. The most important issue that the forces don't act instantaneous as Newton assumed.

IMO when you compare Newton with SR and you consider our solar system the Newton considers only one coordinate system with the Sun at rest. In SR you can consider an observer at rest on the surface on earth, or an observer at rest at the center of earth, or an observer at jupiter etc. All these observer are at rest in their frames and clocks moving in these frames run slower. IMO such an approach is not very practical. For our galaxy Newton also would assume one coordinate system with a BH at its center.

IMO anything related to SR Newton would not consider i.e. no moving clocks and no worldlines.

The more I read about worldline the less I understand why 'you' call it a worldline. It is not a line like a string, it is something else with the underlying issue if a worldline IS something.

Nicolaas Vroom.

[[Mod. note -- We now have direct experimental tests of intercomparison of moving clocks. Some early classic experiments involving flying atomic clocks in around the world are described in

@article{Hafele-Keating-1972,
  author = "J. C. Hafele and Richard E. Keating",
  title  = "Around-the-World Atomic Clocks: Predicted Relativistic Time Gains",
  journal = "Science",
  year   = 1972, month = "14 July",
  volume = 177, issue= 4044, pages = "166--168",
  doi    = "10.1126/science.177.4044.166",
  X-url  = "http://moby.mib.infn.it/~oleari/public/elementi_fis_teorica/material
e_didattico/Hafele-Keating-predict_observ.pdf",
  X-note = "flying atomic clocks around the world to check special/general relat
ivity",
}

@inbook {
        Alley-1983-GR-rods-and-clocks,
        author = "Carroll O. Alley",
        title = "Proper Time Experiments in Gravitational Fields
                 With Atomic Clocks, Aircraft, and Laser Light Pulses",
        pages = "363--427",
        editor = "Pierre Meystre and Marlan O. Scully",
        booktitle = "Quantum Optics, Experimental Gravitation,
                     and Measurement Theory",
        publisher = "Plenum",
        address = "New York",
        year = 1983,
        snote = "++good discussion on `rods and clocks' experiments
                 testing special/general relativity",
        X-url = "https://link.springer.com/chapter/10.1007/978-1-4613-3712-6_18",
        }

There are also lots of references on relativity effects in GPS/Glonass/Galileo/etal. -- jt]]

8 Twins and space station

Nope. It means that moving clocks (once synchronized) become unsynchronized.

>	At the same time this also means that these moving clocks continuously require some form of synchronisation to be 'usefull' ?

I guess that depends on what you mean by "useful."

>	The issue is that you can explain that just by studying the inner workings of a clock and based on the assumption that the speed of light in one coordinate system is the same in all directions.

Also that it's the same in all inertial frames.

>	That means if you have clocks with move all in the x direction with different speeds, these speeds are important for the ticking rate and not the speed of light.

APPARENT ticking rate as observed from different inertial frames.

> > >

In fact you should do one experiment to test both. 1) One clock stays for one year on earth, 2) One clock travels for half a year away and for half a year back (fast) 3) One clock which travels for one year around the earth (stays in orbit) when they meet which clock shows longest duration (highest # of ticks) and which the shortest?

> >	It most likely won't be done because (1) every knowledgeable person knows what will happen, (2) satisfying the whims of doubters is a waste of time and money, and (3) spacecraft are better used discovering what we DON'T know.

>

What I try to compare is the behaviour of a twin type experiment with an as much as possible identical GPS type of experiment. That means one clock stays at home and the two other two clocks move under different circumstances. For any of these clocks, if they run behind, then IMO this is a physical issue and require continuous synchronisation. What I'm supposed to understand that all these clocks run normal.

You seem to be choking at that.

> > >

Specific what type of experiment do you have in mind to demonstrate "time dilation"? Is it one above?

> >	(1) The postulates of special relativity are well-established experimentally,

>	IMO we should start from the experiments and try to explain them (as simple as possible)

SR and GR do that quite well.

> >	(2) the derivation of the LT is logically correct and predicts TD and the twin paradox, and (3) there is abundant experimental evidence to support various aspects of both SR and GR and none to refute them.

>	I'm not claiming that SR and GR are wrong. I have my doubts if you need all of them to explain the trajectories of the stars in our Galaxy or the planets in our solar system.

Generally speaking, these motions are too slow to require relativistic corrections.

> >	So why would you continue to doubt?

>	Part of the problem is the more I try to read, the more I try to understand and discuss the issues involved are becoming less clear. Sometimes the responds indicate that Einstein had it wrong. Also many books have it wrong.

Listen to Tom Roberts and others in this group who have been around a long time. They have it right.

>	As such I have problems with this sentence:

> > >

"Length contraction", like "time dilation" is purely a geometrical projection, in both SR and GR. So the object being observed is not physically affected. But still, such projections can have physical consequences (e.g. a ladder fits through a narrow doorway) etc.

>	What is "Length contraction"? Is it physical? yes or no?

That depends on what you mean by "physical." It's what measurement shows.

>	What is "Time dilation"? Is it physical?

That depends on what you mean by "physical." It's what measurement shows.

>	What are twin type experiments? Are they physical?

Of course.

>

Part of the problem is what and how do you measure something. If you use a rod (example) but the length changes during the measuring process then you have to be careful. To measure the rate of a clock with an other clock you also have to be careful. In some sense Newton's Law is very straight forward. The book Newton's Prinicpia emphasizes this. Newton's Law starts in some sense that at each instant for the whole of the Universe there is a now. At that same instant you can calulate the sum of all the forces active for each object considered. Is the sum is zero the particle will continue to move in the same direction. If it's not the direction will change. Using that concept you can calculate the trajectories of the planets around the sun. The most important issue that the forces don't act instantaneous as Newton assumed. IMO when you compare Newton with SR and you consider our solar system the Newton considers only one coordinate system with the Sun at rest. In SR you can consider an observer at rest on the surface on earth, or an observer at rest at the center of earth, or an observer at jupiter etc. All these observer are at rest in their frames and clocks moving in these frames run slower.

That's a bit simplistic. The "anyone can consider himself at rest" rule is valid in inertial frames. But there is no truly infinite inertial frame.

>	IMO such an approach is not very practical. For our galaxy Newton also would assume one coordinate system with a BH at its center.

That depends on how big a volume and how short a time is under consideration.

>

IMO anything related to SR Newton would not consider i.e. no moving clocks and no worldlines. The more I read about worldline the less I understand why 'you' call it a worldline. It is not a line like a string, it is something else with the underlying issue if a worldline IS something. Nicolaas Vroom. [[Mod. note -- We now have direct experimental tests of intercomparison of moving clocks. Some early classic experiments involving flying atomic clocks in around the world are described in @article{Hafele-Keating-1972, author = "J. C. Hafele and Richard E. Keating", title = "Around-the-World Atomic Clocks: Predicted Relativistic Time Gains", journal = "Science", year = 1972, month = "14 July", volume = 177, issue= 4044, pages = "166--168", doi = "10.1126/science.177.4044.166", X-url = "http://moby.mib.infn.it/~oleari/public/elementi_fis_teorica/material e_didattico/Hafele-Keating-predict_observ.pdf", X-note = "flying atomic clocks around the world to check special/general relativity",

9 Twins and space station

No. As I keep saying, moving clocks do NOT "run slow" -- that is a shortcut phrase used in some popular writings that glosses over the actual situation.

If moving clocks actually did "run slow", then when multiple observers are involved the clock would have to "run slow" at a different rate for each observer, which is manifestly impossible -- a clock can tick at just one rate.

One can say that a stationary observer will MEASURE a moving clock to tick slower than an identical clock at rest. But that is quite different from the moving clock actually "running slow".

Bottom line: clocks tick at their usual rate, regardless of how they might be moving or where they might be located (relative to anything). This includes gravity.

However, clocks that follow different trajectories through spacetime can experience different elapsed proper times between meetings, and observers moving relative to a clock can measure different values for its tick rate. This is all just geometry, and there is no effect on the clock itself.

>	Part of the problem is the more I try to read, the more I try to understand and discuss the issues involved are becoming less clear. Sometimes the responds indicate that Einstein had it wrong. Also many books have it wrong.

Yes, relativity is subtle. And all too many popular books do indeed get it wrong, in that they use shortcuts to avoid complex discussions that would confuse non-experts. Clearly you are an instance of that.

Get a GOOD book on Special Relativity: Taylor and Wheeler, _Spacetime_Physics_. When you understand that, get a GOOD book on General Relativity: Misner, Thorne, and Wheeler, _Gravitation_.

>	What is "Length contraction"? Is it physical?

Depends on what you mean by "physical". Is the moving object affected? -- NO [#]. Does it have physical consequences? -- YES [@].

LC: A current-carrying wire remains electrically neutral in its rest frame, but the difference in LC between the electrons and ions in the wire yields an electrostatic force in a moving frame that is interpreted as a magnetic force in the wire frame.

TD: Pions traverse kilometer-long beamlines at Fermilab and CERN because for such high-energy pions the projection of their lifetime onto the beamline is significantly longer than the beamline's length.

>	What is "Time dilation"? Is it physical?

See above. the answer is the same as for "length contraction".

>	What are twin type experiments? Are they physical?

Certainly. But they don't really display "time dilation". They demonstrate a different geometrical property: different paths can have different path length between intersections. For timelike paths of the twins, path length is elapsed proper time, and that is what their clocks display.

>

[...]

Understand the above before trying to go on.

Tom Roberts

10 Twins and space station

> >

"Length contraction", like "time dilation" is purely a geometrical projection, in both SR and GR. So the object being observed is not physically affected. But still, such projections can have physical consequences (e.g. a ladder fits through a narrow doorway only if the geometrical projection of its length onto the doorway is smaller than the latter's width).

Some more thoughts about the same issues:

I do not understand the above. Suppose when I try to bring a ladder through a doorway and what ever I try I do not succeed. How is it possible that when the ladder has a high speed and I suppose the doorway (house/wall) is at rest, that you can perform this experiment?

Consider an additional experiment. The LHC ring has a length of 27 km. Suppose I build a track within that ring and on the track I place 270 trains of each 100 meter. The trains are marked #1 to #270. The train #1 is the leader. All the trains are started and all the operators are asked to touch the train before him. The speed increases.
My question is will it be possible to insert more trains in the ring. If there is length contraction then there should be space between the end of train #270 and the front of train #1.

A whole different issue is what would Newton think about "Time dilation" i.e. moving clocks and "Length contraction".

IMO he would not understand "Time dilation" in the sense that something can happen with the concept "time" nor with "Length contraction" in the sense that the length changes as a function of speed.

Moving clocks IMO are an issue for Newton. When clocks have different speeds their ticking rates can be different. Other conditions can also influence their behaviour. For example: height above the earth i.e. gravity.

The subject of Newton is the solar system. The whole solar system is his reference frame with the sun at the center (approx).
IMO Newton would assume that the speed of light is the same in all directions within this frame. A clock at rest in this frame (i.e. near the sun) would have the highest ticking rate. All the other ones (i.e one placed at the center of our earth) would have a lower rate. The explanation for this behaviour is mechanical and depents about the innerworkings of the clock. When internal, the clock uses lightsignals to count, then the speed of the clock relatif to the fixed speed of light influences the ticking rate.

Nicolaas Vroom

11 Twins and space station

This is just a concept reaction. The rotating shaft I believe has a higher material shear strength than a slower rotating shaft. I though it a property of inertia, but maybe it is this paradox, the Ehrenfest_paradox. My first examination of the topic occurred while looking at high speed ocean racer propeller shafts. I heard that when engine horsepower goes up and shaft rotation goes up, the shaft diameter does not necessarily have to increase.

The load force per rpm is a function. And the shear strength of the shaft, per rpm, is a function. It basically means build engines with high power at high rpm if you want smaller shaft diameters as compared to high power at lower rpm. This story is from a metalurgist. He attributed this effect as inertial effect in general.

For another perspective, in flying aircraft in turbulence, the aircraft airframe can survive easier at a higher speed as compared to a slower speed aircraft.

This is sort of a disjointed comment. Putting a sudden load on the oceanracer propeller and shaft by the throttleman is more survivable with the higher rpm.

The relativistic formulation is difficult for me, but when there is relative inertia it is a general relativity scenario, as shown in the wiki topic.

Accelerating a body whose inertia is slower than another body, has a nonlinear ??? And I loose the argument here, but have a shaft surface accelerating, as rotating, as compared to a linear acceleration. So the shaft has a second acceleration function. Changing shaft rpm accelerates the rotational acceleration. Seen by Einstein and generalized.

12 Twins and space station

13 Twins and space station

14 Twins and space station

In any experiment I should try to minimize the number of observers. The clocks are the observers. The preferred number of real observers is 1.
In a twin type experiment you also need one observer. The pilots are not part of the experiment. The pilots follow a strict flight plan. (in fact one) The only thing that the observer does is to reset both clocks at the beginning of the experiment and perform the readings of both clocks at the end.

>	How could an observer who is just passing by possibly affect a clock's tick rate??? -- from the clock's perspective, the phrase "moving clocks run slow" would imply that they do.

Observers are not supposed to touch the clocks during the experiment and are not physical part of the experiment. The immer workings of the clock (light rays) affect the ticking (rate).

>	One can say that a stationary observer will MEASURE a moving clock to tick slower than an identical clock at rest. But that is quite different from the moving clock actually "running slow".

Why ? I would add: actually "running slow" compared to a clock at rest.

What you are doing is comparing the ticking rate of a clock at rest versus a the ticking rate of a moving clock both in the same frame at rest.

>	Bottom line: clocks tick at their usual rate, regardless of how they might be moving or where they might be located (relative to anything). This includes gravity.

Maybe I use the wrong wording, but if my understanding is correct than the laws of physics are the same in each reference frame. Using that same reasoning the physical speed of light is the same in each reference system.
But that says nothing about the value of the speed of light in each frame which in a moving frame should be measured by moving rods and clocks. I agree with you that clocks tick at their usual rate each in their own reference frame, but that does not say anything.
What you should do is compare (the same and different clocks) in the same reference frame. First all at rest and decide which type is the most stable and then moving clocks and unravel how moving clocks influence their behaviour.

>	However, clocks that follow different trajectories through spacetime can experience different elapsed proper times between meetings, and observers moving relative to a clock can measure different values for its tick rate. This is all just geometry, and there is no effect on the clock itself.

I think we use different wordings for the same observations. I think what you call proper time is the same as clock reading. If I start an experiment and and I reset my clock and at the end of the experiment my clocks shows 10 counts than something mechanical or physical has caused this counting process.
If you do the same and when we meet your clock shows 8 counts than also something mechanical or physical has taken place. However because the results are not the same the mechanical, chemical or physical processes involved are not the same. This is not geometry.

>	Get a GOOD book on Special Relativity: Taylor and Wheeler, _Spacetime_Physics_. When you understand that, get a GOOD book on General Relativity: Misner, Thorne, and Wheeler, _Gravitation_.

Discussed separately

> >	What is "Length contraction"? Is it physical?

>	[@] Rotating a ladder etc That is a rotation in a space-space plane; both LC and TD are similar rotations, but in a space-time plane; all can have physical consequences.

IMO both LC and TD should be handled separately. Rotating a ladder is a physical process but has no long-term consequences on the length of the ladder.
Moving a clock is a physical process but can have long-term consequences i.e. the temporarily moved clock runs behind a clock at rest.

Nicolaas Vroom

15 Twins and space station

That is what confuses you. You THINK you understand SR when you only have one clock, but having many clocks refutes your "understanding."

>	In a twin type experiment you also need one observer. The pilots are not part of the experiment. The pilots follow a strict flight plan. (in fact one) The only thing that the observer does is to reset both clocks at the beginning of the experiment and perform the readings of both clocks at the end.

"Both" clocks? You said previously that you only needed one.

Never mind. The point is that with another observer moving wrt both the first observer AND the pilot, that second observer will get a different reading than the first observer got. Thus, as Tom said:

> >	How could an observer who is just passing by possibly affect a clock's tick rate??? -- from the clock's perspective, the phrase "moving clocks run slow" would imply that they do.

>	Observers are not supposed to touch the clocks during the experiment and are not physical part of the experiment. The immer workings of the clock (light rays) affect the ticking (rate).

And since light always travels at c, the clock is NOT affected by movement. This would violate the firs postulate if it did.

> >	One can say that a stationary observer will MEASURE a moving clock to tick slower than an identical clock at rest. But that is quite different from the moving clock actually "running slow".

>	Why ? I would add: actually "running slow" compared to a clock at rest.

Your insisting that you can only have one observer leads you astray.

>	What you are doing is comparing the ticking rate of a clock at rest versus a the ticking rate of a moving clock both in the same frame at rest.

???

> >	Bottom line: clocks tick at their usual rate, regardless of how they might be moving or where they might be located (relative to anything). This includes gravity.

>

Maybe I use the wrong wording, but if my understanding is correct than the laws of physics are the same in each reference frame. Using that same reasoning the physical speed of light is the same in each reference system. But that says nothing about the value of the speed of light in each frame which in a moving frame should be measured by moving rods and clocks. I agree with you that clocks tick at their usual rate each in their own reference frame, but that does not say anything. What you should do is compare (the same and different clocks) in the same reference frame. First all at rest and decide which type is the most stable and then moving clocks and unravel how moving clocks influence their behaviour.

It is assumed that the clocks are identical, so they all have the same behavior. The point is that a clock in frame A measures a clock in frame B as running slow, but the clock in frame B measures the clock in frame A as running slow. It is irrational to say that moving clocks run slow since (1) they each measure the other as running slow and (2) there is no way to determine which clock is actually moving.

> >	However, clocks that follow different trajectories through spacetime can experience different elapsed proper times between meetings, and observers moving relative to a clock can measure different values for its tick rate. This is all just geometry, and there is no effect on the clock itself.

>

I think we use different wordings for the same observations. I think what you call proper time is the same as clock reading. If I start an experiment and and I reset my clock and at the end of the experiment my clocks shows 10 counts than something mechanical or physical has caused this counting process.

How could it? You are presuming that there is something absolute about movement. There is not. Believing that motion is absolute is a violation of the principle of relativity (the first postulate).

16 Twins and space station

I do noy think so. Light-travel time has nothing to do with this physical experiment.

>	Terrell showed that a moving sphere, which appears as a circle in projection, would NOT appear as an ellipse (nor any other non-circular form) when moving. See the Wikipedia article on Terrell rotation. Isn't the trains-at-CERN experiment analogous to Terrell rotation?

Terrell rotation (as far as I understand it) is involved in what I call: the race track train experiment. In that case a train travels around a race track. Two specific conditions can be considered.
1) When the train moves away the observed length is smaller than the train is. This can be explained when the train starts to move away from you, you see the back of the train move earlier than the front of the train. That means the train appears shorter.
2) When the train approaches you the observed length is longer than the train is. This can be explained as when the train at high speed is very close to you at that instant you can also see the back of the train but that image comes from further away than the length of the train actual is. i.e. the train seems longer than it really is.

Both effects are clearly optical illusions.

This effect does not appear when the distance towards the train is constant.

When the train moves along a circular track there is an other physical phenomena: you observe the train at a position where the train not is but at a retarded position. This angle depents about the speed of the train, but is also an optical illusion.

>	Is it really a "genuine paradox"?

A paradox IMO is if there are two contradictory theories which try to explain the same experiment. A paradox is tricky when it involves a thought experiment, specific if the thought experiment contradicts a theory i.e. SR.

The LHC experiment tries to contradict SR in the sense that SR predicts length contraction. The issue is what exactly is meant with length contraction. Is it physical (like heat increases and cold decreases) yes or no. If it is physical than the LHC experiment should demonstrate this.

> >	[[Mod. note -- The author's gedanken-experiment with trains moving around the LHC ring is essentially the Ehrenfest paradox. This is a genuine paradox in relativity, and is nicely discussed in the Wikipedia article, https://en.wikipedia.org/wiki/Ehrenfest_paradox -- jt]]

The Ehrenfest paradox is more a 2D problem?
The issue is: how does a fast rotating disc physical behave? The issue is radius versus circumference. What happens when you rotate a disc? Does the radius change?
Does the circumference change? Both? Neither?
The only way to decide IMO is by means of experiment.

What you can also do is place a ring of rods on top of the disc, each fixed midway with at one point on the surface of the disc. Each rod touches two other rods. Now you can again turn the disc and ask the questions:
1. Will each rod always touch the two other rods or
2. will there appear a certain space between each rod.
(If this is the case there is length contraction involved) The problem is also in this case the radius can change which results that if this behaviour is real it disappears.

In the experiment inside the LHC the radius is fixed, making it slightly simpler.

Nicolaas Vroom

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OK. That's just words with no impact on the physics. This is not an old interpretation of QM in which observers are special (even magical).

>>	How could an observer who is just passing by possibly affect a clock's tick rate??? -- from the clock's perspective, the phrase "moving clocks run slow" would imply that they do.

>	Observers are not supposed to touch the clocks during the experiment and are not physical part of the experiment. The immer workings of the clock (light rays) affect the ticking (rate).

Right. Though no real clocks use light rays in their inner workings. So if a clock's tick rate were to actually change, that would mean its inner workings changed, implying that the laws of physics governing its inner workings changed, and that would violate the PoR.

I repeat: clock tick rates DO NOT CHANGE. But observers measuring the tick rate of a moving clock can obtain a value different from when the clock is at rest, due to the difference in measurement procedures required.

>>	One can say that a stationary observer will MEASURE a moving clock to tick slower than an identical clock at rest. But that is quite different from the moving clock actually "running slow".

>	Why ? I would add: actually "running slow" compared to a clock at rest. What you are doing is comparing the ticking rate of a clock at rest versus a the ticking rate of a moving clock both in the same frame at rest.

This is as much an issue of language as of physics. Your three lines here are OK, because they mention more than the clock. But when one says "this clock runs slow", one is discussing the clock ONLY, without regard to anything else; it implies that the laws of physics that govern its ticking are DIFFERENT when it is moving than when it is at rest, and that violates the PoR. In SR and GR, no clock ever "runs slow", they always tick at their usual rate, regardless of how they might be moving or where they might be located. Note also that "runs slow" is a COMPARISON which does not mention to what the clock's tick rate is being compared -- subtle subjects like physics require MUCH better precision in thought and word than that.

One can say: "this observer measures that clock to run slower than her own, when she uses the usual measurement procedure". That's essentially what you did here.

>>	Bottom line: clocks tick at their usual rate, regardless of how they might be moving or where they might be located (relative to anything). This includes gravity.

>

Maybe I use the wrong wording, but if my understanding is correct than the laws of physics are the same in each reference frame. Using that same reasoning the physical speed of light is the same in each reference system. But that says nothing about the value of the speed of light in each frame which in a moving frame should be measured by moving rods and clocks.

If the vacuum speed of light is c relative to a given inertial frame, then standard clocks and rulers at rest in that frame MUST measure c for the vacuum speed of light. After all, that's what those words mean. So if the vacuum speed of light is c in every inertial frame, this EXPLICITLY answers the question implicit in your last sentence -- the vacuum speed of light will be measured to be c by (inertially) moving rods and clocks.

>	I agree with you that clocks tick at their usual rate each in their own reference frame, but that does not say anything.

Sure it does. It shows that "moving clocks run slow" is WRONG -- they do indeed tick at their usual rate, each in their own rest frame.

>	What you should do is compare (the same and different clocks) in the same reference frame. First all at rest and decide which type is the most stable and then moving clocks and unravel how moving clocks influence their behaviour.

Already done, in many ways with many different clocks. Inertial motions of clocks do NOT affect their tick rate. Motion relative to a frame can (will) affect how instruments at rest in that frame MEASURE a moving clock's tick rate, but the tick rate of the clock ITSELF is unaffected.

Yes. Every clock displays its own elapsed proper time, along its own trajectory through spacetime.

>	If I start an experiment and and I reset my clock and at the end of the experiment my clocks shows 10 counts than something mechanical or physical has caused this counting process.

I assume you reset both clocks at the start, move one clock away and then back again, and compare the clock readings -- a twin paradox.

You forgot the possibility of a geometrical explanation, and that's how SR and GR model this.

>	If you do the same and when we meet your clock shows 8 counts than also something mechanical or physical has taken place.

OR GEOMETRICAL.

>	However because the results are not the same the mechanical, chemical or physical processes involved are not the same. This is not geometry.

Yes, it is geometry of spaceTIME.

It is common for different paths to have different path lengths.

In triangle ABC, the direct path from A to B has a shorter path length than the path ACB between the same points.

In the twin paradox, the two twins follow different paths through spacetime, and their paths have different path lengths (aka elapsed proper time along the path). After all, with instantaneous accelerations and inertial motions of the traveling twin this _IS_ a triangle in spaceTIME.

>	Moving a clock is a physical process but can have long-term consequences i.e. the temporarily moved clock runs behind a clock at rest.

You are confusing differential measurement with integral measurements. Differential measurements, like comparisons of tick rates, have no memory of the histories of the clocks. Integral measurements, like comparing displayed values, do have memory of the histories of the clocks.

Tom Roberts

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I understand this experiment. My understanding is when the trains contract than the space in between expands. Expands ofcourse is the wrong wording. It is the distance between the train that becomes larger. This increase is a physical consequence of the state (length) of the train, but not of the space inbetween.

>	No serious person claims that "length contraction" is some physical effect with regard to the material of the object or whatever.

But what is then the reason to study rigid versus non-rigid objects. Such a distinction only make sense if their is a physical distinction between the two materials in the sense that: A moving rigid object does not show physical "length contraction"; a moving no-rigid object shows physical "length contraction"

>	This should be obvious from the fact that it depends only on the frame of reference.

When you study (perform an experiment) the length of a moving train (rod) from the view point of a frame at rest versus from a moving frame and the first shows no length contraction and the second does etc than you should only use a frame at rest.

I have a problem to study any physical process using different reference frames because such a frame has nothing to do with the process it self. In fact you should study the total process from the simplest coordinate system because that will give the simplest laws (equations) to describe the process.

>	Thus, it shouldn't make any difference whether I use the front and back of a train as a fiducial length, or the front and back of a gap between trains.

This is only true when there is no length contraction of the train.

[Moderator's note: Make sure that you don't assume that that which you are trying to prove is true. -P.H.]

Nicolaas Vroom

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It is obviously not physical. Instead of the trains running around the track, we could have two observers circling around the track---at different speeds. What they will observe will be different. Thus, there is no physical contraction. In SR with non-accelerated motion, there is not even a way to tell who is "really" moving. Length contraction here is obviously an illusion.

>

What you can also do is place a ring of rods on top of the disc, each fixed midway with at one point on the surface of the disc. Each rod touches two other rods. Now you can again turn the disc and ask the questions: 1. Will each rod always touch the two other rods or 2. will there appear a certain space between each rod. (If this is the case there is length contraction involved)

Do the same experiment with unaccelerated motion. Do gaps appear or not? Make the rods alternately coloured, red and blue. Which contract? Replace the blue rods with empty space. My claim: a length defined by empty space behaves the same as a length defined by some physical object.

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Not so. An "illusion" could not have physical consequences, but "length contraction" does. For instance the magnetic forces from current-carrying wires, the correspondence between fixed-target and intersecting-beam cross-sections, and the frequency/wavelength of free-electron lasers.

Of course "length contraction" is most definitely NOT an illusion in SR, it is a geometrical projection. Just like "time dilation", which is a similar projection but onto the time coordinate rather than a length coordinate. Such projections have more substance than illusions, and have physical consequences in appropriate situations, but aren't a physical change in the object, either.

Tom Roberts

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In English, "illusion" has VERY different implications and connotations than "projection"; in this situation most of them are wrong. In relativity, "length contraction" and "time dilation" are geometrical projections, not "illusions".

Tom Roberts

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I expect what you mean: as measured in their own frame. The issue is when an observer measures both a clock at rest versus a moving clock (based on final readings) the clock at rest ticks the fastest.

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What I meant with this sentence is when length contraction is physical, then the space between the first and last train should increase and it should be physical possible to place more trains on the track.

>	It is obviously not physical.

Does that mean that the space between the first and last train does not increase?

>	Instead of the trains running around the track, we could have two observers circling around the track---at different speeds. What they will observe will be different.

That means the train are at rest? When the trains are at rest a moving observers maybe will make different observations i.e. measurements but that has no physical consequences.

>	Thus, there is no physical contraction. In SR with non-accelerated motion, there is not even a way to tell who is "really" moving. Length contraction here is obviously an illusion.

IMO when you to study illusion, the track should have a horse race shape. That means two parts should be straight. The observer stays and the center of one half circle.
The observer sees, when the train moves away, that the train becomes shorter. When the train approaches the train becomes longer. The most interesting part is when the moving away train and the approaching train are at the same distance from the observer. In that case the moving away train becomes shorter and the approaching train becomes longer (observed). All of this are illusions.
What SR claims (with different wording) that the moving train also becomes physical shorter (along the whole track). That is the question. It is either (physical) true or not.

In this discussion only one frame is considered and no moving observers are involved.

Nicolaas Vroom

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Is there "length contraction" involved in these 3 examples? If yes then please explain one.

>

Of course "length contraction" is most definitely NOT an illusion in SR, it is a geometrical projection. Just like "time dilation", which is a similar projection but onto the time coordinate rather than a length coordinate. Such projections have more substance than illusions, and have physical consequences in appropriate situations, but aren't a physical change in the object, either.

The problem with time dilation is that IMO the physical implication is that moving clocks run slower, in the same frame, as a clock at rest. As such time dilation is not an illusion. Length contraction is a much more difficult issue to identify the physical implications.

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(1) For a wire carrying a current involving moving electrons, in the rest frame of the wire it remains electrically neutral (the power supply generating the current ensures this is so). So a nearby charged particle at rest in that frame experiences no EM force from the wire and its current. But a charged particle moving parallel to the wire at the same speed as the electrons sees the ions of the wire (i.e. the atomic nuclei) as "length contracted", and the electrons as not, so in its frame there is a net positive charge on the wire, and it feels an EM force. In the wire rest frame we call this "magnetic force", while in the moving frame it is "electrostatic force". This is much more general that my simple description, and when worked out numerically it is correct; I believe that Perkins's book on E&M goes into this in detail.

(2) scattering an unpolarized particle beam from an unpolarized target is cylindrically symmetric, and we measure the differential cross-section as a function of polar angle. For a fixed-target experiment the target is at rest in the lab; for a colliding-beam experiment the center-of-mass is at rest in the lab. To reconcile these two measurements at a given center-of-mass energy, one must invoke "length contraction".

(3) a free-electron laser consists of an energetic electron beam traveling through a magnetic field that alternates transverse directions in space, typically every 10-20 cm over a length of several meters (the magnets are at rest in the lab). As the beam is "wiggled" by the magnetic field, it oscillates with the frequency it sees the field alternate -- this generates radiation of that frequency, and for quantum reasons this can be a laser. In the lab this radiation is measured, and to account for the observed frequency, that 10-20 cm alternation must be reduced by the "length contraction" formula in the beam rest frame.

Tom Roberts

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My dilemma being the cause to coherency. Is length contraction the relation that makes a spherical emission change to beam? A stationary wiggler will emit, but will it display a focusing, i.e. coherency if the particle moves?

The pure electron emission is my model here. Coherency as a solid state channel effect is a con-founder for me. Here a mass causes resonation making mass cause focus. A common relation for coherency would be nice to find.

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No. You've inadvertently created a paradox that doesn't exist. By invoking gravitons you've defined your example via gravitational field; yet you paradoxically use Newtonian mechanics when you specify the position of the source as the matter itself.

When you decide to use a field method then you must specify where in the field the boson originates. When you do that you'll see there is no paradox as the graviton must originate at the leading edge of the field.

If you use Newtonian mechanics then you're assuming instantaneous interactions; and you still have no paradox.

Brad

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I do not think the discussion is about a paradox. The issue is about is the question: is length contraction something physical. The reply of Phillip is (if I'm allowed) No. it is an illusion.
I agree with him.
The reply Tom is: There are certain cases where you can invoke "length contraction" to explain certain physical phenomena. I agree with him in the case of the moving train experiment. In that case the observed length contraction is a visible illusion but it can still have physical consequences.
Consider the Sun and a (fast moving) comet. The shape of the comet is not round but has the shape of a rocket. The direction of the rocket is in the direction of the path of the comet i.e. rocket.
In that situation length contraction has physical consequences when you assume that, using Newtonian mechanics, gravity does not act instantaneous. In GR I assume this is the same.

Nicolaas Vroom.

34 Twins and space station

There _IS_ no such "experiment". As a gedanken, you cannot possibly obtain an answer different from SR, unless you made a mistake in applying SR to the situation.

>	See: https://www.nicvroom.be/VB%20Train%20operation.htm

I do not run such programs on my machines.

>	The simulation shows that the observed moving away train, is shorter than the real length of the train. The simulation also shows that the observed approaching train, is longer than the real length of the train.

"Length contraction" applies between INERTIAL FRAMES. The essence of that situation is NOT inertial motion.

>	The question is have these effects any physical consequences.

"Length contraction" certainly does, with or without gravitation present (see my three examples). Those "effects" of yours are merely aspects of a gedanken.

>	I think the three examples mentioned are similair in the sense that no real length contraction is involved.

"length contraction' is never "real" in the sense of affecting the object itself. But it is "real" in the sense that it can have physical consequences (observable effects), as discussed for those three examples.

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The simulation is not in conflict with SR in the sense of your definition. see (*) below.
One assumption of the simulation is that the real (physical) length of the object does not change.
What the simulation shows the the clock on the moving train runs slower than the clock at rest. That is a real (physical) effect. The simulation also allows you to demonstrate what the observer sees when real length contraction is involved.

> >	See: https://www.nicvroom.be/VB%20Train%20operation.htm

>	I do not run such programs on my machines.

The screen dumps, with the description, give a reasonable impression.

> >	The simulation shows that the observed moving away train, is shorter than the real length of the train. The simulation also shows that the observed approaching train, is longer than the real length of the train.

>	"Length contraction" applies between INERTIAL FRAMES. The essence of that situation is NOT inertial motion.

The moving train also defines an inertial frame. In that sense the simulation shows a difference what an observer sees at rest between a frame at rest versus a moving frame.
IMO the example par 3-17 contraction or rotation Spacetime physics 2nd does the same.
The text is unambiguous: "In the laboratory frame the cube is Lorentz contracted in the direction of motion" Also in that example different inertial frames are involved.

> >	The question is have these effects any physical consequences.

>	"Length contraction" certainly does, with or without gravitation present (see my three examples). Those "effects" of yours are merely aspects of a gedanken.

No they are not. They are in agreement with 'my' definition. See (**)

> >	I think the three examples mentioned are similair in the sense that no real length contraction is involved.

(*)

>	"length contraction' is never "real" in the sense of affecting the object itself.

I would rewrite this sentence as: (**) "length contraction' in SR is never real (physical) in the sense of affecting the object itself.

>	But it is "real" in the sense that it can have physical consequences (observable effects), as discussed for those three examples.

The problem with the three problems is to find what is common between those three examples. I do not think it is dicussed in the book Spacetime physics.
When you do a search in Google with: "moving magnets and length contraction" you get a very mixed result.

Nicolaas Vroom

>	Tom Roberts

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