What is a reference frame?

Discussion in 'Physics & Math' started by James R, Oct 13, 2004.

  1. James R Just this guy, you know? Staff Member

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    39,397
    MacM,

    *sigh*

    It seems everything needs to be explained to you in precise detail, and even then you usually end up not understanding. But, I'm reasonably patient, so I'll give it a go.

    Your problem here is that you aren't using terms such as "acceleration" and "inertial" in their technical senses, whereas I am.

    We are considering a situation where a train accelerates from rest to some constant speed along a railway track.

    In a reference frame in which the tracks are stationary, the train is observed to accelerate. In a reference frame in which the train is stationary, the tracks accelerate. It is important to use the proper definition of acceleration. Acceleration is simply the second derivative of the displacement with respect to time. Therefore, in the train frame, it is not arguable that the tracks do not accelerate. The tracks are displaced over a period of time, and this occurs in such a way that the mathematically defined acceleration is non-zero in this frame.

    That's one issue. You want to mix this up with notions of inertial and non-inertial reference frames, which is a separate issue completely.

    Both observers agree that the track frame is an inertial frame, while the train frame is a non-inertial frame. An inertial frame is one in which all the laws of physics hold in their unaltered forms, which is not true in non-inertial frames.

    An observer on the train "feels" the acceleration. He feels like he is pushed towards the back of the train. That is due to the inertial force which arises in his non-inertial reference frame. This inertial force does not exist in the track reference frame.

    You asked what force makes the tracks accelerate, from the train's point of view. The answer is: partly the force of the train's engine pushing the tracks backwards, and partly the inertial force on the tracks due to the non-inertial frame of reference.

    In light of this explanation, let's look at what you wrote.

    I hope you now see why this is wrong. In the train frame, the track "actually" accelerates, just as in the Earth frame, the train "actually" accelerates. That is just a matter of knowing what acceleration is. Your vague statement about "forces of acceleration" would in fact refer to the inertial forces discussed above, if you understood the concept, but instead you mix up concepts.

    I hope it is clear to you why this is wrong. Clearly, you misinterpreted what I said. I was careful; you were sloppy. Hence your mistake.

    I hope that, after my clear and careful explanation, we no longer have an argument. On the other hand, with you nothing can be taken for granted.

    You continually make assertions that I contradict myself, but you can never actually point out any post where that happens. Funny, that.
     
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  3. MacM Registered Senior Member

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    I'll not debate symantics but only state:

    1 - I was trying to be very clear about the differance between the view of accelertion of the tracks by the train vs actual acceleration.

    2 - You have said nothing here that I didn't know; accept that you can so casually mix acceleration into an inertial frame and not cause confusion.

    3 - I responded as I did because you infact had corrected me in the past for having said A see B accelerate and B therefore sees A accelerate. You very specifically said I was wrong and was showing that I did not understand physics, that in the one frame it wasn't accelerating. That there was no force of accelertion.

    4 - I can only ask that you stick with a consistant description of these terms. I am making a note of this ost for future reference since you have a habiot of forcing me to go back and spend time looking for statments that have been made. I am currently searching back over threads 1 1/2 years old to post where ou commented on it being imperative that I know calculsu to understand Relativity.

    Perhaps that is part of your plan to keep me occuied but I will not dedicated full time I will continue to post. So that will not gain you much. In fact it is going to backfire since I will find it and I will post it.

    ********************************************
    Well here is for starters. I'll be posting more as I find them.

    [post=293004]James R., Invokes Absolute Universal Space-Time[/post]


    [post=674627]James R Tells MacM His Test is Void Because there is No Absolute Universal Time[/post]


    *******************************************
     
    Last edited: Oct 25, 2004
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  5. James R Just this guy, you know? Staff Member

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    That is false. Link to the relevant post, if it exists.

    Good. Maybe you won't misrepresent what I've said next time.

    You shouldn't make claims you can't support.
     
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  7. Pete It's not rocket surgery Registered Senior Member

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    Are you sure you have the right post?
    James neither stated nor implied anything about the nature of time in that post.

    It seems that you *think* this implies absolute time.

    Can you explain why?
     
  8. MacM Registered Senior Member

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    10,104
    See above. You will be corrected this time. You have done this to many times to let it go.
     
  9. one_raven God is a Chinese Whisper Valued Senior Member

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    13,433
    James,
    Thanks for that description.
    It has cleared up a few things in my mind, visually.
    I appreciate it.

    geistkiesel,
    Believe me, I have problems with Relativity, just as you do.
    I am not sold on it's reflection of reality, but this particular aspect has no other objective way to look at it.

    The Earth is not anchored to anything.
    The reason there is no preferred refernce frame, is because we have nothing to consider "at rest" to measure relative movement to.
    In the view you presented above, we would have to accept that the Earth is stationary, and the Sun and universe revolves around IT.
    Einstein (nor James) was contending that the train is doing the work to move the Earth.
    Relativity simply has to be true regarless of where you are standing, in order for it to be a valid theory.
    If you are thinking like I was, you are now wondering if that means that Einstein skewed things to MAKE relativity valid.
    Not quite.
    In order for ANY theory that is designed to quantify movement universally it would have to do one of two things:
    1.) Depict a preferred reference frame and assume it is at absolute rest (you already see the problems inherent in that, the universe not only does not revolve around the Earth, but it would be a helluva diffcult task to determine an orbit trajectory for a probe taking off on Mars, by taking the revolution of the entire universe around Earth into account as a preferred refernce).
    2.) Assume that nothing is at rest since we do not have anything to measure as a backdrop. In other words EVERYTHING is in motion relative to one another. If space DID have some sort of static infinitely rigid fabric (such as reference frame 1 in James' above explanation) to consider at absolute rest, AND we had a way of observing that fabric this wouldn't be a problem.

    Since we know the Earth orbits the Sun, let's make believe we view the Sun as the prefrred frame of reference...
    When your car is driving down the road at 60 MPH, if your car is viewed from the Sun and the Earth were invisible it would look as if it is hurtling through space in loop-the-loop fashion with the center of it's orbit moving at over 12,000 MPH (no, I am not sure of the actual number, but it really doesn't matter) in a big eliptic circle.

    From the Sun's perspective, a geocentric preferred reference frame seems like a pretty silly thing, doesn't it?
    That is all relativity is saying.

    (someone please correct me if I got any of this incorrect)
     
  10. MacM Registered Senior Member

    Messages:
    10,104
    I would like to see a different explanation other than a universal time for a defintion called "Global Universal Space-Time".

    He is telling me my view requires a Universal Time and that it does not exist.
     
  11. Pete It's not rocket surgery Registered Senior Member

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    10,167
    Ummm... where was that post again?
     
  12. James R Just this guy, you know? Staff Member

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    39,397
    Sorry, MacM, but taking quotes out of context doesn't cut it.

    I guess I'll have to explain this one to you as well. I wrote:

    Note that I said "global (universal) spacetime" and not, as you assert "global (universal) time".

    Note also the word "universal" in brackets, which is there to make it clear that I was talking about the spacetime of the universe as a whole, as distinct from spacetime in a particular local region.

    Your claim that I have somehow agreed with your silly notion that some absolute standard of time exists throughout the universe, is spurious.

    Try again.
     
  13. Pete It's not rocket surgery Registered Senior Member

    Messages:
    10,167
    ???

    Space-time is not Time, Mac.

    For example, it is easily demonstrated that in SR, the space-time interval between two events is absolute, but the time interval is not.


    "Universal space-time" does not imply universal time. That's crazy talk.
     
  14. 2inquisitive The Devil is in the details Registered Senior Member

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    3,181
    Whom is accelerating is easily determined. Remember the solution to the Twins Paradox?
     
  15. MacM Registered Senior Member

    Messages:
    10,104
    I actually don't disagree here. But my point would be his repeated nit picking of my use of terms and admonitions to be precise and yet he also lacks clarity many times.

    He should have simple referred to the expansion of space-time not causing the affect but instead he throws in the reference to "Global" universal space-time.

    Now seperating time out of universal space-time and saying it doesn't exist is nothing more than a play on words. Space-Time are irrevocably linked in Relativity. We haven't been discussing it in time dilation but associsted with the relavtive velocity is the Lorentz contraction issue as well. Time dilation is linked to the same "Global Space-Time" to which he refers.
     
  16. MacM Registered Senior Member

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    10,104
    I will. But first understand I did not assume you believed in UT but your post unnecessarily refers to "global (universal) space-time and you should have forgone the reference and simply stated the expansion of space time had no mass affect.

    I'm still looking for the direct quote regarding your admonition with respect to acceleration.

    Also, I do not see this as taken out of context. I did not crop any information which altered your statement. That paragraph stands alone in its full meaning. Another innuendo by you. That I have quoted you out of context when I have not.

    I have corrected my link which inadvertantly dropped "Space-"
     
    Last edited: Oct 25, 2004
  17. MacM Registered Senior Member

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    10,104
    I agree but James R switched horses here and now says the train and track both accelerate.
     
  18. James R Just this guy, you know? Staff Member

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    Talk is cheap. Prove it.
     
  19. Pete It's not rocket surgery Registered Senior Member

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    10,167
    I thought the sentence was perfectly understandable. The distinction between local spacetime and global spacetime is an important one. If you didn't understand, you should have said so.

    There's no play on words here, Mac.

    If Space and Time were individual entities, absolutes in their own right, there would be no need to combine them into space-time. That's the whole point of space-time. It is required by the fact that space and time are two elements of the same entity. Space-time is the underlying entity that manifests itself as space and time.

    A space-time interval is absolute (not frame dependent) - all observers will agree on the space-time separation between two events. But the way that interval manifests as space and time is frame dependent - the distance and time between the events varies with perspective (reference frame).
     
  20. Pete It's not rocket surgery Registered Senior Member

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    10,167
    There's really no switching horses here Mac...

    I understand that you're upset that think you've received mixed messages from James, but you're doing yourself a disservice by whining about it and blaming James's communication.

    This is an opportunity to learn. When you receive a mixed message from someone, I think it's better to ask for clarification than to try and leverage it as some point-scoring exercise.

    Why don't you look at the issue again, and try to reach some common ground with James. Here's some questions to think about:
    What does "accelerate" mean?
    What does "inertial frame" mean?
    What does acceleration in an inertial frame feel like? Why?
    What does acceleration in a non-inertial frame feel like? Why?
    What does acceleration in an inertial frame feel like? Why?
    What does being stationary in an inertial frame feel like? Why?
    What does being stationary in a non-inertial frame feel like? Why?


    Which of the preceding situations are interchangeable?
    Why would you choose to consider yourself in one situation rather than another, if the two are interchangeable?
     
  21. Pete It's not rocket surgery Registered Senior Member

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    10,167
    You're thinking of the Special Relativity solution. Strictly speaking, SR deals only with inertial reference frames, which means that we can't use SR to treat the stay-at-home twin as accelerating.

    But there are solutions to the Twin paradox in which the stay-at-home twin accelerates, and the other does not. This requires the use of non-inertial frames, meaning that General Relativity is needed to fully analyse it.

    In practical terms, we used non-inertial frames all the time. For instance, as I'm sitting in my chair, I'm in a non-inertial reference frame. I maintain that I'm stationary in this frame, and that the pencil falling off my desk is accelerating, being pulled by gravity. In the non-inertial frame of the pencil in free fall, there is no gravitational force, and I'm the one accelerating - I'm being pushed up by the chair seat against my backside, hard enough to accelerate me at 10m/s/s.

    Either point of view is fine, and can be used to produce the same results for any practical question.

    Here is a great resource for Twin Paradox explanations, and show why all the explanations amount to the same thing:
    Relativity FAQ - the broken vase
     
    Last edited: Oct 26, 2004
  22. Pete It's not rocket surgery Registered Senior Member

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    10,167
    This might be the heart of the disagreement - a simple terminology difference.

    Mac agrees that the train sees the tracks change velocity.
    The concept Mac is articulating is exactly the same concept James is articulating when he says that the train sees the tracks accelerate.

    Mac also notes that the tracks do not experience a real force to effect this change.
    The concept Mac is articulating is exactly the same concept James is articulating when he says that the track's reference frame is inertial.


    Mac,
    James is agreeing with you in concept, but correcting your terminology.
    James is using the word "accelerate" to mean what you agreed is true - that for the train, the tracks change velocity.
    James is also pointing out that the term "inertial frame" is generally used to articulate the other fact you mentioned - that the tracks feel no objective force.
     
  23. one_raven God is a Chinese Whisper Valued Senior Member

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    13,433
    Can someone please explain the difference between an inertial frame and a non-inertial frame?

    Thanks
     

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