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Comment by chc | Hacker News Reader

But that still isn't the same thing as "forever." Time does not go back infinitely — according to current theory, the universe definitely has a beginning in time.

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The whole dimension-tautology has always irked me. Time, as a unit, is simply the "time" it takes for light to travel a certain distance. Distance, OTOH, is the "distance" it takes for light to travel under a certain amount of time. AFAIK one of the authors of this theory is a proponent of a variable speed of light. So if at some point in time light didn't move, then time didn't move, then that would mean that our universe existed since forever...? Not a physicist, but sometimes I feel weird when I am reminded of the fact that the Big Bang had a definite starting point 13.8B years ago (as you have done so), and then think about how time is really... a function of length and length is just a function of time.

> Time, as a unit, is simply the "time" it takes for light to travel a certain distance.

Without time there is no interaction between two atoms. There is no movement. It's not only about light traveling.

> The only reason for time is so that everything doesn't happen at once. -Albert Einstein

The Einstein quote is great. I would add: The only reason for time is so that everything can't be understood at once. 'Stretching' and 'scaling' as the foundational principle of distances and time makes the idea of a starting point needless. I wrote about it here: http://hashsign.co.uk/contemporary/by/thoughts/pop-pop-bang-...

>Time, as a unit, is simply the "time" it takes for light to travel a certain distance.

I believe that time is currently defined in terms of the period of a Cesium isotope.

No, time is defined in terms of atomic properties. It is not a function of length. (Length is, however, a function of time). A second is defined as the duration of 9,192,631,770 periods of the radiation corresponding to the transition between the two hyperfine levels of the ground state of the caesium-133 atom.

A meter is defined in terms of the speed of light and the definition of a second. A meter is exactly the length of the path traveled by light in a vacuum during a time interval of 1/299,792,458 seconds.

Ah, thanks for that. I look pretty stupid now...I have actually heard of that definition but somehow I was thinking of time being ultimately defined by Planck units (time required for light to travel a Planck length).

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