Those wishing to preserve the possibility of time travel must discard any hope of traveling to the past, which we will henceforth refer to as “past-travel,” due to the paradoxical problems involved in the journey. Of the many paradoxes that plague theories of past-travel, the double occupation paradox proves most severe, not only due to its underlying physics violations, but also the fact that those who argue against the paradox have failed to acknowledge its severity. The paradox holds that a time machine cannot past-travel because it would collide with its past self, as no two things can inhabit the same place at the same time. Some have argued that the machine should move physically on a 3D plane, but this has been refuted on matters pertaining to personal identity. However, even if we accept discontinuous travel, neither the time traveler nor the machine can past-travel because the process would attempt to duplicate matter and energy already existing in the past, thereby violating the law of conservation and other principles of physics. Moving forward, we will examine a scenario that demonstrates the paradox’s effects.
An Einstein Problem
Suppose 1950 is the present, and physicist Albert Einstein secretly invented a time machine in 1949 shaped like a British telephone box and christened the “Time-Dial.” Whether the machine is stationary or not, as we will see shortly, is irrelevant. Presently, Einstein plans two trips into the past; his reasons for traveling are also irrelevant. The first trip is to exactly one year ago, after he first completed the Time-Dial, and the second is to 1550 A.D. We will ignore the other paradoxes involved in these voyages and focus solely on how double occupation results in neither of these trips occurring, or at least ending pleasantly for Albert.
As a scientist, Einstein should recognize the issues that arise in past-travel involving three concepts in physics. First is the conservation of matter, a law which states that matter can neither be created nor destroyed (“Conservation Law”). Second is Einstein’s own equation for mass-energy equivalence, E=MC2, demonstrating that mass is a quantitative measure of energy and vice versa (“Einstein’s Mass-Energy Relation”). Third is the second law of thermodynamics, entropy, which many consider the physical manifestation of the “arrow of time” (Nahin 158). With these principles in mind, Einstein sits down and contemplates his journey before entering the Time-Dial.
The Nature of Antimatter
While Einstein contemplates the physical concepts involved in his journey, we return to the 21st century to analyze the debate surrounding the paradox. In regards to double occupation, philosopher William Grey argues that a stationary time machine will collide with itself on the way to the past, and a moving time machine would result in identity discontinuity; thus, past-travel is impossible on both accounts (Grey). Philosopher Phil Dowe counters by asserting that particles traveling to...