Re: preventing the heat death of the universe?

These are tricky ideas to explain, but we'll try...

The second-law of thermodynamics does indeed seem to tell us that the universe is destined to eventually experience a heat death. For people who don't like this idea, the bad news is that many physicists think that the second law of thermodynamics is in some sense the most reliable law of nature we have identified. The reason is that whilst our other theories of nature, such as relativity or electromagnetism seem likely to be subsumed into more complete theories in the future, the second law of thermodynamics is essentially a statistical statement. Strictly speaking it's predictions refer only to large systems (ones containing many particles) and are about the overwhelmingly most likely future behaviour, but are not absolutely certain.

The basic idea behind the second law is that if the energy of the system of particles (a box of gas, for example) is initially mainly possessed by one particle, say, then the physical interactions are likely to spread the energy around the other particles as well. If the energy is already spread fairly evenly around, then it is most unlikely that they'll all conspire to give the majority of energy to just a single particle. This very basic argument can be generalised to the universe as a whole. We say that the entropy of the universe is always increasing (the "entropy" is a measure of how spread around the energy is). Once energy is spread out amongst lots of particles (ie. as heat), there's simply no practical way to turn it back into useful energy (we only get energy from heat when we also have something cold that the heat can be spread to). The implication is that even if energy is sometimes concentrated in one part of a physical system, for example when we accelerate a car and therefore give it a lot of kinetic energy, a lot more energy must be spread around in order to make this possible -- in the example of the car this occurs by the production of heat, for example that given to the exhaust gases. No matter how efficient the car engine a certain amount of heat loss is unavoidable.

The Carnot heat engine is a actually a theoretical concept -- it tells us the requirements we must meet in order to create the most efficient possible engine. Unfortunately no real engine can ever do as well as the Carnot engine, and even a Carnot engine can't avoid increasing entropy! As far as we can tell, there's in fact no way to avoid the second law in the long run, and we have no idea of any method or technology which could be developed to do so.

So, coming back to the idea of "heat death", if the whole energy of the universe is going to end up spread amongst as many particles as possible, then it should be given to particles of zero rest mass. Particles of light, photons, for instance, can in principle have energy as low as you like. Particles which have a rest mass always have at least that amount of energy. There are various ways in which the energy of the universe may end up spread primarily in very low energy photons. For example, in the far far future much of the matter in the universe should be accreted by black holes. Hawking in the 1970s showed that black holes in the far future, when the universe is very cold, should actually slowly evaporate, producing vast numbers of very low energy photons.

In terms of its relevance to the future of humanity (and any other intelligent beings), I'm not sure it's really something to worry about. Even if we can keep our own planet fit for habitation and solve the problems of our future energy and resource requirements etc.; if we aren't wiped out by any cosmic catastrophes, such as being hit by a comet, we will still have to deal with our Sun evolving to a red giant. If that is solved (eg. by moving ourselves to another solar system), eventually all the stars (and other sources of usable energy) will die out. This may happen in hundreds of billions of years, but still long long before the heat death I described above. At the same time, if our current measurements of the expansion of the universe are correct, then the universe will be a far more lonely place than it is now -- most of the other galaxies being vastly far away from us. If intelligent beings are still around then, it's possible they could carry on living and thinking, but only if they can do so using progressively less energy (and ultimately, progressively more slowly).

So, this isn't a very cheery picture, but the timescales we are talking about are so immense compared to the entire history of human civilisation to date (say 10000 years at most), that it seems to me there's plenty of time for us to do great things (and avoid doing tragic ones), and much better to focus on those!