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# Relativity/uniform frames of reference

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Dear Uncle Ben,

This question is silly but maybe it is a path to some kind of understanding.  Would it be possible to see the effect of time dilation as regards youthful appearance from one uniform frame of reference to another  as in---- person standing by side of road  who looks  in train window passing by and sees friend looking ten years old  when person knows she embarked at age twenty?

This scenario, of course, has to involve vastly enlarged relativity effects from a science fiction fairy tale but that aside---- would it be possible as the light travels from the train window and  friend's face to the  eyes of  inertial person by the  road , as light always carries images,  would a ten year old (and thus ten years younger face) be seen?

No acceleration involved. ONly thing leaving the moving frame is the light.

Thank very much,
Joan

Answer
Joan, viewing relativistic effects from a frame of reference in which an object is moving is a tricky business. For example, if a rectangular block is whizzing by, it looks as if it is rotated! I have not studied that effect myself, but it is well-known.

So to observe a person whizzing by is hard to understand.

What might be easier to understand might be a radio-emitting clock going by, and you pick up the ticks.

It is true that the rate of ticking relative to you is slower than its proper rate (relative to itself).
But even there you have to account for the Doppler effect:  the rate will be higher as it approaches and lower when it recedes. You can read about the relativistic Doppler effect in relativity books.

After correcting for the Doppler effect, you would find that the clock ticks slower than your stationary clock.

So it isn't exactly simple.

Ben
Questioner's Rating
 Rating(1-10) Knowledgeability = 10 Clarity of Response = 10 Politeness = 10 Comment Dear uncle Ben , Sorry I took so long to answer. Thank you for your answer. It is clear and logical. My question was fanciful but when you answer as if it were real I see better what relativity effects are. Some day I would like to ask you about the twin paradox.

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#### Uncle Ben

##### Expertise

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.

##### Experience

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.

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

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

Past/Present Clients
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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