How does a body "know" how to move??!!

[Strange]

I may be in a position to resolve one aspect of "motion" that is bugging me - about being able to "freeze" a moving object and being able to discern its velocity. ... Can this not be done - by an external observer at least - by looking for relativistic length contraction?

 

 

Intersting point. I guess that is true. Clever idea.

 

(Although, as already noted, Zeno's paradox is resolved by calculus, which allows you to determines the instantaneous velocity.)

[Rasher Null]

Thank you Strange for your generous comment ). I never thought I would be relieved by the strangeness of special relativity! But in a vaguely informationy sort of way it is a relief.

[geordief]

Well leaving out quantum scale things (for now at least), shall we say that an "object" is something that has a position? We could call it an "aardkarv" even - but some name would be useful if the definition of motion is based on a "change of position", because presumably that means a change of position of something and it would be convenient to have a name...

FANFARE!!

 

I may be in a position to resolve one aspect of "motion" that is bugging me - about being able to "freeze" a moving object and being able to discern its velocity. ... Can this not be done - by an external observer at least - by looking for relativistic length contraction?

So ,if you know the spatial dimensions of an object at rest you can determine its motion wrt yourself (along all 3 spatial axes) by measuring its length contraction or expansion in the corresponding direction?

 

So motion (wrt an observer) is a function of how "distorted" the object appears to an observer?

 

Out of interest If the object is approaching directly at .9c would the contraction (or elongation -I am confused) be the inverse of the contraction of the same object receding at .9c?

[Strange]

Out of interest If the object is approaching directly at .9c would the contraction (or elongation -I am confused) be the inverse of the contraction of the same object receding at .9c?

 

 

No, it purely depends on speed not direction. A moving object is always contracted when observed from another frame of reference.

 

If it is coming directly towards you, it might be tricky to measure its length in that direction, though!

[studiot]

 

 

No, it purely depends on speed not direction. A moving object is always contracted when observed from another frame of reference.

 

If it is coming directly towards you, it might be tricky to measure its length in that direction, though!

 

What would be the contraction at right angles to its direction of motion?

[Tim88]

[..]

 

In terms of my query - which perhaps can be better phrased as essentially "what IS motion?" - there is still a lot of room for ruminating (including philosophizing on POLA of course ,as alluded to by Strange). But I still want to get my head around motion as a "property" ... objects seem to possess it yet they don't in isolation, because it requires time to be expressed. Contrast this with the property of mass - which you can deduce by isolating and totting up the mass of the constituent particles say. I hope you can see where I can coming from, even if you don't think it is very interesting.

[..]

 

I hope you are realizing that you are asking philosophical questions in a physics forum. The answers you get are therefore just opinions most of which cannot be proven and don't really belong here. For example, Newton appears to have considered motion to be a property just as you reason here; Strange thinks differently, and his disagreement with Newton is his right. Such philosophical statements about motion and time are not physics proper. It's important to understand that!

What is great about scienceforums (just my impression as newcomer ) is that it also has a philosophy section.

[Strange]

Saying that motion is observer dependent and not inherent in the object is physics, not philosophy. It is testable and measurable and so it is science, not opinion.

 

What would be the contraction at right angles to its direction of motion?

 

 

Zero.

[Tim88]

Saying that motion is [..] not inherent in the object is physics, not philosophy. It is testable and measurable and so it is science, not opinion.

[..]

 

You said instead that it is not a property of the object. [edit:] In fact, you exclaimed "Absolutely not" to the above cited reflection "objects seem to possess [motion] yet they don't in isolation" by Rasher.

 

It does sound as if you don't understand the philosophical basis (also related to the non-universal definition of words). Let's try this (emphasis mine:

 

 

A property [..] is this, that if a place is moved, whatever is placed therein moves along with it; and therefore a body, which is moved from a place in motion, partakes also of the motion of its place.

 

Was I wrong in thinking that you probably disagree with that?

Edited October 10, 2016 by Tim88

[Strange]

It does sound as if you don't understand the philosophical basis (also related to the non-universal definition of words).

 

 

The philosophical basis of what?

 

If you are saying that, ultimately, everything is philosophical (after all, the nature of scientific knowledge is a philosophical question) then that is obviously correct and completely unhelpful.

 

Given that we can and do draw a distinction between scientific knowledge (evidence, testability, etc) and opinion, I don't see how it can be correct to say that something that is determined by observation, experiment and theory is an "opinion".

 

 

Let's try this (emphasis mine:

 

 

Was I wrong in thinking that you probably disagree with that?

 

I don't really know what that means. Perhaps some context for where it came from, who said it, why, etc. would be helpful. (Or probably not if you are going to hijack this and turn it into yet another of your "what is relativity" threads - how many do you have going now?)

[Rasher Null]

Regarding contraction I am assuming that if the observer had a few cameras dotted about he would be able to discern the direction and speed of motion of the object....

 

If so then the object is effectively carrying around a vector of velocity for all to see without it having to actually move, but - though of course this is a vector of relative velocity and so one cannot say without reservation that motion is a sole intrinsic property of the body. However, it would seem to support a case for saying that objects have a sort of velocity property. If one were to wish this to be the case, then length scaling in 3 dimensions would be the neatest and simplest way to express this/ One could almost deduce length scaling from first principles!

 

Is it philosophy? mmm maybe partly, but given that philosophy is about thinking, and scientific knowledge has to be assimilated by the brain, I think it helps to become more accepting of scientific knowledge to shoehorn in a bit of philosophical thinking into scientific discussions sometimes.... *runs away*

[studiot]

 

 

 

Zero.

 

So what would be the contraction at 45^o to the direction of motion?

[Strange]

One could almost deduce length scaling from first principles!

 

If your "first principles" includes the fact that the speed of light is invariant. This wasn't obvious until Maxwell, at which point it was only a matter of time until someone deduced the principles of special relativity. (Given time, Maxwell himself might have done it.)

 

So what would be the contraction at 45^o to the direction of motion?

 

Here (and in fact, regarding geordief's question regarding elongation) you need to distinguish the physical effect from what is observed. The physical contraction would be described by the vector sum (so at 45^o would be 0.7 of the contraction in the direction of motion).

 

However, what you observe may be different (for example, an object moving towards you will appear elongated - effectively because of the Doppler effect; i.e. the different time it takes for light from the front and back to reach you).

https://en.wikipedia.org/wiki/Terrell_rotation

[studiot]

 

If your "first principles" includes the fact that the speed of light is invariant. This wasn't obvious until Maxwell, at which point it was only a matter of time until someone deduced the principles of special relativity. (Given time, Maxwell himself might have done it.)

 

Here (and in fact, regarding geordief's question regarding elongation) you need to distinguish the physical effect from what is observed. The physical contraction would be described by the vector sum (so at 45^o would be 0.7 of the contraction in the direction of motion).

 

However, what you observe may be different (for example, an object moving towards you will appear elongated - effectively because of the Doppler effect; i.e. the different time it takes for light from the front and back to reach you).

https://en.wikipedia.org/wiki/Terrell_rotation

 

 

So it does depend on direction as well as speed.

 

 

Rasher Null

 

If one were to wish this to be the case, then length scaling in 3 dimensions would be the neatest and simplest way to express this/ One could almost deduce length scaling from first principles!

 

One can indeed.

But there are many possibilities of formulae for this.

Unfortunately not many treatments of relativity discuss why we use the quadratic we do other than to say 'it fits nature' (ie our observations).

[Tim88]

[..] Given that we can and do draw a distinction between scientific knowledge (evidence, testability, etc) and opinion, I don't see how it can be correct to say that something that is determined by observation, experiment and theory is an "opinion".

 

 

I don't really know what that means. Perhaps some context for where it came from, who said it, why, etc. would be helpful. (Or probably not if you are going to hijack this and turn it into yet another of your "what is relativity" threads - how many do you have going now?)

 

The context is what I referred to in the post that you reacted on: Newton's definition of motion (incl. "property"), which sounds quite similar to that of Rasher (which is why I looked it up), and to which you replied "Absolutely not". Clearly at least one of us misunderstood the OP! I'll verify next.

 

BTW I have zero "what is relativity threads" as we probably all know that (or at least swansont does), but one thread on two opposing interpretations of relativity.

Think of a body moving as in Newton's first law - in a straight line at a fixed speed say. I have been wondering where is the information for the body to "know" how to move? One could argue that the motion itself is the "information", but that is somewhat unsatisfactory from an information theory point of view. [..]

 

Rasher, in view of the above, can you please clarify, did you mean with "how does the particle "know" that it is moving:

 

a. How does the PARTICLE "know" that it is moving (NOT restricting ourselves to direct HUMAN observations).

or

b. What do HUMANS OBSERVE about how a particle "knows" that it is moving (and please limit the meaning of the word "properties" to those things that are both accessible to and invariant in direct HUMAN observations).

or

c. Neither a) or b) but.....

 

To clarify the difference with atoms as illustration:

 

How does a gold bar "know" that it cannot be divided endlessly into ever smaller portions of gold? Is "consisting of atoms" a property of a gold bar?

 

a. A gold bar "knows" that it is made of atoms; and yes of course that's a property of gold.

or

b. Gold bars did not "know" that they are made of atoms until the 20th century; and until that time atoms were NOT a property of gold.

Edited October 10, 2016 by Tim88

[Strange]

The context is what I referred to in the post that you reacted on: Newton's definition of motion (incl. "property"), which sounds quite similar to that of Rasher (which is why I looked it up), and to which you replied "Absolutely not". Clearly at least one of us misunderstood the OP! I'll verify next.

 

I'm afraid I have absolutely no idea what you are talking about. I have clearly failed to understand you and/or the OP.

[Rasher Null]

Re Tim & Strange exchange : I think Tim is saying that Strange appears to be making contradictory statements as to whether he believes velocity is an intrinsic property of objects?? My position is that I don't know for sure and in exactly what way but the fact that objects effectively carry a velocity (relative!) vector around with them supports, to some extent at least, the notion that velocity might be an intrinsic property of objects - as opposed to space, say. As to whether the import of such thinking has scientific possibilities or is mere philosophic interpretation I am unsure about.

 

Tim: I do wish I never used "know" in the thread title because it is too loaded. What I was really asking is if there is a property of velocity that belongs to an object. I am also interested in whether motion can be dissected and analysed further in various ways; how motion scales up from the quantum scale and so forth. Perhaps asking how an intrinsic velocity might operate is where I'm at?

 

 

 

One can indeed.

But there are many possibilities of formulae for this.

Unfortunately not many treatments of relativity discuss why we use the quadratic we do other than to say 'it fits nature' (ie our observations).

 

Noted with interest. Thanks!

Edited October 10, 2016 by Rasher Null

[Tim88]

[..] Tim: I do wish I never used "know" in the thread title because it is too loaded. What I was really asking is if there is a property of velocity that belongs to an object. I am also interested in whether motion can be dissected and analysed further in various ways; how motion scales up from the quantum scale and so forth. Perhaps asking how an intrinsic velocity might operate is where I'm at? [..]

Hi Rasher, I did not claim anything contradictory in Strange's words, but I noticed a difference in interpretation of what you are asking. And I meant "know" in the way you meant it.

 

Thus: can you please clarify, did you mean with "is there a property of velocity that belongs to an object"

 

a. Can we understand velocity as a property of an object (NOT restricting ourselves to direct measurements but also possible inferences)?

or

b. Is velocity an invariant property of an object ("property" restricted to meaning "directly measurable")?

or

c. Neither a) or b) but.....

 

For the difference, the gold bar illustration stands (second half).

Edited October 10, 2016 by Tim88

[studiot]

 

 

Noted with interest. Thanks!

 

Part 2 in the extract I posted here, post#135 contains the beginning of the mathematics.

 

http://www.scienceforums.net/topic/98845-models-for-making-sense-of-relativity-physical-space-vs-physical-spacetime/page-7

 

But your whole view of motion is too simplistic.

 

I did ask what you meant by motion.

 

Consider the fluid motion we are discussing this in another thread.

 

Is fluid motion a property of the fluid as a whole (ie the flow) or a property of each individual fluid element or some combination?

 

http://www.scienceforums.net/topic/99281-turbulence/

[Rasher Null]

Interesting posts, I will have to spend some time thinking... I will be back though

[Strange]

Re Tim & Strange exchange : I think Tim is saying that Strange appears to be making contradictory statements as to whether he believes velocity is an intrinsic property of objects??

 

I am not sure how my statements could be seen as contradictory but, to clarify what I mean: motion cannot be intrinsic to an object because it is observer dependent. It is defined by the relationship between the object and the observer.

[Phi for All]

I hope you are realizing that you are asking philosophical questions in a physics forum.

It does sound as if you don't understand the philosophical basis (also related to the non-universal definition of words). Let's try this (emphasis mine:

 

!

Moderator Note

Please take your own advice here. This is NOT the Philosophy section. You have several threads open there to explore those questions.

 

Responding to this note here would be off-topic, but you can Report This Post if you object.

[steveupson]

Think of a body moving as in Newton's first law - in a straight line at a fixed speed say. I have been wondering where is the information for the body to "know" how to move? One could argue that the motion itself is the "information", but that is somewhat unsatisfactory from an information theory point of view.

 

Direction.

 

I think that I understand perfectly what your question is about. Historically, this type of information medium was once attributed to the aether, and it’s probably best described (not any more imprecisely than you described it in your question) as Mach’s principle.

 

Many modern theories discount the existence of an aether or ether and yet these theories have no explanation of how the physical property (attribute, quality, quantity, whatever) that we call direction manifests itself.

 

 

 

 

From wiki:

 

 

“Three-dimensional space (also: 3-space or, rarely, tri-dimensional space) is a geometric setting in which three values (called parameters) are required to determine the position of an element (i.e., point).”

 

 

and:

 

 

“In geometry, a position or position vector, also known as location vector or radius vector, is a Euclidean vector that represents the position of a point P in space in relation to an arbitrary reference origin O. Usually denoted x, r, or s, it corresponds to the straight-line distances along each axis from O to P…"

 

 

 

 

When you examine the mathematical representation of position, it is simply a length and a direction (referenced from another position), although the meaning of the thing we call direction is never quantified. We begin with the axiom that xyz are orthogonal to one another and leave it at that. Hence, every direction in two dimensions is expressed as either the ratio between two perpendicular lengths (Pythagoras) or as the ratio between the lengths of the diameter and circumference of a circle (π).

 

In either case, the mathematical representation relies on length, which implies that a metric has already been applied to the quantity (mathematical representation of direction) before its even evaluated. No one seems to be sure about this, and for some reason (probably because no one knows exactly how this can be) it’s very controversial to talk about this.

 

 

 

What I was really asking is if there is a property of velocity that belongs to an object. I am also interested in whether motion can be dissected and analysed further in various ways; how motion scales up from the quantum scale and so forth. Perhaps asking how an intrinsic velocity might operate is where I'm at?

 

 

Since we all seem to believe that velocity is a derivative of position, and since the position of a point is a relationship involving distance and direction from some other reference point, then all points must be related to one another by direction. All particles that are in motion have a future that lies in a specific direction. Since all these directions are relative to one another, no particle can change direction without affecting the direction of all other particles. Although this doesn't mean that there's an absolute direction, it does seem to imply that there is a universal direction, or system of directions. In spacetime it might be time that produces universal direction.

 

There’s a paper that I keep posting as a reference to some of this which no one has yet to understand why I keep posting it. It concerns the mechanism by which force manifests itself. Although the paper talks about the analysis of a static system, the exact same principles are also present in dynamic systems.

 

2015

The Physical Origin of Torque and of the Rotational Second Law

Daniel J. Cross

 

I wish someone would think this through wrt kinetics. My take on it is that it is fundamental. And although their analysis seems correct, their conclusions are speculative. It would be equally correct (mathematically) to simply change the direction rather than the position of the force, thus a non-rigid body is not necessarily a requirement.

 

(welcome Rasher Null, and since you're new here, let me warn you that everyone everywhere calls me a crank (without actually doing the math) so take everything that I say with a grain of salt)

[Rasher Null]

For the sake of simplicity I would prefer to stick to small rigid/near rigid balls than wade into fluids.

 

 

 

 

 

 

Since we all seem to believe that velocity is a derivative of position,

Because of the existence of instantly observable length contraction of moving bodies, it would seem that in actual fact, time is not needed to measure velocity!!?? Thus objects could be said to possess some mysterious attribute that causes a change of position over time.

I am not actually fretting over the nature of direction (though maybe I should it seems?) - I would say I want to know in what way two bodies - that are structurally identical but have different velocities - are intrinsically different, if in fact they are.

Thanks for your welcome Steve , fellow crank!?

Edited October 26, 2016 by Rasher Null

[Rasher Null]

"Time is not needed to measure velocity".

 

Profound? Promising? Provocative? Probably neither?

Edited October 31, 2016 by Rasher Null

[Strange]

"Time is not needed to measure velocity".

 

Profound? Promising? Provocative? Probably neither?

 

 

Wrong?