The following section has a brief description of some ideas that have been suggested over the years for interstellar travel, ideas based on the sciences that do exist today.
Just when you thought it was confusing enough, those physicist had to come up with wormholes. Here’s the premise behind a "wormhole." [graphic] Although Special Relativity forbids objects to move faster than light within spacetime, it is known that spacetime itself can be warped and distorted. It takes an enormous amount of matter or energy to create such distortions, but distortions are possible, theoretically. To use an analogy: even if there were a speed limit to how fast a pencil could move across a piece of paper, the motion or changes to the paper is a separate issue. In the case of the wormhole, a shortcut is made by warping space (folding the paper) to connect two points that used to be separated. These theories are too new to have either been discounted or proven viable. And, yes, wormholes do invite the old time travel paradox problems again.
Here’s one way to build one:
First, collect a whole bunch of super-dense matter, such as matter from a neutron star. How much?- well enough to construct a ring the size of the Earth’s orbit around the Sun. Then build another ring where you want the other end of your wormhole. Next, just charge ‘em up to some incredible voltage, and spin them up to near the speed of light -- both of them.
No problem? Well if you could do all that, and notice you already had to be where you wanted to go to, I’m sure you could think of more clever ways to travel. Don’t expect any wormhole engineering any time soon. There are other ideas out there too - ideas that use "negative energy" to create and to keep the wormhole open.
Here’s what a naturally occurring wormhole might look like if it passed in front of another star. This painting is from Pat Rawlings.
Here’s the premise behind the Alcubierre "warp drive": Although Special Relativity forbids objects to move faster than light within spacetime, it is unknown how fast spacetime itself can move. To use an analogy, imagine you are on one of those moving sidewalks that can be found in some airports. The Alcubierre warp drive is like one of those moving sidewalks. Although there may be a limit to how fast one can walk across the floor (analogous to the light speed limit), what about if you are on a moving section of floor that moves faster than you can walk (analogous to a moving section of spacetime)? In the case of the Alcubierre warp drive, this moving section of spacetime is created by expanding spacetime behind the ship (analogous to where the sidewalk emerges from underneath the floor), and by contracting spacetime in front of the ship (analogous to where the sidewalk goes back into the floor). The idea of expanding spacetime is not new. Using the "Inflationary Universe" perspective, for example, it is thought that spacetime expanded faster than the speed of light during the early moments of the Big Bang. So if spacetime can expand faster than the speed of light during the Big Bang, why not for our warp drive? These theories are too new to have either been discounted or proven viable.
Any other sticky issues?
Yes... First, to create this effect, you’ll need a ring of negative energy wrapped around the ship, and lots of it too. It is still debated in physics whether negative energy can exist. Classical physics tends toward a "no," while quantum physics leans to a "maybe, yes." Second, you’ll need a way to control this effect to turn it on and off at will. This will be especially tricky since this warp effect is a separate effect from the ship. Third, all this assumes that this whole "warp" would indeed move faster than the speed of light. This is a big unknown. And fourth, if all the previous issues weren’t tough enough, these concepts evoke the same time-travel paradoxes as the wormhole concepts.
[Our gratitude to Michael Pfenning for pointing out an error in our older explanation of the Alcubierre warp drive.]
It has been shown that is theoretically possible to create a continuously propulsive effect by the juxtaposition of negative and positive mass and that such a scheme does not violate conservation of momentum or energy. A crucial assumption to the success of this concept is that negative mass has negative inertia. Their combined interactions result in a sustained acceleration of both masses in the same direction. This concept dates back to at least 1957 with an analysis of the properties of hypothetical negative mass by Bondi, and has been revisited in the context of propulsion by Winterberg and Forward in the 1980’s.
Regarding the physics of negative mass, it is not known whether negative mass exists or if it is even theoretically allowed, but methods have been suggested to search for evidence of negative mass in the context of searching for astronomical evidence of wormholes.
A "space drive" can be defined as an idealized form of propulsion where the fundamental properties of matter and spacetime are used to create propulsive forces anywhere in space without having to carry and expel a reaction mass. Such an achievement would revolutionize space travel as it would circumvent the need for propellant. A variety of hypothetical space drives were created and analyzed by Millis to identify the specific problems that have to be solved to make such schemes plausible. These hypothetical drives are just briefly introduced here. Please note that these concepts are purely hypothetical constructs aimed to illustrate the remaining challenges. Before any of these space drives can become reality, a method must be discovered where a vehicle can create and control an external asymmetric force on itself without expelling a reaction mass and the method must satisfy conservation laws in the process.
[Note: This section is excerpted from Millis' "Challenge to Create the Space Drive," in the AIAA Journal of Propulsion and Power, Vol.13, No.5, pp. 577-582, Sept.-Oct. 1997. This 6 page report uses 7 hypothetical space drive concepts to highlight the unsolved physics and candidate next steps toward creating a propellantless space drive. It also contains figures for each concept which are not currently available electronically.]
Hypothetical Differential Sail: Analogous to the principles of an ideal radiometer vane, a net difference in radiation pressure exists across the reflecting and absorbing sides. It is assumed that space contains a background of some form of isotropic medium (like the vacuum fluctuations or Cosmic Background Radiation) that is constantly impinging on all sides of the sail.
Hypothetical Diode Sail: Analogous to a diode or one-way mirror, space radiation passes through one direction and reflects from the other creating a net difference in radiation pressure.
Hypothetical Induction Sail: Analogous to creating a pressure gradient in a fluid, the energy density of the impinging space radiation is raised behind the sail and lowered in front to create a net difference in radiation pressure across the sail.
Hypothetical Diametric Drive: This concept considers the possibility of creating a local gradient in a background scalar property of space (such as gravitational potential) by the juxtaposition of diametrically opposed field sources across the vehicle. This is directly analogous to negative mass propulsion. The diametric drive can also be considered analogous to creating a pressure source/sink in a space medium as suggested with the Induction Sail.
Hypothetical Pitch Drive: This concept entertains the possibility that somehow a localized slope in scalar potential is induced across the vehicle which causes forces on the vehicle. In contrast to the diametric drive presented earlier, it is assumed that such a slope can be created without the presence of a pair of point sources. It is not yet known if and how such an effect can be created.
Hypothetical Bias Drive: This concept entertains the possibility that the vehicle alters the properties of space itself, such as the gravitational constant, G, to create a local propulsive gradient. By modifying Newton’s constant to have a localized asymmetric bias, a local gradient similar to the Pitch Drive mechanism results.
Hypothetical Disjunction Drive: This concept entertains the possibility that the source of a field and that which reacts to a field can be separated. By displacing them in space, the reactant is shifted to a point where the field has a slope, thus producing reaction forces between the source and the reactant. Although existing evidence strongly suggests that the source, reactant, and inertial mass properties are inseparable, any future evidence to the contrary would have revolutionary implication to this propulsion application.