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Q & A: What is time?

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Most recent answer: 12/22/2009
Time is such a strange concept. What is time? I have heard that in some sense time is defined by the speed of light and the expansion of the universe. How does this work? Thanks, Trev
- Trev (age 13)
United Kingdom
My old college physics professor said " time is vot you measure mit your vatch" (he had an accent). He was right.  What he didn't explain was how your watch is calibrated.   In the old days time was calibrated in terms of the rotation period of the earth. The day was divided into 24 hours, the hour into 60 minutes and the minute into 60 seconds.  As timekeeping apparatus became more accurate it was realized the earth's rotation had fluctuations in it and needed to be replace by a more accurate standard.  These days the standard SI second is defined by the number of oscillations of a certain radiative transition in Cesium atoms.  

So, to answer your question, time has nothing to do with the speed of light or the expansion of the universe.  It is defined by the number of ticks of an atomic clock. It is a definition, not a derived quantity.   See   for more details.


(published on 07/22/2009)

Follow-Up #1: Can you create a time machine?

Is it possible to create a time machine and what would the consiquence be if you sent something back into time? Would someting dramistic happen like what happened when an atom is spitted? thanks, bowen
- bowen (age 13)
Altanta, georgia usa
So far, it's been done only in movies and science fiction stories.   The current consensus is that it's impossible.   What would happen if you could do it is anybody's guess.


(published on 12/22/2009)

Follow-up on this answer.