Principle of Causality

The principle of causality asserts that cause always precedes effect. This principle prevents us from receiving a message before it is sent, from making a decision that affects the past, or from building a time machine to visit historical events. The principle of causality and the second law of thermodynamics are conceptually very similar. Although the principle of causality is more qualitative in nature (not quite defining what constitutes a cause and what constitutes an effect) and the second law of thermodynamics is more quantitative (declaring that the well-defined entropy can only increase), they both say that time has a directionality that allows certain processes to proceed only into the future and never into the past.

But the relativity of simultaneity requires that future and past be relative quantities. Simply by changing from one reference frame to another it is possible to change the future into the past. Therefore, if it ever becomes possible to perform a hyperspace jump to transport someone instantaneously from one point in space to another, time travel also becomes possible. In fact, one does not even have to jump instantaneously through space to perform time travel, all one has to do is be able to travel at superluminal velocities.

Of course it is obvious that superluminal velocities would allow a person to outrun light rays and look back at historical events through a high-powered telescope. For example, a person could rush ahead of the light long ago emitted from the battle of the Alamo and photograph the battle actually taking place. But this is not what real time travel means. Real time travel means that a person could actually go back in time and personally participate in the battle, possibly getting himself killed or saving the life of Davy Crocket.

Clearly such a scenario constitutes a violation of the principle of causality. But you do not have to send observers faster than light to violate causality. You can send any physical object. Or you can simply send information.

If you could send a message faster than light, you could send it through time, future or past. Therefore, you could receive a message before it was sent, a clear violation of the principle of causality.

Because most people believe in the principle of causality and most scientists believe in the second law of thermodynamics, the theory of relativity asserts that the speed of light is the ultimate speed. Nothing – no person, no object, no information – can travel faster that the speed of light.

Questions:

Q1. Why does special relativity assert that no physical object can travel faster than the speed of light? (A) The energy required to “bust the light barrier” would be infinite. (B) The principle of causality would be violated. (C) Time and space would be interchanged. (D) Two of these. (E) Three of these.
E 515321

1. Time Travel – How hyperspace jumps would make time travel possible.

2. Superluminal Velocities – How superluminal velocities violate causality.

ModPhy1/Unit1/SpecialRelativity/RelativeView/Time/Simultaneity/