1. Has sometimes been used to suggest
Now, some Brief Philosophy!!!! (or Religion?) The 2nd Law & Life on Earth
The existence of low-entropy organisms like us
Has sometimes been used to suggest
that we live in violation of the 2nd Law!
British cosmologist Sir Roger Penrose has considered this in his book
“The Road to Reality: a Complete Guide
to the Laws of the Universe” (2005). 1

2. In “The Road to Reality: a Complete Guide to the Laws of the Universe”, Penrose points out that “it is a common misconception to believe that the Sun’s energy is the main ingredient needed for our survival”.
He says,
“What is important is that the energy source be far from thermal equilibrium”.
For example, a uniformly illuminated sky supplying the same amount of energy as the Sun, but at a much lower energy, would be useless to us.
Fortunately the Sun is a hot sphere in an otherwise cold sky. So, it is a low entropy source, which keeps our entropy low. 2

3. The 2nd Law & Life on Earth
The optical photons supplied by the Sun contain much more energy than the IR photons leaving us, since εph = hν.
Since the energy reaching us is contained in fewer photons,
the Sun is a low entropy source.
Plants utilize the low entropy energy, to reduce their entropy through photosynthesis.
We keep our entropy low by breathing oxygen produced by plants, and by eating plants, or animals ultimately dependent on plants. 3

4. More Brief Philosophy!!!! The 2nd Law & the Arrow of Time!
One of the Deepest Philosophical Paradoxes of Modern Physics!
Microscopic Physics is Time Reversal Symmetric. But, Nature is Not!!
The fundamental, Microscopic Theories of Physics (classical & quantum mechanical) don’t care which way time goes! Newtonian Mechanics, Electromagnetism, Relativity, & Quantum Mechanics are all Time Reversal Symmetric. 4

5. Arrow of Time Why does time have a direction?
(Always forward & never back!)
It’s NOT a feature of the microscopic laws of physics!!
Those work equally well forwards or backwards in time; they are “Invariant under time reversal”! (t  -t, pi  -pi)
A common answer to why there is an arrow of time is:
Entropy & The 2nd Law of Thermodynamics
It says that entropy increases (in closed systems) as a function of time. So, the time derivative of the entropy must be positive:
That is:
The 2nd Law says that entropy is NOT
invariant under time reversal! 5

6. Entropy & The 2nd Law of Thermodynamics
Tell us that the entropy must be such that
So, The 2nd Law is NOT invariant under time reversal!
Reconciling this fundamental Law of Thermodynamics, (known to be correct) with the time reversal symmetry of the underlying microscopic physics is a
Deep, Difficult, Philosophical Problem.
Starting from well before the Boltzmann’s time until the present, many very smart people have tried to come up with a satisfactory explanation. So far, there is no agreement among the experts on such an explanation.
To outline some solutions would require an entire lecture! One is downloadable from the lecture page. No time to cover it in class. 6

7. Now, a brief (hopefully) humorous discussion L

8. Some Popular (joke!) Versions of The Laws of Thermodynamics
1st Law: You can’t win.
2nd Law: You can’t break even.
3rd Law: There’s no point in trying.

9. Other Popular Versions of
The Laws of Thermodynamics
Version 1
Zeroth Law: You must play the game.
First Law: You can't win the game.
Second Law: You can't break even in the game.
Third Law: You can't quit the game.
Version 2
First Law: You can't win the game;
You can only break even.
Second Law: You can only break even at absolute zero.
Third Law: You can't reach absolute zero!

10. Version 3 Version 4 Zeroth Law: You must play the game.
First Law: You can't win the game.
Second Law: You can't break even except
on a very cold day.
Third Law: It never gets that cold!
Version 4
Zeroth Law: There is a game.
First Law: You can't win the game.
Second Law: You must lose the game.
Third Law: You can't quit the game.

11. “Things get worse under pressure!!”
A joke!!
“Murphy's Law”
of Thermodynamics:
“Things get worse under pressure!!”

12. Murphy’s “Law”: “Anything that can go wrong,
will go wrong!”. L

13. Murphy’s “Law”: “Anything that can go wrong,
will go wrong!”. L
Mrs. Murphy’s “Corollary”: “Murphy was an optimist!”
There are many other variations of these kinds of “Laws”!
Whole books have been written compiling these kinds of
“Laws” applied to various situations.