# Tag Archives: arrow of time

## Reversing Reality

“Time is out of joint.”

I’ve long puzzled over the idea that physics is reversible. That its laws, with some caveats, work the same if time runs forwards or backwards. It’s even been suggested that, except for entropy, time could run backwards just as easily as forwards.

But this seems contrary to our everyday experience. With some exceptions, we can tell if a film or video clip is shown in reverse. Objects that fall, break, or grow (such as plants or crystals), look different seen in reverse.

I think there is more going on there than just entropy.

## Smolin: Time Reborn

I’ve been reading science texts almost as long as I’ve been reading anything. Over those years, many scientists and science writers have taught me much of what I know about science. (Except for a Computer Science minor, and general science classes, most of my formal education was in the Liberal Arts.)

Recently I read Time Reborn (2013), by Lee Smolin, a theoretical physicist whose personality and books I’ve enjoyed. I don’t always agree with his ideas, but I’ve found I do tend to agree with his approaches to, and overall sense of, physics.

However in this case I almost feel Smolin, after long and due consideration, has come around to my way of thinking!

## Entropy Isn’t Fundamental!

particles & their momenta

Over the decades I’ve seen various thinkers assert that entropy causes something — usually it’s said that entropy causes time. Alternately that entropy causes time to only run in one direction. I think this is flat-out wrong and puts the trailer before the tractor. (Perhaps due to a jack-knife in logic.)

The problem I have is that I don’t understand how entropy can be viewed as anything but a consequence of the dynamical properties of a system evolving over time according to the laws of physics. Entropy is the result of physical law plus time.

It’s a “law” only in virtue of the laws of physics.

## The Puzzle of Entropy

I’ve been chiseling away at Cycles of Time (2010), by Roger Penrose. I say “chiseling away,” because Penrose’s books are dense and not for the fainthearted. It took me three years to fully absorb his The Emperor’s New Mind (1986). Penrose isn’t afraid to throw tensors or Weyl curvatures at readers.

This is a library book, so I’m a little time constrained. I won’t get into Penrose’s main thesis, something he calls conformal cyclic cosmology (CCC). As the name suggests, it’s a theory about a repeating universe.

What caught my attention was his exploration of entropy and the perception our universe must have started with extremely low entropy.

## Time and Thermodynamics

There is something about the articles that Ethan Siegel writes for Forbes that don’t grab me. It might be that I’m not in the target demographic — he often writes about stuff I explored long ago. I keep an eye on him, though, because sometimes he comes up with a taste treat for me.

Such as his article today, No, Thermodynamics Does Not Explain Our Perceived Arrow Of Time. I jumped on it because the title declares something I think many have backwards: the idea that time arises from entropy or change. Quite to the contrary, I think entropy and change are consequences of time (plus physics).

Siegel makes an interesting argument I hadn’t considered before.

## Time and Two Doors

In the last week or so I read an interesting pair of books: Through Two Doors at Once, by author and journalist Anil Ananthaswamy, and The Order of Time, by theoretical physicist Carlo Rovelli. While I did find them interesting, and I’m not sorry I bought them (as Apple ebooks), I can’t say they added anything to my knowledge or understanding.

I was already familiar with the material Ananthaswamy covers and knew of the experiments he discusses — I’ve been following the topic (the two-slit experiment) since at least the 1970s. It was nice seeing it all in one place. I enjoyed the read and recommend it to anyone with an interest.

I had a little trouble with the Rovelli book, perhaps in part because my intuitions of time are different than his, but also because I found it a bit poetic and hand-wavy.

## Got Time?

For the last week or so, on a physics blog I follow, I’ve been part of a debate about the nature of time. It’s been interesting and fun, but the conversation has reached that point where folks are mainly maintaining their positions, and it seems that the matter has stalled.

Some of the on-going assertions bemused me so much, that I was about to tender one more rebuttal comment… When I remembered what a wiser person, “back in the day” (before the web), said about online debates: State your view. Support it further if you need to address points raised. But once you’ve covered it well enough, just stop. After that, you’re just wasting your time; it’s rare that anyone changes their mind on the internet. Including yours.

Fair enough. I can natter on about it to myself on my own blog, though…