# Relativity and shared realities (split from clocks, rulers...)

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This is the most sensible post I have read for a while. +1

But:

The weird thing is about your statement in bold, because you could say that the same happens to length: the object is observed as being contracted.

However it seems to me (from another thread) that nothing "physically happened " to the ruler, it remains the same length all the time. Simply it is been observed as contracting.

Do you perhaps think a moving train in front of your eyes is NOT contracted, but you only see it contracted (kind of an optical illusion)?

Stretch your arm and show two fingers. A very fasty moving train in front of your arm will fit exactly between your two fingers. You physically feel the rear of the train touching one finger simultaneously with the front of the train touching the other finger. Now tell me whether you consider that contracted train physically real or not?

And yes, the passenger in that train will have a physically very small you standing on a physically very small embankment: a split second you fit perfectly between two fingers of the train passenger.

Seems you don't understand how relativity of simultaneity works.

You cannot consider one set of simultaneous events as real, but not another set of simultaneous events.

Edited by VandD

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The weird thing is about your statement in bold, because you could say that the same happens to length: the object is observed as being contracted.

However it seems to me (from another thread) that nothing "physically happened " to the ruler, it remains the same length all the time. Simply it is been observed as contracting.

The "physical happening" is the acceleration which must happen if the speed changes. However, this does not compress the ruler.

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Do you perhaps think a moving train in front of your eyes is NOT contracted, but you only see it contracted (kind of an optical illusion)?

Stretch your arm and show two fingers. A very fasty moving train in front of your arm will fit exactly between your two fingers. You physically feel the rear of the train touching one finger simultaneously with the front of the train touching the other finger. Now tell me whether you consider that contracted train physically real or not?

And yes, the passenger in that train will have a physically very small you standing on a physically very small embankment: a split second you fit perfectly between two fingers of the train passenger.

Seems you don't understand how relativity of simultaneity works.

You cannot consider one set of simultaneous events as real, but not another set of simultaneous events.

O yes, and a passenger on the train measures that YOU are contracted, because YOU are moving and he is at rest.

And yes I don't understand how Relativity works, based on what is written in literature.

Or maybe I understand something and I think it is a wrong interpretation of accurate mathematical equations.

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

Edited by michel123456
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This is the most sensible post I have read for a while. +1

But:

The weird thing is about your statement in bold, because you could say that the same happens to length: the object is observed as being contracted.

However it seems to me (from another thread) that nothing "physically happened " to the ruler, it remains the same length all the time. Simply it is been observed as contracting.

Thanks

People who claim that "nothing physically happened" will have a difficult time to explain how after "nothing happened" there is a difference in clock readings - by pure definition something physical happened. And by straightforward definition too, the average clock rates were different during the separation according to all reference systems.

Note that, nevertheless, this has historically been the subject of much debate; I recall having searched for and found, in the peer reviewed literature, articles that claim that "nothing happens" and articles that "length contraction is real" (from memory, I don't recall the exact phrasings but they were definitely in disagreement!). also, neglecting a possible effect of the force of acceleration, of course the proper length does not change, and neither does the proper clock rate.

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All frames exist simultaneously. Different observers observe the clock as ticking at different rates and the stick as being at different lengths over the same period as each other.

Every possible length and tick rate already "exists" at all times. When something changes frames, it is not physically changing, it is simply changing which length and tick rate is measured by which frame.

Yes (for the bold part above)

As I have stated numerous times before: the observer does not dictate what is "really" happening. Every observer in any state of motion will have another "view" of what "reality is".

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Yes (for the bold part above) [When something changes frames, it is not physically changing]

As I have stated numerous times before: the observer does not dictate what is "really" happening. Every observer in any state of motion will have another "view" of what "reality is".

Although I agree with your statement, I was contemplating to add a clarification to my earlier post which, by pure chance, disagrees with the bold statement with which you agreed. The reason is subtle but essential for understanding.

I consider "physical change" to mean a change according to measurements with any reference system in which the laws of physics hold - called "inertial frames" or "Observers". As words like "reality" are insufficiently predefined for discussions as this one, I label such a physical change as "real".

Now -please forgive a little nitpicking- in SR it is impossible for things to change inertial frames; no doubt what was meant, is that when something changes velocity relative to inertial frames, it is not physically changing.

However, we know from SR that according to measurements with any inertial frame, the length and emission frequency or clock frequency of things change when they change speed. The change of clock frequency has been confirmed by experiments, and if we assume that the speed of light is unaffected by the motion of the source, then necessarily this implies a similar change of length.

This is important for physical insight: the different "view" of Observers in different state of motion is immediately understood as due to their different states of motion - there is no magical difference in view without physical cause for that difference in view.

That also immediately clarifies why it is not the same if you accelerate, or if the other accelerates: if you change velocity and recalibrate your instruments to the new "rest system" (indeed, your clocks are not automagically synchronized to that system!), then -using the above definition of "real"- the change in lengths and clock rates of the other system are merely appearance due to real changes in your measurement system. However, the changes in your measurement system remain "relative" in the sense that different Observers attach different values to them.

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People who claim that "nothing physically happened" will have a difficult time to explain how after "nothing happened" there is a difference in clock readings - by pure definition something physical happened. And by straightforward definition too, the average clock rates were different during the separation according to all reference systems.

What is meant is that it is not a mechanical effect from the motion. It's not friction, or vibration, or any other effect that could be mitigated with a different assembly. It is not an effect like the change in frequency of a pendulum clock, which explicitly depends on g (and the length) if you moved it to a different elevation, or changed temperature, etc.

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O yes, and a passenger on the train measures that YOU are contracted, because YOU are moving and he is at rest.

Yes, for a split second he feels me between his two fingertips.

And you think it's madness because you don't accept that for the train passenger I'm the one moving? Is that it?

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What is meant is that it is not a mechanical effect from the motion. It's not friction, or vibration, or any other effect that could be mitigated with a different assembly. It is not an effect like the change in frequency of a pendulum clock, which explicitly depends on [...] if you [..] changed temperature, etc.

Yes, agreed; however:

[..] It is not an effect like the change in frequency of a pendulum clock, which explicitly depends on g (and the length) if you moved it to a different elevation [..]

There the discussion would come into muddy (and GR) waters, as it's *somewhat* like that. Both changes don't depend on the kind of material or assembly. In Einstein's original version of GR it was even pretended to be the same effect (edit: from a different perspective), substituting acceleration with gravitation.

Edited by Tim88
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O yes, and a passenger on the train measures that YOU are contracted, because YOU are moving and he is at rest.

And yes I don't understand how Relativity works, based on what is written in literature.

Or maybe I understand something and I think it is a wrong interpretation of accurate mathematical equations.

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

You are right, there is a logical difficulty.

I can illustrate it with a little story:

Two space travellers are moving toward each other.

They want to know whose ship is the longest. They decide to do the following

experiment: they place a clock at each end of the ships and each of them

synchronizes its two clocks. They take place in the middle of the ship.

When they will cross the other, they 'll throw a paint spot to the other

ship on each end (This moment can be calculated in advance, so the two shoots

can be done at the same moment).

After that they will compare the impacts.

But they are astonished about the experiment and they decide to meet to

compare the spaceships.

The two ships have the same length. Where are the paint spots?

These are the two versions of the experiment as viewed by A and B:

(Note that each of them consider himself at rest)

There are two important facts:

- the synchronization is done by light signals. Therefor, each observer

considers that his clocks indicate the same time, but that the other's

clocks are not synchronized (the front clock runs after and the back one

runs forward wrt the middle of the ship).

- the bodies are contracted in the direction of the movement.

VERSION A VERSION B

1. A is at rest. B is at rest

B's clocks are not synchronized. A's clocks are not synchronized.

B shoots first at the back and A shoots first at the back and

misses A. misses B.

v====B====| --> |=====B=====|

|=====A=====| <-- |====A====^

2. B is smaller than A, so A doesn't A is smaller than B, so B doesn't

hit B. hit A.

|====B====| --> v=====B=====v

^=====A=====^ <-- |====A====|

3. Now B shoots at the front and Now A shoots at the front and

misses A. misses B.

|====B====v --> |=====B=====|

|=====A=====| <-- ^====A====|

4. When they meet, the lengths are the same and the result corresponds

with both 's experience (they both missed the other).

|=====B=====| |=====B=====|

|=====A=====| |=====A=====|

The crucial moment is of course point 2.

At the same time, A pretends that B is smaller, but B states the contrary.

Are they both right?

Obviously, both versions are consistent, so both can be true.

But logically, the two versions are mutually exclusive, so they can't be

true together, at the event E, which is the moment A and B are

at the same place (point 2).

At that moment, the reason why the spots miss the target is

- either because one of the ships is smaller than the other (1)

- or because the clocks are not synchronous (2)

(1) can not be right for both ships, but (2) implies that the ship in which

the clocks are not synchronized is moving, so at least one of the observers has

to accept that he is not really stationary.

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There the discussion would come into muddy (and GR) waters, as it's *somewhat* like that. Both changes don't depend on the kind of material or assembly. In Einstein's original version of GR it was even pretended to be the same effect (edit: from a different perspective), substituting acceleration with gravitation.

But we're discussing SR, so this is moot. You can't do any measurement that tells you who is moving, and there is no mechanical effect slowing the clocks.

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The crucial moment is of course point 2.

At the same time, A pretends that B is smaller, but B states the contrary.

Are they both right?

Yes.

Obviously, both versions are consistent, so both can be true.

They are both true.

But logically, the two versions are mutually exclusive, so they can't be

true together, at the event E, which is the moment A and B are

at the same place (point 2).

At that moment, the reason why the spots miss the target is

- either because one of the ships is smaller than the other (1)

- or because the clocks are not synchronous (2)

(1) can not be right for both ships, but (2) implies that the ship in which

the clocks are not synchronized is moving, so at least one of the observers has

to accept that he is not really stationary.

For ship A astronaut the two A-shot events occur simultaneously with event E. But none of the B-shot events occurs simultaneously with event E.

For ship B astronaut the two B-shots occur simultaneously with event E, but none of the A-shots.

Relativity of simultaneity at work. Welcome to SR

At event E both astronauts consider a different 3D space of simultaneous events existing 'now' at event E. The different 3D spaces of simultaneous events are only 3D reality sections of the bigger 4D spacetime reality (see also Einstein's quotes here.)

One more thing: movement is a relative thing: for A, B moves. For B, A moves. Period.

Edited by VandD
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tim88

......................clocks loose their ticks...............

Paste function not working.

Clocks don't loose some ticks, they just never make them.

A big source of confusion is identifying which frame a clock is in and not trying to take time differences between clocks in different frames.

Edited by studiot
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They are both true.

You must have meant: they are both wrong.

Because as you have all agreed when the ships are put side by side, they have the same length.

And at no moment in your graphs the ships have change length.

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But we're discussing SR, so this is moot. You can't do any measurement that tells you who is moving, and there is no mechanical effect slowing the clocks.

You introduced gravitation in this thread, not me; and it's not "moot" at all. We can tell who is accelerating just as we can tell who is at a different gravitational potential. I would be surprised it anyone here was thinking about mechanical effects; but one never knows of course!

[...]

Clocks don't loose some ticks, they just never make them.

[..]

Ah well spotted, that was somewhat sloppy phrasing - thanks for the correction!

Similar nitpicking:

[...]

A big source of confusion is identifying which frame a clock is in [..]

Once more, all clocks are in all frames. To avoid confusions and calculation errors one has to either stick to one frame, or consciously do a transformation from one frame to another.

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You introduced gravitation in this thread, not me

I think you'll find that you're the only one who has mentioned it thus far, should you choose to look.

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You must have meant: they are both wrong.

Because as you have all agreed when the ships are put side by side, they have the same length.

And at no moment in your graphs the ships have change length.

Interesting total disagreement!

On this I agree with bvr, and I suspect that the disagreement with VandD is about language, or philosophical (in which case it cannot be fixed by physics).

When the ships are put side by side at the same velocity then they are measured to be the same length. Expanding on my posts #29 and #31:

According to SR the length of objects is contracted when they are in motion (in itself that is a *relative" observation); and this is consistent with the measured clock retardation due to motion of atomic clocks. By the way, relativity of simultaneity is irrelevant for that absolute effect, as the final comparison is done at the same location.

I think you'll find that you're the only one who has mentioned it thus far, should you choose to look.

I replied to your "the change in frequency of a pendulum clock, which explicitly depends on g". g is the gravitation constant. However, I now take it that you merely intended to introduce it as a kind of spring. And then we totally agree on that.

Edited by Tim88
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Because this thread is going away from the OP, I would like to resume the question.

From what I understand, everybody except me agree that all observers are correct when observing that the ruler/ship/train is contracting.

My objection to that is NOT that they are actually observing that phenomenon.

My objection is that it is not possible for the ruler/ship/train to be physically large/short/shorter all at the same time. To me, there must be only one single physical entity.

This "physical entity" is the one that you draw in the diagram.

It is the ruler/ship/train as measured at rest in its own frame.

Because if you have no starting point, no proper length, then the ruler/ship/train has multiple lengths and it is impossible to draw it in your diagram at the first place.

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

And to add: this proper length never changes. The effect of shortening is an effect caused by observation from another state of motion. Again, the observer can have no effect on the physical entity.

Interesting total disagreement!

On this I agree with bvr, and I suspect that the disagreement with VandD is about language, or philosophical (in which case it cannot be fixed by physics).

When the ships are put side by side at the same velocity then they are measured to be the same length. Expanding on my posts #29 and #31:

According to SR the length of objects is contracted when they are in motion (in itself that is a *relative" observation); and this is consistent with the measured clock retardation due to motion of atomic clocks. By the way, relativity of simultaneity is irrelevant for that absolute effect, as the final comparison is done at the same location.

I replied to your "the change in frequency of a pendulum clock, which explicitly depends on g". g is the gravitation constant. However, I now take it that you merely intended to introduce it as a kind of spring. And then we totally agree on that.

And, what a coincidence, it appears that this length is the maximum possible from all other states of motion! It is a "special" length.

Edited by michel123456
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[..]

- either because one of the ships is smaller than the other (1)

- or because the clocks are not synchronous (2)

(1) can not be right for both ships, but (2) implies that the ship in which

the clocks are not synchronized is moving, so at least one of the observers has

to accept that he is not really stationary.

An "Observer" in SR is in fact a reference system that is supposed, by definition, to be stationary. Each Observer, by convention, assumes the other to be in motion. The Observers must admit (in fact they already claim) that they are not both really stationary; however none of them is obliged to accept that his conventional assumption is wrong. Once more, that is no different in Newtonian mechanics.

Because this thread is going away from the OP, I would like to resume the question.

From what I understand, everybody except me agree that all observers are correct when observing that the ruler/ship/train is contracting.

My objection to that is NOT that they are actually observing that phenomenon.

My objection is that it is not possible for the ruler/ship/train to be physically large/short/shorter all at the same time. To me, there must be only one single physical entity.

This "physical entity" is the one that you draw in the diagram.

It is the ruler/ship/train as measured at rest in its own frame.

Because if you have no starting point, no proper length, then the ruler/ship/train has multiple lengths and it is impossible to draw it in your diagram at the first place.

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

And to add: this proper length never changes. The effect of shortening is an effect caused by observation from another state of motion. Again, the observer can have no effect on the physical entity.

[..]

Bold face mine. I think that I already replied to that in post #31. According to some the ruler contracts while according to others it expands; thus I'm an exception to "everybody". Observation follows physical laws, in particular the laws of optics. How do you explain that the observation from another state of motion results in a shortening effect? Or that clocks really retard on others, if nothing really happens?

PS I know of two explanations for "one physical entity" that are consistent with SR: 4D block universe (which is a kind of 4D ether) and Lorentz ether (which is 3D). We are free to choose the one that appeals most to us.

Edited by Tim88
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Actually, Tim, the frame of the clock(s) is of vital importance and most certainly not present at all events, which they would be if they were present in all frames.

Michel,

There is a big difference between length contraction and time dilation.

An example for both of you.

A rocket ship (observer A) leaves Earth (observer B) and take with it a clock and an one metre rule, marked off in 1cm graduations (0 through 100) and travelling at 0.8c.

Both A and B will always 'see' the one hundred and one graduations on the ruler at all points in the journey.

But they will differ in their assessment of the length of that ruler.

As to the clocks, say A sends back a light flash every year, A will not see anything in the first three years, that is A's first flash will arrive at B when B's clock reads 3 years.

The rocket clock is no longer in B's frame and B cannot measure on it.

Edited by studiot
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Actually, Tim, the frame of the clock(s) is of vital importance and most certainly not present at all events, which they would be if they were present in all frames.

[..]

I cannot parse that sentence; however Einstein's 1905 paper may be useful for clarification. He used just one frame for two clocks, see §4 of http://www.fourmilab.ch/etexts/einstein/specrel/www/ And although this is slightly off-topic, by chance the title there is "physical meaning [..]"

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Bold face mine. I think that I already replied to that in post #31. According to some the ruler contracts while according to others it expands; thus I'm an exception to "everybody". Observation follows physical laws, in particular the laws of optics. How do you explain that the observation from another state of motion results in a shortening effect? Or that clocks really retard on others, if nothing really happens?

PS I know of two explanations for "one physical entity" that are consistent with SR: 4D block universe (which is a kind of 4D ether) and Lorentz ether (which is 3D). We are free to choose the one that appeals most to us.

Concerning the bold black text above: The ruler can expand? More than what is measured at rest in its own frame?

Concerning the blue text : many many reasons (like you said the laws of optics) make us observe the world with paramorphosis. Relativity is another effect.

About clocks, there is nothing weird into observing a clock retarding on others.

The weird thing happens when the clocks are reunified because the symmetry of Relativity is broken: it should have been that both clocks see the other as ticking slower. While in the famous twin paradox, there is no such a symmetry. There is a "preferred" clock at rest and an "unpreferred" clock in motion. This second clock observes exactly the contrary of what Relativity tells us.

Edited by michel123456
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tim88

I cannot parse that sentence

Sorry, Tim, I see what you mean. Try this.

Actually, Tim, the frame of the clock(s) is of vital importance which are most certainly not present at all events in all frames, which they would be (could be) if they were present in all frames. I was not referring to Einsteins paper but the paintball games posts in this thread.

Concerning the bold black text above: The ruler can expand? More than what is measured at rest in its own frame?

Concerning the blue text : many many reasons (like you said the laws of optics) make us observe the world with paramorphosis. Relativity is another effect.

About clocks, there is nothing weird into observing a clock retarding on others.

The weird thing happens when the clocks are reunified because the symmetry of Relativity is broken: it should have been that both clocks see the other as ticking slower. While in the famous twin paradox, there is no such a symmetry. There is a "preferred" clock at rest and an "unpreferred" clock in motion. This second clock observes exactly the contrary of what Relativity tells us.

In a way it is a shame that this thread had been split from another since it seems to have killed its progenitor.

The twins offer a good opportunity to simply work through both time dilation and length contraction.

In neither thread is anyone prepared to work properly through this.

Furthermore the example I quoted above is the commencement of a simple analysis where the numbers drop out very easily.

It also shows what happens to both clocks, only one of which (the rocket clock) is present at all important events. (Do you know what an 'event' is?)

It is the sad fact that so many published 'explanations' of the twins tells less than half the story that IMHO confuses so many people.

(I have to own to a bit of this myself in my very simple explanation of the asymmetry)

I thought Mordred had already explained why you mistakenly think one clock 'ticks' faster and how you were misinterpreting spacetime diagrams, but obviously I was wrong since you persist in this view.

Edited by studiot
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You must have meant: they are both wrong.

.

No, they are both correct. The problem is that you refuse to see why.

Because as you have all agreed when the ships are put side by side, they have the same length.

They have not the same length when they cross in 2. (see below). You probably missed that.

2. B is smaller than A, so A doesn't A is smaller than B, so B doesn't

hit B. hit A.

|====B====| --> v=====B=====v

^=====A=====^ <-- |====A====|

They have both the same length when they get back together and are at rest relative to each other.

And at no moment in your graphs the ships have change length.

Wrong. See 2.

Edited by VandD
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No, they are both correct. The problem is that you refuse to see why.

They have not the same length when they cross in 2. (see below). You probably missed that.

They have both the same length when they get back together and are at rest relative to each other.

Wrong. See 2.

Yes I missed that and I still do. The observed object has the same length in both cases (but I must miss something)

(...)

I thought Mordred had already explained why you mistakenly think one clock 'ticks' faster and how you were misinterpreting spacetime diagrams, but obviously I was wrong since you persist in this view.

I have understood that a clock doesn't tick "so faster than I thought it was", i.e. that the clock that clicked slower only regains its original clicking rate (maybe wrong wording here). As for the rest I don't understand where I am misunderstanding spacetime diagrams.

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