Every time I read an article about great scientific discoveries, I read about someone performing a reaction that was not time-reversible. That is, the reaction has a specific direction in time it must travel or else the numbers don’t add up. How is this possible in a universe that has the conservation of mass-energy law? Unless I’ve read something wrong, for a reaction to be considered non-reversible something must either be totally destroyed or created from absolutely nothing. From what I know, both scenarios are impossible. Since mass and energy are identical, not even a nuclear reaction, where mass is seemingly lost but instead is converted to energy, is considered non-reversible. This is bugging me, the avid amateur physicist I am, and I’d like someone (I’m thinking Chronos) to help me here.
Time-Reversibility:
http://einstein.drexel.edu/courses/Physics-480_1995/chapter_3a/ode/node9.html
Non-reversibility probably refers to Hamiltonian dynamics, but it’s a WAG since I don’t know what you are talking about.
Not sure what you heard is in the same context but it could be referring to the second law of thermodynamics. It’s simple to say that the trend of the universe is toward increasing entropy but it’s one of the most complex and difficult concepts to fully understand.
One illustration is a diver hitting the water and making a nice disorderly splash. There is no law that says all the water and molecules won’t all get together to form a “reverse” splash and push the diver back out the way he cam but it’s extremely unlikely. Explosions, both combustion and nuclear, are similar in that it’s hard to get back to the material you started with by backing up on the same path.
Any mention of irreversable reactions involves the second law of thermodynamics. Entropy always increases with time.
Any reaction that involves entropy is irreversible. This includes almost any reaction involving heat. Heat always flows from a warmer region to a cooler region. This heat flow is irreversible, but it does not imply loss of mass/energy. Just a loss of usable energy.
Put another way, energy can’t be created or destroyed, but it can change forms, and some changes are easier than others. Specifically, it’s easy to turn just about any form of energy into heat, but it’s more difficult to convert heat to other forms. The net result is that the total heat of the Universe is always increasing, while the total of all other forms of energy is decreasing.
Interesting. But the heat “density” would decrease with expanding space, right?
Or stays the same. In an isolated system.
sorry for the nit pic, but heck, I’m still paying off student loans so I might as well get some use out of those college years
Thermodynamics… This is one of those subjects that suffers MOST from oversimplification. That is, simplifying thermodynamics to pithy statements introduces so much error that it makes them wrong.
“Entropy always increases.”
No, entropy will tend to increase * all other things being equal *. Since all other things are not always equal, entropy does NOT have to increase. This is an oversimplifcation of the REAL principle, which is threefold.
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Free energy of the universe must remain constant. Free energy is equal to enthalpy (heat) and entropy (sometimes simplified as “disorder”).
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The universe is cooling off. Thus, as the enthalpy availible in the universe DECREASES, there must be a coincident INCREASE in entropy to compensate. However, if an event causes the universe to heat up (increasing enthalpy) then it may result in a localized decrease in entropy.
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Entropy is temperature dependent (actually, enthalpy is as well, but MUCH MUCH less so). This is actually a corrolary of point 2. If you remove enough enthalpy from a system to the universe, you can generate a decrease in entropy. Thus, if you remove enough heat from water, you can cause ice to spontaneously form, which occurs at 273 K. Thus, you can make an apparent violation of the second law of thermodynamics, so long as you heat up the universe at the same time.
Phobos
Yes, the heat density is decreasing, but this does not change the amount of usable heat. You can only use heat to do work when there is a difference in heat between two bodies.
I’ll grant you your nit pic. You are correct. I should have said, “In a closed system, entropy never decreases with time.”.
[positively anal nit pic]
In a closed system of any significant size, the odds that entropy will increase are vanishingly small.
[/positively anal nit pic]
No offense to the thermodynamics explanations, but I don’t think that’s what he’s hearing about.
A lot of experimentation is going on right now to find exactly what he’s talking about – physical effects (generally quantum-mechanical) that are not invariant under time-reversal. This is important because clearly, there is an ‘arrow’ of time, as you discussed above, in which entropy always remains the same or increases, and which determines which way we can remember things (backward in time) and which way we have to guess at (forward), and non-reversibility would help explain these things.
Basically, there are three transformations being considered: C is a transformation where all charges are replaced by their opposites, P is a reversal of parity, where left becomes right, and T is a reversal of time. Current theory predicts that while any combination of two of these may be broken (e.g. reversing parity and time may change the laws of physics) the set of three, done all together (CPT) will behave just like normal.
I don’t know of any particular experiment that is looking for symmetry breaking in T (because time is notoriously hard to reverse =P) so most of the projects are looking for violations of CP, which would imply an OPPOSITE breaking in T in order to keep the whole thing unbroken. (I mostly know about this because my university is building the detector arrays for one of these experiments, called BaBar. They’re looking to find CP violation in weak nuclear interactions using baryons and anti-baryons, which are denoted by B and B-bar, hence the name BaBar. This passes for wit among particle physicists.) The weak nuclear force is known to do odd things with parity, so they’re hoping that by looking at decay ratios between baryons and anti-baryons (which, being oppositely charged but otherwise identical, are a perfect way to implement transformation C) they can determine whether T is broken, and in what way.
Neat stuff, although I’m not sure whether it will have any practical applications in the near future…
Those things must be incredibly small.
Finally, an interesting physics thread. The 2ndLOTD (Second law of thermodynamics) can be stated in many simple ways but the implications are broad enough to base a career on.
Another way of stating it is there can be no process in a closed system where the only result is to move heat from a cold place to a hot place. You might think your fridge does that but not so. It does take heat from the cold inside, making it colder, and move it outside making the room warmer but only works because outside energy is added to the system, adding more heat to the room than was removed from the inside.
This is a pet peeve of mine because one of my hobbies is debunking free energy and over unity devices. The frustrating part is trying to explain faulty logic to someone who has little or no understanding of physics. They often buy into the notion of traditional science being the equivalent of religious dogma. Sometimes I miss going onto USENET, but just a little. We’re continuing into an age where even a person with a good education doesn’t understand the mechanics of technology, making Clarke’s adage even truer, “any sufficiently advanced technology is indistinguishable from magic.”
<wanders to the BBQ pit with flame-thrower in hand.>
DrMatrix said:
"Any mention of irreversable reactions involves the second law of thermodynamics. Entropy always increases with time. "
Entropy does not have to increase. Entropy will, pretty much by definition, PROBABLY increase. The larger the system, the larger the probability; with macroscopic systems the probability that the entropy will increase approaches unity…
-Luckie
I always hated thermo, while Stat Mech comes naturally…
Thanks, all. PaulT, I think you’ve hit on what I heard about. After all, how much of an advance would it be for someone to prove one of the Laws of Thermodynamics? That’s high school stuff. And Padeye, I feel your frustration. I have to explain the 2nd Law to Creationist idiots who think it ‘disproves evolution’. After the fifth futile attempt at reason I begin to claw for my … well, some things are better left unsaid. These things can be subpoenaed. (Secret fact that torpedoes the Creationist argument: It’s the sun, stupid! The Earth will not be a closed system as long as the sun still shines. Just try getting them to listen …) And jayron 32, thanks for the list. I know people who could use it in just that format (not least the religion freaks).
Although if you want to get ttechnical (I can, BTW) you can bring thermodynamics into it, there is a simpler explanation. Chemical reactions come in two flavors–things that go one direction and only one direction, and things that form an equilibrium. For example, if you have a weak nucleophile attacking in the reaction, the odds are that sometimes it just pops right back off. You get an equilibrium–a certain amount of the molecules have the nucleophile on them, a certain amount don’t. If you remove some of one, the other will convert until the same proportion is restored.
Non-revesible reactions generally involve (as mentioned) large thermodynamic differences. If you have a very strong nucleophile, once it’s on there, it sure as heck isn’t coming back off–at least, without the input of a * lot * of energy. So it’s a one-way street.
I know this is simplifying things, but sometimes it’s worthwhile to have the simpler answer available as well.