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25-04-2012, 07:02   #76
roosh
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Originally Posted by Morbert View Post
Your deduction implicitly assumes absolute rest. Your reasoning, which I have pointed out in the previous post, and which you have reiterated above, contains the following:

1)Consider two observers in motion with respect to each other.
2)Therefore at least one observer must be in motion with respect to absolute rest.

2) does not follow from 1) unless you assume absolute rest exists.
I think the problem might lie in the assumptions being made about absolute motion.

A common representation of absolute motion appears to say that absolute motion is relative to an absolute reference frame; this effectively says that absolute motion is relative, which, of course, would be a contradiction in terms.

If we think of absolute motion as a simple "yes or no", or "either, or" question, then there is no need to assume an absolute reference frame. Indeed, this would be more in keeping with the idea of the all important, qualifying adjective "absolute".

We can simply ask the questions "is X moving?" or "is Y moving?"


This is something which most people take to be common sense; when driving down the road in a car most people would come to the conclusion that it is the car that is actually moving.

Of course, the earth could be moving too, and the car along with it; but it is possible for the car to subsequently move again. Here, there is no need for an absolute reference frame, it is a simple question of whether or not something is actually moving. Something most people take to be common sense.



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Originally Posted by Morbert View Post
I don't think you are following what you are saying. We have the following propositions:

P: Einstein's Relativity
Q:Absolute rest

It is, of course, true that (P ∧ Q) ∧ (P ∧ ¬Q) leads to a contradiction. This, however, has nothing to do with what either of us is saying. I am saying, if Einstein's relativity implies absolute rest, then Einstein's relativity and "no absolute rest", must lead to a contradiction. I.e.

(P → Q) → ¬(P ∧ ¬Q)

The contrapositive of this is

(P ∧ ¬Q) → ¬(P → Q)

I.e. If Relativity and "No absolute rest" are consistent, then it is not true that relativity implies absolute rest. And this is the case. I can, for example, assume relativity is true, and that there is no absolute rest, and they will be perfectly consistent with each other.

In short, the fact that relativity is perfectly consistent with the assumption that there is no absolute rest means relativity does not imply absolute rest.
Relativity is effectively a description of the universe; to say that a description of the universe where absolute rest exists is compatible with a description of the universe where absolute rest does not exist, is contradictory.

But to say that there should be a contradiction between relativity and "no absolute rest" is an empty statement, particularly when the consequences of "no absolute rest" are functionally equivalent to treating specific reference frames as being at absolute rest.

To say that relativity is compatible with "no absolute rest" is an equally empty statement if "absolute rest" is tacitly assumed; what needs to be demonstrated is the difference between the two, and how relativity is compatible with both. Simply stating that absolute rest isn't assumed isn't sufficient, when the issue is to demonstrate that it isn't tacitly assumed.
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25-04-2012, 15:40   #77
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Originally Posted by roosh View Post
I think the problem might lie in the assumptions being made about absolute motion.

A common representation of absolute motion appears to say that absolute motion is relative to an absolute reference frame; this effectively says that absolute motion is relative, which, of course, would be a contradiction in terms.

If we think of absolute motion as a simple "yes or no", or "either, or" question, then there is no need to assume an absolute reference frame. Indeed, this would be more in keeping with the idea of the all important, qualifying adjective "absolute".

We can simply ask the questions "is X moving?" or "is Y moving?"
"Moving", using the concept of notion in relativity, is a coordinate statement. I.e. It implies you are making a statement in the context of a coordinate system. So if you ask "Is X moving?", you are asking "Is X moving, according to an arbitrary coordinate system?". Hence, if you don't also supply the coordinate system, the question is meaningless. So "X is moving and Y is still." implies an arbitrary coordinate system, as does "Y is moving and X is still". They are both equally true statements.

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This is something which most people take to be common sense; when driving down the road in a car most people would come to the conclusion that it is the car that is actually moving.
Science has repeatedly shown that intuition and common sense is not a reliable litmus test for what is and isn't physical. Just look at Quantum Mechanics for a variety of examples.

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Of course, the earth could be moving too, and the car along with it; but it is possible for the car to subsequently move again. Here, there is no need for an absolute reference frame, it is a simple question of whether or not something is actually moving. Something most people take to be common sense.
To reiterate my above paragraph. You cannot use the word "moving" without some reference to a coordinate system. "Moving" does not exist independently from coordinate systems.

Quote:
Relativity is effectively a description of the universe; to say that a description of the universe where absolute rest exists is compatible with a description of the universe where absolute rest does not exist, is contradictory.

But to say that there should be a contradiction between relativity and "no absolute rest" is an empty statement, particularly when the consequences of "no absolute rest" are functionally equivalent to treating specific reference frames as being at absolute rest.

To say that relativity is compatible with "no absolute rest" is an equally empty statement if "absolute rest" is tacitly assumed; what needs to be demonstrated is the difference between the two, and how relativity is compatible with both. Simply stating that absolute rest isn't assumed isn't sufficient, when the issue is to demonstrate that it isn't tacitly assumed.
This bit in red is the important bit. You are absolutely correct. "No absolute rest" is physically equivalent to the assumption that there is absolute rest. Hence, you cannot say relativity assumes absolute rest, since we can equivalently say relativity assumes no absolute rest. So instead, what we say is statements about absolute rest are not a part of relativity.
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01-05-2012, 02:36   #78
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Originally Posted by Morbert View Post
"Moving", using the concept of notion in relativity, is a coordinate statement. I.e. It implies you are making a statement in the context of a coordinate system. So if you ask "Is X moving?", you are asking "Is X moving, according to an arbitrary coordinate system?". Hence, if you don't also supply the coordinate system, the question is meaningless. So "X is moving and Y is still." implies an arbitrary coordinate system, as does "Y is moving and X is still". They are both equally true statements.
Fair enough, but that doesn't mean that an absolute reference frame has to be assumed for the purpose of absolute motion, because to suggest that absolute motion is relative would be a contradiction in terms.


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Science has repeatedly shown that intuition and common sense is not a reliable litmus test for what is and isn't physical. Just look at Quantum Mechanics for a variety of examples.
That's fair enough, but we're talking about what can be deduced from relative motion, and absolute motion is one such thing that can be deduced.


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To reiterate my above paragraph. You cannot use the word "moving" without some reference to a coordinate system. "Moving" does not exist independently from coordinate systems.
You can, we do it everyday of our lives; we talk about the earth orbiting the sun and the earth rotating, as opposed to the sun orbiting the earth; we talk about us moving as we walk down the street, as opposed to us walking on the spot while the world moves past us as though on a giant conveyor belt; we talk about an escalator moving up or down, as opposed to the entire universe shifting in such a way to give the same effect.

All of these use the word "moving" without reference to a co-ordinate system; that the movement manifests as relative motion is secondary.

That we cannot determine which is the true case does not mean that both are equally true, it simply means we cannot determine which is the true case; but we can deduce that one of them must be i.e. one of them must actually be moving, in an absolute sense.


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This bit in red is the important bit. You are absolutely correct. "No absolute rest" is physically equivalent to the assumption that there is absolute rest. Hence, you cannot say relativity assumes absolute rest, since we can equivalently say relativity assumes no absolute rest. So instead, what we say is statements about absolute rest are not a part of relativity.
To say that relativity assumes no absolute rest is an empty statement, because the assumption of absolute rest is implicit. That the two are functionally equivalent demonstrates the point, it doesn't invalidate it.
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01-05-2012, 15:12   #79
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Originally Posted by roosh View Post
Fair enough, but that doesn't mean that an absolute reference frame has to be assumed for the purpose of absolute motion, because to suggest that absolute motion is relative would be a contradiction in terms.

That's fair enough, but we're talking about what can be deduced from relative motion, and absolute motion is one such thing that can be deduced.

You can, we do it everyday of our lives; we talk about the earth orbiting the sun and the earth rotating, as opposed to the sun orbiting the earth; we talk about us moving as we walk down the street, as opposed to us walking on the spot while the world moves past us as though on a giant conveyor belt; we talk about an escalator moving up or down, as opposed to the entire universe shifting in such a way to give the same effect.

All of these use the word "moving" without reference to a co-ordinate system; that the movement manifests as relative motion is secondary.
All of those statements make implicit reference to a coordinate system. Walking down the street is moving "with respect to the street". Orbiting the sun is moving "with respect to the sun". Since all statements of moving make reference to a coordinate system, we cannot therefore infer an "absolute" reference frame where objects are "truly" moving, without the added assumption that absolute rest exists.

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That we cannot determine which is the true case does not mean that both are equally true, it simply means we cannot determine which is the true case; but we can deduce that one of them must be i.e. one of them must actually be moving, in an absolute sense.
You cannot even deduce that one of them must be a "truer" case, because physics itself, not just experimental apparatus, is invariant in either case. I.e. To speak of a "correct" reference frame is physically meaningless.

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To say that relativity assumes no absolute rest is an empty statement, because the assumption of absolute rest is implicit. That the two are functionally equivalent demonstrates the point, it doesn't invalidate it.
That the two are physically equivalent completely invalidates your point, as I showed earlier. If you cannot show a logical contradiction between relativity and "no absolute rest", then you cannot say relativity implicitly assumes absolute rest. This is why I have repeatedly asked you for a contradiction between relativity and "no absolute rest".
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03-05-2012, 06:27   #80
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Originally Posted by Morbert View Post
All of those statements make implicit reference to a coordinate system. Walking down the street is moving "with respect to the street". Orbiting the sun is moving "with respect to the sun". Since all statements of moving make reference to a coordinate system, we cannot therefore infer an "absolute" reference frame where objects are "truly" moving, without the added assumption that absolute rest exists.
We make plenty of unqualified statements such as "I am moving"; "I didn't move, you moved"; [on a train] "we're moving"; [in traffic] "we're not moving"; and many, many more.

None of which refer to a reference frame; and none of which assume that absolute rest exists. Indeed, the idea of absolute rest isn't necessary for the concept of absolute motion. To say that absolute motion is motion relative to absolute rest is to say that absolute motion is relative motion, which is a contradiction in terms.

Absolute motion is the idea of motion that isn't qualified as being relative to anything; it is a "yes or no", "either, or" question.


While we don't need to assume that any object is in a state of absolute rest, we can use the concept as a purely hypothetical, mathematical construct for the purpose of deduction; we can't measure velocities relative to it, but we can make certain deductions about the physical world by considering it. Something you mention is entirely compatible with Einsteinian relativity.


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Originally Posted by Morbert View Post
You cannot even deduce that one of them must be a "truer" case, because physics itself, not just experimental apparatus, is invariant in either case. I.e. To speak of a "correct" reference frame is physically meaningless.
The issue might lie in stating that we cannot say that one reference frame is more true than any other, and I wouldn't necessarily disagree with that; what we can deduce however, is that, at least one object must absolutely be in motion. This would have certain deductive consequences that would not, necessarily, be revealed by the mathematical reference frames.



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Originally Posted by Morbert View Post
That the two are physically equivalent completely invalidates your point, as I showed earlier. If you cannot show a logical contradiction between relativity and "no absolute rest", then you cannot say relativity implicitly assumes absolute rest. This is why I have repeatedly asked you for a contradiction between relativity and "no absolute rest".
The statement that "relativity doesn't implicitly assume absolute rest" is a bare assertion fallacy; it's an empty statement.

What you need to demonstrate is the difference between "no absolute rest" and "absolute rest" and how relativity includes one but not the other; because if there is no difference between them, then they are the same thing i.e. what you are asserting isn't absolute rest, is actually absolute rest.

Bear in mind that the difference between inertial motion [that isn't absolute rest] and absolute rest pertains to the path length of a photon in a light clock.
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03-05-2012, 10:27   #81
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Originally Posted by roosh View Post
We make plenty of unqualified statements such as "I am moving"; "I didn't move, you moved"; [on a train] "we're moving"; [in traffic] "we're not moving"; and many, many more.
Come on, that's the English language, not physics.

Can you write down a single physical formula for motion which is equivalent to the statement that something is "absolutely moving"?

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Originally Posted by roosh View Post
None of which refer to a reference frame; and none of which assume that absolute rest exists. Indeed, the idea of absolute rest isn't necessary for the concept of absolute motion. To say that absolute motion is motion relative to absolute rest is to say that absolute motion is relative motion, which is a contradiction in terms.
Motion by definition is something like "change in position with respect to time." Change in position is inherently relative because "position" is a physical property that depends on where you choose to measure from.

Absolute motion would be motion with respect to an absolutely defined (valid everywhere in the universe) set of position measurements. The motion is relative to those measurements which can be used by anybody anywhere to see if an object is absolutely moving or not absolutely moving.

For instance, if somebody is "moving" down a street then by that motion we mean that if we measure from the start of the street, or from one metre from the start of the street, that their distance from that point changes.

If on the other hand we were walking beside this person at the exact same pace in the same direction then they have no motion away from us, so then they are not moving if we choose to measure all motion as motion with respect to our own position.

There is no physical reason to say "motion with respect to the start of the street" is more fundamental than "motion with respect to my own position." However if there was a notion of absolute motion, ie if there was an absolute set of coordinates that someone walking down the street could use to measure the motion of somebody walking beside them, then they could try to say things like "this person is really moving, absolutely."

Notice that when I say things like "someone walking down the street" that is not saying "someone is absolutely moving down the street." Someone walking down the street is an English sentence that in proper physical terms means just that another person placed at the start of the street measures the distance between them and the someone as changing.
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03-05-2012, 11:37   #82
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Originally Posted by antiselfdual View Post
Come on, that's the English language, not physics.

Can you write down a single physical formula for motion which is equivalent to the statement that something is "absolutely moving"?
The english language, like language in general, evolved to allow us to communicate about our experience of the universe; Physics is just an extension of that, and where it uses the english language in its formulation, it isn't completely divorced from it.

As for writing down an equation, I don't see why it is necessary; we only need to be able to make certain deductions, that don't necessarily involve any measurements or calculations.


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Originally Posted by antiselfdual View Post
Motion by definition is something like "change in position with respect to time." Change in position is inherently relative because "position" is a physical property that depends on where you choose to measure from.
Motion is also a state, the absolute nature of which cannot be determined by experiment.

Absolute means
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Philosophy
  • a value or principle which is regarded as universally valid or which may be viewed without relation to other things
So absolute motion would be the state of motion that may be viewed without relation to other things.


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Originally Posted by antiselfdual View Post
Absolute motion would be motion with respect to an absolutely defined (valid everywhere in the universe) set of position measurements. The motion is relative to those measurements which can be used by anybody anywhere to see if an object is absolutely moving or not absolutely moving.
This is saying that absolute motion is relative motion, just motion relative to an absolute reference frame. Absolute motion ins't relative motion, not least because it is possible to experimentally determine relative motion. It would also be a contradiction in terms.

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For instance, if somebody is "moving" down a street then by that motion we mean that if we measure from the start of the street, or from one metre from the start of the street, that their distance from that point changes.

If on the other hand we were walking beside this person at the exact same pace in the same direction then they have no motion away from us, so then they are not moving if we choose to measure all motion as motion with respect to our own position.
If someone is "moving" down the street then we mean that we can measure the change in distance from a given point, but also, we mean that the person is actively "doing the moving", while the street is passive.

We don't say, as the univers shifted around the person, and the road behaved like a treadmill, so as to cause a change in distance between the person and the point of reference, as they pumped their legs; this is because we attribute the movement to the person walking

Even if we cannot determine which scenario actually happened, we know that it must have been one or the other, otherwise there would have been no relative motion to measure; the person would have remained at rest relative to the street.

Yes we can define a frame of reference which labels either as moving, but unless one actually moves, then there would be no relative motion to warrant those labels.


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Originally Posted by antiselfdual View Post
There is no physical reason to say "motion with respect to the start of the street" is more fundamental than "motion with respect to my own position." However if there was a notion of absolute motion, ie if there was an absolute set of coordinates that someone walking down the street could use to measure the motion of somebody walking beside them, then they could try to say things like "this person is really moving, absolutely."
We cannot say that "motion with respect to the start of the street" than "motion with respect to my own position" - let's call that A, and the reverse B - because we cannot determine if that is the case; what we can say, however, is that either A is mor fundamental than B, or B is more fundamental than A i.e. one is more fundamental than the other, we just cannot determine which one.

Of course, both A and B could be true, but that would just further compound the idea of absolute motion.


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Originally Posted by antiselfdual View Post
Notice that when I say things like "someone walking down the street" that is not saying "someone is absolutely moving down the street." Someone walking down the street is an English sentence that in proper physical terms means just that another person placed at the start of the street measures the distance between them and the someone as changing.
It is an english sentence that attempts to characterise our experience of the physical world; it ascribes the act of motion to the person walking; notice that you don't say that the road is moving [like a treadmill], this is because we view the road as being passive in the scenario where relative motion occurs between us and the road.

Of course, the road can be moving, as indeed it is, if the earth is in orbit or rotating; but being in motion with it, we can subsequently move again, along the surface of the earth.

Again, we can define reference frames that label either the road or ourselves as moving, but unless one of us us actually moving, then there would be no relative velocity to cause us to label either as moving. We can't determine which it is, but we can determine it must be one or the other.

Example
Just to try and highlight the example again; you start off at rest relative to the road; in order for relative motion to occur either your walking must propel you forward, or you walk as though on a treadmill; those two scenarios account for the relative motion that is measured, but both are two different examples which entail relative motion.

I don't doubt that you understand the difference.
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03-05-2012, 18:01   #83
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I've been following this thread for a while now and have gotten a little lost. Roosh what do you mean by absolute motion? I saw the definition you wrote earlier, but it doesn't really make the idea clear. Its one thing to define something, its another thing to explain something and show that you fully understand it.

For me the English word absolute, means in a physical sense, something that is observed to agree with all observers no matter how, when, where or what the measurement is taken or the theory is described . There are few instances in science where it is actually used, the only one I can think of is absolute zero.

Any way from what I can see from what you say by absolute motion, this is an agreed upon reference frame, that the entire universe has decided is moving. No matter where the observer is or what they are doing they will all say it is moving, even if they are moving along with the absolute frame.

My understanding could be wrong, but to me that seems a little unintuitive.


As regard to relative motion

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Originally Posted by roosh View Post
If someone is "moving" down the street then we mean that we can measure the change in distance from a given point, but also, we mean that the person is actively "doing the moving", while the street is passive.

We don't say, as the univers shifted around the person, and the road behaved like a treadmill, so as to cause a change in distance between the person and the point of reference, as they pumped their legs; this is because we attribute the movement to the person walking
This is true, however in a relativistic sense. Although we know that we are doing the moving down the street, both scenarios are equal. Its just a matter of how you set up your initial coordinate system.

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Originally Posted by roosh View Post
Even if we cannot determine which scenario actually happened, we know that it must have been one or the other, otherwise there would have been no relative motion to measure; the person would have remained at rest relative to the street.
This is not entirely true. We could set up a coordinate system where both the street and the person is moving, with the exact same consequences. Although it would be pointless and greatly increase the difficulty of the problem it can still be done.

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Yes we can define a frame of reference which labels either as moving
True

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but unless one actually moves, then there would be no relative motion to warrant those labels.
Also true, but it would be a terribly very boring universe, in an already boring universe with only a man and a street.

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We cannot say that "motion with respect to the start of the street" than "motion with respect to my own position"
Again true, they are two different scenarios.

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Originally Posted by roosh View Post
- let's call that A, and the reverse B - because we cannot determine if that is the case; what we can say, however, is that either A is mor fundamental than B, or B is more fundamental than A i.e. one is more fundamental than the other, we just cannot determine which one.
Again true, but not in the sense that you think. Neither is "fundamental," that is a bad choice of words. Both are scenarios that we arbitrarily set up to determine the outcome of the same event. Both are correct, both are equivalent and both are equal. Just a different way of looking at the same thing.


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Originally Posted by roosh View Post
Of course, both A and B could be true
In fact both are true.

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Originally Posted by roosh View Post
but that would just further compound the idea of absolute motion.
Again I need further clarification of what you mean.*

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Originally Posted by roosh View Post
It is an english sentence that attempts to characterise our experience of the physical world; it ascribes the act of motion to the person walking; notice that you don't say that the road is moving [like a treadmill], this is because we view the road as being passive in the scenario where relative motion occurs between us and the road.
English has no place in science

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Originally Posted by roosh View Post
Of course, the road can be moving, as indeed it is, if the earth is in orbit or rotating; but being in motion with it, we can subsequently move again, along the surface of the earth.
Think of a car and a tree. The tree sees a car coming straight at it at an alarming speed. The car sees a tree coming at it at an alarming speed. We know that the car is doing the "actual" moving ( I think this is what you mean by "absolute motion" ) however both are right. The tree didn't uproot and run at the car.

Introducing the earth is an unnecessary complication, but we can still use it if we want.

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Originally Posted by roosh View Post
Again, we can define reference frames that label either the road or ourselves as moving, but unless one of us us actually moving, then there would be no relative velocity to cause us to label either as moving. We can't determine which it is, but we can determine it must be one or the other.
This further increases my belief that you mean the "thing" doing the motion when you say "absolute motion." We are lucky (and unlucky) to have a stable reference frame that we call the earth, from which we can make all of our measurements. Most day to day measurements wrt to motion are made relative to this stable frame. As a result humans have a fundamental built in intuition about what moves wrt it. This is not to say that its correct. Essentially everything not moving (trees, buildings, mountains etc) is in the same reference frame as the earth, everything else is in a different frame. Its how we compare these reference frames that matters.

We use the earth like the street in the earlier argument. For most its a good approximation of a flat infinite plane.

Hope this all makes sense

*Just had an idea, by absolute motion, do you mean the thing that is actually doing the motion?
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03-05-2012, 19:46   #84
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Instead of repeating what citrusburst and antiselfdual have said, I will ask you these question:

If you believe there is a state of absolute motion, do you believe it is possible to not be in a state of absolute motion? What would you call this state?

Hopefully these questions should illustrate why you are (perhaps unknowingly) presupposing a state of absolute rest in your understanding of motion.
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04-05-2012, 03:56   #85
roosh
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Cheers Citrus, it's refreshing to not just get the usual reply that "actually moving" doesn't make sense. I will reply to individual points in your post, but just to say that you are correct in your interpretation, by "absolute motion" I mean "the thing that is actually doing the motion"

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Originally Posted by citrus burst View Post
I've been following this thread for a while now and have gotten a little lost. Roosh what do you mean by absolute motion? I saw the definition you wrote earlier, but it doesn't really make the idea clear. Its one thing to define something, its another thing to explain something and show that you fully understand it.

For me the English word absolute, means in a physical sense, something that is observed to agree with all observers no matter how, when, where or what the measurement is taken or the theory is described . There are few instances in science where it is actually used, the only one I can think of is absolute zero.

Any way from what I can see from what you say by absolute motion, this is an agreed upon reference frame, that the entire universe has decided is moving. No matter where the observer is or what they are doing they will all say it is moving, even if they are moving along with the absolute frame.

My understanding could be wrong, but to me that seems a little unintuitive.
I'm not sure if what you say later in the post supercedes this, but I will address it for the sake of clarity.

By "absolute motion" I am not suggesting that there is an agreed upon reference frame, that the entire universe has decided is moving.

My understanding of the term absolute is
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Philosophy
  • a value or principle which is regarded as universally valid or which may be viewed without relation to other things
That is, the concept of a reference frame doesn't necessarily come into it. It is a statement about the absolute nature of motion, as opposed to the measurement of what that motion is relative to.

The experimental test of the principle of relativity states that
Quote:
Any uniformly moving observer in an inertial frame cannot determine his "absolute" state of motion by a co-moving experimental arrangement.
Tests of SR

The term "absolute motion" I take to refer to a "state of motion", without necessarily qualifying it as being relative to something.


Contextual understanding
For some reason the idea seems to cause some confusion, but I think some contextual examples should help clarify the meaning.

I see "absolute motion" as being a "yes or no" question, or an "either, or" question e.g.:
"Is X moving?"
"Is Y moving?"
"Is X or Y moving?"

None of these make reference to a reference frame, they don't qualify the statement with "relative to Z"; they are questions about the absolute nature of motion i.e. which one is doing the moving.


Everyday examples
An everyday example would be driving down the road; we can ask the question "is my car moving?" or "is the road moving?". Yes, the car is moving relative to the road, and the road is moving relative to the car - that is relative motion; but the question of which is actively "doing the moving", without qualifying the statement by adding "relative to X", is a question of "absolute motion".


Even if we cannot determine which one is actively "doing the moving", we can deduce that, at least one of them has to be.


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Originally Posted by citrus burst View Post
As regard to relative motion


This is true, however in a relativistic sense. Although we know that we are doing the moving down the street, both scenarios are equal. Its just a matter of how you set up your initial coordinate system.
While we can define different co-ordinate labeling systems which label us as moving relative to the road, and the road as moving relative to us, and both those are perfectly equal; the question of "which one is doing the moving" introduces a disparity, which has logical consequences.

They represent two different scenarios which result in the same, observed, relative motion, but we can make deductions about the physical consequences, that aren't necessarily revealed by co-ordinate labeling systems,


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Originally Posted by citrus burst View Post
This is not entirely true. We could set up a coordinate system where both the street and the person is moving, with the exact same consequences. Although it would be pointless and greatly increase the difficulty of the problem it can still be done.
Apologies, I should have included that third possibility, but I find that it can, sometimes, help to deal with just the two contrasting possibilities and say that "at least" one of them has to be the case.

The point is, essentially, wherever there is relative motion, at least one of the relatively moving bodies has to, actually, be doing the moving.


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Originally Posted by citrus burst View Post
Also true, but it would be a terribly very boring universe, in an already boring universe with only a man and a street.
Very true; but it would narrow the number of possible answers to the question, "how many roads must a man walk down, before you can call him a man?



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Again true, they are two different scenarios.
With different deductive consequences.


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Again true, but not in the sense that you think. Neither is "fundamental," that is a bad choice of words. Both are scenarios that we arbitrarily set up to determine the outcome of the same event. Both are correct, both are equivalent and both are equal. Just a different way of looking at the same thing.
Again, apologies, I was trying to stick with the terminology that antiselfdual had used, for the purpose of explanation.

While both scenarios are potentially two different ways of looking at the same thing, I wouldn't necessarily say that they are equal or equivalent; they each have different deductive consequences.

The mathematical reference frames may be equivalent, or equally valid, but the mathematical reference frames don't allow us to make certain deductions, that considering the two physically different scenarios does.


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In fact both are true.
I would say that both mathematical reference frames, which label each as moving relative to the other, are true; however, we cannot determine which physical scenario is the true one, or even if both are true; these physical scenarios are independent of the co-ordinate referencing system, despite the fact that the co-ordinate labeling system is entirely dependent on the physical scenario.

While we cannot determine which of A, B, or C is correct, we can deduce that either A, B, or C has to be correct.

And each one has different deductive consequences.


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Again I need further clarification of what you mean.*
You are pretty much spot on with the "addendum"; I am referring to "the thing that is actively, or actually, doing the motion"

Hopefully the above also helps to clarify it.


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English has no place in science
Apparently so!



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Think of a car and a tree. The tree sees a car coming straight at it at an alarming speed. The car sees a tree coming at it at an alarming speed. We know that the car is doing the "actual" moving ( I think this is what you mean by "absolute motion" ) however both are right. The tree didn't uproot and run at the car.
I would agree that we can define a reference frame which says the car is moving relative to the tree and a different reference frame which says the tree is moving relative to the car; I would also agree that it is the car that is doing the "actual" moving.

However, we apparently cannot determine if the car is doing the "actual" moving; for all we know, the earth could be doing the "actual" moving and the cars wheels could just be rotating in the opposite direction to a giant conveyor belt, at the exact speed to offset the speed of the conveyor belt moving in the opposite direction.

It think, however, that we can deduce that it is, at the very least, one or the other which is true; of course, the cars wheels could be turning at a faster pace than the conveyor belt and both could be true.


All three scenarios would have different deductive consequences though, which is probably easier to determine if we introduce a light clock for each observer.


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Introducing the earth is an unnecessary complication, but we can still use it if we want.
I would agree with that, that is why I generally tend to avoid it; we can equally consider just the relative motion between the earth and the sun, but the deductions would be the same.



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This further increases my belief that you mean the "thing" doing the motion when you say "absolute motion." We are lucky (and unlucky) to have a stable reference frame that we call the earth, from which we can make all of our measurements. Most day to day measurements wrt to motion are made relative to this stable frame. As a result humans have a fundamental built in intuition about what moves wrt it. This is not to say that its correct. Essentially everything not moving (trees, buildings, mountains etc) is in the same reference frame as the earth, everything else is in a different frame. Its how we compare these reference frames that matters.

We use the earth like the street in the earlier argument. For most its a good approximation of a flat infinite plane.
That is pretty much exactly what I mean, "the 'thing' doing the motion".

My understanding though, is that everything in the universe is in every reference frame, it just depends on what the motion is measured relative to. That is, the sun is in the reference frame that labels the earth as "at rest", but the sun is labeled as "in motion relative to the Earth", and vice versa.

The Lorentz transform then allows us to determine the co-ordinates of a different refrence frame, using the co-ordinates of any given reference frame,


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Hope this all makes sense
Cheers Citrus, it did make perfect sense; I hope that what I said also made sense.


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*Just had an idea, by absolute motion, do you mean the thing that is actually doing the motion?
Pretty much.
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04-05-2012, 03:59   #86
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Instead of repeating what citrusburst and antiselfdual have said, I will ask you these question:

If you believe there is a state of absolute motion, do you believe it is possible to not be in a state of absolute motion? What would you call this state?

Hopefully these questions should illustrate why you are (perhaps unknowingly) presupposing a state of absolute rest in your understanding of motion.
If I can just turn the question around: if we can deduce that at least one observer, of two relatively moving observers, has to actually be moving i.e. has to be in a state of absolute motion, is it possible to be in a state of non-absolute motion; what would you call this state of non-absolute motion?

This should hopefully demonstrate that it is a deduction, not an assumption.
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04-05-2012, 14:23   #87
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If I can just turn the question around: if we can deduce that at least one observer, of two relatively moving observers, has to actually be moving i.e. has to be in a state of absolute motion, is it possible to be in a state of non-absolute motion; what would you call this state of non-absolute motion?

This should hopefully demonstrate that it is a deduction, not an assumption.
The problem is we can't deduce such a thing. "Motion", in this case, means any coordinate system we use has to label at least one of the observers as in motion. That is all it means. You cannot deduce absolute motion, in the sense of an absolute velocity, from this.

What is happening is you are deducing a specific concept of motion (The idea that no reference frame labels both observers as at rest), and then unknowingly switching your definition of motion to mean an absolute velocity.
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04-05-2012, 14:30   #88
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So after thinking about this my own explanation of this argument is the following:

What you're saying is basically that if X perceives Y to be moving, and itself fixed, and we could also switch to the Y reference frame in which Y is fixed and X is moving, then it must be true that one of X or Y is "actually" moving, which you call "absolute" motion.

The first point here is that your definition of "absolute motion" is not the same as the standard physics definition of absolute motion (as you yourself have said and which I did not realise at first). So I will therefore term what you are describing as "intrinsic motion." You argue that if both X and Y observe the other to be moving then one of them must have an "intrinsic motion" as otherwise no motion could have occurred.

The problem is that this notion is completely unphysical, because even if we suppose that object X in some situation has an intrinsic motion we are always able to find a reference frame in which X is not moving (actually we can find an infinite number of reference frames in which X is not moving as well as an infinite number of reference frames in which X is moving). In fact we can describe all of the laws of physics in the reference frame in which X is not moving, and they will be as valid there as in a reference frame in which X is moving. Therefore it makes absolutely no physical difference if we regard X as to have "intrinsic motion" or not.

As a result I think your argument that "something must actually be moving" is in some sense pure semantics, because there is no way you can draw a physical distinction between X having intrinsic motion or Y having intrinsic motion.
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06-05-2012, 08:27   #89
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The problem is we can't deduce such a thing. "Motion", in this case, means any coordinate system we use has to label at least one of the observers as in motion. That is all it means. You cannot deduce absolute motion, in the sense of an absolute velocity, from this.

What is happening is you are deducing a specific concept of motion (The idea that no reference frame labels both observers as at rest), and then unknowingly switching your definition of motion to mean an absolute velocity.
I think that is where the confusion lies; the purpose isn't to deduce the existence of an absolute velocity, it is to deduce something about absolute motion; that is, to deduce something about the nature of motion as opposed to its measurement.

When we talk about motion with respect to a reference frame, we are necessarily talking about relative motion, not absolute motion. Absolute motion doesn't necessarily require a reference frame because it pertains to the nature, or state of motion.

In general, where there is relative motion between two bodies we can determine three physically distinct scenarios which account for the relative motion, all of which make different statements pertaining to the nature of motion of the bodies involved. We can't deduce which scenario is the true one, but we can deduce that one of them must be, and with it that one of the bodies must absolutely be in motion.
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06-05-2012, 09:36   #90
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So after thinking about this my own explanation of this argument is the following:

What you're saying is basically that if X perceives Y to be moving, and itself fixed, and we could also switch to the Y reference frame in which Y is fixed and X is moving, then it must be true that one of X or Y is "actually" moving, which you call "absolute" motion.

The first point here is that your definition of "absolute motion" is not the same as the standard physics definition of absolute motion (as you yourself have said and which I did not realise at first). So I will therefore term what you are describing as "intrinsic motion." You argue that if both X and Y observe the other to be moving then one of them must have an "intrinsic motion" as otherwise no motion could have occurred.

The problem is that this notion is completely unphysical, because even if we suppose that object X in some situation has an intrinsic motion we are always able to find a reference frame in which X is not moving (actually we can find an infinite number of reference frames in which X is not moving as well as an infinite number of reference frames in which X is moving). In fact we can describe all of the laws of physics in the reference frame in which X is not moving, and they will be as valid there as in a reference frame in which X is moving. Therefore it makes absolutely no physical difference if we regard X as to have "intrinsic motion" or not.

As a result I think your argument that "something must actually be moving" is in some sense pure semantics, because there is no way you can draw a physical distinction between X having intrinsic motion or Y having intrinsic motion.
The argument only seems like semantics because there appears to be some dispute over what the concept of absolute motion means; I'm trying to use a more rigorous definition that accurately reflects the idea of "absolute" and pertains to the nature of motion. The standard scientific definition, which attempts to define absolute motion as being relative [to an absolute referece frame], is self-contradictory, and only relates to the measurement of motion. The term "intrinsic" is probably helpful in this regard.

It's interesting that you should say that the intrinsic motion of a body is completely unphysical and then qualify it by saying that we will always be able to find a reference frame in which the body is not moving; when it is, of course, the abstract, mathematical reference frames which have no intrinsic physicality. The intrinsic motion of an object is entirely phyiscal, and is not dependent on mathematical reference frames.

Indeed, once we establish the idea of intrinsic motion we can then proceed to see what deductive consequences that it has; which, of course, will be entirely phyiscal.
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