## Tuesday, May 9, 2017

### WHAT IS ARROW OF TIME?

I had read that laws of physics are symmetric in time.
If so then why we always see time moves forward?
I have the impression that most physicists think arrow of time must be caused by entropy.
Because it seems generally entropy is always increasing in the universe.
I know that entropy is a measure of disorder and it seems increasing the temperature of any gas/liquid/solid increases its entropy.
If so can we say increasing temperature of anything must be slowing down time for that thing?
(Can we try to measure slowing of time by keep heating a transparent gas and keep measuring speed of light when light passing in that gas?
Or are there other kinds of systems we can use as clock when getting heated up?)
My guess is answer is no that time would not slow down with increasing temperature.

Could there be another explanation for arrow of time?
If we are moving forward in time what keeps us from moving backward in time?
Is it Grandfather Paradox (which must apply to anything traveling backwards in time)?
Or is it what is called causality (cause and effect)?
(I think Grandfather Paradox is just another description of causality or more like a special case of it.)
I think causality is the real reason why we cannot move backwards in time, and causality itself is the arrow of time.
So I think if any physical system/experiment can break causality, then there will be something moving backwards in time in there.