
#1
Mar608, 09:08 PM

P: 22

It seems to me that all of the talk about "absolute" acceleration is a complete nonissue, in terms of relativistic effects. For, if there is no such thing as absolute position, then there can be no such thing as absolute change in position, whether this change is understood in the sense of uniformity (nonaccelerated) or nonuniformity (i.e. accelerated).
As far as relativity is concerned, then, isn't there always complete symmetricity as far as relatively moving observers are concerned, no matter the uniformity of this motion? I always see references to gforces being the determining factor as far as who is "truly" accelerating. But isn't this the precise reason why Einstein always used such things as "practically rigid rods" and "ideal clocks" in his thought experiments? In other words, things that are "practically rigid" or "ideal" are, by definition, impervious to the stresses caused by external forces, are they not? It seems that people who invoke such things as the "twin paradox" are overlooking these essential considerations of Einstein. But my real confusion arises when I see that wikipedia (in http://en.wikipedia.org/wiki/Twin_paradox) attributes this same kind of fallacious thinking about absolute acceleration to Einstein himself! So what's the deal? Is wikipedia lying? Or did Einstein really contradict his earlier work in his later years? Or am I just totally nuts? 



#2
Mar608, 11:05 PM

P: 3,966

In this example of the twin's paradox one twin accelerates away on his outward journey to 0.8c in an Easterly direction. If we call this "absolute" acceleration then we imply that we are absolutely sure that the accelerated twin's clock rate is slower than that of the Earth twin, but this is not true. To an observer that has always been moving at 0.8c in an Easterly direction relative to the Earth it looks like the accelerated twin has deaccelerated to a stop and therefore his clock rate should now be going faster than that of the Earth twin. As you can see there is nothing absolute here and it is impossible to determine whose clock rate has actually changed until the twins are brought together again. If the Earth twin that remained behind (Edward) decides to chase after his spacebound sibling (Adam) then it turns out that it is Edward that aged the least when he catches up with his brother. If Edward behaves himself and stays at home like he supposed to in the classical twins paradox then it is Adam that ages the least when they meet again on Earth. It is impossible to determine what the absolute clock rates are on the outward journey and clearly if one observer sees an object as accelerating while another sees it as deaccelerating then the notion of absolute acceleration is nonsense.
It is probably best to view the twins paradox in terms of the path length through spacetime. The twin that takes the shortest (straightest) path through spacetime ages the most and any observer will agree with this measurement. However, it is only fair to point out that a notion of absolute rotation (which is a form of acceleration) is valid. As far as "practically rigid rods" are concerned, it is well known that an absolutely infinitely rigid rod is not compatible with relativity. 



#3
Mar708, 02:21 AM

P: 3,542

 Proper time is what a clock measures.  Acceleration is what a accelerometer measures. 



#4
Mar708, 07:03 AM

Mentor
P: 16,476

Absolute acceleration? 



#5
Mar708, 10:17 AM

Sci Advisor
P: 8,470

In any case, there would be numerous experimental ways you could determine if you were accelerating or not in flat spacetime. For example, you could hold out a ball in front of you so that it is still in your coordinate system, then let go of it; if it continues to stay at the same coordinates then you're moving inertially, if it begins to move then you're accelerating. Many laws of physics that hold in inertial coordinate systems would not hold in accelerating onesthe speed of light is not even constant in accelerating coordinate systems! So, there is really no ambiguity about whether one is accelerating or not. 



#6
Mar708, 11:11 AM

Math
Emeritus
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Thanks
PF Gold
P: 38,881

But acceleration is different Force= mass * acceleration so we can feel acceleration. 



#7
Mar1408, 06:20 PM

P: 22

All of you guys seem to be missing one crucial thing: general relativity. When I invoked the term, "acceleration," that should have clued you in. After all, what does the following statement mean...
In other words, special relativity is just an arbitrary case of the general theory. It is of no fundamental importance whatsoever. 



#8
Mar1408, 07:21 PM

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#9
Mar1408, 10:09 PM

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#10
Mar1508, 07:50 AM

P: 863

I think the misunderstanding about acceleration being absolute here is more fundamental.
With motion we can't say who is moving and who is at rest. Any observer regardless off motion can rightfully claim to be at rest. However, if an object accelerates then ALL other observers will agree that that object accelerated. In that way it is absolute. Yet observers will disagree on how much and even the direction the object accelerated. In this way acceleration is not absolute. When we talk about acceleration being absolute we are only talking about the fact that all observers can agree on what accelerated, not on how much or in what direction it accelerated. You are right that there is no absolute change of position. 



#11
Mar1508, 01:22 PM

P: 95

I'm quite unhappy with accelerationsolution of Twin Paradox. Let's modify Twin Paradox to a "chasing" form:
Twins A and B are initially at rest in the same place. A accelerates instantly to 0.8c and continues steadily thereafter. B waits some time and then accelerates instantly to 0.8c to the same direction, entering A's reference frame. A and B compare their clocks. Both have experienced identical acceleration, but nevertheless B's clock is behind. Really? 



#12
Mar1508, 01:53 PM

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#13
Mar1508, 03:18 PM

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P: 16,476

If you have to use a geometric analogy to explain the acceleration solution then why not just go completely with geometry to begin with and use the spacetime interval solution?




#14
Mar1508, 03:41 PM

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#15
Mar1508, 03:53 PM

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www.phy.syr.edu/courses/modules/LIGHTCONE/LightClock/ 



#16
Mar1608, 03:35 PM

Mentor
P: 16,476

First, it is easy to visualize the geometry, it is just distance measured as a family of hyperbolas instead of a family of circles. But even if it is not easy to visualize it is easy to use and calculate. Second, the point isn't to provide analogies, the point is to resolve the paradox. This exact same post comes up on this forum at least weekly, so obviously talking about acceleration is a poor way to resolve the twin paradox. Most students fail to understand that the acceleration only breaks the symmetry of the situation and instead come away mistakenly thinking that acceleration causes time dilation. Of the students who do grasp the symmetrybreaking explanation it still leaves them unable to make correct predictions about even slight variations in the paradox like Ookke brought up. It also generally leaves them unable to quantify how much time should have elapsed for each twin. The spacetime interval explanation suffers from none of these problems. It is clear, quantitative, and generally applicable. The resulting tool provides a motivation for an understanding of fourvectors and Minkowski geometry. I think it is a travesty that the acceleration explanation is still used when it is demonstrably such a poor teaching tool. 


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