Warp Drives

“Warp drives” originated in the science fiction shows “Star Trek” in the 1960s, but in 1994, the Mexican Miguel Alcubierre published a paper arguing that under certain conditions exceeding light speed was not forbidden by Einstein’s General Relativity. Alcubierre reached his solution by assuming it was possible, then working backwards to see what was required while rejecting those awkward points that arose. The concept is that the ship sits in a bubble, and spacetime in front of the ship is contracted, while that behind the ship is expanded. In terms of geometry, that means the distance to your destination has got smaller, while the distance from where you started gets longer, i.e. you moved relative to the starting point and the destination. One of the oddities of being in such a bubble is you would not sense you are moving. There would be no accelerating forces because technically you are not moving; it is the space around you that is moving. Captain Kirk on the enterprise is not squashed to a film by the acceleration! Since then there have been a number of proposals. General relativity is a gold mine for academics wanting to publish papers because it is so difficult mathematically.

There is one small drawback to these proposals: you need negative energy. Now we run into definitions, and before you point out the gravitational field has negative energy it is generated by positive mass, and it contracts the distance between you and target, i.e. you fall towards it. If you like, that can be at the front of your drive. The real problem is at the other end – you need the repulsive field that sends you further from where you started, and if you think gravitationally, the opposite field, presumably generated from negative mass.

One objection often heard to negative energy is if quantum field theory were correct, the vacuum would collapse to negative energy, which would lead to the Universe collapsing on itself. My view is, not necessarily. The negative potential energy of the gravitational field causes mass to collapse onto itself, and while we do get black holes in accord with this, the Universe is actually expanding. Since quantum field theory assumes a vacuum energy density, calculations of the relativistic gravitational field arising from this are in error by ten multiplied by itself 120 times, so just maybe it is not a good guideline here. It predicts the Universe has long since collapsed, but here we are.

The only repulsive stuff we think might be there is dark energy, but we have no idea how to lay hands on it, let alone package it, or even if it exists. However, all may not be lost. I recently saw an article in Physics World that stated that a physicist, Erik Lentz, had claimed there was no need for negative energy. The concept is that energy could be capable of arranging the structure of space-time as a soliton. (A soliton is a wave packet that travels more like a bubble, it does not disperse or spread out, but otherwise behaves like a wave.) There is a minor problem. You may have heard that the biggest problem with rockets is the mass of fuel they have to carry before you get started. Well, don’t book a space flight yet. As Lentz has calculated it, a 100 m radius spacecraft would require the energy equivalent to hundreds of times the mass of Jupiter.

There will be other problems. It is one thing to have opposite energy densities on different sides of your bubble. You still have to convert those to motion and go exactly in the direction you wish. If you cannot steer as you go, or worse, you don’t even know for sure exactly where you are and the target is, is there a point? Finally, in my science fiction novels I have steered away from warp drives. The only times my characters went interstellar distances I limited myself to a little under light speed. Some say that lacks imagination, but stop and think. You set out to do something, but suppose where you are going will have aged 300 years before you get there. Come back, and your then associates have been dead for 600 years. That raises some very awkward problems that make a story different from the usual “space westerns”.

Why not explain.

Suppose you write a story that introduces “science” or magic that has to be there for the plot to work.  Thus in the Nibelungenlied, Siegfried must help Gunther win Brunhild for the plot to progress. Now, in a contest of strength watched by all and sundry, Siegfried cannot be seen doing it, hence the need for a cloak of invisibility. Star Wars would not be the same story without the force. Star Trek would be quite different without teleportation and warp drive. One issue facing a writer is, when should one try to explain what underpins these devices?

One possibility is, “never”. Thus for centuries, people listed to the Nibelungenlied without worrying a jot about how the cloak worked; it was magic! The Star Trek “science” is slightly different. Teleportation in the sense used there was not really important other than as a means of getting on with the story, so as long as the story was good, who cared? It just removed the need for tiresome issues with shuttles, and it was accepted (or not) without further discussion. Warp drive was something slightly different. Again, that was needed to get the story moving, and to permit the crew to return to Earth, but it has an interesting side issue. According to relativity, or more particularly the representation of space-time in relativity, moving faster than light requires moving through time in a negative direction so you can arrive before you set off. As far as I am aware, this peculiarity was never made use of.

This introduces another reason not to explain: the explanation is just too complicated. Think of going back in time through warp speed. This depends critically on the concept of space-time, without which the equations of general relativity are sufficiently difficult that they beat Einstein (who, at first, thought “space-time” just plain wrong, but he later adopted it because there seemed to be no other way of making progress). The author does not need to get bogged down into that discussion! For example, one view might be that just because the maths are more easily solved using space-time, it does not mean that spacetime is a physical object. Think of the geometry problems you solve by making a construction, or a differential equation using a substitution. Making a construction on a sheet of paper does not make it real on whatever the diagram represents! It just makes it easier to solve the problem. Similarly, modern quantum mechanical problems are addressed through using something called Hilbert space. Nobody I know has suggested that Hilbert space is a real thing, and if it were, it is not really compatible with relativistic space-time. See why you do not want to get involved? Why dig a hole for yourself and lose readers?

Notwithstanding that, there is a problem with ignoring it. Thinking of warp drive, you can either do what Star Trek did and use it as a way of getting from A to B to shorten travel times. Now you are incompatible with relativity, so no explanation is a good idea, but you consign the concept to the “convenient” and turn your stories into the “ordinary”. Be careful, or all you end up with is a space western, and the author has lost a huge number of possible plots!

Take another reason. Think of the force in Star Wars. I remember watching the first three movies, and I, along with everyone else I know, accepted that, in these movies anyway, there was something called the force that a very few could access after a lot of training. I did not care what caused it. But then, a number of years later, Lucas made three more movies, and explained the origin of the force. In my opinion, the explanation was ridiculous, and it only detracted from the movie. To summarize what I am suggesting, sometimes it is better not to try to explain something. Details can add to a story, but silly details subtract from it.