What space contracts in Special Relativity?

[stevendaryl]

With respect to you, I don't think this is a useful discussion.

Well, you asked where in SR it says that moving objects shrink, and I answered that question.

 

[Sugdub]

I think it's more accurate to say there there are an infinite number of different ways to describe the configuration of the rod, one for each reference frame.

Let's step away from something as complicated as a rod and look at something simpler, like a point charge. Figure 1 shows the electric field produced by a stationary point charge...Figure 2 shows the electric field produced by a moving charge... Clearly there's a difference between the two electric fields -- a difference that arises from the point charge's motion.

An observer at rest relative to the point charge will always see Figure 1, while an observer moving relative to the will always see Figure 2. Is it correct to say that there are two (or more) different configurations of the electric field? I don't think so. Better to say that there are two different descriptions of the electric field...

(my emphasis)
The electric field is not observable, so observers won't see it, irrespective of their relative motion. The theoretician who decides to select an IRF in which the electric charge is at rest will represent the electric field as shown in Figure 1. Conversely a theoretician who selects another IRF in which the charge is in relative motion will invoke a representation alongside Figure 2 for the electric field induced by the same charge. SR imposes this constraint. But eventually the predictions made by both theoreticians concerning the outcome of observations and measurements will be identical, assuming they invoke Lorentz-invariant laws. The SR theory impacts our formal representation of the electric field, however changing IRF has no observable consequences.

Sure, from a moving POV the charge distribution/rod looks different. But nothing happened to the rod, because there's no physical difference between inertial motion and rest.

None of these arguments demonstrate more than the fact that things are different when measured (described ?) from a moving POV. Which we expect. But the rod is unchanged by this...

(my emphasis)
If the theoretician selects an IRF showing the rod at rest, he/she will describe the rod using its proper length. Should another theoretician select another IRF showing the rod in inertial motion, then he/she will invoke another representation of the same rod with a contracted length and altered internal forces as well (btw forces are not observable). SR imposes this constraint. All such changes will impact the theoretical analysis which will then differ between both theoreticians. Indeed the SR theory impacts how theoreticians formally describe the rod. But ultimately their respective predictions concerning what can be seen or measured by any observer will be identical, assuming they invoke Lorentz-invariant laws. The SR theory impacts our formal representation of the rod, however changing IRF has no observable consequences.

Conversely, changing observation conditions, for example changing the relative speed between an observer and the observed rod, I mean a genuine change in the experimental conditions, will definitely trigger a change in the outcome of the measurement performed by the observer. This change is not dealt with by SR, it is not formalized through a change of the IRF. As soon as you use words like “see”, “looks”, “view” or “POV”, you need to specify the relative motion of the “observer” to whom these words relate, in respect to the rod. You also need to specify which physical mechanism (light rays, ...) propagates the relevant information toward the “observer”, including of course the propagation speed. Eventually you will overlay a difference between what the moving “observer” can “measure” or “see” as compared to what he/she would have actually measured if at rest in respect to the rod. In other words the “observer” becomes part of the scenario. Overall what can be seen by the moving observer is a compound of an SR-compatible description of the rod and a Doppler effect overlay.

I recommend that in a first instance you eliminate any direct or indirect reference to the “observer” in order to first find an agreement on whether the rod shrinks or not (or better: to determine what this expression actually means), irrespective of it being “observed”, therefore eliminating the mix between the SR-impact on the formal description of the rod and the Doppler-effect on the outcome of measurements by a moving observer.

In a second raw, it will be interesting to consider how the rod “looks” from the “POV” of a moving “observer”. But for this second step, you will be keen in avoiding the usual confusion between the velocity of the rod in respect to the IRF you select for the formal representation of the scenario on the one hand, and the velocity of the rod in respect to the observer (who is part of the scenario) on the other hand. The representation frame (IRF) chosen by the theoretician for performing his/her analysis is a different concept than the observation frame chosen by those who perform the experiment.

Hopefully both contributors can accept my wording.

 

[Mentz114]

Well, you asked where in SR it says that moving objects shrink, and I answered that question.

Thank you. But as I've said, the object does not shrink, the measurement made is subject to relativistic length contraction. It is not necessary for anything to shrink in order for the measurement to give a smaller value than the rest length. So relativity does not say that an object shrinks as an effect of inertial motion.