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Time Travel through Warp Drive:

Principles of Warp Drive

            Long considered to be a science fiction gimmick that could never actually be achieved, recent scientific work by Alcubierre has given evidence that faster-than-light warp travel may actually be achievable, although it would require far more advanced technology than we have today. Furthermore, if such travel could be achieved, it can be altered to act as a time machine.

                Warp travel allows objects to travel faster than light in the same way that allows far galaxies to be receding from the Earth at speeds greater than light. Instead of moving a ship (or whatever) through spacetime, a warp drive operates by creating a ‘bubble’ of space (which includes the ship) and propels this ‘bubble’ through spacetime by contracting the space between the ship and its destination (decreasing the distance) and expanding the space between the ship and its origin (increasing the distance). In this way, the ship, viewed from outside, appears to move from origin to destination at the chosen speed.

           

Use as a Time Machine

            Rather than simply travelling through space, however, one person by the name of Everett has shown that there is another aspect of such travel. The properties of warp travel given above are also consistent with a situation in which the spacetime axes inside the bubble have been changed with respect to those outside the bubble. Specifically, the axis which represents time inside the warp bubble is bent on an angle towards the direction of travel outside the warp bubble. (A similar phenomenon is seen at the event horizon of black holes, where escape is impossible because the time axis has been tipped over so that it points towards the black hole.) This leads to the very passage of time inside the bubble (or black hole) being equivalent to outside motion as seen from the outside universe.

            This has an important consequence for the question of time travel. If the axis of time inside the warp bubble has been bent towards a direction in space outside the warp bubble, then is it possible that a direction of space inside the bubble is equivalent to the direction of time outside the bubble? The answer would appear to be yes, and if so, it follows that it is possible to change one’s movement through time – as seen by an observer outside – by changing one’s speed in the direction of space inside the bubble that corresponds to time outside the bubble.

            As a demonstration of how this works, consider a situation in which there are only two dimensions – a space dimension (left and right) and a time dimension (forwards and backwards). Now, imagine that a ship in the space was using warp drive to travel to the left, so that the time axis inside the bubble was transposed over the ‘space’ axis outside the bubble (This represents infinite speed of the bubble as seen from outside, as none of the time dimensions overlap – a situation which I am inclined to believe is impossible, although I have no justification for this, but it nonetheless makes for a simpler example). In this new set of axes, going forward in time inside the bubble is equivalent to going left in the space outside, going left is equivalent to going backwards in time outside, and going right is equivalent to going forwards in time outside. Hence, once the bubble is formed, travelling ‘left’ or ‘right’ can involve travel backwards and forwards in time, respectively. Although travel inside the bubble – and hence travel in the time dimension outside – is limited by the speed of light, by accelerating to relativistic speeds inside the bubble time dilation can take hold and decrease the subjective time required for the time traveller(s) to move through time as seen outside the bubble.

Advantages and Disadvantages of Time Travel via the Warp Drive

            One of the main advantages of using the warp-drive method of time travel, especially over the wormhole method, is that it represents far greater freedom than that offered by wormholes: while a wormhole only allows travel from two locations in spacetime that must be linked via other means, warp travel allows freedom to move anywhere in spacetime, both past and future (including the ability to go back in time to before the construction of the first time machine).

            Another advantage is that, if warp travel becomes a primary means of interstellar faster than light transport, it immediately follows that time travel will become readily available. However, this is a bit of a double-edged sword: If it is possible to use time travel to affect the past, this makes having large numbers of warp-propelled ships dangerous unless properly controlled (all it takes is one lunatic with whatever-century’s technology to go back and…) Furthermore, another thing that makes it double-edged is that anything preventing time travel may also result in prevention of warp travel, although warp travel that does not attempt to travel through time should escape the beam of vacuum fluctuations from Stephen Hawking’s Chronology Protection conjecture.

            There are a number of other problems, too. One is more of an inconvenience then a problem: Once created, the warp bubble cannot be controlled from within. This means that either the bubble must be controlled from an external control source (possibly returning to a travel arrangement similar to a wormhole network by having ships being encapsulated in bubbles and pushed out at the origin, and being caught and removed from the bubble at the destination, but without the disadvantage of having to transport wormhole mouths – the warp bubble control apparatus could probably be built on-site from scratch or with prefabricated materials) or the bubble’s entire lifespan must be predetermined in some way before being created by the ship for travel. Either way, this makes travel dangerous, as once the bubble is formed the ship has no way to respond if an emergency arises.

            There is another problem with the construction of the warp bubble: By compressing space in the direction of travel, the ship may also compress matter between the ship and its destination. This compression could result in turning even sparse interstellar dust into a dangerous travel hazard, and disturbing solar systems (which may be inhabited) by the compression of space reducing distances in the system. The reverse also occurs for the expansion of space behind the ship. Overall, the ship must take care that in moving from point A to point B, it does not cause too much damage to matter in the spacetime between these points.

            The two points above deal equally to use as FTL travel and time travel, but there is one inconvenience that is specific to time travel: When the warp bubble is formed with altered time axes, time inside the bubble is still pointed in a direction corresponding to a given direction in outside space. In the time (as seen from inside the bubble) that it takes for the time traveller to move to the desired time in space outside the bubble, the bubble (and the ship inside) will have been constantly moving through outside space. Because of this, a time traveller who neglects this aspect attempting to travel in time may find themselves at the correct time, but light years away from the actual location they wanted to be. This, however, can be circumvented by making half of the journey with a warp bubble heading one way, and then turning the bubble around (or creating a new one) to return to the desired location in space, while the time traveller also reverses direction inside the bubble to continue travelling in the desired direction. This process is, of course, similar in operation to the time traveller who travels into the future using relativistic effects by travelling into space at near-lightspeed, turning around, and returning ‘home’.

            The final problem that must be mentioned is the negative energy requirement of warp drive. To create a bubble large enough to contain a ship would require a magnitude of negative energy sufficient to neutralise all of the mass and energy of the known universe several times over. Furthermore, this energy must be confined to an area of thickness of 10 to the power of –32 metres: only a few orders of magnitude greater then the Planck length, 10 to the power of –35 metres (the smallest unit of distance with meaning: space, like everything else, is quantised) (ref).

            There is one proposal that can reduce the negative energy requirement. Since the creation of the bubble involves creation and destruction of space outside the bubble, why not warp the space in the bubble? Specifically, a bubble requiring less negative energy can be produced by creating a bubble which appears larger on the inside than it does on the outside. This led to a proposal by the scientist Van den Broeck to create a bubble which, while large enough to contain a ship and any other required space on the inside, is only a few orders of magnitude greater than the Planck length in the outside universe, substantially decreasing the required magnitude of negative energy, although it is still confined to an extremely small area and has an extremely high negative energy density in this area.

            However, this approach does have a few limitations. The first is that, with the warp bubble being smaller than the wavelength of all common forms of electromagnetic radiation, the ship will not be able to see outside the bubble (although the lack of control noted above means they couldn’t do anything about it anyway, making this unimportant for safety purposes). More importantly, this technique shrinks the bubble down to a scale where it is subject to the laws of quantum gravity, of which we know little. These laws may prevent the warp bubble from reaching such a small size, making this form of warp drive impossible. Even worse, the form of the warp drive is effectively a bubble of spacetime connected to the greater universe through a tiny neck (the warp bubble as seen from outside). The warp bubble may form as described, only for the neck to ‘pinch off’ like a wormhole, leaving the ship stranded in the bubble with no known way of returning to the normal Universe. (ref)

Chris Weekes

Oct 01, 2001

References:

  1. Lawrence H Ford & Thomas A Romer, Negative energy, wormholes, and warp drive, Jan 2000, Scientific American, vol 282, issue 1, pgs 46-53
  2. Parsons, Paul, A warped view of time travel, Oct 1996, Science, vol 274, issue 5285, pg 202
  3. John G. Cramer, The Alternate View: The Micro-Warp Drive, Feb 2000, Analog Science Fiction and Fact, vol 120, issue 2, pgs 58-60 and 78

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