Thursday, November 6, 2014

Gravity alone does not explain the Arrow of Time

Not sure if you all have seen the recent article by Julian Barbour about an arrow of time arising from a purely gravitational system. If not, check out the following articles in Physics or Wired.
First off, the title of the articles contradict the substance of the articles.
Julian Barbour has shown that a system of 1000 objects interacting only via gravity can start dispersed, then clump together, and then disperse again. That's it. This is not exciting work. This was a similar problem to one that I was assigned in a freshman level computer programming class...just with ~100 objects rather than 1000 particles.

Second, Julian Barbour has shown that there is no arrow of time of for such systems, i.e. there is no way to tell the future from the past. (This is very different  than let's say 'life', which only runs in one direction. You are born, you remember the past, and you eventually die.)

As such, Julian Barbour has re-proven something that has been known for quite awhile:  In a system of particles that only interact via gravity, there is no arrow of time.

How can scientists and journalists mess this one up so badly?  Thoughts?


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