Based on Einstein’s special theory of relativity, if we are able to move information or matter from one point to another faster than the speed of light, there would be some inertial frame of reference (i.e., a frame of reference moving at a constant velocity) in which the signal or object is moving backward in time. Let us understand why this is the case.

Consider sending a signal from one location to another. The first event is sending the signal. The second event is receiving the signal. As long as the signal travels at or below the speed of light, according to the “relativity of simultaneity,” the first event will always precede the second event in all inertial frames of reference. Although this squares with our everyday observation of reality, that cause precedes effect, you may have a question. What is the relativity of simultaneity?

The relativity of simultaneity is a concept introduced by Einstein in the special theory of relativity. The simultaneity of an event is not an absolute to all observers, but depends on the observer’s frame of reference. For example, if one observer is midway on a train car, and a second observer is at rest on the platform at the train station, they will see the simultaneity of an event differently. As the two observers pass, assume the observer in the train takes a picture using a flashbulb. From the viewpoint of the observer within the train, the light reaches both the front and rear of the train car at the same time. However, the observer on the platform sees a different situation. From the observer on the platform’s viewpoint, first the flashbulb goes off, and then the light reaches the back of the train car, since it was moving toward the fixed observer on the platform. Lastly, the observer sees the light reach the front of the train car, since it was moving away from the observer. The effect is more pronounced as the speed of the train approaches the speed of light.

Based on the relativity of simultaneity, if a signal propagates faster than the speed of light, there would always be some frames of reference where the signal arrives before it was sent. To illustrate this, let us go back to the above example and assume the train is traveling close to the speed of light. The observer is now closer to the end of the train car when the flashbulb flashes. Let us also assume the light exceeds the speed of light in a vacuum. For example, we could assume the interior of the train car contains a negative energy vacuum, which some in the scientific community believe would allow light to travel faster than it would in a normal positive vacuum. Given these two inertial frames of reference, the train moving close to the speed of light, and the observer situated closer to the rear of the train car when the flashbulb goes off, it would appear that the light reached the end of the train car prior to the light from the flashbulb reaching the observer on the platform. Why is this? (You might want to draw this out on a piece of paper to visualize the light paths.) The light inside the train instantaneously reaches the back of the train car, and then travels a short distance in the inertial frame of the observer, who records the event. This is witnessed ahead of the light reaching the observer from the source, since now the observer is farther away from the source. Therefore, the observer first witnesses the light reach the back of the train, and then observes the light from the source (i.e., flashbulb goes off). From the viewpoint of the observer at the station, the effect preceded the cause. If the light within the train did not travel faster than the speed of light in a vacuum, the effect of reverse causality would be lost.

From inside the train car, nothing changes for the observer seated midway in the car. The faster-than-light signals reach the front and back at the same time. In summary, the observer on the platform witnesses reverse causality. The light signal reaches the back of the train car before the light from the flashbulb reaches the observer on the platform. This thought experiment, illustrating reverse causality, suggests the observer on the platform witnesses an event taking place in the past (i.e., light reaching the end of the train car), since the flashbulb light at the source will reach the observer on the platform later (i.e., the future).

Does anything travel faster than the speed of light in the real world? Maybe! Some quantum physicists believe the phenomena of quantum entanglement (i.e., two particles that have interacted to the point that the physical state of one particle is dependent on the other) exhibits effects that travel faster than the speed of light. However, this is controversial, and more data is required to make an irrefutable case that this is true.

This post was based on material taken from my new book, How to Time Travel. It is available from Amazon in a paperback or Kindle editions. Click How to Time Travel to browse the book free.