Share this thread: 
#37
Sep2308, 06:39 AM

P: 1,060

Hello neopolitan.
Quote: So if you were designing this clock, what would it do? Why would i need such a clock. Why not just calculate A's time. Should you wish for a display of the results of this time calculation then that is easy enough. Matheinste. 


#38
Sep2308, 11:33 AM

Emeritus
Sci Advisor
PF Gold
P: 9,417

So far this definition of simultaneity agrees with the one I posted in #18, but have you really thought about what it means? Do you agree or disagree with my claim that the blue lines are simultaneity lines of the rocket when it's moving away from Earth? What about the simultaneity lines when the rocket has turned around? 


#39
Sep2308, 06:03 PM

P: 801

While such a clock would not be "needed", it is interesting to note how it would have to work. It would have to be rigged so that acceleration would cause it to speed up. Great question, neopolitan. Why am I the only person to even attempt to answer it? Al 


#40
Sep2308, 07:32 PM

P: 1,060

Hello Al68.
This is purely a reply to your remark in #40. To disagree with a point of view is absolutely acceptable as is my right to put forward my view, which on this subject is very much mainstream. I know you do not mean to criticise religion as such ( but it would not bother me if you do ) but the implication is that answers i have given to the best of my admittedly limited ability are given as a matter of unthinking, blind faith, in the manner of a crusade. My answers and those of many others on this question may in your eyes be incorrect but do not deserve to be called religious with all the underlying disparaging connotations of the word. The word itself is of course not offensive but its implications in your remark are. Matheinste 


#41
Sep2308, 09:05 PM

P: 801

Al P.S. I don't believe your answer on this question is incorrect, just incomplete. 


#42
Sep2308, 09:17 PM

P: 421

Jumping time is a misconception due to the instantaneous reversal of direction. In reality, it would be a gradual process and the earth signal frequency would increase accordingly. 


#43
Sep2308, 09:32 PM

P: 645

The something moving from one event to another event is the information which I have discussed previously, a signal. A signal moving from "A" to "B". My contention is that even if "B" undergoes a change of frame, the calculations which "B" uses should not be used in such a way to indicate that this signal sent by "A" at (20,0) was simultaneous with any event at "B" earlier than (12,0). That is a consequence of the implication in your diagram (Twins.PNG) that according to "B", "A" suddenly ages 25.6 years. With the information that "B" has to hand, there is no need to make such a ridiculous claim  even if it may be standard simultaneity fare. The 25.6 years is based on realigning the frames with the end result, so that "A" is a nice 40 years old when "B" gets there. However, it is not real. The clock I discussed with Matheinste won't suddenly scroll forward from 7.2 to 32.8 years. And here is why not ... the 32.8 year figure is based on "A" not moving at all during the 20 years. That means that the clock would have to somehow predict the future. This is totally separate from the issue that the calculation behind the 32.8 years is based on a combination of situations, the bastard son of two frames, and that the calculation totally ignores how information flows in the universe. "B" should, at the turnaround, make a projection that "A" has aged a total of 20 years. Not 32.8 years. cheers, neopolitan PS Perhaps you might like to create a chart which maps the "A" events which are, according to "B", simultaneous with "B" events. Make all the events ageing events, ie '"B" has aged x days, this is simultaneous, according to "B" with "A" having aged y days' and plot y against x. In my version, there will be a straight line (with a little bump in the middle if I am going to be pedantic), since "B" effectively maintains the same speed the whole time (0.8c) and I will not be ignoring the information that "B" receives. In your version, there will be three straight lines  (0,0) to (12,7.2), (12,7.2) to (12,32.8) and (12,32.8) to (24,40). Which sounds more representative of a realworld situation? PPS phyti has approximately the right sort of diagram. His figures are for a shorter trip and show the situation in a different way, but at each end of phyti's diagonal lines are the simultaneous events which I suggest you chart, Fredrik. 


#44
Sep2308, 10:21 PM

P: 801

Al 


#45
Sep2308, 10:33 PM

Emeritus
Sci Advisor
PF Gold
P: 9,417

This discussion doesn't seem to be going anywhere, so I might withdraw from it after this. 


#46
Sep2308, 10:38 PM

P: 801

Al 


#47
Sep2308, 10:42 PM

P: 1,473

I have a different take in the twins thing  first of all, solve the problem by reducing it to two one way trips  and double the result to get the total age difference  so on the outward bound one way trip use two clocks, one on earth, one at the destination  they are synchronized in the earth frame and always read the same  the distance to the traget is vt where t is the lapsed time in the earthtarget frame, and ct is the temporal distance the earth frame has moved during the one way trip  so the space time path followed by the traveler is a composite of the the space and temporal increments  during this interval, the travelers clock logs a time t' and the temporal distance for the traveler is ct'
so (ct)^2 = (ct')^2 + (vt)^2 from which you get Gamma and the amount of time that difference between the two frames, that is t' = Gamma(t) You don't have to send signals, you don't need to get involved with turn around accelerations, and it doesn't make any difference whether the earthtarget frame is moving or the traveler  all you need is Minkowski orthogonality of space and time and realize that each frame has taken a different space time path so since one frame has moved only in time and the other frame has experienced both a space and time increment  the intervals will be equal, but the components of the interval in each case will be different 


#48
Sep2308, 10:56 PM

P: 645

Fredrik and Al,
Perhaps we can break the deadlock if you explicity state the assumptions of your conclusion(s). Here are the assumptions of my conclusion, as far as I can tell:
I do think that the last is important, and is possibly the sticking point. Please present your assumptions, then have a go at mine, if you so wish. cheers, neopolitan PS as yogi points out, signals are not really strictly necessary, assumption 4 would eliminate the need for assumption 1. But I keep the signals to force some real universe thinking in the example. 


#49
Sep2308, 11:11 PM

P: 801

Al 


#50
Sep2308, 11:38 PM

P: 801

I actually see what you're saying, I think. I answered your question about how a clock would work that is supposed to show earth time on the ship. And I agree with you that there is a discontinuity associated with instantaneous turnaround, but it's not real because instantaneous turnaround is not possible. Such a clock would simply run fast relative to clock "B" during any acceleration (toward earth). And it would always show the time on earth simultaneous with any moment on the ship (or a comoving inertial frame). I would use your assumptions, although #1 wouldn't be necessary. Al 


#51
Sep2408, 01:37 AM

P: 645

Acceleration qua acceleration has no effect on timing, only as a consequence of the altered relative speeds. (And simultaneity is only altered as a consequence of altered relative velocities.) cheers, neopolitan 


#52
Sep2408, 02:55 AM

P: 1,060

Hello neopolitan
As regards the turnaround, it need not be abrupt, it does not matter how long it takes. For the sake of my present post let us assume that it is takes a finite time. Now this finite time could be almost instantaneous, fairly fast………very slow, imperceptibly slow.. If the turnaround took a long time and was smooth and continuous and the clock showed a very slow advancement ( the one with B telling A’s time ), would you accept this. If you would, then let us crank the rate of turnaround up a notch and ask if you would accept this also ( assuming you accept the first degree of slowness ). Obviously you see where I am going. If the discontinuity bothers you, by the way I think it makes for a bad scenario and would not have used it myself, lets just do away with it. If that was your worry would you accept scenarios with increasingly fast but smooth turnarounds and if not, at which point would they cease to be acceptable, assuming you accept accelerated rates of clock advancement of A’s clock from B’s point of view, at at all? Matheinste. 


#53
Sep2408, 04:29 AM

Emeritus
Sci Advisor
PF Gold
P: 9,417




#54
Sep2408, 06:07 AM

P: 303

Regards, Bill 


Register to reply 
Related Discussions  
Twin paradox  Special & General Relativity  43  
Twin Paradox  Special & General Relativity  5  
Twin paradox  Special & General Relativity  1  
Twin Paradox  Introductory Physics Homework  39 