Metanet Forums

http://en.wikipedia.org/wiki/Ilya_Prigogine

http://en.wikipedia.org/wiki/Entropy_%2 ... 9#Overview

I recently had something absolutely fascinating explained to me about modern physics, specifically the work of Ilya Prigogine. I doubt I will recount everything to (scythe's) satisfaction but I can try. Basically, classical mechanics, relativity and various other aspects of physics have always relied on probabilities to explain reversible and irreversible phenomena -- that, given infinite chances, a stone dropped in a pond would create ripples going inward instead of outward, to pick one example. However, this would never actually be witnessed in nature. It's an indeterministic, random view of physical events. On the face of it, the inward ripples seem ridiculous, but, say physicists, given enough instances they would happen.

Prigogine realized that this was possibly bullshit, and he had a counter-argument involving the insertion of a time element into mechanics and relativity (and thermo; see the second link). There isn't one in Newton or Einstein or Boltzmann's equations (when I say there isn't a time element, I don't mean that there isn't a variable t -- I mean that there isn't an inherent causality. You can run the equations forward or backward disregarding conventional/naturally observed cause and effect*). As near as I can tell, Prigogine's new equations would say that inward ripples cannot exist because time flows in one direction and the cause (stone dropped in pond) could therefore never yield the effect (inward ripples rather than outward).

Thoughts? I didn't express this perfectly but I think that's the gist of Prigogine's ideas. I'm currently looking for a copy of his seminal work, Order out of Chaos.


*Think back to your basic physics problem, a mass sliding down an incline. Under classical mechanics, you can reverse the problem and have the mass slide up the incline while still following every one of Newton's laws -- even though this is clearly impossible.

EDIT: two out of the three people I've shown this to don't understand it :(

So classical physics didn't have a time element, which people didn't really care about (or notice), and this guy just realized that there needed to be one to explain why things didn't happen?

ENT474 wrote:So classical physics didn't have a time element, which people didn't really care about (or notice), and this guy just realized that there needed to be one to explain why things didn't happen?

The lack of a time element was generally explained away (in Schrodinger, or Einstein's work) by 'probability'* -- things that Newton's hypotheses made possible that didn't seem natural could happen, given enough time, through chance. But they weren't observable in nature.

*http://en.wikipedia.org/wiki/Determinism

Prigogine, on the other hand, supported indeterminism*. He recognized time reversibility* as a somewhat fundamental problem with the work of his predecessors. Basically, he didn't buy the deus ex machina that anything could happen, eventually. He looked for another explanation.

*http://en.wikipedia.org/wiki/Indeterminism#Science

*http://en.wikipedia.org/wiki/Time_reversibility


This subsection sums everything up pretty well, actually: http://en.wikipedia.org/wiki/Prigogine# ... _Certainty

Related: Einstein takes an interesting viewpoint on this which appeared in some random magazine called Eureka (monthly publication by the Times in the UK); here is a section of it; I can probably scan the whole thing if anyone wants (it's not freely available on the internet).

As Stephen Hawking says:

The laws of science do not distinguish between the forward and backward arrows of time. However, there are at least three arrows of time that do distinguish the past from the future.

Thermodynamic arrow: direction of time in which disorder increases
(paper falls off the shelf & becomes disordered)
--------->--------->--------->--------->--------->--------->--------->--------->--------->--------->
Psychological arrow: direction of time in which we remember the past & not the future
(we remember watching the paper fall off the shelf...)
--------->--------->--------->--------->--------->--------->--------->--------->--------->--------->
Cosmological arrow: direction of time in which the Universe expands rather than contracts
(forget the paper, this is much bigger than that!)
--------->--------->--------->--------->--------->--------->--------->--------->--------->--------->
(pictures of lurid coloured paper falling off a non-lurid-coloured shelf)


Hawking believed that the three arrows of time all pointed in the same direction as long as the Universe continued to expand. But he also thought that the Universe must start to contract at some point. In that case the cosmological arrow would reverse - and the other two would follow. Then, people would live their lives backward. Time would go backwards.

Thermodynamic arrow:

--------->--------->--------->--------->--------->--------->--------->--------->--------->--------->
<---------

Psychological arrow:

--------->--------->--------->--------->--------->--------->--------->--------->--------->--------->
<---------<---------

Cosmological arrow:

--------->--------->--------->--------->--------->--------->--------->--------->--------->--------->
<---------<---------<---------

(pictures of lurid coloured paper climbing onto a non-lurid-coloured shelf)


But, as Stephen Hawking explains...

I realised that I had made a mistake: the no boundary condition implied that disorder would in fact continue to increase during the contraction. The thermodynamic and psychological arrows of time would not reverse when the Universe begins to recontract, or inside black holes.

His discovery of the no boundary proposal eventually persuaded him that, even if the Universe were to

contract, only the cosmological arrow would reverse; the other two wouldn't.

So, Hawking concluded that

any kind of time travel to the past... is probably impossible. However... this doesn't make all time travel impossible. I do believe in time travel. Time travel to the future.

(later on)

All of which, Hawking suggests, brings us to what's known as an anthropic answer to the question: "Y do we observe the thermodynamic, subjective and cosmological arrows pointing in the same direction?" If the Universe were eventually to contract, we who are asking the question wouldn't be present. At that distance in the future, Hawking points out, the Universe will have reached a state of nearly total disorder, all the stars burned out, the protons and neutrons in them decayed into light particles and radiation. There will be no strong thermodynamic arrow of time at all and, without that, we can't exist.

One reason is that humans have to eat, and food is a more ordered from of energy than the heat into which our bodies convert it. In the contracting phase of the Universe there could be no intelligent life. The subjective arrow of time is our experience of the Universe. Without us around, that arrow arguably would not exist at all. The answer to the question why we observe all three arrows of time pointing in the same direction is that this is the only situation in which we could be around to ask the question.

That was hella interesting. I remain fascinated by this subject -- still can't find a copy of Prigogine's book.

*strums harp*

The flow of time is always cruel... Its speed seems different for each person, but no one can change it... A thing that doesn't change with time is a memory of younger days..