# Precise instrument (w,h,l) after length contraction still working?

1. Feb 25, 2012

### digi99

To understand length contraction better, would an precise instrument where width, height and length must be exact equal for working when standing still in frame A, still working in its rest frame B at a higher speed in frame A with length contraction ?

With other words, is length contraction just real in the meaning of time depended but not real in another physical explanation (e.g. on atom level) ?

Or is it general to say when two persons moving (same size), both sees the other length contracted, both can be descriped as an physical explanation (e.g. on atom level) in their own rest frames (will be experienced) ... so you see just all happened in your own frame and all is just real for you only inside your frame ?

2. Feb 25, 2012

### ghwellsjr

The instrument will work exactly the same before and after any amount of acceleration that changes its speed, even though it has experienced a change in its dimension along the direction of the change in speed, right down to the atomic level, otherwise the instrument will serve a different purpose, that of determining a state of absolute motion.

3. Feb 26, 2012

### Naty1

This complements ghwellsjr post:

The instrument shows nothing 'unique' in your own inertial frame...As observed from another inertial frame, its contracts along with everything in your frame but you are unaware of such changes. Everything seems normal to you, like the ticking rate of your local clock (proper ...time)

These are REAL changes ..Lorentz contractions ARE 'real'.....and coincide with changes in time...those may be a bit easier to 'understand' since we know, for example, GPS clocks in orbit vary from those on earth [two different coordinate times] and have to be frequently synchronized.....
and 'cumulative' elapsed times shows up when such different clocks are brought together and compared even though the tick rates are again identical.

It's a rather 'crazy' world out there.

4. Feb 26, 2012

### digi99

Thanks Ghwellsjr and Naty1 (saw your responce when I finished this answer).

To be sure, the length, width and height are equal in the rest frame from the device B, as they were in frame A when standing still.

So when the device has a speed in frame A, it has length contraction for the observer in frame A because of the Lorentz transformation, but in the rest frame from the device itself nothing has changed in dimensions (all dimensions width, length, height are still equal). And for the working of the device is only its rest frame important and the physics laws valid in it.

The clock runs slower in frame B, and also in frame A on locations where the device has its speed (clock attached to device), the observer in frame A sees length contraction, but the size of the device in its rest frame is unchanged.

A meter stick attached to the device is smaller (moving direction) for an observer in frame A, a meter stick right on the moving direction is not smaller (only its width) for an observer in frame A. These meter sticks are all unchanged in the rest frame B of the device (as when standing still in frame A) because observations are frame depended (you never know what really is real).

Right (than much wiser in physics) ?

Last edited: Feb 26, 2012
5. Feb 26, 2012

### Staff: Mentor

Actually, no frame is important. The laws of physics are valid in all reference frames, and the correctly expressed laws of physics which govern the working of the device will predict that it functions in all frames.

6. Feb 26, 2012

### digi99

Thanks DaleSpam, when you develop it, its standing still of course in your frame or any other frame and in all are width, length and height still equal. When it has a speed in any of the frames, only the observer sees length contraction, because observers see things different, but the device functions in any frame ... when standing still in a frame you see its design (width, length and height equal).

Learn a lot now and very close to understanding of the SR. I have only to figure out for myself or duration is different ... it is a puzzle everybody have to solve for him/her self for good understanding.

Could you answer my question "Right (than much wiser in physics) ?", ignore the sentence "(you never know what really is real)" thats clear already (we see things only different but they are real).

7. Feb 27, 2012

### digi99

Because nobody confirmed this I have to correct it (I suppose the answer would be no), so I am sure what is right (it is really not easy to reach that final stadium with all info). So I try again:

The length, width and height are equal in the rest frame B from the device, as they are too in frame A when standing still (and in any frame when standing still in that frame).

So when the device has a speed in frame A (or another frame), it has length contraction for the observer in frame A because of the Lorentz transformation, but in the rest frame from the device itself nothing has changed in dimensions (all dimensions width, length, height are still equal). And for the working of the device is it only important that it works in a frame where all physics laws are valid as in any frame is, the development is done in a frame where the device is standing still.

The clock runs slower in frame A on locations where the device has its speed (clock attached to device), the observer in frame A sees length contraction, but the size of the device in its rest frame is unchanged and the clock in that frame is running normal (but the same ticks as observed in frame A where it is visible running slower). Observers from other frames sees things different like the length contraction and time dilation.

A meter stick attached to the device is smaller (moving direction) for an observer in frame A, a meter stick right on the moving direction is not smaller (only its width) for an observer in frame A. These meter sticks are all unchanged in the rest frame B of the device (as when standing still in frame A) because observations are frame depended (things can be different seen but are real).

So a moving meter stick is smaller but it has no meaning to measure something with it, because the meter stick in the rest frame of the device is unchanged, and in frame A one measures all with an own meter stick, standing still, and with that meter stick you find length contraction (if you could measure it) .

I am a little bit sure I see it now finally right. Than I will finally start to read my books completely.

Right (than much wiser in physics) ?

Last edited: Feb 27, 2012
8. Feb 28, 2012

### digi99

Yes I understand what you mean now (this sentence was wrong "but the same ticks as observed in frame A where it is visible running slower"), and together with the statement "things will be different seen by observers" (like vibration direction photons in light, length contraction, time dilation) time dilation and different duration is today also personal accepted for me (spent much time today to stop or accept the subject), so that point is finally passed and nice to think/continu further about it (I like the subject). I know now what I was doing wrong in frame thinking.

So if in a frame of reference the device has a speed (suppose a car), for the driver are dimensions and clock as defined in the frame. For the observer standing still in the frame is the device observed as length contracted and time dilated in all directions. So for the observer is the clock slower running, inside the device is everything normal as standing still in the frame, but time and distances reported from inside to an observer have to be recalculated via Lorentz transformations.

The short derivation for Lorentz in Wiki shows clearly that duration must be different while light observing (even if you could think "but this is not light" but is always the speed of photons).

So if on the device with speed many light sources are attached, in other directions as the moving direction, time is dilated (length not contracted) and the relativistic Doppler effect is observed (frequentie changed).

I would think exact in the moving direction, light would be length contracted too and a light wave would be smaller, so no frequentie change and so no Doppler (just as for the device, length contracted / lower time = speed measured by observer) ?

I find also strange that in the derivation of Lorentz is not considered that in all directions the lightspeed is C at any moment ...

Last edited: Feb 28, 2012
9. Feb 29, 2012

### digi99

So you mean you in your inertial frame where the instrument is standing still, is unaware of such changes observed by other observers from other frames. But in fact for the other observers is the device length contracted in the moving directions, and for the other observers not its height. But in your own inertial frame you report your local time to many observers. Depended on the speed difference between device and observer that local time will be recalculated in their own local times because of speed depended time dilations observed. Lorentz transformations are in fact for the observers because of speed differences. It goes more about relative duration differences.

Last edited: Feb 29, 2012