At rest in Einsteinian relativity

[Dale]

Let's say that we have a door and a frame, and the door is at rest relative to the frame in the open position. How can the door change to the closed position without either the frame or the door moving; surely we can deduce that either the door, or the frame, or both, actually move so that the door goes from being open to closed?

You completely missed the point of the analogy. The point is that the designation of a door being open or closed doesn't even make sense without comparison to the door frame.

Tell me, in your opinion, if there is no door frame, can the door be considered to be open or closed?

 

[Aurin]

I'm sure I'll violate all sorts of precision in my choice of words, so please forgive me in advance.

As I understand it, being at rest isn't a separable entity of kinematics. It's just a "special case" of motion where velocity is equal to 0 and that value is no more or less significant than a non-zero value of velocity. For any given object we can attribute a frame providing it any value for its velocity we choose.

As I also understand it, an object relative to a frame that suddenly changes direction, moving opposite to its original path, had at one instantaneous point attained a velocity equal to zero and thus may have been at rest in that frame for a moment. By extension, other frames never witnessed the object changing direction, but only a change in speed witnessed as speeding up or slowing down.

So if asked "what does it mean to be at rest in SR?" like this thread's original purpose, my response would surely be that an object's velocity is equal to 0 relative to the frame that provides that velocity. Is there a disjoint in that answer?

 

[russ_watters]

I see how the logic works now:

1. Start with the assumption that absolute motion and rest exist.
2. If two objects are at rest relative to each other, they may be in absolute motion or absolute rest.
3. If one starts moving relative to the other, now we can be certain that at least one is in absolute motion since you can't simultaneously have both in absolute rest.
4. Conclude therefore that absolute motion and rest exist.

The line of logic is absolutely fine in the middle, but the conclusion is #3, not #4. #4 is just an echo of the starting assumption and the test does not delve into the issue of if the starting assumption is correct.

It also frustrates me that people cling so dearly to the assumption of absolute motion/rest, particularly when every waking moment of our lives we deal in relative motion and often even multiple simultaneous relative velocities in several frames at once (ie, when driving or playing sports). None of our experiences mandate the existence of an absolute reference frame, so I have trouble even understanding where the default assumption of an absolute reference frame would come from. Perhaps it comes from the thought of a fish-tank universe with God looking through the glass. Perhaps he's there, but whether he is or not, there is no basis for assuming he is nor any value in making the assumption. If one acknowledges that he and his frame are completely undetectable, then speculation about his existence is of no value in a scientific setting. It is purely a religious belief.

But...at the risk of confusing things, I will point out that there are objective, absolute realities that have to be acknowledged here. Consider the case of a rocket launching off the earth. The rocket burns fuel, so it must be said that it is moving (accelerating) away from the Earth and not the other way around. But the absoluteness of the fuel burning doesn't require an absoluteness of motion/rest.

 

[harrylin]

[..]None of our experiences mandate the existence of an absolute reference frame, so I have trouble even understanding where the default assumption of an absolute reference frame would come from. Perhaps it comes from the thought of a fish-tank universe with God looking through the glass. Perhaps he's there, but whether he is or not, there is no basis for assuming he is nor any value in making the assumption. If one acknowledges that he and his frame are completely undetectable, then speculation about his existence is of no value in a scientific setting. It is purely a religious belief. [..]

That comment seems to be a direct attack on Newton. So, if you like, I'll add your comment to the new thread that I just started on that topic.

 

[yoron]

My take on it :)

Can you define a 'motion'?
Do you need another frame of reference to do it?

If you are on 'something' being in a uniform motion, lifting/accelerating from it, does it matter what speed it has? For what acceleration/lift you need from that object?

Think about that one for a while before answering.

If uniform motion, as measured relative something else, doesn't crave any more 'energy/momentum' relative, or if you like, no matter what 'speed/velocity' you define it to have, or make you expend more energy accelerating from it, what is a 'uniform motion'?

Einstein differed between two things, 'uniform motion' and accelerations. He called one 'relative motion', the other a 'gravity'.
=

To simplify, define a speed/velocity of something (relative some 'inertial' definition). Accelerate from it. find the energy expended. Then make the object you accelerate/lift from beget double the speed/velocity, relative that same inertial frame of reference, to then become in a 'uniform motion' again. Then lift/accelerate from it again..

Will the energy expended by you accelerating/lifting differ between those two scenarios? If it won't, what do you think a uniform motion means?

 

[Mentz114]

This statement

It is not posible to distinguish between a state of rest and uniform motion

has been wrongly interpreted by the OP as meaning that a body is in one of these states or the other. But that is not what it means. It means they are the same state.

It may be said that a body is either in a state of uniform motion or accelerated motion, and that covers it.

What the statement above is saying is that we should abandon the idea of 'rest'.

The OP keeps insisting that there is actual motion, and I think he is referring to accelerated motion, which is certainly 'actual' in the sense that it can be detected.

 

[mangaroosh]

Without reference to a third frame of reference, you cannot say which of them moved.

But you could presumably conclude that one of them did, in fact, move, no?

Now you've introduced another frame of reference ( your grid) and you can tell which one moved relative to that frame. Your movements are always expressed realtive to some frame. It's not possible to define movement except as relative movement.

Is it not more accurate to say that you cannot measure movement except relative to some frame?

I would have thought that "absolute motion" would be a simple "yes" or "no" answer to the question, "did X move"? Measurement is, by necessity, relative, so I think the idea of measuring absolute motion would be a contradiction in terms, no?

If an absolute reference frame could be detected, which of course it can't, it might be possible to define something as "absolute velocity", which would simply be velocity relative to the "absolute frame".

But getting back to the door and the frame; if we imagine an infinite number of reference frames in the universe, wouldn't all observers agree which one actually moves i.e. the frame or the door; and could they not deduce that one of them, has to actually move in order for the door to go from being open, to being closed?

 

[Dale]

But you could presumably conclude that one of them did, in fact, move, no?

Good example of the logical fallacy "begging the question".

http://www.fallacyfiles.org/begquest.html

Your question presumes that the word "move" without any qualifiers has meaning. This is the very point under dispute, so you cannot assume it.

To make your question not fallacious you would need to qualify the word "move" in the question by specifying what it is moving relative to since both sides of the argument agree that relative motion is well defined and meaningful.

 

[mangaroosh]

You completely missed the point of the analogy. The point is that the designation of a door being open or closed doesn't even make sense without comparison to the door frame.

Tell me, in your opinion, if there is no door frame, can the door be considered to be open or closed?

I understood the analogy but I don't think it fully addressed the question. I agree with the point being made, and would say that a door cannot be considered open or closed in the absence of a frame.

But we're taking the example where there is a door and a frame; the door starts off in the open position and then subsequently changes to the closed position.

In your opinion, how can the door go from being open to closed without either the frame moving or the door moving; surely one of them, at least, has to move in order for the door to close?

We may not be able to determine which one moves, and in the doors frame of reference it might be the frame that is labelled as moving (although it cannot be determined if that is the case), and in the frames reference it might be the door that is labelled as moving (although that too cannot be determined); surely though we can agree that one of them, at least, has to move for the door to close?

 

[mangaroosh]

I see how the logic works now:

1. Start with the assumption that absolute motion and rest exist.
2. If two objects are at rest relative to each other, they may be in absolute motion or absolute rest.
3. If one starts moving relative to the other, now we can be certain that at least one is in absolute motion since you can't simultaneously have both in absolute rest.
4. Conclude therefore that absolute motion and rest exist.

The line of logic is absolutely fine in the middle, but the conclusion is #3, not #4. #4 is just an echo of the starting assumption and the test does not delve into the issue of if the starting assumption is correct.

#1 above can be dropped.

2. Two objects are at rest relative to each other e.g. a door and a door frame

3. The door is in the open position.

4. The door changes to the closed position.

5. Either the door or the frame, or both, moved.

It also frustrates me that people cling so dearly to the assumption of absolute motion/rest, particularly when every waking moment of our lives we deal in relative motion and often even multiple simultaneous relative velocities in several frames at once (ie, when driving or playing sports). None of our experiences mandate the existence of an absolute reference frame, so I have trouble even understanding where the default assumption of an absolute reference frame would come from. Perhaps it comes from the thought of a fish-tank universe with God looking through the glass. Perhaps he's there, but whether he is or not, there is no basis for assuming he is nor any value in making the assumption. If one acknowledges that he and his frame are completely undetectable, then speculation about his existence is of no value in a scientific setting. It is purely a religious belief.

It does appear as though people seem to be hung up on the idea of an absolute reference frame; I would have thought that the question of absolute motion was a simple "yes" or "no" question that didn't require an absolute reference frame e.g. "is X moving"?

You mention that we deal with relative motion every waking moment of our lives, but for most people - and this is where part of my struggle comes from - it makes perfect sense to ask "am I moving", without necessarily asking "relative to what?".

Take the example of walking down the street; most people would be able to engage in a conversation to answer the question "is it me that is moving, or is the Earth moving beneath my feet?". We would all measure the movement relative to the street, that isn't a problem, but the question of "which one is actually moving?" is a question that most people would, at least, agree makes sense to ask. They wouldn't say, "what do you mean 'actually moving', that doesn't make sense!", they would usually answer uneqivocaly, "don't be stupid! I'm actually moving, of course". If it were explained to them that it cannot actually be determined that they are moving, then they would presumably agree that either they must be moving, or the ground beneath them, and all the buildings, must be moving.

But...at the risk of confusing things, I will point out that there are objective, absolute realities that have to be acknowledged here. Consider the case of a rocket launching off the earth. The rocket burns fuel, so it must be said that it is moving (accelerating) away from the Earth and not the other way around. But the absoluteness of the fuel burning doesn't require an absoluteness of motion/rest.

But surely the fact that there is relative motion demonstrates that something must actually be moving, in some absolute sense - not necessarily relative to an absolute reference frame. The motion will always be measured relative to something, but either the Earth or the rocket has to be moving.Just to re-iterate the real life example of walking down the street, because I think it helps to anchor the question in everyday terms, and might help clarify where my issue lies.

If you are walking down the street, or even just walk across the room you are in; as you notice your motion relative to the objects in the room or on the street, ask yourself, is it me that is moving, or is the street, or the floor, moving beneath my feet like a treadmill?

 

[Mentz114]

But you could presumably conclude that one of them did, in fact, move, no?
Is it not more accurate to say that you cannot measure movement except relative to some frame?

I would have thought that "absolute motion" would be a simple "yes" or "no" answer to the question, "did X move"? Measurement is, by necessity, relative, so I think the idea of measuring absolute motion would be a contradiction in terms, no?

If an absolute reference frame could be detected, which of course it can't, it might be possible to define something as "absolute velocity", which would simply be velocity relative to the "absolute frame".

But getting back to the door and the frame; if we imagine an infinite number of reference frames in the universe, wouldn't all observers agree which one actually moves i.e. the frame or the door; and could they not deduce that one of them, has to actually move in order for the door to go from being open, to being closed?

So you've deduced that in order for something to change its state of motion a force must be applied.

How is this relevant to distinguishing 'rest' from unaccelerated motion ? Even if you knew the entire acceleration history of two comoving bodies in uniform motion, it does not alter the fact that now it does not matter to which we ascribe motion. They are now in a reciprocal state connected by a Lorentz transformation.

I don't think you like relativity and you cannot understand what it is and what it is used for. You are horribly misinterpreting the theory. Have you seen my post#106 ?
Do you agree with Newtons laws of motion ?

Finally, by your definition of 'actually moving', anything that has ever experienced acceleration is 'actually moving'. That probably includes all the matter in the universe so it is a useless concept, without any deductive or physical significance, and I wish you'd stop going on about it.

 

[Dale]

I agree with the point being made, and would say that a door cannot be considered open or closed in the absence of a frame.

And by analogy, an object cannot be considered moving or at rest in the absence of a reference frame.

But we're taking the example where there is a door and a frame; the door starts off in the open position and then subsequently changes to the closed position.

And by analogy there is motion and a reference frame.

In your opinion, how can the door go from being open to closed without either the frame moving or the door moving; surely one of them, at least, has to move in order for the door to close?

Has to move relative to what? As we have discussed neither the door nor the door frame can be considered moving or at rest in the absence of a reference frame. Until you specify that, your question is incomplete.

If you specify some inertial frame then at least one of them moved relative to the inertial frame. You could instead simply specify that they moved relative to each other. Of course, if you specify some non-inertial frame then you may find that neither moved relative to that frame. You have to specify what the motion is relative to otherwise the question is meaningless. Not only is it meaningless, but it is fallacious.

 

[russ_watters]

In your opinion, how can the door go from being open to closed without either the frame moving or the door moving; surely one of them, at least, has to move in order for the door to close?

As always, your formulation - by saying "move" without qualifiers - assumes the existence of absolute motion. It isn't necessary. The door and frame have to move relative to each other in order to open or close, but there is no need or value for assuming one or the other is the "one" to move in an absolute sense. Or, both could be moving with respect to a 3rd frame and the door may not necessarily close. You've assumed the existence of this third frame, that it is the universal frame and that one is stationary relative to it and the other is moving. None of that is necessary.

#1 above can be dropped.

2. Two objects are at rest relative to each other e.g. a door and a door frame

3. The door is in the open position.

4. The door changes to the closed position.

5. Either the door or the frame, or both, moved.

Again, with #5, stating "moved" without qualifiers. Indeed in order to close, one must move relative to the other. This allows us to conclude nothing about an absolute frame other than that it isn't necessary for this thought experiment. Pile up enough thought experiments where it isn't necessary and you have pretty good circumstantial evidence that it doesn't exist and no evidence whatsoever that it does.

It does appear as though people seem to be hung up on the idea of an absolute reference frame; I would have thought that the question of absolute motion was a simple "yes" or "no" question that didn't require an absolute reference frame e.g. "is X moving"?

Since motion requires comparison of reference frames, absolute motion must therefore require an absolute reference frame be one of those frames. That's by definition. That's what "absolute motion" means!

You mention that we deal with relative motion every waking moment of our lives, but for most people - and this is where part of my struggle comes from - it makes perfect sense to ask "am I moving", without necessarily asking "relative to what?".

As I said earlier in the thread, the reason it is rarely ever stated in everyday life is that people just intuitively understand it. Our brains are programmed to "get" it and use it. That's the reason I am so incredulous that you don't. In my thought experiments with the police officer measuring the speed of a car, the two different methods of measuring the speed of a car would never, ever, ever confuse anyone: There's three "observers" and two different speed measurments happening in both cases, but no one would ever get confused about which one is being referred to.

Take the example of walking down the street; most people would be able to engage in a conversation to answer the question "is it me that is moving, or is the Earth moving beneath my feet?". We would all measure the movement relative to the street, that isn't a problem, but the question of "which one is actually moving?" is a question that most people would, at least, agree makes sense to ask. They wouldn't say, "what do you mean 'actually moving', that doesn't make sense!", they would usually answer uneqivocaly, "don't be stupid! I'm actually moving, of course". If it were explained to them that it cannot actually be determined that they are moving, then they would presumably agree that either they must be moving, or the ground beneath them, and all the buildings, must be moving.

Doubt it. Simply by pointing out that the Earth is rotating - which everyone knows - would anyone be able to be convinced that the relative motion of a person and the Earth wrt each other truly is relative.

But surely the fact that there is relative motion demonstrates that something must actually be moving, in some absolute sense ...

I can't fathom a way you could have worded that any more exactly self-contradictory other than to cut out intermediate words. Do that and you just said 'relative = absolute'. Wow.

...in some absolute sense - not necessarily relative to an absolute reference frame.

Double wow. Now you're saying 'absolute is not necessarily absolute.' Honestly, I'm having a lot of trouble taking you seriously now.

If you are walking down the street, or even just walk across the room you are in; as you notice your motion relative to the objects in the room or on the street, ask yourself, is it me that is moving, or is the street, or the floor, moving beneath my feet like a treadmill?

And if you are truly being honest here and not trolling us, you would conclude: it doesn't matter.

 

[yoron]

Motion is always relative something, an acceleration can be seen as displacements of a infinite assemble of 'uniform motions', but it is also the single definition in where you can 'prove' a motion in that you locally will find something new coming into play, a 'gravity' :)

What you seem to mean is that as there is something called a motion, there must also be a absolute reference frame from where it exist, a 'gold standard' of it. The easiest way to define such a notion would be to turn it around, don't you agree?

If there is no 'motion', then why can we displace ourselves relative something else?

That's a really interesting question.

 

[harrylin]

[..]
It also frustrates me that people cling so dearly to the assumption of absolute motion/rest, particularly when every waking moment of our lives we deal in relative motion and often even multiple simultaneous relative velocities in several frames at once (ie, when driving or playing sports). None of our experiences mandate the existence of an absolute reference frame, so I have trouble even understanding where the default assumption of an absolute reference frame would come from. Perhaps it comes from the thought of a fish-tank universe with God looking through the glass. Perhaps he's there, but whether he is or not, there is no basis for assuming he is nor any value in making the assumption. If one acknowledges that he and his frame are completely undetectable, then speculation about his existence is of no value in a scientific setting. It is purely a religious belief.

Concerning classical mechanics I now answered your remark in the new thread on Newton's relativity. You could similarly start a topic about Bell's assumption of such a frame which he based on his theorem in the QM forum (note that he confusingly called it a "preferred frame" without suggesting disagreement with SR). As far as relativity is concerned, Langevin (I doubt that he was religious) gave his SR-based arguments (in 1911 it was still called "relativity") for the existence of a Lorentz ether here, starting from p.47:
http://en.wikisource.org/wiki/The_Evolution_of_Space_and_Time

[..] I will point out that there are objective, absolute realities that have to be acknowledged here. Consider the case of a rocket launching off the earth. The rocket burns fuel, so it must be said that it is moving (accelerating) away from the Earth and not the other way around. [..]

I also mentioned that fact of observation in post #88. Perhaps Mangaroosh did not understand what I wrote, but will understand what you wrote.

 

[bahamagreen]

The first eight derivatives of position are:

velocity
acceleration
jerk (jolt, surge, lurch)
jounce (snap)
crackle
pop
lock
drop

With acceleration we have the first "absolute" in terms of qualitative existence, but the quantitative value will be measured differently by observers in various inertial FOR.

What about the higher derivatives? Will there never be one where all the absolute attributes become quantitatively universal for all inertial FOR?

If acceleration is the "floor" for finding even a qualitative absolute presence, is there a way to demonstrate that this is the result and expectation from dimensional geometry?

 

[Dale]

With acceleration we have the first "absolute" in terms of qualitative existence, but the quantitative value will be measured differently by observers in various inertial FOR.

Well, acceleration is a vector quantity, so you will always have disagreement about the components of vectors. However, you can make it into an invariant by making a scalar (rank 0 tensor) from it. The invariant quantity is called the proper acceleration. It is the magnitude of the 4-acceleration in any frame and is equal to the magnitude of the 3-acceleration in the momentarily co-moving inertial frame.

 

[mangaroosh]

I've recently been accused of trolling, with specific reference to this thread, so I just wanted to clarify where I'm coming from, and hopefully dispell that perception, because I think it can have a negative overall effect on the discussion.

Where I'm coming from
I'm really just hoping to give an honest representation of my understanding, and discuss any potential issues with it, because this issue represents one of the stumbling blocks I have to learning relativity. It is essentially something I would have taken to be common sense, and something I believe the majority of lay people would also take to be common sense. I know this doesn't mean that it is correct, but hopefully it might clarify why there is resistance to the idea that it is incorrect. Essentially, because there is a deeply ingrained belief which was taken to be self-evident, which is now being challenged as incorrect.

Representative
I would also be fairly confident that my understanding is highly representative of the vast majority of lay people out there; if you ask them about their experience of walking down the street, and ask them "are they actually moving, or is it the ground and buildings that are actually moving, while they 'walk on the spot' (as on a treadmill)", I have no doubt whatsoever that the vast majority of people will not respond by saying "actually moving" doesn't make sense; instead, they will unequivocally reply, that they are the ones who are "actually moving".

For the vast majority of people, this would be taken as common sense, regardless of its accuracy; but this represents just one of the stumbling blocks to learning relativity for lay people. It is a genuine block to learning relativity, because it requires discarding a deeply ingrained belief. Personally I find that a reasoned discussion, based on logic is the best way to circumnavigate, or unlearn erroneous beliefs. A certain level of self-awareness is also required to identify the level of attachment that one holds to their beliefs also.

Even Salvestrom's comment, in this thread, indicates that I am not alone in my understanding of this

if it depends on our point of view shouldn't we perhaps be move cautious in proclaiming "actually moving" has no meaning? I'm pretty sure my fingers are moving as I'm typing

I think we all know what it is like discussing things on an internet forum; it is easy to get a little wound up when you think someone is trolling, or just being intentionally dense; as part of the human condition we tend to get a little wound up when our accepted beliefs are perceived to be challenged. I know that I have let the frustration get the better of me at times, and it will undoubtedly do so again, but hopefully we can all look beyond that to the logic in the posts and discuss the issue primarily on that basis.


(Why can't we all just get along? )

 

[mangaroosh]

Good example of the logical fallacy "begging the question".

http://www.fallacyfiles.org/begquest.html

Your question presumes that the word "move" without any qualifiers has meaning. This is the very point under dispute, so you cannot assume it.

To make your question not fallacious you would need to qualify the word "move" in the question by specifying what it is moving relative to since both sides of the argument agree that relative motion is well defined and meaningful.

Hopefully I can type this in such a way as it doesn't come across confrontational, because that isn't my intention. I think this might repersent a good starting point though, because it addresses some fundamental issues.


Essentially the question is about whether or not "actual movement", or "move without any qualifiers" makes sense; it could probably be classified as asking whether or not "absolute motion" makes sense or exists.

You mention that it must be speficied, relative to what "movement" occurs, in order for the question not to be fallacious, but I think that is assuming the conclusion that "absolute motion" doesn't make sense and that only "relative motion" does.

My understanding is that "absolute motion" would be a simple "yes" or "no" answer to the question "is X moving"; if it must be specified relative to what it is moving then that would be "relative motion", not absolute. Motion will always be measured relative to something, but the question of which object is moving doesn't require that specification.


Real world example
Take a walk down the street, or across the room; notice the ground moving beneath your feet, and the scenery moving by; then stop walking and notice the difference; start walking again and notice the chages.

To me, and I dare say, the majority of people out there, we would conclude that "I am the one who is doing the moving".

Now, my understanding of the PoR is that we cannot actually determine who is doing the moving i.e. we cannot determine the absolute nature of the motion; we can even define reference frames which label either one or the other as movin; again, I just see this as an extension of the same principle i.e. we cannot tell who is doing the moving.

But I think we have to conclude that one, or the other, is actually doing the moving.


Clarification
You'll have to forgive me for this, because the idea is very intuitive to me, and almost self-evidently true, so I'm trying to see if you can relate to what I am getting at.

I presume you've been on a treadmill before, and know the experience of jogging on the spot while the band moves beneath your feet; if you contrast this with walking down the street where you wouldn't really say that the ground, and the scenery are moving as though on a conveyor belt; you would presumably say that you are the one causing the relative motion between you, the ground and the surrounding environment.

Again, we could define reference frames which label either one or the other as moving, but surely we cand deduce that one of them has to be "causing the relative motion" or "doing the moving" - apologies, I'm searching for a term that might adequately describe what I am trying to say.

 

[Austin0]

Well, acceleration is a vector quantity, so you will always have disagreement about the components of vectors. However, you can make it into an invariant by making a scalar (rank 0 tensor) from it. The invariant quantity is called the proper acceleration. It is the magnitude of the 4-acceleration in any frame and is equal to the magnitude of the 3-acceleration in the momentarily co-moving inertial frame.

Hi what is 3 acceleration regarding an inertial frame? On the face of it ,it seems to be a contradiction of terms.
Are you saying that it is possible for all inertial frames to calculate a shared quantitative value for an observed accelerating system?
Thanks

 

[mangaroosh]

So you've deduced that in order for something to change its state of motion a force must be applied.

How is this relevant to distinguishing 'rest' from unaccelerated motion ? Even if you knew the entire acceleration history of two comoving bodies in uniform motion, it does not alter the fact that now it does not matter to which we ascribe motion. They are now in a reciprocal state connected by a Lorentz transformation.

I don't think you like relativity and you cannot understand what it is and what it is used for. You are horribly misinterpreting the theory. Have you seen my post#106 ?
Do you agree with Newtons laws of motion ?

Finally, by your definition of 'actually moving', anything that has ever experienced acceleration is 'actually moving'. That probably includes all the matter in the universe so it is a useless concept, without any deductive or physical significance, and I wish you'd stop going on about it.

It's not a question of liking relativity, it is a matter of trying to develop a better understanding of the world, including relativity, and trying to reconcile relativity with my existing worldview; or develop my worldview to incorporate relativity. As it stands I am being told a number of things about relativity that are, on the surface perhaps, at odds with my existing worldview - which is based on my empirical experiences to date. These reprsent stumbling blocks to learning relativity, and I find that logical discussions are the best way to develop that understanding and address those stumbling blocks.Coming from a non-scientific background, I can only try to offer my basic understanding, and try to relate it to my everyday experiences. I'm not sure that I would say that "anything that has ever experienced acceleration is 'actually moving'", because someone explained to me that, when we take the equivalence principle into account, an observer would not be able to determine if they were experiencing acceleration or gravity; they said that, an observer who experiences acceleration would not be able to determine if they were actually accelerating or at rest in a gravitational field.

Everyday example
The deduction is more something that I believe the majority of lay people would take to be common sense; if you are walking down the street, and the ground is moving beneath your feet with the scenery changing, the majority of people, I beleive, would not question the fact of whether they were "actually moving", it would seem self-evidently true that they are, in fact, "actually moving" and that that is the reason for the relative motion between them and the surrounding scenery.

Of course it is possible that the Earth is actually moving too, as we would say it is rotating and orbiting the sun, as our solar system moves through the galaxy, which itself moves through the universe. It's possible that they start off at rest relative to the earth, but due to the Earth's movement through the universe they too are in motion; they can then start moving relative to the Earth i.e. they can start walking and actually start moving, as opposed to the Earth's rotation changing, and behaving like a treadmill, to manifest in the relative motion between them.

Significance
I would think that, if "actual motion" or "absolute motion" were a reality then it would have certain deductive and physical significance, I would think.

 

[mangaroosh]

And by analogy, an object cannot be considered moving or at rest in the absence of a reference frame.

And by analogy there is motion and a reference frame.

That is fair enough, but we can have an infinite number of reference frames where two objects start off at rest relative to each other, and then start moving relative to each other. The question we have is, how, or why, does the relative motion manifest between them.

To answer, because they start moving relative to each other doesn't address the question, because the question is, "how does that happen?"

Has to move relative to what? As we have discussed neither the door nor the door frame can be considered moving or at rest in the absence of a reference frame. Until you specify that, your question is incomplete.

If you specify some inertial frame then at least one of them moved relative to the inertial frame. You could instead simply specify that they moved relative to each other. Of course, if you specify some non-inertial frame then you may find that neither moved relative to that frame. You have to specify what the motion is relative to otherwise the question is meaningless. Not only is it meaningless, but it is fallacious.

The things is, we start off by considering only the relative motion between the door and the frame; they start off at rest relative to each other in the closed position, say; they move relative to each other such that they are subsequently in the open position.

So far we have only considered relative motion. We know that we can define a reference frame in which one is "at rest" and the other "in motion"; this appears to be an extension of the PoR and the fact that we cannot determine the absolute nature of the motion of either the door or the frame.

But what we do have is the scenario where the door has gone from being closed to being open; and we're trying to determine if this scenario can help us deduce the existence of absolute motion; to suggest that we have to specify relative to what the door is moving is fallacious, because that would be relative motion, not absolute motion.

To my mind, absolute motion would be a simple "yes" or "no" answer to the questions: "did the door move?", "did the frame move?".

We cannot actually determine which answer is correct in each case, but I believe we can deduce that it has to be "yes" with regard to at least one of them.


Everday example
Again, just to try and illustrate what I am trying to get at; try going to the nearest door and open it; ask yourself the question, did I move the door, or did the entire room move while the door remained still?


A more intuitive example might be to put your hand together, palm to palm, then move your left hand away; then ask the question, was it your left hand that actually broke contact with the right, and move away, or did the right break contact with the left and move away?

 

[mangaroosh]

As always, your formulation - by saying "move" without qualifiers - assumes the existence of absolute motion. It isn't necessary. The door and frame have to move relative to each other in order to open or close, but there is no need or value for assuming one or the other is the "one" to move in an absolute sense. Or, both could be moving with respect to a 3rd frame and the door may not necessarily close. You've assumed the existence of this third frame, that it is the universal frame and that one is stationary relative to it and the other is moving. None of that is necessary.

Absolute motion isn't being assumed, it is, possibly, being deduced.

If you open a door, yes the door moves relative to the frame, and the frame moves relative to the door; and we cannot detect the absolute motion of either the door or the frame, but surely we can deduce that one must absolutely move. How else could there be relative motion between the door and the frame?

There is no reference to an absolute reference frame there; it is only implied if we insist that absolute motion be relative to an absolute reference frame, but that would appear to be a contradiction in terms; because absolute motion is, by definition, not relative.

Again, with #5, stating "moved" without qualifiers. Indeed in order to close, one must move relative to the other. This allows us to conclude nothing about an absolute frame other than that it isn't necessary for this thought experiment. Pile up enough thought experiments where it isn't necessary and you have pretty good circumstantial evidence that it doesn't exist and no evidence whatsoever that it does.

I'm not sure about the insistence on the necessity for an absolute reference frame; a "yes" or "no" question doesn't necessarily require one, but I think it would still have deductive significance.

And we only need one exception to disprove the rule, don't we?

Since motion requires comparison of reference frames, absolute motion must therefore require an absolute reference frame be one of those frames. That's by definition. That's what "absolute motion" means!

Again, I'm not sure about the insistence on the necessity for an absolute reference frame; it would surely be a "yes" or "no" question, and, therfore, wouldn't require one. Defining "absolute motion" relative to an absolute reference frame makes it relative motion, not absolute, by definition.

But we are still comparing reference frames; we're comparing a reference frame which labels the door as "at rest" and the frame as "in motion" to a reference frame which has the opposite labels. It's from that, that I think we deduce absolute motion.

That we can have two such reference frames re-iterates the fact that we cannot determine the absolute nature of the motion of either, as per the PoR; but I think we can deduce that, at least one of them must, absolutely be in motion; we just can't tell which one it is.

As I said earlier in the thread, the reason it is rarely ever stated in everyday life is that people just intuitively understand it. Our brains are programmed to "get" it and use it. That's the reason I am so incredulous that you don't. In my thought experiments with the police officer measuring the speed of a car, the two different methods of measuring the speed of a car would never, ever, ever confuse anyone: There's three "observers" and two different speed measurments happening in both cases, but no one would ever get confused about which one is being referred to.

I'm not sure I would agree that people intuitively understand it, or that "Our brains are programmed to "get" it and use it"; I have very little doubt that if you were to go out and survey 100 people, with no scientific background, and asked them the question: "when walking down the street, are you actually moving, or is the road and the surrounding scenery actually moving, as though on a conveyor belt?", the vast majority of people will answer that they are the ones who are actually moving, and won't say that "actually moving" doesn't make sense. Salvestrom's post #47 would suggest that I am not alone in that understanding. That is why I am incredulous that people have such a tough time relating to what I'm saying.

As for the radar gun example, I fully understand that, and amn't confused by it at all; the issue pertains to the determination of the absolute nature of the motion; that example demonstrates that relative motion can easily be determined; but when speaking of determingin "absolute motion" we talk about determinging whether or not an observer is "in motion" or "at rest"; does that mean that the term "at rest" has two different meanings? Also, relative to what can an observer not determine their motion?

Doubt it. Simply by pointing out that the Earth is rotating - which everyone knows - would anyone be able to be convinced that the relative motion of a person and the Earth wrt each other truly is relative.

Genuinely, try it, go out and ask lay people who are walking down the street, or across a room if they are actually moving, or if it is the room/street that is actually moving as though on a conveyor belt.

When you say that everyone knows the Earth is rotating, what do you mean? Surely we can define a frame of reference in which the Earth is not rotating.

I'm not sure what you mean by "would anyone be able to be convinced that the relative motion of a person and the Earth wrt each other truly is relative"; how do you mean "truly relative"?

I can't fathom a way you could have worded that any more exactly self-contradictory other than to cut out intermediate words. Do that and you just said 'relative = absolute'. Wow.

If I had worded it as you have worded it above, then that would have been not only more self-contradictory, but actually contradictory. That isn't what was said though; what was said is that, I think absolute motion can be deduced from relative motion.

If two things are moving relative to each other then, surely, one of them has to absolutely be in motion. It's possible that both of them are absolutely in motion, but one of them, at least, has to be, surely. Otherwise, how would there be relative motion?

The conclusion that, I think, can be drawn from that is not that relative = absolute; which appears to be the necessary conclusion that absolute motion has to be defined relative to an absolute reference frame; but rather that absolute motion and relative motion are compatible i.e. when an object moves absolutely, its motion will be measured relative to something else; but absolute motion cannot be measured, because measurement is, by necessity, relative; that would be my take on it anyway.

Double wow. Now you're saying 'absolute is not necessarily absolute.' Honestly, I'm having a lot of trouble taking you seriously now.

No, what is being said here is that absolute ≠ relative; that is absolute motion can not, by definition, be relative to something. I would have though it was a "yes" or "no" question, without the need for an absolute reference frame.

And if you are truly being honest here and not trolling us, you would conclude: it doesn't matter.

I won't look up the name of that particular fallacy, but I'm sure, being a reasonable person, you will acknowledge that that is a fallacious argument.

I do actually think that it does matter, as I think it might have a certain deductive significance.

 

[mangaroosh]

Motion is always relative something, an acceleration can be seen as displacements of a infinite assemble of 'uniform motions', but it is also the single definition in where you can 'prove' a motion in that you locally will find something new coming into play, a 'gravity' :)

What you seem to mean is that as there is something called a motion, there must also be a absolute reference frame from where it exist, a 'gold standard' of it. The easiest way to define such a notion would be to turn it around, don't you agree?

If there is no 'motion', then why can we displace ourselves relative something else?

That's a really interesting question.

I would say that motion is always measured realtive to something, and that would be relative motion; my understanding of the notion "absolute", is that it would be a simple "yes" or "no" question, without the necessity of an absolute reference frame to define it.

To say that "absolute motion" has to be relative to an "absolute reference frame" seems to be somewhat of a contradiction in terms; because it appears to be saying that absolute = relative.

Your turnaround above is an interesting way of putting it. How can there be relative motion between two objects if neither one moves?

Adding the qualifier "relative to the other" seems both tautological in one sense, and circular in another; depedning on the context in which it is used.

 

[harrylin]

[...] I would also be fairly confident that my understanding is highly representative of the vast majority of lay people out there; if you ask them about their experience of walking down the street, and ask them "are they actually moving, or is it the ground and buildings that are actually moving, while they 'walk on the spot' (as on a treadmill)", I have no doubt whatsoever that the vast majority of people will not respond by saying "actually moving" doesn't make sense; instead, they will unequivocally reply, that they are the ones who are "actually moving".

For the vast majority of people, this would be taken as common sense, regardless of its accuracy; but this represents just one of the stumbling blocks to learning relativity for lay people.[..]

The point that you raise now (although you may have raised it before) can be replied in quite the same manner with Newton's relativity as with Einstein's relativity. That strengthens my impression that you try to jump too far in one go, and I hope that you can appreciate the following common answer as could have been given by both of them.

You gave the treadmill example. Why do you think that the Earth is not a big treadmill? Surely you know that the Earth is rotating, and all the streets and buildings along with it. Thus, if someone is running (if it were possible) in the contrary direction at exactly the rotation speed, this person will still be carried along in the Earth's orbit about the Sun. However, he or she will not share in the Earth's circular motion about its axis. Is that hard to understand or counter intuitive?

 

[harrylin]

Absolute motion isn't being assumed, it is, possibly, being deduced. [..]

Why would you do that in a discussion of Einsteinian relativity? He did not use such a concept, and different people even attach different meanings (and thus different concepts) to that label. For example Newton used a definition that is totally incompatible with your definition of the same words; and of course, they are not defined in Einsteinian relativity. BTW, there is now an active topic on Newtonian relativity.

 

[mangaroosh]

The point that you raise now (although you may have raised it before) can be replied in quite the same manner with Newton's relativity as with Einstein's relativity. That strengthens my impression that you try to jump too far in one go, and I hope that you can appreciate the following common answer as could have been given by both of them.

You gave the treadmill example. Why do you think that the Earth is not a big treadmill? Surely you know that the Earth is rotating, and all the streets and buildings along with it. Thus, if someone is running (if it were possible) in the contrary direction at exactly the rotation speed, this person will still be carried along in the Earth's orbit about the Sun. However, he or she will not share in the Earth's circular motion about its axis. Is that hard to understand or counter intuitive?

Hey Harry, I don't have any problem with that whatsoever, and I know that we could probably define a reference frame which treats the Earth as being like a big treadmill; but, given that scenario, the conclusion we would reach is that it is the Earth that is actually in motion.

I don't have any trouble with the idea that we cannot determine absolute motion , but, based on my everyday experiences, and what most people, I'm sure, would take to be commone sense, we can surely deduce that one or the other - me/you or the Earth - has to actually be moving, in order for relative motion to occur between us.

 

[harrylin]

[..]
Everday example
Again, just to try and illustrate what I am trying to get at; try going to the nearest door and open it; ask yourself the question, did I move the door, or did the entire room move while the door remained still?
[..]

The answer to that question is - again - practically the same in Newton's mechanics as in Einstein's. Do you know the answer??

It is also related to what I (#88) and Russ (#103) tried to explain to you earlier, as I mentioned in post #115 (the attribution there is wrong, I replied to Russ). What did you understand from that??

 

[mangaroosh]

Why would you do that in a discussion of Einsteinian relativity? He did not use such a concept, and different people even attach different meanings (and thus different concepts) to that label. For example Newton used a definition that is totally incompatible with your definition of the same words; and of course, they are not defined in Einsteinian relativity. BTW, there is now an active topic on Newtonian relativity.

I use it because it is pertinent to the discussion at hand, and because the concept appears to be referenced through the test of the principle of relativity, and the idea that the absolute nature of motion cannot be determined i.e. absolute rest or absolute motion cannot be determined.

It also represents one of the primary stumbling blocks that I, and no doubt many lay people, have when it comes to undertsanding relativity, because it seems to run counter to what a lot of people would deem common sense, or would think is self-evidently true.

 

[harrylin]

Hey Harry, I don't have any problem with that whatsoever, and I know that we could probably define a reference frame which treats the Earth as being like a big treadmill; but, given that scenario, the conclusion we would reach is that it is the Earth that is actually in motion.

Exactly - and that is understood since Galileo!
Now, Einstein tried with general relativity to make the viewpoint that the Earth is in rest equally valid; but while that is mathematically doable, I don't think that this is generally accepted nowadays in the philosophical sense. And I fear that a discussion of that may be simply beyond your current level. However, that's exactly what one would expect a discussion on "at rest in Einsteinian relativity" to be about.

I don't have any trouble with the idea that we cannot determine absolute motion , but, based on my everyday experiences, and what most people, I'm sure, would take to be commone sense, we can surely deduce that one or the other - me/you or the Earth - has to actually be moving, in order for relative motion to occur between us.

Such a concept does not exist in Einsteinian relativity, and I doubt if most people's common sense is like yours - quite the contrary I think.

 

[harrylin]

I use it because it is pertinent to the discussion at hand, and because the concept appears to be referenced through the test of the principle of relativity, and the idea that the absolute nature of motion cannot be determined i.e. absolute rest or absolute motion cannot be determined.

It also represents one of the primary stumbling blocks that I, and no doubt many lay people, have when it comes to undertsanding relativity, because it seems to run counter to what a lot of people would deem common sense, or would think is self-evidently true.

- A concept that does not exist in a theoretical framework can hardly be pertinent to the discussion of that framework; Einstein rephrased the PoR such that that concept was not mentioned.
- I do note that your definition of those words is incompatible with their meaning; consequently you can not understand it. A fitting place to bring it up would be the thread on Newtonian relativity.

 

[mangaroosh]

Exactly - and that is understood since Galileo!
Now, Einstein tried with general relativity to make the viewpoint that the Earth is in rest equally valid; but while that is mathematically doable, I don't think that this is generally accepted nowadays in the philosophical sense. And I fear that a discussion of that will be simply beyond your current level.

Would you agree then that "actual motion" makes sense?

Such a concept does not exist in Einsteinian relativity, and I doubt if most people's common sense is like yours - quite the contrary I think.

I genuinely don't believe that it is just me and slavestrom (judging from post #47) that think "actual motion" makes sense, or at least, might make sense. I would be willing to wager, that if you ask 100 lay people, with no formal background in science (and perhaps even some with a formal background) whether it was they that was actually moving when walking down the street, or if it was the Earth that was actually moving like a conveyor belt/treadmill, the majority of people would not say "actually moving" doesn't make sense; I'm certain the vast majority of people would answer that it was they, themselves that was actually moving - they would say that the Earth was also moving through the universe, but they would say that they were actually moving too, and that is how the relative motion was manifest.

Such a concept might not exist in Einsteinian relativity, but it is a concept which seems to be commone sense to me, and I dare say, many others; as such, it appears to represent a block to learning relativity; one which I believe can only be overcome through rational discourse on the subject.

- A concept that does not exist in a theoretical framework can hardly be pertinent to the discussion of that framework; Einstein rephrased the PoR such that that concept was not mentioned.
- I do note that your definition of those words is incompatible with their meaning; consequently you can not understand it. A fitting place to bring it up would be the thread on Newtonian relativity.

If the purpose of the conversation is to expand the understanding of that theoretical framework, and that concept represents a stumbling block to doing so, then it is potentially pertinent to the discussion; in the context of this discussion I think it definitely is pertinent.

While the PoR may have been reprhased, such that the concept wasn't explicitl mentioned, I think it is still implict; particularly with the stated test of the principle.


Which definitions were incompatible with which meanings?

 

[harrylin]

Would you agree then that "actual motion" makes sense?

Sorry, I have no idea of its meaning! I wait for you reply to post #128.

 

[Michael C]

Hopefully I can type this in such a way as it doesn't come across confrontational, because that isn't my intention. I think this might repersent a good starting point though, because it addresses some fundamental issues.


Essentially the question is about whether or not "actual movement", or "move without any qualifiers" makes sense; it could probably be classified as asking whether or not "absolute motion" makes sense or exists.

You mention that it must be speficied, relative to what "movement" occurs, in order for the question not to be fallacious, but I think that is assuming the conclusion that "absolute motion" doesn't make sense and that only "relative motion" does.

My understanding is that "absolute motion" would be a simple "yes" or "no" answer to the question "is X moving"; if it must be specified relative to what it is moving then that would be "relative motion", not absolute. Motion will always be measured relative to something, but the question of which object is moving doesn't require that specification.

Could you define what you mean by "absolute motion"?

 

[mangaroosh]

Everday example
Again, just to try and illustrate what I am trying to get at; try going to the nearest door and open it; ask yourself the question, did I move the door, or did the entire room move while the door remained still?


A more intuitive example might be to put your hand together, palm to palm, then move your left hand away; then ask the question, was it your left hand that actually broke contact with the right, and move away, or did the right break contact with the left and move away?

The answer to that question is - again - practically the same in Newton's mechanics as in Einstein's. Do you know the answer??

It is also related to what I (#88) and Russ (#103) tried to explain to you earlier, as I mentioned in post #115 (the attribution there is wrong, I replied to Russ). What did you understand from that??

Is the answer to the question about the hands the same in both, because you only reference the question about the door and the frame.

My guess would be that it depends on the frame of reference, as to which one is labelled as moving; if that is the answer then it just brings us back to the original question of how can there be relative motion without one or the other actually moving.

I'm just wondering if you can relate at all to what I am saying, because people seem to be making this a lot more difficult than is necessary; I don't think it is a ridiculously difficult concept to grasp. As I mentioned, salvestrom's post, earlier in the thread, seems to suggest that I am not alone in my understanding; even your post about the Earth rotating you seem to hint at it.

You say that surely I know that the Earth is rotating, but you say that "Einstein tried with general relativity to make the viewpoint that the Earth is in rest equally valid; but while that is mathematically doable, I don't think that this is generally accepted nowadays in the philosophical sense"; the thing is, we can define a reference frame in which the Earth is not rotating, is that not correct? How can I surely know that it is then? and is this not tantamount to saying that the Earth actually is rotating i.e. it actually is moving?

Sorry, I have no idea of its meaning! I wait for you reply to post #128.

I'm sorry, I'm confused by your earlier reply to the following post.

Hey Harry, I don't have any problem with that whatsoever, and I know that we could probably define a reference frame which treats the Earth as being like a big treadmill; but, given that scenario, the conclusion we would reach is that it is the Earth that is actually in motion.

Exactly - and that is understood since Galileo!

Are you not agreeing to the conclusion that the Earth is actually in motion?

 

[mangaroosh]

Could you define what you mean by "absolute motion"?

I'm not entirely sure.

I'm trying to relate my understanding to real world experiences, because, as a lay person, that is all I can really do.

You are no doubt familiar with walking across a room; you notice the ground move beneath your feet, and the surroundings move as well.

I'll also assume that you've been on a treadmill before, and know what that experience is like; where the belt moves beneath your feet and you jog on the spot; or an automated walkway at the airport, where you stand still and the belt moves you.

I'm sure you can discern a difference between those two kinds of experiences.


In the first one, walking across the room, would you say that it is you that is actually moving, or is it the room that is moving and you are just walking on the spot, like on a treadmill?


I know that we can tell the difference which one is the true state of affairs, but would you not be inclined to agree that it has to be one or the other, no?

 

[Michael C]

I'm not entirely sure.

I'm trying to relate my understanding to real world experiences, because, as a lay person, that is all I can really do.

You are no doubt familiar with walking across a room; you notice the ground move beneath your feet, and the surroundings move as well.

I'll also assume that you've been on a treadmill before, and know what that experience is like; where the belt moves beneath your feet and you jog on the spot; or an automated walkway at the airport, where you stand still and the belt moves you.

I'm sure you can discern a difference between those two kinds of experiences.

No, I can discern no difference in what my legs or feet feel. In any situation where I am walking on a horizontal surface that isn't accelerating, I move in the same way and exert the same energy in order to move at a given speed relative to that surface. Whether I'm walking at 4 km/h on a floor, a treadmill, or the deck of a boat moving at constant velocity, the physical experience is identical.

In the first one, walking across the room, would you say that it is you that is actually moving, or is it the room that is moving and you are just walking on the spot, like on a treadmill?

I know that we can tell the difference which one is the true state of affairs, but would you not be inclined to agree that it has to be one or the other, no?

No, I would not agree at all. In all these case my muscles are doing the work of keeping me moving relative to the surface on which I am walking. In all these cases, if I stop moving my muscles, I will stop moving relative to that surface. That's all.

You need to think about your definition of "motion". Here's a pretty standard definition of motion, from Wikipedia:

"Motion is a change in position of an object with respect to time"​

This definition only makes sense with respect to a particular reference frame: without defining the reference frame, we cannot define "position". Can you state a definition of motion that does not need to assume a reference frame?

 

[Dale]

Personally I find that a reasoned discussion, based on logic is the best way to circumnavigate, or unlearn erroneous beliefs. ... I know that I have let the frustration get the better of me at times, and it will undoubtedly do so again, but hopefully we can all look beyond that to the logic in the posts and discuss the issue primarily on that basis.

That is exactly what you have received, multiple times. What could be more logical than pointing out the exact logical fallacy you are committing?

Essentially the question is about whether or not "actual movement", or "move without any qualifiers" makes sense; it could probably be classified as asking whether or not "absolute motion" makes sense or exists.

Yes.

You mention that it must be speficied, relative to what "movement" occurs, in order for the question not to be fallacious, but I think that is assuming the conclusion that "absolute motion" doesn't make sense and that only "relative motion" does.

The existence of relative motion is a premise that we can both agree upon, so it is not fallacious to assume. Your goal is to start with that premise (and perhaps some other mutually agreeable premises) and prove that absolute motion exists.

To do so will require some series of statements, each of which are logically implied by the premises and the previous statements. The concluding statement will be something like "therefore x moved" or "therefore absolute motion exists". The premises, the logically implied statements, and the conclusion are collectively called a proof.

If you make such a statement before the final statement, then you are committing the fallacy known as begging the question. That is the problem with all of your attempted proofs up to now.

Take a walk down the street, or across the room; notice the ground moving beneath your feet, and the scenery moving by; then stop walking and notice the difference; start walking again and notice the chages.

Please limit your examples to relevant ones. Specifically, inertial motion. It is not a topic of disagreement that non inertial motion (proper acceleration) is "absolute" in some sense.

But I think we have to conclude that one, or the other, is actually doing the moving.

Why? Please try your proof again, now that you have been made aware of the nature of the fallacy perhaps you can avoid it.

 

[Dale]

Hi what is 3 acceleration regarding an inertial frame? On the face of it ,it seems to be a contradiction of terms.
Are you saying that it is possible for all inertial frames to calculate a shared quantitative value for an observed accelerating system?
Thanks

The 3 acceleration is the ordinary acceleration. I.e. It is the vector defined by
\left( \frac{d^2 x}{dt^2}, \frac{d^2 y}{dt^2}, \frac{d^2 z}{dt^2} \right)

All frames agree on the magnitude of the four acceleration which is equal to the magnitude of the three acceleration in the momentarily comoving inertial frame.

 

[harrylin]

Is the answer to the question about the hands the same in both, because you only reference the question about the door and the frame.

Not sure, nor did I consider it, which is why I did not include it.

My guess would be that it depends on the frame of reference, as to which one is labelled as moving; if that is the answer then it just brings us back to the original question of how can there be relative motion without one or the other actually moving.

That's perfectly wrong, as we hoped you would know from classical mechanics. It boils down to the difference between:

- being in relative motion,
and
- initiating a relative motion/a change in relative motion.

It appears that you confound the two; perhaps with "true motion" you meant the second one?

I'm just wondering if you can relate at all to what I am saying, because people seem to be making this a lot more difficult than is necessary; I don't think it is a ridiculously difficult concept to grasp. As I mentioned, salvestrom's post, earlier in the thread, seems to suggest that I am not alone in my understanding; even your post about the Earth rotating you seem to hint at it.

You say that surely I know that the Earth is rotating, but you say that "Einstein tried with general relativity to make the viewpoint that the Earth is in rest equally valid; but while that is mathematically doable, I don't think that this is generally accepted nowadays in the philosophical sense"; the thing is, we can define a reference frame in which the Earth is not rotating, is that not correct? "

Right

How can I surely know that it is then? and is this not tantamount to saying that the Earth actually is rotating i.e. it actually is moving?

Descriptions according to which is the Earth is truly rotating are much simpler than those according to which the Earth is in rest.
Moreover, Einstein's 1916 GR according to which it is equally possible to say that the Earth is in rest, has IMHO problems with consistency. In any case, the common attitude has become again to take the point of view relative to inertial reference systems, so that one regards the Earth as in motion.

[..] Are you not agreeing to the conclusion that the Earth is actually in motion?

I would agree with saying that the Earth is actually rotating, if that is what you mean. The main reason is simple: we would have to arbitrarily assign enormous velocities (as well as very low ones) to distant star light, as well as other weird effects; a simpler set of laws of nature makes much more sense to me.

 

[jtbell]

I would agree with saying that the Earth is actually rotating, if that is what you mean. The main reason is simple: we would have to arbitrarily assign enormous velocities (as well as very low ones) to distant star light, as well as other weird effects; a simpler set of laws of nature makes much more sense to me.

We don't even have to look that far. How would we explain the Foucault pendulum or the Coriolis effect if the Earth weren't rotating?

 

[harrylin]

We don't even have to look that far. How would we explain the Foucault pendulum or the Coriolis effect if the Earth weren't rotating?

Ah yes, I was looking unnecessarily far away.

 

[bahamagreen]

This is an interesting discussion.

It seems that if two objects are in relative motion, that is a "qualified" kind of motion; but in the end one may state that there is a motion.
The fundamental question is about the rigor of the logical extension, that this "qualified" motion, or some portion of it must logically rise to "unqualified" motion, motion inferred by relative motion but not itself relative to anything, "actual" motion, "pure" motion, absolute motion...

I understand the OP's inference, just like most people firmly understand that 2+2=4... and 160K+160K=320K, no doubt about that... yet when adding velocities simple addition is not correct.

It may actually be the case that the logic even if sound is simply not suitable for representation of measured reality; in the same way that addition in math does not get the right answer when applied to addition of velocities.

I just wonder if there is a way to indicate this disjunction in a satisfying way?

 

[mangaroosh]

That is exactly what you have received, multiple times. What could be more logical than pointing out the exact logical fallacy you are committing?

I was more trying to address the perception, which seems to abound, that I am trolling, because I know how such a perception can impact on how people engage in discussion.

I appreciate the point you are making, and that is partly the kind of discussion I think is helpful, but there seems to be either, an unwillingness, or an inability, to consider points that most people, or most lay people perhaps, would consider to be common sense, and self evidently true. While I might not be explaining myself perfectly, I don't think I would have this much trouble trying to get a point across with most other people.To address the point you make, however; I think it only appears to be begging the question if the conclusion, that moving on it's own does not make sense, is assumed. I have been trying to give everyday examples that, I think, make the distinction quite clearly, and that most people would understand; which, I feel demonstrates that it does indeed make sense to ask "which one is actually moving".

If we take the example that Harry raised, about the rotating earth. There we have the relative motion between the Earth and other objects in the universe, the sun for example. Historically, there have been two dominant scenarios which could have accounted for the relative motion; the first was that the sun moved around the earth; the other that the Earth is actually rotating.

This is a perfect example of where the relative motion is attributable to the fact that either the sun is moving, or the Earth is moving, and which should demonstrate the fact that "actually moving" makes sense.

Now, we might not be able to actually determine which is the case, but we surely can deduce that, at least, one of them has to be the case.

It is of course possible that both of them are actually moving, but it isn't a possibility that neither of them are actually moving. The movement will always manifest as relative motion, and while we may not be able to determine the absolute nature of the motion, as per the test of the PoR, I think we can deduce that the nature of the motion has to be absolute.

The existence of relative motion is a premise that we can both agree upon, so it is not fallacious to assume. Your goal is to start with that premise (and perhaps some other mutually agreeable premises) and prove that absolute motion exists.

To do so will require some series of statements, each of which are logically implied by the premises and the previous statements. The concluding statement will be something like "therefore x moved" or "therefore absolute motion exists". The premises, the logically implied statements, and the conclusion are collectively called a proof.

If you make such a statement before the final statement, then you are committing the fallacy known as begging the question. That is the problem with all of your attempted proofs up to now.

I'm not saying that assuming relative motion is fallacious, I'm saying that stating that motion has to be defined relative to something is, because that is assuming the conclusion that only relative motion makes sense.

I've tried to give real world examples which demonstrates that motion will always manifest as relative motion, but that there are two possibilities which can give rise to that relative motion; either one object moves, or the other object does.

It's like the train example, where you are on a train and the train beside you starts to pull away; you can't tell if it is your train that is moving or if it is the other train; then you realize that it was the other train that was actually moving. I think most people would understand that point.

Of course, we might not be able to determine which train was actually moving, but I think we can deduce that one of them actually was moving.

Again, both could be moving, as is the case if the Earth is actually moving, but that would just compound the point about actual motion, or absolute motion.

Please limit your examples to relevant ones. Specifically, inertial motion. It is not a topic of disagreement that non inertial motion (proper acceleration) is "absolute" in some sense.

Inertial motion isn't excluded, it's just easier to highlight when we consider two objects at rest relative to each which then start moving, relative to each other. If we have two inertial objects/observers, moving relative to each other, we can still conclude that one of them at least has to actually be moving.

Again, taking two trains that pass each other; we can have, at least, two scenarios; one where one train is stopped, on the surface of the earth, and one train is moving on the surface of the earth. There will be relative motion between the two trains because one of them is actually moving; or where both trains are moving on the surface of the earth; again, there will be relative motion because both trains are actually moving.

We can say that the movement of [EDIT] the wheels of [/EDIT] one of the trains actually makes them remain stationary i.e. the ground is moving beneath the train, like a conveyor belt and the motion of the wheels simply causes it to stay put; but in that case we can deduce that it is the Earth that is actually moving. We might not be able to determine which is the case, but we can surely deduce that it must be one or the other.

Why? Please try your proof again, now that you have been made aware of the nature of the fallacy perhaps you can avoid it.

Stating that motion has to be defined relative to something is, in itself fallacious, because it assumes the conclusion that only relative motion makes sense.I've given a number of different examples which most people would have no trouble understanding, which should hopefully demonstrate that "actually moving", or "absolute motion" perhaps, makes perfect sense.

Again, is the Earth actually rotating, or are the other objects actually revolving around the earth? Both cases would result in relative motion, but both are contrasting reasons as to why the relative motion manifests.

That we cannot determine which is correct is reflected in the idea that we can define a reference frame in which either is the case; but surely, we can deduce that, at least, one or the other must be the case; it is of course possible that everything in the universe is actually moving, and nothing is in a state of absolute rest, but that would just compound the idea of absolute motion.That would be my understanding, anyway.

 

[russ_watters]

Now, we might not be able to actually determine which is the case, but we surely can deduce that, at least, one of them has to be the case.

It is of course possible that both of them are actually moving, but it isn't a possibility that neither of them are actually moving. The movement will always manifest as relative motion, and while we may not be able to determine the absolute nature of the motion, as per the test of the PoR, I think we can deduce that the nature of the motion has to be absolute.

Again, the logic is that if two objects are in relative motion with respect to each other, then at most one can be in absolute rest and the other must therefore be in absolute motion. Well sorry, but it doesn't matter how many times it is repeated in different contexts: the logic still doesn't follow. Why? Simply because it starts with the premise that absolute motion/rest exists. You cannot prove the existence of absolute motion/rest with a thought experiment that starts with the assumption that it exists. It is circular logic that reduces to "absolute motion exists because absolute motion exists".

Stating that motion has to be defined relative to something is, in itself fallacious, because it assumes the conclusion that only relative motion makes sense.

Nonsense. Absolute motion is a special case/subset of relative motion. Motion (speed) is the change in relative distance over the change in a time interval. If one of the references is absolute, then it is absolute motion. Otherwise, it is relative motion.

And more to the point, a definition can be whatever the consensus desires it to be. That's all definitions are!

 

[mangaroosh]

Not sure, nor did I consider it, which is why I did not include it.

That's fair enough, I'm just trying to offer simple, everyday examples to demonstrate my understanding; that is one such one, which I think might help to do so.

That's perfectly wrong, as we hoped you would know from classical mechanics. It boils down to the difference between:

- being in relative motion,
and
- initiating a relative motion/a change in relative motion.

It appears that you confound the two; perhaps with "true motion" you meant the second one?

Right

Descriptions according to which is the Earth is truly rotating are much simpler than those according to which the Earth is in rest.
Moreover, Einstein's 1916 GR according to which it is equally possible to say that the Earth is in rest, has IMHO problems with consistency. In any case, the common attitude has become again to take the point of view relative to inertial reference systems, so that one regards the Earth as in motion.

I would agree with saying that the Earth is actually rotating, if that is what you mean. The main reason is simple: we would have to arbitrarily assign enormous velocities (as well as very low ones) to distant star light, as well as other weird effects; a simpler set of laws of nature makes much more sense to me.

This a great example, and one which represents an area of confusion with relativity, and which, also, can hopefully help me clarify the point I am trying to make.

My understanding was that we can't say which reference frame is correct in Einsteinian relativity; that the reference frame where the Earth is not rotating, is equally valid to the one where it is rotating; but you seem to be suggesting that there is a preferred reference frame here. It would be one I personally would agree with, but I was lead to believe that that would be at odds with Einsteinian relativity.


You mention, above, that you agree that the Earth is actually rotating, and that is precisely what I mean. It is the difference between the Earth actually rotating and the other objects in the universe revolving around the earth, which leads to the enormous velocities for distant starlight. It is the difference in the example where you're not sure if it is your own train or the train beside you that is moving out of the station; it is either the train that is moving, or the entire Earth that changes it's motion to manifest in the relative motion. Both examples give rise to the observed relative velocity, but I would have thought that we can deduce that either one or the other must be true.

 

[mangaroosh]

Again, the logic is that if two objects are in relative motion with respect to each other, then at most one can be in absolute rest and the other must therefore be in absolute motion. Well sorry, but it doesn't matter how many times it is repeated in different contexts: the logic still doesn't follow. Why? Simply because it starts with the premise that absolute motion/rest exists. You cannot prove the existence of absolute motion/rest with a thought experiment that starts with the assumption that it exists. It is circular logic that reduces to "absolute motion exists because absolute motion exists".

OK, I see what you're making, but that re-formulation doesn't necessarily represent the point.

We don't start by saying that "at most, one can be in absolute rest"; we start without any reference to absolute rest, or any assumption of absolute rest; we take the example of two objects at rest relative to each other, the absolute nature of whose motion we cannot determine, as per the PoR.

Relative motion occurs between them; again, no assumption of absolute rest or absolute motion, as neither of them has to be at absolute rest.

To say that, "in order for relative motion to occur, one of them has to move", is something that would make sense to, I dare say, most people.

For example, if you have a parked car and want it moved, there are two alternatives; move the car, or moving everything else around the car. Again, this is something I think most people would understand.

So, for our two observers at rest relative to each other; for relative motion to manifest, one of them has to move.

Regardless of which one it is, it will still manifest as relative motion; but there are two possible explanations for why the relative motion manifests; my understanding is that we can deduce that either one, or the other, has to be correct; even if we cannot determine which one it actually is.


Alternatives
Here, we can consider the alternative explanations for why relative motion occurs between the observers. Take the example of the train pulling out of the station, where, for a moment, you can't tell if it is your train that is moving, or if it is the other train.

Incidentally, that is an example that doesn't qualify the term moving, but makes sense to most people.

The two alternatives there are; 1)your train is moving; 2)the other train is moving; both would cause relative motion to manifest, but both are different explanations of it.

If we assume, for the sake of the point, that it is the other train that leaves the station (without assuming that it "actually moves", it simply leaves the station). With the second option above, the explanation for why the train leaves the station is that it is the other train that is actually moving.

The first alternative above requires that it is the earth, the buildings and your train that are actually moving, while the other train rotates its wheels to remain stationary, much like a person running on a treadmill jogs "on the spot".


Both examples would account for the relative motion, but each one requires something to actually move.

That we can define a reference frame for both cases simply reiterates the idea that we cannot determine the absolute nature of motion of either i.e. we cannot determine which one is correct; but surely we can deduce that either one, or the other, is correct.

Rotating earth
Harrry's example might be more palatable, or easier to understand. Is the Earth actually rotating, or is everything in the universe revolving around the earth?

Both would account for the relative motion, but they are two different scenarios.

Nonsense. Absolute motion is a special case/subset of relative motion. Motion (speed) is the change in relative distance over the change in a time interval. If one of the references is absolute, then it is absolute motion. Otherwise, it is relative motion.

And more to the point, a definition can be whatever the consensus desires it to be. That's all definitions are!

and the consensus on the definition of the term "absolute" is

not qualified or diminished in any way; total


Philosophy
a value or principle which is regarded as universally valid or which may be viewed without relation to other things:

http://oxforddictionaries.com/definition/absolute?q=absolute

So to define absolute motion as being relative to an absolute reference frame would be a contradiction in terms.


I would have thought the question of absolute motion would have been a "yes" or "no", or either or, answer to the question "did X move?" or "did Y move?".


Move

[no object, usually with adverbial of direction] go in a specified direction or manner; change position:
- she moved to the door
- I heard him moving about upstairs

http://oxforddictionaries.com/definition/move?q=move


The above definition of "move" might help me to clarify my understanding. If we take the example of "she moved to the door".

"She moved to the door" would represent one reference frame.

"the door moved to her" would represent the other.


In both cases the movement is attributed to either the door, or "her". We cannot determine which one is correct, so when we ask the questions:
Did she move to the door? we answer, we can't tell.
Did the door move to her? we answer, we can't tell.


But from those, to my understanding anyway, we can deduce that either she moved to the door, or the door moved to her.

Either way the act of movement is ascribed to one or the other, in an absolute sense; even though it manifests relatively.


That would be my understanding anyway, and it appears to represent a stumbling block to learning relativity. Bahamagreen might perhaps be able to put it more clearly that I can, but I think the examples I've given are fairly intelligible to most people.

 

[Michael C]

Move

http://oxforddictionaries.com/definition/move?q=move


The above definition of "move" might help me to clarify my understanding. If we take the example of "she moved to the door".

"She moved to the door" would represent one reference frame.

"the door moved to her" would represent the other.


In both cases the movement is attributed to either the door, or "her". We cannot determine which one is correct, so when we ask the questions:
Did she move to the door? we answer, we can't tell.
Did the door move to her? we answer, we can't tell.


But from those, to my understanding anyway, we can deduce that either she moved to the door, or the door moved to her.

Please define what you mean by "moved" in the last phrase. You gave a link to a dictionary definition of "move" which gives a lot of literary meanings of the word which have no place in this discussion. The only definition there that could be the start of a rigorous physical definition is this one: "change position". As I already said, "change position" only makes sense relative to a specific frame of reference.

Talking about people "moving" can confuse the issue, since a human is a complex collection of parts that can move relative to one another. Let's just imagine an inert lump of rock in space. How do you define "moving" for this rock? According to you, the rock must be either "actually moving" or "not moving". What is the difference between the two possibilities?

 

[Mentz114]

... and it appears to represent a stumbling block to learning relativity.

It is not. Special relativity is about Minkowski spacetime, the Lorentz transformation, electrodynamics and many things but has no use for your idea of 'really moved'. It is absolutely not relevant to SR.

 

[Austin0]

Move

http://oxforddictionaries.com/definition/move?q=move "She moved to the door" would represent one reference frame.

"the door moved to her" would represent the other.


In both cases the movement is attributed to either the door, or "her". We cannot determine which one is correct, so when we ask the questions:
Did she move to the door? we answer, we can't tell.
Did the door move to her? we answer, we can't tell.


But from those, to my understanding anyway, we can deduce that either she moved to the door, or the door moved to her.
Either way the act of movement is ascribed to one or the other, in an absolute sense; even though it manifests relatively.


That would be my understanding anyway, and it appears to represent a stumbling block to learning relativity. Bahamagreen might perhaps be able to put it more clearly that I can, but I think the examples I've given are fairly intelligible to most people.

In the door case and many others you have mentioned, the answer is clear cut because acceleration is involved. She is moving in a series of complex accelerations. Everybody agrees. ;-)
Do I understand your thoughts correctly in assuming when you say absolute you simply mean actual , with no implication of a quantifiable value to the motion?
If this is the case you must have realized by now that absolute is a poor choice of word for you because it has such a specific definition in science.