Measuring the absolute velocity of inertial frames.

Discussion in 'Physics & Math' started by geistkiesel, Jun 2, 2005.

  1. geistkiesel Valued Senior Member

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    All inertial frames A,B and C are moving relative to each other and I will refer to each simply as "frame".
    This is a very, very simple model. Initially C is moving the same direction and speed as A. Therefore, the measured relative velocity Vab =Va + Vb = VcB, as C is equivalent here to the A frame.

    What is the C frame going to do? Two things only, that is, C will make two measuremnts. 1. The relative velocity of C wrt B and the relative velocity of C wrt A where A is assumed at rest wrt C.

    The C frame is going to adjust its speed until the relative velocity of C wrt B is zero. That is C is going to change speed until C is moving exactly the same speed as B.

    Now consider what we do when C reaches the B velocity Vc - Vb = 0. We will have gone from a relative velocity of Vcb = Vab to Vcb = 0. t the very same time span the relative velocity of C wrt A is Vac = Va + Vc, however, we assume the A frame is at rest with rest to the C frame.

    In order to keep the relative motion of A wrt B a constant A and B do not change speeds. However, using the C frame as we are, we simulate the A frame motion change using the C frame to actually change speeds until the "simulated" speed of the A frame is identical to the speed of the B frame, or when C is moving at the B frame speed..

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    Geistkiesel ​
     
    Last edited: Jun 2, 2005
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  3. James R Just this guy, you know? Staff Member

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    Let Vab mean "The velocity of a relative to b".
    Let Va mean "The absolute velocity of a".

    By definition, we have things like this:

    Vab = Va - Vb
    Vac = Va - Vc
    Vbc = Vb - Vc

    And therefore

    Vab = Vac + Vcb

    Suppose we measure Vac and Vcb. Then we know Vab. Does this help us find Va, for example? Well

    Va = Vab + Vb

    so, we need to know Vb to find Va.

    Ultimately, to find Va, we will always need to know one or more of the absolute velocities Vb or Vc, regardless of how we do it.

    Problem: There is no way to directly measure Vb or Vc, since we don't know where the "absolute standard of rest" is. Asking for Vb is equivalent to asking for

    V(b)(universal standard) = Vb - V(universal standard)

    But we don't know V(universal standard), since we can't identify any object in the universe to be absolutely at rest.

    Therefore, all this talk about Va and Vb as "absolute" velocities is a complete waste of time. All Va really is is a velocity relative to an imagined universal standard, which doesn't actually exist.
     
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  5. geistkiesel Valued Senior Member

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    Very good Jameas R you have made an enormkopus stride. However, slow down just a skosh, and look at what we are doing.

    Do yourself a favor here. See if there is another way to phrase the statement we need to find Vb to find Va and turn is around, We need to know Va to find Vb., OK, are you still with me?

    Absoltely correct, you need to know Vc.

    You may have overlooked a possibility, an important possibility,and you may have restricted youraelf to thinking in terms of a universal standard. There may be universal standards a such as my V = 0 FOR, but that is another problem.

    But finish your statement.


    Check your attitude at the door if you want to conserve the waste of time. You aren't reading the posts clearly enough.

    Question James R., when the C frame which was originally moving identically to A, whre Va - Vc = 0 and we construct the relative velocity expression, does not Va - Vc = 0 = Vac.Initially?

    One step at a time if this is favorable to you.

    Is the relative velocity Vac = Va - Vc? I assume you answered in the affirmative (of course trhe sign will depend on the realtrive directions of the C and A frames).

    Now do tyou agree that as Vc is being adjusted that the Vac is changing with each Vc adjustment? Again I assunme you answered in the affirmative.

    Now when C adjusts its velocity until the measured relative velocity Vbc = 0, or Vc - Vb = 0 and Vb = Vc.Again I assume you will agree, and probably want to say but we don't know what Vb is!. I would say you are correct there also.
    lets continue.

    Will you agree that when we mweasure Vc = Vb we also can meaxsiure Vac? the relative velocity of A wrt C frame? yes you will say, probably getting a little impatient now, nervous even?.

    Here is combination math/physics question for the professor\:

    In the expression Vac, we started with the value Vac = 0, OK?

    As the C frame kept adjusting the C velocity looking for the B velocity the Vac is changing also, correct? Yes you say, get on with it!@!&*%-#@.

    Patience, James R.

    Now, When the observer on C assumes through out that A is at rest wrt C then will not the expression Vac = Vc be an accurate statement, physically as well as mathematically?. Don't look for some esoteric foofoo to obfuscate the matter. The answer which you may be loath to agree to is yes, Vac is equivalent to Vc when A is assumed at rest wrt to C.

    This has some intuitive dovetailing also. In deep space when C is right along side of A, the ships both seem at rest do they not? yes of course. So when C adjust its motion adn begins moving in the direction of the B ship which he can easily do, C can considers the A ship at rest wrt C.

    What is the measured Vac when Vc = Vb as measured ?
    Easy question isn't it? Vac = Vc = Vb. correct? Yes, yes, yes you say, the excitiment is heard developiong as you see the implication that is about to explode in your understanding and yopur typing speed takes on a fervent rush.

    Vac = Vc, therefore,
    Va + Vb = Vab,wher Vab is a known and measured constant value, and since Vb = Vc we can shuffle the algebra and substutute for Vb, as

    Va = Vab - Vc.

    Vab is measured.
    Vc is measured, and notwithstanding the measuremnt Vac is a realtive velocity there is some added information going along withe expression Vac. There is a history of Vac developing from Vac = 0 to Vac = Vc where |Vc| > 0.
    QED.
    And as an observatrion the Vac = Va gained from Vac = 0 initially, is simply the Vbc lost as C adjusted its velocity when "decreasing" the Vbc.

    Taking the differential as I did in the post, Va + Vb = Vab, and
    d(Va) + d(Vb) = d(Vab) = 0.

    and d(Va) = - d(Vb).

    A long as the Vab is held constant the negative of the relative velocity changes of inertial frames moving relative to each other are equal, equivalent. Does not this say that relative velocity need not be uniform velocity under the conditions where the relative velocity of accelerating frames is constant?


    It really isn't that difficult, learning to think physically as opposed to algorithmically, which has its place, I agree, but it isn't the only place.
    A wise man once published in a leading scientific journal remarks regarding the necessity of people to, "open their minds to new ideas". I agree to this, but I offer you james R, caution. Caveat Emptor: not to open the mind so wide that you let your half-dozen or so brain cells fall out.

    Good work james R.
    Geistkiesel ​
     
    Last edited: Jun 2, 2005
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  7. Neddy Bate Valued Senior Member

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    If the C craft is equipped with an accelerometer and a clock, or more simply, a radar gun like the traffic police use, then it should be able to measure the relative velocities of the A and B crafts. However, I fail to see how C can determine the absolute velocities of A and B. The shortcoming arises from the fact that C is not able to determine its own absolute velocity in the first place. Space is a vacuum and therefore there is nothing for an absoilute speedometer to measure against.

    There is something about these absolute velocity theories that seems to have a small glimmer of promise. Since c can be viewed as an absolute maximum velocity in the universe, it is not totally illogical to believe that there would also be an absolute minimum velocity of zero. For example, some state of rest in which all objects eventually find themselves after all of their kineteic energy has been lost to friction against dust particles, perhaps. Of course the dust particles could also be considered to be in motion, and the 'rest state' would be nothing more than a state of co-moving with the dust. Still, would the laws of thermodynamics not demand that moving objects eventually slow down and stop, given enough time?

    At any rate I am not sure it would be useful to know where the absolute rest state is. What would be the usefulness of knowing, for example, that when the Earth is at the vernal equinox. people on the equator at noon are actually at rest with respect to the vacuum of space? It seems nonsensical to me.
     
  8. 2inquisitive The Devil is in the details Registered Senior Member

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    by Neddie Bate:

    "What would be the usefulness of knowing, for example, that when the Earth is at the vernal equinox. people on the equator at noon are actually at rest with respect to the vacuum of space? It seems nonsensical to me."
    ===============================================================

    I can give you real world example where knowing the position of the vernal equinox
    was essential. GPS. When GPS was first formulated, an imaginary line connecting the
    vernal equinox and the center of the Earth was used to determine the velocities of
    both the orbiting satellites and the surface velocity of the Earth as this line was passed. Time dilation due to differential velocities was calculated this way. Most
    engineers and physicists working in the GPS field now use the International Celestial
    Reference frame, adopted on Jan. 1, 1998. It is more precise as an 'approximate' absolute reference frame. Notice I did say approximate, but essential for astronomers
    to calculate movement in the universe. For example, the Milky Way galaxy and hundreds of other galaxies are converging on a point near the Virgo Cluster. We are
    moving toward that point, that point is not moving towards us while we are 'at rest.'
     
  9. geistkiesel Valued Senior Member

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    2,471

    If A acelerated to somr velocity and kept a history of deceleration the when it measures the Vab = 0 nand it knows the change in its iwn vel;ocity A should be able to do this by himself

    at t < 0
    Vab = Vac as Vc = Va
    Va + Vb = Vab
    Vb + Vc = Vbc
    Before we start, this is all we know. For us outside observers Vb = 480 units
    Vc = 240 units Vab = 720 units, where the Va and Vb measured wrt Ve the earth embankment, unknown to A and B.

    At t = 0 Neddy accelerates the C frame , coasts , measures Vbc and repeats the cycle until at t = 1,
    Vbc = 0 and Vb = Vc and the measured at this instant, Vac = 240 units.
    Neddy has changed his velocity to the identical velocity lof the B frame and ergo Neddy knows Vac = 720 units. However, Neddy knows that he measured the Vac at 240 when assuming Va = 0 wrt Vc

    Neddy reasons thus. The Vab = 720 and I he therefore substracts 240 units to arrive at a number 480. which is the difference Neddy knows he had to increase his velocity to catch Vb, therefore reaons Neddy, B was moving faster than himself.
    The onlypossibility is that Nedy was moving 240 units, B was moving 480 units of absolute velocity with respect to each other.
    I cannot agree here. The assumption about a 'maximum' velocty wrt the universe is a non sequitor and unrelated to the parameters of the current problem. It seems that your assumptions have an inherent bias.
    Your use of the 'if' statements seems biased also. If the objects lose kinetic energy to stellar dust then eventally the object will be at zero velocity wrt the dust mass. I have heard similar arguments when one respondent asked "how do you know the universe is not moving?"

    If the universe is moving as a unit, then what wouldit be moving in relation to? Absolutely nothing.
    Howver, the maximum velocity assumption is totally unjustified from aphysical perspective. Thee s no data or other evdence remotely suggesting the maximum velocity assumption. Likewise, there is nothing in the problem to suggest otherwise.

    Look at it like this. Two space ships meet in space, the same make and model ships. Each navigator knows the spip have a designed maximu veloct of .5c, though usually cruise ay less than .1 c. They meet in a collision course amd each measures the relatuve motion at .2c. Using SR each could assume their own ship at rest and the other moving at .2c. However, each would know the assumption to be arbitrary. Each could also use the average of the relative motion or .2c/2 and come to a closer answer of the absolute velocity of each ship rather than adhere tosome strictly theoretically model imposed by the assumptions of SRT.
    Only if there was something to stip the object. The object in uniform motion will continue moving ntil acted on by an outside force. Thelaws of thermodynamics are not designed to be imposed in such an arrangment described here.

    That the reality of people is of less interst than the theoretical value of measuring the speed of light seems nonsensical. Who cares what the noontime value of the people at some time of the day unless it has some value to know this.

    However, a navigator of space ship in deep space may wish to calculate a useful speed and direction for a heading home and then the absolute motion of the ship becomes critical. Neddy, I've said it before and i will say it again,your response here is designed to deny the existence of abslute zero velocity and added on top of this is the bias that even if zero velocity were achevasble it has no utility, therefore discard the concept intellectually and treurn to the silliness of SRT, which for the current utility valule s worthless except tot hiose getting paid to teach the stuff to others.

    Explain to us Neddy, the concept of speed with respect to absolutely nothing. AE introduced us to the concept, he having never made the measurment and only intellectually designed the concept to fit with the rest of his constructed mental model of sheer insensible silliness.

    AS you struggle to destroy model counter intuitive to the counterintuitve SR model you destroy reason, rationality, observed phenomena, mesurmemnet and above all neddy, SR negates the concept of motion itelf. How so you maywonder? If a moving platform always assumes his motion wrt the motion of light is always C. The only way this can occur is if the motion of the observer is assumed zero. This is the effect of the equivalence principal is it not? Tell me neddy are the platfoerms from which the speed of light measured ever moving equivalently with the vacuum? Ofcopurse thery are not.

    You sai abiove that there is nothing to measure the C velocity against. Sure there is, the A ship. The only possible problem I see here is if the unverse is drifting with respect to the nothingness of the vacuum. Here, Neddy, is where the "ho give a fuck if the uivese is moving wrt to the nothing" comes in? The se of nothing is fundamental to SR as this is the standard of all moton theortically imposed by the postuilate of the speed of l;ight being constant, which means the only way this postulate can survive with any physical reality is by asuming the motion of the platfom to be equivalent to the vacuum when measuring the speed of light.
    Geistkiesel​
     
  10. Neddy Bate Valued Senior Member

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    2,548
    You have misjudged my intentions. I care much more about people than I do about the speed of light.

    The concept of speed with respect to absolutely nothing is the concept of "absolute speed". This is something which I do not subscribe to, but it is something that you have advocated.

    I am not struggling, and I do not wish to destroy anything. When have I ever advocated that people should less reasonable, less rational, disregard observed phenomona, disregard measurement? You are in error once again.

    And by the way, is it any more meaningful to say that the vacuum is moving, than it is to say that the vacuum is not moving?
     
  11. James R Just this guy, you know? Staff Member

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    39,397
    geistkiesel:

    Vac = Va - Vc, by definition.

    The assumption that A is at rest with respect to C is that Vac = 0, not that Va = 0. Because Va is the absolute velocity of A, remember? If you measure the velocity with respect to C, you've measured only Vac, not Va.

    My point is, you haven't succeeded in measuring Va, Vb or Vc. All you ever measure in your scheme is Vac or Vab or Vbc - relative velocities. And from relative velocities alone, you cannot derive absolute velocities. You need to know at least one absolute velocity at the start. And that, you cannot determine in any way.
     
  12. geistkiesel Valued Senior Member

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    2,471

    I have measured the relative velocity of C wrt A assumed at rest. C is moving, remember? Again C is moving, the only Frame of reference in the triad with recognizable motion due to its periodic spurts of acceleration when seekingh the B frame velocity to merge to.Remember?

    How manytimes have I seen a post from you describing the relative motion of inertial frame where one observer assumes her motion is at rest wrt the other nertial frame. Would it make any difference in your evalutaion of the A vs C motion to say the observer on the A frame assumed her motion was at rest wrt C and that she radiated her periodic measurements of the relative motion of Vac to the C frame observer? Not really, as either observer can do this and the result is the same..

    Did you read the part about the differnetial of the absolute velociy terms? This will straighten you ut in the matter. I leave it to your convenience to respond appropriately.
    Prove it, prove this statement. I just did it James R. I did it right in fornt of you so how are you going to prove i didn't do it? You are throwing theoretical darts that just don't stick anymore hoping that at least one will work. This is a reminder of ashotgun defense - throw everything you have at he target and hope for a hit ohn target.

    Prove your statement. You have just utterd a theoretical bit of nothing as far as I am concerned and I am not particilalry concerned. Just prove it or leave the matter alone.

    If the C frame observer is assuming the totality of the changed motion it is measuring while moving away from from the A frame then the expression (corrected by the substitution of the '+' for '-' as the A and C frame are moving anti-parallel) you so clearly stated, Vac = Va + Vc = Vc as Va = 0 being in a state of rest wrt Vc. This is pretty basic James R and as far as the C frame observer knows, he is on the frame that moved away from the A frame. The assumption wrt C of the state of rest of A being continuous, can be verified by a continuous neasurement of Vab the relative motion of the A and B frames remaining uniformly in motion.

    Your problem here James R is you can't handle this matter with SR, nothing personal mind you, as no one can. This better than SR, much better.

    Geistkiesel​
     
  13. geistkiesel Valued Senior Member

    Messages:
    2,471

    Chill Neddy, I was referring to the issue in the context of SR and its effect. The very few exchanges we have done her has made me think nothing but the highest of you as a person , as an intelligence and one caring about that which he speaks. You are actually a welcome lighthouse in this often foggy world of SR dissidency.


    Not really Neddy. If you look closley it was the analysis of the system that led me to conclude the ansolute velocity zero. I didn't start from a theoretical assumption and then produce the state of absolute motion. Big difference Neddy, huge difference.


    I am a poor man and a poor man with words. Neddy, I have never considered you anything but a gentleman and a very intelligent one at that. Everyone lnows that gentlemen do not destroy. When I use the 'you", well that written by a poor tired man's hasty choice of words. Allow me, please , to offer an addendum and substitute "one" for "you".
    I think it is more meaningful to say that the vacuum ain't nuttin' and where I come from 'nuttin' don't move, as far as I have been able to tell.

    Have there been any reports of firey nothing vacua recently, or ever, 'aving crashed and left a deep 'ole in 'er majesty's merry 'ol? {Aye use m'self a Sir Michael Caines drolling Liverpool brough 'ere.}

    Geistkiesel​
     
  14. James R Just this guy, you know? Staff Member

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    39,397
    geistkiesel:

    But your whole aim is to deduce Va, A's absolute velocity. You can't just assume that Va = 0, without proof.

    If you measure the velocity of C wrt A, then you've measured Vca, and

    Vca = Vc - Va.

    You'd have Vca = Vc only if Va was "absolutely" zero, and you don't know a priori that it is. You'd need to show that, and you can't.

    The equation Vca = Vc - Va works regardless of the values of Vc and Va. The only measureable here is Vca, and from that alone you can't deduce Vc or Va.

    That's an assumption like Vac = 0. There's no need to assume Va = 0. Relativity requires no such assumption of "absolute velocity". All velocities in relativity are relative (of the form Vxy); none are absolute (like Vx).

    Right. As soon as you use the term "wrt" you're talking about Vac or Vbc or whatever - one velocity with respect to another. You're not talking about absolutes any more.

    I've already told you, twice. Don't you understand? It is impossible to derive Va, Vb or Vc from measurements of Vab, Vbc and Vac alone. You need to know at least one of Va, Vb or Vc from the start. Unless you can show how we can determine the absolute velocity of at least one object in the universe, you have no way to determine any absolute velocities, anywhere, anytime.

    Can you establish the absolute velocity of one object in our universe, geistkiesel? No, you can't.

    I've completely demolished your argument. Perhaps you need time to digest that fact.

    I repeat: A being in a state of rest with respect to C tells us that Vac = 0. It doesn't tell us what Va is, or what Vc is.

    If C is at rest with respect to A, then C can't simultaneously be moving away from A. That much seems very clear to me. Think about it.

    We're not even talking SR here. This is simple Galilean relativity at this stage. Let's worry about SR after we've sorted out your basic misconceptions of reference frames, shall we?
     
  15. geistkiesel Valued Senior Member

    Messages:
    2,471


    http://ourworld.cs.com/Sandgeist/absvel/AbsoluteMotionPage.html
    You are perceptive JR but slow down man. That is the purpose, but instead of actually lowering the velocity of A I will simulate that reduction using the C frame. Now as we are only discussing the change in velocity, I can assume Va at rest wrt Vc.
    Do not worry I am not assuming Va is an absolute velocity; it is merely a relative velocity wrt C. Now think just in terms of velocity. You can always reorient your thinking and attack the post at any time.

    No JR, not quite. Look at what I am doing here, there is more so don't think you are going to wipe me out here, you aren't, that may come later, but it isn't here, Why not? Because I am only measuring the change in relative velocity. Actually, I am making many instantaneous velocity measurements as the C velocity increases with respect to A. If A were moving a zillion units/sec nothing would change as long as the Va was constant.

    When I took the differential I assumes the Va' = 0 as A is not changing speeds.

    I did it with the same power of assumption any observer in an inertial frame can make. I measure the relative velocity, over a period of time while C is increasing velocity in his effort to catch the B frame velocity.

    Remembers James R, C started at rest wrt A. C started moving away, The C frame assumed the velocity of A was ay rest, with respect C. As C only measures relative velocity so the increases in Vac as C catches the B frame is a legitimate measurement. I can legitimately assume the A frame is at rest wrt C as the A frame makes no change in velocity. A long as the A and B frame are maintaining a constant velocity Vab, which they do, the acceleration of C wrt A is such that as the Vac increases the Vbc decreases. The relative rate of increase and decrease is, wall lets see.
    This is something you discoverd on your own later in the post.

    Vac = Va + Vc. As the relative velocity is increasing we take the differential, hence (V'a) = -(Vc' ) = V'ac. Now as the C frame is changing velocity and the A frame is not, we see immediately that -Vc' = Vac' as V'a = 0 as Va is constant and relative velocity of the C frame wrt the A frame, so the
    Games begin with the values below. James R you are jumping ahead too fast. There are relationships you haven’t acknoledged except to remark "they are all relative velocities.
    Vac = 0 at t < 0
    Vab = Vbc t < 0.

    It will all be clear.


    Here is a collection of Abs90lute velocity links.
    You are perceptive JR but slow down man. That is the purpose, but instead of actually lowering the velocity of A I will simulate that reduction using the C frame. Now as we are only discussing the change in velocity, I can assume Va at rest wrt Vc.

    Do not worry I am not assuming Va is an absolute velocity; it is merely a relative velocity wrt C. Now think just in terms of velocity. You can always reorient your thinking and attack the post at any time.

    No JR, not quite. Look at what I am doing here, there is more so don't think you are going to wipe me out here, you aren't that may come later, but it isn't here, Why not? Because I am only measuring the change in relative velocity. Actually, I am making many instantaneous velocity measurements as the C velocity increases with respect to A. If A were moving a zillion units/sec nothing would change as long as the Va was constant.

    When I took the differential I assumes the Va' = 0 as A is not changing speeds.

    I did it with the same power of assumption any observer in an inertial frame can make. I measure the relative velocity, over a period of time while C is increasing velocity in his effort to catch the B frame velocity.

    Remembers James R, C started at rest wrt A. C started moving away, The C frame assumed the velocity of A was ay rest, with respect C. AS C only measures relative velocity so the increases in Vac as C catches the B frame is a legitimate measurement. I can legitimately assume the A frame is at rest wrt C as the A frame makes no change in velocity. A long as the A and B frame are maintaining a constant velocity Vab, which they do, the acceleration of C wrt A is such that as the Vac increases the Vbc decreases. The relative rate of increase and decrease is, wall lets see.

    Vac = Va + Vc. As the relative velocity is increasing we take the differential, hence (Va) = -(V' ) = (V'ac). Now as the C frame is changing velocity and the A frame is not, we see immediately that -Vc' = Vac' as V'a = 0 as Va is constant and relative velocity of the C frame wrt the A frame, so the
    Games begin with the values below. James R you are jumping ahead too fast. There are relationships you haven’t acknoledged except to remark "they are all relative velocities.
    Vac = 0 at t < 0
    Vab = Vbc t < 0.

    It will all be clear.





    I measure the Vac from the C observer. It is only C that is moving with velocity changes, this is why V'a = 0. V'ac = V’a.



    .
    OK, technically perhaps wrt SR, but whio has mentioned SR? I casn write, even in SR expressions, Va + Vb = Vab. If I am in an SR convention, I should mention thast the Va terms and the Vb term is only representative of the awarness that the relative velcoity of A with respect to B, Vab, is constituted by two velocityies Va and Vb, though here we know nothing of their individual values. WE can assum under some conditions that the Va is measured wrt theembankment as is Vb, though this does not assist the observers on A nd B in the slightest when making measurements.

    This situation is not exactly an SR condition. I didn't assume Vac = zero. C started out moving along side of the A frame and then, at t = 0, C began increasing speed with respect to Va > 0.isn't accelerating, therefore everybody in the universe knows that C is moving with respect to Va at rest., wrt C.


    Right. As soon as you use the term "wrt" you're talking about Vac or Vbc or whatever - one velocity with respect to another. You're not talking about absolutes any more.. I was never talking about absolutes at all, yet.


    JR I haven't made any absolute measurements so far, and I will not! Because the change in Vac as C increases velocity with A not changing velocity, therefore I keep a running account of the measured Vac, using radar or what ever, so far, OK?

    I start with a known Vbc = Vab when C is at rest wrt A, OK?

    Now as C increases in speed are measured an ever decreasing relative velocity Vbc, results and an ever-increasing velocity Vac, ok? This is all we have done so far. Let us make one assumption here that Vab = 720 units. Which means we start with the value, Vbc = 720 at t < 0. This is the relative velocity I will monitor as Vc -> Vb.

    At some point Vc = Vb, correct? BY increasing C velocity C has found the Vc = Vb. Even though we start Vbc = 1000 and end with Vc = Vb. We have more information. B is moving atthe sam speed as C and we know now how fast C is wmoving wrt A.. There have been no arbitrary assumptions regarding Va =0. WE observe the phyical reality here. C is acclerating and doinf all the motion changes, while A remainings at constant speed wrt B.

    You are not the quintessential objective college professor at the moment' I see you getting worked up emotionally, even angry, and I fear that you might become so angry as to let you reason and unbiased analysis become corrupted. So, chill for a moment.



    I haven/t made any arguments for you to demolish. Perhaps you need time to digest that. When I look aback over this post I see you really didn’t grasp what I was going to do. You weren't even aware that C was at absolute rest wrt A as the ships were moving side by side, until C began increasing the relative velocity of Vc wrt Va.

    I ask you, for a straight -forward answer to a straight-forward question.
    If automobile X is unmoving relatively to automobile Y where Vxy = 0, the the cars are moving at the same velocity, what ever that is.

    You and I are side by side, moving nose = nose and you begin to accelerate in spurts and I remain at some constant speed, certainly not zero and you and I both measure the changes in velocity of your car. Let us say you reach a relative velocity of 50 units above that zero relative value we began with. I say to you on my cell phone. "JR, you are going 50 units faster than me, and you naturally agree.

    See we came to an agreement. I swear I just got to this part of the post and I note you have been paying attention.
    When I will say some I will tell you. You are anticipating something I am not going to do JR. So please roll back into your mind the objectivity you require here.





    You are exactly correct. Think about this: At no time have I made any assumptions about any frames absolute velocity. The at rest status of A wrt C is for the changes in velocity only as you deduced above.
    Vac started from a position Vac = 0 to Vac = k > 0. Remember, these are relative velocities so far. But then C begins to move! so Vc is not 0 anymore wrt A.



    WE shall do exactly that. However, from here you a have some correction to make to your thinking to clear up the assumptions you made erroneously.

    JR I have been waiting for someone to dsee what you just said for months. Thank you sir!!!.

    Did I say i was going to deduce the Vc or Va?
    No you did!!.

    I measure the Vac from the C observer. It is only C that is moving with velocity changes, this is why V'a = 0. V'ac = V’ac.



    . JR these are initial values you are comoplaining about. Remember, C begins to move wrt A and wrt B.

    This situation is not exactly an SR condition. I didn't assume Vac = zero. C started out moving along side of the A frame and then, at t = 0, C began increasing speed with respect to Va, Va isn't accelerating, therefore everybody in the universe knows that C is moving with respect to Va at assumed rest., wrt C. because James R, C is changing velocity, A is not!!!.




    I've already told you, twice. Don't you understand? It is impossible to derive Va, Vb or Vc from measurements of Vab, Vbc and Vac alone. You need to know at least one of Va, Vb or Vc from the start. Unless you can show how we can determine the absolute velocity of at least one object in the universe, you have no way to determine any absolute velocities, anywhere, anytime.[/quote]
    JR I haven't made any absolute measurements so far, and I will not! Because the change in Vac as C increases velocity with a not changing velocity, therefore I keep a running account of the measured Vac, using radar or what ever, so far, OK?

    I start with a known Vbc = Vab when C is at rest wrt A, OK?

    Now as C increases speed are measures an ever decreasing relative velocity Vbc, and an ever-increasing velocity Vac, ok? This is all we have done so far. Let us make one assumption here that Vab = 720 units. Which means we start with the value, Vbc = 720 units t < 0. This is the relative velocity I will monitor as Vc -> Vb.

    At some point Vc = Vb, correct? BY increasing C velocity C has found the Vc = Vb. Eve though we start Vbc = 720 and end with Vc = Vb. we have more information. B is moving at the same speed of C that has a measured relative velocity wrt A.

    You are not the quintessential objective college professor at the moment' I see you getting worked up emotionally, even angry, and I fear that you might become so angry as to let you reason and unbiased analysis become corrupted. So, chill for a moment.
    But OK. Two phiotons are emitted simultaneousl prom point P. The photons move equal distances in equal durations of time. If the photons are not parallel their mutual uintersectionsw marks the point P, If one of the photons is reflected 180 degreesd after moving adistance ct,. the photon will arrive at P after mioving anrother distance ct,.
    From thepositulates of light we see that the photons are moving in straight-lines (moving uisotropically) and we see the photons moving at a constant speed of light C, wrt P, or equivalently wrt the "vacua". The invariance in all the attibutes of light mentioned guarantee the invarinace in spatial location of P. Even if some EM disturbance perturbs the direction of motion of the light the original point isn't erased, it is just more difficult to locate.

    No ,oviong frame can alter the invariance of P. No observer is granted special staus to perturb the motions of light. Certianly not in a local environment for short bursts oif time.



    I haven/t made any arguments for you to demolish. Perhaps you need time to digest that. When I look aback over this post I see you really didn’t grasp what I was going to do. You weren't even aware that C was at absolute rest wrt A as the ships were moving side by side, until C began increasing the relative velocity of Vc wrt Va. But now it seems we are starting to dovetail in thinking at a very fast pace. You will see what I mean.

    I ask you, for a straight -forward answer to a straight-forward question.
    If automobile X is moving relatively to automobile Y where Vxy = 0, then the cars are moving at the same velocity, what ever that is.

    You and I are side by side, moving nose = nose and you begin to accelerate in spurts and I remain at some constant speed, certainly not zero and you and I both measure the changes in velocity of your car. Let us say you reach a relative velocity of 50 units above that zero relative value we began with. I say to you on my cell phone. "JR, you are going 50 units faster than me, and you naturally agree.

    See we came to an agreement. I swear I just got to this part of the post and I note you have been paying attention. You are totally correct.

    When I make an assumption I will tell you, but we are almost home. You are anticipating something I am not going to do JR. So please roll back into your mind the objectivity you require here. Don't you wonder why i have agreed to with you on two major points where you thought you had me?





    You are exactly correct. Think about this: At no time have I made any assumptions about any frames absolute velocity. The at rest status of A wrt C is for the changes in velocity only as you deduced above.
    Vac started from a position Vac = 0 to Vac = k > 0. Remember, these are relative velocities so far.



    We shall do exactly that. However, from here you a have some correction to make to your thinking to clear up the assumptions you made erroneously. Which are easy to fix. I would just ignore them myself I don't see them coming up again as a protest from either of us.

    JR, old boy, you cannot know how long I have been trying to get people to see what you just said. Hell, I didn;'t even have to talk you into it!!!. You made my day JR, you really made my day.

    I guess you just might be one smart cookie after all. This is amazing, this is great because I really get bored these days, being to be the smartest one in the room,. more often than not, I see though that now my relief has showed up, YESSS!

    Geistkiesel​
     
  16. Neddy Bate Valued Senior Member

    Messages:
    2,548
    Yes, now I see that there is indeed some usefulness in defining a reference frame of approximate absolute rest. It sounds like astronomers have defined a universal coordinate system, out of convenience, which automatically factors out the motion of the earth around the sun, and perhaps other motion factors which affect their data collection.

    However, in theory, it is still possible that the whole system might be co-moving -- a state that would be indistiguishable from being at absolute rest. You might aggue that this notion is gradually reduced to absurdity as we catalog more and more celestial bodies, and I tend to agree. The point is that of all of the celestial bodies observed to be in motion, we cannot know which motion is real and which is relative other than defining it to be so.
     
  17. James R Just this guy, you know? Staff Member

    Messages:
    39,397
    geistkiesel:

    I have trouble making any sense of your last post. You have repeated several sections of my reply. You have quoted statements made by you and attributed them to me, then congratulated yourself.

    Sorting through the mess, you now seem to be backing away from your claim that you can measure an absolute velocity. Now you say you haven't actually measured Va, Vb or Vc at all, but only relative velocities. Which makes me wonder what your point was in the first place.

    Looks to me like we're done here, too.
     
  18. 2inquisitive The Devil is in the details Registered Senior Member

    Messages:
    3,181
    by Neddie Bate:

    "Yes, now I see that there is indeed some usefulness in defining a reference frame of approximate absolute rest. It sounds like astronomers have defined a universal coordinate system, out of convenience, which automatically factors out the motion of the earth around the sun, and perhaps other motion factors which affect their data collection."
    ===============================================================

    Yes, you are correct that astronomers picked an convenient point and frame of reference for their purposes. It corresponds to the barycenter of our solar system.
    Doppler shifts of hundreds of celestial objects were catalogued as reference points
    to use in this frame of reference, also, a kind of universal map. By taking measurements of the changes in Doppler shifts of these reference points, an observer
    traveling through the galaxy could determine his velocity and direction of travel relative
    to the barycenter of our solar system. On smaller scales, this reference frame is also
    used to keep track of velocity and direction of travel within our solar system, used by
    both GPS and NASA's interplanetary missions. Spacecraft don't travel in straight lines
    from the Earth, for instance, so SR is not useful. Newtonian Celestial Mechanics is the
    system used by NASA, as it is less complicated than GR, so I have read.
     
  19. James R Just this guy, you know? Staff Member

    Messages:
    39,397
    Newton is ok for many purposes, but NASA uses the full apparatus of relativity when it needs to.
     
  20. geistkiesel Valued Senior Member

    Messages:
    2,471
    James R, I posted this before I read your response to the last exchange we had. At this instant I don't even know if you have responded. In the spirit of each suggesting the other get his head turned around, and properly situated, this is what I did. My result is consisitent with my previous observation and your observgation also.Hey James R let us talk physics, OK? The figure below describes the full scenario of our previous discussion.

    1. The time period when the A and C frame were at the same velocty, speed and direction, while B is moving in the opposite drection. Here Vac = 0. Vab = Vbc.

    2. Is when C start moving in the direcion of motion of B. C is increasing his velocity wrt A who is not accelerating. Wile C is decreasing his velocity wrt B. This is the observation you pointed out to me.
    The figure shows relative velocity (rv) vs time (t) for Vbc and Vac. .

    Please Register or Log in to view the hidden image!



    And James R, let the numbers do the walking. Originally C and A were side by side, with Va = Vb,ergo them Vac = 0 Then, (frame 2 above), C begins to move in the direction of the B motion, while A makes no changes in velocity. So All during the 2nd frame above,, Vac was increasing as you state, yet all the increase was due to the motion of the C frame, correct? I thinbk you were screaming this at me in the other post.
    All we know is Vab = 720 uniuts, as an example. Now, let uis assume that the measured Vac = 240 units when C catches the velocity of B. The relative velocity of Vbc is zero, and therefore, Vc = Vb.

    Let me ask you,: what veolocity must the A frame add to his velocity in order to match the B velocity? According to the experiment A should add 240 units, which means that B must have been moving at 480 Units. there are a infinite number of posibilities where B couild be moving 240 units faster than A but only one posibility where the relative velocity is 720 units.

    Maybe this was too fast. James R, how much velocity did C acquire wrt A in order to arrive at the identical velocity of B? Our example we said 240 unit were added. C had to increase 240 units to null out the B velocity, corect?


    Wait, wait, wait!!!

    If the combined velovcity of A and B is 720, and and iif B is moving 240 more than A then by simple algebara, Vb = 480.

    Are there any other possible results?

    What if A, from the get go did all the motion changes that C went through here? How many speed units would A have had to increase in order to be moving at the ext speed as B? 240 units. Would A have been able to record a measured total change in velocity it added when chasing the B velocity? Of course, but we don't have to worry about that here, because C did all the changing for A.

    Now A has the information that B is moving 240 units faster than A and that their combined relative velocity is 720. If the combined velocity of B and A is 720 and B is moving 240 more than A then A is moving at 240 units.
    Similarly Vac has been increasing from Vac = 0 at t = 1, to Vac = Vc at t = 3.
    This last statement is unambiguously true. A did not accelerate during the measurement period when Vbc -> 0. So whether I assume that A is at rest wrt C, or whether A is really at rest wrt the changed velocity, A is absolutely at rest wrt to C, both options are legitiment, and coincidentally, both result in the same answer.

    We agreed on an expression last time that Va + Vb = Vab ; we disagreed milldly on the expression for Vac, as I am saying Vac = Va + Vc. Now as this expression tells what the relative velocity of the two frames is measured, and as Va = 0, Vac = Vc. hence, Vac = Vc = Vb. Now substituting,Vc for Vb in Va + Vb = Vab and
    Va = Vab - Vc.
    All velocity components of the inertial frames, A and B, moving relative to each other, have been measured.

    You can respond anyway you choose, but if A was assumed at rest wrt C, the only frame of reference, moving uniformly, or physically .

    The3 only problem I see trhat you have is discovering for yourself the intrinsic errior in the SRT that is manifestly incomplete.
    Geistkiesel​
     
  21. Billy T Use Sugar Cane Alcohol car Fuel Valued Senior Member

    Messages:
    23,198
    Nice pictures.
    I am confused how C, traveling at 240 revelative to A, catches B that has always been travelling at 720 relative to A, but won't stress this point. Perhaps after accelerating away from A, C soon had velocity much greater than 720 and is deaccelerting as it closes on B (C just happen to have 240 relative A when it first came along side B and will soon fall behined B again as B continues with its steady 720 with respect to A. That is, C must speed up again and not brake so hard next try. After a few tries, I will grant that C can be along side B and both could be traveling at 720 units with respect to A. (I sure would not want C's pilot to be in charge of a moon landing!)

    Please Register or Log in to view the hidden image!



    None of the above bothers me (or seems to have any point, other than serving as some sort of smoke screen of details.) What bothers me is your:
    "and therefore, Vc = Vb"
    This is an assumed/ inferred/ implied statement of what you are trying to demonstrate, namely that absolute velocities and rest frames exist. An unbiased, (non-presumptive of the existance of absolute velocity) statement would be:
    "and therefore, Vcz = Vbz, where z is any inertial frame one wants to chose."

    You can not prove the existance of the absolute velocity or absolute rest reference frame, by notational slight of hand (droping the "z" in my restatement of your "and therefore....." statement.

    Your version (without the z) does not acknowledge that it is true with respect to any inertial frame. By omitting the z, you are trying to infer that Vc and Vb have some unique meaning. When I ask you what is their numerical value (using the set of numerical values you have assumed) you are not able to state it.

    I can state that there exists some frame, z where it is 10, another frame z' where it is 1000 or yet another frame, z,'' where it is zero, but this does not prove that any of these frames (z, z' or z'') are at absolute rest. The only thing that even vaguely suggest that such a frame exists is your slight of hand notation trick that drops the z. That implies, by notation alone, not physics, that the velocities of C and B are knowable in some not yet shown, frame that is at absolute rest. Slight of hand notational omission of the z, is not a proof of this. If you disagree, please tell me the value of either Vc or Vb in your example.
     
    Last edited by a moderator: Jun 7, 2005
  22. geistkiesel Valued Senior Member

    Messages:
    2,471
    Billy T, slow down. C is originally movihg at the same velocity as A. C must accelerate in order to match the velocity of B, which is moving relative to C. It isn'ta question of which frame A or B is at rest with respect tothe other, the issue is how much velocity is A and B both contributing to the relative, total velocity of the A and B frames moving relative to each other. Or how much faster is B moving wrt A, such that their relative velocity is 720 units?
    You didn't read the post closely enough. I said very clearly that C began moving in the direction of the B frame and increased the C speed while measuring the relative velocity of C wrt B. C is continuously increasing its speed, making a relative velocity measuremnet wrt B. At some instant C has increased its own speed such that the Vbc = 0. Now if the relative velocity of C wrt B is zero, then by definition |Vc| = |Vb|.​

    With what ever common frame you wish to pick, the velocity of C wrt B is identical, from measurement, as opposed to theory. I'll pick a reference frame VabsoluteFrame = 0.​

    C starts out at some unknown speed equal to A. It then increases its speed in the driection of B. As C measures the relative velocity of C wrt b that relative velocity decreases, while the relative velocity of C wrt A increases. Remember we assumed C was moving wrt A that was at rest wrt C. This is reasonable as it was only C that is changing speed here.​

    Yes the velocity of C avd B are knowable as the Vab is known.
    Originally the Vbc = Vab as C was at rest wrt A. Then C began moving and measuring the realtive velocit of C wrt B. TThis relative velocity decreased until the moment when Vcb = o. This result in any of your z notation means that C and B are moving at the same speed. Do you find this confusing or difficult to understand?
    All the time that C was measuring the Vbc relative velocity, C was also measuring the Vac relative velocity that started from Vac = 0 when C was moving exactly alongside the A frame.

    OK, you want the exact velocity of B and C. When we measure the relative velocity of Vbc, this means that the velocity of B wrt C, correct? This is the meaning of relative velocity. Now, with respect to A, C was moving at some measured 240 units, which is just a number I picked out out the air as an example. So if it took C 240 units to increase the C speed in order to physically achieve the B absolute velocity, then A must necessarily have been moving 240 units slower than B. The total velocity of B and A combined or, in other words, the relative velocity is 720,

    Now ask, what is the velocity of B wrt A, if A is moving 240 less than B and the combined relative velocity of A and B is 720? The velocity of B then is Vb = Vab - Va or 720 - 240 = 480.

    There may be slight of hand here, but not quite as you saw it, or maybe as you saw it. I confess my continuing error in substituting Va for Vb in the Vab expreassion did not offer much in the clarifying the experiment here.

    OK Billy T I was remiss in the previous posts. When I said that Vc = Vb and then substituted into the Va + Vb = Vab expression, I used the wrong reference point. It is Va that should be substituted for, not Vb. If B is moving faster than A by an amount of 240 units it should be then , Va + Vb = Vab, Vb = Vab Then Vb = Vab - Va = 729 - 249 = 480.
    We do know that A could not have been at rest wrt B and still be "moving " 240 less than B as the relative velocity of A and B is 720.
    Thanks for pointing out my oversight.

    Bottom line: The measurement of the relative velocity of C wrt A at the instant the measured Vc= Vb, is the measurement of the velocity A must increase in order to be at the same velocity as B -- B is moving 240 faster than A is the interpretation of the measurement. The combined velocity of of A wrt B is 720.
    If you can find another frame such that the combined velocity of A wrt B (both moving anti-parallel) sums to 720 where B is moving 240 units faster than A then please demonstrate what yiu mean, OK?
    Geistkiesel​
     
    Last edited: Jun 8, 2005
  23. James R Just this guy, you know? Staff Member

    Messages:
    39,397
    geistkiesel:

    Yes.

    Vbc = Vb - Vc,

    assuming that the absolute velocities Vb and Vc exist. Your aim is to determine Vb or Vc (or Va), the absolute velocities.

    In maths, this statement is: Vca = 240.

    So, Vca = Vc - Va = 240.

    Your aim is to find Vc or Va, remember.

    In maths, this means:

    Vab = -240 = Va - Vb, or (equivalently) Va = Vb - 240.

    You still need to determine Va or Vb.

    What do you mean by "the total velocity of B and A combined"? Do you mean

    Va + Vb?

    Since we don't know Va or Vb yet, you can't give a value for this.

    All we know so far is

    Vc - Va = 240
    Va = Vb - 240

    From that, we can deduce things like Vc - (Vb - 240) = 240, which means Vc - Vb = 0 = Vcb, but again, this doesn't help us determine Vc or Vb or Va.

    You're going around in circles and getting nowhere.
     

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