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Speed test in SR

  1. Aug 31, 2006 #1
    SR tells us that at the speed of light, time stops, your length contracts to nothing, and your resistance to acceleration becomes infinite requiring infinite energy... according to an stationary observer.

    But! How do we know that observer is stationary, and it is not him who requires infinite energy?

    Einstein did not like the ether because if you could travel the speed of light then you could measure EM radiation as simply E or M feilds at constant magnitude. Thus, his first postulate is that the laws of physics are the same for all intertial frames. However, even in SR, at the speed of light, couldn't you tell how fast you are moving based on your resistance to acceleration. If you require infinate energy to change speed, perhaps you are going the speed of light. (or, more realistically, if you require a great deal of energy to change speed, perhaps you are moving very near to the speed of light).

  2. jcsd
  3. Aug 31, 2006 #2
    Not quite, you are reading some bad books. Though it becomes more difficult to gain speed as you approach c, paradoxically it is impossible to tell from inside your system that you are approaching c. The mathematical reason is shown here:

    Last edited: Aug 31, 2006
  4. Sep 3, 2006 #3
    The reason one can't "tell" they are approaching c is because of direct application of SR's first postulate (the equivalence of inertial frames, physics looks the same in all of them). There is no mathematical reason to show here as it is a definition from a postulate. The velocity addition formulas you linked show none of this reasoning and so don't really answer the question.
  5. Sep 3, 2006 #4


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    You are close, but still missing the point. "Approaching c" is a meaningless phrase. You may have a speed of .99c with respect to some other observer and a speed of .0001c with respect to still another. There are always infiitely many observer speeds possible for you to relate to. Futhermore you are allowed to attribute the speed, whatever it is, to them, rather than to yourself. So it's quite reasonable that you are allowed to consider yourself at rest. This is nothing but a conclusion from the Galilean relativity which you cite. ANY steady speed is undetectable, not just .999999c.
  6. Sep 3, 2006 #5
    Yes, of course. I was responding to the wording of clj4. I put the "" around "tell" instead of "aproaching c" because I was focussing more on why you couldn't measure your speed (ie why you don't have an absolute speed).
  7. Sep 3, 2006 #6
    I recommend that you read the complete post, it simply refers to "pervect's" approach. It is a mathematical refinement on "pervect's'" approach that avoids use of "relativistic mass" , that's all, so if you have any philosophical complaints, please take them up with "pervect" . You also seem to be missing the point of the discussion: why can't a rocket accelerate continously wrt its launch pad (the only frame of reference in the exercise) attain c? It is a simple exercise, you should have no trouble solving it.
    Last edited: Sep 3, 2006
  8. Sep 3, 2006 #7


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    I'm not quite sure which post of mine is supposed to be the issue here :-).

    Anyway, I will say that posts like the OP's post are one reason I don't like the concept of relativistic mass. There is not any way to tell one's velocity by how fast one is moving. If relativistic mass appears to conflict with this idea, my strong suggestion to anyone thinking this is that one forgets about relativistic mass altogether, and sticks with invariant mass.

    There is nothing about the concept of relativistic mass, if applied correctly, that should lead one to the idea that one can measure one's absolute speed. The fact that it confuses some people in this respect is just one of many reasons I don't particularly care for the concept.

    If one is confused in this manner, I suggest considering the situation from the POV of invariant mass. If one is not familiar with invariant mass, I suggest that one learn about it, and then consider the situation from the POV of invariant mass. What I don't suggest is writing endlesss confused posts. (OK, the posts by a single individual are probably NOT endless, but taken as a group, it seems like an unending sea of confusion :-().
  9. Sep 4, 2006 #8
    None really, apparently clj4 misread the poster's question and thought he was asking why someone couldn't attain light speed. This wasn't what the poster was asking.

    Even after pointing out your misreading, it still seems you are misreading the question. Oh well, no harm done.

    So, to get this back on topic:
    As the thread title "Speed Test" suggests, and in the original poster's own words, he wants to know "couldn't you tell how fast you are moving based on your resistance to acceleration"?

    I'm not sure if relativistic mass was really the issue here. The poster seemed to think that because it takes more and more energy to increase the speed of an object as v approaches c, that somehow the object could measure this in its own rest frame. This of course is not possible.

    Silverdiesel, if it was indeed the notion of relativistic mass that lead you to thinking this, it probably would be best to just forget about relativistic mass altogether (as pervect suggested).

    Correct and direct from SR's first postulate.
    I think that is the main point here.
  10. Sep 4, 2006 #9
    Wouldn't it be simpler and more straightforward to ask silverdiesel what he was asking?
  11. Sep 5, 2006 #10
    You made a mistake. Oh well, no harm done. I don't understand why you are defensive and feel it necessary to insist you never misread the post.

    As the thread title "Speed Test" suggests, and in the original poster's own words, he wants to know "couldn't you tell how fast you are moving based on your resistance to acceleration"? As explained, no speed test is possible because all inertial frames are equivalent. There is no such thing as an absolute velocity according to relativity.

    Silverdiesel, if I too am misreading your post, please feel free to be more specific so we can try to answer your question.
  12. Sep 5, 2006 #11
    Why not let him speak? Why do you always have to put words in people's mouths? More than half of the time you get it wrong anyways.
    Last edited: Sep 5, 2006
  13. Sep 5, 2006 #12
    No one is preventing him from speaking. And to the contrary, people are asking him to clarify if necessary.

    I am not putting words in people's mouths. I am quoting the poster's actual words:

    As the thread title "Speed Test" suggests, and in the original poster's own words, he wants to know "couldn't you tell how fast you are moving based on your resistance to acceleration"?

    I don't understand why you become so defensive and start vaguely stating that I am wrong more than half the time anyway. Resorting to ad hominem attacks over a simple misreading on your part? This is just silly.
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