Clocks and rulers

[StringJunky]

If we send a clock up into space or take it on a high speed journey then bring it back it will not be synchronised with a clock that was in the same position at the start. If we send a ruler out in the same type of experiment, has its length changed when it is returned to its starting position? If it hasn't changed why not, if a clock does? What's different about them that one is permanently changed but the other isn't?

[Strange]

Good question!

 

You should compare the length a ruler with the rate at which the clock ticks - i.e. the length of the units they measure. In that case you will find them both consistent when they return from their journey.

 

The equivalent of the elapsed time measured by the clock is the total distance travelled by the ruler.

[StringJunky]

Good question!

 

You should compare the length a ruler with the rate at which the clock ticks - i.e. the length of the units they measure. In that case you will find them both consistent when they return from their journey.

 

The equivalent of the elapsed time measured by the clock is the total distance travelled by the ruler.

Any possibility of elaborating a bit more?

[Sensei]

If object is in the same frame of reference as other object,

it's velocity vector is matching velocity vector of other object,

therefor Lorentz factor [math]\gamma=\frac{1}{\sqrt{1-\frac{v^2}{c^2}}}[/math]

with same v, are the same value.

And basically be both equal 1.

 

Analyze situation for Lorentz factor 1.0 at the beginning,

then in one object it's going to some higher value (because it's accelerating),

and then after some time, it's going back to initial 1.0 value (because it's decelerating).

While the other object, reference Earth for example, it's all the time 1.0.

Edited July 29, 2016 by Sensei

[Mordred]

If we send a clock up into space or take it on a high speed journey then bring it back it will not be synchronised with a clock that was in the same position at the start. If we send a ruler out in the same type of experiment, has its length changed when it is returned to its starting position? If it hasn't changed why not, if a clock does? What's different about them that one is permanently changed but the other isn't?

You need to think of what is meant by synchronized and clock rate.

 

The clock will no longer be synchronized. However both length and clock rate will return to match the original conditions.

 

Albiet the clock wont be synchronized.

[StringJunky]

You need to think of what is meant by synchronized and clock rate.

 

The clock will no longer be synchronized. However both length and clock rate will return to match the original conditions.

 

Albiet the clock wont be synchronized.

Gotcha. The ruler can't leave a trace what length it was before.

[swansont]

Mordred's point is the key: the analogous effect to length contraction is a change in frequency, or the duration of a standard unit of time, e.g. one second. Time is the integral of frequency (counting the ticks of a clock). If you count the number of meter stick lengths, you will get the same conceptual result. The lengths measured by the two observers will not agree.

[StringJunky]

The lengths measured by the two observers will not agree.

I know, if the two lengths are compared simultaneously in their respective positions, but I was wondering why the ruler doesn't stay at the length it was in space like the clock stays at the same time position but, as Mordred pointed out, the clock does resume its original frequency back on Earth but just happens to leave a trace of what it was in space. It appears that a ruler can't do that

[Endy0816]

Odometer?

[Delta1212]

Yeah, an odometer is a better comparison for a clock. A ruler is more like a metronome. The metronome has a tick rate that changes with time dilation, but doesn't record the number of ticks, and when returned to the original frame, the change in tick rate will return to normal with no discernible "record" of the elapsed time.

 

Rulers don't record distance. An odometer would maintain a record of the distance traveled at the length contracted distance and not leap to a large number to reflect the change in the distance of the path it had already traveled as measured in the new frame, in the same way that a clock doesn't jump forward to reflect the change in the elapsed time as measured by the frame it is returning to.

 

Your confusion seems to be that you are conflating the change in the length of the units used to measure travel through a temporal/spatial dimension with the duration/distance that is covered while moving through it. The units change to whatever the present frame measures with no memory of any previous frames. Recorded elapsed time/distance does not change.

 

A clock measures both the units and the elapsed time. A ruler measures units but not distance traveled.

 

Edit: For a concrete example:

 

A clock measures your point in time in reference to other points in time. So if you start at 2:00 and after some time passes it is 3:00, you know that you are one hour in the future from when it was 2:00.

 

If, however, you and your clock spends time dilated to half the tick rate, then returns to the initial frame when it reads three, your clock and you will have travelled one hour into the future but you will actually be two hours into the future in that frame. That gives you a one hour offset that you retain.

 

Similarly, if you travel at a speed such that distances are length contracted to half, if you start at point A and travel 50 miles south, then return to your original frame you will have travelled 50 miles but discover that you are 100 miles from your starting point in your current frame, in the same way that 1 hour has elapsed but you are 2 hours into the future in that frame.

 

We just tend not to measure distance in the same way that we measure time as a normal convention, and so the parallels are not as immediately intuitive.

Edited July 30, 2016 by Delta1212

[michel123456]

If we send a ruler out in the same type of experiment, has its length changed when it is returned to its starting position?

 

A simple answer would be great.

 

1. Yes it has changed

2. No it has not changed.

Please.

[ajb]

A simple answer would be great.

 

1. Yes it has changed

2. No it has not changed.

 

Please.

There is no change... if you have a pair of identical rulers and send one on this round trip, and then compare it with the rule left at home (assuming the rulers are now at rest with respect to each other) then they will be the same length.

 

As Mordred points out the same is true of clocks - they both tick at the same rate when brought back together.

Edited July 30, 2016 by ajb

[michel123456]

There is no change... if you have a pair of identical rulers and send one on this round trip, and then compare it with the rule left at home (assuming the rulers are now at rest with respect to each other) then they will be the same length.

(Hijacking again)

So. The situation is: we have a ruler, we send it to space and while traveling we are measuring it changing length, then upon coming back in our frame we are measuring that miraculously it has recovered its dimensions.

And if there were an astronaut with the ruler at hand all the time, we could even get a signed testimony that the ruler didn't change during the travel.

What makes us believe that the ruler did change at all and that it was not simply a kind of observational paramorphosis?

Edited July 30, 2016 by michel123456

[ajb]

What makes us believe that the ruler did change at all and that it was not simply a kind of observational paramorphosis?

Because all mesurements are relative - any choice of intertial frame is as good as another.

[michel123456]

Because all mesurements are relative - any choice of intertial frame is as good as another.

So you say there is no one single reality.

Is that it?

[ajb]

So you say there is no one single reality.

That sounds a stonger statement that I want to make. The point is that how we measure things depends on the (inertial) frames we use. There is a notion of proper length which is the loosely the 'true' length, this is the length as measured in the rest frame of the object we wish to mesasure. But from other frames the length will be shorter.

[michel123456]

That sounds a stonger statement that I want to make. The point is that how we measure things depends on the (inertial) frames we use. There is a notion of proper length which is the loosely the 'true' length, this is the length as measured in the rest frame of the object we wish to mesasure. But from other frames the length will be shorter.

There is proper mass too.

there is also the fact that all observers will get the information delayed (because of c is not infinite), which means that only the frame of the object itself observes what is happening now. All the other frames will see what was happening.

Edited July 30, 2016 by michel123456

[ajb]

There is proper mass too.

Yes, we tend to call that just mass - other books may call it rest mass.

 

there is also the fact that all observers will get the information delayed (because of c is not infinite), which means that only the frame of the object itself observes what is happening now. All the other frames will see what was happening.

This is true - and the reason for simultaneity of events being a subtle issue.

Edited July 30, 2016 by ajb

[studiot]

 

What makes us believe that the ruler did change at all and that it was not simply a kind of observational paramorphosis?

 

We should not call in either the old ghostbuster squad or the new one whenever we are faced with an einstinian relativity issue.

 

We should call on common experience and realise that such changes also occur for other, well known, reasons.

 

We should also not be surprised at the results when we take two variables that are not independent and plot an effect against them as though they were.

Space and time are not independent.

 

Consider the following.

 

But the end of a ruler against a wall.

 

Hang a pendulum clock on the wall above the ruler. Synchronise it with your watch.

 

Now heat both clock and ruler (but not the watch) by some means, then allow to cool to original temperature.

 

The ruler no longer abuts the wall and the clock and watch are no longer in synchronism.

[swansont]

(Hijacking again)

So. The situation is: we have a ruler, we send it to space and while traveling we are measuring it changing length, then upon coming back in our frame we are measuring that miraculously it has recovered its dimensions.

And if there were an astronaut with the ruler at hand all the time, we could even get a signed testimony that the ruler didn't change during the travel.

What makes us believe that the ruler did change at all and that it was not simply a kind of observational paramorphosis?

 

 

We can do the same thing with a clock. And have done it under slightly different circumstances. Even though the clock didn't change according to the persons traveling with it, it comes back with a different amount of accumulated time. That's why we're confident it would happen with length, too, if we could measure length and distance as precisely.

[J.C.MacSwell]

There is proper mass too.

there is also the fact that all observers will get the information delayed (because of c is not infinite), which means that only the frame of the object itself observes what is happening now. All the other frames will see what was happening.

No. What happens at a distance takes time to be observed. You don't observe what happens now in your frame, at any distance, instantaneously.

[StringJunky]

No. What happens at a distance takes time to be observed. You don't observe what happens now in your frame, at any distance, instantaneously.

Only the observed object is in the now isn't it? All observers see it in the past, whatever the distance.

Edited July 30, 2016 by StringJunky

[J.C.MacSwell]

Only the observed object is in the now isn't it? All observers see it in the past, whatever the distance.

Not sure what you are asking. If you observe something at a distance you see it in the past. It may still exist.

[imatfaal]

Exactly - frames of reference pertain to relative movement but not necessarily physical proximity. They are a common coordinate system - any two objects which are both at rest with respect to each other - ie neither of their coordinates are changing - are in the same frame. They can be distance from each other and simultaneity becomes complicated

 

x-posted

[StringJunky]

Not sure what you are asking. If you observe something at a distance you see it in the past. It may still exist.

I basically repeated what you said, I realised after.... duh!.