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

[michel123456]

Of course, so what?

If you don't accept he feels with his hands in front of him the contracted car, rear and front simultaneously, then you still haven't understood an inch about SR.

Oh I thought naively that when you said Mr Magoo was moving, he was moving with the car...

So you say that Mr Magoo moves and still can reach with his hands the car still at rest. That is quite a challenge. Usually when you move, you know, distance increases...

But even in this case of extending his arms like Mr Fantastic, I guess his hands must join the frame of the car in order to measure it. For example putting a solid ruler next to the car. And you will tell me then that the solid ruler will contract. Because the ruler, the hands, Mr Magoo are under the laws of physics. I am afraid this example leads nowhere.

[Mordred]

There is a distinction people miss. They tend to focus on the object. Lets simplify this.

 

Take a ruler with scale 1:1 now take another ruler B with scale 1:1/2.

 

now observer A uses ruler A. Observer B uses ruler B. Ruler B matches his Euclidean spacetime geometry. Ruler A matches Observer A's Euclidean geometry.

 

They pass the same object to each other from one location to the other.

 

A measures a 1 foot cube.

B also measures a one foot cube.

 

How did this happen? did the object change or did spacetime geometry itself change?

[michel123456]

There is a distinction people miss. They tend to focus on the object. Lets simplify this.

 

Take a ruler with scale 1:1 now take another ruler B with scale 1:1/2.

 

now observer A uses ruler A. Observer B uses ruler B. Ruler B matches his Euclidean spacetime geometry. Ruler A matches Observer A's Euclidean geometry.

 

They pass the same object to each other from one location to the other.

 

A measures a 1 foot cube.

B also measures a one foot cube.

 

How did this happen? did the object change or did spacetime geometry itself change?

I don't understand, please clarify.

Of course, so what?

If you don't accept he feels with his hands in front of him the contracted car, rear and front simultaneously, then you still haven't understood an inch about SR.

 

But don't be desperate. You are not the only one I encounter during the last 25 years thinking that because the length contraction and time dilation is a reciprocal thing between the car passenger and Mr Magoo, it can only be possible if its a visual optical illusion effect.

Do you think Einstein would have spent his time with such a stupid optical illusion effect?

Accepting and understanding are 2 different things.

And it is not a "stupid optical illusion" effect. It is an effect that rules the laws of physics. The question here is to convince me (& others) that the effect of contracting is real, not in the sensibility of the observer (nobody is arguing against that), but in the sensibility of the observed.

I keep saying that the observed doesn't change because it is been observed and measured by an hilarious number of comic heroes.

And reversely the comic heroes are not contracting because they are been measured by the guy in the car.

Edited September 5, 2016 by michel123456

[Mordred]

I don't understand, please clarify.

Ok lets stick with "observer rest frame"

 

[latex]a^2+b^2=c^2 [/latex]

 

time within a reference frame is absolute. Rest frame only...

 

The spacetime geometry is Euclidean flat. There is no spacetime curvature. All the Newtonian physics apply. Optical illusions due to distance change.

 

However the same is true in Observer B's rest frame. Within a rest frame all rules of Pythagoras geometry and trig apply. Time is normalized as 1.

 

These two geometry spacetime conditions are physically Real in every sense.( Geometry relations, don't treat spacetime as a material thing)

 

It isn't until you compare Euclidean geometry A to Geometry B and vise versa do you realize the two do not match. Time and the x axis are distinctly different.

 

So we have to translate the two distinct spacetime Euclidean geometries. This is what relavity allows us to do.

 

Your rest frame geometry compared to my rest frame geometry every observer has his own rest frame Euclidean geometry this is real in every sense.

 

So it isnt that different observers change anothers rest frame geometry. That is false...

 

It is the comparison of my rest frame compared to your rest frame that changes. Not the rest frames itself. Spacetime itself is not absolute. Different observers has his own Euclidean rest frame with different lengths x axis only and time This is real it is also what people have a hard time accepting...

 

An outside observer doesn't change my rest frame. It is the comparison between the two reference rest frames that changes.

Edited September 5, 2016 by Mordred

[VandD]

I don't understand, please clarify.

Accepting and understanding are 2 different things.

And it is not a "stupid optical illusion" effect. It is an effect that rules the laws of physics. The question here is to convince me (& others) that the effect of contracting is real, not in the sensibility of the observer (nobody is arguing against that), but in the sensibility of the observed.

I keep saying that the observed doesn't change because it is been observed and measured by an hilarious number of comic heroes.

And reversely the comic heroes are not contracting because they are been measured by the guy in the car.

 

I don't think you will ever understand SR, but let's see whether this helps to trigger your mind:

 

The 4D train (or car) indeed doesn't change. When Mr Magoo moves relative to the train he feels with his hands a different 3D section through that 4D train. And that section through the 4D train is shorter than a 3D section through the train if Mr Magoo is at rest relative to the train.

 

You pretend there is only a 3D real train and Mr Magoo moving relative to the train 'observes' a distortion of that 3D train. That's wrong. You completely miss the fact that the different section throught the 4D train is a equaly valid different set of simultaneous events.

And if you call one set of simultaneous events 'real', then all the other 3D sections are real as well. You simply have no valid physical reason for believing the contrary.

Edited September 5, 2016 by VandD

[swansont]

This is wrong.

Mr Magoo is (being observed) contracting as well. He feels nothing.

No, it's not wrong.

 

Other people observing him have no effect on him. He is always in his own frame.

 

I have a deal to propose for you:

I will agree with that if you agree that in no way an observer can make things happen in an object just by looking at it.

AFAICT nobody has suggested otherwise. I know I have been adamant that this view is wrong.

 

 

Excellent point about Newton mechanics: nobody is arguing that a body "has" kinetic energy. Nobody is asking "who is right?" because the simple answer is that kinetic energy is relative.

The question is why is nobody asking this of KE, but they are asking it of the length? Why is one obvious but the other is disbelieved/rejected?

 

But I argue that the body to which this kinetic energy is linked is a body that "exists" and "has" dimensions. It is not a body that "has" multiple lengths at the same time according to the state of motion of its observers. And if another observer arrives, the body doesn't change length.

And exactly as in the case of kinetic energy, there is a "zero" point, which corresponds to the frame at rest. In this frame, kinetic energy is zero, the length is maximum, the ticking rate is minimum, and rest mass is defined.

And the body doesn't change its speed or KE, either. It simply has those values when measured in the other observer's frame. That is the "reality" for that other observer. Further — and this is really important, since this is a physics discussion — those are the values that other observer needs to use to do physics. Using the rest values for relative variables is nonsensical.

[Tim88]

Maybe a simple statement may help.

 

" Every observed measurement is real from the perspective of the observer."

 

without throwing in time dilation or length contraction we can see this when one measures speed or percieved size of an object or even the percieved color (frequency of light) the percieved blackbody temperature will even be affected.

 

Its funny no one has any problem accepting observer perceptions in these cases. Yet the observers in relativity follow precisely the same rules. With or without time dilation.

[..]

 

I don't think that anyone has a problem with the reality of observation of phenomena; for sure the phenomena are real. For example, at turnaround the traveler observes an instant increase of the light and radio frequencies coming from Earth, compared to the ship's own clock frequency. Disagreements only arise when claims are made that go beyond the phenomena themselves.

[michel123456]

No, it's not wrong.

 

Other people observing him have no effect on him. He is always in his own frame.

 

AFAICT nobody has suggested otherwise. I know I have been adamant that this view is wrong.

 

 

The question is why is nobody asking this of KE, but they are asking it of the length? Why is one obvious but the other is disbelieved/rejected?

 

And the body doesn't change its speed or KE, either. It simply has those values when measured in the other observer's frame. That is the "reality" for that other observer. Further — and this is really important, since this is a physics discussion — those are the values that other observer needs to use to do physics. Using the rest values for relative variables is nonsensical.

OK let's discuss length only.

 

When the observer extends his hands and physically feels the length of the car being contracted, we are talking about the following (please correct me if I get it wrong)

_the fact of touching simultaneously the 2 ends of the car is in fact 2 different events. That is because the left end of the car has a different 4D coordinate than the right end of the car. The T coordinate, as observed by you, is the same for both events, but the spatial coordinates are different.

So we have a set of 2 events.

For the moving observer, the 2 events are simultaneous

For the observer in the car, the 2 events are not simultaneous.

 

What about the length of the car?

For the observer in the car, as measured at rest, the car has its own proper length.

For the observer moving , the measured length correspond to the projection of a rotation of this length: it is contracted. Think of a circle with diameter=length. According to the angle of the diameter, its orthogonal projection will be less or equal, never larger.

Anyway, what the moving observer measures is the projection of the length, not the proper length.

 

The 3D length between the two 4D events doesn't change.

Edited September 6, 2016 by michel123456

[swansont]

OK let's discuss length only.

 

When the observer extends his hands and physically feels the length of the car being contracted, we are talking about the following (please correct me if I get it wrong)

_the fact of touching simultaneously the 2 ends of the car is in fact 2 different events. That is because the left end of the car has a different 4D coordinate than the right end of the car. The T coordinate, as observed by you, is the same for both events, but the spatial coordinates are different.

So we have a set of 2 events.

For the moving observer, the 2 events are simultaneous

For the observer in the car, the 2 events are not simultaneous.

Which should not be surprising as the length is different.

 

What about the length of the car?

For the observer in the car, as measured at rest, the car has its own proper length.

For the observer moving , the measured length correspond to the projection of a rotation of this length: it is contracted. Think of a circle with diameter=length. According to the angle of the diameter, its orthogonal projection will be less or equal, never larger.

Anyway, what the moving observer measures is the projection of the length, not the proper length.

 

The 3D length between the two 4D events doesn't change.

 

And?

 

If the stationary observer wants to do any physics involving the train, which length should she use? Let's say she wants to cross the track before the next train comes. Does she use the proper length?

[Tim88]

[..] it is not a "stupid optical illusion" effect. It is an effect that rules the laws of physics. The question here is to convince me (& others) that the effect of contracting is real, not in the sensibility of the observer (nobody is arguing against that), but in the sensibility of the observed.

I keep saying that the observed doesn't change because it is been observed and measured by an hilarious number of comic heroes.

And reversely the comic heroes are not contracting because they are been measured by the guy in the car.

 

I notice that we are now zooming in on the issue that relativistic effects cannot be a mere illusion.

 

"The question here is to convince me (& others) that the effect of contracting is real, [..] in the sensibility of the observed" is a faulty question - similar to asking someone to convince you that a chameleon is brown.

 

IMHO, the main reason why it is difficult to really understand SR, is because people often debate two erroneous interpretations, neither of which matches SR:

1. all measurements of length contraction and time dilation of a measured object inform us about the physical reality of that object

2. all measurements of length contraction and time dilation of a measured object are mere illusions.

 

1) can easily be disproved by the fact that it depends on with which reference standards they are compared; and we cannot prefer one over the other.

2) can easily be disproved by the fact that clock retardation is measurable as a physical fact about which everyone has to agree; mere illusions cannot do that.

 

The solution, of course, has to be found in the fact that every measurement is a comparison between the measured object and a reference object.

We then find that a change of velocity must affect the object that changed velocity. With the Lorentz ether interpretation, true changes happened to lengths and clock rates, but the true values are not accessible to us. With the Minkowski Spacetime (or Block Universe) interpretation, the object's trajectory in Spacetime receives a bend at that location in Spacetime.

 

Consequently when next things such as lengths and frequencies are compared, they will be measured differently from before due to a physical effect on the accelerated object.

Thus, if it is the observed that changes velocity, there is a real effect on the observed (although measured differently from different "perspectives"); while if it is the observer that changes velocity, the apparent effect on the observed is due to a real effect on the observer.

 

A most useful exercise from which I personally gained much understanding, is to do a calculation of the effect of velocity on the mutual observations of lengths and clock rates, as calculated from an assumed "rest system" S0, e.g.

S0 C1-----L----C2

S1 C'1-----L----C'2 ->v

 

A more elaborated calculation that leads to additional insight, is to analyse what happens if S1 starts at rest and then attains velocity v.

[michel123456]

 

I notice that we are now zooming in on the issue that relativistic effects cannot be a mere illusion.

 

"The question here is to convince me (& others) that the effect of contracting is real, [..] in the sensibility of the observed" is a faulty question - similar to asking someone to convince you that a chameleon is brown.

 

IMHO, the main reason why it is difficult to really understand SR, is because people often debate two erroneous interpretations, neither of which matches SR:

1. all measurements of length contraction and time dilation of a measured object inform us about the physical reality of that object

2. all measurements of length contraction and time dilation of a measured object are mere illusions.

 

1) can easily be disproved by the fact that it depends on with which reference standards they are compared; and we cannot prefer one over the other.

2) can easily be disproved by the fact that clock retardation is measurable as a physical fact about which everyone has to agree; mere illusions cannot do that.

 

The solution, of course, has to be found in the fact that every measurement is a comparison between the measured object and a reference object.

We then find that a change of velocity must affect the object that changed velocity. With the Lorentz ether interpretation, true changes happened to lengths and clock rates, but the true values are not accessible to us. With the Minkowski Spacetime (or Block Universe) interpretation, the object's trajectory in Spacetime receives a bend at that location in Spacetime.

 

Consequently when next things such as lengths and frequencies are compared, they will be measured differently from before due to a physical effect on the accelerated object.

Thus, if it is the observed that changes velocity, there is a real effect on the observed (although measured differently from different "perspectives"); while if it is the observer that changes velocity, the apparent effect on the observed is due to a real effect on the observer.

 

A most useful exercise from which I personally gained much understanding, is to do a calculation of the effect of velocity on the mutual observations of lengths and clock rates, as calculated from an assumed "rest system" S0, e.g.

S0 C1-----L----C2

S1 C'1-----L----C'2 ->v

 

A more elaborated calculation that leads to additional insight, is to analyse what happens if S1 starts at rest and then attains velocity v.

This is enlightenment! (the bold part above). I agree 100%. That is fully more understandable. Thank you.

But is that Relativity?

Because that is not what I have understood from this long conversation. The exact contrary to speak frankly.

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

Edited for enabling 2nd thoughts...after enthusiasm rebound.

 

 

And?

 

 

Your question suggests that my description is correct.

 

If the stationary observer wants to do any physics involving the train, which length should she use? Let's say she wants to cross the track before the next train comes. Does she use the proper length?

I guess not. But if she wants to embark merchandise, she should use proper length. IOW it depends on the question.

Edited September 6, 2016 by michel123456

[swansont]

I guess not. But if she wants to embark merchandise, she should use proper length. IOW it depends on the question.

Embark merchandise?

 

Anyway, it's physics. It's got to be a physics question.

[VandD]

 

Anyway, what the moving observer measures is the projection of the length, not the proper length.

 

The 3D length between the two 4D events doesn't change.

 

You cannot measure a projection of a length. What an observer mesures is the distance between two simultaneous events. If there is relative movement, the observer measures NOT the distance between the same simultaneous events as an observer 'at rest' would do. Do you understand this, or did you never read about relativity of simultaneity? No wonder you don't understand different 3D sections through 4D spacetime...

Edited September 6, 2016 by VandD

[Tim88]

This is enlightenment! (the bold part above). I agree 100%. That is fully more understandable. Thank you.

But is that Relativity?

Because that is not what I have understood from this long conversation. The exact contrary to speak frankly.

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

Edited for enabling 2nd thoughts...after enthusiasm rebound.

 

I did not notice a direct contradiction to my "bold" statement in this long thread; however I did not read all comments. Note that SR, just like QM, is a theory about phenomena; the popular version (Einstein's) is on purpose stripped of speculations about "reality". In that sense, strictly speaking, no direct answer on the questions about "reality" "are" SR.

 

However, in the sentences that led to that statement I briefly indicated that both the "Lorentzian" and the "Minkowskian" interpretation agree that nothing (or at least, nothing that we are here concerned with) happens to the inertial system, but that something happens to the non-inertial system that affects mutual measurements (if before and after the change of state the system is used as inertial reference system). And I know no other interpretations that consistently match SR.

 

Maybe you can identify posts that seem to disagree with that?

 

[edit: glitches in phrasing]

Edited September 6, 2016 by Tim88

[michel123456]

No. What you show is not what is meant by time dilation (time slowing).

The time dilation is here:

 

For RED simultaneity: When red clock indicates 2015, the green clock indicates 2013.4

I.ow. when the red clock ticked from 2007 to 2015 (8 years), the green clock ticked from 2007 to 2013.4 (=6,4 years). This is also what gamma tells us for v=.6c:

8 / 1.25 = 6.4

 

For GREEN simultaneity: When green clock indicates 2015, the red clock indicates 2013.4

I.o.w. when the green clock ticked from 2007 to 2015 (8 years), the red clock ticked from 2007 to 2013.4 (=6,4 years). This is also what gamma tells us for v=.6c:

8 / 1.25 = 6.4

 

A. s. o.

For example: (not made bold in the diagram)

For GREEN simultaneity: When green clock indicates 2013.4, the red clock indicates ... 2012.12. Can you find which green simultaneity line I'm talking about?

When the green clock ticked from 2007 to 2013.4 (6.4 years), the red clock ticked from 2007 to 2012.12 (=4.12 years). This is also what gamma tells us for v=.6c:

6.4 / 1.25 = 4.12

And where is length contraction?

[VandD]

And where is length contraction?

Don't you know? I thought you knew everything about length contraction?

[VandD]

I see you still made no progress...

 

I show you symmetry of length contraction in the other thread.