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Q & A: Time reversal invariance

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Most recent answer: 05/07/2014
My understanding is that there is no particular reason that time must always move forward, that equations involving time are fully reversible, etc. I have seen a number of physicists discuss the idea that we never see a glass go from broken to whole, or an cracked egg go back into its shell, etc. My question is since our perceptions and memory are based on a kind of accretion of data, how would we ever perceive time moving any direction other than forward? Couldn't time reverse, or only tick by for one second and then hold steady for a millenia and tick another second and we would perceive it as smoothly moving forward? In other words, when physicists claim that the arrow of time is uni-directional what external data could they rely on to support the claim?
- Ivan Record (age 37)
Bloomington, IN, United States

Hi Ivan,

This is a pretty tricky question and has worried people for a long time.  Suppose you made a movie of the Earth-Moon-Sun system. If you played the movie backwards you couldn't tell the difference in the sense that the bodies would still obey Newton's Laws.  Billiard ball collisions are the same.  Similarly electromagnetic reactions are unchanged under time reversal.   But in the weak interactions, for example beta decay, there seems to be a tiny-tiny asymmetry.  It is an extremely difficult experiment and was only observed in the last few years.  Here is an article about it:  . 

I don't think your 'stop-motion' model of time works.  Space-time would have to be quantized.  I'm not sure how but I think general relativity would rear its ugly head into the problem.   I know of no experiments on this topic.



Like Lee says, this is a problem people have worried about for a long time. On your "stop-time" idea, I'm not sure that would even mean anything. It has no observational consequences. On the question of why various arrows of time (psychology, entropy increase, quantum measurement, cosmological expansion,...) are aligned the way they are, there has been a lot of thought and progress. For example, it's understood that the increase of local entropies and the "measurement" increase of quantum entanglement between remote regions are mathematically equivalent. Nevertheless, the most basic question of why the almost perfectly reversible micro-laws somehow make up a very asymmetrical universe is not really answered.  Mike W.

(published on 05/07/2014)

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