Nineteenth century physicists were also extremely interested in the concept of energy. In physics, "energy" is a bookkeeping device for keeping track of the amount of work that has been done on or by a system. In popular fiction, "energy" became a buzzword, and finally a kind of substance. Science fiction writers and pseudoscientists today talk about things made of "pure energy." Since temperature, for instance, is a direct measure of internal kinetic energy, to say something is "pure energy" makes no more sense than to say something is "pure temperature." This is gibberish, without any connection to the world we live in. Energy is not a substance or a thing. It is a number (with units of work) and it is not the number of anything.
Another ever-popular idea from 19th century fiction was, "what if?" What if the South had won the Civil War? What if Napoleon had not invaded Russia? Characters were struck by lightning or something and woke up in another world in which history had taken some other turn. It makes a good novel, but it has nothing to do with the world we live in. In its purest form, this is a na�ve "concrete" interpretation of the abstract concept of probability the claim that if the coin is 50% likely to fall heads, and 50% likely to fall tails, it has to do both the universe splits into two universes, in one of which the coin falls heads, in the other of which it falls tails. Since almost everything that happens in the universe is a matter of probability, and since an almost limitless number of processes are taking place in each split second, one is talking about an almost limitless number of "new" universes being born every split second. Of course, one can talk about anything. There is, however, nothing in the real world corresponding to the concept of parallel worlds. In modern physics one does encounter what are called "multiple vacua." The present state of the universe is not the only possible state. For all we know the whole universe could suddenly make a "phase transition" to a different vacuum, in which all the fundamental constants and laws of nature would be totally different, and things as we know them would cease to exist. But this possibility has nothing whatsoever to do with the concept of "parallel universes," which are instead more often confused with "higher dimensions," "coexistent worlds," etc. A good 20th century science fiction novel dealing with parallel worlds is Keith Laumer's Worlds of the Empirium; see also H. Beam Piper's "Pratime" series.
What if we could transfer your mind into the body of a spider? Or my mind into the body of a whale? Makes for a good 19th or 20th century fantasy story or satire, but what on earth are we talking about? Not brain transplants. Some swami makes a mumbo-jumbo incantation -- the movie All of Me features a classic example -- and minds shuffle around among bodies like cards. What does the word "mind" mean anyway? What the writers did is to take the ancient religious concept of "soul," rename it "mind," and go on from there. Again, there's no connection to reality at any point. But it's now a common teaching in pseudoscience that you can "learn" -- there's that word again, and it's just $250 for easy lessons if you act today! -- To detach your mind from your body and send it out to visit distant places. Sounds like a primitive scenario to account for the very natural phenomenon of dreaming.
REALITY AS MENTAL IMAGE
Another popular theme of 19th century fantasy fiction was to have a character realize that his inner thoughts could reshape the whole universe by thinking that everyone should have two heads, he somehow causes everyone to instantly and from then on have two heads. H. G. Wells wrote a fine story along these lines, entitled The Man Who could Work Miracles, and 20th century writers have used it too, as did Ursula LeGuin in The Lathe of Heaven.
As far as reality is concerned, however, it is one of the most obvious facts of our experience that the universe goes on totally independent of our thoughts, desires, dream, and fancies. The fact that a dimwitted cook doesn't know what temperature water boils at, or thinks water boils at 50� F, does not alter the boiling temperature of the water on the cookstove. The original 19th century nonsense has been re-treaded many times, and most recently has shown up in crackpot "popularizations" of quantum physics or in books about "mystical physics," in which it is claimed that physicists have shown that "your mind creates your own reality," and similar vague gobbledygook. What goes on in the human brain has no more effect on what goes on inside the Sun than what goes on inside the Sun has on what goes on inside the human brain. How various processes affect one another is the precise thing that physicists do study. There is no question, experimentally, that thoughts alone do not affect processes going on outside the body.
CIVILIZATION AS PERIODIC
When in the 19th century people began to realize the earth was millions of years old (actually it's billions of years old), but knew little or nothing about biology and evolution, they said things like, "But recorded history only goes back a few thousand years! What were humans doing the rest of that time?" In fact, humans didn't even exist for most of earth's history, as was clearly understood by the end of the 19th century. But writers and pseudoscientists were going strong with the theme of civilizations rising and falling, rising and falling. Half a million years ago some great civilization had TV and microwave ovens and girls whom could kick off clothes even faster than Bo Derek. But that civilization fell, and humans reverted to savagery, and then a long climb began up to our present civilization, which will also fall, etc. There is no question but that this idea is totally wrong as it applies to the past. There is not one shred of archaeological evident of any past civilization with a technology anything like our own. The archaeological record is wholly consistent with the usual idea that urban civilization is only about 10,000 years old at most. (The human race itself, in its present form, is only around 50,000 years old and for most of its existence lived a semi-nomadic, hunter-gatherer lifestyle.)