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ENTROPY OF THE UNIVERSE What is Entropy? The actions of anything, from the flight of an insect to the movements of the largest galaxies, are caused by energy changes. If we are to comprehend the universe, it is vital to understand energy and the rules governing it. In particular, it is vital to understand the laws of thermodynamics. The first of these laws tells us there is a fixed amount of energy in the universe. Energy can be changed from one form to another, but never created or destroyed. E
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  ENTROPY OF THE UNIVERSE What is Entropy? The actions of anything, from the flight of an insect to the movements of the largest galaxies, arecaused by energy changes. If we are to comprehend the universe, it is vital to understand energy andthe rules governing it. In particular, it is vital to understand the laws of thermodynamics.The first of these laws tells us there is a fixed amount of energy in the universe. Energy can bechanged from one form to another, but never created or destroyed. Energy which appears to havedisappeared has in fact been converted into a form we cannot detect - for example, sound energyseems to be lost, but really turns into minute quantities of heat.Although energy cannot be destroyed, it is of little use to anyone if it cannot make things happen.Unfortunately, the second law of thermodynamics tells us all energy changes decrease the amount of useful energy in the universe.Consider a box of small magnets. If the small magnets are lined up in the same direction, as a groupthey can attract other metal objects. If they are not lined up in the same direction, individual magnetscancel each other's effect and cannot do useful work. The same is true of energy - it is useful when itis ordered, but when it is disordered, its effects cancel each other out. For example, although thetwelve men below all have the same strength, the 'ordered' six can push a lorry (useful work) and theother six cannot.Entropy is a measure of the lack of order in the energy. There is no definite value of entropy for agiven system (as there is for, say, mass), as entropy is a purely statistical measure. When there is zeroentropy, all the energy can be used. As the entropy increases, available energy decreases until, withmaximum entropy, no useful energy is available.All systems, therefore, tend towards a state with maximum entropy. In most cases heat is the energyform with most entropy, so all energy tends to become heat. As a heat difference has some order (theheat flows in one direction, which can be used to do work), any heat differences will decrease. Thusan object with maximum entropy is completely homogenous (same throughout) in terms of temperature and has no energy but heat. Specific Entropy So far I have discussed the fact that, although the energy in the universe remains constant, less andless of it can be used to do work, as entropy increases. Though the entropy of a small system is easy  to calculate (compare the 'useful' energy to the heat energy within it), measuring entropy on the scaleof the universe presents many problems - 1.We cannot physically measure the 'useful' energy or the heat energy evenin nearby stars, let alone the rest of the universe.2.Although we can roughly infer the heat energy of a region of space, due toits radiation, we cannot measure the level of heat accurately.3.Even if we could measure a region's entropy accurately, we couldn't scaleit up to the size of the universe. We know neither if that region of spacereflects the universe's entropy as a whole, nor the size of the universe toscale it up to. The specific entropy of the universe helps solve these problems. Before I define it, I will explain howthe idea was developed, and in so doing, why it makes sense.All hot objects tend to give out electromagnetic radiation (light) - the hotter the object is, the more photons (units of radiation) are given off. As heat increases anywhere, so the number of photons willincrease. Therefore photon numbers are a good indicator of the amount of entropy. However,counting them all is impossible. Instead we can count their numbers in a given volume of space. As photons travel very fast and move through a vacuum, the number of photons is likely to be fairlyuniform throughout space.This idea runs into problems when we realise that in most models of the universe space is not static but expanding. Even if the number of photons in the universe stays the same, their numbers in a fixedvolume would vary - we need to count the numbers of photons relative to something which remainsconstant. The proton (a fundamental particle found in the centre of atoms) was chosen for this task.The photon to proton ratio is called the specific entropy and is used to approximate the true entropyof the universe.The specific entropy is an important tool in understanding the universe, because it gives a consistant,accurately defined and easily measurable idea of the universe's entropy. Entropy of Gravity To clarify, gravitationally dimpled means space-time is distorted a lot. The laws of relativity statethat mass distorts space-time, the larger the mass, the greater the distortion. Therefore, to dimple theuniverse matter must move together - the explanation is no more than the original problemformulated in a different way.If gravity contradicts entropy, then surely all other forces should also do so. Electromagnetism is probably the best known of these other forces, so let's consider this. When an electromagnetic forceis used it does so by exchanging a force particle. In this case, that particle is a photon.   Now let's think back to the specific entropy - the ratio of photons to protons. When electromagnetismis used, photons are produced, so the specific entropy must rise. Yet what is true of one force should be true of them all - there should be an entropy rise due to an increase in any force carrying particles.Specific entropy therefore should be the ratio of force carrying particles to protons - the photon to proton ratio was an over-simplification. As any force acts, the specific entropy rises, and so does theuniverse's entropy.Yet there are problems with this view of gravity, The problem lies in accepting the books' claim thatstar formation causes an entropy contradiction, because matter is getting more organised. This isclearly irrelevant, because the laws of thermodynamics deal purely with energy change. The secondlaw definitely allows star formation - gravitational energy becomes energy with greater entropy(usually heat) as the star forms and the excess heat is radiated away as light to decrease the heatdifference created.The arrangement of matter has really no bearing on the energy changes and thus entropy. Even wherematter seems to exert a force to create energy changes, it is not caused by the matter, but by theenergy in the matter. Pressure, for example, can be used to do work, but is caused by the kineticenergy of the particles, not the particles themselves.On the same line, Although Boltzmann did his initial experiments on gases and extrapolated the principles he learnt from them when formulating the second law, mixing gases is not an example of the second law itself. Rather, it shows the statistical processes involved in governing the second law,and the books should claim only this.The insights gained in both gravity and entropy are clearly important. In particular, giving gravity agood theoretical basis is vital, as gravity is the only force strong enough to act between stars and sohold the universe together  . Conclusions about entropy We have discussed what specific entropy is and how gravitational entropy works, but we have notexplained what entropy actually tells us about the universe we live in.The conventional view is that we live in a low entropy universe. This idea is supported by the factthat our planet hosts complex life, the stars are pouring out energy, in fact all the visible universe isfantastically intricate. As entropy increases, more and more energy will become unavailable heat,stars will burn up and be swallowed up by black holes; the universe as we know it will die a heatdeath.Specific entropy, on the other hand, gives a vastly different view. Over its life span, the sun will produce an increase of about 106 photons for each proton within it. The universe at present has 1010 photons per proton. This is ten thousand times as much specific entropy as the entire sun has or willever produce. When we compare the amount of entropy that is produced now to the vast quantitieswhich were produced in the past, we must conclude (in all but the smallest details) that the 'heatdeath' has already occurred.By counting the abundance of different elements in our sun, scientists believe there can only have been (at most) two or three stars before the current set of stars. This indicates that the entropyincrease today is only marginal, compared to what it was in the past, and that entropy was increased by a different mechanism then compared to now.  However, the question remains - whatever state of entropy we have at present, the entropy of a black hole is much greater, so why isn't there a black hole here? Although they take time to form, therehave been at least 15 million years available (current estimate according to the big bang theory,though estimates vary from a thousand years to infinitely long). The best explanation is the anthropic principle. If there was a black hole here, no humans would be there to question its absence - so therecan't be a black hole.The last question could only be answered within the framework of a universe model (in this case the big bang theory). Few further conclusions can be drawn from entropy alone without having an ideaof how the universe has evolved. For this reason, we will now look at how entropy works in thecontext of different models of the universe. Evidence for Models of the Universe A good model of the universe has to explain a number of physical effects. I do not want to dwell onthese, yet I will briefly mention all the main effects and how the big bang theory explains them, asthis is the most accepted theory.The red-shift is the most important physical evidence, and is discussed in greater depth further on. Inessence, it shows us that all the stars in the sky are moving away from us. This implies that all starsare receding from each other, so the universe was denser in the past than it is now. This laid thegroundwork for models showing an expanding universe.The infra-red background radiation is uniform heat radiation found everywhere in space. The big bang theory states it is the light from the big bang red-shifted to a fantastic extent.The helium abundance has its srcin in nuclear synthesis (formation of the elements in the stars).Stars create all elements naturally during their lifetime, in the proportions that are close to thoseexisting in the universe (which in turn suggests they formed these elements), but there is too large a proportion of helium, lithium and other light elements. The big bang theory claims these were formedin the heat of the big bang.Additionally, a good model of the universe should have several particular features. It should ideallyonly use existing and experimentally proven physical laws (there is no point in creating new lawsthat may or may not exist). The big bang theory is in accordance with this, in that it logically followson from Einstein's theory of relativity and a small number of assumptions. Secondly, the theory behind a good model should not contravene any accepted fundamental physical laws (a model, for example, which demands something moving faster than light-speed would be suspect). Finally, itshould be capable of making predictions which can then be tested (it is all very well to theorise aboutthe moon being made of green cheese, but if the theory cannot be proven or disproven, it is useless).Although I so far have only mentioned how the big bang model would explain these effects, manycomprehensive models of the universe explain most if not all of them equally well. However, as themost important factor in understanding the universe is without doubt the red-shift, I feel I shouldspend a little time exploring it further.
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