Philosophy Discussion Forums | A Humans-Only Philosophy Club

[David Cooper]

I think I still dispute that David's definitions of basic terms like "acceleration" are merely different and not wrong. As I've told him, I think the definition of terms like that are standard bits of physics, irrespective of SR or LET. The trouble is, his obsession with the idea that everyone has been brainwashed by SR leads him to apply that accusation even to people who have never heard of SR. Since the definition of a term like "acceleration" pre-dates SR, I presume he would even claim that people who lived before Einstein was born, and who accept the standard definitions of such terms, were brainwashed by a theory that didn't exist yet.

You know full well what I mean by acceleration because I've spelt it out clearly. I prefer my usage of the word because it is less ambiguous than yours - this leads to greater clarity. Everyone is entitled to use words in they way they want to so long as they spell out how they intend to be interpreted, and I have done so. I don't complain about you using words in other ways that go against what the man in the street understands - you aren't wrong to do so, and the man in the street isn't wrong to use words differently from you. I prefer to lean in the direction of the language of the man in the street so that more people can follow what's being said.

[David Cooper]

This is the twin paradox, and its solution within SR can be found in many scientific and popular articles. The main points to be considered are:

1. My trip consists of two frames.
2. Simultaneity observed with my clock in frames T1 and T2 is not the same as simultaneity observed with your clock in frame D.
3. Because of all this, when we are reunited your clock shows the same reading for me in frame T2 as for you in frame D. Only my own clock is not in sync with your clock any more.

Perhaps someone can explain this better.

Your clock is not in sync with mine any more for the simple reason that yours ran slow (on average). If you look at what you actually did during your trip, you simply discarded lots of ticks that you saw coming from my clock (while correcting for Doppler shift) and didn't count them in your T1 and T2 measurements, but all of my clock's ticks reached you during your trip, revealing that my clock was actually ticking faster than yours, and this was also confirmed by the final scores. These, the most important measurements, were made simultaneously (from and to points where the two clocks were together).

Now, there are ways to argue against such simultaneity, but they involve switching models. If you use a set zero model, neither of the clocks were ticking at all, so there is no time and nothing simultaneous. If you use a set 1 model, both of the clocks were ticking at the same rate throughout, but one of them took a shorter path into the future through the "time" dimension, and that makes such simultaneous events impossible to measure within such a universe because the clocks don't meet up at the same Newtonian time due to an event-meshing failure. However, we're actually supposed to be working in a set 2 model context here where time runs (clocks do tick), and where event-meshing failures don't happen, which means that some clocks must run faster than others some of the time. You don't break that by switching to a set zero model - you have to stick to the rules of set 2.

[Steve3007]

David, I'm going to deal with this section first because it appears to show that you haven't got your head around the concept of relative motion, and as a result see us as swapping places when we haven't done so. It appears to me to be a good example of your attachment to absolute motion and, possibly, lack of precision about the definition of acceleration getting you in a muddle.

It's from this post:
viewtopic.php?p=320597#p320597

Let's do something else with our clocks.

OK.

We repeat the first experiment and see that your clock ticked more slowly than mine (on average).

OK. To be clear: In this experiment I'm the traveler, in the sense that I'm the one who has to accelerate at some points, while you remain in the same inertial reference frame.

I walk away from you and come back. So I travel at constant speed in one direction and then turn around (i.e. accelerate) and travel at constant speed in the other direction. Let's say the first direction is to the right. Let's call the speed 'v'. So my speed, relative to you and relative to the earth on which we stand is v for half the journey and -v for the other half. Therefore, on turning back, my acceleration causes my speed, relative to an inertial reference frame, to change by 2v to the left. From v to -v. Your speed relative to the earth is zero throughout. You have undergone no acceleration.

Yes?

I then move my clock at the speed you carried your clock at during the first leg of that first trip and you race on ahead of me with yours, then wait for me to catch up.

OK. You move at speed v to the right and I move at speed 2v to the right, both relative to the Earth, on the first leg. So our speed relative to each other is v, as before. I then reduce my speed relative to the earth to zero. I stop. So, as before, I accelerate to the left such that my speed changes by 2v. You do not accelerate. So, in terms of our relative speeds and our accelerations this situation is exactly the same as the first one.

Yes?

Again my clock has ticked more than yours, so we don't learn much from that.

Not surprising, because the situation is exactly the same and as we know the laws of physics are the same when measured against any inertial reference frame. Measured against an inertial reference frame traveling to the right at speed v, relative to the earth, our movements in this second experiment are identical to our movements in the first experiment measured against an inertial reference frame that is stationary relative to the Earth.

Do you follow this?

But let's try it again with me walking at the same speed, but this time you stop for a while, then race after me. Again, your clock has run slower than mine.

OK. So, relative to the earth, you're moving at speed v to the right for the whole journey, with no acceleration. Relative to the earth, my speed is zero and then it increases to 2v by my act of acceleration, this time accelerating to the right. In the previous two examples I accelerated to the left. So, in terms our our separation, our relative speeds and our accelerations, again, this situation is exactly the same as the previous two, except that right and left are swapped. So, again, no surprise that we get the same results.

But look at what's just happened. During this third experiment, my clock was moving through space at the same speed as yours was during the first leg of the first experiment, and your clock was moving through space during the first leg of the third experiment at the same speed as my clock did during the first experiment, so they've swapped places.

No. They haven't swapped places at all. The only things that have physical significance are their spatial separations, their relative velocities and their accelerations. Those have not changed. The second experiment is identical to the first, and the third experiment is the same except that left and right are swapped, which obviously doesn't make any difference. The reason why you think that they've swapped places is because you have this notion that their velocities "through space" are significant when all that is actually significant is their separation, accelerations and relative velocities.


OK, I'll gradually deal with the other points through this weekend.

[Steve3007]

A bit more.

I think I still dispute that David's definitions of basic terms like "acceleration" are merely different and not wrong...

You know full well what I mean by acceleration because I've spelt it out clearly.

You have absolutely not spelled it out at all. I have succinctly stated numerous times that the definition of acceleration is rate of change of velocity with respect to time.

I prefer my usage of the word because it is less ambiguous than yours - this leads to greater clarity.

This is the opposite of the truth. The reason why terms like "acceleration" have well defined specific meanings in physics is so that they are absolutely unambiguous. It is impossible to mistake the short, simple definition of acceleration for anything else. But generally, in everyday life, words are used ambiguously, meaning different things in different contexts.

Everyone is entitled to use words in they way they want to so long as they spell out how they intend to be interpreted, and I have done so.

No you haven't. I have said many times "acceleration means...". You have not.

I don't complain about you using words in other ways that go against what the man in the street understands - you aren't wrong to do so, and the man in the street isn't wrong to use words differently from you. I prefer to lean in the direction of the language of the man in the street so that more people can follow what's being said.

The "man in the street" has numerous different purposes for words and generally isn't concerned with precision of language. When discussing physics we don't have that luxury. As a software engineer, like me, you should know this. You don't talk to your computer using vague, ambiguous English, do you? You use precisely defined, unambiguous computer languages that are defined according to agreed published standards that anybody who is interested can look up. If you want to be properly understood you need to apply similar standards here.

[Halc]

You know full well what I mean by acceleration because I've spelt it out clearly.

You have absolutely not spelled it out at all. I have succinctly stated numerous times that the definition of acceleration is rate of change of velocity with respect to time.

I think he spelled it out quite clearly actually. The definition you give here is one used by the two of us and by Einstein and physics in general. David has a different one, which is more or less 'rate of increase of objective speed', which isn't even a vector. Problem is, David is using this very different definition when interpreting the words of others, including the words of Einstein.

David's definition fits with any theory that has a concept of objective speed, but is meaningless in a theory that doesn't. The physics definition is not thus dependent on a specific view, which is why physics uses it.

I don't complain about you using words in other ways that go against what the man in the street understands

The man in the street isn't talking absolute speed. He is giving an implied reference, and that makes it correspond to the physics definition.

[Steve3007]

I think he spelled it out quite clearly actually. The definition you give here is one used by the two of us and by Einstein and physics in general. David has a different one, which is more or less 'rate of increase of objective speed', which isn't even a vector.

Yes, I can see by the context that that is how he's using it, at least mostly. But I haven't seen him explicitly define it as such. As far as I can see, he has never said anything like "I will be using the word 'acceleration' in this sense...". If he's going to use a word in a non-standard way then I think it's all the more important to explicitly do that, and not just assume that we can figure how how he's using it from the context.

When the "man in the street" uses the word "acceleration", in my experience, he sometimes simply uses it as a vague, approximate synonym for speed. Sometimes as a vague, approximate synonym for power. And sometimes he uses it accurately. We can get away with that ambiguity in a casual conversation about the performance of our cars, but it won't do if we want to have any hope of clearly and unambiguously stating what we mean when talking about a subject like this, where it's necessary to think very, very carefully and precisely about exactly what we mean when we discuss such things as measuring spatial location, velocity and time.

[Steve3007]

The definition you give here is one used by the two of us and by Einstein and physics in general.

One thing I've considered it important to point out is that "acceleration" has nothing, in itself, to do with Einstein or SR. If we say that, I think we feed the bizarre idea of his that the whole of physics is corrupted by the evil dogma of SR, including the physics that existed before SR was born.

[Steve3007]

As an example of the way that the simple definition of the word "acceleration" is, in David's mind, all wrapped up with this idea that everyone has been brainwashed by the establishment, look again at what he said to you here:

viewtopic.php?p=318925#p318925

Interesting. Acceleration is a vector rate of change in velocity, not a change in speed. There is no difference between acceleration and deceleration. The ISS for instance is always accelerating, yet its speed remains constant (relative to Earth). This is high-school physics, not even touching on relativity.

It's a contradiction that you've been heavily brainwashed into believing is acceptable, but the part about clocks speeding up and slowing down at the same time takes it a much more obvious step into the irrational.

Your simple statement of a fact of physics, that has nothing at all to do with Einstein and is simple high school Newtonian mechanics, is seen to be all part of this "brainwashing" process. The result is that you can't simply use words in accordance with their standard definitions without being accused of being "brainwashed".

[Tamminen]

This is the twin paradox, and its solution within SR can be found in many scientific and popular articles. The main points to be considered are:

1. My trip consists of two frames.
2. Simultaneity observed with my clock in frames T1 and T2 is not the same as simultaneity observed with your clock in frame D.
3. Because of all this, when we are reunited your clock shows the same reading for me in frame T2 as for you in frame D. Only my own clock is not in sync with your clock any more.

Perhaps someone can explain this better.

No, you got it pretty much correct. David only considers the ordering of events from the one preferred frame (unspecified in your example, but it could have been any of them, but probably a 4th one).

Not sure what you meant by "your clock shows the same reading for me in frame T2". Your T clock shows a time on it when you get back to the D clock, and it is a different number, so that isn't the same reading. I think you mean that if you stop when you get home, your clock will resume ticking at the same pace as the D clock, which is correct.
This seems to be the nature of reality. It really does take less time to go from Tuesday to Wednesday by a moving route than by sitting on your behind the whole day.

In fact I meant that in spite of the clock in frame D ticking more slowly than T2's clock, from T2's perspective, the time in D's clock from T2's perspective is the same as from D's perspective when we meet. Only my own clock has a different time. Do you see this in the same way?

As to the "missing ticks" that David speaks of, they can be explained by the accelerations required for frame changes. Right?

[Halc]

One thing I've considered it important to point out is that "acceleration" has nothing, in itself, to do with Einstein or SR. If we say that, I think we feed the bizarre idea of his that the whole of physics is corrupted by the evil dogma of SR, including the physics that existed before SR was born.

Sure. Not sure if it was clearly spelled out until Newton did all his work. He produced the three laws, and I'm not sure how acceleration was defined before that.

[Halc]

In fact I meant that in spite of the clock in frame D ticking more slowly than T2's clock, from T2's perspective, the time in D's clock from T2's perspective is the same as from D's perspective when we meet. Only my own clock has a different time. Do you see this in the same way?

The two clocks do not read the same time. That means they differ, yes, but not that one of them is 'the different one'. 'different' is a relation between two things, not a property of the T clock.

As to the "missing ticks" that David speaks of, they can be explained by the accelerations required for frame changes. Right?

Yes, acceleration of T towards object X makes the time simultaneous with T in the new frame be very much into the future of the moment that was simultaneous with T in the old frame, and acceleration away from X would do the opposite.

This is very similar to a foot race where A and B are going forward with A in the lead, but they're widely separated, with A to the left and B to the right, relative to the current definition of 'forward'. Change that forward definition by the track taking a turn to the right (toward's B), and suddently A is behind B. How did B get ahead? He didn't really move forward, the definition of forward just changed is all. If they're in each others presence at the turn, then the turn makes no difference as to who's ahead, just like acceleration makes no difference to what time D currently reads if it is in T's presence.
In this analogy, David equivalently says there is only one definition of forward, and if you're not going that way, you are racing at least partially to the side, not to the finish. The turn makes no difference as to whether A or B is more objectively forward than the other. An assessment of who is winning taken at a different angle is an invalid assessment, not corresponding to which direction is really forward.

With the numbers I proposed (5 years out and 5 back for T, 20 elapsed at D), at the halfway point, just before acceleration in frame T1, D clock would read 2.5 years, and just after acceleration, D clock would read 17.5 years. This assumes acceleration is done in a brief time. The traveler was already dead from fatal G forces in our example, so might as well do it proper.
This is not what is observed from the turnaround point, but what events are simulateous with the turnaround event in different frames. If T looks back at D the time he turns around, he sees time 0.67 on the D clock because light from that moment is just now reaching the turnaround point. On the trip back, the clock at D would appear to the traveler to move quickly from 0.67 to 20. As David says, none of the D clock ticks go unwitnessed by the traveler. The rate difference between the two frames T1 and T2 is due to the Doppler effect.

[Tamminen]

The two clocks do not read the same time. That means they differ, yes, but not that one of them is 'the different one'. 'different' is a relation between two things, not a property of the T clock.

Yes, of course, and the T clock reads a different time than the D clock at our meeting, but that was not my point. I asked what is the time reading of the D clock seen from (1) T2's perspective and (2) D's perspective when they meet. The perspectives of different frames can lead to different readings of the same clock relative to some event if the clock is far away, due to the difference in simultaneity, but not now that they meet. This means that acceleration does not change the readings of a clock seen from different frames when the clocks of the frames meet. The "missing ticks" can only be caused by a distant acceleration. I am thinking of this to remove all possible contradictions that David says there are. Am I right? Did you get what I mean?

Your description of the effect of acceleration on time was clarifying.

[Halc]

The two clocks do not read the same time. That means they differ, yes, but not that one of them is 'the different one'. 'different' is a relation between two things, not a property of the T clock.

Yes, of course, and the T clock reads a different time than the D clock at our meeting, but that was not my point. I asked what is the time reading of the D clock seen from (1) T2's perspective and (2) D's perspective when they meet. The perspectives of different frames can lead to different readings of the same clock relative to some event if the clock is far away, due to the difference in simultaneity, but not now that they meet.

Right, so now everybody present at the meeting agrees that T's clock reads, what, 10, and D's clock reads 20. That's a frame independent fact for anybody present at that event.

This means that acceleration does not change the readings of a clock seen from different frames when the clocks of the frames meet.

A frame is not an event. Different accelerations, velocities and frames do not change the readings of various clocks all present at some event from the perspective of that event.

The "missing ticks" can only be caused by a distant acceleration.

Yes, it is called the 'moment of acceleration' (google that), sort of like the moment of inertia. It is acceleration multiplied by 'leverage' distance that causes clocks elsewhere to change how it is synced.

I am thinking of this to remove all possible contradictions that David says there are. Am I right? Did you get what I mean?

Yes, I got what you mean.

[David Cooper]

This seems to be the nature of reality. It really does take less time to go from Tuesday to Wednesday by a moving route than by sitting on your behind the whole day.

That's what happens with set 1 models - some paths into the future are more direct than others. That doesn't work in other models though, so they mustn't be mixed. In set zero models, there is no movement at all, while in set 2 and 3 models, some clocks run slow (with contradictions in set 2 models because any clock that runs slower than another also has to run faster than it at the same time).

[David Cooper]

David, I'm going to deal with this section first because it appears to show that you haven't got your head around the concept of relative motion, and as a result see us as swapping places when we haven't done so. It appears to me to be a good example of your attachment to absolute motion and, possibly, lack of precision about the definition of acceleration getting you in a muddle.

It's a good example of you not getting your head around the thought experiment, as will soon become clear.

OK. To be clear: In this experiment I'm the traveler, in the sense that I'm the one who has to accelerate at some points, while you remain in the same inertial reference frame.

I walk away from you and come back. So I travel at constant speed in one direction and then turn around (i.e. accelerate) and travel at constant speed in the other direction. Let's say the first direction is to the right. Let's call the speed 'v'. So my speed, relative to you and relative to the earth on which we stand is v for half the journey and -v for the other half. Therefore, on turning back, my acceleration causes my speed, relative to an inertial reference frame, to change by 2v to the left. From v to -v. Your speed relative to the earth is zero throughout. You have undergone no acceleration.

Yes?

Fine. We can also have someone else travel with you on the way out who doesn't undergo any accelerations, and someone else come back with you who doesn't undergo any accelerations, and they can confirm that your clock ticks at the same rate as theirs while they are travelling with you, thereby removing any magical role for the accelerations to affect the timings - this shows that the way the clocks tick relative to each other is entirely controlled by their speed of movement through space.

I then move my clock at the speed you carried your clock at during the first leg of that first trip and you race on ahead of me with yours, then wait for me to catch up.

OK. You move at speed v to the right and I move at speed 2v to the right, both relative to the Earth, on the first leg. So our speed relative to each other is v, as before. I then reduce my speed relative to the earth to zero. I stop. So, as before, I accelerate to the left such that my speed changes by 2v. You do not accelerate. So, in terms of our relative speeds and our accelerations this situation is exactly the same as the first one.

Yes?

Fine (ignoring the relativistic speed addition aspect which reduces the 2v speed by a fraction).

Again my clock has ticked more than yours, so we don't learn much from that.

Not surprising, because the situation is exactly the same and as we know the laws of physics are the same when measured against any inertial reference frame. Measured against an inertial reference frame traveling to the right at speed v, relative to the earth, our movements in this second experiment are identical to our movements in the first experiment measured against an inertial reference frame that is stationary relative to the Earth.

Do you follow this?

Of course, but there is a hidden difference.

But let's try it again with me walking at the same speed, but this time you stop for a while, then race after me. Again, your clock has run slower than mine.

OK. So, relative to the earth, you're moving at speed v to the right for the whole journey, with no acceleration. Relative to the earth, my speed is zero and then it increases to 2v by my act of acceleration, this time accelerating to the right. In the previous two examples I accelerated to the left. So, in terms our our separation, our relative speeds and our accelerations, again, this situation is exactly the same as the previous two, except that right and left are swapped. So, again, no surprise that we get the same results.

And yet there's a hidden difference.

But look at what's just happened. During this third experiment, my clock was moving through space at the same speed as yours was during the first leg of the first experiment, and your clock was moving through space during the first leg of the third experiment at the same speed as my clock did during the first experiment, so they've swapped places.

No. They haven't swapped places at all.

Yes they have. If my clock was ticking faster than yours during the first leg of the first experiment, when we do the third experiment, your clock must be ticking faster than mine during the first leg. You deny that, but I know it to be true because I had the wit to leave a third clock behind which was with me throughout the first experiment, and you were stationary relative to it throughout the fist leg of the third experiment. That third clock hasn't accelerated at any time, so it was ticking at a constant rate throughout all three experiments. In the same way, during the first experiment, you were accompanied by a fourth clock on the first leg, and it kept on going when you turned back, moving at a constant speed which it maintained right through the second and third experiments. Throughout the first leg of the third experiment, that fourth clock and I were co-moving, so it was ticking at the same rate as my clock. This proves what I said at the start of this paragraph: if my clock was ticking faster than yours during the first leg of the first experiment, when we do the third experiment, your clock must be ticking faster than mine.

If you don't nail the details like this, you accidentally change the proposed speed of light relative to the system without realising that you're doing so, but we need to analyse these experiments with greater rigour than that to avoid making careless mistakes.

The only things that have physical significance are their spatial separations, their relative velocities and their accelerations. Those have not changed. The second experiment is identical to the first, and the third experiment is the same except that left and right are swapped, which obviously doesn't make any difference. The reason why you think that they've swapped places is because you have this notion that their velocities "through space" are significant when all that is actually significant is their separation, accelerations and relative velocities.

And it is no surprise to me that you do make such careless mistakes, because almost everyone in the SR camp makes the exact same mistakes every time due to their abject failure to check what they're doing to the speed of light relative to the system. The third and fourth clocks show up their error.