Relativity - Oh dear, here we go again!
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Paulhoff
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I like my father's version better, Keep It Simple, Stupid.
(Yeah, KISS: Keep It Short and Simple.)
As I have said before, humans just like to make things complicated. And some of them have a very overactive speech center in their brain.
Paul
Paulhoff
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Apparently there are separate areas in the brain for generating speech and generating writing* but there is another area that formulates what you want to communicate that generates input for both of these areas.
*There are stroke sufferers who know exactly what they wish to say but are unable to say it, but they can write it down for you. (I assume they are able to type it as well)
when you read something, the vocal cords move. it's called subvocalization.
in linguistics, there is a theory that words are simply speech figures. obviously some languages like Chinese are closer to ideographic/logographic, while Romance languages are way different.
in linguistics, there is a theory that words are simply speech figures. obviously some languages like Chinese are closer to ideographic/logographic, while Romance languages are way different.
Before complete failure is declared, let me ask a “KISS” question . . .
Synchronised clocks A and B travel away from the Earth in opposite directions, at the same speed and for the same distance, then turn around and return to Earth. In other words, everything about their trips are exactly the same, except that they are in opposite directions. When the clocks are back on Earth again, would A observe that B still showed the same time as A?
Synchronised clocks A and B travel away from the Earth in opposite directions, at the same speed and for the same distance, then turn around and return to Earth. In other words, everything about their trips are exactly the same, except that they are in opposite directions. When the clocks are back on Earth again, would A observe that B still showed the same time as A?
Ziggurat
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Yes.
Thanks. I don‘t understand why the answer isn‘t “No”. From the frame of A, B travelled at a different speed so why was there no time dilation?
Ziggurat
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You're making an assumption that there's a single frame which you can label "A". But there isn't, and you cannot make that assumption. "A" changes reference frames. Here's a spacetime diagram of what you describe, as seen from three different reference frames:
The red line is earth. Green and blue are A and B, respectively. Panel 1 shows the earth reference frame. Panel 2 shows the reference frame in which B is stationary for the outbound journey. Panel 3 shows the reference frame in which A is stationary for the outbound journey. Note that if you consider the entire journeys, the A and B journeys are actually symmetric in all three reference frames (though the symmetry isn't as simple in panels 2 and 3). In panel 2, for example, both A and B will have time dilation for part of their journeys and no time dilation for part of their journeys, and which part comes first is different, but it's still symmetric (can you see why?). In fact, the journeys are symmetric for any reference frame, not just the three I picked. Therefore all obervers should agree that A and B experience the same amount of time. This is different from the twin paradox, where the journeys of the twins are not symmetric for any reference frame.
I will attempt an explanation without the spacetime diagrams and without acceleration.
To avoid the problems associated with acceleration, we need four spaceships, A, B, X, and Y.
The analagous scenario would be as follows:
Firstly the setup:
A and B are moving towards each other at 0.8c.
They synchronise their clocks as they pass.
The Earth happens to be where A and B pass and an Earthling sychronises his watch with those of A and B.
The rest is analogous to the previous example:
Now A and B are moving away from each other at 0.8c
After a certain time interval as seen from Earth, A and B are passed by X and Y respectively, also at 0.8c
As X passes A, X synchronises his watch with A's watch.
As Y passes B, Y synchronises his watch with B's watch.
Eventually X and Y pass each other.
Symmetry demands that they pass at the Earth.
They compare the times on their clocks.
The symmetry of the situation demands that the times on the clocks of X and Y are the same.
The Earthling will confirm that the times on X and Y's clocks are less than on his own clock.
Thus both time dilation and it's independence on acceleration are confirmed.
To avoid the problems associated with acceleration, we need four spaceships, A, B, X, and Y.
The analagous scenario would be as follows:
Firstly the setup:
A and B are moving towards each other at 0.8c.
They synchronise their clocks as they pass.
The Earth happens to be where A and B pass and an Earthling sychronises his watch with those of A and B.
The rest is analogous to the previous example:
Now A and B are moving away from each other at 0.8c
After a certain time interval as seen from Earth, A and B are passed by X and Y respectively, also at 0.8c
As X passes A, X synchronises his watch with A's watch.
As Y passes B, Y synchronises his watch with B's watch.
Eventually X and Y pass each other.
Symmetry demands that they pass at the Earth.
They compare the times on their clocks.
The symmetry of the situation demands that the times on the clocks of X and Y are the same.
The Earthling will confirm that the times on X and Y's clocks are less than on his own clock.
Thus both time dilation and it's independence on acceleration are confirmed.
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Paulhoff
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You asked if the time would be the same, and it is. There was time dilation, it was the same for both.
Paul
Fredrik
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I have already answered this more than once. See e.g. #256.
No one says there wasn't. Time dilation is present in this scenario. But there's also a "simultaneity shift" when the rocket turns around. I have explained all of this already.
The way ynot figures it is that, in A's frame, B's clock should show less time has passed for B and, from B's frame, A's clock should show less time has passed for A.
Both the "shifting frames" and the "simultaneity shift" concepts are confusing for someone struggling with the basics.
I don't know if I will fare any better though.
Fredrik
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Ynot, I'm answering BillyJoe, but you're the one who really needs to read this!
To be more specific, there's no way that ynot will ever understand this unless he understands all of these three things:
1. The scenario is perfectly symmetrical, so any correct statement abut A and B will still be correct if A is replaced with B and vice versa. (This of course includes 2 and 3 below).
2. In both of A's frames, B is aging slower than A. (This is time dilation).
3. When A turns around, he goes from a frame where B is younger to a frame where B is older. (This is not time dilation, it's relativity of simultaneity).
Ynot seems to understand 1 and 2 already, so he needs to focus on 3.
I understand that of course, but there's no way that he (or anyone else) will ever understand time dilation without also understanding simultaneity.
To be more specific, there's no way that ynot will ever understand this unless he understands all of these three things:
1. The scenario is perfectly symmetrical, so any correct statement abut A and B will still be correct if A is replaced with B and vice versa. (This of course includes 2 and 3 below).
2. In both of A's frames, B is aging slower than A. (This is time dilation).
3. When A turns around, he goes from a frame where B is younger to a frame where B is older. (This is not time dilation, it's relativity of simultaneity).
Ynot seems to understand 1 and 2 already, so he needs to focus on 3.
The problem is that "simultaneity shift" is really not possible. You can't go instantly from zero to 0.8c. Of course it works at any acceleration and final speed, but then it is less obvious that time dilation isn't caused by acceleration. I think ynot needs a leg up first before he tackles this, but I could be wrong.
Fredrik
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That stuff about how infinite acceleration isn't possible, is irrelevant, but you know that already. You may worry that others might not understand this, but I think it's pointless to worry about that, because anyone who can't get past a detail like that, has a lot to learn about physics in general before it makes sense to even begin to try to understand relativity. Newton's laws would be a better starting point.
I understand why you'd like to eliminate acceleration from these relativistic problems: You have noticed that some people incorrectly conclude that acceleration must be the cause of some "weirdness" in e.g. the twin "paradox" problem, and you have found a quick and easy way to prove them wrong. It's a good thing that you're able to prove them wrong without actually solving the problem, because now they don't have to understand the full solution to see that they're wrong. However, what you've said only explains why acceleration isn't a relevant part of these problems, and why A and B will be the same age.
That A and B will be the same age is implied by 1, and ynot already understands that. He understands both 1 and 2. His problem is that he thinks that 2 contradicts 1, and the only reason for that is that he doesn't understand 3.
3 is the reason why 2 doesn't contradict 1, so there's no way that anyone can understand this problem without understanding 3. (And that's true regardless of whether they consider ynot's formulation of the problem or yours).
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I’m feeling guilty that my busy life outside this forum doesn’t allow me to adequately read and respond to posts, let alone study the good information that has been provided. I understand how this can be annoying to those that have given this thread their time and effort (sorry). Rather than continue with “yeah-but” and “what-if” responses, I will study what has been provided when I get the time. Thanks to all for your help and patience. I might not “get it” yet but it certainly hasn’t been a total waste of time for me, and it seems it has been helpful to others.
