Superluminal interstellar travel using space-time warp drives

Difficulties of Warp Drive

Whilst the warp drive (as proposed by Miguel Alcubierre) does seem to be relatively plausible, there are many hurdles that on is required to overcome before such an idea could really be put into practice. The main problems are:

1. Mass-energy requirement

If the aforementioned quantum inequalities thought of by certain physicists stay strong, then the energy requirements for some warp drives may actually be completely inappropriate (or, at least by today's standards). An energy requirement of -1064kg, for example, may be needed to transport a relatively small space ship across our Milky Way galaxy. This certain amount is much larger the postulated mass of the entire observable universe.

In 1999, Chris Van den Broek, from Belgium, attempted to address this particular problem. By contracting the 3+1-dimensional surface area (as used by Miguel Alcubierre) of the bubble being transported by the drive, while simultaneously expanding the three-dimensional volume contained inside, Van der Broeck was capable of reducing the total energy required to transport small atoms to fewer than three solar masses. At a later time, because he slightly altered the Van der Broeck metric, Krasnikov reduced the necessary total amount of negative mass to only a few milligrams.

Of late, in 2012, Harold White and his team made the announcement that by modifying the geometry of the exotic matter, one could reduce the mass-energy requirements for a macroscopic (i.e. large enough for crew or equipment) from the relative equivalent of the gas giant Jupiter to a much smaller value (around 700kg; the mass of the probe Voyager 1). They stated that their intent was to perform small-scale experiments in generating warp fields. White said that this would work if the shape of the warp bubble was no longer the conventional spherical shape, but actually more like a doughnut shape. In addition, if the intensity of the space warp could be oscillated over time, the required energy is lessened even more. According to White, a specifically modified Michelson-Morely interferometer could be used to test the idea.

2. Placement of matter

Krasnikov said that if the tachyonic matter cannot be discovered or ends up being unusable, then a solution could be to arrange for masses along the path of the space vessel to be put in motion in a way that would mean that the required field was generated. In this case, however, the Alcubierre drive ship could only be capable of travelling routes that had the necessary infrastructure. Because of this, it could be compared to a contemporary railroad. The pilot who is inside the bubble is causally disconnected with the walls of the bubbles and will not be able to carry out actions outside the bubble. The bubble cannot be used for the precursor journey to any distant star because the pilot of the ship cannot position infrastructure in front of the bubble's path whilst it is moving. For example, journeying to the star Vega (25ly away) needs everything to be arranged so that the bubble moving toward Vega with a superluminal velocity would appear; so, such arrangements would always have to take 25 years or more.

It has been contended that arrangements, such as Alcubierre's, are not practicable because matter placed en route of the spacecraft's envisioned path must be positioned at faster than light speed, and that constructing an Alcubierre drive needs an Alcubierre drive even if the metric that permits it is physically important. An analogous objection will apply to any projected technique of an Alcubierre drive's construction.

3. Survivability inside the bubble

Crew members, possibly, may not be capable of steering the warp-enabled spacecraft, due to the fact that the ship could not send signals ahead of the bubble.

Extremely high temperatures generated by Hawking radiation could result from travelling at FTL speeds. This radiation could be capable of destroying anything living inside the warp bubble when traversing at superluminal velocities.

4. Damaging effect on destination

Some have argued that when a ship using an Alcubierre drive slows down from faster than light speed, the release of the particles that the bubble had gathered in transit would occur. These particles may be released in energetic outbursts, much like the shockwave created by a sonic boom. In the instance of particles that are forward-facing, these outbursts could have enough energy to destroy anything in front of the ship.

5. Wall thickness

Regardless, the energy required for this form of propulsion is still not entirely known. A warp bubble travelling at 10 times the speed of light (10c) must have a wall thickness of no more than 10-32 meters. This is close to the limiting Planck length (1.6 × 10−35). A bubble that is big enough to envelop a 200 meter long ship would need a total amount of exotic matter equal to 10 billion times the observable universe' mass. Straining the exotic matter to a tremendously thin band of 10-32 meters is thought to be impractical. Krasnikov's so called superluminal subway also is effected by the same constraints. Not so long ago, Chris Van den Broeck constructed a modification of Alcubierre's model. This newer model requires considerably less exotic matter but it places the spaceship in a curved space-time 'bottle', which has a neck of about 10-32 meters. Hypothesized in some cosmological theories, cosmic strings implicate great energy densities in extensive, narrow lines. Nevertheless, all known physically reasonable cosmic string models have positive space-time warping effects and densities. These outcomes seem to make it rather improbable that one could construct an Alcubierre warp drive by using exotic matter generated as a result of quantum effects.

Allen Everett's calculations show that warp bubbles could be used to generate time-like curves in general relativity. This means that the theory predicts that they could be used for backwards time travel. While it is conceivable that the fundamental laws of physics might permit closed time-like curves, the chronology protection conjecture hypothesizes that in all instances where the classical theory of general relativity allows them, quantum effects would interfere to eradicate the possibility, making these particular space-times difficult to realize. A few results in semi-classical gravity appear to back up the conjecture, comprising a calculation dealing precisely with quantum effects in warp drive space-times which suggested that warp bubbles would be semi-classically insecure. However, in the end, the conjecture can only be decided by a complete theory of quantum gravity.

In a series of online lectures, Miguel Alcubierre wrote "beware: in relativity, any method to travel faster than light can in principle be used to travel back in time (a time machine)." In the next slide he brings up the chronology protection conjecture, and writes "The conjecture has not been proven (it wouldn't be a conjecture if it had), but there are good arguments in its favor based on quantum field theory. Notice that the conjecture does not prohibit faster than light travel. It just states that if a method to travel faster than light exists, and one tries to use it to build a time machine, something will go wrong: the energy accumulated will explode, or it will create a black hole."