hey i read stephen hawkins book "a breif histroy of time" a while back now n i was talkin at work about some of his theories, n i was explain the thermodynamic arrow of time, the whole reversl of low-high order to high to low order, like he metaphorically explained usin the glass falling off the table, but what i cudnt explain is why wud this happen, whats the logic behind it? thanks all :m:

Caveat emptor: I am not at all sure about the following.

Thermodynamics says that the energy in a closed system becomes less useful for doing work as time passes. If you have a cold container full of ice and a container of very hot water. you can cause a piston to move by putting it on the ice first, contracting the gas inside. Then you put it on the container of hot water and the gas expands. You can keep this up for a while, but the ice melts and the hot water cools down. Whgen you have water at the same temperature in both containers, the system no longer has usable energy.

The above is referred to as increasing entropy, entropy being a measure of unusable energy. There is a also concept of order and disorder related to entropy. A manufactured object like a drinking glas is matter in a highly orgainize state. If the glass falls off a table and breaks on a cement floor, the glass fragments are no longer highly organized. This accident is also referred to as increasing entropy.

In a contracting universe, entropy of both kinds is expected to increase on the average, although there would be regions in which it decreases as it does now.

thanx alot explain alot, atm im only studying phsyics at A levels so im picking up the basics, but im so interested in astrophysics n i cnt help but stick my nose in

No offense Dave, but your posts are almost unreadable because of all the web abbreviations. You may have the most insightful questions ever, but it won't do you any good if people can't read what you type. :)

As for your question, thermodynamics says that the entropy of a closed system will always increase. That's another way of saying that things end up with a greater amount of disorder than when they started, on average. The example of the glass falling and breaking is meant to show what this means with a vivid mental picture. Before the glass falls, it is all one piece, very organized. In other words, low entropy. When you drop the glass, it shatters into thousands of pieces and becomes disordered, it has lots of entropy. You never see a shattered glass spontaneously jump back up on the table and become whole again because that would mean the entropy of the system (the glass) would be decreasing, which would break one of the laws of thermodynamics.

Hopefully this answers you question, it's been a couple years since I learned thermodynamics in chemistry, and I haven't gotten to it yet in physics. If you still don't understand something, just ask, there are lots of knowledgeable people here.

:)

fanx man, ill try and type in normal language but its so difficult coz im so used to slang terms.

so the glass smashes and the entropy rises from low to high as it smashes, so in real terms say the universe is expaninding there is a little entrophy, and as it contracts due to some more powerful force the forces within the universe become inert therefore there is alot of entrophy, so am i correct in beleiving that if the universe contracts everythin will happen in reverse? you will have to forgive if im being dumb but im only 17 and a beginner at the whole advanced physics scene, thanks for your time :D

doh, sorry i just re-read what you said, so reversal due to a decreasin entrophy breaks the law of thermodynamics as we believe at this time, but what wud be the side effects of a contracting universe? would gravity become useless? im jus really intrigued as to what the hypothetical effects would be if they exist

I am not sure what the experts say, so the following is my opinion, which has a good chance of being valid.

If the universe started to contract, it would not have much effect within our solar system for a long time. Just as we do not notice the expansion locally, we would not notice contraction. I doubt if it would be noticed in our local group of galaxies.

The decrease in entropy would be on a much greater scale than within our local cluster of galaxies.

The experts do not think that the universe will ever contract, although there was a time when it was considered possible for gravity to revese the expansion.

Since cosmology is on the frontiers of physics, the experts are more likely to be way off base here than in other disciplines. However, I would not bet against them if I had to put up real money.

A wise man once said.The race is not always won by the swiftest runner and the fight is not always won by the fiercest warrior, but that is the way the smart money always bets.