md65536

----- To show that transfer of information is not important, we can further modify the experiment by putting markers (flags, or planets) at the location where A and B meet, and where B and C meet. Then B and C are measuring the proper time between passing the two markers, which they could do reliably. (Perhaps observing the markers counts as transfer of information, but it doesn't matter; whether an event is observed or not does not change the prediction of the event.) Observer A is measuring the proper time between B passing, and then C passing. Without those events, there is no other meaning to the proper time that A is measuring here. However, again, SR does not care whether an event is observed, in its prediction of timing. So all these details that I'm arguing are unnecessary for the EFFECT of twin paradox time dilation, such as synchronizing clocks, transferring information, observing events, these are important for RELIABLY MEASURING an experiment to confirm the twin paradox... but the effect still occurs exactly the same whether it is measured properly or not.

Another way to put it is that the relative simultaneity of an event at B and one at C depends on gamma (ie velocity) and on separation distance. If the separation distance * gamma is negligible, the disagreement of simultaneity is negligible. If B and C are at the same location as an event at the moment of the event occurs, everyone will agree on that. We could defer to an expert as referee perhaps? Yes, you're correct, elapsed proper time is all that matters in this experiment. If we remove the "transfer of information" described in post #1, then I am claiming that with gamma=2, observer A measures 4 units of proper time between its two events, B measures 1 unit between its two events, and C measures 1 unit between its two events, and this is the same value calculated in a similar twin paradox experiment, using the same calculations of the Lorentz transformation. If this is wrong, where is it wrong? I'm calculating 3 proper times between 3 events; are the 3 events valid, and if so, what are the conflicting proper times that you calculate between those events? In my opinion, claiming that transferring information from B to C changes the proper time that one of the observers measures, is a crackpot theory. Do you suggest that C's clock is invalid until it gets information from B, or that the information physically changes the clock, or other?

Integration is essentially an infinite sum of parts. The parts are independent. In this case it is the sum of infinitesimal proper times along the path of the clock. Those parts are independent, meaning a clock has no memory. You could swap in another clock for part of the path, and the two clocks would behave identically over that part. If you split a path into N parts, the proper time of one clock following all N parts is the same as the sum of the proper times of N clocks each following one of the N parts. That also doesn't matter in the effect, as per: No information is passed here. The proper times between events measured by any of the observers will not change based on whether or not B sends any information to C. A clock will age 1 year per year, regardless of when it was last set, or what value it was set to.

There are 3 events: E1: "A and B pass" (twins part), E2: "B and C pass" (equivalent to turnaround), and E3: "C and A pass" (equivalent to twins reunite). No pair of clocks passes through the same two events. There is nothing to disagree on. There are 3 clocks measuring 3 individual proper times; everyone agrees on all 3 proper times measured. In the example I've used, with gamma=2, A measures the proper time between E1 and E3 as 4 units, B measures E1 to E2 as 1 unit, and C measures E2 to E3 as 1 unit. The sum of B's and C's proper times is 2 units, corresponding to a traveling twin aging half as much as an inertial twin. Where is the error in that? Relativity of simultaneity doesn't come into consideration, because we're not comparing the simultaneity of any separated events. We're treating "B passes C" and "C passes B" as one event, ie. simultaneous for all observers, which is acceptable if they pass at a negligible distance. In the example, the proper time measured by A is 4 units, the proper time measured by B is 1, and the proper time measured by C is 1. How is it not possible to compare these values? Eg. 4 > 1, all agree. ------- Perhaps to HELP explain this version of the paradox, or perhaps it will only make it more complicated, note that A ages half as fast according to B and C, and vice versa. So according to A, while A ages 4 units, C ages only 2. According to C, A ages 4 over the experiment while C ages 8. This is perfectly consistent with SR. A and C begin the experiment separated and with a relative velocity, so they don't agree on the timing of the start of the experiment (however we don't care, since it doesn't matter how much C has aged before it passes B). Using composition of velocities, the Lorentz factor between B and C is 7. This is consistent: C measures A aging 3.5 units between events E1 and E2, while it ages 7 units itself, and thus it measures B aging 1 unit -- in agreement with the proper time that B measures between E1 and E2.

To be clearer, a better title for this topic might be "Acceleration is not essential in the twin paradox effect". Yes, that all sounds right. If clocks are "synchronized" does that mean they remain synchronized for some non-zero duration? I avoided using the word because I thought that, though from your description it should be clear that the synchronization is only at a single instant. No, the clock reading doesn't have to be transferred from B to C. All 3 clocks will measure proper time correctly, whether they're ever synchronized or not. The 3 observers don't even have to know about each other, they just have to properly time the events (at the right locations). For example, the 3 proper times could be measured independently, and then later the results are sent to another observer to compare. These could be sent by light signal, and I don't think you can say that the photons switch frames or accelerate in order to do this?? Do you mean that because no observer changes frame, each pair of observers has a reciprocal relationship, or whatever? That's true, but... To elaborate further on where the asymmetry might be "seen", suppose that in the example observer A (clock one) ages 4 years before meeting C, while B (clock two) ages 1 year before meeting C, and C (clock 3) ages 1 year from there until meeting A (gamma=2). Then, when B and C meet, B can say "I have aged 1 year, while A has aged 0.5 years," but C will say "I agree B has aged 1 year, but A has aged 3.5 years since B left it." The difference in their measurements is the same as the difference in measurements before and after a twin's instantaneous turnaround.

What I'm trying to show is that some of the aspects that some people think are important in the twin paradox---even to the point that they "cause" the paradox---are not essential to the phenomenon. An example from your quote here is "if you eliminate acceleration you eliminate the asymmetry." Maybe that's true in the typical twin experiment, but it is not true in general because there are other ways to achieve asymmetry. I elaborated in post #3 at "Here's another way of putting this", using a 3-observer setup where there is neither symmetry nor acceleration. Yes, but the turn around doesn't cause the time dilation, it causes the frame switch, which doesn't actually require that a physical object switch frames. In the experiment, using a single traveling observer, having the observer switch frames is essential, and so acceleration is essential. However that's essential to implement the experiment, not the effect. If you change the experiment, you can reproduce the same effect without the non-essential aspects. Here's another example of my point: You might say the twins must come together at rest to meaningfully compare clocks. And that might be true in some sense but comparing clocks is not essential to the effect, it just makes the experiment reliable. For example, if you run the experiment in two parts, where twin A is stationary for a day, then a year later you have twin B travel at near c and return, B will age less than a day per one Earth day, just as if the two twins ran the experiment together and actually compared their ages. You don't have to have the twins actually together, IF you can reliably measure proper times in a properly run experiment. So poor reasoning might go like this: The clocks have to start synchronized, and that's what having twins accomplishes. If the clocks aren't sync'd, there's no meaningful measurement that can be made between the clocks. Therefore the experiment doesn't work except for twins. Then you can keep going: There must be some physical difference between twins and non-twins. So I will try to explain/figure out the twin paradox by speculating that twins are somehow "entangled", and that causes time dilation. This is an extreme exaggeration but it's the type of reasoning that leads to pseudoscience, astrology etc. The mix between causation and correlation. In the twin paradox, turning around correlates with the effect, but it doesn't physically cause it.

I do get the point of the twin paradox and am making a further point. The "simplest" version of the experiment is set up to show the effects of time dilation in a humanly understandable situation. It does not mean however that ALL OF the details of the simplest version are in any way essential. Some are there just to make it easier to comprehend. 1) The twins don't have to start at rest. 3) They don't have to end at rest. 2) Forcing one twin to turn around is no different than using 2 observers who each implement one leg of the traveling twin's trip. Here's another way of putting this: Suppose that A, B, and C are all inertial observers. Let there be an event E1 where A and B are coincident, a later event E2 where B and C are coincident, and a later event E3 where A and C are coincident. (We could run this experiment in parallel with a normal twin experiment, where E1 is "twins part", E2 is "traveler instantly turns around", and E3 is "twins reunite".) Even if no clocks are synchronized and no information is passed between observers, anyone who calculates A's proper time between E1 and E3 will find it is greater than the sum of B's proper time between E1 and E2 plus C's proper time between E2 and E3. This is no longer exactly the twin paradox, but it uses the exact same time dilation amounts, and has the "switch between inertial frames" without needing to switch anything physical. The paradox is clearly there in the modified experiment. It is false to claim that any missing aspects (eg. having one observer physically turn around) are the "reason" for the paradox, when the exact same effects can be found without those aspects. In other words, if we modify the experiment to remove any non-essential aspects, and get the same exact result, then those aspects are not the reason for the effect. I don't think it is possible to fully understand an experiment if one is unable to separate the material and immaterial aspects of it.

I'll try this again! What's important in the twin paradox, which results in the asymmetry between the twins, is that one of the twins remains in an inertial frame while the other uses primarily two different inertial frames. It is tempting then to think that a mechanical switch between the frames somehow "causes" the relativistic effects---and further that the only way to switch frames is to accelerate---but this is not true. This can be shown by running the experiment with 3 moving clocks, none of which needs to accelerate during the experiment. Start with 2 passing clocks, A and B, which are each set to zero at passing. Let B travel some distance at velocity v, where it passes clock C traveling in the opposite direction at the same speed, and have C set its clock to match B's as they pass. When C and A meet they'll record the same difference in proper time as if the experiment was run with clock B instantly turning around as it passes clock C. What this shows is that the effect of time dilation does not depend on physically turning around, it only depends on what happens while in the various inertial frames. If B and C pass at a distance of 0, their meeting can be considered a single event that is simultaneous according to all observers. However at that moment, their different frames are important (not any mechanical effect of switching anything between their frames) as they have very different measures of simultaneity relative to A. C will have measured A's aging as greater than B has measured A's aging, even though they are at the same place at the moment of their passing. Because there is no difference in relativistic effects whether we abstractly switch from B to C, or physically cause B to turn around, it cannot be said that the mechanics of turning around has any physical bearing on the paradox. If we use a single traveling observer, then the resulting frame switch is what's important, but any acceleration used to cause the frame switch should not be confused as being the cause of the relativistic effects.

Thanks! It didn't make perfect sense to me! I spend too much time trying to figure out the details, and rarely get to the point of understanding enough to know which details are important, and which don't need to be mentioned at all! Proper time is the time between two events measured by a clock that passes through those two events. In other words it's the time according to a given single clock. In the twin paradox it's "aging"; you age along with a local clock and you can't separate yourself from that clock and age according to some distant clock. Each clock can have its own proper time, which depends on its path. For example, between the events "twins separate" and "twins reunite", the paths and the aging of the two twins are different, even though they both record a proper time between the same pair of events. Not necessarily. Red could have very low mass (eg. a neutrino) and the Lorentz transformation would still apply. But she could be stationary in a system that is moving around her; yes she is accelerated by stuff that hits her but that can still happen with a moving system that moves past her. By the equivalence principle, she is stationary in her frame of reference.

There seems to be two separate conversations here, based on two separate ideas brought up by OP... Edit: I should have used [latex]\tau[/latex] instead of t' to make it clear I'm talking about proper time, on which all observers agree. Consistency would make this less confusing. :S Suppose that t=2 and t'=1. Then t and t' are not equal the same way that t' and t are not equal, but switching their order does not change their values. They're not symmetrical. One is definitively older. Say that t=2 and t'=1 and the pulsar ticked 10 times. Then the inertial twin measured 2 ticks of its own clock, 1 tick of its twin's, and 10 ticks of the pulsars. In this frame, the pulsar appeared to tick at a rate of 5 ticks per tick of proper time (measured on a local clock). The traveling twin measured 1 ticks of its own clock, 2 ticks of the other twin's, and 10 ticks of the pulsars. In this frame, the pulsar appeared to tick on average at a rate of 10 ticks per tick of local time. The twins age according to local clocks, which tick at different rates relative to the other twin's, so we say the pulsar ticks at different rates in different frames, not that the twins age the same according to some universal clock. You can also replace "tick" with "second" (or "year") if it's more intuitive. No one seemed to care but I was sloppy here and feel compelled to correct myself. Each of the observers will age a different amount in this experiment, and so there's no sense in having them travel at different speeds for the same fixed time, unless we measure that time in the inertial (stay at home) twin's frame. By that observer's clock, all others would turn around at a fixed time, ie. simultaneously. For the other observers, the turn-around wouldn't be simultaneous; each traveler would turn at a different time. So in the other observers' frames, they wouldn't measure being halfway between Earth and the next faster observer. As usual everything's consistent in SR; the observers wouldn't measure being half the speed as the next, because if an observer's speed relative to Earth is v, then the next faster observer will have a speed of a composition of v and v (Edit: wrong once again!) say vb relative to this observer, where the composition of v and vb is 2*v, so vb doesn't equal v, (vb should be greater)... we must use relativistic composition of velocities. tl;dr What I described is measured only in the inertial observer's frame. It seems like to do what you described you would always have to choose a single observer for which the symmetry holds.

No, they don't experience the same thing. In the typical twin setup, the twin that turns around experiences its change of direction halfway through the round trip. The inertial twin only experiences or observes this much closer to the end of the round trip, due to delay of light. You could do this by having N observers all travel for a fixed time, then return for an equal amount of time, where the N+1'th observer travels at half the velocity of the N'th. Then you could easily calculate their relative aging, all in agreement with SR (where your hypothetical argument is wrong). The 21st observer would travel at about a millionth the speed of the 1st observer. If the first is very close to c, the 21st would be close to 80km/hr, and would experience negligible relativistic effects. Edit: Even the 7th traveling observer at about 1.5% c would measure less than half a second difference per hour relative to the inertial observer. Relativistic effects diminish quickly as velocity decreases. Yes. They'd also count an identical number of observed ticks of any clock. They'd both agree that Twin A's clock ticked a certain number t, and that Twin B's ticked a certain number t', but t and t' are not equal in the typical twin experiment. Check out http://en.wikipedia.org/wiki/Twin_paradox#Viewpoint_of_the_traveling_twin for a description of using the equivalence principle to consider the traveling twin to be stationary. This is done in terms of an equivalent gravitational field (the same twin experiences proper acceleration either way). Then continue reading the next section http://en.wikipedia.org/wiki/Twin_paradox#Difference_in_elapsed_time_as_a_result_of_differences_in_twins.27_spacetime_paths to see the calculations involved in considering acceleration over a non-negligible time. In my opinion this is a more complex situation that does nothing to help understand the twin paradox, but does show that the simplest cases are consistent with more complex ones in SR. Notice that there are 6 phases in this version, 4 of which involve acceleration, but that the 2 other phases are exactly the same as the simple case where acceleration is neglected. In other words, the acceleration phases do not affect what happens during the constant v phases, and the twin paradox is readily apparent with only those constant v phases.

Astronaut Chris Hadfield apparently spends a ton of time on the internet while in space, answering questions on Reddit and posting videos of low-g experiments and junk. So people have experienced what you just described. http://www.reddit.com/search?q=hadfield

But there are absolute and apparent horizons, and the latter can be different depending on how it observed. Is there any possibility of a "coordinate black hole" where one observer sees a black hole and another doesn't? (I've speculated on related junk and you didn't like it, sorry if I'm going back in that direction.) I assume that an absolute black hole would be measurable with proper time etc, so that if a particle determines that it's in a black hole, then there's not really any debate -- it is in a black hole. So if there are relative black holes, I think it would require that one observer could measure that another is in a black hole, while that other observer does not measure being in a black hole. Is there anything like that in theory, ie. "extreme relative curvature"?

Stationary in space is relative to a chosen frame of reference. Why can't "stationary in time" be exactly the same, with a convention where you are always at time 0 in your chosen frame of reference and the time of past events is constantly increasing relative to you? Edit: Nevermind, even if chose coordinates where you stayed at time 0, it doesn't mean you stay fixed relative to any event, while it does make sense to stay stationary in space relative to some object.

So to explain "membrane", you switch to a new term "parameter", use it as if it's a thing without explaining what it is, but still use "membrane" along with some other undefined terms that seem to be added in too. And instead of explaining previously made claims, you just pile on new ones! No worry that this speculation on gravity doesn't explain gravity, it could also explain all geological activity! I'm glad you're not daunted by people who say they come to the forums to laugh at people. If you're trolling, well done. Personally I think it's terrible that all the energy around here goes into fighting the people who don't give up on a bad argument, and none goes into encouraging scientific imagination in those who do. Those who would be put off by someone laughing at their ideas are encouraged to stay away, or to not even bother trying to think of scientific ideas.

Asking a question about an idea and having it be ignored doesn't prove the idea is wrong. Especially when the question is based on an assumption, eg. that the source of a push force would have to be a single point and thus move fast. Some of the ideas I think have been shown to be wrong. Others have only been questioned, and the requirement that the ideas be backed up has been neglected. I guess this is what "not even wrong" means, where ideas and claims are just piled on, without even enough explanation or specification enough that they can be proved right or wrong. Also I guess that's why the forum rules are as they are, because you can't explain every step of how to usefully reason about science, when any attempt to do so is ignored.

How does it being a conservative field have anything to do with that? Edit: Never mind, I think I see your point now.

You're doing this wrong, over and over again. Don't make up an idea and show that it explains one or two things that are already explained. If you want to explain something new, go find something that existing theory doesn't explain. Perhaps some connection between gravity and other forces or phenomena. Or, if you're going to try to explain what accepted theories of gravity already explains, you're going to have to explain it ALL. You can do that by showing that your theory is equivalent to what already exists. For example, you can show that a "pushing" theory is equivalent to a "pulling from a point" theory by showing that if something is pushing from all points EXCEPT where some mass is, the resulting force vector is equal to a pull from that one point. You can handle distances in different ways, preferably in a way that you don't have to worry about the shape of space at infinite distances. Then, you could figure out a testable difference between your pushing and an otherwise equivalent pulling theory. Since there are no known testable differences, no mechanism that shows a pulling force, and certainly no known pushing mechanism, then unless you found one you would come to the conclusion that it is impossible to say for certain that gravity is either a pushing or a pulling force. And that conclusion fits with our current understanding of gravity: it is an effect of spacetime curvature, and that explains it, without needing a mechanism for either pushing or pulling force. Based on what is known, there's no point in arguing that it's either one or the other. To say either for certain, you're going to have to find something new. Also, it is foolish to assume that the effects of tides or orbits are unexplained by existing theory with respect to gravity. Why not look up or ask about what things are currently not explained by accepted theories? (Why not look up anything? Why make foolish claims that could easily be checked?)

I was thinking something that is also true but along an opposite line of thinking... Many people think that words and maths are mutually exclusive or somehow in conflict with each other. Both words and maths have meaning only because what they represent is consistent. If you use the word "electron" it has meaning because it consistently represents something understandable. Similarly, if I combine 4 baskets with 10 apples each, then the count of apples is reliably and consistently represented by using multiplication. Just like I can't just use made-up words to convey meaning, you can't just use made-up math to describe a hypothesis, or expect that math alone explains how things behave. Either words or math must be carefully chosen to properly represent the meaning. Maths represent many different consistent relationships between many different types of things, that can be symbolically represented, and the consistency of those relationships lets you make conclusions based on the maths. Some words represent relationships too. The consistent relationships of physical aspects of the world are often much more efficiently described with math. Whether using words or maths, the value in either is what meaning (and its quality and quantity) that you can convey.

But isn't "a small region of extreme spacetime curvature" an answer to the question of how a photon traveling linearly at c could be confined to a quasi-stationary space? There is a known mechanism in the case of black holes, but couldn't the problem be shifted from "how can something travel linearly and be confined to a quasi-stationary location?" to describing such a mechanism in other cases?

Split from http://www.scienceforums.net/topic/74079-matter-are-em-wave-packets/ Doesn't exactly this happen with black holes? Is it conceivable that any particle of matter could have the equivalent of a black hole in it?

I think you're all nuts, maddened by the desire for the other to be wrong while losing grip on anything right. Why are you arguing whether it's a pulling or pushing force? There's nothing physical doing a pushing or pulling that makes a detectable difference either way. If I were to say "General relativity's wrong because it says that gravity is due to spacetime curvature, but gravity is already known to be due to a force pulling on masses," then I'd probably get a lot of "It's not actually a pulling force" arguments. But you'll construct ridiculous arguments to show that it is a pulling force if someone says it isn't. Spacetime neither pulls nor pushes from a single location. It's essentially a mathematical or conceptual convenience to call it either a push or a pull, unless you've all discovered something new here. ------------------------------ Edit: Okay so I reread some of the examples I was seeing, and as usual I haven't bothered to comprehend them until after I posted. Note that swansont's arguments in this thread are not in favour of physical "pull" forces but only arguing against "push" forces, and only seemed ridiculous due to my misinterpretation of them, and my mistakenly grouping them in with other ridiculous arguments for "pull" gravity.

Imagine exploring the 3d world as a 2d sensor, like an eye or a digital camera's sensor. Imagine you have no understanding of 3d geometry. You see a projection of the 3d world onto your 2d senses. What you see is a 2d image at any moment, but it rotates and distorts as you move through the 3d world. Lengths appear longer or shorter as you move closer or farther from them (along an extra dimension that you can't directly perceive). Lengths appear to shorten as they rotate toward lining up with that 3rd axis. Things rotate, revealing different sides of things, as walking through a doorway reveals the other side of a wall. This is also how animations of 4d objects appear: they twist and distort and lengths change. (They are 4d objects projected into a 3d space---a projection that is weird to us---which is then projected onto a 2d surface making an image---a projection which is familiar to us.) What would be interesting is to build a virtual 4d world, something like the 4d equivalent of a simple house, and project it onto 3d geometry and then explore it like a video game. What I'm wondering is whether the brain would be able to assemble a consistent mental image of the world. Like, if we see a perspective image of a cube, we effortlessly process it into a mental image. We see distorted edge lengths but are able to intuitively understand that they're the same length. Would our brains be able to understand the geometrical relationships of points in the 4d world, just by experiencing it? Is geometrical understanding possible due to simply training of the brain, or are our brains hardwired to understand 3d?

Well uh, to be less harsh... I think it's well-written and an interesting work of scientific imagination, a start. I think that to refine an idea sometimes requires major transformative changes, like a sculptor changes a block of marble by getting rid of everything that doesn't fit in the final work... The details I think still need to be figured out here.

Just because I have OCD, here's how I think that the twin 'E' can leave at velocity 0.866c, and appear to return at velocity -0.866c, and trick twin 'R' (who remains at rest) into thinking that E has aged twice as much as R (the original hypothesis of the thread): After a time of 1 year at R, E has appeared to age 0.268 years, and appears to have reached a rest (R's frame) distance of 0.232 LY. If at that moment E disappears (turned off its signal), and an imposter E' at a rest distance of 3.232 LY appears, timed to seem simultaneous to R, then E' can travel at velocity -0.866c for a time of 3.732 years, appearing to age that much while R ages 1 year. So E and the imposter E' are seen aging a total of 4 years while R ages 2. R has been tricked, as I have, by not realizing that E' had to travel so much more on the inbound trip than E traveled on the outbound trip. The trick might work if R is unable to distinguish between the two different rest distances, and is dum.