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Einstein's 1905 postulates entail that, if two clocks are in relative motion, either clock is slow as judged from the other clock's system (SYMMETRIC time dilation). An obvious implication is:

Time SPEEDS UP for the traveler.

That is, any traveler who checks stationary clocks against his spaceship's clocks will find stationary clocks slow and his spaceship's clocks FAST. This also means that the traveler sees himself aging FASTER than stationary people.

Physicists teach the opposite of what Einstein's postulates entail:

Time SLOWS DOWN for the traveler.

Examples:

Brian Greene: https://youtu.be/Q1y3YnPgaY4?t=1157

Albert Einstein 1911: "The clock runs slower if it is in uniform motion..." http://einsteinpapers.press.princeton.edu/vol3-trans/368

Richard Feynman: "Now if all moving clocks run slower, if no way of measuring time gives anything but a slower rate, we shall just have to say, in a certain sense, that time itself appears to be slower in a space ship." http://www.feynmanlectures.caltech.edu/I_15.html

Brian Greene: "If you're moving relative to somebody else, time for you slows down." https://m.youtube.com/watch?v=QnmnLmwBmfE

Neil deGrasse Tyson: "...Einstein's special theory of relativity, which gives the precise prescription for how time would slow down for you if you are set into motion." http://abcnews.go.com/Entertainment/back-future-30th-anniversary-neil-degrasse-tyson-talks/story?id=32191481

Jim Al-Khalili: "Einstein showed that for anything (or anyone) travelling at speeds approaching that of light...time literally runs more slowly." https://scienceinschool.org/2009/issue11/timetravel

Why do physicists teach the opposite of what Einstein's 1905 postulates logically entail?

Pentcho Valev
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1 Answers1

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An obvious implication is:

Time SPEEDS UP for the traveler.

That is, any traveler who checks stationary clocks against his spaceship's clocks will find stationary clocks slow and his spaceship's clocks FAST. This also means that the traveler sees himself aging FASTER than stationary people.

You have this somewhat backwards. The key issue is that speeds are relative and relative quantities require the specification of a reference frame from which they are measured. It is best if the specification of the reference frame is explicit, but often it is just implicit and you must look at context clues.

A clock is always at rest relative to itself, so when a clock is described as “moving” then implicitly we are not using the clock’s reference frame. We are instead using the reference frame of some external observer and comparing the clock to that frame. So whereas you said “traveler who checks stationary clocks against his spaceship's clocks” the comparison implies when talking of a moving clock is instead “stationary observer who checks the traveler’s clock against his stationary clocks”.

If you find the implied frame difficult to determine, then the invariant distinction is which observer is using a single clock at both events and which observer is using a pair of synchronized clocks. The time will always be less on the single clock than on the pair of synchronized clocks.

Dale
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