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The '''Second Law of Thermodynamics''' is a fundamental truth about the tendency towards disorder in the absence of intelligent intervention.  This principle correctly predicts that heat will never flow from a cold body to a warmer one, unless forced to do so by a man-made machine.  As the self-described [[atheist]] scientist [[Isaac Asimov]] admitted:

The '''Second Law of Thermodynamics''' is a fundamental truth about the tendency towards disorder in the absence of intelligent intervention.  This principle correctly predicts that heat will never flow from a cold body to a warmer one, unless forced to do so by a man-made machine.  As the self-described [[atheist]] scientist [[Isaac Asimov]] admitted:

{{Cquote|Another way of stating the Second Law then is: The universe is constantly getting more disorderly.  Viewed that way, we can see the Second Law all about us. We have to work to straighten a room, but left to itself it becomes a mess again very quickly and very easily. Even if we never enter it, it becomes dusty and musty. How difficult to maintain houses, and machinery, and our bodies in perfect working order: how easy to let them deteriorate. In fact, '''''all we have to do is nothing, and everything deteriorates, collapses, breaks down, wears out, all by itself -- and that is what the Second Law is all about'''''.|||Isaac Asimov, Smithsonian Institute Journal, June 1970, p. 6, emphasis added}}

{{Cquote|Another way of stating the second law then is: The universe is constantly getting more disorderly.  Viewed that way, we can see the second law all about us. We have to work to straighten a room, but left to itself it becomes a mess again very quickly and very easily. Even if we never enter it, it becomes dusty and musty. How difficult to maintain houses, and machinery, and our bodies in perfect working order: how easy to let them deteriorate. In fact, '''''all we have to do is nothing, and everything deteriorates, collapses, breaks down, wears out, all by itself -- and that is what the second law is all about'''''.|||Isaac Asimov, Smithsonian Institute Journal, June 1970, p. 6, emphasis added}}

The Second Law of Thermodynamics is the result of the intrinsic uncertainty in nature, manifest in [[quantum mechanics]], which is overcome only by intelligent intervention.  As explained in the [[Epistle to the Hebrews (Translated)#1:10-11|Hebrews 1:10]], the universe shall "wear out" like a "garment", ''i.e.'', [[entropy]] is always increasing.

The Second Law of Thermodynamics is the result of the intrinsic uncertainty in nature, manifest in [[quantum mechanics]], which is overcome only by intelligent intervention.  As explained in the [[Epistle to the Hebrews (Translated)#1:10-11|Hebrews 1:10]], the universe shall "wear out" like a "garment", ''i.e.'', [[entropy]] is always increasing.

This Law makes it impossible to build a [[perpetual motion machine]] - the increase in entropy inevitably derails the system even if energy remains constant.

This law makes it impossible to build a [[perpetual motion machine]] - the increase in entropy inevitably derails the system even if energy remains constant.

The Second Law of Thermodynamics disproves the [[atheistic]] [[Theory of Evolution]] and [[Theory of Relativity]], both of which deny a fundamental uncertainty to the physical world that leads to increasing disorder.

The Second Law of Thermodynamics disproves the [[atheistic]] [[Theory of Evolution]] and [[Theory of Relativity]], both of which deny a fundamental uncertainty to the physical world that leads to increasing disorder.

==Entropy and disorder ==

==Entropy and disorder ==

In this context "increasing disorder" means the decline in organization that occurs without intelligent intervention.  Imagine your old room at your parent's house.  Remember how easy it was to let the room turn into a uniform mess, (disorder) and remember how hard it was to clean it up until it fit a specific, non-uniform design (order).  Not cleaning up would always result in an increase of entropy in your room!

In this context "increasing disorder" means the decline in organization that occurs without intelligent intervention.  Imagine your old room at your parent's house.  Remember how easy it was to let the room turn into a uniform mess (disorder) and remember how hard it was to clean it up until it fit a specific, non-uniform design (order).  Not cleaning up would always result in an increase of entropy in your room!

The flow of energy (by heat exchange) to places with lower concentrations is called the "heat flow."

The flow of energy (by heat exchange) to places with lower concentrations is called the "heat flow."

The often-heard argument that this law disproves an eternal universe is true, because in that case, maximum entropy would have been reached already. A counterargument to this would be to suggest that the universe is still in the process of approaching maximum entropy.

The often-heard argument that this law disproves an eternal universe is true, because in that case maximum entropy would have been reached already. A counterargument to this would be to suggest that the universe is still in the process of approaching maximum entropy.

There are many different ways of stating the Second Law of Thermodynamics. An alternative statement of the Law is that heat will tend not to flow from a cold body to a warmer one without intelligent intervention, or work being done, as in the case of a [[refrigerator]].  Other statements include that it is impossible for an [[engine]] to convert [[heat]] perfectly (I.e. at 100% efficiency) into [[work]]. These statements are qualitative and stating the Second Law in terms of [[entropy]] makes the law quantitative.

There are many different ways of stating the Second Law of thermodynamics. An alternative statement of the law is that heat will tend not to flow from a cold body to a warmer one without intelligent intervention, or work, being done, as in the case of a [[refrigerator]].  Other statements include that it is impossible for an [[engine]] to convert [[heat]] perfectly (I.e. at 100% efficiency) into [[work]]. These statements are qualitative and stating the Second Law in terms of [[entropy]] makes the law quantitative.

This Law makes it impossible to build a [[perpetual motion machine]] - the increase in entropy inevitably derails the system even if its [[energy]] remains constant (as described by the [[Conservation of Energy|first law of thermodynamics]]).

This law makes it impossible to build a [[perpetual motion machine]] - the increase in entropy inevitably derails the system even if its [[energy]] remains constant (as described by the [[Conservation of Energy|first law of thermodynamics]]).

The Second Law can be expressed mathematically as:

The Second Law can be expressed mathematically as:

where <math>\frac{dS_{universe}}{dt}</math> is the rate of change of [[entropy]] of the [[universe]] with respect to time.

where <math>\frac{dS_{universe}}{dt}</math> is the rate of change of [[entropy]] of the [[universe]] with respect to time.

Another way of viewing the Second Law is in terms of probability.  In nature, there is no one to clean up the universe- only chances.  The chance of something becoming orderly is essentially zero, which it is a certainty that things will become more disorderly.

Another way of viewing the Second Law is in terms of probability.  In nature there is no one to clean up the universe, only chances.  The chance of something becoming orderly is essentially zero, which it is a certainty that things will become more disorderly.

On a universal scale, a tidy room would be a universe which has pockets of above average concentrations of energy (if you - incorrectly - assume relativity [[E=mc²]] this includes matter as well.)

On a universal scale a tidy room would be a universe which has pockets of above average concentrations of energy (if you - incorrectly - assume relativity [[E=mc²]] this includes matter as well.)

==Second Law compared with other physical laws==

==Second Law compared with other physical laws==

Thermodynamics occupies an unusual place in the world of [[science]], particularly at the high school and undergraduate levels.  The Second Law is the one that is especially peculiar (in fact, the other Laws are comparatively mundane.  The first Law is just a statement that heat is a form of energy, and that energy, whether in the form of heat or not, is conserved.  This was a very nontrivial result at first, but, with the understanding of heat and temperature that later developed, it's quite unremarkable.  The third Law is a statement that absolute zero can't be reached by any finite number of Carnot cycles.<ref>[https://en.wikiversity.org/wiki/Carnot_engine Carnot Engine]</ref>  While true, its significance pales in comparison to that of the Second Law).

Thermodynamics occupies an unusual place in the world of science, particularly at the high school and undergraduate levels.  The Second Law is the one that is especially peculiar.  (In fact, the other laws are comparatively mundane.  The first law is just a statement that heat is a form of energy, and that energy, whether in the form of heat or not, is conserved.  This was a very nontrivial result at first, but, with the understanding of heat and temperature that later developed, it's quite unremarkable.  The third law is a statement that absolute zero can't be reached by any finite number of Carnot cycles.<ref>[https://en.wikiversity.org/wiki/Carnot_engine Carnot Engine]</ref>  While true, its significance pales in comparison to that of the Second Law.)

Perhaps what makes the Second Law so remarkable is that it describes ''irreversible'' phenomena.  In particular, it describes the observed fact that heat energy, in bodies that are not being externally manipulated by compression, etc., flows only from a warmer body to a cooler one.  When a warmer body is placed in contact with a cooler one, heat energy will flow (always preserving total energy, of course) from the warmer one to the cooler one.  The warmer one will cool off as it releases its energy, and the cooler one will warm up.  This process will continue until the two bodies reach the same temperature, or "thermal equilibrium".

Perhaps what makes the Second Law so remarkable is that it describes ''irreversible'' phenomena.  In particular, it describes the observed fact that heat energy, in bodies that are not being externally manipulated by compression, etc., flows only from a warmer body to a cooler one.  When a warmer body is placed in contact with a cooler one, heat energy will flow (always preserving total energy, of course) from the warmer one to the cooler one.  The warmer one will cool off as it releases its energy, and the cooler one will warm up.  This process will continue until the two bodies reach the same temperature, or "thermal equilibrium".

Until the development of [[statistical mechanics]], no one knew why this was so, or what temperature actually meant.  What was known was simply that a body with a higher temperature would send heat to a body with a lower temperature, no matter what the bodies were made of.

Until the development of [[statistical mechanics]], no one knew why this was so, or what temperature actually meant.  What was known was simply that a body with a higher temperature would send heat to a body with a lower temperature, no matter what the bodies were made of.

The fact that this kind of heat flow is irreversible makes the whole field of thermodynamics lie outside of the realm of classical Newtonian mechanics or Relativistic mechanics.  In Newtonian or Relativistic mechanics, every phenomenon can go in reverse order.  The catchy phrase [[Arrow of time]] (or "time's arrow") was coined by [[Arthur Eddington]] to denote this one-way behavior not shared by other theories of physics.<ref>''The Nature of the Physical World'', Arthur Eddington, MacMillan, 1929, ISBN 0-8414-3885-4</ref>

The fact that this kind of heat flow is irreversible makes the whole field of thermodyamics lie outside of the realm of classical Newtonian mechanics or Relativistic mechanics.  In Newtonian or Relativistic mechanics, every phenomenon can go in reverse order.  The catchy phrase [[Arrow of time]] (or "time's arrow") was coined by [[Arthur Eddington]] to denote this one-way behavior not shared by other theories of physics.<ref>''The Nature of the Physical World'', Arthur Eddington, MacMillan, 1929, ISBN 0-8414-3885-4</ref>

The field of statistical mechanics attributes the increase in entropy to the statistical tendencies of huge aggregates of particles at the molecular or atomic level.  While Newtonian and Relativistic mechanics can, in principle, precisely describe assemblages of any number of particles, in practice they are not directly applied to the behavior of bulk material.  That is, they are not applied to a number of particles on the order of Avogadro's number.

The field of statistical mechanics attributes the increase in entropy to the statistical tendencies of huge aggregates of particles at the molecular or atomic level.  While Newtonian and Relativistic mechanics can, in principle, precisely describe assemblages of any number of particles, in practice they are not directly applied to the behavior of bulk material.  That is, they are not applied to a number of particles on the order of Avogadro's number.

Statistical mechanics makes no assumptions about the microscopic cause of the seemingly random behavior.  Statistical mechanics was developed in the 19^th century prior to quantum mechanics, and does not depend on the [[Heisenberg uncertainty principle]].  The quantum mechanical uncertainty goes away at the microscopic level once the system is observed and the wave function collapses.

Statistical mechanics makes no assumptions about the microscopic cause of the seemingly random behavior.  Statistical mechanics was developed in the 19^th century prior to quantum mechanics, and does not depend on the [[Heisenberg uncertainty principle]].  The quantum mechanical uncertainty goes away at the microscopic level once the system is observed and the wave function collapses.

It needs to be emphasized that the peculiarities of thermodynamics, with the phenomenon of irreversibility and the "arrow of time", ''in no way'' contradict the (reversible) processes of classical Galilean, Newtonian, Lagrangian, or Hamiltonian physics, or of special or general relativity.  Those formulations are precise in the regimes in which the behavior of individual particles are analyzed.  When two gas molecules collide in the [[Kinetic theory]], that collision is perfectly reversible.  It is only when one goes into a problem domain in which the bulk behavior of huge numbers (on the order of Avogadro's number) of particles are analyzed, without regard for the individual particles, that thermodynamics and statistical mechanics come into play.

It needs to be emphasized that the peculiarities of thermodynamics, with the phenomenon fo irreversibility and the "arrow of time", ''in no way'' contradict the (reversible) processes of classical Galilean, Newtonian, Lagrangian, or Hamiltonian physics, or of special or general relativity.  Those formulations are precise in the regimes in which the behavior of individual particles are analyzed.  When two gas molecules collide in the [[Kinetic theory]], that collision is perfectly reversible.  It is only when one goes into a problem domain in which the bulk behavior of huge numbers (on the order of Avogadro's number) of particles are analyzed, without regard for the individual particles, that thermodynamics and statistical mechanics come into play.

==Elementary probability and statistics==

==Elementary probability and statistics==

If you flipped a coin 20 times and it came up heads each time, you would consider that to be a remarkable occurrence (Perhaps so much so that you would inspect the coin to be sure it didn't have heads on both sides). If you tried it and got tttthtththhhththhthh, you would probably not consider it remarkable, yet each of these outcomes is equally probable: about 1 in 10⁶.  If you shuffled a deck of cards and found them all in exact order from the 2 of clubs to the ace of spades, you would consider that to be very remarkable- but if you got the distribution shown in the illustration on page 314 of Alfred Sheinwold's ''5 Weeks to Winning Bridge'', you would probably consider it just "random".  Yet each of these orderings has the same probability of occurring&mdash;1 in 52 factorial, which is about 10⁶⁶.<ref>Actually, this is ignoring the fact that Bridge players sort their cards by suit, and the diagram shows the result of the sorting.</ref>

If you flipped a coin 20 times and it came up heads each time, you would consider that to be a remarkable occurrence.  (Perhaps so much so that you would inspect the coin to be sure it didn't have heads on both sides.)  If you tried it and got tttthtththhhththhthh, you would probably not consider it remarkable.  Yet each of these outcomes is equally probable: about 1 in 10⁶.  If you shuffled a deck of cards and found them all in exact order from the 2 of clubs to the ace of spades, you would consider that to be very remarkable.  But if you got the distribution shown in the illustration on page 314 of Alfred Sheinwold's ''5 Weeks to Winning Bridge'', you would probably consider it just "random".  Yet each of these orderings has the same probability of occurring&mdash;1 in 52 factorial, which is about 10⁶⁶.<ref>Actually, we're ignoring the fact that bridge players sort their cards by suit, and the diagram shows the result of the sorting.</ref>

For small sets such as coin tosses or card shufflings, what constitutes "random" vs. "well ordered" is in the eye of the beholder.  If a room in your house started in a state that most people would consider "neat and tidy", and you went into that room every day, picked up a random object, and threw it against a random wall, after a month most people would consider the room a mess.  But once again, this is hard to quantify.  The kinds of statistical analyses that are required for the study of thermodynamics have to be much more careful than this.  The kind of folksy quotations in popular articles about messy rooms, as in the Isaac Asimov quote, may sell magazines, but they don't shed much light on how thermodynamics works.

For small sets such as coin tosses or card shufflings, what constitutes "random" vs. "well ordered" is in the eye of the beholder.  If a room in your house started in a state that most people would consider "neat and tidy", and you went into that room every day, picked up a random obect, and threw it against a random wall, after a month most people would consider the room a mess.  But, once again, this is hard to quantify.  The kinds of statistical analyses that are required for the study of thermodynamics have to be much more careful than this.  The kind of folksy quotations in popular articles about messy rooms, as in the Isaac Asimov quote, may sell magazines, but they don't shed much light on how thermodynamics works.

What is needed is an analysis of ''aggregate properties'', not individual items.  There is a need to quantify the results.  For the case of the coin toss, it might be asked how many times the result of heads happen.  The probabilities can be worked out; they are a "Gaussian distribution", also known as a "bell curve".  The probability of heads 0 times out of 20 is 1 in 1048576.  Getting heads exactly 1 time is .00002, and so on, as shown in this table.  Notice that getting heads exactly 10 times is the most probable outcome, but its probability is still only 18%.  If the experiment was done a larger number of times, the probability of exactly 50% heads would still be higher than any other, but it would be quite small.  What is important is the probability of getting a certain number of heads ''or less''.

What is needed is an analysis of ''aggregate properties'', not individual items.  We need to quantify the results.  For the case of the coin toss, we might ask how many times we got heads.  The probabilities can be worked out; they are a "Gaussian distribution", also known as a "bell curve".  The probability of heads 0 times out of 20 is 1 in 1048576.  Getting heads exactly 1 time is .00002, and so on, as shown in this table.  Notice that getting heads exactly 10 times is the most probable outcome, but its probability is still only 18%.  If we did the experiment a larger number of times, the probability of exactly 50% heads would still be higher than any other, but it would be quite small.  What is important is the probability of getting a certain number of heads ''or less''.

{| class="wikitable"

{| class="wikitable"

|-

|-

::The properties of an aggregate of measurements, when the individual measurements are not predetermined, tend toward the "most probable" distribution.

::The properties of an aggregate of measurements, when the individual measurements are not predetermined, tend toward the "most probable" distribution.

The measurements could be things like whether a coin came up heads, whether a card in a deck has a certain value, or the energy of a gas molecule.  The fundamental truth of this, for reasonable numbers of things like coins or cards, is quite sensible on the intuitive level.  These principles were worked out, by Fermat and others, in the 17^th and 18^th centuries.  The same principles apply when the numbers are enormous, on the order of Avogadro's number, but some intuitive conclusions can be misleading.

The measurements could be things like whether a coin came up heads, whether a card in a deck has a certain value, or the energy of a gas molecule.  The fundamental truth of this, for reasonable numbers of things like coins or cards, is quite sensible on the intuitive level.  These principles were worked out, by Fermat and others, in the 17^th and 18^th century.  The same principles apply when the numbers are enormous, on the order of Avogadro's number, but some intuitive conclusions can be misleading.

When dealing with thermodynamics, it is a matte of the statistical aggregate behavior of macroscopic pieces of matter, so the number of items must be increased from 10, or 52, to something like [[Avogadro's number]].  Therefore, the number of possible situations, instead of being 10⁶ or 10⁶⁶, is something like 10^{Avogadro's number}- that is, 10^10²³.  The enormity of such a number makes a huge amount of difference.  If a coin is flipped Avogadro's number of times, it will come up heads about half the time, as before.  But, for all practical purposes, it can be said that it will come up heads ''exactly'' half the time.  The number of heads might be off by a few quintillion (this is the "law of large numbers"), but that won't make any practical difference.

When dealing with thermodynamics, we are dealing with the statistical aggregate behavior of macroscopic pieces of matter, so we have to increase the number of items from 10, or 52, to something like [[Avogadro's number]].  So the number of possible situations, instead of being 10⁶ or 10⁶⁶, is something like 10^{Avogadro's number}, that is, 10^10²³.  The enormity of such a number makes a huge amount of difference.  If you flip a coin Avogadro's number of times, it will come up heads about half the time, as before.  But, for all practical purposes, we can say that it will come up heads ''exactly'' half the time.  The number of heads might be off by a few quintillion (this is the "law of large numbers"), but that won't make any practical difference.

*You can't ask any questions about individual items&mdash;air molecules don't have labels like "Jack of Diamonds".  You can only ask questions about the aggregate behavior of macroscopic pieces of space.

*You can't ask any questions about individual items&mdash;air molecules don't have labels like "Jack of Diamonds".  You can only ask questions about the aggregate behavior of macroscopic pieces of space.

*An example is the question of how likely it is that all the air molecules in a room will move to one corner, asphyxiating everyone.<ref>Actually, conservation of momentum requires that we consider half the molecules going to one corner and the other half to the opposite corner.</ref>  This is sometimes worked out in physics classes.  But the conclusion has to be that this occurrence, or anything remotely resembling it, might have a probability on the order of 1 in 10^10²³&mdash;it just doesn't happen.

*An example is the question of how likely it is that all the air molecules in a room will move to one corner, asphyxiating everyone.<ref>Actually, conservation of momentum requires that we consider half the molecules going to one corner and the other half to the opposite corner.</ref>  This is sometimes worked out in physics classes.  But the conclusion has to be that this occurrence, or anything remotely resembling it, might have a probability on the order of 1 in 10^10²³&mdash;it just doesn't happen.

==Isn't it cheating to say that heat flows from a warmer body to a colder one, when all we really know is that it "probably" flows that way?==

==Aren't we cheating when we say that heat flows from a warmer body to a colder one, when all we really know is that it "probably" flows that way?==

Well, yes, and no.  The laws of probability and statistical mechanics are precisely true, since they are mathematical theorems.  They are as precise as that 2+2=4 exactly.  It's their application to the real world of molecules and such that is merely "probably" correct.    When it is said that heat flows downward in macroscopic objects (ice cubes, teapots, Carnot engines<ref>[https://en.wikiversity.org/wiki/Carnot_engine Carnot Engine]</ref>), what actually happens is that it flows downward with a probability of 99.999<put in Avogadro's number of 9's here>999 percent.

Well, yes, and no.  The laws of probability and statistical mechanics are precisely true, since they are mathematical theorems.  They are as precise as that 2+2=4 exactly.  It's their application to the real world of molecules and such that is merely "probably" correct.    When we say that heat flows downward in macroscopic objects (ice cubes, teapots, Carnot engines<ref>[https://en.wikiversity.org/wiki/Carnot_engine Carnot Engine]</ref>), what we really mean is that it flows downward with a probability of 99.999<put in Avogadro's number of 9's here>999 percent.

Everyone, scientists and lay people alike, accept this as true.  It is the basis for just about everything, including breathing.  No one ever questions the reading on the pressure gauge of a gas canister on the grounds that it is only probably correct, or is surprised when one blows into a balloon and it expands.

Everyone, scientists and lay people alike, accept this as true.  It is the basis for just about everything, including breathing.  No one ever questions the reading on the pressure gauge of a gas canister on the grounds that it is only probably correct, or is surprised when one blows into a balloon and it expands.

The main argument against [[evolution]] using the second law of thermodynamics is that [[evolution]] requires a decrease in entropy (disorder). However, the second law of thermodynamics states that entropy increases, so the two are contradictory. These resources misrepresent the Second Law of Thermodynamics, ignoring the fact the [[earth]] is not an isolated system (energy is added from the [[sun]] for example).

The main argument against [[evolution]] using the second law of thermodynamics is that [[evolution]] requires a decrease in entropy (disorder). However, the second law of thermodynamics states that entropy increases, so the two are contradictory. These resources misrepresent the Second Law of Thermodynamics, ignoring the fact the [[earth]] is not an isolated system (energy is added from the [[sun]] for example).

== The First and Second Laws of Thermodynamics and the universe having a beginning ==

== The 1st and 2nd law of thermodynamics and the universe having a beginning ==

''See also:'' [[Atheism and the origin of the universe]]

''See also:'' [[Atheism and the origin of the universe]]