Any plausible explanation for dynamical length contraction?

In summary: Oh... By dynamical length contraction I mean the object really shortens as it moves compared to an absolute frame. By length contraction I mean the apparent effect in SR, when actually nothing contracts at all, no squeezing is happening, it's just a kinematic...
  • #71
Nugatory said:
The observed physical phenomenon is that the string breaks, but only in one frame can we explain this as dynamical length contraction of the string.
Actually, what "contracts" is not the actual length of the string but its "unstressed" length, the length it would have if no force were being applied to it. But a force is being applied to it, from the spaceships, and in the frame in which the ships are originally at rest, that force keeps it at the same length (i.e., prevents it from contracting as it otherwise would since it is moving).
 
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  • #72
alexandrinushka said:
does a dynamical length contraction contradict any current physical law?
No. John Bell's paper on how to teach special relativity, the one in which he introduced the Bell spaceship paradox, had this as one of its main points.
 
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  • #74
alexandrinushka said:
I would therefore be grateful if you could share the paper you are talking about.
Unfortunately I can't find a link to it. I thought it was in the comment thread I linked to earlier but it doesn't seem to be.

The basic argument of the paper was the same as the one I gave in my Insights article.
 
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  • #75
alexandrinushka said:
if you don't mind, I might at some point PM you for more details
That's fine.
 
  • #76
PeterDonis said:
Actually, what "contracts" is not the actual length of the string but its "unstressed" length,
And here I thought that I was being so clever by not putting any dangerous adjectives in front of the word "length" :smile:

But seriously, kidding aside, one of the virtues of Bell's thought experiment is that it requires us to think about the actual physics, what would stress and strain gauges measure and why.
 
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  • #77
alexandrinushka said:
It has been refuted by Petkov
Petkov correctly points out that presentism (the view that only the "present", defined as all events in spacetime that are simultaneous with some chosen event, exists) is not consistent with SR (because SR says that the "present", so defined, is frame-dependent, and physics can't depend on frame-dependent quantities). Here he agrees with Stein.

His "refutation" of Stein is based on his claim that Stein failed to consider that SR shows that spacetime is four dimensional. (I haven't read Stein's article so I can't comment on whether that claim of Petkov's is correct.) But the statement that spacetime is four dimensional is much weaker than the block universe claim. My Insights article gives an example of why: the alternative I proposed in that article, to consider the past light cone of a chosen event as "real" (or "fixed and certain" as I phrased it in the article), is not even considered by Petkov, and since a light cone is also four-dimensional, it is perfectly consistent with the four dimensionality of spacetime as shown by SR. So Petkov might have "refuted" Stein, but he has not established that the block universe is the only possibility consistent with SR.
 
  • #78
phyti said:
Here is an example of physical length contraction.

By the way, what do you mean by "physical"?
(I ask because it is ambiguous [to me] in the literature of "length contraction".)

You might be interested in my GeoGebra visualization of the "causal diamond" or "light-clock diamond" (the intersection of light-cones) https://www.geogebra.org/m/pr63mk3j (which is based on https://www.geogebra.org/m/XFXzXGTq the spacetime diagram of the Michelson-Morley apparatus).
 
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  • #79
PeterDonis said:
Petkov correctly points out that presentism (the view that only the "present", defined as all events in spacetime that are simultaneous with some chosen event, exists) is not consistent with SR (because SR says that the "present", so defined, is frame-dependent, and physics can't depend on frame-dependent quantities). Here he agrees with Stein.
Actually, presentism can't be refuted as long as an advocate of it accepts that the 'true present' is inherently undetectable, while still being 'real'. That is, one simply posits, as an axiom that there exists an undetectable foliation of spacetime (taken as mathematical model) defining the 'real' present. An axiom is not subject to refuation, only demonstration of conflict with reality. However, in this case, the indistinguishability of LET from BU in physical predictions, implies this axiom cannot be refuted by observations. [Note, my definition of presentism is slightly different from yours - there is no claim that the true present, defining what really exists moment to moment, is related to simultaneity for any observer, defined by any particular convention. It might be, but we could never know which observer, or which convention.]
 
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  • #80
robphy said:
By the way, what do you mean by "physical"?
(I ask because it is ambiguous [to me] in the literature of "length contraction".)

You might be interested in my GeoGebra visualization of the "causal diamond" or "light-clock diamond" (the intersection of light-cones) https://www.geogebra.org/m/pr63mk3j (which is based on https://www.geogebra.org/m/XFXzXGTq the spacetime diagram of the Michelson-Morley apparatus).
A measurable change in dimension in the direction of motion vs. an apparent visual image change.
Here is another example specific to the MMX.
 

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  • #81
PeterDonis said:
Petkov correctly points out that presentism (the view that only the "present", defined as all events in spacetime that are simultaneous with some chosen event, exists) is not consistent with SR (because SR says that the "present", so defined, is frame-dependent, and physics can't depend on frame-dependent quantities). Here he agrees with Stein.
His "refutation" of Stein is based on his claim that Stein failed to consider that SR shows that spacetime is four dimensional. (I haven't read Stein's article so I can't comment on whether that claim of Petkov's is correct.) But the statement that spacetime is four dimensional is much weaker than the block universe claim. My Insights article gives an example of why: the alternative I proposed in that article, to consider the past light cone of a chosen event as "real" (or "fixed and certain" as I phrased it in the article), is not even considered by Petkov, and since a light cone is also four-dimensional, it is perfectly consistent with the four dimensionality of spacetime as shown by SR. So Petkov might have "refuted" Stein, but he has not established that the block universe is the only possibility consistent with SR.
So wait. The past light cone is real. The intersection between the past light cone and future light cone is real (we call if "the present moment"), but the future light cone is not real yet. Right? How is this different from the growing block universe?
 
  • #82
alexandrinushka said:
The past light cone is real.
I said "fixed and certain" in my Insights article, but we can use "real" to mean that for this discussion.

alexandrinushka said:
The intersection between the past light cone and future light cone is real (we call if "the present moment")
No. The past light cone does not include its apex, the single point called "the present moment", and nor does the future light cone. We can take the "present moment" to be "real" if we take it to become fixed and certain as soon as we experience it. But that does not mean it's an intersection of the past and future light cones.

alexandrinushka said:
the future light cone is not real yet.
Yes.

alexandrinushka said:
How is this different from the growing block universe?
Because, first, what becomes "real" from moment to moment in the view I was suggesting in the Insights article is not the entire future light cone of any given moment, or even just the portion of it that is in the past light cone of the next moment, but the points that are in the past light cone of the next moment but weren't in the past light cone of the previous moment. This includes points that were not in the future light cone of the previous moment (though just an infinitesimal amount of them).

And second, because the growing block universe says that what becomes "real" from moment to moment is not a tiny set of points that is in the past light cone of the next moment but wasn't in the past light cone of the previous moment, but an entire spacelike 3-surface's worth of points.
 
  • #83
PeterDonis said:
the single point called "the present moment",
I tend to use “here and now” instead because “the present moment” is too easy to misconstrue as a spacelike surface of simultaneity instead of a single event. Taking about these things is always difficult since the language hasn’t evolved to have commonly accepted words for all of these concepts.
 
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  • #84
The only adequate language for "modern physics" is math ;-)).
 
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