Absolute motion's point of reference

[Dale]

Then according to you two twins moving away and them moving back and meeting, and according to what you say they can change direction without rotation, so who will be younger?

The one which underwent non-zero proper acceleration.

Anyway a pencil can change directions, but a horse and buggy it is against common sense and physics to claim motion in 2 directions

Show me a detailed derivation where the horse and buggy doesn't make sense. You are imagining a problem with relativity that does not exist.

 

[yoelhalb]

The one which underwent non-zero proper acceleration.

Why should anyone of them?

Show me a detailed derivation where the horse and buggy doesn't make sense. You are imagining a problem with relativity that does not exist.

Imagine A,B,C are togheter, C is a horse and buggy.
Now A and B move apart in a uniform motion, and C starts accelration till it meets A.
Since a horse and buggy can go only in one direction it must be that A is the one that moves.

 

[Dale]

Why should anyone of them?

Because otherwise they will not be able to reunite.

Imagine A,B,C are togheter, C is a horse and buggy.
Now A and B move apart in a uniform motion, and C starts accelration till it meets A.
Since a horse and buggy can go only in one direction it must be that A is the one that moves.

This is not even approximately a derivation. Please do an actual derivation using explicit expressions for the various worldlines, transformations, and derived quantities of interest. Which derived quantity do you think is wrong? E.g. the tension between the horse and buggy should always be positive, do you think you get a negative tension in some frame, if so then derive the tension in that frame.

 

[yoelhalb]

Again A,B,C are together at one point.
C is a horse and a buggy.
Now A and B start moving apart, each one seeing the other one moving 100 m/s, A moves to the left and B to the right (from each others perspective), like this A<----------->B.
In the same second C also started an acceleration of 1 m/s² to the left.
Since it is clear the direction of the horse and buggy is clear it follows that every one must agree that C is moving left only.
Now if A is the point of reference then C should never be to his right, but just to his left, and will never meet him again as long C is not rotating, since according to A's perspective A is at the point of reference and where the motion started.
But if B's claim that he his the point of reference then C should be next to B, (in the first second he will be 1 m apart, the second 3 m, etc), until after a long time he will meet A.
Now we have clear proof who is moving.

 

[Dale]

You really don't seem to understand what a derivation is.

So I will ask you again: what are the worldlines of A, B, and C? And which derived quantity is concerning you?

You have asserted several times that a horse and buggy cannot go backwards, but have not said why you believe that and have not shown that whatever is troubling you is actually predicted by relativity. If you cannot even form a coherent argument how do you expect to have a rational discussion.

 

[JesseM]

According to what you write it follows that C (from A's point of view) started with its direction to the right (since they are all together in the beginning and then C moves to the right side of A and A is the frame of reference), and then he changed directions and met A, that essentially means that he changed direction without rotating.
With this you are actually destroying the answer on the twin paradox.

Actually although this can really be, there are some instances that such a claim is invalid, I have no clue if a spaceship can be claimed to be backing up, but it is against physics and common sense to claim that a buggy can pull the horse, (and yes there might be something like that in space), actually special relativity in its answer on the twin paradox claims this to be true for any motion.
So there are situations that the direction is clear for all, and A's claim makes no sense and you would never believed it if some one would tell you such a story in real life, and I don't see why we have to believe it just to support an hypothesis that can never be tested.

Your argument has nothing specifically to do with relativity at all! In basic Newtonian physics, suppose that in the frame of the ground a car is accelerating down the road so its speed relative to the road is increasing. Then if I am an inertial observer moving at constant velocity down the same road, if the accelerating car's velocity relative to the road goes from below mine to above mine, then in my frame the car's direction will change without it turning around. It's not hard to see why this must true--when the accelerating car's speed relative to the road is lower than mine, if I am in front the distance between me and the accelerating car is increasing, so in my frame (where I am at rest) the accelerating car must be moving away from me; but then when the accelerating car's speed exceeds mine while I am still in front, the distance between me and the accelerating car is now decreasing, so in my frame the accelerating car must now be moving towards me.

For a more mathematical demonstration, suppose the accelerating car's position as a function of time in the ground frame is given by x(t) = (1.5 meters/second^2)*t^2, and my own position as a function of time in the ground frame is given by x(t) = (27 meters/second)*t, so I have a constant speed of 27 m/s and we both start at position x=0 meters at time t=0. You said you were familiar with derivatives, can you calculate the instantaneous velocity as a function of time (i.e. first derivative of x(t) with respect to t) for the accelerating car, and therefore figure out the time t at which the accelerating car's velocity exceeds that of my car?

Then in Newtonian physics, the coordinates of events in my own rest frame x',t' are related to the coordinates x,t in the ground frame by the following simple transformation:

x' = x - (27 m/s)*t
t' = t

And the reverse transformation:

x = x' + (27 m/s)*t'
t = t'

So if the accelerating car has x=(1.5 m/s^2)*t^2 in the ground frame, we can substitute x=x' + (27 m/s)*t' and t=t' to conclude x' + (27 m/s)*t' = (1.5 m/s^2)*t'^2, which means x'(t') in my frame is x'(t') = (1.5 m/s^2)*t'^2 - (27 m/s)*t'. Again, can you take the first derivative of this to find the velocity as a function of time of the accelerating car in my own rest frame?

 

[yoelhalb]

Your argument has nothing specifically to do with relativity at all! In basic Newtonian physics, suppose that in the frame of the ground a car is accelerating down the road so its speed relative to the road is increasing. Then if I am an inertial observer moving at constant velocity down the same road, if the accelerating car's velocity relative to the road goes from below mine to above mine, then in my frame the car's direction will change without it turning around. It's not hard to see why this must true--when the accelerating car's speed relative to the road is lower than mine, if I am in front the distance between me and the accelerating car is increasing, so in my frame (where I am at rest) the accelerating car must be moving away from me; but then when the accelerating car's speed exceeds mine while I am still in front, the distance between me and the accelerating car is now decreasing, so in my frame the accelerating car must now be moving towards me.

It has with the principle of relativity.
Before special relativity there was claimed to be absolute motion (such as the ether) so you would never claim that the car changed directions.
Such a claim was only introduced by Einstein, and it can never be proved, and as I show it is against common sense.

 

[Dale]

as I show it is against common sense.

You certainly have not shown any such thing. You have merely asserted it with no proof, derivation, nor even an explanation about why you might think such an absurd thing.

 

[yoelhalb]

You certainly have not shown any such thing. You have merely asserted it with no proof, derivation, nor even an explanation about why you might think such an absurd thing.

Then please explain it to me.
I will tell you the story and you will explain me just what is going on.
Imagine 3 objects are at together A,B,C.
Then A and B are moving away with a uniform motion A<----------->B, at 100 m/s.
C also starts to accelerate to the left, C is a horse and buggy, accelerating from C's view 1 m/s² from the initial point.
So in the first second after the beginning of the motion, (C has moved 1 m from the initial point) will C be 1 m to the left of B or 1 m to the left of A?, and how will A and B interpret this.

 

[JesseM]

It has with the principle of relativity.
Before special relativity there was claimed to be absolute motion (such as the ether) so you would never claim that the car changed directions.

Even before relativity, there'd be nothing stopping you from having a road moving inertially relative to the ether frame (after all the Earth is not the center of the universe, so we wouldn't expect the surface of the Earth to remain at rest relative to the ether), and a car moving relative to the road at just the right velocity so it was at rest in the ether frame. In this case, if you have a second accelerating car initially at rest relative to the road, but then accelerating in a constant way so that its velocity relative to the road eventually exceeded the inertial car's velocity, then naturally the accelerating car will turn around without rotating in the inertial car's frame--and here we have set things up so the inertial car's rest frame is the ether frame, so the accelerating car turns around without rotating in the ether frame too.

 

[Mentz114]

Then A and B are moving away with a uniform motion A<----------->B, at 100 m/s.

I presume you mean moving apart. In which case they must have experienced some acceleration before the state of motion you describe.

This period is crucial in working out if C will overtake A. But if the system was completely specified there would be no doubt about the positions of A,B and C, and all observers will agree.

 

[yoelhalb]

Even before relativity, there'd be nothing stopping you from having a road moving inertially relative to the ether frame (after all the Earth is not the center of the universe, so we wouldn't expect the surface of the Earth to remain at rest relative to the ether), and a car moving relative to the road at just the right velocity so it was at rest in the ether frame. In this case, if you have a second accelerating car initially at rest relative to the road, but then accelerating in a constant way so that its velocity relative to the road eventually exceeded the inertial car's velocity, then naturally the accelerating car will turn around without rotating in the inertial car's frame--and here we have set things up so the inertial car's rest frame is the ether frame, so the accelerating car turns around without rotating in the ether frame too.

My question is different then your example, since in my example both started at the same place, and my question is not because he passes him but because A must claim him backing up, (also a car can back up without rotating and is not the same as a horse and buggy), here is what your example might look like on Earth before relativity.
Imagine two horse and buggies are initially at the same spot, then suddenly one horse and buggy accelerates backward and then suddenly he passes the other horse and buggy, all without rotation.
(However nobody would make such a claim before relativity, and you would never believe such a story).

This is analogous to what I am speaking, A,B,C are initially together, then A and B move away with linear motion and A is to the left, if A is the point of reference then how can C (who is accelerating to the left) be to the right side of A.
Thus, clearly showing that although A moves with a linear motion B is the point of reference.

You might claim that C will never be to A's right, but this is not true, consider two ships moving away with a uniform motion do you think the water between them will be emptied out?.
So C might stay to his right, and thus proving that a is not the frame of reference, (Actually this is what I started the whole thread that there must be some global reference and not that every body can claim to be his own point of reference).

 

[ghwellsjr]

Then please explain it to me.
I will tell you the story and you will explain me just what is going on.
Imagine 3 objects are at together A,B,C.
Then A and B are moving away with a uniform motion A<----------->B, at 100 m/s.
C also starts to accelerate to the left, C is a horse and buggy, accelerating from C's view 1 m/s² from the initial point.
So in the first second after the beginning of the motion, (C has moved 1 m from the initial point) will C be 1 m to the left of B or 1 m to the left of A?, and how will A and B interpret this.

You claimed in post #67 that you already know about derivatives but now it is clear that you do not. If C is accelerating at 1 m/s^2, then after 1 second, C will have moved 1/2 m from the initial point, not 1 m. You seem to be getting the position confused with the speed which is 1 m/s after 1 second.

And your example has nothing to do with relativity. We can't even figure out what your issue is. You give us a multiple choice question where all the answers are incorrect. Or maybe I should say, the only way one of your answers could be correct is if we interpret the question in a way that I'm sure you didn't mean.

The way I think you mean is: After one second, A has moved to the left 50 m, B has moved to the right 50 m, and C has moved to the left 1 m. But then why are you asking us if C is 1 m to the left of A or 1 m to the left a B?

So you must have meant that A is moving to the left and is 100 m from the starting point and B is stationary so then the correct answer would be: C is 1 m to the left of B.

But you could have meant that B is moving to the right and is 100 m from the starting point and A is stationary so then the correct answer would be: C is 1 m to the left of A.

(Keep in mind, I am using your incorrect understanding of the actual position of C after 1 second.)

Can you see why your example is so confusing? You have been presenting this example since post #17, and you still haven't presented it in an unambiguous way that would allow us to respond in any meaningful way.

 

[ghwellsjr]

My question is different then your example, since in my example both started at the same place, and my question is not because he passes him but because A must claim him backing up, (also a car can back up without rotating and is not the same as a horse and buggy), here is what your example might look like on Earth before relativity.
Imagine two horse and buggies are initially at the same spot, then suddenly one horse and buggy accelerates backward and then suddenly he passes the other horse and buggy, all without rotation.
(However nobody would make such a claim before relativity, and you would never believe such a story).

This is analogous to what I am speaking, A,B,C are initially together, then A and B move away with linear motion and A is to the left, if A is the point of reference then how can C (who is accelerating to the left) be to the right side of A.
Thus, clearly showing that although A moves with a linear motion B is the point of reference.

You might claim that C will never be to A's right, but this is not true, consider two ships moving away with a uniform motion do you think the water between them will be emptied out?.
So C might stay to his right, and thus proving that a is not the frame of reference, (Actually this is what I started the whole thread that there must be some global reference and not that every body can claim to be his own point of reference).

You seem to think that special relativity is saying that every person, horse, buggy, ship, car, etc. can all claim to be a different point of reference all at the same time. But it does not say that. It says you can pick anyone to be the point of reference and analyze what everyone else is doing from that reference frame. Then, if you want, you can pick another one to be the point of reference and analyze everything from that reference frame and there are ways to convert the answers you get from one reference frame into another reference frame. The number you get for speed, positions and times can be all different in each reference frame but they will be consistent with each other with regard to the order of events. You can even pick a frame of reference for which there is no object.

So let's do it for your example. If we decide that the initial starting point where A, B, & C are stationary is the frame of reference, then we could say that A moves to the left at 50 m/s, B moves to the right at 50 m/s and C accelerates to the left at 1 m/s^2. In this case, after 1 second, C would be to the right of A by 49 m and to the left of B by 51 m. (Again, I'm using your incorrect understanding of position due to acceleration.)

Or we could decide to use the frame of A's motion as the reference frame. Then after 1 second, C would be to the right of A by 49 m and B would be to the right of C by 51 m.

Or we could decide to use the frame of B's motion as the reference frame. Then after 1 second, C would be to the left of B by 51 m and A would be to the left of C by 49 m.

Do you see that these all give the same answers, even though we assume different reference frames?

(These speeds are good enough because they are so slow, it would be a little more complicated if the speeds approached the speed of light.)

 

[Dale]

Then please explain it to me.

Sorry, I cannot read your mind and I cannot make sense out of nonsense. Post 83 by ghwellsjr outlines the same problems that I am having with your scenario. It is not up to me to try to guess your intentions and do both sides of the argument. If you have a point to make then it is up to you to convey it clearly and unambiguously.

To make your point you need to do the following:
1) explicitly state the worldlines of A, B, and C in some inertial frame (as I have requested 3 times now).
2) explain what condition prevents a horse and buggy from going backwards.
3) show that that condition arises in your example.

If you cannot do 1) and 3) then you should still at least be able to do 2). You have provided no explanation for why a horse and buggy cannot go backwards other than asserting "common sense". So, explain, what prevents a horse and buggy from going backwards, do you imagine that the horse explodes, if so what causes the explosion, if not then what else could prevent it from going backwards?

 

[JesseM]

My question is different then your example, since in my example both started at the same place, and my question is not because he passes him but because A must claim him backing up, (also a car can back up without rotating and is not the same as a horse and buggy), here is what your example might look like on Earth before relativity.

In my example the accelerating car's wheels were still rolling forwards when it was initially going backwards in the rest frame of the inertial car at rest in the ether, it wasn't "backing up" in the traditional sense of making its wheels go backwards. That's because in this frame the road was itself moving backwards (think of a treadmill), so even though the accelerating car was going forwards relative to the road, it was still going backwards in the frame of the inertial car until its speed relative to the road matched that of the inertial car, at which they were both at rest relative to the ether, and after that the accelerating car's continued acceleration would cause it to start moving forward relative to the ether.

Nothing about this example would change if you imagined that we replaced the two cars with two horse-and-buggies, and imagined that both started at the same position on the road. It would still be true that if the road was moving backwards at speed v relative to the ether, and the inertial horse-and-buggy was moving forward at speed v relative to the road, then the inertial horse-and-buggy would be at rest relative to the ether. And if the accelerating horse-and-buggy started out at rest relative to the road, then it would start out moving backwards relative to the ether. If it later accelerated until it was moving at a speed greater than v relative to the road, then it would be moving forward relative to the ether. Do you disagree?

 

[yoelhalb]

You seem to think that special relativity is saying that every person, horse, buggy, ship, car, etc. can all claim to be a different point of reference all at the same time. But it does not say that. It says you can pick anyone to be the point of reference and analyze what everyone else is doing from that reference frame. Then, if you want, you can pick another one to be the point of reference and analyze everything from that reference frame and there are ways to convert the answers you get from one reference frame into another reference frame. The number you get for speed, positions and times can be all different in each reference frame but they will be consistent with each other with regard to the order of events. You can even pick a frame of reference for which there is no object.

So let's do it for your example. If we decide that the initial starting point where A, B, & C are stationary is the frame of reference, then we could say that A moves to the left at 50 m/s, B moves to the right at 50 m/s and C accelerates to the left at 1 m/s^2. In this case, after 1 second, C would be to the right of A by 49 m and to the left of B by 51 m. (Again, I'm using your incorrect understanding of position due to acceleration.)

Or we could decide to use the frame of A's motion as the reference frame. Then after 1 second, C would be to the right of A by 49 m and B would be to the right of C by 51 m.

Or we could decide to use the frame of B's motion as the reference frame. Then after 1 second, C would be to the left of B by 51 m and A would be to the left of C by 49 m.

Do you see that these all give the same answers, even though we assume different reference frames?

(These speeds are good enough because they are so slow, it would be a little more complicated if the speeds approached the speed of light.)

The question here is simple (it is a logical and not a mathematical question), since C was initially together with A, and since C is an horse and buggy heading to the left, then if A is the point of reference then C should not never arrive to his right.
To illustrate this in real life, consider the typical relativity example.
You are in a train and there is a train next to it, then both trains start to move apart, so you claim that the other train moves away from you, but the people on the other train claim that you are moving.
Now let's change the example and instead of another train this time a horse and buggy is next to your train, and again your train and the horse and buggy move apart, so you think that the horse and buggy has moved.
But then you look out and you see that while your train and the horse and buggy still move apart, the horse is still facing your train, that means in other words that the buggy is pulling the horse away from you.
Can this be?.
So you are actually the one who moves.

 

[yoelhalb]

In my example the accelerating car's wheels were still rolling forwards when it was initially going backwards in the rest frame of the inertial car at rest in the ether, it wasn't "backing up" in the traditional sense of making its wheels go backwards. That's because in this frame the road was itself moving backwards (think of a treadmill), so even though the accelerating car was going forwards relative to the road, it was still going backwards in the frame of the inertial car until its speed relative to the road matched that of the inertial car, at which they were both at rest relative to the ether, and after that the accelerating car's continued acceleration would cause it to start moving forward relative to the ether.

Nothing about this example would change if you imagined that we replaced the two cars with two horse-and-buggies, and imagined that both started at the same position on the road. It would still be true that if the road was moving backwards at speed v relative to the ether, and the inertial horse-and-buggy was moving forward at speed v relative to the road, then the inertial horse-and-buggy would be at rest relative to the ether. And if the accelerating horse-and-buggy started out at rest relative to the road, then it would start out moving backwards relative to the ether. If it later accelerated until it was moving at a speed greater than v relative to the road, then it would be moving forward relative to the ether. Do you disagree?

So you say now that objects are not being moved apart by a force internal to the object, but rather by an external force such as the road, water or wind (for ships), and the object itself might actually be moving in the opposite direction.
(This is similar to what the Greek's thought about the stars and planets rotating every day around the world, that the universe carries them around the world, even though the planets have their own motion).

So now let's imagine this with a simple example, A and B are initially together, then A and B are being moved apart by an external force A<------------>B
this can be true even if A and B are both horse and buggies facing the opposite direction of the motion, (e.g. A faces the right, and B the left).
The reason is because of an external force, that's what you explained.
Now imagine the external force (road, water, wind, or universe) changes its direction and instead of moving apart the objects it reunites them, (without any acceleration or rotation, actually in our example we don't rotation since the horse are anyway facing the direction of unity).
Now WHO of them is younger?

 

[ghwellsjr]

The question here is simple (it is a logical and not a mathematical question), since C was initially together with A, and since C is an horse and buggy heading to the left, then if A is the point of reference then C should not never arrive to his right.
To illustrate this in real life, consider the typical relativity example.
You are in a train and there is a train next to it, then both trains start to move apart, so you claim that the other train moves away from you, but the people on the other train claim that you are moving.
Now let's change the example and instead of another train this time a horse and buggy is next to your train, and again your train and the horse and buggy move apart, so you think that the horse and buggy has moved.
But then you look out and you see that while your train and the horse and buggy still move apart, the horse is still facing your train, that means in other words that the buggy is pulling the horse away from you.
Can this be?.
So you are actually the one who moves.

The problem with your examples is that you don't say enough about what is going on. When you say that A and C move apart without specifying which one (or both) is accelerating and then want to draw some conclusions based on which way a horse and buggy are facing, it shows that you don't understand some basic principles of physics which have nothing to do with relativity.

I want you to consider another example: You get in a stopped train at the railroad station. You sit in a seat. The shades are pulled down so you can't see out the windows. You consider this to be your reference frame. After a while, you feel a new force pushing you backwards into your seat. Now you know that you are accelerating. That means you are starting to move forward. As long as you continue to feel the force pushing you back into your seat, you know you are gaining speed. After a while, the force pushing you into the back of your seat diminishes until it is gone. Now you know that you have stopped accelerating and you are traveling at a constant speed. But you also know that as soon as you first felt the force, you were no longer stationary in your initial reference frame. You have been and continue to be moving in your initial reference frame. You don't need to look at anything outside your train to know that you are now moving with respect to your initial condition. Do you understand and agree with all of this?

 

[yoelhalb]

Sorry, I cannot read your mind and I cannot make sense out of nonsense. Post 83 by ghwellsjr outlines the same problems that I am having with your scenario. It is not up to me to try to guess your intentions and do both sides of the argument. If you have a point to make then it is up to you to convey it clearly and unambiguously.

To make your point you need to do the following:
1) explicitly state the worldlines of A, B, and C in some inertial frame (as I have requested 3 times now).
2) explain what condition prevents a horse and buggy from going backwards.
3) show that that condition arises in your example.

If you cannot do 1) and 3) then you should still at least be able to do 2). You have provided no explanation for why a horse and buggy cannot go backwards other than asserting "common sense". So, explain, what prevents a horse and buggy from going backwards, do you imagine that the horse explodes, if so what causes the explosion, if not then what else could prevent it from going backwards?

My question is that it is impossible to happen ny internal forces, yet it is possible to happen by external forces (even while it is itself accelerating on the opposite direction).
Imagine the horse and buggy are not on Earth but traveling in water, then the water can surely take them backwards.
But if this is true, then the external force can also take them back without any acceleration or rotation, now when they will meet together who will be younger?.

 

[Dale]

My question is that it is impossible to happen ny internal forces

Do you mean the force between the buggy and the horse, or do you mean the internal forces holding the buggy together or the internal forces holding the horse together?

What is wrong with the internal forces in the case of a backwards moving horse and buggy? Can you draw a free body diagram or cite some force law that causes the internal forces to have a problem?

Let me be clear. You are suggesting that you are smarter than all of the most brilliant minds on the planet for the last century. You should at least be able to do the things we would expect of a freshman-level undergraduate student such as draw a free-body diagram, cite a force law, and derive an expression for the critical internal force as a function of the velocity. This is a very minimal requirement I am asking here considering the enormity of your claim.

 

[JesseM]

So you say now that objects are not being moved apart by a force internal to the object, but rather by an external force such as the road, water or wind (for ships), and the object itself might actually be moving in the opposite direction.

No, I said nothing about any force applied by the surface the objects are moving on. For example, a car or wagon moving along a road need not be receiving any sideways force from the road--this is the ideal case of rolling without slipping. And we could also just imagine some rockets directly above each horse-and-buggy, not in contact with the surface at all! In the case of the horse-and-buggy moving at constant speed relative to the road (with a forward speed relative to the road that's exactly equal to the backwards speed of the road in the frame of the ether, so this horse-and-buggy is at rest in the ether frame), the rocket above it has its rockets off, so it's not accelerating and is at rest relative to the ether too. Meanwhile, the other horse-and-buggy starts out at rest relative to the road (and is therefore moving backwards relative to the ether), then the horse starts accelerating relative to the road until this horse-and-buggy is moving forward in the ether frame; similarly, we can imagine the rocket above it initially has its engines off and is just coasting backwards at the same speed relative to the ether as the horse-and-buggy below it, then when this horse starts running forward relative to the road, the rocket turns on its engines and starts accelerating forward relative to the road too, thus moving in exactly the same way as the horse-and-buggy. In neither case is there any need for either of them to rotate in order to change their direction of motion in the ether frame.

(This is similar to what the Greek's thought about the stars and planets rotating every day around the world, that the universe carries them around the world, even though the planets have their own motion).

How is it similar? My example relies only on standard Newtonian physics, it doesn't require any non-Newtonian assumptions like the one that says forces can't act at a distance, or the one that says an object needs constant pushing to travel at constant velocity.

So now let's imagine this with a simple example, A and B are initially together, then A and B are being moved apart by an external force A<------------>B

The road in my example isn't responsible for "moving them apart"--they simply start out with different initial velocities, one initially at rest relative to the ether and one moving backwards relative to the ether. Do you understand that in Newtonian physics an object moving at constant velocity will continue to move at that velocity until some force is applied to it? So if an object is initially moving backwards in the ether frame it will continue to move that way unless some force pushes it forward (decreasing its speed in the backward direction), like the horse's legs pushing against a road or a rocket's engine firing.

this can be true even if A and B are both horse and buggies facing the opposite direction of the motion, (e.g. A faces the right, and B the left).

In my example I was assuming both horse and buggy were facing in the forward direction, it's just that one had an initial velocity in the direction opposite to the one it was facing.

The reason is because of an external force, that's what you explained.

No, there was nothing in my post about an external force.

Now imagine the external force (road, water, wind, or universe) changes its direction and instead of moving apart the objects it reunites them, (without any acceleration or rotation, actually in our example we don't rotation since the horse are anyway facing the direction of unity).
Now WHO of them is younger?

If two objects move apart and come back together symmetrically in any frame (i.e. each one has the same speed at any given time in that frame, though the direction of their motions will be opposite), then they will be the same age when they reunite.

 

[yoelhalb]

If two objects move apart and come back together symmetrically in any frame (i.e. each one has the same speed at any given time in that frame, though the direction of their motions will be opposite), then they will be the same age when they reunite.

Actually according to relativity evry one of them claims to be at rest and the other one moving, so according to A then B's clock is getting slower and according to B then A's clock is getting slower, then who of them will be younger when they reunit?.

 

[yoelhalb]

Do you mean the force between the buggy and the horse, or do you mean the internal forces holding the buggy together or the internal forces holding the horse together?

What is wrong with the internal forces in the case of a backwards moving horse and buggy? Can you draw a free body diagram or cite some force law that causes the internal forces to have a problem?

Let me be clear. You are suggesting that you are smarter than all of the most brilliant minds on the planet for the last century. You should at least be able to do the things we would expect of a freshman-level undergraduate student such as draw a free-body diagram, cite a force law, and derive an expression for the critical internal force as a function of the velocity. This is a very minimal requirement I am asking here considering the enormity of your claim.

All brilliant minds have believed in Aristotle's teachings for thousands of years, and it turned out to be wrong.
And Galileo has not disproved Aristotle with any diagrams or functions, just by putting it to test in real life, and with though experiments.
(Actually what it was found is, that all the brilliant minds never thought that Aristotle can be wrong, even though it was never proved.
Actually had you ever thought that the principle of relativity might be wrong, remember this is evidence based science, on the other hand the principle of relativity can never be proved).
Surely you are right that since there is an established way to present an argument I have to adhere to it, so can you please show me where I can see more on those diagrams.
Thanks.

What I am saying about internal force, I mean the usual force that a horse pulls a buggy with, which is the normal reason for a horse and buggy to be considered moving, and for this motion to change direction it has to rotate, and to speed up it has to accelerate.
Any other reason to the motion of an horse and buggy such as the road moving or the wind or the water (for ships) etc. I call here external, and for this type of motion you can get to speed without any acceleration and you can also move back without any rotation (for example two ships are being moved away by the water and they can also be reunited by the water without any rotation).
And I am asking, in this case who of them will be younger?.

 

[ghwellsjr]

The problem with your examples is that you don't say enough about what is going on.

So that's what this is all about--the Twin Paradox? This is the first time you have suggested that two of your objects come back together. Why didn't you say this in your first post where you mentioned an example?

When two objects (whether they be trains, boats, horses, buggies, clocks, people or anything else) start together and are at the same age (or have the same time on their clocks) and then one or both of them move in any direction at any speed for any distance with any rotations but they eventually come back together (even if it isn't their initial starting point) and they compare their ages (or clocks), there will be one and only one answer as to their ages (or the times on their clocks). This is reality. Now in order for you to tell what they will measure, it doesn't matter whether you analyze the problem using Special Relativity or any other consistent physical theory, you will get the same answer that they get. But you cannot ask us to tell what answer they get unless you tell us how they move. That is the reason why we are not getting anywhere in helping you.

You need to say which object is moving in which direction and for how long, etc, etc, etc. Now there is one special case where you don't have to give any details and that is when only one object accelerates while the other remains in the initial starting condition. In this case the one that accelerated will always be younger than the one that didn't accelerate. And, again, this has nothing to do with Special Relativity. You can analyze the problem the same way people analyzed the problem before Einstein came along and they and you will get the same answer. It's the way the world works.

 

[yoelhalb]

So that's what this is all about--the Twin Paradox? This is the first time you have suggested that two of your objects come back together. Why didn't you say this in your first post where you mentioned an example?

When two objects (whether they be trains, boats, horses, buggies, clocks, people or anything else) start together and are at the same age (or have the same time on their clocks) and then one or both of them move in any direction at any speed for any distance but they eventually come back together (even if it isn't their initial starting point) and they compare their ages (or clocks), there will be one and only one answer as to their ages (or the times on their clocks). This is reality.

Even if they move in uniform motion only?.
So we clearly know who was moving, and with claiming this you actually break the principle of relativity.

 

[JesseM]

If two objects move apart and come back together symmetrically in any frame (i.e. each one has the same speed at any given time in that frame, though the direction of their motions will be opposite), then they will be the same age when they reunite.

Actually according to relativity evry one of them claims to be at rest and the other one moving, so according to A then B's clock is getting slower and according to B then A's clock is getting slower, then who of them will be younger when they reunit?.

I should have written:

If two objects move apart and come back together symmetrically in any inertial frame (i.e. each one has the same speed at any given time in that frame, though the direction of their motions will be opposite), then they will be the same age when they reunite.

The SR law of time dilation, which says that clocks with a greater velocity in some frame run slower in that frame, only applies in inertial frames. In non-inertial frames, a clock with a greater coordinate velocity may sometimes run faster than a clock with a lesser coordinate velocity, you can't count on time dilation obeying the same rules in a non-inertial frame (but again, if you know the coordinate transformation from an inertial frame to the non-inertial frame, you can always deduce how laws of physics like time dilation work in the non-inertial frame by applying the coordinate transformation to the known equations expressing these laws in the inertial frame). Since A and B must accelerate in order to move apart and come back together, then although it is possible to define non-inertial rest frames for each one, there is no reason for each one of them to predict that the other one's clock must have elapsed less time.

Now that that's cleared up, are you going to address any of my other points in post #92? Do you finally see how it's true in classical Newtonian mechanics as well as relativity that objects can change their direction of motion without rotating or changing the direction that force is being applied to them, or are you still confused on this point?

 

[ghwellsjr]

What do you mean, "even if they move in uniform motion only?" Uniform motion means nonaccelerating. If they were both in uniform motion, say, two passengers on the same moving train, sitting next to each other, they would age at the same rate. But if one of them got up and went to the bathroom and came back and sat down while the other one remained seated, the one that went to the bathroom would be younger.

And like I say, this has nothing to do with Special Relativity. You can analyze this same problem using any theory of physics that works. They will all get the same answer. If you don't like the answer, you need to complain to Mother Nature, not to Einstein. He isn't making the results come true, he is only offering the simplest way to analyze Mother Nature.

If you don't like relativity, what other theory of physics would you like to propose to analyze your problems?

 

[JesseM]

What I am saying about internal force, I mean the usual force that a horse pulls a buggy with, which is the normal reason for a horse and buggy to be considered moving, and for this motion to change direction it has to rotate, and to speed up it has to accelerate.

But I already showed you this was wrong. You complained that in my example the road was itself applying a force, but this is always true of a horse and buggy, the horse can only move forward because of a sideways friction force applied to the horse's hooves by the road (a horse on a totally frictionless surface, like the smoothest ice imaginable, would be unable to change the motion of its center of mass--if its center of mass was originally at rest relative to the frictionless surface, then the horse would be unable to start moving forward relative to the frictionless surface by walking or running). No purely "internal" forces can cause a horse and buggy to change velocity. If you don't like examples where some "external" surface is applying a force, why not instead consider a buggy pulled along by a rocketship whose nose is pointing away from the buggy and whose exhaust nozzle is facing back towards the buggy? The rocket, unlike the horse, can provide a forward pull on the buggy without the need for any other object to apply a force on it, the rocket accelerates forward by accelerating its exhaust in the opposite direction. Clearly unless this rocket is rotated relative to the buggy, the rocket can only apply a forward force to the buggy, never a backward one, but nevertheless if the buggy starts out moving inertially backwards in the frame of the ether before the rocket is activated, then after the rocket starts thrusting it can change direction relative to the ether without any change in the orientation of the rocket.

 

[yoelhalb]

I should have written:

The SR law of time dilation, which says that clocks with a greater velocity in some frame run slower in that frame, only applies in inertial frames. In non-inertial frames, a clock with a greater coordinate velocity may sometimes run faster than a clock with a lesser coordinate velocity, you can't count on time dilation obeying the same rules in a non-inertial frame (but again, if you know the coordinate transformation from an inertial frame to the non-inertial frame, you can always deduce how laws of physics like time dilation work in the non-inertial frame by applying the coordinate transformation to the known equations expressing these laws in the inertial frame). Since A and B must accelerate in order to move apart and come back together, then although it is possible to define non-inertial rest frames for each one, there is no reason for each one of them to predict that the other one's clock must have elapsed less time.

Now that that's cleared up, are you going to address any of my other points in post #92? Do you finally see how it's true in classical Newtonian mechanics as well as relativity that objects can change their direction of motion without rotating or changing the direction that force is being applied to them, or are you still confused on this point?

Who says they must accelerate?.
Isn't it possible that a strong wind took one ship (for example) at a steady velocity?.
Imagine two ships in water, they meet and then they move apart, there needs to be no acceleration involved.
And what in case when both accelerated first?.
And even if only one of them accelerated for 10 minutes and then traveled for 100 years, who is then younger?.

 

[yoelhalb]

What do you mean, "even if they move in uniform motion only?" Uniform motion means nonaccelerating. If they were both in uniform motion, say, two passengers on the same moving train, sitting next to each other, they would age at the same rate. But if one of them got up and went to the bathroom and came back and sat down while the other one remained seated, the one that went to the bathroom would be younger.

And like I say, this has nothing to do with Special Relativity. You can analyze this same problem using any theory of physics that works. They will all get the same answer. If you don't like the answer, you need to complain to Mother Nature, not to Einstein. He isn't making the results come true, he is only offering the simplest way to analyze Mother Nature.

If you don't like relativity, what other theory of physics would you like to propose to analyze your problems?

Imagine two trains are moving in opposite directions coming from different locations, then they meet together, and continue their motion, this does not need any acceleration, and according to relativity every one might claim resting (but not according to regular physics in which you would find a reference to the some absolute point).
So every one claims to be at rest and the other trains clock to slow down.
Now the train who was moving backs up and they reunite, who of them is younger?.

 

[yoelhalb]

What do you mean, "even if they move in uniform motion only?" Uniform motion means nonaccelerating. If they were both in uniform motion, say, two passengers on the same moving train, sitting next to each other, they would age at the same rate. But if one of them got up and went to the bathroom and came back and sat down while the other one remained seated, the one that went to the bathroom would be younger.

And like I say, this has nothing to do with Special Relativity. You can analyze this same problem using any theory of physics that works. They will all get the same answer. If you don't like the answer, you need to complain to Mother Nature, not to Einstein. He isn't making the results come true, he is only offering the simplest way to anaylize Mother Nature.

If you don't like relativity, what other theory of physics would you like to propose to analyze your problems?

Let's put it different, is it possible for to have two people in linear motion without any acceleration or no?.
If no then we don't need relativity, since we always know who is moving.
If yes then in this situation when they reunite who is younger?.

 

[JesseM]

Who says they must accelerate?.
Isn't it possible that a strong wind took one ship (for example) at a steady velocity?.
Imagine two ships in water, they meet and then they move apart, there needs to be no acceleration involved.

Now you're changing the scenario! Before you said they moved apart and then reunited again:

So now let's imagine this with a simple example, A and B are initially together, then A and B are being moved apart by an external force A<------------>B
this can be true even if A and B are both horse and buggies facing the opposite direction of the motion, (e.g. A faces the right, and B the left).
The reason is because of an external force, that's what you explained.
Now imagine the external force (road, water, wind, or universe) changes its direction and instead of moving apart the objects it reunites them, (without any acceleration or rotation, actually in our example we don't rotation since the horse are anyway facing the direction of unity).
Now WHO of them is younger?

If two objects just move apart at constant speed, they can never meet at the same location to compare clocks, so there is no frame-independent truth about which one is older, because of the relativity of simultaneity. If you and I are both 20 when we meet and then we move apart inertially at 0.6c, then in my rest frame the event of my turning 40 is simultaneous with the event of your turning 36, but in your rest frame the event of my turning 40 is simultaneous with the event of you turning 45, so our two frames disagree on which of us has aged more at the moment I turn 40.

 

[yoelhalb]

Now you're changing the scenario! Before you said they moved apart and then reunited again:

If two objects just move apart at constant speed, they can never meet at the same location to compare clocks, so there is no frame-independent truth about which one is older, because of the relativity of simultaneity. If you and I are both 20 when we meet and then we move apart inertially at 0.6c, then in my rest frame the event of my turning 40 is simultaneous with the event of your turning 36, but in your rest frame the event of my turning 40 is simultaneous with the event of you turning 45, so our two frames disagree on which of us has aged more at the moment I turn 40.

I have not changed the story, the ships can be brought back together without acceleration or rotation, just be the wind or the water.
Anyway if the universe is round as Einstein proposed and we can come back to where we started, then who will be younger.
Anyway only one of them can be younger at a given time, and even if we don't have to know which one of them is younger there is only one who is younger.

 

[JesseM]

I have not changed the story, the ships can be brought back together without acceleration or rotation, just be the wind or the water.

Once again you seem not to understand the basic meaning of "acceleration"! Any change in speed or direction is acceleration, it doesn't matter whether the reason for this change in direction is something "internal" like firing a rocket or "external" like the wind or water applying a force on the object.

Anyway if the universe is round as Einstein proposed and we can come back to where we started, then who will be younger.

In this case spacetime is curved so no coordinate system covering each object's entire path can qualify as "inertial", and again there is no requirement that time dilation work the same way in a non-inertial frame. So even if you define a non-inertial coordinate system where object A is at rest throughout the journey while object B moves away and then returns, there's no reason to predict that B ages more slowly throughout the journey in this non-inertial coordinate system, it's only in inertial coordinate systems that objects in motion always age more slowly than objects at rest.

Anyway only one of them can be younger at a given time, and even if we don't have to know which one of them is younger there is only one who is younger.

If they're far apart there doesn't have to be any frame-independent truth about which is younger. Again, in relativity there is no frame-independent notion of a "given time", different frames have different definitions of which sets of events occur at the "same time" and which occur at "different times" (i.e. whether a given pair of events are assigned the same time-coordinate or different time-coordinates), so two events that happened at the same time in one frame can have happened at different times in another frame (unless both events happened at both the same place and the same time in one frame, in which case all frames agree the events happened at the same place and time). Did you look at the link I gave you on the "relativity of simultaneity"? Here is another one: http://www.pitt.edu/~jdnorton/Goodies/rel_of_sim/index.html