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Posts posted by md65536

  1. The question is: Why do photons move with the maximum possible velocity permitted in nature? Because it is their nature. Then, there may be a relation between Relativity and Electromagnetism. This is what Einstein were trying to do, if I am not wrong.

    I believe that anything can be considered to be moving at c. It is more of a standard speed than a maximum speed.

    c is essentially a constant linking distance and time.

    I'm pretty sure this view requires some misinterpretations of accepted science! Hopefully someone can correct me.


    The details are:

    - All energy, unobstructed, travels at c.

    - Energy and mass are equivalent. One might consider mass to be "made of" energy, however I remember reading comments that suggest this is misleading?

    - Therefore all particles or energy that can be considered to be moving < c according to some frame of reference (including their rest frame), must oscillate or change directions. If the energy moved in a single direction, it would move at c.

    - If energy is moving at c, but oscillates or changes direction, then after a time t its total displacement will be less than the total distance traveled by the energy. So if you consider a moving particle, the energy that makes up that particle is moving at c, but the particle itself can move at speeds < c. As an analogy, if a sailing ship moves from point A to B in a time of t, its (average) velocity is ||B-A||/t, but if it is tacking in the wind its speed through the water can be much greater.



    So, if you are considering a mass moving in a straight line, its velocity is its displacement over a given time, while the speed of its constituent energy is the total distance that energy oscillates over the same time (which will always be c).


    With this view, the maximum speed occurs when displacement and distance are the same, ie. movement in a straight line.

    In which case, light etc. travels at c in a vacuum because it travels in a strictly straight line in a vacuum.

  2. The thought experiment you propose sounds like "Bell's Spaceship Paradox":




    Is this the situation you're thinking about?

    Yes, I think that the link provided gives an answer to the question.


    In the "Analysis" section, they set up a good simplification (treating Frank and Bert in our examples as point masses).


    The applicable bit is: "This implies that xA(t) − xB(t) = a0 − b0, which is a constant, independent of time. In other words, the distance L remains the same. This argument applies to all types of synchronous motion."


    They go on to show that "when switching the description to the comoving frame, the distance between the spaceships appears to increase by the relativistic factor [gamma]. Consequently, the string is stretched."



    In other words: If the front and back of the ship can be accelerated using synchronous motion, then yes, the length of the ship will remain the same, and will appear to remain the same for observers which see the front and back synchronized.


    It should be easy to show the following: Iff all points on the ship can be accelerated using synchronous motion, observers which see it synchronized would see no length-contraction deformations of the ship. Otherwise, sections in between the synchronized points would stretch in accordance with the wikipedia article.

  3. I'm replying to a post in another thread that I think this thread is based on? My reply seems more applicable here.

    Yes. More like the impossibility of "visualizing" a fourth spatial dimension, since three axes fully describe "space" as a 3-D matrix. (What direction is signified by a fourth axis?... not "time" as a fourth spatial dimension.))

    This will make little sense until you understand the ontological debate about space and time.

    I have provided links (many times) to the ontology of the spacetime debate. Little response. The International Society for the Advanced Study of Spacetime has presented many scientific/philosophical papers on the subject over the last decade of conferences on the subject. Mostly ignored as I have presented links in this forum.


    I suggested that you read my favorite author on this Euclidean/ non-Euclidean debate, (as pertains to relativity) but you have not commented on the paper linked above. Too busy to read new information? I don't know. Just guessing. You must be busy as a science website administrator.

    Please put this thread in the philosophy section where it can be formally ignored by physicists who don't care about the ontology of time.. or space.. or "spacetime."


    I couldn't find any actual links that you're referring to. Can you repost the links, or a link to the post containing the links? Links to specific papers and even references to sections within them would be appreciated... no one wants to wade through the ISASS site trying to find writings that back up your views.


    Is the author you're referring to Dennis Dieks?

    Have you read any of Hans Reichenbach's work? I myself haven't, but I see references to him in stuff that makes sense to me.

    I don't get Dieks, personally. For example, in the first section of http://www.phys.uu.nl/~wwwgrnsl/dieks/becoming.pdf, he references Reichenbach and an idea (Conventionalism) that makes sense, but then concludes from it an idea that I can't make sense of (a "global shifting Now").



    Conventionalism (http://en.wikipedia.org/wiki/Philosophy_of_time#Conventionalism) seems like a useful idea for your "ontology of time", because it seems to provide a means to sidestep GR, perhaps treating it as an arbitrary interpretation of time that is agreed on by convention. However, defining an authoritative distance (a fixed diameter of the Earth, etc) seems to be aiming in the exact opposite direction (Asbolutism or something).


  4. No. Bert would require Frank to finish adjacent to the bow of the Old Model Spaceship, which would have required him to move in the opposite direction at much higher than light speed, yet remain at rest with respect to him at the same time.

    I defy you to find any rate of acceleration that wouldn't produce a similar contradiction (ie. Frank moving backward or seen moving backward by Bert, at any speed) by your reasoning.



    Meanwhile I'll try to explain your hypothetical scenario differently...


    In the scenario, the New ship is synchronized to Bert at the back's clocks.

    Bert sees Frank at the front begin to move at the same time that Bert does.

    Bert also sees the ship reach full speed just as he's pulling alongside the back of Old ship.

    Bert writes: "Cabooseman's log, stardate... whatever. We have begun our journey at the same time that Old ship came by, and we are side-by-side."


    Old ship was contracted to 0.5 LY length, but now it is back to 1 LY length. The front of Old ship should be beside the front of New ship, just as full speed has been reached.

    However, Bert will not immediately see this, because light from the front of the ship is still delayed by 1 LY.

    What Bert will see might be described as a wave traveling up the length of the length of the Old ship, changing the ship from length-contracted to rest length, but not all at once.

    I believe this would take a year to complete. Only after a year, does Bert see the front of Old ship coincide with the front of New ship. He sees Frank wave and give a thumbs up, 1 year into the journey.


    In other words, Bert sees the front of the ship continuing to move forward, catching up to Frank after a year of Bert's time.


    This is consistent with what Frank sees.

    Frank's clocks are synchronized to Bert's. According to Frank, he starts moving long before Bert does.

    Frank might write in his log: "Captain's log... I have begun our journey long before the back of Old ship is scheduled to catch up to Bert."


    Frank matches the Old ship's velocity, but this happens a very long way away, so Frank doesn't observe this happening immediately.

    Frank will observe the Old ship continuing to appear to be moving toward Frank, even though Frank knows he's moving forward too.

    I don't know how to explain this properly. One might say: If Frank were now at rest relative to Old ship, the light from the observations of Old ship would still be catching up to Frank at a velocity of c. No matter what velocity Frank takes on, the observations will still catch up to Frank, so Frank can never see himself outrunning (or at rest relative to) those observations... In other words, Frank will still see the Old ship continue to catch up.


    I think it's valid to say that Frank does not actually reach full speed AND enter the rest frame of the distant Old ship simultaneously. This is not a contradiction, because the 2 are separated by a great distance when Frank reaches full speed.



    I'll stop with the details here because I know I'm going to screw them up.

    Frank sees himself start the journey early, and after a long time (1 year I guess it must be) he sees Old ship's front catch up and come to rest next to him.

    He waves to Bert and gives a thumbs up.



    Different observers see different events happening simultaneously.

    There is no contradiction or impossible situation here.



    Addendum: Bert does not join the rest frame of the back of Old ship at the same time he joins the rest frame of the front of Old ship, even if it's the same frame.


    Frank does not join the rest frame of the distant front of Old ship at the same time that Frank reaches full speed.

    Things from one frame can switch to a different shared frame at different times, if those things are separated by distance.

  5. If at noon, you begin moving away from me at v=.866c, and then immediately (negligible delay) you send a signal to tell me to start, I will receive that signal after 0.5s by your clock, which is after 0.5 *gamma = 1 second according to my clock, as observed by you. So again it works out.

    This description and calculation is so wrong its embarrassing.

    I got the relativity of motion and of light wrong.

    I applied time dilation backwards.



  6. Can anything accelerate while all it's parts stay in exactly the same shared rest frame at all times?

    Yes, iff it can be treated as a point particle.




    In general, no. I think we all agree that the sharing of frames is relative, and that if all parts of the velocity-changing ship share the same frame according to one observer (point) location on the ship, then other observers on the same ship will not see all the parts of the ship remain in the same frame.


    The instant acceleration is just to simplify the thought experiments.

    If you could build a ship where all of its parts can accelerate the same way (ie every point location on the ship is part of a propulsion system that accelerates the same way as every other point), and you synchronize the time at all locations on the ship to one point location's clock, and you have all parts of the ship coordinated to accelerate and decelerate at the same specific times according to their own local clocks, then the whole ship will remain in the rest frame of that one point location, according to that one point location. But the same observation won't be seen by any other part of the ship.


    Is there an absolute limit on acceleration that depends on length?

    I don't think so.


    If it's possible to build a ship that stays in the rest frame of a single observer's location on the ship, then according to that location, the ship need not experience any stresses while accelerating, at any rate.


    Therefore, the ship need not be torn apart due to the impossibility of synchronizing its parts.


    If it doesn't get torn apart according to one observer, it won't be torn apart according to another observer. Other observers will necessarily see the ship affected by length contraction, and its various parts moving at different times, however it must be that other observers also observe the ship experiencing no stress. Any deformations (of space or material) observed will balance each other to allow no stress.


    There are likely engineering reasons that make such a ship impossible or infeasible to build, but I don't think SR itself limits acceleration.


    What happened? Is Frank in the same rest frame as Old Model Spaceship? Is he in the same rest frame as Bert? Is Brand New Spaceship still intact?

    Yes, Frank is in the same rest frame as Old Model Spaceship, and the same rest frame as Bert. The ship can remain intact.


    When you say the ship was synchronized, that must be done according to a specific clock.

    In this example it seems like it's synchronized to Bert's clock.


    Your example is certainly puzzling, but I think you're failing to consider changes to relative simultaneity that occur during frame switches.

    I'm sure that if this was considered, then you'd find that if Bert was next to the stern of Old Model Spaceship immediately after finishing accelerating, then Frank would be next to the bow immediately after accelerating, even though the timing of various events would be different for the different viewpoints.


    I think your outcome implies that Frank and Bert remained synchronized both to each other and to the Old ship???, which is impossible.

  7. Quite a few people on the internets seem to simply not "believe" in SR, and are trying to figure out time while holding onto that belief.

    I don't get why anyone would try to figure it out while denying the reality of SR. I can see questioning the evidence of SR, but given the evidence, trying to figure out time according to ideas incompatible with SR seems to involve figuring out something incompatible with reality. How can you figure out time while reasoning about ideas that are not based in reality? It is essentially reasoning based on fantasy. You can claim anything you want to, if you're not concerned with reality.


    For fun I thought I'd try it (while still considering the forum rules). Perhaps consider the topic to be "any fantastic ideas about time" and feel free to suggest wildly different directions. This is for fun but the value of it might be in inspiring new ideas?



    Chronular Fantasia Theory (CFT)


    Assumption 1a: SR is wrong wrong wrong!

    Assumption 1b: (Corollary) Reality is also wrong.




    1. Events are clearly made up of a series of individual instants. Instants can be considered infinitely thin (in the time dimension) slices of reality.


    2. Since instants have no depth in the time dimension, it is impossible for any 2 instants to overlap each other. Therefore no 2 instants can occur at the same time. One must always precede or succeed another. That is, no part of an event may happen at the same time as another part of any event. Therefore, no 2 events can happen at the same time.


    3. Since all instants are separate, they can be put in a definite order, like a stack of library index cards. The passing of time involves going through these index cards in order, and implementing the "instant" described on the index card.


    4. Since any 2 particles moving would have to be considered separate events, it's clear that an instant can only involve at most one particle. So an instant must be a single change that occurs in only one place in the universe.




    The passing of time is the sequential processing of tiny individual changes to single particles.

    Events that you witness, such as a blade of grass blowing in the wind, involve many particles each waiting their turn to move, in order. These changes are interspersed with all the other minuscule changes in the universe.

    It only seems to happen all at once because of the super high rate at which the instants occur. I'm talking about millions of instants per minute!



    5. It must be that all instants occur at fixed intervals in the time dimension. This is what makes it appear that all clocks tick at the same fixed rate.

  8. If I am correct, one might possibly time travel if you could get some very specialized equipment to record the pattern shifts of a particular field of atoms in space, and you'd also have to have a some very specialized equipment to somehow run this cycle again, but, this would be a new instance of a past moment. Kinda like that movie, Ground Hog Day.

    I've always thought that if you could take a system and restore its state to a previous state, you'd effectively have it time travel to that past state.

    If it could be done so that there is no way to tell the original state from the restored state, there would be no difference between this, vs. the system time traveling to the past.



    Unfortunately, it is not possible to do this with systems where distances are involved, due to lack of simultaneity.

    Why? Basically, the idea requires saving and restoring the state of a system as it exists in a single instant. But a single instant is different according to different locations within the system.


    Or as you suggest, recording the patterns of atoms in space... this could only be done according to the timing of one observer, I think.


    I think that what this practically means, is that the system can return to a previous state only according to a single observer.

    Since doing this would not restore the same past state according to other participants in the system, it would involve restoring the system to a different state than the past one, and thus it would be impossible to replay the system the same as it was the first time around. Even without the uncertainty principle (which I kinda suspect is really just a consequence of or similar to lack of simultaneity), you could not return to the past (ie. a past state) and relive it.




    Or perhaps I'm wrong or there's another way around it. But I think time travel to the past is impossible for any system with more than 1 dimension.

  9. What is not given is that when the star moves across the line of sight, still it will not acquire velocity of the source. In short photons emitted by the star in the direction perpendicular to the motion of the star will not move along with the star. If this was not the case then we would miss the photons and we will not be able to see the star when it is moving perpendicular to the line of our sight.


    Am I correct?

    Yes. The propagation of light is isotropic, meaning the same in all directions.


    You can't directly observe signals sent perpendicularly to you, but you can receive information sent from different events (the simplest being if the light signals are also reflected to you at both the source and destination), and will always calculate the speed of light between any 2 points in a vacuum, to be c.



  10. I agree with swansont. I can't find a way to force a single observer to see the ship's length change, without there being a simple way to compensate.


    Most other observers would see the length between the observer and the object change. It's easy for me to mix up frames and get stuck thinking that their rest length must change.

    Is a rest length between two relatively moving points even defined?




    Sharing a frame with something that changes velocity, will depend on timing. Due to lack of simultaneity, there is no absolute sharing of a frame by multiple objects. Whether 2 objects are in the same frame or not depends on how they are observed.


    In clock synchronization you account for this delay. If we are 1 light-second apart, and I send you a signal at noon, you know you receive it at 12:00:01

    If at noon, you begin moving away from me at v=.866c, and then immediately (negligible delay) you send a signal to tell me to start, I will receive that signal after 0.5s by your clock, which is after 0.5 *gamma = 1 second according to my clock, as observed by you. So again it works out.


    But from my perspective, if my clock is synchronized to yours, and I know that at noon you'll begin moving...

    Suppose for now I'm just observing signals from you, and not trying to stay in your rest frame.

    If gamma = 1 (no movement, just a signal), I would receive it at 12:00:01.

    But with gamma = 2, your frame switch would change the synchronization of our clocks. When it's 12:00:00.05 for me, I think it's noon for your clocks, according to me??? (This is only for an instant where you've instantly accelerated to 0.866c but before you've covered a considerable distance.) I would receive your signal after 0.5s, again at 12:00:01.


    In previous posts I must have been incorrectly dealing with the synchronization change.


    So it seems the details of a complicated viewpoint balance to match the outcome of a simple viewpoint, which should be expected.







    But there are always further complications to consider!



    According to the above,

    When it's noon my time, it's noon your time and you are 1 LS away.

    When it's 12:00:00.05 my time, it's noon your time (in your new frame), and you are 0.5 LS away.

    However, due to the travel time of light, I won't actually observe any disruption in your passage of time, and I will see you instantly jump from 1 LS away to 0.5 LS away, at 12:00:01.


    I think...


  11. If we are neglecting the acceleration time as per the OP, why?


    Because to maintain clock synchronization, the object would have to get a head start on the observer.

    A signal to begin a synchronized start would have to come at the same time as the observation of the object beginning that start, and these would be delayed by the speed of light.

    So you're changing the rest distance between object and observer, and are thus forced to change the lead time that the object acts with.

    But I think this reasoning is wrong!???




    I'll try to work through an example with "signals" instead of "observations", which usually gives some insights.


    Suppose observer O and remote point P are 1 LY apart, and P is sending 1 signal every day.

    At some arbitrary time, P will move in the direction away from O, at a speed of 0.866c, for a given number of days, and then stop.

    O knows this much information and will attempt to remain stationary relative to O.


    At rest, O is receiving signals sent "a year ago" from P. There are currently 365 signals "en route" from P to O.


    When it happens, P begins moving away from O at 0.866c.

    This length contracts the initial distance between them to 0.5 LY.

    Due to an update in simultaneity, only a half year has passed at P relative to O. There are now only about 365/2 ~= 182 signals currently "en route" from P to O.


    According to causality, O will not be able to observe any change in the signals, including their rate, or their source distance, for at least half a year. It must receive those 182 signals exactly as if no length contraction has taken place.


    O then receives the signal to begin moving, 0.5 years after P begins moving.

    O instantly accelerates and is now at rest relative to P.

    Let's suppose this can happen in much less than a day, and timed so that O never observes any of the daily signals being length-contracted.


    I would assume that the rest distance between them will now be greater than the original rest distance, because P has been moving away during its head start.

    But is this a mistake??? The rest distance now applies to a different frame, so I don't think I can easily claim this.




    Okay, I can't figure my way past this part.

    In a "stationary observer's frame", P has had a 0.5 year head start at 0.866c, so the rest distance between O and P is now 1.433 LY, however this stationary observer would not currently observe O or P at rest, so would probably observe that distance under length contraction.


    What is the rest distance according to O and P, who are currently at rest relative to each other?

    It would have to be the same distance for both of them, so how is this possible, when P observes O starting much later???

    I'm guessing it would have to still be 1 LY, but I can't figure out how this is so according to P's perspective! blink.gif


    Okay so according to P's perspective...

    P starts moving. Distance to O contracts to 0.5 LY, and O moves away at 0.866c.

    O continues moving away until it is 1 LY away, and then it remains at rest relative to P for as long as P remains in this inertial frame.


    I don't have a firm grasp on why this is.




    But yes, I now agree:

    1. If an observer can sync its movements perfectly with a distant object, the observer will always appear to be at rest relative to the object.

    2. In such cases, the rest distance between object and observer seems to remain fixed.


    A distance can be maintained, from the viewpoint of the observer or the object, but not both.

    Agreed, but I was wondering if it was even possible from just one of those viewpoints, because it didn't seem intuitive in my example.


    I suppose the solution is typical: If one looks at it in the simplest way, the right answer may come easily. If one looks at it in the most complicated way, the wrong answer seems to come out, until all the effects of SR are properly accounted for, at which point the right answer again works out.

  12. It will always be 1 light-second away if it stays in the same reference frame, i.e. it is at rest with respect to the observer.

    Is this true even if the frame isn't an inertial frame?


    If the observer and object change velocity (relative to some other frame) at "the same time", even if separated by say a light year, does their relative simultaneity remain fixed as long as they don't move relative to each other? And thus we can talk about "simultaneously" for the entire frame, no matter how big it is?


    If the object and observer moved independently, but in sync... Suppose the object was allowed to choose whether to move along with the observer's reference frame, or stay stationary relative to the other frame.

    If the observer moved at 0.866 c for 1 second and then stopped, but the object was 1 LY away, the observer would not be able to tell whether or not the object moved, until about a year later. Does this mean that any possible visible length-contraction effects would appear one year later?


    I assume the answer is "no", and that length contraction would be apparent immediately. In which case, for the object to be synchronized with the observer, it would actually have to start its movement early... (by half a year in this case, with gamma=2, I think).

  13. Can they be tested? Not yet. Unfortunately. My hopes are that whoever reads my post may get a different perspective. Who knows? Maybe someone who has the capabilities, may read that and try it in a model.

    My experience is that no one cares.

    Turning an idea into a theory will likely take a LOT of work, and no one's going to volunteer to do that work unless you can express an idea that sparks someone's desire to care.

    If anyone's going to be voluntarily putting work in, it will probably have to be you. In the course of doing this work, you'll learn a lot about existing science, which will greatly change your ideas. You'll learn how to better express your ideas, and how to evaluate them. Chances are you'll throw away more than you keep (always adding more ideas and always throwing away most of them).


    Improving the standard model could take a lifetime of dedicated work, without any guarantee of success.


    When we non-formally-trained-scientists start out, we don't know how to express our ideas, we don't know of or understand the existing ideas we're competing against, and we don't know how to work with our ideas. All in all, it's very little to offer someone who already has all those abilities.



    But uh... keep working at it! Great scientific ideas will come from non-scientists. It is our challenge to improve the ideas until we can convince someone to care. A good idea might still inspire others, while inspiring yourself to work on it.


    Whether you spend a few minutes thinking about it now and then, or turn it into a serious lifelong work, you won't know the value of the outcome until you do it.

  14. If your model predicts some observations simply or more accurately than the standard model, that's great, but for a model to replace the standard model it would pretty much have to predict all known observations that fit the standard model. The weird or complicated stuff is usually there because of specific observations.


    In your example, the round-earth model is better than the flat-earth model, but the round-earth model still predicted all the observations that had been made which had fit the flat-earth model.



    (One possible way around this would be the discovery of some new evidence that trumps all existing observations. If for example someone from antiquity had a round-earth model that predicted that the oceans would fall off the planet, this flawed model should still be preferable to a flat-earth model given the evidence of someone traveling into space and observing that the Earth is in fact round. I can't imagine what possible observations might disprove the standard model. "Many things can be answered" alone would not do that. For "many things can be answered", you'd want a model that is pretty compatible with the standard model and all its complications.)

  15. So I got completely confused in this other thread...


    Suppose an observer moves along the x axis in some known way at speeds where length contraction is significant.

    Is it possible to synchronize the movement of another object on the x axis, at some distance from the observer, such that the object always appears the same distance away according to the observer?



    For example, suppose the observer and object are relatively at rest separated by 1 light second, and then at a predetermined time the observer moves in the direction of the object at v=0.866c for one second (with negligible acceleration time) and then stops. Is it possible to move the object in such a way that it always appears 1 light second away according to the observer, despite any length contraction that the observer might experience?

  16. Continued...


    In this example the ship is 1 LY from Rear to Front, and there is a travel distance of 1 LY from Front to Destination. v = 0.866c; gamma = 2.

    Assume negligible acceleration time.



    Okay so if we let the Front get a half??? 1.15??? year start (according to Rear), and stop one year some time in advance (again according to Rear but this time in a different frame), Rear never sees the length of the ship change ???????, although it is certainly affected by length contraction.


    So if the distance to Destination contracts to 1 LY according to Rear, but the ship also remains at least 1 LY long, then what gives?



    Okay so after the Front's head start, Rear travels at 0.866c for 0.577 years local time, covering 1 LY of rest distance in the Destination's frame (ie. the total distance of the trip).

    At the start of its trip, the distance to the Destination (rest distance 2 LY) contracts by gamma = 2 to 1 LY, and continues to decrease as the rocket approaches. In other words, it contracts to closer than the front of the ship. BUT the observations of this still take 1 LY to reach Rear.


    Before Rear can observe the Destination contracted to closer than the Front, after 0.577 years rocket time it stops, undergoing a frame shift, and changing its simultaneity relative to Destination. Any observations of the Destination being closer than the Front are unobservable, because they happen too far away to be seen in the time that Rear remains in its moving frame. The frame shift essentially changes what has not yet been observed.

    This brings up an interesting consequence of SR: If an event is predicted to happen, but then becomes unobservable due to an update to simultaneity, then that event didn't happen. Only observable events actually happened.


    So in conclusion: The ship will also be affected by length contraction (relative to the observer on the ship), BUT if the front is ever calculated to be beyond the destination, it will only happen for a short enough duration to make it unobservable, which means it never actually happens.




    If you read all that... sorry...

  17. I couldn't help thinking some more about this problem.



    Let's simplify the problem further.

    Suppose we have an observer at the back of the ship, and the front of the ship is one unit away (1 LY say). We'll consider the front and back of the ship as 2 separate entities, with nothing in between; we'll ignore the rest of the ship so we don't have to worry about how it behaves.

    Suppose one unit beyond the front of the ship is a destination.


    The destination is 2 units away from the observer. Length contraction of gamma = 2 will bring the destination to the same distance as the front of the ship; any higher gamma and the destination will be closer than the front of the ship.



    The ship can't start moving as a single unit at a single time according to all observers. At best the start time can be synchronized according to some location. This can be done by sending a signal to start, to both the front and back of the ship. Suppose this is done from the middle of the ship. From that perspective, equidistant to front and back, both start at the same time. But according to the observer at the back, she gets the signal to start before she observes the front of the ship starting.


    Another alternative is that the front of the ship sends the signal to start. Then, the observer at the back observes the signal to start at the same time that she observes the front of the ship also starting. In this case, the observer sees the ship starting as a single unit.


    Using this synchronization method, it's clear that there are only 2 cases: The observer will begin moving before observing the front of the ship starting to move, or at the same time as observing the front of the ship starting to move.


    Case 1: Observer begins moving first.

    Until the observer sees the front of the ship moving, the front of the ship will be in the same inertial frame as the destination.

    The length of the ship will contract by a factor of gamma, the same ratio that the distance to the destination contracts. The front of the ship will never be seen to be beyond the destination.


    ... Then there are a bunch of details I don't want to try to figure out and am skipping, in the timing of this.


    Case 2: Observer begins moving at the same time as the front.

    With gamma=2, what the observer sees is that the distance to the destination contracts to half its length, and the front of the ship appears to have instantly reached the destination, at which point it stops and becomes part of the destination's inertial frame (thus again, now length-contracting by the same amount with the front never appearing to be beyond the destination).


    This is consistent with what other observers would see: From another perspective, the front appears to have a head start, the ship stretches, and the front of the ship reaches the destination early.



    Again there's a bunch of details that can be worked out, but this is enough to resolve the paradox:

    Unless the front of the ship passes the destination (according to all observers), then an observer elsewhere on the ship will necessarily see her own ship length-contract (for at least part of the duration) along with the destination, such that she never observes the front of the ship passing the destination.





    Edit: I think I got this at least partly wrong. It should be possible to set this up so that the rear observer never observes any change in the length of the ship.

    I think I was forgetting about the travel time of light.


    Dammit complicated relativity, you are ruining me.

    Note: the following turned into a huge mess when I tried to correct errors in the original.



    Alright say for example the ship is 1 LY long, and the front travels a further 1 LY to reach its destination, at v=0.866c with gamma = 2.

    The time it takes, according to an observer in the destination's frame, is 1 LY /0.866c = 1.15 years.

    According to the front of the ship, it has to travel a length-contracted distance of 0.5 LY at 0.866c, taking half as long or 0.577 years local rocket time.


    If the front of the ship gets a 1.15 year (destination frame time) head start, the length of the ship becomes 1.15 year * 0.866c = 1 LY longer according to the rear, but the distance is length-contracted by a factor of gamma=2 before the rear starts moving, to a distance of 1 LY. Sorry... confused...

    If the front of the ship gets a 0.5 year (destination frame time) head start, the length of the ship contracts by a factor of 2 to 0.5 LY, so it takes 0.5 years for observations of this to reach the rear. But if the rear observer begins moving at the same time as this observation reaches her, she is now at rest relative to the front and observes herself moving in sync with the front, keeping an observed ship length of 1 LY.

    In the destination's frame, the rocket is 1 LY + 0.5 year * 0.866c = 1.433 LY long. If the front stops while the rear is still moving, their distance according to the rear will contract to half that, 0.7165 LY. (Ugh, sorry these calculations got a lot more complicated than I expected..) Now if the front stops by a time of t earlier than the rear, then the rear will continue to move forward at 0.866c while the observation of the front having stopped travels backward at c... we want (1+0.866)*t = 0.7165 LY. t = 0.3839 years... If the front stops 0.3839 years before the rear, then the rear will observe this after 0.3839 years... at which time it stops. Having traveled 0.3839 years*0.866c = .332 LY, the rocket is now 1.433 LY - 0.332 LY = 1.10 LY long.

    SO! I don't know if I've just thoroughly confused myself and everyone trying to read that,

    or if I've demonstrated that if the scenario is set up so that the rear observer observes being in sync with the front (and thus never experiencing extreme length contraction relative to the front), then the rear cannot remain at 1 LY away from the front (ie. it's impossible to keep the ship always appearing to be 1 LY long).

    I'll try to figure out the rest of the details sometime later. I'm probably still wrong here so far, somewhere. Feel free to correct me!

  18. Then he stumped me: it should therefore be theoretically possible to arrive at a speed sufficiently close to c such that the entire universe seems to contract to shorter than the length of his spaceship. The front end would essentially have shot passed his destination and the back end would essentially have regressed behind his point of origin. How does one resolve this paradox?

    With these extremes you can't treat the spaceship as one unit. You have to take relativity of simultaneity into account. Different parts of the ship would experience the situation and its timing differently.


    To simplify the situation, we can consider the viewpoint only from the back of the ship (to begin with, at least).

    Let us set up the timing such that from this viewpoint, the observer never moves relative to the front of the ship. We can make the ship very long (a lightsecond, say) if it helps.

    To simplify, rather than considering the entire universe we could consider only some distant "destination" point, for the front of the ship to reach.

    Let's assume instant acceleration and deceleration.


    I think the paradox resolves as follows:

    With extreme enough velocity, the destination would indeed contract to a length less than the length of the ship.

    However, the travel time (according to the observer's clocks) to reach the destination would be shorter than the time that it takes light to travel from the front of the ship to the back (someone please correct me if that's not certain).


    When the ship instantly accelerates, it experiences an "update to simultaneity" relative to the destination.

    Before the observer can observe anything happening at the front of the ship, the ship has reached its destination, and the observer experiences another "update", and at no point has the observer observed the front of the ship being farther ahead than the destination.



    If the ship instead passes the "destination point" and keeps going, the observer simply observes it all happening with different timing than the front of the ship would.

    If the front of the ship crashes into the destination while the observer continues moving forward, the length of the ship would be decreased to the length between observer and destination before the observer could witness anything impossible happening.




    I have a feeling this explanation is missing something in the details though...

  19. The problem I see lies in the units... Let's say the unit for time is seconds and the unit for space is in meters... How can "Everything" be defined in Meter Seconds? You need to define your "Everything" better. Is there a use for this equation your suggesting?

    Perhaps Infinity I is multidimensional. Perhaps Everything is Meters3 Seconds. Then it makes sense.


    However, a decimal with an infinite number of zeroes followed by a one in not equal to an infinite number of zeroes followed by a zero.


    I think it's equal.

    It is because of the mathematics shown above, that no two points will ever be able to come together due to scaling.

    Perhaps I missed something but how is it that scaling by a factor of 0 is disregarded?




  20. In other words, on the one hand you seem to agree that earth does not actually move closer to the sun, as length contracted measurements would have it. On the other hand, all that physics seems to be an argument for the opposite, i.e., that the 19 million km distance, as seen from the fly-by frame is accurate in 'the real world' and that, by the above "magic" somehow the heat diminishes with the shorter distance and time of exposure.


    Again, I do not deny that the voyager "will see the earth as being closer to the sun." But it still looks like a distortion effect of relativity to me. Same with earth's diameter. It does not shrink just because extremely high speed frames of reference from which it is measured make it look like a 1/8 sized mini-earth.

    What you are talking about is known as "rest distance".

    You are arguing that rest distance is more definitive of the concept of distance, than is relativistic distance.

    You can argue for that, except that it leaves the distance between moving objects undefined, which is not very useful.

    It might be okay for describing a static universe, or ideas that we've known for 100 years to be false.


    In fact, all of your "ontological studies" seem to advocate a return to early 1800s understanding of time and space, back when the metre was defined as a fraction of the Earth's circumference. Your ontological arguments involve undoing 200 years of understanding and progress, but I don't see anywhere that they offer a means forward beyond what was known back then.



    Edit: Erased discussion-stifling comments.

  21. Actually, we don't fry, because of how length contraction works.

    I would have thought that it could also be answered in terms of time dilation.


    With gamma = 8, the earth would receive "8 minutes worth of radiation in 1 minute of observer's time" (ie. 1 AU's worth of radiation in the time it takes light to travel 1/8th AU) due to length contraction ???,

    but the sun would emit 1/8th the radiation per minute of the observer's time, due to time dilation.

  22. That is true but you still age at the rate that is normal for you and you still spend time in time, just a slowed down version in relation to others who are not moving in relation to you. When i think of time travel i think of simply stepping into a machine and arriving at a destination as though you are going to the mall or the next room. You have a valid point but to me that is not visiting the future independent of time but still with in the "Now" of everyone else. You simply age slower, there is no disconnect with time that time travel implies.

    Traveling relative to a universal time is impossible because there is no universal time. Time travel "independent of time" makes no sense. Existing anywhere but (your) "now" makes no sense.


    The time travel you describe (stepping into a machine or going to the mall or next room) and the time travel that A Tripolation describes can be united. If you travel close enough to the speed of light, then even a small duration for you can mean a large duration for whatever you're moving relative to. Whatever you do, you will age along with your own clocks, and the actions you described (stepping into a machine, going to the mall or next room) take time. Even if you spend only a second in the time machine, if you travel fast enough, you could have years pass outside the time machine.


    Not only does the technology not exist, the energy requirements would be ridiculous, and it involves changes in velocity that would probably turn you into a "quark soup" or worse.


    At 0.99999 c, 1 second in such a time machine would get you only about 3 minutes 43 seconds into the future.


    0.99999999999999 c for 1 second would get you 81 days into the future (if my spreadsheet remains accurate to those decimal places). Each extra "99" tacked on gets you about 10x farther into the future, so

    0.9999999999999999999999 c for 1 second should get you over 2 thousand years into the future.


    Or just spend a minute at 0.99999999999999 c to get 13 years into the future.


    No guarantees at all on the accuracy of these numbers.


    given time dilation/relativity that perhaps in the


    future we may be able to "jump" forwards in time

    To jump forward in time by only exploiting special relativity, you'd have to travel at the speed of light.



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