Can someone explain the purpose of spice to me?

it extends lifespan, helps to get visions / precognitive abilities.

It doesn't help to fold space, it's just necessary for the Guild to calculate the space routes, precognition, you know.

EXTREME PREJUDICE--
http://www.youtube.com/watch?gl=US&v=XwNydRob4pI

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It's also a nifty metaphor for oil.

It's only found in the desert, where it's buried under the sand and guarded by a bunch of messianic religious fanatics.

It fuels the interstellar economy and is a crucial resource for any empire's national security.

It is exploited by empires and the proceeds of its extraction are used to prop up brutal dictators and treacherous guilds.

It's also an element in a sci-fi/fantasy novel, so the logic of its origins and practicalities of its usage are secondary to its symbolic value.

the spice is a naturally occuring product , made by the worms , which is highly addictive , but does give cognative powers to those who use it in refined/concentrated forms. long term usage makes the eyes turn deep blue as shown in the films.
there is a book called ; the dune encyclopedia , it should answer all the questions you can ask.

The most important aspect of spice is that it extends life significantly. But it has other properties. I has no negative side effects other than the fact that once you are addicted to it, you cannot stop taking it or you will die. It makes you happier and gives you energy as well.

It has more mystical properties however, all explained scientifically. For one thing Spacing Guild members will take it in large quantities and they will artificially evolve beyond human form. Once they gain a certain control over their DNA they are able to use the spice in precognition, which allows for FTL plotting without computers, but they can't see the future because there DNA is no longer human (correct me if I got this wrong). Advanced Benegesserit don't always use spice to gain precognition, they just need a substance known as "truth serum" which allows them to consciously manipulate and read their DNA, coupled with their greatly improved computational capacity they are able to see the future to a degree. It just so happens that the form of truth serum on Arakkis is made from highly concentrated spice.

Paul personally realizes that he has gained precognition when he realizes he has become addicted to the spice. To begin with, he already had advanced mental capacity to compile and compute information. But because he gains this genetic sight through his spice addiction he is able draw from the experiences of past generations of both women AND men, which gives him such immense abilities of precognition and power. Advanced Bene Gesserit are only able to draw upon the experiences of past generations of women. It is not until Paul takes Dune's truth serum that he is able to reach the height of his power.

The spice extends life. The spice expands consciousness. The spice is vital to space travel.

....

Essentially, while there are other methods of space travel mentioned in some of the books, you have to figure that since this is the Known Universe, and not a galaxy (Which is typical in most science fiction stories), the distance you'd have to cover is incredible. What would be the point of trying to do any business if it took over a hundred years to get to point A to point B?

But in Dune, it's different. Folding Space makes it instant. It becomes a total non-issue. The only catch is that only the Spacing Guild has the capabilities to pull it off. They can only pull it off with the spice.

There are attempts made in the novels to come up with either a synthetic form of spice or to produce spice off of Arrakis, but until the later novels its all fruitless.

The spice may be great for the Bene Gesserit, and awesome for making you live for years and years, but that's just a side benefit. It's the fact that the Guild is the only ones capable of folding space that makes it so valuable.

To put it another way...

Imagine if there was only one county (not country) on Earth that had both coal and oil. They could set the price of it at will, and as a further incentive, they could be the only ones to provide steam engines, ships and planes. We'd still have other ways to transport our goods (by sail and horseback), but when you look at the sheer speed that planes, ships, trains and automobiles can ship things, it would be largely impractical.

THAT is the purpose of the spice. "He who controls the spice controls the universe". Without it, they'd be set back farther than we, in the present time, would be if we went back to the days of Adam and Eve.

"Tell me, Mr. Anderson, what good is a phone call, if you are unable to speak?" - Agent Smith

At minimum dosage, it's an immune booster. "Health and long life is the gift of the spice." More and it begins to effect prescience which is why the Bene Gesserit used it. Surrounded by it like the Fremen and it begins to alter physicality - the blue in blue eyes. The Guild Navigators are examples of its ultimate effect - probably near immortal, prescient and physically so altered at the cellular level that they are no longer truly human.

No one is really sure how the spice was discovered, probably just stumbled upon it. Appentantly it took awhile for its effects to be realized and used.

Before the Navigators came on line, the Holtzman Effect was used - a form of space folding or warping. However, it was tricky and dangerous. I believe it's basically what the Navigators use, but much more reliable and efficient.

With interstellar travel, there's no such thing as an extra week on space travel. Getting to the next nearest star, even pushing your craft to 99% light speed (that requires a hugh energy source), would take much more than four years and another four years plus to get home. Our galaxy is o/a 60K light years across. If Uncle Albert is right about relativistic time effects and he appears to be, if you transverse the galaxy at near or even at light speed, back home 120K years or more have passed. Right now folding, warping or using black holes is the hand wavium of choice for fictional space travel. Anyone's guess what the real deal will be, if there ever is one.

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