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Thermodynamics and pathological science.

DeLorean time machine from the film ' Back to the future'

Time traveling? DeLorean time machine from the film 'Back to the future'

Chemistry and Physics offer more than mere equations to the serious student. Even philosophical concepts can be derived from the thermodynamic laws. Can you believe it?

Let’s read, for example, a Statement of the Second Law of Thermodynamics;

It is impossible for an engine to perform work by cooling a portion of matter to a temperature below that of the coldest part of the surroundings.

OK, maybe this is not exactly what you and I had in mind when speaking of philosophy, but this law is powerful indeed, and has an incredible predictive potential!

Let me try and rephrase the statement in a ‘less scientific’ way: if the whole universe spontaneously goes from state A to state B, it WON’T be able to go back from state B to state A. Is the philosophical concept clearer now?

The Second Law thus states (and it has been demonstrated, something that philosophers cannot easily do) that some kinds of phenomena cannot possibly happen, no matter how ingenious the way to achieve them appears!!!

Every now and then, the myth of perpetual motion machines and other examples of pathological science (to use the words of I. Langmuir, Chemistry Nobel Prize-winner) come back. These are probably the only exceptions to the Second Law of thermodynamics, since these fake statements usually resurface more or less, in their exact previous versions!

So, why is the perpetual motion machine impossible? After all, we ‘produce’ energy in engines, or do we? Well, we effectively don’t… we simply transfer energy from a source (usually, hotter) to another source (usually, colder). And during this transfer, we use the energy transferred to produce some kind of work, while the remaining part is  ‘wasted’ as heat.

Since it is impossible to completely avoid producing heat (see Figure 3.5 on page 3-23 of the Physical chemistry textbook, where ‘reversible’ means ‘practically impossible’), we can COMPLETELY transform only work into heat, but we cannot do the opposite.

Transfer of heat - Physical Chemistry

Spontaneous processes DO produce heat, we cannot avoid ‘losing work’ during each and every operational step of our machine. It is not possibile to ‘gather heat’ in some reservoir and hope to use all of it to produce an equal amount of work again. Thus, the wonderful opportunity to obtain a perpetual motion machine goes (irreversibly, of course!) into the dumpster.

Obviously, what I’ve just said works only if I’m able to demonstrate it; Pages 3-24 and 3-25 give a tremendously easy way to understand the inequality of Clausius, who first proposed this interpretation of spontaneity.

Now: I promised you philosophical concepts, and here I am to fulfill my promise… can TIME be reversed? Basically, the answer is negative: the Second Law tells us that no DeLorean will take us back in time to collect a pet dinosaur, since all the spontaneous processes cannot be reversed (their entropy wouldn’t obey the Law). Right?

Well, modern physicists believe this Law to be statistically valid, but not absolutely. What should this mean? Only time will tell…

Written by: Gaetano Giammarino

Gaetano Giammarino has authored 11 more articles.

He has received a PhD in Computational Chemistry at Università degli Studi della Basilicata, where he teaches General Chemistry in the Faculty of Engineering.

One Response to Thermodynamics and pathological science.

  • Another thought provoking entry which goes to the heart of why perpetual motion is impossible. Time reversal is microscopically possible. I like the demo whereby a bucket of glycerol is mixed after using a syringe to write a word in the glycerol with ink. Say MCH is written. Mix it until the ink is smeared out, then reverse the mixing and the word reforms. This is microscopic reversibility. However the word MCH is slightly fuzzy, because of irreversible component.

    So microscopic time reversal of collisions works, but macroscopically the second law takes over.

    Ludwig Boltzmann talked about this in his time dependent Boltzmann equation (in the 1870’s). He had to make the choice (anzatz) that BEFORE a collision, the particles are uncorrelated. That choice leads to an”arrow if time” and led to the Boltzmann H-theorem, which is the Kinetic Theory formulation of the second law.

    Pointcare was listening to Boltzmann and stated that he found that for all FINITE systems, that eventually all the particles would return to their initial conditions if you wait long enough–the Pointcare recursion time. This is a statement of ergoticy. Boltzmann replied: “You should live so long!”

    When I was a post doc in the Kamerlingh Onnes Labs in Leiden Holland, I was browsing the books in the Erenfest Reading room and found the hand written PhD thesis of Pointcare, just sitting on the shelves. It is in French.

    Time can be reversed in the equations to see if they are odd or even under the reversal. Momentum is odd to time reversal, energy is not. This is a symmetry only, along with coordinate reversal (Parity) and charge reversal (Charge conjucation). No systems have been found that are odd to the three symmetries of PCT.

    But in the end, the statistical averaging over many processes leading to the second law means time travel is for Sheldon and his crew in Big Bang Theory.

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