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  1. #1 common paradoxes in physics 
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    Greetings forum members.


    As we head into Easter, I was hoping to get a briefing on the common and latest paradoxes of physics theory......something for me to ponder while enduring the madness of children and their school holiday easter break.

    I am not looking for any long winded paradoxes, just a few short statements I can perhaps follow-up on the internet with.

    Cheers for any assistance.


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  3. #2 Qm and Relitivity 
    Forum Freshman vistotutti's Avatar
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    I'm not sure if it qualifies as a paradox, but the current Quantum theory that describes the very small, and Einsteinian Relativity, which describes the very large are incompatible.
    Both theories work very well, but, to date, they seem to contradict one another.

    There is a Nobel prise waiting for someone who can reconcile the situation.


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  4. #3  
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    Try and think about quantum entanglement and time dilation. Should keep you stimulated.

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    There are two major kinds of paradoxes. The first is where the result is entirely unexpected, but really only violates common sense rather than actually being contradictory. A common example is the birthday paradox. In a room with 23 people, there's a greater than 50% chance someone shares a birthday. These aren't really paradoxes.
    The second kind is one where the results actually somehow seem to contradict themselves. If a barber shaves all the men that do not shave themselves, does he shave himself? Usually these arise out of some form of misunderstanding or abuse of something.
    Known physics has plenty of the first kind, but none of the second. (I say known physics, because the grandfather paradox may be either type in the end, but at the moment, we don't even know if it's possible to test.)
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  6. #5 Where was the big bang? 
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    We think the universe is 13-14 billion years old. With our current technology we can see almost that far back in time. Thats a lot of stars and galaxies all expanding outward from a common point. Using basic geometry, shouldnt we be able to trace the path of things back to where it all began?
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  7. #6  
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    Nope. Unfortunately, after measuring everything, everything is moving away from everything else. There is no center. When you run everything backwards, everything came from everywhere, which is a lot bigger now than it was then.
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  8. #7 Re: Where was the big bang? 
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    Quote Originally Posted by Maldwyn
    We think the universe is 13-14 billion years old. With our current technology we can see almost that far back in time. Thats a lot of stars and galaxies all expanding outward from a common point. Using basic geometry, shouldnt we be able to trace the path of things back to where it all began?
    We know where it all began: everywhere.
    Everywhere was just a lot smaller back then.
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  9. #8 Re: Qm and Relitivity 
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    Quote Originally Posted by vistotutti
    I'm not sure if it qualifies as a paradox, but the current Quantum theory that describes the very small, and Einsteinian Relativity, which describes the very large are incompatible.
    Both theories work very well, but, to date, they seem to contradict one another.

    There is a Nobel prise waiting for someone who can reconcile the situation.
    In what way are they incompatible? Can you elaborate a bit more please ?
    The hand of time rested on the half-hour mark, and all along that old front line of the English there came a whistling and a crying. The men of the first wave climbed up the parapets, in tumult, darkness, and the presence of death, and having done with all pleasant things, advanced across No Man's Land to begin the Battle of the Somme. - Poet John Masefield.

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  10. #9 Re: Qm and Relitivity 
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    Quote Originally Posted by leohopkins
    Quote Originally Posted by vistotutti
    I'm not sure if it qualifies as a paradox, but the current Quantum theory that describes the very small, and Einsteinian Relativity, which describes the very large are incompatible.
    Both theories work very well, but, to date, they seem to contradict one another.

    There is a Nobel prise waiting for someone who can reconcile the situation.
    In what way are they incompatible? Can you elaborate a bit more please ?
    At the quantum scale, things get very "choppy". There are sudden discrete changes in energy. In GR space time curvature is sensitive to energy. So when you try to apply GR at the Quantum scale space time curvature becomes very erratic, which leads to sudden large erratic changes in gravity. IOW, GR only works at small scales if you assume a smooth space time curvature all the way down, But QM predicts that it will not be smooth at that scale.
    "Men are apt to mistake the strength of their feelings for the strength of their argument.
    The heated mind resents the chill touch & relentless scrutiny of logic"-W.E. Gladstone


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  11. #10 Re: Qm and Relitivity 
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    Quote Originally Posted by vistotutti
    I'm not sure if it qualifies as a paradox, but the current Quantum theory that describes the very small, and Einsteinian Relativity, which describes the very large are incompatible.
    Both theories work very well, but, to date, they seem to contradict one another.

    There is a Nobel prise waiting for someone who can reconcile the situation.

    So, quantum theory that describes the very small is incompatible with with Einstein relativity which describes the very large. They contradict one another. Can I ask "how" they contradict one another, and why you think ideally they shouldn't? Maybe perhaps we are not getting the full picture of both quantum theory and einstein relativity that would otherwise allow us to form a similar appreciation for both at either end of the space-time scale of magnitude. Are there features therefore of quantum theory and einstein relativity that are not complete to our understanding?
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  12. #11 Re: Qm and Relitivity 
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    Quote Originally Posted by theQuestIsNotOver
    Quote Originally Posted by vistotutti
    I'm not sure if it qualifies as a paradox, but the current Quantum theory that describes the very small, and Einsteinian Relativity, which describes the very large are incompatible.
    Both theories work very well, but, to date, they seem to contradict one another.

    There is a Nobel prise waiting for someone who can reconcile the situation.

    So, quantum theory that describes the very small is incompatible with with Einstein relativity which describes the very large. They contradict one another. Can I ask "how" they contradict one another, and why you think ideally they shouldn't? Maybe perhaps we are not getting the full picture of both quantum theory and einstein relativity that would otherwise allow us to form a similar appreciation for both at either end of the space-time scale of magnitude. Are there features therefore of quantum theory and einstein relativity that are not complete to our understanding?
    There is no paradox there, just an incompatibility. They do not really contradict one another but rather are different in character. There is no current viable quantum theory of gravity. General relativity is a classical, deterministic theory which, given accurate initial and boundary conditions provides a precise prediction of the outcome. Quantum theory is stochastic and predicts only probabilities.

    It is also not correct to say that general relativity only predicts the very large and quantum theory the very small. In principle they each apply at all scales. In practice general relativity only seems to be important for large bodies at large scales and quantum effects important at small scales for small bodies. But there are situations, as near the singularity of a black hole or in the earliest times following the big bang when both effects appear to be important, and in those situations our current understanding of physical theory is simply inadequate.

    It is not that the theories provide different and contradictory predictions for the same phenomena.

    Our understanding is not complete. It may never be complete. There is research in progress to try to develop a unified theory that encompasses all of the fundamental theories of physics -- general relativity, quantum chromodynamics and the electroweak theory -- in a single consistent package. Thus far that research has not produced the requisite theory. If you have heard of string theory, or its successor M Theory, then you have heard of one research path being pursued to attempt to developa unified "theory of everything". So far, no brass ring.
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  13. #12  
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    Thank you for your reply. It is quite accurate.

    Just continuing from what you say about future developments of research and theory, what do you think will come first, the "theory itself of all space-time laws (et al)" or the research results prompting a theory itself of all space-time laws (et al). I guess I am asking, "is it possible to theorise what we already perceive of reality, or do we need to research reality first in order to properly theorise it". I know it sounds an absurd question, but the seriousness of it stems from the idea of possibly theorising a theory based on our "absolute" ability to be logical/theoretical/reasoning (that is, if we have a limit to being humanly logical in alliance to our absolute ability to perceive reality as humans).

    (late edit)

    I guess what I am asking is whether after all the research we do into space-time, using all the measurement devices of mathematics we use for measuring our ideas of space and time in developing theories of reality, that someone can come along with a new theory of space and time that via pure thoery explains how all the different features of currently known laws of space and time can be understood in a holistic context. For instance, what if someone came along with a theory that made use of hyper/sub dimensions of space and time, subset dimensions of space and time that pieced together our three dimensions space and one dimensional time theoretical reality as we know it better than the siongula-time 3-d space theory matrix can.
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  14. #13  
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    Quote Originally Posted by theQuestIsNotOver
    Thank you for your reply. It is quite accurate.

    Just continuing from what you say about future developments of research and theory, what do you think will come first, the "theory itself of all space-time laws (et al)" or the research results prompting a theory itself of all space-time laws (et al). I guess I am asking, "is it possible to theorise what we already perceive of reality, or do we need to research reality first in order to properly theorise it". I know it sounds an absurd question, but the seriousness of it stems from the idea of possibly theorising a theory based on our "absolute" ability to be logical/theoretical/reasoning (that is, if we have a limit to being humanly logical in alliance to our absolute ability to perceive reality as humans).
    I don't follow you.

    As I see it there are two potential paths for deeper understanding of fundamental physics.

    One is the formulation a basic theory that unifies gravitation and the quantum field theories (electroweak and the strong force). That would be what is called a "theory of everything". There is a lot of work going on this area, mostly not making a lot of progress. I am a bit skeptical that the currently fashionable string theory cum M theory will ever result in a real physical theory. There are a lot of conjectures, of a truly fundamental nature, that have been assumed to be true but have not been proved. The foundations are rather shaky. But if it does succeed then the reductionist program will have found its Holy Grail. I do support a general reductinist approach.

    The other potential path is not so clearly reductionist. There is some rather responsible speculation that a good deal of physical law may be the result of what are called "emergent phenomena", that is phenomena that only come to light when many entities are involved. The notion of entropy for instance only makes sense for many-body systems. There is some, not a lot, work going on to understand this approach as well.

    My personal opinion is that there ought to be a lot more work oriented at filling some rather large holes in our understanding of what we have now. The quantum field theories rely heavily on a process called 'renormalization" that is critical. It is also ad hoc and not well defined mathematically. In other words there are rules for performing the arithmetic, but there is no firm mathematical description that provides any real understanding of what is going on. Roughly speaking what one does is perform a perturbatin calculation, discard some of the terms, and find that the remaining terms provide a very accurate prediction. The kicker is that the discrded terms are infinite. It is also the case that M theory has never been precisely defined, The biggest open problem is M theory is actually to clearly define what M theory is. Similarly the so-called AdS/CFT correspondence of the string theorists is at this point only a conjecture. It needs eithe proof or refutation. It is time to step back and make sense of some of the theories that we have that are on shaky foundations.

    But the bottom line is that I really don't know what will be next, Neither does anyone else. That is what makes research exciting.
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    (I thought you might not follow me, which is why I added the late edit, just, as it seems, you made your reply, so here it is again



    I guess what I am asking is whether after all the research we do into space-time, using all the measurement devices of mathematics we use for measuring our ideas of space and time in developing theories of reality, that someone can come along with a new theory of space and time that via pure thoery explains how all the different features of currently known laws of space and time can be understood in a holistic context (yet as pure theory, no research). For instance, what if someone came along with a theory that made use of hyper/sub dimensions of space and time, subset dimensions of space and time that pieced together our three dimensions space and one dimensional time theoretical reality as we know it better than the singular-time 3-d space theory matrix that we use can. What if that theory was based on our ultimate human biological ability to use our nueronal matrix to be logic/reasoning, as it would need to be, for how could we theorise all of reality in not using 100% of our ability to reason.
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  16. #15 Re: Qm and Relitivity 
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    Quote Originally Posted by DrRocket
    Quote Originally Posted by theQuestIsNotOver
    Quote Originally Posted by vistotutti
    I'm not sure if it qualifies as a paradox, but the current Quantum theory that describes the very small, and Einsteinian Relativity, which describes the very large are incompatible.
    Both theories work very well, but, to date, they seem to contradict one another.

    There is a Nobel prise waiting for someone who can reconcile the situation.

    So, quantum theory that describes the very small is incompatible with with Einstein relativity which describes the very large. They contradict one another. Can I ask "how" they contradict one another, and why you think ideally they shouldn't? Maybe perhaps we are not getting the full picture of both quantum theory and einstein relativity that would otherwise allow us to form a similar appreciation for both at either end of the space-time scale of magnitude. Are there features therefore of quantum theory and einstein relativity that are not complete to our understanding?
    There is no paradox there, just an incompatibility. They do not really contradict one another but rather are different in character. There is no current viable quantum theory of gravity. General relativity is a classical, deterministic theory which, given accurate initial and boundary conditions provides a precise prediction of the outcome. Quantum theory is stochastic and predicts only probabilities.

    It is also not correct to say that general relativity only predicts the very large and quantum theory the very small. In principle they each apply at all scales. In practice general relativity only seems to be important for large bodies at large scales and quantum effects important at small scales for small bodies. But there are situations, as near the singularity of a black hole or in the earliest times following the big bang when both effects appear to be important, and in those situations our current understanding of physical theory is simply inadequate.

    It is not that the theories provide different and contradictory predictions for the same phenomena.

    Our understanding is not complete. It may never be complete. There is research in progress to try to develop a unified theory that encompasses all of the fundamental theories of physics -- general relativity, quantum chromodynamics and the electroweak theory -- in a single consistent package. Thus far that research has not produced the requisite theory. If you have heard of string theory, or its successor M Theory, then you have heard of one research path being pursued to attempt to developa unified "theory of everything". So far, no brass ring.


    I forgot to mention, "thanks for your honesty".

    I guess what I am trying to say is if indeed we are looking for new ways to tackle the idea of a grand unified theory, joining quantum mechanics with gravity, maybe we need to approach the theory with a new axiom-basis for our quantitative values for space and time. I say that only because it is a hope, in knowing that physics and the learning institutions physics is taught are both very rigid structures hardly willing to budge let alone make the concession that their current appreciation of space if not time maybe according to an inappropriate or flawed fundamental assessment/assumption of the idea of space and time themselves.
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  17. #16  
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    You shouldn't confuse research physics with institutional physics. Certain truths are just too complex to be taught full formed right from the beginning. This applies to pretty much every single field of study. I'm not saying the way things are taught are perfect, but explaining the whole truth (as it is currently understood, which is a whole 'nother can of worms) right from the start would be information overload, even to first year undergraduates.

    In math, you wouldn't try to teach 1st graders the Peano axioms. Instead, you teach them 1+1=2. Similarly, in physics, you don't teach all the finest details right up front because those students haven't even learned how to learn yet. (Note that while I don't know how things should be taught, I'm pretty much of the opinion that rote memorization isn't the right way.)
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  18. #17 Re: Qm and Relitivity 
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    Quote Originally Posted by theQuestIsNotOver
    I guess what I am trying to say is if indeed we are looking for new ways to tackle the idea of a grand unified theory, joining quantum mechanics with gravity, maybe we need to approach the theory with a new axiom-basis for our quantitative values for space and time. I say that only because it is a hope, in knowing that physics and the learning institutions physics is taught are both very rigid structures hardly willing to budge let alone make the concession that their current appreciation of space if not time maybe according to an inappropriate or flawed fundamental assessment/assumption of the idea of space and time themselves.
    Magimaster is quite correct. You need to be very careful not to confuse research physics with known and accepted physics.

    Before you start to concentrate on research physics and the speculation that comes with it, you need to thoroughly understand the current state of knowledge in "accepted" physics. You need to understand what it says, what it does not say, and the limitations of the current theory. You cannot understand the need for further work until you understand the foundation on which that work is to be built. That foundation has some problems, but it has some very strong points as well -- strong points that must be preserved in any viable extension.

    What we know about physical law permits very accurate predictions of natural phenomena in all but the most extreme and unusual circumstances. The theory is very good, and is fully adequate for all engineering applications. Anything that simply contradicts known physics in the known domains of validity is bogus. It is also true that there are chinks in the armor, and those chinks need to be understood, exploited, and better theory devised to fill them. That may result in radical new ways of looking at natural law, but it cannot simply repeal that law and replace it with anything that produces radically different results in known conditions.

    It is an unfortunate aspect of what is currently being published in books for the general public that some aspects of speculative research physics, unproved aspects, are being presented as though they are clearly true. That is particularly the case with much that is written about or based on string theory. String theory is a viable avenue of research, but there are important aspects that are simply plausible conjectures but are often presented as though they have been proved. The result is that to many people these speculations seem to have become viewed as known physics -- they quite simply are not.

    Unfortunately in order to figure out that some notions are simply conjectures one has to do quite a bit of independent research, or know some honest professional physicists who are knowledgable in the area. Some of the misconceptions have been put into print by well-known and respected physicists, but people who are bit too enthusiastic in pushing their own research perspectives.

    I can assure you that "institutional" research physicists are not the least be reticent to consider new ideas. That is the nature of research. The problem seems tp be that recently that are not reticent enough about discussing that research in public without the necessary caveates adn qualifiers.
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  19. #18 Re: Qm and Relitivity 
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    Quote Originally Posted by Janus
    Quote Originally Posted by leohopkins
    Quote Originally Posted by vistotutti
    I'm not sure if it qualifies as a paradox, but the current Quantum theory that describes the very small, and Einsteinian Relativity, which describes the very large are incompatible.
    Both theories work very well, but, to date, they seem to contradict one another.

    There is a Nobel prise waiting for someone who can reconcile the situation.
    In what way are they incompatible? Can you elaborate a bit more please ?
    At the quantum scale, things get very "choppy". There are sudden discrete changes in energy. In GR space time curvature is sensitive to energy. So when you try to apply GR at the Quantum scale space time curvature becomes very erratic, which leads to sudden large erratic changes in gravity. IOW, GR only works at small scales if you assume a smooth space time curvature all the way down, But QM predicts that it will not be smooth at that scale.
    Well, yeah but you cant apply the laws of physics of the "large" to the very small, and vice-versa. (because although one serving the other and vice versa, they are independant systems) Think about it.
    The hand of time rested on the half-hour mark, and all along that old front line of the English there came a whistling and a crying. The men of the first wave climbed up the parapets, in tumult, darkness, and the presence of death, and having done with all pleasant things, advanced across No Man's Land to begin the Battle of the Somme. - Poet John Masefield.

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  20. #19 Re: Qm and Relitivity 
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    Quote Originally Posted by leohopkins
    Well, yeah but you cant apply the laws of physics of the "large" to the very small, and vice-versa. (because although one serving the other and vice versa, they are independant systems) Think about it.
    That is simply wrong.
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    Here's a quick paradox.
    You've got a rod that's 1 light year long and weighs 1 kg(it's hypothetical :-) )
    You push the end for 1 second with a force of 1 newton thus moving the rod 1 meter.
    A week(or month) later, a friend who's been camped out half a light year down the rod decides to cut it in half!
    so here's the paradox, either you break the law of conservation of energy(six months after the initial push the new end of the rod at the half light year mark moves one meter, hence you've somehow managed to lose half the energy that was required by the initial push as the mass is now half the original).
    Or you make the bold claim that the entire rod moves simultaneously and hence you've managed to transmit a force faster than the speed of light(as this would require the end of the rod 1 light year away to move at the same rate as the end originally pushed).
    So which generally accepted claim do you break?
    nothing(including information) can travel faster than the speed of light
    ,or,
    The first law of thermodynamics, energy can be transformed but neither created or destroyed.

    looking forward to the replies
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  22. #21  
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    Quote Originally Posted by Chotta
    Here's a quick paradox.
    You've got a rod that's 1 light year long and weighs 1 kg(it's hypothetical :-) )
    You push the end for 1 second with a force of 1 newton thus moving the rod 1 meter.

    First off, since a = f/m and d= at^2/2, exerting one Newton for 1 sec on a 1 kg object moves it 1/2 meter not one meter.

    Secondly, you cannot even assume that you are moving the end 1/2 meter, since you are only moving the end and not the whole mass of the rod. How much the end of the rod moves depends on how much it compresses from your exertion of force, which in turn depends on the material it is made of, and since the rod cannot be perfectly rigid the end will in all cases move more than the 1/2 meter.

    So rather than assuming 1 Newton for 1 sec, let's just assume 1 Newton over 1 meter. Thus you are expending 1 Joule of energy. The rod compresses by 1meter in length, and the compression wave travels through the rod. This compression will also try to decompress the rod back out to its regular length. Which means the the end you pushed will tend to snap back at you. How much it snaps back at you depends on the total mass of the rod. IOW, the energy expended by you is stored by the compression of the rod.

    If the rod remains whole that energy will be expended moving that mass resulting in a cetain final velocity of the rod.

    It the rod is severed at the midpoint before the compression wave reaches that point, that energy will be expended on the remaining mass of the rod , resulting in a higher velocity

    So the answer is the both the speed of light limit and energy is conserved.
    "Men are apt to mistake the strength of their feelings for the strength of their argument.
    The heated mind resents the chill touch & relentless scrutiny of logic"-W.E. Gladstone


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  23. #22  
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    I'd say most of the energy you exert will end up as thermal energy in the rod.
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  24. #23  
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    Quote Originally Posted by Janus
    Quote Originally Posted by Chotta
    Here's a quick paradox.
    You've got a rod that's 1 light year long and weighs 1 kg(it's hypothetical :-) )
    You push the end for 1 second with a force of 1 newton thus moving the rod 1 meter.

    First off, since a = f/m and d= at^2/2, exerting one Newton for 1 sec on a 1 kg object moves it 1/2 meter not one meter.

    Secondly, you cannot even assume that you are moving the end 1/2 meter, since you are only moving the end and not the whole mass of the rod. How much the end of the rod moves depends on how much it compresses from your exertion of force, which in turn depends on the material it is made of, and since the rod cannot be perfectly rigid the end will in all cases move more than the 1/2 meter.

    So rather than assuming 1 Newton for 1 sec, let's just assume 1 Newton over 1 meter. Thus you are expending 1 Joule of energy. The rod compresses by 1meter in length, and the compression wave travels through the rod. This compression will also try to decompress the rod back out to its regular length. Which means the the end you pushed will tend to snap back at you. How much it snaps back at you depends on the total mass of the rod. IOW, the energy expended by you is stored by the compression of the rod.

    If the rod remains whole that energy will be expended moving that mass resulting in a cetain final velocity of the rod.

    It the rod is severed at the midpoint before the compression wave reaches that point, that energy will be expended on the remaining mass of the rod , resulting in a higher velocity

    So the answer is the both the speed of light limit and energy is conserved.
    Correct. To add to thta one would note that what happens when you apply a force to the end of the rod is that you start the transmission of a stress wave (compression in this case) that will travel at something very close to the speed of sound in the material of the rod. Since the rod cannot move as a rigid body (that would indeed violate special relativity) you cannot move the rod a large distance quickly without a very large force to impart a very high level of compressive stress. You don't move the rod one meter, but rather you compress the rod sufficiently to move one end of the rod one meter, and that may take a lot more force than 1 Newton, depending on the Young's modulus of the material. So the way the problem is stated is in fact not consistent with the physics -- you can't say that 1 N for 1 sec on 1 kg moves the rod 1 m, since you are no longer dealig with rigid bodies.
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