Clocks and rulers

[Strange]

Thank you.

 

Now, is there another reality than the ruler?

I mean, isn't the ruler already a 4D object, that is to say a ruler existing in space-time? Or is it a mystical "projection" of an unimaginable 4D object?

 

The ruler that you measure (and that someone else measures differently) is a 3D projection. Which is why you all get different results. (See Markus's tincan analogy if you can't see that.)

 

The "true" (invariant) view of the ruler is the one described by GR. Not your preferred frame.

[michel123456]

 

The ruler that you measure (and that someone else measures differently) is a 3D projection.

 

No. See AJB's answer above. post#71

 

 

Which is why you all get different results. (See Markus's tincan analogy if you can't see that.)

 

 

Markus analogy has an object, anyway something is there as the focus point. One object, one single reality. What I see is that different results are obtained from different point of views and that is most natural. None of them describes the object accurately.

The "true" (invariant) view of the ruler is the one described by GR. Not your preferred frame.

Maybe GR gives you a mathematical description. 4D.

But the world we live in is already 4D. The tin can, the ruler are the way they are. They are not "projections".

If they were projections one should imagine that there exist an "expanded tin can" in 4D. There is no such thing. The tin can is already that.

Yes, this is the proper length - which is the length as measured in the rest or comoving frame, i.e., the frame in which the ruler is stationary.

Thank you.

Edited August 2, 2016 by michel123456

[Strange]

No. See AJB's answer above. post#71

 

Yes. When you measure the length, you are just measuring one dimension (of three). You are not looking at it as a 4D object.

 

 

But the world we live in is already 4D.

 

If that were true, you would already be (intuitively) aware of the invariant quantities. As it is, you are making up an answer that satisfies you (even though it is not useful, in any scientific sense).

[swansont]

Looking at length (or color) might just be a distraction. There are other more illuminating properties one might consider. How about kinetic energy? Surely if there is a "reality" for length, there is one for motion. Is the best frame to measure the kinetic energy of an object its own rest frame? That seems silly, because the value will always be zero. How is that helpful in any way?

[Mordred]

good example. Using rest frames is essentailly what SR does. However SR doesnt work in every circumstance. Hence we have GR. Sure you can get a good approximation. However no two objects are perfectly in the same frame. (Despite your personal belief)

 

Using primarily rest frames has been tried. It leads to unnecessary difficulties particukarly if both objects aren't at rest.

 

Its not an original thought that the rest frame should be a preferred frame. You can look through the history but this thinking was proven wrong.

 

Well here is one counter argument.

 

"Inertial frames are privileged because they do not have physics whose causes are outside of the system, while non-inertial frames do."

 

this argument states the privelege frame should be the exact opposite....

 

You can find the paper in the references.

 

https://en.m.wikipedia.org/wiki/Preferred_frame

 

Eather theories also attempted a preferred frame.

 

Then along came GR...

 

"Einstein went on to develop general relativity and the equivalence principle, in which inertial-gravitational frames are no longer privileged,[1]:215223 because their geodesics explain these inertial-gravitational effects without an external cause.[2]"

Edited August 2, 2016 by Mordred

[michel123456]

Looking at length (or color) might just be a distraction. There are other more illuminating properties one might consider. How about kinetic energy? Surely if there is a "reality" for length, there is one for motion. Is the best frame to measure the kinetic energy of an object its own rest frame? That seems silly, because the value will always be zero. How is that helpful in any way?

Properties of what?

What is the subject of observation?

isn't it something there?

One single "reality" (how else to describe it). One single tin can with a bunch of properties.

Or are there many different "things"? One for each observer.

Maybe you don't care. But I do.

Then I will go directly to reach the frame of the object, and make the recognition that a tin can is actually a tin can, that a contracted ruler is nothing but a regular ruler, and that reality is pretty much as observed by Newton and Galilei. It is not the projection of anything else, it is not contracted, it is not weird.

What some other observer far away may observe as contracted when he looks at me is not part of the things that happen, it is part of the things that appear to happen.

Edited August 2, 2016 by michel123456

[Delta1212]

Things don't have to be far away to appear contracted. They can be right next you. They can pass by right in front of your nose. They can collide with you. You can directly interact with them.

 

And when you do interact with them, they will behave as if they are the length you measure them to be in your frame, not their rest frame.

[J.C.MacSwell]

Properties of what?

What is the subject of observation?

isn't it something there?

One single "reality" (how else to describe it). One single tin can with a bunch of properties.

Or are there many different "things"? One for each observer.

Maybe you don't care. But I do.

Then I will go directly to reach the frame of the object, and make the recognition that a tin can is actually a tin can, that a contracted ruler is nothing but a regular ruler, and that reality is pretty much as observed by Newton and Galilei. It is not the projection of anything else, it is not contracted, it is not weird.

What some other observer far away may observe as contracted when he looks at me is not part of the things that happen, it is part of the things that appear to happen.

I think there is an underlying assumption that, whatever the thing is being measured, it is all the same reality, equally real in it's rest frame or another. Sufficiently accurate measurement in any frame of convenience should allow you to describe it in any other assuming you are able to correlate between the two frames.

 

The bold is of course not a very practical approach for objects at relativistic speeds. It won't get you any data collecting work at CERN, even if you show up with a nice camera.

[Strange]

Properties of what?

What is the subject of observation?

isn't it something there?

One single "reality" (how else to describe it). One single tin can with a bunch of properties.

Or are there many different "things"? One for each observer.

Maybe you don't care. But I do.

Then I will go directly to reach the frame of the object, and make the recognition that a tin can is actually a tin can, that a contracted ruler is nothing but a regular ruler, and that reality is pretty much as observed by Newton and Galilei. It is not the projection of anything else, it is not contracted, it is not weird.

What some other observer far away may observe as contracted when he looks at me is not part of the things that happen, it is part of the things that appear to happen.

 

 

And so, as swansont points out, you will determine the kinetic energy of all objects to be zero, because that is what it is in their own frame of reference. That makes it pretty meaningless.

[swansont]

Properties of what?

What is the subject of observation?

isn't it something there?

One single "reality" (how else to describe it). One single tin can with a bunch of properties.

Or are there many different "things"? One for each observer.

Maybe you don't care. But I do.

Then I will go directly to reach the frame of the object, and make the recognition that a tin can is actually a tin can, that a contracted ruler is nothing but a regular ruler, and that reality is pretty much as observed by Newton and Galilei. It is not the projection of anything else, it is not contracted, it is not weird.

What some other observer far away may observe as contracted when he looks at me is not part of the things that happen, it is part of the things that appear to happen.

What is the kinetic energy of the can? The "real" value. The rest value is zero.

 

How is that useful?

[MigL]

Say you are looking at a distant object that is moving relativistically with respect to your frame.

All the properties of that object that you can measure from your frame, will affect you.

Any that you cannot measure from your frame, have no effect on you whatsoever, and might as well not even exist.

 

If that object is moving towards you and shining a visible light in your direction, that blue shifted light WILL burn you, even if it's only visible light in its rest frame.

If that object is moving away from you, its light will be red shifted and you may not even be able to detect it, even if its shining as bright as the Sun in its rest frame.

 

Maybe you have a different understanding of 'reality', but, what affects you ( i.e. what you measure ) is what is 'real'.

Not what you think may be there !

[Markus Hanke]

No. See AJB's answer above. post#71

 

 

AJB has correctly pointed out that the ruler is a hyper-slice out of an ensemble of 4D world lines. As such, it is a projection of a 4D object onto a 3D hypersurface. How the length of the ruler is measured depends on the angle at which that surface is inclined with respect to the coordinate axis.

[michel123456]

About clocks now.

 

Is there a frame for which, instead of time dilation, time contraction can be observed?

[Mordred]

That last post doesn't make much sense. When you contract something your changing the volume/density. Yes time is a coordinate in GR but its not a volume related coordinate or a spatial coordinate. (ct,x,y,z)

Edited August 3, 2016 by Mordred

[michel123456]

That last post doesn't make much sense. When you contract something your changing the volume/density. Yes time is a coordinate in GR but its not a volume related coordinate or a spatial coordinate. (ct,x,y,z)

Sorry for that.

I mean: the contrary of time dilation.

[J.C.MacSwell]

Sorry for that.

I mean: the contrary of time dilation.

 

About clocks now.

 

Is there a frame for which, instead of time dilation, time contraction can be observed?

As an example:

 

On a rotating reference frame, without gravitational effects, points further from the axis of rotation should observe time running faster for points closer to the axis.

[swansont]

In SR, no, but in GR, yes. If you go higher in a potential well, clocks speed up.

[michel123456]

 

 

As an example:

 

On a rotating reference frame, without gravitational effects, points further from the axis of rotation should observe time running faster for points closer to the axis.

So the answer is yes if i understand clearly.

 

In SR, no, but in GR, yes. If you go higher in a potential well, clocks speed up.

Again the answer is yes.

So there is more than time dilation, there is also the contrary.

 

In this case, when clocks appear to speed up, is there also a frame that observes the contrary of length contraction? (I had a negative answer on this previously)

Edited August 3, 2016 by michel123456

[J.C.MacSwell]

In SR, no, but in GR, yes. If you go higher in a potential well, clocks speed up.

The non inertial frame I described should work in SR, should it not?

[MigL]

Time dilation or 'contraction' is only WRT another clock.

It is not, nor can it be, observed on a single clock.

[swansont]

The non inertial frame I described should work in SR, should it not?

 

 

Yes. Since gravity and acceleration are indistinguishable, it's functionally the same scenario as in GR. You could solve the equations as a potential or using kinematics and you will get the same answer. The experiment has been done; it was similar to the Pound-Rebka tower experiment, but with a centrifuge

http://www.nature.com/nature/journal/v198/n4886/abs/1981186b0.html

"This expression may be obtained either in terms of the time dilatation of special relativity or in terms of the pseudo-gravitational potential difference between source and absorber."

 

Including any non-inertial frame, with only kinematic effects you will have one clock running faster than another. The non-inertial frame can't claim to be at rest, so it will run slow relative to an inertial frame. There may be some caveats here (there may still be inertial frames that run slower), but it's true that you can have clocks running faster in this situation.

[michel123456]

 

So the answer is yes if i understand clearly.

 

Again the answer is yes.

So there is more than time dilation, there is also the contrary.

 

In this case, when clocks appear to speed up, is there also a frame that observes the contrary of length contraction? (I had a negative answer on this previously)

Maybe I have to rephrase my question.

Aren't length contraction and time dilation always linked together? When time dilation is observed then length contraction must be observed too. And vice versa?

Edited August 4, 2016 by michel123456

[Markus Hanke]

Aren't length contraction and time dilation always linked together? When time dilation is observed then length contraction must be observed too. And vice versa?

 

 

Yes, they are manifestations of the same phenomenon. Take the atmospheric muon experiment for example - an observer on Earth sees the muon come at him at speed, and thus sees its average decay time to be dilated. In the rest frame of the muon, it is the Earth that approaches at high speed, and hence the distance to the Earth's surface is length-contracted. These are just two sides of the same coin, they are not separate phenomena. The physical outcome then is something both observers agree on - the muon reaches the surfaces and is detected there, which is an empirical finding in both frames.

[swansont]

 

 

Yes, they are manifestations of the same phenomenon. Take the atmospheric muon experiment for example - an observer on Earth sees the muon come at him at speed, and thus sees its average decay time to be dilated. In the rest frame of the muon, it is the Earth that approaches at high speed, and hence the distance to the Earth's surface is length-contracted. These are just two sides of the same coin, they are not separate phenomena. The physical outcome then is something both observers agree on - the muon reaches the surfaces and is detected there, which is an empirical finding in both frames.

 

One should note that in this example time dilation is the phenomenon's explanation in one frame, while length contraction is the explanation in the other. The link is that they are both consequences of c being invariant.

[imatfaal]

 

 

Yes, they are manifestations of the same phenomenon. Take the atmospheric muon experiment for example - an observer on Earth sees the muon come at him at speed, and thus sees its average decay time to be dilated. In the rest frame of the muon, it is the Earth that approaches at high speed, and hence the distance to the Earth's surface is length-contracted. These are just two sides of the same coin, they are not separate phenomena. The physical outcome then is something both observers agree on - the muon reaches the surfaces and is detected there, which is an empirical finding in both frames.

 

 

One should note that in this example time dilation is the phenomenon's explanation in one frame, while length contraction is the explanation in the other. The link is that they are both consequences of c being invariant.

 

And correct me if I am wrong - there are a myriad of other inertial frames in which the observatory could be situated which will measure greater or lesser amounts of time dilation of the muon balanced with lesser or greater length contraction of the atmosphere all of which when the maths is done and the transforms performed will give the same prediction percentage of muons surviving the 10km journey.

 

But there is NO frame in which there is no time dilation AND no length contraction- in that frame practically none of the muons should make it to the earth's surface - but they do! This magic preferred frame is both a physical impossibility and worthless in that it would not give the correct predictions.