Fabric of Space-time

Conjectures about the “fabric of space-time” and “tears in the space-time continuum” abound in science fiction movies. An entertaining 2001 film involved a protagonist who derived an equation for the time and place of a temporary tear in the fabric of space-time. Several characters jumped off the Brooklyn Bridge through the temporary space-time tear, acting as a portal to another dimension to the year 1876, and then returned through the next temporary tear by jumping off the bridge again days later. In addition, film characters have used the phrase “speed of gravity” in an ambiguous way. What can you say about the physics here?

Answer

Space-time is not a piece of cloth, nor does space-time behave like a real material. The metaphor “fabric of spacetime” allows one to visually model space-time with its coordinates of space and time in a way similar to the simpler two-dimensional construction of woven cloth. There is no way for space-time to rip or tear, although mathematically there can be singularities of various dimensions and other mathematical properties that some people have stretched into real physical properties in the name of dramatic license.

As far as being able to go back in time as a time traveler, the time dimension of space-time is not like the spatial dimensions of space-time. Mathematically, the passage of time operator in quantum mechanics is anti-unitary, while the spatial displacement operator is unitary. In addition, no one has shown convincingly that a fundamental particle can progress either forward or backward in time differently than we now experience and understand that is, time passes at its normal rate for all particles and collections of particles. One possible explanation for all particles behaving the same with regard to the passage of time relies on the direction of time being built into the quantum state definition of a fundamental particle, with the opposite direction of time for its antiparticle.

The phrase “speed of gravity” is meaningless in the simplest interpretation, being an error in stating the acceleration of gravity. Most films confuse the concepts of speed and acceleration in the same manner that most people do. Unfortunately for society, the scriptwriters did not learn their beginning physics, and we all continue to suffer from their intuitive mistakes in describing the behavior of nature. Or perhaps a more positive view should be taken in that the entertainment value is improved by ignoring the laws of physics!