Hard SF : SF Tech Issues : Faster Than Light
As picky as I can be about some things, I'm generally not bothered by the fact a book says a spaceship leaves one star system and arrives at another star system a week later. I don't know why. Perhaps it's because having to use a spaceship to travel and needing some length of time to get from one place to the other does not leave the feeling of something magical and anti-scientific.
I do prefer that the book not give me some supposed explanation of how the spaceship manages to ignore the laws of nature.
The issues affecting FTL are on the sophisticated side and there have been too many schemes presented by SF writers to rationalize their FTL travel to deal with them all. Readers who want to know more might try the following sources for explanations written for non-scientists: the books mentioned in this site's Science Books page or articles in the Scientific American articles pages. Physicist and SF author John Cramer also has articles from his Alternative View column originally published in Analog available free online - click here
"The speed of light" is generally quoted as being 186,000 miles / sec. or 300,000 km / sec. These are approximations for the speed between astronomical bodies. That is, it's the speed at which photons pass through the vacuum of space. Photons travel at somewhat different speeds through air, water and other media. There are also a few special circumstances where photons do or "sort of" travel at a somewhat different speed in the same medium. As far as space travel is concerned, there is no photon speed that crosses the distances between stars in a matter of weeks. And the laws of nature are not even as generous to spaceships as they are to photons.
According to Einstein's Theory of Relativity, objects with a non-zero rest mass require energy to accelerate towards the speed of light. There are relationships between curvature of space-time, mass and gravity; mass and energy; velocity and energy; etc. Especially when an object is moving at a large percentage of the speed of light it experiences "relativistic effects". Its speed results in it having more energy which is also expressed as mass. Having more mass, it requires more power to accelerate further. Even if the object could be accelerated to 99.99% of the speed of light, it would then be so massive it would require nearly infinite power to go faster. At the speed of light, the amount of power to go faster becomes literally infinite.
Photons naturally travel at light speed because they have a rest mass of zero. However, photons do express mass as long as they aren't at rest. Photons can have different amounts of energy, but increasing the energy does not cause a photon to go faster than 186,000 mps. (Actually, there are special situations where photons may move a bit faster, but not so much to take less than years between stars.)
Einstein's equations can be employed using different starting assumptions. (Not all of these starting assumptions are necessarily valid.) Depending on what you begin with, some solutions suggest that a kind of particle dubbed "tachyons" could exist that travel faster than photons. Scientists do not know whether tachyons really exist. Experiments designed to make tachyons in particle colliders or otherwise find them have so far failed to uncover them.
Even if they exist, that does not mean we can make practical use of them. The solutions that include tachyons indicate that low-energy tachyons travel fastest and additional energy must be supplied to slow one down towards light speed. A tachyon at the lowest possible energy would have infinite speed. This is related to the fact their mass would be an imaginary number.
I don't know what a “typical” energy level or speed would be considered to be. What I've read leaves me with the impression physicists are neither sure of this or factors that would affect it. They seem to more often say something like “if tachyons had this characteristic, the following would also be true.”
It would seem there would be two avenues to try to use tachyons for FTL.
Find a way to transform all of the particles in a spaceship into tachyon-type particles and have the tachyon spaceship fly through space – then convert it back to normal matter when nearing the ship’s destination. I don't know whether any theory suggests tachyons can be a kind of particle that you could build a spaceship out of, so this may simply not be an option. There is no particular reason to think normal particles could be converted to tachyons or vice versa. It is also necessary to ask how long such a process would take.
Consider converting the ship back to normal matter when you reach your destination. Even if one could transform a tachyon into a normal matter particle, how do you convert an intact spaceship? Tachyons always travel at > 186,000 miles / sec. So even if you can convert the entire ship in 1/1000 second, the tachyon ship would have traveled more than 186 miles during the process. (If your tachyon ship is traveling at 5 times the speed of light, so it takes nearly 1 year to reach the nearest star, the ship will travel 930 miles in 1/1000 sec.) With this speed and distance, the assembly of the normal matter ship will be difficult.
Physicists seem to be uncertain whether tachyons would have electrical charge. If they don't have charge, a tachyon ship could not be held together using charge as normal matter atoms are. So we need to ask what force would hold tachyons together in a complex structure such as a spaceship.
The tachyons could be used as a faster means of matter transmitting (teleportation). This is only really applicable after you sent a teleportation receiving station to the destination by slower-than-light means. There are many questions surrounding matter transmitting regardless of what particles are transmitted; click here for articles.
From what I've read (and what I understand of that), it seems that using tachyons for faster-than-light communications is believed to be impossible because of ways in which a tachyon field would behave. I assume that if tachyon communications is impossible, tachyon teleportation would be also.
Some more info and links are available in Tachyons article.
Often those wanting some form of FTL travel suggest what amounts to shortcuts so the traveled distance only requires a relatively brief period of time to transverse. A spaceship is pictured as somehow crossing a barrier from our familiar universe into "hyperspace", "a parallel universe", "another dimension", "a space-time discontinuity", "a wormhole" or some other place. Later the ship has a way to exit from this secret passageway at a location chosen by the crew. Some of these shortcuts, like "another dimension", simply are not places or areas that could constitute a shortcut.
According to quantum theory, particles do not act in a deterministic way. There are a number of possible outcomes to a quantum event and each possible outcome has a certain probability of occurring. Some quantum theorists believe each of the possible outcomes actually takes place, with each resulting in a slightly different universe branching off. While this is only one of the schools of thought in quantum physics, it is a scientific theory with considerable support that suggests "parallel universes". We don't know if these actually exist or there is a way to go from one to another.
If we could, there would still be issues. Assuming we did not go to another universe based on the universe many billions of years ago, the laws of physics in that other universe would be the same as ours. Unless you went to one based on the universe over 1 billion years ago, the stars would not be that much closer together than what our universe has expanded to. Going to a "far past" universe might have advantages in different physical laws or closer stars, but may not be as feasible to get to as more similar universes. On the other hand, if we went to a similar universe's Earth and then had to fly through that universe to reach their Rigel and then come back to our Rigel, we wouldn't save much. Such parallel universes are only useful for FTL if there is some mysterious shortcut connection between "our" Earth and some location in "their" universe plus another connection between "their" location and "our" Rigel which does not involve extended time and energy for the entire trip. We should ask why there would be shortcuts from "our" Earth to "their" Rigel, but not from "our" Earth to "our" Rigel. Even if we can go to such parallel universes, it is questionable whether the resulting trip would really be shorter than travel in our universe.
It is possible there are multiple universes in another sense. It could be that "our universe" is one of a number of "bubbles" embedded in and defined by a higher level universe. Again, we don't know that this is the case or that if it were we could get outside our bubble. Going to another "bubble" probably wouldn't be useful as far as FTL is concerned (the bubble probably would not have connecting relationships). Going outside our bubble into the over-universe may have issues as a result of what would likely be fundamentally different physical laws for the two levels of universes.
Can the over-universe make a useful shortcut? Who knows? Even if in some sense it was a shortcut, the effect of the over-universe's laws of physics on us could have consequences. If going to the over-universe and doing whatever we have to do there to be able to return to our universe at Rigel made our bodies experience many decades of aging (perhaps to death), we might not consider it worthwhile even if the calendars on Rigel didn't say it had been that many years. If the process didn't age us, but the calendars on Rigel said it was many decades later, that would limit its usefulness. Only when we know if it's possible and what the practical implications are can we know if there's any point in doing it.
It's a nice name, but what does it mean? If it means a higher level of space, it might mean the over-universe mentioned above. It's not a term that seems to be used often by many scientists. Aside from other "shortcuts" discussed here, I'm not aware of other "places" suggested by the laws of physics.
Some scientists have used the word “hyperspace” when discussing the curled-up extra dimensions theorized to exist according to string theory and its derivatives. These additional dimensions would be “curled up” at microscopic levels that have prevented scientists from observing the extra dimensions or their consequences. It doesn’t seem a macroscopic spaceship could “turn a corner” to travel along the lines of those other dimensions. If a spaceship could somehow enter the extra dimensions curled around one point in the familiar three dimensions, the curled up state of the other dimensions might preclude moving from there to the curled up dimensions around a different point in our three dimensions. Assuming that were possible, it does not mean the trip will be any shorter. To take an analogy, suppose you are at Second Avenue and Fourth Street and want to get to Fifth Avenue and Twentieth Street. Normally, that would mean traveling in two dimensions (North-South and East-West). You could try using an additional dimension (Up-Down), but going up or down doesn’t necessarily get you closer to Fifth and Twentieth.
Wormholes have become a more popular way in fiction to take a shortcut. Most physicists seem to believe that quantum wormhole flash in and out of existence in the quantum foam similarly to virtual particles. Some physics models question their existence. There is no solid evidence as to whether they do or don’t exist.
Naturally-occurring quantum wormholes are too small for even a photon to pass through them and are generally believed to flash out of existence after a very brief length of time. To make practical use of a wormhole would require humans to artificially enlarge them and (according to most models) for humans to artificially stabilize them to keep the wormhole open long enough to make use of it. In theory, this could be done. However, there are factors that may prevent actual implementation.
Models of traversable wormholes are based on “metrics” describing extraordinary curvatures of space-time. Most of these can only be made stable with the use of significant amounts of negative mass-energy – which may not be implementable. Alternative metrics have been suggested that would be self-stabilizing, but the ability to create such space-time topologies is speculative at this time.
Even assuming a wormhole was large enough and stable would not necessarily make it safe for travel. The extreme and peculiar space-time curvatures mean forces and stresses inside the wormhole that could be harmful to a traveler. Harnessing those forces and stresses put additional constraints on a wormhole that might prevent implementation. Within our current scientific understanding, the more optimistic expectations for wormholes picture their implementation involving longer travel times and greater risks than are generally depicted in SF.
For a more extensive discussion of the alternatives and issues, and links to other resources, see the Wormhole section.
Suppose, you wanted to go to a star 100 light-years away and your spaceships only go at 1/10 light speed. That would take 1000 years. Now, suppose, you had a way to send a spaceship back in time 1000 years before it started its journey. It would arrive at the other star the same year it began its journey through time and then space.
It's not as neat a solution as the shortcuts. There will still be 1000 years of wear and tear on the ship. If you send people on the ship and wanted the same people to arrive at the other star just as young as they are now, you'd have to have some sort of "suspended animation" that would work for 1000 years without harming the people. Or you could have a ship crewed by people who live longer than 1000 years who will be 1000 years older when they arrive. Or you need to have a ship designed to house people over the course of many generations, and have distant descendents arrive at the other star.
Time is a known quality of the universe, and we know there are extreme conditions that can significantly affect time (or at least how time is experienced by particular people and objects). The issues and limitations of devising "time travel" make the above scenario unlikely. Please see the suggested reading in the Science Books and Scientific American articles pages.
Another suggested method is to use some peculiar aspects of Relativity. In theory, one could make a spaceship move “faster than light” by making space-time behind the ship expand and making space-time in front of the ship contract. In one sense, the spaceship would not be moving – it would be space-time outside the ship warping. The expansion and contraction of space-time itself is not subject to the speed of light limit.
In models proposing such space warping, the spaceship is in a “warp bubble”. At least if the warp bubble is bringing the ship to its destination at what is in effect faster than light speed, the ship inside the bubble is unable to interact with anything outside the bubble. That includes being unable to steer the ship or alter its effective speed. Presumably, controlling the space warping / ship’s movements would have to be done by something other than the ship – for instance, by machines at the places the ship departs from and arrives at. How a ship gets out of the warp bubble once it arrives at the destination may be an unresolved issue. Factors such as this raise serious questions about its practicality.
The initial model for such a “warp drive” presented physicists with a general idea how one might attempt to do this, but had a literally impossible energy requirement. Later versions of the concept have found ways to reduce the energy requirements, but generally have impractical requirements given in terms of solar masses. The energy requirement is related to the surface area of the warp bubble that faces the external universe. A bubble with an external area on the scale of a spaceship needs enormous energy. One way around this has been proposed. In principle, space-time could be warped in such a way that a large internal chamber could be in an outer shell with a microscopic external surface. If such a space-time bottle with an external area smaller than an atom could be made with a spaceship inside it, the warp bubble it required would have a low energy requirement. The scenario presented for this proposes a surface area too small for light to enter – making it impossible for those inside to know anything about the outside world no matter what speed they traveled at. While the idea can be intriguing, there are questions about both the space-time bottle and the warp bubble.
See also: Warp Drive article
One of the strange aspects of quantum physics is a phenomenon called “quantum entanglement”. When two particles have a certain kind of relationship between them, an action that affects the properties of one of the particles can instantaneously affect the properties of the other particle. This is an accepted element of quantum physics, and the effect is not restricted by the speed of light. Therefore, some have suggested using this for faster than light communications or teleportation.
Although the interaction between the two entangled particles occurs faster than light, there are issues in quantum physics that prevent meaningful communications (or teleportation) from being implemented by only using the two entangled particles. In order to have meaningful communications, physicists have devised a means by which a third (non-entangled) particle at the “sending end” is needed and the sending and receiving ends (each with one of the two entangled particles) must be in communications by some other means in order to provide information required to interpret each change in the entangled particles’ properties. This second means of communication that must be employed cannot be quantum entangled particles, and therefore will be no faster than the speed of light. As a result, the effective speed of the communications or teleportation will be the speed of the second means of communications minus the time required to manipulate the entangled particles for that part of the communication. This means the use of entangled particles might have a useful purpose (such as preventing others from intercepting your messages), but it does not fulfill the objective of being faster than light.
See article: Quantum Entanglement
For someone else's review of
the proposed methods and issues of FTL:
http://math.ucr.edu/home/baez/physics/Relativity/SpeedOfLight/FTL.html
Or if you're up for something
longer (and downloadable):
http://www.physicsguy.com/ftl/
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