The Doctor uses his TARDIS (Time And Relative Dimension(s) In Space) to traverse the universe (2). The TARDIS achieves space travel by entering the "time vortex" which is not composed of normal space-time. Because it is not composed of the same material as the normal universe, it is not bound by the laws of physics. For example, within the time vortex, Einstein's equation relating energy and mass is E=mc³ rather than E=mc² (6). While the time vortex's ability to escape the laws of physics allows the Doctor to conveniently travel from one point in space-time to another, space travel within the usual universe is much more difficult.

Many methods of time travel can also be applied to space travel. For long distance voyages at high speeds, the effects of time dilation are particularly noticeable. For example, for a mission lasting 50 years, during which the average velocity was 99.9% the speed of light, the traveler would return to their place of origin 1120 years later from the perspective of that location. This unfortunate consequence of high speed travel means that the traveler would return to find their friends, family and potentially their civilization long gone. Travel using Kerr black holes is equally dangerous in its irreversibility. While worm holes allow for bidirectional travel, they are likely very small and prone to collapse, making them impractical as well (5). The Doctor may have the luxury of the time-vortex, however, it would appear that long distance travel constrained by the laws of physics is currently impractical.