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Relativity/twin paradox


Dear Uncle Ben, I'm not sure if I can ask you a question so soon after my first  re the visibility of relativity effects from the non moving frame.  Your answer led me to another question  and if it is too soon  i understand.,

You said that it would be possible to actually  observe pulses emitted  from the moving frame (train) that would be slower than  those emitted from the bystander in the non moving frame.

Translating this to effects of aging you could assume that a bodily clock (heart) in the moving frame  could (theoretically and fantastically ) could emit a pulse too and it would be slower as seen by the bystander in the non moving frame.

But this silly scenario would have to be vice versa and the bystander heart would be emitting slower beats to the observer on the  moving train-- slower than his..  Silly it is but relatavistically speaking  I think this is the way it goes--

But how could it be possible -- two hearts beating slower and thus you could say aging slower at the same time and in real time?
Tho never the twain will meet in a fantasy of the two traveling forever  -- no acceleration or deceleration train stopping, etc.-- there will be  two physical states of slower hearts that exist in actuality--on down the road.  is this possible?  
Thank you very much,

Joan, the twin paradox has bothered people a lot since Relativity was introduced.

The answer is that while the twins are separating, there certainly is a mutuality between them.  Let A be the stay-at-home and B the traveller. Then clocks on one relative to the other run slow because one is moving with respect to the other. The "proper"rates, that is, the rate of each clock with respect to itself, are still what they always were.

This is the hard thing to accept about relativity: A clock has more than one rate according to the frame of reference the rate is measured from. That is really what "relativity" means. Motion is relative, length is relative, time is relative. "Absurd", says the critic!  

Easy example: You are in Boston, maybe, and I am in Albany. My brother is in Springfield. To me, he is in the east. To you he is in the west.  Which is it? East or west?  Obviously both are true, because positions are relative to where zero is.  That is easy.  What is hard is that time intervals are relative to frames in which the clock is moving.  That is mind-bending.

Back to the twins: When B turns around to come home to A, that breaks the symmetry. B changes direction; A does not. That is why B comes home younger than A. Change in velocity is not relative!

We may never understand this in the way we understand my brother's position, but experiment shows that it is true.  You can read about experiments in which an atomic clock is flown around the world. When flown around the world and compared, the clock had lagged behind a stationary clock. See Wikipedia.

Relativity promises to make correct predictions.  It does not promise we will understand them intuitively.



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Uncle Ben


I can answer questions regarding Einstein's Theory of Relativity, particularly in Special Relativity. I will not answer homework questions or mathematical problems that require special symbols.


I have taught physics at the college level, undergraduate and graduate, for many years including Special Relativity. I have taught at Johns Hopkins, Case-Western, and MIT. I have also served as a staff member of the Commission on College Physics, which was supported by the National Science Foundation to recommend improvements in the curriculum of college physics departments in the US. I am also the author of a textbook titled Vector Calculus, which was used at MIT in the teaching of electromagnetic theory and relativity. My research interests were mainly in solid state physics, especially the properties of metals at low temperatures. I am listed in the publication known as American Men of Science.

I have dozens of papers published in the Physical Review and in the American Journal of Physics.

I hold a Ph.D. degree in physics from the Johns Hopkins University.

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Johns Hopkins University, Case-Western Reserve University, Massachusetts Institute of Technology, Empire State College, Georgetown University, Commission on College Physics, and UNESCO.

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